Echo plane spectrum imaging system and method

Abstract

The invention relates to an echo plane spectral imaging system and method, in which an MRI system coil insert for use within a bore of a primary MRI system, the coil insert comprising at least one gradient coil for creating a spatially varying magnetic field along a respective axis and arranged to be electrically driven at an ultrasonic frequency.

Description

[0001] This application is a divisional application of the invention patent application filed on February 26, 2021, with application number 202110222737.8 and entitled "MRI Device". Technical Field

[0002] This invention relates to MRI apparatus, and more particularly to MRI system coil inserts for use with MRI systems, MRI system apparatuses including MRI systems with coil inserts or MRI systems with other components, and echo-plane spectroscopy imaging systems including MRI system apparatuses and methods for operating MRI systems. Background Technology

[0003] MRI (Magnetic Resonance Imaging) systems are widely used for imaging subjects and acquiring spectral information in the case of MRSI (Magnetic Resonance Spectroscopic Imaging). An MRI system typically includes a magnet assembly for creating a large static magnetic field B0, a set of radio frequency coils or antennas for generating an alternating magnetic field B1 and collecting magnetic resonance signals (in other words, acquiring magnetic resonance data), and a set of gradient coils that allow spatial coding in the B0 field to achieve tomographic imaging. Furthermore, in the case of MRSI, gradient coils are also used to allow spectral coding.

[0004] Spatial encoding of magnetic resonance signals is typically achieved through rapid transformations of three magnetic field gradients (X, Y, Z), which are created by gradient coils positioned around the scanner aperture, within which the object to be examined is placed.

[0005] Typically, in existing systems, gradient coils are driven within the 0-10kHz range. When driving gradient coils within the audible range (20Hz-20kHz), considerable effort has been made to reduce noise caused by the Lorentz force during gradient transformation. Such methods include using materials to suppress the generated noise.

[0006] To increase the spatiotemporal resolution of MRI, gradient systems can be driven to be "faster" and "stronger." In other words, higher gradient transition rates (T per m per s) and higher gradient strengths (mT per m). At the current gradient, performance may be primarily limited by uncomfortable peripheral nerve stimulation (PNS) caused by the rapid transition of strong magnetic field gradients. Gradient transitions induce electric fields and currents in conductive tissues such as muscles and nerves, and can lead to nerve depolarization and ultimately nerve stimulation. Summary of the Invention

[0007] Therefore, it is desirable to develop MRI devices and methods for operating MRI devices that can produce enhanced performance without increasing physical discomfort to patients due to audible effects and / or peripheral nerve stimulation. Similarly, it is desirable to develop MRI devices and methods for using MRI devices that can reduce audible effects and / or peripheral nerve stimulation and / or other causes of discomfort, regardless of the presence of performance enhancements in the MRI procedure itself. It is also of interest if scans can be completed more quickly, minimizing patient discomfort and / or obtaining results less affected by the time spent acquiring the scan.

[0008] According to a first aspect of the invention, an MRI system coil insert for use within a bore of a main MRI system is provided, the coil insert comprising at least one gradient coil for creating a spatially varying magnetic field along a corresponding axis and being arranged to be electrically driven at an ultrasonic frequency.

[0009] This allows for examinations using the main MRI system in conjunction with the insertion device. Using ultrasound frequencies has several advantages. First, because the Lorentz force acts for a shorter time, this results in less force on the coil itself. This means the coil can be lighter compared to driving it at a more conventional frequency, thus allowing for physical differences in the coil and the structures supporting them. Second, the gradient switching is inaudible to the patient. Third, it has been found that peripheral nerve stimulation (PNS) in examined patients may be lower when using ultrasound frequencies, supposedly because the nerves do not have enough time to respond to the switching field.

[0010] The coil insert can have a central region without gradient coil windings. This allows for a window within the insert through which the patient can see when their head is inside the insert. This can be facilitated by using a lighter coil or applying less force to the coil.

[0011] The insert is typically cylindrical. The insert may have a main axis. This main axis can be arranged to align with the main axis of the bore of the main MRI system used with the insert. In this case, the axes of the insert and the main MRI system can be aligned by being parallel to each other or coincident with each other.

[0012] The at least one gradient coil may include a Z-gradient coil for creating a spatially varying magnetic field along the main axis of the insert. The at least one gradient coil may include an X-gradient coil or a Y-gradient coil for creating a spatially varying magnetic field transverse to the main axis of the insert.

[0013] The coil insert may include a first gradient coil and a second gradient coil, wherein the first gradient coil is used to create a spatially varying magnetic field along a corresponding first axis, and the second gradient coil is used to create a spatially varying magnetic field along a corresponding second axis.

[0014] The first gradient coil may be a Z-gradient coil, which is used to create a spatially varying magnetic field along the main axis of the insert. The second gradient coil may be an X-gradient coil or a Y-gradient coil, which is used to create a spatially varying magnetic field transverse to the main axis of the insert.

[0015] Even when the coil insert includes a first gradient coil and a second gradient coil, the coil insert may still have a central region without gradient coil windings.

[0016] The coil insert may include a third gradient coil for creating a spatially varying magnetic field along a corresponding third axis transverse to the first and second axes. However, it is typically preferred to provide one or two gradient coils without a third gradient coil, as this helps to provide a central region free of gradient coil windings.

[0017] The main MRI system will have its own gradient coils, which can be used in conjunction with the gradient coils of the insert during operation. Therefore, for example, in cases where the insert lacks gradient coils for creating a spatially varying magnetic field along a specific axis, the gradient coils of the main MRI system can be used to achieve spatial encoding along that axis.

[0018] The Z-gradient coil may include at least one set of windings.

[0019] The Z-gradient coil may include a first set of windings disposed at a first end of the insert and a second set of windings disposed at a second end of the insert. In one embodiment, the region between the first set of windings and the second set of windings may not contain a Z-gradient coil winding.

[0020] It has been determined that, although creating a central region without windings may result in a smaller linear magnetic field, this field is tolerable in the current type of insert, especially when the insert is used for examinations involving the patient's head. In this case, the insert need not have a large axial length.

[0021] In some cases, the Z-gradient coil may include more than two sets of windings. For example, four or more sets of windings may be provided.

