Operating an MRI apparatus

Abstract

A method of operating a magnetic resonance imaging (MRI) apparatus includes exciting a body coil of the MRI apparatus to emit a radio-frequency signal, determining a center frequency of a resonance curve of the body coil, and calculating a magnet target frequency based on the determined center frequency. A magnet is ramped to the magnet target frequency.

Claims

1. A method of operating a magnetic resonance imaging (MRI) apparatus, the method comprising: computing a first magnet target frequency based on a first tolerance window; and computing a second magnet target frequency based on a second tolerance window; selecting a magnet target frequency from the first magnet target frequency and the second magnet target frequency; and ramping a magnet of the MRI apparatus to the selected magnet target frequency.

2. The method of claim 1, wherein the first tolerance window has a bandwidth of at most 5 kHz.

3. The method of claim 1, wherein the second tolerance window has a bandwidth of at least 10 kHz.

4. The method of claim 1, wherein the selected magnet target frequency is the first magnet target frequency when a magnet frequency is to be determined using a probe during a ramp procedure.

5. The method of claim 4, wherein the probe comprises a field probe or an array shim device.

6. The method of claim 1, wherein the selected magnet target frequency is the second magnet target frequency when a magnet frequency is to be estimated using a current sensor.

7. The method of claim 1, wherein the selected magnet target frequency is the second magnet target frequency when a magnet frequency is to be estimated using a Hall sensor or a fiber-optic magnetometer.

8. A magnetic resonance imaging (MRI) apparatus comprising: a first frequency determination module configured to identify a first magnet target frequency based on a first tolerance window; a second frequency determination module configured to identify a second magnet target frequency based on a second tolerance window; a selection module configured to select a magnet target frequency from the first magnet target frequency and the second magnet target frequency; and a ramp module configured to initiate a ramping procedure to ramp a magnet of the MRI apparatus to the selected magnet target frequency.

9. The MRI apparatus of claim 8, wherein a field strength of the magnet of the MRI apparatus is at most 1.0 Tesla.

10. The MRI apparatus of claim 8, wherein the magnet is a superconductive magnet.

11. The MRI apparatus of claim 8, further comprising a current sensor arranged to measure magnet current, wherein the current sensor comprises an ammeter shunt with a long-term measurement accuracy in the region of 500-10,000 ppm.

12. The MRI apparatus of claim 8, wherein a bandwidth of the body coil comprises at most 50 kHz.

13. In a non-transitory computer-readable storage medium that stores instructions executable by one or more processors to operate a magnetic resonance imaging (MRI) apparatus, the instructions comprising: computing a first magnet target frequency based on a first tolerance window; and computing a second magnet target frequency based on a second tolerance window; selecting a magnetic target frequency from the first magnet target frequency and the second magnet target frequency; and ramping a magnet of the MRI apparatus to the selected magnet target frequency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

(2) FIG. 1 shows a simplified circuit diagram of a superconductive low-field magnetic resonance imaging (MRI) apparatus according to an embodiment;

(3) FIG. 2 shows a simplified block diagram of an MRI apparatus according to an embodiment;

(4) FIG. 3 illustrates a determination of a magnet target frequency using an embodiment of a method;

(5) FIG. 4 shows a simplified circuit diagram of a superconductive low-field MRI apparatus according to an embodiment;

(6) FIG. 5 shows a simplified block diagram of an MRI apparatus according to an embodiment;

(7) FIG. 6 illustrates an exemplary determination of a target frequency based on a narrow tolerance window;

(8) FIG. 7 illustrates an exemplary determination of a target frequency based on a wide tolerance window;

(9) FIG. 8 shows a simplified block diagram of an MRI apparatus according to an embodiment;

(10) FIG. 9 shows a simplified circuit diagram of a superconductive low-field MRI apparatus according to an embodiment;

(11) FIG. 10 shows an exemplary graph of magnet field decay obtained using an embodiment; and

(12) FIG. 11 shows a first graph of exemplary patient scan center frequencies obtained using an embodiment of a method.

