U.S. patent application number 16/388931 was filed with the patent office on 2020-10-22 for simultaneous proton resonance frequency shift thermometry and t1 measurements using a single reference variable flip angle t1 method.
The applicant listed for this patent is University of Utah Research Foundation. Invention is credited to Dennis L. Parker, Allison Payne, Bryant Svedin.
Application Number | 20200333417 16/388931 |
Document ID | / |
Family ID | 1000004065685 |
Filed Date | 2020-10-22 |
![](/patent/app/20200333417/US20200333417A1-20201022-D00000.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00001.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00002.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00003.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00004.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00005.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00006.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00007.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00008.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00009.png)
![](/patent/app/20200333417/US20200333417A1-20201022-D00010.png)
View All Diagrams
United States Patent
Application |
20200333417 |
Kind Code |
A1 |
Svedin; Bryant ; et
al. |
October 22, 2020 |
SIMULTANEOUS PROTON RESONANCE FREQUENCY SHIFT THERMOMETRY AND T1
MEASUREMENTS USING A SINGLE REFERENCE VARIABLE FLIP ANGLE T1
METHOD
Abstract
A computer implemented method for measuring T.sub.1 in an
anatomical region of interest during a dynamic procedure includes
acquiring a reference MR image of the anatomical region of interest
using a first flip angle. A first set of dynamic MR images of the
anatomical region of interest are acquired using a second flip
angle. The reference MR image and the first set are used to
calculate a reference T.sub.1 value for tissue in the anatomical
region of interest. During an intervention where the T.sub.1 value
may change, a second set of dynamic MR images of the anatomical
region of interest is acquired using the second flip angle. The
reference MR image and the second set are used to calculate an
estimated T.sub.1 value. The reference T.sub.1 value, the estimated
T.sub.1 value, and the first and second flip angles may then be
used to correct the estimated T.sub.1 value.
Inventors: |
Svedin; Bryant; (West
Jordan, UT) ; Parker; Dennis L.; (Centerville,
UT) ; Payne; Allison; (Salt Lake City, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Utah Research Foundation |
Salt Lake City |
UT |
US |
|
|
Family ID: |
1000004065685 |
Appl. No.: |
16/388931 |
Filed: |
April 19, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 33/4828 20130101;
G01R 33/5615 20130101; G01R 33/4824 20130101 |
International
Class: |
G01R 33/561 20060101
G01R033/561; G01R 33/48 20060101 G01R033/48 |
Claims
1. A computer implemented method for measuring T.sub.1 in an
anatomical region of interest during a dynamic procedure, the
method comprising: acquiring a reference magnetic resonance (MR)
image of the anatomical region of interest using a first flip
angle; acquiring a first set of dynamic MR images of the anatomical
region of interest using a second flip angle that is distinct from
the first flip angle, wherein the reference MR image and the first
set of dynamic MR images are each acquired while the anatomical
region of interest was at substantially the same temperature; using
the reference MR image and the first set of dynamic MR images to
calculate a reference T.sub.1 value for tissue in the anatomical
region of interest; during an intervention where the T.sub.1 value
may change, acquiring a second set of dynamic MR images of the
anatomical region of interest using the second flip angle; using
the reference MR image and the second set of dynamic MR images to
calculate an estimated T.sub.1 value; and using the reference
T.sub.1 value, the estimated T.sub.1 value, and the first and
second flip angles to correct the estimated T.sub.1 value to take
into account the change in T.sub.1 value due to the intervention to
obtain an updated T.sub.1 value for the dynamic MR images.
2. The method of claim 1, further comprising: generating a
T.sub.1-weighted image of the anatomical region of interest using
the dynamic MR images and the updated T.sub.1 value.
3. The method of claim 1, further comprising: using phase
information from the dynamic MR images to generate proton resonance
frequency (PRF)-based temperature images of aqueous based tissues
in the anatomical region of interest.
4. The method of claim 1, further comprising: producing a
temperature map of the anatomical region of interest using the
PRF-based temperature images.
5. The method of claim 1, wherein acquisition of dynamic MR images
is performed using a multi-echo pseudo golden angle stack of stars
(SOS) acquisition.
6. The method of claim 1, wherein the dynamic MR images are
reconstructed from k-space data collected during the acquisition
using a k-space weighted image contrast (KWIC) reconstruction
method.
7. The method of claim 6, wherein the KWIC reconstruction method
uses a sliding window of projections moved by a plurality of
productions between each reconstruction performed to generate the
dynamic MR images.
8. The method of claim 1, wherein the second flip angle used for
acquisition of the dynamic MR images is the Ernst angle of the
adipose tissue prior to the temperature change.
9. The method of claim 1, wherein the thermal therapy procedure is
MR-guided focused ultrasound (MRgFUS).
10. An article of manufacture for measuring T.sub.1 in an
anatomical region of interest during a thermal therapy procedure,
the article of manufacture comprising a non-transitory, tangible
computer-readable medium holding computer-executable instructions
for performing a method comprising: receiving a reference magnetic
resonance (MR) image of the anatomical region of interest using a
first flip angle; receiving a first set of dynamic MR images of the
anatomical region of interest using a second flip angle that is
distinct from the first flip angle, wherein the reference MR image
and the first set of dynamic MR images are each acquired while the
anatomical region of interest was at substantially the same
temperature; using the reference MR image and the first set of
dynamic MR images to calculate a reference T.sub.1 value for tissue
in the anatomical region of interest; receiving a second set of
dynamic MR images of the anatomical region of interest using the
second flip angle, wherein the second set of dynamic MR images are
acquired during an intervention where the T.sub.1 value may change;
using the reference MR image and the second set of dynamic MR
images to calculate an estimated T.sub.1 value; and using the
reference T.sub.1 value, the estimated T.sub.1 value, and the first
and second flip angles to correct the estimated T.sub.1 value to
take into account the change in T.sub.1 value due to the
intervention to obtain an updated T.sub.1 value for the dynamic MR
images.
11. The article of manufacture of claim 10, wherein the method
further comprises: generating a T.sub.1-weighted image of the
anatomical region of interest using the dynamic MR images and the
updated T.sub.1 value.
