Modular pet detector system for radiotherapy systems

Abstract

Disclosed herein are variations of a modular positron emission tomography (PET) detector system. In some variations, the modular PET detector system is part of a radiotherapy system. The modular PET detector system comprises a rotatable ring and a PET cassette reversibly coupled to the rotatable ring. The rotatable ring may have a cooling circuit comprising a cooling conduit having a fluid therein. The PET cassette may be reversibly coupled to the rotatable ring such that the PET cassette is in thermal contact with the cooling circuit. The thermal contact or interface between the PET cassette and the cooling circuit promotes the transfer of heat without transferring any fluid between the cooling circuit and the PET cassette. The circuit board of the PET cassette may be aligned perpendicularly with respect to a photon detection surface, which may reduce forces on the cassette when the PET cassette is coupled to the rotatable ring.

Description

MODULAR PET DETECTOR SYSTEM FOR RADIOTHERAPY SYSTEMSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 511,098 filed June 29, 2023, the disclosure of which is hereby incorporated by reference in its entirety.BACKGROUND

[0002] A positron emission tomography (PET) system comprises an array of detectors and supporting electronics that are mounted on a gantry, typically a circular gantry. The gantry may be stationary, as is in the case of most diagnostic PET imaging systems, or may be rotatable, as in the case of a radiotherapy system for biology -guided radiotherapy (BgRT).

[0003] Repairing, replacing, and otherwise servicing a PET system and its individual components can be cumbersome and challenging. For example, the weight of a PET detector arc that has an angular span of 90° may be more than 200 pounds. In some cases, servicing a single detector may require the removal of the entire PET detector arc from the gantry, which may be bulky and difficult to maneuver, and may involve the disconnection and re-connection of hundreds of electrical wires, buses, and tubing for cooling fluid(s). Accordingly, improved PET detector systems are desirable.SUMMARY

[0004] Disclosed herein are variations of a modular positron emission tomography (PET) detector system. The modular PET detector system comprises a rotatable ring and a PET cassette reversibly coupled to the rotatable ring. In some variations, the rotatable ring may be configured to rotate at a rate of about 60 RPM or more. A cooling circuit may be mounted onto the rotatable ring and may comprise a cooling conduit having a fluid therein. The PET cassette may comprise at least one photon detection module and may be reversibly coupled to the rotatable ring such that the PET cassette is in thermal contact with the cooling circuit. In one variation, the thermal contact or interface between the PET cassette and the cooling circuit may promote the transfer of heat without transferring any fluid between the cooling circuit and the PET cassette. The circuit board connected to the photon detection module may be aligned perpendicularly with respect to a photon detection surface, such that deflection forces on the cassette are reduced when the PETcassette is coupled to the rotatable ring. This may help reduce or eliminate the risk of the circuit board becoming dislodged and / or damaged during high-speed rotation of the ring. The modular PET detector systems described herein may be included as a sub-system of a radiotherapy system that comprises a therapeutic radiation source.

[0005] Described herein are variations of a modular PET detector system. In one variation, a modular PET detector system may comprise a rotatable gantry, where the rotatable gantry has a cooling circuit mounted thereon, and where the cooling circuit comprises a cooling conduit having a fluid therein, and a PET cassette reversibly coupled to the gantry and in thermal contact with the cooling circuit without transfer of the fluid between the cooling conduit and PET cassette. The PET cassette may comprise a cooling bar and at least one photon detection module mounted on the cooling bar. The cooling bar may comprise tungsten. The cooling circuit may further comprise a plurality of heat sinks. The PET cassette may further comprise a radiation shield, and optionally, the cooling bar may also function as a radiation shield. In some variations, the PET cassette may further comprise at least one alignment pin for engagement with a socket on the rotatable gantry. The modular PET detection system may comprise at least two PET cassettes, for example, three PET cassettes, five PET cassettes, seven PET cassettes or more. The rotatable gantry may be configured to rotate at least at 60 RPM. In some variations, the modular PET detector system may comprise a therapeutic radiation source. The therapeutic radiation source may comprise a linear accelerator (linac) and a magnetron, and optionally, a multi-leaf collimator. In some variations, the cooling conduit may comprise a first segment and a second segment parallel to the first segment, and the first segment may be configured to cool the at least one photon detection module.

[0006] One variation of a modular PET detector system may comprise a rotatable gantry configured to rotate at least at 60 RPM, and a PET cassette reversibly coupled to the rotatable gantry. The PET cassette may comprise a photon detection surface and at least one photon detection module having a printed circuit board that is aligned perpendicularly with respect to the photon detection surface, such that forces on the cassette are reduced when the PET cassette is coupled to the gantry. The alignment of the printed circuit board perpendicularly with respect to the photon detection surface may help further reduce radiation damage on the PET cassette when the PET cassette is coupled to the gantry. In some variations, the modular PET detector system may comprise a therapeutic radiation source. The therapeutic radiation source maycomprise a linear accelerator (linac) and a magnetron, and optionally, a multi-leaf collimator. For example, the modular PET detector system may be part of a radiotherapy system. In some variations, the PET cassette may further comprise at least one alignment pin for engagement with a socket on the rotatable gantry. The modular PET detection system may comprise at least two PET cassettes, for example, three PET cassettes, five PET cassettes, seven PET cassettes or more. The rotatable gantry may comprise a cooling circuit mounted thereon, and the cooling circuit may comprise a cooling conduit having a fluid therein.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 A is a block diagram of a variation of a radiotherapy system.

[0008] FIG. IB is a perspective schematic view of one variation of a radiotherapy system.

[0009] FIG. 1C is a perspective component view of one variation of a radiotherapy system.

[0010] FIG. 2A is a perspective view of one variation of a rotatable gantry ring and a PET detector arc comprising a plurality of PET cassettes.

[0011] FIG. 2B is a component view of a section of the gantry ring of FIG. 2A.

[0012] FIG. 2C is a component view of another variation of a section of the gantry ring ofFIG. 2A, with an inset of the region enclosed by dashed lines.

[0013] FIG. 2D is a component view of another section of the gantry ring of FIG. 2A corresponding to the variation depicted in FIG. 2B.

[0014] FIG. 2E is a component view of another variation of a section of the gantry ring of FIG. 2A corresponding to the variation depicted in FIG. 2C.

[0015] FIG. 2F is a schematic, conceptual front view of one variation of a gantry ring.

[0016] FIG. 3 A is a perspective view of one variation of a PET cassette.

[0017] FIG. 3B is an exploded perspective view of one variation of a PET cassette.

[0018] FIG. 3C is another exploded perspective view of one variation of a PET cassette.

[0019] FIG. 3D is a perspective view of a subset of the internal assemblies of one variation of a PET cassette.

[0020] FIG. 3E depicts an elevational view of one variation of a cooling bar.

[0021] FIG. 3F depicts a close-up cross-sectional perspective view of one variation of a cable management assembly of a PET cassette.

[0022] FIG. 3G is an elevational view of one variation of an EMI housing for any of the PET cassettes described herein.

[0023] FIG. 3H is an elevational view of another variation of an EMI housing for any of the PET cassettes described herein.

