Abstract:
A system and method is provided for acquiring a medical image of a portion of a vascular structure of a subject using a magnetic resonance imaging (MRI) system. A magnetization preparation RF module is applied to a portion of a subject including a vascular structure using the MRI system. A readout procedure is performed to collect image data, wherein the readout procedure includes a phase encoding scheme configured to provide a desired delay time after the application of the magnetization preparation RF module to allow a partial recovery of signal within the vascular structure following application of the magnetization preparation RF module when sampling a central region of k-space during the readout procedure. The image set is reconstructed into an image of the vascular structure wherein blood within the vascular structure is reflected as a gray-blood image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on, claims priority to, and incorporates herein by reference, U.S. Provisional Patent Application Ser. No. 61/614,575 filed on Mar. 23, 2012, and entitled “SYSTEM AND METHOD FOR IMAGING OF THE VASCULAR COMPONENTS USING MAGNETIC RESONANCE IMAGING.” 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a system and method for performing magnetic resonance imaging and, more particularly, to a system and method for acquiring images using magnetic resonance imaging that allow clinical review of a subject&#39;s vascular components, including the blood vessel wall, lumen and intra-wall components. 
       BACKGROUND OF THE INVENTION 
       [0003]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment M z  is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ; also referred to as the radiofrequency (RF) field) which 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 , which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B 1  is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited. 
         [0004]    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 experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. 
         [0005]    Magnetic resonance angiography (MRA) and, related imaging techniques, such as perfusion imaging, use the NMR phenomenon to produce images of the human vasculature or physiological performance related to the human vasculature. There are three main categories of techniques for achieving the desired contrast for the purpose of MR angiography. The first general category is typically referred to as contrast enhanced (CE) MRA. The second general category is phase contrast (PC) MRA. The third general category is time-of-flight (TOF) or tagging-based MRA. 
         [0006]    Regardless of the particular category of MRA technique utilized, these angiographic techniques typically focus on modifying the contrast associated with the blood flowing within the lumen in the vessel. As such, such imaging techniques often produce so-called “black-blood” or “white-blood” images, which refers to the appearance of the blood as having a “black” (i.e. dark) or “white” (i.e. bright) contrast relative to stationary or tissues that have otherwise not had their associated contrast manipulated using one of the aforementioned techniques. 
         [0007]    Many areas of clinical interest focus on the vascular system. For example, atherosclerosis is an inflammatory process in which lipids and calcifications form within the vessel wall to form “plaques.” Atherosclerosis is a major cause of cardiovascular mortality and morbidity in the Western world. As described above, traditional clinical imaging techniques produce “black-blood” or “white-blood” images. “White-blood” images enable the clinician to focus on the degree of luminal impingement by atherosclerotic plaques on the vascular lumen by visualizing an unusual constraint on the vascular lumen. “Black-blood” images are designed to best view the expected appearance of healthy, non-diseased vascular wall. In this regard, traditional MRA techniques provide indirect and incomplete interrogation of vascular disease and vascular plaques by focusing on the appearances of blood and the healthy vascular wall. 
         [0008]    Unfortunately, in traditional MRA images, such as “black-blood” or “white-blood” images, the features and boundaries of the vessel lumen, vessel wall, and the ability to accurately discriminate between the vessel lumen and superficial and juxtaluminal intraplaque calcification, as well as other intraplaque components including hemorrhage, are sub-optimally portrayed or generally unavailable to the clinician. For example, the substantial contrast of the black or white blood generally “overpowers” and obscures any subtle contrast of surrounding vascular structures and components. That is, by focusing the enhancing contrast mechanisms in traditional MRA images heavily on either the blood within the vessel or on visualizing healthy vascular wall, direct interrogation of the diseased vascular wall, vascular plaques, and plaque contents through use of a tailored image contrast have generally been foregone in favor of signal properties of the blood proximate to these structures. Accordingly, because the vascular wall characteristics of the in a diseased state and specifically the vascular plaque, plaque contents, and the like, are of greater clinical interest for clinical analysis of conditions such as atherosclerosis and many other vascular conditions, important features can be lost, missed or overpowered in the images produced using traditional MRA techniques. 
