Title: Scoutless whole-body imaging with fast positioning
Abstract: A method and apparatus are provided for forming a magnetic resonance angiographic image of a human body. A plurality of spatially non-selective radio-frequency pulses and a plurality of different combinations of phase-encoding gradients are applied to the human body, that are temporally non-coincident with the radio-frequency pulses and where each combination includes a pulse value in a slice selective direction and a pulse value in an in-plane direction and magnetic resonance imaging data are detected. A slice processor and/or a thickness processor identify the presence, location and/or thickness of a body portion of the human body. Identification of a body portion in a first imaging volume becomes the basis of application of a second plurality of spatially non-selective radio-frequency pulses to a second imaging volume of the human body.
Patent Number: 6,901,282 Issued on 05/31/2005 to Edelman
| Inventors:
|
Edelman; Robert R. (Highland Park, IL)
|
| Assignee:
|
Evanston Northwestern Healthcare, Inc. (Evanston, IL)
|
| Appl. No.:
|
357967 |
| Filed:
|
February 4, 2003 |
| Current U.S. Class: |
600/420; 324/307; 324/309; 382/128; 382/130; 600/415 |
| Intern'l Class: |
A61B 005/05.5 |
| Field of Search: |
600/410,415,419,420,421,422,425,431
324/307,306,309,318,322,312
382/128,131
|
References Cited [Referenced By]
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| 5167232 | Dec., 1992 | Parker et al.
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| 5239591 | Aug., 1993 | Ranganath.
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| 5368033 | Nov., 1994 | Moshfeghi.
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| 5433196 | Jul., 1995 | Fiat.
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| 2003/0052875 | Mar., 2003 | Salomie.
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| 2003/0053669 | Mar., 2003 | Suri et al.
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Other References
"A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems",
CDRH Magnetic Resonance Working Group, U.S. Food and Drug Administration—Center
for Devices and Radiological Health website, released for comment on Feb. 7, 1997.
"Magnetic Resonance Angiography (MRA), A Brief Overview", Greg Brown, SMRT Royal
Adelaide Hospital (www.users.on.net/vision/papers/basicMRA/MRA_Intro.htm, prepared
in Sep. 1997.
"CAR Standards for Magnetic Resonance Imaging" reviewed by Magnetic Resonance
Imagining Expert Advisory Panel: Pierre Bourgouin, MD, Chair; John May, MD; Blake
McCarthy, MD; Pierre Milette, MD; Peter Poon, MD and approved Jun. 1999 (www.car.ca/standards/mri.htm).
"Automated Bolus Chase Peripheral MR Angiography: Initial Practial Experiences
and Future Directions of This Work-In-Progress", Vincent B. Ho, MD; Peter L. Choyke,
MD; Thomas K.F. Foo, PhD; Marueen N. Hood, BS; Donald L. Miller, MD; Julianna M.
Czum, MD; Alex M. Aisen, MD, Journal of Magnetic Resonance Imaging 10:376-388 (1999).
|
Primary Examiner: Mercader; Eleni Mantis
Attorney, Agent or Firm: Welsh & Katz, Ltd.
Parent Case Text
This application is a continuation-in-part of U.S. patent application Ser. No.
09/999,583 filed on Oct. 25, 2001 (pending).
Claims
1. An apparatus for forming a magnetic resonance angiographic image of a human
body comprising:
means for applying a first plurality of spatially non-selective radio-frequency
pulses to a first imaging volume of the human body;
means for applying a first plurality of combinations of magnitude of phase-encoding
gradients in slice-selective and in-plane directions through the first imaging
volume of the human body in conjunction with application of the first plurality
of spatially non-selective radio-frequency pulses;
means for detecting first magnetic resonance imaging data from the first imaging
volume of the human body based upon the first plurality of spatially non-seleqtive
radio-frequency pulses and the applied first plurality of combinations of magnitude
of phase-encoding gradients;
means for identifying a location of a body portion of the human body within the
first imaging volume based upon the detected first magnetic resonance imaging data;
means for applying a second plurality of spatially non-selective radio-frequency
pulses to a second imaging volume of the human body based upon the identified location
of the body portion;
means for applying a second plurality of combinations of magnitude of phase-encoding
gradients in slice-selective and in-plane directions through the second imaging
volume of the human body in conjunction with application of the second plurality
of spatially non-selective radio-frequency pulses;
means for detecting magnetic resonance imaging data from the second imaging volume
of the human body based upon the second plurality of spatially non-selective radio-frequency
pulses and second plurality of combinations of magnitude of phase-encoding gradients;
and
means for forming the magnetic resonance angiographic image of the second imaging
volume.
