U.S. patent application number 16/736663 was filed with the patent office on 2021-07-08 for dynamic b0 shimming for improved fat saturation in magnetic resonance imaging (mri).
The applicant listed for this patent is SYNAPTIVE MEDICAL INC.. Invention is credited to Philip J. Beatty, Chad Tyler Harris, Curtis Nathan Weins.
Application Number | 20210208223 16/736663 |
Document ID | / |
Family ID | 1000004624273 |
Filed Date | 2021-07-08 |
United States Patent
Application |
20210208223 |
Kind Code |
A1 |
Beatty; Philip J. ; et
al. |
July 8, 2021 |
DYNAMIC B0 SHIMMING FOR IMPROVED FAT SATURATION IN MAGNETIC
RESONANCE IMAGING (MRI)
Abstract
A fat saturation method for a magnetic resonance imaging system
having a main magnet providing a magnetic field B.sub.0 The method
includes: driving a shim coil assembly with a first set of shimming
currents to sufficiently alter a B.sub.0 field inhomogeneity of the
magnetic field B.sub.0 within a region that includes a first
imaging volume of interest such that water saturation inside the
region is reduced from before the first set of shimming currents
are applied; applying a fat saturation pulse to the region;
identifying the first imaging volume of interest from the region;
driving the shim coil assembly with a second set of shimming
currents to alter the B.sub.0 field inhomogeneity of the magnetic
field B.sub.0 within the first imaging volume of interest such that
the B.sub.0 field inhomogeneity within the first imaging volume of
interest is reduced; and obtaining magnetic resonance signals from
the first imaging volume of interest.
Inventors: |
Beatty; Philip J.; (Toronto,
CA) ; Harris; Chad Tyler; (Toronto, CA) ;
Weins; Curtis Nathan; (Toronto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SYNAPTIVE MEDICAL INC. |
Toronto |
|
CA |
|
|
Family ID: |
1000004624273 |
Appl. No.: |
16/736663 |
Filed: |
January 7, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 33/3875 20130101;
G01R 33/4828 20130101 |
International
Class: |
G01R 33/3875 20060101
G01R033/3875; G01R 33/48 20060101 G01R033/48 |
Claims
1. A method to perform fat saturation on a magnetic resonance
imaging (MRI) system having a main magnet providing a magnetic
field B.sub.0, the method comprising: driving a shim coil assembly
of the MRI system with a first set of shimming currents to
sufficiently alter a B.sub.0 field inhomogeneity of the magnetic
field B.sub.0 within a region that includes a first imaging volume
of interest such that water saturation inside the region is reduced
from before the first set of shimming currents are applied;
applying a fat saturation pulse to the region; identifying the
first imaging volume of interest from the region; driving the shim
coil assembly of the MRI system with a second set of shimming
currents to alter the B.sub.0 field inhomogeneity of the magnetic
field B.sub.0 within the first imaging volume of interest such that
the B.sub.0 field inhomogeneity within the first imaging volume of
interest is reduced from before the second set of shimming currents
were applied; and obtaining magnetic resonance (MR) signals from
the first imaging volume of interest using an imaging pulse
sequence.
2. The method of claim 1, wherein obtaining magnetic resonance
signals from the first imaging volume of interest comprises:
applying gradient pulses to encode MR signals from the first
imaging volume of interest; and in response to the gradient pulses,
acquiring the MR signals from the first imaging volume of
interest.
3. The method of claim 1, wherein the imaging pulse sequence
comprises at least one of: a gradient-echo pulse sequence, a
spin-echo pulse sequence, a steady-state free precession (SSFP)
pulse sequence, and an echo-planar imaging (EPI) pulse
sequence.
4. The method of claim 1, wherein driving the shim coil assembly of
the MRI system with a first set of shimming currents comprises:
driving the shim coil assembly of the MRI system with a first set
of shimming currents to substantially minimize water saturation
within the region.
5. The method of claim 1, further comprising: identifying a second
imaging volume of interest from the region; subsequently driving
the shim coil assembly of the MRI system with a third set of
shimming currents to alter the B.sub.0 field inhomogeneity of the
magnetic field B.sub.0 within the second imaging volume of interest
such that the B.sub.0 field inhomogeneity within the second imaging
volume of interest is reduced from before the third set of shimming
currents were applied; and obtaining magnetic resonance signals
from the second imaging volume of interest using the imaging pulse
sequence.
