MRI System and RF Transmission Antenna Configuration

Abstract

To provide an MRI system RF transmission antenna constitution, an MRI system including such an MRI system RF transmission antenna constitution, and combination therapy and / or an imaging system.SOLUTION: An MRI system RF transmission antenna constitution 3 includes an antenna 5 provided with a coaxial cable 51 of constant length having a conductive core wire 52 and a conductive outer shield 53 where a core wire passes in the middle and extends. In the MRI system RF transmission antenna constitution 3, the core wire has: a supply point 52a configured for electric connection to an RF source; and at least one interruption part 53a which is partially provided in the conductive outer shield along the coaxial cable of constant length so that it divides the conductive outer shield 53 into shields parts that are apart in at least two axial directions and, consequently, at least one of the shield parts acts as a radiation element when the RF source is connected to the supply point 52a.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an MRI system (including an MRSI system) including an RF transmission antenna configuration, an RF transmission antenna configuration for use within an MRI system (including an MRSI system), and, in some embodiments, a combined system such as an MR-Linac and a PET-MR system, where an MRI system is used in combination with another system.

Background Art

[0002] An MRI system typically includes a main MRI scanner configuration, a patient support or bed on which the patient lies during the scan, and, in at least some cases, a separate local RF (radio frequency) configuration (or body-part-specific RF configuration) positioned within the region of a particular body part that is desired to be scanned.

[0003] The main MRI scanner configuration typically includes a main magnet, a gradient coil, an RF transmission antenna / coil, and a receive coil, all of which are disposed within a main unit having a bore in which the patient is positioned during the scan. When present, the body-part-specific RF configuration is also typically positioned within the bore during the scan. The body-part-specific RF configuration may comprise at least one receive coil and / or at least one RF transmission antenna / coil.

[0004] As is well known, MRI (Magnetic Resonance Imaging) systems are widely used to image a subject and can also be used in combined systems such as MR-Linac and PET-MR systems, and in MRI hyperthermia treatments such as MRI-guided laser interstitial thermotherapy. These combine magnetic resonance imaging with other techniques, for example, for treatment in MR-Linac, MRI hyperthermia, or to provide functional imaging in, for example, PET-MR. A further subset of MRI systems is MRSI (Magnetic Resonance Spectroscopic Imaging) systems that can obtain spatially localized spectra from within a sample or patient.

[0005] In MRI operation, the magnet generates a large static magnetic field B0, the RF transmission antenna / coil generates an alternating magnetic field B1, and the receive coil is configured to collect (i.e., acquire magnetic resonance data) the magnetic resonance signal, whether provided within the main unit or within a receive coil specific to a body part. The gradient magnetic field coils are used such that spatial encoding on the B0 magnetic field enables tomographic imaging.

[0006] When an MRI system is operated using a receive coil provided within the main unit of the scanning device, the resulting resolution and accuracy can be limited in some cases. As a result, a separate, so-called local receive coil, for example, a coil specific to a body part as described above, will be used with the aim of improving the imaging of the selected location / body part. In at least some instances, it can be beneficial to be able to provide the RF transmission antenna / coil within a local or body-part specific RF configuration, either together with or independently of the local receive coil.

[0007] Providing a local RF transmission antenna / coil can be problematic in at least some cases because conventional RF transmission antennas / coils may be unsuitable for inclusion in RF configurations specific to a body part that are configured to closely match the shape of a particular body part.

[0008] A further issue to consider is the SAR (Specific Absorption Rate) (heating effect) seen in the imaged subject due to signals being applied to the subject to generate the desired B1 magnetic field. Typically, the heating effect is caused by the electric field generated within the subject as a by-product of generating the desired B1 magnetic field. In general, it is desirable to minimize the SAR level generated for any given intensity of B1 magnetic field.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] Therefore, it is desirable to provide an MRI system RF transmission antenna configuration aimed at addressing at least one of these problems, as well as an MRI system, combination therapy and / or imaging system including such an MRI system RF transmission antenna configuration.

Means for Solving the Problems

[0011] According to a first aspect of the present invention, there is provided an MRI system RF transmission antenna configuration comprising an antenna having a conductive core wire and a coaxial cable of a certain length having a conductive outer shield extending therethrough, the core wire being divided into a supply point configured for electrical connection to an RF source and the conductive outer shield into at least two axially spaced shield portions, such that when the RF source is connected to the supply point, at least one of the shield portions acts as a radiating element, and having at least one interruption portion provided partially within the conductive outer shield along the coaxial cable of a certain length.

[0012] Such a configuration facilitates providing an antenna that is both effective and flexible, and as a result, they can be more easily shaped as desired. This enables, for example, including such an antenna shaped to more closely conform to the subject or a part of the subject to be examined in the configuration. Moreover, such a configuration facilitates providing an antenna that minimizes the SAR level observed for a given magnitude of the B1 magnetic field generated within the subject. This can be considered as a result of minimizing the electric field within the subject for a given magnitude of the B1 magnetic field generated within the subject.

[0013] The antenna can be configured as a monopole antenna in which a supply point is provided within a region at one end of the coaxial cable of a certain length.

[0014] Preferably, the antenna is configured as a dipole antenna in which the supply point is provided towards the midpoint of the coaxial cable of a certain length such that the coaxial cable of a certain length has a first coaxial cable portion on one side of the supply point and a second coaxial cable portion on the opposite second side of the supply point.

