Normal Skull and Variants

Normal anatomy of the skull is shown in Fig. 2.1 . The most common differential problem on plain skull x-rays is distinguishing cranial sutures from vascular grooves and fractures. The main sutures are coronal, sagittal, and lambdoid. A suture also runs in a rainbow shape over the ear. In the adult, sutures are symmetric and very wiggly and have sclerotic (very white) edges. Vascular grooves are usually seen on the lateral view and extend posteriorly and superiorly from just in front of the ear. They do not have sclerotic edges and are not perfectly straight.

Normal Skull.

(A) Lateral, (B) anteroposterior (AP), (C) AP Towne projection, and (D) AP Waters view.

A few common variants are seen on skull x-rays. Hyperostosis frontalis interna is a benign condition of females in which sclerosis, or increased density, is seen in the frontal region and spares the midline ( Fig. 2.2 ). Large, asymmetric, or amorphous focal intracranial calcifications should always raise the suspicion of a benign or malignant neoplasm. Occasionally, areas of lucency (dark areas) are found where the bone is thinned. The most common normal variants that cause this are vascular lakes or biparietal foramen. Asymmetrically round or ill-defined holes should raise the suspicion of metastatic disease ( Fig. 2.3 ).

Hyperostosis Frontalis Interna.

A normal variant, most common in female patients, in which increased density of the skull occurs in the frontal regions. Notice that sparing of the midline is present (arrows).

Multiple Myeloma.

Multiple asymmetric holes in the skull are seen only with metastatic disease. Metastatic lung or breast carcinoma can look exactly the same as this case of multiple myeloma.

Paget disease can affect the bone of the skull. In the early stages, very large lytic, or destroyed, areas may be seen. In later stages, increased density (sclerosis) and marked overgrowth of the bone, causing a cotton wool appearance of the skull, may be seen ( Fig. 2.4 ). Always be aware that both prostate and breast cancer can cause multiple dense metastases in the skull and that both diseases are more common than Paget disease.

Paget Disease.

The fluffy cotton wool densities overlying the skull are caused by bone expansion. Note also that the calvaria is very thick (arrow) . The base of the skull has become softened; the cervical spine and foramen magnum look as though they are pushed up, but in reality the skull is sagging around them.

Normal Anatomy

Box 2.1 gives a methodology to follow or checklist of items to use when examining a computed tomography (CT) scan. Both CT and magnetic resonance imaging (MRI) are capable of displaying anatomic slices in a number of different planes. The identical anatomy of the brain can appear quite different on CT and magnetic resonance (MR) images ( Fig. 2.5 ). The normal anatomy of the brain on CT and MR images is shown in Figs. 2.6 and 2.7 . You should be able to identify some anatomy on these images. There are many very complex imaging sequences used during MRI, depending upon the clinical question or suspected pathology. You are not expected to be familiar with all of these, but you should realize that success in making a diagnosis depends on your indicating the clinical problem accurately so that the radiologist can prescribe the correct imaging sequences.

Axial Images of the Brain on Computed Tomography (CT) and Magnetic Resonance Imaging (MRI).

(A) On a noncontrast CT scan. the skull is easily seen, the brain is varying shades of gray, and cerebrospinal fluid (CSF) is dark. (B) On a T1 MRI scan. the skull is difficult to see, the brain is gray, and the CSF is dark. (C) On a T2 image, the CSF is white.

Normal Axial Images of the Brain at Three Different Levels.

Noncontrast computed tomography (A1, B1, C1), T1 magnetic resonance images (MRI) (A2, B2, C2), and T2 MRI images (A3, B3, C3).

Normal T1 Magnetic Resonance Imaging Anatomy of the Brain in Sagittal (A) and Coronal (B) Projection.

Intracranial Calcifications

Intracranial calcifications can be seen occasionally on a skull x-ray, but they are seen much more often on CT. Intracranial calcifications may be due to many causes. Normal pineal and ependymal calcifications may occur. Scattered calcifications can occur from toxoplasmosis, cysticercosis, tuberous sclerosis ( Fig. 2.8 ), or granulomatous disease. Unilateral calcifications are very worrisome because they can occur in arteriovenous malformations, gliomas, and meningiomas.

Tuberous Sclerosis.

Scattered calcifications are seen about the ventricles in the posterior parietal regions. Other diseases that could show this appearance include intrauterine TORCH ( t oxoplasmosis, o ther agents, r ubella, c ytomegalovirus, h erpes simplex) infections.

ANT , Anterior.

Headache

Headaches are among the most common of human ailments. They can be due to a myriad of causes and should be characterized by location, duration, type of pain, provoking factors, and age and sex of the patient. In the primary care population, less than 0.5% of acute headaches are the result of serious intracranial pathology. Simple headaches, tension headaches, migraine headaches, and cluster headaches do not warrant imaging studies. A good physical examination is essential, including evaluation of blood pressure, urine, eyes (for papilledema), temporal arteries, sinuses, ears, neurologic system, and neck. In a patient with a febrile illness, headache, and stiff neck, a lumbar puncture should be performed. In only a few circumstances is imaging indicated ( Box 2.2 ).

In general, imaging is indicated when a headache is accompanied by neurologic findings, syncope, confusion, seizure, and mental status changes or after major trauma. Sudden onset of the “worst headache of one’s life” (thunderclap headache) should raise the question of subarachnoid hemorrhage. Sudden onset of a unilateral headache with a suspected carotid or vertebral dissection or ipsilateral Horner syndrome should prompt a CT or MR angiogram.

Sinus headaches can usually be differentiated from other causes because they worsen when the patient is leaning forward or when pressure is applied over the affected sinus. Indications for CT use in sinus headaches are presented in Box 2.3 .

Hearing Loss

Hearing loss is characterized as conductive, sensorineural, or mixed. Conductive loss results from pathology of the external or middle ear that prevents sound from reaching the inner ear. Sensorineural loss results from abnormalities of the inner ear, including the cochlea or auditory nerve. CT is the best technique for evaluating conductive loss and the bony structures of the middle ear. Not all patients with conductive loss need a CT scan. Indications include complications of otomastoiditis, preoperative and postoperative evaluation of prosthetic devices, cholesteatoma, and posttraumatic hearing loss. Sensorineural hearing loss may be sudden, fluctuating, or progressive and may also be associated with vertigo. It can be due to viral infection, eardrum rupture, acoustic neuroma, or vascular occlusive disease. Evaluation is best done by MRI, with and without intravenous contrast.

Head Trauma

On skull x-rays, fractures are dark lines that have very sharp edges and tend to be very straight ( Fig. 2.9 ). If a fracture is present over the middle meningeal area, an associated epidural hematoma may be found. If a depressed fracture is present, the lucent fracture lines can be stellate or semicircular ( Fig. 2.10 ). In either of these cases, substantial brain injury may be present and a CT scan, including bone windows, is indicated.

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