U.S. patent application number 13/684199 was filed with the patent office on 2013-04-04 for arterial spin-labeled (asl) multiplexed echo planar imaging (m-epi).
The applicant listed for this patent is David Feinberg. Invention is credited to David Feinberg.
Application Number | 20130085379 13/684199 |
Document ID | / |
Family ID | 47993247 |
Filed Date | 2013-04-04 |
United States Patent
Application |
20130085379 |
Kind Code |
A1 |
Feinberg; David |
April 4, 2013 |
ARTERIAL SPIN-LABELED (ASL) MULTIPLEXED ECHO PLANAR IMAGING
(M-EPI)
Abstract
An MRI system and method for imaging perfusion in an arterial
spin labeled (ASL) process in which multiplexed echo-planar imaging
(M-EPI) is used rather than conventional EPI, to thereby speed up
imaging and generate sets of images that show different phases of
perfusion and provide additional benefits. A single multiband RF
excitation pulse can be used to excite multiple slices for imaging,
or a time sequence of multiband pulses can be used to further
increase the number of slices.
Inventors: |
Feinberg; David;
(Sebastapol, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Feinberg; David |
Sebastapol |
CA |
US |
|
|
Family ID: |
47993247 |
Appl. No.: |
13/684199 |
Filed: |
November 22, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US11/57161 |
Oct 20, 2011 |
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13684199 |
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13632941 |
Oct 1, 2012 |
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PCT/US11/57161 |
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13397634 |
Feb 15, 2012 |
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13632941 |
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61394929 |
Oct 20, 2010 |
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61443215 |
Feb 15, 2011 |
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61444031 |
Feb 17, 2011 |
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61444039 |
Feb 17, 2011 |
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Current U.S.
Class: |
600/419 |
Current CPC
Class: |
G01R 33/56366 20130101;
G01R 33/5616 20130101; A61B 5/0263 20130101; G01R 33/543 20130101;
A61B 5/0042 20130101; G01R 33/4835 20130101 |
Class at
Publication: |
600/419 |
International
Class: |
A61B 5/026 20060101
A61B005/026 |
Claims
1. A magnetic resonance imaging (MRI) method of imaging perfusion
in a patient using arterial spin labeling (ASL) and multiplexed
echo planar imaging (M-EPI) comprising: positioning a patient in an
MRI scanner; applying arterial spin labeling to a selected portion
of the patient's anatomy; applying a first multiband RF excitation
pulse that essentially simultaneously excites multiple selected
slices of the patient's anatomy; applying magnetic gradients to the
patient; essentially simultaneously acquiring a first set of
magnetic resonance (MR) signals generated for the multiple selected
slices in response to the first multiband RF excitation pulse in an
M-EPI process; said acquiring being selectively timed relative to
arterial, capillary and/or venous perfusion in the patient related
to the applying of arterial labeling; and computer-processing the
first set of MR signals to generate a first set of ASL perfusion
images of the slices and display the imaged on a computer
display.
2. The method of claim 1 further including: applying a second
multiband RF excitation pulse after applying the first one, to
thereby again excite the multiple selected slices but in a
subsequent phase of said perfusion, and essentially simultaneously
acquiring a second set of MR signals generated for the multiple
selected slices in response to the second multiband RF excitation
pulse; wherein the computer system is further configured to
computer-process the second set of MR signals to generate a second
set of ASL perfusion images of the slices, and wherein the first
and second set of images show respective different phases of the
perfusion.
3. The method of claim 2 further including applying additional
multiband RF excitation pulses in a time succession after the first
and second RF excitation pulses to excite the slices at respective
different times, acquiring additional sets of MR signals
essentially simultaneously for said slices at times related to the
respective additional RF excitation pulses, and including said
additional sets of MR signals in said computer-processing to
thereby generate additional sets of slice images showing respective
additional phases of the perfusion.
4. A magnetic resonance imaging (MRI) system for imaging perfusion
in a patient using arterial spin labeling (ASL) and multiplexed
echo planar imaging (M-EPI) comprising: an MRI scanner having an
imaging volume for a patient; an arterial spin labeling source
configured to apply ASL pulses to the patient in the imaging
volume; RF coils configured to selectively apply a first multiband
RF excitation pulse that essentially simultaneously excites
multiple selected slices of the patient in the imaging volume;
magnetic gradient coils configured to apply magnetic gradients to
the patient in the imaging volume; MR signal acquisition coils
configured to essentially simultaneously acquire magnetic resonance
(MR) signals generated for the multiple selected slices in response
to the first multiband RF excitation pulse in an M-EPI process;
said acquiring being selectively timed relative to arterial,
capillary and/or venous perfusion in the patient related to the
applying of arterial labeling; and a computer system configured to
computer-processing the MR signals to generate a first set of ASL
perfusion images of the slices.