[0024] FIG. 4 is a conceptual schematic of one variation of a gantry cooling conduit or circuit.DETAILED DESCRIPTION

[0025] Described herein is a modular PET detector system that detects positron emission paths emitted by a PET tracer that is localized to the tumor(s) during a radiation delivery session (e.g., a treatment session). Timely detection of positron emission paths that originate within the tumor may provide real-time location data, which may be used to guide the delivery of therapeutic radiation. The modular PET detector system may comprise a stationary frame and a rotatable ring, and one or more PET cassettes, which may be reversibly coupled to the rotatable ring. The modular PET detector system disclosed herein may have an improved cooling circuit that facilitates the reversible coupling and decoupling of the PET cassette without any fluid transfer. To achieve timely detection and reduce latency, the rotatable ring may rotate at a rate of at least 60 RPM. Thus, it is desirable to improve the stability and function of the one or more PET cassettes under high-speed rotation and in the presence of high-energy radiation, as well as facilitate the serviceability of the PET detector system.

[0026] The modular PET detector systems described herein may be used with various radiotherapy systems, for example, any radiotherapy system comprising a therapeutic radiation source configured to emit high-energy photons and a radiation detector (also referred to as a radiation imager) located opposite the therapeutic radiation source. FIG. 1 A depicts a functional block diagram of a variation of a radiotherapy system that may be used with one or more of themethods described herein. Radiotherapy system 100 comprises one or more therapeutic radiation sources 102 and a patient platform 104. The therapeutic radiation source may comprise an X-ray source, electron source, proton source, a neutron source, and / or any suitable particles including carbon atoms. For example, a therapeutic radiation source 102 may comprise a linear accelerator (linac), Cobalt-60 source, and / or an X-ray machine. The therapeutic radiation source may be movable about the patient platform so that radiation beams may be directed to a patient on the patient platform from multiple firing positions and / or angles. In some variations, a radiotherapy system may comprise one or more beam-shaping elements and / or assemblies 106 that may be located in the beam path of the therapeutic radiation source. For example, a radiotherapy system may comprise a linac 102 and a beam-shaping assembly (106 disposed in a path of the linac radiation beam. The beam-shaping assembly may comprise one or more movable jaws and one or more collimators. At least one of the collimators may be a multi-leaf collimator (e.g., a binary multi-leaf collimator, a 2-D multi-leaf collimator, etc.), which may comprise a plurality of independently movable leaves. The linac and the beam-shaping assembly may be mounted on a gantry or movable support frame that comprises a motion system configured to adjust the position of the linac and the beam-shaping assembly. In some variations, the linac and beamshaping assembly may be mounted on a support structure comprising one or more robotic arms, C-arms, gimbals, and the like. The patient platform 104 may also be movable. For example, the patient platform 104 may be configured to translate a patient linearly along a single axis of motion (e.g., along the IEC-Y axis), and / or may be configured to move the patient along multiple axes of motion (e.g., 2 or more degrees of freedom, 3 or more degrees of freedom, 4 or more degrees of freedom, 5 or more degrees of freedom, etc.). In some variations, a radiotherapy system may have a 5-DOF patient platform or a 6-DOF patient platform that is configured to move along the IEC-Y axis, the IEC-X axis, the IEC-Z axis, as well as pitch, yaw, and / or roll.

[0027] In the variation shown in FIG. 1 A, the radiotherapy system 100 also comprises a controller 101 that is in communication with the therapeutic radiation source 102, beam-shaping elements or assemblies 106, patient platform 104, and one or more image sensors 108 (e.g., one or more imaging systems, such as the modular PET detector system described herein). The controller 101 may comprise one or more processors and one or more machine-readable memories in communication with the one or more processors, which may be configured to execute or perform any of the methods described herein. The one or more machine-readable memories may store instructions to cause the processor to execute modules, processes and / orfunctions associated with the system, such as one or more treatment plans, the calculation of radiation fluence maps based on treatment plan and / or clinical goals, segmentation of fluence maps into radiotherapy system instructions (e.g., that may direct the operation of the gantry, therapeutic radiation source, beam-shaping assembly, patient platform, and / or any other components of a radiotherapy system), and image and / or data processing associated with treatment planning and / or radiation delivery. In some variations, the memory may store treatment plan data (e.g., treatment plan firing filters which may be transform functions that convert imaging data into fluence maps, fluence maps, beamlet sequence, planning images, treatment session PET pre-scan images and / or initial CT, MRI, and / or X-ray images), and instructions for delivering the derived fluence map (e.g., instructions for operating the therapeutic radiation source, beam-shaping assembly and patient platform in concert). In some variations, the controller 101 may comprise a processor for each of the therapeutic radiation source, beam-shaping elements, patient platform, and / or image sensors, with corresponding machine-readable memories. For example, in one variation, there may be an MLC processor and corresponding machine readable memory that generates and / or stores MLC instructions that specify the position, motion, location, timing (e.g., transition or travel time, dwell time, etc.) of each of the leaves of the MLC. The MLC instructions may be generated by a treatment planning system and stored in the memory associated with the MLC processor. Alternatively, or additionally, the MLC instructions may be generated by the MLC processor (e.g., in real-time) by the MLC processor, based on updated fluence maps and / or fluence maps generated based on imaging data (e.g., biology-guided radiotherapy which generates fluence maps from newly- acquired biologically-related imaging data, such as PET data). The controller of a radiotherapy system may be connected to other systems by wired or wireless communication channels. For example, the radiotherapy system controller may be in wired or wireless communication with a radiotherapy treatment planning system controller such that fluence maps, firing filters, initial and / or planning images (e.g., CT images including cone beam CT images and fan beam CT images, MRI images, PET images, 4-D CT images), imaging data, patient data, and other clinically-relevant information may be transferred from the radiotherapy treatment planning system to the radiotherapy system. The delivered radiation fluence, any dose calculations, and any clinically-relevant information and / or data acquired during the treatment session may be transferred from the radiotherapy system to the radiotherapy treatment planning system. Thisinformation may be used by the radiotherapy treatment planning system for adapting the treatment plan and / or adjusting delivery of radiation for a successive treatment session.

[0028] FIG. IB depicts one variation of a radiotherapy system 100. Radiotherapy system 100 may include a gantry 120 rotatable about a patient treatment region 112, one or more PET detectors 108a, 108b mounted on the gantry, a therapeutic radiation source 102 mounted on the gantry, a beam-shaping module 106 disposed in the beam path of the therapeutic radiation source, and a patient platform 104 movable within the patient treatment region 112. In some variations, the gantry 120 may be a continuously-rotatable gantry (e.g., able to rotate through 360° and / or in arcs with an angular spread of less than about 360°). The gantry 120 may be configured to rotate from about 20 RPM to about 70 RPM about the patient treatment region 112. For example, the gantry 120 may be configured to rotate at about 60 RPM. The gantry may also be configured to rotate at a slower rate, e.g., 20 RPM or less, 10 RPM or less, 1 RPM or less. The one or more PET detectors 108a, 108b may be time-of-flight (TOF) PET detectors that measure the arrival times of positron annihilation photons at the PET detectors with high precision (e.g., high temporal sensitivity). This precise temporal data may be used by the controller to calculate an estimated location where the positron annihilation event occurred, which may help determine the location of a target region (e.g., a cancer lesion or tumor that has taken up a PET tracer). The beam-shaping module 106 may include a movable jaw and a dynamic multi-leaf collimator (MLC) having one or more independently movable leaves. The beam-shaping module may be arranged to provide variable collimation width in the longitudinal direction of 1 cm, 2 cm, or 3 cm at the system iso-center (e.g., the rotation center of the system and on the central treatment or imaging plane, the center of a patient treatment area). The jaw may be located between the therapeutic radiation source and the MLC or may be located below the MLC. Alternatively, the beam-shaping module may include a split jaw where a first portion of the jaw is located between the therapeutic radiation source and the MLC, and a second portion of the jaw is located below the MLC and coupled to the first portion of the jaw such that both portions move together. The therapeutic radiation source 102 may be configured to emit radiation at predetermined firing positions (e.g., firing angles 0° / 360° to 359°) about the patient treatment region 112. For example, in a system with a continuously-rotatable gantry, there may be from about 50 to about 100 firing positions (e.g., 50 firing positions, 60 firing positions, 80 firing positions, 90 firing positions, 100 firing positions, etc.) at various angular positions (e.g., firing angles) along a circle circumscribed by the therapeutic radiation source as it rotates. Thefiring positions may be evenly distributed such that the angular displacement between each firing position is the same.