6. The method of claim 1, further comprising: placing a subject in
the main magnet such that the region covers a portion of the
subject.
7. The method of claim 6, wherein the portion of the subject
includes at least one of: an abdominal organ of the subject, a
breast of the subject, a neck of the subject, an extremity of the
subject, and a head of the subject.
8. The method of claim 1, wherein prior to obtaining the MR signals
from the first imaging volume of interest: applying a first
slice-select gradient while applying a first radio-frequency (RF)
pulse such that protons from the first imaging volume of interest
are excited by the first RF pulse.
9. A magnetic resonance imaging (MRI) system, comprising: a main
magnet providing a magnetic field B.sub.0; a shim coil assembly; a
gradient coil assembly; a radio-frequency (RF) coil; a controller
in communication with the shim coil assembly, the gradient coil
assembly, and the RF coil, the controller configured to perform
operations of: driving the shim coil assembly of the MRI system
with a first set of shimming currents to sufficiently alter a
B.sub.0 field inhomogeneity of the magnetic field B.sub.0 within a
region that includes a first imaging volume of interest such that
water saturation inside the region is reduced from before the first
set of shimming currents are applied; applying a fat saturation
pulse to the region; identifying the first imaging volume of
interest from the region; driving the shim coil assembly of the MRI
system with a second set of shimming currents to alter the B.sub.0
field inhomogeneity of the magnetic field B.sub.0 within the first
imaging volume of interest such that the B.sub.0 field
inhomogeneity within the first imaging volume of interest is
reduced from before the second set of shimming currents were
applied; and obtaining magnetic resonance (MR) signals from the
first imaging volume of interest using an imaging pulse
sequence.
10. The MRI system of claim 9, wherein obtaining magnetic resonance
signals from the first imaging volume of interest comprises:
applying gradient pulses to encode MR signals from the first
imaging volume of interest; and in response to the gradient pulses,
acquiring the MR signals from the first imaging volume of
interest.
11. The MRI system of claim 9, wherein the imaging pulse sequence
comprises at least one of: a gradient-echo pulse sequence, a
spin-echo pulse sequence, a steady-state free precession (SSFP)
pulse sequence, and an echo-planar imaging (EPI) pulse
sequence.
12. The MRI system of claim 9, wherein driving the shim coil
assembly of the MRI system with a first set of shimming currents
comprises: driving the shim coil assembly of the MRI system with a
first set of shimming currents to substantially minimize water
saturation within the region.
13. The MRI system of claim 9, wherein the operations further
comprise: identifying a second imaging volume of interest from the
region; subsequently driving the shim coil assembly of the MRI
system with a third set of shimming currents to alter the B.sub.0
field inhomogeneity of the magnetic field B.sub.0 within the second
imaging volume of interest such that the B.sub.0 field
inhomogeneity within the second imaging volume of interest is
reduced from before the third set of shimming currents were
applied; and obtaining magnetic resonance signals from the second
imaging volume of interest using the imaging pulse sequence.
14. The MRI system of claim 9, wherein the region covers a portion
of a subject placed in the main magnet.
15. The MRI system of claim 14, wherein the portion of the subject
includes at least one of: an abdominal organ of the subject, a
breast of the subject, a neck of the subject, an extremity of the
subject, and a head of the subject.
16. The MRI system of claim 9, wherein prior to obtaining the MR
signals from the first imaging volume of interest: applying a first
slice-select gradient while applying a first radio-frequency (RF)
pulse such that protons from the first imaging volume of interest
are excited by the first RF pulse.
17. A method to compute sets of shimming currents for a magnetic
resonance imaging (MRI) system having a main magnet that provides a
magnetic field B.sub.0, the method comprising: obtaining a field
map of a portion of the magnetic field B.sub.0; based on the field
map, computing values of a first set of shimming currents that
substantially minimize water saturation within a region that covers
a portion of a subject placed in the main magnet; obtaining
location information of a first imaging volume of interest, the
first imaging volume of interest included by the region; and based
on the field map, and the location information of the first imaging
volume of interest, computing values of a second set of shimming
currents that substantially minimize magnetic field inhomogeneity
within the first imaging volume of interest.
18. The method of claim 17, further comprising: obtaining a fat map
of the portion of the subject placed in the main magnet; and
obtaining a water map of the portion of the subject placed in the
main magnet.
19. The method of claim 18, wherein the values of a first and
second sets of shimming currents are computed based on the field
map, the fat map, and the water map.