[0022] The windings can be arranged to allow for segmented gradients along the Z-axis. In other words, instead of providing a continuous gradient, such as a linear gradient, along the Z-axis, a series of gradient segments are provided along the axis. This means that, overall, a smaller maximum absolute magnetic field can be used to establish a gradient along the Z-axis. This, in turn, can help minimize peripheral nerve stimulation (PNS) of the patient being examined. This arrangement is more useful for longer inserts, i.e., those with a longer Z-axis. Such inserts can be used to examine larger areas of the patient; they can be whole-body inserts.

[0023] In other words, the windings can be arranged to allow a spatially non-monotonic gradient along the Z-axis.

[0024] The windings can be arranged to allow for a spatially multi-lobed gradient along the Z-axis.

[0025] The windings can be arranged to allow a gradient that oscillates in space along the Z-axis.

[0026] The windings can be arranged to allow a gradient along the Z-axis that has a spatial polynomial or sinusoidal variation.

[0027] It will be understood here that when referring to these gradient patterns, we are referring to the change in amplitude / magnitude of the magnetic field seen along the Z-axis.

[0028] The Z-gradient coil windings can be arranged in two layers, wherein the turns in the first layer are configured to be misaligned with the turns in the second layer. This can help improve the achievable spatial coding.

[0029] The first set of windings of the Z-gradient coil can be arranged in two layers, wherein the turns in the first layer are misaligned with the turns in the second layer. The second set of windings of the Z-gradient coil can also be arranged in two layers, wherein the turns in the first layer are misaligned with the turns in the second layer.

[0030] The X-gradient coil or Y-gradient coil may include at least one set of windings.

[0031] The X-gradient coil or Y-gradient coil may include a pair of windings disposed on radially opposite sides of the insert. Each winding in the pair may include a plurality of helically wound turns, wherein each complete turn has an inner arcuate segment, a first end segment extending outward to an outer arcuate segment, and a second end segment extending inward from the outer arcuate segment to a corresponding inner arcuate segment of a subsequent turn. Each inner arcuate segment may follow the sidewall of the insert. Each outer arcuate segment may follow the sidewall of the insert.

[0032] When a Z-gradient coil is provided and the Z-gradient coil includes a first set of windings and a second set of windings as defined above, an X or Y-gradient coil may be provided and the X or Y-gradient coil may be axially placed in the insert and located between the first set of windings and the second set of windings of the Z-gradient coil.

[0033] The X or Y gradient coil may include a third and a fourth set of windings, which are disposed on radially opposite sides of the insert and axially located between the first and second sets of windings of the Z gradient coil. Therefore, the third and fourth sets of windings can be disposed in regions where no Z gradient coil windings are present. Note that the third and fourth sets of windings are the same as the pair of windings described above; in this embodiment, they are the third and fourth sets of windings in the sense that they define the first and second sets of windings relative to the Z gradient coil.

[0034] The circumferential spacing can be provided between the third and fourth sets of windings, such that the insert has an area where neither the Z-gradient coil nor the X or Y-gradient coil is present. This area can be arranged as a window through which the patient can see when their head is placed in the insert.

[0035] Each of the third and fourth winding groups may include a plurality of spirally wound coils, wherein each complete coil has an inner arc segment, a first end segment and a second end segment, the first end segment extending outward to an outer arc segment, and the second end segment extending inward from the outer arc segment to a corresponding inner arc segment of a subsequent coil.

[0036] Each inner arcuate segment may follow the sidewall of the insert. Each outer arcuate segment may follow the sidewall of the insert.

[0037] Such a device can help maximize the axial length of the insert, along which the X or Y gradient coils can generate a linear magnetic field. It also allows for maximizing the circumferential spacing between the third and fourth sets of windings, thereby enabling the creation of a window for the patient.

[0038] The insert may be provided with a partially shielded coil for the at least one gradient coil, or it may not be provided with a shielded coil for the at least one gradient coil. When the insert includes a first gradient coil and a second gradient coil, the insert may be provided with a partially shielded coil for the first gradient coil and the second gradient coil, or it may not be provided with a shielded coil for the first gradient coil and the second gradient coil.

[0039] The insert may include at least one capacitor electrically connected to the at least one gradient coil to cause the respective gradient coil to resonate at a predetermined ultrasonic frequency. This contributes to the efficiency of driving the gradient coil at the predetermined frequency. It may also mean that a lower current from a high-impedance signal source can be used to drive the gradient coil, which can reduce inductive coupling with other coils / metallic objects in its area.

[0040] In the case where the insert includes a first gradient coil and a second gradient coil, at least one first capacitor may be electrically connected to the first gradient coil to cause the first gradient coil to resonate at a first predetermined ultrasonic frequency, and at least one second capacitor may be electrically connected to the second gradient coil to cause the second gradient coil to resonate at a second predetermined ultrasonic frequency.

[0041] The first predetermined ultrasonic frequency can be the same as the second predetermined ultrasonic frequency.

[0042] However, preferably, the first predetermined ultrasonic frequency is different from the second predetermined ultrasonic frequency.

[0043] This allows the two gradient coils to operate at different frequencies, which in turn can lead to sampling of more spatial frequencies during imaging, as this combination will generate Lissajous coding rather than circular or spiral coding. This, in turn, can increase the potential for accelerated imaging.

[0044] The potential problem has been identified by using two frequencies: the potential for producing audible sounds. It has been determined that this is due to a "beat" generated between the sounds created at the two frequencies.

[0045] Preferably, the frequency difference between the first predetermined ultrasonic frequency and the second predetermined ultrasonic frequency is an inaudible frequency. That is, an infrasonic frequency or an ultrasonic frequency. Typically, in practice, the first predetermined ultrasonic frequency and the second predetermined ultrasonic frequency are selected such that the frequency difference between them is at the infrasonic frequency.

[0046] According to another aspect of the present invention, an MRI system coil insert device is provided, comprising a coil insert as defined above and a signal generator device for electrically driving the at least one gradient coil at an ultrasonic frequency.

[0047] In the case where the at least one gradient coil includes a Z-gradient coil, the windings of the coil and the signal generator device can be arranged to provide a segmented gradient on the Z-axis.

[0048] The windings and the signal generator device can be arranged to provide a spatially non-monotonic gradient along the Z-axis.

[0049] The winding and the signal generator device can be arranged to provide a spatially multi-lobed gradient along the Z-axis.

[0050] The winding and the signal generator device can be arranged to provide a spatially oscillating gradient along the Z-axis.

[0051] The winding and the signal generator device can be arranged to provide a gradient along the Z-axis that has a polynomial or sinusoidal variation in space.

