Abstract:
Radiation damping (RD) is employed to hasten the recovery of longitudinal magnetization after RF excitation and signal readout in a magnetic resonance measurement cycle. A switch driven by the pulse sequence that performs the measurement cycle energizes a feedback RF coil driven by an amplified and phase shifted portion of the received MR signal. The recovery of longitudinal magnetization is thus under direct control of the MR system and enables the reduction of the otherwise inefficient waiting times that are required for natural T1 recovery of the excited spin magnetization. This enables shortened acquisition times, improved sensitivity, better spatial and temporal resolution, and reduction of motion artifacts that result from long acquisition times.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the controlled recovery of longitudinal magnetization after the readout of MR signals from a pulse sequence. 
         [0002]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or longitudinal magnetization, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment, or transverse magnetization. An MR signal is emitted by the excited spins after the excitation signal B 1  is terminated and this MR signal may be received and processed to form an image. 
         [0003]    When utilizing these signals to produce images, magnetic field gradients (G x , G y  and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. There are numerous known measurement cycles that may be performed under the direction of prescribed pulse sequences. 
         [0004]    An important objective in MR imaging and spectroscopy is to reduce the total scan time needed to perform the sequence of measurement cycles. It is possible to reduce the number of measurement cycles and their duration, but this generally results in the loss of image quality. 
         [0005]    As shown in  FIG. 3 , a measurement cycle is comprised of a magnetization excitation and preparation portion  1 , followed by an MR signal readout portion  2 . If the pulse sequence can be ended at this point and immediately repeated, the measurement cycle can be very quickly performed and repeated to acquire all the needed data in a very short scan time. However, such a prompt repetition of the pulse sequence is not possible without a substantial loss of acquired MR signal strength and consequent loss of image signal to noise ratio (SNR). This is due to the fact that the longitudinal magnetization that is excited to produce the MR signal needs a substantial recovery time before the measurement cycle is repeated. This recovery time is known as the T 1  relaxation time and in biologic subjects it ranges from 500 to 2500 ms in duration. As a result, the repetition time (TR) between successive repeats of the measurement cycle must be extended well after the MR signal is acquired as shown in  FIG. 3 . 
         [0006]    Methods are known for reducing this magnetization recovery time. One approach is referred to generally as driven equilibrium (DE) or fast recover (FR). It is implemented by applying a 180° RF refocusing pulse after the MR signal readout  2 , followed by another 90° RF pulse precisely at the moment the resulting echo signal refocuses. This combination of RF pulses flips the transverse magnetization back to the longitudinal axis in preparation for the repeat of the pulse sequence. The most successful uses of this method are incorporated into a spin echo sequence, so that the flip-back pulse occurs when the echo is refocusing. This is key to the FR or DE method since any phase error accrued will cause the magnetization to miss the z axis, even possibly going to the −z axis. Therefore, successful implementation in a gradient echo sequence requires the addition of a 180° refocusing pulse after the readout and then the flip-back pulse. This negatively impacts SAR, minimum TE and introduces T2 weighting. SSFP sequences (True FISP) can be viewed as using a “flip-back” pulse to achieve high levels of steady state magnetization even with short TR and high flip angles. SSFP, however, leaves room for improvements since it does not achieve full equilibrium magnetization and suffers from phase accrual, causing the “flip-back” to miss the z axis. Thus only short TR are possible, and banding artifacts occur for off-resonance spins. 