12. The article of manufacture of claim 10, wherein the method
further comprises: using phase information from the dynamic MR
images to generate proton resonance frequency (PRF)-based
temperature images of aqueous based tissues in the anatomical
region of interest.
13. The article of manufacture of claim 10, wherein the method
further comprises: producing a temperature map of the anatomical
region of interest using the PRF-based temperature images.
14. The article of manufacture of claim 10, wherein acquisition of
dynamic MR images is performed using a multi-echo pseudo golden
angle stack of stars (SOS) acquisition.
15. The article of manufacture of claim 10, wherein the dynamic MR
images are reconstructed from k-space data collected during the
acquisition using a k-space weighted image contrast (KWIC)
reconstruction method.
16. The article of manufacture of claim 15, wherein the KWIC
reconstruction method uses a sliding window of projections moved by
a plurality of productions between each reconstruction performed to
generate the dynamic MR images.
17. The article of manufacture of claim 10, wherein the second flip
angle used for acquisition of the dynamic MR images is the Ernst
angle of the adipose tissue prior to the temperature change.
18. The article of manufacture of claim 10, wherein the thermal
therapy procedure is MR-guided focused ultrasound (MRgFUS).
19. A system for measuring T.sub.1 in an anatomical region of
interest during a thermal therapy procedure, the system comprising:
a plurality of imaging coils used to: acquire a reference magnetic
resonance (MR) image of the anatomical region of interest using a
first flip angle; acquire a first set of dynamic MR images of the
anatomical region of interest using a second flip angle that is
distinct from the first flip angle, wherein the reference MR image
and the first set of dynamic MR images are each acquired while the
anatomical region of interest was at substantially the same
temperature; during an intervention where the T.sub.1 value may
change, acquire a second set of dynamic MR images of the anatomical
region of interest using the second flip angle; a control computer
configured to: use the reference MR image and the first set of
dynamic MR images to calculate a reference T.sub.1 value for tissue
in the anatomical region of interest; use the reference MR image
and the second set of dynamic MR images to calculate an estimated
T.sub.1 value; and use the reference T.sub.1 value, the estimated
T.sub.1 value, and the first and second flip angles to correct the
estimated T.sub.1 value to take into account the change in T.sub.1
value due to the intervention to obtain an updated T.sub.1 value
for the dynamic MR images.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to performing
simultaneous proton resonance frequency (PRF) shift and T.sub.1
measurements with equivalent temporal resolution using a single
reference variable flip angle (VFA) method. The techniques
described herein facilitate, among other things, simultaneous
thermometry in both aqueous and fatty tissue.
BACKGROUND
[0002] Magnetic resonance temperature imaging (MRTI) has been used
to monitor several kinds of thermal therapies including
radiofrequency, microwave, laser and MR-guided focused ultrasound
(MRgFUS). MRgFUS has been used to noninvasively treat breast,
prostate, liver and brain cancers as well as essential tremor and
Parkinson's disease. In order to ensure treatment safety and
efficacy, it is desirable that MRTI be able to monitor the entire
heated tissue volume with enough temporal resolution to accurately
follow the most rapid temperature changes. The different types of
thermal therapy pose different challenges in terms of the needed
coverage and acquisition speed. MRgFUS is especially challenging
because the ultrasound often traverses a large volume of normal
tissue before being focused to a point where rapid heating occurs.
Thus, ideally, for MRgFUS, MRTI would monitor the focus and the
near and far-fields with full coverage and high temporal
resolution.
[0003] The PRF shift method has been widely adopted due to its
linearity over the temperature range of interest, the constant of
proportionality being largely independent of tissue type (except
adipose tissue), and its ability to produce temperature maps with
the spatial and temporal resolution required to monitor treatments
in real time. There have been several successful implementations of
fully 3D thermometry methods using the proton resonance frequency
(PRF) shift thermometry technique. However, many FUS treatment
targets are surrounded by or have adipose tissue in the near field
(e.g., breast, abdominal targets) and PRF thermometry is unable to
monitor temperature changes in adipose tissue. Thus, it is desired
to provide a method of that allows for simultaneous thermometry in
both aqueous and adipose tissue.
SUMMARY
[0004] Embodiments of the present invention address and overcome
one or more of the above shortcomings and drawbacks, by providing
methods, systems, and apparatuses related to the simultaneous
proton resonance frequency shift thermometry and T.sub.1
measurements using a single reference variable flip angle T.sub.1
method. Briefly, the techniques described herein allow for rapid
simultaneous PRF and T.sub.1 thermometry techniques, providing
T.sub.1 images with the temporal resolution equivalent to the
magnitude image. The disclosure that follows presents the theory
behind single reference VFA T.sub.1 calculations and the optimal
sequence parameters for simultaneous PRF and single reference VFA
T.sub.1, determined using Monte Carlo simulations of noisy signal
as a function of flip angle.
[0005] According to some embodiments, a computer implemented method
for measuring T.sub.1 in an anatomical region of interest during a
dynamic procedure includes acquiring a reference magnetic resonance
(MR) image of the anatomical region of interest using a first flip
angle. A first set of dynamic MR images of the anatomical region of
interest are acquired using a second flip angle that is distinct
from the first flip angle. The reference MR image and the first set
of dynamic MR images are each acquired while the anatomical region
of interest was at substantially the same temperature. The
reference MR image and the first set of dynamic MR images are used
to calculate a reference T.sub.1 value for tissue in the anatomical
region of interest. During an intervention where the T.sub.1 value
may change, a second set of dynamic MR images of the anatomical
region of interest is acquired using the second flip angle. The
reference MR image and the second set of dynamic MR images are used
to calculate an estimated T.sub.1 value. The reference T.sub.1
value, the estimated T.sub.1 value, and the first and second flip
angles may then be used to correct the estimated T.sub.1 value to
take into account the change in T.sub.1 value due to the
intervention to obtain an updated T.sub.1 value for the dynamic MR
images.
[0006] In other embodiments, an article of manufacture for
measuring T.sub.1 in an anatomical region of interest during a
thermal therapy procedure comprises a non-transitory, tangible
computer-readable medium holding computer-executable instructions
for performing the method discussed above.