[0029] FIG. 1C depicts a perspective component view of the radiotherapy system 100. As shown there, the beam-shaping module may further include a primary collimator or jaw 107 disposed above a binary MLC 122 (partially obscured). A binary MLC is a multi-leaf collimator where each leaf may be individually and independently retained at, and moved between, an open position and a closed position. The radiotherapy system may also include an MV X-ray detector 103 located opposite the therapeutic radiation source 102. The rotatable ring may be coupled to a stationary frame 110. Optionally, the imaging system of the radiotherapy system 100 may further include a kV CT imaging system on a ring 111 that is attached to the rotatable gantry such that rotating the gantry also rotates the ring 111. The kV CT imaging system may include a kV X-ray source 109 and an X-ray detector located across from the X-ray source 109. The therapeutic radiation source or linac 102 and the PET detectors 108a (not depicted; located opposite 108b) and 108b may be mounted on the same cross-sectional plane of the gantry (i.e., PET detectors are co-planar with a treatment plane defined by the linac and the beam-shaping module), while the kV CT scanner and ring may be mounted on a different cross-sectional plane (i.e., not co-planar with the treatment plane). The radiotherapy system 100 of FIGS. IB and 1C may have a first imaging system that includes the kV CT imaging system and a second imaging system that includes the PET detectors. Optionally, a third imaging system may include the MV X-ray source (e.g., therapeutic radiation source, linac) and MV detector. The imaging data acquired by one or more of these imaging systems may include X-ray and / or PET imaging data, and the radiotherapy system controller may be configured to store the acquired imaging data and calculate a radiation delivery fluence using the imaging data, for example, in a biology-guided radiotherapy (BgRT) session. In some variations, the acquired imaging data, such as the acquired PET imaging data, may be used to calculate the location(s) of one or more target regions, and the controller may use this location information to control the delivery of the therapeutic radiation to more precisely irradiate the one or more target regions. The PET detectors may be time-of-flight (TOF) PET detectors, and the acquired PET imaging data may be used to directly calculate the locations of positron annihilation events, which may indicate the real-time location of a target region. Some variations may further include patient sensors, such as position sensors and the controller may be configured to receive location and / or motion data from the position sensor and incorporate this data with the imaging data to calculate a radiationdelivery fluence and / or adjust radiation delivery parameters or instructions. For example, data from one or more patient sensors and / or on-board imaging systems may be used to determine the location(s), or a range of likely location(s), of one or more target regions. The location(s) and / or range of likely location(s) may be used to adjust radiation delivery during the treatment session to guide the therapeutic radiation to the real-time location of one or more target regions. Additional descriptions of radiotherapy systems that may be used with any of the systems or methods described herein are provided in U.S. Pat. No. 10,695,586, filed November 15, 2017.Gantry

[0030] FIG. 2A depicts one variation of a rotatable ring of a radiotherapy system gantry. The ring 200 may be a drum (e.g., a shortened cylinder) having a first end surface 220, a second end surface 222, and a length 224 therebetween. The opening 226 of the ring 200 may be referred to as a bore. The bore may be the space through which the patient platform is positioned and moved. The length 224 may be the thickness of the drum, which may generally correspond to the length of the bore. One or more of the components of the radiotherapy system described above may be mounted on the drum, along the length 224 within the bore 226. For example, first and second PET detector arcs (only one PET detector arc 201 is depicted in FIG. 2A), each comprising a plurality of modular PET cassettes, may be mounted on the ring 200 within the bore 226. The ring 200 may be configured to rotate about a central axis 211, which may also be referred to as the axis of rotation. The modular PET cassettes of each PET detector arc may be radially mounted on the ring (i.e., along arrow 213). The radial mounting of the PET cassettes may help to alleviate the mechanical stresses on the PET cassettes as the ring rotates. In some variations, the attachment or mounting mechanisms between the PET cassettes and the rotatable ring may be configured such that the forces generated during rotation (e.g., centripetal and / or centrifugal forces) may act to further secure the PET cassettes to the ring. For example, the attachment or mounting mechanisms may comprise screws that are oriented radially outward so that during gantry rotation, the centripetal and / or centrifugal forces may act to press the screws into the ring, which may secure the PET cassette more securely to the ring.

[0031] While the variation shown in FIG. 2A depicts a PET detector arc 201 having 7 modular PET cassettes 210, any suitable number of modular PET cassettes may be used depending on the configuration and size of the system. For example, a PET detector arc may have a single modular PET cassette, may have 2, 3, 4, 5, 6, 7, 8, or even more PET cassettes. In variations inwhich two PET detector arcs are provided, each arc may have the same or different number of modular PET cassettes. For example, in one variation, a radiotherapy system may have two PET arcs that are mounted opposite to (i.e., across from) each other, where each arc has an angular span of approximately 90° of the gantry ring, and each PET detector arc may have 7 PET cassettes (for a system total of 14 PET cassettes). Alternatively, one PET detector arc may have more or less modular PET cassettes than the other. In some variations, each PET cassette may have 4 photon detection modules comprising one or more multi-pixel photon counters or MPPCs (which will be described below).

[0032] The ring 200 may comprise a cooling circuit mounted within and along the surfaces 220, 222 and the length 224 of the gantry ring (which may be referred to as a gantry or ring). FIG. 2B depicts a section of the ring 200 configured for modular PET cassettes in which all but one of the modular PET cassettes have been removed, which reveals one variation of a cooling circuit 202. FIG. 2C depicts a section of the ring 200 having another variation of a cooling circuit 202. The cooling circuit 202 may comprise a cooling conduit 203 having a fluid within the conduit. The cooling conduit may comprise, for example, tubing or piping. In some variations, the tubing or piping may be flexible or inflexible and may comprise polymeric (e.g., plastic) and / or metal material(s). In some variations, the cooling conduit may comprise a channel that is located along or within a portion (e.g., a wall, length, surface) of the rotatable ring. In one variation, the cooling conduit 203 may span the length of a PET detector arc 201. Each PET detector arc may have its own corresponding cooling circuit, which may or may not be fluidly connected to the cooling circuit of the other PET detector arcs. The cooling conduit 203 may span any suitable length and be placed in any suitable location that allows for cooling of the PET cassettes. For example, the cooling conduit may span at least a portion of the circumference of the ring (e.g., inner circumference along the bore 226, outer circumference, or any location between the inner and outer circumference) of the rotatable ring 200.

