This audit explores the potential of dual-energy computed tomography post-processing in enhancing the detection and characterisation of gallstone-related disease in the acute Emergency Department setting. Gallstone diseases, including cholelithiasis, choledocholithiasis and gallstone pancreatitis, pose a diagnostic challenge due to variable stone composition and radiopacity on conventional CT as well as post processed DECT. The traditional reliance on ultrasound and magnetic resonance cholangiopancreatography (MRCP) for gallstone evaluation is limited by resource availability particularly in the after hours setting, potentially delaying definitive clinical decisions.
DECT and its post processing technology has emerged as a promising adjunct to conventional CT in acute upper abdominal pain of suspected hepatobiliary origin in those whose clinical status cannot wait for an ultrasound or MRCP, and where CT is otherwise indicated (such as in patients who are poorly differentiated clinically and require exclusion of other differential diagnoses). At our institution, CT examinations of the abdomen are variably acquired either on a SECT or DECT scanner depending on scanner availability. In patients who are scanned on our dual-source DECT scanner, many gallstones remain difficult to detect. DECT atomic number applications may enhance gallstone visibility by exploiting its ability to differentiate materials based on differences in atomic number. By utilizing post-processed dual-energy datasets, radiologists may be able to enhance the visualization of radiolucent gallstones and sludge, which are predominantly composed of cholesterol.
Our study cohort demonstrated improvements in predominantly cholesterol gallstone detection post application of an atomic number map compared to conventional CT. Spectral Rho/Z projection algorithm converts the dual-energy CT studies into mass density (rho) and effective atomic number (Z(eff)) information. In addition, utilizing colour LUTs (Lookup Tables) for liver fat overlay proved helpful in enhancing gallstone visibility. The overall combination of Rho/Z maps with liver fat colour LUT overlay, enhanced conspicuity of predominantly cholesterol stones in 18 patients, whose gallstones were poorly seen and easy to miss prior to post-processing. All poorly visualised gallstones and sludge were of low density on conventional CT, in keeping with predominantly cholesterol composition as intuitively expected. Of the patients with poorly visualised gallstones on conventional CT that were enhanced on post processed DECT, 50% underwent an MRCP and all underwent an ultrasound examination, which were potentially unnecessary if DECT atomic number maps could have been made available to the primary reporting radiologist at the time of the initial report.
One patient with cholelithiasis on conventional DECT had an additional diagnosis of choledocholithiasis with a 10mm stone, which was enhanced on post processed DECT. All additional cases of choledocholithiasis (n = 5) were all occult on both conventional and post-processed DECT images with stone size < 10 mm in all cases. The increased conspicuity of choledocholithiasis on post processed DECT in a single patient is hypothesised to relate to stone size which was 10 mm in this case and its cholesterol composition. Whereas, The additional 5 cases of choledocholithiasis, all occult on both conventional and post-processed DECT images, had a stone size < 10 mm in all cases. Thus, the occult nature of choledocholithiasis in these patients most likely relates to the small stone size. Hence, a significant limitation of our described DECT post processing technique is its poor spatial resolution and inability to detect choledocholithiasis if stone size < 10 mm in our small cohort. Further research and improvements in DECT atomic number map spatial resolution will be required to extrapolate DECT to accurately diagnose choledocholithiasis in those with relatively radiolucent and small sized calculi.
In two cases of pigmented stone composition, post processed DECT did not enhance stone conspicuity. This likely relates to the stone composition and MRCP, ultrasound and ERCP are superior in this regard. 1 patient did not have gallbladder or biliary calculi however the post-processing workup serendipitously highlighted an obstructing pancreatic ductal stone, which was initially missed amongst obscuring regional inflammatory changes secondary to acute necrotising pancreatitis. The colour maps in this setting proved useful, negating the noise from the regional inflammatory change.
Brakel and colleagues have examined the correlation between CT number (HU) and stone composition. Their prediction rule suggests that pure cholesterol stones typically have a HU < 140. In our cohort, we also observed that the group of poorly visualised gallstones on conventional DECT generally demonstrated higher atomic number and lower HU (Fig. 1, circled). This would support that increased cholesterol content correlates to lower stone density, and thus their conspicuity. The highest density stone in this group measured 117HU and supports 140HU as a suitable threshold on Rho/Z application to highlight cholesterol content.
Despite these promising results, several considerations merit attention. The study's retrospective nature, small sample size and reliance on a single-centre audit limits extrapolation of these results to broader clinical settings. Another limitation is that effective atomic number maps are only validated for natural body materials such as fat, soft tissue and bone. Results for materials with higher atomic numbers may be inaccurate. Additionally, the liver fat overlay has not been validated for extrapolation to analysis of cholelithiasis. The reconstructed atomic number maps are also susceptible to CT artefacts and spatial resolution is poor, particularly with small structures.
Future research should involve larger prospective studies to validate these findings across more diverse patient populations. Additionally, ongoing advancements in DECT post processing technology will further enhance its diagnostic accuracy and clinical utility in gallstone-related diseases. What is promising, is the ability of DECT to exploit atomic number differences and therefore enhance conspicuity of patients with predominantly cholesterol gallstones through application of atomic number maps. In a subset of patients who would have already undergone CT, this could reduce reliance on ultrasound and MRCP examinations for patients with cholelithiasis and cholecystitis. Whilst utility in the diagnosis of choledocholithiasis remains in doubt, further research in this area should be considered if the spatial resolution of DECT atomic number maps can be enhanced.