Method for the magnetic resonance examination of a measurement object and to a radio-frequency unit of a magnetic resonance imaging scanner

Abstract

A method for the magnetic resonance examination of a measurement object is described, in which a measurement sequence is used in which the magnetic resonance response to the transmitted signal during transmission is measured. It is provided that a correction signal corresponding to the transmitted signal be generated and be used for correction of the response signal. To this end, the correction signal is modulated by a phase value and an amplitude value. The phase value and the amplitude value are automatically and iteratively customized for optimum correction of the response signal by an optimization method using a respective present state value of the measurement signal. Further, a radio-frequency unit (1) is described that can be used to carry out the method according to the invention.

Claims

1. A method for the magnetic resonance examination of a measurement object, comprising: radiating a transmitted signal into the measurement object, picking up a response signal emitted by the measurement object in reaction to the transmitted signal, the response signal being at least intermittently picked up during a period of radiation of the transmitted signal, generating a measurement signal by correction of the response signal and reconstructing a piece of information about the measurement object from the measurement signal, generating a correction signal corresponding to the transmitted signal and using the correction signal for correction of the response signal and iteratively carrying out the following steps for correction of the response signal automatically: ascertaining a present state value of the measurement signal, then ascertaining at least one of a present phase value or a present amplitude value for the correction signal using an optimization method, the optimization method using the present state value of the measurement signal as an input, then modulating a present section of the correction signal that corresponds to a present section of the transmitted signal with the at least one of the present phase value or the present amplitude value, then generating a present section of the measurement signal by subtraction of the present section of the correction signal from a present section of the response signal that is picked up at a same time as the present section of the transmitted signal is radiated, and also using the respective present section of the measurement signal for reconstruction of the information about the measurement object.

2. The method as claimed in claim 1, wherein the present state value of the measurement signal is the measure of at least one of a transmitted signal component or a signal strength in the measurement signal.

3. The method as claimed in claim 1, wherein the optimization method further comprises using at least one further previously ascertained state value of the measurement signal as an input.

4. The method as claimed in claim 1, wherein the optimization method further comprises ascertaining at least one of the respective present phase value or amplitude value by a gradient method.

5. The method as claimed in claim 4, further comprising in each iteration, altering a variable on which at least one of the phase value or the amplitude value is dependent, reading a direction in which the variable has been altered in a previous iteration, choosing a step size for an alteration of the variable, comparing the present state value of the measurement signal with the present state value that was previously ascertained and altering the variable in the read direction in an event of an improvement in the present state value and altering the variable in a direction opposite to the read direction in an event of a worsening of the present state value.

6. The method as claimed in claim 1, further comprising picking up the response signal completely during a period of radiation of the transmitted signal.

7. The method as claimed in claim 1, wherein the information about the measurement object is reconstructed from the measurement signal by using at least 90% of a total pickup time of the response signal.

8. The method as claimed in claim 1, wherein the modulating of the present section of the correction signal that corresponds to the present section of the transmitted signal with at least one of the present phase value or the present amplitude value is effected in voltage-controlled fashion using at least one of a phase modulator (13), an amplitude modulator (14) adjustable in voltage-controlled fashion, or a digital voltage controller.

9. The method as claimed in claim 1, wherein the measurement signal is amplified using a signal amplifier (9), the signal amplifier (9) having a dynamic bandwidth that is outside a signal strength of an amplitude-modulated correction signal.

10. The method as claimed in claim 1, further comprising first checking the present state value of the measurement signal and, in the event of a first limit value being exceeded, carrying out the steps for correction of the response signal until the state value of the measurement signal has reached a second limit value.

11. The method of claim 10, wherein the second limit value is identical to the first limit value.

12. The method of claim 10, wherein at least one of the first or the second limit value is greater than an estimate of a maximum state value of the measurement signal.

13. The method as claimed in claim 1, wherein in the event of the present state value of the measurement signal exceeding a third limit value, generating a most recently generated section of the measurement signal again.

14. The method as claimed in claim 1, wherein an on-resonant pulse is radiated into the measurement object in order to generate the present section of the measurement signal.