[0007] According to another aspect of the present invention, a
system for measuring T.sub.1 in an anatomical region of interest
during a thermal therapy procedure comprises a plurality of imaging
coils and a control computer. The imaging coils are used to perform
at least 3 acquisitions. First, a reference MR image of the
anatomical region of interest is acquired using a first flip angle.
Second, a first set of dynamic MR images of the anatomical region
of interest is acquired using a second flip angle that is distinct
from the first flip angle. The reference MR image and the first set
of dynamic MR images are each acquired while the anatomical region
of interest was at substantially the same temperature. Third,
during an intervention where the T.sub.1 value may change, a second
set of dynamic MR images of the anatomical region of interest is
acquired using the second flip angle. The control computer uses the
reference MR image and the first set of dynamic MR images to
calculate a reference T.sub.1 value for tissue in the anatomical
region of interest. The reference MR image and the second set of
dynamic MR images are used to calculate an estimated T.sub.1 value.
Then, the reference T.sub.1 value, the estimated T.sub.1 value, and
the first and second flip angles may be used to correct the
estimated T.sub.1 value to take into account the change in T.sub.1
value due to the intervention to obtain an updated T.sub.1 value
for the dynamic MR images.
[0008] Additional features and advantages of the invention will be
made apparent from the following detailed description of
illustrative embodiments that proceeds with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0010] The foregoing and other aspects of the present invention are
best understood from the following detailed description when read
in connection with the accompanying drawings. For the purpose of
illustrating the invention, there are shown in the drawings
embodiments that are presently preferred, it being understood,
however, that the invention is not limited to the specific
instrumentalities disclosed. Included in the drawings are the
following Figures:
[0011] FIG. 1 shows the measured T.sub.1 versus true T.sub.1 using
the single reference VFA method without correction (depicted in
solid blue) and with correction using Equation 6 (depicted in
dashed red);
[0012] FIG. 2 shows the experiment setup for the first ultrasound
heating at a first point in the first cadaver breast (labeled "a"),
the second ultrasound heating in the first cadaver breast (labeled
"b") and the third ultrasound heating in the second cadaver breast
(labeled "c");
[0013] FIG. 3 shows the standard deviation of the T.sub.1
measurement for each flip angle combination using the single
reference VFA method for a baseline T.sub.1 of 300 ms and TR=10 ms
for .DELTA.T.sub.1=0 ms (labeled "a"), .DELTA.T.sub.1=100 ms
(labeled "b"), .DELTA.T.sub.1=200 ms (labeled "c"), and
.DELTA.T.sub.1=300 ms (labeled "d");
[0014] FIG. 4 shows relative SNR of PRF measurements for a tissue
with T.sub.1=800 ms and single reference VFA T.sub.1 measurements
with a baseline T.sub.1=300 ms and the reference flip
angle=5.degree. as a function of dynamic flip angle choice this
figure also shows the SNR of the PRF method for the same tissue as
a function of flip angle.
[0015] FIG. 5 shows separated water and fat images for the first
cadaver breast (labeled "a") and the second cadaver breast (labeled
"c"); Also shown are the flip angle maps obtained using the
pre-saturation pulse technique for the first cadaver breast
(labeled "b") and the second cadaver breast (labeled "d").
[0016] FIG. 6 shows images of the maximum temperature time point
for the gelatin phantom (labeled "a"), the first sonication
trajectory (labeled "b") and the second sonication trajectory
(labeled "c") in the first cadaver breast and the first sonication
trajectory (labeled "d") in the second cadaver breast;
[0017] FIG. 7 shows the PRF temperature (.degree. C., black) and
T.sub.1 change (ms, red) versus time in in the same voxel in the
gelatin phantom (labeled "a"), in the same aqueous tissue voxel in
the first cadaver breast (labeled "b"), in adjacent fat and aqueous
voxels in the second cadaver breast (labeled "c"), and in a fat
voxel in the second cadaver breast (labeled "d");
[0018] FIG. 8 shows the PRF temperature change versus T.sub.1
change within only the heated region in the gelatin phantom
(labeled "a") and aqueous tissue in the first cadaver breast during
the first ultrasound sonication (labeled "b");
[0019] FIG. 9A shows average T.sub.1 value during baseline scans
for the gelatin phantom (labeled "a"), first cadaver breast
(labeled "b"), and the second cadaver breast (labeled "c"), as well
as the standard deviation of T.sub.1 value as a percent of average
value during baseline scans for the gelatin phantom (labeled "d"),
first cadaver breast (labeled "e") and the second cadaver breast
(labeled "f");
[0020] FIG. 9B shows the standard deviation of T1 measurements in
adipose tissue as a percent of a baseline value in three healthy
volunteers in sagittal (labeled "a") and coronal (labeled "c")
orientations and the standard deviation of PRF temperature
measurements in aqueous tissues in sagittal (labeled "b") and
coronal (labeled "d") orientations;
[0021] FIG. 10 shows an example method measuring T.sub.1 in an
anatomical region of interest during a thermal therapy procedure
such as MRgFUS, according to some embodiments of the present
invention; and
[0022] FIG. 11 shows an example MRI system that may be used in
acquisition of the reference and dynamic images, according to some
embodiments of the present invention.
DETAILED DESCRIPTION
[0023] The present disclosure describes systems and methods for
Simultaneous Proton Resonance Frequency Shift Thermometry and
T.sub.1 Measurements Using a Single Reference Variable Flip Angle
T.sub.1 Method. More specifically, the technology disclosed herein
acquires a single reference image at the lower flip angle and all
dynamic images at the higher flip angle. T.sub.1 is calculated
using a single reference VFA method, which accounts for the
reference image temperature remaining constant. The single
reference VFA technique discussed herein provides a reliable way to
simultaneously measure PRF temperature and T.sub.1 change and
overcomes PRF's inability to simultaneously monitor temperature in
aqueous and adipose tissues. Real time temperature in fat will
increase patient safety and treatment efficacy. The techniques
described herein are especially applicable to interventional
treatments in inhomogeneous tissue types containing large amounts
of adipose tissue such as the breast or abdominal targets.
