Abstract:
A magnetic resonance imaging (MRI) system includes a transmitter that produces an RF excitation pulse that is applied to a subject positioned in the MRI system to induce emission of at least one of an NMR signal and an ESR signal therefrom, and that produces a reference signal indicative of the phase of the RF excitation pulse. A first analog-to digital converter has an input for receiving the reference signal that is synchronous with the RF excitation pulse. One or more additional analog-to-digital converters/processors have inputs for receiving the at least one of NMR signals and ESR signals produced by a subject placed in the MRI system and produce one or more complex digital signal therefrom. A normalizer is connected to receive and normalize the digital reference signal and a mixer is connected to receive the normalized digital reference signal and the digital signal. Accordingly, the mixer is operable to multiply the normalized complex digital reference signal with the complex digital signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on provisional application 60/743,739 filed Mar. 24, 2006 and entitled “System and Method for Direct Digitization of NMR Signals” and claims the benefit thereof. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    -- 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates generally to magnetic resonance imaging (MRI) systems and, more particularly, to a system and method for receiving and directly digitizing imaging data signals. The invention is capable of utilizing commodity analog-to-digital (A/D) converters and adaptable software-processing algorithms to improve image quality while reducing manufacturing costs. 
         [0004]    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 ) that is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment, M t . A signal is emitted by the excited spins at or around the Larmor frequency after the excitation signal B 1  is terminated that is then received and processed to form an image. The Larmor frequency at a 1.5 Tesla (T) polarizing field strength is around 64 MHz. 
         [0005]    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 MRI signals is received using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. 
         [0006]    To transmit the RF signals that are used to excite the desired spins and receive the resulting MRI data signals, transmit and receive coils or a common transceiver coil is used. The receiver coil (or transceiver coil) receives the MRI data signals excited during the imaging process and provides the data signals to various hardware processing components. 
         [0007]    In particular, the signal produced by the subject being imaged in response to excitation by the RF excitation pulses is picked up by a receiver coil and applied through a preamplifier to a receiver amplifier. The receiver amplifier further amplifies the signal by an amount determined by a digital attenuation signal received from the pulse sequence server. Since the received signal is at or around the Larmor frequency and these hardware-based receive systems cannot provide adequate sampling at such high frequencies, this high frequency signal is down-converted in a two-step process by a down converter that first mixes the imaging signal with the carrier signal and then mixes the resulting difference signal with a reference signal. In this regard, these hardware systems typically down convert the received analog signals to an intermediate frequency that is less than the MRI imaging frequency and then mix it with an analog reference signal. 
         [0008]    Only after this conversion and mixing is the signal finally digitized by an analog-to-digital (A/D) converter that samples and digitizes the down-converted/mixed analog signal. Once digitized, the signal is applied to a digital detector and signal processor that produces 16-bit in-phase “I” values and 16-bit quadrature “Q” values corresponding to the received signal. Therefore, only after a variety of significant analog processing steps are the analog signals finally digitized and processed to reconstruct the resulting image. 
         [0009]    To carry out these mixing and digitizing processes, hardware systems are employed that are specifically tailored to the particular MRI system with which the mixing and digitizing hardware is to be associated. For example, once the constraints of a particular MRI system are identified (i.e., 1.5 Tesla or 3 Tesla and capable of only echo-planar imaging processes or capable of other fast-spin-echo techniques, such as gradient- and spin-echo processes), hardware that is specifically designed to prepare (i.e., synchronize and digitize) the imaging data received under those constraints is coupled therewith. That is, the hardware is specifically designed and tailored to perform down-conversion, mixing, and analog-to-digital conversion under the specific constraints and parameters (i.e., sampling frequency and MRI frequency) necessary for a given MRI system. 
         [0010]    While these hardware-based systems yield suitable results, they are extremely rigid since they are specifically designed and tailored for a particular MRI system. In this regard, although a wide variety of components, such as analog-to-digital converters, are produced as commodity (i.e., low-cost and/or mass-produced) components for use in mass-market devices (e.g., cellular phones and the like), these components cannot be readily utilized in MRI systems without redesigning or reconfiguring the hardware of the receiver system to accommodate the specific functionality of a given commodity component. Furthermore, as various hardware designs and components attain higher bandwidth and dynamic range, these MRI systems cannot harness these capabilities to yield higher quality images without hardware-level redesigns and reconfigurations of the receiver system. 
