Contributors: Alison Dickie, Fraser McConnell

 Species: Canine   |   Classification: Techniques

Introduction Requirements Preparation Procedure Aftercare Outcomes Further Reading

Introduction

  • The basic radiographic projections used to image the skull have been described Radiography: skull (basic) Radiology: skull and mandible.
  • The complex anatomy of the skull means that interpretation can be difficult. Superimposition can be a major problem when trying to evaluate specific structures.
  • Many of the special projections attempt to project specific structures with a minimum of superimposition.
  • Careful positioning is vital to produce standard views of the structure being examined therefore general anesthesia is necessary.
  • The production of good quality radiographs using appropriate film-screen combinations with careful radiographic and processing techniques is also vital to aid interpretation.
  • The skull is bilaterally symmetrical and so comparison between the structures on each side can help identify abnormalities.
  • Views which project both sides on the same film make comparison easy. Views which require each side to be imaged independently need to be positioned carefully to ensure each side is projected at the same angle therefore allowing comparison.
  • The angle of the projection may have to be altered to take into account the skull conformation of the dog.
  • Poor positioning may lead to distortion of the structure being examined and also superimposition making interpretation impossible.
  • Radiographic changes may be subtle, localized or absent even in the presence of significant disease.

Uses

  • Investigation of specific areas of the skull, eg temporomandibular joints, tympanic bullae, teeth, nasal chambers, etc.
  • Identification of bony abnormalities, eg fractures Fracture: overview , dysplasia, neoplasia.
  • Identification of bony involvement in association with other conditions, eg otitis media Otitis media , soft tissue neoplasia Nasal cavity: neoplasia.

Advantages

  • Radiography is widely available in general veterinary practice.
  • Radiography is well suited to imaging bone and gas filled structures.

Disadvantages

  • Radiography involves the use of ionizing radiation.
  • The skull can rotate in several planes making accurate positioning a challenge.
  • Superimposition and complex anatomy make radiographic interpretation of the skull difficult.
  • Radiographic changes may be subtle, localized or absent.
  • Limited information on soft tissues.

Alternative Techniques

  • MRI Magnetic resonance imaging: brain and CT are both ideal for imaging the head.
  • They produce individual slices which allow each structure to be viewed independently and both sides can be easily compared.
  • MRI is superior for soft tissue structures and CT for bony structures.
  • Cost and availability are currently the major factors limiting their use.
  • Ultrasound can be useful for examining the soft tissue structures of the head and can image through open fontanelles and also through the thin bone wall of the tympanic bulla.
  • Advantages of ultrasound include its wide availability, low cost, non-invasive nature and no need for sedation.

Decision Taking

Criteria for choosing test

  • Is radiography the most appropriate diagnostic procedure for the patient?
  • Will it give clinically relevant additional information?
  • Will the management of the case be affected by the radiological findings?
  • Which projection/combination of projections is most appropriate?

Choosing the right projection

Ventrodorsal projection

  • Produces similar projection to dorsoventral (basic skull).
  • Can compare both sides of skull on single view.
  • Calvarium and sinuses closer to film therefore less magnification and distortion of these regions than in dorsoventral projection.
  • However, more difficult to position symmetrically so dorsoventral view usually preferable.

Ventrodorsal open mouth view

  • Allows visualization of the nasal chambers, frontal sinuses and maxilla.
  • Largely superceded by dorsoventral intraoral view (basic skull) as more difficult to position and increased distortion due to angulation of beam relative to film.
  • Can be useful if flexible casette or non-screen film required for dorsoventral intraoral (occlusal) projection is not available.
  • Ventrodorsal positioning is more difficult and symmetry is required to allow comparison between sides.

Rostrocaudal open mouth projection

  • Allows visualization of the tympanic bullae, odontoid process of the axis and foramen magnum.
  • Projects these structures ventral to the temporal bone.
  • Both tympanic bullae projected onto a single film allowing comparison.
  • Careful positioning required to ensure symmetry.

Lateral oblique view

  • Also known as the nose up view.
  • Allows visualization of the lower temporomandibular joints and tympanic bullae.
  • Superior visualization of the temporomandibular joints free from superimposition than with the 45°DV oblique.
  • Long axis of condyloid process is projected perpendicular to film allowing good visualization of the joint space.
  • Each joint or bulla projected independently so views of both sides of the skull are necessary and careful positioning is required to ensure the same angle of obliquity thus allowing comparison.
  • Can examine each temporomandibular joint in open and closed positions.

