For many decades, the central nervous system (CNS) of living individuals could be examined only indirectly, for example by using x-rays to study changes in the bones surrounding the CNS or the blood vessels around or within it. In addition, these imaging studies involved projecting all the x-ray density under investigation in the head (a three-dimensional structure) onto a two-dimensional sheet of film. As a result, the images of structures actually separated in space (e.g., the middle cerebral and anterior cerebral branches in Fig. 9.17A ) are superimposed on each other in these studies.

The past 40 years have seen revolutionary changes in clinical imaging, partly a result of the use of computers to reconstruct two-dimensional “slices” at various levels of a patient’s head (i.e., tomography ) and partly a result of the ability to construct images based on parameters other than x-ray density.

The most commonly used clinical imaging techniques at present are x-ray computed tomography ( CT ) and magnetic resonance imaging ( MRI ). CT provides images based on x-ray density, so structures that attenuate x-rays, such as bone, appear light; areas filled with air or cerebrospinal fluid, which do not attenuate x-rays as much, appear much darker. Appropriate techniques can accentuate brain, bone, or blood ( Figs. 9.1 to 9.3 ). MRI ( Fig. 9.4 ), in contrast, provides images based on chemical concentrations (most commonly emphasizing the concentration of free water). This chapter provides a series of examples of the use of CT and MRI to demonstrate normal anatomy in clinical imaging. In addition, although traditional angiographic techniques are no longer used very often, they still yield the most highly detailed images of the cerebral vasculature, so examples of angiograms are also provided.

An axial (approximately horizontal) CT scan, demonstrating the relative x-ray densities of some cranial structures. The range of x-ray densities in the head, from bone through brain to air, is much greater than the human visual system can discriminate as a series of gray shades. In this case, the computer was adjusted so that the gray scale was applied to the x-ray density of brain and cerebrospinal fluid. Hence, bone is uniformly white, and fluid is black (as air would be); gray matter is very slightly more x-ray dense than white matter, so the two can be differentiated from each other.

(Provided by Dr. Raymond F. Carmody.)

An axial (approximately horizontal) CT scan, adjusted so that the gray scale is distributed over the entire range of cranial and intracranial x-ray densities. Air is black, but little soft-tissue or fluid detail can be seen. However, the details and relative densities of different bones (e.g., sphenoid versus temporal) are apparent.

(Provided by Dr. Raymond F. Carmody.)

Blood flowing through arteries and veins can be seen more easily if an iodinated, x-ray dense contrast agent is injected intravenously before the CT scan.

(Provided by Dr. Raymond F. Carmody.)

Parasagittal MRI. The concentration of water ranges from very low (air, bone) to intermediate levels (brain, muscle) to very high (cerebrospinal fluid [ CSF ], perilymph), allowing all of these to be differentiated. The lack of signal from certain spaces that contain a lot of water—blood vessels—is explained later.

(Provided by Dr. Raymond F. Carmody.)

(A–D) A series of seven CT images at different levels of a normal brain. (Provided by Dr. Raymond F. Carmody.)

(A) The planes of the “slices” shown in B–H .

(B) Foramen magnum.

(C) Pituitary gland and fourth ventricle. (The streaks cutting across the cerebellum and pons are artifacts resulting from the presence of dense bone nearby. The density in each frontal lobe results from nearby bone in the orbital roofs.)

(D) Base of the diencephalon.

(E–H) Normal CT images.

(E) Inferior thalamus.

(F) Midthalamus.

(G) Just above the thalamus.

(H) Above the corpus callosum.

(A–D) A series of seven CT images from the same patient shown in Fig. 9.5 . In this case an iodinated intravenous contrast agent was administered before the CT study, making blood vessels visible. (Provided by Dr. Raymond F. Carmody.)

(A) The planes of the “slices” shown in B–H .

(B) Foramen magnum.

(C) Cavernous sinus. The infundibulum and choroid plexus appear x-ray dense because the blood–brain barrier is lacking at these sites, allowing the contrast agent to leak out.

(D) Circle of Willis.

(E–H) Contrasted CT images.

(E) Near the bottom of the thalamus.

(F) Interventricular foramen.

(G) Just above the thalamus.

(H) Above the corpus callosum. The falx (dura mater) is outside the blood–brain barrier, so contrast agent leaks out here.

(A–F) The use of CT to demonstrate intracranial pathology. By convention, all axial scans (both CT and MRI) are oriented with anterior toward the top of the page and the patient’s left on the right side, as though you were looking up from the patient’s feet. (Provided by Dr. Raymond F. Carmody.)

(A) CT measures x-ray density, so structures and substances more dense than brain stand out and are light. In this 31-year-old woman, blood in a recent left intracerebral hemorrhage (1) is apparent, spreading through subarachnoid space on either side of the falx cerebri (2) and through the lateral ventricle (3) .

(B) Structures and substances less dense than brain also stand out, but are dark. In this case, two old infarcts, one (1) in part of the right middle cerebral artery territory and another (2) in the left posterior cerebral territory, are apparent in this 67-year-old woman.

(C) Over a period of weeks, intracranial blood breaks down and becomes less dense than brain. The chronic left subdural hematoma in this 60-year-old man has re-bled and contains a mixture of old (1) and new (2) blood. Pressure from the hematoma bows the falx cerebri toward the right (3) and squeezes out CSF on the left, so that the subarachnoid space (4 , 6) and lateral ventricle (5) apparent on the right can no longer be seen on the left.

(D) An epidural hematoma (1) in a 1-year-old boy with a head injury. Because dura mater adheres tightly to the skull, these hematomas usually have a characteristic convex shape (versus the long crescent shape of subdural hematomas such as the one in C ). Contused and swollen tissue (2) can also be seen at the site of injury.

(E) The same patient as in (D) but with the CT contrast window set to show bone detail. Now the basal and occipital skull fractures (1 , 2) are visible, but the hematoma is not.

(F) The same patient as in D . Multiple bone-window images such as that in E were combined to make a three-dimensional reconstruction of the skull, showing the occipital fracture in detail.

In T1-weighted images (left) , white matter is lighter than gray matter and cerebrospinal fluid ( CSF ) is dark. Conversely, in T2-weighted images (right) , white matter is darker than gray matter, and CSF is bright and prominent. In both, air and dense bone, which contain relatively few hydrogen nuclei, are dark. The appearance of flowing blood depends on a number of technical parameters, but in many instances (e.g., the T2-weighted image on the right), perturbed nuclei have left the area before the imaging measurement is made, so the blood vessel appears dark, as though no hydrogen nuclei were present.

(Provided by Dr. Raymond F. Carmody.)

Use of diffusion-weighted imaging for early detection of stroke damage. (A) On the day of a stroke, CT images fail to reveal significant damage. (B) On the same day, diffusion-weighted MRI shows areas of edema and restricted diffusion in part of the right middle cerebral artery distribution. (C) Three days later, damage in the edematous areas shows up in CT images.

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