20. The method of claim 17, wherein the first set of shimming
currents substantially maximize fat saturation within the region
that covers the portion of the subject.
Description
BACKGROUND
[0001] The present specification relates to magnetic resonance
imaging.
SUMMARY
[0002] In one aspect, some implementations provide a method to
perform fat saturation on a magnetic resonance imaging (MRI) system
having a main magnet providing a magnetic field B.sub.0. The method
includes driving a shim coil assembly of the MRI system with a
first set of shimming currents to sufficiently alter a B.sub.0
field inhomogeneity of the magnetic field B.sub.0 within a region
that includes a first imaging volume of interest such that water
saturation inside the region is reduced from before the first set
of shimming currents are applied; applying a fat saturation pulse
to the region; identifying the first imaging volume of interest
from the region; driving the shim coil assembly of the MRI system
with a second set of shimming currents to alter the B.sub.0 field
inhomogeneity of the magnetic field B.sub.0 within the first
imaging volume of interest such that the B.sub.0 field
inhomogeneity within the first imaging volume of interest is
reduced from before the second set of shimming currents were
applied; and obtaining magnetic resonance (MR) signals from the
first imaging volume of interest using an imaging pulse
sequence.
[0003] Implementations may include one or more of the following
features.
[0004] Obtaining magnetic resonance (MR) signals from the first
imaging volume of interest may include: applying gradient pulses to
encode MR signals from the first imaging volume of interest; and in
response to the gradient pulses, acquiring the MR signals from the
first imaging volume of interest.
[0005] The imaging pulse sequence may include at least one of: a
gradient-echo pulse sequence, a spin-echo pulse sequence, a
steady-state free precession (SSFP) pulse sequence, and an
echo-planar imaging (EPI) pulse sequence.
[0006] Driving the shim coil of the MRI system with a first set of
shimming currents may include: driving the shim coil of the MRI
system with a first set of shimming currents to substantially
minimize water saturation within the region and maximizing fat
saturation inside the first imaging volume of interest. Driving the
shim coil assembly of the MRI system with a first set of shimming
currents may include: driving the shim coil assembly of the MRI
system to substantially minimize water saturation throughout the
region.
[0007] The method may further include: identifying a second imaging
volume of interest from the region where fat saturation has been
applied; subsequently driving the shim coil of the MRI system with
a third set of shimming currents to alter the B.sub.0 field
inhomogeneity of the magnetic field B.sub.0 within the second
imaging volume of interest such that the B.sub.0 field
inhomogeneity within the second imaging volume of interest is
reduced from before the third set of shimming currents were
applied; and obtaining magnetic resonance signals from the second
imaging volume of interest using the imaging pulse sequence.
[0008] The method may further include: placing a subject in the
main magnet such that the region covers a portion of the subject.
The portion of the subject may include at least one of: an
abdominal organ of the subject, a breast of the subject, a neck of
the subject, an extremity of the subject, and a head of the
subject.
[0009] Prior to obtaining the MR signals from the first imaging
volume of interest: applying a first slice-select gradient while
applying a first radio-frequency (RF) pulse such that protons from
the first imaging volume of interest are excited by the first RF
pulse.
[0010] Some implementations provide a magnetic resonance imaging
(MRI) system that includes: a main magnet providing a magnetic
field B.sub.0; a shim coil assembly; a gradient coil assembly; a
radio-frequency (RF) coil; a controller in communication with the
shim coil assembly, the gradient coil assembly, and the RF coil.
The controller configured to perform operations of: driving the
shim coil assembly of the MRI system with a first set of shimming
currents to sufficiently alter a B.sub.0 field inhomogeneity of the
magnetic field B.sub.0 within a region that includes a first
imaging volume of interest such that water saturation inside the
region is reduced from before the first set of shimming currents
are applied; applying a fat saturation pulse to the region;
identifying the first imaging volume of interest from the region;
driving the shim coil assembly of the MRI system with a second set
of shimming currents to alter the B.sub.0 field inhomogeneity of
the magnetic field B.sub.0 within the first imaging volume of
interest such that the B.sub.0 field inhomogeneity within the first
imaging volume of interest is reduced from before the second set of
shimming currents were applied; and obtaining magnetic resonance
(MR) signals from the first imaging volume of interest using an
imaging pulse sequence.
[0011] Obtaining magnetic resonance signals from the first imaging
volume of interest may include: applying gradient pulses to encode
MR signals from the first imaging volume of interest; and in
response to the gradient pulses, acquiring the MR signals from the
first imaging volume of interest.