5. The system of claim 4 in which: the RF coils are further
configured to apply a second multiband RF excitation pulse after
applying the first one, to thereby again excite the multiple
selected slices but in a subsequent phase of said perfusion, the MR
signal acquisition coils are further configured to essentially
simultaneously acquire a second set of magnetic resonance (MR)
signals generated for the multiple selected slices in response to
the second multiband RF excitation pulse; and the computer system
is further configured to computer-process the second set of MR
signals to thereby generate a second set of ASL perfusion images of
the slices, wherein the first and second set of images show
respective phases of the perfusion.
6. The system of claim 5 in which: the RF coils are further
configured to apply a sequence of additional multiband RF
excitation pulses after applying the first and second RF excitation
pulses, to thereby excite the multiple selected slices in
respective subsequent phase of said perfusion; the MR signal
acquisition coils are further configured to essentially
simultaneously acquire additional sets of MR signals generated for
the multiple selected slices in response to the respective
additional multiband RF excitation pulses; and the computer system
is further configured to computer-process the additional sets of MR
signals to thereby generate additional sets of ASL perfusion images
of the slices that show additional respective phases of the
perfusion.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY
REFERENCE
[0001] This patent application is a continuation-in-part of (a) PCT
International Application No. PCT/US11/57161, filed Oct. 20, 2011,
which claims the benefit of U.S. Provisional Application No.
61/394,929, filed Oct. 20, 2010, and (b) U.S. patent application
Ser. No. 13/632,941, filed Oct. 1, 2012, which is a continuation of
U.S. patent application Ser. No. 13/397,634, filed Feb. 15, 2012,
which claims the benefit of U.S. Provisional Application Nos.
61/443,215, filed Feb. 15, 2011, 61/444,031, filed Feb. 17, 2011,
and 61/444,039, filed Feb. 17, 2011. This patent specification
incorporates by reference the entire contents of each of these
applications, including their drawings and the appendices attached
thereto.
FIELD
[0002] This patent specification is in the field of magnetic
resonance imaging (MRI). More specifically it pertains to imaging
tissue perfusion with MRI.
BACKGROUND AND SUMMARY OF THE DISCLOSURE
[0003] Arterial spin labeling (ASL) is a technique for imaging
tissue perfusion with MRI. It is generally limited by
signal-to-noise ratio (SNR) due to the small (e.g., 3%) fraction of
blood in tissue volumes so the MR signal is proportionately small.
Another limitation is that tissue blood is moving through different
vascular compartments including arterial, capillary and venous
compartments, so the time window of imaging can be constrained to a
few hundred milliseconds for the capillary compartment. ASL labels
the blood with an inversion pulse, followed by a delay and then
readout of image slices. The is repeated with and without inversion
labeling so that differences between the two acquired signals would
tend to null the static signal whereas the inversion labeled blood
would be separated and remain in the image. Echo planar imaging
(EPI) has been used as a readout image for the ASL perfusion
technique but is limited in the number of slices that can be imaged
due to the normal physiological time course of labeled blood moving
through the different vascular compartments. For that reason
repeated ASL labeling inversion pulses may be utilized to acquire
many images and slices, consistent with physiological time
constraints. ASL with conventional multi-slice 2D EPI has not been
able to satisfactorily image perfusion in the entire brain because
of timing limitations. 3D imaging techniques, including 3D GRASE,
have therefore been developed as an alternative to 2D EPI. However,
2D images have certain desirable characteristics compared with 3D
images, and it would be desirable to find an effective way to
accurately record perfusion in 2D images.
[0004] In order to retain the benefits of 2D perfusion images, it
has been discovered that it is not only practical but also brings
about surprising benefits to replace conventional EPI in ASL
techniques with multiplexed EPI that uses multiband (MB) RF
excitation pulses. The technique of using MB RF excitation pulses
in EPI imaging can be called M-EPI in this patent specification.
ASL perfusion imaging with M-EPI can be several times faster than
ASL with conventional EPI because M-EPI records images for several
2D slices essentially simultaneously, and the location of the blood
is the same for all images, whereas this is not true in
conventional, time sequential acquisition of multiple EPI images.
Further improvements can be achieved by using a time sequence of
multiband excitation pulses, so that the number of slices that are
imaged in a single pulse sequence is the product of the number of
slices for each multiband RF pulses times the number of multiband
RF pulses in a single pulses sequence. For example, this product
can be 2.times.2, 2.times.8, 3.times.8, etc., where the first
number is the number of slices per multiband RF pulse and the
second is the number of multiband pulses used in time succession in
a single pulse sequence. Of note, a single multiband RF pulse can
be used so the number of slices M=MB where MB is the number of
simultaneously excited slices using a single multiband RF pulse.
