Chapter 11: Cranium, Facial Bones, & Paranasal Sinuses

Detailed Chapter Overview

Chapter 11 presents a comprehensive guide to the radiography of the skull, one of the most anatomically complex and technically demanding areas of the body. The chapter systematically covers the three main subdivisions: the cranium, the facial bones, and the paranasal sinuses. A core theme is the absolute necessity for precision in positioning, as even minor errors in rotation or tilt can lead to misdiagnosis due to the superimposition of numerous small structures. The chapter begins with a detailed anatomical survey of the 8 cranial bones and 14 facial bones, emphasizing key landmarks, sutures, and foramina. A significant portion is dedicated to defining and utilizing standard cranial positioning lines (OML, IOML, GML) to ensure consistency and repeatability. The paranasal sinuses are explored in depth, highlighting the critical importance of using an erect position and a horizontal x-ray beam to demonstrate air-fluid levels, a key indicator of sinusitis. The chapter provides meticulous, step-by-step instructions for a wide array of projections, explaining the rationale behind specific CR angulations and positioning techniques like the AP Axial (Towne), PA Axial (Caldwell), and Parietoacanthial (Waters) methods. For each projection, exacting evaluation criteria are established, training the radiographer to critically analyze their images for proper landmark alignment, absence of rotation, and clear visualization of the specific anatomical structures of interest.

In-Depth Study Guide

The Cranium (Neurocranium): The Brain's Protective Vault

The cranium is composed of eight bones that directly encase and protect the brain. These bones are joined by immovable, fibrous joints called sutures.

1. Frontal Bone (1)

The frontal bone forms the anterior aspect of the cranium, commonly known as the forehead.

• Vertical (Squamous) Portion: Forms the forehead. Contains the **frontal sinuses**. Key landmarks include the **glabella** (smooth area between the eyebrows), and the **supraorbital margins (SOM)**, which form the superior rim of the orbits.
• Horizontal (Orbital) Portion: Forms the roof of the orbits.
• Articulations: Articulates with the right and left parietals, the sphenoid, and the ethmoid bones.

2. Parietal Bones (2)

These two large, square-shaped bones form the bulk of the lateral walls and roof of the cranium.

• Landmarks: The widest part of the entire skull is located between the **parietal tubercles (eminences)** on each side.
• Articulations: They articulate with each other at the **sagittal suture**, with the frontal bone at the **coronal suture**, with the occipital bone at the **lambdoidal suture**, and with the temporal bones at the **squamosal sutures**. The anterior fontanel in an infant (the "soft spot") is located at the junction of the sagittal and coronal sutures, an area called the **bregma** in adults. The posterior fontanel is at the junction of the sagittal and lambdoidal sutures, an area called the **lambda**.

3. Occipital Bone (1)

Forms the posteroinferior base of the cranium.

• Foramen Magnum: The most prominent feature, a large opening through which the medulla oblongata passes to connect with the spinal cord.
• Occipital Condyles: Two oval processes on either side of the foramen magnum that articulate with the C1 vertebra (atlas) to form the atlanto-occipital joint.
• External Occipital Protuberance (Inion): A prominent bump on the posterior surface, used as a key positioning landmark.

4. Temporal Bones (2)

Complex bones located on each side of the skull between the occipital bone and the sphenoid bone. They house the delicate organs of hearing and balance.

• Zygomatic Process: Projects anteriorly to articulate with the zygomatic bone, forming the zygomatic arch.
• External Acoustic Meatus (EAM): The external opening of the ear canal. A critical positioning landmark.
• Mastoid Process: A prominent process posterior and inferior to the EAM, containing air cells.
• Styloid Process: A slender, pointed projection inferior to the EAM.
• Petrous Portion (Petrous Pyramids/Pars Petrosa): The most important part radiographically. This is a very dense, pyramid-shaped portion of bone that houses the middle and inner ear canals. Its location and density are key considerations in skull radiography, as it can obscure other structures if not projected correctly.

5. Sphenoid Bone (1)

Considered the "keystone" of the cranium because it is the only bone that articulates with all seven other cranial bones, holding them all together.

• Body: The central portion, which contains the two **sphenoid sinuses** and a deep depression called the **sella turcica**.
• Sella Turcica: A saddle-shaped structure on the superior surface of the body that protects the pituitary gland. It is a key structure demonstrated in a lateral skull projection. The posterior aspect of the sella is the **dorsum sellae**.
• Wings: Two pairs of wings extend from the body. The **lesser wings** are triangular and superior, while the **greater wings** extend laterally.
• Foramina: Contains numerous foramina for the passage of cranial nerves and blood vessels, including the foramen rotundum, foramen ovale, and foramen spinosum.

6. Ethmoid Bone (1)

A light, spongy bone situated between the orbits, forming part of the cranial floor, the orbits, and the roof of the nose.

• Cribriform Plate: The horizontal plate, which contains many small foramina for the passage of olfactory nerves from the nasal cavity to the brain.
• Crista Galli: A prominent, triangular process projecting superiorly from the cribriform plate.
• Perpendicular Plate: Projects inferiorly to form the superior portion of the bony nasal septum.
• Labyrinths (Lateral Masses): Contain the numerous small **ethmoid air cells (sinuses)**.

The Facial Bones: Structure of the Viscerocranium

These 14 bones provide the structure of the face.