15. A radio-frequency unit (1) of a magnetic resonance imaging scanner comprising: a generator (2) for generating a radio-frequency signal, a signal divider (4) that has a signal input operatively connected to an output of the generator (2) and that has a first output and a second output, a transmission coil (5) that generates a transmitted signal and that is operatively connected to the first output of the signal divider (4), a receiving coil (6) that picks up a response signal forming a magnetic resonance signal from a measurement object, a modulator (8) that is operatively connected to the second output of the signal divider (4), the modulator (8) comprising a phase modulator (13) and/or an amplitude modulator (14), a signal combiner (7) that is operatively connected to the receiving coil (6) and the modulator (8) that generates a measurement signal, a control unit (11) operatively connected to the signal combiner (7) and the modulator (8), the control unit being configured such that the radio-frequency unit (1) carries out the method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is now described in more detail using a few exemplary embodiments, but is not restricted to these few exemplary embodiments. Further exemplary embodiments arise through combination of the features of single or multiple protective claims with one another and/or with single or multiple features of the exemplary embodiments.

(2) In the drawings:

(3) FIG. 1 shows an inventive radio-frequency unit of a magnetic resonance imaging scanner,

(4) FIG. 2 shows a simplified circuit diagram of a phase modulator that can be used in FIG. 1,

(5) FIG. 3 shows a profile of the modulating phase value as a function of an input voltage of the phase modulator shown in FIG. 2,

(6) FIG. 4 shows a profile of the modulating phase value as a function of an input voltage of an alternative phase modulator,

(7) FIG. 5 shows a simplified circuit diagram of an amplitude modulator that can be used in FIG. 1,

(8) FIG. 6 shows a profile of the modulating amplitude value as a function of an input voltage of the amplitude modulator shown in FIG. 5,

(9) FIG. 7 shows a simplified circuit diagram of an alternative amplitude modulator that can be used in FIG. 1,

(10) FIG. 8 shows a profile of the modulating amplitude value as a function of an input voltage of the amplitude modulator shown in FIG. 7,

(11) FIG. 9 shows a flowchart for a method according to the invention for the MR examination of a measurement object,

(12) FIG. 10 shows a sequence diagram from an MR measurement in which the method depicted in FIG. 9 can be used,

(13) FIG. 11 shows a flowchart for an alternative method according to the invention for the MR examination of a measurement object,

(14) FIG. 12 shows a sequence diagram for an MR measurement in which the method depicted in FIG. 11 can be used,

(15) FIG. 13 shows a measurement graph that shows the convergence behavior of a correction method configured according to the invention when the examined measurement object is moved in an MRI scanner, and

(16) FIG. 14 shows an alternative sequence diagram for an MR examination according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(17) In the description of different exemplary embodiments of the invention that follows, elements with a matching function are provided with matching reference numerals even if their design or shape is different.

(18) FIG. 1 shows an inventive radio-frequency unit 1 of a magnetic resonance imaging scanner that is not depicted, which radio-frequency unit can be used to carry out a method according to the invention. An RF generator 2 that can be used to generate an RF signal is connected to a low-noise signal amplifier 3 (low noise amplifier) that transmits the amplified RF signal to the signal divider 4. In the signal divider 4, the RF signal is split into a transmit signal v.sub.TX(t), which is transmitted to the transmission coil 5, and a corresponding correction signal c*v.sub.TX(t). The correction signal has the same signal profile as the transmit signal, but has its amplitude modulated with the factor c, for example with c=0.28.

(19) The transmit signal is converted by the transmission coil 5, so that a transmitted signal can be radiated into a measurement object (not depicted). The receiving coil 6 can then be used to pick up an MR signal emitted by the measurement object in reaction to the transmitted signal. A response signal s.sub.Rx(t) picked up using the receiving coil 6 is then transmitted to the signal combiner 7. In the signal combiner 7, the correction signal A*c*v.sub.Tx(t)*e.sup.j modulated by the modulator 8 is subtracted from the response signal. The subtracted signal is then amplified by the ultra-low-noise signal amplifier 9 (ultra low noise amplifier). The resultant measurement signal s(t)+v.sub.leak(t) is transmitted to an analog-to-digital converter 10 that digitizes the measurement signal.

