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United States Patent |
6,135,954
|
Foo
,   et al.
|
October 24, 2000
|
Method for peripheral MR angiography
Abstract
A method of peripheral MR angiography is provided for imaging an artery or
other vessel, wherein the vessel is of such length that MR data must be
acquired at each of a plurality of scan stations spaced along the vessel.
In accordance with the method, a contrast agent is intravenously injected,
in order to provide a bolus which successively flows to each of the scan
stations. After acquiring an initial subset of the MR data associated with
a given scan station, the bolus is tracked to determine whether it has
arrived at the next-following scan station. If so, at least some of the MR
data associated with the next scan station are then acquired. However, if
it is found that the bolus has not yet arrived at the next scan station,
acquisition of further data at the given scan station is continued.
Inventors:
|
Foo; Thomas K. F. (Rockville, MD);
Ho; Vincent B. (North Bethesda, MD);
Bernstein; Matthew A. (Rochester, MN)
|
Assignee:
|
General Electric Company (Milwaukee, WI)
|
Appl. No.:
|
118411 |
Filed:
|
July 17, 1998 |
Intern'l Class: |
A61B 005/055 |
Field of Search: |
600/407,409,410,420,425,427
324/307,309,300,306
424/9.3
|
References Cited
U.S. Patent Documents
5417213 | May., 1995 | Prince | 600/420.
|
5422576 | Jun., 1995 | Kao et al. | 324/309.
|
5590654 | Jan., 1997 | Prince | 600/420.
|
5928148 | Jul., 1999 | Wang et al. | 600/420.
|
Primary Examiner: Lateef; Marvin M.
Assistant Examiner: Imam; Ali M.
Attorney, Agent or Firm: Skarsten Law Offices, Cabou; Christian G., Price; Phyllis Y.
Claims
What is claimed is:
1. A method of peripheral MR angiography for imaging structure pertaining
to a vessel, wherein MR data are to be acquired at each of a succession of
scan stations positioned along the vessel or vascular territory, said
succession including at least first and second scan stations, said method
comprising the steps of:
intravenously injecting a contrast agent, to provide a bolus disposed to
flow to said first scan station, and to thereafter flow to said second
scan station;
acquiring an initial portion of a first set of MR data associated with said
first scan station;
monitoring said second scan station to determine whether said bolus has
arrived at said second scan station;
acquiring at least some of the data of a second MR data set associated with
said second scan station, if it is determined from said monitoring step
that said bolus has arrived at said second scan station;
suspending acquisition of said first MR data set at said first scan
station, if it is determined from said monitoring step that said bolus has
arrived at said second scan station; and
continuing to acquire data of said first MR data set, if it is determined
from said monitoring step that said bolus has not yet arrived at said
second scan station.
2. The method of claim 1 wherein:
said initial portion of said first set of MR data comprises central k-space
data.
3. The method of claim 2 wherein said monitoring step comprises:
placing an MR detector responsive to said contrast agent proximate to each
of said scan stations, each of said detectors being in closely spaced
relationship with said vessel and disposed to detect MR signal excited in
a small volume of said vessel; and
setting each of said detectors to generate a signal when the amount of said
contrast agent at its corresponding proximate scan station exceeds a
specified threshold.
4. The method of claim 2 wherein said monitoring step comprises:
rapidly acquiring MR data from a region proximate to said vessel and to
said second scan station; and
constructing a real-time image from said rapidly acquired MR data for use
by an operator to determine whether vasculature at said second scan
station has become filled with said contrast agent.
5. The method of claim 2 wherein said monitoring step comprises:
placing an MR detector responsive to said contrast agent in closely spaced
relationship with said vessel and proximate to said second scan station;
setting said detector to generate a signal when the amount of said contrast
agent at said second scan station exceeds a specified threshold;
rapidly acquiring MR data from a region proximate to said vessel and to
said second scan station and constructing a real-time image therefrom; and
detecting a signal generated by said detector and visually observing said
rapidly acquired image, in combination, to determine arrival of said bolus
at said second scan station.
