Magnetic Resonance Imaging

For a wide variety of conditions related to the CNS, MRI is the modality of choice (Fig. 8-13). Its sensitivity is excellent in evaluation of all types of spinal diseases, including tumors, abscesses, and disk disease. The ability of MRI to evaluate brain tumors and conditions such as stroke, cranial tumors, and infection surpasses that of CT. Because MRI does not image the dense petrous bone, it is excellent in evaluating the brainstem and anomalies of the posterior fossa (Fig. 8-14). Evaluation of demyelinating disease such as multiple sclerosis (MS) is substantively enhanced by MRI. It is, however, important to note that the high sensitivity associated with MRI in the diagnosis of myelopathy may also prove to be a limitation at times. The ease with which MRI studies depict expansion and compression of the spinal cord may potentially lead to false-positive examinations and inappropriately aggressive therapies. These issues can be minimized by experienced observers meticulously correlating clinical and radiologic findings. Despite rapid evolution in technology, MRI currently has a small role in the evaluation of trauma, mainly limited to evaluation of spinal cord compression and, in some instances, vertebral fractures.

Magnetic resonance angiography (MRA) is used to evaluate the vascular anatomy of the head and neck (Fig. 8-15). It has been proven to accurately locate vascular occlusions within the large arteries of the brain that may lead to a stroke. MRA also demonstrates the major veins and dural sinuses within the brain and plays a large role in the diagnosis and treatment of cerebral venous thrombosis. To date, it has not replaced conventional cerebral angiography but is often used as an adjunct modality.

Functional MRI (fMRI) may be used to correlate and map motor functions to specific locations in the brain. This procedure identifies neural activation on the basis of hemodynamic changes within the brain. It provides useful information in demonstrating alterations in normal CNS function caused by structural changes in individuals with MS. fMRI is also helpful in identifying the regions of the cortex affected by neoplastic lesions within the brain and in assessing patients with a history of seizures and epilepsy. Currently, researchers are also using this modality to achieve a better understanding of the physiologic basis of autism by seeking information about neuroanatomic as well as physiologic changes resulting from this disorder.

Diffusion-weighted imaging (DWI) involves monitoring the extent to which water can freely move in any volume element (voxel). When the motion of water molecules within a voxel is restricted, greater magnetization will result, and the associated voxels will appear brighter. Reduced water diffusivity has been correlated with more aggressive tumor behavior and is sometimes seen with high cellular density and tumor recurrence as well as ischemic stroke (Fig. 8-16). Diffusion-tensor imaging (DTI) capitalizes on the expected relative increase in restricted water molecules among neurologic fiber tracts by acquiring a three-dimensional map of the fiber tracts with a postprocessing technique referred to as tractography. The higher information content of the voxels makes DTI extremely sensitive to subtle pathology in the brain, which may be preventing the natural movement of water in the brain.

Perfusion-weighted imaging (PWI) involves the use of paramagnetic contrast agents to assess the cerebral hemodynamic status of a patient. By utilizing acquisition times that are short enough to capture the changes in signal intensity as a bolus of contrast material passes through the brain, PWI provides useful information on the vascular supply of various tumors, which is only implied from conventional MRI. The relative cerebral blood volume (rCBV) within a tumor can also be measured and compared with the contralateral normal white matter, typically displayed on color maps. The perfusion curves that are generated provide additional prognostic information by distinguishing between the characteristics of benign and malignant tumors. Investigative studies that are now under way indicate that intraarterial MRI perfusion methods could be useful in better understanding the perfusion characteristics of some tumors and may also be useful in monitoring the delivery of therapeutics (Fig. 8-17).