[0052] When the insert includes a first gradient coil and a second gradient coil, the signal generator device can be arranged to drive the first gradient coil at a first selected ultrasonic frequency and the second gradient coil at a second selected ultrasonic frequency. The first frequency and the second frequency can be the same as each other. The first frequency and the second frequency can also be different from each other.

[0053] Preferably, the frequency difference between the first selected ultrasonic frequency and the second selected ultrasonic frequency is an inaudible frequency. That is, an infrasound frequency or an ultrasonic frequency. Typically, in practice, the first selected ultrasonic frequency and the second selected ultrasonic frequency are chosen such that the frequency difference between them is at an infrasound frequency.

[0054] The first selected frequency can be the same as the first predetermined frequency. The second selected frequency can be the same as the second predetermined frequency.

[0055] Therefore, it should be noted that the first and second coils can be driven at selected frequencies regardless of whether capacitors are provided to make the coils resonate at those frequencies, although it is preferred to provide capacitors and drive the coils at their respective resonant frequencies.

[0056] According to another aspect of the invention, an MRI system apparatus is provided, the MRI system apparatus comprising an MRI system having a main aperture, and an MRI system coil insert as defined above for use within said aperture.

[0057] According to another aspect of the present invention, an MRI system apparatus is provided, the MRI system apparatus comprising an MRI system having a main aperture and an MRI system coil insert apparatus as defined above having a coil insert arranged for use within the aperture.

[0058] Although the above features have been introduced in the context of MRI system coil inserts, the features and ideas defined above may be used in the MRI system itself, where the context permits.

[0059] Therefore, according to another aspect of the invention, an MRI system including a coil device is provided, the coil device including at least one gradient coil for creating a spatially varying magnetic field along a corresponding axis and arranged to be electrically driven at an ultrasonic frequency.

[0060] Generally, where the context permits, the optional features described above are also optional features in this aspect of the invention. For the sake of brevity, all content will not be repeated here, but some content will be explicitly illustrated by way of examples.

[0061] The MRI system may include a signal generator device for electrically driving the at least one gradient coil at an ultrasonic frequency.

[0062] In one set of embodiments, the at least one gradient coil includes a Z-gradient coil, and the windings of the coil and the signal generator device are arranged to provide a segmented gradient on the Z-axis.

[0063] In other words, the winding and the signal generator device can be arranged to provide a spatially non-monotonic gradient along the Z-axis.

[0064] The winding and the signal generator device can be arranged to provide a spatially multi-lobed gradient along the Z-axis.

[0065] The winding and the signal generator device can be arranged to provide a spatially oscillating gradient along the Z-axis.

[0066] The winding and the signal generator device can be arranged to provide a gradient along the Z-axis that has a polynomial or sinusoidal variation in space.

[0067] The MRI system may include at least one capacitor electrically connected to the at least one gradient coil to cause the respective gradient coil to resonate at a predetermined ultrasound frequency.

[0068] The coil device may include a first gradient coil and a second gradient coil, wherein at least one first capacitor is electrically connected to the first gradient coil to cause the first gradient coil to resonate at a first predetermined ultrasonic frequency, and at least one second capacitor is electrically connected to the second gradient coil to cause the second gradient coil to resonate at a second predetermined ultrasonic frequency.

[0069] The first predetermined ultrasonic frequency can be the same as the second predetermined ultrasonic frequency.

[0070] However, preferably, the first predetermined ultrasonic frequency is different from the second predetermined ultrasonic frequency.

[0071] Preferably, the frequency difference between the first predetermined ultrasonic frequency and the second predetermined ultrasonic frequency is an inaudible frequency. That is, an infrasound frequency or an ultrasonic frequency. Typically, in practice, the first predetermined ultrasonic frequency and the second predetermined ultrasonic frequency are selected such that the frequency difference between them is at an infrasound frequency.

[0072] When the coil arrangement includes a first gradient coil and a second gradient coil, the signal generator arrangement can be configured to drive the first gradient coil at a first selected ultrasonic frequency and the second gradient coil at a second selected ultrasonic frequency. The first frequency and the second frequency can be the same as each other. Alternatively, the first frequency and the second frequency can be different from each other.

[0073] Preferably, the frequency difference between the first selected ultrasonic frequency and the second selected ultrasonic frequency is an inaudible frequency. That is, an infrasound frequency or an ultrasonic frequency. Typically, in practice, the first selected ultrasonic frequency and the second selected ultrasonic frequency will be selected such that the frequency difference between them is at an infrasound frequency.

[0074] The first selected frequency can be the same as the first predetermined frequency. The second selected frequency can be the same as the second predetermined frequency.

[0075] According to another aspect of the present invention, a method for operating an MRI system apparatus as defined above is provided.

[0076] According to another aspect of the present invention, an echo-plane spectral imaging system is provided, comprising an MRI system apparatus having an acquisition unit for acquiring magnetic resonance data and a reconstruction unit for reconstructing image and spectral information from the acquired magnetic resonance data. The acquisition unit includes: RF transmitting devices are configured to transmit single-band RF pulses or multi-band RF pulses; A first gradient coil is arranged to be driven at an ultrasonic frequency to create a spatially varying magnetic field along a first corresponding axis; a second gradient coil is arranged to be driven at an ultrasonic frequency to create a spatially varying magnetic field along a second corresponding axis; and A signal generator device for electrically driving the first gradient coil at a first selected ultrasonic frequency and driving the second gradient coil at a second selected ultrasonic frequency. The signal generator device is configured to apply multiple linear frequency modulated (CFM) pulses to the first gradient coil and multiple CFM pulses to the second gradient coil during a readout period as part of acquiring magnetic resonance data, thereby achieving spectral encoding and spatial encoding on the first and second corresponding axes; and The acquisition unit is arranged to read out magnetic resonance data during the readout period, and the reconstruction unit is arranged to reconstruct an image and spectral information related to the image from the magnetic resonance data read out during the readout period.

[0077] This allows for spatial encoding within the duration of each linearly frequency-modulated pulse and spectral encoding within the duration of the readout period. By using ultrasound frequencies and a series of linearly frequency-modulated pulses, MRSI data (i.e., MRI images and associated spectral data) can be obtained for examination periods that are significantly shorter than those achieved in the prior art. This avoids the loss or obfuscation of information over time, such as the flow or chemical changes of metabolites within the subject during the examination period.