[0015] The shield within the first coaxial cable portion can be electrically, typically galvanically, connected to the shield within the second coaxial cable portion. The core wire within the first coaxial cable portion and the core wire within the second coaxial cable portion can be configured such that an RF source is connected between them at the supply point.

[0016] In one set of embodiments where the antenna is configured as a dipole antenna, the conductive outer shield of a coaxial cable of a certain length is divided into at least three axially spaced shield portions, and as a result, at least one interruption is provided partially within the conductive outer shield along the first coaxial cable portion of a certain length and at least one interruption is provided partially within the conductive outer shield along the second coaxial cable portion of a certain length such that when an RF source is connected to the supply point, at least one of the shield portions acts as a radiating element.

[0017] This can thereby provide a particularly simple and effective antenna. Typically, the intermediate portion of the conductive outer shield acts as a radiating element within such a configuration. This intermediate portion is composed of a portion of the outer shield of the first coaxial cable portion and a portion of the outer shield of the second coaxial cable portion and has ends defined by respective interruptions of the outer shield. The length of this intermediate portion tends to define the field of view of the subject area scanned during use. Thus, in some cases, the length of this intermediate portion can be selected according to the desired field of view.

[0018] In an alternative form, the antenna can comprise three or more coaxial cable portions extending outwardly from the supply point. In such cases, at least one interruption can be provided partially within the conductive outer shield along each coaxial cable portion of a certain length.

[0019] In some cases, a multi - pole antenna may be provided.

[0020] Generally, the radiating element or each radiating element of the antenna configuration can be provided by at least one shield portion of the coaxial cable portion, and the at least one shield portion is separated from another shield portion of the coaxial cable portion by an axial interruption within the conductive outer shield of the coaxial cable portion.

[0021] The antenna configuration can include at least one electrical component connected to at least one of the conductive outer shield and the conductive core wire of a coaxial cable of a certain length in order to control the electrical characteristics of the antenna configuration.

[0022] The at least one electrical component may include at least one of an inductor, a resistor, and a capacitor.

[0023] The at least one electrical component can be selected to adjust the antenna configuration to operate while being driven at a predetermined RF source frequency.

[0024] The at least one electrical component can be selected such that the antenna configuration is adjusted or adjustable to operate while being driven at a plurality of predetermined RF source frequencies and / or within a predetermined RF source frequency range.

[0025] The at least one electrical component can include at least one connecting electrical component that is electrically connected between the conductive outer shield and the conductive core wire of the coaxial cable portion or each coaxial cable portion toward an end outward from the supply point. The at least one electrical component can be provided within a matching circuit provided at a supply point through which a power source can be connected to the antenna configuration.

[0026] The at least one connecting electrical component can be electrically connected between the conductive outer shield and the conductive core wire of the coaxial cable portion or each coaxial cable portion toward an end outward from the supply point.

[0027] At least one connecting electrical component may include at least one of an inductor, a resistor, and a capacitor.

[0028] In a set of preferred embodiments, at least one connecting electrical component includes an inductor.

[0029] Providing at least one connecting electrical component at such a location (or at a plurality of such locations) can help reduce losses by helping to control the relative levels of current flowing in the conductive outer shield and the conductive core wire.

[0030] At least one connecting electrical component can be selected to adjust the antenna configuration for operation while being driven at a predetermined RF source frequency.

[0031] At least one connecting electrical component can be selected such that the antenna configuration is adjusted or adjustable for operation while being driven at a plurality of predetermined RF source frequencies and / or within a predetermined RF source frequency range.

[0032] At least one connecting electrical component may include at least one inductor and at least one capacitor arranged in an LC resonance circuit. At least one connecting electrical component can include at least one switch for selecting an adjusted frequency of the antenna configuration from among a plurality of predetermined RF source frequencies or from within a predetermined RF source frequency range.

[0033] In an alternative form, there may be an open circuit between the conductive outer shield and the conductive core wire of the coaxial cable portion or each coaxial cable portion, towards the end that is outward from the supply point.

[0034] In another alternative form, there may be a short circuit between the conductive outer shield and the conductive core wire of the coaxial cable portion or each coaxial cable portion, towards the end that is outward from the supply point.

[0035] The antenna configuration can include a matching circuit provided at a supply point through which a power source can be connected to the antenna configuration.

[0036] The matching circuit can include at least one matching electrical component. The matching circuit may include at least one of an inductor, a resistor, and a capacitor.

[0037] In a preferred embodiment, the matching circuit includes a capacitor.

[0038] In the currently most preferred embodiment, the connecting electrical component includes an inductor and the matching circuit includes a capacitor. Even more preferably, at least one connecting electrical component includes a respective single inductor and the matching circuit includes a single capacitor.

[0039] This can result in an efficient design that minimizes losses within the coaxial cable portion and minimizes losses within at least one electrical component and the matching circuit.

[0040] At least one coaxial cable of a certain length can have a length within the range of 10 cm to 100 cm, preferably within the range of 20 cm to 60 cm. One length that functions well is 30 cm.

[0041] The radiating element can have a length within the range of 8 cm to 50 cm or even 8 cm to 75 cm, but preferably within the range of 10 cm to 30 cm.