         [0009]    Furthermore, beyond the constraints on clinical information imposed by such “black-blood” and “white-blood” imaging techniques, the imaging protocols, themselves, present some unfortunately constraints in the clinical setting. For example, black-blood imaging has two major limitations for MRI of the vascular wall. First, black-blood imaging restricts MR data acquisition to a short time window in which the vascular lumen and blood pool is completely suppressed. This restriction limits the temporal efficiency of imaging, or, when this short time window is neglected, introduces substantial apodization in the acquired k-space data and blurring and artifacts in the image space due to the evolving magnetization of the blood pool. Second, black-blood imaging often fails to adequately delineate the vessel lumen from intraplaque calcification due to their similar dark appearance. 
         [0010]    A limitation of white-blood imaging techniques (such as 2D and 3D time-of-flight magnetic resonance angiography) for vessel wall imaging is that the extremely hyperintense vessel lumen adjacent to the inner boundary of the vessel wall and the hyperintense regions of perivascular fat adjacent to the outer boundary of the vessel wall severely obscure the boundaries of the relatively hypointense vessel wall. Furthermore, the large flip angles utilized in white-blood imaging largely suppresses the MRI signals in vessel wall. This obscuration of the vessel wall severely limits the ability of white-blood imaging techniques for precise vessel wall area measurement and component characterization of atherosclerotic plaques. 
         [0011]    Therefore, it would be desirable to have a system and method for non-invasively producing images that allow clinicians to accurately delineate the shape and boundaries of the vessel lumen and vessel wall and discriminate between the vessel lumen and juxtaluminal intraplaque calcification, as well as depict other intraplaque components including hemorrhage. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention overcomes the aforementioned drawbacks by providing a magnetic resonance imaging technique that elicits image contrast in which the blood pool is of a moderate, but distinct, signal intensity from the vessel wall to thereby facilitate unambiguous display and accurate analysis of the vessel wall and associated features and structures. Furthermore, the present invention overcomes the limitations of black-blood imaging by permitting the use of linear phase encode schedules that enhance the imaging point spread function and enable clear separation of superficial and deep-seated intraplaque calcification from the vascular lumen. Also, gray-blood and black-blood image sets (“dual-contrast” imaging) may be simultaneously obtained by modifying the phase-encoding schedule to one that utilizes a mixture of centric and linear phase-encoding ordering. 
         [0013]    In accordance with one aspect of the invention, a method is disclosed for acquiring a medical image of a portion of a vascular structure of a subject using a magnetic resonance imaging (MRI) system. The method includes selecting a magnetization preparation radio frequency (RF) module to be applied to a portion of the subject including the vascular structure using the MRI system. The method also includes determining a delay time between an application of the magnetization preparation RF module and a sampling of a central region of k-space during a readout procedure using the MRI system, wherein the delay time is configured to permit partial recovery of signal within the vascular structure following the completion of the magnetization preparation RF module. The method also includes selecting a phase encoding scheme for the readout procedure configured to provide the determined delay time between the application of the magnetization preparation RF module and the sampling of a central region of k-space during the readout procedure. The method further includes performing a pulse sequence using the MRI system to effectuate the selected magnetization preparation RF module, determined delay time, and readout procedure including the selected phase encoding scheme to acquire at least a gray-blood image set and reconstructing the gray-blood image set into a gray-blood image. 
         [0014]    In accordance with another aspect of the invention, a magnetic resonance imaging (MRI) system is disclosed that includes a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system, a plurality of gradient coils configured to apply a gradient field to the polarizing magnetic field, and a radio frequency (RF) system configured to apply an excitation field to the subject and acquire MR image data therefrom. A computer system is provided that is programmed to apply a magnetization preparation RF module to a portion of a subject including a vascular structure using the MRI system and perform a readout procedure to collect image data. The readout procedure includes a phase encoding scheme configured to provide a desired delay time after the application of the magnetization preparation RF module to allow a partial recovery of signal within the vascular structure following application the magnetization preparation RF module when the sampling of a central region of k-space during the readout procedure. The computer is further programmed to reconstruct the acquired image set into an image of the vascular structure wherein blood within the vascular structure is reflected as a gray-blood image. 