         [0007]    Radiation damping (RD) is a magnetic resonance phenomenon that has long been known in high field, high resolution liquid-state NMR where the RF coil sensitivity and quality factor is very high. Understood for decades, RD is an undesirable phenomenon whereby the nuclear magnetization acts back on itself via the induced currents in the radio-frequency (RF) coil used for detection. It is a manifestation of the detection process in that the induced currents in the coil (which is what the imager detects) themselves create a magnetic field known as the radiation damping field. The RD field rotates the spin magnetization back to its equilibrium direction at a characteristic rate that is distinct from longitudinal or T1 relaxation. Unlike T1 relaxation, which restores the length of the magnetization vector to its equilibrium value, the RD field does not alter the length of the magnetization vector. Rather, the RD field is an oscillating RF field that is on-resonance with the spins, since it was created by the precessing magnetization. It therefore has a resonant excitation effect on the spins, in a manner similar to an external RF excitation pulse. The RD effect is larger in high-resolution spectroscopy at high fields using small samples, and it is considered a nuisance in this regime (it shortens recovery and thus broadens spectral lines). Therefore, methods have been developed for reducing the RD effect through an external feedback device. The external device senses the current induced by the spins in the circuit and uses an external feedback device to cancel or reduce this current and thus its effect on the spinning magnetization. Such a solution is disclosed in U.S. Pat. No. 5,767,677. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is a method and apparatus for controlling the duration of the magnetization recovery period in a magnetic resonance measurement cycle by employing the radiation damping (RD) phenomenon. More specifically, the received MR signal is coupled to a feedback circuit which imposes a phase shift and amplifies the resulting RD feedback signal. This RD feedback signal is applied to an RF coil disposed near the subject of the MR examination and its field quickly restores longitudinal magnetization in the subject. Referring particularly to  FIG. 4 , this radiation damping indicated by reference number  3  is applied after the MR signal readout  2  and it enables the next measurement cycle to be performed in a much shorter TR without loss of signal or image SNR. 
         [0009]    This invention recognizes that the feedback circuit can be used to enhance the RD effect and that the increased RD effect can be employed to enable controlled and vastly accelerated recovery of the longitudinal magnetization. In preliminary experiments, we have found that we can achieve 100% of the recovery of the longitudinal magnetization by activating the feedback device for only 10 ms. On the other hand, natural T1 recovery in this sample (e.g., brain tissue) takes more than 3000 ms to achieve a 95% recovery by natural T1 relaxation. The feedback enhanced RD effect can thus be employed to substantially reduce the TR of existing pulse sequences. 
         [0010]    An object of the present invention is to shorten total scan time by shortening the TR of pulse sequences used to acquire data. Using the feedback enhanced RD effect, a scan performed with the RARE imaging sequence (also known as Fast Spin Echo (FSE) or Turbo Spin Echo (TSE) may be shortened. In this scan a long TR is typically used to allow full recovery of the longitudinal magnetization by natural T1 processes. For sequences with a limited number of slices, this leads to excessive dead time. For example, the excitation and encoding period of a typical 12-echo sequence is about 150 ms. If a 256 image matrix is desired, this requires 256/12=22 excitations to acquire the imaging matrix. At a TR=3 s per excitation this requires over a minute of scan time. If the same longitudinal recovery is achieved in 10 ms of enhanced RD feedback, then the TR can be set to 160 ms, providing an imaging time of less than 2 s. Since the efficiency of the recovery is improved the image sensitivity would be identical, but with a vast saving in imaging time, and improved efficacy for movement in difficult patient populations. 
         [0011]    Another object of the present invention is to increase the sensitivity of existing pulse sequences and thereby increase the SNR of the image reconstructed from the data they acquire. For example, in a typical spoiled gradient-recalled echo pulse sequence such as FLASH, a short TR is used to obtain acceptable imaging times, but the long (approximately 1 s) T1 recovery of tissue requires a low flip angle RF excitation pulse be used to ensure longitudinal magnetization is preserved for the duration of the scan. This greatly reduces the sensitivity of the image. For a proton density-weighted FLASH image, typically a TR of 50 ms and flip angle of 10° is used to preserve the high level of steady state magnetization needed to maintain proton density contrast. By providing full T1 recovery using the present invention within the 50 ms TR, a 90 degree RF excitation may be used, which improves the sensitivity and SNR by a factor of 7. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of an MRI system which employs the present invention; 
           [0013]      FIG. 2  is a block diagram of the transceiver used in the MR system of  FIG. 1  which employs a preferred embodiment of an RD feedback circuit that employs the present invention; 
           [0014]      FIG. 3  is a pictorial representation of a conventional pulse sequence used to perform an MR measurement cycle on the MR system of  FIG. 1 ; 
           [0015]      FIG. 4  is a pictorial representation of a pulse sequence that employs the present invention; 
           [0016]      FIG. 5  is a graphic representation of a pulse sequence that employs the present invention; 
           [0017]      FIG. 6  is a flowchart setting forth the steps of a radiation damping calibration technique in accordance with the present invention; and 
           [0018]      FIG. 7  is a pictorial representation of a pulse sequence used perform a radiation damping technique of  FIG. 6 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Referring particularly to  FIG. 1 , the preferred embodiment of the invention is employed in an MRI system. The MRI system includes a workstation  10  having a display  12  and a keyboard  14 . The workstation  10  includes a processor  16  which is a commercially available programmable machine running a commercially available operating system. The workstation  10  provides the operator interface which enables scan prescriptions to be entered into the MRI system. 
