Which central ray is used to demonstrate the petrous ridges which completely clear of superimposition of the orbits?

Introduction

Radiographic anatomy for positioning

The facial bones are a series of irregular bones that are attached collectively to the antro-inferior aspect of the skull. Within these bones, and some of the bones forming the cranium, are a series of air-filled cavities known as the paranasal air sinuses. These communicate with the nasal cavity and appear of higher radiographic density than surrounding tissues, since the air offers little attenuation to the X-ray beam. If the sinuses become filled with fluid due to pathology (e.g. blood in trauma), this results in a decrease in density. The sinuses are therefore best imaged by using a horizontal beam, usually with the patient in the erect position, thus demonstrating levels resulting from any fluid collection.

The following comprise the paranasal air sinuses:

• Maxillary sinuses (maxillary antra): paired, pyramidalshaped structures located within the maxillary bone either side of the nasal cavity. They are the largest of the sinuses.

• Frontal sinuses: paired structures located within the frontal bone adjacent to the fronto-nasal articulation. They are very variable in size, and in some individuals they may be absent.

• Sphenoid sinuses: structures that lie immediately beneath the sella turcica and posterior to the ethmoid sinuses.

• Ethmoid sinuses: a labyrinth of small air spaces that collectively form part of the medial wall of the orbit and the upper lateral walls of the nasal cavity.

Radiological considerations

• The facial bones and sinuses are complicated structures, and the radiographer must be aware of their location and radiographic appearances in order to assess the diagnostic suitability of an image. The accompanying diagrams and radiographs outline the position of the major structures and landmarks used for image assessment.

• Facial projections must demonstrate clearly the likely sites of facial fracture, especially in the mid-facial area. These include the orbital floor, lateral orbital wall and zygomatico-frontal suture, lateral antral wall, and zygomatic arch.

• The signs of fracture in these areas may be subtle, but if they form part of a complex facial fracture they will be very important.

• Facial fractures may be bilateral and symmetrical.

Equipment

Given the subtle pathologies often encountered in this region, resolution is an important consideration. The highest-quality images will be obtained using a skull unit with the cassette holder in the vertical position. The facility to tilt the object table offers considerable advantages for positioning, patient comfort and immobilization.

If no skull unit is available, then a vertical tilting Bucky or stationary grids can be used. A high grid lattice with more than 40 grid lines per centimetre will give far superior results in terms of resolution, and their use is to be recommended.

Cassette size

Since the sinuses are grouped close together, 18 X 24-cm cassettes will provide enough space to visualize the region. A 24 X 30-cm cassette may be required to provide enough coverage for entire facial region.

Collimation

It is essential to use a small field of radiation to exclude all structures except those immediately adjacent to the sinuses, thereby reducing scatter to a minimum and improving image quality. A slightly larger field will be required if all of the facial bones need to be included. Certain skull units offer the advantage of using a circular field collimator, which is more suited to this region.

Opaque legends

Given the tight collimation required, the clip-type side-marker will often be excluded from the field. Therefore, individual side markers that can be attached directly to the cassette face should be available.

Screens

Cassettes with a high-speed intensifying screen/receptor should be employed due to the radiosensitivity of the eyes and other adjacent structures. The loss of resolution is compensated for by gains from using the skull unit, appropriate grid selection and small focal spot size.

Preparation of patient and immobilization

Patient preparation

It is important to remove all items likely to cause artefacts on the final image. These may include metal dentures, spectacles, earrings, hair clips, hair bunches/buns and necklaces. Hearing aids should be removed after full instructions have been given to and understood by the patient.

Immobilization

Short exposure times attainable on modern equipment have led to immobilization not being used in many cases. It should be noted that errors often occur as the patient may move between being positioned and the radiographer walking back to the control panel. If the patient appears to be unstable in any way, it is recommended that the head is immobilized using Velcro straps or other appropriate devices.

Recommended projections

TMJ, temporo-mandibular joint.