[0078] Each linear frequency modulated pulse can have a length of less than 100 milliseconds, preferably less than 10 milliseconds. In one embodiment, each linear frequency modulated pulse has a length of approximately 1 millisecond. In another embodiment, each linear frequency modulated pulse has a length of approximately 0.5 milliseconds.

[0079] The signal generator device can be configured to apply at least 10 linear frequency modulated pulses, preferably at least 50 pulses, during the readout period. In one embodiment, 200 linear frequency modulated pulses can be applied during the readout period.

[0080] Therefore, the readout period can have approximately, for example, 100 milliseconds (e.g., 200 linear frequency modulated pulses every 0.5 milliseconds) or 200 milliseconds (e.g., 200 linear frequency modulated pulses every 1 millisecond). This, in turn, can result in bandwidths of 2 kHz and 1 kHz, and resolutions of the spectral data of approximately 10 Hz and 5 Hz, respectively.

[0081] The acquisition unit can be arranged to acquire magnetic resonance data during multiple readout periods to generate a set of magnetic resonance data.

[0082] The acquisition unit can be arranged such that the RF transmitting device transmits a single-band RF pulse or a multi-band RF pulse before the start of each readout period, and the acquisition can be arranged to apply a corresponding plurality of linear frequency modulated pulses to the first gradient coil and a corresponding plurality of linear frequency modulated pulses to the second gradient coil during each readout period, thereby realizing spectral coding and spatial coding on the first corresponding axis and the second corresponding axis.

[0083] The reconstruction unit can be arranged to reconstruct an image and spectral information associated with the image from the set of magnetic resonance data read out during the plurality of readout periods.

[0084] The first selected ultrasonic frequency and the second selected ultrasonic frequency can be the same frequency.

[0085] Preferably, the first selected ultrasonic frequency is different from the second selected ultrasonic frequency.

[0086] Preferably, the frequency difference between the first selected ultrasonic frequency and the second selected ultrasonic frequency is an inaudible frequency. That is, an infrasound frequency or an ultrasonic frequency. Typically, in practice, the first selected ultrasonic frequency and the second selected ultrasonic frequency will be selected such that the frequency difference between them is at an infrasound frequency.

[0087] The acquisition unit may include at least one first capacitor and at least one second capacitor, wherein the at least one first capacitor is electrically connected to the first gradient coil to cause the first gradient coil to resonate at a first predetermined ultrasonic frequency, and the at least one second capacitor is electrically connected to the second gradient coil to cause the second gradient coil to resonate at a second predetermined ultrasonic frequency.

[0088] The first predetermined ultrasonic frequency can be the same as the second predetermined ultrasonic frequency.

[0089] However, preferably, the first predetermined ultrasonic frequency is different from the second predetermined ultrasonic frequency.

[0090] Preferably, the frequency difference between the first predetermined ultrasonic frequency and the second predetermined ultrasonic frequency is an inaudible frequency. That is, an infrasound frequency or an ultrasonic frequency. Typically, in practice, the first predetermined ultrasonic frequency and the second predetermined ultrasonic frequency are selected such that the frequency difference between them is at an infrasound frequency.

[0091] The first selected frequency can be the same as the first predetermined frequency. The second selected frequency can be the same as the second predetermined frequency.

[0092] The acquisition unit may further include a third gradient coil, which is arranged to be electrically driven to create a spatially varying magnetic field along a third corresponding axis. The signal generator device may be arranged to electrically drive the third gradient coil and specifically provide spatial encoding on the third corresponding axis, thereby allowing selection of a slice of the object being examined from which the aforementioned magnetic resonance data or a set of magnetic resonance data is acquired during a corresponding readout period or multiple corresponding readout periods.

[0093] The acquisition unit can be arranged to acquire additional magnetic resonance data, or another set of magnetic resonance data, in subsequent operations to improve the signal-to-noise ratio of the acquired data.

[0094] The acquisition unit can be arranged to acquire additional magnetic resonance data, or another set of magnetic resonance data, in subsequent operations by using spatial coding on a third corresponding axis to select different slices of the examined object.

[0095] The MRI system device may include an MRI system having a main aperture and an MRI system coil insert device for use within the aperture.

[0096] The insert may include the first gradient coil and the second gradient coil.

[0097] The insert may include a coil insert according to the first aspect of the invention.

[0098] The third gradient coil can be a gradient coil of the MRI system itself. This allows the gradient coil to be further away from the patient, helping to keep any windows in the insert unobstructed.

[0099] Typically, the MRI system will also include corresponding gradient coils, which are arranged to create a spatially varying magnetic field along the first corresponding axis and to create a spatially varying magnetic field along the second corresponding axis. However, in performing the acquisition of magnetic resonance data according to the above-described other aspect of the invention, in at least some cases, these other corresponding gradient coils may not be used.

[0100] In other embodiments, there may be no insert, but instead the first gradient coil and the second gradient coil of the above-described other aspect of the invention may be provided in the body of the MRI system.

[0101] In other implementations, gradient coils can be used in the main MRI system and even in the insert on the same axis. Therefore, Z-gradient coils, for example, in the insert and in the main MRI system, can be used for encoding.

[0102] According to another aspect of the present invention, a method for echo-plane spectral imaging using an MRI system apparatus is provided, the MRI system apparatus having an acquisition unit for acquiring magnetic resonance data and a reconstruction unit for reconstructing images and spectral information from the acquired magnetic resonance data. The method includes: Apply a single-band or multi-band RF pulse to the object being inspected; The first gradient coil is electrically driven at a first ultrasonic frequency to create a spatially varying magnetic field along a first corresponding axis, and the second gradient coil is electrically driven at a second ultrasonic frequency to create a spatially varying magnetic field along a second corresponding axis. As part of acquiring magnetic resonance data, multiple linear frequency modulated pulses are applied to the first gradient coil and the second gradient coil during the readout period, thereby achieving spectral encoding and spatial encoding on the first and second corresponding axes, respectively. Magnetic resonance data is read out during the readout period; and The image and spectral information associated with the image are reconstructed from the magnetic resonance data read out during the readout period.