Computed Tomography

CT continues to play a significant role in the evaluation of the CNS. It is rapid, noninvasive, and reasonably accurate. Because CT has excellent contrast resolution, it readily differentiates among the sulci, ventricles, and the gray and white matter within the brain (Fig. 8-18). It is particularly useful in evaluating cerebral bleeding after trauma because it readily reveals the extent of hematomas, if present. In many institutions, it is common practice to routinely perform CT examinations of the head and neck after significant trauma. CT of the brain is also performed to evaluate the ventricles, as in the case of hydrocephalus; to demonstrate cortical atrophy, as in the case of dementia; and to identify neoplasms within the brain, which may or may not create a mass effect or shifting of the midline of the brain. In cases of infarction, edema, abscess, or cyst formation, tissue density is decreased (Fig. 8-19). Tissue density is increased by fresh blood or recent hemorrhage and calcifications within the brain. Other related roles include routine anatomic evaluation such as assessment of shunt (an artificial passageway) functioning and assessment of the bony cerebral and visceral skull. Contrast-enhanced CT of the brain is used to demonstrate anomalies in the cerebral vasculature and to delineate neoplastic growth within the cranial vault. It should be noted, however, that CT-related artifacts that may occur in association with bone at the margins of the posterior fossa often require the use of MRI to optimally evaluate pathology in this region of the brain. As discussed earlier, postmyelographic CT of the spine is also fairly prevalent. CT myelograms demonstrate encroachments of the spinal cord and nerve roots, as does MRI, but bone detail is best visualized with CT. CT is also used to evaluate vertebral fractures, as discussed in Chapter 2.

Sonography

Sonography is useful in evaluating the brains of neonates before closure of the fontanels because the fibrous tissue covering the fontanels provides a ready window into the brain (Fig. 8-20). Sonography readily detects cerebral hemorrhage and hydrocephalus. In premature infants, brain tissue around the ventricles (periventricular germinal matrix) is prone to hemorrhage, often resulting in intraventricular hemorrhage. In addition, premature infants are at an increased risk for periventricular white matter infarction. Portable sonography is a noninvasive method of evaluating cerebral anatomy and can be performed in the neonatal intensive care unit (NICU) to evaluate the status of affected infants.

Duplex Doppler sonography is a noninvasive modality useful in evaluating carotid blood flow. It demonstrates vascular occlusion and ulceration within the carotid arteries, most commonly at the point of bifurcation. Although it cannot provide the same level of detail compared with traditional angiography, it is a cost-effective and radiation-free alternative that is useful for evaluating patients experiencing carotid artery transient ischemic attacks (TIAs).

Nuclear Medicine

In nuclear medicine, radionuclide brain scans are primarily used to confirm brain death in patients with the appropriate clinical signs, generally after trauma or an intracranial bleed (Fig. 8-21). Technetium-labeled flow agents in combination with single photon emission computed tomography (SPECT) may be used experimentally to assess tumors, areas of stroke and ischemia, and Alzheimer disease. To complement the more traditional anatomic evaluation of other modalities, PET scanning allows imaging of the body’s normal chemical processes and provides physiologic evaluation (Fig. 8-22). In high-risk patients with identified intracranial pathology, PET and SPECT both assist in differentiating tumors from infections.

Vascular and Interventional Radiology

Cerebral angiography is used to demonstrate vessel anatomy within the neck and brain (Fig. 8-23). Because of the increased use of MRA and CT angiography (CTA) vascular studies, conventional angiography is most frequently utilized when an interventional therapeutic procedure is warranted. It is used to diagnose and treat neoplastic lesions, stenosis, aneurysms, arteriovenous malformations (AVMs), and congenital anomalies of the CNS. Cerebral blood flow may be visualized by placing the catheter in the aortic arch or in selected vessels such as the carotid or vertebral arteries. Therapeutic devices such as stents and shunts and therapeutic medications are also placed by the interventionalist via catheters, often eliminating the need for surgical intervention.

Interventional techniques include angioplasty, stent or shunt placement, thrombolysis, or embolization. Percutaneous transluminal angioplasty (PTA) and intraarterial thrombolysis are used to open stenotic vessels and improve blood flow within the vessel treated. Laser-tipped angioplasty and percutaneous atherectomy may also be performed to open stenotic vessels, especially in the neck. Spheres, coils, or beads are used to embolize or occlude cerebral vasculature, as in the case of aneurysms or AVMs. If an AVM is suspected, contrast-enhanced MRI, MRA, and CTA, as well as traditional myelography images can be utilized to initially identify abnormal spinal vasculature and guide spinal arteriography and intervention, potentially limiting the number of direct vascular injections required.