X-ray anatomy of the skull and brain

The main and proven method of radiological examination of the skull is survey radiography (X-ray of the skull). It is usually performed in two standard projections - direct and lateral. In addition to them, axial, semi-axial and targeted radiographs are sometimes required. Survey and targeted images are used to establish the position, size, shape, contours and structure of all the bones of the skull.

On survey radiographs in the direct and lateral projections, the cranial and facial skull are clearly outlined. The thickness of the bones of the vault varies from 0.4 to 1 cm. In the area of the temporal fossa it is the smallest, which is manifested as enlightenment on the lateral radiograph. At the same time, in the area of the parietal and occipital tubercles the bones are thicker. Against the background of the fine-mesh structure of the bones of the vault, various enlightenments are noticeable. These include tree-like branching grooves of the meningeal arteries, wide canals and stellate branches of the diploic veins, small rounded or crescent-shaped enlightenments of the pachion fossae and indistinct outlines of digital impressions (mainly in the frontal part of the skull). Naturally, the air-containing sinuses (frontal, ethmoid, paranasal, sinuses of the sphenoid bone) and pneumatized cells of the temporal bones demonstratively appear on the images.

The base of the skull is clearly visible on lateral and axial images. Three cranial fossae are defined on its inner surface: anterior, middle and posterior. The border between the anterior and middle fossae is the posterior edges of the lesser wings of the sphenoid bone, and between the middle and posterior - the upper edges of the pyramids of the temporal bones and the back of the sella turcica. The sella turcica is a bony receptacle for the pituitary gland. It is clearly visible on a lateral image of the skull, as well as on targeted images and tomograms. The images are used to assess the shape of the sella, the condition of its anterior wall, bottom and back, its sagittal and vertical dimensions.

Due to the complex anatomical structure of the skull, X-ray images show a rather mixed picture: images of individual bones and their parts are superimposed on each other. In this regard, linear tomography is sometimes used to obtain an isolated image of the required section of a particular bone. If necessary, CT is performed. This is especially true for the bones of the base of the skull and the facial skeleton.

The brain and its membranes weakly absorb X-rays and do not produce a discernible shadow on normal images. Only calcium deposits, which under normal conditions are sometimes found in the pineal gland, vascular plexuses of the lateral ventricles and the falx, are reflected.

Radiation anatomy of the brain

The main methods of intravital study of the structure of the brain are currently CT and especially MRI.

Indications for their implementation are determined jointly by the attending physicians - a neurologist, neurosurgeon, psychiatrist, oncologist, ophthalmologist and a specialist in the field of radiation diagnostics.

The most common indications for radiological examination of the brain are the presence of signs of cerebrovascular accident, increased intracranial pressure, general cerebral and focal neurological symptoms, and impairments of vision, hearing, speech, and memory.

Computer tomograms of the head are performed with the patient in a horizontal position, isolating images of individual layers of the skull and brain. No special preparation for the examination is required. A complete examination of the head consists of 12-17 slices (depending on the thickness of the layer being isolated). The level of the slice can be judged by the configuration of the ventricles of the brain; they are usually visible on tomograms. Often, in CT of the brain, a method of enhancement is used by intravenous administration of a water-soluble contrast agent.

Computer and magnetic resonance tomograms clearly distinguish the cerebral hemispheres, brain stem and cerebellum. It is possible to differentiate gray and white matter, contours of convolutions and furrows, shadows of large vessels, cerebrospinal fluid spaces. Both CT and MRI, along with layered imaging, can reconstruct a three-dimensional display and anatomical orientation in all structures of the skull and brain. Computer processing allows obtaining an enlarged image of the area of interest to the doctor.

When studying brain structures, MRI has some advantages over CT. Firstly, MR tomograms more clearly distinguish structural elements of the brain, differentiate white and gray matter, all stem structures more clearly. The quality of magnetic resonance tomograms is not affected by the shielding effect of the skull bones, which worsens the image quality in CT. Secondly, MRI can be performed in different projections and obtain not only axial, as in CT, but also frontal, sagittal and oblique layers. Thirdly, this study is not associated with radiation exposure. A special advantage of MRI is the ability to display vessels, in particular vessels of the neck and base of the brain, and with gadolinium contrast - and small vascular branches.