[0024] The VFA method for measuring T.sub.1 uses the spoiled
gradient recalled steady state signal as set forth in Equation
1:
S = M 0 ( 1 - E 1 ) sin ( .alpha. ) 1 - E 1 cos ( .alpha. ) E 2 ( 1
) ##EQU00001##
where E.sub.1=exp(-TR/T.sub.1), E.sub.2=exp(-TE/T.sub.2*), M.sub.0
is the equilibrium magnetization, .alpha. is the flip angle, TR is
the pulse repetition time, TE is the sequence echo time, and
T.sub.1 and T.sub.2 are the longitudinal and observed transverse
relaxation times, respectively. Equation 1 is derived with the
assumption that either TR>>T.sub.2* or adequate spoiling is
used to ensure that negligible transverse signal remains before
subsequent excitations. If these conditions are not true, Equation
1 will not accurately describe the signal and significant errors in
T.sub.1 calculation will occur. T.sub.1 is calculated from the
linearized form of Equation 1.
S sin ( .alpha. ) = E 1 S tan ( .alpha. ) + M 0 ( 1 - E 1 ) E 2 ( 2
) ##EQU00002##
[0025] The signal from applying two different flip angles (.alpha.
and .beta.) can be used in Equation 2 to calculate the slope m,
which is equal to an estimate of E.sub.1 (E.sub.1est):
E 1 est = m = y 2 - y 1 x 2 - x 1 y 2 = S 2 sin ( .beta. ) , y 1 =
S 1 sin ( .alpha. ) , x 2 = S 2 tan ( .beta. ) , x 1 = S 1 tan (
.alpha. ) ( 3 ) ##EQU00003##
T.sub.1 is then calculated from the slope using Equation 4:
T 1 = - TR ln ( m ) ( 4 ) ##EQU00004##
[0026] The single reference VFA method acquires a single reference
image at the lower flip angle .alpha. and then acquires dynamic
images at the higher flip angle .beta.. Estimates of T.sub.1 using
the original VFA method (T.sub.1est) will have a systematic error
because the T.sub.1 of the dynamic images will change with
temperature, while the reference image is constant. The actual
T.sub.1 of the dynamic images can be calculated with a simple
correction. The signals from the reference and dynamic images are
given Equation 5:
S r = M 0 ( 1 - E 1 ) sin ( .alpha. ) 1 - E 1 cos ( .alpha. ) E 2 S
d = M 0 ( 1 - E 1 d ) sin ( .beta. ) 1 - E 1 d cos ( .beta. ) E 2 (
5 ) ##EQU00005##
where E.sub.1d=exp(-TR/(T.sub.1+.DELTA.T.sub.1)), and
.DELTA.T.sub.1 is the change in T.sub.1 due to temperature change.
Substituting Equation (5) into Equation (3), where S.sub.d=S.sub.2
and S.sub.r=S.sub.1, yields:
T 1 + .DELTA. T 1 = - TR ln ( 1 - .gamma. 1 - .gamma. cos ( .beta.
) ) .gamma. = 1 - E 1 1 - E 1 cos ( .alpha. ) 1 - E 1 est cos (
.alpha. ) 1 - E 1 est cos ( .beta. ) ( 6 ) ##EQU00006##
Equation 6 requires knowing the baseline T.sub.1 value. The value
for E.sub.1 can be calculated using the signal from the reference
image (S.sub.r) and the baseline images of the dynamic images
(S.sub.d) before the heating begins. The baseline images are taken
while the tissue in the region of interest is substantially at the
same temperature as it was during acquisition of the reference
image. In this context "substantially the same temperature" refers
to a temperature difference of less than or equal to 5 degrees
Fahrenheit. The true T.sub.1+.DELTA.T.sub.1 is calculated by first
calculating the E.sub.1est, which is incorrect, and then applying
the correction in Equation (6) using the baseline E.sub.1 value,
the TR, and the flip angles used.
[0027] The potential systematic error is shown in FIG. 1. More
specifically, FIG. 1 shows the measured T.sub.1 versus true T.sub.1
using the single reference VFA method without correction (depicted
in solid blue) and with correction using Equation 6 (depicted in
dashed red). Baseline T.sub.1 was 700 ms. This example shows that
the calculated T.sub.1est overestimates the true T.sub.1 as T.sub.1
increases from the baseline value and will underestimate the true
T.sub.1 if T.sub.1 decreases from the baseline.
[0028] Flip Angle Sensitivity Simulations
[0029] To simulate the sensitivity of single reference VFA T.sub.1
measurements to noise and to determine the optimal choice of flip
angles for the single reference VFA method, noisy measurements were
simulated using a Monte Carlo technique for a range of flip angles.
The effects of T.sub.2 decay were ignored and TR was set to 10 ms.
The reference steady state signal values were calculated for flip
angles from 1 to 90.degree. in 1.degree. increments for T.sub.1=300
ms using Equation 1. Dynamic steady state signal values were
calculated for the same flip angles and for T.sub.1 values from 200
to 900 ms in 10 ms increments. White Gaussian noise was added to
each reference and dynamic signal value 1000 different times.
T.sub.1 values were calculated for every combination of flip angles
using equation 3 and then corrected using equation 6. The standard
deviation (.sigma..sub.T.sub.1) at each flip angle combination was
calculated from the 1000 noisy estimates.
[0030] Flip Angle Correction/B1 Mapping
[0031] Various techniques known in the art may be used for
performing the B1 mapping. One example method is described in Chung
S, Kim D, Breton E, Axel L. Rapid B(1)(+) Mapping Using a
Pre-Conditioning RF Pulse with TurboFLASH readout. Magnetic
resonance in medicine: official journal of the Society of Magnetic
Resonance in Medicine/Society of Magnetic Resonance in Medicine
2010; 64(2):439-446. The method uses the ratio between two proton
density images, I.sub.PD where one of the images has been
pre-saturated by a slice selective pulse, I.sub.PRE. The images
were acquired with a centric reordered turbo fast low angle shot
(turboFLASH) MRI sequence to decrease scan time. The ratio of the
two images is related to the nominal flip angle of the saturation
pulse, .theta..sub.nom, by
I PRE I PD = cos ( .kappa. ( r ) .theta. nom ) ( 7 )
##EQU00007##
where .kappa.(r) is a flip angle scale factor at position r, which
by definition is the actual flip angle divided by nominal flip
angle. Equation 7 can be rewritten as
.kappa. ( r ) = 1 .theta. nom cos - 1 I PRE I PD ( 8 )
##EQU00008##
[0032] In the example implementation described below, the flip
angle of the pre-saturation pulse was set to
.theta..sub.nom=60.degree. for this work. The data acquisition was
started as quickly as possible (10 ms) after the pre-saturation to
minimize the effects of T.sub.1 relaxation, which are ignored. A
centric k-space reordering also minimizes the effects of T.sub.1
relaxation, which can be minimized further by segmenting the
acquisition into multiple "shots." The original method by Chung
acquired a single 2D slice, which could be acquired rapidly in a
single shot. During the example implementation, 3D maps were
acquired which required segmenting the data acquisition to maintain
the proton density weighting. Both images were filtered with a
Hamming window along the phase encoding direction before
reconstruction.
