What is the role of SPECT (Single Photon Emission Computed Tomography) for diagnosing Coronary Artery Disease (CAD)?

SPECT for Diagnosis of Coronary Artery Disease

SPECT myocardial perfusion imaging is recommended for diagnosing coronary artery disease in patients with moderate to high pre-test probability (15-85%) of obstructive CAD, where it effectively identifies myocardial ischemia, quantifies disease extent, and provides robust prognostic information with a normal scan predicting very low cardiac event rates (1% per year). 1

Patient Selection Algorithm

Use SPECT when:

• Pre-test probability of obstructive CAD is 15-85% 1, 2
• Patient cannot exercise (use pharmacologic stress with adenosine or dipyridamole) 1
• Baseline ECG is abnormal (left bundle branch block, paced rhythm, Wolff-Parkinson-White syndrome, left ventricular hypertrophy, or digitalis use) where exercise ECG is non-interpretable 1
• Previous non-invasive test was non-diagnostic 2

Do NOT use SPECT as first-line when:

• Pre-test probability is low to moderate (5-50%)—use coronary CT angiography instead 2
• Pre-test probability is very high (>85%) with severe refractory symptoms—proceed directly to invasive coronary angiography 2

Diagnostic Performance

SPECT demonstrates:

• Sensitivity: 87-89% for detecting angiographically significant CAD 1
• Specificity: 73-75% for obstructive disease 1
• Negative predictive value: 83% in meta-analysis of 114 studies 1
• Excellent prognostic value: normal scan = 1% annual rate of cardiac death or myocardial infarction 1

The test identifies reversible perfusion defects (ischemia) versus non-reversible defects (infarction), distinguishes single-vessel from multivessel disease, and quantifies extent and severity of coronary stenosis 1, 3

Technical Advantages of Modern SPECT

Newer-generation cadmium-zinc-telluride (CZT) cameras provide: 1

• Substantially reduced radiation dose exposure
• Shorter acquisition time
• Increased diagnostic accuracy
• Absolute quantification of myocardial blood flow
• Improved detection of multivessel CAD 1

Technetium-99m labeled agents (sestamibi, tetrofosmin) are preferred over thallium-201 due to improved image resolution, higher count density, and more favorable radiation dosimetry 2, 3

Critical Enhancement: Coronary Artery Calcium Scoring

When performing SPECT, measure coronary artery calcium score (CACS) from the non-contrast CT used for attenuation correction—this is a Class I recommendation. 1, 2 This combination improves detection of both non-obstructive and obstructive CAD, providing important risk stratification even when perfusion is normal 1

Hybrid SPECT/CCTA: Superior Diagnostic Accuracy

Hybrid SPECT/coronary CT angiography substantially outperforms either test alone: 1, 2, 4

• Sensitivity: 96% (vs SPECT alone 93%, CCTA alone 98%)
• Specificity: 95% (vs SPECT alone 79%, CCTA alone 62%)
• 92% agreement on revascularization decisions compared to SPECT plus invasive angiography 1

The hybrid approach combines anatomical coronary stenosis information with functional perfusion evidence, yielding net reclassification improvement of 1.57 compared to pre-test probability plus exercise ECG 4

Common Pitfalls to Avoid

False-positive results occur more frequently with: 1

• Abnormal resting ECG
• Left ventricular hypertrophy
• Electrolyte imbalance
• Atrial fibrillation
• Women (lower sensitivity and specificity) 1

Absolute contraindications include: 2

• High-risk unstable angina
• Complicated acute coronary syndrome or acute MI within 2 days
• Significant arrhythmias
• Significant hypotension (systolic BP <90 mmHg)

Comparison with Alternative Modalities

PET is preferred over SPECT when both are available due to superior diagnostic accuracy, lower radiation exposure, and ability to quantify absolute myocardial blood flow 1, 2. However, SPECT remains widely used due to broader availability, versatility with exercise or pharmacologic stress, lack of absolute contraindications, and cost-effectiveness 1, 5

Stress echocardiography and cardiac MRI are reasonable alternatives with Class I recommendations for the same patient population (moderate-high pre-test probability), chosen based on local expertise, availability, and patient-specific factors 1, 2

Clinical Decision-Making Based on Results

Inducible ischemia on SPECT identifies patients with short-term benefit from revascularization, while left ventricular ejection fraction predicts cardiac death 1. The extent and severity of perfusion abnormalities guide decisions between medical therapy versus invasive angiography with consideration for revascularization 1, 3

Important limitation: SPECT detects flow-limiting stenoses but misses non-obstructive coronary atherosclerosis not causing ischemia 1. This is why adding CACS from attenuation correction CT is valuable for comprehensive risk assessment 1, 2

1, 2, 3, 5, 4