Method for performing magnetic resonance angiography with dynamic k-space sampling

MRA data is acquired from a large region of interest by translating the patient to successive stations at which successive portions of the three-dimensional MRA data set are acquired. Patient movement is chosen to track a bolus of contrast agent as it passes through the region of interest and a 3D acquisition is performed at a succession of stations. The order and shape in which k-space is sampled differs from station-to-station to achieve maximum image contrast.

BACKGROUND OF THE INVENTION
 The field of the invention is magnetic resonance angiography ("MRA"), and
 particularly, studies of the human vasculature using contrast agents which
 enhance the NMR signals.
 Diagnostic studies of the human vasculature have many medical applications.
 X-ray imaging methods such as digital subtraction angiography ("DSA") have
 found wide use in the visualization of the cardiovascular system,
 including the heart and associated blood vessels. One of the advantages of
 these x-ray techniques is that image data can be acquired at a high rate
 (i.e. high temporal resolution) so that a sequence of images may be
 acquired during injection of the contrast agent. Such "dynamic studies"
 enable one to select the image in which the bolus of contrast agent is
 flowing through the vasculature of interest. Images showing the
 circulation of blood in the arteries and veins of the kidneys, the neck
 and head, the extremities and other organs have immense diagnostic
 utility. Unfortunately, however, these x-ray methods subject the patient
 to potentially harmful ionizing radiation and often require the use of an
 invasive catheter to inject a contrast agent into the vasculature to be
 imaged. There is also the issue of increased nephro-toxicity and allergic
 reactions to iodinated contrast agents used in conventional x-ray
 angiography.
 Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance
 (NMR) phenomenon to produce images of the human vasculature. When a
 substance such as human tissue is subjected to a uniform magnetic field
 (polarizing field B.sub.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.sub.1) which is in the x-y plane and which is near the Larmor frequency,
 the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y
 plane to produce a net transverse magnetic moment M.sub.t. A signal is
 emitted by the excited spins, and after the excitation signal B.sub.1 is
 terminated, this signal may be received and processed to form an image.
 When utilizing these signals to produce images, magnetic field gradients
 (G.sub.x G.sub.y and G.sub.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 NMR signals are digitized and processed to
 reconstruct the image using one of many well known reconstruction
 techniques.
 MR angiography (MRA) has been an active area of research. Two basic
 techniques have been proposed and evaluated. The first class,
 time-of-flight (TOF) techniques, consists of methods which use the motion
 of the blood relative to the surrounding tissue. The most common approach
 is to exploit the differences in magnetization saturation that exist
 between flowing blood and stationary tissue. Flowing blood, which is
 moving through the excited section, is continually refreshed by spins
 experiencing fewer excitation pulses and is, therefore, less saturated.
 The result is the desired image contrast between the high-signal moving
 blood and the low-signal stationary tissues.
 MRA methods have also been developed that encode motion into the phase of
 the acquired signal as disclosed in U.S. Pat. No. Re. 32,701. These form
 the second class of MRA techniques and are known as phase contrast (PC)
 methods. Currently, most PC MRA techniques acquire two images, with each
 image having a different sensitivity to the same velocity component.
 Angiographic images are then obtained by forming either the phase
 difference or complex difference between the pair of velocity-encoded
 images.
 To enhance the diagnostic capability of MRA a contrast agent such as
 gadolinium can be injected into the patient prior to the MRA scan.
 Excellent diagnostic images may be acquired using contrast-enhanced MRA if
 the data acquisition is properly timed with the bolus passage.
