CT Urography

CT urography (CTU) is a multiphase contrast enhanced CT that images the kidneys, ureters and bladder, including the renal parenchyma and the opacified collecting system.

A non-contrast phase (optional, for stones and calcification) is followed by a nephrographic phase at 80 to 100 seconds (portal venous, for renal masses) and an excretory phase at 8 to 15 minutes (opacified pelvicalyceal system, ureters and bladder).

The excretory phase is obtained after a split bolus of contrast (for example, 30 to 50 mL at the start and 70 to 80 mL after 8 to 10 minutes, with a single scan at 100 seconds after the second injection) or by a single bolus with a delayed scan.

Compression of the lower abdomen, saline infusion (250 mL) or furosemide (10 mg) improve distension and opacification. Slices of 0.6 to 1.25 mm are reconstructed in axial, coronal and curved planar views, with maximum intensity projections and volume rendering of the collecting system.

Effective dose is about 10 to 20 mSv for a three phase study, less with a split bolus and low dose techniques.

CTU has replaced intravenous urography as the main test for haematuria and upper tract urothelial disease. It has a sensitivity of 94 to 98% and a specificity of 95 to 99% for upper tract urothelial cancer, over 95% for renal masses over 1 cm, and about 95% for stones in the non-contrast phase.

Visible haematuria is caused by malignancy in 10 to 20% of patients (bladder cancer in most, renal cell carcinoma and upper tract cancer in the rest), while non-visible haematuria carries a malignancy rate of 2 to 5%.

Indications. Visible (macroscopic) haematuria, and non-visible haematuria in patients with risk factors (age over 40 to 50, smoking, occupational exposure to dyes or chemicals, previous pelvic irradiation, cyclophosphamide). Suspected upper tract urothelial carcinoma and surveillance of known disease. Staging of renal and bladder cancer.

Renal masses needing characterisation. Congenital anomalies (duplex system, ectopic ureter, pelviureteric junction obstruction, horseshoe kidney). Ureteric strictures and obstruction of uncertain cause, retroperitoneal fibrosis, and ureteric injury.

Urinary fistula (vesicovaginal, ureterovaginal), urinoma and leak after surgery or trauma. Recurrent infection with suspected structural cause, and renal transplant donor assessment (arterial and venous anatomy, collecting system). Not for first line evaluation of renal colic (non-contrast low dose CT is used), not for pregnancy, and not for patients with renal impairment where ultrasound or MR can answer.

Contraindications and precautions. Previous severe contrast reaction (premedication or MR urography), renal impairment (estimated glomerular filtration rate under 30 mL each minute: avoid, because opacification is poor and the contrast load is high), pregnancy (avoid; MR urography), dehydration, and myeloma. Metformin (follow local advice depending on renal function).

Furosemide is avoided in patients with severe obstruction or dehydration. Hydration with 500 mL of water before the scan is encouraged. Children: use ultrasound and MR; CT only when essential, with low dose settings.

Preparation and technique. The patient drinks 500 to 1000 mL of water in the hour before the scan and empties the bladder immediately before (a full bladder obscures the distal ureters, though it helps bladder assessment; practice varies). A cannula of 20 G is placed. Non-contrast phase from the diaphragm to the symphysis (optional where stones are not suspected).

Contrast is injected at 2 to 3 mL each second, and the nephrographic phase is taken at 80 to 100 seconds. The excretory phase follows at 8 to 15 minutes (or 5 to 7 minutes after furosemide), with the patient prone for part of the delay to move contrast into the ureters, or with abdominal compression.

A single bolus combined phase technique with a split bolus delivers the nephrographic and excretory phases in one acquisition, lowering the dose. Images are reviewed in all planes and with 3D urogram reconstructions.

Normal appearances. The kidneys are 10 to 12 cm long with smooth contours and uniform cortical thickness of 1.5 to 2 cm. In the nephrographic phase the cortex and medulla enhance uniformly (80 to 120 HU).

The collecting system fills with contrast, with sharp calyceal cups, smooth pelvis and ureters of 3 to 8 mm that are opacified in segments because of peristalsis (a full length opacification of both ureters is not always achieved).

The ureters pass over the pelvic brim, cross the iliac vessels, and enter the bladder at the vesicoureteric junction. The bladder wall is thin and smooth, under 3 mm when distended. No filling defects, masses, stones or hydronephrosis.

Interpretation.

