U.S. patent application number 09/967644 was filed with the patent office on 2003-04-03 for oscillating dual-equilibrium steady state angiography using magnetic resonance imaging.
This patent application is currently assigned to THE BOARD OF TRUSTEES OF THE LELAND STANDFORD JUNIOR UNIVERSITY. Invention is credited to Overall, William R..
Application Number | 20030062893 09/967644 |
Document ID | / |
Family ID | 25513097 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030062893 |
Kind Code |
A1 |
Overall, William R. |
April 3, 2003 |
OSCILLATING DUAL-EQUILIBRIUM STEADY STATE ANGIOGRAPHY USING
MAGNETIC RESONANCE IMAGING
Abstract
A method of oscillating dual-equilibrium steady-state
angiography (ODESSA), utilizes a modified steady state free
precession (SSFP) pulse sequence. The SSFP sequence is modified
such that flowing material reaches a steady state which oscillates
between two equilibrium values, while stationary material attains a
standard, non-oscillatory steady state. When alternating sequences
are employed, subtraction of adjacent echoes results in large,
uniform signal from all flowing spins and zero signal from
stationary spins. Venous signal can be suppressed based on its
reduced T.sub.2. ODESSA arterial signal is more than three times as
large as that of traditional phase-contrast angiography (PCA) in
the same scan time, and also compares favorably with other
techniques of MR angiography. Pulse sequences are implemented in
2D, 3D, and volumetric projection modes. Angiograms of the lower
leg, generated in as few as 5 s, show high arterial SNR and full
suppression of other tissues.
Inventors: |
Overall, William R.; (Menlo
Park, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
THE BOARD OF TRUSTEES OF THE LELAND
STANDFORD JUNIOR UNIVERSITY
900 Welch Road, Suite 350
Palo Alto
CA
94304
|
Family ID: |
25513097 |
Appl. No.: |
09/967644 |
Filed: |
September 28, 2001 |
Current U.S.
Class: |
324/306 ;
324/307; 324/309 |
Current CPC
Class: |
G01R 33/5613 20130101;
G01R 33/563 20130101 |
Class at
Publication: |
324/306 ;
324/309; 324/307 |
International
Class: |
G01V 003/00 |
Claims
What is claimed is:
1. A method of magnetic resonance angiography comprising the steps
of: a) placing an object to be imaged in a static magnetic field,
b) applying an RF pulse and axial gradients to the object, c)
detecting first signals from nuclei in the object, d) applying an
RF pulse, axial gradient, and a bipolar flow-encoding pulse, e)
detecting second signals from nuclei in the object, f) repeating
steps b) and c) to establish a steady state of first signals, g)
repeating steps d) and e) to establish a steady state of second
signals, and h) subtracting the steady state of first signals from
the steady state of second signals to obtain signals of non-static
material.
2. The method as defined by claim 1 wherein steps b) and c) and
steps d) and e) are repeated sequentially in order to produce a
steady state which oscillates between first and second signals.
3. The method as defined by claim 1 wherein steps b) and c) are
repeated a multiple of times for each of steps d) and e).
4. The method as defined by claim 1 wherein step b) includes
applying a three-lobed compensation pulse to mitigate the effects
of eddy currents, concomitant Maxwell gradients, and
acceleration.
5. The method as defined by claim 1 wherein step d) the bipolar
flow-encoding pulse follows step e).
6. The method as defined by claim 1 wherein in step d) the bipolar
flow-encoding pulse precedes step e).
7. The method as defined by claim 1 wherein step h) provides two
dimensional image signals.
8. The method as defined by claim 1 wherein step h) provides three
dimensional image signals.
9. The method as defined by claim 1 wherein in step d) the bipolar
pulse is applied on any axis.
10. The method as defined by claim 1 wherein in step d) bipolar
pulse is applied on a combination of axes.
11. The method as defined by claim 1 wherein in step d) the bipolar
pulse is changed at each iteration or groups of iterations.
12. The method as defined by claim 1 wherein in steps b) and d) RF
phase is constant.
13. The method as defined by claim 1 wherein in steps b) and d) RF
phase is incremented for each RF pulse.
14. A method of magnetic resonance angiography comprising the steps
of: a) placing an object to be imaged in a static magnetic field,
b) applying a first refocused steady state precession pulse and
gradient sequence to obtain first signals from nuclei in the
object; c) applying a second refocused steady state precession
pulse and gradient sequence including a bipolar flow encoding pulse
to obtain second signals from nuclei in the object, d) repeating
step b) to establish a steady state of first signals, e) repeating
step c) to establish a steady state of second signals, and f)
subtracting the steady states of first signals and second signals
to obtain signals of non-static material.