[0103] Note that, generally speaking and with any necessary modifications to the wording, all other features defined above after any aspect of the invention can be used as further features of all other aspects of the invention defined above. For the sake of brevity only, these other features will not be repeated after each aspect of the invention. Attached Figure Description

[0104] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 The MRI system setup is shown schematically. Figure 2 It schematically shows that it can be Figure 1 The MRI system coil insert shown is of the type used in the MRI system device and has a Z-gradient coil. Figure 3 A portion of an alternative MRI system coil insert is schematically shown, with only the Z-gradient coil and either the X or Y-gradient coil of the insert shown. Figure 4 A more detailed comparison with a portion of a conventional X or Y gradient coil is shown. Figure 3 The winding pattern of the X or Y gradient coil is shown in the figure; Figure 5 This diagram illustrates a set of alternative windings for a Z-gradient coil used to create a segmented Z-axis gradient in an alternative MRI system coil insert; and Figure 6 Examples are shown that can be used Figure 1 The timing diagram shows the echo-plane spectroscopy imaging technique performed by the type of MRI system device shown. Detailed Implementation

[0105] Figure 1 An MRI system apparatus is schematically illustrated, comprising a main MRI system 1 and an MRI system coil insert 2 in this embodiment. In this embodiment, the structure and operation of the main MRI system 1 are largely conventional. Therefore, although several aspects of the main MRI system are shown in the figures and will be discussed below, other aspects of the MRI system 1 are not shown or described in detail, but such details are, of course, well-known and understood in the field of MRI examination.

[0106] The MRI system insert 2 is arranged to be supplied and used in conjunction with existing MRI systems. This is certainly commercially advantageous because it means that existing MRI systems can be adapted to utilize the ideas of this invention, rather than requiring the development of an entirely new MRI system. That is, in an alternative, Figure 1 The MRI system device shown can be constructed from scratch, and in other alternatives, the features and functions of insert 2 can be incorporated into the body of MRI system 1 if desired.

[0107] exist Figure 1 For clarity, the MRI system coil insert 2 is shown externally to the main MRI system 1. However, during operation, the insert 2 will be moved to a position inside the main port B of the main MRI system 1. Figure 1 The position is shown by the dashed line. Figure 2 Schematic illustration Figure 1 The type of insert 2 shown is installed on the bed 3 of the MRI system 1, thereby... Figure 2 The position shown can be slidably moved between the position inside the aperture B of the MRI system 1. Figure 2 The position shown is the same as Figure 1 The position of the insert 2 shown corresponds to the position outside the hole B of the MRI system 1.

[0108] A variety of different types of inserts can be provided.

[0109] Figure 2 The insert 2' shown includes an insert Z gradient coil 2z, which includes a first set of windings 2z1 and a second set of windings 2z2, with a set of windings provided at each end of the insert 2.

[0110] The absence of a Z-coil winding in the central region between the first winding 2z1 and the second winding 2z2 results in a suboptimal field; however, it has been determined that these defects are tolerable in at least some cases. For example, these defects are tolerable because the space between the first windings 2z1 and 2z2 in an insert designed for examining a person's head is relatively small.

[0111] Figure 1 The insert 2 shown includes a similar insert Z-gradient coil 2z, which includes a first set of windings 2z1 and a second set of windings 2z2, and the insert 2 also includes an insert X-gradient coil 2x, which includes a first set of windings 2x1 and a second set of windings 2x2. Figure 3 The windings of these groups are shown more clearly in an isolated state.

[0112] exist Figure 1 and Figure 3 Insert 2 and Figure 2 In the insert 2', each Z-winding group 2z1, 2z2 itself comprises two sets of helically wound turns 2z1a, 2z1b, 2z2a, 2z2b. The second layer 2z1b, 2z2b is wound on top of the corresponding first layer 2z1a, 2z2a but slightly misaligned with the corresponding first layer 2z1a, 2z2a, so that the conductors in the outer group can provide a reverse current direction. It has been found that this improves the shielding of the gradient field to minimize eddy currents in the surrounding conductive material.

[0113] By combining Figure 3 consider Figure 4 You can see more clearly Figure 1 and Figure 3 The winding pattern of the X gradient coil 2x of the insert 2 is 2x1 and 2x2.

[0114] Figure 4 The inner layer of the X-gradient coil, which is wound in a more conventional manner, is shown. Figure 4 Compared to the prior art, the X-insertion gradient coil 2x has multiple windings 2x1 and 2x2. Each winding 2x1 and 2x2 is spirally wound, and each complete turn includes inner arc segments 2x1a and 2x2a, which are connected to outer arc segments 2x1c and 2x2c via first end segments 2x1b and 2x2b. The turn is completed by second end segments 2x1d and 2x2d, which lead inward and backward to the beginning of the next turn. The next turn begins from the corresponding inner arc segments 2x1a and 2x2a of the subsequent turn, and so on.

[0115] It can be seen that the arc of each inner arc segment has the same radius as the arc of the other inner arc segments. Similarly, the arc of each outer arc segment has the same radius as the arc of the other outer arc segments. The radius of the arc of the inner segment is smaller than the radius of the arc of the outer segment.

[0116] This winding arrangement results in a longer linear region in the axial direction of the insert compared to prior art winding arrangements, and allows for a larger circumferential spacing between the ends of the two sets of windings (2x1, 2x2) compared to prior art line profiles. There are trade-offs in the properties of the field generated by this winding, but it has been found that the benefits outweigh these issues in this system.

[0117] Figure 1 and Figure 3 The insert 2 shown is designed for use during examination of a patient's head (especially the brain). Therefore, the insert 2 is arranged to position the patient's head within the aperture of the insert. The arrangement of the Z-gradient coil 2z and X-gradient coil 2x in the insert allows for the provision of a window W (schematically shown in the image) to be placed within the insert 2. Figure 3 And its position is schematically shown in Figure 1 (As indicated in the middle), the user can see through this window when the user's head is inserted into insert 2. If needed, this window W can be an opening in the insert, or it can be an opening filled with transparent material.

[0118] In addition, Figure 2 In the type of insert 2' shown without the X-gradient coil, it will again be possible to create one or more windows W through which the user can see when their head is inside the insert 1. Of course, providing such a window helps patients avoid claustrophobia.

[0119] The MRI system device includes a signal generator device 4, which is configured to drive the gradient coils 2z and 2x of the insert 2.

[0120] Commercially, this signal generator device 4 can be supplied together with the insert 2, and the signal generator device 4 and the insert 2 can be considered together as an MRI system coil insert device that can be used in the existing MRI system 1. Of course, again, in other alternative embodiments, if a complete system is developed, a separate signal generator device may not be necessary, but can be incorporated into the system located within the main MRI system.