         [0011]    Accordingly, the original manufacturers of MRI systems cannot readily adapt to the fast-paced and ever-changing world market of commodity components because in-depth hardware redesigns and reconfigurations would be required for each and every new chip or board that is selected. Accordingly, though commodity components might provide significant manufacturing savings, the design costs associated with adapting to varying component constraints preclude the realization of such savings. 
         [0012]    Similarly, end users cannot simply replace the hardware-based receiver components that were originally included in an MRI system with new components that yield higher bandwidth and/or dynamic range. In this case, though an end user may wish to reap the benefits of newly available components that yield improved image quality, the constraints of the hardware-based receiver system preclude the integration of after-market components into an MRI system. 
         [0013]    Therefore, it would be desirable to have a system and method for facilitating the adaptability necessary to accommodate changing component constraints. Furthermore, it would be desirable to allow end users to upgrade the receiver system of a given MRI system to improve image quality without undue reconfiguration and redesign. Additionally, it would be desirable to provide a system and method to facilitate direct detection of MRI imaging data. That is, it would be desirable to have a system and method capable of receiving and digitizing MRI imaging data signals without the need for intermediate analog processing steps, such as intermediate frequency processing. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention overcomes the aforementioned drawbacks by providing a system and method for utilizing receiver components in MRI receiver systems that are capable of achieving direct detection/digitization of received nuclear magnetic resonance (NMR) signals. As will be described, the present invention, though described with respect to NMR signals, is also capable of receiving and processing other signals, such as electron spin resonance (ESR) signals. Furthermore, the present invention allows A/D conversion systems manufactured as commodity items for integration into mass-market devices such as cellular phones, satellite communications systems, and the like to be utilized in MRI receiver systems. In this regard, the present invention allows traditional A/D converters that are specifically designed for MRI applications to be replaced or forgone in favor of low-cost A/D conversion systems that can be readily mass-produced. 
         [0015]    In accordance with one aspect of the invention, a magnetic resonance imaging (MRI) system is disclosed that includes a transmitter that produces an RF excitation pulse that is applied to a subject positioned in the MRI system to induce emission of at least one of an NMR signal and an ESR signal therefrom, and that produces a reference signal indicative of the phase of the RF excitation pulse. The system also includes an RF receiver that has a plurality of digitization channels. One of the channels is configured to digitize the reference signal. Another of the channels is coupled to a coil that receives the at least one of the NMR signal and the ESR signal and is configured to directly digitize the at least one of the NMR signal and the ESR signal. The digitized reference signal forms a reference array and the digitized signal forms a data array. A software mixer then mixes data from the reference array and the data array to generate an imaging data array from which an image can be reconstructed. 
         [0016]    In accordance with another aspect of the invention, a method of processing NMR signals is disclosed that includes receiving NMR signals at its Larmor frequency and directly digitizing the NMR signals. The method also includes receiving and digitizing a reference signal produced by an RF transmitter. The method further includes storing the digitized reference signal as a reference array and the digitized NMR signals as a data array. The method further includes mixing the data in the reference array and the digitized NMR data in the data array to generate a phase-accurate NMR data array. The resulting NMR data is k-space data used to reconstruct an MR image of the imaging subject. 
         [0017]    In accordance with yet another aspect of the invention, a kit configured to retrofit an RF transceiver system of an MRI system is disclosed that includes an RF receiver circuit having a plurality of analog-to-digital conversion channels. At least one of the channels is configured to digitize a reference phase signal generated by the MRI system. Another of the channels is configured to digitize at least one NMR signal received from a subject in the MRI system. The digitized reference phase signal is arranged as a reference phase array and the digitized NMR signal is arranged as one or more data array(s). The kit also includes a processor configured to normalize the reference phase array to create a complex conjugated array and generate phase-accurate NMR data by mixing the complex conjugated array with the NMR data array(s). 
         [0018]    In accordance with another aspect of the invention, an MRI system is disclosed that includes an RF coil assembly configured to transmit RF signals and receive NMR signals at or around the Larmor frequency. The MRI system includes an RF receiver circuit having a plurality of digitization channels, each of which is configured to digitize the NMR signal at its original Larmor frequency. 