[0042] The ratio of the length of the radiating element to the total length of the coaxial cable can be within the range of 0.2:1 to 0.9:1. Thus, at these extremes, the radiating element is 1 / 5 of the cable length and 9 / 10 of the cable length. Preferably, the ratio of the length of the radiating element to the length of the coaxial cable is within the range of 0.4:1 to 0.8:1. A ratio that functions well is about 2:3, and thus the radiating element is 2 / 3 of the total cable length.

[0043] Generally, by increasing the spacing between the shield breaks in the dipole antenna as defined above, a more uniform current distribution on the radiating element is promoted. In contrast, it has been found that this can tend to increase the current flowing in the standing wave on the inner conductor in at least some cases. As mentioned above, providing connection electrical components can help to control this.

[0044] The distance between the feed point of each coaxial cable section and the break in the outer conductive shield can be selected such that, in combination with the frequency at which the antenna is to be driven, the distance is less than or equal to 3 / 4 of the wavelength as seen on the radiating element.

[0045] According to another aspect of the invention, there is provided a method of manufacturing an MRI system RF transmission antenna configuration as defined above, the MRI system RF transmission antenna configuration comprising a dipole antenna having two coaxial cable sections, each of the two coaxial cable sections having a respective break provided partially within the outer shield along the length of each coaxial cable section, the method comprising: a) selecting a desired length L of the radiating element of the antenna corresponding to the distance between each break in the outer shield; b) modeling an antenna comprising two coaxial cable sections each having a break in the outer shield at a distance L / 2 from the feed point and an end of the coaxial cable of length X beyond each break, a source connected to the feed point, at least one first connection electrical component provided towards the distal end of the first coaxial cable section, and at least one second connection electrical component provided towards the distal end of the second coaxial cable section; c) determining a value of length X and the characteristics of at least one first connection electrical component and at least one second connection electrical component, the value and characteristics being i) The flatness of the current distribution on the radiating element, ii) Minimizing losses in the inner core wire and connection components, and iii) Determining steps to optimize and determine a desired input impedance at the source, d) Steps to design the transmitting antenna configuration according to the above design.

[0046] At least one first connection electrical component may include an inductor or may be composed of an inductor. At least one second connection electrical component may include an inductor or may be composed of an inductor.

[0047] The steps of determining the value of length X and the characteristics of at least one first connection electrical component and at least one second connection electrical component may include or may be composed of the steps of determining the value of length X and the inductance value of each inductor.

[0048] According to another aspect of the present invention, a method of manufacturing an MRI system RF transmitting antenna configuration as defined above is provided. The MRI system RF transmitting antenna configuration includes a dipole antenna having two coaxial cable portions. Each of the two coaxial cable portions is provided with respective interruptions partially within the outer shield along the length of each coaxial cable portion. The method includes a) Selecting a desired length L of the radiating element of the antenna corresponding to the distance between each interruption within the outer shield, b) Two coaxial cable portions each having an interruption within the outer shield at a distance L / 2 from the supply point and an end of a coaxial cable of length X beyond each interruption, a supply source connected to the supply point, a first inductor as a connection electrical component provided towards the distal end of the first coaxial cable portion, and a second inductor as a connection electrical component provided towards the distal end of the second coaxial cable portion, and modeling an antenna including them. c) determining the value of the length X and the value of the inductance of each inductor, the values being i) the flatness of the current distribution on the radiating element, ii) minimizing losses in the inner core wire and connection components, and iii) optimizing and determining a desired input impedance at the source; and d) configuring the transmitting antenna according to the above design.

[0049] The flatness of the current distribution can be measured as a coefficient of variation of the current along the radiating element, calculated as the value obtained by dividing the standard deviation by the average of the current amplitudes along the radiating element.

[0050] As an example with a typical length L (e.g., about 20 cm), the target coefficient of variation of the current along the radiating element should desirably be less than 0.2, preferably less than 0.15, and even more preferably less than 0.10.

[0051] Losses can be measured as the percentage of input power wasted. Preferably, the percentage of input power wasted is less than 20%, more preferably 10% or less, and even more preferably 5% or less.

[0052] The desired input impedance is one that is easy for the source to match and, optionally, minimizes losses in any matching circuit. The real part of the impedance is important in achieving optimal operation. Preferably, the real part of the input impedance is at least 5 ohms, more preferably at least 10 ohms. Preferably, the real part of the input impedance is 150% or less of the impedance of the source. Ideally, the real part of the input impedance is at least 10 ohms, such that a single electrical component in the matching circuit is sufficient.

[0053] The method can include a step of selecting an operating frequency or an operating frequency range at which the antenna should be driven during use, and a step of performing at least one of steps a), b), and c) while taking into account the selected operating frequency or operating frequency range.

[0054] The antenna configuration can include an array of antennas, each having a respective coaxial cable of a fixed length as defined above. For example, the antenna configuration can include eight antennas.

[0055] The antenna configuration can include a support structure for supporting at least one coaxial cable of a fixed length.

[0056] The antenna configuration can be configured as an antenna configuration specific to a body part, and in a configuration selected for scanning each respective body part, can include a support structure for supporting at least one coaxial cable of a fixed length.

[0057] The antenna configuration can include at least one RF receiving coil.

[0058] The support structure can be configured to support at least one RF receiving coil.

[0059] The antenna configuration specific to a body part can include at least one RF receiving coil supported on the support structure in a configuration selected for scanning each respective body part. The at least one RF receiving coil can include respective coaxial cables of a fixed length configured in a loop.