         [0015]    In accordance with yet another aspect of the invention, a method is disclosed for acquiring a medical image of a portion of a vascular structure of a subject using a magnetic resonance imaging (MRI) system. The method includes applying a magnetization preparation RF module to a portion of a subject including a vascular structure using the MRI system and performing a readout procedure to collect image data. The readout procedure includes a phase encoding scheme configured to provide a desired delay time after the application of the magnetization preparation RF module to allow a partial recovery of signal within the vascular structure following application the magnetization preparation RF module when the sampling of a central region of k-space during the readout procedure. The method also includes reconstructing the acquired image set into an image of the vascular structure wherein blood within the vascular structure is reflected as a gray-blood image. 
         [0016]    The foregoing and other advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a block diagram of an MRI system for use with the present invention. 
           [0018]      FIG. 2  is a schematic representation of a transceiver system for use with the MRI system of  FIG. 1 . 
           [0019]      FIG. 3  is a flow chart of the steps performed in accordance with one exemplary implementation of the present invention. 
           [0020]      FIGS. 4A and 4B  are diagrams illustrating timing versus phase encoding strategies in accordance with the present invention. 
           [0021]      FIG. 5  is a collection of images illustrating black-blood, white-blood, and gray-blood images in side-by-side comparison. 
           [0022]      FIG. 6  is a collection of images illustrating black-blood and dual-contrast images in a side-by-side comparison. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring particularly to  FIG. 1 , an example of a magnetic resonance imaging (“MRI”) system  100  is illustrated. The workstation  102  includes a processor  108 , such as a commercially available programmable machine running a commercially available operating system. The workstation  102  provides the operator interface that enables scan prescriptions to be entered into the MRI system  100 . The workstation  102  is coupled to four servers: a pulse sequence server  110 ; a data acquisition server  112 ; a data processing server  114 ; and a data store server  116 . The workstation  102  and each server  110 ,  112 ,  114 , and  116  are connected to communicate with each other. 
         [0024]    The pulse sequence server  110  functions in response to instructions downloaded from the workstation  102  to operate a gradient system  118  and a radiofrequency (“RF”) system  120 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  118 , which excites gradient coils in an assembly  122  to produce the magnetic field gradients Gx, Gy, and Gz used for position encoding MR signals. The gradient coil assembly  122  forms part of a magnet assembly  124  that includes a polarizing magnet  126  and a whole-body RF coil  128 . 
         [0025]    RF waveforms are applied to the RF coil  128 , or a separate local coil (not shown in  FIG. 2 ), by the RF system  120  to perform the prescribed magnetic resonance pulse sequence. Responsive MR signals detected by the RF coil  128 , or a separate local coil (not shown in  FIG. 1 ), are received by the RF system  120 , amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  110 . The RF system  120  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  110  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  128  or to one or more local coils or coil arrays (not shown in  FIG. 1 ). 
         [0026]    The RF system  120  also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the MR signal received by the coil  128  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The magnitude of the received MR signal may thus be determined at any sampled point by the square root of the sum of the squares of the and components: 
         [0000]        M =√{square root over ( I   2   +Q   2 )}  Eqn. (2);
 
         [0027]    and the phase of the received MR signal may also be determined: 
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         [0028]    The pulse sequence server  110  also optionally receives patient data from a physiological acquisition controller  130 . The controller  130  receives signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server  110  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
         [0029]    The pulse sequence server  110  also connects to a scan room interface circuit  132  that 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  132  that a patient positioning system  134  receives commands to move the patient to desired positions during the scan. 
         [0030]    The digitized MR signal samples produced by the RF system  120  are received by the data acquisition server  112 . The data acquisition server  112  operates in response to instructions downloaded from the workstation  102  to receive the real-time MR data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server  112  does little more than pass the acquired MR data to the data processor server  114 . However, in scans that require information derived from acquired MR data to control the further performance of the scan, the data acquisition server  112  is programmed to produce such information and convey it to the pulse sequence server  110 . For example, during prescans, MR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  110 . Also, navigator signals may be acquired during a scan and used to adjust the operating parameters of the RF system  120  or the gradient system  118 , or to control the view order in which k-space is sampled. By way of example, the data acquisition server  112  acquires MR data and processes it in real-time to produce information that may be used to control the scan. 
         [0031]    The data processing server  114  receives MR data from the data acquisition server  112  and processes it in accordance with instructions downloaded from the workstation  102 . Such processing may include, for example: Fourier transformation of raw k-space MR 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 MR data; the generation of functional MR images; and the calculation of motion or flow images. 