         [0019]    In accordance with still another aspect of the invention, an MRI system is disclosed that includes an RF receiver circuit having a plurality of digitization channels, one of which is configured to digitize a reference signal and produce phase-accurate MRI data. 
         [0020]    Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a block diagram of an MRI system for use with the present invention; 
           [0022]      FIG. 2  is a schematic representation of a transceiver system for use with the MRI system of  FIG. 1  and incorporating a receiver system in accordance with the present invention; 
           [0023]      FIG. 3  is a block diagram showing the components of a receiver system in accordance with one embodiment of the present invention; and 
           [0024]      FIG. 4  is a flow chart setting forth the steps of a technique for receiving and processing MRI information in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    Referring to  FIG. 1 , an 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. 
         [0026]    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 . It is contemplated that the functionality of the data store server  23  may be performed by the workstation processor  16  and associated disc drive interface circuitry or may be a stand-alone computer system. In any case, functionalities of the remaining three servers  18 ,  20 , and  22  are performed by separate processors mounted in a single enclosure and interconnected using a backplane bus. The pulse sequence server  18  employs a commercially available microprocessor and a commercially available quad communication controller. The data acquisition server  20  and data processing server  22  both employ the same commercially available microprocessor and the data processing server  22  further includes one or more array processors, for example, based on commercially available parallel vector processors. 
         [0027]    The workstation  10  and each of the servers  18 ,  20  and  22  are connected to a serial communications network. This serial network conveys data that is downloaded to the servers  18 ,  20 , and  22  from the workstation  10  and it conveys tag data that is communicated between the servers  18 ,  20 ,  22  and the workstation  10 . In addition, a high speed data link is provided between the data processing server  22  and the workstation  10  in order to convey image data to the data store server  23 . 
         [0028]    The pulse sequence server  18  functions in response to program elements downloaded from the workstation  10  to operate a gradient system  24  and an RF system  26 . Gradient waveforms necessary to perform a prescribed scan are produced and applied to the gradient system  24  which excites the coils of a gradient coil assembly  28  to produce the magnetic field gradients G x , G y , and G z  used for position-encoding MRI signals. The gradient coil assembly  28  forms part of a magnet assembly  30  which includes a polarizing magnet  32  and an RF coil  34 . As will be described below, though a whole-body RF coil  34  is shown, it is contemplated that the present invention may be equally applicable to a wide variety of coil arrangements, such as various local coil configurations and the like. 
         [0029]    Furthermore, through a whole-body RF coil  34  and gradient coil assembly  28  is shown within one type of MRI system, it is contemplated that other MRI systems, such as open MRI systems, may be utilized with the present invention. Furthermore, though illustrated as designed for imaging humans, the present invention may also be used with MRI systems designed to image animals and other objects. Furthermore, though described with respect to MR imaging, the present invention may also be used with other MR systems, such as MR spectroscopy systems or systems designed for non-medical purposes. 
         [0030]    During an imaging process, RF excitation waveforms are applied to the RF coil  34  by the RF system  26  to perform the prescribed magnetic resonance pulse sequence. Responsive thereto, MRI or imaging data signals are detected by the RF coil  34  and received by the RF system  26  and processed under direction of commands produced by the pulse sequence server  18 . As will be described in detail below, 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. In this regard, the generated RF pulses may be applied to the whole body RF coil  34  or to one or more local coils or coil arrays. 
         [0031]    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 “gate” the performance of the scan with the subject&#39;s respiration or heart beat. 
         [0032]    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. 
         [0033]    It should be apparent that the pulse sequence server  18  performs real-time control of MRI system elements during a scan. As a result, it is necessary that its hardware elements be operated with program instructions that are executed in a timely manner by run-time programs. The description components for a scan prescription are downloaded from the workstation  10  in the form of objects. The pulse sequence server  18  contains programs which receive these objects and converts them to objects that are employed by the run-time programs. 
         [0034]    The digitized MRI 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 description components downloaded from the workstation  10  to receive the real-time imaging 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 imaging data to the data processing server  22 . However, in scans that require information derived from acquired imaging 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, imaging 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 MRI signals used to detect the arrival of contrast agent in an MRI scan. In all these examples, the data acquisition server  20  acquires MRI data and processes it in real-time to produce information that is used to control the scan. 