[0033] MRgFUS Experiments
[0034] An experiment was performed in a Siemens Prisma 3T MRI
scanner (Siemens Healthcare, Erlangen, Germany) using the
MRI-compatible phased-array transducer (256 elements, 1 MHz
frequency, 10 cm radius of curvature; Imasonic, Besancon, France
and Image Guided Therapy, Pessac, France) from a breast-specific
MRgFUS system to evaluate the single reference VFA method. Two
human cadaver breasts from different donors preserved with formalin
were positioned such that the ultrasound focus was approximately 3
cm deep in the cadaver breast.
[0035] FIG. 2 shows the experiment setup for the first ultrasound
heating (labeled "a"), the second ultrasound heating in the first
cadaver breast (labeled "b") and the third ultrasound heating in
the third cadaver breast (labeled "c"). The ultrasound transducer
is outlined with the blue line at the bottom of each image and the
ultrasound propagation envelope is shown with the red lines. The
yellow ovals are the approximate locations for each of the
electronically steered focal locations.
[0036] In the example of FIG. 2, the single reference VFA angle was
used in conjunction with a multi-echo linearly rotated
stack-of-stars (RSOS) imaging sequence with an in-plane
pseudo-golden angle increment. One example implementation of the
SOS method is described in Block K T, Chandarana H, Milla S, Bruno
M, Mulholland T, Fatterpekar G, Hagiwara M, Grimm R, Geppert C,
Kiefer B, Sodickson D K. Towards Routine Clinical Use of Radial
Stack-of-Stars 3D Gradient-Echo Sequences for Reducing Motion
Sensitivity. J Korean Soc Magn Reson Med 2014; 18(2):87-106.
However, it should be understood that other types of acquisition
sequences (e.g., other spiral-based techniques and non-spiral
based) may be used in implementing the single reference VFA angle
technique described herein.
[0037] The images shown in FIG. 2 were reconstructed with a dynamic
(KWIC) reconstruction to increase the temporal resolution. An
example implementation of KWIC is described in Svedin B T, Payne A,
Bolster B D, Jr., Parker D L. Multiecho pseudo-golden angle stack
of stars thermometry with high spatial and temporal resolution
using k-space weighted image contrast. Magn Reson Med 2017. As with
the use of SOS, the use of KWIC for reconstruction is intended to
be exemplary; and other reconstruction methods may be used to
reconstruct the various images used in the techniques described
herein.
[0038] The 3D imaging volume was prescribed in a sagittal
orientation (voxel size=1.3 mm isotropic; field of
view=208.times.208.times.20.8 mm; matrix=160.times.160.times.16;
1514 radial projections; TR=10.5 ms; TE=2.46.sub.1.23*n, n=0 to 5
ms; readout bandwidth=1200 Hz/pixel). The reference image flip
angle was .alpha.=5.degree. and the dynamic images flip angle
(.beta.=15.degree.) was set to the Ernst angle for the fat tissue.
The first cadaver breast was heated with two ultrasound
sonications, each in a different location using electronic
steering, while imaging with the RSOS sequence. The second cadaver
breast was heated with one ultrasound sonication. The ultrasound
sonicated a linear pattern composed of four discrete points (each
separated by 2 mm, 50 ms per point) at 75 acoustic watts for 30
seconds total. A breast shaped homogenous gelatin phantom was also
sonicated with the same parameters. B1 maps for each phantom were
acquired using the method described above (voxel size=2.6 mm
isotropic zero-filled to 1.3 mm isotropic; field of
view=208.times.208.times.20.8 mm; 40 lines per TR; TR=5000 ms;
TE=1.31 ms).
[0039] Image Reconstruction
[0040] The gpuNUFFT algorithm was used to reconstruct the
non-Cartesian data. The dynamic images were reconstructed using a
sliding symmetric KWIC window with 13 central projections and 377
total projections and the sliding window was advanced 13
projections between reconstructions. The effective temporal
resolution of the KWIC reconstructed images was 2.18 s. The
reference images were reconstructed without a KWIC window and using
all of the collected data. Separate water and fat images were
created using the 3-point Dixon method from the first three echoes
for the reference image and each dynamic image. T.sub.1 values were
calculated using the single reference VFA method described above
using the separated water/fat images and acquired B1 maps. The
change in T.sub.1 from a trajectory matched baseline value was
calculated for each image. The phase information from each echo was
combined using a weighted linear least squares fit and the PRF
temperatures were calculated using the trajectory matched baseline
described in Svedin B T, Payne A, Bolster B D, Jr., Parker D L.
Multiecho pseudo-golden angle stack of stars thermometry with high
spatial and temporal resolution using k-space weighted image
contrast (KWIC). Magn Reson Med 2017. The change in T.sub.1 is
compared to PRF temperature values in aqueous tissue.
[0041] Results
[0042] The Monte Carlo simulations of the single reference VFA
precision are shown in FIG. 3 for several values of .DELTA.T.sub.1.
Specifically, FIG. 3 shows the standard deviation of the T.sub.1
measurement for each flip angle combination using the single
reference VFA method for a baseline T.sub.1 of 300 ms and TR=10 ms
for .DELTA.T.sub.1=0 ms (labeled "a"), .DELTA.T.sub.1=100 ms
(labeled "b"), .DELTA.T.sub.1=200 ms (labeled "c"), and
.DELTA.T.sub.1=300 ms (labeled "d"). The red x is the smoothed
estimate of the flip angle choices that give the minimum standard
deviation.