[0121] The signal generator device 4 is arranged to drive the gradient coils 2z and 2x in the insert 2 at an ultrasonic frequency. That is, the frequency is above the audible range.

[0122] In the case where there are two or more gradient coils in insert 2, for example in Figure 1 and Figure 3 In some cases, the same ultrasonic frequency can be used to drive each of the gradient coils 2z and 2x in the type of insert shown.

[0123] Alternatively and preferably, in the presence of two gradient coils (e.g., the two gradient coils 2z and 2x in insert 2), the signal generator device 4 is arranged to drive the two gradient coils at correspondingly different ultrasonic frequencies.

[0124] For example, in some implementations, a single ultrasonic frequency can be used and set to 20.2 kHz. In other implementations using two different frequencies, such as 22 kHz and 19.9 kHz, one frequency is used to drive the X-gradient coil 2x and the other is used to drive the Z-gradient coil 2z. In practice, as will be appreciated, there is considerable freedom in choosing which frequencies to use.

[0125] However, it has been found particularly advantageous if two distinct frequencies are first selected, and then if the difference between these two distinct frequencies is itself an inaudible frequency. This is because, as described in the introduction, preferred spatial coding can be achieved when the two frequencies are different, and a frequency difference within the inaudible range avoids the generation of audible “beat” signals created between the sounds generated at the two selected inaudible frequencies. Typically, two frequencies can be selected such that the frequency difference is below the range of human hearing. In other words, the frequency difference is below 20 Hz.

[0126] like Figure 1 As schematically illustrated, signal generator device 4 is arranged to drive each gradient coil 2x, 2z in the insert via corresponding capacitors Cx, Cz. The values ​​of these capacitors are selected so that the corresponding gradient coils 2x, 2z resonate at the frequency to be driven by signal generator device 4. This facilitates the use of a high-impedance signal generator and the delivery of lower current to the gradient coils 2x, 2c, which in turn helps to reduce coupling between them.

[0127] Note that the idea of ​​making a gradient coil resonate at a predetermined frequency includes situations where the gradient coil is an electrical entity driven by a signal generator, and the gradient coil consists of multiple discrete electrical entities that can be driven individually. Therefore, in the case of multiple independent windings in the gradient coil, each winding will resonate by providing an appropriate capacitor.

[0128] In this device, no RF shielding is provided in the insert 2 or between the insert 2 and the main MRI system. This helps to minimize losses caused by the generation of eddy currents, which would otherwise occur when the gradient coils 2x and 2z of the insert are driven at ultrasonic frequencies. Meanwhile, the insert, specifically the gradient coils 2x and 2z, is relatively transparent to signals to be transmitted and received by the main MRI system. Therefore, with the insert 2 positioned within the aperture B of the main MRI system, operation of the main MRI system can continue.

[0129] Generally, minimizing the amount of conductive material near the insert gradient coils 2x, 2z (or any gradient coils operating at ultrasonic frequencies) helps improve efficiency by minimizing the effects of eddy currents. This problem is mitigated as the spacing between the ultrasonically driven gradient coils and the metallic objects increases. Therefore, in this device, it is permissible to tolerate the fact that active gradient field shielding is suboptimal or even nonexistent for the main MRI system, and that other metallic objects are present in the main MRI system. This desire to minimize the amount of metal near the ultrasonically driven gradient coils i) makes the feasibility of using such frequencies less obvious, and ii) makes it more convenient to include them in the insert rather than in the main MRI machine in at least some cases.

[0130] Figure 5 An alternative form of the Z-gradient coil is schematically shown, which can be configured as an alternative form of the insert. Similarly, again, in the alternative, this form of Z-gradient coil can be configured within the body of the newly constructed MRI machine 1.

[0131] Here, the Z-gradient coil comprises four sets of windings 2z1-2z4, each set having the same construction as the multiple sets of Z windings 2z1, 2z2 described above. Again, these windings are arranged to be driven by the signal generator device 4 and are arranged to resonate at a selected drive frequency by including one or more capacitors (not shown).

[0132] In this case, instead of establishing a monotonically changing Z-gradient field in the insert via a Z-gradient coil as in the traditional case, the above-mentioned... Figures 1 to 4 In the described insert, a piecewise Z-gradient is created. That is, Figure 5The insert is arranged to create a more complex and varied Z-gradient field, rather than a monotonically varying one. Specifically, this can be established as multilobed and, for example, with a polynomial or sinusoidal spatial variation along the Z-axis. This, in turn, means that the maximum magnitude of the difference in the gradient field from one end of the axis to the other can be controlled, while still providing sufficient gradient variation per unit length along the insert. It has been determined that providing a segmented Z-gradient field may be advantageous, particularly in cases where longer inserts will be used (i.e., inserts for examining areas larger than the head), such as when the insert is used for whole-body examinations or when gradient windings are incorporated into a whole-body MRI machine. Furthermore, it has been determined that the SENSE (sensitivity-encoded) reconstruction technique, well-known in MRI technology, can be successfully deployed using a... Figure 5 The information obtained when acquiring magnetic resonance data using a segmented Z-gradient of the type of insert shown is illustrated. Furthermore, it has been determined that using such a segmented Z-gradient can reduce the occurrence of PNS episodes in the examined object, or alternatively allow for the use of a stronger Z-gradient before obtaining PNS episodes.

[0133] After a brief introduction to the main MRI system 1, the operation of the MRI system device will now be described.

[0134] Generally, a main MRI system 1 includes: an acquisition unit I for acquiring magnetic resonance data; and a reconstruction unit R for reconstructing images and spectral information related to those images from the acquired magnetic resonance data. Figure 1 In the illustrated device, the insert 2 and the signal generator device 4 form part of the acquisition unit I. That is, they cooperate with the elements of the acquisition unit I of the main MRI machine to acquire magnetic resonance data, which can then be reconstructed by the reconstruction unit R.

[0135] Generally, the acquisition unit I in the main MRI machine 1 includes a magnet, a coil assembly 5, and a control system 6. The control system 6 has an output unit 61 for controlling the operation of the magnet and coil assembly 5, and a receiver unit 62 for receiving information from the magnet and coil assembly 5. In this apparatus using the coil insert 2, the output unit 61 also sends a control signal to the signal generator unit 4, and in some embodiments, the receiver unit 62 may also receive output from the insert 2, although this is optional. In the apparatus now described, the reception of magnetic resonance data is performed within the main MRI machine 1 itself.

