Vicarious visualization of gall bladder on Tc-99m ethylene dicysteine renal dynamic study

INTRODUCTION

The application of Technetium-99m ethylene dicysteine (99mTc-EC), as potential renal imaging agent was first proposed in 1990 and was based on the observation that brain perfusion agent ethylene cysteine dimer is rapidly cleared through kidneys after being metabolized to ethylene dicysteine. 99mTc-EC has since evolved to be the scintigraphy agent of choice for assessment of renal function.[1]

Various pre-clinical and clinical studies have established 99mTc-EC as a better tubular agent than 131I-Orthoiodohippurate (OIH) as well as 99mTc-Mercaptoacetyl triglycine (MAG3). It has a high first pass extraction of ~70% and low plasma protein and red blood cell binding (~30% and ~5-6%, respectively). Apart from these merits, the key characteristics 99mTc-EC are low or negligible extra-renal clearance and high kidney-to-background ratio.[123] Herein we report a rare vicarious excretion of 99mTc-EC through hepatobiliary route.

CASE REPORT

A 10-year-old patient with history of dual cadaveric renal transplant presented with abdominal pain of 1 month duration. The ultrasound study showed a collection in the pelvis. The patient underwent renal scintigraphy at our department for evaluation of renal function. 185 MBq of activity was administered. Dynamic images were acquired anteriorly, on a Symbia T6 dual head SPECT/CT system (Siemens) at 2 s/frame for 60 s followed by 15 s/frame for 20 min. Delayed static images were acquired for 100 s at 256 × 256 matrix. An urinoma was detected in the delayed image of the renal dynamic scan pushing the bladder. Uptake in gall bladder (GB) was incidentally observed on delayed images [Figure 1] indicating hepatobiliary excretion of 99mTc-EC. GB activity was confirmed on SPECT/CT images acquired at 15 s/frame, 32 frames in step and shoot mode, 12° angulation and 80 kVp; 52 mAs; 1.5 pitch [Figure 2].

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Figure 1

Delayed static image (ant/post) in a renal dynamic scan showing an urinoma in the pelvis pushing the bladder. Also noted is uptake in the right hypochondriac region just below the liver

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Figure 2

Fused SPECT/CT, CT only and SPECT only transaxial images localizing the ethylene dicysteine uptake to the gall bladder

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DISCUSSION

Unexpected visualization of GB with non-hepatobiliary radiopharmaceuticals has been reported in many studies and is not always pathological. Altered bio-distribution of a radiopharmaceutical might be a result of its low radiochemical purity.[456] However, GB visualization in the present study cannot be attributed to the quality of 99mTc-EC because GB activity was not seen in any other patient injected with the same preparation of 99mTc-EC. Liver or GB activity has been frequently reported on delayed scintigraphy images with 99mTc-MAG3. Around 10% of 99mTc-MAG3 is known to be cleared through hepatobiliary system and can affect the interpretation of the study.[78910] This is unlike 99mTc-EC that might show some low-level uptake in liver (<6%) in anuric patients but even in such patients no significant GB activity is seen.[1] Prvulovich et al. have shown that even in patients with chronic renal failure the hepatobiliary excretion of 99mTc-EC remains low enabling clear delineation of right kidney from liver.[2] It is, therefore, a very rare case when a significant amount of GB uptake is seen on a 99mTc-EC scan.