Transcript SMASH - Research Imaging Institute

SMASH
Contrast-enhanced 3D MR
Angiography with Simultaneous
Acquisition of Spatial Harmonics: a
Pilot Study by Dr. Sodickson and Dr.
McKenzie, et al
Anthony C. Lanctot
2nd yr Diagnostic Medical Physics
[email protected] 567-6638
The Pilot Study Published in
Radiology Journal Oct, 2000
• A partially parallel image acquisition
technique—S.M.A.S.H.
• Simultaneous Acquisition of Spatial
Harmonics
• Used to increase the spatial and/or
temporal resolution in contrast material
enhanced 3-D MR angiography of the
abdominal aorta and renal arteries.
Pilot Study
• Eight healthy subjects
• Breath-hold duration halved at constant
spatial resolution• Spatial Resolution doubled at fixed breathhold duration• 30-55% reduction in SNR
• Preserved or improved image quality
• With any given MR imaging system, the
spatial and temporal resolution is generally
limited by the gradients or gradient safety
considerations. SMASH overcomes these
limitations without the need for changes in
the gradient system.
Combinations of component coil
signals in a radio-frequency coil array
are used to substitute for omitted
gradient steps.
This allows some portion of the MR
image data to be acquired in parallel.
Parallel Imaging
• A reduced data set in phase encoding
direction(s) of k-space is acquired to shorten
acquisition time.
• In principle, MRI imaging techniques can be
applied to any imaging sequence and k-space
trajectory.
• SENSE: sensitivity encoding /IPAT: integrated
Parallel Acquisition Techniques/ GRAPPA: kspace technique/ also SPEEDER/ASSET/ and of
course SMASH.
Imaging Protocol
 Contrast-enhanced
first-pass 3D MR
angiography performed on 1.5 T Phillips
 A custom six-element surface-coil array,
each 8 x 18 cm
 Initial low-resolution scout images
obtained
 Breath-hold non-enhanced images
obtained
 Provides invivo coil sensitivity references
Imaging Protocol
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Administered 2-mL test bolus of Magnevist with
a 30mL saline flush—this is to identify bolus
timimg with the signal intensity measurements
on dynamic traverse images with 2D T1W
spoiled gradient-echo sequence.
 12.3 msec repitition time/1.1 msec echo time/20°
flip angle/350mm FOV/82x128 matrix/10mm
section thickness/50mm saturation slabs/1 sec
per dynamic image temporal resolution
Imaging Protocol
 Two
contrast material injections of about
18mL each with 30mL saline flush
 These are seperated by about 15 minutes.
 One injection is used for conventional
angiography imaging.
 The other is for SMASH imaging.
Contrast-Enhanced Studies
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3D T1W radio-frequency spoiled gradient echo
imaging sequence
7.0 msec rep time 1.5 msec echo time
30° flip angle 350 x 350 mm FOV
128x 256 matrix size
20 4mm thick sections interpolated to 40 2mm
thick sections
Center of k-space coincides with the peak of the
bolus material
No EKG gating Breath-hold duration 24 secs
Reduced-Breath-hold SMASH
 FOV
and the matrix size in the right to left
phase encoding direction were reduced by
a factor of two.
 Only 64 phase encoding steps are used.
 With twice the k-space interval.
 Results in a 12 sec breath-hold time.
Inc Spatial Resoltion SMASH
 FOV
was halved
 Number of Phase encoding steps stayed
the same at 128.
 Resulted in a doubled Intrinsic Spatial
Resolution.
 Breath-hold time the same as the
reference hold time of 24 secs.
Image Reconstruction
Reference images were reconstructed
using a sum of squares combination of
component coil images.
Here L is the number of array elements, ky = 2 /(field of view), and
R0(x,y) is an arbitrary
function multiplying all the target spatial harmonics exp(im kyy).
Final in-plane matrix 128x256 reduced breath-hold matrix and
256x256 increased spatial resolution SMASH images
The top row shows typical 0th harmonic target functions used in the original phased sum
(left) and coil-by-coil (right) implementations of SMASH. (The solid lines are the real part
of the complex target function, the dotted lines are the imaginary part.) The second row
shows the matching 1st harmonic target functions. The bottom row shows images formed
from simulated SMASH fits to the corresponding sets of target functions.
Wiley-Liss, Inc
Figure 1. Maximum intensity projections of 3D image sets demonstrating the use of SMASH for reduction of breath-hold
duration
Three different Subjects: A,B, and C.
A-C: Maximum
Intensity
Projections of
Reference
Images: breathhold duration 24
seconds.
D-F:
Corresponding
maximum
intensity
projections of
SMASH
images: breathhold time 12
seconds.
Key Point: Image
Quality is
preserved on the
accelerated
SMASH images
despite the
shorter breathhold duration.
Sodickson, D. K. et al. Radiology 2000;217:284-289
Copyright ©Radiological Society of North America, 2000
Results
The SMASH reconstructions showed negligible
artifact throughout the 3D volume in all cases.
A slight increase in edge enhancement artifact in
the distal abdominal aorta due to the timing of
the shorter acquisitions is seen in Figure 1.
Otherwise, Image Quality is preserved on the
accelerated SMASH images despite the shorter
breath-hold time.
Figure 2. Maximum intensity projections of 3D image sets demonstrating the use of SMASH for spatial resolution enhancement
A-C: Maximum
intensity
projections of
reference
images (matrix:
128x256x20):
breath-hold time
24 seconds.
Sodickson, D. K. et al. Radiology 2000;217:284-289
Copyright ©Radiological Society of North America, 2000
Results
In Figure 2: Increased Spatial Resolution at the
edges of structures such as the aorta, which are
oriented perpendicular to the right-to-left phase
encoding directioncan be seen on the SMASH
images.
In the increased spatial resolution series as a
whole the mean “sharpness” of the abdominal
aorta was 0.74 +/- 0.05 compared with 0.66 +/0.05 on the reference images.
This corresponds to a net 12% increase in
vessel sharpness on the SMASH images.
Plot of the mean SNR relative to the
reference image:
Sodickson, D. K. et al. Radiology 2000;217:284-289
Contrast-to-Noise Ratio Results
Reduced Breath-hold versus
reference images:
Sodickson, D. K. et al. Radiology 2000;217:284-289
Figure 5. Maximum intensity projections of three successive time-resolved SMASH 3D image sets obtained during a single
breath hold
Sodickson, D. K. et al. Radiology 2000;217:284-289
Copyright ©Radiological Society of North America, 2000
Final Analysis and Discussion:
• The reduced breath-hold series has an approx.
40% reduction in the average Signal-to-noise
and Contrast-to-noise ratios.
• The increased spatial resolution series resulted
in an approx. 55% reduction in these.
• The measured SNR and CNR in this study
approached the optimum theoretically predicted
results for a SMASH acceleration factor of 2.
Final Analysis and Discussion:
• The SNR penalty associated with SMASH
reconstruction may be offset by using
faster contrast material injection rates as
the temporal resolution is improved.
• Although these results are consistent with
theoretical expectations, their statistical
significance remains to be assessed in
studies with larger number of subjects.
Final Analysis and Discussion:
• With SMASH ( Simultaneous Acquisition of
Spatial Harmonics), temporal resolution can be
increased without sacrificing spatial resolution,
making SMASH a particularly promising
approach for time-resolved MR angiography.
• Further studies will be undertaken to asses the
value of SMASH for improving image quality and
increasing the accuracy of stenosis
quantification in clinical applications of contrastenhanced 3D MR angiography.
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