Chapter 22: Imaging of the Brain

Brain imaging provides essential diagnostic information and is very useful for research on the brain. Images of the skull, the brain and its vessels, and spaces in the brain containing cerebrospinal fluid can aid immeasurably in the localization of lesions. In concert with physical examination and history, imaging studies can provide important clues to diagnosis. In emergency cases, images of unconscious patients may be the only diagnostic information available.

Computed tomography (CT), magnetic resonance imaging (MRI), and other similar imaging methods are usually displayed to show sections of the head, the sagittal, coronal (frontal), and horizontal (axial) planes are commonly used (Fig 22–1).

Skull x-rays provide a simple method for imaging calcium and its distribution in and around the brain when more precise methods are unavailable. Plain films of the skull can define the extent of a skull fracture and a possible depression or determine the presence of calcified brain lesions, foreign bodies, or tumors involving the skull. They can provide images of the bony structures and foramens at the base of the skull and of the sinuses. Skull x-ray films can also provide evidence for chronically increased intracranial pressure, accompanied by thinning of the dorsum sellae, and abnormalities in the size and shape of sella turcica, which suggest large pituitary tumors. Skull films are sometimes used to screen for metal objects before beginning MRI of the head.

Cerebral Angiography

Angiography (arteriography) of the head and neck is a neurodiagnostic procedure used when a vessel abnormality such as occlusion, malformation, or aneurysm is suspected (Figs 22–2, 22–3, 22–4, 22–5, 22–6, 22–7, 22–8, 22–9, 22–10, 22–11, 22–12; see also Chapter 12). Angiography can also be used to determine whether the position of the vessels in relation to intracranial structures is normal or pathologically changed. Aneurysms, arteriovenous fistulas, or vascular malformations can be treated by interventional angiography using balloons, a quickly coagulating solution that acts as a glue, or small, inert pellets that act like emboli.

Angiograms consist of a series of images showing contrast material introduced into a major artery (eg, via a catheter in the femoral) under fluoroscopic guidance. Arterial-phase films are followed by capillary and venous-phase films (see Figs 22–6, 22–7, 22–8, 22–9, 22–10). Right and left internal carotid and vertebral angiograms may be complemented by other films (eg, by an external carotid series in cases of meningioma or arteriovenous malformation). The films are often presented as subtracted, that is, as reversal prints superimposed on a plain film of the skull.

CT, also called computed axial tomography (CAT), affords the possibility of inspecting cross sections of the skull, brain, ventricles, cisterns, large vessels, falx, and tentorium. It has become a primary tool for demonstrating the presence of abnormal calcifications, brain edema, hydrocephalus, many types of tumors and cysts, hemorrhages, large aneurysms, vascular malformations, and other disorders.

CT scanning is noninvasive and fast. Although it has high sensitivity, its specificity is relatively limited. Correlation with the clinical history and physical examination is an absolute requirement. In the case of a subarachnoid hemorrhage, for example, although a CT scan may quickly localize the areas containing blood (Fig 12–21), additional CT images (Fig 12–22), magnetic resonance imaging, or angiography is often required to determine whether the cause was an aneurysm or an arteriovenous malformation.

The CT scanning apparatus rotates a narrow x-ray beam around the head. The quantity of x-ray absorbed in small volumes (voxels [volume elements, or units]) of brain, measuring approximately 0.5 mm² × 1.5 mm or more in length, is computed. The amount of x-ray absorbed in any slice of the head can be thus determined and depicted in various ways as pixels (picture elements) in a matrix. In most cases, absorption is proportional to the density of the tissue. A converter translates the numeric value of each pixel to a gray scale. Black-and-white pictures of head slices are then displayed, with black representing low-density structures and white representing high-density structures. The thickness of the slices can vary, from 1.5 mm to 1 cm. The gray scale can also vary; although a setting at which brain tissue is distinguished best is commonly used, in some cases bone, fat, or air need to be defined in great detail.

A series of 10 to 20 scans, each reconstructing a slice of brain, is usually required for a complete study. The plane of these sections is the orbitomeatal plane, which is parallel to both Reid's base plane and the intercommissural line used in stereotactic neurosurgery (Fig 22–13). Usually, a "scout view" similar to a lateral skull roentgenogram is taken with a CT scanner to align the planes of sections (Fig 22–14). With the modern technology now available, each scan takes only a few seconds. Examples of normal and abnormal CT scans are shown in Figures 22–15 and 22–16.

CT scanning of the posterior fossa may provide only limited information because of the many artifacts caused by dense bone. Images reformed by a computer from a series of thin sections allow visualization in any desired plane, for example, midsagittal (see Fig 6–15) or coronal. Coronal sections are often extremely useful for structures lying at the base of the brain, in the high convexity area, or close to the incisura. Detailed examination of orbital contents requires planes at right angles to the orbital axis.

Tissue density can change pathologically. Areas of hyperemia or freshly clotted hemorrhage appear more dense (Fig 12–19); edematous tissue appears less dense (Fig 12–14).

Magnetic resonance imaging (MRI) depicts protons and neutrons in a strong external magnetic field shielded from extraneous radio signals; no radiation is used.