DETAILED DESCRIPTION

(13) In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

(14) FIG. 1 shows a greatly simplified circuit diagram of one embodiment of a superconductive low-field MRI apparatus 1 (e.g., an MRI apparatus or an MRI system). The MRI apparatus 1 includes various modules and units, most of which will be known to the skilled person and need not be explained here. The MRI apparatus 1 includes a main magnet 10 that generates a very homogenous main magnetic field B0. An MPSU 10P is used to supply current I.sub.10 to the magnet 10 during a ramp-up procedure when the magnet 10 is ramped to a previously determined target frequency. A switch assembly 17 including a superconducting switch in parallel with a bypass resistor is shown connected across the main magnet coil. The switch is closed during the ramp-up procedure so that a small amount of current passes through the bypass resistor. In this exemplary embodiment, an ammeter shunt S is used to measure the magnet current I.sub.10 during ramping so that a power supply controller 15 may estimate the magnet frequency and compare the estimated magnet frequency to the target frequency f.sub.T so that the ramp-up procedure may be halted when the target frequency f.sub.T has been attained. At this point, the switch 17 is opened again.

(15) FIG. 2 shows a simplified block diagram of one embodiment of the MRI apparatus 1 indicating the main magnet 10 and the body coil BC. The usual arrangement of additional coils such as shim coils, local coil, pickup coil, and a number of gradient coils may be assumed to be present. The diagram shows a body coil excitation unit 11 configured to excite the body coil BC to emit an RF signal at a chosen frequency f.sub.BC. A reflected RF signal f.sub.BC′ is detected, and a resonance curve of the reflected RF signal f.sub.BC′ is analyzed in frequency determination module 14 (e.g., a processor) to identify a corresponding center frequency f.sub.c. The center frequency f.sub.c is stored in a memory module 13 (e.g., a memory device) that may be realized as a memory module of the body coil BC, or as a memory module of a control unit (e.g., a controller, the processor, or another processor) of the MRI apparatus 1. The reflection coefficients of the system provide that the center frequency f.sub.c of the reflected body coil signal f.sub.BC′ is lower than the body coil frequency f.sub.BC.

(16) A target frequency computation module 12 (e.g., the processor or another processor) determines a magnet target frequency f.sub.T based on the identified center frequency f.sub.c. Depending on the type of ramp-up sequence that is to be carried out, an offset df may be added to the frequency f.sub.c. In an exemplary process flow, the center frequency f.sub.c may be identified, for example, by the manufacturer or at some point during the lifetime of the MRI apparatus 1. Either way, the center frequency f.sub.c is stored in the memory module 13. Before carrying out a ramp-up sequence, the center frequency f.sub.c is retrieved from the memory module 13 and adjusted as necessary or as desired by a suitable offset df to give the target frequency f.sub.T, and the magnet is ramped to the target frequency f.sub.T.

(17) A ramp control module is provided to initiate a subsequent ramping procedure at a suitable time. The magnet power supply unit 10P accordingly supplies current I.sub.10 to the magnet 10 during the ramp-up procedure in order to ramp the main magnet 10 to that target frequency f.sub.T.

(18) The units and modules described above may be realized as part of a central control system of the MRI apparatus 1.

(19) FIG. 3 shows an exemplary resonance curve of a body coil. The Y-axis shows reflection coefficients between 0 and 1. The X-axis shows frequency offset in kHz, with 0 corresponding to the minimum reflection coefficient. Such a curve is obtained by averaging the reflected body coil signal. The shape of the resonance curve 30 is determined largely by the Q-factor of the body coil. For the comparatively low magnet frequency of a low-field MRI system, the body coil has a high Q-factor during an imaging sequence (e.g., with a patient inside the body coil) on account of the low ohmic losses arising from lower conductivity. In a low-field MRI system, therefore, the quality of an imaging procedure is dependent on how well the magnet frequency and body coil frequency are matched. The lowest point or minimum of the resonance curve, corresponding to the center frequency f.sub.C of the reflected body coil signal, is identified and used to arrive at a magnet target frequency for a subsequent ramp-up sequence.

