US-12625207-B2 - MRI apparatus and amplifying apparatus

US12625207B2US 12625207 B2US12625207 B2US 12625207B2US-12625207-B2

Abstract

In one embodiment, an MRI apparatus comprising an amplifying apparatus configured to supply an amplified RF signal to a load, wherein the amplifying apparatus comprises a plurality of parallel element circuits, each of which includes two amplification circuits installed in parallel and an impedance conversion circuit provided between the load and an output terminal of at least one of the two amplification circuits. The impedance conversion circuit is configured in such a manner that; a polarity of reactance as viewed from an output terminal of one of the two amplification circuits toward the load is opposite to a polarity of reactance as viewed from an output terminal of another of the two amplification circuits toward the load; and impedance as viewed from the output terminal of at least one of the two amplification circuits toward the load via the impedance conversion circuit differs between the plurality of parallel element circuits.

Inventors

Aoi SAKAMITSU
Kosuke Hayashi
Mitsuyuki Murakami
Hirofumi YAMAKI
Masahiko Ono

Assignees

CANON MEDICAL SYSTEMS CORPORATION

Dates

Publication Date: 20260512
Application Date: 20240520
Priority Date: 20230531

Claims (11)

1 . An MRI apparatus comprising: an RF coil configured to apply an RF (Radio Frequency) signal of a Larmor frequency to an object; and an amplifying apparatus configured to amplify the RF signal and supply the amplified RF signal to a load that includes at least the RF coil and the object, wherein: the amplifying apparatus comprises a plurality of parallel element circuits, each of which includes two amplification circuits installed in parallel and an impedance conversion circuit provided between the load and an output terminal of at least one of the two amplification circuits; the impedance conversion circuit is configured in such a manner that, in each of the plurality of parallel element circuits, a polarity of reactance as viewed from an output terminal of one of the two amplification circuits toward the load is opposite to a polarity of reactance as viewed from an output terminal of another of the two amplification circuits toward the load; and the impedance conversion circuit is further configured in such a manner that impedance as viewed from the output terminal of at least one of the two amplification circuits toward the load via the impedance conversion circuit is different between the plurality of parallel element circuits.
2 . The MRI apparatus according to claim 1 , wherein: the amplifying apparatus includes a total of n amplification circuits (n is a positive even number) by being provided with n/2 parallel element circuits as the plurality of parallel element circuits; the amplifying apparatus further comprises a divider configured to divide an input signal to be inputted to the amplifying apparatus into n input signals for respective amplification circuits, n phase adjustment circuits provided between the divider and respective input terminals of the n amplification circuits, a combiner configured to combine respective output signals outputted from the n amplification circuits and supply a combined signal to the RF coil as the RF signal, and n impedance conversion circuits provided between the combiner and respective output terminals of the n amplification circuits; each of the n phase adjustment circuits is configured to adjust a phase of a k-th amplification circuit (k is a natural number from 1 to n) of the n amplification circuits to a phase corresponding to a (n−k)/2n wavelength; each of the n impedance conversion circuits is configured to adjust a phase of the k-th amplification circuit to a phase corresponding to a (k−1)/2n wavelength; and the n phase adjustment circuits and the n impedance conversion circuits are configured in such a manner that a sum of a phase of a phase adjustment circuit and a phase of a impedance conversion circuit in each of the n amplification circuits corresponds to a (n−1)/2n wavelength.
3 . The MRI apparatus according to claim 2 , wherein each phase adjustment circuit and each impedance conversion circuit are configured to adjust a phase based on a transmission line length.
4 . The MRI apparatus according to claim 2 , wherein each phase adjustment circuit and each impedance conversion circuit are configured to adjust a phase by using an LC circuit composed of a capacitor C and an inductor L.
5 . The MRI apparatus according to claim 2 , wherein, in each of the plurality of parallel element circuits, a transmission line between an output terminal of the amplification circuits and the combiner has a wavelength that generates a phase difference of 180° between impedance as viewed from the output terminal of the another of the two amplification circuits toward the load and impedance as viewed from the output terminal of the one of the two amplification circuit toward the load.
6 . The MRI apparatus according to claim 2 , wherein the n amplification circuits are disposed in the amplifying apparatus in such a manner that heat of the n amplification circuits is dispersed based on relationship between heat generation amount and a phase with respect to impedance as viewed from the output terminal of each of the amplification circuits toward the load.
7 . The MRI apparatus according to claim 2 , wherein, in each of the plurality of parallel element circuits, the two amplification circuits are disposed to be not adjacent to each other within the amplifying apparatus.
8 . The MRI apparatus according to claim 2 , wherein the n amplification circuits are disposed within the amplifying apparatus in such a manner that a polarity of reactance as viewed from an output terminal of each of the amplification circuits toward the load is opposite to a polarity of reactance as viewed from an output terminal of an adjacent amplification circuit toward the load.
9 . The MRI apparatus according to claim 2 , further comprising: n variable phase shifters provided between the divider and the n amplification circuits; and a phase control circuit configured to perform feedback control on the n variable phase shifters in such a manner that a first phase at an input terminal of the combiner that receives an output signal from one of the n amplification circuits becomes a same phase as a second phase of another input terminal of the combiner that receives an output signal from another of the n amplification circuits.
10 . An MRI apparatus comprising: an RF coil configured to apply an RF signal of a Larmor frequency to an object; and an amplifying apparatus configured to amplify the RF signal and supply the amplified RF signal to a load that includes at least the RF coil and the object, wherein: the amplifying apparatus comprises a plurality of parallel element circuits, each of which includes two amplification circuits installed in parallel and an impedance conversion circuit provided between the load and an output terminal of one of the two amplification circuits; the impedance conversion circuit is configured in such a manner that, in each of the plurality of parallel element circuits, a polarity of reactance as viewed from an output terminal of one of the two amplification circuits toward the load is opposite to a polarity of reactance as viewed from an output terminal of another of the two amplification circuits toward the load; and the plurality of parallel element circuits are disposed in such a manner that the one of the two amplification circuits and the another of the two amplification circuits are alternately arranged.
11 . An amplifying apparatus configured to amplify an RF signal of a Larmor frequency to be applied to an object and supply the amplified RF signal to a load that includes at least an RF coil and the object, wherein: the amplifying apparatus comprises a plurality of parallel element circuits, each of which includes two amplification circuits installed in parallel and an impedance conversion circuit provided between the load and an output terminal of at least one of the two amplification circuits; the impedance conversion circuit is configured in such a manner that, in each of the plurality of parallel element circuits, a polarity of reactance as viewed from an output terminal of one of the two amplification circuits toward the load is opposite to a polarity of reactance as viewed from an output terminal of another of the two amplification circuits toward the load; and the impedance conversion circuit is further configured in such a manner that impedance as viewed from the output terminal of an amplification circuit toward the load via the impedance conversion circuit is different between the plurality of parallel element circuits.

