Cardiac MRI
Cardiac MRI is a full, non-invasive study of cardiac structure, function, tissue characteristics, perfusion and flow. It is the reference test for ventricular volumes, mass and ejection fraction, and the only imaging test that characterises myocardial tissue in detail.
The study uses a 1.5 or 3 tesla scanner with a cardiac coil, electrocardiogram gating, and breath hold sequences of 8 to 15 seconds. The examination takes 30 to 60 minutes. Patients must lie still, follow breath hold commands and have a regular rhythm for the best images.
The core sequences are cine steady state free precession images in long axis and short axis planes (for volumes, function and wall motion), T2-weighted or mapping sequences (for oedema), T1 and extracellular volume mapping (for fibrosis and infiltration), first pass perfusion imaging with stress and rest (adenosine, regadenoson or dobutamine), late gadolinium enhancement (for scar and fibrosis, 10 to 15 minutes after gadolinium 0.1 to 0.2 mmol each kg),
and phase contrast flow imaging (for shunts and valves).
Cardiovascular disease is the leading cause of death worldwide.
Cardiac MRI changes diagnosis in 20 to 30% of patients referred with unexplained heart failure or suspected myocarditis, and it is accurate for ischaemia (sensitivity about 90% and specificity about 80 to 90% for stress perfusion) and for infarct size and viability.
It has no radiation.
Indications.
- Cardiomyopathy: dilated, hypertrophic, restrictive, arrhythmogenic, non-compaction, and cardiomyopathy of unknown cause.
- Myocarditis and pericarditis; acute chest pain with normal coronary arteries.
- Ischaemic heart disease: stress perfusion for ischaemia, viability (late gadolinium enhancement) before revascularisation, and infarct complications.
- Infiltrative disease: amyloidosis, sarcoidosis, iron overload, Fabry disease, and haemochromatosis.
- Cardiac masses and thrombus (tumour, thrombus, vegetation).
- Congenital heart disease: anatomy, shunts, function and surgical repair.
- Valve disease: quantification of regurgitation and stenosis when echocardiography is inadequate.
- Aortic disease and pulmonary veins before ablation.
- Pericardial disease: constriction, effusion and thickening.
- Cardiotoxicity from chemotherapy and monitoring.
- Risk stratification before implantable defibrillator placement in selected patients.
- Athlete's heart versus cardiomyopathy.
Not for unstable patients needing urgent treatment, or when echocardiography answers the question.
Contraindications and precautions.
- The usual MRI safety screen. Many modern pacemakers and defibrillators are conditional, and need device checks and monitoring. Older devices, abandoned leads and non-conditional devices are contraindications.
- Gadolinium: renal precautions apply. Macrocyclic agents are used in reduced renal function.
- Stress agents: adenosine and regadenoson are avoided in severe asthma, high grade atrioventricular block, severe aortic stenosis, hypotension and recent infarction. Dobutamine is avoided in severe arrhythmia and obstruction.
- Caffeine and theophylline are stopped for 12 to 24 hours before adenosine or regadenoson stress.
- Arrhythmia (atrial fibrillation, frequent ectopy) degrades images. Rate control may be needed.
- Claustrophobia, inability to breath hold, and large body size.
- Pregnancy: non-contrast MRI is acceptable if essential.
Preparation and technique.
The patient fasts for 4 hours for stress studies and avoids caffeine for 12 to 24 hours. Electrocardiogram electrodes are placed, and a cannula is inserted (two for stress studies). Blood pressure and heart rate are monitored.
Localisers are followed by cine imaging in the usual long axis and short axis planes covering the ventricles. Mapping and oedema imaging follow. For stress perfusion, the stress agent is given and first pass gadolinium imaging is taken during peak stress, then a rest perfusion is acquired after about 10 minutes.
Late gadolinium enhancement images are taken 10 to 15 minutes after the final dose, with the inversion time set to null normal myocardium. Phase contrast imaging of the aortic and pulmonary valves quantifies flow.