This projection shows the floor of the orbits in profile, the nasal region, the maxillae, the inferior parts of the frontal bone and the zygomatic bone. The zygomatic arches can be seen, but they

Facial bones

Occipito-mental

are visualized end-on, with their entire length superimposed over a small part of the image.

The occipito-mental (OM) projection is designed to project the petrous parts of the temporal bone (which overlie the region and would cause unwanted noise on a facial bone image) below the inferior part of the maxilla.

Position of patient and cassette

• The projection is best performed with the patient seated facing the skull unit cassette holder or vertical Bucky.

• The patient’s nose and chin are placed in contact with the midline of the cassette holder. The head is then adjusted to bring the orbito-meatal baseline to a 45-degree angle to the cassette holder.

• The horizontal central line of the Bucky/cassette holder should be at the level of the lower orbital margins.

• Ensure that the median sagittal plane is at right-angles to the Bucky/cassette holder by checking the outer canthi of the eyes and that the external auditory meatuses are equidistant.

Direction and centring of the X-ray beam

• The central ray of the skull unit should be perpendicular to the cassette holder. By design, it will be centred to the middle of the cassette holder. If this is the case and the above positioning is performed accurately, then the beam will already be centred.

• If using a Bucky, the tube should be centred to the Bucky using a horizontal beam before positioning is undertaken. Again, if the above positioning is performed accurately, and the Bucky height is not altered, then the beam will already be centred.

• To check that the beam is centred properly, the cross-lines on the Bucky or cassette holder should coincide with the patient’s anterior nasal spine.

Essential image characteristics

• The petrous ridges must appear below the floors of the maxillary sinuses.

• There should be no rotation. This can be checked by ensuring that the distance from the lateral orbital wall to the outer skull margins is equidistant on both sides (marked a and b on the image opposite).

Common faults and remedies

• Petrous ridges superimposed over the inferior part of the maxillary sinuses: in this case, several errors may have occurred. The orbito-meatal baseline may not have been positioned at 45 degrees to the film: a five- to ten-degree caudal angulation could be applied to the tube to compensate for this.

• As this is an uncomfortable position to maintain, patients often let the angle of the baseline reduce between positioning and exposure. Always check the baseline angle immediately before exposure.

Modified mento-occipital

Patients who have sustained trauma will often present supine on a trolley, in a neck brace, and with the radiographic baseline in a fixed position. Modifications in technique will therefore be required by imaging the patient in the antero-posterior position and adjusting the beam angle to ensure that the petrous bones are projected away from the facial bones.

Position of patient and cassette

• The patient will be supine on the trolley and should not be moved. If it is possible to place a cassette and grid under the patient's head without moving the neck, then this should be undertaken. If this is not possible, then place the cassette and grid in the cassette tray under the patient.

• The top of the cassette should be at least 5 cm above the top of the head to allow for any cranial beam angulation.

• A 24 X 30-cm cassette is recommended.

Direction and centring of the X-ray beam

• The patient should be assessed for position (angle) of the orbito-meatal line in relation to the cassette.

• If the baseline makes an angle of 45 degrees back from the vertical (chin raised), then a perpendicular beam can be employed centred to the midline at the level of the lower orbital margins.

• If the orbito-meatal baseline makes an angle of less than 45 degrees with the cassette because of the neck brace, then the difference between the measured angle and 45 degrees should be added to the beam in the form of a cranial angulation. The centring point remains the same.

• For example, if the orbito-meatal baseline was estimated to be 20 degrees from the vertical as the chin was raised, then a 25-degree cranial angulation would need to be applied to the tube to maintain the required angle (see diagram).

Notes

• As the cranial angulation increases, the top of the cassette should be displaced further from the top of the head.

• These images suffer greatly from poor resolution resulting from magnification and distortion from the cranial angulation. It may be worth considering postponing the examination until any spinal injury can be ruled out and the patient can be examined without the neck brace or moved on to a skull unit if other injuries will allow.

Patient imaged supine with 45-degree baseline

Patient imaged supine with 20-degree baseline and 25-degree cranial angulation

This projection demonstrates the lower orbital margins and the orbital floors en face. The zygomatic arches are opened out compared with the occipito-mental projection but they are still foreshortened.

