Computed tomographic (CT) scanning is a noninvasive, computer-assisted, radiologic means of examining anatomic structures (Figure 2-5). It permits the detection of structural intracranial abnormalities with precision, speed, and facility. It is thus of particular use in evaluating patients with progressive neurologic disorders or focal neurologic deficits in whom a structural lesion is suspected, patients with dementia or increased intracranial pressure, and patients with suspected stroke or head injuries. Intravenous administration of an iodinated contrast agent improves the detection and definition of vascular lesions and those associated with a disturbance of the blood–brain barrier. Contrast-enhanced scans may provide more information than unenhanced scans in patients with known or suspected primary or secondary brain tumors, arteriovenous malformations (AVMs), aneurysms, cerebral abscesses, chronic isodense subdural hematomas, or infarctions. Because the contrast agents may affect the kidneys adversely, they should be used with discrimination. Other adverse effects of the contrast agents in common use are pain, nausea, thermal sensations, and anaphylactoid reactions that include bronchospasm and death.
Contrast-enhanced CT brain scans from a 62-year-old man, showing the normal anatomy. Images are at the level of the lateral ventricles (left) and midbrain (right) (same patient as in Figure 2-6).
CT scan can distinguish infarction from intracranial hemorrhage; it is particularly sensitive in detecting intracerebral hematomas (see Figure 13-20), the location of which may provide a guide to their cause. CT scan occasionally demonstrates a nonvascular cause of the patient’s clinical deficit, such as a tumor or abscess.
CT scans can indicate the site of a brain tumor, the extent of any surrounding edema, whether the lesion is cystic or solid, and whether it has displaced midline or other normal anatomic structures. It also demonstrates any acute hemorrhagic component.
CT scans are important for detecting traumatic intracranial (epidural, subdural, subarachnoid, or intracerebral) hemorrhage and bony injuries. They also provide a more precise delineation of associated fractures than do plain x-rays.
CT scanning may indicate the presence of a tumor or hydrocephalus (enlarged ventricles), with or without accompanying cerebral atrophy. The occurrence of hydrocephalus without cerebral atrophy in demented patients suggests normal pressure or communicating hydrocephalus. Cerebral atrophy can occur in demented or normal elderly subjects.
E. Subarachnoid Hemorrhage
In patients with subarachnoid hemorrhage, the CT scan generally indicates the presence of blood in the subarachnoid space and may even suggest the source of the bleeding (see Figure 6-5). If the CT scan findings are normal despite clinical findings suggestive of subarachnoid hemorrhage, the CSF should be examined to exclude hemorrhage or meningitis. CT angiography (see later) may demonstrate an underlying vascular malformation or aneurysm.
MAGNETIC RESONANCE IMAGING
Magnetic resonance imaging (MRI) involves no ionizing radiation. The patient lies within a large magnet that aligns some of the protons in the body along the magnet’s axis. The protons resonate when stimulated with radiofrequency energy, producing a tiny echo that is strong enough to be detected. The position and intensity of these radiofrequency emissions are recorded and mapped by a computer. The signal intensity depends on the concentration of mobile hydrogen nuclei (or nuclear-spin density) of the tissues. Spin–lattice (T1) and spin–spin (T2) relaxation times are mainly responsible for the relative differences in signal intensity of the various soft tissues; these parameters are sensitive to the state of water in biologic tissues. Pulse sequences with varying dependence on T1 and T2 selectively alter the contrast between soft tissues (Figure 2-6).
Brain MR images from a 62-year-old man, showing the normal anatomy. (A and B) Gadolinium-enhanced T1-weighted (CSF dark) images; (C and D) T2-weighted (CSF white) images. Images are at the level of the lateral ventricles (A and C) and midbrain (B and D). A midsagittal T1-weighted image is shown in (E). Brain images are from the same patient as in Figure 2-5.