[0012] The imaging pulse sequence may include at least one of: a
gradient-echo pulse sequence, a spin-echo pulse sequence, a
steady-state free precession (SSFP) pulse sequence, and an
echo-planar imaging (EPI) pulse sequence.
[0013] Driving the shim coil assembly of the MRI system with a
first set of shimming currents may include: driving the shim coil
of the MRI system with a first set of shimming currents to
substantially minimize water saturation within the region.
[0014] The operations may further include: identifying a second
imaging volume of interest from the region; subsequently driving
the shim coil assembly of the MRI system with a third set of
shimming currents to alter the B.sub.0 field inhomogeneity of the
magnetic field B.sub.0 within the second imaging volume of interest
such that the B.sub.0 field inhomogeneity within the second imaging
volume of interest is reduced from before the third set of shimming
currents were applied; and obtaining magnetic resonance signals
from the second imaging volume of interest using the imaging pulse
sequence.
[0015] The region may cover a portion of a subject placed in the
main magnet. The portion of the subject may include at least one
of: an abdominal organ of the subject, a breast of the subject, a
neck of the subject, an extremity of the subject, and a head of the
subject.
[0016] Prior to obtaining the MR signals from the first imaging
volume of interest: applying a first slice-select gradient while
applying a first radio-frequency (RF) pulse such that protons from
the first imaging volume of interest are excited by the first RF
pulse.
[0017] Some implementations provide a method to compute sets of
shimming currents for a magnetic resonance imaging (MRI) system
having a main magnet that provides a magnetic field B.sub.0. The
method includes: obtaining a field map of a portion of the magnetic
field B.sub.0; obtaining a fat map of a portion of a subject placed
in the main magnet; obtaining a water map of the portion of the
subject placed in the main magnet; based on the field map, the fat
map, and the water map, computing values of a first set of shimming
currents that substantially minimize water saturation within a
region that covers the portion of the subject; obtaining location
information of a first imaging volume of interest, the first
imaging volume of interest included by the region; and based on the
field map, the fat map, the water map, and the location information
of the first imaging volume of interest, computing values of a
second set of shimming currents that substantially minimize the
magnetic field inhomogeneity within the first imaging volume of
interest.
[0018] Implementations may include one or more of the following
features.
[0019] The first set of shimming currents may substantially
maximize fat saturation within the region that covers the portion
of the subject.
[0020] The details of one or more aspects of the subject matter
described in this specification are set forth in the accompanying
drawings and the description below. Other features, aspects, and
advantages of the subject matter will become apparent from the
description, the drawings, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A to 1B illustrate an example of an apparatus for
performing magnetic resonance imaging (MRI).
[0022] FIGS. 2A to 2C illustrate examples of shimming for a volume
of coverage and slice selection when both fat and water signals are
present.
[0023] FIGS. 3A to 3C illustrate additional example of shimming for
the volume of coverage as illustrated in FIG. 2.
[0024] FIG. 4A to 4C illustrate examples of timing diagrams for
performing MR imaging in accordance with some implementations.
[0025] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0026] Various embodiments and aspects of the specification will be
described with reference to details discussed below. The following
description and drawings are illustrative of the specification and
are not to be construed as limiting the specification. Numerous
specific details are described to provide a thorough understanding
of various embodiments of the present specification. However, in
certain instances, well-known or conventional details are not
described in order to provide a concise discussion of embodiments
of the present specification.
[0027] Magnetic resonance (MR) imaging based on proton resonance
can provide excellent soft tissue contrast. However, protons from
fat tissue and water molecules (in non-fat tissue) experience
slightly different (for example, 3.5 ppm) resonant frequencies in a
magnetic field. Fat saturation methods can suppress fat signals,
thereby improving MR image quality for various applications.
Indeed, fat saturation can be added to many MR sequences with
minimal increase in scan time and minimal reduction in signal to
noise ratio (SNR). However, FatSat can be more challenging at lower
field, where the resonant frequencies of fat and water are closer
together. This can result in unrealistically long spectrally
selective excitation pulses or an intractable problem when the
B.sub.0 field cannot be sufficiently shimmed. As noted, the peak
corresponding to fat proton is about 3.5 ppm away from the water
proton. In comparison, a volume of interest can have, for example,
10 ppm B.sub.0 variation. To use this volume for MR imaging, the
frequency range for suppressing all of the fat signal would overlap
with the range for the water signal, rendering it impractical to
excite one but not the other.
