A Sectional Study of the Head

Key Points of Thorough Image Analysis

The basic steps of image evaluation are similar for CT and MRI of the brain. Let us run through the analysis of a head CT to familiarize you with a good standard approach. The first aspect to pay attention to is the gross morphologic appearance of the brain (Fig. 11.1a, b): How wide are the inner and outer cerebral fluid spaces? For example, how well can you see the cerebellar gyri and sulci? Is the temporal horn of the lateral ventricle of normal caliber? Does the appearance of the brain correlate with the age of the patient or does it suggest prior disease, or abnormalities induced by toxins, lifestyle, or drugs? The next point of interest is the appearance of the actual brain parenchyma: Is the gray-white matter differentiation normal? Are sulci and gyri of the same width bilaterally? Are the median structures really in the middle, do the lateral ventricles appear symmetric? Are there any focal parenchymal lesions?

We also evaluate the CSF space surrounding the brain within the skull, which is, of course, restricted to start with: Are the cisterns of normal caliber or obliterated, particularly infratentorially (Fig. 11.1c)? Finally, we analyze the image after intravenous contrast administration: Is there evidence for abnormal contrast enhancement in any focal lesions or the meninges? Enhancement of the pituitary gland, the choroid plexus and the vessels is, of course, normal.

f The normal appearance of the brain, perhaps more than i that of any other organ, changes profoundly from birth to death. Sound knowledge of the age-appropriate normal appearance is crucial to detect significant abnormalities without overcalling normal phenomena.

I Evaluate the Brain Status!

I Evaluate the Brain Status!

Head Gland Calcified Images

Fig. 11.1a This typical head CT of a 25-year-old demonstrates narrow ventricles and well-defined sulci and fissures. Calcification in the pineal gland area (arrow) is normal and develops early in life. b The typical head CT of an 85-year-old, on the other hand, shows significantly wider ventricles and external CSF spaces. In a 25-year-old this pattern would, of course, be highly pathological. One would have to consider, for example, HIV-

encephalopathy, prior meningitis, and alcohol- or drug-induced toxic injury of the brain. Temporary volume loss of the brain during high-dose steroid therapy or forced dehydration could result in this appearance. c The size of the infratentorial cisterns (arrows) tells us something about the pressure in the posterior fossa. If they become obliterated, herniation of the brainstem becomes a possibility.

Fig. 11.1a This typical head CT of a 25-year-old demonstrates narrow ventricles and well-defined sulci and fissures. Calcification in the pineal gland area (arrow) is normal and develops early in life. b The typical head CT of an 85-year-old, on the other hand, shows significantly wider ventricles and external CSF spaces. In a 25-year-old this pattern would, of course, be highly pathological. One would have to consider, for example, HIV-

encephalopathy, prior meningitis, and alcohol- or drug-induced toxic injury of the brain. Temporary volume loss of the brain during high-dose steroid therapy or forced dehydration could result in this appearance. c The size of the infratentorial cisterns (arrows) tells us something about the pressure in the posterior fossa. If they become obliterated, herniation of the brainstem becomes a possibility.

I See an Abnormality—What Do I Do Now?

Did you notice a diffuse abnormality of the brain parenchyma or a circumscribed lesion? In diffuse parenchymal disease changes of brain volume (atrophy, edema) and changes of the meninges (meningitis, subarachnoidal hemorrhage) are the most frequently encountered. If a focal lesion has been detected several aspects have to be evaluated:

• Is the lesion located inside (arising from) the brain parenchyma (intra-axial) or outside of the brain (extraaxial)? In particular, extra-axial meningiomas and sellar tumors have to be differentiated from parenchymal lesions.

• Is the lesion solitary or multifocal? Primary brain tumors tend to be solitary lesions. Multiple lesions suggest metastatic disease, infectious etiology, or a vascular or embolic process. Imaging findings need to be correlated with the clinical findings to arrive at the correct diagnosis since many of these entities may look similar on your study.

• Is the lesion hypodense or hyperdense (CT), or hypo-intense, isointense, or hyperintense (MRI) relative to surrounding parenchyma? Meningiomas tend to be hyperdense on plain CTand isointense in noncontrast MRI.

• Is the lesion homogeneous or heterogeneous? Hemorrhage frequently appears very heterogeneous. Some primary brain tumors can calcify partially and develop necrosis and secondary hemorrhage, which also gives them a very heterogeneous appearance.

• Is the lesion surrounded by edema or is there no significant associated surrounding parenchymal reaction? Pronounced surrounding edema indicates rapid growth and is often seen in metastases and high-grade gliomas, while the absence of edema generally indicates a more benign biological behavior. • Does the lesion accumulate contrast medium and, if so, centrally or peripherally? Loss of the blood-brain barrier is always pathological. Ringlike enhancement, for instance, may be indicative of an abscess or a highly malignant glioblastoma.

Are You Ready for Your First Case?

Paul and Giufeng have been assigned to neuroradiology for a week. They follow all the radiological rounds for the neurologists and neurosurgeons and—if the neurointerven-tionalist has his friendly day—some of those really sophisticated neuroradiological interventions. Gregory is quite pleased with Giufeng's lively interest in his favorite field, and Paul plows through his neuroanatomy book every evening to impress Giufeng and hold his own in the face of Greg's pestering attacks.

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