WO2014027271A2

WO2014027271A2 - Magnetic field probe system with synchronous excitation for use in a magnetic resonance imaging system

Info

Publication number: WO2014027271A2
Application number: PCT/IB2013/056307
Authority: WO
Inventors: Miha Fuderer; Arne Reykowski; Charles Albert Saylor; George Randall DUENSING
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-08-13
Filing date: 2013-08-01
Publication date: 2014-02-20
Also published as: WO2014027271A3

Abstract

A magnetic field probe system (24) for determining an actual magnetic field pattern of a magnetic resonance (MR) imaging system, comprising: at least one field probe body (26) that contains an amount of a magnetic resonance-active species of nuclei (28); a radio frequency (RF) transmit antenna (30) arranged for resonant excitation of the amount of the resonance-active species of nuclei (28); a radio frequency (RF) generator (34) provided to supply RF power to the RF transmit antenna (30); a radio frequency (RF) receive antenna (32) provided for receiving magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28); an RF control unit (42) provided for adjusting the RF power supplied to the RF transmit antenna (30) by using a detected value of the emitted RF energy to maintain a transverse magnetization of the amount of a magnetic resonance-active species of nuclei (28); a method of using a field probe system (24) for determining a magnetic field pattern of a magnetic resonance (MR) imaging system (10), comprising steps of arranging at least one field probe body (26) that contains an amount of a magnetic resonance-active species of nuclei (28) at a location within an examination space (16) of the MR imaging system (10); exciting the amount of the magnetic resonance-active species of nuclei (28) at a resonance frequency; acquiring a magnetic resonance RF energy emitted from the amount of the magnetic resonance-active species of nuclei (28); adjusting at least one out of phase and frequency of the applied RF power, by using a detected value of the emitted RF energy to maintain a transverse magnetization of the amount of a magnetic resonance-active species of nuclei (28); and determining a magnetic field strength at the location of the at least one field probe body (26).

Description

Magnetic field probe system with synchronous excitation for use in a magnetic resonance imaging system

FIELD OF THE INVENTION

The invention pertains to a magnetic field probe system for use in a magnetic resonance (MR) imaging system, a method of using a field probe system for determining a magnetic field pattern of an MR imaging system, and an MR imaging system employing a magnetic field probe system.

BACKGROUND OF THE INVENTION

In the art of magnetic resonance (MR) imaging with MR systems having a main magnet for generating a substantially static magnetic field, it is known that several potential sources exist for an occurrence of image distortions, image artifacts during image reconstruction and an inability to acquire data in a quantitative rather than a qualitative manner. Such sources may be spatial inhomogeneities of the magnetic field or nonlinearities that are inherent to the MR system. Nonlinearities inherent to the system can be measured, calibrated and compensated if so required. However, there is also a class of nonlinearities that are patient-dependent, sequence-dependent or time -varying for various reasons and that can therefore not be predicted or corrected by calibration. For instance, fast switching of coils of a magnetic gradient coil system commonly used in MR systems for encoding may induce eddy currents which in turn distort the magnetic gradient field. Another example of non-static nonlinearities is a field inhomogeneity due to rapid changes in the susceptibility of a patient.

A magnetic field probe is a device intended to dynamically measure the actual magnetic field strength during an MR measurement sequence. Having a plurality of such devices arranged within a bore of an MR scanner of an MR system allows for characterizing the precise magnetic field pattern generated by a switching of the field-gradients, in addition to any other intended or unintended causes of magnetic field variations. With the knowledge of the actual magnetic field pattern, i.e. magnetic field strength and direction, that existed at the time of acquiring MR signals, MR images or spectra may be reconstructible with fewer artifacts and less distortion. This concept has been suggested before and is described, for instance, in document EP 1 582 886 Al. Various embodiments of magnet field probes for MR

applications have been reported in document EP 1 847 845 Al .

