Magnetic resonance scan

By employing a strong electromagnetic field, small magnetic changes can be stimulated in body tissues and the signal generated when the dipole is lost can be detected and measured. This non-invasive method of imaging is excellent at staging certain urological tumours, including prostate cancer. Lymph nodes can also be well demonstrated.

Using catheters Micturating cystoureterogram

A catheter is passed into the bladder and the urine drained out. Contrast is used to fill the bladder. The catheter is removed and the patient positioned to allow radiographical screening of the KUB during voiding. This allows an assessment of ureteric reflux to be made.

Retrograde ureterogram

This is usually performed under general anaesthesia in the operating theatre. A ureteric catheter is passed, via a cysto-scope, into the ureteric orifice and contrast injected directly up the ureter. Using radiographical screening the flow of contrast into the ureter is demonstrated and any obstruction or filling defects shown.

Antegrade uretogram

Contrast is injected, via a percutaneous nephrostomy tube while radiographical screening, to demonstrate any hold up to the free flow of contrast down the ureter.

Urethrogram

This is performed to assess a urethral stricture or, in cases of trauma, to the urethra, to assess rupture. Contrast is injected down a metal probe or catheter passed into the meatus and radiographs are taken.

Vasogram

This test demonstrates vasal obstruction in an infertile (azoospermic) male. Under a general anaesthetic the vas is identified either percutaneously or after a scrotal incision. A fine needle is passed into the lumen and contrast is injected antegradely. A radiograph will demonstrate contrast in the seminal vesicles if there is no obstruction.

Functional tests Nuclear isotope renography

Mercaptoacetyltriglycine renogram

The compound mercaptoacetyltriglycine (MAG 3) labelled with an isotope of technetium is now the most commonly used agent for renography. An intravenous bolus is given

Curve B

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Figure 19.4. Renogram curve.

Curve B

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Figure 19.4. Renogram curve.

and the uptake and rate of excretion by the kidneys is measured, by the rise and fall of radioactive activity, using a gamma camera placed posteriorly against the patient's back. In Fig. 19.4, curve A shows a rapid uptake by the kidney to a maximum with a smooth decay as the isotope is excreted with the urine. Curve B shows a reduced uptake suggesting reduced renal function. However, no fall in the curve is seen. This is characteristic of an obstructed kidney, where the urine containing the isotope is not excreted. Classically, this pattern is seen in a PUJ obstruction. Occasionally, urine production is poor and the isotope is excreted slowly or the calyces and renal pelvis are dilated and baggy and take a long time to fill. An intravenous diuretic is routinely given to increase urine (and isotope) excretion and determine whether the delay in excretion is due to an obstruction. The relative differential function of the kidneys can be assessed from the renogram (e.g. 60% right and 40% left). However, to assess absolute function of each kidney, the GFR must be measured and this is not usually assessed using MAG 3.

Chromium ethylenediaminetetraacetic acid clearance Ethylenediaminetetraacetic acid (EDTA) labelled with a radioactive an isotope of chromium is given intravenously and measurement of blood and urine concentration gives a close approximation to the GFR. The agent is filtered at the glomerulus only, with little or no tubular secretion occurring. It therefore provides a quick and convenient test of GFR.

Dimercaptosuccinic acid renogram

This agent is both filtered and resorbed by the kidney. It has high cortical fixation and is therefore the agent of choice for renal cortical imaging in cases of acute pyelonephritis and renal scarring. A non-functioning obstructed kidney can also have its tubular function assessed by a static renogram using technetium-labelled dimercaptosuccinic acid (DMSA).

Bone scan

Radioisotope bone scanning, using technetium-99m tracer, is the standard method for assessing potential bone metastases

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Figure 19.5. Urine flowmetry: (a) obstructed flow or poor detrusor muscle contraction and (b) non-obstructed flow.

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