When is a cardiac MRI (Cardiac Magnetic Resonance Imaging) indicated after a cardiac arrest?

Cardiac MRI After Cardiac Arrest

Cardiac MRI is indicated for neuroprognostication 2-6 days after cardiac arrest in comatose patients, where extensive diffusion restriction on brain MRI can help predict poor neurological outcomes when combined with other established predictors. 1

Primary Indication: Neuroprognostication

The American Heart Association guidelines establish that brain MRI (not cardiac MRI) is the primary imaging modality recommended after cardiac arrest for prognostic purposes. 1

Timing and Technique

• Optimal timing: 2-6 days post-arrest for diffusion-weighted imaging (DWI) sequences 1
• DWI changes become most apparent after 48 hours, with most studies examining patients 3-7 days post-arrest 1
• Earlier imaging may miss evolving injury patterns 1

Specific MRI Findings That Predict Poor Outcome

In patients treated with targeted temperature management (TTM):

• Presence of >10% brain volume with apparent diffusion coefficient (ADC) <650 × 10⁻⁶ mm²/s predicts poor outcome with 0% false positive rate 1
• Low ADC at putamen, thalamus, or occipital cortex levels (0% false positive rate, though thresholds vary by region) 1

In patients NOT treated with TTM:

• Diffuse DWI abnormalities in cortex or brainstem at median 80 hours post-arrest (0% false positive rate) 1
• Extensive DWI changes involving cortex, basal ganglia, and cerebellum (0% false positive rate) 1
• Whole-brain ADC <665 × 10⁻⁶ mm²/s (0% false positive rate) 1

• 10% brain volume with ADC <650 × 10⁻⁶ mm²/s (0% false positive rate) 1

Critical Guideline Recommendation

It may be reasonable to consider extensive restriction of diffusion on brain MRI at 2-6 days after cardiac arrest in combination with other established predictors to predict poor neurologic outcome (Class IIb, LOE B-NR). 1

Multimodal Prognostication Approach

Brain MRI should NEVER be used in isolation for prognostication—the AHA emphasizes using multiple modalities together: 1

• Clinical examination (motor response, pupillary reflexes)
• Neurophysiological testing (EEG reactivity, somatosensory evoked potentials)
• Brain imaging (MRI with DWI sequences)
• Biomarkers (though NSE and S-100B should not be used alone) 1

Pediatric Considerations

In children, brain MRI using conventional imaging and DWI in the first 3-7 days after ROSC may supplement clinical assessment including serial neurological examinations, EEG, and SSEPs. 1

• Abnormalities in basal ganglia on conventional imaging and brain lobes on DWI within first 2 weeks associated with unfavorable outcome 1
• DWI lesions in cerebral cortex and basal ganglia within first week associated with unfavorable outcome 1
• Some children with regional lesions still had favorable outcomes, limiting specificity 1

Cardiac MRI for Structural Heart Disease

Cardiac MRI (as opposed to brain MRI) has a distinct role in identifying the underlying cardiac pathology in sudden cardiac arrest survivors without coronary artery disease. 2, 3

Diagnostic Utility

• Cardiac MRI contributed to diagnosis in 49% of sudden cardiac arrest survivors without coronary disease and was decisive in 30% 3
• Most frequent findings: dilated cardiomyopathy, myocarditis/sarcoidosis, occult myocardial infarction, hypertrophic cardiomyopathy 3
• Can identify pathologic or arrhythmogenic substrates and detect potentially reversible myocardial edema 2

Prognostic Value

• Presence of late gadolinium enhancement and extent correlate with major adverse cardiac events 3
• Right ventricular ejection fraction is an independent predictor of recurrent arrhythmia or death 3
• Risk of major adverse cardiac events doubled in those with a cardiac MRI diagnosis 3

Important Caveats and Pitfalls

Logistical considerations:

• MRI frequently requires sedation, introducing procedural risks 1
• Intra-hospital transport carries inherent risks in unstable patients 1
• Selection bias exists—only clinically stable patients typically undergo MRI 1
• Requires expertise in acquisition and interpretation; subject to interobserver variability 1

Timing is critical:

• Too early (<48 hours): May miss evolving DWI changes 1
• Too late (>7 days): Some DWI abnormalities may resolve 1
• Optimal window: 2-6 days for neuroprognostication 1

Never use MRI findings alone:

• The decision to perform coronary angiography should NOT include consideration of neurologic status due to unreliability of early prognostic signs 1
• Prognostication requires integration of clinical exam, EEG, SSEPs, and imaging over time 1

What Takes Priority: Brain vs. Cardiac Imaging

In the immediate post-arrest period, focus on identifying treatable causes:

• Emergency coronary angiography takes precedence for ST-elevation or suspected acute coronary syndrome 1
• Brain CT early (<24 hours) can identify treatable intracranial causes (hemorrhage, mass, hydrocephalus) but insufficient for neuroprognostication 1
• Brain MRI for neuroprognostication should wait until 2-6 days 1
• Cardiac MRI for structural assessment can be performed once patient stabilized to guide long-term management and ICD decisions 2, 3