15. The method as defined by claim 14 wherein steps b) and c) are
repeated sequentially in order to produce a steady state which
oscillates between first and second signals.
16. The method as defined by claim 14 wherein step b) is repeated a
multiple of times for each step c).
17. The method as defined by claim 14 wherein step b) includes
applying a three-lobed compensation pulse to mitigate the effects
of eddy currents, concomitant Maxwell gradients, and
acceleration.
18. The method as defined by claim 14, wherein in step c) the
bipolar flow-encoding pulse follows second signal detection.
19. The method as defined by claim 14 wherein in step c) the
bipolar flow encoding pulse precedes second signal detection.
20. The method as defined by claim 14 wherein step f) provides two
dimensional image segments.
21. The method as defined by claim 14 wherein step f) provides
three dimensional image signals.
22. The method as defined by claim 14 wherein in step c) the
bipolar pulse is applied on any axis.
23. The method as defined by claim 14 wherein in step c) the
bipolar pulse is applied on a combination of axes.
24. The method as defined by claim 14 wherein in step c) the
bipolar pulse is changed at each iteration or groups of
iterations.
25. The method as defined by claim 14 wherein in steps b) and c) RF
phase is constant.
26. The method as defined by claim 14 wherein in steps b) and c) RF
phase is incremented for each sequence.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to magnetic resource
imaging (MRI), and more particularly the invention relates to
magnetic resonance angiography (MRA) using an oscillating
dual-equilibrium steady state free precession process.
[0002] Magnetic resonance angiography (MRA) is a clinically
relevant non-invasive alternative to traditional X-ray angiography.
A number of approaches have been used to isolate signal from
vessels. Inflow-based methods, such as time-of-flight and
spin-tagging, waste significant scan time while waiting for inflow
to occur and may suffer from reduced signal in distal portions of
the artery being imaged. Flow-independent techniques make use of
the inherent T.sub.1 and T.sub.2 relaxation times of arterial and
venous blood to generate the desired contrast. However, these
techniques suffer from incomplete suppression of background signal
and may require careful selection of numerous scan parameters for
ideal performance.
[0003] Phase contrast angiography (PCA) makes use of the phase
accrual of moving transverse spins during the application of a
gradient. This technique can suppress background signal very well;
however, the resulting blood signal strength is proportional to
flow in the direction of flow sensitivity. As a result, three
acquisitions are often required to get uniform signal from the
extent of the vessel. Care must also be taken to match the velocity
sensitivity of the sequence to the flow rate in the vessel of
interest. Finally, because signal phase is proportional to
velocity, thick-slab images suffer from signal loss in pixels where
multiple vessels overlap.
[0004] T.sub.1-shortening contrast agents, primarily gadolinium
compounds, have also been used successfully to produce angiographic
images. These techniques boost blood signal while keeping
background signal from other spins constant; this background signal
can reduce image contrast. Moreover, several doses of contrast
agent are often required in order to produce the necessary
contrast, and acquisitions must be carefully timed in order to
isolate arterial signal.
BRIEF SUMMARY OF THE INVENTION
[0005] The invention is a new angiographic process, which is
denoted oscillating dual-equilibrium steady state angiography
(ODESSA). This method incorporates velocity-dependent phase shifts
into a modified steady state free precession (SSFP) process in
order to select a blood vessel signal. Refocused SSFP pulse
sequences, which refocus all magnetization over the RF repetition
interval TR, provide a means of rapid imaging while maintaining
high signal. Until recently, these methods have been largely
ignored for imaging applications because of their sensitivity to
off-resonant precession. Recently, high-performance imaging systems
have allowed refocused SSFP sequences with very short TRs, which
minimize these unwanted artifacts.
[0006] More particularly, the invention utilizes short-TR refocused
SSFP sequences to rapidly produce images with angiographic
contrast. In order to isolate signal from blood, an oscillating
steady state is generated for flowing material through the periodic
addition of a bipolar flow-encoding pulse, such as every second TR,
for example. Static tissue is unaffected by this bipolar pulse, and
thus approaches a single steady state. Addition of adjacent echoes
produces anatomic contrast similar to that of standard SSFP
sequences. Subtraction of adjacent echoes results in suppression of
signal from stationary tissue, while the signal from flowing blood
retains the high SNR expected of SSFP sequences. Furthermore, the
subtracted signal from flowing blood is uniform in magnitude and
phase over a wide range of velocities. Therefore, scans with
multiple directions of velocity sensitivity may not be necessary,
and thick-slab projection imaging is possible. Contrast between
arterial and venous blood can be adjusted through manipulation of
the RF tip angle. The technique can also be used in conjunction
with a T.sub.1-shortening contrast agent to further enhance
SNR.