 The non-invasiveness of MRA makes it a valuable screening tool for
 cardiovascular diseases. Screening typically requires imaging vessels in a
 large volume. This is particularly true for diseases in the runoff vessels
 of the lower extremity. The field of view (FOV) in MR imaging is limited
 by the volume of the B.sub.0 field homogeneity and the receiver coil size
 (typically, the FOV&lt;48 cm on current commercial MR scanners). The anatomic
 region of interest in the lower extremity, for example, is about 100 cm
 and this requires several FOVs, or stations, for a complete study. This
 requires that the patient be repositioned inside the bore of the magnet,
 the patient be re-landmarked, scout images be acquired and a preparation
 scan be performed for each FOV. All of these additional steps take time
 and, therefore, are expensive. When contrast enhanced MRA is performed,
 the repositioning also necessitates additional contrast injections.
 Recently gadolinium-enhanced bolus chase techniques have been reported
 which overcome this difficulty, K. Y. Ho, T. Leiner, M. H. de Hann, J. M.
 A. van Engleshoven, "Gadolinium optimized tracking technique: a new MRA
 technique for imaging the peripheral vascular tree from aorta to the foot
 using one bolus of gadolinium (abs)." Proc. 5th Meeting of ISMRM, p203,
 1997. As described in U.S. Pat. No. 5,928,148 issued on Jul. 27, 1999 and
 entitled "Method For Performing Magnetic Resonance Angiography Over A
 Large Field of View Using Table Stepping," bolus chase techniques employ a
 stepping table and an imaging strategy that allows imaging location
 synchronized to the location of peak contrast enhancement. As the contrast
 bolus transits through the lower extremity, images encompassing the entire
 lower extremity are acquired. Mask subtraction is used to suppress
 background and produce angiograms. Both 2D and 3D fast gradient echo
 sequences can be used to acquire data. 2D bolus chase MRA offers high
 in-plane resolution and a fast frame rate. Using 2D MRA, the entire lower
 extremity can be imaged with a small dose of gadolinium. But 2D
 acquisition lacks depth resolution, which may be required for depiction of
 vascular bifurcations, such as in the aorta, iliac and femoral arteries.
 The depth information can be obtained with additional 2D projections or a
 3D acquisition. In prior bolus chase 3D MRA studies, a long acquisition
 time (2 min) is required and consequently, a large contrast dose (40 mL)
 is used to maintain the contrast at peak level over the entire scan time.
 In addition, long acquisition times may result in venous enhancement in
 the distal station, due to venous return at the calf and ankle, which
 makes interpretation of the images difficult.
 SUMMARY OF THE INVENTION
 The present invention is a method for acquiring NMR data from a large
 region of interest by acquiring NMR data from a series of smaller fields
 of view which collectively span the large region of interest. The patient
 is automatically translated, or stepped, to a new position within the bore
 of the magnet by moving the patient table between the acquisition of each
 field of view. To enhance the image, a contrast agent is injected in the
 patient and flows through the successive fields of view. To take maximum
 advantage of the contrast agent, the order in which k-space sampling is
 performed to acquire each field of view is changed. For the first field of
 view a descending sampling order is used and for the final field of view
 an ascending sampling order is used. An edge-center-edge sampling order is
 used for intermediate fields of view. To shorten acquisition time at each
 field of view, partial k-space sampling tailored to each field of view is
 performed.
 A general object of the invention is to obtain maximum image contrast in a
 bolus tracking MRA scan. Even when using a 3D acquisition which does not
 enable the bolus of contrast agent to be tracked fast enough as it travels
 through successive fields of view, contrast enhancement is achieved by
 judiciously changing the k-space sampling order to sample the center of
 k-space at peak contrast. This is accomplished by delaying the sampling of
 central k-space during the acquisition of the first field of view and by
 sampling central k-space early during the acquisition of the last field of
 view.
 Another general object of the invention is to shorten the acquisition time
 at each field of view during a bolus tracking MRA scan. The acquisition
 time at each field of view is reduced without loss of SNR and spatial
 resolution by using a partial k-space sampling which is tailored to the
 particular anatomy at each field of view. With this method, the outer edge
 of k-space with less image signal contribution is undersampled, and this
 undersampling is tailored along both the slice encoding and phase encoding
 directions to minimize artifacts at each field of view.
