Abstract:
A system and method is provided for magnetic resonance angiography (MRA) that includes performing a pulse sequence using the MRI system, the pulse sequence including a phase-based flow encoding to collect a time-series of image data from the portion of the vasculature of the subject and identifying at least a portion of the time series of image data corresponding to a period of reduced flow through the portion of the vasculature. The portion of the time series of image data is subtracted from the time series of image data to create a time series of images of the portion of the vasculature having background tissue surrounding the portion of the vasculature substantially suppressed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of, and claims priority to, U.S. provisional patent application Ser. No. 61/586,310 by Robert R. Edelman et al. filed on 13 Jan. 2012. 
    
    
     This invention was made with government support under HL096916 awarded by National Institutes of Health. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a system and method for performing magnetic resonance angiography (MRA) and, more particularly, to a system and method for MRA that utilizes phase-based flow encoding but does not require acquisition of two separate data sets for the purposes of image subtraction, thereby improving imaging efficiency and reducing artifacts. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     Magnetic resonance angiography (MRA) uses the NMR phenomenon to produce images of 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 time-of-flight (TOF) MRA. The third general category is phase contrast (PC) MRA. 
     To perform CE MRA, a contrast agent, such as gadolinium, is injected into the patient prior to the magnetic resonance (MR) angiogram to enhance the diagnostic capability of the MR angiogram. While CE MRA is a highly effective means for noninvasively evaluating suspected vascular disease, the technique suffers from several additional drawbacks. First, the contrast agent that must be administered to enhance the blood vessel carries a significant financial cost. Second, contrast agents such as gadolinium have recently been shown to be causative of an often catastrophic disorder called nephrogenic systemic fibrosis (NSF). Third, CE MRA does not provide hemodynamic information, so that it is not always feasible to determine if a stenosis is hemodynamically significant. Fourth, the signal-to-noise ratio (SNR) and, therefore, spatial resolution is limited by the need to acquire data quickly during the first pass of contrast agent through a target vessel. For these reasons, there have been substantial efforts to move away from CE MRA imaging protocols in favor of non contrast-enhanced (NCE) MRA protocols. 
     Fortunately, TOF and PC MRA imaging techniques do not require the use of a contrast agent. The 3D TOF techniques were introduced in the 1980&#39;s and they have changed little over the last decade. The 3D TOF MRA techniques commonly used for cranial examinations and have not been replaced despite recent advances in time-resolved contrast-enhanced 3D MRA. An alternative technique known as pulsed arterial spin labeling (PASL) was first applied to image intracranial circulation years ago; however, image quality never approached that of 3D TOF and the method has had little clinical utility. Moreover, electrocardiographic (ECG) gating was required. The use of TOF MRA is generally limited to imaging of intracranial circulation, however, because of sensitivity to patient motion and flow artifacts. 
     Finally, phase contrast (PC) MRA is largely reserved for the measurement of flow velocities and imaging of the vasculature. Phase contrast sequences are the basis of MRA techniques utilizing the change in the phase shifts of the flowing protons in the region of interest to create an image. Spins that are moving along the direction of a magnetic field gradient receive a phase shift proportional to their velocity. Specifically, in a PC MRA pulse sequence, two data sets with different amounts of flow sensitivity are acquired. This is usually accomplished by applying gradient pairs, which sequentially dephase and then rephase spins during the sequence. The first data set is acquired using a “flow-compensated” pulse sequence or a pulse sequence without sensitivity to flow. The second data set is acquired using a pulse sequence designed to be sensitive to flow. The amount of flow sensitivity is controlled by the strength of the bipolar gradient pulse pair use in the pulse sequence because stationary tissue undergoes no effective phase change after the application of the two gradients, whereas the different spatial localization of flowing blood is subjected to the variation of the bipolar gradient. Accordingly, moving spins experience a phase shift. The raw data from the two data sets are subtracted to yield images that illustrate the phase change, which is proportional to spatial velocity. 
     Thus, to perform PC MRA pulse sequences, a substantial scan time is generally required and the operator must set a velocity-encoding sensitivity, which varies unpredictably depending on a variety of clinical factors. The necessity of acquiring two data sets negatively affects the temporal resolution by a factor of two. Moreover, the two acquisitions use different gradient waveforms (e.g. a flow-encoded data set uses a bipolar gradient whereas a flow-rephased data set uses three gradient lobes). Consequently, gradient-induced eddy currents are not identical for the two acquisitions, which results in spatially and time-varying background phase shifts despite the use of image subtraction. Such background phase shifts cause errors in the velocity measurement and necessitate the use of complex phase correction algorithms. 