Occipito-mental 30 degrees caudad

Position of patient and cassette

• The projection is best performed with the patient seated facing the skull unit cassette holder or vertical Bucky.

• The patient's nose and chin are placed in contact with the midline of the cassette holder. The head then is adjusted to bring the orbito-meatal baseline to a 45-degree angle to the cassette holder.

• The horizontal central line of the Bucky or cassette holder should be at the level of the symphysis menti.

• Ensure that the median sagittal plane is at right-angles to the Bucky or cassette holder by checking that the outer canthi of the eyes and the external auditory meatuses are equidistant.

Direction and centring of the X-ray beam

• The tube is angled 30 degrees caudally and centred along the midline, such that the central ray exits at the level of the lower orbital margins.

• To check that the beam is centred properly, the cross-lines on the Bucky or cassette holder should coincide approximately with the upper symphysis menti region (this will vary with anatomical differences between patients).

Essential image characteristics

• The floors of the orbit will be clearly visible through the maxillary sinuses, and the lower orbital margin should be demonstrated clearly.

• There should be no rotation. This can be checked by ensuring that the distance from the lateral orbital wall to the outer skull margins is equidistant on both sides.

Common faults and remedies

• Failure to demonstrate the whole of the orbital floor due to under-angulation and failure to maintain the orbito-meatal baseline at 45 degrees. For the patient who finds difficulty in achieving the latter, a greater caudal tube angle may be required.

Note

On many skull units, the tube and cassette holder are fixed permanently, such that the tube is perpendicular to the cassette. This presents a problem for this projection, as the baseline should be 45 degrees to the cassette. This would not be the case when the 30-degree tube angle is applied. The patient must therefore be positioned with their orbito-meatal line positioned at 45 degrees to an imaginary vertical line from the floor (see image opposite). Although such an arrangement makes positioning and immobilization more difficult, it does have the advantage of producing an image that is free of distortion.

Modified reverse occipito-mental 30 degrees for the severely injured patient

It is possible to undertake a reverse OM30°^(i.e. an MO30°T) with the patient supine on a trolley, provided that the patient can raise their orbito-meatal baseline to 45 degrees. Problems arise when the baseline is less than 45 degrees, as additional cranial angulation causes severe distortion in the resultant image. This results from the additional cranial angulation that must be applied to the tube. Clements and Ponsford (1991) have proposed an effective solution to this problem, which is described below.

Position of patient and cassette

• The patient is supine on the trolley with the head adjusted, such that the median sagittal plane and orbito-meatal baseline are perpendicular to the trolley top.

• A gridded cassette is positioned vertically against the vertex of the skull and supported with foam pads and sandbags, such that it is perpendicular to the median sagittal plane.

Direction and centring of the X-ray beam

• The tube is angled 20 degrees to the horizontal (towards the floor) and centred to the symphysis menti in the midline.

• A 100-cm focus-to-film distance (FFD) is used, but it may be necessary to increase this for obese or large patients, as the tube will be positioned close to the chest. Remember to increase the exposure if the FFD is increased.

Essential image characteristics

• The floors of the orbit will be visible clearly through the maxillary sinuses, and the lower orbital margin should be demonstrated clearly.

• There should be no rotation. This can be checked by ensuring that the distance from the lateral orbital wall to the outer skull margins is equidistant on both sides.

Note

If the orbito-meatal baseline is raised by any degree, then there will have to be a corresponding correction of the tube angle to compensate. This may be required if the patient is in a rigid neck brace, when the neck must not be moved.

Positioning for reverse OM30; this will result in image distortion

Positioning for modified projection

Lateral facial bones showing foreign body

Lateral

In cases of injury, this projection should be taken using a horizontal beam in order to demonstrate any fluid levels in the paranasal sinuses. The patient may be positioned erect or supine.

Position of patient and cassette

Erect

• The patient sits facing the vertical Bucky or cassette holder of the skull unit. The head is rotated, such that the side under examination is in contact with the Bucky or cassette holder.