         [0032]    Images reconstructed by the data processing server  114  are conveyed back to the workstation  102  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 1 ), from which they may be output to operator display  112  or a display  136  that is located near the magnet assembly  124  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  138 . When such images have been reconstructed and transferred to storage, the data processing server  114  notifies the data store server  116  on the workstation  102 . The workstation  102  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0033]    As shown in  FIG. 1 , the radiofrequency (“RF”) system  120  may be connected to the whole body RF coil  128 , or, as shown in  FIG. 2 , a transmission section of the RF system  120  may connect to one or more transmit channels  202  of an RF coil array  204  and a receiver section of the RF system  120  may connect to one or more receiver channels  206  of the RF coil array  204 . The transmit channels  202  and the receiver channels  206  are connected to the RF coil array  204  by way of one or more transmit/receive (“T/R”) switches  208 . 
         [0034]    Though illustrated as having multiple transmit channels  202  and multiple receiver channels  206  connected to multiple transmit/receive switches  208 , the present invention is not limited to traditional or parallel imaging systems. However, as will be further made apparent below, the dual-contrast imaging technique to be described may particularly benefit from parallel imaging acceleration in the phase-encoding direction. 
         [0035]    Also, the receiver channel  206  may also be an assembly of coils separate from the transmit coil array. In such a configuration, the T/R switches  208  are not needed. The transmit coil elements are detuned or otherwise rendered dysfunctional during the receive operation, and the receiver coil elements are similarly detuned or otherwise rendered dysfunctional during operation of the transmit coils. Such detuning may be accomplished with appropriate control logic signals. 
         [0036]    Referring particularly to  FIG. 2 , the RF system  120  includes one or more transmit channels  202  that produce a prescribed RF electromagnetic field. The base, or carrier, frequency of this RF field is produced under control of a frequency synthesizer  210  that receives a set of digital signals from the pulse sequence server  110 . These digital signals indicate the frequency, amplitude, and phase of the RF carrier signal produced at an output  212 . The RF carrier is applied to a modulator and, if necessary, an up converter  214  where its amplitude and phase is modulated in response to a signal, R(t), also received from the pulse sequence server  110 . The signal, R(t), defines the envelope of the RF 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. 
         [0037]    The magnitude of the RF pulse produced at output  216  is attenuated by an attenuator circuit  218  that receives a digital command from the pulse sequence server  110 . The phase of the RF pulse may also be altered using phase shifters (not shown). The modulated RF pulses are then applied to a power amplifier  220  that drives one element of the RF coil array  204 , or several such elements that are electrically coupled. Multiple transmit channels then drive other elements of the multichannel transmit coil array. 
         [0038]    The MR signal produced by the subject is picked up by the RF coil array  202  and applied to the inputs of the set of receiver channels  206 . A preamplifier  222  in each receiver channel  206  amplifies the signal, which is then attenuated, if necessary, by a receiver attenuator  224  by an amount determined by a digital attenuation signal received from the pulse sequence server  110 . The received signal is at or around the Larmor frequency, and this high frequency signal may be down converted in a two step process by a down converter  226 . In an example of such a process, the down converter  226  first mixes the MR signal with the carrier signal on line  212  and then mixes the resulting difference signal with a reference signal on line  228  that is produced by a reference frequency generator  230 . The MR signal is applied to the input of an analog-to-digital (“ND”) converter  232  that samples and digitizes the analog signal. As an alternative to down conversion of the high frequency signal, the received analog signal can also be detected directly with an appropriately fast analog-to-digital (“A/D”) converter and/or with appropriate undersampling. The sampled and digitized signal may then be applied to a digital detector and signal processor  234  that produces in-phase (I) and quadrature (Q) values corresponding to the received signal. The resulting stream of digitized I and Q values of the received signal are output to the data acquisition server  112 . In addition to generating the reference signal on line  228 , the reference frequency generator  230  also generates a sampling signal on line  236  that is applied to the A/D converter  232 . 
