The Influence of Region-of-interest Selection on Ejection Fraction Calculation in a Septate Gallbladder

A septate gallbladder is a rare congenital anomaly characterized by the presence of a thin septum dividing the gallbladder lumen transversely or longitudinally into two intercommunicating distinct compartments. It is usually diagnosed incidentally during imaging studies or surgical procedures.[123] The clinical significance of a septate gallbladder remains uncertain, as most cases are asymptomatic and do not require specific treatment. However, some may experience recurrent right upper quadrant pain, epigastric pain, nausea, and/or dyspepsia. Compared to a normal gallbladder, a septate gallbladder may be more prone to increased pressure, bile stasis, and gallstone formation secondary to altered bile flow dynamics.[3456]

Accurate calculation of the gallbladder ejection fraction (GBEF) using hepatobiliary scintigraphy (HBS) is crucial for the diagnosis and management of biliary disorders.[7] In patients with anatomical variations such as septate gallbladder, accurate interpretation of gallbladder contractility using HBS remains challenging. The presence of a septate gallbladder can complicate the selection of the appropriate region of interest (ROI) for GBEF calculation.[8] Our case underlines an instance of GBEF overestimation due to inaccurate ROI selection. The initial ROI approach unknowingly excluded the gallbladder isthmus and led to a GBEF >40, potentially leading to an incorrect impression of normal gallbladder function. By recognizing the septate nature of the gallbladder and applying a comprehensive ROI, the accurate GBEF value was obtained, confirming the diagnosis of biliary dyskinesia or chronic cholecystitis [Figure 1]. Meticulous attention to gallbladder morphology, correlation with anatomic imaging, and careful review of the selected ROI are essential when interpreting HBS studies. Although septate gallbladder is a rare entity, its presence should be considered. The nuclear medicine physicians should be aware of the challenges posed by such variations and adapt their ROI selection approach accordingly to ensure accurate diagnosis and appropriate patient management.

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Figure 1 Representative sequential images of the abdomen from HBS obtained at 30 minutes and 1 hour (a and b respectively) after the intravenous injection of 9.9 mCi of technetium 99m mebrofenin as per protocol in a 62-year-old woman with chronic nausea and epigastric pain. Subsequently, delayed images were acquired at 30 minutes post oral administration of Ensure (c) to calculate the GBEF. The image labeled 60 min (b), is the pre-Ensure image. Hepatobiliary scintigraphy revealed delayed biliary to bowel transit time. However, the diagnosis of biliary dyskinesia or chronic cholecystitis was questioned by a normal GBEF value of 47%. Upon careful review and correlation with previous ultrasound images, the diagnosis of septate gallbladder with two distinct compartments was confirmed (f). Furthermore, we found that the initial ROI used to calculate the GBEF included only the fundus of the gallbladder (d), resulting in an overestimated GBEF value that falls within the normal range. Arrows in a, c, and f point to the GB septum. Considering the septate nature of the gallbladder, a separate ROI was drawn to encompass the fundus and isthmus of the gallbladder (e). The recalculated GBEF value was 37%, which is compatible with findings of biliary dyskinesia or chronic cholecystitis

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