−60 to −100
−400 to −600
Phases of a CT: Administration of intravenous contrast provides functional information and gives rise to various “phases” determined by time, as contrast material passes through the vascular system and into the renal collecting system (Table 4.2).
CT phases and their objective
Time post injection
Identification of calculi, baseline soft tissue density measurement
Enhancement of arterial vessels
Enhancement of venous vessels, soft tissue density measurement (assess for contrast enhancement)
Enhancement of collecting system
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 .
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 .
The non-contrast scan is preferable as contrast obscures accurate identification of the stone in the excretory phase, and requires significantly less radiation . 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 .
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.
Tips in Assessing CT for Urolithiasis
Assess kidneys for hydronephrosis, perinephric stranding, and anatomical abnormalities
Check for co-existing renal stones, ipsi- or contralateral
Track ureters proximal to distal
Assess bladder for stones & other pathology
Stone density can be assessed to determine stone type preoperatively, which may also influence management. The density of common stone types have been described and are presented in Table 4.3 [5, 8, 14–16, 18].
Stone types and approximate density
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 . 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].