Does the patient need a CT or MRI?

Neuroimaging Selection for Altered Mental Status

For patients with altered mental status, a non-contrast head CT is the most appropriate initial imaging study, particularly in acute settings, while MRI is preferred for specific clinical scenarios requiring higher sensitivity for subtle abnormalities.

Clinical Decision Algorithm for Neuroimaging

When to Order CT Head Without Contrast:

• First-line imaging for acute altered mental status 1
• Suspected intracranial hemorrhage, trauma, or elevated intracranial pressure
• Patients with anticoagulant use or coagulopathy
• Hypertensive emergency
• Rapid assessment needed (unstable patients)
• Persistent altered mental status despite management of suspected medical cause 1

When to Order MRI Brain:

• When higher sensitivity is needed after negative CT but persistent unexplained symptoms 1, 2
• Known intracranial pathology requiring follow-up
• Suspected small cortical contusions, subtle white matter lesions, or diffuse axonal injury
• Suspected metabolic disorders (carbon monoxide poisoning, Wernicke encephalopathy) 1
• Subacute or chronic head trauma with persistent symptoms 1, 2
• New onset psychosis with atypical presentation or abnormal neurological findings 1

Evidence-Based Rationale

The American College of Radiology (ACR) Appropriateness Criteria strongly recommends non-contrast head CT as the initial imaging test for acute altered mental status 1. CT provides rapid assessment for acute hemorrhage, mass effect, hydrocephalus, and large infarcts with minimal motion artifact risk in unstable patients 1.

MRI offers superior sensitivity for detecting subtle abnormalities but requires longer acquisition times and may be impractical in acute settings 2, 3. The ACR notes that "MRI is complementary to CT in the evaluation of suspected progression of intracranial mass lesions, infection, and ischemia" 1.

Special Considerations

For New Onset Psychosis:

The diagnostic yield of neuroimaging in patients with new onset psychosis without neurological deficits is very low (0-1.5%) 1. However, the American Psychiatric Association suggests that brain MRI may provide helpful information in patients with unclear clinical pictures, atypical presentations, or abnormal neurological findings 1.

For Suspected Medical Causes:

In patients with altered mental status attributed to medical illness or toxic-metabolic causes, neuroimaging may not be required initially 1. However, if symptoms persist despite appropriate management, non-contrast head CT is recommended to evaluate for possible neurological causes 1.

Pitfalls to Avoid

1. Overreliance on CT alone: While CT is excellent for detecting acute hemorrhage, it has limited sensitivity for subtle abnormalities. Consider MRI when symptoms persist despite negative CT 3.

2. Unnecessary imaging: Neuroimaging is not always required for conditions with clear medical causes (e.g., intoxication, hypoglycemia) unless symptoms persist despite treatment 1.

3. Misinterpreting normal variants: Emergency physicians may have higher rates of both false positives and false negatives compared to radiologists when interpreting head CTs 4. Ensure radiologist review of all studies.

4. Inadequate clinical correlation: The diagnostic yield of neuroimaging increases when guided by specific clinical findings rather than performed indiscriminately 5.

By following this evidence-based approach to neuroimaging selection, clinicians can optimize diagnostic accuracy while minimizing unnecessary radiation exposure and healthcare costs.