[0136] As mentioned above in the alternatives section, the components of insert 2 and the signal generator device can be incorporated into the MRI machine itself.

[0137] It should be understood that an MRI machine will typically include one or more "computers" for controlling operation and processing received data. Each such computer may include a processor, memory, and at least one data storage device. The control system 6 may be computer-implemented. The reconstruction unit R may be computer-implemented.

[0138] The magnet and coil assembly 5 includes: a main magnet 51 for creating a static field; X, Y, and Z gradient coils 52; an RF transmitting coil 53; and an RF receiving coil 54.

[0139] In operation, the output unit 61 transmits the drive current to the gradient coil 52, causing the transmitting coil 53 to output an appropriate radio frequency transmission pulse, while the receiver unit 62 receives the input from the receiving coil 54.

[0140] In this embodiment, the output unit 61 also provides a control trigger signal to the signal generator device 4 to allow it to generate gradient drive signals for driving the insert gradient coils 2x and 2z at appropriate timing.

[0141] In principle, any combination of the gradient coil 52 of the main MRI system 1 and the gradient coils 2x and 2z of the insert 2 can be used to give the desired coding effect.

[0142] Most typically, in the case where the insert 2 includes an X-gradient coil 2x and a Z-gradient coil 2z, these coils can be used in combination with the Y-gradient coil 52y of the main MRI machine 1.

[0143] Therefore, in a specific embodiment, the gradient coil 52y of the main MRI machine 1 can be used for spatial coding to select a specific slice for the object to be examined, and the Z gradient coil 2z and X gradient coil 2x in the insert 2 can be used for spatial coding within that slice.

[0144] When the insert only includes a Z-gradient coil (e.g., in...) Figure 2 As shown in the insert, the X gradient coil 52x and Y gradient coil 52y in the main MRI machine 1 can be used in conjunction with the Z gradient coil 2z in the insert 2.

[0145] In other cases, two gradient coils on a specific axis of the main MRI machine 1 and the insert 2 can be used together.

[0146] For example, when the X gradient coil 52x, Y gradient coil 52y, and Z gradient coil 52z from MRI machine 1 are used in combination with the Z gradient coil 2z of the insert, the following can be used: Figure 2 The insert shown is an example. This can provide different options for spatial encoding.

[0147] In the above case, the gradient coil 52 in the main MRI machine 1 will be driven at a conventional frequency, while the gradient coil in the insert 2 will be driven at an ultrasound frequency.

[0148] Figure 6 A timing diagram of echo-planar spectral imaging (EPSI) is shown, which can be performed using an MRI system device of the type described above. Specifically, when combined with the above regarding... Figure 1 and Figure 3 When the described types of inserts are used together, Figure 1 The MRI system device can be used as an echo-plane spectral imaging system, and according to Figure 6 The timing diagram operation is shown in the figure.

[0149] In this technique, the Z-gradient coil 2z of the insert 2 is used in conjunction with the X-gradient coil 2x of the insert and the Y-gradient coil 52y of the main MRI machine 1. Timing Figure 6 The illustration shows when execution Figure 6 The signals applied to these gradient coils during the technique. It should be understood that "Gz insert" refers to the signal applied to the Z gradient coil 2z of the insert, "Gx insert" refers to the signal applied to the X gradient coil 2x, and "Gy main body" refers to the signal applied to the Y gradient coil 52y of the main MRI machine.

[0150] Furthermore, in the timing diagram RF, it indicates the signal applied using the transmit coil 53 of the main MRI machine 1.

[0151] At the beginning of each repetition period (TR), a multiband pulse is applied by the RF transmit coil 53, and a slice selection pulse is applied to the Y gradient coil 52y of the MRI machine 1. Then, in the subsequent readout period, a corresponding plurality of linearly frequency-modulated pulses are applied, on the one hand, to the Z gradient coil 2z of the insert 2, and on the other hand, to the X gradient coil 2x of the insert 2. In this embodiment, each linearly frequency-modulated pulse has a length of 500 microseconds, and a total of 200 such linearly frequency-modulated pulses are applied to each of the Z gradient coil 2z and the X gradient coil 2x during the readout period.

[0152] In this embodiment, each linear frequency modulated pulse has the same waveform as each other linear frequency modulated pulse, and the linear frequency modulated pulses on the Z gradient coil 2z and the X gradient coil 2x are applied in phase with each other. However, other possibilities are also available.

[0153] The purpose of applying linear frequency modulated pulses is to capture a large K-space (the amplitude of each axis defines the spatial resolution on that axis) and the distance between K-space points / circles (the attenuation amplitude defines the field of view).

[0154] Using this technique, all spatial coding can be defined within each linear frequency modulated (LFM) pulse, while spectral information is encoded over a group of LFM pulses (i.e., over 200 LFM pulses in this embodiment). The number of LFM pulses, their lengths, and the total length of the LFM pulses determine the spectral bandwidth and resolution. Therefore, in the current embodiment, with each LFM pulse being 500 microseconds long, the bandwidth will be 2 kHz, and with a total LFM pulse length of 100 milliseconds, this will result in a resolution of approximately 10 Hz (excluding relaxation effects and other small order effects).

[0155] In another implementation, the length of each linear frequency modulation (LFM) pulse can be one millisecond. In this case, if 200 LFM pulses are applied again, this would result in a bandwidth of 1 kHz and a resolution of approximately 5 Hz, instead of the approximately 10 Hz resolution in the above embodiment.

[0156] Each linear frequency modulated pulse comprises the application of a finite time-varying amplitude of an ultrasonic frequency, with corresponding insert gradient coils 2z, 2x arranged to operate at that ultrasonic frequency. In this embodiment, the two ultrasonic frequencies are identical, although they may differ in other cases, resulting in the other benefits mentioned above.

[0157] In this embodiment, as indicated in the timing diagram, four lenses are used to improve the accuracy of the results. A second multiband pulse in MB2 is set to initiate the second lens, and an appropriate slice selection signal is applied to the Y-gradient coil 52y of the main MRI machine 1. Subsequently, during the readout period of this second lens, a second linearly modulated pulse sequence is applied to the Z-gradient coil 2z and X-gradient coil 2x of the insert 2. The entire process is repeated in the third and fourth lenses, allowing the acquired magnetic resonance data to be fed to the reconstruction unit R for image reconstruction and image-related spectral data. The reconstruction system R can be configured within software for performing SENSE (sensitivity-encoded) reconstruction.

