Neuroimaging Guidelines

Headache

With the development of more sophisticated neuroimaging technology, practitioners must become familiar with the appropriate selection of diagnostic imaging procedures for patients presenting with headache.

Prior to the development of CT scan and MRI, electroencephalogram (EEG) was the modality used to evaluate patients with headache (1). With the advent of superior imaging procedures, this technique is no longer recommended. American Academy of Neurology guidelines now state that (2):

The electroencephalogram (EEG) is not useful in the routine evaluation of patients with headache. This does not exclude the use of EEG to evaluate headache patients with associated symptoms, suggesting a seizure disorder such as atypical migrainous aura or episodic loss of consciousness. Assuming head imaging capabilities are readily available, EEG is not recommended to exclude a structural cause of headache.

CT Scan

CT scans pass x-ray beams through the head allowing a computer to create a cross-sectional representation. Contrast highlights blood vessels, the pituitary gland, choroid plexus and dura. When the blood-brain barrier (BBB) is compromised, contrast will also highlight lesions causing defects in the BBB, such as infectious lesions, tumors and infarctions.

The CT scan is superior to the MRI in the evaluation of acute parenchymal, subdural and subarachnoid bleeds. For subacute and chronic bleeds, however, the MRI is the study of choice.Over time, the probability of recognizing an aneurysmal hemorrhage on CT scan declines (1) (Table I):

Table I

Non-contrast CT scans also play an important role in the evaluation of patients with acute head trauma by delineating the bony calvarium, including the osseous skull base and temporal bones.

Disadvantages of using a CT scan include exposure to radiation and potential allergic reactions from contrast, such as anaphylaxis.

MRI

Magnetic resonance imaging is an excellent imaging modality to evaluate most intracranial processes. The MRI measures the energy produced when protons in biologic tissue energized by a magnetic field move from the energized to the relaxed state.

Table II

MRI is superior to CT scan in the evaluation of most intracranial processes that may be associated with headache (Table II). MRI with gadolinium is useful when looking for primary and metastatic brain tumors, brain abscess and other infectious lesions and demyelinating diseases and cranial nerve and meningeal abnormalities, because gadolinium-based contrast will highlight areas where the BBB is compromised.

MRI is also valuable in the evaluation of vascular lesions of the brain but ideally in conjunction with an MRA or cerebral angiogram.

One disadvantage of MRI is the requirement that the patient not move at all during the procedure. Also, patients who are morbidly obese, claustrophobic or contain metal clips or fragments may not be candidates for MRI.

MRA

Magnetic resonance angiography picks up fast-moving blood and, by suppressing background signals, permits visualization of a vascular flow map. It has less resolution than conventional angiography, however, and may not pick up small lesions such as aneurysms < 3 mm in size. It also cannot detect slow-moving blood well and may miss occlusive or near-occlusive cerebrovascular disease.

Nevertheless, combined with MRI, MRA has many indications (Table III).

Table III

Cerebral Angiogram

Cerebral angiography involves threading a catheter, usually through the femoral artery, and then injecting contrast material into a blood vessel.

It may be useful in imaging small vessel disease such as cerebral vasculitis, small aneurysms, AVMs and cerebrovascular occlusive disease. It is generally the modality of choice when therapy is planned and is the modality through which intravascular therapeutic procedures are performed.

Cerebral angiography is generally considered the gold standard for evaluating cerebral aneurysms following a subarachnoid hemorrhage.

The major risk of cerebral angiography is the 4% chance of stroke and the potential neurotoxic effects of the hyperosmolar contrast material (3).

Positron Emission Tomography and Single Photon Emission Tomography

PET and SPECT scans measure brain perfusion or metabolism by detecting the uptake of biologically active radioligand tracers as they participate in biological processes (4).

These studies differ in terms of equipment and sensitivity. Although PET scans are often more sensitive than SPECT studies, they require more expensive technology and close proximity to the radionucleotide source (due to the short half-life of the radioisotopes).

The indications for the use of PET or SPECT in the study of brain function continue to expand. Several of the widely accepted indications are listed below (Table IV).

Table IV

Conclusion

In conclusion, chest radiography is indicated only when there is a clinical suspicion of chest disease based on findings on a prior medical history and physical examination.

We would like to acknowledge the editorial assistance of:

Richard Toran, M.D. 
Chief of Neurology
Newton-Wellesley Hospital
Assistant Professor of Neurology
Tufts School of Medicine

References

1. Evans RW. Diagnostic testing for the evaluation of headaches. Neurologic Clinics 1996; 14(1):1-26.
   
2. American Academy of Neurology: Practice parameter: The electroencephalogram in the evaluation of headache. Neurology 1995; 45:1411-1413.
   
3. Dillon WP: Chapter 362. Neuroimaging in neurologic disorders. In Fauci S, Braunwald E, Isselbacher KJ et al (eds): Harrisons Principles of Internal Medicine. New York, McGraw-Hill, 1998, p 2293.
   
4. Gilman, S. Imaging the brain: first of two parts. NEJM 1998; 338(12):812-820.

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