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 – 10^o, 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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