01Purpose and principlesWhat the assessment is for and the core concepts behind it.
CMR is particularly valuable when echo images are inadequate or when myocarditis, infarction pattern, infiltrative disease, cardiomyopathy, ventricular thrombus or viability is the question. Breath-hold, rhythm and patient tolerance affect quality.
Nuclear imaging trades radiation exposure for sensitive perfusion and molecular information. SPECT is widely available; PET offers higher resolution and quantitative flow but is less available.
Select the modality by the unanswered decision, contraindications and local expertise. A technically limited study should be labelled as such and redirected rather than over-interpreted.
Key points
- CMR provides accurate biventricular volumes, function and tissue characterisation without ionising radiation.
- Late gadolinium enhancement pattern helps distinguish ischaemic scar from many non-ischaemic cardiomyopathies but is not a diagnosis in isolation.
- Stress CMR, SPECT and PET can demonstrate inducible perfusion abnormality; PET can quantify myocardial blood flow in specialist practice.
- SPECT compares relative tracer uptake, so attenuation artefact and balanced multivessel ischaemia can conceal disease.
- Check the exact implanted device and leads for MR conditions; 'pacemaker' is not automatically an absolute contraindication.
- Use the lowest-risk suitable gadolinium agent and screen renal function and pregnancy according to the product and MHRA safety guidance.
- Justify ionising-radiation studies, optimise dose and avoid duplicate functional tests that will not change management.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Subendocardial or transmural late enhancement following a coronary territory supports infarction; transmural extent affects likelihood of functional recovery.
Mid-wall, subepicardial or patchy enhancement may suggest myocarditis or cardiomyopathy, but distribution, oedema and clinical context are required.
Stress abnormality that normalises at rest supports inducible ischaemia; a fixed defect may be scar or artefact.
Large inducible burden, marked biventricular dysfunction or a mechanical complication requires prompt specialist action.
New airway, breathing or circulatory compromise during injection requires immediate emergency management and documentation.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Cine CMRFirst step - Why
- Quantify chamber volumes, mass and biventricular ejection fraction.
- Interpretation and limitations
- Highly reproducible when contours and rhythm are adequate; compare serial studies using consistent methods.
- 02
Tissue-characterisation CMR - Why
- Assess oedema, fibrosis, infiltration, thrombus and infarct pattern using mapping and late enhancement.
- Interpretation and limitations
- Pattern narrows mechanism but must be integrated with timing, biomarkers, coronary anatomy and clinical phenotype.
- 03
Stress perfusion CMR - Why
- Detect inducible myocardial hypoperfusion without ionising radiation.
- Interpretation and limitations
- Check stress adequacy and distinguish dark-rim artefact from a reproducible coronary or subendocardial defect.
- 04
SPECT myocardial perfusion imaging - Why
- Assess relative perfusion, scar and gated ventricular function.
- Interpretation and limitations
- Review raw data, attenuation and motion; normal relative perfusion can miss balanced global reduction.
- 05
PET perfusion or metabolic imaging - Why
- Quantify flow or assess selected inflammatory, infiltrative and viability questions.
- Interpretation and limitations
- Preparation is indication-specific; failed suppression or tracer protocol can make the study non-diagnostic.
04Clinical next stepsHow the result changes management or prompts escalation.
01Preferred routeSuspected cardiomyopathy or myocarditisFirst stepPreferredUnexplained ventricular dysfunction, arrhythmia or myocardial injury+
- 1Obtain ECG, biomarkers and comprehensive echocardiography and address acute instability.
- 2Use CMR when tissue characterisation or accurate biventricular assessment will change diagnosis or management.
- 3Review device compatibility, renal function, pregnancy possibility and tolerance before scanning.
- 4EscalationEscalate high-risk arrhythmia, severe dysfunction or active inflammation findings through specialist pathways.
02AlternativeFunctional ischaemia imagingAlternativeCTCA is uncertain or functional significance is required+
- 1Choose stress CMR, SPECT or PET from renal function, device, radiation, rhythm, body habitus and availability.
- 2Follow the modality-specific stress preparation and document adequate stress.
- 3Report ischaemic burden, scar, ventricular function and technical limitations.
- 4Correlate with coronary anatomy before invasive or revascularisation decisions.
03EscalationNon-diagnostic or unsafe studyEscalationDevice conditions cannot be met, renal or pregnancy risk is unacceptable, or images are inadequate+
- 1Stop and clarify the exact safety constraint rather than applying a blanket modality ban.
- 2AlternativeSelect a non-contrast CMR sequence set or alternative echo, CT, SPECT or PET when it answers the question safely.
- 3Use the relevant MHRA and product-specific contrast advice.
- 4Document residual uncertainty and the next test or clinical decision owner.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Verify device model, leads, MR conditions and pre/post-scan device checks where applicable.
- Assess renal function and pregnancy status when contrast or ionising radiation is planned.
- During pharmacological stress monitor rhythm, blood pressure, symptoms and oxygen saturation with resuscitation capability.
- Record administered activity, radiation dose metrics or gadolinium product and dose.
- Ensure important findings and any reaction are directly communicated and entered in the patient record.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Pattern beats presence
Late enhancement is informative because of its distribution, not merely because enhancement exists.
CMR can detect small infarcts
Subendocardial scar may be visible even when global ejection fraction is preserved.
PET flow reserve adds physiology
A global reduction can support diffuse coronary or microvascular dysfunction when focal relative defects are absent.
Fixed SPECT defect is not always scar
Attenuation, motion and subdiaphragmatic activity must be reviewed before calling infarction.
Device labels are conditional
Many systems are MR conditional only when the generator-lead combination, scanner and programming requirements are met.
07Common pitfallsFrequent interpretation and management errors.
- 01
Treating any non-ischaemic late-enhancement pattern as diagnostic of myocarditis.
- 02
Calling a normal relative SPECT study reassuring without considering balanced ischaemia or poor stress.
- 03
Refusing all CMR solely because a cardiac device is present.
- 04
Giving gadolinium without checking product-specific renal and pregnancy precautions.
- 05
Ordering both SPECT and stress CMR without a distinct unanswered decision.