The soft-tissue contrast available with MRI makes it more sensitive than CT scanning in detecting certain structural lesions. MRI provides better contrast than CT scans between the gray and white matter of the brain. It is superior for visualizing abnormalities in the posterior fossa and spinal cord, for subacute and chronic hemorrhage, and for detecting lesions associated with multiple sclerosis or those that cause seizures. In addition to its greater sensitivity, it is also free of bony artifact and permits multiplanar (axial, sagittal, and coronal) imaging with no need to manipulate the position of the patient. Because there are no known hazardous effects, MRI studies can be repeated in a serial manner if necessary. Occasional patients cannot tolerate the procedure without sedation because of claustrophobia.
Gadopentetate dimeglumine (gadolinium-DPTA) is an effective enhancing MRI contrast agent that is stable and well-tolerated intravenously. It is useful in identifying small tumors that, because of their similar relaxation times to normal cerebral tissue, may be missed on unenhanced MRI. It also helps to separate tumor from surrounding edema, identify leptomeningeal disease, and provide information about the blood–brain barrier. Gadolinium has been associated with nephrogenic systemic fibrosis in patients with renal insufficiency, so it should be used judiciously in this setting.
INDICATIONS FOR USE & COMPARISON WITH CT SCAN
Within a few hours of vascular occlusion, cerebral infarcts may be detected by MRI. Breakdown in the blood–brain barrier (several hours after onset of cerebral ischemia) permits the intravascular content to be extravasated into the extracellular space. This can be detected by T2-weighted imaging and fluid-attenuated inversion-recovery (FLAIR) sequences. Diffusion-weighted MRI also has an important role in the early assessment of stroke, as is discussed later, whereas CT scans may be unrevealing for up to 48 hours. Thereafter, the advantage of MRI over CT scanning lessens, except that MRI detects smaller lesions and provides superior imaging of the posterior fossa.
Nevertheless, to determine quickly whether hemorrhage has occurred, CT scanning without contrast is usually the preferred initial study in acute stroke. Hematomas of more than 2 to 3 days’ duration, however, are better visualized by MRI. Although MRI can detect and localize vascular malformations, angiography is necessary to define their anatomic features and plan effective treatment. In cases of unexplained hematoma, a follow-up MRI obtained 3 months later may reveal the underlying cause, which is sometimes unmasked as the hematoma resolves.
Both CT scans and MRI are useful in detecting brain tumors. MRI is the preferred technique because of its greater soft tissue sensitivity, the absence of bone artifacts at the vertex or in the posterior fossa, and the ability to employ advanced imaging techniques such as MR spectroscopy and diffusion and perfusion imaging that better characterize a lesion. MRI or CT scan may detect secondary effects of tumors, such as cerebral herniation, but MRI provides more detailed and sensitive anatomic information. Neither technique, however, permits the type of a tumor to be determined with certainty.
In the acute phase after head injury, CT scan is preferable to MRI because it requires less time, is superior for detecting intracranial hemorrhage, and may reveal bony injuries. Similarly, spinal MRI should not be used in the initial evaluation of patients with spinal injuries because nondisplaced fractures are often not visualized. For follow-up purposes, however, MRI is helpful for detecting parenchymal pathology of the brain or spinal cord.
In patients with dementia, either CT scan or MRI can help in demonstrating treatable structural causes, but MRI is more sensitive in demonstrating abnormal white matter signal and associated atrophy.
Lesions in the cerebral white matter or the cervical cord are best detected by MRI, as lesions may not be visualized on CT scans. The lesions on MRI may have signal characteristics resembling those of ischemic changes, however, and clinical correlation is therefore always necessary. Gadolinium-enhanced MRI permits lesions of different ages to be distinguished. This ability facilitates the diagnosis of multiple sclerosis: The presence of lesions of different ages suggests a multiphasic disease, whereas lesions of similar age suggest a monophasic disorder, such as acute disseminated encephalomyelitis.
