Cardiac MRI and Coronary CT Angiography
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Direct answer
Cardiac MRI and coronary CT angiography are non-invasive tests that answer different questions. Cardiac MRI characterises tissue through oedema, late enhancement and T2* iron measurement, making it the test of choice for myocarditis, infiltrative disease and cardiac iron overload. Coronary CT angiography rules out obstructive coronary disease in stable chest pain, since a normal study has a high negative predictive value.
What you must remember
- Late gadolinium enhancement: a subendocardial pattern that follows a coronary territory indicates infarct scar, whereas patchy mid-wall or epicardial enhancement suggests non-ischaemic disease.
- Lake Louise criteria: (the 2018 update adds T1 and T2 mapping) T2 signal for oedema, early gadolinium enhancement for hyperaemia and late enhancement for scar are combined to support a diagnosis of myocarditis.
- Cardiac iron: T2 MRI measures myocardial iron, and a T2 below 20 milliseconds indicates cardiac iron overload, with values below about 10 milliseconds carrying high risk of heart failure.
- Amyloidosis: diffuse subendocardial enhancement with poor myocardial nulling suggests cardiac amyloidosis.
- Coronary CTA: a normal study has a high negative predictive value for obstructive coronary disease, which makes it useful for low-to-intermediate probability chest pain.
- Calcium score: the Agatston score quantifies coronary calcium; a score of zero makes obstructive disease unlikely in most people, but it cannot exclude soft non-calcified plaque.
- Device safety: older non-MR-conditional pacemakers and defibrillators remain a relative contraindication to cardiac MRI, while MR-conditional systems allow scanning under protocol.
Choosing the test by the clinical question
Each cardiac imaging test answers one clinical question best, so an examiner usually describes the question rather than naming the test. Matching the question to the test is the core skill. The table pairs common questions with the test that answers them.
| Clinical question | Preferred test | Key finding |
|---|---|---|
| Acute myocarditis | Cardiac MRI | Oedema on T2 with late enhancement |
| Ischaemic versus non-ischaemic scar | Cardiac MRI | Subendocardial versus mid-wall enhancement |
| Cardiac iron in thalassaemia | Cardiac MRI | T2* below 20 ms |
| Infiltrative disease | Cardiac MRI | Diffuse subendocardial enhancement |
| Stable chest pain, low-intermediate risk | Coronary CT angiography | Normal study excludes obstruction |
| Plaque burden in asymptomatic risk | Calcium score | Agatston score |
Cardiac MRI is a tissue test, so its findings describe the cause of a cardiomyopathy or myocardial injury. Coronary CT angiography is an anatomy test, so its findings describe the vessel lumen and plaque rather than how much flow is limited.
Coronary CT needs iodinated contrast and a slow, steady heart rate to reduce motion artefact. Beta blockers are often given beforehand for that reason. Results are always read alongside the ECG, troponin and the clinical picture.
Where the imaging question gets tricked
Most errors come from answering a question the test was never designed to answer.
- Trap: using the calcium score to exclude obstruction in a patient with typical symptoms Correct: the score measures plaque, not stenosis, so coronary CT angiography or functional testing is needed.
- Trap: mistaking mid-wall enhancement for an ischaemic infarct Correct: ischaemic scar is subendocardial and follows a coronary territory, while patchy mid-wall enhancement suggests myocarditis, sarcoidosis or a non-ischaemic cardiomyopathy.
- Trap: avoiding cardiac MRI in every patient with a device Correct: MR-conditional devices and protocols allow many patients to be scanned, although older non-conditional systems remain a relative contraindication.
Frequently asked questions
Which cardiac MRI findings suggest acute myocarditis?
Acute myocarditis is suggested by T2-weighted oedema, early gadolinium enhancement and late enhancement, which together form the Lake Louise criteria. Patchy mid-wall or epicardial enhancement is the typical pattern, and it does not follow a single coronary territory.
What does late gadolinium enhancement distinguish?
Late gadolinium enhancement separates ischaemic from non-ischaemic myocardial injury by its pattern. Subendocardial enhancement in a coronary territory suggests infarct scar, while mid-wall or epicardial patchy enhancement suggests non-ischaemic disease such as myocarditis or sarcoidosis.
What does a coronary calcium score of zero mean?
A coronary calcium score of zero makes obstructive coronary disease unlikely in most people, which is why the score is used for risk stratification in asymptomatic patients. It does not exclude non-calcified soft plaque, so a zero score does not rule out all coronary disease.
Which MRI test measures myocardial iron in thalassaemia?
T2 cardiac MRI measures myocardial iron, with a T2 below 20 milliseconds indicating cardiac iron overload. Values below about 10 milliseconds carry a high risk of heart failure, so serial T2* imaging guides chelation therapy.