U.S. patent application number 11/108417 was filed with the patent office on 2005-09-15 for method and apparatus for anatomically tailored k-space sampling and recessed elliptical view ordering for bolus-enhanced 3d mr angiography.
This patent application is currently assigned to Cornell Research Foundation, Inc.. Invention is credited to Prince, Martin R., Wang, Yi, Watts, Richard.
Application Number | 20050203377 11/108417 |
Document ID | / |
Family ID | 26807473 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050203377 |
Kind Code |
A1 |
Watts, Richard ; et
al. |
September 15, 2005 |
Method and apparatus for anatomically tailored k-space sampling and
recessed elliptical view ordering for bolus-enhanced 3D MR
angiography
Abstract
Current bolus chase magnetic resonance angiography is limited by
the imaging time for each station. Tailoring the density of k-space
sampling along the anterior-posterior direction of the coronal
station allows a substantial decrease in scan time that leads to
greater contrast bolus sharing among stations and consequently a
significant improvement in image quality. Fast arterial-venous
transit in the carotid arteries requires accurate, reliable timing
of the acquisition to the bolus transit to maximize arterial signal
and minimize venous artifacts. The rising edge of the bolus is not
utilized in conventional elliptical-centric view ordering because
the critical k-space center must be acquired with full arterial
enhancement. The invention provides a recessed elliptical-centric
view ordering scheme is introduced in which the k-space center is
acquired a few seconds following scan initiation. The recessed view
ordering is shown to be more robust to timing errors in a patient
studies.
Inventors: |
Watts, Richard; (New York,
NY) ; Wang, Yi; (Pittsburgh, PA) ; Prince,
Martin R.; (New York, NY) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402-0938
US
|
Assignee: |
Cornell Research Foundation,
Inc.
|
Family ID: |
26807473 |
Appl. No.: |
11/108417 |
Filed: |
April 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11108417 |
Apr 18, 2005 |
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10109888 |
Apr 1, 2002 |
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60279691 |
Mar 30, 2001 |
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Current U.S.
Class: |
600/410 |
Current CPC
Class: |
A61B 5/055 20130101;
G01R 33/5601 20130101 |
Class at
Publication: |
600/410 |
International
Class: |
A61B 005/05 |
Goverment Interests
[0001] The invention was made with government support under
R01HL60879 by the National Institutes of Health. The government has
certain rights in the invention.
Claims
1. A method of MR imaging a region of interest in a body containing
arterial features comprising: a) applying a static magnetic field
to the region of interest; b) applying a magnetic gradient along at
least first and second dimensions to produce a spatially
distributed magnetic response of varying frequency and varying
magnitude; c) sampling the magnetic response at a spacing interval
corresponding to a sampling rate which is less than a Nyquist rate;
d) performing a Fourier transform on the result of step (c) to
provide a first imaging result; e) providing a duplicate of image
information of the first imaging result to provide a second imaging
result; and f) combining the first and second imaging results to
isolate arterial features in the region of interest.
2. The method of claim 1, wherein step (b) further comprises
applying a magnetic gradient along a third dimension, and said step
(c) comprises sampling at a sub-Nyquist rate along a thickness
dimension of said first, second and third dimensions.
3. The method of claim 1, wherein said step (e) comprises repeating
steps (a)-(d) in the field of interest to provide the second
imaging result, and introducing a contrast agent to the region of
interest when repeating steps (a)-(d).
4. The method of claim 3, wherein only a single dose of a contrast
agent is introduced when repeating steps (a)-(d).
5. A method according to claim 1, further comprising: performing MR
imaging at a first station using k-space sampling prior to
performing said steps (a)-(f), wherein sampling at the first
station is delayed such that sampling of a center of the k-space is
timed to coincide with a contrast agent passing through the region
of interest.
6. The method according to claim 5, wherein said k-space sampling
at the first station is performed at the Nyquist rate and spacing
corresponding to the Nyquist rate.
7. The method according to claim 5, wherein said k-space sampling
at the first station is performed at less than the Nyquist rate and
at a spacing interval larger than a spacing interval of the Nyquist
rate.
8. The method according to claim 1, wherein said step of combining
the first and second imaging results comprises reconstructing
portions of the first imaging result and the second imaging result
that correspond to arterial features to isolate the arterial
features in the region of interest.
9. The method according to claim 3, wherein said step of combining
the first and second imaging results comprises determining a
background area from one of said first and second imaging results
and overlapping the background area over the other imaging result
to isolate arterial features in the region of interest in the other
imaging result.
10. A method of MR imaging a region of interest in a body
containing arterial features comprising: a) applying a static
magnetic field to the region of interest; b) applying a magnetic
gradient along at least first and second dimensions to produce a
spatially distributed magnetic response of varying frequency and
varying magnitude; c) sampling the magnetic response at a spacing
interval which corresponds to a sampling rate which is less than a
Nyquist rate, but taking a number of samples that corresponds to
the Nyquist rate, thereby expanding a sampling range of the
frequency; d) performing a Fourier transform on the result of step
(c) to provide a first imaging result; e) providing a duplicate of
image information of the first imaging result to provide a second
imaging result; and f) combining the first and second imaging
results to isolate arterial features in the region of interest.
11-19. (canceled)
20. The method of claim 5, wherein the sampling of the k-space
center is timed to coincide with a substantial peak of the contrast
agent passing through the region of interest.
21-22. (canceled)
23. A method of MR imaging a region of interest comprising:
administering a contrast agent to the region of interest; applying
a static magnetic field to the region of interest; applying a
magnetic gradient along at least one of first and second dimensions
to produce a distributed magnetic response having a spatial
distribution in k-space; sampling the magnetic response, wherein
sampling is performed such that sampling of a center of the k-space
is delayed from a start of sampling of the magnetic response, such
that sampling of the center of the k-space is timed to coincide
with a substantial peak of the contrast agent passing through the
region of interest.
24. The method of claim 23, wherein the sampling of the k-space
center is preceded by sampling of a recessed-edge of k-space.
25. The method of claim 24, wherein the sampling of k-space center
is followed by sampling of an edge of k-space substantially
corresponding to a maximum spatial frequency of the magnetic
response in said region of interest.