[0007] The invention and objects and features thereof will be more
readily apparent from the following description and appended claims
when taken with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIGS. 1(a) and 1(b) illustrate SSFP processes for constant
TR precession and constant RF tips and for either the RF tips or
the precession is periodic, respectively.
[0009] FIGS. 2(a) and 2(b) illustrate 2D and 3D oscillating
equilibrium SSFP sequences, respectively, in accordance with an
embodiment of the invention.
[0010] FIGS. 3(a) and 3(b) illustrate steady state magnetism for
stationary and for moving spins, respectively, in pulses sequences
of FIGS. 2(a) and 2(b).
[0011] FIGS. 4(a) and 4(b) are graphs of magnitude and phase versus
velocity induced precession for ODESSA echoes and for the complex
sum and difference of the echoes, respectively.
[0012] FIG. 5 is a graph of difference magnitude versus tip angle
of blood using ODESSA and PCA.
[0013] FIG. 6 is a graph difference magnitude versus TR for ODESSA
and for PCA.
[0014] FIG. 7 is plots of magnitude and phase versus
velocity-induced precession angle for ODESSA and PCA.
[0015] FIG. 8 illustrates ODESSA signal magnitude versus free
precession and velocity-induced precession.
[0016] FIG. 9 is graphs of magnitude and phase observed in a flow
phantom.
[0017] FIGS. 10(a), 10(b) are images of axial ODESSA knee images
showing anatomic sum image and anatomic difference image,
respectively.
[0018] FIGS. 11(a), 11(b) are images of 3D ODESSA coronal slice in
the lower leg of a human showing anatomic sum image and
angiographic difference image, respectively.
[0019] FIG. 12 is a 3D image of a lower-leg arterial tree generated
from ODESSA data.
[0020] FIG. 13 is a plot of normalized scan efficiency of the
ODESSA process and gradient-echo imaging versus concentration of
contrast agent.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0021] FIGS. 1(a) and 1(b) illustrate the key differences between a
conventional SSFP sequence and a SSFP sequence employing ODESSA, in
accordance with the invention.
[0022] A "standard" refocused SSFP pulse sequence (FIG. 1a)
consists of equal a-tips separated by an RF repetition time (TR).
During each TR interval, transverse spins precess by an angle
.theta.=.theta..sub.F+.the- ta..sub.I. .theta..sub.F represents
free precession from various sources of off-resonance, and
.theta..sub.I represents precession induced by the pulse sequence.
Put another way, .theta..sub.I represents those components of
intra-TR precession which the pulse sequence designer can
manipulate. In a standard refocused SSFP sequence, magnetization
after the nth tip ({right arrow over (M)}.sub.n) approaches a
steady state {right arrow over (M)}.sub.ss for large n:
{right arrow over (M)}.sub.n={right arrow over (M)}.sub.n+1={right
arrow over (M)}.sub.ss for all n sufficiently large. (1)
[0023] In contrast, an "oscillating" steady state sequence (FIG.
1b) repeats a pattern of tips and precession periodically in order
to produce useful contrast. In this paper, we explore only those
sequences with a repetition interval of 2-TR, in which case the
steady state conditions are
{right arrow over (M)}.sub.2n-1={right arrow over
(M)}.sub.2n+1={right arrow over (M)}.sub.ss1
{right arrow over (M)}.sub.2n={right arrow over (M)}.sub.2n+2
={right arrow over (M)}.sub.ss2 for all n sufficiently large.
(2)
[0024] Hence, two distinct steady states, {right arrow over
(M)}.sub.ss1 and {right arrow over (M)}.sub.ss2, are produced.
[0025] Oscillating steady states generated by periodically varying
RF phase are well-documented. Altering the RF phase linearly (e.g.,
phase schedules of {0.degree., 180.degree. . . . } or {0.degree.,
90.degree., 180.degree., 270.degree. . . . }) shifts the response
of the steady state with respect to .theta. (12, 17). Other RF
phase schedules can be used to reduce sensitivity to off-resonant
precession or to selectively image water and fat.