• The arm on the same side is extended comfortably by the trunk, whilst the other arm may be used to grip the Bucky for stability. The Bucky height is altered, such that its centre is 2.5 cm inferior to the outer canthus of the eye.

Supine

• The patient lies on the trolley, with the arms extended by the sides and the median sagittal plane vertical to the trolley top.

• A gridded cassette is supported vertically against the side under examination, so that the centre of the cassette is 2.5 cm inferior to the outer canthus of the eye.

Notes

In either case, the median sagittal plane is brought parallel to the cassette by ensuring that the inter-orbital line is at right- angles to the cassette and the nasion and external occipital protuberance are equidistant from it.

Direction and centring of the X-ray beam

• Centre the horizontal central ray to a point 2.5 cm inferior to the outer canthus of the eye.

Essential image characteristics

• The image should contain all of the facial bones sinuses, including the frontal sinus and posteriorly to the anterior border of the cervical spine.

• A true lateral will have been obtained if the lateral portions of the floor of the anterior cranial fossa are superimposed.

Notes

• This projection is often reserved for gross trauma, as the facial structures are superimposed.

• If a lateral is undertaken for a suspected foreign body in the eye, then additional collimation and alteration in the centring point will be required.

Zygomatic arches: infero-superior

This projection is essentially a modified submento-vertical (SMV) projection. It is often referred to as the 'jug-handle projection', as the whole length of the zygomatic arch is demonstrated in profile against the side of the skull and facial bones.

Position of patient and cassette

• The patient lies supine, with one or two pillows under the shoulders to allow the neck to be extended fully.

• An 18 X 24-cm cassette is placed against the vertex of the skull, such that its long axis is parallel with the axial plane of the body. It should be supported in this position with foam pads and sandbags.

• The flexion of the neck is now adjusted to bring the long axis of the zygomatic arch parallel to the cassette.

• The head in now tilted five to ten degrees away from the side under examination. This allows the zygomatic arch under examination to be projected on to the film without superimposition of the skull vault or facial bones.

Direction and centring of the X-ray beam

• The central ray should be perpendicular to the cassette and long axis of the zygomatic arch.

• A centring point should be located such that the central ray passes through the space between the midpoint of the zygomatic arch and the lateral border of the facial bones.

• Tight collimation can be applied to reduce scatter and to avoid irradiating the eyes.

Essential image characteristics

• The whole length of the zygomatic arch should be demonstrated clear of the skull. If this has not been achieved, then it may be necessary to repeat the examination and alter the degree of head tilt to try and bring the zygomatic arch clear of the skull.

Radiological considerations

Depressed fracture of the zygoma can be missed clinically due to soft-tissue swelling, making the bony defect less obvious. Radiography has an important role in ensuring that potentially disfiguring depression of the cheekbones is not missed.

Notes

• Both sides may be examined on one cassette using two exposures.

• It is important for the radiographer to have a good understanding of anatomy to correctly locate the position of the zygomatic arch and thus allow for accurate positioning and collimation.

• In some individuals, variations in anatomy may not allow the arch to be projected clear of the skull.

This is a frequently undertaken projection used to assess injuries to the orbital region (e.g. blow-out fracture of the orbital floor) and to exclude the presence of metallic foreign bodies in the eyes before magnetic resonance imaging (MRI) investigations. The projection is essentially an under-tilted occipito-mental with the orbito-meatal baseline raised 10 degrees less than in the standard occipito-mental projection.

Collimation used for foreign-body projection

Orbits: occipito-mental (modified)

Position of patient and cassette

• The projection is best performed with the patient seated facing the skull unit cassette holder or vertical Bucky.

• The patient's nose and chin are placed in contact with the midline of the cassette holder. The head is then adjusted to bring the orbito-meatal baseline to a 35-degree angle to the cassette holder.

• The horizontal central line of the vertical Bucky or cassette holder should be at the level of the midpoint of the orbits.