 The foregoing and other objects and 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 herein for
 interpreting the scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring first to FIG. 1, there is shown the major components of a
 preferred MRI system which incorporates the present invention. The
 operation of the system is controlled from an operator console 100 which
 includes a keyboard and control panel 102 and a display 104. The console
 100 communicates through a link 116 with a separate computer system 107
 that enables an operator to control the production and display of images
 on the screen 104. The computer system 107 includes a number of modules
 which communicate with each other through a backplane. These include an
 image processor module 106, a CPU module 108 and a memory module 113,
 known in the art as a frame buffer for storing image data arrays. The
 computer system 107 is linked to a disk storage 111 and a tape drive 112
 for storage of image data and programs, and it communicates with a
 separate system control 122 through a high speed serial link 115.
 The system control 122 includes a set of modules connected together by a
 backplane. These include a CPU module 119 and a pulse generator module 121
 which connects to the operator console 100 through a serial link 125. It
 is through this link 125 that the system control 122 receives commands
 from the operator which indicate the scan sequence that is to be
 performed. The pulse generator module 121 operates the system components
 to carry out the desired scan sequence. It produces data which indicates
 the timing, strength and shape of the RF pulses which are to be produced,
 and the timing of and length of the data acquisition window. The pulse
 generator module 121 connects to a set of gradient amplifiers 127, to
 indicate the timing and shape of the gradient pulses to be produced during
 the scan. The pulse generator module 121 also receives patient data from a
 physiological acquisition controller 129 that receives signals from a
 number of different sensors connected to the patient, such as ECG signals
 from electrodes or respiratory signals from a bellows. And finally, the
 pulse generator module 121 connects to a scan room interface circuit 133
 which receives signals from various sensors associated with the condition
 of the patient and the magnet system. It is also through the scan room
 interface circuit 133 that a patient positioning system 134 receives
 commands from the pulse generator module 121 to move the patient to the
 sequence of desired positions to perform the scan in accordance with the
 present invention. The operator can thus control the operation of the
 patient positioning system 134 through the keyboard and control panel 102.
 The gradient waveforms produced by the pulse generator module 121 are
 applied to a gradient amplifier system 127 comprised of G.sub.x, G.sub.y
 and G.sub.z amplifiers. Each gradient amplifier excites a corresponding
 gradient coil in an assembly generally designated 139 to produce the
 magnetic field gradients used for position encoding acquired signals. The
 gradient coil assembly 139 forms part of a magnet assembly 141 which
 includes a polarizing magnet 140 and a whole-body RF coil 152. A
 transceiver module 150 in the system control 122 produces pulses which are
 amplified by an RF amplifier 151 and coupled to the RF coil 152 by a
 transmit/receive switch 154. The resulting signals radiated by the excited
 nuclei in the patient may be sensed by the same RF coil 152 and coupled
 through the transmit/receive switch 154 to a preamplifier 153. The
 amplified NMR signals are demodulated, filtered, and digitized in the
 receiver section of the transceiver 150.
 The transmit/receive switch 154 is controlled by a signal from the pulse
 generator module 121 to electrically connect the RF amplifier 151 to the
 coil 152 during the transmit mode and to connect the preamplifier 153
 during the receive mode. The transmit/receive switch 154 also enables a
 separate RF local coil to be used during the receive mode. In the
 preferred embodiment of the invention a single, stationary local coil
 described below and shown in FIG. 7 is switched into operation. In the
 alternative, a multi-segment local coil described below and shown in FIG.
 8 may also be used. In this case, the separate segments of the local coil
 are switched into operation by the transmit/receive switch 154 as the
 patient is translated to successive stations during the scan.