See said U.S. patent application Ser. No. 13/397,634, filed Feb.
15, 2012 (including its Appendices A-C). The technique of using a
sequence of RF excitation pulses to image multiple 2D slices in a
single pulse sequence can be called simultaneous image refocusing
(SIR) or simultaneous echo refocusing (SER).
[0005] The replacement of EPI with M-EPI in ASL imaging has several
advantages. For example, the complex interdependence of slice
saturation effects on arterial input functions and recorded MR
signal is greatly improved as far fewer time sequential echo trains
are utilized. The time sequential echo trains can be reduced by a
factor of N (the number of simultaneously images slices) in M-EPI
compared with conventional EPI. With ASL using M-EPI combined with
SIR, the entire brain can be scanned in, e.g., in 400 ms or less
time with 30 to 60 images, as determined by N images=SIR.times.MB
factors. If SIR=1, then N=MB alone, and this is one example
included in the scope of the new process described in this patent
specification. Ultimately, all slices can be acquired in a single
echo train if the SIR and/or MB factors are large enough for the
product to create enough slices to cover the entire brain or at
least the portion of interest. It can be very important to cover
the entire brain or organ, or at least the portion of interest,
with slices to record the entire state of perfusion. This has not
been possible for the entire brain with ASL based on conventional
EPI due to the limited time window for measuring the hemodynamic
perfusion effects because it was not believed possible to acquire
enough conventional EPI images to cover the entire brain. Equally
limiting to ASL using conventional EPI in this relationship to
hemodynamics, is that the EPI images are time sequential and so
even during a limited total scan time to make 10 images within an
acceptable time window, each image is at a different temporal delay
with respect to the initial blood labeling inversion pulse, so that
each image actually has a different inflow time and this gives
error in calculating their combined measurement of perfusion in the
brain.
[0006] Another benefit of using ASL with M-EPI, is that the time to
scan the brain can be reduced to such a great amount, for example
in the range of 6 to 30 times faster than with EPI, that multiple
measurements of MR signals can be made after the labeling inversion
pulse. This can result in a series of inflow time-dependent
measurements which can sample the blood as it moves through
different vascular compartments. The MR signal data can be combined
to improve SNR or accuracy and used to model the different transit
times for useful hemodynamic parameters to describe disease states
within the brain or other organ. Many different variants of ASL
labeling schemes can be combined with M-EPI and/or SIR.
[0007] Note that ASL imaging techniques typically utilize two
identical acquisitions differing in one being a Labeling sequence
(L) in which the blood is labeled and a control sequence (C) in
which the blood is not labeled but all other parameters are
unchanged so that L-C gives a measure of blood labeled with
subtraction of other sources of signal that are constant in the two
sequence acquisitions.
BRIEF DESCRIPTION OF THE DRAWING
[0008] FIG. 1 illustrates MR signal acquisition in arterial spin
labeling (ASL) using conventional echo-planar imaging (EPI).
[0009] FIGS. 2a and 2b illustrate a comparison between ASL using
conventional multi-slice EPI (FIG. 2a) and ALS using a multi-band
RF excitation pulse to carry out M-EPI in which MR signals for
multiple slices are acquired essentially simultaneously.
[0010] FIG. 3 illustrates a similar comparison related to phases of
perfusion in tissue and changes in the value of an ASL signal with
time. The upper portion of FIG. 4 illustrates the case of using ASL
with conventional EPI to acquire MR signals for multiple slices
while the lower portion illustrates the case of using a single
multiband RF excitation signal to essentially concurrently acquire
MR signals for the same number of multiple slices. In each case,
acquisition time is related to changes in perfusion with time.
[0011] FIG. 4 illustrates ALS using a time sequence of multiband RF
excitation pulses to acquire, in time sequence, blocks of MR
signals where each block is for multiple, essentially
simultaneously acquired slices but different blocks correspond to
different perfusion phases.
[0012] FIG. 5(A) illustrates the use of EPI limited by time of
physiological phase which limits number of ASL images.
[0013] FIG. 5(B) illustrates the use of M-EPI that increases the
number of obtainable images within the same time window to achieve
much greater coverage of the entire brain.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0014] FIG. 1 illustrates conventional multi-slice 2D EPI in which
several slice images are acquired one after the other in time. The
different blood inflow times in consecutively acquired slice images
limit the accuracy of blood flow quantization. Different vascular
compartments are filled at different times, resulting in
undesirable coupling to spatial slice position.
[0015] FIGS. 2a and 2b illustrate important differences between ASL
multi-slice EPI imaging and ASL multiplexed EPI imaging. The
example of FIG. 2a illustrates that MR signals for multiple 2D
slice images are obtained in time sequence over a time span that is
much longer than that seen in FIG. 2b, where MR signals for the
same number of multiple 2D slice images are obtained using M-EPI
essentially simultaneously.
