Abstract:
A magnetic resonance (MR) imaging system equipped with real-time imaging capability and method of interactively prescribing image contrast are disclosed herein. The MR imaging system includes a sequence controller for constructing MR imaging pulse sequences and a waveform memory for storing waveform segments. The MR imaging system allows an operator to interactively prescribe image contrast mechanism prior to and/or during real-time imaging. The use of image contrast waveform segments, only as needed, minimizes unnecessary MR scan time.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The patent application is a continuation-in-part of U.S. Pat. application Ser. No. 09/200,158 by Debbins, et al., entitled “MR imaging System with Interactive Image Contrast Control”, filed Nov. 25, 1998 now U.S. Pat. No. 6,166,544. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to magnetic resonance (MR) imaging systems. More particularly, the present invention relates to an MR imaging system equipped for real-time imaging and which allows interactive modification of the image contrast of MR images produced therein. 
     When an object of interest, such as human tissue, is subjected to an uniform magnetic field (polarizing field Bo along the z direction in a Cartesian coordinate system denoted as x, y, and z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the object, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is the x-y plane and which is near the Larmor frequency, the net aligned moment, M 2 , may be rotated, or “tipped” at a certain tipping angle, into the x-y plane to produce a net transverse magnetic moment M 1 . A signal is emitted by the excited spins after the excitation signal B 1  is terminated and this signal may be received and processed to form an MR image. 
     When utilizing these signals to produce MR images, magnetic field gradients (G x , G y  and G z ) are also employed. Typically, the object to be imaged is scanned by a sequence of measurement cycles in which these gradient waveforms vary according to the particular localization method being used. The resulting set of received NMR signals (also referred to as MR signals) are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
     When viewing an MR image of a structure of interest, such as an anatomical section, the MR imaging system operator may desire to view an MR image in which one or more types of tissue comprising the anatomical section is contrasted with respect to the remaining types of tissue comprising that anatomical section. Moreover, the operator may desire to modify the image contrast of an MR image acquisition in progress or to prescribe the image contrast prior to an MR image acquisition. 
     Each MR pulse sequence responsible for an MR image is comprised of at least one set of (regular) waveform segments—the imaging waveform segments. In addition, the MR pulse sequence includes certain features or architecture to provide image contrast in the MR image: (1) image contrast mechanisms can be inherent in the imaging waveform segments; (2) one or more parameters associated with the MR pulse sequence can be modified and/or specified by the operator, thereby affecting image contrast; or (3) one or more sets of image contrast waveform segments can be included along with the imaging waveform segments to comprise the MR pulse sequence. In this last case where image contrast waveform segments are utilized, such image contrast mechanisms are made possible by a corresponding magnetization preparation applied to the anatomical section prior to the application of the imaging waveform segments. Briefly, magnetization preparation involves preparing the spin state in the bore such that the anatomical section to be imaged is in a certain magnetized state immediately before the regular image scanning commences. 
     In conventional MR imaging systems, every MR pulse sequence responsible for a specific image contrast is typically constructed and stored in the MR imaging system prior to scanning. For example, an MR pulse sequence may comprise a specific image contrast waveform segment permanently linked to an imaging waveform segment. Then when the operator desires this specific image contrast, this all-inclusive pulse sequence is evoked and executed in its entirety. In another example, the MR pulse sequence may be constructed prior to scanning from a specific selection of short (or more basic components comprising the) waveform segments. 
     The drawback to these types of pulse sequence architectures is that the operator must wait until the image acquisition in progress is completed before newly desired image contrast mechanism(s) can be evoked. Moreover, even if the amplitudes, periods, or other parameters relating to a portion of die MR pulse sequence (e.g., the image contrast waveform segment) can be modified while the image acquisition is in progress (e.g., amplitude is set to zero), there is only negligible reduction in acquisition time because the modified portion of the pulse sequence must still be executed along with the rest of the pulse sequence. 
     Thus, there is a need for an MR imaging system capable of providing interactively prescribable image contrast mechanisms in real-time. There is a further need for an MR imaging system capable of modifying image contrast mechanisms in MR pulse sequences, as desired, through dynamic construction of MR pulse sequences of run-time. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to a method for interactively prescribing an image contrast of a magnetic resonance (MR) image produced in a magnetic resonance (MR) imaging system. The method includes storing a plurality of waveform segments in a waveform memory. Each of the waveform segments is associated with a distinct memory address and at least one of the waveform segment includes an image contrast mechanism. The method further includes selecting from the plurality of waveform segments stored in the waveform memory, and constructing an MR pulse sequence in real-time by dynamically connecting selected waveform segments at run-time. The method further includes acquiring MR data in real-time by execution of the MR pulse sequence to generate a current MR image that includes a desired image contrast. 