 The NMR signals picked up by the RF local coil are digitized by the
 transceiver module 150 and transferred to a memory module 160 in the
 system control 122. When the scan is completed and an entire array of data
 has been acquired in the memory module 160, an array processor 161
 operates to Fourier transform the data into an array of image data. This
 image data is conveyed through the serial link 115 to the computer system
 107 where it is stored in the disk memory 111. In response to commands
 received from the operator console 100, this image data may be archived on
 the tape drive 112, or it may be further processed by the image processor
 106 and conveyed to the operator console 100 and presented on the display
 104.
 For a more detailed description of the transceiver 150, reference is made
 to U.S. Pat. Nos. 4,952,877 and 4,922,736 which are incorporated herein by
 reference.
 While many pulse sequences may be used to practice the present invention,
 in the preferred embodiment a 3D gradient-recalled echo pulse sequence is
 used to acquire the NMR data. Referring particularly to FIG. 2, an RF
 excitation pulse 220 having a flip angle of 45.degree. is produced in the
 presence of a slab select gradient pulse 222 to produce transverse
 magnetization in the 3D volume of interest as taught in U.S. Pat. No.
 4,431,968. This is followed by a phase encoding gradient pulse 224
 directed along the z axis and a phase encoding gradient pulse 226 directed
 along the y axis. A readout gradient pulse 228 directed along the x axis
 follows and a partial echo (60%) NMR signal 230 is acquired and digitized
 as described above. After the acquisition, rewinder gradient pulses 232
 and 234 are applied to rephase the magnetization before the pulse sequence
 is repeated as taught in U.S. Pat. No. 4,665,365.
 As is well known in the art, the pulse sequence is repeated and the phase
 encoding pulses 224 and 226 are stepped through a series of values to
 sample the 3D k-space in the field of view. As will be described in more
 detail below, it is the order and the extent to which these phase encoding
 gradients are stepped, and hence the order and shape in which k-space is
 sampled, which is the subject of the present invention. In the preferred
 embodiment 32 phase encodings are employed along the z axis and 256 phase
 encodings are employed along the y axis for a full sampling of k-space.
 Sampling along the k.sub.x axis is performed by sampling the echo signal
 230 in the presence of the readout gradient pulse 228 during each pulse
 sequence. It will be understood by those skilled in the art that only a
 partial sampling along the k.sub.x axis may be performed and the missing
 data is computed using a homodyne reconstruction or by zero filling. This
 enables the echo time (TE) of the pulse sequence to be shortened to 1.0 ms
 and the pulse repetition rate (TR) to be shortened to 4.5.
 Referring to FIG. 3, an examination of the vasculature of a patient's legs
 can be performed by dividing up the region of interest into three
 overlapping fields of view indicated at 250, 252 and 254. As shown in FIG.
 4, this is accomplished by moving a patient table 256 to three successive
 locations, or stations, within the bore of the magnet to align the
 respective centers of the field of view with the isocenter 258 of the MRI
 system. A local RF coil 262 such as a 4-coil array consisting of anterior
 and posterior subunits is positioned at the system isocenter and it
 remains stationary as the patient is translated by the table 256 to three
 different stations. At each station of the table 256 the pulse sequence of
 FIG. 2 is employed to acquire NMR data from which an image of the field of
 view may be reconstructed.
 As shown in FIG. 7, a coil holder 200 has a closed end 202 which is
 attached to the carriage cover of the MRI system and supports an upper
 plate 204 and lower plate 206 which extend into the bore of the magnet.
 The end of the upper plate 204 supports the upper RF coil unit 262A which
 rests on the patient and slides smoothly over the patient during table
 translation. A rectangular opening 207 in the upper plate 204 provides
 space for the patient's feet. The lower plate 206 supports the lower RF
 coil unit 262B on its outer end which is positioned beneath the patient.
 The patient lies on an elevated table 208 and the lower plate 206 extends
 beneath the elevated table 208. The elevated table 208 rests on the
 sliding table 210 in the MRI system and is spaced therefrom by legs 212 to
 provide space for the lower plate 206. The patient may thus be translated
 between the RF coil units 262A and B which remain stationary at the system
 isocenter and in close proximity for optimal signal reception at all table
 stations.