26. The method of claim 25, wherein a first time interval separates
sampling of the k-space center and the recessed edge of the k-space
and a second time interval separates sampling of k-space center and
the edge of the k-space, wherein during the first time interval,
sampling occurs in decreasing order of k-space radius from the
recessed edge to the k-space center while sampling every Mth point
(M>1), and wherein during the second time interval, sampling
occurs in increasing order of k-space radius to sample each k-space
point not sampled during the first time interval.
27. The method of claim 23, wherein sampling the magnetic response
produces N points, said method further comprising: forming an array
[K] of said N points ordered in said array in order of ascending
radius from the center of the k-space using index N; setting a
recessed radius k.sub.R having a corresponding index N.sub.R-1 in
said array to start said sampling; setting a recess time
T.sub.recess corresponding to a time between start of sampling and
peak contrast agent in the region of interest; setting a sequence
repetition time T.sub.R as a time to acquire one sample; wherein
sampling order of the k-space prior to sampling of the k-space
center is determined according to Index=N.sub.R-1-(N.sub.R/(T-
.sub.recess/TR))*n for n, 0.ltoreq.n<(T.sub.recess/TR).
28. The method of claim 27, wherein (NR/(T.sub.recess/TR))=2.
29. A method of MR imaging a region of interest comprising: a)
administering a contrast agent to the region of interest; b)
applying a static magnetic field to the region of interest; c)
applying a magnetic gradient along at least one of first and second
dimensions to produce a distributed magnetic response having a
spatial distribution in k-space including a low spatial frequency,
an intermediate spatial frequency and a high spatial frequency; d)
sampling the magnetic response in order of the intermediate
frequency, the low spatial frequency and the high spatial
frequency, said high spatial frequency corresponding to a maximum
radius value of the magnetic response in the k-space for said
region of interest.
30. The method of claim 29, wherein a first time interval separates
sampling of the intermediate spatial frequency and the low spatial
frequency, and a second time interval separates sampling of the low
spatial frequency and the high spatial frequency, wherein during
the first time interval, sampling occurs in decreasing order of
spatial frequency from the intermediate spatial frequency to the
low spatial frequency, sampling every Mth point (M>1), and
wherein during the second time interval, sampling occurs in
increasing order of spatial frequency to sample each point not
sampled during the first time interval.
31. The method of claim 29, wherein the sampling of the low spatial
frequency substantially corresponds to a peak of the contrast agent
passing through the region of interest.
32. The method of claim 29, wherein sampling the magnetic response
produces N points, said method further comprising: forming an array
[K] of said N points ordered in said array in order of ascending
radius from the center of the k-space using index N; setting a
recessed radius k.sub.R having a corresponding index N.sub.R-1 in
said array to start said sampling; setting a recess time
T.sub.recess corresponding to a time between start of sampling and
peak contrast agent in the region of interest; setting a sequence
repetition time T.sub.R as a time to acquire one sample; wherein
sampling order of the k-space prior to sampling of the k-space
center is determined according to Index=N.sub.R-1-(N.sub.R/(T-
.sub.recess/TR))*n for n, 0<n<(T.sub.recess/TR).
33. The method of claim 32, wherein (NR/(T.sub.recess/TR))=2.
34-36. (canceled)
37. The method of claim 25 wherein said first and second time
intervals correspond to a time for imaging during application of a
single magnetic gradient.
38. The method of claim 30 wherein said first and second time
intervals correspond to a time for imaging during application of a
single magnetic gradient.
39. The method of claim 27, wherein after sampling of the k-space
center, sampling occurs in increasing order of k-space radius to
sample each point not sampled during said interval
0<n<(T.sub.recess/TR).
40. The method of claim 32, wherein after sampling of the k-space
center, sampling occurs in increasing order of k-space radius to
sample each point not sampled during said interval
0<n<(T.sub.recess/TR).
Description
FIELD OF INVENTION
[0002] The present invention relates to MR angiography using a
reduced sample set to decrease the time needed to acquire image
data for arterial features. As a result of the time reduction, the
MR data acquisition can be performed in conjunction with the
arrival and flow of a contrast agent through the arteries, thereby
reducing the amount of contrast agent introduced in a patient. The
invention further relates to method for acquiring MR to better
delineate arterial features from a background and venous artifacts.
In particular, modification of k-space view ordering more
accurately times data acquisition to contrast arrival and peak
contrast enhancement in a region of interest.
DESCRIPTION OF THE RELATED ART
[0003] Generally, contrast-enhanced MRA offers many advantages in
imaging, including reduced flow artifacts and reduced scanning
time.
[0004] In one application, bolus chase MR angiography has
revolutionized imaging of peripheral vascular disease by allowing
rapid imaging of the arterial anatomy of the entire lower half of
the body with a single infusion of a gadolinium based contrast
agent. This has been previously discussed in Meaney, J F M, Prince
M R, Floating Table Bolus Chase Peripheral Vascular MR Angiography,
U.S. Pat. No. 5,924,987, issued Jul. 20, 1999; Ho K Y, Leiner T, de
Haan M W, Kessels A G, Kitslaar P J, van Engelshoven J M.,
Peripheral vascular tree stenoses: evaluation with moving-bed
infusion-tracking MR angiograph;. Radiology 1998; 206(3):683-692;
Wang Y, Lee H M, Khilnani N M, et al. Bolus-chase MR digital
subtraction angiography in the lower extremity. Radiology 1998;
207(1):263-269; and Meaney J F, Ridgway J P, Chakraverty S, et al.
Stepping-table gadolinium-enhanced digital subtraction M R
angiography of the aorta and lower extremity arteries: preliminary
experience. Radiology 1999; 211(1):59-67. Two strategies have
emerged for obtaining arterial-phase images at three successive
stations while avoiding excessive venous enhancement. One strategy
is to image relatively slowly with a long infusion as further
discussed in Ho K Y, Leiner T, van Engelshoven J M. MR angiography
of run-off vessels. Eur Radiol 1999; 9(7):1285-1289. The infusion
rate must be fast enough to give sufficient arterial enhancement
but slow enough so that after extraction of gadolinium contrast in
capillaries, venous enhancement is minimal. Typically an infusion
rate of 0.3-0.5 ml/sec is suggested for an imaging duration of 45
seconds per station. However, there is little sharing of contrast
dose between stations, and venous enhancement can still occur.