[0033] The cooling circuit 202 may further comprise a plurality of heat sinks 204. The heat sinks 204 may be in thermal contact with the cooling conduit 203 and mounted on or to the rotatable ring 200. In some variations, the heat sinks are in fluid communication with the cooling conduit(s) while in other variations, the heat sinks and cooling conduit(s) are not in fluid communication but rather, transfer heat by direct surface contact and / or air transfer. The heat sinks may offer a larger surface area than the cooling conduit to help promote efficient heatexchange with PET cassettes mounted on the rotatable ring 200. The cooling circuits corresponding to each PET detector arc may each have their own heat sinks or may have the same heat sinks. In some variations, the heat sinks 204 may have wedge interfaces for reversible thermal coupling with a PET cassette 205. The heat sinks may be any suitable shape and size. The shapes may be geometric or non-geometric and be regular or irregular. Examples of suitable shapes include, but are not limited to, circular, square, trapezoidal, rectangular, triangular, or fin- shaped. Alternatively, the heat sink may comprise a matrix or array of smaller geometric shapes, such as plates, square pins, or round pins. In some variations, the heat sinks 204 may have an undulating and / or textured surface that may help increase the surface area contact with a PET cassette. For example, the surface of the heat sink that interfaces with a PET cassette may have a plurality of protrusions and / or concavities that may interleave with a corresponding plurality of concavities and / or protrusions on the structures providing more surface area contact between the PET cassette cooling bars and the heat sinks. The PET cassette 205 may be reversibly coupled to the rotatable ring 200 and in thermal contact with the cooling circuit 202 without transfer of the fluid between the cooling conduit 202 and the PET cassette 205.

[0034] In some variations, the heat sinks may be slidable or movable along the cooling conduit and / or walls (e.g., walls 220, 222) of the gantry ring so that their position may be adjusted as desired to accommodate one or more PET cassettes, and then the heat sinks may be secured (i.e., no longer slidable or movable) when the desired position and / or engagement with the PET cassette is attained. The adjustability (e.g., movability, slidability) of the heat sinks may facilitate the installation of a PET cassette onto the gantry and help to ensure that there is a desired level of engagement or contact between the PET cassette and the gantry ring and / or heat sinks. In some variations, there may be grooves, rails, protrusions, and / or ridges along the cooling conduit and / or along the surfaces of the gantry ring to guide the positioning of the heat sinks, which may help attain a general alignment with PET cassettes while maintaining thermal engagement between the heat sink(s) and the cooling conduit. After the slidable heat sinks are in their desired location, they may be secured using any suitable mechanism, including, for example, one or more of the following: screws (e.g., drive screws), nails, brackets, pins, locks, braces, tabs, and / or wedges. Slidable heat sinks may facilitate PET cassette installation, for example, by allowing the heat sinks to slide or shift position when the PET cassette is initially inserted and adjusting their position along the PET cassette to attain a desired engagement between the gantry ring and the PET cassette is attained and / or a desired level of contactbetween the PET cassette and the heat sinks. Then the location(s) of the heat sink(s) may be secured using any of the mechanisms described above, which may help retain the PET cassette(s) in the gantry ring. In some variations, if the contact between the PET cassette and the heat sink(s) becomes too loose, the securing mechanism may be tightened or reinforced to increase the contact between the PET cassette and the heat sink(s).

[0035] One variation of a system comprising adjustable or slidable heat sinks is depicted in FIG. 2C. In this variation, heat sinks 204 may comprise slidable wedges and a plurality of heat sink attachment mechanism such as drive screws 240 that adjustably couple the heat sinks 204 to a wall of the gantry ring, while still contacting the cooling conduit(s) of the gantry ring. In this variation, there are one or more drive screws 240 connected to each heat sink 204. Larger heat sinks may be connected with a plurality of drive screws 240, while smaller heat sinks may be connected with a single drive screw 240. In some variations, the drive screws 240 may be mounted to a base plate 241 that is affixed to the wall of the gantry ring. The heat sinks 204 may have one or more openings to accommodate the drive screw(s) 240, and may optionally have one or more protrusions or grooves that correspond with grooves or protrusions (respectively) along the cooling conduit and / or on the wall of the gantry ring. In one variation, the drive screws 240 are initially loosened before placement of a PET cassette 205 into the gantry ring. After the PET cassette is placed or installed into the gantry ring, the slidable heat sinks 204 may be drawn up through the use of the drive screws 240 to establish thermal contact between PET cassette 205, the heatsinks 204, and the cooling conduit 202. In some variations, the slidable heatsinks 204 may be guided by a tongue and groove interface between the heatsink 204 and the cooling conduit 202.

[0036] In some variations, the cooling conduit 203 may further comprise two segments, which may be parallel or in series to each other. Each segment of the cooling conduit 203 may span the length of an arc 201. Fluid within the first cooling conduit segment 206 may transfer heat from the heat sinks that are connected to the first conduit segment 206, and enter the second cooling conduit segment 207 and transfer heat from the heat sinks that are connected to the second conduit segment 207. The heat transferred into the cooling fluid from the PET cassettes via the heat sinks may be dissipated by the air circulated between the rotatable ring and the stationary frame. In the variations depicted in FIGS. 2B and 2C, air circulation may be improved with one or more fans 208 on the rotatable ring. In variations in which fans are used, any number of fansmay be used as desirable. Alternatively, or additionally, the cooling fluid may flow into a reservoir of cooled fluid and / or may be routed through a chiller.

[0037] As depicted in FIGS. 2D and 2E (where FIG. 2D corresponds with the variation depicted in FIG. 2B and FIG. 2E corresponds with the slidable heat sink variation depicted in FIG. 2C), the rotatable ring 200 may further comprise mechanical and electrical interfaces for coupling with one or more PET cassettes, which may be used alone or in combination with the coupling interfaces described above. For example, a mechanical interface may comprise at least one alignment pin socket 209 on the outer portion of the rotatable ring 200. Alternatively, or additionally, the mechanical interface may comprise any number and any type of fasteners, such as snap hooks, buckles, or clips, and may include, alone or in combination, any of the attachment or securing mechanisms described herein. The mechanical interface may have any suitable geometry that is configured to mate with a corresponding interface on the PET cassette, such as grooves, protrusions, slots, and the like. In this example, the mechanical interface may comprise cassette alignment pin socket 230 and one or more screw openings 232 on a base plate 241 fixed to the gantry ring. The mechanical interface may facilitate alignment and engagement of a PET cassette. The gantry ring may comprise a pair of mechanical interfaces (i.e., an alignment pin socket and one or more screw openings) to attach the ends of each of the plurality of PET cassettes to the gantry ring. A mechanical interface having pin socket(s) and screw opening(s) is configured to receive the pin(s) and screw(s) in an outward radial orientation, which may help further secure mechanical engagement of the PET cassette to the gantry ring while the gantry ring is rotating. An electrical interface may comprise a commercial ODU connector receiver (not depicted in the drawing). The electrical interface may include electrical sockets, wires, electrical ribbon cable or buses (i.e., groups of wires) that transfer PET data from the PET cassettes to the controller of the system (e.g., controller 101) and / or transfer control signals between the PET cassettes and the system controller. The electrical communications may be unidirectional or bidirectional.

[0038] As described above, one or more of the attachment mechanisms described herein may be fortified and / or strengthened by the centripetal / centrifugal forces generated during rotation. This may help lower the disengagement likelihood of the PET cassettes from the gantry while the system is in use (i.e., while the gantry is rotating). For example, FIG. 2F is a schematic view of a front view of the gantry ring 200, a few example PET cassettes 205 (e.g., of a PET detectorarc 201 that comprises a plurality of PET cassettes) coupled to the gantry ring. The arrow 213 represents the radial direction, which in this view may be along the z-axis, relative to the x-axis and the y-axis (which is into, or perpendicular to, the plane of the page.) The PET cassette 205 may slide radially into and out of the gantry ring 200. When the PET cassette is coupled to the gantry ring using any of the mechanical interfaces described herein, its radial orientation (i.e., along arrow 213 in this view, but more generally, in a direction toward the center of the gantry ring on the axis of rotation) aligns with the centripetal / centrifugal force generated by rotation of the gantry ring 200. Consequently, the PET cassette-to-ring engagement may be further secured when the centripetal / centrifugal force presses the PET cassette against the mechanical interface of the rotatable ring. In the variations of FIGS. 2D and 2E, the alignment sockets and screw openings are configured to receive pins and screws orientated such that the tips of the pins and screws are pointing radially outward (i.e., away from the center of the gantry ring).

