Is a computed tomography (CT) head scan necessary if a magnetic resonance imaging (MRI) head scan has already been performed?

From the Guidelines

If you have already had an MRI of the head, you typically do not need a CT scan of the head for the same condition, as MRI generally provides more detailed images of brain tissue and is superior for detecting most neurological conditions. However, there are specific situations where a CT might still be needed even after an MRI: CT scans are better for visualizing acute bleeding, bone fractures, and calcifications; they are faster and more suitable for emergency situations; and they may be necessary for patients who cannot undergo further MRI scans due to implanted metal devices.

Key Considerations

• The decision to perform a CT after an MRI should be made by your healthcare provider based on your specific clinical situation, the findings from the MRI, and what additional information is needed for diagnosis or treatment planning 1.
• MRI is more sensitive than CT for subtle findings adjacent to the calvarium or skull base, such as small cortical contusions and subdural hematomas 1.
• CT scans are highly sensitive for the detection of findings that may require neurosurgical intervention in the acute phase, such as acute bleeding or bone fractures 1.

Clinical Scenarios

• If the clinical focus has transitioned from short-term management to long-term prognostication in the acute phase, then an early MRI may be of greater value, particularly in patients who have mild TBI with normal CTs 1.
• In patients with mild TBI and positive CT, the appropriateness of routine repeat CT may depend on the size and type of intracranial findings, as well as the patient's clinical condition and risk factors 1.
• The risk of delayed traumatic ICH is low, even on anticoagulant or antiplatelet medication, and does not merit routine observation and repeat CT, according to some studies 1.

From the Research

CT Head Scans after Head Trauma

• CT scanning is the current first imaging technique to be used after head injury, in those settings where a CT scan is available 2.
• The value of CT is the demonstration of scalp, bone, extra-axial hematomas and parenchymal injury 2.
• If the CT does not demonstrate pathology adequate to account for the clinical state, MRI is warranted 2.

Indications for CT Scanning

• Features most predictive for intracranial injuries included: disorientation, abnormal alertness, somnolentia and neurological deficits 3.
• Patients with no loss of consciousness and in normal physical condition need only clinical assessment 3.
• Indications to head CT scanning are determined by decision rules, such as the Canadian Computed Tomography Head Rule 3.

Role of MRI in Head Trauma

• MRI is more sensitive for all posttraumatic lesions other than skull fracture and subarachnoid hemorrhage, and can demonstrate parenchymal spinal cord injury 2.
• MRI will be used increasingly to study early head injury because of its ability to measure cerebral blood flow, cerebral blood volume and the location and extent of cerebral edema 2.
• Follow up is best done with MRI as it is more sensitive to parenchymal change than is CT 2.

Comparison of CT and MRI

• CT and MRI are important in diagnosing spine injury, but given the differences between certain conditions, the utility of obtaining both studies in all patients warrants scrutiny 4.
• In patients with known vertebral body injury on CT, additional injury to the posterior elements on CT or MRI in certain patients was found to be lower than in others 4.
• MRI should be strongly considered for any patient with certain conditions regardless of neurologic status 4.