     Accordingly, several attempts have been made to utilize a “referenceless” phase contrast MRI technique. For instance, Nielsen J F, Nayak K S, Referenceless phase velocity mapping using balanced SSFP. Magn. Reson. Med, 2009 May; 61(5): 1096-102, uses a balanced SSFP pulse sequence with the requirement that the echo time (TE) be equal to one-half of the repetition time (TR). The use of a bSSFP sequence is not desirable because of an intrinsic sensitivity to artifacts from off-resonance effects and increased artifacts from gradient-induced eddy currents compared with a spoiled gradient-echo acquisition. Moreover, the method requires the manual placement of regions of interest near blood vessels and additional processing with a phase correction algorithm. Even with all these steps, the background phase correction is not uniform. 
     Therefore, it would be desirable to have a system and method for decreasing the acquisition time of phase-based, flow-encoding, imaging techniques that does not correspondingly increase artifacts in the resulting images. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks by providing a system and method for 
     In accordance with one aspect of the invention, a method is provided for acquiring a magnetic resonance angiography (MRA) image of a portion of a vasculature of a subject using a magnetic resonance imaging system. The method includes performing a pulse sequence using the MRI system, the pulse sequence including a phase-based flow encoding to collect a time-series of image data from the portion of the vasculature of the subject. The method also includes identifying at least a portion of the time series of image data corresponding to a period of reduced flow through the portion of the vasculature. The method further includes subtracting the portion of the time series of image data from the time series of image data to create a time series of images of the portion of the vasculature having background tissue surrounding the portion of the vasculature substantially suppressed. 
     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, a radio frequency (RF) system configured to apply an excitation field to the subject and acquire MR image data therefrom, and computer system. The computer system is programmed to perform a pulse sequence using the MRI system, the pulse sequence including a phase-based flow encoding to collect a time-series of image data from the portion of the vasculature of the subject. The computer system is also programmed to identify at least a portion of the time series of image data corresponding to a period of reduced flow through the portion of the vasculature. The computer system is further programmed to subtract the portion of the time series of image data from the time series of image data to create a time series of images of the portion of the vasculature having background tissue surrounding the portion of the vasculature substantially suppressed. 
     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 
         FIG. 1  is a block diagram of an MRI system for use with the present invention. 
         FIG. 2  is a schematic representation of a transceiver system for use with the MRI system of  FIG. 1 . 
         FIG. 3  is a flow chart of the steps performed in accordance with one exemplary implementation of the present invention. 
         FIG. 4  is a diagram illustrating a pulse sequence performed by the MRI system of  FIG. 1  in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring particularly to  FIG. 1 , the invention is employed in an MRI system. The MRI system includes a workstation  10  having a display  12  and a keyboard  14 . The workstation  10  includes a processor  16  that is a commercially available programmable machine running a commercially available operating system. The workstation  10  provides the operator interface that enables scan prescriptions to be entered into the MRI system. 
     The workstation  10  is coupled to, for example, four servers, including a pulse sequence server  18 , a data acquisition server  20 , a data processing server  22 , and a data store server  23 . The servers  18 ,  20 ,  22 , and  23  may be physically embodied as individual server systems or may be representations of one or more physical server systems. In one configuration, the data store server  23  is performed by the workstation processor  16  and associated disc drive interface circuitry and the remaining three servers  18 ,  20 ,  22  are performed by one or more separate servers. 
     The workstation  10  and each of the servers  18 ,  20 ,  22 , and  23  are connected to a communications network. This network conveys data that is downloaded to the servers  18 ,  20 ,  22 , and  23  from the workstation  10  and conveys data that is communicated between the servers  18 ,  20 ,  22 , and  23  and between the workstation  10  and the servers  18 ,  20 ,  22 , and  23 . 
     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 the prescribed scan are produced and applied to the gradient system  24  that excites gradient coils in an assembly  28  to produce the magnetic field gradients G x , G y , and G z  used for position encoding NMR signals. The gradient coil assembly  28  forms part of a magnet assembly  30 , which includes a polarizing magnet  32  and a whole-body RF coil  34 . 