[0158] If needed, further runs as described above can be performed to obtain more data in order to improve the signal-to-noise ratio.

[0159] Alternatively or additionally, further operations can be performed for different “Y-slices” of the object. That is, a different set of offset frequencies for slice selection can be applied to the RF coil 54 of the MRI machine 1 to select a second batch of slices for examination, etc.

[0160] It will be noted that, using this technique, spatial coding is performed in each linear frequency modulated pulse, and for a set of slices, both spatial coding and spectral coding can be performed in a total of approximately 100 milliseconds, using only one shot, or in a total of approximately 400 milliseconds, where... Figure 6 The illustrated embodiment uses four lenses.

[0161] This means that image and spectral data collected within a very short timeframe can be considered. This helps avoid information loss or result corruption that can occur in conventional techniques, which require much longer encoding times. In more traditional echo-plane spectral imaging techniques, data is encoded one voxel at a time in the region of interest, where spatial encoding is performed to efficiently select a specific voxel to begin with, and then the voxel is spectrally sampled before moving to the next voxel. This results in a much longer examination period of approximately several seconds.

Claims

1. An echo-plane spectral imaging system, comprising an MRI system apparatus, the MRI system apparatus having an acquisition unit for acquiring magnetic resonance data and a reconstruction unit for reconstructing image and spectral information from the acquired magnetic resonance data. The acquisition unit includes: RF transmitting devices are configured to output single-band RF pulses or multi-band RF pulses; A first gradient coil is arranged to be driven at an ultrasonic frequency to create a spatially varying magnetic field along a first corresponding axis. And a second gradient coil, arranged to be driven at an ultrasonic frequency, thereby creating a spatially varying magnetic field along a second corresponding axis; as well as A signal generator device for electrically driving the first gradient coil at a first selected ultrasonic frequency and driving the second gradient coil at a second selected ultrasonic frequency. The signal generator device is configured to, as part of acquiring the magnetic resonance data, apply a plurality of linear frequency modulated pulses as pulses with time-varying amplitudes to the first gradient coil and to the second gradient coil during the readout period, thereby achieving spectral encoding and spatial encoding on the first and second corresponding axes. as well as The acquisition unit is arranged to read out magnetic resonance data during the readout period, and the reconstruction unit is arranged to reconstruct an image and spectral information related to the image from the magnetic resonance data read out during the readout period, wherein the first selected ultrasonic frequency is different from the second selected ultrasonic frequency.

2. The echo plane spectrum imaging system according to claim 1, wherein each linear frequency modulated pulse has a length of less than 10 milliseconds.

3. The echo plane spectrum imaging system according to claim 1 or claim 2, wherein the signal generator device is configured to apply at least 10 linear frequency modulated pulses during the readout period.

4. The echo plane spectrum imaging system according to claim 1 or claim 2, wherein the acquisition unit is arranged to acquire magnetic resonance data during multiple readout periods to generate a set of magnetic resonance data, and the acquisition unit is arranged to cause the RF transmitting device to output a single-band RF pulse or a multi-band RF pulse before the start of each readout period, and the acquisition is arranged to apply a corresponding plurality of linear frequency modulated pulses to the first gradient coil and to the second gradient coil during each readout period to achieve spectral coding and spatial coding on the first corresponding axis and the second corresponding axis.

5. The echo plane spectrum imaging system according to claim 1 or claim 2, wherein the frequency difference between the first selected ultrasonic frequency and the second selected ultrasonic frequency is an inaudible frequency.

6. The echo plane spectral imaging system according to claim 1 or claim 2, wherein the acquisition unit includes at least one first capacitor and at least one second capacitor, the at least one first capacitor being electrically connected to the first gradient coil to cause the first gradient coil to resonate at a first predetermined ultrasonic frequency, and the at least one second capacitor being electrically connected to the second gradient coil to cause the second gradient coil to resonate at a second predetermined ultrasonic frequency.

7. The echo plane spectrum imaging system according to claim 6, wherein the first selected frequency is the same as the first predetermined frequency, and the second selected frequency is the same as the second predetermined frequency.

8. The echo plane spectral imaging system according to claim 1 or claim 2, wherein the acquisition unit further comprises a third gradient coil arranged to be electrically driven to create a spatially varying magnetic field along a third corresponding axis, and the signal generator device is arranged to electrically drive the third gradient coil to provide spatial encoding on the third corresponding axis, thereby allowing selection of a slice of the object being examined, from which the magnetic resonance data or the set of magnetic resonance data is acquired during a corresponding readout period or a plurality of corresponding readout periods.

9. The echo-plane spectral imaging system according to claim 1 or claim 2, wherein the MRI system device comprises an MRI system having a main aperture and an MRI system coil insert device for use within the aperture, the insert comprising a first gradient coil and a second gradient coil.

10. The echo planar spectral imaging system according to claim 1 or claim 2, wherein the MRI system device comprises an MRI system having a main aperture and an MRI system coil insert, the MRI system coil insert comprising at least one gradient coil for creating a spatially varying magnetic field along a corresponding axis and being arranged to be electrically driven at an ultrasonic frequency.

11. A method for echo-plane spectral imaging using an MRI system, the MRI system comprising an acquisition unit for acquiring magnetic resonance data and a reconstruction unit for reconstructing image and spectral information from the acquired magnetic resonance data. The method includes: Apply a single-band or multi-band RF pulse to the object being inspected; The first gradient coil is electrically driven at a first ultrasonic frequency to create a spatially varying magnetic field along a first corresponding axis, and the second gradient coil is electrically driven at a second ultrasonic frequency to create a spatially varying magnetic field along a second corresponding axis. As part of acquiring magnetic resonance data, during the readout period, multiple linearly frequency-modulated pulses with time-varying amplitudes are applied to the first gradient coil and multiple linearly frequency-modulated pulses with time-varying amplitudes are applied to the second gradient coil, thereby achieving spectral encoding and spatial encoding on the first and second corresponding axes; During the readout period, magnetic resonance data is read out; as well as An image and spectral information associated with the image are reconstructed from magnetic resonance data read out during the readout period, wherein the first selected ultrasound frequency is different from the second selected ultrasound frequency.

12. The echo plane spectrum imaging method according to claim 11, wherein the frequency difference between the first selected ultrasonic frequency and the second selected ultrasonic frequency is an inaudible frequency.

Images