[0026] An oscillating steady state can also be obtained by
periodically varying the amount of precession induced within TR,
.theta..sub.I. If the amount of precession induced during odd and
even TR intervals (.theta..sub.I1 and .theta..sub.I2, respectively)
is manipulated such that .theta..sub.I1=.theta..sub.I2 for static
tissue and .theta..sub.I1.noteq..theta..sub.I2 for moving tissue,
then an oscillating steady state occurs only for moving tissue. The
complex difference between adjacent echoes in the steady state,
defined as
{right arrow over (D)}.sub.ss={right arrow over (M)}.sub.ss2-{right
arrow over (M)}.sub.ss1, (3)
[0027] can then be used to isolate signal from moving tissue.
[0028] 2D and 3D sequences which generate this oscillating steady
state are shown in FIG. 2. Two TR intervals are shown. Each axis
has zero net gradient area over the TR interval. During odd TRs (at
left), a bipolar flow-encoding pulse follows readout on any axis. A
triphasic pulse after even readouts, though not necessary, is
included to mitigate imaging system nonidealities. The numbered
locations correspond to spin states shown in FIG. 3. All three
gradient axes are fully refocused for static spins over each TR. RF
chopping is employed in this sequence; that is, the transmitter and
receiver phase is incremented by 180.degree. at each excitation. A
bipolar flow-encoding gradient is included on a user-selectable
axis after each odd readout. This flow-encoding gradient ideally
has no effect on static spins, but causes precession .theta..sub.I1
proportional to velocity v for moving spins: 1 I1 = m 1 v = v 0 TR
G FE ( ) , ( 4 )
[0029] where m.sub.1 is the first moment of the flow-encoding
gradient waveform G.sub.FE(t), and .gamma. is the gyromagnetic
ratio.
[0030] For static spins, .theta..sub.I1=.theta..sub.I2=0, and a
standard steady state is produced. Therefore, {right arrow over
(D)}.sub.ss=0, for static spins. Moving spins, however, precess by
.theta..sub.I1+.theta..su- b.F during odd TR intervals, and precess
by .theta..sub.F during even intervals. This produces an
oscillating steady state, with nonzero {right arrow over
(D)}.sub.ss.
[0031] Since the flow-encoding pulse is included only for its
effect on the steady state, it can be located either before or
after readout with little effect on performance. Two relatively
minor factors led to the placement of the pulse after readout.
First, this placement minimizes eddy current effects from the
bipolar pulse during readout, thereby reducing image distortion.
Also, this placement allows a shorter TE, and therefore better
signal from spins with short T.sub.2*.
[0032] After even echoes, a three-lobed compensation pulse is
present. While not strictly necessary, this pulse is intended to
mitigate the effects of certain imaging system nonidealities. It is
designed to have the same squared area as the flow-encoding pulse,
causing the first-order concomitant-gradient Maxwell terms to be
identical in even and odd TR intervals. Therefore, any concomitant
gradient-induced precession will be identical for each TR and will
not contribute to an oscillation in the steady state.
[0033] The compensation pulse can be further designed to match the
eddy current effects of the bipolar pulse. Eddy current effects
within a TR tend to cancel themselves out, but those eddy currents
persisting beyond the next RF pulse can cause unequal precession
from TR to TR. While the optimal compensation strategy would be
machine-dependent, good matching to first order can be achieved by
aligning the final ramps of the two pulses (flow-encoding and
compensation) within each TR. Other methods of making the pulse
sequence less sensitive to these effects are discussed below.
[0034] The steady-state ODESSA signal was derived analytically
through a matrix inversion assuming tip angle .alpha.=30.degree.
.theta..sub.F=0, TR=6 ms, T.sub.1=1000 ms, and T.sub.2=220 ms.
These relaxation parameters are based on experimental measurements
of fully-oxygenated arterial blood. The result (FIG. 4a) shows a
marked difference between even and odd echoes for all nonzero
.theta..sub.I1. Odd echoes (during which the flow-encoding pulse
occurs) exhibit nearly zero signal for all moving spins, while even
echoes exhibit uniformly high signal. The signal profile for odd
echoes suggests that they might be used alone in order to produce
images with black blood contrast. The ODESSA signal (FIG. 4b),
which is the complex difference of the two echoes, is uniformly
high for all nonzero precession, with a sharp notch producing no
signal for stationary spins. Furthermore, the phase of the ODESSA
signal is near zero at all velocities. FIG. 4 illustrates simulated
signal (top: magnitude, bottom: phase) vs. velocity induced
precession .theta..sub.I1 (.alpha.=30.degree. TR=6 ms, T.sub.1=1000
ms, and T.sub.2=220 ms). Signal from each of the two ODESSA echoes
is plotted in (a); the complex sum and difference of these echoes
is shown in (b). Even echoes exhibit signal attenuation in the
presence of flow, while odd echoes show signal enhancement. ODESSA
signal difference is zero for static spins and uniformly large for
moving spins. The horizontal axis can be converted to velocity
units through Eq. [4].