Ultrasound scanning can also be used to examine the brain, but only in early childhood, when the fontanelle is preserved. It is above the fontanelle membrane that the ultrasound detector is placed. In adults, one-dimensional echography (echoencephalography) is mainly performed to determine the location of the midline structures of the brain, which is necessary for recognizing volumetric processes in the brain.

The brain receives blood from two systems: two internal carotid and two vertebral arteries. Large blood vessels are visible on CT scans obtained under conditions of intravenous artificial contrast. In recent years, MR angiography has rapidly developed and gained general recognition. Its advantages are non-invasiveness, ease of implementation, and the absence of X-ray radiation.

However, a detailed study of the vascular system of the brain is only possible with angiography, and preference is always given to digital image registration, i.e. performing DSA. Vascular catheterization is usually performed through the femoral artery, then the catheter is inserted into the vessel under study under fluoroscopy control and a contrast agent is injected into it. When it is injected into the external carotid artery, its branches are displayed on the angiograms - the superficial temporal, middle meningeal, etc. If the contrast agent is injected into the common carotid artery, then the vessels of the brain are differentiated in the images along with the branches of the external carotid artery. Most often, they resort to carotid angiography - the contrast agent is injected into the internal carotid artery. In these cases, only the vessels of the brain are visible on the images. At first, the shadow of the arteries appears, later - the superficial veins of the brain and, finally, the deep veins of the brain and venous sinuses of the dura mater, i.e. sinuses. To examine the vertebral artery system, a contrast agent is injected directly into this vessel. This examination is called vertebral angiography.

Angiography of the brain is usually performed after CT or MRI. Indications for angiography include vascular lesions (stroke, subarachnoid hemorrhage, aneurysms, lesions of the extracranial part of the main vessels of the neck). Angiography is also performed when it is necessary to perform intravascular therapeutic interventions - angioplasty and embolism. Contraindications include endocarditis and myocarditis, decompensation of the heart, liver, kidneys, very high arterial hypertension, shock.

Brain examination by radionuclide diagnostic methods is limited mainly to obtaining functional data. It is generally accepted that the value of cerebral blood flow is proportional to the metabolic activity of the brain, therefore, by using the appropriate radiopharmaceutical, for example, pertechnetate, it is possible to identify areas of hypo- and hyperfunction. Such studies are carried out to localize epileptic foci, to detect ischemia in patients with dementia, and to study a number of physiological functions of the brain. In addition to scintigraphy, single-photon emission tomography and especially positron emission tomography are successfully used as a method of radionuclide visualization. The latter, for technical and economic reasons, as noted earlier, can only be performed in large scientific centers.

Radiation methods are indispensable in the study of blood flow in the brain. They are used to establish the position, caliber and outlines of the cranial branches of the aortic arch, the external and internal carotid arteries, the vertebral arteries, their extra- and intracerebral branches, veins and sinuses of the brain. Radiation methods allow recording the direction, linear and volumetric speed of blood flow in all vessels and identifying pathological changes in both the structure and functioning of the vascular network.

The most accessible and very effective method of studying cerebral blood flow is ultrasound examination. Naturally, we are talking only about ultrasound examination of extracranial vessels, i.e. neck vessels. It is indicated in dispensary and clinical examination at the very first stage. The examination is not burdensome for the patient, is not accompanied by complications, and has no contraindications.

Ultrasound examination is performed using both sonography and, mainly, Dopplerography - one-dimensional and two-dimensional (color Doppler mapping). No special preparation of the patient is required. The procedure is usually performed with the patient lying horizontally on his back. Guided by anatomical landmarks and palpation results, the location of the vessel being examined is determined and the body surface above it is covered with gel or vaseline oil. The sensor is installed above the artery without squeezing it. Then it is gradually and slowly moved along the artery, examining the image of the vessel on the screen. The study is carried out in real time with simultaneous recording of the direction and speed of blood flow. Computer processing ensures that a color image of the vessels, Dopplerogram and corresponding digital indicators are obtained on paper. The study is necessarily carried out on both sides.