Review of techniques for ultrasonic determination of kidney stone size

INTRODUCTION: Imaging is a vital cog in the wheel of diagnosis and management of patients suspected with renal and ureteric calculi, and it is imperative to choose the appropriate investigation that is accurate as well as safe for the patient. At present, computed tomography (CT) is the gold standard for a patient suspected to have stone disease. However, CT scan is associated with the hazards of radiation and high cost. Ultrasonography (US) is cheap and also devoid of any radiation hazard to the patient. But, at the same time, its usage is limited by decreased sensitivity and specificity, inaccuracy in measuring stone size, and observer dependency. In this article, we review the techniques to improve the accuracy of US in measuring stone size. ACCURACY OF US: According to a review, the sensitivity and specificity for renal calculi are 45% and 88%, respectively, and for ureteric calculi, they are 45% and 94%, respectively. The sensitivity of US decreased when the size of the stone is <3 mm and also in a nondilated system, and the sensitivity increased as the size of the stone increased. TOOLS TO IMPROVE ACCURACY: There are factors that can be adjusted to increase the accuracy of stone measurement. The main factors are changes in gain and depth and alternate modes such as flash angle imaging, harmonic mode, and S mode. Also measures such as use of shadow for size measurement can help in improving the accuracy of stone size measurement. A new automated computerized stone-sizing program improves the accuracy of stone size calculation and reduces user variability. CONCLUSION: US is an ideal first-line imaging modality for nephrolithiasis due to its advantages such as low cost, absence of radiation, and easy availability. However, the only limitation is its reduced sensitivity and specificity when compared with CT. The addition of newer modes can improve the accuracy in stone size measurement.