 An alternative local coil structure which moves with the patient may also
 be used. Referring to FIGS. 8A and 8B, this alternative local coil is a
 coil array comprised of five coil segments 214-218, each comprised of four
 coil elements. The coil segments 214-218 are positioned on the patient and
 distributed along the entire region of interest to be imaged--in this
 case, the legs and feet. The coil elements are supported by fabric (not
 shown) which is sewn into a pant-like garment that clothes the patient's
 legs.
 Each coil segment 214-218 has four coil elements that acquire NMR data from
 the FOV. Two of the coil elements are positioned on top, or anterior, of
 the lower extremity as seen in FIG. 8B, and the other two elements are
 positioned below, or posterior, of the lower extremity. The coil elements
 connect together to form one of the coil segments 214-218, and each is
 separately connected through terminals 219 to the above-described
 transmit/receive switch 154. Coupling between coil segments 214-218 is of
 little concern, because only one segment is operative at any moment during
 the scan. As the patient is translated to bring the successive coil
 segments 214-218 into the FOV of the MRI system, the aligned coil segment
 is connected to the system transceiver module 150. MRA data is acquired
 using that coil segment and the patient is then translated to the next
 station and the next coil segment is enabled.
 The multi-element coil segments may be constructed using a number of
 well-known methods. Coupling between the separate coil elements is
 minimized to increase the SNR of the data acquired using the coil segment.
 Such decoupling is achieved using the methods taught in U.S. Pat. No.
 4,825,162 issued to Roemer, et al., and entitled "Nuclear Magnetic
 Resonance (NMR) Imaging With Multiple Surface Coils" or U.S. Pat. No.
 4,721,913 issued to Hyde, et al. and entitled "NMR Local Coil Network".
 When a 3DFT acquisition is performed at each FOV, the successive
 acquisitions may not be fast enough to track with the bolus of contrast
 agent as it passes through successive FOVs. In the example shown in FIG.
 9, it may require nearly 50 seconds to acquire 3D NMR data from three
 FOVs, and the contrast indicated by curve 264 may be at peak level for
 only 30 seconds. During the acquisition of the first FOV, the contrast is
 increasing as indicated at 265 and is peak only during the final moments
 of the acquisition. On the other hand, during the acquisition of the third
 FOV the contrast is declining as indicated at 266, and is peak only during
 the initial part of the acquisition.
 It is well known in the art that for most objects the bulk of the signal
 power is contained in the NMR samples taken near the origin of (k.sub.y,
 k.sub.z) space, and it is these samples which contribute most
 significantly to the appearance of the reconstructed image. This results
 from the fact that the NMR signals acquired during the scan are Fourier
 transformed along the k.sub.x, k.sub.y, k.sub.z directions to produce
 intensity values for an image in real (x,y,z) space. It is the nature of
 this transformation that the samples near the origin (k.sub.y =0, k.sub.z
 =0), or the central part of k-space, contribute a disproportionate share
 to the signal power of the reconstructed image. Accordingly, it is a basic
 idea of this invention that the sampling orders for each FOV acquisition
 be selected to sample the central region of k-space when the contrast is
 at its peak value.
 Referring again to FIG. 9, if the distance from the center of k-space
 (k.sub.y =0, k.sub.z =0) is represented by the frequency radius
EQU k.sub.r =k.sub.y.sup.2 +k.sub.z.sup.2 +L ,
 each of the three FOVs is sampled in a different order. The first FOV is
 sampled in a descending sampling order indicated by dashed line 267 in
 which the peripheral views of k-space (i.e. large k.sub.r) are sampled
 first and the central views of k-space (i.e. small k.sub.r) are sampled
 last. As a result, the central part of k-space is sampled later in the FOV
 acquisition when the contrast 264 is at peak value. On the other hand, the
 third FOV is sampled in an ascending sampling order indicated by dashed
 line 268 in which the central views of k-space are sampled first and the
 peripheral views of k-space are sampled last. This sampling order is also
 known in the art as "centric view order" and the preferred method used
 herein is described in U.S. Pat. No. 5,122,747 which is incorporated
 herein by reference. As a result of the ascending sampling order, the
 central part of k-space is sampled early in the acquisition of the third
 FOV while the contrast 264 is still at its peak.