MRI is sensitive in detecting white matter edema and probably permits earlier recognition of focal areas of cerebritis and abscess formation than CT scan. Diffusion MR imaging is particularly helpful in detecting areas of reduced diffusion, typical of purulent abscess and encephalitis.
MRI is contraindicated by the presence of intracranial ferromagnetic aneurysm clips, metallic foreign bodies in the eye, demand-mode pacemakers, and cochlear implants. Many implanted devices are also contraindications for MRI. Patients requiring close monitoring are probably best studied by CT if possible. Furthermore, MRI is difficult in patients with claustrophobia, extreme obesity, uncontrolled movement disorders, or respiratory disorders that require assisted ventilation or carry any risk of apnea. Advances in MRI-compatible mechanical ventilators, pacemakers, and monitoring equipment, however, now allow many critically ill patients to be scanned safely.
DIFFUSION-WEIGHTED MAGNETIC RESONANCE IMAGING
This technique, in which contrast within the image is based on the microscopic motion of water protons in tissue, provides information that is not available on standard MRI or CT. It is particularly important in the assessment of stroke because it can discriminate cytotoxic edema (which occurs in strokes) from vasogenic edema (found with other types of cerebral lesion) and thus reveals cerebral ischemia early and with high specificity. Diffusion-weighted MRI permits reliable identification of acute cerebral ischemia during the first few hours after onset, before it is detectable on standard MRI. This is important because it reveals the true volume of infarcts prior to treatment with thrombolytic agents. However, because diffusion-weighted imaging will be positive in the setting of cytotoxic edema of any cause (eg, brain abscess, highly cellular tumors), clinical correlation is always required. When more than one infarct is found on routine MRI, diffusion-weighted imaging permits the discrimination of acute from older infarcts by the relative increase in signal intensity of the former.
DIFFUSION TENSOR MAGNETIC RESONANCE IMAGING
This technique produces neural tract images by measuring the diffusion of water in tissue. It is important in determining the severity and extent of cerebral involvement after head injury, localizing brain tumors, and planning surgical procedures. White matter changes may be detected that are not seen on conventional MRI.
PERFUSION-WEIGHTED MAGNETIC RESONANCE IMAGING
Blood flow through the brain may be measured using either an injected contrast medium (eg, gadolinium) or an endogenous technique (in which the patient’s own blood provides the contrast). Cerebral blood-flow abnormalities can be recognized and the early reperfusion of tissues after treatment can be confirmed. Cerebral ischemia may be detected very soon after clinical onset. Comparison of the findings from diffusion-weighted and perfusion-weighted MRI may have a prognostic role and is currently under study. The distinction of reversible from irreversible ischemic damage is important in this regard. Perfusion-weighted imaging also contributes in distinguishing between various types of brain tumors such as gliomas and metastases.
POSITRON EMISSION TOMOGRAPHY
Positron emission tomography (PET) is an imaging technique that uses positron-emitting radiopharmaceuticals, such as 18F-fluoro-2-deoxy-D-glucose or 18F-L-dopa, to map brain biochemistry and physiology. PET thus complements other imaging methods that provide primarily anatomic information, such as CT scan and MRI, and may demonstrate functional brain abnormalities before structural abnormalities are detectable. PET has proved useful in several clinical settings and is now combined with CT or MR scanners in hybrid machines. When patients with medically refractory epilepsy are being considered for surgical treatment, PET CT scan can identify focal areas of hypometabolism in the temporal lobe as likely sites of the origin of seizures. PET can also be useful in the differential diagnosis of dementia, because common dementing disorders such as Alzheimer disease and frontotemporal dementia exhibit different patterns of abnormal cerebral metabolism. In vivo imaging of amyloid-β (Aβ) with PET facilitates the early diagnosis of Alzheimer disease and provides prognostic information for patients with mild cognitive impairment. PET can help distinguish between clinically similar movement disorders, such as Parkinson disease and progressive supranuclear palsy, and can provide confirmatory evidence of early Huntington disease. It may also be of value in grading gliomas, selecting tumor biopsy sites, and distinguishing recurrent tumors from radiation-induced brain necrosis. It has been an important tool with which to investigate the functional involvement of different cerebral areas in behavioral and cognitive tasks and is used frequently in patients with suspected metastatic disease. However, PET is more expensive than MR or CT alone, requires administration of radioactive isotopes, and thus exposes subjects to radiation.
