European Nuclear Medicine Guide
See: 7.1.1. and 7.2.1
[99mTc]Tc-MAG3 (betiatide, chelated to technetium), also known as
Benzoyl mercaptoacetyltriglycine
[99mTc]Tc-Mertiatide (Nephromag/Technescan MAG3)
[99mTc]Tc-diethylenetriaminepentaacetic acid, also known as
[99mTc]Tc-DTPA
[99mTc]Tc-Pentetate
Note: [99mTc]Tc-MAG3 is preferred over [99mTc]Tc-DTPA, especially in cases with increased serum creatinine, because the faster renal extraction fraction of [99mTc]Tc-MAG3 results in superior interpretability (44).
Renovascular hypertension is defined as an elevated blood pressure caused by renal hypoperfusion, resulting from stenosis of the renal artery and activation of the renin–angiotensin-aldosterone system. Atherosclerotic renovascular disease represents the most common type of renal artery stenosis, whereas only 10% of cases are due to fibromuscular dysplasia (45). The prevalence of renal artery stenosis in hypertensives is probably less than 1%, but may be as high as 14–24% in patients with severe or drug- resistant hypertension (46). Duplex ultrasound is the most commonly used screening tool for atherosclerotic renovascular disease due to its lower cost and non-invasive nature. Magnetic resonance angiography and computed tomography angiography are further sensitive methods for detection of renovascular stenosis. However, captopril renal scintigraphy was reportedly highly accurate in predicting clinical outcomes (47).
Captopril renal scintigraphy is known to be sensitive for detecting renovascular hypertension (RVH), although some limitations (e.g., in the presence of bilateral renal artery stenosis, renal failure, or diffuse microangiopathy) should be considered.
Captopril renal scintigraphy examines the functional aspects of RVH related to renal hypoperfusion. In this context, angiotensin II plays a key role in regulating the glomerular filtration rate (GFR) when renal perfusion pressure is low due to renal artery stenosis. Captopril, an angiotensin-converting enzyme (ACE) inhibitor, disrupts this adaptive mechanism, leading to a reduction in GFR and a prolongation of average tubular transit times, which also become more widely distributed. These changes can be visualized during dynamic renal scintigraphy using both glomerular and tubular tracers.
In most centres, captopril (25–50 mg orally) is administered on day 1, and dynamic renal scintigraphy is conducted 60 minutes later. A captopril dosage of 50 mg may be preferred if diastolic pressure is higher than 80 mmHg, while a dosage of 25 mg may be chosen in an elderly patient or in cases of suspected bilateral stenosis. If the renogram shows a normal pattern, no additional baseline scan is needed, since the chance of a haemodynamically relevant RVH is very low. If the pattern is abnormal, a second renal scintigraphy without prior administration of captopril is performed on a different day. The scintigraphic and renographic images are then visually assessed to interpret the changes induced by captopril.
General preparation follows the recommendations for renal dynamic scintigraphy with the respective radiopharmaceutical (chapters 7.1 and 7.2). Additionally, before performing a captopril renal scan, it is essential to discontinue ACE inhibitors and diuretics for several days (up to one week, depending on the medication's half-life). If necessary, antihypertensive treatment can be replaced with alpha-blockers or calcium antagonists or doxazosin. However, bilateral symmetrical abnormalities have been reported in patients taking calcium antagonists. For this reason, it is reasonable to avoid calcium antagonists when there is an alternative (48). If hypertension is severe, it is not necessary to discontinue all antihypertensive medications before the procedure (48).
Patient preparation, including the discontinuation of certain drugs, should be similar for the captopril scan and the comparative scan without captopril.
Blood pressure and heart rate should be monitored and recorded before captopril medication and radiopharmaceutical administration, every 5–15 min thereafter, and at the end of the study. The systemic effect of captopril should be recorded and briefly described in the report. It is recommended to have an intravenous line present in case of severe response to captopril, especially in patients with transient ischaemic attack, recent myocardial infarction, carotid disease or angina pectoris. After the study patients should not be discharged before they reach at least 70% of their usual blood pressure.
The following paragraphs provide a brief description of relevant findings. Refer to the SNMMI guideline (48) for additional diagnostic criteria and much more detailed guidance.
In patients with normal or minimally reduced renal function (creatinine < 1.7 mg/dL), captopril renography has a sensitivity and specificity of about 90% for diagnosis of renovascular hypertension (48).
In cases of RVH, the post-captopril study typically shows reduced relative uptake of the affected kidney (resulting in lower split renal function) both with tubular and glomerular agents. With tubular agents, parenchymal transit of the radiopharmaceutical in the affected kidney is delayed by captopril, resulting in delayed Tmax, diminished tracer excretion with increased 20 min/peak ratio and increased parenchymal retention. With glomerular agents, marked parenchymal retention induced by captopril is also highly suggestive of RVH (5).
False positive results indicating renal artery stenosis can occur with multiple renal arteries or in states of dehydration or sodium depletion, which may activate the renin system (5). For this reason, diuretics should be discontinued several days before the test. Bilateral symmetrical changes after captopril are indicative of such unspecific effects unrelated to RVH (5). Conversely, false-negative results may arise in cases of haemodynamically significant stenosis or in patients with poorly functioning kidneys at baseline or in bilateral microangiopathy. However, the results are most reliable when interpreted in the context of predicting the success of revascularization as a treatment for hypertension.
See 7.1.7 and 7.2.7.
Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."