[0005] Another strategy is to image sufficiently rapidly to keep up
with the contrast bolus so that the same bolus is imaged at all
three stations. This has been discussed by Wang, et al as noted
above and further in Wang Y, Lee H M, Avakian R, Winchester P A,
Khilnani N M, Trost D., Timing algorithm for bolus chase MR digital
subtraction angiography. Magn Reson Med 1998; 39(5):691-696 and Ho
V B, Choyke P L, Foo T K, et al. Automated bolus chase peripheral
MR angiography: initial practical experiences and future directions
of this work-in-progress. J Magn Reson Imaging 1999; 10(3):376-388.
Faster imaging allows the contrast to be injected at a greater rate
to maximize arterial enhancement but requires high performance
gradients and accurate bolus timing for all three stations. Using a
3D sequence, the time to image each station (typically .about.20s)
is significantly longer than transit time of the contrast agent
through that station (FOV/bolus velocity .about.40 cm/5
cm/s.about.8s). This reduces sharing of contrast agent between
stations. As a consequence, the contrast dose required to give
adequate vessel enhancement must be increased. Known MR angiography
techniques use sampling according to the Nyquist sampling theorem
to prevent aliasing in the MR image. However, sampling at the
Nyquist rate and at the spacing interval required by the Nyquist
theorem increases the acquisition time at a particular station,
making it difficult to image multiple stations using a single
administration of a contrast agent.
[0006] The rapid transit time of a contrast agent through the
peripheral vascular anatomy also poses timing problems in other
regions, such as the carotid artery. The carotid artery is one of
the most common sites for athlerosclerotic vascular disease, which
accounts for the majority of cerebrovascular events in western
society. Although 2D and 3D time-of-flight MRA have revolutionized
the evaluation of the carotid arteries, these MRA techniques have
important limitations related to motion and flow artifacts, long
acquisition times, and the difficulty in imaging the entire vessel
down to the aortic arch. Contrast-enhanced carotid MRA, in addition
to reduced scan time, also improves visualization of plaque
ulcerations and improved field of view. For the carotid artery,
contrast-enhanced MRA allows imaging the artery to the great vessel
origins.
[0007] However, a major challenge in contrast enhanced imaging of
the carotid arteries is due to rapid arterial-venous transit. The
duration of the arterial-only phase (typically .about.10s) is
usually substantially shorter than the acquisition time required
for a high-resolution 3D dataset (.about.30s).
[0008] Conventional MRA employs elliptical-centric view ordering of
the k-space. This standard view order concentrates the high signal
k-space center at the beginning of a scan in a region of interest.
However, if the k-space center is acquired during the rising edge
of arterial enhancement, reduced arterial signal intensity and
arterial ringing artifacts may result. Delaying the acquisition of
the k-space center (while using the conventional centric-order
view) too far into the scan produces a significant venous artifact
due to rapid contrast transit.
SUMMARY OF THE INVENTION
[0009] The present invention uses a variable k-space sampling
method that minimizes scan time by tailoring the acquisition
according to the vascular anatomy of the station, hence increasing
dose sharing between stations. In particular, in a three station
scan of the lower extremities (abdomen, thigh, and calf), at least
the central station is sampled such that the number of samples
taken is less than would be taken at the Nyquist rate. The spacing
is more sparse than that required by the Nyquist sampling theorem,
to prevent the well-known imaging artifact of wrap-around
(aliasing). The MR images of the region of interest are duplicated.
Subtraction of a pre-contrast mask removes non-enhancing tissue
(ref. Wang), while the limited extent of vessels allows duplicated
vessels to be removed using volume of interest imaging.
[0010] Additionally, to more accurately time data acquisition to
bolus passage, preferably peak bolus passage, the k-space center is
delayed into the scan interval for a region of interest, using a
recessed view ordering scheme. This view ordering does not waste
arterial phase time, because as the scan commences, low signal
level high spatial frequency components are acquired.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Exemplary embodiments of the invention are set forth below
with reference to the attached drawings where:
[0012] FIG. 1 is a timing diagram showing view ordering by k-space
radius, k.sub.r according to a first embodiment of the present
invention as applied to lower peripheral extremities;
[0013] FIG. 2 shows how the statistical parameter used to evaluate
the image quality is derived from arterial data;
[0014] FIG. 3 shows a comparison of the image quality figures of
merit obtained for all studies and stations of the peripheral
extremities;
[0015] FIGS. 4A-4C illustrate maximum intensity projection (MIP)
images from a 52-year old female showing claudication of the left
leg. FIG. 4A shows a coronal view. FIG. 4B shows a sagittal view
and FIG. 4C shows a sagittal after removal of ghost image in
station 2 according to an embodiment of the invention;
[0016] FIG. 5 illustrates the arterial signal to background ratio
vs. superior-inferior position for median studies for a
conventional protocol and a protocol of the present invention;
[0017] FIGS. 6A and 6B illustrate maximum intensity projections of
the median data presented in FIG. 5. FIG. 6A. corresponds to
projections for a conventional protocol, FIG. 6B illustrates
projections for protocol with reduced anterior-posterior
field-of-view in the second station according an embodiment of the
present invention;
[0018] FIG. 7 illustrates a view ordering scheme (for a carotid
artery), where the recessed center-edge view order (gray) better
matches the acquisition of the k-space center to the peak arterial
concentration prior to venous enhancement than the standard
elliptical-centric acquisition (black);
[0019] FIG. 8 is a histogram of the duration of arterial
enhancement (in a carotid artery) prior to venous return in a 22
patient study, where measurements were made using time-resolved
(1-2 seconds/phase) thick-slab 2D breath hold acquisitions with
complex subtraction;
[0020] FIG. 9 illustrates examples of the extent of ringing in
coronal MIPs (top) and source images (bottom). FIG. 9(a) shows no
ringing visible (11% of cases), FIG. 9(b) shows moderate ringing
visible in the source images (74% of cases), and FIG. 9(c) shows
substantial ringing, visible in the MIP (16% of cases). Arrows
indicate venous artifacts;
[0021] FIG. 10 shows a carotid study of a 62 year-old female.
Oblique 2D real-time images (FIG. 10(a)) at 4 seconds, and (FIG.
