Myocardial Perfusion Imaging (SPECT)

Myocardial perfusion imaging with single photon emission computed tomography (SPECT) shows the distribution of blood flow in the left ventricular muscle at stress and at rest.

Normal myocardium takes up more tracer when flow rises with stress, while muscle supplied by a narrowed artery takes up less. Comparing the stress and rest images separates ischaemic muscle from scar.

Technetium-99m sestamibi or tetrofosmin is used in most centres. A one day technique gives 250 to 400 MBq for the first injection and a dose about three times larger for the second.

Thallium-201 (74 to 111 MBq) is a potassium analogue that redistributes over hours, and is used for viability and in some centres for stress imaging. Gated acquisition links the images to the electrocardiogram and gives left ventricular ejection fraction, volumes and wall motion.

Stress is by exercise on a treadmill or bicycle. In patients who cannot exercise adequately, a vasodilator (adenosine, dipyridamole or regadenoson) or dobutamine is used.

The tracer is injected at peak stress, and imaging follows 30 to 60 minutes later because technetium tracers stay fixed in the muscle. Rest images follow a second injection.

The one day study takes 3 to 4 hours. The effective dose is about 8 to 12 mSv for technetium tracers and 15 to 20 mSv for thallium.

Coronary artery disease is the leading cause of death worldwide. Perfusion imaging has a sensitivity of about 85 to 90% and a specificity of 70 to 85% for obstructive coronary disease, with the lower specificity due mainly to attenuation.

A normal study carries a yearly risk of cardiac death or infarction under 1%, and the extent and severity of defects predict outcome and the benefit of revascularisation.

Indications.

• Suspected coronary disease with chest pain or an equivalent symptom and an intermediate likelihood of disease.
• Known coronary disease with new or changed symptoms.
• Functional importance of a stenosis of intermediate severity seen on angiography or CT angiography.
• Risk assessment before non-cardiac surgery in patients with poor exercise capacity and several risk factors.
• After myocardial infarction: residual ischaemia and risk stratification.
• Viability in poor left ventricular function before revascularisation (rest redistribution thallium, or technetium with a nitrate).
• Recurrent symptoms after stenting or bypass surgery.
• Left bundle branch block or a paced rhythm (vasodilator stress is used).
• Heart failure of unknown cause, to look for ischaemic disease.
• Women and diabetic patients with atypical symptoms.
• Coronary artery involvement after Kawasaki disease, and transplant vasculopathy.

Not for screening of people without symptoms and at low risk, and not during ongoing unstable chest pain.

Contraindications and precautions.

• Exercise stress: acute coronary syndrome within 48 hours, uncontrolled arrhythmia, severe symptomatic aortic stenosis, decompensated heart failure, acute pulmonary embolism, acute myocarditis or pericarditis, aortic dissection and severe uncontrolled hypertension.
• Vasodilator stress: active wheeze or severe airway disease, second or third degree heart block or sick sinus syndrome without a pacemaker, systolic blood pressure under 90 mmHg, and unstable angina. Caffeine, theophylline and dipyridamole block the effect of adenosine.
• Dobutamine: severe arrhythmia, severe hypertension, aortic aneurysm and recent infarction.
• Pregnancy: avoid unless essential. Breast feeding: interrupt for about 12 hours after technetium tracers.
• Caffeine (coffee, tea, cola, chocolate) is stopped for 12 to 24 hours and theophylline for 48 hours before vasodilator stress.
• Beta blockers, nitrates and calcium channel blockers are stopped for 24 to 48 hours when the aim is to diagnose coronary disease and it is safe to do so. They are continued when the aim is to assess ischaemia on treatment.

Preparation and technique.

The patient fasts for 3 to 4 hours (a light fatty meal after the injection speeds clearance of liver and bowel activity). Intravenous access, blood pressure, a 12-lead electrocardiogram and symptoms are monitored. Resuscitation equipment and aminophylline are available.

Exercise continues to at least 85% of the predicted maximum heart rate, and the tracer is injected at peak, with exercise continued for 1 to 2 minutes.

Adenosine is given at a constant rate for 4 to 6 minutes, with the tracer injected at the mid point, and low level exercise reduces side effects. Regadenoson is given as a single bolus.

Images are taken with a gamma camera over a 180 degree arc from the right anterior oblique to the left posterior oblique, or over 360 degrees. Rest and stress sets are processed into short axis, vertical long axis and horizontal long axis slices.

CT attenuation correction, prone imaging or an upright camera are used in patients prone to attenuation artefact. Stress first imaging, with rest images only if the stress images are abnormal, cuts the dose.

Normal appearances.

Uptake is uniform over the left ventricle, with a normal decrease at the apex and the basal septum (the membranous part). The right ventricle is faint. The stress and rest images match.

Gated ejection fraction at rest is over 50%, and wall motion and thickening are normal. The left ventricular cavity is the same size at stress and at rest.