(20) The digitized measurement signal is then transmitted to a computation unit 11. In the exemplary embodiment described here, the computation unit 11 is an external personal computer (PC). The computations to be performed by the computation unit 11 can also be performed by a reconstruction unit, for example, however, which also performs the reconstruction of the desired information about the measurement object from the digitized measurement signal, such as particularly an image reconstruction.

(21) The computation unit 11 sends the computation result U.sub.,U.sub.A to the modulator 8. The variables U.sub.,U.sub.A are voltage values in digital form that can be provided by the DC voltage power supply module 12. The DC voltage power supply module 12 supplies a phase modulator 13 operated in analog fashion with the voltage U.sub. and supplies the amplitude modulator 14, which is likewise operated in analog fashion, with the voltage U.sub.A. The phase modulator supplied with the voltage U.sub. impresses a phase onto the correction signal c*v.sub.Tx(t), and the amplitude modulator supplied with the voltage U.sub.A modulates the phase-shifted correction signal with the amplitude A, so that ultimately the modulator 8 delivers the modulated correction signal A*c*v.sub.Tx(t)*e.sup.j.sup. to the signal combiner 7 already described above.

(22) FIG. 2 shows a simplified circuit diagram of a voltage-controlled phase modulator 13 that can be used in FIG. 1. The phase modulator 13 is an analog phase shifter of reflection type. It can be used to impress a variable phase onto an input signal RF.sub.in applied to a first connection 16 of a 90 hybrid coupler 15, so that the output signal RF.sub.out at a second connection 17 has a phase shift of in relation to the input signal. To achieve this, the impedances at a third connection 18 and at a fourth connection 19 of the 90 hybrid coupler are varied. This is effected by using varactor diodes 20, 21, whose respective capacitance C.sub.Var is adjustable by virtue of the phase V.sub.pha of a DC voltage source 22 being altered, for example by the DC voltage power supply module 12. In the exemplary embodiment described here, C.sub.Var is monotonously rising relative to the voltage V.sub.pha. At 25 V, C.sub.Var may be 75 pF, for example.

(23) FIG. 3 shows the phase shift attained with the phase modulator 13 depicted in FIG. 2 as a function of the control voltage V.sub.pha. The phase/voltage characteristic 23 shown in FIG. 3 may be important if, although specifically a particular phase shift is meant to be attained, merely voltage values V.sub.pha are prescribable.

(24) FIG. 4 shows the phase/voltage characteristic 24 of an alternative phase modulator 13 that can be used in FIG. 1. The phase/voltage characteristic 24, like the phase voltage characteristic 23 described above, is strictly monotonously rising and, furthermore, has an almost linear profile. The alternative phase modulator 13 therefore reacts less sensitively to voltage changes than the phase modulator 13 shown in FIG. 2.

(25) FIG. 5 shows a simplified circuit diagram of an amplitude modulator 14 that can be used in FIG. 1. In the case of the amplitude modulator 14 shown in FIG. 5, the signal leaning from a second connection 26 is attenuated in comparison with a signal arriving at a first connection 25. The degree of attenuation is adjustable by a DC voltage source 27 with invariable voltage V.sub.fix and a DC voltage source 28 with variable voltage V.sub.att. To use the amplitude modulator 14 shown in FIG. 5 in the RF unit shown in FIG. 1, V.sub.fix can be set to 4.5 V, for example, and V.sub.att can be varied between 0 V and 8 V, for example. The voltage-controlled amplitude modulator 14 is based on a network that is formed by the three pin diodes 29, 30 and 31. The pin diodes 29, 30 and 31 behave as voltage-controlled resistors. If V.sub.att is raised, the resistance in the RF path between the first connection 25 and the second connection 26 decreases.