(20) The target frequency may be set as the identified center frequency f.sub.C that was identified in the resonance curve of the body coil reflection. However, an offset may instead be added to the target frequency. The magnitude of the offset may be chosen based on the shape of the resonance curve and/or on various parameters of the ramp-up sequence. For example, by identifying a maximum reflection coefficient as indicated in FIG. 3, a resulting offset df may be identified. Generally, it is desired to set the magnet target frequency to be higher than the body coil frequency. Therefore, the target frequency f.sub.T may be expressed as:
f.sub.T=f.sub.c+df  (1)

(21) Alternatively, a fraction of the offset may be used (e.g., 25% of the offset). In this case, the target frequency f.sub.T may be expressed as

(22) $\begin{array}{cc}{f}_T=f_c+\frac{\mathrm{df}}{4}& \left(2\right)\end{array}$

(23) To give an example, the center frequency f.sub.c of the reflected signal may be determined to be 20.0 MHz. Adding a suitable offset such as 50 kHz, the target frequency f.sub.T for the next ramp event is determined to be 20.05 MHz using the above equation. In this way, the target frequency f.sub.T may be identified based on a desired accuracy of the intended ramping procedure. The method of one or more of the present embodiments of using an echo experiment to determine the magnet target frequency is associated with a favorably high degree of accuracy (e.g., with an error of less than 1.0 kHz). In another example, the center frequency f.sub.c of the reflected signal may be determined to be 30.1 MHz. Adding a suitable offset such as 10 kHz, the target frequency f.sub.T for the next ramp event is determined to be 30.11 MHz using the above equation.

(24) Excitation of the body coil BC, measurement of the reflected body coil signal f.sub.BC′, and computation of the center frequency f.sub.c and the target frequency f.sub.T may be performed entirely independently of the main magnet field B0, so that the method of one or more of the present embodiments may be carried out when the magnet 10 is ramped down.

(25) FIG. 4 shows a greatly simplified circuit diagram of a superconductive low-field MRI apparatus 1. The MRI apparatus includes the main magnet 10 that generates a very homogenous main magnetic field B0. FIG. 5 shows a simplified block diagram of one embodiment of an MRI apparatus 1 indicating the main magnet 10. The usual arrangement of additional (e.g., a body coil BC, a shim coil, and a number of gradient coils) may be assumed to be present. An MPSU 10P is used to supply current I.sub.10 to the magnet 10 during a ramp-up procedure. A switch assembly 17 including a superconducting switch in parallel with a bypass resistor is shown connected across the main magnet coil. The superconducting switch is closed during the ramp-up procedure so that a small amount of current passes through the bypass resistor. When the desired magnetic field strength has been reached, the switch is opened.

(26) In this exemplary embodiment, a current sensor S is configured as an ammeter shunt and includes a shunt 18 and a galvanometer 19 arranged to measure current through the shunt 18. The measured current I.sub.10′ corresponds essentially to the current I.sub.10 through the magnet 10. Adjusting for the slight loss through the shunt, the magnet current I.sub.10 may be determined to an accuracy of 500-5,000 ppm. The current sensor components will age over time, and this aging may be quantified and used to adjust the measured current value as appropriate.

(27) The MRI system 1 may be delivered with a set of parameters such as a body coil RF bandwidth W.sub.BC and tolerance windows W1, W2 for use in computation of a target frequency. Any such parameters may be stored in a suitable memory module. FIG. 5 shows two frequency determination modules 20, 21, each of which is realized to identify a magnet target frequency f.sub.T1, f.sub.T2 based on a tolerance window W1, W2, a body coil RF bandwidth W.sub.BC, and an appropriate input variable. To compute the higher first target frequency f.sub.T1, the input variable to the first frequency determination module 20 is a measured RF signal f.sub.BC′ reflected from the body coil BC when a probe is placed inside the body coil BC during ramp-up. To compute the lower second target frequency f.sub.T2, the input variable to the second frequency determination module 21 is magnet current I.sub.10′ measured by the ammeter shunt S.

