Scott B. King1, Jarod Matwiy1, Calvin Bewsky1, Hung-Yu Lin1, and Masoom A. Haider2,3
1Medical Devices, National Research Council Canada, Winnipeg, MB, Canada, 2Dept of Medical Imaging, Sunnybrook Health Sciences Center, Toronto, ON, Canada, 3Department of Medical Imaging, University of Toronto, Toronto, ON, Canada
Synopsis
The pancreas is
difficult to image because of its central location deep in the abdomen, often with
overlapping artifact in parallel MRI reconstruction through the mid abdomen and
drop off of signal. In this
new concept for improved SNR and parallel MRI in pancreas MRI, a surface array is
pushed/indented into the abdomen, bringing smaller array elements closer to the
pancreas. Compared to the benchmark array, the Indenting Array demonstrated
>2x SNR and 40% improved R=3 A-P parallel MRI with g-factor = 1 within the
pancreas. This new “indenting” array
design could have a significant impact on pancreas diagnostic MRI.Introduction
The pancreas can be a
difficult organ to image because of its central location deep in the abdomen.
There is frequently overlapping artifact in parallel imaging reconstruction
through the mid abdomen and drop off of signal. Most array
coils used for body imaging contain array elements that are 12-15 cm in
diameter, evenly distributed to cover the body [1,2]. Using smaller array
elements (6cm-8cm in diameter) could increase SNR near the periphery of the
body, but would not increase SNR deep inside the abdomen. We report on a new concept for improved SNR
and parallel MRI in pancreas MRI, whereby a surface array of smaller elements
are pushed/indented into the abdomen, thereby bringing the smaller array
elements (6cm-8cm in diameter) closer to the deep lying pancreas, with the added benefit of displacing the transverse
colon and stomach away from the pancreas.
Methods
The anterior array design
philosophy was to use two array sections, a smaller “indenting” surface array
and a larger outer surface array for extended FOV coverage. The Indenting Array was shaped to contour the
abdomen and sit inferior to the rib cage, containing 5-elements approximately
6-8 cm in diameter with an in-plane size of 22.1cm x 11.9cm (Fig.1b). The Outer Array contained 8-elements with
dimensions ranging from 12-18 cm with an in-plane size of 48cm x 32cm (Fig. 1a). A system of “indenting” is shown in Figure
1c. Overlapping and preamplifier decoupling minimized coupling between array elements. Each of the 13 anterior elements tuned to
123.2 MHz included 1-active trap, 1-passive trap, 1-fuse, and a 1.4 Ω input
impedance preamp (Hi-Q.A., Carleton Place, ON, Canada). The 13-Channel Pancreas Array was combined
with a 9-Channel OEM Spine Matrix for posterior coverage. The 13-Channel Pancreas
Array was safety validated and interfaced to a Siemens 3T 32-Channel TIM Trio
MRI. The OEM 6-Channel Body Matrix combined
with the same 9-Channel Spine Matrix was used for benchmark comparisons. Imaging
experiments were carried out on healthy subjects following an approved REB
protocol. The OEM “sn” sequence was used
for SNR/g-factor evaluation, and 3D VIBE for clinical evaluation. SNR/g-factor
analysis was performed using Musaik (Schmid and Partner Engineering AG,
Zurich).
Results
The pancreas body was initially at a depth
of 95mm from the surface and OEM array, but with the indenting array system,
the pancreas body was only 54mm below the smaller elements of the indenting
array. Compared to the benchmark array,
the 13-Ch Indenting Array demonstrated >2x SNR improvement (Fig.2 a,c) and 40% improved R=3 A-P parallel MRI with a g-factor near 1 (Fig.2 b,d) within the pancreas.
This >2x SNR gain was leveraged for much better quality high
resolution (704x264 matrix) VIBE imaging (Fig. 3a) and R=3 A-P parallel MRI (Fig.3c)
compared to the benchmark array. Zoomed 3D VIBE images (Fig. 3a,b - bottom) obtained
on the 3T Trio system at a FOV of 40x30cm, 5mm slice thickness, 18s acquisition
time for 24 slices with a matrix of 704x264, have in-plane resolution of 0.57mm
x 1.13mm. The image with the Indenting Array (Fig.3a) shows a sharp image
through the inferior pancreatic head without noise or speckle artifact with
clear definition of the lobulated architecture of the pancreatic head. The image
with the manufacturers array (Fig.3b) shows noise and speckle artifact through
the pancreas limiting detailed evaluation. When parallel MRI is pushed to R=3 A-P,
images at 512x250 with in-plane
resolution 0.78mm x 1.2mm and 12sec acquisition time with the Indenting
Array (Fig.3c) are free of artifact seen in the corresponding images with the
manufacturers array (Fig.3d).
Conclusions
This new “indenting”
array design has demonstrated 2-fold SNR improvement in pancreatic MRI, and
could have a significant impact on pancreas diagnostic MRI. This
has the potential to improve detection of small pancreatic cancers, small
distal common bile duct stones and periampullary tumors.
Acknowledgements
This research was funded by the Ontario Institute of Cancer
Research. The authors thank CIMTEC for their support.References
[1]Zhu et al. MRM 52:869-877
(2004). [2]Hardy et al. MRM
55:1142-1149 (2006).