[0060] According to another aspect of the present invention, there is provided an MRI system RF transmission antenna device including an MRI system RF transmission antenna configuration as defined above and an RF source connected to a feed point.

[0061] The antenna configuration can be adjusted to operate at a predetermined RF source frequency, and the RF source can be configured to drive the antenna configuration at the above-mentioned predetermined frequency.

[0062] The antenna configuration can be adjusted or can be made adjustable to operate at a plurality of predetermined RF source frequencies and / or within a predetermined RF source frequency range, and the RF source can be configured to drive the antenna configuration at the above-mentioned plurality of predetermined RF source frequencies and / or within a predetermined RF source frequency range.

[0063] According to another aspect of the present invention, an MRI system is provided that includes an MRI system RF transmission antenna configuration as defined above.

[0064] According to another aspect of the present invention, an MRI system is provided that includes a main MRI scanner configuration, a patient support, and an MRI system that includes an MRI system RF transmission antenna configuration as defined above, which is electrically connected to the main MRI scanner configuration.

[0065] According to another aspect of the present invention, an MR-Linac system is provided that includes an MRI system as defined above and a medical linear accelerator system.

[0066] According to another aspect of the present invention, a PET-MR system is provided that includes an MRI system as defined above and a positron emission tomography system.

[0067] According to another aspect of the present invention, a hyperthermia MR system is provided that includes an MRI system as defined above and a hyperthermia system.

[0068] Generally, and with any necessary verbal modifications, all of the additional features defined above in accordance with any aspect of the present invention are applicable as additional features of all other aspects of the present invention defined above. It should be noted that these additional features are not recited again after each aspect of the invention, solely for the purpose of brevity.

[0069] Here, by way of mere example, embodiments of the present invention will be described with reference to the accompanying drawings.

Brief Description of the Drawings

[0070]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0071] FIG. 1 shows an MRI system including a main scanner configuration 1, a patient support 2 configured to support a patient when within the scanner configuration 1, and an MRI system RF transmission antenna configuration 3 separated from the main scanner configuration and including an array 31 of RF transmission antennas 5 which in this embodiment is a body part specific RF transmission antenna configuration 3. Such RF transmission antennas 5 can be used alone, but more commonly the antennas 5 are provided in an array 31 as in this embodiment. As an example, eight antennas 5 may be provided in the array 31. Further details of each RF transmission antenna 5 are further described below. Most simply, the RF transmission antenna configuration 3 can comprise only the antenna 5. However, in this embodiment, the RF transmission antenna configuration 3 also has other component parts.

[0072] In this embodiment, the body part specific RF transmission antenna configuration 3 also comprises a local reception coil 32 and can be considered a body part specific RF configuration 3. More generally, such an RF transmission antenna configuration 3 (or RF configuration) may be referred to as a local RF transmission antenna configuration (or local RF configuration).

[0073] As suggested above, in some cases the MRI system is used in combination with other systems, for example, to provide an MR-Linac system comprising an MRI system and a medical linear accelerator system, or in another example, to provide a PET-MR system comprising an MRI system and a positron emission tomography system, or in another example, to provide a thermotherapy MR system comprising an MRI system and a thermotherapy system. In such cases, the MRI system shown in FIG. 1 can be complemented by a linear accelerator system, a positron emission tomography system, or a thermotherapy system S, shown only in dotted lines in a highly schematic form in FIG. 1.

[0074] The main MRI scanner configuration 1 may be a fully conventional main MRI scanner configuration including a main magnet 11 typically being a superconducting electromagnet, a main unit RF transmission coil 12, a gradient magnetic field coil 13, and a main unit reception coil 14. These components are provided within the body of the MRI scanner configuration 1 having a main bore B where the patient support 2 is provided therein or, more typically, the patient support 2 for transporting the patient can be moved thereto until reaching the surgical position.

[0075] At least during use, the RF configuration 3 specific to the body part is also provided within this main bore B. If present, at least a part of a medical linear accelerator system, a positron emission tomography system, or a hyperthermia system S can also be located within this main bore during operation.

[0076] During operation, it is located within the main bore B of the main MRI scanner 1, and the RF configuration 3 is electrically connected to the main scanner configuration 1, so that the magnetic resonance signal picked up by the reception coil 32 can be supplied to the main scanner configuration 1 for processing. The RF drive signal can also be provided from the main MRI scanner 1 to the RF configuration 3 to drive the antenna 5 during operation, or the RF drive signal may be provided from a separate signal source (not shown).

[0077] The signal picked up by the reception coil 32 can be used alone or in combination with the signal picked up by the main unit reception coil 14 in the processing and generation of images. In some cases, the main MRI scanner configuration 1 without its own main unit reception coil 14 can be used with this type of RF configuration 3.

[0078] More specifically, in an alternative form, at least one antenna 5 of this type, and more typically an array of antennas 5 of this type, can be provided within the local RF arrangement 3 and can be used with an MRI system main scanner without using any of its own main unit RF transmission coils 12. In another alternative form, at least one antenna 5 of this type can be provided to act as the main RF transmission coil 12 within the MRI system main scanner arrangement. Such an alternative arrangement may then not have a local RF arrangement 3 if desired.

[0079] The structure and operation of the MRI scanner arrangement are well developed and understood, and the concept in this embodiment relates to the RF arrangement 3 for use with such an MRI scanner arrangement, and more specifically, to the structure and operation of the antenna 5 of this type. Therefore, no further explanation of the structure and operation of the MRI scanner arrangement 1 is necessary, and the remainder of this specification relates to the RF arrangement 3, and in particular, to its antenna or each antenna 5.