• Ensure that the median sagittal plane is at right-angles to the Bucky or cassette holder by checking that the outer canthi of the eyes and the external auditory meatuses are equidistant.

Direction and centring of the X-ray beam

• The central ray of the skull unit should be perpendicular to the cassette holder and by design will be centred to the middle of the image receptor. If this is the case and the above positioning is performed accurately, then the beam will already be centred.

• If using a Bucky, the tube should be centred to the Bucky using a horizontal beam before positioning is undertaken. Again, if the above positioning is performed accurately and the Bucky height is not altered, then the beam will already be centred.

• To check that the beam is centred properly, the cross-lines on the Bucky or cassette holder should coincide with the midline at the level of the mid-orbital region.

Essential image characteristics

• The orbits should be roughly circular in appearance (they will be more oval in the occipito-mental projection).

• The petrous ridges should appear in the lower third of the maxillary sinuses.

• There should be no rotation. This can be checked by ensuring that the distance from the lateral orbital wall to the outer skull margins is equidistant on both sides.

Notes

• If the examination is purely to exclude foreign bodies in the eye, then tight 'letter-box' collimation to the orbital region should be applied.

• A dedicated cassette should be used for foreign bodies This should be cleaned regularly to avoid small artefacts on the screens being confused with foreign bodies.

• If a foreign body is suspected, then a second projection may be undertaken, with the eyes in a different position to differentiate this from an image artefact. The initial exposure could be taken with the eyes pointing up and the second with the eyes pointing down.

Nasal bones: lateral

Position of patient and cassette

• The patient sits facing an 18 X 24-cm cassette supported in the cassette stand of a vertical Bucky.

• The head is turned so that the median sagittal plane is parallel with the cassette and the inter-pupillary line is perpendicular to the cassette.

• The nose should be roughly coincident with the centre of the cassette.

Direction and centring of the X-ray beam

• A horizontal central ray is directed through the centre of the nasal bones and collimated to include the nose.

Radiological considerations

Nasal fracture can usually be detected clinically and is rarely treated actively. If a fracture causes nasal deformity or breathing difficulty, then it may be straightened, but lateral projections will not help. Considering the dose of radiation to the eye, this projection should be avoided in most instances.

Notes

• A high-resolution cassette may be used if detail is required.

• This projection may be useful for foreign bodies in the nose. In this case, a soft-tissue exposure should be employed.

• In the majority of cases, severe nasal injuries will require only an occipito-mental projection to assess the nasal septum and surrounding structures.

• The projection can also be undertaken with the patient supine and the cassette supported against the side of the head.

Position of patient and cassette

• The patient lies in the supine position. The trunk is rotated slightly and then supported with pads to allow the side of the face being examined to come into contact with the cassette, which will be lying on the tabletop.

Mandible: lateral 30 degrees cephalad

• The median sagittal plane should be parallel with the cassette and the inter-pupillary line perpendicular.

• The neck may be flexed slightly to clear the mandible from the spine.

• The cassette and head can now be adjusted and supported so the above position is maintained but is comfortable for the patient.

• The long axis of the cassette should be parallel with the long axis of the mandible and the lower border positioned 2 cm below the lower border of the mandible.

• The projection may also be performed with a horizontal beam in trauma cases when the patient cannot be moved.

• In this case, the patient will be supine with the median sagittal plane at right-angles to the tabletop. The cassette is supported vertically against the side under examination.

Direction and centring of the X-ray beam

• The central ray is angled 30 degrees cranially at an angle of 60 degrees to the cassette and is centred 5 cm inferior to the angle of the mandible remote from the cassette.

• Collimate to include the whole of the mandible and temporo-mandibular joint (TMJ) (include the external auditory meatus (EAM) at the edge of the collimation field).

Essential image characteristics

• The body and ramus of each side of the mandible should not be superimposed.

• The image should include the whole of the mandible, from the TMJ to the symphysis menti.

Radiological considerations

Do not mistake the mandibular canal, which transmits the inferior alveolar nerve, for a fracture.

Common faults and remedies

• Superimposition of the mandibular bodies will result if the angle applied to the tube is less than 30 degrees or if the centring point is too high.