• Upper tract urothelial carcinoma: filling defect in the opacified collecting system, focal wall thickening of the renal pelvis or ureter (over 5 mm), irregular narrowing of the ureter, a soft tissue mass, hydronephrosis with or without an obstructing lesion, and a "goblet" sign (dilated ureter distal to a tumour); the commonest sites are the pelvis and distal ureter; multifocal disease and synchronous bladder tumours are frequent.
• Bladder cancer: focal wall thickening, sessile or papillary mass, filling defect in the excretory phase, and perivesical stranding, extension and lymph nodes in invasive disease; cystoscopy remains the main test for the bladder.
• Renal cell carcinoma: enhancing solid mass (hypervascular in clear cell carcinoma with washout in the nephrographic phase, hypovascular in papillary tumours), necrosis, calcification, invasion of the renal vein and inferior vena cava (tumour thrombus), nodes and metastases; small enhancing masses under 4 cm are usually cancer or oncocytoma; angiomyolipoma contains fat (lower than -20 HU).
• Cystic renal masses: simple cyst (water density, thin wall, no enhancement), complex cysts with septa, thick wall, nodularity or enhancement (suspicious for malignancy), and haemorrhagic or proteinaceous cysts (hyperdense, non-enhancing).
• Stones: hyperdense calculi on the non-contrast phase (over 130 HU), with hydronephrosis, perinephric stranding and obstruction; uric acid stones are less dense; stones may be masked by contrast on excretory images.
• Obstruction: pelviureteric junction obstruction (dilated pelvis with narrowing at the junction and a normal ureter), vesicoureteric junction obstruction, extrinsic compression (retroperitoneal fibrosis with medial deviation of ureters, tumour, lymph nodes, aortic aneurysm), and ureteric stricture (post-operative, post-radiation, tuberculosis).
• Infection and inflammation: pyelonephritis (wedge shaped zones of reduced enhancement), abscess (rim enhancing collection), emphysematous pyelonephritis (gas), pyonephrosis (dilated collecting system with debris), tuberculosis (calyceal destruction, calcification, strictures, "putty kidney"), and schistosomiasis (calcified bladder and ureteric walls).
• Congenital: duplex collecting system (with ureterocele, ectopic insertion and the "drooping lily" appearance), horseshoe kidney, ectopic or malrotated kidney, megaureter, and medullary sponge kidney (contrast blush in the papillae).
• Papillary necrosis: contrast filling of clefts and sloughed papillae in the calyces (diabetes, analgesic nephropathy, sickle cell disease).
• Leak, urinoma and fistula: extravasation of contrast into the perinephric or pelvic space on delayed images, and fistula to the vagina, bowel or skin.
• Pre-operative: renal artery and vein anatomy (multiple arteries, early branching, retroaortic left renal vein), and collecting system anatomy for nephrectomy and stone surgery.
• Incidental: adrenal, liver, pancreatic lesions, aortic aneurysm, and pelvic masses.

Critical values and findings needing immediate action.

• Obstructed infected kidney (pyonephrosis, emphysematous pyelonephritis): urgent decompression (nephrostomy or stent) and antibiotics.
• Suspicious urothelial filling defect or mass: urgent urology referral for cystoscopy, ureteroscopy with biopsy, and staging.
• Renal mass with features of malignancy: multidisciplinary review for staging, surgery or ablation.
• Renal vein or vena cava tumour thrombus: urgent urological oncology.
• Urine leak or ureteric injury: urgent stent, drainage or repair.
• Obstructing stone with solitary kidney or renal failure: emergency drainage.
• Retroperitoneal fibrosis with obstruction: stenting and specialist review.
• Active haemorrhage from renal tumour (Wunderlich syndrome): resuscitation and embolisation.

Limitations and artefacts.

• Poor or incomplete opacification of the ureters occurs in 15 to 30% (peristalsis, obstruction, poor renal function), and a short delay misses lesions in non-opacified segments; further imaging (repeat delayed scan, MR urography, retrograde pyelography, ureteroscopy) is needed when a segment is not seen.
• Flat urothelial tumours and carcinoma in situ are not detected; cystoscopy and ureteroscopy remain essential, and CTU cannot exclude bladder cancer.
• Small tumours under 5 mm, early wall thickening, and tumours in non-distended segments are missed; false positives arise from blood clots, air bubbles, stones, papillary necrosis, pyeloureteritis cystica and mixing of contrast and urine.
• Dense contrast in the bladder or collecting system hides stones and small mural lesions; unenhanced images are needed.
• Radiation dose is high with multiphase techniques; split bolus and dual energy techniques reduce it, and ultrasound or MR should be used in young patients and in those needing repeated surveillance.
• Contrast nephropathy risk in patients with reduced renal function; and contrast reactions.
• Small renal masses under 1 cm are not characterised reliably.
• Bladder distension and compression artefacts may distort the ureters.

Special groups.

• Smokers and people with occupational exposure (dyes, rubber, leather): higher risk of urothelial cancer.
• Patients with Lynch syndrome: upper tract urothelial cancer risk, and surveillance.
• Patients after treatment of urothelial cancer: surveillance of the upper tract with CTU (every 1 to 2 years) alongside cystoscopy.
• Pregnancy: use ultrasound and MR urography.
• Children: ultrasound, micturating cystourethrogram and MR urography; CT is rare.
• Patients with renal impairment: MR urography without gadolinium or retrograde studies.
• Regions: urinary schistosomiasis causes bladder cancer (squamous), calcified bladder and obstructive uropathy; tuberculosis of the urinary tract, stones, infection and prostatic obstruction are common, while CT availability is limited; ultrasound and cystoscopy are the main tests.

Follow up tests. Cystoscopy with biopsy (white light and enhanced techniques), ureteroscopy with biopsy and selective cytology, urine cytology and urinalysis, urine culture, MR urography, retrograde pyelography, renal ultrasound, blood tests (creatinine, estimated glomerular filtration rate, full blood count, calcium, urate), metabolic stone evaluation (24-hour urine),

staging CT of the chest, abdomen and pelvis, nuclear renography for function and drainage, renal mass biopsy in selected cases, serology and urine microscopy for schistosomiasis, tuberculosis tests, and referral to urology, nephrology or oncology.