10(b)) immediately prior to initiation of the 3D scan. Arrows
indicate enhancement of the pulmonary arteries, aortic arch and the
carotid arteries. FIG. 10(c) shows a maximum intensity projection
from the subsequent coronal 3D dataset; and
[0022] FIG. 11 shows a carotid study of a 74 year-old male acquired
using the recessed view-ordering scheme. FIG. 11(a) shows a coronal
maximum intensity projection showing coverage from the aortic arch
to the Circle of Willis. FIG. 11(b) shows selected volume oblique
view showing multiple stenoses at the left carotid bifurcation.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0023] Imaging Protocol for Peripheral Extremity Study
[0024] A protocol for bolus-chase MR digital subtraction
angiography according to an embodiment of the present invention was
compared to the protocol used and published previously by Wang, Y.,
Lee, H M, Avakian R, Winchester P A, Khilnani N M, and Trost D
discussed above and Lee H M, Wang Y. Dynamic k-space filling for
bolus chase 3D MR digital subtraction angiography. Magn Reson Med
1998; 40(1):99-104.
[0025] The differences between the protocols are outlined in Table
1, and are discussed in more detail below.
1TABLE 1 Summary of parameters for a conventional protocol and the
inventive protocol. Conventional Protocol Inventive Protocol Timing
of Station 1 Complete filling at Complete filling halfway start of
acquisition through acquisition View Order for Station 1
Edge-Center Recessed Edge-Center View Order for Station 2
Edge-Center-Edge Edge-Center-Edge View Order for Station 3
Center-Edge Center-Edge Reduced A/P No Yes FOV for station 2
Receiver bandwidth 64 kHz 32 kHz
[0026] The k-space sampling rate along the anterior-posterior (A/P)
direction is reduced 2-fold relative to the Nyquist rate for the
central station where the femoral and popliteal arteries have
limited A/P extent. The rf excitation volume remains the same to
ensure that all vessels are fully visible. Halving the sampling
rate results in a wrap-around ghost of the vessels. Since the
background is eliminated with subtraction and arteries are sparse,
these effects do not cause artifacts that obstruct visualization of
the arteries. The acquisition time for this station is thereby
halved while maintaining resolution.
[0027] This reduced sampling could also be applied to the first and
last stations, where the vascular A/P extent at any longitudinal
location is still limited. However, dynamic view ordering optimally
times the acquisition of the critical k-space center for the first
station towards the end, and for the last station towards the
beginning. Notably, the center of the k-space acquisition at the
first region of interest (first station) is delayed. This
modification in the shift of the k-space center acquisition is
generally applicable to improve MRA for any region of interest,
which will be described in more detail below. With respect to FIG.
1, the temporal separation of the centers of k-space for the three
stations is thus primarily determined by the scan time of the
middle station. Hence only the k-space sampling of the middle
station needs to be reduced.
[0028] Timing information was provided by a test injection of 5 cc
of Gd at 2 cc/s, during which coronal 2D projectional images of the
central station were acquired with a temporal resolution of 1.8 s.
Complex subtraction of a mask image acquired prior to the contrast
arrival was used to visualize the vessels. From these data, the
bolus arrival, transit and departure at the central station were
determined. The timing for the bolus-chase sequence was estimated
as discussed by Wang Y, Lee H M, Avakian R, Winchester P A,
Khilnani N M, Trost D. Timing algorithm for bolus chase MR digital
subtraction angiography. Magn Reson Med 1998; 39(5):691-696. In
particular, for the conventional protocol, 3D imaging of the first
station was started only when the full enhancement of the entire
station was observed on the timing run. For the present embodiment,
the delay to the first station was chosen such that at least the
last 60% of the data (the k-space center) were acquired with
filling of the entire station.
[0029] A fraction of the time saved by the improved first station
timing and the reduced A/P FOV was used to decrease the receiver
bandwidth. This also compensated for the loss in SNR of the central
station due to the reduced k-space sampling. Using the conventional
protocol the 64 kHz bandwidth and 60% fraction echo gave TE/TR=1.1
ms/4.8 ms. For this embodiment, 32 kHz bandwidth gave TE/TR=1 0.3
ms/6.2 ms. The 29% increase in scan time was considered worthwhile
to give a {square root}2 improvement in SNR.
[0030] The pulse sequence was implemented on a GE Echospeed Signa
MR imager, using the body coil, and is based on a standard
multiphase 3D gradient echo sequence. The following imaging
parameters were used: Flip angle 45.degree., coronal acquisition
matrix of 256.times.192.times.(32-4- 0) (corresponding to k.sub.z
superior inferior direction, ky left-right direction, and kz A/P
directions respectively) with 60% fractional echo along S/I, 40 cm
FOV with 0.5-0.8 phase (L/R) FOV. With these parameters, each full
station was acquired in 20-30 s, and the reduced acquisition in
10-15 s.
[0031] The pulse sequence allows a user-specified view order and
reduced k.sub.z (A/P) sampling for each station. Each acquisition
is ordered according to k-space radius, with k.sub.r={square
root}(k.sub.y.sup.2+k.s- ub.z.sup.2). The view orders available for
each station used were 1. edge-center, 2. center-edge, 3.
edge-center-edge, or 4. edge-center, with the center recessed from
the end of the scan by {fraction (1/4)} of the acquisition time.
The latter has been found to reduce the effects of poor breathhold
in the first station (abdomen) for patients with difficulty in
holding their breath for the full acquisition.
[0032] The view ordering and timing of the bolus-chase sequence is
illustrated schematically in FIG. 1. For the first station
(abdomen), full acquisition with recessed edge-center view order;
for the second (thigh) the spacing .delta.k.sub.z is doubled and
the number of k.sub.z samples is reduced by a factor of 2, with
edge-center-edge view ordering; for the third (lower leg), full
acquisition with center-edge view ordering. Using this procedure,
the total temporal separation of the k-space centers for the three
stations is reduced to 3/4 of the full acquisition time, plus the
time to move the patient between stations (approximately 2 seconds
for each of the 2 repositions). With view ordering only, the
corresponding temporal separation is 1{fraction (1/4)} of the full
acquisition time plus movement, and with no view ordering it is 2
full acquisitions plus reposition time.
[0033] Two repeated acquisitions for all three stations were
performed. The first was a mask run, with no injection of contrast
agent. The second was timed to the injection of 35-40 cc of Gd-DTPA
(Magnevist, Berlex, Wayne, N J; Omniscan, Nycomed, Princeton, N.J.)
at 1.5-2 cc/s. The total contrast dose was kept constant, and not
scaled to the patient weight. Complex subtraction of the mask data
from the bolus data was used to remove the background tissue
signal.