[0016] FIG. 3 illustrates some important consequences of
differences between ASL using conventional EPI and the new approach
of using ASL with M-EPI. With the old approach, the acquisition
time of MR signals for the illustrated multiple images (top part of
FIG. 3) extends over an excessively long portion of the delay time
inherent in perfusion through different vascular compartments
(arteries and capillaries). However, with the new approach of ASL
using M-EPI (with or without using SIR as well), the time to
acquire the same number of slice images is much shorter and can
reflect perfusion parameters and their relationships much more
accurately. While an example of 36 slice images is shown in FIG. 3
for the case of the prior technique of ASL using conventional EPI,
in fact typically only 8 to 10 slice images were acquired in
practice to give a reasonable estimate of perfusion parameters but
for less than the entire brain or organ of interest. Using selected
timing the images for ASL with M-EPI can be acquired for the
intra-capillary phase (as illustrated in FIG. 3, middle portion),
or for the intra-arterial phase, or for some other phase.
[0017] FIG. 4 illustrates one of the important benefits of the new
ASL with M-EPI approach, in which perfusion parameters for the
entire brain or organ of interest can be obtained so rapidly that
scans of the organ can be taken repeatedly to thereby view dynamic
changes and exchanges of blood between arterial, capillary and
venous compartments. As seen in FIG. 4, MR signals are taken in 14
time-sequential blocks, where the MR signals for 36 slices are
essentially simultaneously acquired in each block. Because the
blocks are sequential in time, each block can result in images for
36 slices that show a respective phase of perfusion. FIG. 4
illustrates a case in which imaging starts as intra-capillary
perfusion accelerates, but the MR signal acquisition can be timed
to cover any desired phase in arterial, capillary or venous
perfusion.
[0018] FIGS. 5(A) and 5(B) illustrate an important difference
between previously known ASL EPI imaging and the use of M-EPI to
obtain additional slices to cover a larger region of the brain than
would be achievable using current EPI imaging.
[0019] In FIG. 5(A), which illustrates the previously known
process, the time duration is limited to the window of time in
which blood is primarily within the vascular compartment. Using
EPI, typically N=8 images can be acquired in this time window which
can then only cover N.times.slice thickness for the extent of
slices covering the brain.
[0020] FIG. 5(B) illustrates that the use of M-EPI can increase the
extent of slice coverage to equal N.times.M where M is the number
of simultaneously acquired slices. In other words, M-EPI gives much
greater slice coverage, even possibly whole brain coverage within
the physiological limited time window of choosing a particular
vascular compartment such as capillary perfusion phase. Current ASL
EPI techniques are limited to a few slices due to the limited time
window so this would greatly improve the utility of the
technique.
[0021] ASL M-EPI imaging as described in this patent specification
may be performed with or without SIR. Expressed more generally, an
MRI system using the teachings of this patent specification can
generate ALS MR signals for multiple slices using any one of (a) RF
excitation pulses that are multiband pulses to essentially
simultaneously excite plural slices of the patient's anatomy, (b) a
time sequence of two or more multiband RF excitation pulses each of
which essentially simultaneously excites multiple slices and the
time sequence of which excites a number of slices equal to the
product of the bands in each multiband pulse times the number of
multiband pulses in the sequence, and (c) a time sequence of RF
excitation pulses that are not multiband pulses. Different MRI
scanner hardware may have different RF coils and gradients so that
it may be desirable to perform M-EPI in these more limited ways, in
which ASL M-EPI (without SIR) would still have advantage of a
reduced scan time.
[0022] An explanation and illustrations of M-EPI and SIR pulse
sequences, and MRI scanners using them, can be found in the PCT
application and the U.S. application that re incorporated by
reference in this patent specification. In addition, the following
papers may provide useful background and are hereby incorporated by
reference: [0023] 1. Barbier E L, et al., Perfusion Imaging Using
Dynamic Arterial Spin Labeling (DASL), Magnetic Resonance in
Medicine 45:1021-1021 (2001); [0024] 2. Wang Y, Regional
reproducibility of pulsed arterial spin labeling perfusion imaging
at 3T, NeuroImage 54 (2011) 1188-1195; [0025] 3. Wang J, Reduced
susceptibility effects in perfusion fMRI with single-shot spin-echo
EPI acquisitions at 1.4 Tesla, Magnetic Resonance Imaging 22 (2004)
1-7; and [0026] 4. Donahue M J, et al., Cerebral blood flow, blood
volume, and oxygen metabolism dynamics in human visual and motor
cortex as measured by whole-brain multi-modal magnetic resonance
imaging, Journal of Cerebral Blood Flow & Metabolism (2009) 29,
1856-1866.
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