[0035] While FIG. 4 was produced using specific sequence and tissue
parameters, the overall characteristic of the signal (both sum and
difference) is maintained over a wide range of .alpha., TR,
T.sub.1, and T.sub.2. One case of interest is that of venous blood,
whose T.sub.2 can range from 35 ms to more than 100 ms, depending
on oxygenation and effective refocusing interval. This reduced
T.sub.2 results in lower overall signal, as seen in FIG. 5, but the
sharp stopband and uniform signal is maintained. Thus, the ODESSA
sequence intrinsically suppresses venous signal in favor of
arterial signal. The relative proportion of venous signal can be
manipulated through proper selection of tip angle. FIG. 5
illustrates simulated difference magnitude vs. tip angle .alpha.
(.theta..sub.I1=180.degree., TR=6 ms, T.sub.1=1000 ms,
T.sub.2,arterial=220 ms, T.sub.2,venous=50 ms). The ODESSA signal
peak (solid lines) is broader than that of PCA (dashed line), and
occurs at a higher tip angle. Unlike PCA, ODESSA signal is
dependent on T.sub.2; therefore, contrast is generated between
arterial and venous blood. This contrast can be manipulated through
judicious choice of the ODESSA tip angle. Here, a tip angle of
30.degree. is used to generate high contrast between venous and
arterial signal.
[0036] The ODESSA difference profile is particularly desirable for
a number of reasons. First, it provides uniform signal for all flow
rates which correspond to more than about 10.degree. of precession
(as calculated by Eq. [4]). Thus, spins moving within a broad range
of speeds and directions will all exhibit equal signal. This allows
a tortuous vessel to be imaged using flow encoding only along its
primary direction. As long as the flow component in the direction
of the flow-encoding gradient does not fall below the 10.degree.
threshold, it will provide uniform signal regardless of its
orientation. Furthermore, since signal phase above this threshold
does not vary significantly, signal from vessels which are
superimposed will add in-phase. This facilitates projection imaging
as well as imaging of thick slabs. Finally, the sequence is very
sensitive to small amounts of induced precession, allowing imaging
of very slow flow without the necessity of large flow-encoding
gradients.
[0037] Summation of adjacent echoes, {right arrow over
(M)}.sub.ss1+{right arrow over (M)}.sub.ss2, provides anatomic
contrast very similar to that of a standard refocused SSFP sequence
with NEX=2. The sum signal from motionless tissue is identical to
that from the standard refocused SSFP sequence, while signal from
moving arterial blood is slightly attenuated (79% of the static
blood signal, as seen in FIG. 4b).
[0038] FIG. 6 illustrates simulated difference magnitude vs. TR
(.theta..sub.I1=180.degree., =1000 ms, T.sub.2=220 ms). ODESSA
(solid line) and PCA (dashed line) magnitudes are calculated using
the maximizing tip angle for each TR. ODESSA signal is maximized by
reducing TR, while PCA signal is maximized by increasing TR.
[0039] ODESSA signal strength increases as TR decreases (FIG. 6);
therefore, reductions in TR while holding per-TR readout time
constant will result in both higher SNR and greater scan
efficiency. Furthermore, the assumption of negligible free
precession .theta..sub.F is only valid at very short TRs.
Therefore, this sequence should be designed to operate with the
shortest possible repetition time. Signal strength is maximized for
arterial blood at a tip angle near 35.degree. for TR=6 ms (FIG. 5),
while signal from venous blood is maximized at smaller tips.
Therefore, a relatively large tip of .alpha.=30.degree. is employed
throughout this study in order to maximize arterial signal. If less
contrast between venous and arterial blood is desired, a smaller
tip in the range of 15.degree. can be employed.
[0040] FIG. 3 illustrates pictorial diagrams of steady state
magnetization for (a) stationary and (b) moving spins in an ODESSA
sequence. Numbered vectors correspond to labeled time points in
FIG. 2. Induced precession during the 2-3 interval results in an
asymmetric steady state for moving spins. Odd echoes, acquired at
time point 2, experience signal attenuation due to flow. Even
echoes, acquired at time point 4, experience signal enhancement due
to flow.
