Imaging of the Central Nervous System

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74 Imaging of the Central Nervous System

Andrew D. Perron, Carl A. Germann

• The emergency physician needs to be able to accurately interpret and act on certain findings on computed tomography (CT) because many disease processes require immediate action.

• Even with a brief educational intervention, emergency physicians can significantly improve their ability to interpret cranial CT scans.

• Use the mnemonic “blood can be very bad” (where blood = blood, can = cisterns, be = brain, very = ventricles, and bad = bone) to quickly and thoroughly review a cranial CT scan for pathology.

• Magnetic resonance imaging offers excellent anatomic resolution and provides greater discrimination of various soft tissues than CT does.

Perspective

Imaging of the central nervous system (CNS) has assumed a critical role in the practice of emergency medicine for the evaluation of intracranial emergencies, both traumatic and atraumatic. A number of studies have revealed a deficiency in the ability of emergency physicians (EPs) to interpret head computed tomography (CT) scans.¹^–⁶ However, a number of these same studies also show that with even brief educational effort, EPs can gain considerable proficiency in cranial CT scan interpretation.^2,³ This is important because in many situations the EP must interpret and act on head CT results in real time without assistance from other specialists such as neurologists, radiologists, or neuroradiologists.^7,⁸ Advantages of using these technologies for diagnosing CNS pathology in the emergency department (ED) include widespread availability at many institutions, speed of imaging, patient accessibility, and sensitivity in detecting many emergency pathologic processes.

Computed Tomography

Basic Principles

The fundamental principle behind radiography is the following statement: x-rays are absorbed to different degrees by different tissues. Dense tissues such as bone absorb the most x-rays and hence allow the fewest to pass through the body part being studied to reach the film or detector. Conversely, tissues with low density (e.g., air, fat) absorb almost none of the x-rays, and most pass through to the film or detector. Conventional radiographs are two-dimensional images of three-dimensional structures; they rely on a summation of the density of tissues penetrated by x-rays as they pass through the body.

With CT scanning, an x-ray source and detector, situated 180 degrees across from each other, move 360 degrees around the patient while continuously detecting and sending information about the attenuation of x-rays as they pass through the body. Very thin x-ray beams are used to minimize the degree of scatter or blurring that limits conventional radiographs. In CT a computer manipulates and integrates the acquired data and assigns numeric values on the basis of subtle differences in x-ray attenuation. Based on these values, a gray-scale axial image is generated that can distinguish between objects with even small differences in density.

Attenuation Coefficient

The tissue contained within each image unit (called a pixel) absorbs a certain proportion of the x-rays that pass through it (e.g., bone absorbs a lot, air almost none). The ability to block x-rays as they pass through a substance is known as attenuation. For a given body tissue, the amount of attenuation is relatively constant and is known as that tissue’s attenuation coefficient. In CT, these attenuation coefficients are mapped to an arbitrary scale of between −1000 Hounsfield units (HU) (air) and +1000 HU (bone) (Box 74.1). This scale is the Hounsfield scale (in honor of Sir Jeffrey Hounsfield, who received a Nobel Prize for his pioneering work with this technology).

Windowing

Windowing allows the CT scan reader to focus on certain tissues within a CT scan that fall within set parameters. Tissues of interest can be assigned the full range of blacks and whites rather than a narrow portion of the gray scale. With this technique, subtle differences in tissue densities can be maximized. The image displayed depends on both centering of the viewing window and the width of the window. Most CT imaging includes windows that are optimized for brain, blood, and bone (Fig. 74.1).

Fig. 74.1 CT scan windowing.

A, Brain. B, Blood. C, Bone.

Artifact

CT of the brain is subject to a few predictable artifactual effects that can potentially inhibit the ability to accurately interpret the images. Besides motion and metal artifact (self-explanatory), the two most common effects are called beam hardening and volume averaging. It is important to understand these artifactual effects and to be able to identify them because they can mimic pathology, as well as obscure actual significant findings.

Beam hardening is a phenomenon that causes an abnormal signal when a relatively small amount of hypodense brain tissue is immediately adjacent to dense bone. The posterior fossa, where extremely dense bone surrounds the brain, is particularly subject to this phenomenon. It appears as either linear hyperdensities or hypodensities that can partially obscure the brainstem and cerebellum. Although beam hardening can be reduced with appropriate filtering, it cannot be eliminated.