         [0035]    The data processing server  22  receives imaging data from the data acquisition server  20  and processes it in accordance with description components downloaded from the workstation  10 . Such processing may include, for example: Fourier transformation of raw k-space MRI data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a back-projection image reconstruction of acquired MRI data; the calculation of functional MR images; the calculation of motion or flow images; and the like. 
         [0036]    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 database memory cache (not shown) from which they may be delivered to the operator display  12  or another display  42 , such as may be 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. 
         [0037]    As shown in  FIG. 1 , the RF system  26  may be connected to the whole body RF transceiver coil  34 , or as shown in  FIG. 2 , a transmitter section  46  of the RF system  26  may connect to one RF coil  48  and a receiver section  49  may connect to a separate RF receive coil  50 . Often, the transmitter section  46  is connected to a whole-body RF coil  34  and each receiver section is connected to a separate local coil  50 . Multi-channel receivers have become available in recent years to practice SENSE, SMASH, and GRAPPA imaging methods. 
         [0038]    Continuing with respect to  FIG. 2 , the RF system  26  includes a transmitter  46  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  52  that receives a set of digital signals from the pulse sequence server  18  of  FIG. 1 , as well as a reference signal that is produced by a reference frequency generator  53 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  54  of the frequency synthesizer  52 . The RF carrier is applied to a modulator and up converter  56  where the amplitude is modulated in response to a signal, R(t), that is also received from the pulse sequence server  18  of  FIG. 1 . The signal R(t) defines the envelope of the RF excitation pulse to be produced and is generated 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. 
         [0039]    The magnitude of the RF excitation pulse produced at the output  58  of the modulator/up converter  56  is attenuated by an exciter-attenuator circuit  60  that receives a digital command from the pulse sequence server  18  of  FIG. 1 . The attenuated RF excitation pulses are applied to a power amplifier  62  that drives the RF coil  48 . 
         [0040]    The NMR signal produced by the subject being imaged in response to excitation by the RF excitation pulses is picked up by the receiver coil  50  and coupled to a receiver processing system  64 . The received NMR signal is at or around the Larmor frequency. This frequency will depend on the strength of the polarizing field B 0 , but in any case, it is a very high frequency (e.g., approximately, 42 MHz/Tesla). Referring now to  FIG. 3 , the receiver processing system  64  includes a section implemented in hardware indicated at  66  and a section implemented in software indicated at  68 . As will be described, the present invention has moved some of the functionality typically embodied in hardware into the software section  68 . As a result, the present invention allows for direct digitization of NMR signals and moves functions to software to allow greater adaptability and functionality that can be harnessed by original manufacturers to realize production cost savings and utilized by end users to yield increased image quality. 
         [0041]    Continuing with respect to  FIG. 3 , the received NMR signals acquired by the receiver coil  50  are passed to a preamplifier  70 . Thereafter, the imaging data signals are passed to amplifiers/filters  72 ,  74  that are dedicated to respective receive channels  76 ,  78  of the hardware system  66 . Additionally, a variety of filters may be associated with each receive channel  76 ,  78  to limit their bandwidth around the Larmor frequency. Additionally or alternatively, though  FIG. 3  illustrates that the receiver coil  50  includes two receive coils  50   a  and  50   b  that provide the NMR signal to respective preamplifiers  70   a ,  70   b  and receive channels  76 ,  78 , it is contemplated that a multiplicity of individual receive coils may be dedicated to respective receive channels. Furthermore, though  FIG. 3  shows a single board  80  having three channels  76 ,  78 ,  82 , it is contemplated that each board  80  may include a number of channels, for example, four. Furthermore, it is contemplated that multiple boards may be included to provide 128 channels or virtually any number of receive channels. 
         [0042]    For example, it is contemplated that each channel  76 ,  78 ,  82  may be a quad-channel receiver. Each receive channel  76 ,  78 ,  82  includes an analog-to-digital (A/D) converter  84  and a processor  86 . In particular, the receiver may be an Echotek receiver. Echotek is a registered trademark of Mercury Computer Systems, Inc. of Chelmsford, Mass. In this regard, for example, each board  80  may be an ECDR-GC314 board, whereby the A/D converters  84  are three 14-bit, 200 MHz Analog Devices AD6645 converters clocked at 100 MSPS and the processors  86  are Graychip 4016 quad digital receivers. ECDR-GC314 boards are also commercially available from Mercury Computer Systems, Inc. of Chelmsford, Mass. However, it is contemplated that for MRI applications, the standard bandwidth of each anti-alias filter, which is 50 MHz (half of the sampling rate), may be reduced by a decimation of 1024 to increase the dynamic range by 5 bits. Accordingly, each A/D converter  84  acts as a 19-bit converter. In general, the bandwidth of a given chip may be decreased by 4 N , where N is the number of bits of dynamic range increase of the assembly over the dynamic range of the A/D converter. Hence, the system may be adapted to utilize a wide variety of chips that are currently available, such as a Field Programmable Gate Array (FPGA), or even chips that have yet to be developed. 
