Non-contrast CT scans have an extremely high sensitivity (94–97 %) and specificity (96–100 %) in detecting urinary calculi [20, 21]. As a result, any suspicion of renal colic should be assessed with a non-contrast scan, even if other diagnoses are more probable [21].

Although, strictly speaking, a non-contrast CT scan is not a dynamic study, and therefore it lacks functional information, it will demonstrate secondary signs of obstruction, including ureterohydronephrosis and perinephric stranding [7, 17, 22]. This may influence the urgency of treatment, particularly in the context of fever or impaired renal function [17].

The non-contrast scan is preferable as contrast obscures accurate identification of the stone in the excretory phase, and requires significantly less radiation [4]. Stone size and location is accurately determined by non-contrast CT scanning, and will have the greatest influence on management. Comprehensive anatomical information and alternative diagnoses may require a contrast scan [4].

It is helpful to track the ureters on axial slices in a cranio-caudal direction, paying particular attention to the areas where stones are likely to impact (pyeloureteric junction and intramural ureter). The authors prefer to use a workstation to facilitate this, however hard copy films are equally easy to interpret. Coronal reconstructions may be helpful.

Urological indications for a CT include micro- or macroscopic hematuria, flank pain, recurrent urinary tract infections or constitutional symptoms (fatigue, unintentional weight loss, pyrexia of unknown origin). However, most renal masses are discovered incidentally. A four phase CT is required to assist in the diagnosis, staging, treatment planning, and surveillance of renal masses. The non-contrast phase provides a baseline density measurement. The cortico-medullary phase (25–75 s delay) results in renal cortex enhancement with limited medullary enhancement providing information regarding renal vasculature that is important for surgical planning and also helps differentiates pseudo-tumors from neoplasms. The cortico-medullary phase is not the ideal phase for evaluating a renal mass however, as a small renal mass may enhance similar to that of renal medulla during this phase and hypovascular neoplasms such as papillary lesions may not be readily identifiable during this phase [3, 9, 11]. The nephrogenic phase (>80 s delay) is important for the evaluation of a renal mass as it provides maximal homogenous parenchymal enhancement [10]. The delayed or excretory phase (3–5min delay) allows contrast excretion into the collecting system aiding in identification of calyceal or renal pelvis involvement and detection of urothelial tumors [10, 19].