6. The method of claim 2, wherein said vessel comprises an artery residing
in an imaging subject, an MR scanning device and a table supporting said
imaging subject being associated with said method, and wherein:
said table is operated to selectively position said imaging subject with
respect to said scanning device, in order to acquire MR data associated
with a given one of said scan stations.
7. A method for acquiring an MR image of a vessel within a subject, wherein
the vessel is of such length that MR data must be acquired at each of a
succession of scan stations spaced along the vessel, said method
comprising the steps of:
injecting a contrast agent into said vessel, to provide a bolus disposed to
flow successively to each of said stations;
establishing relative movement between said subject and an MR scanning
device to successively enable each of said stations to acquire MR data;
upon enabling a given one of said stations other than the last station in
said succession, acquiring an initial portion of a first MR data set
associated with said given station;
monitoring the station next following said given station in said
succession, to determine whether said bolus has arrived at said next
following station; and
enabling said next following station and acquiring an initial portion of a
second MR data set associated therewith, while suspending acquisition of
said first data set at said given station, if it is determined from said
monitoring step that said bolus has arrived at said next following
station, and otherwise acquiring further data of said first data set at
said given station.
8. The method of claim 7 wherein:
said initial portions of said first and second data sets respectively
comprise central k-space data.
9. The method of claim 8 wherein:
said acquired further data of said first data set comprises reacquired
central k-space data, and k-space data of higher spatial frequencies,
selectively.
10. The method of claim 9 wherein:
upon enabling said last station, a complete set of MR data associated
therewith is acquired; and
data acquisition is thereafter completed at each of the other stations in
said succession.
11. The method of claim 7 wherein said subject is supported upon a moveable
table, and wherein:
said table is operated to selectively position said subject with respect to
said scanning device, in order to successively enable each of said
stations to acquire MR data.
12. The method of claim 7 wherein said monitoring step comprises:
placing an MR monitoring means responsive to said contrast agent in closely
spaced relationship with said vessel, and proximate to each of said
stations; and
setting the monitoring means proximate to a particular station to generate
a signal, when the amount of said contrast agent at said particular
station exceeds a specified threshold.
13. The method of claim 7 wherein said monitoring step comprises:
rapidly acquiring MR data from a region proximate to said vessel and to
said next following station; and
constructing a real-time image from said rapidly acquired MR data for use
by an operator to determine whether vasculature at said next following
station has become filled with said contrast agent.
14. The method of claim 13 wherein:
said real-time image comprises a fast two-dimensional projection image.
15. The method of claim 13 wherein:
said step of acquiring MR data from a region proximate to said vessel
includes employing a gradient non-linearity compenasation technique to
determine the actual location of said region.
16. The method of claim 15 wherein:
said compenasation technique comprises a GRADWARP technique.
Description
BACKGROUND OF THE INVENTION
The invention disclosed and claimed herein generally pertains to magnetic
resonance (MR) angiography, i.e., to MR imaging of an artery or like
vessel carrying blood or other fluid. More particularly, the invention
pertains to a method of the above type wherein MR data are acquired at
each of a number of scan locations or stations, which are spaced along a
vessel of comparatively great length. Even more particularly, the
invention pertains to a method of the above type wherein an amount of
contrast agent, or bolus, moves along the vessel or other conduit, from
station-to-station, and measures are taken to ensure that MR data is
acquired at a particular station only or substantially when the bolus is
located there.
It is now a well known practice in MR angiography to insert a volume of
contrast agent, such as gadolinium chelate, into blood flowing along a
vessel. The volume or mass of contrast agent is referred to as a bolus,
and has the effect of shortening the T.sub.1 time of the blood. Thus, an
MR image of the blood, acquired by a fast gradient echo or like technique,
will show up very well with respect to adjacent stationary tissue of the
vessel structure.