(26) FIG. 6 shows an attenuation/voltage characteristic 32 that corresponds to the amplitude modulator 14 shown in FIG. 5. The y axis plots the attenuation in dB, and the x axis plots the set voltage V.sub.att in volts. As can be seen from FIG. 6, the attenuation/voltage characteristic 32 has a monotonously rising profile, is strictly monotonous between V.sub.att=2.5 V and V.sub.att=8.0 V and, in this range, has an almost linear dependency of the attenuation on the voltage V.sub.att.

(27) FIG. 7 shows a simplified circuit diagram of an alternative amplitude modulator 14 that can be used in FIG. 1. FIG. 8 shows the corresponding strictly monotonously rising attenuation/voltage characteristic 33. The alternative amplitude modulator 14 shown in FIG. 7 is an attenuator with a hybrid coupler 34 that has a symmetrical configuration. The amplitude modulator 14 is matched, so that reflections are prevented and the analog correction circuit has a wider useful bandwidth than the amplitude modulator 14 shown in FIG. 5. Pin diodes 35, 36 have been connected up in parallel with 50 resistors 37, 38. This configuration already provides adequate attenuation and is less sensitive to voltage changes than the amplitude modulator 14 depicted in FIG. 5 in a detail from a usable attenuation range.

(28) FIG. 9 shows an exemplary embodiment of an inventive method for the MR examination of a measurement object in the form of a flowchart. The method can be carried out using the RF unit 1 shown in FIG. 1. A sequence diagram from an MR measurement for which the method depicted in FIG. 9 can be used is depicted in FIG. 10.

(29) First, initialization of the method takes place in step 100. In this step, initial values are determined for the voltages U.sub. and U.sub.A that are intended to be used to operate the phase modulator 13 and the amplitude modulator 14 at the beginning of the measurement. These values may be stored in a memory or else determined by a prior measurement. In this context, it is made certain that the ultra-low-noise signal amplifier 9 connected downstream of the signal combiner 7 is not saturated. Further, initial values are stipulated for the optimization method. In particular, initial step sizes are stipulated, i.e. the voltage values U.sub. and U.sub.A in the exemplary embodiment described here. These voltage values prescribe that, after the first iteration of the optimization method, U.sub. is changed by U.sub. and U.sub.A is changed by U.sub.A. Further, it is possible to stipulate whether the initial change is intended to be effected in a positive or a negative direction, i.e. whether U.sub.,A.fwdarw.U.sub.,A+U.sub.,A or whether U.sub.,A.fwdarw.U.sub.,AU.sub.,A. These initial values may also be prescribed or can be determined by a prior measurement.

(30) After the initialization 100, a first section 39 of the measurement selected by the user and known to a person skilled in the art is then performed with a duration of TR in a step 101 (cf. FIG. 10). To this end, a transmitted signal of duration TR is radiated into the measurement object and picked up using the receiving coil 6 by applying particular gradient sizes G.sub.x,y,z that can be ascertained by a person skilled in the art. As already described above, the correction signal modulated with and A is subtracted from the response signal picked up, is amplified and is digitized. The measurement data ascertained in this manner are transmitted to a reconstruction unit for buffer-storage and later use for the reconstruction of the desired information about the measurement object. The same measurement data are also transmitted to the computation unit 11.

(31) In a subsequent step 102, a check is performed to determine whether the measurement is at an end. This is the case when the measurement sequence provides no further measurement sections 39, 40, 41. In this case, the method is terminated in step 103. To this end, the information that the user requires about the measurement object, such as particularly an MR image, is reconstructed particularly from the data supplied to the reconstruction unit.

(32) If the measurement is not at an end, the computation unit 11 then carries out step 104. In this step 104, a check is performed to determine whether the signal strength of that section of the measurement signal 39, 40, 41 that is ascertained in step 101 exceeds a limit value that is stipulated in step 100. In this case, the limit value can be determined as already described above, for example by virtue of a respective measurement with a measurement object and without a measurement object being performed and a difference for the respective measurement result being formed and a supplement of 50%, for example, being added to this difference. The signal strength of the measurement section 39, 40, 41 is ascertained by virtue of the p2 standard of the measurement data of the measurement section that are available in digital form and conditioned as a vector being computed.