It is desirable to have the actual magnetic field strength available at all points of time of an MR measurement sequence that is carried out at a subject, so that the measurement sequence of the MR system can be planned without any accounting for timing or other requirements of magnetic field probe measurements.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a magnetic field probe system for determining an actual magnetic field pattern of a magnetic resonance (MR) imaging system having at least a main magnet for generating a substantially static magnetic field and a gradient coil system for generating a gradient magnetic field.

In a preferred embodiment, the magnetic field probe system comprises:

at least one field probe body that contains an amount of a magnetic resonance-active species of nuclei;

a radio frequency (RF) transmit antenna that is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei and that is provided to apply an RF magnetic field to the amount of the resonance-active species of nuclei for resonant excitation;

a radio frequency (RF) generator that is provided to supply RF power to the RF transmit antenna to resonantly excite the amount of the magnetic resonance-active species of nuclei;

a radio frequency (RF) receive antenna (RF) antenna that is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei and that is provided for receiving magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei;

an RF control unit that is provided for adjusting at least one out of phase and frequency of the RF power supplied to the RF transmit antenna (30), based on a detected value of the one out of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei, and for maintaining a magnetization of the amount of the magnetic resonance-active species of nuclei that is transverse to a direction of the magnetic field of the main magnet. The phrase "transverse magnetization", as used in this application, shall be understood particularly as a component of a magnetization vector of the amount of the magnetic resonance-active species of nuclei at right angles to a direction of the main magnetic field. A precession of a vector representing the transverse magnetization at the Larmor frequency is responsible for a detectable MR imaging signal. In the absence of externally applied RF energy, the transverse magnetization will decline to zero with a characteristic time constant T2.

This decline of the transverse magnetization is due to a dephasing of the protons and is sometimes named spin-spin or transverse relaxation. The time constant T2 is a measure of time until phase coherence is lost among nuclei spinning perpendicular to the main field. It is therefore obvious to the one of skills in the art that, if the transverse magnetization of the amount of a magnetic resonance-active species of nuclei is maintained by adjusting the phase and/or the frequency of the RF power used for exciting the nuclei on the basis of the phase and/or the frequency of the magnetic resonance RF energy that is emitted by the amount of the magnetic resonance-active species of nuclei, the actual magnetic field strength can readily be determined at any point of time from receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei. In particular, it is not necessary to wait for a previous excitation to decline. The transverse magnetization of the amount of a magnetic resonance-active species of nuclei may therefore be maintained at least for the duration of an MR measurement sequence.

The RF power supplied to the RF transmit antenna to resonantly excite the amount of the magnetic resonance-active species of nuclei may consist of a train of RF pulses, wherein the RF control unit is provided for adjusting at least one out of phase and frequency of a subsequent RF pulse to be supplied to the RF transmit antenna by

extrapolating at least one out of phase and frequency detected from a received MR RF pulse that was emitted in response to a preceding supplied RF pulse. The phrase "RF pulse", as used in this application, shall be understood particularly as a wave packet of a radio frequency that has a duration that is by far longer than a periodic cycle of the radio frequency. The phrase "RF pulse train", as used in this application, shall be understood particularly as a sequence of RF pulses that are substantially separated by time gaps, and shall also encompass such sequences wherein at times during the time gaps, an RF amplitude is not zero but lower than an amplitude of the RF pulses. The RF control unit may be provided for detecting one out of phase and frequency of the magnetic resonance RF energy emitted in response to a first RF pulse. Based on the detected value, the control unit may be provided for adjusting the at least one out of phase and frequency of the RF power of a second RF pulse that is to follow the first pulse to be supplied to the RF transmit antenna. In order to maintain the transverse magnetization of the amount of the magnetic resonance- active species of nuclei, the adjustment may be carried out such that the RF energy of the second pulse supports a precession of spins of the nuclei and is substantially synchronous with it. By that, the transverse magnetization can effectively be maintained during a sequence of MR images to be carried out by the MR imaging system.