[0039] Optionally, some interfaces between the PET cassettes and the gantry may comprise one or more sensors that indicate whether there are any changes in the engagement strength between the PET cassettes and the gantry. For example, there may be pressure sensors that detect the force with which the PET cassettes are pressing against the gantry, and if there are unexpected changes or larger-than-acceptable deviations from the acceptable pressure, the system may generate a notification that informs the user of these pressure changes and / or which PET cassette(s) are experiences these pressure changes.

[0040] FIGS. 3A-3F depict one variation of a PET cassette 300. Starting with FIG. 3A, one variation of a PET cassette 300 is shown comprising an electromagnetic interference (“EMI”) housing 304, a cover 317 disposed over a photon detection surface, and an electrical and mechanical interface 314 configured to engage and connect with a corresponding interface on the rotatable ring of a gantry. The photon detection surface, when coupled to the gantry ring, may face the inner portion of the rotatable ring (for example, as depicted in FIG. 2A, the patient treatment region 112). FIG. 3B depicts an exploded perspective view of the PET cassette 300, in a rotated view from FIG. 3 A (i.e., rotated approximately 180°), without the housing 304. As shown there, a PET cassette 300 may comprise a plurality of photon detection modules 302. In this variation, the PET cassette has 4 photon detection modules, though any suitable number of photon detection modules may be used. The photon detection surface of the PET cassette (which is beneath cover 317) may comprise the photon detection surfaces of each of the photondetection modules. The PET cassette 300 may also comprise a cooling assembly that comprises one or more cooling bars 303. The cooling bars 303 may be arranged in positions to correspond with the position of the heat sinks of the cooling circuits on the gantry ring. There may be multiple cooling bars 303 that are configured to cool different PET cassette components and may be thermally independent from each other. Cooling bars 303 may be sized and shaped to correspond with the size and shape of the gantry heat sinks such that they may contact each other for the transfer of heat from the PET cassette to the cooling circuit that is located on the gantry ring (also referred to as a gantry ring cooling circuit). For example, the heat sinks may have a wedge shape with an angled surface and the cooling bars 303 may have a corresponding wedge shape with a complementary angle. The cooling bar(s) may have a wedge interface to improve contact with the heat sinks on the gantry. Alternatively, or additionally, the heat sinks may have a concave or convex surface, and / or an opening or a protrusion, and correspondingly, the cooling bars may have a convex or concave surface, and / or a protrusion or an opening. Variations and details of the cooling bar(s) of a PET cassette are described further below. The electrical and mechanical interface 314 may comprise one or more alignment pins and one or more electrical connectors. The alignment pins may mechanically engage with corresponding recesses in the gantry ring, and the electrical connectors may comprise ports and / or plugs that electrically communicate with corresponding ports and / or plugs of the gantry ring.

[0041] FIG. 3C depicts an exploded perspective view of the PET cassette 300. FIGS. 3B and 3C are inverted from the orientation of FIG. 3A (i.e., the photon sensing surface is at the bottom in FIGS. 3B-3C), with the cover 317 and EMI housing 314 removed. A photon detection module 302 may further comprise one or more photon detectors 308 (within a housing of the photon detection module) and at least one circuit board 306 connected to the photon detection modules 302 for processing the data acquired by the multi-pixel photon counters or MPPCs of the photon detection modules. In this variation, each photon detection module 302 may be connected to two circuit boards 306, i.e., a circuit board pair. As depicted in FIG. 3C, the photon detection module 302 and the circuit board 306 may be connected to each other with at least one ribbon cable 307. The photon detection module may comprise a MPPC 308 that detects incident photons and an optical detector board 309 that processes and outputs electrical signals from the MPPC 308. The circuit board 306 may comprise a processor that analyzes the photon data and machine-readable memory that stores the photon data. In some variations, the processor and other electronic components on the circuit board may be configured to acquire, process, and store time-of-flight(TOF) PET detector data, such as arrival times of positron annihilation photons at the MPPCs. Photon data that includes arrival time, location (e.g., the specific MPPC pixel, specific photon detection module, photon detection module gantry location or angle, etc.) may be used to calculate an estimated location of where a positron annihilation event occurred, which may help more precisely locate a target region with the patient. The example herein depicts a printed circuit board, but it should be understood that in other variations, the circuit board 306 may be a breadboard, a stripboard, a system-on-a-chip, or any board that mechanically supports and electrically connects electronic components. The circuit board(s) 306 may be mounted such that they are perpendicular to the photon detection surface 301. This vertical arrangement of the circuit board(s) may help reduce the proportion of the board(s) that is exposed to radiation, which may help reduce radiation damage to the components mounted on the board(s).

[0042] The PET cassette may comprise a cooling assembly having one or more cooling bars that may thermally contact the various components within the cassette. The cooling bars may be arranged to distribute and route heat from the PET cassette components to the thermal interfaces with the cooling circuit of the gantry ring. In one variation, a PET cassette may have one or more cooling bars 310 that may be mounted to one or more photon detection modules (which may be referred to as photon detection module cooling bars), which may be arranged to cool (i.e., transfer heat from) the MPPCs of the photon detection modules. The PET cassette may further comprise one or more circuit board cooling bars 311 that may be arranged to cool (i.e., transfer heat from) the circuit boards. In some variations, the one or more circuit boards 306 may be mounted to the circuit board cooling bar 311. The cooling bars 310 and / or 311 may be made of tungsten. Alternatively, they may contain one or more elements of other high-density, radiation-shielding metals like lead, tin, antimony, or bismuth. In some variations, the photon detection module cooling bar 310 may be isolated from the circuit board cooling bar 311 and may be connected to different gantry heat sinks and gantry cooling circuit segments. That is, the photon detection module cooling bar(s) and the circuit board cooling bar(s) may be thermally isolated from each other and / or may not be connected within the PET cassette. For example, the photon detection module cooling bar(s) and the circuit board cooling bar(s) may be thermally isolated from each by an air gap provided by one or more spacers and / or foam made of heat insulating materials. This may facilitate tiered thermal regulation when the PET cassette is mounted on the gantry, as described further below.