[0041] For static tissue, excitations with alternating RF polarity
cause the magnetization to oscillate about the longitudinal (z)
axis. Recovery occurs identically in each TR, and the steady state
is symmetric about this axis. Subtraction of the transverse
component (after compensating for RF chopping) results in zero
signal difference. When tissue is in motion, it precesses unequally
in even and odd TR intervals, and the symmetry of the steady state
is perturbed. For all moving spins, the magnetization is
"tipped-up", or close to the longitudinal axis, during odd echoes
(time point 2). Therefore, the exact amount of velocity-induced
precession has minimal effect on the steady state magnetization.
This vector is tipped into the transverse plane during even echoes
(time point 4), resulting in signal enhancement. Because precession
due to motion occurs when the magnetization is primarily
longitudinal, phase does not accrue, and the signal difference
vector remains along y.
[0042] An interesting point to note from FIG. 3 is that recovery
plays a relatively minor role in the steady state for flowing
spins. The combined effect of recovery and induced precession
between time points 2 and 3 is small because the vector is
tipped-up during this time. As a result, the counterbalancing
effect of recovery during the 4-1 interval must also have a small
effect on the direction and magnitude of the magnetization.
Therefore, the presence of this tipped-up state is crucial to the
performance of the ODESSA sequence.
[0043] To illustrate the importance of the tipped-up state,
consider two minor 1-5 sequence variations which alter the steady
state. The new steady states do not include a tipped-up state, and
as a result, the unique properties of ODESSA are lost.
[0044] As a first modification, we note that the ODESSA sequence
utilizes an unbalanced acquisition, with flow-encoding occurring
only on odd-numbered TRs. One might also use flow encoding
gradients which alternate in polarity each TR in order to generate
an oscillatory steady state. The inherent symmetry of this sequence
means that the steady state will also be symmetric about the
longitudinal axis. Thus, no tipped-up state is present, and
recovery must have a larger effect in order to counterbalance
induced precession. This causes a net reduction in the steady state
vector magnitude for flowing spins, as can be seen in FIG. 7.
[0045] FIG. 7 illustrates comparison of several variations on the
ODESSA sequence. Simulated signal (top: magnitude, bottom: phase)
is plotted vs. velocity induced precession .theta..sub.I1(TR=6 ms,
T.sub.1=1000 ms, T.sub.2=220 ms, and tip angle .alpha. set at the
maximizing value for arterial blood from FIG. 5). ODESSA signal
magnitude is plotted with (solid line) and without (dashed line) RF
chopping. ODESSA signal phase is unaffected by chopping. The signal
from an ODESSA sequence with alternating-sign flow-encoding pulses
is plotted with the dotted line. Signal generated by a PCA sequence
with the same TR and using the maximizing tip from FIG. 5 is also
shown, for comparison.
[0046] Another possible sequence modification is to use equal RF
tips, rather than the chopped RF utilized in ODESSA. Because each
tip is in the same direction, magnetization is no longer tipped
back to the longitudinal axis just prior to induced precession. As
a result, the exact amount of precession has a more pronounced
impact on the steady state. In this case, the signal magnitude is
not uniform for all velocities (FIG. 7); instead, it increases with
velocity similarly to phase contrast angiography, discussed
next.
[0047] Comparisons were made between theoretical ODESSA signal and
the signal predicted in a complex-difference PCA experiment. For
flow with velocity corresponding to .theta..sub.I1=180.degree.,
ODESSA produces over 300% of the signal of the optimal PCA sequence
in the same scan time with TR<6 ms (FIG. 7). At lower velocities
(.theta..sub.I1<180.degr- ee.), ODESSA signal is unchanged, but
PCA signal reduces dramatically. For instance, at a velocity
corresponding to .theta..sub.I1=45.degree., the ODESSA signal is
greater than 5 times the PCA signal. As TR decreases, this
advantage increases (FIG. 6).
[0048] Because signal is maintained over a range of induced
precession, smaller flow-encoding gradients can be used without
loss of signal. Therefore, slow flow which would require
prohibitively large flow-encoding gradients in PCA studies could be
imaged with ODESSA using smaller encoding gradients. Additionally,
when flow is primarily along one axis, separate scans with
flow-encoding gradients on each axis are not necessary. ODESSA
signal can also be tailored to use T.sub.2 differences between
arterial and venous blood as a contrast mechanism. Finally, ODESSA
does not suffer from signal loss due to intra-voxel velocity
dispersion as PCA does, allowing rapid acquisition of projection
images.
[0049] While ODESSA signal is zero for all spins with
.theta..sub.11=.theta..sub.12, even a small difference between
induced precession in even and odd echoes can produce a large
signal difference. This sensitivity is desirable in order to
produce uniform signal for all flowing material, but it also
requires that extreme care be taken so that nonidealities in the
imaging equipment do not cause an oscillation in the steady state.