[0080] The RF arrangement 3 also includes a support structure 33 for supporting an array of antennas 5 and also a local reception coil 32, and is suitably shaped for placement around a portion of the subject to be scanned. At least some of the reception coils 32 can be essentially flexible, allowing them to be shaped around the region of interest of the subject to be scanned. These reception coils 32 may, in some cases, include reception coils made from loops of coaxial cable.

[0081] Each of the antennas 5 of this embodiment is flexible and is configured such that when provided on the support structure 33, it can be shaped to conform to a portion of the subject to be scanned.

[0082] In other cases, each antenna 5 need not be shaped to match a particular part of the subject being scanned, provided that it is still provided within the RF configuration 3 in close proximity to the area being scanned. Having the antenna or each antenna 5 within the area of the part of the subject being scanned and / or shaping it to closely match the shape of the part of the subject being scanned can help reduce the amount of power required to generate the desired B1 magnetic field within the region of interest of the subject.

[0083] FIG. 2 shows a schematic cross-section of one of the antennas 5 included in the array 31 of this embodiment. The antenna 5 comprises a coaxial cable 51 of constant length having an overall length of 300 mm in this embodiment. The coaxial cable 51 of constant length comprises a conductive core 52 and a conductive shield 53 surrounding it. The coaxial cable of constant length itself can simply be a standard length, for example a 50 ohm or 75 ohm coaxial cable, and such a cable typically has an outer insulating casing over the shield 53 and an inner insulating spacer between the core 52 and the shield 53, and typically these may be made of a plastic material.

[0084] A supply point 52a to which an RF drive source can be connected is provided on the core 52. In this embodiment, the supply point 52a is provided towards the midpoint of the core 52, and the antenna 5 is configured as a dipole antenna.

[0085] If the supply is connected to the supply point in the middle of the core of the coaxial cable of constant length and nothing else is done, it can be predicted that the coaxial cable will not act as an antenna because the shield 53 shields the core 52 and greatly impedes radiation.

[0086] However, the inventors have found that when a gap (or break) is provided within the outer shield 53 at the selected location, an effective antenna can be realized. In this embodiment, two breaks 53a are provided within the outer shield 53 at positions along a coaxial cable 51 of a certain length. In this embodiment, each break 53a within the shield 53 is provided at a distance of 100 mm from the midpoint of the coaxial cable 51 of a certain length. Thus, in this embodiment, there is a distance of 200 mm between the two breaks 53a within the outer shield 53. The exact length of the breaks 53a is generally not considered particularly important, but they may be, for example, about 3 mm. Any outer insulating casing and inner insulating spacer of the coaxial cable 51 may also be absent within the break 53a, or, for example, for convenience, the outer casing may be absent at the break, and the inner spacer may be present within the break 53a. Functionally important is the break 53a within the outer conductive shield 53.

[0087] The coaxial cable 51 of a certain length can be considered to include two portions, namely, a first portion 51a on the first side of the supply point 52a and a second portion 51b on the second side of the supply point 52a. Thus, the first break 53a is provided within the first coaxial cable portion 51a, and the second break 53a is provided within the second coaxial cable portion 51b. The outer shield 53 within the first coaxial cable portion 51a is electrically connected to the outer shield 53 within the second coaxial cable portion 51b, and as a result, there is a central portion 53b of the shield that is electrically continuous between the two breaks 53a. This central portion 53b of the shield acts as a radiating element during the operation of the antenna. There are two ends 53c of the shield beyond the central portion 53b, but these typically carry current and are not typically useful radiating elements.

[0088] In this embodiment, the first inductor 54a is provided toward the distal end of the first portion 51a of the coaxial cable and is electrically connected between the outer shield 53 (particularly the end portion 53c) and the inner core wire 52 at a position facing this distal end. The second inductor 54b is provided toward the distal end of the second coaxial cable portion 51b and is electrically connected between the outer shield 53 (particularly the end portion 53c) and the inner core wire 52 at a position facing this distal end.

[0089] In other embodiments, different forms of antennas may be provided. For example, the antenna can be configured as a monopole antenna in which a supply point is provided toward one end of a coaxial cable of a certain length, or a plurality of antennas may be provided. In any such case, each of the coaxial cables of a certain length or the coaxial cables of a certain length are provided with respective interruptions within the shield.

[0090] During operation, when an RF drive voltage from a supply source is applied at the supply point 52a, that is, when this voltage source is connected between the core wire in the first coaxial cable portion 51a and the core wire in the second coaxial cable portion 51b, current flows within the core wire 52 and the outer shield 53 and also through the inductors 54a and 54b. The central portion 53b of the outer shield 53 acts as a radiating element for transmitting an RF signal outward from the antenna 5 toward the subject to be scanned.

[0091] The characteristics of the coaxial cable itself, the inductors 54a, 54b, the length of the central section 53b, and the length of each end section 53c of the outer shield can be selected so as to give desirable transmission characteristics to the antenna 5.

[0092] FIG. 3 schematically shows again the dipole antenna shown in FIG. 2. Here, a supply cable 6 connected to the inner core wire 52 via a matching circuit 55 is shown.

[0093] In this embodiment, where appropriate values are selected for inductors 54a and 54b, the matching circuit can include a single capacitor connected in parallel with supply cable 6.

