Specialised projections of the skull

Chapter 20 Specialised projections of the skull

Elizabeth Carver

In the 21st century, the majority of hospitals in the Western world have access to specialised imaging modalities. Of these, computed tomography (CT) and magnetic resonance imaging (MRI) have largely replaced plain radiography in the diagnosis of diseases which were originally only assessed with plain radiography. Unfortunately, plain radiography frequently only provides information when disease is very advanced; CT provides more detailed and high-quality information, and MRI has the advantage of providing information on neurological and other soft tissues (with no patient dose from ionising radiation) before any bony effects are seen.

Information on plain radiography is still provided in this book, as support for radiographers working in areas with limited or no access to MRI and CT.

Sella turcica (pituitary fossa)

To clarify the use of terms in this chapter, the name ‘pituitary fossa’ refers to the depression within the sella turcica in which lies the pituitary gland. The sella turcica itself forms the top of the central portion of the sphenoid bone, lying over the sphenoid sinus in the midline.

An enlarged and eroded sella can be a sign of a pituitary tumour or raised intracranial pressure, but this appearance is an effect of long-term disease.

For all projections of the sella turcica image receptor (IR) is vertical; an antiscatter grid is employed

Lateral sella turcica (Fig. 20.1A,B)

Positioning

• The patient is initially seated facing the IR

• The trunk is brought as close as possible to the receptor unit and the patient is asked to sit with their spine as erect as possible. This helps the patient turn their head more easily into the required lateral position

• The head is turned through 90° to bring the side of the head in contact with the IR

• The median sagittal plane (MSP) is parallel to the IR; there should be no tilt or rotation of the head. This can be assessed by checking the midline of the cranium over the top and symmetry of the frontal bone and orbits

Figure 20.1 Lateral sella turcica.

Beam direction and focus receptor distance (FRD)

Horizontal, at 90° to the IR

Centring

Midway between the posterior tubercle of the first cervical vertebra and the glabella or 2.5 cm anterior to the external auditory meatus (EAM), along the orbitomeatal baseline (OMBL), and 2.5 cm above this point

The second centring point will have variable efficacy due to variations in the skull size of the individual patient.

Collimation

Sphenoid bone from lesser wing (anterior clinoid processes) anteriorly and posterior clinoids posteriorly

Criteria for assessing image quality

• Anterior and posterior clinoid processes, sella turcica, sphenoid sinus and dorsum sellae are demonstrated

• Superimposition of both sides of the floor of pituitary fossa and floor of the anterior cranial fossa

• Anterior clinoid processes are superimposed

• Posterior clinoid processes are superimposed

• Sharp image demonstrating outline of the clinoids and pituitary fossa in contrast with the temporal bones and sphenoid sinus

Common errors	Possible reasons
Both pairs of clinoid processes overlapped, seen one above the other	Tilted skull
Both pairs of clinoid processes overlapped, seen side by side	Rotated skull
’Double’ floor of sella turcica; two lines over floor area	Tilted skull or floor is eroded on one side by tumour

Occipitofrontal (OF) sella turcica (Fig. 20.2A,B,C)

The OF 20° projection will demonstrate the floor of the sella turcica, seen as asymmetry of the floor if the floor is eroded on one side. The OF 30° projection has limited value, demonstrating the dorsum sellae through the foramen magnum; the dorsum will appear as low density if it is eroded.

Figure 20.2 (A) OF sella turcica; (B) OF 20° sella turcica; (C) OF 30° sella turcica.

Positioning

• The patient is seated facing the bucky, their forehead in contact with it

• The OMBL and MSP are perpendicular to the IR

Beam direction and FRD

(a) Initially horizontal, a 20° caudal angle will demonstrate the floor of the pituitary fossa through the ethmoid and sphenoid sinuses

(b) A 30° cranial angle will demonstrate the dorsum sellae through the foramen magnum

Centring

(a) With 20° caudal angle: Above the external occipital protuberance (EOP) to emerge through the nasion

(b) With 30° caudal angle: Below the EOP, on the neck, to emerge through the glabella

Collimation

OF 20°: Lesser wing of sphenoid, sphenoid and ethmoid sinuses

OF 30°: Ethmoid sinus, foramen magnum

Criteria for assessing image quality

OF 20°

• Lesser wing of the sphenoid, sphenoid and ethmoid sinuses are demonstrated

• Medial aspects of the superior border of petrous ridge are shown superimposed on the inferior orbital margin

• Lesser wing of sphenoid seen medially and symmetrically across the upper portion of the orbits

• Medial borders of the orbits are equidistant from nasal septum

• Floor of pituitary fossa is seen as a horizontal line across the ethmoid sinuses. In cases of erosion of the floor of the fossa, this line may deviate from horizontal orientation

• Sharp image showing the fine line indicating the floor of the pituitary fossa in contrast with the air-filled ethmoid sinus

OF 30°

• Foramen magnum is demonstrated

• Dorsum sellae is seen in the centre of the foramen magnum

• Sharp image showing dorsum sellae in contrast with the less dense foramen magnum

Common errors – OF 20°	Possible reasons
Petrous ridge seen above the level of the inferior orbital margins	Inadequate angle selected or OMBL is incorrectly positioned (chin down too far)
Petrous ridge seen below the level of the inferior orbital margins	Angle selected is too great or OMBL is incorrectly positioned (chin not far enough down)
Common errors – OF 30°	Possible reasons
Foramen magnum appears short or is not evident. Dorsum sellae may be visible above the portion of the foramen magnum that is seen	Angle selected is inadequate or OMBL is positioned incorrectly (chin not far enough down)
Large foramen magnum seen but curve of the posterior arch of C1 is seen in its lower third, rather than the anvil shape of dorsum sellae	Angle selected is too great or OMBL is not positioned correctly (chin too far down)

Mastoids

Lateral oblique mastoids (Fig. 20.3A,B)

Positioning for this projection is identical to that for lateral oblique temporomandibular joints (TMJs), although the centring point differs. The pinna of the ear must also be cleared from the mastoid area. Both sides are examined for comparison.

Figure 20.3 Lateral oblique mastoids.

IR is vertical; an antiscatter grid is employed

Positioning

• The patient is initially seated, facing the IR

• The trunk is brought as close as possible to the receptor unit and the patient is asked to sit with their spine as erect as possible. This helps the patient turn their head more easily into the required lateral position

• The head is turned through 90° to bring the mastoid on the side under examination over the IR. The location of this bone can be detected by palpating the mastoid process, which lies inferiorly and posteriorly to the EAM

• The pinna of the ear on the side nearest the IR is then gently pulled forward and the head rests against the IR to keep the pinna forward. This clears the image of the pinna from the area of interest

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Tags: Medical Imaging Techniques, Reflection and Evaluation

Mar 3, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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