It is also well known that certain clinical assessments require imaging a
vascular territory of comparatively great length. Using MR for these
evaluations, therefore, necessitates the acquisition of MR data over
several stations or scan locations, which are located at intervals along
the vessel path of flow. To acquire data at a particular station, the
patient is selectively positioned with respect to an MR scanner, typically
by movement of a table supporting the patient. Data are then acquired from
a series of slices taken through a region or section of the patient, which
comprises the particular scan location or station. Thereafter, the patient
is shifted, relative to the scanner, so that data may be acquired from
another section of the patient, comprising another scan location or
station. MR angiography employing this procedure in conjunction with an
injection of a contrast bolus may be referred to as bolus chasing
peripheral MR angiography.
At present, when a contrast agent is used in connection with a peripheral
MR angiography exam, the first scan station is selected to be the section
of the patient, along a vessel of interest, at which the bolus arrives
first. When the scan at the first station is completed, the acquisition
normally moves to the next scan station. However, the most appropriate
time to move to the next station is not precisely known. For example, in
the case of slow blood flow, the distal vasculature at the next scan
station may not have had adequate time to fill with contrast material. On
the other hand, if flow rate is greater than anticipated, the contrast
agent may tend to move into stationary tissue adjacent to the next scan
station, before data acquisition commences. In either case, contrast
between moving fluid and stationary tissue may be significantly reduced at
the next scan station. Moreover, undesirable effects, resulting either
from flow rate which is too slow or too great, may tend to become
progressively worse as imaging proceeds to subsequent scan stations, and
as the total number of scan stations increases.
SUMMARY OF THE INVENTION
The invention is generally directed to a method of peripheral MR
angiography for imaging structure associated with a vessel, such as a
comparatively long artery within a patient, wherein MR data are to be
acquired at each of a plurality of scan stations positioned along the
vessel. The method includes the step of inserting a contrast agent into
blood via intravenous injection, to provide a bolus which flows to first
and second stations, in succession. The method further comprises acquiring
an initial portion of a first MR data set associated with the first
station, and monitoring the second station to determine whether the bolus
has arrived there. If it is determined from the monitoring step that the
bolus has in fact arrived at the second station, at least some of the MR
data of a second data set associated therewith are then acquired. However,
if it is determined that the bolus has not yet arrived at the second
station, acquisition of further data at the first station is continued.
In a preferred embodiment of the invention, the portion of MR data
initially acquired at the first scan location comprises central k-space
data, and the vessel comprises an artery residing in an imaging subject.
Moreover, an MR scanning device and a table supporting the imaging subject
are associated with the MR method, the table being operated to selectively
position the imaging subject with respect to the scanning device, in order
to acquire the MR data sets respectively associated with the first and
second scan stations.
In a useful embodiment of the invention, the monitoring step comprises
placing an NMR monitoring means which is responsive to the contrast agent
in closely spaced relationship with the vessel and proximal to the second
station, and setting the monitoring means to generate a signal when the
amount of contrast agent at the second station exceeds a specified
threshold.
In another useful embodiment of the invention, the monitoring step
comprises rapidly acquiring MR data from a region which is proximal to the
vessel and also to the second scan station, and then rapidly
re-constructing an image from the rapidly acquired data. An operator may
then readily determine the amount of contrast agent at the second scan
station simply by visual inspection of the rapidly acquired image.
OBJECTS OF THE INVENTION
An object of the invention is to provide a more optimum method for
peripheral MR angiography, which is directed to an artery or other vessel
of substantial length.
Another object is to provide a method of the above type, wherein data
acquisition at each scan station, in a succession of scan stations spaced
along the vessel, is substantially synchronized in time with arrival at
the scan station of a bolus of contrast agent.
Another object is to provide a method of the above type which tracks
arrival of the bolus at successive scan station along the vessel path of
flow.
Another object is to provide a method of the above type, wherein an
assessment is made as to whether the bolus has or has not moved from the
current scan station to a subsequent scan station, and data acquisition
either shifts to the subsequent station, or continues at the current
station, in accordance with the assessment.
These and other objects and advantages of the invention will become more
readily apparent from the following description, taken together with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing basic components of an MR system for
use in practicing embodiments of the invention.