         [0039]    Referring to  FIG. 3 , a flow chart setting forth the steps of a method  300  for operating an MRI system, such as described above with respect to  FIGS. 1 and 2 , is provided. The process begins at process block  302  with a flow-suppressing magnetization preparation module. Examples of such flow-suppressing magnetization preparations include motion-sensitized driven equilibrium which are described in Koktzoglou I, Li D. Diffusion-prepared segmented steady-state free precession: Application to 3D black-blood cardiovascular magnetic resonance of the thoracic aorta and carotid artery walls and J Cardiovasc Magn Reson. 2007; 9(1):33-42. PubMed PMID: 17178678 or Balu N, Yarnykh V L, ChuB, Wang J, Hatsukami T, Yuan C. Carotid plaque assessment using fast 3D isotropic resolution black-blood MRI. Magn Reson Med. 2011 March; 65(3):627-37. doi: 10.1002/mrm.22642. Epub 2010 Oct. 12. PubMed PMID: 20941742; PubMed Central PMCID: PMC3042490, both of which are incorporated herein by reference. The flow-suppressing magnetization preparation is applied and is then followed at process block  306  by a gradient-echo based readout utilizing a linear phase-encoding scheme. 
         [0040]    As illustrated by intervening process block  304 , a delay time is selected between the flow-suppression preparation at process block  302  and the acquisition of central k-space at process bock during a readout at process block  306 . Process block  304  is illustrated as optional because it may be embodied as a period of time that elapses between performance of the magnetization preparation at process block  302  and beginning of the readout at process block  306 . However, as will be explained, the delay time at process block  304  may be achieved by introducing an “effective delay time” between the magnetization preparation at process block  302  and the sampling of central regions of k-space as part of the readout at process block  306 . 
         [0041]    The delay time, whether a separate delay time between the magnetization preparation at process block  302  and the commencing readout at process block  306  or an effective delay time between the magnetization preparation at process block  302  and sampling of central regions of k-space as part of the readout at process block  306 , may be selected to permit partial recovery of signal within the blood vessel lumen. In the latter case, the delay time may be selected taking into account the k-space sampling pattern so as to delay sampling of central k-space to a desired level, but can be selected so as to not unduly extend the duration of the scan. Because the flow suppressing magnetization-preparation at process block  302  minimally affects the longitudinal magnetization of stationary tissue, the delay time at process block  304  can be selected to allow slight recovery of longitudinal magnetization of the flowing blood pool. 
         [0042]    Accordingly, by balancing the k-space sampling pattern of the readout of process block  306  with the delay time of process bock  304 , the vascular wall appears hyperintense compared to the blood pool in the image reconstructed at process block  308 . This image appearance, which is referred to herein as “gray-blood” image contrast, provides clear and unambiguous delineation of the vessel wall, lumen and intraplaque components such as, but not limited to, calcification and hemorrhage. 
         [0043]    Referring to  FIG. 4A , a timing and phase-encoding diagram is provided that contrasts the present gray-blood imaging technique against traditional black-blood imaging techniques. Specifically,  FIG. 4A  illustrates an exemplary linear phase encoding schedule that may be used with the present invention. In  FIG. 4A  sampling in accordance with the present invention is illustrated by the gray sampling points  402  in contrast to the traditional black-blood sampling points  404 . As shown, conventional black-blood imaging uses a centric phase-encode schedule in which central k-space  406  is acquired shortly after application of the blood suppressing magnetization preparation  408 . However, by selecting a gray-blood k-space sampling pattern  402  that, in this case, inherently or “effectively” provides the desired delay time  410  between the application of the flow-suppressing magnetization preparation  408  and the acquisition of the central k-space region  406 , a gray-blood phase-encode schedule  402  is performed that yields a desired gray-blood image. Of course, alternative k-space sampling patters are contemplated that can be combined with dedicated delay times  410 , for example, when no k-space samplings are performed, to still yield the desired contrast information at the time of sampling the central k-space region  406 . However, dedicated delay times serve to extend the overall duration of the scan. In any case, a combination of delay time  410  and a desired gray-blood k-space sampling pattern  402  yields a gray-blood image that provides substantial clinical advantages, for example, when compared to traditional black- or white-blood images. 
         [0044]    Specifically, the gray-blood imaging technique of the present invention overcomes the limitations of black-blood imaging by allowing the user to either shorten the acquisition time through the use of longer echo train lengths or to mitigate undesirable signal apodization during the echo train. The latter improves the imaging point spread function and reduces blurring in the reconstructed images. More importantly, gray-blood images provide clinicians with images that substantial improve the ability to identify intraplaque calcification and provide clear separation of juxtaluminal (or superficial) calcification from the vessel lumen, which both appear dark with standard black-blood imaging. With respect to “white-blood” imaging, gray-blood imaging is advantageous in that it provides clear discrimination of the vessel wall from the adjacent vessel lumen, and suppresses image artifacts associated with flowing blood spins. 
