What is the indication for a technetium-99m (Tc-99m) ammonia scan?

Indications for Technetium-99m Ammonia Scan

N-13 ammonia is a PET perfusion tracer used for diagnosing coronary artery disease and assessing myocardial viability, though it remains investigational by the FDA and is not as widely available as other cardiac imaging agents. 1

Primary Clinical Indications

Diagnosis of Coronary Artery Disease

• N-13 ammonia PET demonstrates abnormal coronary perfusion patterns in the majority of patients with coronary artery disease, with sensitivities ranging from 87% to 97% and specificities from 78% to 100%. 1
• The tracer is used with pharmacologic stress (typically dipyridamole or adenosine) to identify ischemia in patients with suspected or known coronary disease. 1
• PET perfusion imaging with N-13 ammonia has demonstrated significantly higher diagnostic accuracy compared to thallium-201 SPECT for detecting angiographically documented coronary artery disease. 1

Myocardial Viability Assessment

• N-13 ammonia is specifically indicated for assessing myocardial viability through perfusion and metabolic imaging in patients with ischemic heart failure or left ventricular dysfunction being considered for revascularization. 2
• PET viability assessment can be performed regardless of heart rate (even at 100 bpm or higher), as it evaluates myocardial metabolism and perfusion independent of cardiac rhythm. 2
• The technique provides quantitative assessment of myocardial blood flow with operator-independent accuracy. 2

Advantages Over SPECT Imaging

• N-13 ammonia PET offers superior image quality compared to SPECT, particularly for detecting disease in the inferior and posterior walls where thallium-201 SPECT has greater interpretative errors due to photon attenuation. 1
• Two comparative studies involving 281 patients demonstrated significantly higher diagnostic accuracy of PET perfusion imaging with pharmacologic stress compared to thallium SPECT in the same patients. 1

Practical Limitations

Availability and Infrastructure

• The major limitation of N-13 ammonia is that it requires an on-site or nearby cyclotron for production, as it has a short half-life (approximately 10 minutes). 1
• This requirement significantly limits availability compared to generator-produced tracers like rubidium-82 or technetium-99m agents. 1
• N-13 ammonia remains investigational by the FDA, though it is used clinically at centers with appropriate infrastructure. 1

Alternative PET Perfusion Tracers

• Rubidium-82 is the only FDA-approved PET perfusion tracer and is obtained from a generator, obviating the need for a cyclotron, though the generator cost is high. 1
• O-15 water is another PET perfusion tracer option but also requires cyclotron production. 2

Patient Preparation and Imaging Protocol

• Patients should fast for 6 hours before the scan if combined with FDG imaging for viability assessment. 2
• Blood glucose should be controlled, as hyperglycemia can reduce FDG uptake in viable myocardium and lead to false-negative viability results. 2
• The patient must lie still for 5-20 minutes during image acquisition. 2

Common Pitfalls to Avoid

• Do not delay viability assessment waiting for heart rate control when PET is the chosen modality—heart rate does not affect PET perfusion or metabolic imaging quality. 2
• Ensure adequate patient preparation with fasting and glucose control when viability assessment with FDG is planned. 2
• Do not assume N-13 ammonia is readily available—verify cyclotron access before ordering, as rubidium-82 or technetium-99m agents may be more practical alternatives. 1