2. The apparatus for forming a magnetic resonance angiographic image as in claim
1 further comprising means for injecting a contrast agent into the human body.
3. The apparatus for forming a magnetic resonance angiographic image as in claim
1 further comprising means for encoding in-plane spatial information using one
of the group consisting of vastly undersampled projection reconstruction, Hadamard
encoding and wavelet encoding.
4. The apparatus for forming a magnetic resonance angiographic image as in claim
1 further comprising means for deleting any imaging slices within the first imaging
volume without the identified body portion of the human body.
5. A method of forming a magnetic resonance angiographic image of a human body
comprising the steps of:
applying a first plurality of spatially non-selective radio-frequency pulses
to a first imaging volume of the human body;
applying a first plurality of combinations of magnitude of phase-encoding gradients
in slice-selective and in-plane directions through the first imaging volume of
the human body in conjunction with application of the first plurality of spatially
pon-selective radio-frequency pulses;
detecting first magnetic resonance imaging data from the first imaging volume
of the human body based upon the first plurality of spatially non-selective radio-frequency
pulses and the first plurality of combinations of magnitude of phase-encoding gradients;
identifying a location of a body portion of the human body within the first imaging
volume based upon the detected first magnetic resonance imaging data;
applying a second plurality of spatially non-selective radio-frequency pulses
to a second imaging volume of the human body based upon the identified location
of the body portion;
applying a second plurality of combinations of magnitude of phase-encoding gradients
in slice-selective and in-plane directions through the second imaging volume of
the human body in conjunction with application of the second plurality of spatially
non-selective radio-frequency pulses;
detecting magnetic resonance imaging data from the second imaging volume of the
human body based upon the second plurality of spatially non-selective radio-frequency
pulses and second plurality of combinations of magnitude of phase-encoding gradients;
and
forming the magnetic resonance angiographic image of the second imaging volume.
6. The method of forming a magnetic resonance angiographic image as in claim
5 further comprising injecting a contrast agent into the human body.
7. The method of forming a magnetic resonance angiographic image as in claim
5 further comprising encoding in-plane spatial information using one of the group
consisting of vastly undersampled projection reconstruction, Hadamard encoding
and wavelet encoding.
8. The method of forming a magnetic resonance angiographic image as in claim
5 further comprising deleting any imaging slices within the first imaging volume
without the identified body portion of the human body.
9. The method of forming a magnetic resonance angiographic image as in claim
5 further comprising moving the human body relative to a stationary radiofrequency
antenna that detects the first and second magnetic resonance imaging data.
10. An apparatus for forming a magnetic resonance angiographic image of a human
body comprising:
a body coil adapted to apply a first plurality of spatially non-selective radio-frequency
pulses to a first imaging volume of the human body and a second plurality of spatially
non-selective radio-frequency pulses to a second imaging volume of the human body;
a controller adapted to apply a first plurality of combinations of magnitude
of phase-encoding gradients in a first slice-selective and a first in-plane directions
through the first imaging volume of the human body in conjunction with application
of the first plurality of spatially non-selective radio-frequency pulses;
a slice processor adapted to identify a presence and location of a body portion
of the human body within the first imaging volume and to delete any imaging slices
within the first imaging volume without the identified body portion of the human
body;
the controller adapted to apply a second plurality of combinations of magnitude
of phase-encoding gradients in a second slice-selective and a second in-plane directions
through the second imaging volume of the human body in conjunction with application
of the second plurality of spatially non-selective radio-frequency pulses based
upon the identified location of the body portion;
a receiver adapted to detect magnetic resonance imaging data from the first and
second imaging volumes of the human body based upon the first and second plurality
of spatially non-selective radio-frequency pulses and applied first and second
plurality of combinations of magnitude of phase-encoding gradients;
a signal processing subsystem adapted to position the human body and acquire
imaging data of the first and second imaging volumes; and
an imaging processor for forming the magnetic resonance angiographic image of
the second imaging volume.
