Skull and Cranial Bones Radiographic Anatomy and Positioning

Comprehensive practice flashcards covering skull and cranial bone osteology, topography, classifications, landmarks, and radiographic positioning techniques.

How many bones compose the cranium and the facial bone divisions of the skull?

The cranium is composed of $8$ bones, and the facial skeleton is composed of $14$ bones, for a total of $22$ bones in the skull.

Which four bones form the skullcap of the cranium (calvaria)?

The frontal bone, right parietal bone, left parietal bone, and occipital bone.

Which four bones form the floor of the cranium?

The right temporal bone, left temporal bone, sphenoid bone, and ethmoid bone.

Identify the cranial bones labeled A through F on this lateral view of the cranium.

A: Frontal bone; B: Parietal bone; C: Occipital bone; D: Temporal bone; E: Sphenoid bone (greater wing); F: Ethmoid bone.

Which four cranial bones articulate with the frontal bone?

The right parietal bone, left parietal bone, sphenoid bone, and ethmoid bone.

What key anatomical landmarks are found on the orbital (horizontal) and squamous portions of the frontal bone?

The frontal tuberosities (eminences), superciliary ridges (arches), supraorbital grooves (SOG), supraorbital margins (SOM), supraorbital notches (foramina), glabella, orbital plates, ethmoid notch, and nasal (frontal) spine.

How many cranial bones articulate with each parietal bone, and what anatomical landmark marks the widest portion of the skull?

Each parietal bone articulates with $5$ cranial bones. The widest portion of the entire skull is located between the parietal tubercles (eminences).

How many bones articulate with the occipital bone, and which occipital structure forms the atlanto-occipital joint?

The occipital bone articulates with $6$ bones. Its lateral condylar portions (occipital condyles) articulate with the superior articular facets of the atlas ($C1$) to form the atlanto-occipital joints.

What is the prominent midline prominence on the external surface of the squamous occipital bone?

The external occipital protuberance, also termed the inion.

What are the three primary portions of the temporal bone?

(1) The squamous portion, (2) the mastoid portion (including the mastoid process/tip), and (3) the petrous portion (also termed petrous pyramid, pars petrosa, or petromastoid portion).

How many cranial bones articulate with each temporal bone, and what primary sensory organs does it house?

Each temporal bone articulates with $3$ cranial bones. Its petrous portion houses the organs of hearing and equilibrium.

How many cranial bones articulate with the sphenoid bone?

All other $7$ cranial bones articulate with the sphenoid bone, making it the central anchor of the cranium.

What structures compose and surround the sella turcica of the sphenoid bone?

The anterior clinoid processes, posterior clinoid processes, dorsum sellae, and clivus.

What are the three small paired foramina found in the greater wings of the sphenoid bone?

(1) Foramen rotundum, (2) foramen ovale, and (3) foramen spinosum.

How many cranial bones articulate with the ethmoid bone, and what are its major structural parts?

The ethmoid bone articulates with $2$ cranial bones (the frontal and sphenoid). Its major parts are the crista galli, cribriform plate, perpendicular plate, and lateral labyrinths (masses) with the superior and middle nasal conchae.

What are the four primary cranial sutures and the bones they separate?

Coronal suture (separates frontal bone from parietal bones), Sagittal suture (separates right and left parietal bones), Lambdoidal suture (separates parietal bones from occipital bone), and Squamosal sutures (separate parietal bones from temporal bones).

What are the four major cranial suture junction points in the adult skull?

Bregma (junction of coronal and sagittal sutures), Lambda (junction of sagittal and lambdoidal sutures), Pterion (junction of frontal, parietal, sphenoid, and temporal bones), and Asterion (junction of occipital, parietal, and temporal mastoid portions).

How many fontanels are present in the infant cranium, and what are their names?

There are $6$ fontanels: the anterior fontanel, posterior fontanel, right and left sphenoid fontanels, and right and left mastoid fontanels.

Where are sutural (Wormian) bones most commonly located?

Within the lambdoidal suture.

What are the three primary anatomical divisions of the ear?

(1) External ear (auricle/pinna, tragus, external acoustic meatus), (2) Middle ear (tympanic membrane, auditory ossicles, tympanic cavity proper, epitympanic recess), and (3) Internal ear (osseous and membranous labyrinths).

What are the three auditory ossicles, their common names, and their function?

Malleus (hammer), Incus (anvil), and Stapes (stirrup). They transmit sound vibrations from the tympanic membrane to the oval window.

With which two anatomical structures does the middle ear communicate?

The nasopharynx (via the Eustachian or auditory tube) and the mastoid air cells/antrum (via the aditus).