[0043] Similar to the standard VFA method, the measurement
precision is best when the two flip angles chosen are on opposite
sides of the Ernst angle. The precision varies rapidly when varying
the smaller flip angle and is relatively forgiving when varying the
larger flip angle. The optimal flip angle combination changes with
A T.sub.1, but the choice of reference flip angle remained
constant. For the simulated scan parameters, the optimal choice of
reference flip angle was 5.degree.. As A T.sub.1 increases, the
optimal choice for the dynamic flip angle approaches but does not
go lower than the original baseline Ernst angle.
[0044] The relative SNR of PRF measurements for a tissue with
T.sub.1=800 ms and single reference VFA T.sub.1 measurements with a
baseline T.sub.1=300 ms and the reference flip angle=5.degree. as a
function of dynamic flip angle choice is shown in FIG. 4. In FIG.
4, relative PRF SNR is shown in black, while T.sub.1 measurement
SNR is shown in red. Relative T.sub.1 SNR is equal to the standard
deviation divided by T.sub.1+.DELTA.T.sub.1 and then normalized to
the maximum value of .DELTA.T.sub.1=0 ms.
[0045] FIG. 5 shows separated water and fat images for the first
cadaver breast (labeled "a") and the second cadaver breast (labeled
"c"). Also shown are the flip angle maps obtained using the
pre-saturation pulse technique for the first cadaver breast
(labeled "b") and the second cadaver breast (labeled "d"). In each
image labeled "a" and "b", the water images are shown on the left,
while the fat image are on the right. The color maps labeled "b"
and "d" show the B1 map acquired using the pre-saturation pulse
technique for the first and second cadaver breasts, respectively.
Cadaver breast 1 had a mix of fibroglandular tissue and fat while
cadaver breast 2 was comprised primarily of fat.
[0046] Images of the PRF temperature, the T.sub.1 change in water
voxels and the T.sub.1 change in fat voxels for the gelatin phantom
and cadaver breasts during ultrasound sonication are shown in FIG.
6 at the time of the peak temperature rise. More specifically, FIG.
6 shows images of the maximum temperature time point for the
gelatin phantom (labeled "a"), the first sonication trajectory
(labeled "b"), the second sonication trajectory in the first
cadaver breast and the second cadaver breast (labeled "c" and "d,"
respectively). The left column of images in FIG. 6 shows the PRF
temperature maps in aqueous tissue. The middle column of images
shows the T.sub.1 change in aqueous tissue voxels. Finally, the
right column of images in FIG. 6 shows the T.sub.1 change in
adipose tissue voxels. The PRF temperature and T.sub.1 change in
water voxels are masked to only be shown in water voxels and the
T.sub.1 change in fat is only shown for fat voxels.
[0047] FIG. 7 shows the PRF temperature (.degree. C., black) and
T.sub.1 change (ms, red) versus time in each model. The plot
labeled "a" depicts the measurements obtained both from the same
voxel in the gelatin phantom; the plot labeled "b" depicts the
measurements from the same aqueous tissue voxel from the first
ultrasound sonication in the first cadaver breast. The plot labeled
"c" presents the measurements from neighboring voxels across the
water/fat boundary from the second ultrasound sonication in the
first cadaver breast where the PRF values are from the aqueous
voxel and the T.sub.1 change is from the fat voxel. Note that the
PRF temperature and T.sub.1 change show agreement of the temporal
evolution shape in FIGS. 7A-7C. The plot labeled "d" in FIG. 7
shows the T.sub.1 change in a fat voxel from the second cadaver
breast where PRF temperature measurements are impossible due to the
lack of aqueous tissue.
[0048] FIG. 8 shows the PRF temperature change versus T.sub.1
change within only the heated region in the gelatin phantom
(labeled "a") and aqueous tissue in the first cadaver breast during
the first ultrasound sonication (labeled "b"). The slope of the
T.sub.1 change versus temperature from a linear fit to the data was
42.85 ms/.degree. C. (R2=0.81) for the gelatin and 22.96
ms/.degree. C. (R2=0.5) for the aqueous tissue in the first cadaver
breast.
[0049] FIG. 9A shows average T.sub.1 value during baseline scans
for the gelatin phantom (labeled "a"), first cadaver breast
(labeled "b"), and the second cadaver breast (labeled "c"). FIG. 9A
also shows the standard deviation of the T.sub.1 value as a percent
of the average T.sub.1 value during baseline scans for the gelatin
phantom (labeled "d"), first cadaver breast (labeled "e") and the
second cadaver breast (labeled "f").
[0050] FIG. 9B provides the results of healthy volunteer scans for
sagittal (labeled "A" and "B") and coronal (labeled "C" and "D")
orientations for three volunteers. Volunteer #1 is shown in the
left most column of images, volunteer #2 is shown in the middle
column, and volunteer #3 is shown in the right most column. In each
column, Images A and C show the standard deviation through time of
T.sub.1 measurements in adipose tissue as a percent of the baseline
value. Images B and D show the standard deviation through time of
the PRF temperature (.degree. C.) in aqueous tissue.
DISCUSSION
[0051] This disclosure describes how simultaneous PRF temperature
and T.sub.1 relaxation values can be obtained using a single
reference VFA method. The acquisition of these two values allows
for a simultaneous measure of temperature in aqueous and adipose
tissue. One significant advantage of this method is the fact that
the steady state signal does not need to be adjusted between each
dynamic image, which would increase scan time. This allows the
T.sub.1 measurement to have the same temporal resolution as the PRF
temperature. The ability to measure PRF temperature and T.sub.1 was
demonstrated during MRgFUS sonications in a gelatin phantom and two
cadaver breasts from two separate subjects. The hybrid nature of
this technique can be seen in FIG. 6 where an ablation region can
be seen across the water and fat boundary. The ability of the PRF
temperature and T.sub.1 change to provide equivalent measures is
shown within aqueous tissues, where the size and shape of both the
measures as well as the temporal response show good qualitative
agreement.
[0052] The ability for simultaneous 3D thermometry methods will
increase the monitoring accuracy of mixed tissue type targets. The
described method may be implemented using a rapid image update
reconstruction method, which greatly improves upon previous
simultaneous techniques. The multi-echo SOS acquisition with KWIC
reconstruction allowed for improved data acquisition for both
T.sub.1 and PRF measurements. The SNR for T.sub.1 measurements is
improved with shorter TE and the PRF SNR is optimal when TE=T2*.