[0028] The present specification discusses a system and method to
dynamically change shimming currents to adjust B.sub.0
inhomogeneity. A first set of shimming currents can be used to
achieve a first adjustment of B.sub.0 inhomogeneity for preparing
the entire volume or slab for MR imaging. For the purpose of fat
saturation, this first set of shimming currents may reduce the
amount of water excitation during fat saturation preparation when
spectrally selective fat excitations are applied to the entire
volume or slab. For example, the number of water protons excited
within the entire volume or slab can be minimized while the number
of fat protons excited within the entire volume or slab can be
maximized as an objective function for an optimization procedure.
After signal preparation, a second set of shimming currents can be
used to achieve a second adjustment of B.sub.0 inhomogeneity. This
second set of shimming currents may reduce the overall magnetic
field inhomogeneities within the volume of interest for high
performance imaging.
[0029] In slice-by-slice imaging, it is sufficient to excite (and
saturate) the fat signals in each slice separately before acquiring
imaging data on that slice. However, if water signal is excited and
saturated in another slice in the imaging volume, such saturation
could degrade image quality. Indeed, for each imaging slice for
subsequent slice-by-slice imaging, a different set of shimming
currents can be used to assert a specific adjustment of B.sub.0
inhomogeneity for a particular slice when the particular slice is
being selected. For example, the new set of shimming currents may
be set to maximize the number of in-plane voxels with fat that are
in the excitation frequency range, while minimizing the number of
water voxels in the coverage volume that are in the excitation
frequency range. Similar to the single volume of interest case
described previously, a second set of shimming currents for each
slice/slab may be applied during the acquisition process so as to
minimize field inhomogeneity for that particular slice/slab.
[0030] FIGS. 1A-1B show a perspective view and a cross-sectional
view of an example of a magnetic resonance imaging (MRI) system 100
in which a solenoid magnet 105 is provided in a cylindrical shape
with an inner bore 101. Coil assembly 107, including transmit coil
106 and gradient coil 104, is provided within solenoid magnet 105.
Coil assembly 107 may generally be shaped as an annular structure
and housed within the inner bore of solenoid magnet 105. In some
implementations, annular coil assembly 107 only includes gradient
coil 104. Gradient coil 104 generally provides field gradients in
more than one directions, such as, for example, all three
orthogonal spatial directions. Thus, gradient coil 104 may refer to
three sets of coils, each configured to generate field fluctuations
in a respective direction for the main field in the inner bore of
the solenoid magnet 105. Such field fluctuations may cause
magnetizations from various spatial locations to experience
precessions at different frequencies, enabling encoding of spatial
information of the magnetizations through RF excitation pulses.
[0031] In these implementations, annular coil assembly does not
include transmit coil 106 or any receiver coil. For these
implementations, radio-frequency (RF) excitation pulses are, for
example, transmitted by local coils for imaging the head region 102
of patient 103. In one instance, a head coil in a birdcage
configuration is used for both transmitting RF excitation pulses
and receiving MR signals for imaging the subject. In another
instance, a birdcage coil is used as a transmit coil and a local
coil (such as a surface coil) is used as a receiver coil. In yet
another instance, a surface coil is used for transmitting an RF
excitation pulse into the subject and a phased array coil
configuration is used for receiving MR signals in response. The MRI
system 100 can be used to scan various portions of a subject, for
example, an abdominal organ of the subject, a breast of the
subject, a neck of the subject, an extremity of the subject, and a
head of the subject.
[0032] In some implementations, shim coil assembly 109 are housed
within the gradient coil assembly 104. Shim coil assembly 109 may
include one or more of shim coils. Shim coil assembly 109 may be
powered by a group of power amplifiers. In some cases, the power
amplifiers are housed in a control room and are connected to shim
coil assembly 109 to provide shimming of the magnetic field within
inner bore 101. In driving shim coil assembly 109, power amplifiers
may be controlled by a control unit that generally includes one or
more processors as well as programming logic to configure the power
amplifiers. In some instances, the control unit is housed in a
control room separate from the solenoid magnet 105 of the MRI
system 100. Further, shim coil assembly 109 may not require active
cooling using circulating coolant. In these implementations, an
array of shimming coils can be used to provide adjustment to the
field strength within the inner bore 101 such that the magnet field
within the inner bore 101 becomes more homogenous.
