Positioning and Radiographic Anatomy of the Skull

The importance of plain radiography of the skull has diminished, however, still play a significant role in certain skeletal conditions and, to a limited extent, in trauma, e.g. when a depressed or penetrating injury is suspected. In order to produce high-quality images of the cranium and minimize risk for the patient, the radiographer must have a good understanding of the relevant anatomy, positioning landmarks and equipment used for imaging.

RADIOGRAPHIC ANATOMY Skull As with other body parts, radiography of the skull requires a good understanding of all related anatomy. The anatomy of the skull is very complex, and specific attention to detail is required of the technologist. The skull, or bony skeleton of the head, rests on the superior end of the vertebral column and is divided into two main sets of bones—the 8 cranial bones and the 14 facial bones. 

CRANIAL BONES (8) The eight bones of the cranium are divided into the calvaria (skullcap) and the floor. Each of these two areas primarily consists of four bones: Calvaria (Skullcap) 1. Frontal 2. Right parietal 3. Left parietal 4. Occipital Floor 5. Right temporal 6. Left temporal 7. Sphenoid 8. Ethmoid

Landmarks: Outer canthus of the eye: the point where the upper and lower eyelids meet laterally. Infra-orbital margin/point: the inferior rim of the orbit, with the point being located at its lowest point. Nasion: the articulation between the nasal and frontal bones. Glabella: a bony prominence found on the frontal bone immediately superior to the nasion. Vertex: the highest point of the skull in the median sagittal plane. External occipital protuberance (inion): a bony prominence found on the occipital bone, usually coincident with the median sagittal plane. External auditory meatus: the opening within the ear that leads into the external auditory canal.

Lines Inter-orbital (inter-pupillary) line: joins the centre of the two orbits or the centre of the two pupils when the eyes are looking straight forward. Infra-orbital line: joints the two infra-orbital points. Anthropological baseline: passes from the infra-orbital point to the upper border of the external auditory meatus (also known as the Frankfurter line). Orbito-meatal base line (radiographic baseline): extends from the outer canthus of the eye to the centre of the external auditory meatus. This line is angled approximately 10 degrees to the anthropological baseline.

Planes Median sagittal plane: divides the skull into right and left halves. Landmarks on this plane are the nasion anteriorly and the external occipital protuberance (inion) posteriorly. Coronal planes: these are at right-angles to the median sagittal plane and divide the head into anterior and posterior parts. Anthropological plane: a horizontal plane containing the two anthropological baselines and the infra-orbital line. It is an example of an axial plane. Axial planes are parallel with this plane. Auricular plane:perpendicular to the anthropological plane. Passes through the centre of the two external auditory meatuses. It is an example of a coronal plane. The median sagittal, anthropological and coronal planes are mutually at right-angles.

POSITIONING CONSIDERATIONS Erect versus Recumbent Projections of the skull may be taken with the patient in the recumbent or erect position, depending on the patient's condition. Images can be obtained in the erect position with the use of a standard x-ray table in the vertical position or an upright Bucky) A Bucky is a component of x-ray units that holds the x-ray film cassette (. The erect position allows the patient to be quickly and easily positioned and permits the use of a horizontal beam. A horizontal beam is necessary to visualize any existing air-fluid levels within the cranial or sinus cavities

Patient preparation Before the procedure: all metal objects are removed. Bunches of hair produce artifacts and thus should be untied. If the area of interest includes the mouth, then false teeth containing metal should be removed. clear explanation of movements and film positions to the patient.

Film/Screen combination Hygiene Cranial and facial radiography may require the patient's face to be in direct contact with the technologist's hands and the table/upright Bucky surface. Therefore, it is important that proper hand washing techniques and surface disinfectants be used before and after the examination. Exposure Factors KV mAs FFD Focus Grid Film/Screen combination 80 40 100 Fine Yes Regular

SID)sourse to image distance( The minimum SID with the image receptor in the table or upright Bucky is 40 inches (100 cm). Radiation Protection The best techniques for minimizing radiation exposure to the patient in skull radiography are to (1) use good collimation practices, (2) immobilize the head when necessary, minimizing repeats (3) center properly. Gonadal shielding Generally, with accurate collimation, no detectable contribution to gonadal exposure occurs during radiography of the skull. However, lead shields should be used to reassure the patient

Skull projections Basic views SPECIAL Lateral view Fronto occipital view ( A P view ) Fronto occipital with 30 deg caudad ( TOWNE’S view) Occipito frontal view ( P A view) Occipito frontal with 15 deg caudad ( CALDWELL’S view ) Occipito mental view ( WATER’S view) SPECIAL Submentovertex (SMV) PA axial (Haas method)

Any projection discussed has the following points: Position of the patient & the cassette. Central ray Essential image characteristics

Lateral view LATERAL (ERECT): Position of patient : This position may be used for a cooperative patient. Position of patient : Patient sits facing the bucky and the head is then rotated, such that the median sagittal plane is parallel to bucky and inter orbital line is perpendicular to it. Position the cassette transversely in the erect bucky, such that its upper border is 5 cm above the vertex of the skull

Essential image characteristics : Central ray : midway between the glabella and the external occipital protuberance to a point approx 5 cm superior to the external auditory meatus. Essential image characteristics : The image should contain all cranial bones and the first cervical vertebra. Should superimpose the floor of anterior cranial fossa and posterior cranial fossa. The sella turcica and clinoid processes should also be superimposed.