(33) If the limit value is not exceeded by 50%, it is assumed that the present values of and A that are used for correction are sufficiently optimum and the measurement is continued in step 101 by virtue of the subsequent measurement step 40, 41 being performed.

(34) As soon as and whenever, in the course of the measurement, the limit value is exceeded by at least 50%, however, the optimization step 105 is carried out prior to the start of the next measurement section 40, 41. In this step 105, new values U.sub. and U.sub.A are first of all determined via the computation unit 11. To this end, the signal intensity of the present measurement section 39, 40, 41 is first of all compared with the signal intensity of the most recently picked up measurement section 39, 40, 41. Should the signal intensity have decreased, it is assumed that the most recently performed adaptation of the values U.sub. and U.sub.A has led to an improvement in the quality of the measurement signal, so that the present values of U.sub. and U.sub.A can also be increased in the present iteration step if an increase has taken place previously and can be decreased if a decrease has taken place previously. In the event of a worsening, voltage values are adapted in the opposite direction. In the exemplary embodiment described here, the voltage values U.sub. and U.sub.A are adapted using the constant, initial step sizes U.sub. and U.sub.A. In an alternative exemplary embodiment, U.sub. and U.sub.A are not constant and are readapted according to a particular specification in each iteration. By way of example, it may be expedient if the sizes of U.sub. and U.sub.A are dependent on the extent to which the limit value is exceeded. If it is exceeded to a small extent, then U.sub. and U.sub.A can be chosen to be smaller than if it is exceeded to a great extent, for example.

(35) After the new values U.sub. and U.sub.A are computed, these values are transmitted to the DC voltage power supply module 12.

(36) The measurement is then continued in step 101 by virtue of the subsequent measurement section 40, 41 being formed with values for and A that are adapted according to the respective phase/voltage characteristic 23, 24 and attenuation/voltage characteristic 32, 33, respectively.

(37) FIG. 11 shows an alternative exemplary embodiment of an inventive method for the MR examination of a measurement object. This method can also be carried out using the RF unit 1 shown in FIG. 1. A sequence diagram for an MR measurement in which the method depicted in FIG. 11 can be used is depicted in FIG. 12. In contrast to the method depicted in FIG. 9, the method depicted in FIG. 11 involves the correction being separated from the actual measurement.

(38) Thus, after an initialization 100, a checking step 106 is first of all added in step 106. This checking step 106 involves a checking signal 42 being radiated into the measurement object and measured. This may particularly be an on-resonant square-wave pulse. In step 104, a check is then performed to determine whether the signal strength of that section 42 of the measurement signal that is picked up in step 106 exceeds a previously stipulated limit value. If this is the case, the optimization step 105 is carried out and the checking step 106 is carried out again. This is effected until the limit value is no longer exceeded.

(39) In an alternative exemplary embodiment, an abortion criterion can be defined, for example by prescribing a maximum number of passes of the loop 104, 105, 106 or of any other checking loop in order to avoid the method getting caught in the checking loop. When the checking loop is left, the measurement method would be continued, for example in step 101 in FIG. 11, using the values attained hitherto. Alternatively, the transmitted power can also be briefly reduced in order to restart the checking loop.

(40) Since, in the exemplary embodiment depicted in FIG. 11, the limit value is no longer exceeded, the next measurement section 43 provided in order to reconstruct the desired information about the measurement object is then picked up in step 101 and a check is performed in step 102 to determine whether the measurement is intended to be completed. The measurement section 43 may be a short section comprising just one transmitted pulse 44, but also a section that comprises multiple transmitted pulses 44. If the measurement is intended to be completed, the measurement is terminated in step 103. Otherwise, the checking step 106 is carried out again.

(41) For the method described here, but also generally for a method according to the invention, it may be appropriate not to choose step sizes that are constant over all iterations, but rather to adapt said step sizes. It is thus possible for adaptation to be effected as already described above on the basis of an extent to which the limit value is exceeded. However, it may also be expedient to progressively decrease the step sizes according to a prescribed scheme. This may be advantageous particularly if, during the optimization, first of all only the phase is adapted over multiple iteration steps and the amplitude A is not adapted until in a subsequent step. A mixture of the methods may also be expedient, for example if and A are adapted alternately, with one or more iterations being able to be chosen for the adaptation of and A in each case.