As the pulse sequence is identical in even and odd echoes except
for the flow-encoding and compensation pulses, these pulses must be
designed so that any precession which they induce in static tissue
is identical.
[0050] As described above, the compensation pulse produces
concomitant Maxwell gradients equal to those of the bipolar pulse,
to first order. It is also positioned to mimic the eddy current
behavior of the bipolar pulse. These efforts can be augmented by
careful calibration of the gradient preemphasis network available
on most commercial scanners. A method similar to that described by
Papadakis (Magnetic Resonance in Medicine 2000;44:616-624) was
implemented in the experiments described herein, with good
results.
[0051] If these compensation strategies are not effective, the
variant of the ODESSA pulse sequence without RF chopping may be
utilized, as discussed above, with signal as shown in FIG. 7. This
widens the effective stopband of the ODESSA sequence, allowing a
larger amount of undesired precession without producing large
artifacts. While none of the studies described below utilize this
technique, it might be useful in situations wherein pulse
compensation fails.
[0052] Another source of error which must be considered is free
precession .theta..sub.F. This includes all sources of precession
which manifest themselves equally in even and odd TR
intervals-B.sub.0 inhomogeneity, chemical shift, eddy current
effects, and Maxwell gradients associated with all pulses in the
sequence. ODESSA signal vs. .theta..sub.F and .theta..sub.I1 is
shown in FIG. 8, which illustrates simulated ODESSA signal
magnitude vs. free precession .theta..sub.F and velocity-induced
precession .theta..sub.I1(.alpha.=30.degree., TR=6 ms, T.sub.1=1000
ms, T.sub.2=220 ms). Signal is zero for static spins
(.theta..sub.I1=0). The vertical line at .theta..sub.F=0
corresponds to ODESSA signal plotted in FIG. 4. For moving spins,
modulations in signal magnitude can occur due to free precession.
These modulations can be reduced by carefully shimming the main
field or by combining multiple acquisitions of varying encoding
polarity and magnitude. This plot shows some signal loss due to
.theta..sub.F is possible in flowing vessels. In order to mitigate
this effect, one could utilize two separate acquisitions which
differ only in the polarity of their flow-encoding gradients.
Inversion of the flow-encoding gradient would also invert the sign
of induced precession. Signal from each acquisition could be
summed, or the maximum point-to-point signal could be used to
eliminate areas of signal loss. In the results shown here, careful
shimming was used to reduce off-resonant precession to a level that
made further compensation unnecessary.
[0053] Experiments described below were performed on a 1.5 T GE
Signa research system (GE Medical Systems, Waukesha, Wis.) with 40
mT/m gradients switchable at 150 mT/m/ms.
[0054] As a controlled verification of the ODESSA signal profile
predicted by FIG. 7, experiments were conducted in a flow phantom
containing a single tube of radius 0.8 cm. Slices were oriented
such that the flowing material was in-plane. Both flow and
flow-encoding occurred on the frequency-encoding (x) axis. The
flowing material had T.sub.1=1383 ms and T.sub.2=836 ms. Images
were taken with flow at several known velocities; the signal
recorded from each of these velocities is shown in FIG. 9. FIG. 9
illustrates ODESSA signal (top: magnitude, bottom: phase) observed
in the flow phantom (.alpha.=30.degree., TR=6.4 ms, T.sub.1=1383
ms, T.sub.2=836 ms). Velocity encoding is such that 25 cm/s
corresponds to 180.degree. of precession. The signal advantage of
ODESSA (`x`) over PCA (`o`) is consistent with the predicted by
theory (lines).
[0055] Theoretical signal profiles were calculated by integrating
Eq. [8] over an assumed laminar velocity profile, then scaled to
best match the acquired data. Results correlate quite well with
those predicted by theory. While PCA signal is close to the level
of noise for all velocities, ODESSA signal is nearly an order of
magnitude larger over the entire range of positive velocities.
[0056] Axial knee images were acquired with flow-encoding in the
through-plane direction, as illustrated in FIGS. 10(a) and 10(b).
For this study, a 1.6-cm slice was imaged with matrix size
256.times.256, FOV=32 cm, TR=5.6 ms, TE=1.4 ms, .alpha.=30.degree.,
and flow-encoding gradients such that 30 cm/s corresponded to a
180.degree. phase shift. A 500-.mu.s linear-phase SLR pulse (24)
was used for rapid slice selection.