         [0020]    The workstation  10  is coupled to four servers: a pulse sequence server  18 ; a data acquisition server  20 ; a data processing server  22 , and a data store server  23 . In the preferred embodiment the data store server  23  is performed by the workstation processor  16  and associated disc drive interface circuitry. The server  18  is performed by a separate processor and the servers  20  and  22  are combined in a single processor. The workstation  10  and each processor for the servers  18 ,  20  and  22  are connected to an Ethernet communications network. This network conveys data that is downloaded to the servers  18 ,  20  and  22  from the workstation  10 , and it conveys data that is communicated between the servers. 
         [0021]    The pulse sequence server  18  functions in response to instructions downloaded from the workstation  10  to operate a gradient system  24  and an RF system  26 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  24  which excites gradient coils in an assembly  28  to produce the magnetic field gradients G x , G y  and G z  used for position encoding NMR signals. The gradient coil assembly  28  forms part of a magnet assembly  30  which includes a polarizing magnet  32  and a whole-body RF coil  34 . 
         [0022]    RF excitation waveforms are applied to the RF coil  34  by the RF system  26  to perform the prescribed magnetic resonance pulse sequence. Responsive NMR signals detected by the RF coil  34  are received by the RF system  26 , amplified, demodulated, filtered and digitized under direction of commands produced by the pulse sequence server  18 . The RF system  26  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  18  to produce RF pulses of the desired frequency, phase and pulse amplitude waveform. The generated RF pulses may be applied to the whole body RF coil  34  or to one or more local coils or coil arrays. 
         [0023]    The RF system  26  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the NMR signal received by the coil to which it is connected and a quadrature detector which detects and digitizes the in-phase (I) and quadrature (Q) components of the received NMR signal. The magnitude of the received NMR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components: 
         [0000]        M =√{square root over (I 2   +Q   2 )}, 
         [0000]    and the phase of the received NMR signal may also be determined: 
         [0000]      φ=tan −1   Q/I.    
         [0024]    The pulse sequence server  18  also optionally receives patient data from a physiological acquisition controller  36 . The controller  36  receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. Such signals are typically used by the pulse sequence server  18  to synchronize, or “gate”, the performance of the scan with the subject&#39;s respiration or heart beat. 
         [0025]    The pulse sequence server  18  also connects to a scan room interface circuit  38  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  38  that a patient positioning system  40  receives commands to move the patient to desired positions during the scan. 
         [0026]    The digitized NMR signal samples produced by the RF system  26  are received by the data acquisition server  20 . The data acquisition server  20  operates in response to instructions downloaded from the workstation  10  to receive the real-time NMR data and provide buffer storage such that no data is lost by data overrun. In some scans the data acquisition server  20  does little more than pass the acquired NMR data to the data processor server  22 . However, in scans which require information derived from acquired NMR data to control the further performance of the scan, the data acquisition server  20  is programmed to produce such information and convey it to the pulse sequence server  18 . For example, during prescans NMR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  18 . Also, navigator signals may be acquired during a scan and used to adjust RF or gradient system operating parameters or to control the view order in which k-space is sampled. And, the data acquisition server  20  may be employed to process NMR signals used to detect the arrival of contrast agent in an MRA scan. In all these examples the data acquisition server  20  acquires NMR data and processes it in real-time to produce information which is used to control the scan. 
         [0027]    The data processing server  22  receives NMR data from the data acquisition server  20  and processes it in accordance with instructions downloaded from the workstation  10 . Such processing may include, for example: Fourier transformation of raw k-space NMR data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a backprojection image reconstruction of acquired NMR data; the calculation of functional MR images; the calculation of motion or flow images, etc. 