[0043] FIG. 3G depicts one variation of an EMI housing 304 that may encase the internal components of the PET cassette 300 and protect them against electromagnetic interference. In some variations, the PET cassette may have stand-offs 305 upon which the EMI housing may be mounted to provide electrical grounding to the EMI housing. In some variations, the stand-offs may be made of copper, or any material that provides the desired conductivity between the internal components of the PET cassette and electrical ground. Conductive foam may be used in conjunction with the stand-offs, or may be used instead of the stand-offs. The EMI housing may have one or more tabs 331 for alignment and / or engagement with the internal components of the PET cassette. FIG. 3H depicts another variation of an EMI housing, where the PET cassette may have a conductive gasket 330 where the EMI housing may be mounted and grounded. In this variation, the EMI housing may not have any tabs. Turning back to FIG. 3B, the PET cassette may comprise a relay board 312 and an upper board 313, which may comprise electrical circuits to facilitate the transfer of PET cassette data to the system controller. The mechanical interface may comprise at least one alignment pin that may have a size and shape that is suitable for engaging with the alignment pin socket (e.g., socket 209 depicted in FIGS. 2D and 2E) on the rotatable ring. One or more alignment pins may extend perpendicularly from the EMI housing 304 opposite the photon detection surface 301. An electrical interface may comprise a commercial ODU connector that could engage with the commercial ODU connector receiver on the outer ring of the rotatable ring.PET Cassette Cooling Assembly

[0044] Turning to FIG. 3C, in some variations, a PET cassette cooling assembly may comprise a photon detection module cooling bar 310 that may be configured to conduct the heat generated by at least one photon detection module 302 mounted to the cooling bar 310. The photon detection module cooling bar 310 may span the length of the PET cassette 300. Any of the cooling bars described herein may be made of a material that is able to conduct heat and may optionally have radiation attenuation properties. In such configuration, the cooling bar may be used to transfer heat away from PET cassette components and also to shield the components from radiation. In some variations, the photon detection module cooling bar may be an integral element (e.g., integrally formed, a single unit), while in other variations, the photon detection module cooling bar may comprise multiple elements that fit together to form a bar or other suitable cooling structure. The cooling bar may have any suitable geometry. For example, thephoton detection module cooling bar may be a rectangular prism or hemicylinder. As depicted in FIG. 3C, the photon detection module cooling bar 310 may comprise a first elongate element having a hemicylindrical shape (e.g., having a semi-circle cross-section) and a second elongate element having a rectangular prism shape (e.g., having a rectangle-shaped cross-section). The second elongate element may have a concave surface that has a radius of curvature that approximates the radius of curvature of the first elongate element.

[0045] FIG. 3D depicts a perspective view of the assembly in FIG. 3C. As shown there, the photon detection module cooling bar 310 comprises two elongate hemicylinders 320a and 320b and three elongate elements 322a, 322b, and 322c that each have a rectangular end surface 324a, 324b, 324c, and a concave portion 326a, 326b, 326c such that when the elongate elements 322a, 322b, 322c are arranged together, the concave portions form two cylindrical grooves that are shaped to receive the two elongate hemicylinders. In this variation, as depicted in FIG. 3E, the central elongate element 322b may have two concave portions 326b (left and right). As described above, the cooling bar 310 may be integrally formed (e.g., so that elongate elements 322a, 322b, and 322c are coupled together or otherwise form a unitary element) and comprise one or more cylindrical grooves configured to interface with the elongate hemicylinders. In some variations, the photon detection module cooling bar 310 may have a wedge interface to improve contact with the heat sinks (and associated cooling conduits) on the gantry and may engage, for example, with the heat sinks connected to the first conduit segment 206.

[0046] In some variations, a PET cassette may further comprise one or more cooling bars for the circuit boards. The circuit board cooling bar(s) may be made of one or more thermally conductive materials and arranged relative to the circuit board(s) such that the electronic components are in close proximity to (optionally, even contacting) the circuit board cooling bar. This may facilitate increased heat transfer from the circuit board to the cooling bar. A circuit board cooling bar may contain one or more materials of high thermal conductivity, alone or in combination, such as aluminum, zinc, copper, silver, and / or tungsten. In some variations, the circuit board cooling bar may extend along the length of the PET cassette and in some cases, extend along the entire length of the PET cassette. Optionally, the circuit board cooling bar may be a base or scaffold to which the circuit board(s) may be mounted. The circuit boards may be mounted to the cooling bar to increase the surface area of the circuit board(s) that is facing the cooling bar. The circuit board cooling bar may be thermally isolated from the photon detectionmodule cooling bar so that the heat conducted from the circuit boards is not conducted to the photon detection module (e.g., MPPC chips and associated electronics) and vice versa. Separating the temperature regulation between the circuit boards and the photon detection modules may provide the option to have different temperature ranges for the different components and / or accommodate the different thermal outputs of the circuit boards and the photon detection modules, i.e., tiered cooling. For example, if the photon detection modules generate more heat than the circuit boards, then the photon detection module cooling bars may be made of a more thermally-conductive material than the cooling bars for the circuit boards. The heat from the circuit board cooling bar(s) may be conducted to a different part of the gantry cooling circuit than the heat from the photon detection module cooling bar(s). In the example where the photon detection modules generate more heat than the circuit boards, the photon detection module cooling bar may be connected to a portion of the gantry cooling conduit that has a higher air or fluid flow rate and / or a lower temperature air or fluid, while the circuit board cooling bar may be connected to a portion of the gantry cooling conduit with a lower air or fluid flow rate and / or a higher temperature air or fluid.

[0047] FIGS. 3C and 3D depict one variation of a PET cassette having a circuit board cooling bar 311 that conducts heat from the circuit boards 306 and is also a base or mount to which the circuit boards are attached. FIG. 3 C is an exploded perspective view of the PET module and FIG. 3D is an assembled perspective view of the PET module. In this variation, the circuit board cooling bar 311 may be insulated from the photon detection module cooling bar 310 by a thermal isolator 314, which may be used to create an air gap between the circuit board cooling bar 311 and photon detection module cooling bar 310. Alternatively, or additionally, the circuit board cooling bar 311 may be insulated from the photon detection module cooling bar 310 by a thermal foam, an insulating gasket, and / or stand-offs (e.g., rods, struts, pads, etc.) made with materials of low thermal conductivity. In this variation, the circuit board cooling bar 311 may have a wedge interface to improve contact with the heat sinks (and associated cooling conduits) on the gantry and may engage, for example, with the heat sinks connected to the second conduit segment 207.PET Cassette Radiation Shielding

[0048] In some variations, the circuit board cooling bar(s) and / or the photon detection module cooling bar(s) may act as a radiation shield. In variations where the PET cassettes are used in aradiotherapy system where there is a high-energy therapeutic radiation source (e.g., linac), radiation shielding within the PET cassette may be desirable to reduce the rate of radiation degradation and damage to the components of the PET cassette. The arrangement of the components within the PET cassette may also be selected according to the placement of the PET cassette relative to the direction of (i.e., the field lines of) the therapeutic radiation beam and / or radiation scatter. For example, the PET cassette and / or its components may be aligned along the general direction where the therapeutic radiation may enter the PET cassette. This may help reduce the cross-sectional area that is orthogonal to the field lines and / or direction of the radiation, which may limit the deposited radiation dose to the PET cassette components. In some radiotherapy systems, the general direction of the radiation (e.g., scattered radiation and / or therapeutic radiation) may be orthogonal (i.e., perpendicular) to the photon detection surface of the PET cassette. The circuit board(s) may be oriented perpendicularly with respect to the photon detection surface to reduce radiation damage. Arranging the PET cassettes and / or components to reduce the incidence of undesired radiation from the therapeutic radiation source, either alone or in combination with radiation shielding components in the PET cassette, may help improve the reliability and function of the PET cassette.

[0049] As described above, the one or more cooling bars in the PET cassette may be made of one or more high-density metals or materials. For example, the photon detection module cooling bar 310 and / or the circuit board cooling bar may be made of one or more high-density metals (such as tungsten, lead, tin, antimony, or bismuth). In one example, the photon detection module cooling bar 310 may be made of tungsten and the circuit board cooling bar may be made of aluminum. The tungsten in the photon detection module cooling bar 310 may serve as a shield against photons passing through the photon detection surface 301 and the photon detection module 302. In addition, the perpendicular alignment of the circuit board 306 to the photon detection surface 301 may further reduce radiation damage on the circuit board 306.PET Cassette Cable Management Assembly

[0050] As described herein, PET cassettes may have multiple electrical components that may be arranged in various assemblies and mounted at different locations within the cassette. These assemblies may be electrically connected to each other using one or more electrical wires, cables, ribbon cables (e.g., buses), and the like. For example, there may be one or more ribbon cables that connect a photon detection module to one or more circuit boards. Some variations ofa PET cassette may comprise one or more cable management assemblies that may help direct or route the electrical cables from one electrical component to another electrical component. A cable management assembly may also be configured to stabilize and / or secure the electrical cables, which may help reduce the likelihood of cable damage or dislodgement during gantry rotation. Optionally, a cable management assembly may also provide radiation shielding to the electrical cables, which may help preserve the integrity (e.g., accuracy) of the electrical signal that is transmitted across the electrical cables.