     Another embodiment of die invention relates to an interactive magnetic resonance (MR) imaging system. The system includes means for storing a plurality of waveform segments, and means for selecting from the plurality of waveform segments. Each of the waveform segments is associated with a distinct memory address and at least one of the waveform segment includes an image contrast mechanism. The system further includes means for constructing an MR pulse sequence in real-time by dynamically connecting selected waveform segments at run-time. The system further includes means for acquiring MR data in real-time by execution of the MR pulse sequence to generate a current MR image that includes a desired image contrast. 
     Another embodiment of the invention relates to an interactive magnetic resonance (MR) imaging system. The system includes a memory configured to store a plurality of waveform segments. Each of the waveform segments is associated with a distinct memory address and at least one of the waveform segment includes an image contrast mechanism. The system further includes an operator interface configured to permit an operator to select from the plurality of waveform segments, and a sequence controller coupled to the memory and configured to dynamically connect the selected waveform segments at run-time to construct a MR pulse sequence. The system further includes a MR imaging device coupled to the operator interface, memory, and sequence controller and configured to acquire MR data in real-time and generate a current MR image that includes a desired image contrast. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiment will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
     FIG. 1 is a block diagram of an MR imaging system which employs an embodiment of the present invention; 
     FIG. 2 is an electrical block diagram of a transceiver which comprises a part of the MR imaging system of FIG. 1; 
     FIG. 3 is a more detailed block diagram of a pulse generator which comprises a part of the MR imaging system of FIG. 1; and 
     FIG. 4 is a block diagram of a waveform memory which comprises a part of the MR imaging system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, there is shown the major components of a magnetic resonance (MR) imaging system which includes an embodiment of the present invention. The operation of the system is controlled from an operator console  100  which includes an input device  101 , a control panel  102 , and a display  104 . The console  100  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on the screen  104 . The computer system  107  includes a number of modules which communicate with each other through a backplane. These include an image processor module  106 , a CPU module  108  and a memory module  113 , known in the art as a frame buffer for storing image data arrays. The computer system  107  is linked to a disk storage  111  and a tape drive  112  for storage of image data and programs, and it communicates with a separate system control  122  through a high speed serial link  115 . 
     The system control  122  includes a set of modules connected together by a backplane. These include a CPU module  119  and a pulse generator module  121  which connects to the operator console  100  through a serial link  125 . It is through this link  125  that the system control  122  receives commands from the operator which indicate the scan sequence that is to be performed. The pulse generator module  121  operates the system components to carry out the desired scan sequence. It produces data which indicates the timing, strength and shape of the RF pulses which are to be produced, and the timing of and length of the data acquisition window. The pulse generator module  121  connects to a set of gradient amplifiers  127 , to indicate the timing and shape of the gradient pulses to be produced during the scan. The pulse generator module  121  also receives patient data from a physiological acquisition controller  129  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. And finally, the pulse generator module  121  connects to a scan room interface circuit  133  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  133  that a patient positioning system  134  receives commands to move the patient to the desired position for the scan. 
     The gradient waveforms produced by the pulse generator module  121  are applied to a gradient amplifier system  127  comprised of G x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding gradient coil in an assembly generally designated  139  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly  139  forms part of a magnet assembly  141  which includes a polarizing magnet  140  and a whole-body RF coil  152 . 
     The gradient waveform produced by the pulse generator module  121  are also applied to a transceiver module  150  in system control  122 . The transceiver module  150  produces pulses which are amplified by an RF amplifier  151  and coupled to the RF coil  152  by a transmit/receiver switch  154 . The resulting signals radiated by the excited nuclei in the patient may be sensed by the same RF coil  152  and coupled through the transmit/receive switch  154  to a preamplifier  153 . The amplified NMR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  150 . The transmit/receive switch  154  is controlled by a signal from the pulse generator module  121  to electrically connect the RF amplifier  151  to the coil  152  during the transmit mode and to connect the preamplifier  153  during the receive mode. The transmit/receive switch  154  also enables a separate RF coil (for example, a head coil or surface coil) to be used in either the transmit or receive mode. 