SINGLE-PHOTON EMISSION COMPUTED TOMOGRAPHY
In single-photon emission computed tomography (SPECT), chemicals containing isotopes that emit single photons are administered intravenously or by inhalation to image the brain. SPECT has been used, in particular, to measure perfusion, to investigate receptor distribution, and to detect areas of increased metabolism such as occurs with seizures.
A specific contrast agent (123I ioflupane) used with SPECT detects dopamine transporters (DaT) and helps to distinguish parkinsonian syndromes from essential tremor when this is clinically difficult. A DaT scan is also used to confirm a diagnosis of Parkinson disease in patients with ambiguous symptoms.
FUNCTIONAL MAGNETIC RESONANCE IMAGING
Functional MRI (fMRI) involves pulse sequences that change in signal intensity in response to alterations in the oxygen concentration of venous blood (blood oxygen level–dependent [BOLD]-fMRI), which correlate with focal cerebral activity. Studies are performed with the subject at rest and then after an activation procedure so that the change in signal intensity reflects the effect of the activation procedure on local cerebral blood flow (Figure 2-7). Studies are also performed without a stimulus (“resting state fMRI”) to interrogate the brain’s functional organization. fMRI studies are indicated for preoperative functional mapping of sensorimotor and language areas in the evaluation of patients with brain tumors, as well as in some cases of epilepsy or vascular malformations.
A functional MR brain image obtained from a patient during rapid finger tapping of the left hand. An increase in relative blood flow in the region of the right motor strip is imaged (arrow) and superimposed on a T1-weighted MR scan. (Used with permission from Waxman SG. Correlative Neuroanatomy. 23rd ed. Norwalk, CT: Appleton & Lange; 1996. Copyright © McGraw-Hill.)
MAGNETIC RESONANCE SPECTROSCOPY
Magnetic resonance spectroscopy provides information about the chemical composition of tissue. Proton magnetic resonance spectroscopy (1H-MRS) can determine levels of N-acetylaspartate (exclusive to neurons) or choline, creatinine, and lactate (glia and neurons). Measurements of brain concentrations of these metabolites may be useful in detecting specific tissue loss in Alzheimer disease or other neurodegenerative disorders; in distinguishing brain tumors from non-neoplastic lesions such as abscesses and in classifying brain tumors; in identifying certain inborn errors of metabolism and leukodystrophies; in prognostication of hypoxic-ischemic brain injury; and in localizing the source of seizures in temporal lobe epilepsy. Phosphorus magnetic resonance spectroscopy (31P-MRS) may be useful in the evaluation of metabolic muscle diseases.
The intracranial circulation is visualized most satisfactorily by arteriography, in which the major vessels to the head are opacified and radiographed after injection of contrast material through an arterial or venous catheter. Specifically, a catheter is introduced into the femoral or brachial artery and passed into one of the major cervical vessels. A radiopaque contrast material is then injected through the catheter, allowing the vessel (or its origin) to be visualized. Access to the cranial vessels with a catheter also allows for the delivery of certain therapies. The technique, generally performed after noninvasive imaging by CT scan or MRI, has a definite (approximately 1%) morbidity and mortality associated with it and involves considerable exposure to radiation. It is contraindicated in patients who are allergic to the contrast medium. Stroke may result as a complication of arteriography. Moreover, after the procedure, bleeding may occur at the puncture site, and the catheterized artery (usually the femoral artery) may become occluded, leading to distal ischemic complications. The puncture site and the distal circulation must therefore be monitored.