     The RF excitation waveforms are applied to the RF coil  34  by the RF system  26  to perform the prescribed magnetic resonance pulse sequence. Responsive NMR signals detected by the RF coil  34  are received by the RF system  26 , amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  18 . The RF system  26  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF system  26  is responsive to the scan prescription and direction from the pulse sequence server  18  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole body RF coil  34  or to one or more local coils or coil arrays. 
     The RF system  26  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the NMR signal received by the coil to which it is connected and a quadrature detector that detects and digitizes the in-phase (I) and quadrature (Q) components of the received NMR signal. The magnitude of the received NMR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components. 
     The pulse sequence server  18  also optionally receives patient data from a physiological acquisition controller  36 . The controller  36  receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. Such signals are typically used by the pulse sequence server  18  to synchronize, or “gate”, the performance of the scan with the subject&#39;s respiration or heart beat. 
     The pulse sequence server  18  also connects to a scan room interface circuit  38  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  38  that a patient positioning system  40  receives commands to move the patient to desired positions during the scan. 
     The digitized NMR signal samples produced by the RF system  26  are received by the data acquisition server  20 . The data acquisition server  20  operates in response to description components downloaded from the workstation  10  to receive the real-time NMR data. In some scans, the data acquisition server  20  does little more than pass the acquired NMR data to the data processor server  22 . However, in scans that require information derived from acquired NMR data to control the further performance of the scan, the data acquisition server  20  may be programmed to produce such information and convey it to the pulse sequence server  18 . For example, during prescans NMR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  18 . Also, navigator signals may be acquired during a scan and used to adjust RF or gradient system operating parameters or to control the view order in which k-space is sampled. Furthermore, the data acquisition server  20  may be employed to process NMR signals used to detect or determine subsequent acquisition timings. In all these examples the data acquisition server  20  acquires NMR data and processes it in, for example, real-time to produce information that is used to control the scan. 
     The data processing server  22  receives NMR 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 NMR data to produce two or three-dimensional images, the application of filters to a reconstructed image, the performance of a backprojection image reconstruction of acquired NMR data, the calculation of functional MR images, the calculation of motion or flow images, and the like. 
     Images reconstructed by the data processing server  22  are conveyed back to the workstation  10 . Real-time images may be output to operator display  12  or a display  42  that is located near the magnet assembly  30  for use by attending physicians. Batch mode images or selected real-time images may be stored in a host database on disc storage  44 . When such images have been reconstructed and transferred to storage  44 , 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 or send the images via a network to other facilities. 
     As shown in  FIG. 1 , the RF system  26  may be connected to the whole body RF coil  34 , or as shown in  FIG. 2 , a transmitter section of the RF system  26  may connect to one RF coil  151 A and its receiver section may connect to a separate RF receive coil  151 B. Often, the transmitter section is connected to the whole body RF coil  34  and each receiver section is connected to a separate local coil  151 B. 
     Referring particularly to  FIG. 2 , the RF system  26  includes a transmitter that produces a prescribed RF excitation field. The base, or carrier, frequency of this RF excitation field is produced under control of a frequency synthesizer  200  that receives a set of digital signals from the pulse sequence server  18 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  201 . The RF carrier is applied to a modulator and up converter  202  where its amplitude is modulated in response to a signal R(t) also received from the pulse sequence server  18 . The signal R(t) defines the envelope of the RF excitation pulse to be produced and is produced by sequentially reading out a series of stored digital values. These stored digital values may be changed to enable any desired RF pulse envelope to be produced. 
     The magnitude of the RF excitation pulse produced at output  205  is attenuated by an exciter attenuator circuit  206  that receives a digital command from the pulse sequence server  18 . The attenuated RF excitation pulses are applied to the power amplifier  151  that drives the RF coil  151 A. 
     Referring still to  FIG. 2 , the signal produced by the subject is received by the receiver coil  1528  and applied through a 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 received from the pulse sequence server  18 . 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  that first mixes the NMR signal with the carrier signal on line  201  and then mixes the resulting difference signal with a reference signal on line  204 . The down converted NMR signal is applied to the input of an analog-to-digital (A/D) converter  209  that samples and digitizes the analog signal and applies it to a digital detector and signal processor  210  to produce the I values and Q values corresponding to the received signal. As described above, the resulting stream of digitized I and Q values of the received signal are output to the data acquisition server  20  of  FIG. 1 . The reference signal, as well as the sampling signal applied to the A/D converter  209 , is produced by a reference frequency generator  203 . 