[0033] The embodiments provided in this present specification
allows the operation of multiple RF coils inside an MR magnet. As
described earlier in this specification, an RF coil is a resonant
structure used to either excite the sample, receive signals from
the sample, or perform a combination of both functions during a
magnetic resonance imaging (MRI) acquisition. When operating an RF
coil in the presence of additional RF circuitry, for example, a
close-fitting "receive-only coil," the RF coil can be switched
"off" and "on" for two reasons. During transmission, a resonating
structure located between the transmit coil and the sample results
in a distortion of the transmit field and a reduction in
efficiency. Additionally, transmit coils operate at power levels
that far exceeds the range of receive-only circuits. For
illustration, if such transmit coils are coupled to the
receive-only coils, the power level during transmit events can
destroy the low-power receive-only circuitry. During reception, the
presence of any additional resonating structures in the vicinity of
the receive-only RF coil results in signal degradation and an
increase in overall system noise.
[0034] In this context, transmit coil 106, as an example of a radio
frequency (RF) coil, can be switched "on" and "off" during transmit
and receive operations. RF coils can be configured in an array for
performing excitation, receiving signals or a combination of both
functions. Arrays designed for signal excitation are known as
"transmit coils." Arrays designed for signal reception are known as
"receive coils." Arrays designed for both functions are known as
"transceive coils." Generally, RF arrays are composed of multiple
resonating antennae that are disposed in a judicious manner about
the imaging region such that: (i) efficiency can be maximized
during transmission, (ii) the magnitude of the received signal can
be maximized during reception (signal-to-noise ratio or `SNR`), and
(iii) a combination of both.
[0035] FIGS. 2A to 2C illustrate examples of shimming for a volume
of coverage and slice selection when both fat and water signals are
present. Initially, FIG. 2A shows a profile of B.sub.0
inhomogeneity as a function of slice location (along a z direction
in, for example, inner bore 101 of the magnet of MRI system 100)
across the region (for example, head region 102 of patient 103 for
imaging). The vertical axis corresponds to a frequency offset
measured in parts per million (ppm). The offset is with regard to
the Larmor frequency at a specific field strength. The horizontal
axis corresponds to a spatial measure of the volume (for example,
in the z direction that covers the target slice location). The
shade height at each horizontal location indicates a spread of the
frequency offset within a slice at the particular horizontal
location. As illustrated, B.sub.0 varies by approximately 10 ppm
over the entire coverage region, but can be less (for example, 4
ppm) at the specific target location. B.sub.0 inhomogeneity can be
altered by adjusting shimming current that run through shim coils
(for example, shim coil assembly 109 outside the inner bore 101).
In some cases, shim coil assembly 109 can provide corrections (such
as high order harmonics correction) to alter B.sub.0 inhomogeneity.
FIG. 2B shows an example of a profile of B.sub.0 inhomogeneity
after programming the currents that run through shim coils to
minimize the B.sub.0 inhomogeneity over the specified target slice.
The B.sub.0 inhomogeneity across the specific target slice is
reduced to about 1 ppm. However, the shimming also gives rise to
increased B.sub.0 inhomogeneity outside of the target slice. To
appreciate the effect at the target location and throughout the
region, FIG. 2C shows a plot of resonant frequency offset for both
water and fat chemical species. In this illustration, water and fat
species are separated into distinct frequency bands over the target
slice (where the B.sub.0 inhomogeneity has been reduced to 1 ppm);
but their frequency bands overlap outside of the target slice. This
newly introduced overlap outside the target slice can impose
unintended water saturation during, for example, a multi-slice
image acquisition when the fat saturation preparation pulses burn
out substantial water signals of a particular slice when
neighboring slices are being prepared for image acquisition, as
explained below.
[0036] Referring to FIG. 3A, applying a fat saturation pulse
effectively imposes a band of saturation after shimming for the
target slice. Specifically, signals within the illustrated fat
saturation band can be effectively nullified. As such, this fat
saturation band, when implemented, effectively saturates fat
signals within the target slice. At the same time, however, the
saturation band also effectively nullifies water signals from
outside the target slice but within the imaging volume because
water signals from these regions fall under the saturation band.