• If the shoulder is obscuring the region of interest in the horizontal beam projection, then a slight angulation towards the floor may have to be applied, or, if the patient's condition will allow, tilt the head towards the side under examination.

Notes

• In cases of injury, both sides should be examined to demonstrate a possible contre-coup fracture.

• Tilting the head towards the side being examined may aid positioning if the shoulder is interfering with the primary beam.

Mandible: postero-anterior

Position of patient and cassette

• The patient sits facing the vertical Bucky or skull unit cassette holder. Alternatively, in the case of trauma, the projection may be supine on a trolley, giving an antero-posterior projection.

• The patient's median sagittal plane should be coincident with the midline of the Bucky or cassette holder. The head is then adjusted to bring the orbito-meatal baseline perpendicular to the Bucky or cassette holder.

• The median sagittal plane should be perpendicular to the cassette. Check that the external auditory meatuses are equidistant from the cassette.

• The cassette should be positioned such that the middle of an 18 X 24-cm cassette, when placed longitudinally in the Bucky or cassette holder, is centred at the level of the angles of the mandible.

Direction and centring of the X-ray beam

• The central ray is directed perpendicular to the cassette and centred in the midline at the levels of the angles of the mandible.

Essential image characteristics

• The whole of the mandible from the lower portions of the TMJs to the symphysis menti should be included in the image.

• There should be no rotation evident.

Radiological considerations

• This projection demonstrates the body and rami of the mandible and may show transverse or oblique fractures not evident on other projections or dental panoramic tomography (DPT) (orthopantomography, OPT).

• The region of the symphysis menti is superimposed over the cervical vertebra and will be seen more clearly when using the anterior oblique projection.

Common faults and remedies

Superimposition of the upper parts of the mandible over the temporal bone will result if the orbito-meatal baseline is not perpendicular to the cassette.

Note

A 10-degree cephalad angulation of the beam may be required to demonstrate the mandibular condyles and temporal mandibular joints.

Mandible: postero-anterior oblique

This projection demonstrates the region of the symphysis menti.

Position of patient and cassette

• The patient sits facing the vertical Bucky or skull unit cassette holder. Alternatively, in the case of trauma, the projection may be supine on a trolley, giving an antero-posterior projection.

• The patient's median sagittal plane should be coincident with the midline of the Bucky or cassette holder. The head is then adjusted to bring the orbito-meatal baseline perpendicular to the Bucky or cassette holder.

• From a position with the median sagittal plane perpendicular to the cassette, the head is rotated 20 degrees to either side, so that the cervical vertebra will be projected clear of the symphysis menti.

• The head is now repositioned so the region of the symphysis menti is coincident with the middle of the cassette.

• The cassette should be positioned such that the middle of an 18 X 24-cm cassette, when placed longitudinally in the Bucky or cassette holder, is centred at the level of the angles of the mandible.

Direction and centring of the X-ray beam

• The central ray is directed perpendicular to the cassette and centred 5 cm from the midline, away from the side being examined, at the level of the angles of the mandible.

Essential image characteristics

• The symphysis menti should demonstrated without any superimposition of the cervical vertebra.

Temporal-mandibular joints: lateral 25 degrees caudad

It is usual to examine both temporal-mandibular joints. For each side, a projection is obtained with the mouth open as far as possible and then another projection with the mouth closed. An additional projection may be required with the teeth clenched.

Position of patient and cassette

• The patient sits facing the vertical Bucky or skull unit cassette holder or lies prone on the Bucky table. In all cases, the head is rotated to bring the side of the head under examination in contact with the table. The shoulders may also be rotated slightly to help the patient achieve this position.

• The head and Bucky or cassette holder level is adjusted so the centre cross-lines are positioned to coincide with a point 1 cm along the orbito-meatal baseline anterior to the external auditory meatus.

• The median sagittal plane is brought parallel to the cassette by ensuring that the inter-pupillary line is at right-angles to the table top and the nasion and external occipital protuberance are equidistant from it.