     The NMR signals, also referred to as MR signals, picked up by the RF coil  152  are digitized by the transceiver module  150  and transferred to a memory module  160  in the system control  122 . When the scan is completed and an entire array of data has been acquired in the memory module  160 , an array processor  161  operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  115  to the computer system  107  where it is stored in the disk memory  111 . In response to commands received from the operator console  100 , this image data may be archived on the tape drive  112 , or it may be further processed by the image processor  106  and conveyed to the operator console  100  and presented on the display  104 . 
     Referring particularly to FIGS. 1 and 2, the transceiver  150  produces the RF excitation field B 1  through power amplifier  151  at a coil  152 A and receives the resulting signal induced in a coil  152 B. As indicated above, the coils  152 A and B may be separate as shown in FIG. 2, or they may be a single wholebody coil as  15  shown in FIG.  1 . The base, or carrier, frequency of the RF excitation field is produced under control of a frequency synthesizer  200  which receives a set of digital signals (CF) from the CPU module  119  and pulse generator module  121 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  201 . The commanded 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 generator module  121 . The signal R(t) defines the envelope of the RF excitation pulse to be produced and is produced in the module  121  by sequentially reading out a series of stored digital values. These stored digital values may, in turn, be changed from the operator console  100  to enable any desired RF pulse envelope to be produced. 
     The magnitude of the RF excitation pulse produced at output  205  is attenuated by an exciter attenuator circuit  206  which receives a digital command, TA, from the backplane  118 . The attenuated RF excitation pulses are applied to the power amplifier  151  that drives the RF coil  152 A. For a more detailed description of this portion of the transceiver  122 , reference is made to U.S. Pat. No. 4,952,877 owned by the General Electric Company, which is incorporated herein by reference. 
     Referring still to FIGS. 1 and 2, the MR signal produced by the subject is picked up by the receiver coil  152 B and applied through the preamplifier  153  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 (RA) received from the backplane  118 . 
     The received 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 NMR signal with the carrier signal on line  201  and then mixes the resulting difference signal with the 2.5 MHz reference signal on line  204 . The down converted NMR 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 the received signal. The resulting stream of digitized I and Q values of the received signal are output through backplane  118  to the memory module  160  where they are normalized in accordance with the present invention and then employed to reconstruct an image. 
     In one embodiment of the present invention, an operator interactively controls the image contrast of an MR image prior to or during its acquisition. Such interactive image contrast control is accomplished from the operator console  100  (also referred to as the operator interface) using the input device  101 . The input device  101  is selected from a group including, but not limited to, a mouse, a joystick, a keyboard, a trackball, a touch screen, a light wand, and a voice control. One embodiment of the MR imaging system is capable of imaging in any desired orientation within the structure of interest and is equipped to perform both real-time acquisitions and non real-time acquisitions. In particular, real-time refers to continuous acquisition and reconstruction of MR image data as rapidly as it is acquired and displayed in approximately one second or less, as constrained by system performance. 
     FIG. 3 shows the major components of the pulse generator module used in an embodiment of the MR imaging system. Pulse generator module  121  includes a sequence controller  10  which connects to a waveform memory  12  through a communication link  14 . A graphical user interface  105  and MR images of the structure of interest (not shown in FIG. 3) are displayed on the display  104  of the MR imaging system. The operator interacts with the graphical user interface  105  using the input device  101 . The graphical user interface  105  includes a chemical saturation icon  30 , a inversion recovery icon  32 , a spatial saturation icon  34 , and a flow compensation icon  36 . The link  125  connecting the operator console  100  to the pulse generator module  121  communicates image contrast controls from the operator console  100  to the sequence controller  10 . 
     Waveform memory  12  stores multiple sets of RF and gradient waveform segments, each set corresponding to a distinct imaging mechanism or image contrast mechanism. Each set of RF and gradient waveform segments is assigned a distinct memory address in waveform memory  12  such that any set of RF and gradient waveform segments is accessible from waveform memory  12 . In the preferred embodiment, imaging waveform segments, corresponding to imaging mechanisms, include a set of base imaging waveform segment  16  and a set of flow compensation imaging waveform segment  18 . Image contrast waveform segments, corresponding to image contrast mechanisms, include a set of chemical saturation waveform segment  20 , a set of inversion recovery waveform segment  22 , and a set of spatial saturation waveform segment  24 . It should be understood that the waveforms depicted in the figures are for illustration purposes only and do not represent the actual waveforms in waveform memory  12 . 