 The sampling order of the second FOV is not as critical since the contrast
 264 is at peak value throughout the acquisition. However, in the preferred
 embodiment an edge-center-edge sampling order is used. The first half of
 the acquisition indicated by dashed line 269 samples k-space using a
 descending sampling order, but alternate k.sub.y, k.sub.z samples are
 skipped. For example, odd numbered elements are sampled and even numbers
 are skipped. Once the center of k-space is reached, data acquisition
 continues using an ascending order in which the skipped samples (e.g. even
 numbered samples) are acquired. This is indicated by the dashed line 270
 in FIG. 9. The advantage of this edge-center-edge sampling order is that
 the central portion of k-space is sampled in a temporally compact portion
 of the acquisition, minimizing possible motion artifacts.
 As shown in FIG. 6, the scan is performed under the direction of a stored
 program which directs the pulse generator module 121 to carry out a
 sequence of steps. As indicated by process block 370, the pulse control
 module 121 sends command signals to the patient positioning system 134
 which moves the table to align the first FOV at the system isocenter 258.
 A loop is then entered in which NMR data is acquired for a complete image.
 More specifically, the pulse control module 121 directs the MRI system to
 perform the pulse sequence of FIG. 2 to acquire one view of NMR data as
 indicated at process block 372. Phase encodings are stepped as described
 above and additional views are acquired until the desired k-space has been
 sampled and the acquisition is complete as determined at decision block
 374. During the acquisition of the first FOV, k-space is sampled in
 descending order such that the center of k-space is sampled at the end of
 the acquisition.
 At the completion of the first image acquisition the pulse control module
 121 commands the patient positioning system 134 to move the table 256 to
 the next station to align the next field of view at the system isocenter
 258 as indicated at process block 376. In the preferred embodiment this
 distance is 25 cm. Another image is then acquired at 372 and 374. However,
 the order in which k-space is sampled for the next FOV is different. If it
 is an intermediate FOV, such as the second of three or four to be
 acquired, then an edge-center-edge sampling order is used. In this case
 the center of k-space is sampled near the middle of the acquisition and it
 is sampled in a short time interval to reduce motion artifacts.
 This sequence of acquiring an image and moving the patient table 256
 continues until the last field of view in the scan is to be acquired.
 During the last FOV acquisition the order in which k-space is sampled is
 changed to an ascending view order. The center of k-space is thus sampled
 early in the acquisition to obtain the maximum possible image contrast
 from the bolus of contrast agent. The scan is completed after the last FOV
 is acquired as determined at decision block 378, and image reconstruction
 is performed using the 3D MRA data sets acquired from each FOV.
 Another aspect of the present invention is to tailor the shape and extent
 to which k-space is sampled at each station of the scan. In some
 examinations it is desired to reduce the voxel size in order to improve
 the quality of the maximum intensity projection (MIP) image. This requires
 that more k-space samples be acquired, and this requires a longer
 acquisition at each field of view. In order to accomplish this without
 reducing the size of the field of view or increasing scan time, the shape
 and size of the k-space sampling is tailored for each station in the scan.
 Referring particularly to FIG. 10, we propose a tailored k-space
 acquisition approach by taking advantage of the fact that a majority of
 image signal is located in the center of k-space. In this tailored k-space
 approach, the outer edge of k-space with little image signal is
 undersampled, and k-space sampling along both slice (z) and phase (y)
 directions are tailored to the imaging application. The tailored k-space
 is a balance between the full k-space acquisition that acquires all high k
 views, and the well-known key-hole acquisition that does not acquire any
 high k views.