In an alternative embodiment, the RF power supplied to the RF transmit antenna to resonantly excite the amount of the magnetic resonance-active species of nuclei may consist of a continuous RF wave, and the RF control unit may comprise a phase-locked loop (PLL) circuit that is provided for adjusting of phase and frequency of the RF power supplied to the RF transmit antenna to resonantly excite the amount of the magnetic resonance-active species of nuclei, based on detected values of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei. The spins of the nuclei in the amount of the magnetic resonance-active species of nuclei may be kept at a substantially constant flip angle, and the PLL circuit ensures that the radio frequency fed the RF transmit antenna stays on resonance. The adjusted frequency and phase of the RF power supplied to the RF transmit antenna contain real time information on an actual magnetic field strength at a location of the field probe body.

In a further aspect of the invention, the magnetic resonance-active species of nuclei are chemically bound in an identical way. By that, a chemical shift of resonance frequencies of the magnetic resonance-active species of nuclei can be kept low and a narrow- banded emission spectrum of the amount of the magnetic resonance-active species of nuclei with a well-defined phase and frequency can be achieved, which can alleviate requirements concerning a detection of the value of the one out of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei.

In another preferred embodiment of the magnetic field probe system, the magnetic resonance-active species of nuclei is ¹⁹F (fluorine). ¹⁹F shows a high relative sensitivity compared to 1H and may provide a strong emission of RF energy. The ¹⁹F nuclei may be chemically bound in molecules of hexafluorobenzene C₆F6. By that, an amount of a magnetic resonance-active species of nuclei in which the nuclei are chemically bound in an identical way can readily be provided. In yet another preferred embodiment of the magnetic field probe system, the RF transmit antenna is provided to apply the RF magnetic field to the amount of the resonance-active species of nuclei for resonant excitation at a first time of operation, and is further provided for receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei at another time of operation that is different from the first time of operation. In other words, the RF transmit antenna and the RF receive antenna may physically be one and the same object. This may allow for a compact design of the field probe body and also avoid cross-coupling between the RF transmit antenna and the RF receive antenna.

The magnetic field probe system may comprise a plurality of field probe bodies, each containing an amount of a magnetic resonance-active species of nuclei. By that, an actual magnetic field strength at a plurality of independently selected locations may be derived that can be used for measurement error reduction.

It is another object of the invention to provide a method of using a field probe system for determining a magnetic field pattern of a magnetic resonance (MR) imaging system having at least a main magnet for generating a substantially static magnetic field and a gradient coil system for generating a gradient magnetic field, the method comprising the following steps:

(a) arranging at least one field probe body that contains an amount of a magnetic resonance-active species of nuclei at a location within an examination space of the MR imaging system;

(b) exciting the amount of the magnetic resonance-active species of nuclei by applying RF power at a resonance frequency;

(c) acquiring a magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei in response of the applied RF power;

(d) adjusting at least one out of phase and frequency of the applied RF power, based on a detected value of the one out of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei in response of the applied RF power, such that a magnetization of the amount of a magnetic resonance-active species of nuclei that is transverse to a direction of the magnetic field of the main magnet is maintained; and

(e) determining a magnetic field strength at the location of the at least one field probe body. As is obvious to the one of skills in the art that, if the transverse magnetization of the amount of a magnetic resonance-active species of nuclei is maintained by adjusting the phase and/or the frequency of the RF power used for exciting the nuclei on the basis of the phase and/or the frequency of the magnetic resonance RF energy that is emitted by the amount of the magnetic resonance-active species of nuclei, the actual magnetic field strength can readily be determined at any point of time from receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei. In particular, this is true for points of time when MR signals of a subject are acquired, so that the measured actual magnetic field strength can be considered in an image reconstruction.

In a further aspect of the invention, the applied RF power of the resonance frequency for exciting the amount of the magnetic resonance-active species of nuclei consists of a train of RF pulses separated by time gaps, wherein the time gaps between successive RF pulses are shorter than a transverse relaxation time T2 of the magnetic resonance-active species of nuclei. Preferably, the time gaps are at most of the same order of magnitude as the transverse relaxation time T2 of the magnetic resonance-active species of nuclei. The phrase "same order of magnitude", as used in this application for comparing two time durations, shall be understood particularly such that a ratio of the two time durations is smaller than two or larger than 0.5, respectively. By that, phase coherence among nuclei spinning can be reestablished and the transverse magnetization maintained before dephasing occurs. For further illustration, by way of example a situation is considered in which a magnetic gradient field is ramping up or down, so that the frequency of the magnetic resonance signal will shift. On the basis of a detected and extrapolated frequency and phase, the frequency and phase of a subsequently applied RF pulse may be adjusted to be exactly or substantially in phase with an expected frequency and phase of the magnetic resonance signal to be generated.

