Ultrasound renal tract

Both kidneys, ureters where visible, bladder pre- and post-void

When to use it

Curvilinear transducer of roughly 2-5 MHz for the kidneys and bladder; a linear high-frequency probe is used in children and in slim adults, and in a paediatric renal tract it is often the only probe needed.
The bladder is imaged first with the patient supine and the bladder comfortably full, before the kidneys — deferring it risks the patient voiding during the study and losing the pre-void volume and the ureteric jets.
Each kidney examined in longitudinal and transverse planes with the patient supine and then in the contralateral decubitus or oblique position; bipolar length and cortical thickness recorded, and the upper poles reached by scanning intercostally in suspended inspiration.
Post-void residual measured on the same visit, since an elevated residual reframes bilateral hydronephrosis as outflow obstruction rather than upper tract disease.
Colour Doppler over the trigone for ureteric jets; asymmetric or absent jets support obstruction, and the twinkle artefact behind a calculus makes small stones at the vesicoureteric junction visible when greyscale does not.
Where obstruction is suspected the distal ureter is specifically interrogated through the full bladder, and in pregnancy the physiological right-greater-than-left dilatation must be reported as such rather than as obstruction.

Ureteric calculi are frequently not directly visualised; absence of hydronephrosis does not exclude a stone.