     Although not shown in FIG. 3, image contrast mechanisms available in the MR imaging system, and correspondingly, sets of waveform segments stored in waveform memory  12 , can also include, but are not limited to, one or more of: RF spoiling, inversion recovery, spectrally selective inversion recovery, fluid attenuated inversion recovery, driven equilibrium, fat saturation, water saturation, magnetization transfer, diffusion weighting, inversion tagging, fat/water in-phase or out-of-phase, spatial saturation, T 1  preparation, T 2  preparation, variable TE, variable TR, variable receiver bandwidth, variable flip angle, variable spatial resolution, field of view, slice thickness, slice spacing, multiple slices, multiple passes, no-phase-wrap, asymmetric field of view, fractional Ky (legacy-fractional matrix sampling), fractional Kx (legacy-fractional echo), velocity encoding, respiratory compensation, cardiac compensation, multiple repetitions (legacy-multi-phase), and phase contrast. 
     Preferably, image contrast mechanisms are provided by the image contrast waveform segments, in which each set of image contrast waveform segment is a type of magnetization preparation that would be applied to the subject of interest prior to the application of the imaging waveform segments (e.g., the regular pulse sequence needed to acquire an MR image). However, image contrast mechanisms also be inherent and/or be generated within the imaging waveforn segments. For example, parameters associated with the imaging waveform segments can be modified and/or specified to affect image contrast of the MR image to be acquired. An embodiment of the MR imaging system provides the operator with the ability to modify and/or specify certain parameters (e.g., periods, timing, amplitudes, phases, etc.) associated with imaging waveform segments and/or image contrast waveform segments to further affect image contract. 
     It should be understood that the distinction or categorization of imaging waveform segments and image contrast waveform segments are for descriptive purposes only, and the division between the two may be less well defined. 
     In detail, the preferred embodiment of the present invention uses real-time MR imaging. To interactively prescribe the image contrast in real-time, the operator selects the desired image contrast mechanism by “clicking” on icon  30 ,  32 , or  34  on the graphical user interface  105  (for example, icon  30  for chemical saturation). The operator also selects one imaging mechanism. In the preferred embodiment, the operator selects the flow compensation mechanism by clicking on the flow compensation icon  36 , or the base imaging mechanism by not clicking on any imaging waveform segment icons. Thus, the base imaging waveform segment, corresponding to the base imaging mechanism, is the default imaging mechanism. 
     Sequence controller  10  receives the operator&#39;s selection(s) via link  125  from the operator console  100 . The sequence controller  10  first accesses the image contrast waveform segment corresponding to the selected image contrast mechanism (continuing the example, chemical saturation waveform segment  20 ). Second, the sequence controller  10  accesses the selected imaging waveform segment (continuing the example, base imaging waveform segment  16 ) almost instantaneously in time, to dynamically link or connect  26  selected image contrast waveform segment to the selected imaging waveform segment at run-time, as shown in FIG.  4 . In this manner, a dynamically linked MR pulse sequence, comprised of the selected image contrast waveform segments followed by the selected imaging waveform segment, is constructed as need during execution. Then the sequence controller  10  applies this dynamically linked MR pulse sequence to the gradient amplifier system  127  and transceiver  150  to be executed or “played out” such that MR data can be acquired. 
     Next, the sequence controller  10  accesses the selected image contrast waveform segment (continuing the example, chemical saturation waveform segment) again, almost instantaneously in time, to construct the next dynamically linked  28  MR pulse sequence. Access and execution of the selected image contrast waveform segment and the selected imaging waveform segment occurs repeatedly in this cyclic manner with appropriate modifications to the MR pulse sequence at each cycle or repetition to acquire enough MR data in accordance with the resolution of the proposed MR image to be displayed. For example, an MR image with 256 phase encoding views would require the MR pulse sequence to be executed 256 times in standard spin-warp MR imaging. The final result, after acquisition and reconstruction in real-time, is an MR image with the desired image contrast (in this example, chemical saturation, which suppresses the relatively large magnetization signal from fatty tissue). 
