Small-sized renal calculi – uric acid or non-uric acid?

A 40-year-old male patient, suffering from intense right flank pain, came to the emergency department. He reported neither hematuria nor any other symptoms. A serum creatinine test revealed a normal level of 0.79 mg/dL. His medical history was unremarkable. Urolithiasis was suspected and a TwinBeam Dual Energy (TBDE) CT was performed for further assessment.

TBDE CT images showed two small calculi with smooth surfaces in the right kidney, measuring 3.2 x 1.9 mm and 2.3 x 1.9 mm in size with a density of 950 HU and 607 HU. In the Dual Energy (DE) composition analysis, both calculi were color-coded blue and plotted in the non-uric acid area in the DE diagram. Due to the small size of the calculi, the decision was made to firstly manage the patient medically, considering the potential of spontaneous passing of the calculi. In follow-ups, the patient continued to do well with improved symptoms.

Fig. 3: Both calculi are plotted in the non-UA area in the DE diagram.

Urolithiasis is often a painful urinary disorder and one of the most common causes of visits to emergency departments. [1] It affects around 900,000 people per year in the USA, resulting in annual medical costs of $5.3 billion. [2] CT imaging can provide important information such as the location, size and surface structure of the calculi, as well as the extent of kidney involvement, to plan an optimal patient management. Moreover, the chemical composition of the calculi can be characterized in vivo using DECT, prior to treatment, to plan therapies and to prevent recurrences. [3] A uric acid (UA) calculus, for example, can be treated non-invasively at an early stage with urinary alkalinization. A small calculus can be spontaneously ejected without intervention. DECT is available with a single source CT scanner, a dual source CT scanner or a Photon-counting CT scanner. This case is performed using TBDE on a single source CT scanner, the SOMATOM go.Top. TBDE enables simultaneous acquisition of high and low kV datasets in a single scan. A dedicated DE application is then used to depict attenuation profiles of the calculi, allowing for the differentiation between UA and non-UA stones. The results are plotted in a DE diagram and the calculi are color-coded – red for UA and blue for non-UA. In this case, both calculi were visualized and, despite their very small size, could be characterized as non-UA by the postprocessing software, owing to the motion-free image quality achieved. This made it possible to change the “Resolution” setting from 8 to 3 for an optimal assessment. DECT provides simple and reliable information for the physicians, enabling confident diagnoses and therapy planning for the patients.