FIG. 2 is a schematic diagram illustrating an arrangement for conducting a
peripheral MR angiography exam, in accordance with the invention.
FIG. 3 is a flow chart illustrating an embodiment of the invention.
FIG. 4 is a schematic diagram illustrating effects of gradient
non-linearity in connection with the embodiment of FIG. 3.
FIG. 5 is a flow chart illustrating an alternative embodiment of the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there are shown the basic components of an MR system
or scanner 10 which may be operated to acquire MR data in accordance with
the invention described herein. System 10 includes an RF transmit coil 12,
as well as a cylindrical magnet 14 for generating a main or static
magnetic field B.sub.o in the bore thereof. RF coil 12 is operated to
transmit RF excitation signals into a patient or other subject of imaging
16 residing in the magnet bore, in order to produce MR signals. System 10
further includes gradient coils 18, 20 and 22 for generating G.sub.x,
G.sub.y, and G.sub.z magnetic field gradients relative to orthogonal X-,
Y- and Z-reference axes, respectively. FIG. 1 shows each of the gradient
coils 18, 20 and 22 respectively driven by gradient amplifiers 24, 26 and
28, and RF coil 12 driven by transmit amplifier 30. FIG. 1 further shows
an RF coil 40, which is operated in association with a receive amplifier
38 to acquire MR signals from subject 16. In some arrangements, coil 40
and coil 12 comprise the same RF coil, which is operated in alternate
modes during the imaging sequence. System 10 is further provided with a
pulse sequence control 32, which is operated to control the RF and
gradient amplifiers, and to thereby generate pulse sequences to produce
and acquire sets of MR signals. System 10 also includes system control and
data processing electronics 34, for operating respective components of
system 10 to acquire MR data, in accordance with the invention, and to
construct images thereof. The construction, functions, and
interrelationships of components of MR system 10 are well known and
described in the prior art, such as in U.S. Pat. No. 5,672,969, issued
Sep. 30, 1997 to Zhou et al.
Referring further to FIG. 1, there is shown subject 16 supported on a table
36 or the like, which may be slid or translated along the Z-axis of MR
system 10. Thus, subject 16 may be selectively positioned within the bore
of main magnet 14. The motion of the table is under computer control whose
position along the Z-axis of the magnet bore can be precisely controlled
and is reproducible.
Referring to FIG. 2, there is shown a more detailed view of subject 16,
supported on table 36 within magnet 14. More specifically, FIG. 2 shows
patient or subject 16 having a blood vessel or vascular territory 44 of
substantial length. In this case, the clinical assessment of the
peripheral arteries requires the imaging of an extended field of view
which extends from the abdomen to the lower limbs of the subject. This
evaluation includes portions of the abdominal aorta, iliac arteries,
femoral arteries, popliteal arteries, tibioperoneal arteries and arteries
of the foot. It is desired to acquire MR image data of the extended
vascular territory 44 in its entirety. However, because of the substantial
length of the vascular territory 44, it is necessary to obtain the data by
establishing a plurality of scan locations or stations between subject 16
and components of MR system 10.
In accordance therewith, FIG. 2 shows scan stations 46, 48 and 50, each
comprising a section or region of subject 16. More specifically, scan
station 46 comprises the upper trunk area (abdomen) of subject 16, scan
station 48 comprises the lower trunk area (pelvis/thigh) thereof, and scan
station 50 comprises the lower extremities (calf/foot) thereof. To acquire
MR data associated with a particular scan station, table 36 is moved to
position the particular scan station in specified relationship with main
magnet 14. For example, FIG. 2 shows the midpoint of scan station 46
positioned at isocenter 42 of magnet 14.
In a conventional arrangement, an entire set of MR data, pertaining to the
segment of vessel 44 lying within scan station 46, would be acquired while
such scan station was in the position shown in FIG. 2. Then, table 36
would translate subject 16 leftward, as viewed in FIG. 2, to position the
midpoint of scan station 48 at isocenter 42. After scanning an entire set
of data pertaining to the segment of vessel 44 within scan station 48,
subject 16 would be further translated, to position the midpoint of scan
station 50 at isocenter 42. A set of data pertaining to scan station 50
would then be scanned, to complete the data acquisition procedure. It will
be noted from FIG. 2 that a certain amount of over-lap can occur between
adjacent scan stations.