The major indications for cerebral arteriography are:
Diagnosis of intracranial aneurysms, arteriovenous malformations (AVMs), or fistulas. Although these lesions can be visualized by CT scan or MRI, their detailed anatomy and the vessels that feed, drain, or are otherwise implicated in them cannot reliably be defined by these other means. Moreover, arteriography is required for interventional procedures such as embolization, the injection of occlusive polymers, or the placement of detachable balloons or coils to treat certain vascular anomalies.
Detection and definition of the underlying lesion in patients with subarachnoid hemorrhage who are considered good operative candidates (see Chapter 6, Headache & Facial Pain).
Detection and management of vasospasm after subarachnoid hemorrhage.
Emergency embolectomy in the setting of ischemic stroke due to large-vessel occlusion. In addition, arteriography can define vascular lesions in patients with transient cerebral ischemic attacks or strokes if surgical treatment such as carotid endarterectomy is being considered.
Evaluation of small vessels, when vasculitis is under consideration.
Diagnosis of cerebral venous sinus thrombosis.
Evaluation of space-occupying intracranial lesions, particularly when CT scanning or MRI is unavailable. There may be displacement of the normal vasculature, and in some tumors neovasculature may produce a blush or stain on the angiogram. Meningiomas are supplied from the external carotid circulation. Presurgical embolization of certain tumors reduces their blood supply and decreases the risk of major bleeding during resection.
MAGNETIC RESONANCE ANGIOGRAPHY
Several imaging techniques to visualize blood vessels by MRI depend on the physical properties of flowing blood, thereby allowing visualization of vasculature without the use of intravenous contrast. These properties include the rate at which blood is supplied to the imaged area, its velocity and relaxation time, and the absence of turbulent flow. Magnetic resonance (MR) angiography is a noninvasive technique that is cheaper and less risky than conventional angiography. It has been most useful in visualizing the carotid arteries and proximal portions of the intracranial circulation, where flow is relatively fast. The images are used to screen for stenosis or occlusion of vessels and for large atheromatous lesions. It has particular utility in screening for venous sinus occlusion. Resolution is inferior to that of conventional angiography, and occlusive disease may not be recognized in vessels with slow flow. Moreover, intracranial MR angiograms may be marred by saturation or susceptibility artifacts that result in irregular or discontinuous signal intensity in vessels close to bone. Although current techniques allow visualization of AVMs and aneurysms greater than 3 mm in diameter, conventional angiography remains the “gold standard.” Finally, MR angiography may reveal dissection of major vessels: Narrowing is produced by the dissection, and cross-sectional images reveal the false lumen as a crescent of abnormal signal intensity next to the vascular flow void.
CT angiography is a minimally invasive procedure that requires a CT scanner capable of acquiring numerous thin, overlapping sections quickly after intravenous injection of a bolus of contrast material. Because the images are acquired within a matter of 5 to 10 seconds, CTA is less likely to be affected by patient movement than MR angiography. A wide range of vessels can be imaged with the technique.
CT angiography of the carotid bifurcation is being used increasingly in patients with suspected disease of the carotid arteries. It can also be used for intracranial imaging and can detect stenotic or aneurysmal lesions. However, sensitivity is reduced for aneurysms less than 3 mm, and the method cannot adequately define aneurysmal morphology in the preoperative evaluation of patients. It is sensitive in visualizing the anatomy in the circle of Willis, the vasculature of the anterior and posterior circulations, and intracranial vasoocclusive lesions, but it may not reveal plaque ulceration or disease of small vessels. It is a reliable alternative to MR angiography, but both techniques are less sensitive than conventional angiography.
In patients with acute stroke, CT angiography provides important information complementary to conventional CT scan studies, revealing the site and length of vascular occlusion and the contrast-enhanced arteries distal to the occlusion as a reflection of collateral blood flow. CT perfusion, in which the relative blood flow to an area of the brain is measured as iodinated contrast passes through over time, can provide additional information regarding the proportion of ischemic to infarcted tissue in this setting.