Computed tomographic (CT) scanning is a noninvasive computer-assisted radiologic means of examining anatomic structures (Figure 2-2). 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 with head injuries. Intravenous administration of an iodinated contrast agent improves the ability of CT scan to detect and define lesions, such as tumors and abscesses, associated with a disturbance of the blood–brain barrier. Because the contrast agents may have an adverse effect on the kidneys, 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 scans may provide more information than that obtained by unenhanced scans in patients with known or suspected primary or secondary brain tumors, arteriovenous malformations (AVMs), cerebral abscesses, chronic isodense subdural hematomas, infarcts, or hydrocephalus.
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 2-3).
CT scan is particularly helpful in evaluating strokes because it can distinguish infarction from intracranial hemorrhage; it is particularly sensitive in detecting intracerebral hematomas (Figure 13-18), and the location of such lesions may provide a guide to their cause. Moreover, the 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 hemorrhagic component.
The CT scan is an important means of evaluating patients after head injury, in particular for detecting traumatic intracranial (epidural, subdural, subarachnoid, or intracerebral) hemorrhage and bony injuries. It also provides a more precise delineation of associated fractures than do plain x-rays.
In patients with dementia, CT scan 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.
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 (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) is an imaging procedure that 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 2-3).
Brain MR images from a 62-year-old man, showing the normal anatomy. A-B: gadolinium-enhanced T1-weighted (CSF dark) images; C-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 2-2.
The soft-tissue contrast available with MRI makes it more sensitive than CT scanning in detecting certain structural lesions. MRI provides better contrast than does CT scans between the gray and white matter of the brain. It is superior for visualizing abnormalities in the posterior fossa and spinal cord 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 multiplane (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 because of claustrophobia, but sedation usually alleviates this problem.
Gadopentetate dimeglumine (gadolinium-DPTA) is stable, well-tolerated intravenously, and an effective enhancing MRI agent that 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, it may be possible to detect and localize cerebral infarcts by MRI. Breakdown in the blood–brain barrier (which occurs 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 in a later section. CT scans, on the other hand, may be unrevealing for up to 48 hours. After that period, there is less advantage to MRI over CT scanning except for the ability of the former to detect smaller lesions and its superior imaging of the posterior fossa.
Nevertheless, CT scanning without contrast is usually the preferred initial study in patients with acute stroke to determine whether hemorrhage has occurred. Intracranial hemorrhage is not easily detected by MRI within the first 36 hours, and CT scan is more reliable for this purpose. Hematomas of more than 2 to 3 days' duration, however, are better visualized by MRI. Although MRI is very effective in detecting and localizing vascular malformations, angiography is still 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 very useful in detecting brain tumors, but the absence of bone artifacts makes MRI superior for visualizing tumors at the vertex or in the posterior fossa and for detecting acoustic neuromas. Secondary effects of tumors, such as cerebral herniation, can be seen with either MRI or CT scan, but MRI provides more anatomic information. Neither technique, however, permits the nature of the underlying tumor to be determined with any certainty. Pituitary tumors are often visualized more easily by MRI than CT scan because of the absence of bone or dental metal artifacts.
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 appears to be more sensitive in demonstrating abnormal white matter signal and associated atrophy.
In patients with multiple sclerosis, it is often possible to detect lesions in the cerebral white matter or the cervical cord by MRI, even though such lesions may not be visualized on CT scans. The demyelinating lesions detected by 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 very sensitive in detecting white matter edema and probably permits earlier recognition of focal areas of cerebritis and abscess formation than is possible with CT scan.
Contraindications to MRI include the presence of intracranial clips, metallic foreign bodies in the eye or elsewhere, pacemakers, cochlear implants, and conditions requiring close monitoring of patients. Furthermore, it can be difficult to image 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 and monitoring equipment now allow even 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. 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 infarcts early enough for 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 allows determination of the diffusion of water in tissue in order to produce neural tract images. Such tractography is assuming increasing importance as a clinical and investigative tool, such as in distinguishing different types of dementia, determining the severity and extent of cerebral involvement after head injury, aiding in the precise localization of brain tumors, and planning of surgical procedures. The technique allows the detection of white matter changes that might not be seen on conventional MRI.
Perfusion-Weighted Magnetic Resonance Imaging
Perfusion-weighted imaging measures relative blood flow through the brain by either an injected contrast medium (eg, gadolinium) or an endogenous technique (in which the patient's own blood provides the contrast). It allows cerebral blood-flow abnormalities to be recognized and can confirm the early reperfusion of tissues after treatment. 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. Although its availability is currently limited, PET has proved useful in several clinical settings. When patients with medically refractory epilepsy are being considered for surgical treatment, PET can help 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. 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. PET 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.
The major problems associated with PET are its expense, the requirement that radioactive isotopes are produced near the site of imaging, and the exposure of subjects to radiation.
Single-Photon Emission Computed Tomography
Single-photon emission computed tomography (SPECT) involves the administration intravenously or by inhalation of chemicals containing isotopes that emit single photons in order to image the brain. SPECT has been used, in particular, for perfusion studies, the investigation of receptor distribution, and the detection of areas of increased metabolism such as occurs with seizures. At present the technique is more of academic interest than of clinical relevance, but it is considerably cheaper than PET, and the isotopes in use do not have to be produced near the site of imaging.
Functional Magnetic Resonance Imaging
Functional MRI (fMRI) involves the intravenous administration of contrast material that lowers signal intensity on MRI in relation to blood flow as the material passes through the cerebral vasculature. 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 2-4). An alternative approach involves pulse sequences that show changes in signal intensity from alterations in the oxygen concentration of venous blood (blood oxygen level–dependent [BOLD]-fMRI), which correlate with focal cerebral activity.
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.
(Reproduced from Waxman SG. Correlative Neuroanatomy
. 23rd ed. Norwalk, CT: Appleton & Lange; 1996.)
Magnetic Resonance Spectroscopy
Magnetic resonance spectroscopy is a tool available in some centers; it provides information about the chemical composition of tissue. Proton magnetic resonance spectroscopy (1H-MRS) may be used to 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 diseases such as Alzheimer disease or hypoxic–ischemic encephalopathy, or to classify brain tumors or lateralize 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, a technique in which the major vessels to the head are opacified and radiographed. 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 scanning 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, at the conclusion of the procedure, bleeding may occur at the puncture site, and occlusion of the catheterized artery (usually the femoral artery) may lead to distal ischemic complications. The puncture site and the distal circulation must therefore be monitored with these complications in mind.
The major indications for cerebral arteriography include the following:
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 (Chapter 6).
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, such as when a 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 can be recognized by their blood supply from the external carotid circulation. Presurgical embolization of certain tumors is often performed to reduce their blood supply and decrease the risk of major bleeding during resection.
Magnetic Resonance Angiography
Several imaging techniques that have been used to visualize blood vessels by MRI depend on certain 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 has a reduced cost and reduced risks compared with 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, in vessels with slow flow, it may be difficult to recognize occlusive disease. Moreover, intracranial MR angiograms may be marred by irregular or discontinuous signal intensity in vessels close to the skull base. Although current techniques allow visualization of AVMs and aneurysms greater than 3 mm in diameter, conventional angiography remains the “gold standard” in this context. 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.
Spiral CT angiography is a minimally invasive procedure that requires the CT scanner to be capable of rapidly acquiring numerous thin, overlapping sections after intravenous injection of a bolus of contrast material. It can be performed within minutes and 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. It is a reliable alternative to MR angiography, but both techniques are less sensitive than conventional angiography in this regard.
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.