[0057] Acquisition was delayed for 2 s to allow evolution of the
steady state, resulting in a total imaging time of 4.9 s. The sum
image (FIG. 10a) exhibits the contrast expected from a standard
refocused SSFP sequence at this tip angle. The difference image
(FIG. 10b) shows suppression of static signal to the level of the
image noise. Blood signal, on the other hand, is not significantly
attenuated, with an SNR of approximately 64 in each image.
[0058] An artifact is visible in the phase-encode direction near
the popliteal artery. This is caused by an inconsistent steady
state from phase-encode line to phase-encode line due to pulsatile
flow. Similar flow artifacts are routinely encountered in standard
phase-contrast images; however, the underlying cause is slightly
different. In a phase-contrast sequence, the flow encoding pulse
causes a velocity-dependent phase accrual before readout. If flow
is pulsatile, then this causes a periodic phase modulation in the
k-space data, resulting in a ghosting artifact in the image.
[0059] ODESSA is not as susceptible to this intra-TR phase
modulation because the flow-encoding gradient occurs after readout.
Also, since magnetization is nearly longitudinal when flow encoding
occurs, the steady state is not significantly affected by moderate
variations in flow. However, the popliteal artery has a triphasic
flow waveform, with negative or zero flow for a significant portion
of the cardiac cycle. This results in a periodic steady state,
which causes modulation (in both magnitude and phase) between
phase-encoding lines. The result is a periodic ghosting artifact in
the image, similar to that seen in a standard phase contrast
image.
[0060] The 3D sequence of FIG. 2b was used to generate volumetric
(3D) ODESSA data from the lower leg. For this study, a
256.times.256 matrix was acquired over a 24-cm coronal FOV. 32
slices were acquired over a 15-cm slab. TR was 5.82 ms with tip
angle .alpha.=30.degree.. Flow-encoding gradients were present on
the frequency-encoding axis, with 200 cm/s corresponding to 1800 of
induced precession. Two excitations were acquired asynchronously
with the cardiac cycle in order to reduce pulsatility artifacts;
therefore, total imaging time was 190 s.
[0061] FIG. 11 illustrates coronal slice from a 3D ODESSA study in
the lower leg of a healthy volunteer. FIG. 10(a) is anatomic sum
image, and FIG. 10(b) is angiographic difference image. The
popliteal bifurcation is clearly visualized (SNR=29). This exam
took 190 s; two excitations were used to reduce pulsatility
artifacts.
[0062] Individual slices from this study (FIG. 11) are
qualitatively similar to those acquired in the single-slice study.
A maximum-intensity projection of the image data (FIG. 12) shows
the arterial tree very well. Veins are less conspicuous because of
the reduced T.sub.2 of venous blood; a smaller tip angle could be
employed to increase the relative venous signal if desired.
[0063] This study uses smaller flow-encoding gradients than the
single-slice study above; however, blood signal remains high due to
the near-constant ODESSA signal vs. precession angle (as in FIG.
7). This characteristic allows the pulse sequence designer more
flexibility in choosing the duration of flow-encoding gradients
without sacrificing blood signal.
[0064] We have demonstrated a method of producing angiographic
contrast with a refocused SSFP experiment. In so doing, the
relatively high SNR efficiency of SSFP sequences is maintained.
Simulations predict ODESSA signal to be uniform over a wide range
of velocities, and more than three times as large as that of
conventional phase contrast angiography in the same scan time.
Additionally, ODESSA signal has highly uniform phase, reducing the
effects of intravoxel velocity dispersion and making thick-slab
projection angiography possible. Phantom experiments verify the
advantages predicted by simulation.
[0065] ODESSA can also be used in combination with Ti-shortening
contrast agents such as GdDTPA to further enhance blood signal.
FIG. 13 shows predicted ODESSA signal versus concentration of
contrast. Signal from arterial blood in a standard gradient-echo
acquisition using the same scan time is also shown. For Gd-DTPA
concentrations up to 5 mmol/L, ODESSA provides a significant
contrast improvement over the conventional technique. In fact,
ODESSA provides the equivalent of 1-2 mmol/L of additional contrast
over this range of concentrations.
[0066] The ODESSA pulse sequence can produce non-contrast
angiograms quickly, with high blood signal and virtually no signal
from background tissue. Furthermore, this sequence intrinsically
suppresses venous signal due to its reduced T.sub.2. The technique
compares favorably with other methods of non-contrast MR
angiography, and can provide an additional signal boost in
contrast-enhanced studies.
[0067] While the invention has been described with reference to
specific embodiments, the description is illustrative of the
invention and is not to be construed as limiting the invention.
Various modifications and applications may occur to those skilled
in the art without departing from the true scope and spirit of the
invention as defined by the appended claims.
* * * * *