As stated above, it is common practice in MR angiography to intravenously
inject a contrast agent, such as 20 cc of gadolinium chelate, into blood
52 flowing through vessel 44. This provides a bolus 54 therein. If vessel
44 carries blood from the upper body to the lower limbs of subject 16, the
flow direction is from left to right, as viewed in FIG. 2. After passing
through the cardiac and the pulmonary circulations, the bolus 54 would
arrive first at scan station 46, then arrive at scan station 48, and
finally arrive at scan station 50.
In accordance with a conventional technique known commercially as SMARTPREP
(as described in Foo T K F; Saranathan M; Prince M R; Chenevert T L.
Automated detection of bolus arrival and initiation of data acquisition in
fast, three dimensional, gadolinium-enhanced MR angiography. Radiology
1997; 203: 275-280) by the General Electric Company, a monitor 56 is
placed proximate to vessel 44, upstream to the arterial blood flow for the
field-of-view that constitutes scan station 46, an example of which is
shown in FIG. 2. The monitor 56 periodically detects MR signal excited in
a small volume or region of vessel 44 (not shown in FIG. 2) which is in
closely spaced relationship with monitor 56. The detected MR signal will
reach a specified threshold level when the contrast agent enters the
portion or segment of vessel 44 lying within scan station 46. Thereupon,
scanning of station 46 commences. When such scan is completed, the MR
system will sequentially proceed to acquire data from scan station 48, and
then from scan station 50.
As indicated above, the time required for the bolus 54 to move from one
scan station to the next is not precisely known and varies from
patient-to-patient. Variations in bolus delivery can have a significant
impact on MR image data. Acquiring the MR data prior to the bolus arrival
into the target artery will result in poor (or no) visualization of the
artery. Similarly, acquiring the data after the passage of the bolus can
result in less than optimal visualization of the target artery. Inaccurate
bolus timing (i.e. one's ability to coordinate the bolus with the MR data
acquisition), therefore, could significantly diminish the benefits--and
efficacy--of using a contrast agent in conventional scan techniques as
described above or require the use of increased quantities or dose of the
gadolinium chelate contrast material. Thus, in order to overcome such
disadvantages, and in accordance with an embodiment of the invention,
monitors 58 and 60 are placed in closely spaced relationship with vessel
44, at the positions respectively shown therefore in FIG. 2. More
specifically, monitor 58 is positioned just to the right of the leftward
edge of scan station 48, and monitor 60 is positioned just to the right of
the leftward edge of scan station 50, as viewed in FIG. 2. Thus, monitors
58 and 60 will detect arrival of bolus 54 at scan stations 48 and 50,
respectively. Monitors 58 and 60 may be similar to monitor 54 in operation
and construction. Also, baseline data is obtained for each monitor, prior
to acquisition of image data for the angiography exam. The baseline data
indicates the level of MR signals detected by respective monitors, in the
absence of contrast agent. From such data, a threshold level may be
pre-set for each monitor, to indicate arrival of the bolus at the
corresponding scan location.
Note that in order to monitor the signal at locations 58 or 60 while
acquiring data from 46 or 48, respectively, the table may also move in
order to better visualize 58 or 60. Furthermore, moving the table will
minimize geometric distortion from having 58 or 60 at the edge of a large
image FOV.
By tracking the progress of bolus 54 along vessel 44, as described
hereinafter in further detail, the method of the invention is able to
immediately acquire the most pertinent image data at a scan station, upon
detecting arrival of the bolus at such station. Moreover, the method
enables data acquisition to continue at a scan station, while the bolus is
in transit to the next-following station. Thus, the method of the
invention optimizes use of contrast agent in peripheral MR angiography.