         [0028]    Images reconstructed by the data processing server  22  are conveyed back to the workstation  10  where they are stored. Real-time images are stored in a data base memory cache (not shown) from which they may be output to operator display  12  or a display  42  which is located near the magnet assembly  30  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  44 . When such images have been reconstructed and transferred to storage, the data processing server  22  notifies the data store server  23  on the workstation  10 . The workstation  10  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0029]    Referring particularly to  FIG. 2 , the RF system  26  includes a transmitter that produces a prescribed RF excitation field. The base, or carrier, frequency of this RF excitation field is produced under control of a frequency synthesizer  200  which receives a set of digital signals from the pulse sequence server  18 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  201 . The RF carrier is applied to a modulator and up converter  202  where its amplitude is modulated in response to a signal R(t) also received from the pulse sequence server  18 . The signal R(t) defines the envelope of the RF excitation pulse to be produced and is produced by sequentially reading out a series of stored digital values. These stored digital values may be changed to enable any desired RF pulse envelope to be produced. 
         [0030]    The magnitude of the RF excitation pulse produced at output  205  is attenuated by an exciter attenuator circuit  206  which receives a digital command from the pulse sequence server  18 . The attenuated RF excitation pulses are applied to the power amplifier  151  that drives the RF coil  34  through a transmit/receive (T/R) switch  154 . The T/R switch  154  is operated by the pulse sequence server  18  through control line  156  to couple the power amplifier output to the coil  34  during the RF excitation phases of the pulse sequence and to connect the coil  34  to a receiver during other phases of the pulse sequence. As is well known in the art, separate transmit and receive coils can also be employed, in which case the T/R switch is not required. 
         [0031]    Referring still to  FIG. 2  the MR signal produced by the subject is picked up by the receive coil  34  and applied through a preamplifier  153  and splitter  157  to the input of a receiver attenuator  207 . The receiver attenuator  207  further amplifies the signal by an amount determined by a digital attenuation signal received from the pulse sequence server  18 . The received MR signal is at or around the Larmor frequency, and this high frequency signal is down converted in a two step process by a down converter  208  which first mixes the MR signal with the carrier signal on line  201  and then mixes the resulting difference signal with a reference signal on line  204 . The down converted MR signal is applied to the input of an analog-to-digital (A/D) converter  209  which samples and digitizes the analog signal and applies it to a digital detector and signal processor  210  which produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to samples of the received signal. The resulting stream of digitized I and Q values of the received signal are output to the data acquisition server  20 . The reference signal as well as the sampling signal applied to the A/D converter  209  are produced by a reference frequency generator  203 . 
         [0032]    Referring still to  FIG. 2 , the present invention is implemented by adding a controlled feedback loop indicated generally at  160 . The splitter  157  conveys the acquired MR signal to the receiver as described above and it also conveys that signal to the feedback loop  160 . The feedback MR signal is attenuated to a prescribed level by an attenuator  168  and fed to the input of a phase shifter  170 . The MR feedback signal is then amplified in a low noise amplifier  162  and applied to the input of a switch  164 . Like the T/R switch  154 , the feedback switch  164  is controlled by the pulse sequence server  18  through a control line  166 . The switch  164  is a pin diode driven by an operational amplifier that is turned on by the trigger signal on line  166 . The feedback MR signal is turned on during a prescribed period during the pulse sequence as will be described below. The attenuation and phase shift is set for each prescribed pulse sequence that employs the MR feedback such that the recovery of the longitudinal magnetization is driven to a desired level in a desired time period. The attenuated and phase shifted feedback MR signal is applied to a separate RF feedback coil  172  that is positioned near the subject being examined. 
         [0033]    The RF feedback coil  172  is a conventional MR coil that is designed to operate at the Larmor frequency of the MR system. It produces a uniform magnetic field through the region of interest in the subject being examined. The mutual inductance between the feedback coil  172  and the other active RF coil  34  is minimized. The size of the feedback coil  172  is selected to optimize the strength of the RF field that is fed back to the subject of the examination. 
         [0034]    As indicated generally above, the feedback switch  166  is operated during a pulse sequence after the MR signal is read out to produce a phase shifted and amplified MR feedback signal that exploits the radiation damping (RD) effect. The timing of the radiation damping and the magnitude thereof is under control of the MR system. As a result, the recovery of the longitudinal magnetization is also under the control of the MR system. 
