52 CHAPTER 2 Urological investigations

Radioisotope imaging

A variety of organic compounds can be labeled with a radioactive isotope that emits gamma rays, allowing the radiation to penetrate through tissues and reach a gamma camera placed adjacent to the patient. The most commonly used radioisotope is technetium—99mTc (half-life 6 hours, gamma-ray emission energy 0.14 MeV). The excretion characteristics of the organic compound to which the 99mTc is bound determine the clinical use.

MAG3 renogram

99mTc is bound to mercapto-acetyl-triglycine. Over 90% of MAG3 becomes bound to plasma proteins following intravenous injection. It is excreted from the kidneys principally by tubular secretion (glomerular filtration is minimal).

Following intravenous injection, MAG3 is very rapidly excreted (appearing in the kidney within 15 seconds of the injection and starting to appear in the bladder within about 3 minutes). Approximately two-thirds of the injected dose of MAG3 is taken up by the kidneys with each passage of blood through the kidney. The radioactivity over each kidney thus increases rapidly.

The peak of radioactivity represents the point at which delivery of MAG3 to the kidney from the renal artery is equivalent to excretion of MAG3. The radioactivity starts to decline as excretion outstrips supply. Thus, a time–activity curve can be recorded for each kidney. This time– activity curve is known as a renogram.

Images are collected onto a film at 30-second intervals for the first 3 minutes and then at 5-minute intervals for the remainder of the study (usually a total of 30 minutes).

A normal renogram has 3 phases

•First phase: a steeply rising curve lasting 20–30 seconds

•Second phase: a more slowly rising curve, rising to a peak. If the curve does not reach a peak, the second phase is said to rise continually.

A normal second phase ends with a sharp peak.

•Third phase: a curve that descends after the peak. There can be no third phase if there is no peak.

Description of the renogram

No comment is made about the first phase. The second phase is described as being absent, impaired, or normal. The third phase is described as being absent, impaired, or normal.

The time to the peak depends on urine flow and level of hydration and is a crude measure of the time it takes the tracer to travel through the parenchyma of the kidney and through the renal pelvis. The time to the peak of the renogram normally varies between 2 and 4.5 minutes.

If the renogram continues beyond the time at which the peak should normally occur, then there may be a distal obstruction (e.g., at the PUJ or lower down the ureter). In this situation, an injection of 40 mg of Lasix is given (at about 18 minutes) and if the curves start to fall rapidly, this is taken as proof that there is no obstruction. If it continues to rise, there is

RADIOISOTOPE IMAGING 53

obstruction. If it remains flat (neither rising nor falling), this is described as an “equivocal” result.

Parenchymal transit time can also be measured (parenchymal transit time index [PTTI]). The normal range for PTTI is 40–140 seconds, and averages 70 seconds. PTTI is prolonged (to >156 seconds) in obstruction and in renal ischemia. A normal PTTI excludes obstruction.

Uses

•“Split” renal function (i.e., the % function contributed by each kidney)

•Determine presence of renal obstruction—based on shape of renogram curve and PTTI

•Determine presence of renal obstruction in response to IV Lasix injection

DMSA scanning

Dimercaptosuccinic acid (DMSA) is labeled with 99mTc. It is taken up by the proximal tubules and retained there, with very little being excreted in the urine. A “static” image of the kidneys is thus obtained (at about 3–4 hours after intravenous injection of radioisotope). It demonstrates whether a lesion contains functioning nephrons or not.

Uses

•“Split” renal function (i.e., the % function contributed by each kidney)

•Detection of scars in the kidney (these appear as defects in the cortical outline, representing areas in which the radioisotope is not taken up)

Radioisotope bone imaging

99mTc-labeled methylene-disphosphonate (MDP) is taken up by areas of bone where there is increased blood supply and increased osteoblastic activity. There are many causes of a focal increase in isotope uptake— bone metastases, site of fractures, osteomyelitis, TB, benign bone lesions (e.g., osteoma).

Metastases from urological cancers are characterized by their predilection for the spine and the fact that they are multiple (single foci of metastasis are rare). Prostate cancer classically metastasizes in this way.

UROFLOWMETRY 55

Voided volume (ml)

Figure 2.15 The Bristol flow rate nomogram for men over 50 years. This figure was published in Fitzpatrick J, Societe Internationale D’Urologie Reports. Non-surgical Treatment of BPH, p. 39. Copyright Elsevier 1992.

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