Acquiring multiple echoes allows for echo combination of the phase
data to improve the PRF measurements and using the shorter TE with
water/fat separation for T.sub.1 measurements.
[0053] The precision of the T.sub.1 measurement can further be
optimized with proper choice of flip angles. Using the VFA method
to measure T.sub.1 will amplify noise through its nonlinear nature
of calculating T.sub.1. Using the standard VFA method, the ideal
choice of flip angles will produce .about.71% of the Ernst angle
signal with the two flip angles on different sides of the Ernst
angle. The Monte-Carlo simulations for this single reference VFA
method show that the reference flip angle should be the lower flip
angle and that the optimal choice for the lower angle gives
.about.60% of the Ernst angle signal. A more accurate estimate of
the percent of the Ernst angle signal could be simulated with much
finer flip angle increments or possibly an exact estimate could be
derived using the same propagation of errors method employed by
Schabel M C, Morrell G R. Uncertainty in T(1) mapping using the
variable flip angle method with two flip angles. Phys Med Biol
2009; 54(1):N1-8. The optimal reference flip angle will vary with
TR and T.sub.1 of interest. The optimal choice of dynamic flip
angle, for the simulated parameters, began at 26.degree.
(.about.85% of Ernst signal) and as .DELTA.T.sub.1 increased it
approached the Ernst angle. Simultaneous measurements of PRF and
T.sub.1 need to balance the precision of both measurements (see
FIG. 4). The PRF temperature is optimal when the chosen flip angle
gives the highest SNR in the aqueous tissue, at the Ernst angle of
the aqueous tissue. In general, aqueous tissue will have a higher
T.sub.1 value than adipose tissue, meaning its Ernst angle will be
lower than that of adipose tissue. The precision of VFA T.sub.1
measurements is relatively insensitive to changes in the larger
flip angle as shown in FIGS. 3 and 4. In order to balance the
precision of the PRF and T.sub.1 measurements, the dynamic flip
angle was chosen to be the Ernst angle of the fat tissue before
heating. While this will not provide the maximum PRF SNR, it will
still give more SNR than the conventional VFA method. An example
aqueous tissue T.sub.1=800 ms with TR=10 ms will give .about.88% of
its maximum signal at its Ernst angle.
[0054] The temperature dependence of T.sub.1 has been investigated
previously using conventional techniques. Unlike the PRF shift, the
calibration of T.sub.1 change to temperature is very tissue type
dependent. Comparisons of the standard VFA to inversion recovery
T.sub.1 measurements have been done previously and are generally
accepted as equally accurate although VFA provides lower SNR
measurements. Calibration of T.sub.1 changes with temperature using
this single reference VFA to T.sub.1 changes with temperature using
gold standard T.sub.1 measurements, e.g., inversion recovery,
remains as work to be done. It has also been shown that
irreversible changes to the T.sub.1 value occur after tissue
coagulation/death in aqueous tissue voxels. It is possible that a
similar effect would be observed in adipose tissue. These effects
would cause any T.sub.1 temperature measurement to use a variable
calibration specifically made for ablated tissue in order to be
used as a reliable measure of temperature after ablation. This
technique could potentially also serve as a helpful indication that
the tissue has been ablated.
[0055] FIG. 10 shows an example method for measuring T.sub.1 in an
anatomical region of interest during a thermal therapy procedure
such as MRgFUS, according to some embodiments of the present
invention. Starting at step 1005, an MR system acquires a reference
magnetic resonance (MR) image of an anatomical region of interest
using a first flip angle. One example of an MR system that may be
employed in performing the acquisition is described below with
reference to FIG. 1. The reference MR image is used at step 1010 to
calculate a reference T.sub.1 value for adipose tissue in the
anatomical area of interest. As described above, one way of
calculating this reference T.sub.1 value is using the single
reference VFA method.
[0056] Continuing with reference to FIG. 10, the MR is system is
used at step 1015 to acquire a plurality of dynamic MR images of an
anatomical region of interest using a second flip angle that is
higher than the first flip angle. As described above, the
acquisition of dynamic MR images may be performed using a
multi-echo pseudo golden angle SOS acquisition. Alternatively,
other acquisition techniques generally known in art may be applied.
Furthermore, earlier sections of this disclosure describe how the
dynamic images can be reconstructed from k-space data collected
during the acquisition using a KWIC reconstruction method. But,
again, it should be understood that other reconstruction techniques
known in the art may be used as an alternative to KWIC.
[0057] The reference MR images and the dynamic MR images are used
at step 1020 to calculate a correction to the reference T.sub.1
value that accounts for a change in the reference T.sub.1 value
caused by a temperature change in the adipose tissue during the
thermal therapy procedure. This correction is then applied at step
1025 to the reference T.sub.1 value to yield an updated T.sub.1
value for the dynamic MR images. This updated T.sub.1 value may
then be used, for example, to generate a T.sub.1-weighted image of
the anatomical area of interest based on the dynamic MR images.
[0058] At step 1030, the phase information from the dynamic images
to determine PRF-based temperature images of aqueous based tissues
in the anatomical region of interest. More specifically, one
dynamic image (e.g., the first acquired dynamic image) is used as
the baseline image. Each voxel in each dynamic image is associated
with a PRF value. By comparing the PRF value of the dynamic images
to the corresponding PRF value of the baseline image, a temperature
difference map can be formed. In some embodiments, the baseline
image can be adjusted over time using newly acquired images until
the thermal therapy is completed.
[0059] FIG. 11 shows an example MRI system that may be used in
acquisition of the reference and dynamic images, according to some
embodiments of the present invention. In system 1100, magnetic
coils 12 create a static base magnetic field in the body of patient
11 to be imaged and positioned on a table. Within the magnet system
are gradient coils 14 for producing position dependent magnetic
field gradients superimposed on the static magnetic field. Gradient
coils 14, in response to gradient signals supplied thereto by a
gradient and shim coil control module 16, produce position
dependent and shimmed magnetic field gradients in three orthogonal
directions and generates magnetic field pulse sequences. The
shimmed gradients compensate for inhomogeneity and variability in
an MR imaging device magnetic field resulting from patient
anatomical variation and other sources. The magnetic field
gradients include a slice-selection gradient magnetic field, a
phase-encoding gradient magnetic field and a readout gradient
magnetic field that are applied to patient 11.