[0051] In one variation, a cable management assembly of a PET cassette may comprise one or more cooling bars, clamps, ribbon sockets, and the like. In some variations, the photon detection module cooling bar(s) may comprise contours and grooves that act as an electrical cable management assembly. For example, the cylindrical grooves and / or concave portions of the cooling bar may be sized and shaped to retain electrical connectors, such as wires and / or electrical ribbons. The curvature of the photon detector cooling bar may help position and retain the electrical connectors so that they do not become dislodged or entangled when the PET cassette is being rotated on the gantry. FIG. 3F depicts a close-up cross-sectional perspective view (e.g., corresponding to the region enclosed by dotted lines in FIG. 3D) of one variation where electrical ribbons 307 or wires are routed along the cylindrical groove between the photon detection module 302 and the circuit boards 306. The concave portions of the elongate element 322b and 322c may form, when assembled together, a hemicylindrical groove that matches or corresponds to the curvature of the hemicylindrical cooling bar 320. In this example, the channel or cylindrical groove accommodates the full arc of the hemicylindrical cooling bar 320. The curvature or contours of the cooling bar grooves may serve to route the electrical ribbon 307 without sharp or abrupt angles that can create points or areas of material weakness that can cause the ribbon to break and sever the electrical connection between the photon detection module and the circuit boards. Moreover, the cooling bar grooves may route the electrical ribbon such that the ribbon orientation is generally parallel to the circuit board at its socket or connection point to the circuit board. This may help reduce the pull-out forces on the ribbon and limit torquing forces that may destabilize the electrical connection. In this variation, the photon detection module cooling bar acts as both a thermal conduit to regulate the temperature in the PET cassette and also a mechanism that guides and secures the electrical wiring between components.

[0052] In variations where the PET cassette is subject to rotation, for example, when it is mounted on a fast-rotating gantry of a radiotherapy system (e.g., where the gantry is rotating faster than about 30 RPM, about 50 RPM or more, about 60 RPM), the components in the PET cassette may be subject to greater centripetal and / or centrifugal forces than a stationary PET cassette. Such centripetal and / or centrifugal forces may cause deflection of the circuit board, loosening of electrical and mechanical connections / connectors, and / or vibrations that may affect the function and reliability of the PET cassette. Accordingly, in some variations, the PET cassette components may be assembled to help reduce or eliminate the effect of such centripetal and / or centrifugal forces. For example, deflection of the circuit board 306 may be reduced by aligning the circuit board perpendicularly to the photon detection surface 301. As described above, the cable connections such as the ribbon cables 307 connecting the photon detection module 308 and circuit board 306 may be secured by the cylindrical grooves and the hemicylindrical bar on the photon detection module cooling bars. Optionally, as depicted in FIGS. 3B and 3C, the connection (e.g., cables) between the circuit board to the upper board 313 may be secured by one or more clamps 315. In some variations, the clamps may be attached to the upper board 313 and engage the top edge of circuit board 306 and / or the clamps may be used to retain the connection of the cable to a socket of the circuit board and / or the upper board. In one variation, a two-part clamp may interlock and secure the cable firmly on the circuit board with a screw 316. Other variations may have the connection between the cables and the circuit board secured by a unibody clamp, soldering, or adhesive (such as silicone adhesive or glass bond glue).

[0053] In some variations of a PET cassette, the placement of components, attachment mechanisms, material composition, and / or the overall arrangement of components are selected to address the issues that may arise from high levels of radiation exposure (e.g., from a linac or therapeutic radiation source), temperature fluctuations, and / or forces due to rotation or motion. Alleviating these issues may improve the reliability, serviceability, and / or lifetime of the PET cassette. As an example, PET cassette 300 has circuit board cooling bars that are configured to remove heat from the circuit boards, provide radiation shielding, and / or may also be a mount or scaffold that orient the circuit boards in an arrangement that reduces their radiation exposure, i.e., oriented perpendicular to the photon detection surface. The perpendicular arrangement, which aligns the board along the radially-oriented centripetal / centrifugal force, may also help reduce or eliminate defections of the circuit board during rotation. PET cassette 300 comprisesphoton detection module cooling bars that are configured to remove heat, provide radiation shielding, and / or may be shaped to guide and retain electrical wires and / or cables so that they are not perturbed or dislodged during rotation.Tiered Cooling

[0054] As described above, some variations of a PET cassette comprise independent or thermally-isolated cooling bars that may be made of different materials and / or arranged in different locations relative to the components from which they are transferring heat, which may allow for tuned or tiered cooling. The PET cassette cooling bars that are arranged to remove heat from the hottest components and / or are the most heat-sensitive (i.e., their operating temperature tolerance is relatively lower or a smaller / tighter range) may interface or contact with the gantry heat sinks that can remove that heat quickly and / or efficiently. When the PET cassette is reversibly coupled to the rotatable ring, it comes into thermal contact with the gantry cooling circuit without any fluid transfer between the cooling circuit and the PET cassette. In some variations, for instance, the function of MPPCs may be more sensitive to changes in temperature (e.g., as compared to the circuit board), and in some cases, may function erroneously if the temperature is elevated. For example, the gantry heat sinks that are intended to remove high levels of heat from the PET cassette may be thermally connected to fluid(s) of a cooler temperature and / or fluid moving at a faster rate as compared to gantry heat sinks that remove lower levels of heat from the PET cassette. In some variations, the gantry cooling conduit may be a single fluid circuit having a first segment where the fluid is coolest and a second segment where the fluid is warmer than the fluid in the first segment (because it will have been used to remove heat before it flows to the second segment). FIG. 4 is a conceptual schematic of one variation of a gantry cooling conduit or circuit 400. The arrows 402 denote the direction of the cooling fluid flow in the cooling circuit 400. The cooling fluid may be in liquid form or may be a gas, such as air. The cooling circuit 400 may have a first segment 404 and a second segment 406. In some variations, the fluid in the first segment of the cooling circuit may have just been cooled for example, by a fan or from a cooled reservoir 408. In the variation of a cooled reservoir of fluid, there may be a fluid conduit 409 supplying the fluid from the reservoir 408 to the cooling circuit 400. The PET cassette cooling bar(s) conducting heat from the most heatsensitive components in the PET cassette may contact this first segment 404. Because of the large temperature differential between the fluid in the first segment 404 and the PET cassettecooling bar (or the heat sink of the cooling bar), the heat may be efficiently moved away from the PET cassette. As an example, the first segment and / or the heat sinks that are thermally connected to the first segment may be in contact with the heat sink(s) and / or cooling bar(s) 410 of the photon detection module when the PET cassette is mounted to the gantry. In the variation depicted in FIG. 3D, the end surfaces 324 of the photon detection module cooling bar 310 may be in thermal contact with the first gantry cooling circuit segment 402 when the PET cassette is coupled to the rotatable gantry. The cooling fluid that has moved through the first segment 404 may be warmer than when it first entered the first segment because it now carries the heat transferred to it from a portion of the PET cassette, e.g., heat from the photon detection modules, but may still be at a temperature that can provide cooling to additional components of the PET cassette. In some variations, as the cooling fluid moves into the second segment 406, it may be used to remove heat from the circuit board cooling bar 411. For example, in the variation depicted in FIG. 3D, the end surfaces of the circuit board cooling bar 311 may be in thermal contact with the second gantry cooling circuit segment 406 when the PET cassette is coupled to the rotatable gantry. The cooling fluid may then flow in the direction of arrow 402 back to the cooled reservoir and / or fan (e.g., alone or in combination with chilling by circulated air) for the removal of the heat transferred to the cooling fluid from the PET cassette. This arrangement may prioritize the cooling of the PET cassette components that are in thermal connection with the first cooling segment (e.g., photon detection modules) over the cooling of the PET cassette components that are in thermal connection with the second cooling segment (e.g., circuit boards). In some variations, the cooling circuit on the gantry ring may be pre-filled with cooling fluid, which may reduce the complexity of servicing the PET cassettes of the PET detector arc, since no cooling fluid is exchanged between the PET cassettes and the gantry cooling circuit. Eliminating the need for attaching and detaching fluid conduits between the PET cassette and the gantry cooling circuit may reduce the risk of cooling fluid leaks and damage to radiotherapy system components due to a cooling fluid leak. In some variations, the cooling circuit on the gantry ring may be a closed loop system where the fluid is entirely enclosed within the conduits of the cooling circuit, without any fluid exchange to a PET cassette and / or facility cooling system.