What are the functions and clinical significance of the Eustachian tube?

It equalizes air pressure between the middle ear cavity and the outside atmosphere, and it acts as a pathway for disease organisms to spread from the nasopharynx to the middle ear.

Identify the three semicircular canals of the internal ear shown in this diagram.

Superior semicircular canal, lateral semicircular canal, and posterior semicircular canal.

What are the characteristics of a mesocephalic skull?

A skull of average shape whose width is between $75\%$ to $80\%$ of its length, with the petrous ridges forming an angle of $47^\circ$ to the midsagittal plane (MSP).

What are the characteristics of a brachycephalic skull?

A short, broad skull whose width is $80\%$ or more of its length, with the petrous ridges forming an angle of $\pm 54^\circ$ ($>47^\circ$) to the MSP.

What are the characteristics of a dolichocephalic skull?

A long, narrow skull whose width is less than $75\%$ of its length, with the petrous ridges forming an angle of $\pm 40^\circ$ ($<47^\circ$) to the MSP.

Define the positioning landmarks: nasion, acanthion, and gonion.

Nasion: depression at the bridge of the nose; Acanthion: midline point at the junction of the upper lip and the nasal septum; Gonion: the angle of the mandible.

What is the angular difference between the orbitomeatal line (OML) and the infraorbitomeatal line (IOML)?

There is a $7^\circ$ to $8^\circ$ difference between the OML and IOML.

What are the five common positioning errors encountered in skull radiography?

(1) Rotation, (2) Tilt, (3) Excessive flexion, (4) Excessive extension, and (5) Incorrect CR angle.

How are rotation and tilt differentiated on a lateral skull radiograph?

Rotation causes anterior and posterior separation of symmetric bilateral structures (e.g., mandibular rami), whereas tilt causes superior and inferior separation of symmetric structures (e.g., orbital plates).

What are the central ray (CR) angles and centering point for an AP axial (Towne method) projection of the cranium?

Angle the CR $30^\circ$ caudad to the OML, or $37^\circ$ caudad to the IOML. Center the CR to the midsagittal plane at $2\frac{1}{2}\,\text{inches}\,(6.5\,\text{cm})$ superior to the glabella.

What is the primary evaluation criterion for an AP axial (Towne method) projection of the cranium?

The dorsum sellae and posterior clinoid processes should be clearly visualized projected within the foramen magnum.

What are the positioning requirements and centering point for a routine lateral skull radiograph?

The midsagittal plane (MSP) must be parallel to the IR, the interpupillary line (IPL) perpendicular to the IR, and the CR directed perpendicular to the IR centering $2\,\text{inches}\,(5\,\text{cm})$ superior to the EAM.

Where should the petrous ridges be projected on a $15^\circ$ PA Caldwell skull radiograph?

Over the lower $\frac{1}{3}$ of the orbits (with the CR angled $15^\circ$ caudad exiting at the nasion).

Where are the petrous ridges projected on a $0^\circ$ PA skull radiograph?

The petrous ridges completely fill the orbits, superimposing the superior orbital margins (SOM).

What are the positioning alignment and evaluation criteria for the submentovertex (SMV) projection of the cranium?

The IOML must be parallel to the IR (perpendicular to the CR). The CR centers $1\frac{1}{2}\,\text{inches}\,(4\,\text{cm})$ inferior to the mandibular symphysis. The mandibular condyles must be projected anterior to the petrous pyramids.

What positioning error occurs on an SMV skull projection if the mandibular condyles are projected into the petrous pyramids?

Insufficient extension of the patient's head/neck.

What are the central ray angulation and centering point for the PA axial (Haas method) projection of the skull?

The OML is perpendicular to the IR. The CR is angled $25^\circ$ cephalad and exits $1\frac{1}{2}\,\text{inches}\,(4\,\text{cm})$ superior to the nasion.

How does the PA axial (Haas method) image compare to the AP axial (Towne method) projection?

Both project the dorsum sellae into the foramen magnum, but the Haas method results in a magnified dorsum sellae and magnification of the occipital bone.

What is the central ray centering point for a dedicated lateral sella turcica radiograph?

Center the CR $\frac{3}{4}\,\text{inch}\,(2\,\text{cm})$ anterior and $\frac{3}{4}\,\text{inch}\,(2\,\text{cm})$ superior to the external acoustic meatus (EAM), utilizing close collimation.

Which radiographic skull projection best demonstrates bone erosion or changes associated with a pituitary adenoma?

The lateral projection of the skull / sella turcica.

What is the characteristic radiographic appearance of metastatic osteoblastic lesions in the cranium?

Proliferative bony lesions demonstrating increased radiographic density.