Synopsis
Using 3D dual echo UTE sequence, all cholesterol
gallstones were able to be detected as positive signal in-vitro. Our study may
indicate that UTE has an added value of depicting impacted stones or
hepatolithiasis as positive signal. In initial clinical experiences,
cholesterol gallstones were also successfully visualized as positive signal
with UTE.PURPOSE
The diagnostic accuracy of MRCP for detection of gallstones has been
reported to be comparable to that of ERCP and superior to that of CT or US.
1
However intrahepatic stones or impacted stones in a cystic duct
or a duodenal papilla can be difficult to be detected using MRCP for a lack of high
signal intensity bile surrounding the low signal intensity stones. 2,3
Pigmented stones are sometimes depicted as hyperintense on T1WI; however it does
not apply to cholesterol stones. 4,5 Cholesterol stones are composed of solid-state crystal; therefore it is
difficult to obtain enough MR signals from them, because molecular motions are
restricted and T2 relaxation time is remarkably shortened.
Recent development of ultrashort echo-time
(UTE) with 3D radial sampling of the free-induction decay allows for the
detection of solid object with very short T2 relaxation times. 6-14
The aim of this study is to test whether UTE can help depict
cholesterol stones as positive signals in-vitro and in-vivo.
METHODS
In-vitro
study
Materials Cholesterol gallstones (defined as
cholesterol composition over 70%) obtained from 8 patients were examined. The
stones were cut into half, and then separated into two or three components, if
possible, and were individually analyzed. Consequently, the total
analyzed components were 14.
MR imaging
The gallstones placed in individual plastic bottles filled with
normal saline were examined on a 3.0T clinical MR system. T1WI (3D FFE T1 weighted images with fat suppression) and UTE were performed. In
UTE imaging 3D radial dual-echo sequence was obtained at the first echo (TE
of 70 μsec) (: UTE) and the second echo (2.24 msec). Other parameters were as follows: TR 7.6
msec, FA 15 degree, 248 × 248 matrix、FOV 160mm,
slice thickness 1.0mm. To selectively visualize only short-T2 components, subtraction
images (: Subtraction) between first echo images and second echo images were
processed. T2 map was computed on a
pixel-by-pixel basis by using a mono-exponential decay model from images at TE
of 70, 100, 120 and 150 μsec, too.
CNR of gall stone For a semi-quantitative assessment, CNRs for gallstone to the
surrounding saline were calculated on images of T1WI, UTE and Subtraction.
Statistical evaluation was performed with Student's t-test; P < 0.05 was
considered significant.
In-vivo
study Patients, who had suspected of having gallstone, were examined
using MRI before the cholecystectomy. MR sequence parameters of UTE were as
follows: first echo 0.07μsec , second echo 2.24msec, TR 7.6msec, FA 10 degree, FOV
160 mm, 72× 72 matrix、slice
thickness 2.0mm、40 slices。The scan
time was approximately 5 min.
All patients gave written
informed consent, and this study was approved by the institutional review
board.
RESULTS
In-vitro
Averages (±SD) of CNRs on T1WI, UTE and
Subtraction imaged were -4.3 (±18.4),
14.8 (±13.9),
33.8 (±26.1),
respectively. Statistically significant differences between T1WI and UTE (p = 0.0069),
between T1WI and Subtraction (p = 0.00094) and between UTE and Subtraction (p =
0.038) were measured. On T1WI, 9 segments were negative value of CNR. While one
segment had negative value of - 7.4 on UTE, its value on Subtraction image was
81.4. On subtraction images, all values were positive with minimum value of 6.3.（Fib
1. (Graph )）
Most cholesterol stones were visualized as
low signal on T1WI and high signal on UTE / Subtraction images and the
differences in the inner structures could be observed (Fig. 2).
The signals of stones progressively dropped
to lower values as TE increased from 70μsec to 150μsec. In most cases, the signal
decay was especially overt from 70 to 100μsec.（Fig. 3）
DISCUSSION
Most cholesterol gallstones were successfully visualized as
positive signals as compared to background saline on UTE. On natural UTE
images, substances of short T2 property were visualized as positive signals.
Subtraction method was also useful in making solid-state objects “overt”.
T2 values of cholesterol gallstone were
distributed between 45.4 and 156μsec. Accordingly, the use of TE less than 100μsec
is recommended to obtain signals from cholesterol stones. In addition UTE has
shown its capability to depict in-vivo cholesterol stones as positive signal.
At
this point, SNRs and spatial resolutions are not satisfactory; however, UTE is a
promising adjunct to hydrographic approaches such as MRCP.
CONCLUSION
By using UTE sequence all cholesterol gallstones could be
detected as positive signal. UTE is a promising adjunct to MRI for the assessment
of suspected patient of cholelithiasis.
Acknowledgements
No acknowledgement found.References
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