Referring to FIG. 3, there is shown a flowchart illustrating use of
monitors 56-60 in a method comprising an embodiment of the invention. In
accordance with process blocks 62 and 64, the threshold levels of
respective monitors are initially pre-set, and monitor 56 is operated to
detect the arrival of bolus 54 at the first scan station, i.e., at scan
station 46. Thereupon, a trigger signal is generated to commence
acquisition of image data, in regard to the segment of vessel 44 lying
within each of the n scan stations. While FIG. 2 shows only three scan
stations, it will be readily apparent that in other embodiments the number
of scan stations n may be substantially greater. Moreover, as emphasized
by process block 66 of FIG. 3, initial data acquisition at each scan
station, including the very first station, is limited to central k-space
data, i.e., to k-space data of lower spatial frequencies. Such data is
most significant in image reconstruction, and is usefully acquired over a
period of approximately 5-10 seconds.
After acquisition of the central k-space data for the nth scan station, a
determination is made as to whether such nth scan station is the final
station in the imaging sequence, in accordance with decision block 68. If
not, the monitor signal for scan station n+1 is detected, to determine
whether bolus 54 has arrived there, as collectively indicated in FIG. 3 by
process block 70 and decision block 72. If bolus 54 has reached scan
station n+l, table 36 operates to move subject 16, as previously
described, so that data acquisition can commence at scan station n+1. Such
operation is indicated by process block 74, which is followed by resetting
an associated counter (not shown) from n to n+1, in accordance with
process block 76. Thereupon, central k-space data is acquired at the
updated scan station, in accordance with process block 66.
If it is decided at block 72 that the bolus has not yet reached scan
station n+1, a determination in accordance with decision block 78 must be
made. That is, it must be determined whether data acquisition for the nth
scan station has been completed. If not, the next loop of k-space data
required for scan station n is acquired, as indicated by process block 80.
However, if data acquisition has been completed, the operation of process
block 82 is carried out, that is, central k-space data is reacquired for
the nth scan station, in order to improve signal to noise ratio.
Alternatively, data is acquired at higher spatial frequencies, to increase
spatial resolution. When the operation of either process block 80 or
process block 82 has been completed, the system returns to process block
70, to once again test for arrival of the bolus at scan station n+1.
Referring further to FIG. 3, process block 84 requires that data
acquisition be completed for the last scan station, after such scan
station has been identified by decision block 68. Then, in accordance with
process block 86, all remaining k-space data, which has not yet been
acquired, is obtained for respective scan stations. The operation of
process block 86 would generally require controlled movement of table 36,
to selectively position the subject 16. When acquisition of the remaining
data has been completed, the scanning procedure of FIG. 3 comes to an end.
It will be readily apparent that the respective steps and procedures of
the embodiment, as described herein and shown in FIG. 3, could readily be
implemented by configuring control electronics 34 to direct operation of
system 10 in accordance therewith.
In the embodiment of the invention described above, a problem may be
encountered in placing a monitor over a small volume within a vessel of
interest. As shown in FIG. 2, a monitor and its associated volume are
located near the edge of a scan station, and are therefore at the edge of
the imaging field of view. As a result, the monitor volume may be spaced
so far from the magnet isocenter that it is affected by gradient
non-linearities.
Referring to FIG. 4, there is shown an ideal slice selective slab 88, which
is taken through vessel of interest 44, and is intended to supply MR
signal data for constructing an image. Slab 88 includes a volume 90,
comprising a small region along the path of blood flow which is adjacent
to one of the monitors. FIG. 4 further shows the slice selective slab 88a
from which the MR signal data is actually acquired. Because of gradient
field non-linearity distal from isocenter 42, the ends of slab 88a curve
away from the intended positions thereof. Accordingly, the actual position
of monitor volume 90a is displaced from intended volume position 90. As a
result, the frequency offsets corresponding to the monitor slabs may be
incorrect. This can be compensated for by using the gradient parameters of
a technique known in the art as GRADWARP to pre-calculate the field
distortion. In the current method, the slice selection frequency offsets
and slice selection gradient amplitude may then be changed in accordance
therewith. This allows the prescribed monitor volume to better match the
expected position. The GRADWARP technique is described in the prior art,
such as in U.S. Pat. No. 4,698,591, issued Oct. 6, 1987 to Glover, et. al.
