Perhaps one of the oldest imaging modalities of those that will be described in this chapter, CT remains an important diagnostic tool in the setting of neurologic emergencies. It is widely available, generally accessible in most medical centers at any hour (based on technician availability), and can be performed relatively quickly. Compared to magnetic resonance imaging (MRI), CT is easier to perform in those with claustrophobia or with confusional states.

Calcifications in the brain parenchyma, blood vessels, choroid plexus, pineal gland, and within cystic, neoplastic, or infectious lesions are well differentiated on CT. The advent of spiral CT technology also allows three-dimensional reformatted images of the spine. CT additionally provides useful information about ventricular size, the presence of communicating or obstructive hydrocephalus, large mass lesions, and integrity of hemispheric midline structures. Adjacent sinus structures are also visualized on head CT and may provide a clue to the source of a patient’s headache or facial pain. Additionally, sinus abnormalities may suggest a site of contiguous spread in the setting of central nervous system infections. Dedicated imaging of the sinuses, however, may be necessary.

If MRI evaluation is not immediately available or when patients are unable to tolerate MRI, requesting thin CT slices through the posterior fossa may be helpful. Due to the associated radiation exposure, CT is not recommended in pregnancy. However, in emergent situations, it may be performed while shielding the abdomen from radiation exposure. The use of contrast dye is helpful in evaluating suspected breaches in the blood-brain barrier as can occur with primary or metastatic brain tumors, meningitis, encephalitis, abscesses and other infections, and active areas of demyelination or inflammation. CT contrast may also help in the evaluation of vascular conditions. In ischemic stroke, a filling defect may be seen due to a thrombosed artery, or in patients with increased intracranial pressure, the classic “empty delta sign” indicates a clot at the confluence of venous sinuses. Aneurysms and arteriovenous malformations (AVMs) are better delineated with the use of contrast and may
be missed with a noncontrast study. It is not advisable to administer iodinated contrast to pregnant women, those with renal insufficiency, or if there is an allergy to the contrast agent. If use of contrast is necessary in patients with renal impairment, acetylcysteine and hydration should be used as prophylaxis in attempts to protect the kidneys. Some patients with myasthenia gravis may have an exacerbation of their weakness after the use of CT or MRI contrast and therefore should be monitored closely if use of contrast is necessary.

The advent of MRI has provided increased resolution in imaging structures such as soft tissue, cerebral gray and white matter, the brainstem, cerebellum, spinal cord, cranial nerves, spinal nerve roots, peripheral nerves, and muscle without traditional radiation exposure. Generally speaking, T1-weighted images are useful in evaluating the normal anatomy, and cerebrospinal fluid (CSF) appears black. On T2-weighted images, the CSF is white and this sequence highlights abnormalities in the brain or spinal cord parenchyma. In particular, areas of increased water content or edema look bright, or white. Fluid attenuated inversion recovery (FLAIR) sequences also highlight pathology, but the CSF is black, allowing better contrast for lesions around the ventricles, such as in multiple sclerosis, and near the cortical surface. Diffusion-weighted imaging (DWI) has become an important tool in identifying acute stroke and is based on the diffusion properties of water molecules through normal and injured tissue. Although not specific to acute stroke, patterns of restricted water molecule diffusion will appear bright on the DWI sequence. The apparent diffusion coefficient (ADC) is a measure of the mobility of water and if mobility is reduced, as in injured tissue, the corresponding area will appear dark. Therefore, an area that is bright on DWI and dark on ADC corresponds to an area of acute to subacute injury. These changes may persist for 2 weeks.

Orbital MRI is utilized to visualize the optic nerve and extraocular muscles. To optimally visualize these structures, it is important to specify that the study is done with fat suppression. This eliminates the bright artifact from fat within the orbit.

Aneurysm clips in the current era are designed so that they are compatible with MRI imaging. It is best, however, to verify compatibility prior to ordering an MRI in these patients.

As mentioned earlier, the use of contrast is helpful if a breach of the blood-brain barrier is suspected.

This condition causes fibrosis of the skin and other organs. It is, therefore, essential to weigh the risk of NSF and renal failure against the diagnostic necessity of using dye in these patients.

Evidence for spinal cord or nerve root compression may be evaluated by myelography in patients who are unable to undergo an MRI. Vascular malformations of the cord may also be seen. A lumbar or cervical puncture is performed under fluoroscopic guidance and an iodinated water-soluble contrast agent is injected into the subarachnoid space. X-rays are obtained to visualize the opaque column of dye and to see if there is a block or indentation in the column that may signify an area of
compression. Generally, CT is performed after myelography to identify what is causing the compression (i.e., herniated disc, osteophyte, mass) and to increase the diagnostic yield of the procedure. As with lumbar puncture (LP), a post-dural-puncture headache may occur after myelography. Arachnoiditis and idiosyncratic adverse reactions to the dye may occur.

Ultrasonography remains a very useful tool in the evaluation of patients with transient ischemic attacks and stroke, particularly in the evaluation of the extracranial carotid circulation. It provides valuable information about the presence of arterial stenosis by evaluating flow velocities. This imaging modality is portable, widely available, and can be performed relatively quickly. Although not as well known, transcranial Doppler (TCD) offers an evaluation of the intracranial circulation. Performance of the study relies on sonography through bony windows and may be technically limited by cranial hyperostosis. Carotid and TCD studies can help confirm findings on magnetic resonance angiography (MRA) and vice versa, as areas of high-grade stenosis may be overestimated with either modality alone. TCD is also utilized in monitoring for vasospasm after subarachnoid hemorrhage and can help determine the need for transfusion in sickle cell anemia children when the middle cerebral artery flow velocity escalates.