Lateral – supine Position of patient and cassette: • The patient lies supine, with the head raised and immobilized to include occipit in image. • The head is adjusted, such that the median sagittal plane is perpendicular to the table/trolley and the inter-orbital line is perpendicular to the cassette. • Support the grid cassette vertically against the lateral aspect of the head parallel to the median sagittal plane, with its long edge 5 cm above the vertex of the skull. Radiological considerations: This projection is performed as part of (ATLS) Skull-base FRACTURE very difficult to detect.

Fronto occipital view Positioning : Patient lies supine on a bucky table. Head is adjusted to bring the median sagittal plane at right angles to the film. The external auditory meatuses are equidistance from the cassette The orbito meatal baseline should be perpendicular to the cassette

Central ray : Central ray is directed perpendicular to the cassette along the medial sagittal plane and throw nasion. The field should be set to include the vertex of the skull superiorly and base of the occipital bone inferiorly.

Towne’s view fronto occipital 30 deg caudad Positioning same as AP view Certral ray : Its angled caudally so it makes 30 deg to the orbito meatal plane Centre in the midline such that the beam passes midway between external auditory meatuses. This is a point approx 4 cm above the glabella

Essential image characteristics the sella turcica is projected with in the foramen magnum. Include all the occipital bone and posterior parts of parietal bone, and the lambdoidal suture should be visualized clearly.

Occipito frontal view Central ray : Positioning of patient : Patient is seated facing the erect bucky, so that the median sagittal plane is with the midline of the bucky and perpendicular to it. Neck is flexed to bring orbito meatal line perpendicular to the bucky, this can be achieved by ensuring the nose and forehead are in contact with the bucky. Central ray : Ray is directed perpendicular to the bucky along the median sagittal plane and at the level of nasion. Image should include the vertex superiorly and base of occipital bone inferiorly

Caudal angulation : OF 0 deg : the petrous ridges completely superimposed with orbit OF 10 deg : the petrous ridges appears in the middle third of the orbit OF 20 deg : the petrous ridges appears just below the inferior orbital margin.

Caldwell’s view occipito frontal with 15 deg caudad Positioning same as occipito frontal view Central ray : Ray is directed perpendicular to the bucky alone the median sagittal plane. The tube is rotated 15 deg caudal to the orbito meatal baseline

Occipito mental view WATER’S VIEW Positioning of patient : The patient is made to sit facing the Cassette. Head is adjusted to bring orbito meatal line to 45 deg to the cassette The patient’s nose and chin are placed in contact with the midline of cassette. The patient should open the mouth as wide as possible before exposure.

Central ray : The ray should is perpendicular to the median sagittal plan. The ray should be centre to pass throw the base of nose.

Special skull projection PA axial (Haas method) Submentovertex (SMV)

PA AXIAL PROJECTION: Haas Method Pathology Demonstrated Occipital bone, petrous pyramids, and foramen magnum, with dorsum sellae and posterior clinoids in its shadow

Part Position • Rest patient's nose and forehead against the table/Bucky surface. • Flex neck, bringing OML perpendicular to IR. • Align midsagittal plane to CR and to the midline of the grid or table/Bucky surface. • Ensure that no rotation or tilt exists (midsagittal plane perpendicular to IR).

Central Ray • Angle CR 25° cephalad to OML. • Center CR to midsagittal plane to pass through level of EAMs and exit 1½ inches (4 cm) superior to the nasion. • Center image receptor to projected CR. • Minimum SID is 40 inches (100 cm) Essential image characteratics : • Occipital bone, petrous pyramids, and foramen magnum are shown, with the dorsum sellae and posterior clinoids visualized in the shadow of the foramen magnum.

Submento vertical view Positioning of patient : Patient neck is hyperextended to bring the vertex in contact with the cassette. The median sagittal plane should be right angle to the cassette. The orbito meatal plane should be near as possible and parallel to the cassette.

Essential image characteratics : Central ray : Central ray is perpendicular to orbito meatal line. Center 1½ inch (4 cm) inferior to the mandibular symphysis Essential image characteratics : Should show the angles of mandible clear of the petrous portion of temporal bone. The formina of the middle cranial fossa should be seen symmetrically either side of the midline.