[0060] Further RF (radio frequency) module 20 provides RF pulse
signals to RF coil 18, which in response produces magnetic field
pulses which rotate the spins of the protons in the imaged body of
the patient 11 by ninety degrees or by one hundred and eighty
degrees for so-called "spin echo" imaging, or by angles less than
or equal to 90 degrees for so-called "gradient echo" imaging.
Gradient and shim coil control module 16 in conjunction with RF
module 20, as directed by central control computer 26, control
slice-selection, phase-encoding, readout gradient magnetic fields,
radio frequency transmission, and magnetic resonance signal
detection, to acquire magnetic resonance signals representing
planar slices of patient 11. For example, as described in greater
detail below, in some embodiments, the central control computer 26
directs the various components of the system 1100 to acquire radial
k-space data using a bSSFP sequence with an interleaved-angle
asymmetric radial sampling scheme.
[0061] In response to applied RF pulse signals, the RF coil 18
receives MR signals, i.e., signals from the excited protons within
the body as they return to an equilibrium position established by
the static and gradient magnetic fields. The MR signals are
detected and processed by a detector within RF module 20 and
k-space component processor unit 34 to provide an MR dataset to an
image data processor for processing into an image. In some
embodiments, the image data processor is located in central control
computer 26. However, in other embodiments such as the one depicted
in FIG. 11, the image data processor is located in a separate unit
27. ECG synchronization signal generator 30 provides ECG signals
used for pulse sequence and imaging synchronization. A two or three
dimensional k-space storage array of individual data elements in
k-space component processor unit 34 stores corresponding individual
frequency components which comprises an MR dataset. The k-space
array of individual data elements has a designated center and
individual data elements individually have a radius to the
designated center.
[0062] A magnetic field generator (comprising coils 12, 14 and 18)
generates a magnetic field for use in acquiring multiple individual
frequency components corresponding to individual data elements in
the storage array. The individual frequency components are
successively acquired in an order in which radius of respective
corresponding individual data elements increases and decreases
along a substantially spiral path as the multiple individual
frequency components is sequentially acquired during acquisition of
an MR dataset representing an MR image. A storage processor in the
k-space component processor unit 34 stores individual frequency
components acquired using the magnetic field in corresponding
individual data elements in the array. The radius of respective
corresponding individual data elements alternately increases and
decreases as multiple sequential individual frequency components
are acquired. The magnetic field acquires individual frequency
components in an order corresponding to a sequence of substantially
adjacent individual data elements in the array and magnetic field
gradient change between successively acquired frequency components
is substantially minimized.
[0063] Central control computer 26 uses information stored in an
internal database to process the detected MR signals in a
coordinated manner to generate high quality images of a selected
slice(s) of the body (e.g., using the image data processor) and
adjusts other parameters of system 1100. The stored information
comprises predetermined pulse sequence and magnetic field gradient
and strength data as well as data indicating timing, orientation
and spatial volume of gradient magnetic fields to be applied in
imaging. Generated images are presented on display 40 of the
operator interface. Computer 28 of the operator interface includes
a graphical user interface (GUI) enabling user interaction with
central control computer 26 and enables user modification of
magnetic resonance imaging signals in substantially real time.
Display processor 37 processes the magnetic resonance signals to
provide image representative data for display on display 40, for
example.
[0064] The embodiments of the present disclosure may be implemented
with any combination of hardware and software. In addition, the
embodiments of the present disclosure may be included in an article
of manufacture (e.g., one or more computer program products)
having, for example, computer-readable, non-transitory media. The
media has embodied therein, for instance, computer readable program
code for providing and facilitating the mechanisms of the
embodiments of the present disclosure. The article of manufacture
can be included as part of a computer system or sold
separately.
[0065] The term "computer readable medium" as used herein refers to
any medium that participates in providing instructions to the
processor for execution. A computer readable medium may take many
forms including, but not limited to, non-volatile media, volatile
media, and transmission media. Non-limiting examples of
non-volatile media include optical disks, solid state drives,
magnetic disks, and magneto-optical disks, such as hard disk or
removable media drive. One non-limiting example of volatile media
is dynamic memory. Non-limiting examples of transmission media
include coaxial cables, copper wire, and fiber optics, including
the wires that make up one or more buses. Transmission media may
also take the form of acoustic or light waves, such as those
generated during radio wave and infrared data communications.
[0066] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments will be apparent to those
skilled in the art. The various aspects and embodiments disclosed
herein are for purposes of illustration and are not intended to be
limiting, with the true scope and spirit being indicated by the
following claims.
[0067] An executable application, as used herein, comprises code or
machine readable instructions for conditioning the processor to
implement predetermined functions, such as those of an operating
system, a context data acquisition system or other information
processing system, for example, in response to user command or
input. An executable procedure is a segment of code or machine
readable instruction, sub-routine, or other distinct section of
code or portion of an executable application for performing one or
more particular processes. These processes may include receiving
input data and/or parameters, performing operations on received
input data and/or performing functions in response to received
input parameters, and providing resulting output data and/or
parameters.
[0068] The functions and process steps herein may be performed
automatically or wholly or partially in response to user command.
An activity (including a step) performed automatically is performed
in response to one or more executable instructions or device
operation without user direct initiation of the activity.
[0069] The system and processes of the figures are not exclusive.
Other systems, processes and menus may be derived in accordance
with the principles of the invention to accomplish the same
objectives. Although this invention has been described with
reference to particular embodiments, it is to be understood that
the embodiments and variations shown and described herein are for
illustration purposes only. Modifications to the current design may
be implemented by those skilled in the art, without departing from
the scope of the invention. As described herein, the various
systems, subsystems, agents, managers and processes can be
implemented using hardware components, software components, and/or
combinations thereof. No claim element herein is to be construed
under the provisions of 35 U.S.C. 112(f), unless the element is
expressly recited using the phrase "means for."
* * * * *