• The cassette is placed longitudinally in the cassette holder, such that two exposures can be made without superimposition of the images.

Direction and centring of the X-ray beam

• Using a well-collimated beam or an extension cone, the central ray is angled 25 degrees caudally and will be centred to a point 5 cm superior to the joint remote from the cassette so the central ray passes through the joint nearer the cassette.

Radiological considerations

TMJ images are useful in assessing joint dysfunction by demonstrating erosive and degenerative changes. Open- and closed- mouth projections can be very helpful in assessing whether normal anterior gliding movement of the mandibular condyle occurs on jaw opening. MRI promises greater accuracy, since it also demonstrates the articular cartilages and fibrocartilage discs and how they behave during joint movement.

Notes

• The image should include the correct side-marker and labels to indicate the position of the mouth when the exposure was taken (open, closed, etc.).

• If using a skull unit in which the tube cannot be angled independently of the cassette holder, the inter-pupillary line is at right-angles to an imaginary vertical line drawn from the floor.

• This projection may supplement DPT (OPT) images of the TMJs. Postero-anterior projections may be undertaken by modifying the technique described for the postero-anterior mandible on p. 272.

Paranasal sinuses

Introduction

Plain images of the sinuses are unreliable for diagnosis of inflammatory sinus disease, since many asymptomatic people will have sinus opacification and sinus symptoms may be present in the absence of gross sinus opacification. Acute sinusitis (especially infective) may manifest radiologically as fluid levels in the maxillary antrum, but it is questionable as to whether this alters clinical management. Malignant sinus disease requires more comprehensive imaging by computed tomography (CT) and/or MRI. Some radiology departments will no longer perform plain sinus radiographs.

Recommended projections

Anatomy

As mentioned in the introduction to this chapter, the sinuses collectively consist of the following structures (outlined on the radiographs opposite):

• Maxillary sinuses (maxillary antra): paired, pyramidalshaped structures located within the maxillary bone either side of the nasal cavity. They are the largest of the sinuses.

• Frontal sinuses: paired structures located within the frontal bone adjacent to the fronto-nasal articulation. They are very variable in size, and in some individuals they may be absent.

• Sphenoid sinuses: structures lying immediately beneath the sella turcica and posterior to the ethmoid sinuses.

• Ethmoid sinuses: a labyrinth of small air spaces that collectively form part of the medial wall of the orbit and the upper lateral walls of the nasal cavity.

Occipito-mental

This projection is designed to project the petrous part of the temporal bone below the floor of the maxillary sinuses so that fluid levels or pathological changes in the lower part of the sinuses can be visualized clearly.

Position of patient and cassette

• The projection is best performed with the patient seated facing the skull unit cassette holder or vertical Bucky.

• The patient's nose and chin are placed in contact with the midline of the cassette holder. The head is then adjusted to bring the orbito-meatal baseline to a 45-degree angle to the cassette holder.

• The horizontal central line of the Bucky or cassette holder should be at the level of the lower orbital margins.

• Ensure that the median sagittal plane is at right-angles to the Bucky or cassette holder by checking that the outer canthi of the eyes and the external auditory meatuses are equidistant.

• The patient should open the mouth as wide as possible before exposure. This will allow the posterior part of the sphenoid sinuses to be projected through the mouth.

Direction and centring of the X-ray beam

• The central ray of the skull unit should be perpendicular to the cassette holder and by design will be centred to the middle of the image receptor. If this is the case and the above positioning is performed accurately, then the beam will already be centred.

• If using a Bucky, the tube should be centred to the Bucky using a horizontal beam before positioning is undertaken. If the above positioning is performed accurately and the Bucky height is not altered, then the beam will already be centred.

• To check the beam is centred properly, the cross-lines on the Bucky or cassette holder should coincide with the patient's anterior nasal spine.

• Collimate to include all of the sinuses.

Essential image characteristics

• The petrous ridges must appear below the floors of the maxillary sinuses.

• There should be no rotation. This can be checked by ensuring that the distance from the lateral orbital wall to the outer skull margins is equidistant on both sides.

