343
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Technical
Two-Piece Saara
Wrist Surface
M. Totterman,1
Robert
Heberger,1
Coil
Richard
Miller,2
Deborah
In clinical MR imaging, the ideal surface coil is a coil whose geometry is optimized for inductive loading to the anatomy to be imaged [i ]. In imaging of the wrist, this idealized geometry can be achieved by a coil that closely surrounds the wrist. Whole-volume and partial-volume coils surround the wrist and provide a relatively homogeneous signal oven the imaged area but, as their size is dictated either by the size of the hand on by the distance between pained coils [2-8], their diameter is larger than necessary to fit the size of wrist. This is also true for the slip-oven, circumferential, single-loop, receive-only coil. To minimize the size of the coil, we developed a two-piece surface coil for wrist imaging. Its performance was tested against an identical continuous coil, and its usefulness was evaluated experimentally and clinically.
Materials We
and Methods
first
built
a circumferential,
single-loop,
receive-only
surface
coil with a circumference slightly larger than that of an average-sized male wrist (7.5 x 5.6 mm). We then built a second coil of the same
size, which was split into two sections for reassembly around the wrist. For this purpose, 10-gauge copper wire was formed into two halves of a rectangle. The insulation was stripped from the ends of each half. The corresponding ends of the two halves were connected to each within
other
by
it. This
strip
a small
Plexiglas
of copper
formed
box
that
had
a strip
a bridge
over
the
of copper
gap
between
the two ends. The connections were secured by nylon screws. Both of the coils were tuned for 63.9 MHz and matched for loading of the male wrist.
Next, the signal-to-noise against
the continuous
ratio of the two piece coil was measured coil on a i .5-T
clinical
imager
(Signa,
GE
Medical System, Milwaukee, WI). Two different types of phantoms, one corresponding to the size and shape of the human wrist (7.5 cm in larger diameter) and the other to the size of the head (20 x 20 cm) were used for the test. Both phantoms
doped
with copper
to simulate
The slice
J. Rubens,1
February
S. Blebea1
Judy
For both coils the signal intensity diameter
of the
coil in the head of background Finally,
clinically
coil
in the
phantom. air was
the
tested
wrist
The used
two-piece
coil
suspected
soft-tissue
and
deviation
for noise
calibration.
was
tested
on
volunteers and
for
six
and
The imaging
tumors
along the shorter
along
the
axis
of the
ofthe signal intensity
standard
variety of wrist abnormalities. with
was measured phantom
on six normal
cadaver
wrists,
and
i 8 patients
with a for patients
parameters rheumatoid
patients
were
2000/20, 80; FOV of 8-iO cm; matrix of 256 x i28; slice thickness of 3 mm; and NEX of i .00, 0.75, or 0.50. For patients with suspected soft-tissue injuries, the imaging parameters were 2000/25, 60; FOV of 8 cm; matrix of 256 x 256; slice thickness of i .5 mm; and NEX of 1.00.
Results On phantoms, the performance of the two-piece surface coil was almost identical to that of the same-size continuous coil (Fig. i). In clinical use, the two-piece coil was easily assembled around the wrist and required no coil holder. When the circumferential coil is used for the wrist, the patient must be prone with the arm over head so that the hand is penpendiculan to the gantry. Patients with rheumatoid disease did not tolerate this position well. For those patients, we found that the signal-to-noise ratio of the two-piece coil supported images acquired with 0.50 or 0.75 excitation, thereby minimizing imaging time. Higher resolution images were found to be particularly useful in patients with suspected soft-tissue injury. Images obtained with i .5-mm slice thickness and one excitation provided detailed visualization of the triangular fibrocartilage, articular cartilage, and intrinsic intenosseous ligaments (Figs. 2A and 2B).
Discussion
thickness
Our tests show that a single-loop, receive-only surface coil can be divided into two separate sections without affecting
Received April 2, 1990; accepted after revision July 17, 1990. , Department of Diagnostic Radiology. University of Rochester Medical Center, Rochester, NY 14642. 2 Department of Orthopedics, University of Rochester Medical Center, Rochester, NY 14642. AJR 156:343-344,
and
were filled with saline solution
the loading effects of their (3 mm), interslice interval (FOV) (i8 and 28 cm), number of excitations (NEX), TR/TE, and receiver attenuation value were all kept constant. respective anatomy. (2.5 mm), field-of-view
sulfate
Note
1991 0361-803X/91/1
562-0343
© American
Roentgen
Ray Society
Address
reprint
requests
to S. M. Totterman.
