Ultrasound Assessment of Stones
Although the identification of hydronephrosis on POCUS may be suggestive of ureterolithiasis in a patient with colicky flank pain, stone-specific parameters like location and size are important for counseling and management. Natural history studies have demonstrated that there is an increased likelihood of spontaneous ureteral stone passage for small and distal stones.[48,49] Likewise, the AUA surgical management of stones guidelines makes recommendations for surgical stone treatment based on stone size and location because these factors impact surgical efficacy.
US can be used to directly visualize a stone or shadow, albeit at a lower accuracy than CT. In comparison to CT diagnosis, recent comparisons have reported US sensitivity (52–75%) and specificity (85–96%) for renal calculi, as well as sensitivity (12%) and specificity (93–97%) for ureteral calculi.[50,51] This may be insufficient for patient care, particularly in patients with suspected ureterolithiasis.
Beyond challenges in identifying the presence of uroliths, US also has limitations in other diagnostic criteria used to manage stone patients. In multiple studies, US has been found to overestimate stone size by approximately 2–3 mm compared to CT, particularly in the case of smaller stones or in patients with a larger skin to stone distance.[36,43,52–57] Accuracy in the detection and sizing of stones can be improved by adjusting US settings (e.g. decreasing depth and gain), use of stone-specific gray-scale algorithm (decreased compression/averaging, increased line density/frequency), evaluation of twinkling artefact,[59–63] and measurement of acoustic shadow rather than stone width.[54,64,65] Advanced beamforming methods (e.g. short-lag spatial coherence imaging, mid-lag spatial coherence imaging with incoherent compounding, aperture domain model image reconstruction, and plane wave synthetic focusing) have been described in vitro to improve stone identification and sizing. Studies by Ganesan et al. and Hanqi et al. have specifically found that 1 in 5 patients who underwent US alone might have been inappropriately counseled for observation versus intervention.[43,57] Given this uncertainty, it is not surprising that in a trial randomizing patients to US vs CT for initial evaluation of renal colic, of patients who ultimately underwent urologic intervention, 78% received CT imaging prior to surgery.
Notwithstanding, some stones cannot be directly visualized or do not cast a shadow. Twinkling artifact, or an apparent turbulence of Doppler flow caused possibly by small bubbles trapped on the echogenic stone, may provide additional information and evidence regarding the presence of renal stones as well as the location of ureteral stones.[59–63,67] Doppler twinkling artifact has a reported 96% concurrence with CT-identified stone. A recent study of over 2,000 patients with urolithiasis reported sensitivity of 96.4% (B-US), 97% (twinkling artifact), 99.6% (CT) as well as specificity from 80.8% (B-US), 90.4% (twinkling), 94.2% (CT) for the diagnosis of urolithiasis.
Location may be suggested by positive 'ureteral crossing sign,' or observation of an anechoic tubular structure (dilated ureter) over the iliac vessels, may suggest a distal ureteral stone. Ureteral obstruction may be suggested by Doppler US if there are changes in the ureterovesical jet, specifically lower peak velocity, shorter duration, and decreased frequency in an obstructed ureter as compared to a normal contralateral jet.[69,70] A recent study reported a significant reduction in obstructed jet frequency of 0.7/min (±0.49) compared to an unobstructed frequency of 2.9/min (±1.29).
Curr Opin Urol. 2022;32(4):397-404. © 2022 Wolters Kluwer Health, Inc.