         [0045]    More particularly,  FIG. 5  provides a montage of transversal images of the carotid arterial bifurcation of a human subject displaying the some of the clinical advantages of gray-blood image contrast. Calcification  500  within the arterial wall  502  observed with conventional black-blood imaging  504  cannot be clearly separated from the vessel lumen  506 . On the white blood-image  508 , the calcification  500  is observable, but the vessel wall  502  boundaries cannot be readily identified and distinguished from the vessel lumen  506 . However, on the “gray-blood” image  510 , calcified regions  500  can clearly be identified and distinguished from the arterial lumen  506  and the arterial wall  502 . In addition, the arterial wall  502  can clearly be distinguished from the arterial lumen  506 . 
         [0046]    It is contemplated that simultaneous gray-blood and black-blood image contrast, or “dual-contrast” imaging, may be obtained by modifying the phase-encoding schedule described above to one that utilizes a mixture of centric and linear phase-encoding ordering. Referring to  FIG. 4B , another timing and phase-encoding diagram is provided displaying a hybrid phase encoding schedule permitting the simultaneous acquisition and reconstruction (therefore, the aforementioned “dual contrast imaging”) of a gray-blood image set and a black-blood image set. As described above with respect to  FIG. 4A , the gray sampling points  402  are subject to a delay time  410  and focus on the central k-space region. However, whereas  FIG. 4A  illustrates a black-blood sampling superimposed on the gray-blood sampling,  FIG. 4B  combines conventional black-blood imaging samples  404  with gray-blood imaging samples  402  after application of the blood suppressing magnetization preparation  408 . Specifically, the gray-blood image samples extend along a k-space segment indicated as “C” and the black-blood image samples extend along k-space segments indicated as “A,” “B,” and “D.” In this regard, both the gray-blood and the black-blood image sets can be acquired with no increase in the acquisition time. 
         [0047]    It is contemplated that the reconstruction step illustrated with respect to process block  308  of  FIG. 3  may utilize techniques such as partial Fourier reconstruction methods, such as homodyne or projection onto convex sets, for example, to recover spatial resolution. So long as a blood suppressing magnetization preparation applied at process block  302  is applied in close temporal proximity to the imaging readout at process block  306 , a black-blood image set, such as described above with respect to  FIG. 4B , is obtained by reconstructing the lower spatial frequency lines acquired at the beginning of the echo train along with the higher spatial frequency lines acquired later in the echo train. A gray-blood image set is obtained by reconstructing data acquired in the linearly-ordered segment of the echo train. It is also possible to combine oversampled low spatial frequency lines (by standard or weighted signal averaging) to obtain intermediate contrast weightings and to improve signal to noise ratio. 
         [0048]      FIG. 6  provides images acquired within the dual-contrast, gray- and black-blood imaging methodology described above. The dual-contrast imaging approach simultaneously provides desirable contrast between the vessel wall and the lumen contrast through reconstruction of a black-blood image set, and enhances the clinician&#39;s ability to distinguish and characterize intraplaque constituents through reconstruction of a gray-blood image set. Specifically,  FIG. 6  provides image sets of the carotid arteries acquired concurrently with no increase in scan time using the dual-contrast phase encoding schedule of the present invention. The black-blood image  600  readily shows the vessel wall  602  with clinically desirable contrast between the vessel wall  602  and the arterial lumen  604 , but obscures the boundary between the superficial calcified intraplaque region  606  and the arterial lumen  604 . The gray-blood image  608 , on the other hand, precisely shows the extent of vascular calcification  606 . Furthermore, image sets acquired with dual-contrast imaging (such as shown in  FIG. 6 ) are also spatially registered which facilitates direct use of both images set for plaque component identification without requiring the use fallible image registration methods. 
         [0049]    With respect to gray-blood and dual-contrast imaging, it is contemplated that tailored k-space sampling trajectories, whether Cartesian or non-Cartesian, that frequently or regularly sample the center of k-space can be utilized. For such trajectories, where a plurality of delay times (block  304  of  FIG. 3 ) relative to the acquisition of central k-space can be realized, it is feasible that a plurality of gray blood and black blood images can be generated for vascular wall visualization and plaque component characterization. 
         [0050]    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.