[0034] The present embodiment using user selection and standard
deviation plots produces about a 50% increase in signal to noise
ratio without any motion correction.
[0035] The described process of sample size reduction and image
processing may be implemented by software. In a second embodiment
of the invention, the number of samples taken can correspond to the
number of samples taken according to the Nyquist rate, but spaced
further apart than in Nyquist sampling. This would extend the
spatial frequency, thereby improving image resolution over standard
Nyquist sampling.
[0036] Patient Studies of the Peripheral Extremities
[0037] In order to compare the present invention to a known
protocol, 30 consecutive patients were recruited for evaluation of
clinically suspected peripheral vascular disease at two identical
MR imaging systems from October to December, 1999. The conventional
protocol was used to image 16 patients at one facility (3 female,
13 male; ages 43 to 89 with a mean of 70). The method of the
present invention was used for the remaining 14 (2 female, 12 male;
ages 41 to 76 with a mean of 62) at the other facility. The choice
of location and protocol was based on patient and referring
physician preference but unrelated to medical condition. The
indications leading to referral and the severity of peripheral
vascular disease in the two populations are outlined in Table 2.
The study was IRB approved with all patients giving informed
written consent.
2TABLE 2 Primary indications leading to referral, and findings for
the populations undergoing the conventional and inventive
protocols. Note that in all cases, one or more significant lesions
were found. PTA = Percutaneous transluminal angioplasty.
Conventional Inventive Protocol Protocol Primary Indication
Claudication 12 (75%) 11 (78%) Rest Pain/Ulceration 3 (19%) 2 (14%)
Dissection 1 (6%) 0 (0%) Aneurysm 0 (0%) 1 (7%) Post bypass graft
or PTA/stent 3 (19%) 4 (29%) Severity of Peripheral Vascular
Disease No Significant Lesions 0 (0%) 0 (0%) One Significant Lesion
1 (6%) 3 (21%) Multiple Significant Stenoses 15 (94%) 11 (79%)
[0038] The clinical utility of the images produced depends
primarily on the arterial signal enhancement due to the contrast
agent, and on the background signal. The background signal may be
due to noise or patient motion. Motion is particularly significant
in the first station, where poor, non-reproducible breathhold and
bowel gas movement can significantly reduce the effectiveness of
image subtraction. In subsequent stations, patient motion is
usually less significant.
[0039] To quantitatively assess image quality, the following
procedure was used:
[0040] For each station, complex subtraction of the mask image was
performed. The mean signal magnitude of the subtracted data over
the entire station was calculated. Although this contains the
enhanced arterial signal, the small number of voxels containing
vessels allows this mean signal to be approximated as the
background. For each axial slice (perpendicular to SI) in the
station, the maximum signal intensity was found, which is assumed
to be due to arterial enhancement. The mean peak intensity for all
the slices is divided by the background to give a figure of merit
for the station. This calculation is shown schematically in FIG. 2
and was automated such that no observer bias could be included.
Figures of merit (FOM) were calculated for all stations of all
studies. A two-sample t-test assuming unequal variances was
performed on the figure of merit data from each station to evaluate
the significance of the differences between the results obtained
for the two protocols.
[0041] Results
[0042] FIG. 3 shows the figure of merit data obtained from all
subjects and all stations. The statistical analysis of this data is
given in Table 3, showing that for all stations, significant
improvement (p<0.001) was found using the inventive
protocol.
3TABLE 3 Figure of merit comparison for studies using the reduced
FOV second station. Values quoted are x .+-. .sigma..sub.n-1.
p-values are calculated using a two-sample t-test assuming unequal
variance. Station 1 Station 2 Station 3 Conventional Protocol 8.9
.+-. 1.6 15.9 .+-. 5.1 11.0 .+-. 2.5 (n = 16) Inventive protocol
with 13.8 .+-. 2.2 22.6 .+-. 3.5 24.9 .+-. 7.2 half FOV (n = 14) p
4.1 .times. 10.sup.-7 2.4 .times. 10.sup.-4 3.5 .times.
10.sup.-6
[0043] FIG. 4 shows an example of a scan performed using the
inventive protocol and the reduced A/P FOV in the second station.
The study shows a patient with claudication in the left leg. The
coronal view does not show the ghost image in the second station
because it lies directly on top of the real image (FIG. 4a). The
sagittal view shows that the ghost and real images are separated
(FIG. 4b) and that the ghost can be removed using volume of
interest (VOI) rendering (FIG. 4c). Once the volume of interest has
been defined, projections can be made at any viewing angle. The
ghost image was successfully removed from all studies using the
reduced k.sub.z field of view. Additionally, removing the bladder
from a coronal section was also found to be useful to improve
visualization of vessels on the maximum intensity projection The
high concentration of contrast agent and reduced receiver bandwidth
gives excellent SNR in all three stations, while the short
acquisition time gives negligible venous signal.
[0044] The studies with the median FOM (averaged over all stations)
for the two protocols were selected, and represent typical
examinations. The variation of arterial signal with SI position is
shown in FIG. 5, with the corresponding images shown in FIG. 6. It
can be seen in FIG. 5 that the signal to background ratio is higher
for the invention in all stations. However, it should be noted that
in both cases, the values for the first station are reduced due to
the increased background caused by motion during the breath hold.
FIG. 6A (conventional protocol) shows significantly greater motion
effects than FIG. 6B (inventive protocol), possibly due to the
recessed k-space center in the invention and the different timing
methods.
[0045] Discussion
[0046] The disclosed data demonstrates that the bolus chase
protocol outlined here provides significantly higher quality images
than conventionally known and used protocols. The important
differences between the protocols are: improved timing of the first
station; reduced A/P FOV for the second station; and reduced
receiver bandwidth to give improved SNR. The improvement in image
quality can be mainly attributed to better timing of the
acquisition to the passage of the bolus through the arterial
system, leading to more sharing of the bolus between stations.
[0047] The image quality obtained in bolus-chase studies is
critically dependent on accurate timing of the arterial phase of
the bolus to the acquisition of the k-space centers for each
station. For three-station studies, dynamic view ordering reduces
the minimum separation of the k-space centers of the three stations
to the time to acquire the central station plus the time taken to
move between stations. Reducing the acquisition time of central
station through reducing A/P FOV improves matching of the
acquisition rate to the bolus velocity. In this manner, a full
high-resolution 3D volume can be acquired in the time taken for the
bolus to traverse the station.
