Rakesh Kumar Gupta1, Jaladhar Neelavalli2, Manoj Kumar3, Indrajit Saha4, Pradeep Kumar Gupta5, Jitender Saini6, and Sunita Ahlawat7
1Radiology, Fortis Memorial Research Institute, Gurgaon, India, 2Philips Innovation Campus, Philips India Limited, Bengaluru, India, 3Radiology, National Institute of Mental Health And Neurosciences, BENGALURU, India, 4Philips India Limited, Gurgaon, India, 5Fortis Memorial Research Institute, Gurgaon, India, 6Radiology, National Institute of Mental Health And Neurosciences, Bengaluru, India, 7Surgery, Fortis Memorial Research Institute, Gurgaon, India
Synopsis
Quantification of volume magnetic susceptibility of extracted
gallbladder stones, with 28 different types of textures, was performed using
quantitative susceptibility mapping. Both dia- and para-magnetic stones are
seen and the susceptibility values were found to be comparable in magnitude to
those found in venous vessels and blood products – and hence could be
detected in-vivo using SWI. This is in agreement with a recent work reporting visualization of gall-stones using SWI.
Introduction
Cholelithiasis and choledocolithiasis are common clinical conditions of the
upper abdomen, having an incidence of 4-15% in Asia, 15% in the USA and 22% in
Europe.1–3 Since its introduction 19914, magnetic resonance cholangiopancreatography
(MRCP), a non-invasive imaging method, is routinely used for MRI based
assessment of biliary conditions.5 However, in detecting stones smaller than 5 mm,
MRCP’s sensitivity is only 62-64%.5 A recent work had pointed to susceptibility
weighted imaging (SWI) as a possible an adjunct to MRCP for gall stone
detection, owing to its sensitivity to magnetic susceptibility differences in
SWI phase.6 Indeed ex‑vivo chemical analysis of extracted gallstones
have shown that about 28% of them contain predominantly calcium salts;7 71% contain predominantly cholesterol. Even the
cholesterol stones contain trace amounts of calcium salts.8 While Calcium is diamagnetic relative to water,
the magnetic susceptibility of its salts is determined by the electronic
structure of the compound – and hence its appearance in SWI phase. Cholesterol
too, due to its lipid content, has both chemical shift and susceptibility9 (paramagnetic) difference relative to
parenchyma, leading to a phase signature. However, volume magnetic
susceptibility values of in-vivo gallstones is not known. As a first step
towards understanding the typical magnetic properties of gall stones in the
context of MR imaging - in this work we evaluated magnetic susceptibility
property of gall bladder/duct extracted from patients who underwent
cholecystectomy. This was done using quantitative susceptibility mapping (QSM). Materials and Methods
The study was approved by the local institutional ethics committee. A
total of 32 gall stones were obtained from the surgery department of Fortis
Memorial Research Institute, Gurgaon, India. After washing with distilled
water, the stones were labeled and placed within two agarose gel phantoms (17
stones in phantom-1 and 15 in phantom-2) for MR imaging. One of the stones was
brittle and broke down into 6 pieces, bringing the total number of individual
stone samples within the phantoms to 37. MR
Imaging: Both the phantoms were imaged at 1.5T Philips scanner using
the SWIp sequence with parameters: TR – 52msec, TEs - 12msec, 23msec, 34msec,
45 msec, FA – 10o, BW – 493 Hz/Pix, resolution – 1 x 1 x 1 mm3.
QSM maps were generated from SWIp data using the method described previously.10 Susceptibility values of individual stones were
measured from the minimum or maximum intensity projection images through manual
ROI selection, avoiding the edge voxels. The measurement ROI contained at least
5 voxels within the stone. The range of susceptibilities found is reported and
for the stones which had similar texture, coefficient of variation (COV) measure
was evaluated. It is assumed here that stones of similar texture, will have
similar composition and hence comparable magnetic susceptibility values.Results
Figure - 1 shows the photographs of the stones along with their labels
and the corresponding minimum (or maximum) intensity QSM maps. Of the 37 stone
pieces within the phantoms, measurements could be performed in 33 stones. In 4
of them, measurement could not be performed due to significant partial
voluming. Table – 1 presents the susceptibility values. Of the 32 distinct
stones, 2 of them were found to be paramagnetic and the rest diamagnetic. The
susceptibility values ranged from –0.102 ppm to -0.916 ppm for diamagnetic and
0.203 ppm to 0.486 ppm for paramagnetic ones. Stones having similar texture
were found to have comparable susceptibility values with COV < 0.2. Stone
pieces 15, 16, 17 were the pieces of the same stone and thus are expected to
have comparable susceptibility value. Indeed, mean susceptibility of the three
pieces was -0.488ppm with a COV of 0.057. Stones with labels 2, 11, 20 and 21
have the same texture and had mean susceptibility of 0.820 ppm and a
coefficient of variation of 0.007. Similarly stones B, C, D, E, F having
similar texture had mean susceptibility of -0.369 ppm and COV of 0.20; and
stones K, L, M, O, with similar texture had mean susceptibility of -0.235 ppm
and COV of 0.09.Discussion and Conclusion
Magnetic susceptibilities of gall-stones were quantified through ex-vivo
MRI scanning using QSM. In this study, stones of 28 distinct textures were
included. Stones of both positive and negative magnetic susceptibilities were
observed. The susceptibility values are found to be comparable in magnitude to those
of venous blood, and blood products. Thus SWI may indeed be of help (an adjunct
role to play) in sensitive identification of in-vivo gall-stones. Acknowledgements
We thank Jakob Meineke
and Ulrich Katscher from Philips Research, Hamburg for their support. References
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