11. The apparatus for forming a magnetic resonance angiographic image as in claim
10 comprising a Gss gradient field coil adapted to vary the first and second plurality
of combinations of magnitude of the first and second phase-encoding gradients in
the first and second slice-selective direction after each of the first and second
plurality of spatially non-selective radio frequency pulses.
12. The apparatus for forming a magnetic resonance angiographic image as in claim
10 comprising a Gpe gradient field coil adapted to vary the first and second plurality
of combinations of magnitude of the first and second phase-encoding gradients in
the first and second in-plane directions after each of the first and second plurality
of spatially non-selective radio-frequency pulses.
13. The apparatus for forming a magnetic resonance angiographic image as in claim
10 comprising a gradient controller adapted to very the first and second plurality
of combinations of magnitude of the phase-encoding gradients in the first and second
slice-selective directions and in the first and second in-plane directions after
each of the first and second plurality of spatially non-selective radio-frequency pulses.
14. The apparatus for forming a magnetic resonance angiographic image as in claim
10 further comprising an encoding processor adapted to encode in-plane spatial
information using one of the group consisting of vastly undersampled projection
reconstruction, Hadamard encoding and wavelet encoding.
15. The apparatus for forming a magnetic resonance angiographic image as in claim
10 further comprising a thickness processor adapted to limit imaging processing
to areas with the identified body portions of the human body.
Description
FIELD OF THE INVENTION
The field of the invention relates to computed tomography and more particularly
to magnetic resonance imaging.
BACKGROUND OF THE INVENTION
Arterial diseases and injuries are common and have severe consequences including
amputation or death. Atherosclerosis, in fact, is a major problem in the aged population,
particularly in the developed countries.
Atherosclerosis of the lower extremities (often, otherwise, referred
to as peripheral vascular disease) is a common disorder that increases with age,
ultimately affecting more than 20% of those people over the age of 75. Lesions
resulting from atherosclerosis are often characterized by diffuse and multi focal
arterial stenosis and occlusion.
Peripheral vascular disease often manifests itself as an intermittent
insufficiency or claudication of blood flow in calf, thigh or buttocks. The symptoms
of claudication often result from an inability of the body to increase blood flow
during exercise.
In more extreme cases of peripheral vascular disease, blood flow of even a resting
patient may be insufficient to meet basal metabolic needs of the extremities. Symptoms
of blood flow insufficiency in these areas may include pain in the forefoot or
toes or, in extreme cases, non-healing ulcers or gangrene in the affected limb.
One of the most effective means of diagnosing and treating atherosclerosis is
based upon the use of magnetic resonance angiography (MRA) to create images of
portions of the vascular system. As is well known, MRA is a form of magnetic resonance
imaging (MRI) which is especially sensitive to the velocity of moving blood. More
specifically, MRA generates images by relying upon an enhanced sensitivity to a
magnitude and phase of a signal generated by moving spins present within flowing blood.
MRA, in turn, can be divided into three types of categories: 1) time of flight
(TOF) or inflow angiography; 2) phase contrast (PC) angiography (related to the
phase shift of the flowing proton spins) and 3) dynamic gadolinium enhanced (DGE)
MRA. While the three types of MRA are effective, they all suffer from a number
of deficiencies.
The predominant deficiency of all three types of existing MRA techniques relates
to speed of data collection. For example, patient motion is known to significantly
degrade image quality of TOF MRA. To avoid image degradation, a patient undergoing
DGE MRA is typically required to hold his breath during data collection. PC MRA
relies upon the use of long time-to-echo (TE) intervals for signal sampling that
result in other T2 effects that tend to degrade image quality. Because of the importance
of MRA, a need exists for MRA methods that are less reliant upon time or upon movement
of the patient.
SUMMARY
A method and apparatus are provided for forming a magnetic resonance angiographic
image of a human body. The method includes the steps of applying a plurality of
spatially non-selective radio-frequency pulses of a relatively constant magnitude
to the body applying a plurality of substantially identical, frequency encoding
gradient pulse sequences to the body that correspond in number to the plurality
of radio frequency pulses in a fixed relationship and that are temporally non-coincident
with the radio-frequency pulses applying a plurality of different combinations
of phase-encoded gradients to the body that correspond to the plurality of radio
frequency pulses in a fixed relationship, that are temporally non-coincident with
the radio-frequency pulses and where each combination further comprises a first
pulse value in a slice selective direction and a second pulse value an in-plane
direction and detecting magnetic resonance imaging data from the body based upon
the spatially non-selective radio-frequency pulses and varied phase-encoded gradients.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a magnetic imaging angiography system in accordance
with an illustrated embodiment of the invention;
FIG. 2 depicts a series of imaging stations that may be used by the system of
FIG. 1; and
FIG. 3 depicts a pulse sequence that may be used by the system of FIG. 1.
DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT
FIG. 1 is a block diagram of a magnetic resonance imaging system
10 under
an illustrated embodiment of the invention. While the system
10 is amenable
to a number of different modes of use, one illustrated method provides faster contrast-enhanced
multi-station magnetic resonance angiography (MRA), that eliminates the need for
the acquisition of scout images and/or manual positioning of the patient for collecting
specific imaging volumes. As used herein, the term "scout images" refers to coarse
images collected specifically for the purpose of aligning imaging volumes among
imaging stations.
With conventional moving table MRA, MRA image data is collected at each of a
number (two or more) of imaging Stations (FIG. 2) in order to follow the passage
of an intravenously administered contrast agent. Because the data are typically
acquired over a volume of finite thickness that is smaller than the thickness of
the body part, the three-dimensional (3D) imaging volume must be carefully selected
(i.e., the imaging equipment must be carefully positioned) so as to encompass all
the major arteries. The process of acquiring scout images to locate the arteries
for positioning the equipment and then setting up the 3D acquisition at each imaging
station may require many minutes, which is inefficient, uncomfortable for the patient,
and may result in motion artifacts. Moreover, imperfect positioning of the imaging
equipment may result in the false impression of vessel occlusion.
The system
10 of FIG. 1 uses a unique combination of techniques discussed
in more detail below in order to permit the essentially foolproof acquisition of
MRA data from multiple stations without the need for obtaining scout images to
locate vessels or for manual positioning of each 3D imaging volume within an imaging
space. This may be accomplished by acquiring the imaging data through the entire
thickness of the body, rather than through only a finite thickness of the body.
The imaging data acquired through the thickness of the body by the system
10
may be obtained through relatively thin slices. Because the speed of acquisition
is relatively fast, the total time for data acquisition over the three imaging
stations of FIG. 2 may be less than 30 seconds. Because the data acquisition time
is relatively short, patient motion becomes less of a concern. T1 weighting may
be used in conjunction with the acquired data to discriminate contrast-enhanced
blood vessels. Arteries may be selectively displayed without enhancement of veins.
As shown in FIG. 1, the system
10 for collecting MRA images of a patient
18 may include three subsystems
12,
14,
16. A patient
movement subsystem
16 may be used to move the patient
18 among the
imaging stations of FIG.
2 and to control the movement of a patient transport
table within a scanning zone
20 of the system
10. A signal processing
subsystem
14 may provide the magnetic fields and control transmission and
detection of radio frequency (RF) signals from resonant atoms within the patient
18. A control subsystem
12 may provide programming and control of
the first and second subsystems
14,
16.
The first and second subsystems
14,
16 may be conventional. A body
coil
22 may be used for the transmission of RF pulses and to detect resonant
signals. First, second and third gradient field coils
24,
26,
28
may be used to create and control gradient magnetic fields within the body coil
22. A superconducting magnet
32 and shim coils
30 may be used
to provide a static magnetic field within the scanning zone
20.
In order to prepare the patient
18 for angiography, a contrast agent (e.g.,
gadolinium-chelate)
34 may be injected into the patient
18. The contrast
agent
34 may be administered using any appropriate method (e.g., hypodermic
needle). As the contrast agent
34 passes through the body, the patient movement
subsystem
16 may move the patient
18 through a series of overlapping
positions, such as those shown in FIG.
2.
To collect image data through the thickness of the body, a spatially non-selective
RF pulse may be applied through the body coil
22 without the necessity for
any, or only a relatively low level, slice selective gradient Gss that would otherwise
be applied at the same time as the RF pulse. Because of the relatively constant
frequency of the spatially non-selective RF pulse and the absence of phase-encoding
gradients, the spatially nonselective RF pulse need only be a fraction of the length
of a spatially selective RF pulse. Also, because of the short duration of the spatially
non-selective RF pulse, the minimum repetition time is much shorter. Repetition
rates of less than 3 milliseconds (ms), in fact, are possible using the spatially
non-selective RF pulse.