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of priority of Japanese Patent Application No. 2023-089913, filed on May 31, 2023, the entire contents of which are incorporated herein by reference. FIELD Disclosed Embodiments relate to a magnetic resonance imaging (MRI) apparatus and an amplifying apparatus. BACKGROUND An MRI apparatus is an imaging apparatus that magnetically excites nuclear spin of an object placed in a static magnetic field with a radio frequency (RF) signal having the Larmor frequency and reconstructs an image on the basis of magnetic resonance (MR) signals emitted from the object due to the excitation. Application of the RF signal to the object is performed by: placing the object in a space surrounded by a cylindrical RF coil called a whole body (WB) coil; and applying a high-power RF signal amplified by an RF amplifier to the RF coil, for example. In such an environment, the load of the RF amplifier includes not only the RE coil but also the object. In other words, the load of the RF amplifier also changes due to factors excluding the RF coil, as exemplified by the physique and posture of the object placed inside the RF coil, the relative positional relationship of the object with respect to the RF coil, and the body motion of the object. For example, it is known that the load of the RF amplifier shows various changes due to the factors related to the object other than the RF coil, resulting in not only a resistive load but also an inductive load or a capacitive load. Conventionally, a high-power isolator is provided between the output terminal of the RF amplifier and the RE coil in order to suppress the influence of the above-described load changes. However, the high-power isolator is large in physical size and is expensive. In addition, the high-power isolator has a limit on its maximum output, which affects the output characteristics of the RF amplifier in some cases. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. 1 is a block diagram illustrating an overall configuration of an MRI apparatus according to one embodiment; FIG. 2 is a block diagram illustrating relationship between an amplifying apparatus and the MRI apparatus according to the embodiment; FIG. 3 is a block diagram illustrating a configuration of the amplifying apparatus according to the first embodiment; FIG. 4A is a block diagram illustrating a configuration of a conventional amplifying apparatus as Comparative Example 1; FIG. 4B and FIG. 4C are schematic diagrams showing change in output characteristics of the amplification circuits in Comparative Example 1 for illustrating a conventional problem; FIG. 5 is a block diagram showing a configuration of another conventional amplifying apparatus as Comparative Example 2 for illustrating another conventional problem; FIGS. 6A, 6B, 6C, and 6D are schematic diagrams illustrating the operation and effects of the amplifying apparatus in the case of a capacitive load; FIGS. 7A, 7B, 7C, and 7D are schematic diagrams illustrating the operation and effects of the amplifying apparatus in the case of an inductive load; FIG. 8 is a graph illustrating a drain current of an amplification circuit based on a phase of the load; FIG. 9 is a schematic diagram illustrating relationship between the phase of the load and heat generation in a Smith chart; FIG. 10A and FIG. 10B are schematic diagrams illustrating heat dispersion of the amplification circuits and their layout in the amplifying apparatus; FIG. 11 is a block diagram illustrating a configuration of an amplifying apparatus according to a modification of the first embodiment; FIG. 12 is a schematic diagram illustrating a first layout of a plurality of amplification circuits in the amplifying apparatus; FIG. 13 is a schematic diagram illustrating a second layout of a plurality of amplification circuits in the amplifying apparatus; FIG. 14 is a block diagram illustrating a configuration of an amplifying apparatus according to the second embodiment; FIG. 15 is a schematic diagram illustrating heat dispersion of the amplification circuits and their layout in the amplifying apparatus; FIG. 16 is a schematic diagram illustrating a layout of a plurality of amplification circuits in the amplifying apparatus according to the second embodiment; FIG. 17 is a block diagram illustrating a configuration of an amplifying apparatus according to the third embodiment; and FIG. 18A to FIG. 18D are circuit diagrams, in each of an input-side transmission line and an output-side transmission line of an amplifying apparatus according to the fourth embodiment are achieved by an LC circuit. DETAILED DESCRIPTION Hereinbelow, a description will be given of MRI apparatuses and amplifying apparatuses according to embodiments of the present invention by referring to the accompanying drawings. In one embodiment, an MRI apparatus comprising an RF coil configured to apply an RF (Radio Frequency) signal of a Larmor fre