[0048] Decreasing the sampling rate along k.sub.z in the central
station is an effective way to decrease the acquisition time
without sacrificing resolution or spatial coverage. Exciting the
same volume as the other stations ensures that all vessels are
imaged. The same maximum value of k.sub.z gives the same spatial
resolution as the full acquisition, but the undersampling leads to
a ghost wrap-around image. The removal of the ghost image is
possible because (1) subtraction of the mask removes the
surrounding tissue which would otherwise be wrapped onto the
vessels, and (2) the vessels have limited A/P extent, so the real
and ghost images do not overlap and are separable using standard
volume-of-interest rendering. As long as the vessels at any
cross-sectional location are contained within half the full A/P
field of view then the images will be separable. In our experience,
this is universally the case in the thigh. More generally, an image
volume having x, y, z orthogonal coordinates and a thickness along
z=Tv, for every location x, y within the volume having a thickness
T.sub.s<T.sub.v, three dimensional k space sampling may be
accelerated by a factor F.ltoreq.T.sub.v/T.sub.s by increasing the
spacing of the k space sampling along k.sub.z by a factor of F
while maintaining the sampling frequencies range unchanged from
sampling according to the Nyquist sampling theorem to prevent the
well-known imaging artifact of wrap-around (aliasing).
[0049] For the first station, acquiring only the central 60% of
k-space during full arterial enhancement could lead to a reduction
in the visibility of small vessels. However, in our experience, it
appears that this is not a significant problem. Instead, the image
quality in this station appears to be improved by the modified
timing. One specific advantage is that the earlier acquisition
prevents venous signal obscuring the renal arteries as discussed in
Ruhm S G, Debatin J F. Contrast-enhanced 3D MR angiography of the
thorax, abdomen and lower extremities. Radiologe 1999; 39(2):
100-109, Rofsky N M, Adelman M A. MR Angiography in the Evaluation
of Atheroscierotic Periphal Vascula Disease, Radiology 2000;
214(2):325-338, Grist T M. MRA of the abdominal aorta and lower
extremity. J. Magn Reson Imaging 2000; 11(1):32-43 and Prince M R,
Brist T M, Debatin J F. 3D Contrast MR angiography. Springer,
Berlin. 1999.
[0050] The reduced number of k.sub.z-acquisitions of the second
station would be expected to reduce the SNR by a factor of {square
root}2 in that station. However, the reduced receiver bandwidth and
increased arterial bolus concentration more than compensate for
this loss of SNR.
[0051] It should also be noted that in some patients, the blood
flow rate is significantly reduced due to aneurysms or occlusions.
This can be observed during the 2D timing run. During this study,
4/18 patients (22%) scheduled to have the reduced FOV study were
found to have sufficiently slow blood flow that there was a danger
of the reduced FOV acquisition being faster than the bolus. In
these cases, a full acquisition of the second station was used, but
the data excluded from the study.
[0052] The A/P FOV of the middle station (encompassing femoral
arteries) was reduced by half in this study. Potentially this A/P
FOV can be further reduced for an even faster bolus chase, and
could be replaced with a 2D projection acquisition as most
significant stenoses in femoral arteries may be adequately
visualized in the A/P projection.
[0053] Imaging Protocol for Carotid Study
[0054] To better observe the effects of a recessed center view
order, additional patient studies were made using a GE Signa CVi MR
scanner (General Electric Co, Milwaukee, Wis.). The study was IRB
approved with all patients giving informed written consent.
[0055] In particular, to develop a better understanding of carotid
contrast dynamics, a preliminary study was performed in which 22
patients were imaged using only time-resolved oblique 2D projection
MRA of the aortic arch and carotid arteries. Images were obtained
during a breath-hold at 1-2 second intervals using a fast 2D
spoiled gradient echo sequence. The sequence parameters were:
TR=7.0-9.7, TE=1.8-2.2, bandwidth 15.6-31.3, FOV 24-34 cm, slab
thickness 80-140 cm. Complex subtraction of a precontrast mask was
used to enhance vessel conspicuity.
[0056] The fluoroscopically triggered pulse sequence is based on a
standard multiphase 3D spoiled gradient echo sequence.
[0057] For 3D imaging, two different view-ordering schemes were
evaluated:
[0058] 1. Standard center-edge elliptical-centric;
[0059] 2. A recessed center-edge elliptical-centric view order in
which the very center of k-space is acquired 3 seconds into the
acquisition. It is hypothesized that this view order will better
time the k-space center to the peak arterial concentration prior to
venous return, giving better arterial visualization.
[0060] A comparative illustration of the two view ordering schemes
is illustrated in the bottom portion of FIG. 7. Note that beyond
the recessed region the acquisition timing is identical. The
recessed view ordering of a preferred embodiment is calculated as
follows:
[0061] An array of the ky-kz values to be acquired is generated and
ordered according to the corresponding k-space radius, kr. The four
points (ky, kz), (-ky, kz), (ky, -kz) and (-ky, kz) are degenerate,
with identical values for kr. Due to the sequential nature of the
insertion sort through the ky-kz array, these degenerate points are
always ordered as
[1] (-ky, -kz), [2] (-ky, kz), [3] (ky, -kz) [4] (ky, kz)
[0062] For the simple elliptical-centric case, the order of
acquisition is simply the order of this sorted array--the index of
the array is the acquisition number. For the recessed elliptical
centric case, the index of the sorted array (Index) is calculated
as a function of the acquisition number n, which counts from 0, and
the number of recessed acquisitions (N.sub.R) using the following
calculation.
4 Index = N.sub.R - 2n - 1 n < N.sub.R/2 Index = 2n - N.sub.R
N.sub.R/2 .ltoreq. n < N.sub.R Index = n n .gtoreq. N.sub.R
[0063] The center of the k-space is acquired when n=N.sub.R/2, and
for n.gtoreq.N.sub.R, the acquisition is identical to the
elliptical-centric case. A consequence of this view ordering is
that for each odd index point (in the recessed part of the
acquisition) acquired at a given time prior to the k-space center,
a corresponding even index is acquired at an equal amount of time
after the center. With the array sorting as described, this ensures
that conjugate point (ky, kz) and (-ky, -kz) are acquired at
symmetric time points with respect to the k-space center.