     Thus, each MR pulse sequence to be executed is not stored in its entirety in a memory or buffer. Instead, a given MR pulse sequence is generated or constructed (for only a moment in time) in the sense that real-time or quasi-real-time sequential access of appropriate waveform segment addresses, and correspondingly, the waveform segments, in waveform memory  12  creates a dynamically connected pulse sequence sufficiently long enough for it to be “played out”. 
     It is contemplated that the waveform segments may be stored in more than one waveform memory. It is also contemplated that more than one sequence controller may be included in the MR imaging system of FIG.  1 . Utilizing more than one waveform memory and/or sequence controller can facilitate even shorter scan times. Accessing multiple waveform memories (to fulfill different waveform segment access needs), or conjunctive or alternating sequencing from multiple sequence controllers may be implemented in alternative embodiments of the present invention. 
     In the preferred embodiment, the selected image contrast waveform segment comprises the first portion of the dynamically constructed MR pulse sequence. Moreover, the operator has the option of selecting more than one image contrast mechanism for a proposed MR image or not selecting any image contrast mechanisms for a proposed MR image. In the first case of selecting more than one image contrast mechanism for a proposed MR image, the dynamically linked MR pulse sequence would contain all the selected image contrast waveform segments first followed by the imaging waveform segment. The order of the image contrast waveform segments is determined by predetermined order or some appropriate algorithm already prescribed in the MR imaging system. In the second case of not selecting any imaging contrast mechanism for a proposed MR image, the dynamically linked MR pulse sequence would only contain the selected imaging waveform segment. Hence, the advantage of independently invoking waveform segments to construct the MR pulse sequence, as needed at run-time, becomes apparent in real-time imaging where minimal scan time is of the essence. 
     To further interactively prescribe the image contrast in real-time, the operator can select the desired image contrast and/or imaging mechanisms prior to initiating a scan or the operator can select the desired image contrast and/or imaging mechanisms while a scan is in progress. When the operator selects the desired image contrast and/or imaging mechanisms while a scan is in progress, the MR imaging system can instantaneously (e.g., in less than 100 milliseconds) replace the current sets of waveform segments with new sets of waveform segments (i.e., dynamically construct a new MR pulse sequence) corresponding to the newly mechanisms. Although not shown in the figures, the graphical user interface  105  can additionally include icons for the operator to prescribe how the MR imaging system should deal with the scan in progress. Alternatively, the MR imaging system can include preset rules which dictate what should be done to the scan in progress. 
     For example, the MR imaging system can halt the current scan and/or halt the current MR image from being displayed; instead the sequence controller  10  can immediately initiate a new scan with the newly selected waveform segments and consequently display the latest MR image being desired. Alternatively, the MR imaging system can complete the current scan in progress and display the current MR image acquired; then the sequence controller  10  can initiate a new scan with the newly selected waveform segments. In still another alternative, the newly selected waveform segments can be used to finish the current scan in progress such that the resulting MR image is a conglomeration or hybrid of the current scan that was in progress with the current waveform segments and the new scan (comprising the remainder of the “current scan”) with the newly selected waveform segments. 
     To still further interactively prescribe the image contrast in real-time, the operator can directly modify the image contrast in real-time. Although not shown in the figures, the graphical user interface can include icons configured to allow the operator to directly modify individual gradient and RF amplitudes, pulse widths, and/or relative timing within each waveform segment. The direct modification can be accomplished graphically (e.g., slide bar icons) or explicitly (e.g., specific numerical values). Thus, in this manner, the present invention provides interactive and acquisition time saving image contrast controls to the operator. 
     While die embodiments and application of the invention illustrated in the figures and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. For example, it is contemplated that the invention may be applied to systems other than medical systems which can benefit from the use of interactive image contrast control. In still another example, a second sequence controller and/or a second waveform memory can be utilized to dynamically construct the next MR pulse sequence. The next MR pulse sequence could be associated with the next MR image to be acquired after the current acquisition has been completed. Alternatively, the next MR pulse sequence could be the instantaneous replacement of the current MR pulse sequence because the operator prescribed a different image contrast while the current acquisition was in progress. Accordingly, the invention is not limited to a particular embodiment, but extends to alternatives, modifications, and variations that nevertheless fall within the spirit and scope of the appended claims.