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Downloaded from www.ajronline.org by 184.98.72.104 on 11/02/15 from IP address 184.98.72.104. Copyright ARRS. For personal use only; all rights reserved
n a
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--------------------
t .
0
1
e
2345678
Centimeters
.
0! .
from
Plane
2
1
0
2
1
Center
A
B Fig. 1.-Relative
signal-to-noise
ratios of continuous
coil and two-piece
coil measured
on head phantom
(A
)
and
on wrist
phantom
(B) are almost
identical.
Fig. 2.-A and B, Coronal MR images of wrist at 1.5-mm slice thickness obtained with twopiece receiver coil. Normal volunteer (A), patient with chronic wrist pain (B). Note good visualization of intact triangular fibrocartilage (long arrow), torn triangular fibrocartilage (open arrow), lunotriquetrial ligament (short arrow), scapholunate ligament (small arrowheads), and articular cartilages (large arrowheads).
the performance of the coil as a whole. The resulting coil, when used to image the wrist, can be made smaller than the conventional slip-oven coil and therefore will have a higher signal-to-noise ratio close to the plane of the coil, but also will have a very steep sensitivity profile. The steep sensitivity profile of the coil has both advantages and disadvantages. Because of this steep profile, it is impontant to align the highest signal-to-noise ratio area of the coil with the structures of particular interest. In soft-tissue injuries of the wrist, in which interest is focused on the evaluation of the extrinsic and intrinsic intenosseous ligaments and tnianguban fibrocartilage, the optimal position of the coil is about i cm distal to the radiocarpal joint. In patients with rheumatoid disease, the optimal location is approximately 1 cm more distal. Although one of the potential drawbacks with the single-loop coil is the drop in signal proximal and distal to the plane of the coil, in practice this has not been a problem in the evaluation of wrist disease, in which the longitudinal extent of the region to be imaged is quite limited. The very high signal-to-noise ratio close to the plane of the coil is beneficial in the evaluation of ligament injuries. The area in which the signal-to-noise ratio supports 1 .5-mm slice thickness with in-plane resolution of 0.3 mm is lange enough to coven the area of the triangular fibrocartilage and intenosseous ligaments. In patients in whom high-resolution images are not required, the high signal-to-noise ratio can be used to de-
crease imaging time. With rheumatoid patients who find the imaging position uncomfortable, the short imaging time is important. In all patients, the shortened imaging time increases both patients’ comfort and throughput.
REFERENCES 1 . Schenk JE, Hart HA Jr. Foster TH, Edelstein WA, Hussain MA. High resolution magnetic resonance imaging using surface coils. In: Kressel HY, ed. Magnetic resonance annual. New York: Raven, 1986:123-160 2. Weiss KL, Beltran J, Shamam OM, Stilla RE, Levey M. High-field MA surface-coil imaging of the hand and wrist. I. Normal anatomy. Radiology 1986;160: 143-1 46 3. Weiss KL, Beltran J, Lubbers LM. High-field MR surface-coil imaging of the hand and wrist. II. Pathologic correlations and clinical relevance. Radiology 1986:160:147-152 4. Doornbos J, Grimbergen HAA, Booijen PE, et al. Application of anatomically shaped surface coils in MAI at 0.5T. Magn Reson Med I986;3:270-281 5. Koenig H, Lucas D, Meissner A. The wrist: a preliminary report on highresolution MA imaging. Radiology i986;1 60:463-467 6. Middleton WD, Kneeland JB, Keilman GM, et al. MA imaging ofthe carpal tunnel: normal anatomy and preliminary findings in the carpal tunnel syndrome. AJR 1987;148:307-316 7. Baker LL, Hajek PC, BjOrkengren A, et al. High-resolution magnetic resonance imaging of the wrist: normal anatomy. Skeletal Radiol 1987:16: 128-1 32 8. Zlatkin MB, Chao PC, Osterman AL, Schnall MD, Dalinka MK, Kressel HY. Chronic wrist pain: evaluation with high-resolution MR imaging. Radiology 1989:173:723-729