(42) FIG. 13 plots the signal strength of a respective measurement section (Tx leakage amplitude) over time (Time). The values on the y axis refer to volts, the values on the x axis to seconds. To generate the measurement data, only part of which, for the sake of clarity, is provided with the reference numeral 46, a method is used that is similar to the method depicted in FIG. 11 and FIG. 12. However, a low limit value has intentionally been chosen to check the effectiveness of the optimization method, which means that the method has not been able to break out of the loop 106, 104, 105. At each of the times indicated by an arrow 45, the subject examined made a hand movement. On account of the thereby changing coupling of the receiving coil 6 and the transmission coil 5, new optimum values had to be ascertained for each of and A. From FIG. 13, it can be seen that with a feedback time of 50 ms the optimization method used required between a few hundred milliseconds and a few seconds to converge, corresponding to a number from a few iterations to approximately 40 iterations. Alternatively, other feedback times can also be used, for example 5 ms.

(43) FIG. 14 shows an alternative sequence diagram for an MR measurement of an MR examination according to the invention. In this case, a respective short checking signal 42 lasting a few microseconds is radiated into the measurement sequence at the end of each measurement section 43 and measured. In contrast to the method depicted in FIG. 11, there is no waiting here until convergence is achieved, but rather, in a similar manner to in the case of the method depicted in FIG. 9, a correction is applied continually and the measurement is applied without interruption. In contrast to the method depicted in FIG. 9, however, the measurement data picked up for reconstruction are not used for correcting the response signal, but rather checking pulses developed specifically for the correction are used. Since these pulses are very short, there continues to be an almost optimum acquisition efficiency.

(44) In each of the exemplary embodiments described, the analog-to-digital converter 10 can remain switched on without interruption and can continually pick up measurement data (cf. FIGS. 10, 12 and 14, ADC row). This is advantageous, but not absolutely necessary for the invention. However, there are differences, as already described above, concerning whether and which of the measurement data are used by the optimization method and/or for reconstructing the desired information about the measurement object.

(45) A method for the magnetic resonance examination of a measurement object is described, in which a measurement sequence is used in which the magnetic resonance response to the transmitted signal during transmission is measured. It is provided that a correction signal corresponding to the transmitted signal be generated and be used for correction of the response signal. To this end, the correction signal is modulated by a phase value and an amplitude value. The phase value and the amplitude value are automatically and iteratively customized for optimum correction of the response signal by an optimization method using a respective present state value of the measurement signal. Further, a radio-frequency unit 1 is described that can be used to carry out the method according to the invention.

LIST OF REFERENCE SYMBOLS

(46) 1 RF unit 2 RF generator 3 Signal amplifier 4 Signal divider 5 Transmission coil 6 Receiving coil 7 Signal combiner 8 Modulator 9 Further signal amplifier 10 Analog-to-digital converter 11 Computation unit 12 DC voltage power supply module 13 Phase modulator 14 Amplitude modulator 15 90 hybrid coupler 16 First connection of 15 17 Second connection of 15 18 Third connection of 15 19 Fourth connection of 15 20 Varactor diode 21 Further varactor diode 22 DC voltage source 23 Phase/voltage characteristic 24 Further phase/voltage characteristic 25 First connection of 14 26 Second connection of 14 27 DC voltage source 28 Further DC voltage source 29 Pin diode 30 Further pin diode 31 Further pin diode 32 Attenuation/voltage characteristic 33 Further attenuation/voltage characteristic 34 Hybrid coupler 35 Pin diode 36 Further pin diode 37 Resistor 38 Further resistor 39 First section of a measurement 40 Section of a measurement 41 Further section of a measurement 42 Checking signal 43 Measurement section 44 Transmitted pulse 45 Arrow 46 Measurement datum