 The key-hole acquisition is illustrated in FIG. 10a and it is characterized
 by the elimination of higher k-space samples along one phase encoding axis
 (i.e. the z-axis in the example). With this method only the central
 portion of k.sub.z -space is sampled and the total acquisition time is
 shortened.
 Such a partial sampling of k-space may not be appropriate for each field of
 view during the bolus tracking MRA scan. Accordingly, it is a teaching of
 the present invention that not only the sampling view order be changed
 during the scan, but the shape and size of the k-space sampling be
 tailored to each field of view acquired during the scan. Three additional
 partial k-space sampling shapes are illustrated in FIGS. 10b-d.
 The k-space sampling shape may be asymmetrical as illustrated in FIG. 10b.
 Data sampling takes place primarily in the upper k.sub.z -space and the
 lower k.sub.z -space is only sampled near the center of k-space for better
 SNR and phase calibration. It has been shown that this asymmetric k-space
 sampling can provide a 19% image contrast improvement over the key-hole
 sampling method of FIG. 10a in certain medical applications. The unsampled
 lower k.sub.z -space can be interpolated with phase corrected Hermitian
 conjugation.
 Two symmetrical k-space sampling shapes are illustrated in FIGS. 10c and
 10d. Both the diamond sampling shape of FIG. 10c and the circular sampling
 shape of FIG. 10d provide higher resolution images than the key-hole
 sampling shape of FIG. 10a. In addition, because they are symmetrical
 about the origin of k-space, phase correction and the possible image
 artifacts associated with such corrections may not be required. The
 unsampled "corners" of k-space are zero-filled before the image is
 reconstructed. After all the fields of view have been acquired and the
 scan is completed, the acquired field of view images are registered with
 each other and combined to form a single image of the much larger region
 of interest. The patient is immobilized to the table 256 with straps (not
 shown) to minimize misregistration between the field of view images. In
 addition, a marker made of a contrast agent solution which produces a high
 level signal can be placed along side the patient and used to align each
 field of view image such that they are in registration with each other and
 form a single, contiguous image of the entire region of interest.
 In the preferred method the acquisition of each FOV is timed to occur when
 maximum image contrast is produced by a contrast agent administered
 intravenously to the patient. A test bolus (eg. 5 ml of gadolinium) is
 administered intravenously to measure the bolus timing curve at a
 predetermined region of interest, typically the second FOV. As shown in
 FIG. 5, for example, the curve 274 peaks in the first FOV 250 at
 approximately 23 seconds after the contrast agent is administered, the
 curve 275 peaks in the second FOV 252 at approximately 29 seconds after
 the contrast agent is administered and the curve 276 peaks in the third
 FOV 254 at approximately 35 seconds after contrast administration. This
 establishes when the data should be acquired for best image contrast. In
 the preferred embodiment the data acquisition is timed to begin such that
 when the first FOV has been completely acquired, the onset of the contrast
 peak has reached the second FOV for a few seconds.
 As is well known in the art, "mask" images may also be acquired before
 contrast agent is administered. In the preferred embodiment, the mask
 images are acquired before bolus injection at each of the stations. The
 above described contrast enhanced acquisition is then performed and a
 complex subtraction of corresponding voxels in the mask image from the
 reconstructed image is performed to further enhance image quality.
 The preferred method for practicing the invention includes conducting a
 "scout scan" to confirm that the vasculature of interest is within the FOV
 of at least one of the stations. This is followed by a "timing scan" in
 which information is acquired regarding the arrival of the contrast agent
 bolus, and by a "mask scan" in which the mask image data is acquired. And
 finally, the "bolus tracking MRA scan" is conducted as described above and
 shown in FIG. 6. The following is an exemplary procedure for carrying out
 these steps in a four station scan of a patient's legs.