FIG. 3 depicts a 3D gradient-echo pulse sequence using the spatially non-selective
RF pulse. The RF pulse may remain relatively constant among pulse sequences, as
does the frequency encoding gradient Gfe and the timing of data collection through
the analog-to-digital converter (ADC). The absence of any slice selection gradient
during the RF pulse should be specifically noted in FIG.
3. The absence
of any slice selection gradient during the RF pulse allows the RF pulse to be spatially
non-selective in its effect on resonant atoms.
In order to collect data based upon each spatially non-selective RF pulse of
FIG.
3, the phase-encoding gradient Gss, in the slice direction and the phase-encoding
gradient Gpe in the in-plane direction may be varied by a gradient controller
36
in some predetermined manner. As used herein, varying the phase-encoding gradients
Gss, Gpe means applying a number of phase-encoded gradient combinations among pulse
sequences (after the RF pulse has ended) in the slice selective and in-plane directions
while collecting data for each combination under conditions of a constant frequency-encoding
gradient Gfe and constant three-dimensional spatially non-selective frequency pulses
RF among the pulse sequences.
For example, the full-scale range of the phase-encoding gradients in the slice
and also the in-plane directions may each be divided up into a number of incremental
steps (e.g., 64-256). Data may be collected by selecting a value for the first
phase-encoding gradient while varying a value of the second phase-encoding gradient.
After collecting data over a range of values for the second phase-encoding gradient,
a new value may be selected for the first phase-encoding gradient and the process
may be repeated until a full complement of data has been collected. A full complement
of data may mean collecting data for each combination of phase-encoded gradients
within an imaging area.
As a further, more detailed example, a lowest relative value may be chosen for
the first phase-encoding (e.g., the slice selective) gradient. Next a lowest relative
value of the second phase-encoding (e.g., the in-plane) gradient may be selected
and a first set of data may be collected using these two phase-encoding values
via the use of the sequence of FIG.
3. Following collection of the first
set of data, the phase-encoding value of the second phase-encoding gradient may
be incremented and a second set of data may be collected.
The process of incrementing the second phase-encoding gradient value (and collecting
data sets) may be repeated until a maximum gradient value is achieved for the second
phase-encoding gradient. Once the maximum value is achieved for the second phase-encoding
gradient, the first phase-encoding gradient may be incremented and the process
may be repeated. The process may be repeated by as many steps that it takes to
increment the first phase-encoding gradient from a minimum value to a maximum value.
The process of incrementing phase-encoded gradient levels in both the slice and
in-plane directions may be continued until data collection for a first body portion
(e.g., the body portion referred to as Station
1 in FIG. 2) has been completed.
The imaging system
10 may then wait a predetermined time period measured
by a timer
38 for the contrast medium to reach the next station. Following
the predetermined time period, the system
10 may move to overlapping Station
2 and the process may be repeated. Following data collection and another
predetermined time period, the system
10 may move to Station
3 and
the process may be again repeated. Following collection of imaging data, an imaging
processor
46 may form a set of images that corresponds to the collected data.
Because of the ability of the system
10 to form an image slice across
the entire thickness of the body, it is possible to automatically correlate a slice
collected at one station with a slice collected at another station. The fact that
the stations overlap also simplifies the comparison because correlation may simply
involve identifying the slice of one station that substantially matches a slice
collected through another adjacent station.
In order to further enhance processing efficiency, the system
10 may function
to identify the presence, location and thickness of any body portions of the patient
18 within each slice. Once identified, a thickness processor
40 of
the system
10 may function to limit image processing to the location and
to the thickness of any identified body portions.
As a first step, the system
10 may perform a coarse scan of each slice.
A slice processor
42 may then determine whether the slice passes through
any part of the body of the patient
18. The slice processor
42 may
make this determination by comparing a resonance value of each pixel of the slice
with a threshold value. If the resonance values of each pixel of the slice exceed
the threshold value (indicating that the slice does not pass through any body portions),
then the system
10 may discard the slice.
If it is determined that some part of the slice passes through the patient
18,
then the system
10 may group the pixels of the body portion(s) and identify
an outer boundary of the body portion(s) within the slice. As a first step, a thickness
processor
40 may determine a center of the body part (i.e., the center of
each significant group of pixels that do not exceed the threshold value). This
may be performed using a simple grouping and weighting algorithm.