Interpretation.

• Reversible defect: reduced uptake at stress that fills in at rest, indicating ischaemia. The territory points to the artery (anterior wall, septum and apex for the left anterior descending, inferior wall for the right coronary, and lateral wall for the circumflex). The size is stated as a fraction of the left ventricle, and the severity as mild, moderate or severe.
• Fixed defect: reduced uptake on both sets, indicating scar. A partly reversible defect means scar with ischaemia in the border zone. Hibernating muscle can look fixed with technetium tracers, so viability imaging is used when revascularisation is considered.
• Transient ischaemic dilation: the cavity looks larger at stress than at rest (ratio over about 1.2). It points to severe multivessel disease or extensive subendocardial ischaemia.
• Fall in ejection fraction after stress (over 5 points) or new wall motion abnormality at stress: severe ischaemia with stunning.
• Increased lung uptake with thallium, or increased right ventricular uptake: severe disease and raised filling pressure.
• Balanced ischaemia: three vessel disease may produce uniformly reduced uptake with no clear defect. Clues are cavity dilation, a fall in ejection fraction, ischaemic electrocardiogram changes, symptoms and a poor blood pressure response.
• Left bundle branch block: a septal defect at stress with a normal artery can occur with exercise. Vasodilator stress is the usual method.
• Extent of ischaemia over about 10% of the left ventricle marks a high risk group who benefit from revascularisation. Normal perfusion with a low ejection fraction suggests non-ischaemic cardiomyopathy.
• Non-perfusion findings add information: exercise duration, heart rate recovery, blood pressure response, electrocardiogram changes and symptoms.
• Viability: tracer uptake of over 50 to 60% of the peak in a dysfunctional segment on rest or redistribution images predicts recovery after revascularisation.
• Other patterns: asymmetric uptake in hypertrophic cardiomyopathy, a dilated cavity with patchy uptake in dilated cardiomyopathy, and defects in infiltrative disease.
• Coronary calcification and extracardiac findings on the attenuation CT, and incidental lung or mediastinal lesions, are reported.

Critical values and findings needing immediate action.

• Large reversible defects in several territories, transient dilation, or a low ejection fraction with ischaemia: urgent cardiology review and angiography.
• Prolonged chest pain, ST elevation, sustained arrhythmia, severe hypotension or heart block during stress: stop the test, give oxygen, sublingual nitrate and aminophylline (for vasodilator reactions), and act on it as an acute coronary syndrome if it persists.
• Bronchospasm during vasodilator stress: stop the infusion and give a bronchodilator and aminophylline.
• Anaphylaxis (rare): adrenaline and usual resuscitation measures.

Limitations and artefacts.

• Attenuation by the breast (anterior and lateral wall defects) and the diaphragm (inferior wall defect) mimics a fixed defect. Gated images, prone images and CT correction help.
• Patient motion, a shift of the heart during the scan, and misregistration between the CT and emission images cause false defects.
• High liver or bowel activity next to the inferior wall raises or lowers the counts. Delayed images and water drinking help.
• Submaximal exercise (under 85% of the predicted maximum heart rate) lowers sensitivity.
• Balanced ischaemia and diffuse small vessel disease give false negatives, and microvascular disease gives symptoms with normal perfusion.
• Left bundle branch block, pacing and ventricular hypertrophy give false septal defects.
• Obesity gives poor count statistics and attenuation.
• Gated function is less accurate in arrhythmia and with small ventricles.
• Radiation dose accumulates with repeat studies.
• Perfusion imaging does not show the coronary anatomy, or the plaque burden shown by CT angiography.

Special groups.

• Women: small hearts and breast attenuation lower accuracy. Stress first imaging, prone imaging and PET are options.
• Diabetic patients: silent ischaemia, microvascular disease and a high event rate. A normal study gives less lasting reassurance than in non-diabetic patients.
• Obese patients: PET perfusion or stress echocardiography may work better.
• Older patients: usually need pharmacological stress.
• Kidney failure: high rates of coronary disease and cardiac events, and a vasodilator is needed.
• Heart transplant: vasculopathy is diffuse, and ischaemia may be balanced.
• Children: very rare use, and dose is adjusted by weight.
• Pregnancy: avoid.
• Regions: coronary disease is rising, but rheumatic heart disease, Chagas disease and peripartum cardiomyopathy are also common causes of cardiac symptoms. Perfusion imaging is available in few centres, and exercise electrocardiography, stress echocardiography and clinical risk assessment are used.

Follow up tests.

• Stress echocardiography.
• Coronary CT angiography and calcium score.
• Invasive coronary angiography with pressure wire assessment of a stenosis.
• Stress cardiac MRI perfusion and viability imaging.
• PET myocardial perfusion and viability imaging.
• Resting electrocardiogram and echocardiography.
• Fasting lipids, glucose and HbA1c, renal function and troponin.
• Referral to cardiology, cardiac surgery or heart failure services.