Another method to correctly localize the monitor volume is to use the
saturation effect of the slice selective RF pulses (either a spin echo
orthogonal slice selective gradient or 2-dimensional (2D) cylindrical RF
pulse). The saturation region would indicate the position of the monitor
volume. The correct position can be quickly identified by preceding a
real-time acquisition sequence with a tracking or monitor volume selection
pulse sequence. Thus, a user can move about or around the desired station,
and adjust the monitor volume frequency offsets accordingly.
Referring to FIG. 5, there is shown a flow chart illustrating an embodiment
of the invention, wherein alternative means are used to detect the arrival
of the bolus at respective scan stations. Thus, the need for monitors such
as monitors 56-60, as well as drawbacks associated therewith, are
substantially eliminated. This is similar to the method described by
Wilman A H; Riederer S J; et al. Fluoroscopically triggered
contrastenhanced three-dimensional MR angiography with elliptical centric
view order: application to the renal arteries. Radiology 1997; 205:137-46.
As shown by process block 92, the method of FIG. 5 commences by
periodically acquiring short 2D projection images of the first scan
station, i.e., scan station 46. An operator of MR system 10 physically
views respective images until he sees the vasculature filled with contrast
agent at such scan station. Thereupon, he triggers acquisition of central
k-space data at the first scan station, in accordance with process block
66, which is identical to process block 66 described above in connection
with FIG. 3. The following decision block 68 is likewise identical to
decision block 68 of FIG. 3. Thus, after acquiring the central k-space
data for any scan station other than the last station, a bolus tracking
function, represented by block 94, is initiated. Such function comprises
process blocks 96 and 98 and decision block 100.
In accordance therewith, and following the acquisition of central k-space
data for the nth station, at the end of each k.sub.z encoding loop (in a
3D fast GRE acquisition) a short 2D projection image (preferably using an
echo-train or similar pulse sequence of 100-200 milliseconds acquisition
time) is acquired at scan station n+1 (process block 96). The k.sub.y
encoding value is then updated, and the next k.sub.z loop data is
acquired. Thus, for each k.sub.y encoding value, a fast acquisition is
performed at the next scan station. The fast 2D projection image is
reconstructed and displayed in real-time, at intervals of (T.sub.acq
+n.sub.z TR), in accordance with process block 98. T.sub.acq is the
acquisition time of the fast 2D image, and n.sub.z is the number of slice
encoding values. In accordance with decision block 100, the operator
observes the displayed 2D image to determine whether the vasculature at
the scan station n+1 has filled with contrast material. If so, data
acquisition will move to scan station n+1. Otherwise, data acquisition
will continue at scan station n, in accordance with decision block 78 and
process blocks 80 and 82, described above in connection with FIG. 3. The
remaining blocks shown in FIG. 5 are likewise identical to the
corresponding blocks previously described in connection with FIG. 3.
It is thus seen that in the method of FIG. 5, visual monitoring of the
vasculature at scan station n+1 begins only after a central core of
k-space data has been acquired for scan station n. This insures that data
at the current scan station has sufficient contrast between the
vasculature and the stationary tissue. The acquisition of the fast 2D
projection image at scan station n+1 may be accomplished in one of two
ways. The first is to completely move table 36 to position subject 16 at
scan station n+1. The second is to move the table part way toward such
scan station. The procedure of choice will generally depend on table
motion speed.
Note that the above two approaches or embodiments can also be combined to
provide both a visual and automated method for detection of the contrast
bolus arrival, as described by Kim J; Farb G; Wright G; Sentinel scan:
Test bolus examination in the carotid artery at dynamic gadolinium
enhanced MR angiography. Radiology 1998; 206:283-289.
Obviously, many other modifications and variations of the present invention
are possible in light of the above teachings. It is therefore to be
understood that within the scope of the disclosed concept, the invention
may be practiced otherwise than as specifically described.
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