An amount of applied RF power required to maintain the transverse magnetization can be kept small by keeping a maximum duration of any RF pulse of the train of RF pulses shorter than a minimum of any of the time gaps between successive RF pulses. This is especially advantageous with regard to avoiding RF interference and undesired signal coupling.

In another aspect of the present invention, the RF power at the resonance frequency for exciting the amount of the magnetic resonance-active species of nuclei is applied as a continuous wave, wherein a phase-locked loop (PLL) circuit is employed for adjusting the phase and the frequency of the applied RF power to maintain the magnetization of the amount of the magnetic resonance-active species of nuclei that is transverse to the direction of the magnetic field of the main magnet. Preferably, this method is applied to a magnetic resonance-active species of nuclei with a narrow-banded emission spectrum and a low chemical shift. The spins of the magnetic resonance-active species of nuclei may be kept at a substantially constant flip angle, and the phase-locked loop (PLL) circuit ensures that a frequency of the applied RF power stays on resonance. The frequency and phase of the of the applied RF power contain real time information on the actual magnetic field strength at the location of the amount of magnetic resonance-active species of nuclei.

It is yet another object of the invention to provide a magnetic resonance (MR) imaging system, comprising a main magnet for generating a substantially static magnetic field, a magnetic gradient coil system for generating gradient magnetic fields superimposed to the static magnetic field, an examination space provided to position a subject of interest within, at least one RF transmit antenna that is provided for applying an RF magnetic field at the examination space to excite nuclei of the subject of interest, at least one RF receive antenna that is provided for acquiring MR signals from the excited nuclei of the subject of interest, a magnetic field probe system provided for measuring an actual magnetic field pattern, and an image reconstruction unit provided for reconstructing MR images from the acquired MR signals and for considering a measured actual magnetic field strength in an MR image reconstruction. BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.

In the drawings:

Fig. 1 is a schematic illustration of a part of an embodiment of an MR imaging system in accordance with the invention, comprising a magnetic field probe system,

Fig. 2 illustrates a schematic setup of the magnetic field probe system pursuant to Fig. 1,

Fig. 3 is a schematic illustration of a second embodiment of a magnetic field probe system, and

Fig. 4 schematically depicts a third embodiment of a magnetic field probe system. DETAILED DESCRIPTION OF EMBODIMENTS

This description contains several embodiments of the invention. The individual embodiments are described with reference to particular groups of figures and are identified by a prefix number of the particular embodiment. Features the function of which is the same or basically the same in all embodiments are identified by reference numbers made up of the prefix number of the embodiment to which it relates, followed by the number of the feature.

Fig. 1 shows a schematic illustration of a part of an embodiment of a magnetic resonance (MR) imaging system 110 comprising an MR scanner 112. The MR imaging system 110 includes a main magnet 114 for generating a substantially static magnetic field with a magnetic field strength showing a temporal drift due to unavoidable losses and to varying environmental conditions. The main magnet 114 has a bore that provides an examination space 116 for a subject of interest, usually a patient, to be positioned within. Further, the MR imaging system 110 comprises a magnetic gradient coil system 118 for generating gradient magnetic fields superimposed to the static magnetic field. The magnetic gradient coil system 118 is concentrically arranged within the bore of the main magnet 114, as is well known in the art.

Further, the MR imaging system 110 includes an RF transmit antenna that is provided for applying an RF magnetic field at the examination space 116 to excite nuclei of the subject of interest (not shown), and an RF receive antenna that is provided for acquiring MR signals from the excited nuclei of the subject of interest (also not shown).