[0064] While the above specifically contemplates the case where the
k-space acquisitions in the recessed part of the acquisition are
symmetric about the k-space center, the invention is more generally
applicable depending of the field of view, the enhancement uptake
curve and degree of recess. The amount that the radius is recessed
from the center will also effect the number of acquisitions taken
from the recessed point to the k-space center and the number of
recessed points taken from the k-space center outwards towards the
opposite recessed edge.
[0065] More generally, the algorithm for k-space recession in
real-time triggered contrast enhanced MRA, which matches k-space
sampling to the vessel contrast enhancement profile can be
described as follows.
[0066] As one embodiment of the technique,
[0067] Let N be the total number of points in k-space
k.sub.yk.sub.z.
[0068] Let {K[i]} .A-inverted.i.di-elect cons.[0,N-1] be the array
of k-space points ordered in ascending radius (degenerative points
are ordered with increasing k.sub.z then increasing k.sub.y).
[0069] Let T.sub.recess be the time for recessing the k-space
center (determined by the duration from start of scan to the
arrival of peak enhancement).
[0070] Let k.sub.R be the spatial frequency desired to start for
recession (determined by the enhancement value at the start of
scan), and the corresponding index in the array K[ ] is
N.sub.R-1.
[0071] Let TR be the sequence repetition time, the time to acquire
one point in the ky kz plane.
[0072] Then the number of points in k-space that can be acquired
during recess time is
N.sub.recess=T.sub.recess/TR.ltoreq.N.sub.R/2.
Let M=N.sub.R/N.sub.recess.
[0073] The k-space recession can be performed by allocating k-space
points from the array K[ ] using the following index
assignment:
Index=N.sub.R-1-M*n, for every n.sup.th point to be acquired, with
n=0, 1, . . . N.sub.recess-1.
[0074] Once the k-space center is reached (n=N.sub.recess-1 and
index=0), then k-space sampling is changed to center-edge manner:
allocating k-space points from the remaining unsampled points in
the array K[ ] by sequentially increasing assignment of the index
from 0.
[0075] A second embodiment of the technique corresponds to the case
where the contrast enhancement curve is known or estimated in
advance.
[0076] Then the contrast enhancement curve C(t) (as defined by the
arterial MR signal) can be digitized at the time points of data
sampling to generate an array {C[i]}550 i.di-elect
cons.[0,N-1].
[0077] Let {D[i]}.A-inverted.i.di-elect cons.[0,N-1] be the index
array that sorts array C[ ] in descending order, i.e.,
{C[D[i]]}.A-inverted.i.d- i-elect cons.[0,N-1] is a descending
array.
[0078] Then the recessed centric k-space sampling algorithm that
matches the k-space sampling to the contrast enhancement curve is
to allocate k-space point from the array K[ ] (as defined in the
above embodiment 1) using the following index assignment:
[0079] Index=D[n] for the n.sup.th point to be acquired, with n=0,
1, . . . , N-1. (N is the total number of k-space points, as
defined in embodiment 1).
[0080] Summarizing this scheme, the lowest spatial frequency data
is acquired at the estimated maximum contrast enhancement, the next
lowest spatial frequency data at the next highest contrast and so
on.
[0081] The 3D volume to be imaged is a coronal section prescribed
graphically from an axial scout scan. The top portion of FIG. 7
shows the analysis of a dynamic 2D study. The arterial and venous
signals were measured as a function of time after contrast
injection. The arterial only phase is observed to last 8-10
seconds. Experimental data such as this may be used to accurately
simulate the effects of different triggering delays in 3D
acquisitions.
[0082] The following imaging parameters were used: Flip angle
45.degree., receive bandwidth 32 kHz, TE/TR=2.0/6.0 ms, head/neck
neurovascular array coil, coronal acquisition matrix of
256.times.192.times.(24-32) (corresponding to kx S/I, ky L/R, and
kz A/P directions respectively) with 60% fractional echo along S/I,
24 cm FOV. With these parameters, the 3D dataset was acquired in
20-30s.
[0083] For 2D fluoroscopic monitoring, the same pulse sequence is
used as for the 3D imaging, but with the slice-encoding gradients
removed. Additionally, the 2D volume position and thickness may be
interactively adjusted in real-time independently of the 3D volume.
The imaging volume may be shifted in the frequency- and
slice-encode directions, as well as rotated axially. The scan
parameters (TR, TE, flip angle, acquisition matrix, FOV, receiver
bandwidth) were otherwise identical to those used for the 3D
acquisition. The update rate for an entire set of 2D Fourier data
(one complete image) was approximately one second. Real-time
control was achieved using an additional workstation (Sun Ultra-2,
Sun Microsystems) with a fast fiber-optic link (BIT3 corporation)
to the MR scanner. Using a sliding window technique, the update
time for data transfer, 2D Fourier transform, and display is
approximately 150 ms for a single-coil acquisition and 250 ms for a
4-coil phased-array acquisition. Sliders are used to interactively
adjust the imaging volume, with a button to initiate the 3D
acquisition. The delay between user initiation and the start of the
3D acquisition was 120 ms.
[0084] The fluoroscopic monitoring volume was chosen to be an
oblique thick slab in the plane of the aortic arch, positioned
inferior to the subsequent coronal 3D imaging volume. This allowed
the contrast passage to be followed from the subclavian vein to the
pulmonary arteries, pulmonary veins, left ventricle, aortic arch
and finally the carotid arteries. Monitoring upstream of the
anatomical volume of interest cues the operator prior to contrast
arrival, and may substantially reduce operator-induced delay.
Additionally, this choice of monitoring plane reduces inflow
effects due to blood motion, further decreasing the likelihood of
false triggering. Both the 2D and 3D data was acquired during a
single injection of 20 cc of Gd-DTPA (Magnevist, Berlex, Wayne, NJ)
at 1.5 cc/s.
[0085] Patient Studies on Carotid Artery
[0086] To evaluate the utility of this protocol, 37 consecutive
patients (11 female, 27 male, ages 36-91, mean 67) were recruited
for evaluation of clinically suspected vascular disease. Nineteen
patients were scanned using the elliptical-centric view ordering,
and 18 using the recessed technique of the present invention.
[0087] To quantitatively assess the degree of venous ringing, axial
cross sections were taken through the carotid bifurcation. Note
that there is no ringing in the frequency-encode
(superior-inferior) direction. Signal intensities from the
arteries, the veins, the surrounding tissue and the background
noise were measured and compared for each study. More
qualitatively, the visibility of venous signal on coronal maximum
intensity projections and the axial cross sections was
assessed.