[0094] In other embodiments, matching circuit 55 may include additional or different components. Similarly, in addition to or instead of inductors 54a and 54b provided at the ends of a coaxial cable of a certain length, other components may be provided in these regions. In one alternative, the ends of coaxial cable 51 may remain open-circuited. In another alternative, the ends of coaxial cable 51 may be short-circuited. In further examples, other components such as resistors, capacitors, etc. may be connected and provided between the respective ends of core wire 52 and shield 53.

[0095] In at least some situations, each component provided at the ends of coaxial cable 51, i.e., inductors 54a and 54b or different components used in their place, can be selected to adjust antenna 5 for use at a specific drive frequency. In some situations, these components can be selected to enable adjustment when antenna 5 is driven at a selected frequency among a plurality of different frequencies at different times, or to facilitate adjustment when antenna 5 is driven at frequencies within a predetermined frequency range. In some cases, these components can be configured to include a switching circuit that can switch antenna 5 between a configuration suitable for adjusting antenna 5 to operate at a first predetermined frequency and a configuration suitable for adjusting antenna 5 to operate at a second drive frequency, or can be configured to enable switching between three or more different configurations, each configured such that antenna 5 is adjusted to be driven at its respective drive frequency. Similarly, of course, the drive source, which is either within main MRI scanner 1 or provided separately, can be configured to drive at one or more selected drive frequencies.

[0096] Inductors 54a, 54b and other characteristics of antenna 5 give rise to an input impedance. This input impedance is preferably selected such that it enables the use of a single component, such as a single capacitor within matching circuit 55. By appropriately selecting the inductance values of inductors 54a, 54b, i.e., these inductors, the real part of the input impedance of antenna 5 can be selected to be at least 10 ohms, and in some cases as close as possible to 50 ohms, where the supply cable is a 50-ohm coaxial cable. When the real part of the input impedance is, for example, at least 10 ohms and preferably within the region of 50 ohms, this can correspond to an enhancement of stability in the antenna and can facilitate the use of a single component within matching circuit 55.

[0097] Generally, the values of the inductances of inductors 54a, 54b (or the characteristics of other components provided at their end positions) can be used to control the energization characteristics of antenna 5 and thus its radiation characteristics.

[0098] FIG. 4 is a plot showing the total current along a coaxial cable 51 of a certain length of antenna 5 of the type shown in FIGS. 2 and 3, i.e., the total current flowing through antenna 5 at different points along its length. There are three traces in the plot, each corresponding to a different inductance value of inductors 54a, 54b and showing the respective different current profiles generated. The first trace 401 shows the current with an inductance value of 10.6 nH. The second trace 402 shows the current with an inductance value of 27.8 nH. The third trace 403 shows the current with an inductance value of 33.4 nH.

[0099] To facilitate good transmission characteristics without loss and to minimize unwanted heating of the subject being scanned, it is preferable to make the current profile on outer shield 53 relatively flat.

[0100] FIG. 5 is a plot showing the current level according to the position along the core wire 52 of the coaxial cable 51 of the antenna 5 of the type shown in FIGS. 2 and 3. Again, the plot shows three traces, each with a different inductance value of the inductors 54a, 54b. The first trace 501 shows the core wire current with an inductance value of 10.6 nH. The second trace 502 shows the core wire current with an inductance value of 27.8 nH. The third trace 503 shows the core wire current with an inductance value of 33.4 nH. Here, it can be seen that at some inductance values, much more current flows within the core wire 53, corresponding to greater losses, such as the current within the shield 53 driving the B1 magnetic field generation.

[0101] By appropriately modeling, it is possible to select the preferred values of the inductances of the inductors 54a, 54b that bring about a relatively flat total current while minimizing the current within the core wire 52 to give good transmission characteristics while minimizing losses.

[0102] In this example, for an antenna having a total length of 300 mm and an interruption 53a within the outer shield 53 provided 100 mm from the center, it has been found that an inductance value of about 28 nH results in a relatively flat current on the outside while reducing the core wire current. Further, this configuration brought about an input impedance having a real part of about 50 ohms, which facilitates the use of a single capacitor of 10 pF within the matching circuit 55.

[0103] Generally speaking, it has been found that by moving the interruption 53a within the outer shield 53 outward from the center of a coaxial cable 51 of a certain length, a flatter current distribution is brought about. However, without taking some improvement measures (such as introducing components such as inductors 54a, 54b at the ends of the coaxial cable 51 of a certain length), an increase in the gap between the interruptions 53 within the shield 53 can serve to increase the core wire current.

[0104] To determine the appropriate characteristics of the antenna 5 of other sizes, modeling / simulation software can be utilized to follow the process along the following lines. This process can include the following steps. 1) Determine the length of the central section 53b. Generally, this depends on the field of view desired for imaging. For a general-purpose MRI antenna, 20 cm / 200 mm is a good length as it is large enough to cover most organs. Let this be called the length L. 2) Simulate a section of a coaxial cable longer than L that has a supply source connected to the core wire 52 at the center, two interruptions 53a in the outer shield 53 each positioned at a distance of L / 2 from the center, and two inductors at each end of a coaxial cable 51 of a certain length that connect the core wire 52 to the shield 53. Let the length of the end section 53c from each interruption 53a to each end of the cable section be called X. Thus, the total antenna length is L + 2X. 3) Obtain a combination of the length X and the inductance values of the inductors 54a, 54b that results in the following three beneficial characteristics. i. A "flat" current distribution outside the shield 53 between the interruptions 53a, ii. A minimum amount of loss due to unwanted currents, iii. An input impedance at the supply source that can be easily matched to a supply cable (e.g., typically a 50-ohm coaxial cable).