         [0035]    While the present invention may be used in many different pulse sequences, its use in a FLASH pulse sequence is particularly advantageous. Referring to  FIG. 5 , a FLASH pulse sequence that employs the present invention includes a selective RF excitation pulse  174  that is produced in the presence of a slice select gradient  176 . Phase encoding is applied next by a gradient lobe  178  and a dephasing lobe  180  that forms part of a readout gradient waveform is also applied. An MR signal is then acquired during the application of a readout gradient  182  and this is coupled as described above to the system receiver where it is digitized. Following the data acquisition period indicated at  184 , the phase encoding is rewound by gradient lobe  186  and a rephasing lobe  188  on the readout gradient waveform produces a gradient-recalled MR echo signal immediately thereafter. 
         [0036]    Referring still to  FIG. 5 , the RD feedback switch described above is operated to produce the RD feedback signal as indicated at  190 . In this preferred embodiment the duration of the RD feedback  190  is 10 ms and then the next repetition of the FLASH pulse sequence begins. The longitudinal magnetization is driven to full recovery during this 10 ms RF feedback signal application. Shorter RD feedback signal periods can also be used when it is not necessary to achieve full restoration of the magnetization. For example, the magnetization can be 95% fully restored with a period of only 5 ms. 
         [0037]    The present invention can be used with many different pulse sequences. In some cases it is desirable to destroy any transverse magnetization that remains after the driven magnetization recovery and before the pulse sequence is repeated. Crusher gradients may be applied to accomplish this and the sequence becomes a spoiled FLASH sequence. Or, the gradient waveforms can be altered to fully refocus the spin magnetization prior to its next repetition and the pulse sequence is an SSFP sequence. This can be done, for example, by adding a negative lobe to the slice select gradient waveform as indicated by dotted line  175 . 
         [0038]    The amplitude and phase of the RD feedback is calibrated in a procedure that is performed periodically by maintenance personnel for each receive coil to be used. This calibration procedure provides estimates of the values to be used with the particular receive coil. The same calibration procedure is also performed in a prescan after the receive coil and subject of the examination are in place in the bore of the magnet. This prescan optimizes the RD feedback amplitude and phase shift settings. 
         [0039]    Referring particularly to  FIG. 6 , the calibration process begins by initializing the values of the RD feedback amplitude and phase shift as indicated at process block  200 . When performed during a patient prescan, these initial settings are those determined during the previous service calibration. A loop is then entered in which the phase shift and amplitude of the RD feedback are adjusted until optimal settings are obtained. 
         [0040]    More specifically, a pulse sequence shown in  FIG. 7  is performed in which spins in the region of interest in the subject are excited with a 90° RF excitation pulse  700  as indicated at process block  202 . The saturated spins are then subjected to a period of radiation damping as indicated at process block  204  using the current phase shift and amplitude settings for the RD feedback signal  702 . The radiation damping period is preferably set in the 10 ms to 15 ms range and immediately thereafter the magnetization recovery is measured as indicated at process block  206 . This is a pulse sequence that includes the application of crusher gradients  704  to dephase any remaining transverse magnetization, followed by the application of another 90° RF excitation pulse  706  during a slice selection gradient  708  to excite the region of interest in the patient. 5 ms later, a readout gradient waveform  710  is applied and the magnitude of the FID signal is obtained and stored as a measurement of the effectiveness of the current RD feedback signal settings. 
         [0041]    Referring still to  FIG. 6 , the first part of the calibration process uses the above measurement sequence to determine the optimal phase shift setting. As indicated at decision block  208 , the FID signal magnitude is tested. If the peak signal has not been reached, the phase shift setting is changed as indicated at process block  210  and the measurement sequence is repeated. It has been discovered that RD feedback effectiveness is very sensitive to the phase shift setting and that the proper setting is detected when the FID amplitude reaches a peak and then starts to decline as the phase settings are stepped through a series of values. When the peak phase shift setting is determined, a flag is set as indicated at process block  212  and the second stage of the calibration process is begun as detected at decision block  214 . 
         [0042]    Referring still to  FIG. 6 , the feedback amplitude setting is optimized by repeating the measurement sequence and changing the RD feedback signal amplitude as indicated at process block  216 . Unlike the phase shift, there is no detectable peak setting. Instead, as the RF feedback signal amplitude is increased the measured magnetization recovery improves asymptotically. The amplitude setting is optimized when a continued increase in RD feedback signal amplitude produces less than a preset minimum increase in detected FID signal amplitude as determined at decision block  218 . When this occurs the calibration process is completed by storing the optimal settings for subsequent use during patient scanning as indicated at process block  220 . 
         [0043]    The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.