(28) A selection module 22 (e.g., the processor or another processor) is provided to select one of the computed magnet target frequencies f.sub.T1, f.sub.T2 as appropriate for the next re-ramp procedure. A ramp control module 23 (e.g., the processor or another processor) is provided to initiate a ramping procedure to ramp the main magnet 10 to the selected magnet target frequency f.sub.T1, f.sub.T2. The units and modules may be realized as part of a central control system of the MRI apparatus 1.

(29) FIG. 6 is a graph of amplitude against frequency, illustrating the computation of a target frequency f.sub.T1 for a ramping procedure, based on a narrow tolerance window W1. In this case, the upper boundary of the body coil radio-frequency bandwidth W.sub.BC may be either set as a fixed parameter or may be measured during a tuning procedure or during a manufacturing stage. For example, the body coil RF bandwidth W.sub.BC may be in the order of 50 kHz. This information may be stored in software as a system parameter or may be a parameter that is delivered with the body coil (e.g., in a memory such as an EEPROM or a flash memory that is part of the body coil system). The body coil RF bandwidth W.sub.BC may be read out from such a memory module by a controller of the MRI apparatus.

(30) In this comparatively accurate method, a probe is placed in the body coil BC, and the magnet frequency f.sub.1 is calculated.

(31) The first target frequency f.sub.T1 may then be expressed as
f.sub.T1=f.sub.1+½W.sub.BC−½W1  (3)

(32) To give an example, the frequency f.sub.1 of the magnet may be estimated using a probe to be about 20 MHz. Using a known body coil RF bandwidth W.sub.BC of 30 kHz and the known narrow window W1 of 1 kHz, the target frequency f.sub.T1 for the next ramp event is determined to be 20.014500 MHz using equation (3).

(33) FIG. 7 is a graph of amplitude against frequency, illustrating the computation of a second target frequency f.sub.T2 for a ramping procedure, based on a wide tolerance window W2. The upper boundary of the radio-frequency bandwidth W.sub.BC is known, as explained in FIG. 6 above. The wide tolerance window W2 is also known. The magnet frequency f.sub.2 is estimated using the comparatively inaccurate current I.sub.10′ measured using the shunt S of FIG. 4.

(34) The second target frequency f.sub.T2 may then be expressed as
f.sub.T2=f.sub.2+½W.sub.BC−½W2  (4)

(35) To give an example, the frequency f.sub.2 of the magnet may be estimated using the shunt to be about 20 MHz. Using a known body coil RF bandwidth W.sub.BC of 30 kHz and the known wide window W2 of 10 kHz, the target frequency f.sub.T2 for the next ramp event is determined to be 20.01 MHz using equation (4).

(36) This roughly computed target frequency f.sub.T2 may be used when an autonomous ramp-up sequence is to be performed or when there is no user available to place a field probe, for example.

(37) One embodiment of the MRI apparatus 1 includes a main magnet that generates a very homogenous main magnetic field B0. FIG. 8 shows a simplified block diagram of one embodiment of the MRI apparatus 1 indicating the main magnet 10, the body coil BC, and a receiver coil RC (e.g., a flat receive-only spine coil). The usual arrangement of additional coils such as shim coils, local coil, pickup coil and a number of gradient coils may be assumed to be present. During a patient scan, the body coil BC is excited to emit an RF signal, and the receiver coil RC detects a reflected RF signal 100. The reflected RF signal 100 is processed by a module 38 (e.g., the processor or another processor) that performs appropriate signal processing steps to identify the center frequency f.sub.c of the received signal 100. To exclude any outliers, a filter is applied in block 110, which may compare the present center frequency f.sub.c to the previous center frequency. The present center frequency f.sub.c is only approved if, for example, the present center frequency f.sub.c does not differ from a predecessor by more than ±50 ppm. If approved, the present center frequency f.sub.c is then stored in a memory module 40 (e.g., a memory device). These steps are repeated for successive patient scans, so that ultimately, a collection of center frequencies accumulates in the memory module 40.