• Maxillae (2): The largest immovable bones of the face. They form the upper jaw, most of the hard palate, and contain the large **maxillary sinuses**.
• Zygomatic Bones (2): The cheekbones. The **zygomatic arch** is a key structure formed by the articulation of the zygomatic bone and the temporal bone.
- Nasal Bones (2) & Lacrimal Bones (2):** The nasals form the bridge of the nose. The lacrimals are the smallest facial bones, located in the medial wall of each orbit. - Palatine Bones (2) & Vomer (1):** The palatines form the posterior hard palate. The vomer forms the inferior portion of the bony nasal septum. - Inferior Nasal Conchae (2):** Two thin, scroll-like bones that project from the lateral walls of the nasal cavity. - Mandible (1):** The only movable bone of the adult skull, and the largest facial bone. Key landmarks include the **body**, the vertical **ramus**, the **angle (gonion)**, the **condyloid process** (which forms the TMJ), the **coronoid process**, and the **mental protuberance** (chin).

Positioning Baselines and Terminology

Precision in skull radiography depends on consistent alignment of the head using standardized lines and planes. The **midsagittal plane (MSP)** must be perfectly perpendicular to the IR for AP/PA views and parallel for lateral views to prevent rotation.

• Glabellomeatal Line (GML): Connects the glabella to the EAM.
• Orbitomeatal Line (OML): Connects the outer canthus of the eye to the EAM. This is the primary, most frequently used baseline.
• Infraorbitomeatal Line (IOML): Connects the infraorbital margin to the EAM. There is a consistent 7-degree angle between the OML and IOML.
• Acanthiomeatal Line (AML): Connects the acanthion to the EAM.

Routine Radiographic Projections: A Detailed Approach

AP Axial Projection (Towne Method)

• Purpose: Primarily to demonstrate the occipital bone and the dorsum sellae projected within the foramen magnum. Excellent for evaluating occipital fractures.
• Positioning: Patient is supine. The MSP is perpendicular to the midline of the IR. The head is positioned to place either the OML or the IOML perpendicular to the IR.
• Central Ray: If the OML is perpendicular, the CR is angled **30 degrees caudad**. If the IOML is perpendicular (a more comfortable position for many patients), the CR is angled **37 degrees caudad**. The CR enters about 2.5 inches above the glabella and passes through the foramen magnum.
• Evaluation Criteria: Evidence of no rotation (equal distance from foramen magnum to lateral cranial borders). The dorsum sellae and posterior clinoid processes should be clearly visible within the foramen magnum. Symmetric petrous pyramids should be seen.

PA and PA Axial Projections (Caldwell Method)

• Purpose: To demonstrate the frontal bone, ethmoid sinuses, and orbital structures. The angulation determines the relationship of the petrous pyramids to the orbits.
• PA 0-degree Projection: The OML is perpendicular to the IR. CR exits the nasion. In this view, the very dense **petrous ridges completely fill the orbits**.
• PA Axial (Caldwell Method): The OML is perpendicular to the IR. The CR is angled **15 degrees caudad**, exiting the nasion. This angle projects the **petrous ridges into the lower one-third of the orbits**, allowing for superior visualization of the orbital margins and frontal bone.
• Evaluation Criteria: No rotation (equal distance from lateral orbital margins to lateral cranial cortex). For the Caldwell, the petrous ridges should be in the lower 1/3 of the orbits.

Lateral Projection

• Purpose: To demonstrate the cranial bones, sella turcica, and facial bones in a superimposed profile.
• Positioning: Patient is semiprone or seated erect. The MSP of the head must be perfectly parallel to the IR, and the interpupillary line must be perpendicular to the IR to prevent tilt. The CR is directed perpendicular, entering 2 inches superior to the EAM.
• Evaluation Criteria: No rotation or tilt, evidenced by perfectly superimposed orbital roofs, mandibular rami, and EAMs. The entire sella turcica should be seen in profile.

Parietoacanthial Projection (Waters Method)

• Purpose: The single best projection to visualize the facial bones, particularly the maxillae, zygomatic arches, and orbits. Also the best view for the maxillary sinuses.
• Positioning: Patient is prone or erect. The patient's chin is extended and rested on the IR until the **mentomeatal line (MML) is perpendicular to the IR**. This places the OML at a 37-degree angle to the IR. The CR is directed perpendicularly, exiting the acanthion.
• Evaluation Criteria: The key criterion is that the dense **petrous ridges are projected just below the floor of the maxillary sinuses**. No rotation is present (equal distance between lateral orbital margins and cranium).

Paranasal Sinus Series (Erect, Horizontal Beam)

• Purpose: To assess for sinusitis, polyps, or fractures. The erect position and horizontal beam are mandatory to demonstrate air-fluid levels.
• Lateral: Shows all four sinus groups in profile. The sphenoid sinus is of primary interest.
• PA (Caldwell): Best demonstrates the frontal and anterior ethmoid sinuses.
• Parietoacanthial (Waters): Provides the best visualization of the maxillary sinuses.
• Submentovertical (SMV): Provides a view of the sphenoid, posterior ethmoid, and maxillary sinuses.
• Open-Mouth Waters: The patient performs the Waters position but opens their mouth. This allows the sphenoid sinuses to be visualized through the open oral cavity, free from superimposition.