Moreover, the MR imaging system 110 comprises a magnetic field probe system 124 provided for measuring an actual magnetic field pattern. An image reconstruction unit 122 of the MR imaging system 110 is provided for reconstructing MR images from the acquired MR signals and for considering a measured actual magnetic field strength in an MR image reconstruction.

An MR imaging system control unit 120 of the MR imaging system 110 with a monitoring unit is provided to control functions of the MR scanner 112, as is commonly known in the art.

The magnetic field probe system 124 comprises a plurality of field probe bodies 126 that are arranged at various locations within the examination space 116 of the MR imaging system 110. Each of the field probe bodies 126 contains an amount of a magnetic resonance-active species of nuclei 128 which is ¹⁹F (fluorine nucleus; Fig. 2)). The fluorine nuclei are chemically bound in an identical way in molecules of hexafluorobenzene C₆F6, and thus show a low chemical shift, resulting in a narrow-banded, well-defined magnetic resonance frequency for a given magnetic field strength. The magnetic resonance frequency (Larmor frequency) of ¹⁹F is about 40 MHz/T.

The magnetic field probe system 124 is illustrated in Fig. 2 in more detail, exemplarily showing one of the plurality of field probe bodies 126 only for clarity reasons. The amount of hexafluorobenzene, a liquid at room temperature, is encapsulated in the field probe body 126 of cylindrical shape.

A radio frequency (RF) transmit antenna 130 designed as a helical coil is wrapped around the field probe body 126 and, by that, is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei 128. The RF transmit antenna 130 is provided to apply an RF magnetic field to the amount of the resonance-active species of nuclei 128 for resonant excitation. To this end, an RF generator 134 with a downstream amplifier 38 is provided to supply RF power to the RF transmit antenna 130 to resonantly excite the ¹⁹F nuclei.

The RF transmit antenna 130 is provided to apply the RF magnetic field to the amount of the resonance-active species of nuclei 128 for resonant excitation at a first time of operation. It is further provided for receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 128 at another time of operation that is different from the first time of operation. By that, the RF transmit antenna 130 adopts the function of an RF receive antenna 132 provided for receiving magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 128. In order to separate the RF power supplied to the RF transmit antenna 130 and the RF energy received by the RF transmit antenna 130, the magnetic field probe system 124 comprises a transmit/receive switch 36, one for each field probe body 126 of the plurality of field probe bodies 126. An operation mode of the transmit/receive switch 36 (i.e. transmit mode or receive mode) is controlled by an RF control unit 142 (Fig. 1) that is a part of the MR imaging system control unit 120 synchronous to the supply of RF pulses to the RF transmit antenna 130.

The RF power supplied to the RF transmit antenna 130 to resonantly excite the amount of the magnetic resonance-active species of nuclei 128 consists of a train of RF pulses with pulse duration of about 10 μβ. For a magnetic field strength of 1.5 T at the location of the field probe body 126, each of the RF pulses contains about 600 periods of the resonant radio frequency of about 60 MHz. The RF pulses are separated by time gaps of 190 duration, so that the time gaps between successive RF pulses are shorter than a transverse relaxation time T2 of the magnetic resonance-active species of nuclei ¹⁹F, and a maximum duration of any RF pulse of the train of RF pulses is shorter than a minimum of any of the time gaps between successive RF pulses. Therefore, at 95% of the time the transmit/receive switch 36 is in the receive mode.

After a first RF pulse is provided to the RF transmit antenna 130, the amount of the magnetic resonance-active species of nuclei 128 is resonantly excited at a resonance frequency that depends on the actual magnetic field strength present at the location of the field probe body 126 at the time of application of the RF pulse. In response to the provided first RF pulse, RF energy is emitted by the amount of the magnetic resonance-active species of nuclei 128. After the supply of the first RF pulse to the RF transmit antenna nna 130, the RF generator 134 switches the transmit/receive switch 36 from the transmit mode into the receive mode, and the emitted RF energy is received by the RF transmit antenna 130 that is then acting as the RF receive antenna 132. A signal corresponding to the emitted RF energy is amplified by another amplifier 40 and fed via the RF generator 134 to the RF control unit 142, which is provided to detect both phase and frequency of the signal. Based on the detected value of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 128, the RF control unit 142 is provided to adjust phase and frequency of a second RF pulse to be supplied to the RF transmit antenna 130 subsequent to the first RF pulse such that a magnetization of the amount of a magnetic resonance-active species of nuclei 128 that is transverse to a direction of the magnetic field of the main magnet 114 is maintained by extrapolating phase and frequency as detected from the received RF energy that was emitted in response to the preceding supplied first RF pulse.