[0105] The frequency or frequency range at which the antenna 5 should be driven during the execution of the above process can also be considered.

[0106] The following can be considered in relation to the desired measurement criteria. i. A flat current distribution. The flatness of the current distribution can be described using the coefficient of variation obtained by dividing the standard deviation by the average of the current amplitudes along the length L. The smaller this number, the better.

[0107] Some exemplary values are as follows. For a triangular current distribution that is zero at the discontinuity (which could be considered really bad), this value is 0.58. For the current distribution of a planar dipole having a length of 30 cm at 300 MHz, the CoV over length L is 0.22. For a conventional split dipole antenna that is 30 cm, the CoV for the central 20 cm is 0.17. For the coaxial dipole antenna 5 of the type described above where L = 20 cm and X = 5 cm, the CoV over the central 20 cm is 0.11 (which can be characterized as a very acceptable value). The best possible value as a possibility is 0 which is not actually achievable.

[0108] For example, when taking a typical length of the radiating element portion of an antenna of this type as 20 cm, a CoV higher than 0.2 is considered bad. Any value less than 0.15 is good, and any value less than 0.10 is very good.

[0109] ii. Minimum loss. The loss can be quantified by the percentage of the input power that is wasted. When the currents on the core wire 52 are high, they tend to heat the copper, thereby wasting power. The length X can be increased so as to be much greater than 5 cm. This can serve to reduce the current in the core wire 52, but wastes energy at these longer cable lengths. In a test, an antenna made using a 300 - mm cable having a discontinuity at 125 mm so as to give a 250 - mm field of view and with no inductor connected at the end of the cable was found to waste 33% of the input power, which is quite undesirable. When the gap is provided at 100 mm and no inductor is provided, 11% of the power is wasted, while by including the inductors 54a, 54b, the wasted power drops to 6.5%. Generally, losses above 20% of the input power may be unacceptable, while losses below 20% and down to about 10% are bad but may be acceptable depending on the situation. Losses below 10%, approaching 5% for example, are considered good.

[0110] iii. Suitable input impedance. To make the input impedance perfect, several trial and errors are required. The real part of the input impedance is an important factor in attempting to achieve optimal operation. It is ideal to consider the situation where the supply cable is a 50-ohm coaxial cable having a real part of the input impedance of about 50 ohms. On the other hand, any value exceeding 10 ohms may be effective. Input impedance below 10 ohms is more likely to cause problems. In at least some situations, it is beneficial that only a single component is required in the matching circuit 55. Overall, the most important things are the performance of the antenna 5 and possible losses. Therefore, when the current in the matching circuit 55 is low, even if there are multiple components in the matching circuit 55, this may not be a problem.

[0111] Generally, it is desirable that the real part of the input impedance is at least 5 ohms, more preferably at least 10 ohms. Further, it may be preferable that the real part of the input impedance is close to 50 ohms, or more generally, close to the impedance of the supply cable used to supply the antenna. It may be desirable that only one matching element is required in the matching circuit 55.

[0112] After modeling and selecting appropriate values, the antenna can be manufactured, which can include taking one or more coaxial cables of a certain length, creating appropriate interruptions within the shield, creating a supply point, and adding connection components and a matching circuit. Optionally, tests can be performed to also confirm the B1 magnetic field generated for a given input power and the resulting heating effect in the sample / phantom as a proxy for SAR.

[0113] Of course, when connection components other than the inductor are selected, the same process can be followed.

[0114] An antenna 5 of this type, for example, a dipole antenna of the type shown in FIGS. 2 and 3, has been found to be able to produce better performance during operation than a prior art split dipole antenna. An exemplary prior art split antenna can be found, for example, in DE202007015620U1.

[0115] This performance improvement is observable with respect to the B1 magnetic fields that can be generated and the associated SAR resulting from their use. FIG. 6 shows an example of the B1 magnetic field that can be generated by a prior art split dipole antenna compared to that generated by a coaxial dipole antenna 5 of the type shown in FIGS. 2 and 3, where it can be seen that the B1 magnetic fields generated are similar. Where advantages are seen, it is by considering the plot shown in FIG. 7 that shows the heating effect of each type of antenna, i.e., the effect of the difference in SAR caused by the two antennas 5. The first trace 701 shows the temperature rise caused by a prior art split dipole, and the second trace 702 shows the temperature rise caused by the use of a coaxial dipole of the type described above. Here, it can be seen that the heating effect provided by the split dipole over a certain range of positions within the subject is significantly greater than that resulting from the use of a coaxial dipole of the type shown in FIGS. 2 and 3.

[0116] Furthermore, an antenna of this type is simply constructed and is made from one or more coaxial cables of a certain length and optionally a minimum number of additional electrical components and is essentially flexible.