This nullification is illustrated in FIG. 3B, which shows fat
saturation preparation pulses creating the fat saturation band
shown in FIG. 3A that results in collateral water saturation
outside of the target slice (but within the coverage region of the
image volume). The fat saturation preparation pulses thus suppress
water signals within the fat saturation band but outside the target
slice. This unintended water saturation outside the target slice
can degrade image quality in other slices within the coverage
region. Indeed, the fat saturation preparation pulses are selective
of frequency but not by spatial location. In this example, such a
fat saturation pulse may not be designed to simultaneously saturate
fat signal within the target slice and spare water signal within
the coverage region.
[0037] To mitigate the issue of unintended saturation, the MRI
system 100 can be programmed to have one set of shim currents that
drive shim coil assembly 109 while the fat saturation preparation
pulses are played and use a different set of shim currents for the
shim coils during imaging pulses when MR signals are being acquired
from each selected slice. During the fat saturation preparation
pulse, the shim currents are designed to alter the B.sub.0 field
inhomogeneity as shown in FIG. 3C. Here, the shimming currents may
sacrifice some field homogeneity of the target slice (for example,
an inhomogeneity of 2 ppm across the target slice but without
collateral water saturation outside the target slice but within the
coverage region) in order to move the water frequencies across the
entire coverage region outside of the fat saturation band. In other
words, the objective function of the shimming currents during this
fat saturation phase is to maximize fat signals within the target
slice while minimizing water signals within the entire range. This
objective function is therefore not limited to optimizing a metric
from the target slice alone. Generally speaking, the fat saturation
region can be the union of all subsequent imaging slices (or volume
of interest). After the fat saturation preparation pulses have been
applied, the shim currents can change to a new setting to alter the
field homogeneity across the target slice (for example, fine tuning
the shimming currents to achieve an inhomogeneity of 1 ppm within
the target slice).
[0038] Referring to FIG. 4A, an example of a timing diagram 400 for
performing MR imaging is provided. Timing diagram 400 illustrates a
pulse sequence for global shimming with spoiler gradient 402
following RF preparation pulse 401. Thereafter, RF transmitting
pulse 403 is applied under slice selective pulse 404. Image
encoding gradients 405 is subsequently applied to encode MR
signals. The MR signals are received as signals 406 at the receiver
coil. One set of constant shim currents 407 is set at the beginning
of the scan and held during data acquisition for all imaging
slices, for example, from slice 1 to slice 2.
[0039] FIG. 4B shows an example of a timing diagram 410 of a pulse
sequence for slice-by-slice dynamic shimming. Each slice is given
its own set of shim currents. For example, shim currents 411 apply
to slice 1 and shim currents 412 apply to slice 2. Here, the shim
currents for a target slice are chosen to minimize B.sub.0
inhomogeneity over the target slice.
[0040] FIG. 4C shows another example of a timing diagram 420 of a
pulse sequence according to some implementations. As discussed,
during fat saturation preparation for slice 1, a first set of shim
currents 421 may be applied to suppress fat signals from the target
slice. The first set of shim currents 421 may not maximize fat
suppression within the target slice. Yet, the first set of shim
currents 421 may minimize water saturation, which is unintended,
throughout the volume of interest as a whole. As noted, this
minimization of water saturation within the whole volume may not
correspond to maximization of fat saturation within a target slice.
When the fat saturation preparation pulses are completed, the shim
currents may transition to a second setting 422 that, for example,
can result in a minimization of field inhomogeneity within the
target slice for high-performance imaging. When imaging the next
slice, for example, slice 2, a new set of shim currents 423 may be
applied to suppress fat signals from the second target slice.
Similar to shim currents 421, the shim currents 422 may not result
in maximum fat suppression over the second target slice, yet may
minimize water saturation over the volume of interest as a whole.
After fat saturation and during imaging acquisition for slice 2,
another set of shimming pulses 424 can be used, once again to
minimize field homogeneity over the target slice.
[0041] A variety of MR imaging sequences can incorporate these
signal acquisitions, including, for example, a gradient-echo pulse
sequence, a spin-echo pulse sequence, a steady-state free
precession (SSFP) pulse sequence, and an echo-planar imaging (EPI)
pulse sequence. Moreover, when each slice-select gradient 412A,
412B, or 412C is applied, shim currents 401 may have transitioned
from the first setting of 402 to a corresponding second setting.
The exact timing may not be critical. In some cases, the transition
may immediately precede slice-selection. In other cases, the
transition may immediately follow slice-selection.