 Scout Scan
 A multiple location, sagittal acquisition using a 2D fast gradient echo
 sequence (FOV=40 cm, TE/TR=2/9, receiver bandwidth=16 kHz, 256.times.256,
 1/2 FOV, total acquisition time.about.15 sec) is performed at the first
 station. The patient is translated to the second station and the same scan
 is repeated immediately without any prescan. This process repeats until
 all stations are imaged. For a 4-station scout scan, the total imaging
 time is about 70 sec (allowing 10 seconds for each table motion).
 Timing Scan
 A thick coronal slab timing acquisition is typically performed at the
 second station using a gadolinium contrast agent test dose (3-5 mL, at 1
 mL/sec injection rate followed with a 20 mL saline flush at 1 mL/sec.) and
 a 2D fast gradient echo sequence (FOV=32-40 cm, 80-140 mm thick,
 TE/TR=2/9, flip angle=50.degree., receiver bandwidth=16 kHz, k-space
 sampling matrix=256.times.160-192, 30-40 image frames, total acquisition
 time.about.45-60 sec. to ensure the capture of the contrast peak). From
 this timing acquisition, the arrival time of the contrast agent in the FOV
 is measured.
 Mask Scan
 Similar to the bolus tracking scan described next, except no contrast agent
 is injected. The same pulse sequence parameters and the same table
 movement sequence are used.
 Bolus Tracking MRA Scan
 The contrast agent is administered and a scan is conducted using the fast
 3D gradient echo sequence described above. Each 3D acquisition of a FOV
 was completed in 14 seconds using the following imaging parameters:
 TR/TE=4.5/1, flip angle=45.degree., matrix=256.times.192.times.32, FOV=48
 cm with 1/2 FOV along phase (y) direction, receiver bandwidth=62.5 kHz.
 Total slab thickness ranged between 120-140 mm, resulting in individual
 slice thickness of 4-5 mm. An automatic delay upon completion of each FOV
 acquisition was included. Following each FOV acquisition the table was
 advanced to the next station and the next FOV acquisition was started as
 soon as the table came to a full stop. There was approximately 2 seconds
 between stations, resulting in a 3-station acquisition time of 46 seconds.
 Contrast infusion was performed via a 22 gauge catheter placed in an
 antecubital fossa vein. An MR specific power injector was used. Contrast
 dose of 25 mL (Gadodiomide, Omniscan, Nycomed, Princeton, N.J.) was
 infused 1 mL/sec. The contrast infusion was followed immediately by a 20
 mL saline flush at 1 mL/sec.
 The patient was instructed to remain motionless between the mask and bolus
 tracking acquisitions (less than 2 min). The patient was also instructed
 to hold his/her breath during acquisition of the first FOV during both the
 mask and the bolus acquisitions. Adequate foam rubber and Velcro straps
 were used to immobilize the patient. After completion of the mask
 acquisition, the table was returned to its initial position and contrast
 infusion was started. Timing for contrast infusion and bolus tracking data
 acquisition were determined in the following manner. Data acquisition for
 the first FOV ended when the contrast peak had reached the second FOV for
 5 seconds. The delay between the start of contrast injection and the start
 of the first FOV acquisition was thus set according to the timing scan
 measurement, plus 5 seconds and minus 14 seconds required to acquire the
 NMR data from the first FOV. A pause (approximately 2 seconds) was taken
 between stations to translate the patient to the next station, and the
 next acquisition was started immediately.
 Other data processing techniques may be used to generate angiograms.
 Methods other than complex subtraction, such as digital filters, may be
 used to generate angiograms. The criteria for digital filter design is to
 extract dynamic contrast (or signal variation) from a temporal series of
 images, and vessel contrast is determined by this dynamic contrast. For
 example, matched filters can be used to generate an angiogram that is a
 sum of individual arteriograms generated by complex subtraction.
 Another application for the invention is to image the aorta in the body
 trunk. The aortic arch and the abdominal aorta can be imaged in two
 separate FOVs using a fast 3D acquisition technique and contrast infusion.