[0088] Results
[0089] The distribution of the arterial phase duration for the
preliminary study of 22 patients is shown in FIG. 8. The
arterial-venous transit time is typically around 10 s, but there is
substantial variability between patients, with values of between 4
s and 16 s.
[0090] All 37 patients (100%) scanned with the fluoroscopic
technique demonstrated diagnostic MR arteriograms with sufficient
arterial enhancement for the 3D acquisition. For the nineteen
patients scanned using the conventional elliptical-centric view
order, three (16%) showed substantial venous enhancement on coronal
maximum intensity projections (MIPs). Even in these three cases,
only the edges of the veins were visible, and these edges were
still substantially weaker than the arteries, allowing them to be
easily distinguished. In a further 14 (74%) cases, some ringing was
visible on the source images, but was weak enough that it was not
noticeable on the MIPs. In the remaining 2 cases (11%), no ringing
was visible in either the MIPs or the source images. Volume of
interest MIP rendering further enhanced arterial clarity in all
cases. FIG. 9 shows a spectrum of studies with substantial,
moderate and no ringing artifacts.
[0091] For the eighteen patients scanned using the recessed
elliptical-centric view order, none (0%) showed substantial ringing
on the MIPs. In 14 (78%) cases some ringing was visible on the
source images, while in the remaining 4 (22%) cases no ringing was
visible. In these studies, the ratio of mean arterial signal to
peak venous signal varied from 1.5 (substantial ringing) to greater
than 5, at which point the vein was difficult to distinguish from
the tissue background. In general, ratios of 2 or greater were
found to give good background suppression in MIP images without
selected volume imaging. The ratio of mean arterial signal to peak
venous signal for the elliptical-centric view order was 2.75
(standard deviation 1.48). For the recessed view order this ratio
was increased to 3.07 (standard deviation 1.07). Using a one-tail
t-test, the difference between the means was not significant at the
5% confidence level (p=0.23). This lack of statistical significance
is likely due to the small sample size and the large inter-patient
variability demonstrated in FIG. 8. However, the number of scans
with ratios less than 2 was reduced from 6/19 (32%) to 1/18 (6%).
Assuming a binomial distribution, this difference is significant at
the 5% level.
[0092] The arterial signal intensity in all 37 cases was between 7
and 25 times greater than the signal intensity from the surrounding
neck tissue. This is more than sufficient for excellent vessel
conspicuity against the background tissue. FIGS. 10 and 11 show
examples. In FIG. 10, 2D images prior to initiation of the 3D scan
clearly show pulmonary enhancement several seconds prior to
contrast reaching the aortic arch. FIG. 11 shows data from a
patient with multiple stenoses at the left carotid bifurcation.
[0093] Discussion
[0094] The recessed elliptical-centric view ordering scheme
presented shifts the acquisition of the k-space center a few
seconds into the scan. The time prior to the acquisition of the
center is still used to acquire data, but not the very center of
k-space. This allows the operator to initiate the scan earlier,
with confidence that the k-space center will be acquired close to
peak arterial enhancement and prior to venous enhancement.
[0095] If some data is acquired on the rising edge of contrast
enhancement, the question remains as to whether such data will
cause significant imaging artifacts. It is hypothesized that such
artifacts will be minimal for two reasons.
[0096] 1. The signal power in typical MRA imaging situations is
strongly peaked at the k-space center.
[0097] 2. The recessed acquisition order has 180.degree. symmetry
with respect to the k-space center. For each point (ky, kz)
acquired prior to the center, there is a conjugate point (-ky, -kz)
acquired an equal amount of time following the center.
[0098] The use of recessed elliptical-centric view ordering has
been found to increase the reliability of the technique. This is
reflected in our results: while the mean arterial to venous ratio
is not improved to an extent that is statistically significant with
this small sample size, the number of scans with poor ratios (due
to reduced arterial signal) is reduced.
[0099] The intensity of the venous ringing decreases rapidly as the
venous arrival is delayed later into the acquisition. Larger
vessels have their signal peaked more strongly at the k-space
center, so are expected to be better suppressed than smaller
vessels. The rate at which k-space is traversed during the
acquisition depends inversely on the field of view chosen. In this
study, a large field of view of .about.30 cm is used to image from
the aortic arch to the Circle of Willis, making this particularly
susceptible to venous contamination.
[0100] High contrast to noise ratios (CNR) were obtained in all
cases. This suggests that it would be possible to increase the scan
resolution while still maintaining sufficient CNR for vessel
conspicuity. A further study is underway in which the acquisition
matrix size in the frequency encode (superior-inferior) direction
is doubled. Additionally, increasing the frame rate for the
fluoroscopic part of the acquisition may be achieved by acquiring
fewer phase encodes for each fluoroscopic image. Since high
resolution is not required for monitoring of the aortic arch,
increasing temporal resolution at the cost of decreased spatial
resolution would seem worthwhile.
[0101] Conclusions
[0102] An improved bolus chase MR digital substation angiography
technique is developed that consistently provides high quality
angiograms of the arterial system from the abdomen to the calf. The
robustness against poor breath-hold during imaging the abdomen
region is maintained with recessed reverse centric view order. The
reduced A/P FOV, through modified k-space spacing, allows effective
chasing and maximum sharing of the bolus. With optimal timing at
the start of abdomen station, the calf station is reliably imaged
with adequate SNR and minimal venous signal. This protocol
described here with reduced k.sub.z-sampling has been incorporated
into our routine clinical practice.
[0103] In addition, contrast-enhanced 3D MRA has been shown to be a
fast, reliable, and clinically useful. The recessed
elliptical-centric view-ordering scheme disclosed above better
times the acquisition of k-space center to peak arterial
enhancement. It is more robust to timing errors than conventional
elliptical-centric view-ordering, allowing triggering the scan at
the start of contrast arrival. This permits substantially more of
the arterial-only phase to be utilized, resulting in less venous
signal.
[0104] While preferred embodiments of the invention have been
described above, one skilled in the art would understand that
modifications may be made thereto without departing from the spirit
and scope of the invention. For instance, while the invention has
been discussed with respect to lower extremities and carotid
artery, other body parts may be imaged using the inventive method
and apparatus.
* * * * *