The thickness processor
40 may then calculate the thickness of each body
portion based upon average resonance values of the pixels within the body portions
of the slice. To determine an average value, the processor
40 begins by
selecting a value at a center of the body portion as a first average value and
averaging outwards. As each new pixel value is examined, it is compared with the
average. If it is within a threshold value of the average, it may be incorporated
into the average. If it is not, then the pixel location and value may be segregated
as a potential boundary area of the body.
A line tracing routine may attempt to connect boundary pixel locations that exceed
the threshold (where each boundary pixel lies adjacent other pixel locations that
do not exceed the threshold). If the line tracing routine is able to successfully
trace-a continuous line around the center of the slice, then the line is assumed
to define the boundary of the portion of the body
18 within the slice. The
diameter of the traced boundary line defines the thickness of the body portion
within the slice.
The voltage of the spatially non-selective RF pulse may be adjusted to produce
a relative large flip-angle (e.g., 15-60 degrees). Further, the large number of
phase-encoding steps have been found to provide relatively thin sections despite
the relatively large excitation volume.
To further improve imaging integrity a number of different types of saturation
pulses may be applied. For example, a non-selective RF saturation pulse may be
applied at regular intervals to preferentially reduce signal intensity from non-vascular
structures. Alternatively, a chemical shift-selective RF saturation pulse may be
applied at regular intervals to preferentially reduce the signal intensity from
fat-containing tissue.
Further, gradient or RF spoiling may be used to disperse transverse magnetization.
The dispersion of transverse magnetization may be used as a method of improving
T1 contrast.
In-plane spatial information may be handled in any of a number of ways.
Conventional methods may be used in some cases, or the in-plane spatial information
may be encoded by an encoding processor
44 using non-standard techniques.
Such techniques may include, but are not limited to, vastly undersampled projection
reconstruction (VIPR) Hadamard encoding and wavelet encoding.
A number of previously known imaging techniques may also be used to further enhance
imaging integrity. For example, partial Fourier imaging may be used where appropriate.
Alternatively, parallel imaging (e.g., SENSE or SMASH-like techniques) may also
be used. A rectangular field of view may be imposed to optimize imaging data.
Data may be acquired repeatedly so as to create a series of temporally-distinct
MR angiograms spanning parts or all of the time course of the passage of the contrast
agent through the vessels of interest. This can be accomplished by any of a variety
of techniques, including the use of a very short repetition time, partial k-space
acquisition, or other methods of k-space coverage such as "TRICKS" or "keyhole
imaging" methods.
Further, image enhancement may be used to improve upon the data actually
collected. For example, data may be interpolated along the slice direction to enhance
small objects and eliminate discontinuities.
Before and after images may also be used. Acquisition of a series of "mask"
images may be collected before administration of contrast material. The mask images
may be used to mask out unwanted structures.
Further, the use of measured data values may also be used for image enhancement.
Magnitude or complex data subtraction may be used to highlight contrasted-enhanced
areas over areas without contrast enhancement.
Accurate table positioning has been recognized as an important factor in
image subtraction. However, since the images of adjacent stations can be easily
correlated, image subtraction becomes relatively simply using the data provided
by the system
10.
Image subtraction has been shown to provide improved arterial imaging by eliminating
spurious signal artifacts (e.g., phase wrap, venous in-flow enhancement, etc.).
Image subtraction has been found to have great value in the imaging of the distal
tibioperoneal arteries.
In another embodiment, the patient
18 may remain stationary and the movement
subsystem
16 may move the scanning zone
20. Alternatively, the patient
18 moves, but the radio frequency antenna (e.g., surface coil or phased
array coil)
22 used to receive the signal remains stationary.
A specific embodiment of a method and apparatus for performing magnetic resonance
angiography has been described for the purpose of illustrating the manner in which
the invention is made and used. It should be understood that the implementation
of other variations and modifications of the invention and its various aspects
will be apparent to one skilled in the art, and that the invention is not limited
by the specific embodiments described. Therefore, it is contemplated to cover the
present invention and any and all modifications, variations, or equivalents that
fall within the true spirit and scope of the basic underlying principles disclosed
and claimed herein.
*