(38) The frequency of a received RC signal is related to the magnetic field strength by the relationship

(39) $\begin{array}{cc}f=\frac{\gamma }{2\pi }B0& \left(5\right)\end{array}$
where γ is the gyromagnetic ratio. An analysis module (e.g., the processor or another processor) processes a plurality of the successively collected center frequencies f.sub.c1, . . . , f.sub.c1 to identify a trend. For example, a gradual shift in frequency indicates a gradual decay of the main magnetic field. The analysis module may compare an identified decay trend with an expected decay trend known from the magnet specification, stored, for example, in a memory module 130 (e.g., a memory device). A decision module 41 (e.g., the processor or another processor) may determine whether action is to be taken based on the information provided by the analysis unit 42 (e.g., the processor or another processor). For example, the decision module may issue an alert X if a field strength decay rate is observed to be faster than an expected or specified decay rate.

(40) A ramp control module 43 (e.g., the processor or another processor) is provided to initiate a ramping procedure at a suitable time in order to ramp the main magnet 10 to a target frequency f.sub.T. FIG. 9 shows a greatly simplified circuit diagram of an embodiment of such a superconductive low-field MRI apparatus 1. An MPSU 10P is used to supply current I.sub.10 to the magnet 10 during a ramp-up procedure, when the magnet 10 is ramped to the previously determined target frequency. A switch assembly 17 including a superconducting switch in parallel with a bypass resistor is shown connected across the main magnet coil 10. The switch is closed during the ramp-up procedure so that a small amount of current passes through the bypass resistor. In this exemplary embodiment, an ammeter shunt S is used to measure the magnet current I.sub.10 during ramping so that a power supply controller 15 may estimate the magnet frequency and compare the estimated magnet frequency to the target frequency f.sub.T so that the ramp-up procedure may be halted when the target frequency f.sub.T has been attained. At this point, the switch 17 is opened again.

(41) Returning to FIG. 9, a current measurement may be recorded at the end of a ramp-up sequence (e.g., when the magnet frequency has reached a target frequency). A calibration factor for the current sensor may be derived from a relationship between the current measurement and a subsequent center frequency. A calibration factor C may be expressed as a simple ratio of shunt current to center frequency, such as, for example

(42) $\begin{array}{cc}C=\frac{I_{10}}{f_c}& \left(6\right)\end{array}$

(43) The shunt current measurement and a patient scan center frequency measurement may be temporally close. Since the shunt current measurement may only be made during a ramp-up procedure, the center frequency measurement may be made during the patient scan following the ramp-up.

(44) The calibration factor C may be used in a subsequent ramp-up procedure to correct for aging effects of the shunt S. The calibration factor C may be forwarded to the controller 15 of the MPSU 10P, for example, so that subsequent current readings may be corrected, and the ramp-up procedure may be performed to a higher degree of accuracy.

(45) The units and modules described above may be realized as part of a central control system of the MRI apparatus 1.

(46) FIG. 10 is a graph of magnet field decay showing magnet field strength (in Tesla) against time (in a timescale of months or even years). The diagram shows an expected decay profile 50 and a deduced decay profile 50′ that is established by analyzing a series of patient scan center frequencies. In this exemplary case, the decay profile 50′ shows a faster rate of decay than the expected decay profile 50 and may result in a decision to carry out a re-ramp event sooner than originally scheduled.

(47) FIG. 11 indicates how a collection of patient scan center frequencies may be used to establish a magnet field decay characteristic. The identified center frequencies f.sub.c1, . . . , f.sub.cn are indicated by dots. The graph shows frequency on the X-axis (in MHz) against scan count along the Y-axis. The diagram shows a gradual decrease in the center frequencies f.sub.c1, . . . , f.sub.cn of the detected RC signal. Using an appropriate interpolation or best-fit technique, the frequencies may be translated into a decay profile 50′ of the main magnetic field as shown in FIG. 10.

(48) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(49) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.