As mentioned before, the actual magnetic field strength can readily be determined at any point of time from receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 128. As indicated in Fig. 2, the RF control unit 142 is designed to determine the magnetic field strengths at the locations of the field probe bodies 126 at the time of application of the RF pulse, and to provide the determined magnetic field strengths as an input for correction to the image reconstruction unit 122 of the MR imaging system 110.

In order to enable carrying out the steps described above, the RF control unit 142 is equipped with a software module 146 comprising the steps as a converted program code that is implementable in and executable by the RF control unit 142. Fig. 3 is a schematic illustration of a second embodiment of a magnetic field probe system 224. The magnetic field probe system 224 comprises a plurality of magnetic field probe bodies 226 to be arranged at locations within the examination space 216 of the MR imaging system 210. The field probe bodies 226 are identical to the ones of the embodiment pursuant to Fig. 2. Again, only one of the plurality of field probe bodies 226 is exemplarily shown for clarity reasons.

In contrast to the precedent embodiment, the RF power supplied to excite the amount of the magnetic resonance-active species of nuclei 228 consists of an RF continuous wave. Further, the RF control unit 242 comprises a phase-locked loop circuit 244. Phase- locked loop circuits are usually designed as analog circuits build around a phase comparator between two input signals. In general, however, it is also possible to employ a digital version of a phase- locked loop circuit (digital phase- locked loop circuit).

Again, the RF transmit antenna 230 is used for transmitting RF power to the amount of the resonance-active species of nuclei 228 and for receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 228. The interaction between the RF transmit antenna 230 and the amount of the resonance-active species of nuclei 228 at resonance will cause a change of an input impedance of the RF transmit antenna 230 that is detected and compensated by an impedance match network 248. As the one of skills in the art is familiar with such networks it will not be described in more detail herein. The impedance change will in turn lead to a variation in the reflection of the transmitted RF power at an input port of the RF transmit antenna 230. The reflected signal can be used in conjunction with the phase-locked loop circuit 244 to adjust the transmitter frequency and phase. The phase- locked loop circuit 244 is provided for adjusting phase and frequency of the RF power supplied to the RF transmit antenna 230 to resonantly excite the amount of the magnetic resonance-active species of nuclei 228, based on detected values of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 228. This will keep the transmitter at the resonant frequency of the amount of the magnetic resonance-active species of nuclei 228, and a magnetization of the amount of a magnetic resonance-active species of nuclei 228 that is transverse to a direction of the magnetic field of the main magnet 214 is maintained. The adjusted frequency and phase of the RF power supplied to the RF transmit antenna 230 contain real time information on an actual magnetic field strength at a location of the corresponding field probe body 226 of the plurality of field probe bodies 226. Fig. 4 schematically depicts a third embodiment of a magnetic field probe system 324. Again, the RF power supplied to excite the amount of the magnetic resonance- active species of nuclei 328 consists of an RF continuous wave generated by an RF generator 334 and fed to an RF transmit antenna 330 via an impedance match network 348. Also, the RF control unit 342 comprises a phase-locked loop circuit 344. The magnetic field probe system 324 is identical to the embodiment pursuant to Fig. 3 except for the following features.

