Imaging of the urinary tract - Investigations and procedures

Imaging of the urinary tract

Plain X-ray of the kidneys, ureters and bladder (KUB)

On a plain X-ray radiopaque (calcium-containing, struvite and cystine) stones and renal tissue calcification, calcification of vessels (e.g. an atheromatous aorta) and calcification in tumours will show up. The outlines of the kidneys are unreliably seen because of overlying bowel gas.

Renal ultrasound scan (USS)

This is a useful imaging method of the kidneys. It avoids the use of contrast dyes, which have to be given intravenously, are nephrotoxic, and to which patients occasionally develop an allergic reaction. USS is particularly useful for the following:

·        Renal obstruction, an important reversible cause of renal failure. The pelvicalyceal systems and ureter(s) look dilated except in early obstruction, or if the patient is oligoanuric. Occasionally a cause is seen such as a stone.

·        To assess the size of the kidneys. In renal failure, small kidneys mean chronic renal failure, normal size kidneys usually mean acute renal failure which is potentially reversible. The exceptions are diabetes mellitus, amyloid and multiple myeloma.

·        Assessment of cysts and mass lesions.

·        In refractory pyelonephritis to look for a renal abscess, obstruction or an underlying anatomical abnormality such as a stone.

·        For USS-guided kidney biopsy.

·        Doppler USS - to look for renal blood flow, renal vein thrombosis and renal artery stenosis.

Bladder and prostate USS

Bladder USS can assess residual volumes after bladder emptying. Prostate USS is best done transrectally, and can demonstrate its size, any asymmetry, or suspicious areas. USS-guided prostatic biopsy may be performed.

Intravenous urogram/pyelogram (IVU/IVP)

This is commonly used in the investigation of renal colic, although it is also a useful tool in assessing the anatomy of the urinary tract.

A plain film is taken first as the comparison film, then a slow intravenous injection of an iodine-containing contrast dye is given (prophylaxis with highdose steroids are given to asthmatics and those with a known allergy to iodine, to prevent an allergic reaction). Serial X-rays are then taken, which show the passage of the dye through the renal parenchyma, outlining the kidneys clearly. The dye then normally passes rapidly into the ureters. If there is obstruction, dye will be ‘held-up’ on one or both sides. The exact site of obstruction can often be seen with dilatation above. A filling defect within the ureter suggests a radiolucent stone or tumour. Pre and post-voiding films are taken to look for any filling defects in the bladder (stone or tumour), and to assess bladder emptying.

IVU/IVP should be avoided in significant renal impairment because of nephrotoxicity and poor dye excretion makes the test difficult to interpret. All patients should be well hydrated. Diabetics are particularly at risk of nephrotoxicity, metformin should be stopped prior to giving contrast.

Other urographic studies

These two methods are more invasive than IVU/IVP, but as the dye is given directly into the urinary tract, avoids the risk of nephrotoxicity and allergy. Therapeutic stents may be placed as part of the procedure to relieve obstruction.

· Nephrostomy and antegrade pyelography – for upper tract obstruction, a fine-bore catheter is introduced into the dilated renal pelvis percutaneously under local anaesthetic. This relieves the obstruction and allows urine to drain out. Contrast is then injected through the catheter, to demonstrate the cause and site of obstruction.

·        Retrograde pyelography – following cystoscopy, a catheter is passed into the ureteric orifice normally under general anaesthetic. Contrast is injected and images are obtained. This is useful for defining lower ureteric lesions such as stones, and to look for transitional cell carcinoma of the ureter.

Computed tomography (CT)

In most cases, the diagnostic ability of CT is improved by giving intravenous contrast, but as noted above, certain precautions should be taken.

·        Renal cysts and masses – CT can help differentiate benign cysts from malignancy.

·        It can be used for staging in all types of genitourinary malignancy, including renal cell carcinoma, bladder cancer and testicular tumours.

·        It is able to detect radiolucent stones missed on plain X-ray.

·        In polycystic kidney disease it can be useful if one cyst is thought to be infected or malignant.

Nuclear medicine scans

A non-nephrotoxic radioisotope is given intravenously, which is taken up and excreted by the kidneys. Imaging may be ‘static’ (for anatomical detail), or ‘dynamic’ (for function).

·        Static DMSA scans are more sensitive than IVU to look for scarring and ischaemia.

·        Dynamic DTPA and MAG3 are used to look for renal parenchymal disease and obstruction. To look for obstruction, furosemide is given – the radioisotope will flush out promptly if there is no obstruction.

Magnetic resonance imaging/angiography (MRI/MRA)

MRI is sometimes used to further assess renal cysts and solid lesions. It is also used with gadolinium contrast to perform angiography (MRA) as a non-invasive alternative to renal angiography. Gadolinium is non-nephrotoxic.

Renal angiography

This is mainly performed for suspected renal artery stenosis. A sheath is placed in the femoral artery, and an arterial catheter passed to the aorta. Each renal artery is selectively catheterised and contrast injected. Conventional or digital subtraction angiography (DSA) may be used. DSA uses less contrast, which reduces the risk of contrast nephropathy (acute renal failure secondary to the nephrotoxic contrast). Other complications include cholesterol emboli and arterial dissection. Percutaneous renal angioplasty (PRA) and renal artery stenting can also be performed.