[0042] By way of examples, implementations can provide shim
currents in the second setting that are substantially identical,
even though the target slices are spatially different (for example,
neighboring slices). The shim currents in the second setting can
also vary by slice location as the second setting attempts to fine
tune the B.sub.0 magnetic field by corrections via high order
harmonic terms through the shim coils. Moreover, several
neighboring slices may form a slab or a composite volume for which
a volumetric acquisition can be performed, for example, by an
additional gradient waveform in the z direction. In other words,
slice selection may involve selecting a slab with a
three-dimensional signal acquisition for the slab.
[0043] In contrast to implementations that rely on spectro-spatial
pulses, the implementations of the current specification does not
use gradient coils only. Instead, the implementations of the
current specification uses shim coils for correcting B.sub.0
inhomogeneity. Indeed, other implementations treat the B.sub.0
field as fixed and the optimization problem as an RF pulse only
problem addressed by a spectrally selective RF pulse, or a
spectrally selective RF pulse in combination with gradient
waveforms. Moreover, the first set of shim currents of the present
specification may result in fat saturation preparation pulses of
similar duration as compared to a typical spectrally selective fat
saturation (FatSat) pulse. In contrast, spectro-spatial pulses can
result in significantly longer pulses and may not be appropriate in
many imaging applications. Although the examples discussed above in
the present specification is contrasted with spectro-spatial
pulses, it is understood that the above examples and
spectro-spatial pulses are not mutually exclusive techniques. In
particular, the above examples can incorporate the use of
spectro-spatial pulses with the potential for further improvement
in FatSat quality, at the expense of longer FatSat pulse
durations.
[0044] Implementations disclosed by the present specification are
not limited to MRI systems with a small-bore magnet (for example,
not large enough for whole body scanning) or a low-field magnet
(for example, lower than 1.5 Tesla). In fact, these implementations
are equally applicable to MRI systems with a large-bore magnet (for
example, accommodating whole body scanning) or a high-field magnet
(for example, no lower than 1.5 Tesla).
[0045] Moreover, sets of shimming currents can be computed for
magnetic resonance imaging (MRI) system 100 having a main magnet
that provides a magnetic field B.sub.0 in inner bore 101.
Initially, a field map of the magnetic field B.sub.0 can be
obtained. For example, a field map can be calculated based on the
difference in phase between two different echoes in a double echo
sequence. Additionally or alternatively, two separate acquisitions
with opposite phase encoding directions can be used to calculate a
field map based on the difference in distortion between the two
acquisitions. Thereafter, a fat map of a portion of a subject
placed in the main magnet (for example, inner bore 101) can be
obtained. This fat map can be obtained by globally suppressing
water signals. Likewise, a water map of the portion of the subject
placed in the main magnet (for example, inner bore 101) can also be
obtained by globally suppressing fat signals. In some cases, the
fat map, the water map, and the field map may be jointly obtained
using a chemical shift encoded method. In some cases, the fat map
and the water map can be obtained as fractional maps (for example,
based on the Dixon method to extract in-phase and out-of-phase
signals). Based on the field map, the fat map, and the water map,
values of a first set of shimming currents that substantially
minimize water saturation while providing adequate fat saturation
within a region that covers the portion of the subject can be
obtained. Subsequently, location information of a first imaging
volume of interest, the first imaging volume of interest included
by the region can be obtained. Based on the field map, the fat map,
the water map, and the location information of the first imaging
volume of interest, values of a second set of shimming currents can
be computed that substantially minimize field inhomogeneities
within the first imaging volume of interest.
[0046] As used herein, the terms "comprises" and "comprising" are
to be construed as being inclusive and open ended, and not
exclusive. Specifically, when used in the specification and claims,
the terms "comprises" and "comprising" and variations thereof mean
the specified features, steps or components are included. These
terms are not to be interpreted to exclude the presence of other
features, steps or components.
[0047] As used herein, the term "exemplary" means "serving as an
example, instance, or illustration," and should not be construed as
preferred or advantageous over other configurations disclosed
herein.
[0048] As used herein, the terms "about" and "approximately" are
meant to cover variations that may exist in the upper and lower
limits of the ranges of values, such as variations in properties,
parameters, and dimensions. In one non-limiting example, the terms
"about" and "approximately" mean plus or minus 10 percent or
less.
[0049] The specific embodiments described above have been shown by
way of example, and it should be understood that these embodiments
may be susceptible to various modifications and alternative forms.
It should be further understood that the claims are not intended to
be limited to the particular forms disclosed, but rather to cover
all modifications, equivalents, and alternatives falling within the
spirit and scope of this specification.
* * * * *