Claims

1. An MRI system RF transmission antenna configuration comprising an antenna having a conductive core wire and a coaxial cable of a certain length having a conductive outer shield extending therethrough, wherein the conductive core wire has a supply point configured for electrical connection to an RF source and at least one interruption portion partially provided within the conductive outer shield along the coaxial cable of the certain length, and the at least one interruption portion divides the conductive outer shield into at least two axially spaced conductive outer shield portions, such that when the RF source is connected to the supply point, at least one of the at least two conductive outer shield portions acts as a radiating element. The antenna is configured as a dipole antenna such that the coaxial cable of the certain length has a first coaxial cable portion on one side of the supply point and a second coaxial cable portion on the opposite side of the supply point, and the supply point is provided near the midpoint of the coaxial cable of the certain length, and the at least one interruption portion is configured to be in the first coaxial cable portion or the second coaxial cable portion. A first electrical component is provided near the distal end of the first coaxial cable portion and is electrically connected between the conductive outer shield and the conductive core wire at a position near the distal end of the first coaxial cable portion to control the electrical characteristics of the MRI system RF transmission antenna configuration, and a second electrical component is provided near the distal end of the second coaxial cable portion and is electrically connected between the conductive outer shield and the conductive core wire at a position near the distal end of the second coaxial cable portion to control the electrical characteristics of the MRI system RF transmission antenna configuration. An MRI system RF transmission antenna configuration.

2. The conductive outer shield within the first coaxial cable portion is electrically connected to the conductive outer shield within the second coaxial cable portion, and the conductive core wire within the first coaxial cable portion and the conductive core wire within the second coaxial cable portion are configured such that the RF source is connected therebetween at the supply point. The MRI system RF transmission antenna configuration according to claim 1.

3. At least one of the at least one interruption portion is partially provided in the conductive outer shield along the first coaxial cable portion of the fixed length, and at least another one of the at least one interruption portion is partially provided in the conductive outer shield along the second coaxial cable portion of the fixed length, whereby the conductive outer shield of the coaxial cable of the fixed length is divided into at least three conductive outer shield portions spaced apart in the axial direction. As a result, at least one of the at least three conductive outer shield portions acts as a radiating element when the RF source is connected to the supply point. The MRI system RF transmission antenna configuration according to claim 1 or 2.

4. At least one of the first electrical component and the second electrical component includes an inductor. The MRI system RF transmission antenna configuration according to any one of claims 1 to 3.

5. At least one of the first electrical component and the second electrical component is selected to adjust the MRI system RF transmission antenna configuration for operation while being driven at a predetermined RF source frequency. The MRI system RF transmission antenna configuration according to any one of claims 1 to 4.

6. Comprising a matching circuit provided at the supply point, and a power supply can be connected to the MRI system RF transmission antenna configuration via the matching circuit. The MRI system RF transmission antenna configuration according to any one of claims 1 to 5.

7. The at least one coaxial cable of fixed length has a length in the range of 10 cm to 100 cm. The MRI system RF transmission antenna configuration according to any one of claims 1 to 6.

8. The radiating element has a length in the range of 8 cm to 75 cm. The MRI system RF transmission antenna configuration according to any one of claims 1 to 7.

9. The ratio of the length of the radiating element to the total length of the coaxial cable is in the range of 0.2:1 to 0.9:

1. The MRI system RF transmission antenna configuration according to any one of claims 1 to 8.

10. The MRI system RF transmission antenna configuration is configured as an antenna configuration specific to a body part, and in a configuration selected to scan each body part, it includes a support structure for supporting the at least one coaxial cable of a fixed length. The MRI system RF transmission antenna configuration according to any one of claims 1 to 9.

11. The MRI system RF transmission antenna configuration according to any one of claims 1 to 10, comprising at least one RF reception coil.

12. An MRI system comprising the MRI system RF transmission antenna configuration according to any one of claims 1 to 11.

13. An MR-Linac system comprising the MRI system according to claim 12 and a medical linear accelerator system.

14. A PET-MR system comprising the MRI system according to claim 12 and a positron emission tomography system.

15. A thermal therapy MRI system comprising the MRI system according to claim 12 and a thermal therapy system.

16. A method of manufacturing the MRI system RF transmission antenna configuration according to claim 1, wherein the MRI system RF transmission antenna configuration comprises a dipole antenna having two coaxial cable portions, and each of the two coaxial cable portions is provided with respective interruptions partially within a conductive outer shield along the length of each said coaxial cable portion, the method comprising: a) selecting a desired length L of the radiating element of the antenna corresponding to the distance between each of the interruptions within the conductive outer shield; b) the two coaxial cable portions having interruptions within the conductive outer shield each at a distance L / 2 from the supply point and ends of the coaxial cable of length X beyond each of the interruptions, an RF source connected to the supply point, at least one first electrical component provided near the distal end of the first coaxial cable portion, and at least one second electrical component provided near the distal end of the second coaxial cable portion, modeling the antenna; c) determining a value of the length X and characteristics of at least one of the at least one first electrical component and at least one of the at least one second electrical component, wherein the value of the length X and the characteristics result in: i) flatness of the current distribution on the radiating element, ii) Minimizing losses in the conductive core wire and electrical components, and iii) Determining that the desired input impedance in the RF source is optimized, and d) Creating the MRI system RF transmission antenna configuration. A method comprising these steps.

17. The at least one first electrical component includes an inductor, the at least one second electrical component includes an inductor, and the step of determining the value of length X and the characteristics of the at least one first electrical component and the at least one second electrical component includes determining the value of length X and the value of the inductance of each inductor. The method for manufacturing an MRI system RF transmission antenna configuration according to claim 16.

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