     As mentioned, the present invention provides a system and method for phase-based, flow-encoded, imaging techniques that alleviates the need for performing two separate and distinct data acquisitions for the purposes of image subtraction. Specifically, referring to  FIG. 3 , a method  300  in accordance with the present invention begins by applying a pulse sequence designed for phase-based flow encoding at process block  302 . For example, a gradient-echo pulse sequence  400 , such as illustrated in  FIG. 4 , may be used. The gradient-echo pulse sequence  400  includes an imaging RF pulse  402  timed with a motion compensated gradient  404  along the slice-selection gradient. Thus, one or more bipolar gradients  402  may be applied along one or more directions, such as the slice-selection direction for measurement of through-plane flow in order to encode flow. A readout gradient  406  is applied to acquire the desired NMR signal  408 . 
     Referring again to  FIG. 3 , during the performance of the above-described pulse sequence or similar sequences, the cardiac cycle is monitored, as indicated at process block  304 . As will be described, this monitoring may occur according to one or more methods. For example, non-imaging-based monitoring may be performed to monitor the cardiac cycle, such as electrophysiological monitoring. Also, image-based monitoring may be performed, such as using navigators or other imaging-based tracking methodologies. Regardless of the particular monitoring methods or systems utilized, the cardiac monitoring performed at process block  304  is used to then synchronize data acquisition to the cardiac cycle at process block  306 . That is, the data acquisition illustrated by the acquisition of the NMR signal  408  in  FIG. 4 , is synchronized to the cardiac cycle. This synchronization may be performed in a manner similar to a conventional phase contrast MRI acquisition, such that a series of images (or “frames”) are produced at process block  308  representing sequential temporal phases of blood flow over the heart cycle. 
     At process block  310  a subtraction image is selected. For example, one frame or several frames averaged together to form a common image may be selected to represent a temporal phase of the cardiac cycle for subtraction from the time series of motion encoded image frames. For example, an image frame may be selected or common image frame of multiple averaged images may be selected such that has very low or no flow within a vessel of interest. 
     At process block  312 , the subtraction image frame (or average of several frames) is subtracted from a current image frame. At decision block  314 , a check is made to determine if the current image frame was the last image frame. If not, the current image or image frame is iterated at process block  316  and the new current image or image frame is subtracted at process block  312 . This process is repeated until each of the subsequent frames is subtracted such that the time series of images is produced in which background phase shifts are suppressed. 
     It is noted that when the current image or image frame is iterated at process block  316 , it may desirable to select another subtraction or create a new average of several image frames and, thus, the process may continue to process block  310 . Such an optional process may be desirable, for example, to allow a clinician the option to acquire flow rephased reference data in a minority of frames (typically one or just a few) which allows for correction of the flow velocities in setting where the minimum flow velocity is non-zero. By comparison, with standard phase contrast flow measurement, an equal number of flow encoded and rephased frames would be alternately acquired at process block  302 . 
     Compared with conventional phase contrast MRI, the present invention has several advantages. For example, the present invention improves temporal resolution by a factor of two. Also, by eliminating the need for a three-lobed gradient for the flow-rephased acquisition, the minimum echo time is reduced. Use of a shorter echo time is advantageous for suppressing phase shifts due to magnetic susceptibility effects. Furthermore, because identical gradient waveforms are played out for each frame, gradient-induced eddy currents are substantially equivalent in all frames and do not contribute to background phase shifts. This reduces the need for additional background phase corrections and improves the accuracy of the method compared with previously described approaches. 
     It is noted that the present invention presupposes a condition such that at least one frame be acquired during a phase of the cardiac cycle when flow is minimal or absent. When imaging, for example, the heart or the peripheral arteries, this may be a clinical constraint. That is, the clinic an will need to plan for the collection and identification of the low or now flow image, such as using monitoring systems or acquiring additional images or frames or, at a minimum, selecting a desirable frame. However, if imaging under conditions where the minimum arterial flow velocity never approaches zero, then absolute flow velocity quantification will not be obtained. Nonetheless, under such conditions, one can always determine the pulse wave amplitude over the cardiac cycle, which is often of equal or greater clinical importance in comparison with absolute velocity measurement, and use such to designate or create a desired subtraction image. Alternatively, one can use the process described above to ensure that absolute flow velocities are determined, even when a condition of absent flow does not exist. 
     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.