Imagem do trato urinário

Descrição

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 imaging

Plain x-rays can reveal opaque renal calculi. They may also be helpful in nephrocalcinosis where there is increased uptake of calcium by the kidneys. CT scan of kidneys, ureter and bladder is the preferred imaging modality looking for renal calculi. CT scanning has a higher radiation dose than plain x-ray but it has a much better sensitivity for detecting a stone both directly and indirectly (eg, by detecting dilated ureter or hydronephrosis). Spiral CT takes only five minutes and no preparation is required. Furthermore, CT scanning can also detect lesions other than renal calculi - for example, perinephric abscesses and tumours.¹

Micturating cystourethrogram - this involves instillation of contrast into the bladder following which x-rays are taken whilst voiding. It is used in some children following a urinary tract infection.²

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 non-invasive, reliable and affordable.³It 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.

Renal ultrasonography is useful for the following:

• Lesão renal aguda - mainly looking for post-renal obstruction.

• Doença renal crônica - the presence of small shrunken kidneys suggests irreversible damage. Normal kidney size is approx 11 cm (varies with age, gender and race).³

• Detecting hydronephrosis and hydroureter.

• Some children post following a urinary tract infection.²

• Congenital anomalies - eg, hypoplasia, agenesis, duplex systems.

• Renal cysts, abscesses and neoplasms can be detected - eg, simple cysts, polycystic kidneys.

• Renal calculi can be detected but they can be mistaken for vessels or calcified tumours.³

• 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 ^99m Tc-mercaptoacetyltriglycine scanning (MAG3) and ^99m Tc-diethylenetriamine-pentaacetate (DTPA) uptake scans.¹The 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).⁴MAG3 is also useful in delayed graft function following renal transplant and discriminating between functional renal obstruction and simple dilatation alone.⁴

• Scintigraphy can also be performed with 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. It is also used in some children following a urinary tract infection.²

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 'Plain kidney, ureters and bladder imaging', above.

• Renal and bladder neoplasms - for detection and staging.

• Renal trauma - detecting perinephric haematoma, for example.

Magnetic resonance imaging scanning of the urinary tract

This is used in the following groups of patients:

• Those who are at risk of contrast nephropathy.

• Those who have an allergy to contrast agents.

• Children. Magnetic resonance (MR) urography is being used in children and has the advantage that it provides both functional and morphological imaging. However, this requires the use of complex software.⁵

• Those with renal cell carcinoma.

• Women with chronic urinary tract infections.⁶

Enhanced MRI technology is also proving increasingly important in renal cell carcinoma but is still in an experimental stage - eg, response to chemotherapy.⁷MRI is also superior to 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 into 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 CT or MRI. It is 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.