Volume averaging (also called partial volume artifact) arises when the imaged area contains different types of tissue (e.g., bone and brain). For that particular image unit, the CT pixel produced will represent an average density for all the structures contained within it. In the instance of brain and bone, an intermediate density will be represented that may have the appearance of blood. As with beam hardening, certain techniques can minimize this type of artifact (e.g., thinner slices, computer algorithms), but it cannot be eliminated, particularly in the posterior fossa.

Normal Neuroanatomy As Seen On Head Computed Tomography

As with radiologic interpretation of any body part, working knowledge of normal anatomic structures and location is fundamental to the clinician’s ability to detect pathologic variants. Paramount in head CT interpretation is familiarity with the various CNS structures, ranging from parenchymal areas, such as the basal ganglia, to vasculature structures, cisterns, and ventricles. Additionally, knowing the neurologic function of these regions of the brain helps when correlating CT results with findings on physical examination.

Although detailed knowledge of cranial neuroanatomy and its CT appearance is clearly in the realm of the neuroradiologist, familiarity with a relatively few structures, regions, and expected findings allows sufficient interpretation of most head CT scans by the EP. Figures 74.2 through 74.5 demonstrate key structures of a normal head CT scan.

Fig. 74.2 Head CT—normal anatomy.

A, Posterior fossa. B, Low cerebellum.

Fig. 74.3 Head CT—normal anatomy.

A, High pons. B, Cerebral peduncles.

Fig. 74.4 Head CT—normal anatomy.

A, High midbrain level. B, Basal ganglia region.

Fig. 74.5 Head CT—normal anatomy.

A, Lateral ventricles. B, Upper cortex.

Identifying Central Nervous System Pathology On Cranial Computed Tomography

As long as one is systematic in the search for pathology, any number of techniques can be used when reviewing head CT images. Some recommend a “center-out” technique in which the examiner starts from the middle of the brain and works outward. Others advocate a “problem-oriented” approach in which the clinical history directs the examiner to a particular portion of the scan. In the author’s experience, both these approaches are of limited utility to clinicians who do not frequently review scans. A preferred method, one that has been demonstrated to work in the ED, is to use the mnemonic “blood can be very bad” (Box 74.2).² In this mnemonic, the first letter of each word prompts the clinician to search a certain portion of the CT scan for pathology.

Box 74.2 The “Blood Can Be Very Bad” Mnemonic *

Blood—Acute hemorrhage appears hyperdense (bright white) on computed tomography. The globin molecule is relatively dense and effectively absorbs x-ray beams. As the blood ages, the globin molecule breaks down and loses its hyperdense appearance, beginning at the periphery. Precise localization of the blood is as important as identification of its presence.

Cisterns—Cerebrospinal fluid collections in the brain. The four key cisterns must be examined for blood, asymmetry, and effacement (representing increased intracranial pressure):

• Circummesencephalic—Cerebrospinal fluid ring around the midbrain; it is the first to be effaced with increased intracranial pressure

• Suprasellar (star-shaped)—Location of the circle of Willis; this is a frequent site of aneurysmal subarachnoid hemorrhage

• Quadrigeminal—W-shaped cistern at the top of the midbrain; it is effaced early by rostrocaudal herniation

• Sylvian—Between the temporal and frontal lobes; this is the site of traumatic and distal midcerebral aneurysm and subarachnoid hemorrhage

Brain—Examine for:

• Symmetry—The sulcal pattern (gyri) is well differentiated in adults and symmetric side to side.

• Gray-white differentiation—The earliest sign of cerebrovascular aneurysm is loss of gray-white differentiation; metastatic lesions are often found at the gray-white border.

• Shift—The falx should be midline, with the ventricles evenly spaced to the sides; it can also have a rostrocaudal shift, as evidenced by loss of cisternal space; unilateral effacement of the sulci signals increased pressure in one compartment; bilateral effacement signals globally increased pressure.

• Hyperdensity/hypodensity—Increased density with blood, calcification, intravenous contrast media; decreased density with air or gas (pneumocephalus), fat, ischemia (cerebrovascular aneurysm), tumor.

Ventricles—Pathologic processes cause dilation (hydrocephalus) or compression or shifting; hydrocephalus is usually first evident with dilation of the temporal horns (normally small and slitlike); the examiner must assess the “whole picture” to determine whether the ventricles are enlarged because of lack of brain tissue or increased cerebrospinal fluid pressure.

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