Common faults and remedies

• Petrous ridges appearing over the inferior part of the maxillary sinuses: in this case, several things may have occurred. The orbito-meatal baseline was not positioned at 45 degrees to the film or a five- to ten-degree caudal angulation may be applied to the tube to compensate. As this is an uncomfortable position to maintain, patients often let the angle of the baseline reduce between positioning and exposure. Therefore, always check the baseline angle immediately before exposure.

Note

To distinguish a fluid level from mucosal thickening, an additional projection may be undertaken with the head tilted, such that a transverse plane makes an angle of about 20 degrees to the floor.

Occipito-frontal 15 degrees caudad

This projection is used to demonstrate the frontal and ethmoid sinuses.

Position of patient and cassette

• The patient is seated facing the vertical Bucky or skull unit cassette holder so the median sagittal plane is coincident with the midline of the Bucky and is also perpendicular to it.

• The head is positioned so that the orbito-meatal baseline is raised 15 degrees to the horizontal.

• Ensure that the nasion is positioned in the centre of the Bucky.

• The patient may place the palms of each hand either side of the head (out of the primary beam) for stability.

• An 18 X 24-cm cassette is placed longitudinally in the Bucky tray. The lead name blocker must not interfere with the final image.

Direction and centring of the X-ray beam

• The central ray is directed perpendicular to the vertical Bucky along the median sagittal plane so the beam exits at the nasion.

• A collimation field or extension cone should be set to include the ethmoidal and frontal sinuses. The size of the frontal sinuses can vary drastically from one individual to another.

Essential image characteristics

• All the relevant sinuses should be included within the image.

• The petrous ridges should be projected just above the lower orbital margin.

• It is important to ensure that the skull is not rotated. This can be assessed by measuring the distance from a point in the midline of the skull to the lateral orbital margins. If this is the same on both sides of the skull, then it is not rotated.

Notes

• The degree of angulation may vary according to local preferences. Some departments may prefer to use an OF20°-i projection. In this case, the orbito-meatal baseline is then raised to the angle required by the projection, i.e. 20 degrees. Alternatively a 20-degree caudal angulation could be employed with the orbito-meatal baseline perpendicular to the image receptor.

• An OF10°-i or occipito-frontal projection would not be suitable for demonstration of the ethmoid sinuses, as the petrous ridges would obscure the region of interest.

Lateral

Position of patient and cassette

• The patient sits facing the vertical Bucky or skull unit cassette holder. The head is then rotated, such that the median sagittal plane is parallel to the Bucky and the inter-orbital line is perpendicular to the Bucky.

• The shoulders may be rotated slightly to allow the correct position to be attained. The patient may grip the Bucky for stability.

• The head and Bucky heights are adjusted so that the centre of the Bucky is 2.5 cm along the orbito-meatal line from the outer canthus of the eye.

• Position an 18 X 24-cm cassette longitudinally in the erect Bucky, such that its lower border is 2.5 cm below the level of the upper teeth.

• A radiolucent pad may be placed under the chin for support.

Direction and centring of the X-ray beam

• A horizontal central ray should be employed to demonstrate fluid levels.

• The tube should have been centred previously to the Bucky, such that the central ray will now be centred to a point 2.5 cm posterior to the outer canthus of the eye.

Common faults and remedies

This is not an easy position for the patient to maintain. Check the position of all planes immediately before exposure, as the patient probably will have moved.

Essential image characteristics

• A true lateral will have been achieved if the lateral portions of the floors of the anterior cranial fossa are superimposed.

Note

This projection may also be undertaken with the patient supine and the cassette supported vertically against the side of the face. Again, a horizontal beam is used to demonstrate fluid levels.

Which projection of the cranium demonstrates the petrous ridges within the orbits?

Where are the petrous ridges seen on a Parietoacanthial waters method radiograph?

Where are the petrous ridges seen on a Parietoacanthial projection?

Which of the following positions projections will demonstrate the maxillary sinuses free of superimposition by other structures?