The spatial distribution of elements with an odd number of protons (such as hydrogen) within slices of the body or brain can be determined by their reaction to an external radio frequency signal. The signals are shown as pixels in a matrix, similar to the CT technique. The resolution of the images is comparable to, or exceeds, that of CT scans, and with MRI an image of the brain or spinal cord in any plane can be obtained directly; no reformation is required. Bone is poorly imaged and does not interfere with visualization of nervous tissue; thus, MRI is especially useful for imaging the spinal cord and structures within the posterior fossa.

MRI can also be used to directly and noninvasively evaluate the flow of blood within medium and larger arteries and veins, with no need for intravenous injection of a contrast agent. This makes MRI particularly useful in cerebrovascular studies.

The sequence of radio frequency excitation followed by recording of tissue disturbation (echo signals) can be varied in both duration of excitation and sampling time. The images obtained with short time sequences differ from those obtained with longer time sequences (Fig 22–17). A normal MR image is shown in Figure 22–18; other MR images, both normal and abnormal, are found in Chapter 12 and elsewhere throughout this text.

Magnetic resonance angiography (MRA) uses a water proton signal to provide images of the cerebral arteries and veins. The method does not require the catheterization of vessels or the injection of radiopaque substances and is thus safer than traditional angiography.

The MRI process is relatively slow and takes more time than CT scanning. However, it provides high-quality images of the brain and spinal cord, and is safe for patients who have no ferromagnetic implants. The increasing sophistication of MRI technique (eg, with the use of contrast agents) has broadened its clinical usefulness. MRI provides a primary method of examination, especially in cases of suspected tumors, demyelination, and infarcts. As with CT scanning, successful use of MRI for accurate diagnosis requires correlating the results with the clinical history and physical examination.

In MRI, signals collected from water protons are used to assemble an image of the brain. The resonance signals collected by the computer using nuclear magnetic resonance can also be used to measure the levels of several dozen compounds that are present in the brain, including lactate, creatine, phosphocreatine, and glutamate. Magnetic resonance spectroscopy is routinely used as an experimental tool that provides a noninvasive means of measuring the levels of various molecules within the brain. Magnetic resonance spectroscopy can be used to study the human brain and may be useful in the diagnosis of various neurologic disorders and in studies on putative therapies for diseases affecting the nervous system.

By varying the magnetic field gradients and pulse sequences, it is possible to make MRI sensitive to the rate of diffusion of water within various parts of the brain; this is called diffusion-weighted imaging (DWI). DWI permits the visualization of regions of the brain that have become ischemic within minutes after the loss of blood flow (Fig 22–19).

Neuroanatomy is being revolutionised by functional MRI, which uses MRI pulse sequences that can also be adjusted to produce an image sensitive to local changes in the concentration of deoxyhemoglobin. When neural activity occurs in a particular region of the brain, there is usually an increase in oxygen uptake, which triggers increases in cerebral blood flow and cerebral blood volume. These increases lead to a local reduction in the concentration of deoxyhemoglobin. Changes in deoxyhemoglobin concentration are thus related to the level of neural activity within each part of the brain. By measuring deoxyhemoglobin levels and comparing them when the brain is in a resting state and when it is involved in a particular activity, functional MRI (fMRI) can provide maps that show regions of increased neural activity within the brain. fMRI can be used, for example, to detect changes in brain activity associated with motor activity (eg, tapping of the fingers), sensory activity (ie, stimulation of a sensory organ or part of the body surface), cognitive activity (eg, calculation, reading, or recalling), and affective activity (eg, responding mentally to a fearful stimulus). Examples are shown in Figures 15–18 and 22–20.

Positron emission tomography (PET) scanning has become a major clinical research tool for the imaging of cerebral blood flow, brain metabolism, and other chemical processes (Fig 22–21). Radioisotopes are prepared in a cyclotron and are inhaled or injected. Emissions are measured with a gamma-ray detector system. It is possible, for example, to map regional glucose metabolism in the brain using fluorine 18 (¹⁸F)-labeled deoxyglucose. Images that show focal increases in cerebral blood flow or brain metabolism provide useful information about the parts of the brain that are activated during various tasks. This is another example of functional brain imaging.

It is also possible, using PET scanning, to localize radioactively tagged molecules that bind specifically to certain types of neurons. Using this type of technique, it is possible, for example, to localize dopaminergic neurons in the brain and to quantitate the size of the nuclei containing these neurons.

One disadvantage of PET scanning is the lack of detailed resolution; another is that most positron-emitting radioisotopes decay so rapidly that their transportation from the cyclotron (the site of production) can be rate limiting. Some isotopes, such as ¹⁸F and gamma-aminobutyric acid, have a sufficiently long half-life that they can be shipped by air. Some, such as ruthenium derivatives, can be made at the site of examination.

Advances in nuclear medicine instrumentation and radiopharmaceuticals have opened renewed interest in single photon emission CT (SPECT) of the brain. The increasing use of investigative agents in conjunction with PET imaging has stimulated the development of diagnostic radiopharmaceuticals for SPECT; these are routinely available to clinical nuclear medicine laboratories. A technetium 99m (Tc-99m)–based compound—Tc-99m–hexamethylpropyleneamineoxime (Tc-99m-HMPAO)—is widely used. It is sufficiently lipophilic to diffuse readily across the blood–brain barrier and into nerve cells along the blood flow. It remains in brain tissue long enough to permit assessment of the relative distribution of brain blood by SPECT in 1.0- to 1.5-cm coronal, sagittal, and horizontal tomographic slices. SPECT studies are especially useful in patients with cerebrovascular disease (Fig 22–22).