Description

The urinary tract consists of the kidneys, ureters, bladder, urethra and, in men, the prostate gland.

Imaging of the urinary tract can involve the following tests:

• Plain kidney, ureters and bladder (KUB) testing
• Intravenous urogram
• Ultrasonography
• Nuclear medicine - including mercaptoacetyltriglycine (MAG3) and dimercaptosuccinic acid (DMSA) scans
• Cystography
• Computed tomography (CT) scan
• Magnetic resonance imaging (MRI) scan
• More invasive tests

Plain kidney, ureters and bladder testing

This will reveal opaque renal calculi. It may also be helpful in nephrocalcinosis where there is increased uptake of calcium by the kidneys. However, it does involve radiation and this should be borne in mind before deciding on this investigation in patients. This is especially so as the role of computed tomography (CT) scan in detection of renal calculi has increased - thus it may be better for patients to undergo only one of these investigations. Spiral CT takes only 5 minutes and no preparation is required. Furthermore, CT scanning can also detect lesions other than renal calculi - for example, perinephric abscesses and tumours.¹

Intravenous urogram

This is most useful when looking for obstruction in the urinary tract - for example, hydronephrosis due to the presence of renal calculi. Other uses include diagnosis of medullary sponge kidney. However, it is time-consuming and the use of contrast can lead to contrast nephropathy (good hydration is essential). Thus it is important to have renal function tested beforehand and it should be used with caution in the elderly and in those with renal impairment.

Ultrasonography

Generally ultrasonography is an excellent imaging modality as it is noninvasive, reliable and affordable.² It can be used to investigate the kidney, bladder, and prostate gland. It can also be combined with voiding, providing an indication of the residual volume. This gives an indirect measure of bladder function.

In terms of renal imaging ultrasonography is useful for the following:

• Acute renal failure - mainly looking for postrenal obstruction
• Chronic renal failure - the presence of small shrunken kidneys suggests irreversible damage. Normal kidney size is approx 11 cm (varies with age, gender and race).²
• Detecting hydronephrosis and hydroureter.
• Congenital anomalies - e.g. hypoplasia, agenesis, duplex systems.
• Renal cysts, abscesses and neoplasms can be detected - e.g. simple cysts, polycystic kidneys.
• Renal calculi can be detected but they can be mistaken for vessels or calcified tumours.²
• Renal ultrasound can be combined with Doppler imaging to view the renal artery and vein, which may help detect thrombosis, stenosis or aneurysms.

Nuclear medicine

This involves low amounts of radiation and provides information regarding renal perfusion, function and the contribution each kidney is making to total function.

• These procedures include ^99mTc-mercaptoacetyltriglycine scanning (MAG3) and ^99mTc-diethylenetriamine-pentaacetate (DTPA) uptake scans.¹ The latter is mostly being overtaken by the former. These allow dynamic imaging, as both are filtered by the glomerulus and excreted by the kidney.
• MAG3 scanning is useful in hypertension (looking to see whether those who have renovascular disease will benefit from procedures to improve renal blood flow).³ MAG3 is also useful in delayed graft function following renal transplant and discriminating between functional renal obstruction and simple dilatation alone.³
• Scintigraphy can also be performed with ^99mTc-dimercaptosuccinic acid (DMSA). DMSA is given by intravenous injection and then static imaging is performed 2-4 hours later. This provides information on the contribution each kidney is making to total function. Thus it is useful in situations where there is bilateral scarring.

Cystography

Contrast is inserted into the bladder and images are obtained. The patient can then be asked to void and the extent of vesicoureteral reflux and urinary stress incontinence can be assessed.

Computed tomography scanning of the urinary tract

Again this involves a significant radiation dose, thus needs to be considered with care. It is useful in detecting the following:

• Renal calculi - as discussed under the 'Plain kidney, ureters and bladder testing' section above.
• Renal and bladder neoplasms - for detection and staging.
• Renal trauma - detecting perinephric haematoma for example.

Magnetic resonance imaging scanning of the urinary tract

Used in the following groups of patients:

• Those who are at risk of contrast nephropathy
• Those who have an allergy to contrast agents
• Magnetic resonance (MR) urography is being used in children and has the advantage that it provides both functional and morphological imaging, but this requires the use of complex software⁴

Magnetic resonance imaging (MRI) is superior to computed tomography (CT) scanning in detecting renal cell carcinoma metastases into the renal vein. It may also be better when trying to determine whether renal lesions are simply cysts, neoplastic or haematomas. It can also be used in the detection of renal artery stenosis - magnetic renal angiography (MRA).

More invasive investigations

Ureteropyelography

• Anterograde ureteropyelography - this requires puncture via the skin into the renal pelvis. Via the puncture, contrast is injected and images obtained. This procedure can also be used to relieve obstruction by insertion of a nephrostomy tube.
• Retrograde ureteroscopy - this is performed by insertion of a cystoscope in to the urethra and bladder. This is followed by injection of contrast into the distal ureter, after which images are taken.

Angiography

This can be performed with the aid of computed tomography (CT) or magnetic resonance imaging (MRI). It is very invasive and requires cannulation of the renal arteries. It will provide a definite diagnosis of stenosis and allows angioplasty if necessary. There is also a risk of embolism resulting from trauma to plaques.

Document references

1. Boubaker A, Prior JO, Meuwly JY, et al; Radionuclide investigations of the urinary tract in the era of multimodality imaging. J Nucl Med. 2006 Nov;47(11):1819-36. [abstract]
2. Gheissari A; The place of ultrasound in renal medicine. Saudi J Kidney Dis Transpl. 2006 Dec;17(4):540-8. [abstract]
3. Maisey M; Radionuclide renography: a review. Curr Opin Nephrol Hypertens. 2003 Nov;12(6):649-52. [abstract]
4. Khrichenko D, Darge K; Functional analysis in MR urography - made simple. Pediatr Radiol. 2010 Feb;40(2):182-99. Epub 2009 Dec 12. [abstract]

Acknowledgements