         [0043]    Accordingly, as will be described, by adapting these components to be usable in MRI systems, an MRI receiver system is created that is capable of achieving direct detection/digitization of MRI imaging data. That is, MRI imaging data can be received and digitized without the need for intermediate frequency processing or other analog processing steps. As such, higher quality images can be produced by a given scanner that would not be possible using the receiver components included in the original MRI system. 
         [0044]    The RF system  26  may include two or more channels. In particular, the RF system  26  may include up to N+1 channels, where N is an integer. That is, at least one channel  82  is dedicated to digitizing and processing a reference signal from the reference frequency generator  53  that is used to synchronize the phase of the acquired NMR data, which is digitized and processed by the other channel(s). As stated, each channel  76 ,  78 ,  82  is capable of direct digital detection of RF signals that are “phase coherent” with the RF excitation pulses produced by the transmitter. Accordingly, it is contemplated that the reference frequency generator  53  may be directly digitized by a dedicated channel  82 . 
         [0045]    Hence, the RF system  26  will include at least one additional digitizing channel  76 ,  78 ,  82  than the number of receive channels. Typically, a given MRI system may have anywhere from 2 to 96, or even more, receive channels that may be connected to a corresponding plurality of local coils or to a corresponding plurality of coil elements in a coil array. 
         [0046]    When the NMR signal is received by the receive coil  50 , the NMR signal is at or around the Larmor frequency. In a traditional MRI receiver system, this high frequency signal is down-converted in a one- or two-step process by a down converter that first mixes the NMR signal with the RF carrier signal and then mixes the resulting difference signal with the reference/synchronization signal. Thus, in traditional MR systems, all mixing is performed using analog signals. However, in the present invention, the use of A/D converters  84  with high bandwidth and high dynamic range enables detection at the MRI Larmor frequency as opposed to some intermediate frequency. 
         [0047]    Accordingly, the present invention provides true “direct detection” of MRI imaging data at the same frequency as the RF excitation signal applied to the imaging subject. This is true, for example, even when performing three-dimensional (3D) imaging processes with magnetic field strengths of 3 Tesla (T) or greater. For example, the RF MRI imaging signal that is received may correspond to the proton magnetic resonance frequency when subjected to a static magnetic field strength of 3T or greater and, still, down conversion to an intermediate frequency or similar analog processing is not necessary prior to digitization. 
         [0048]    The A/D converters  84  utilize a signal from a common clock  88  around which to digitize the received signal. After processing in processor  86 , the resulting I and Q digital values are then passed from the hardware section  66  to the software section  68 . The software section  68  includes a processor that is programmed with stored “firmware” to carry out the following functions. These include a normalizer  90  that normalizes the digital I and Q values in the reference array generated from the phase reference signal. In particular, the array of values is normalized using a value (alpha) that is the arctan of I divided by Q. Accordingly, a complex conjugated array of I and Q values is created where: 
         [0000]      α=tan −1    I/Q;    
         [0000]        I   normalized =−sin (α); 
         [0000]        Q   normalzed =cos (α); 
         [0049]    The values in the normalized reference array are then mixed by complex multiplication with the corresponding values in each array of imaging data from each channel in the hardware section  66 . This synchronizes the phases of the acquired I and Q pairs with the reference signal and RF phase of the excitation pulse. As is known, mixing the NMR data with the reference signal, albeit while traditionally performed in hardware and using analog signals, reduces artifacts in a reconstructed image that are attributable to spurious harmonics of hardware mixers and direct current (DC) interference in the imaging data signals. The mixed I and Q pairs then form raw “k-space” data that is sent to the data processing server  22  for image reconstruction. 