[0055] As described above, tiered cooling may prioritize heat transfer from (or cooling of) components that may operate over a narrower (i.e., cooler, tighter) temperature range over the heat transfer from (or cooling of) components that may operate over a wider temperature range.While one variation of a tiered cooling assembly is described above, it should be understood that other arrangements may be suitable for tiered cooling. For example, the gantry cooling circuit may comprise two separate cooling circuits, where the first cooling circuit has a cooling fluid cooled to a first temperature and the second cooling circuit has a cooling fluid cooled to a second temperature. The first temperature may be lower than the second temperature. The cooling fluid for the two cooling circuits may be separate from each other and do not mix with each other and / or may be cooled by separate fans or chillers. This arrangement may help ensure that the heat generated by one set of components of the PET cassette does not affect the cooling for the other set of components of the PET cassette. For example, a gantry may have a first cooling circuit in contact with the cooling bar(s) of the photon detection modules and a separate second cooling circuit in contact with the cooling bar(s) of the circuit boards. If the photon detection modules are overheating, more heat may be transferred to the cooling fluid of the first gantry cooling circuit, but the additional heat in the first cooling circuit will have little if any impact on the second cooling circuit, which may continue to cool and prevent overheating of the circuit boards.PET Cassette and Gantry Assembly

[0056] The PET cassette may be mechanically coupled to the gantry ring by vertically lowering and / or placing the PET cassette into a slot or space in the gantry ring, and secured using the one or more attachment mechanisms described herein. Each PET cassette may be modular in nature and thus coupled or de-coupled from the gantry independently of the other PET cassettes. This may help facilitate servicing of the system where the faulty PET cassettes can be replaced without impacting the functioning PET cassettes. In addition, because the cooling of the PET cassette may not involve any fluid exchange between the PET cassette and the gantry, there is no need to disconnect and / or re-connect fluid lines between the cassette and the gantry. For example, the gantry cooling circuit may be pre-filled with cooling fluid (e.g., at the time of manufacture or installation) and the cooling fluid may remain within the gantry cooling circuit regardless of the servicing and / or replacement of PET cassettes. This may greatly simplify the process of repairing and / or replacing PET cassettes, and reduce the risk of cooling fluid leaks. As described above, there may be one or more attachment sockets on the gantry and one or more corresponding pins on the PET cassette that may facilitate alignment and attachment of the PET cassette with the gantry. For example, FIGS. 2D and 2E depict analignment pin socket 209 to which the pins of the interface 314 depicted in FIG. 3 A may align and attach. Mechanical alignment may comprise electrical alignment and connectivity between the PET cassette and the gantry. Mechanical attachment elements and interfaces may include, alone or in combination, one or more screws, one or more pins, one or more clamps, adhesives, corresponding protrusions and notches that engage by snap-fit, magnetic engagement, and the like.

[0057] While certain variations are described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and / or structures for performing the function and / or obtaining the results and / or one or more of the advantages described herein, and each of such variations and / or modifications is deemed to be within the scope of the inventive variations described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and / or configurations will depend upon the specific application or applications for which the inventive teachings is / are used. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive variations described herein. It is, therefore, to be understood that the foregoing variations are presented by way of example only and that, within the scope of the appended claims and equivalents thereto; inventive variations may be practiced otherwise than as specifically described and claimed. Inventive variations of the present disclosure are directed to each individual feature, system, article, material, kit, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and / or methods, if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

Claims

CLAIMS1. A modular PET detector system for a radiotherapy system comprising: a rotatable gantry, wherein the rotatable gantry has a cooling circuit mounted thereon, and wherein the cooling circuit comprises a cooling conduit having a fluid therein; and a PET cassette reversibly coupled to the gantry and in thermal contact with the cooling circuit without transfer of the fluid between the cooling conduit and PET cassette, wherein the PET cassette comprises a cooling bar and at least one photon detection module mounted on the cooling bar.

2. The system of claim 1, wherein the cooling circuit further comprises a plurality of heat sinks.

3. The system of claim 1, wherein the PET cassette further comprises a radiation shield.

4. The system of claim 1, wherein the cooling bar also functions as a radiation shield.

5. The system of claim 1, wherein the PET cassette further comprises at least one alignment pin for engagement with a socket on the rotatable gantry.

6. The system of claim 1, further comprising at least two PET cassettes.

7. The system of claim 1, wherein the rotatable gantry is configured to rotate at least at 60 RPM.

8. The system of claim 1, further comprising a therapeutic radiation source, wherein the therapeutic radiation source comprises a linear accelerator (linac) and a magnetron.

9. The system of claim 8, further comprising a multi-leaf collimator.

10. The system of claim 1, wherein the cooling bar comprises tungsten.

11. The system of claim 1, wherein the cooling conduit comprises a first segment and a second segment parallel to the first segment, wherein the first segment is configured to cool the at least one photon detection module.

12. A modular PET detector system comprising: a rotatable gantry configured to rotate at least at 60 RPM; anda PET cassette reversibly coupled to the rotatable gantry, wherein the PET cassette comprises a photon detection surface, and at least one photon detection module having a printed circuit board that is aligned perpendicularly with respect to the photon detection surface, such that forces on the cassette are reduced when the PET cassette is coupled to the gantry.

13. The system of claim 12, wherein the alignment of the printed circuit board perpendicularly with respect to the photon detection surface further reduces radiation damage on the PET cassette when the PET cassette is coupled to the gantry.

14. The system of claim 12, further comprising a therapeutic radiation source, wherein the therapeutic radiation source comprises a linear accelerator (linac) and a magnetron.

15. The system of claim 12, wherein the PET cassette further comprises at least one alignment pin for engagement with a socket on the rotatable gantry.

16. The system of claim 12, further comprising at least two PET cassettes.

17. The system of claim 12, wherein the rotatable gantry comprises a cooling circuit mounted thereon, wherein the cooling circuit comprises a cooling conduit having a fluid therein.

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