Head CT or MRI?
Which Should I Order For My Patient?

by Alan Eisenberg, M.D.

The neuroradiologists of Mid-South Imaging and Therapeutics are frequently asked by referring clinicians whether we suggest a head CT or MRI for their patient. The purpose of this communication is to provide insight into the advantages of CT and MRI for specific patient complaints, and our recommendation for diagnostic imaging. It is important, however, to individualize the radiographic workup to the clinical situation. Therefore, please feel free to contact us regarding your specific imaging needs. It is important, however, to individualize the radiographic workup to the clinical situation.

A second commonly asked question
is whether the examination should be ordered to be performed with intravenous contrast administration. In our opinion there are few indications for contrast-enhanced head CT, due to the superior sensitivity of enhanced MRI in most clinical settings. An exception is a contrast enhanced temporal bone CT, which is a very useful exam in specific clinical situations. Patients who are unable to have an MRI, such as due to the presence of a cardiac pacemaker, clipped cerebral aneurysm or severe claustrophobia, will require CT examination.

Headache
Whether to order a CT or MRI depends largely on the degree of clinical suspicion that exists for finding an intracranial lesion. Non-enhanced CT is an excellent screening examination for patients in whom only moderate clinical suspicion exists. MRI is a more sensitive modality, and therefore recommended in cases in which clinical suspicionis high. A specific exception is the patient who presents with an acute, severe headache, in whom subarachnoid hemorrhage is a concern, as MRI is insensitive to blood in the subarachnoid space. Otherwise, the only disadvantage of MRI is its higher cost, and that a small number of patients will find the examination unpleasant due to claustrophobia (and may require "light sedation").

MRI is a more sensitive modality,and therefore recommended in cases in which clinical suspicion is high.

Cost
The exact cost differential will depend upon a number of factors, however, the global Medicare reimbursement of a head MRI is $154 greater than a CT. We generally advise that the MRI not be ordered with contrast unless clinical suspicion is extremely high. There are few significant lesions that will be missed on a non-enhanced study in a patient whose only complaint is headache. Contrast administration adds $371 to the cost of a head MRI and $90 to the cost of a head CT (in Medicare reimbursement). We routinely contrast patients who have certain systemic diseases (such as carcinoma) and patients whose non-enhanced exam suggests an abnormality that requires contrast administration for further delineation.

Dizziness
Like the patient with headache, the need to order the more sensitive MRI depends upon the degree of clinical suspicion. MRI is especially useful in patients in whom cerebellar or brain stem pathology (such as infarction) is suspected, as the posterior fossa is not well evaluated with CT due to beam hardening artifact. In most cases contrast administration is not required. However, some patients may have additional symptoms which necessitate a contrasted posterior fossa/internal auditory canal MRI examination. In particular, a patient with clinical evidence of a vestibular schwannoma (acoustic neuroma), such as neurosensory hearing loss, will benefit from such a study. MRI is especially useful in patients where cerebellar or brain stem pathology is suspected ...

TIA/CVA or Focal Neurologic Deficit
Most patients who present to the emergency room with an acute neurologic deficit initially receive a non-enhanced CT, partially to exclude intracranial hemorrhage prior to anticoagulation. Moderate to large cortical infarctions are well seen on non-enhanced CT beginning approximately 12-24 hours post ictus. Many supratentorial lacunar infarctions are also well visualized on CT. MRI is more sensitive for lacunar and cortical infarction and often preferred in patients whose initial CT scan is negative, and in patients who may have a brain stem or cerebellar infarction.

MRI is also preferred in young patients with neurologic deficits, as non-ischemic etiologies must also be considered. For example, MRI is much more sensitive to the presence of demyelination from multiple sclerosis than CT. Unfortunately, some elderly patients with CVAs are disoriented, and consequently unable to lie motionless for the MRI examination. A contrast enhanced study is not needed, unless the non-enhanced study suggests a possible alternative etiology (such as a tumor). Some elderly patients with CVAs are disoriented, and consequently unable to lie motionless for the MRI examination.

Visual Disturbance
Patients with visual field deficits are best evaluated by MRI. Small occipital lobe cortical infarctions and pituitary tumors are well evaluated with MRI. The orbit is properly evaluated with both CT and MRI. Patients with optic neuritis, however, require an MRI to evaluate for brain demyelination due to multiple sclerosis.

Tinnitus
CT and MRI are complimentary in this clinical setting. MRI is superior in screening for a posterior fossa vascular malformation or mass. However, high resolution CT is better in evaluation of the petrous bone for tumor and vascular anomaly, and is recommended in cases in which a middle ear mass is visualized otoscopically.

Hearing Loss
High resolution CT better evaluates the petrous bone and is preferred in cases of conductive hearing loss. MRI is advised in patients with neurosensory hearing loss. A non-enhanced MRI screening exam (referred to as a diagnostic ear screen) may be performed in patients with only moderate clinical suspicion for a vestibular schwannoma. A contrasted MRI IAC study is preferred in cases with high clinical suspicion for an IAC tumor, as well as those patients in whom the entire brain needs to be evaluated. The "complete" brain/IAC examination adds $371 (Medicare reimbursement) to the cost of the screening MRI exam.

Dementia
Non-enhanced CT is an excellent screening exam in elderly patients with dementia. CT will identify atrophy, hydrocephalus, most mass lesions and advanced small vessel disease. Although MRI is more sensitive, CT is generally adequate, and severely demented patients often do not tolerate MRI. CT will identify atrophy, hydrocephalus, most mass lesions and advanced small vessel disease.

Hormonal disturbance
Patients with hormonal imbalance require an MRI to evaluate for a pituitary tumor. CT is indicated only in patients who cannot have an MRI.

Head Trauma
The patient with acute head trauma who needs an imaging study should have a non-enhanced CT to identify those who require surgical intervention. Patients with unexplained neurologic symptoms who have had significant head trauma may benefit from an MRI. MRI is especially useful in patients who may have sustained diffuse axonal injury.

Seizure
We recommend a contrast enhanced MRI in young and middle-aged patients with new onset seizure. Patients with chronic epilepsy generally do not require contrast, and often have an MRI exam designed to evaluate for mesial temporal sclerosis. Non-enhanced CT is sufficient in many elderly patients with a known history of CVA, in whom clinical suspicion for non-ischemic pathology is low.

Known Brain Tumor Or Possible Brain Metastasis
MRI is more sensitive and specific for the presence of tumor, and recommended in patients in whom the exam is not contraindicated. A contrast enhanced MRI costs $435 (Medicare reimbursement) more than a contrast enhanced CT.

Summary of the recommended study for the more common clinical presentations. Please see text for details.