         [0050]    While in the present invention the mixing is performed in software rather than hardware, the functional result is unchanged. However, unlike hardware-based systems, by performing normalization and mixing in software, the receiver processing system  64  is sufficiently flexible to adapt to variations in the reference frequency provided by the reference frequency generator  53 , which is part of the fundamental hardware design of the MRI scanner. Accordingly, by performing mixing and normalization in software, it is possible to adapt to a wide variety of MRI scanners, each having slightly varied reference signals generated by slightly different reference frequency generators. Similarly, by performing mixing and normalization in software, it is possible to adapt to a wide variety of A/D converters and/or processors. In this regard, the receive system can be easily adapted to accommodate a given board  80  that may be desirable because of increased bandwidth or decreased cost, such as is common in mass-produced commodity components. 
         [0051]    Hence, the present invention allows a variety of commercially available receiver components to be utilized in MRI receiver systems. For example, the present invention allows A/D conversion systems manufactured as commodity items for integration into mass-market devices such as cellular phones, satellite communications systems, and the like to be utilized in MRI receiver systems. In this regard, the present invention allows traditional A/D converters that are specifically designed for MRI applications to be replaced or forgone in favor of low-cost A/D conversion systems that can be readily mass-produced. As such, a significant cost savings can be realized by a scanner manufacturer by having the flexibility to readily incorporate newly available components or components that are already available into a particular scanner design without having to perform significant hardware redesigns. 
         [0052]    Furthermore, it is contemplated that the receiver processing system  64  may be easily incorporated into a kit designed to retrofit any of a variety of traditional MRI systems having integrated hardware receiver systems to allow end users to accommodate the bandwidth, Larmor frequency, and dynamic range available in newly designed or available components. Accordingly, the receiver processing system  64  may be embodied as a kit that can be attached to a given MRI scanner to bypass the internal hardware-based receiver system of the scanner and provide higher quality images. 
         [0053]    Referring now to  FIG. 4 , the functionality of the above-described receiver processing system  64  is set forth as a process  102 . The reference signal is received  104  from the reference frequency generator  53  of the associated MRI system. The reference signal is then digitized and saved as an array of I and Q pairs  106  and normalized  107  to create a new complex array of I and Q pairs, as described above. Simultaneously therewith, NMR data is received at each channel  108 ,  110  (channel  1  through channel N). These NMR signals at each channel are digitized  112 ,  114  and processed to be saved as an array of I and Q pairs. 
         [0054]    Thereafter, the arrays of I and Q pairs for channels  1  through N are mixed with the normalized, complex reference array  116 ,  118 . The software system then returns the data associated with each channel to the MRI system for image reconstruction  120 ,  122 . 
         [0055]    While the above-described systems and methods have been described with respect to NMR signals, it is contemplated that other imaging signals, such as electron spin resonance (ESR) signals may also be utilized. Hence, the present invention provides a system and method for utilizing receiver components in MRI receiver systems that are capable of achieving direct detection/digitization of MRI imaging data. That is, the above-described system and method facilitates the ability to utilize components having an inherently high bandwidth and dynamic range so that MRI imaging data can be received and digitized without the need for intermediate frequency processing. Accordingly, reference frequency mixing can be performed following digitization. Higher quality images can be produced by a given scanner that would not be possible using the receiver components included in the original MRI system. 
         [0056]    Additionally, the present invention provides a system and method for allowing manufacturers to use low-cost, mass-produced components in MRI receiver systems by facilitating the adaptability necessary to accommodate changing component constraints. Furthermore, the above-described systems and methods allow end users to upgrade a receiver system of a given MRI system to improve image quality without undue reconfiguration and redesign. 
         [0057]    Beyond the embodiments and operating environments described above, it is contemplated that the present invention may be used with 3D imaging processes, such as described above, where the imaging scan is directed to the breast, liver, or other area of the patient. Furthermore, it is contemplated that the present invention may be advantageously utilized with systems and processes utilizing a modified reconstruction algorithm, such as cardiac breath-hold applications, to produce an image of relatively high quality in an advantageously reduced time. Similarly, the present invention may be utilized with modified reconstruction algorithms to detect extremity anomalies at the edge of the image. 
         [0058]    Therefore, the present invention has been described in terms of the 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. Therefore, the invention should not be limited to a particular described embodiment.