The RF transmit antenna 330 is used for transmitting RF power to the amount of the resonance-active species of nuclei 328. An RF receive antenna 332 that is designed as a helical coil and wrapped around the field probe body 326 is provided to receive a magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 328. Careful decoupling between the RF transmit antenna 330 and the RF receive antenna 332 is required in order to receive a weak response signal from the amount of the magnetic resonance-active species of nuclei 328 without too much cross-coupling from the transmitted RF power. A signal corresponding to the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 328 is fed into the phase- locked loop circuit 344, and an output of the phase-locked loop circuit 344 is again used for adjusting phase and frequency of the RF power supplied to the RF receive antenna 330 to resonantly excite the amount of the magnetic resonance-active species of nuclei 328, based on detected values of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei 328. This will keep the RF generator at the resonant frequency of the amount of the magnetic resonance-active species of nuclei 328, and a magnetization of the amount of a magnetic resonance-active species of nuclei 328 that is transverse to a direction of the magnetic field of the main magnet 314 is maintained. The adjusted frequency and phase of the RF power supplied to the RF receive antenna 330 contain real time information on an actual magnetic field strength at a location of the corresponding field probe body 326 of the plurality of field probe bodies 326.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

REFERENCE SYMBOL LIST

10 MR imaging system

12 MR scanner

14 main magnet

16 examination space

18 gradient coil system

20 MR imaging system control unit

22 image reconstruction unit

24 magnetic field probe system

26 field probe body

28 amount of magnetic resonance- active species of nuclei

30 radio frequency transmit antenna

32 radio frequency receive antenna

34 RF generator

36 transmit/receive switch

38 amplifier (transmit)

40 amplifier (receive)

42 RF control unit

44 phase- locked loop circuit

46 software module

48 impedance match network

T2 transverse relaxation time

Claims

CLAIMS:

1. A magnetic field probe system (24) for determining an actual magnetic field pattern of a magnetic resonance (MR) imaging system (10) having at least a main magnet (14) for generating a substantially static magnetic field and a gradient coil system (18) for generating a gradient magnetic field, comprising:

at least one field probe body (26) that contains an amount of a magnetic resonance-active species of nuclei (28);

a radio frequency (RF) transmit antenna (30) that is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei (28) and that is provided to apply an RF magnetic field to the amount of the resonance-active species of nuclei (28) for resonant excitation;

a radio frequency (RF) generator (34) that is provided to supply RF power to the RF transmit antenna (30) to resonantly excite the amount of the magnetic resonance- active species of nuclei (28);

a radio frequency (RF) receive antenna (32) that is arranged in close proximity to the amount of the magnetic resonance-active species of nuclei (28) and that is provided for receiving magnetic resonance RF energy emitted by the amount of the magnetic resonance- active species of nuclei (28);

an RF control unit (42) that is provided for adjusting at least one out of phase and frequency of the RF power supplied to the RF transmit antenna (30), based on a detected value of the one out of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28), and for maintaining a magnetization of the amount of a magnetic resonance-active species of nuclei (28) that is transverse to a direction of the magnetic field of the main magnet (14).

2. The magnetic field probe system (24) as claimed in claim 1, wherein the RF power supplied to resonantly excite the amount of the magnetic resonance-active species of nuclei (28) consists of a train of RF pulses, and wherein the RF control unit (42) is provided for adjusting at least one out of phase and frequency of a subsequent RF pulse to be supplied to the RF transmit antenna (30) by extrapolating at least one out of phase and frequency detected from a received RF energy that was emitted in response to a preceding supplied RF pulse.

3. The magnetic field probe system (24) as claimed in claim 1, wherein the RF power supplied to excite the amount of the magnetic resonance-active species of nuclei (28) consists of an RF continuous wave, and wherein the RF control unit (42) comprises a phase- locked loop circuit (44) that is provided for adjusting phase and frequency of the RF power supplied to the RF transmit antenna (30) to resonantly excite the amount of the magnetic resonance-active species of nuclei (28), based on detected values of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28).

4. The magnetic field probe system (24) as claimed in one of the preceding claims, wherein the nuclei of the amount of magnetic resonance-active species of nuclei (28) are chemically bound in an identical way.

5. The magnetic field probe system (24) as claimed in one of the preceding claims, wherein the magnetic resonance-active species of nuclei is ¹⁹F.

6. The magnetic field probe system (24) as claimed in one of the preceding claims, wherein the RF transmit antenna (30) is provided to apply the RF magnetic field to the amount of the resonance-active species of nuclei (28) for resonant excitation at a first time of operation, and is further provided for receiving the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28) at another time of operation that is different from the first time of operation.

7. The magnetic field probe system (24) as claimed in one of the preceding claims, comprising a plurality of field probe bodies (26), each containing an amount of a magnetic resonance-active species of nuclei (28).

8. A method of using a field probe system (24) for determining a magnetic field pattern of a magnetic resonance (MR) imaging system (10) having at least a main magnet (14) for generating a substantially static magnetic field and a gradient coil system (18) for generating a gradient magnetic field, the method comprising the following steps: (a) arranging at least one field probe body (26) that contains an amount of a magnetic resonance-active species of nuclei (28) at a location within an examination space (16) of the MR imaging system (10);

(b) exciting the amount of the magnetic resonance-active species of nuclei (28) by applying RF power at a resonance frequency;

(c) acquiring a magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28) in response of the applied RF power;

(d) adjusting at least one out of phase and frequency of the applied RF power, based on a detected value of the one out of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28) in response of the applied RF power, such that a magnetization of the amount of a magnetic resonance-active species of nuclei (28) that is transverse to a direction of the magnetic field of the main magnet (14) is maintained; and

(e) determining a magnetic field strength at the location of the at least one field probe body (26).

9. The method as claimed in claim 8, wherein the applied RF power of the resonance frequency for exciting the amount of the magnetic resonance-active species of nuclei (28) consists of a train of RF pulses separated by time gaps, wherein the time gaps between successive RF pulses are shorter than a transverse relaxation time T2 of the magnetic resonance-active species of nuclei (28).

10. The method as claimed in claim 9, wherein a maximum duration of any RF pulse of the train of RF pulses is shorter than a minimum of any of the time gaps between successive RF pulses.

11. The method as claimed in claim 8, wherein the applied RF power at the resonance frequency for exciting the amount of the magnetic resonance-active species of nuclei (28) is applied as a continuous wave, and wherein a phase-locked loop (PLL) circuit (44) is employed for adjusting the phase and the frequency of the applied RF power to maintain the magnetization of the amount of the magnetic resonance-active species of nuclei (28) that is transverse to the direction of the magnetic field of the main magnet (14).

12. A magnetic resonance (MR) imaging system (10), comprising: a main magnet (14) for generating a substantially static magnetic field;

a magnetic gradient coil system (18) for generating gradient magnetic fields superimposed to the static magnetic field;

an examination space (16) provided to position a subject of interest within; - an RF transmit antenna (30) that is provided for applying an RF magnetic field at the examination space (16) to excite nuclei of the subject of interest;

an RF receive antenna (32) that is provided for acquiring MR signals from the excited nuclei of the subject of interest;

a magnetic field probe system (24) provided for measuring an actual magnetic field pattern; and

an image reconstruction unit (22) provided for reconstructing MR images from the acquired MR signals and for considering a measured actual magnetic field strength in an MR image reconstruction.

13. A software module (46), provided for carrying out the following steps:

exciting an amount of a magnetic resonance-active species of nuclei (28) of at least one field probe body (26) by applying RF power at a resonance frequency;

acquiring a magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28) in response of the applied RF power;

- adjusting at least one out of phase and frequency of the applied RF power, based on a detected value of the one out of phase and frequency of the magnetic resonance RF energy emitted by the amount of the magnetic resonance-active species of nuclei (28) in response of the applied RF power, such that a magnetization of the amount of a magnetic resonance-active species of nuclei (28) that is transverse to a direction of the magnetic field of the main magnet (14) is maintained; and

determining a magnetic field strength at the location of the at least one field probe body (26);

providing the determined magnetic field strength at the location of the at least one field probe body (26) as an input for correction to an image reconstruction unit (22) of an MR imaging system (10);

wherein the steps are converted into a program code that is implementable in and executable by an MR imaging system control unit (20).