Dorsoventral oblique views

  • Series of angles possible depending on region of interest.
  • Can be performed in sedated animals for general survey but larger amount of superimposition than other views for each region.
  • Each structure is projected independently so views of both sides of the skull are necessary and careful positioning is required to ensure the same angle of obliquity thus allowing comparison.
  • 45° dorsoventral oblique projectionallows visualization of the temporomandibular joints.
  • Larger amount of superimposition than lateral oblique view.
  • Both joints imaged on each film but in different planes so have to repeat from other side to allow comparison.
  • Uppermost joint imaged best and can visualise caudal surface of the condyloid process.
  • 20° dorsoventral oblique projection allows visualization of the frontal sinus nearest the film.
  • Natural angle dorsoventral oblique projection (approximately 10°) allows visualization of the maxillary and mandibular premolar and molar teeth nearest the film and the tooth roots of the teeth furthest from the film.
  • Larger amount of superimposition than open mouth ventrodorsal and dorsoventral 45° oblique views.
  • Angles from 10 - 30° allow visualization of the tympanic bulla and the retroarticular process and mandibular fossa of the temporomandibular joint furthest from the film.

Ventrodorsal 20° oblique projection

  • Allows visualization of the lower tympanic bulla and temporomandibular joint and the upper frontal sinus.
  • Each structure is projected independently so views of both sides of the skull are necessary and careful positioning is required to ensure the same angle of obliquity thus allowing comparison.

Dorsoventral intra-oral (occlusal) projection

  • Variation on the dorsoventral intra-oral view used to image the nasal chambers (basic skull).
  • Allows visualization of the maxillary incisors and the rostral maxilla with minimal distortion.

Ventrodorsal intra-oral projection

  • Two variations of this view.
    • The first allows visualization of the mandible.
    • The second allows visualization of the mandibular incisors and the rostral mandible with minimal distortion

Lateral intraoral dental projection 

  • Allows visualization of selected molar and premolar teeth Dental radiography: overview.
  • Reqiures small flexible cassettes or dental occlusal films.

Open mouth ventrodorsal 45° oblique projection

  • Allows visualization of the maxillary molars, premolars and maxilla nearest the film.

Open mouth dorsoventral 45° oblique projection

  • Allows visualization of mandibular molars, premolars and mandible nearest the film.

Risk assessment

  • All radiographic proceures must be performed in compliance with the Ionising Radiation Regulations 1998.
  • The patient must be able to undergo general anesthesia.

Requirements

Personnel

Other involvement

  • Radiographer, vet nurse, technician or vet to produce radiographs.
  • Radiologist or vet to interpret radiographs
  • Everyone involved must be aware of ionizing radiation regulations and their implications.
  • Also must be aware of importance of positioning and exposure on the quality of the radiograph produced and labeling to identify left and right.

Materials Required

Minimum equipment

  • X-Ray machine.
  • Processing facilities.
  • Positioning aids - sandbags, foam wedges, radiolucent trough, tape, etc.
  • Film labeling facilities.
  • Left / right markers.
  • Cassettes with high detail screens.
  • Small, flexible cassettes for intra-oral views. These usually take the form of screens in light tight envelopes of varying size and can be home made or bought commercially. The screens and film can be cut to the appropriate size.
  • Non-screen film (eg mammography or dental occlusal film) can also be used for intra-oral views. Although it produces high detail images, it requires a large increase in exposure and so is a less desirable alternative.

Ideal equipment

  • High output X-Ray machine.
  • Automatic processor.
  • Rare earth high detail screens.
  • Exposure chart to minimize repeat exposures.

Minimum consumables

  • X-ray film (matched to screen).
  • Anesthetic agents and associated equipment.
  • Radiation monitoring badges for all personnel involved with radiography.
  • Adequately replenished processor chemicals.

Other requirements

  • Useful to have reference text for positioning handy especially when performing unfamiliar special views.
  • Gags are helpful for obliques where mouth needs to be open.

Preparation

Restraint

  • General anesthesia required.
  • Use positioning aids - should never be manually restrained.

Procedure

Approach

Step 1 - Ventrodorsal projection

  • Dog is placed in dorsal recumbency with the thoracic region in a trough and the forelegs drawn caudally.
  • Extend the head and neck so the hard palate is horizontal with the cassette.
  • Maintain in position by passing tape behind the maxillary canine teeth and securing to the table.
  • A foam pad under the neck and another under the nose help maintain this position.
  • The cassette is placed beneath the head.
  • Ensure the median saggital plane is vertical and the interpupillary line is horizontal.
  • Extend the ears laterally to avoid superimposition and ensure the tongue is centrally located.
  • Center on the midline either rostral or caudal to the eyes depending on the region of interest.
  • Endotracheal tube should be removed unless the examination is aimed purely at lateral structures.

Step 2 - Ventrodorsal open mouth projection

  • Dog is positioned as for a ventrodorsal skull view.
  • Tie or tape endotracheal tube and tongue to mandible.
  • Pass tape round mandibular canine teeth and open mouth fully. May be useful to tie to sandbag placed on abdomen.
  • Place cassette beneath dogs head.
  • Direct beam at 20° in a caudoventral direction.
  • Center on the midline of the palate at the level of the 3rd upper premolar.

Step 3 - Rostrocaudal open mouth projection

  • Dog is placed in dorsal recumbency with thoracic region supported in a trough and the forelimbs drawn caudally.
  • Place the cassette behind the head.
  • Bend neck and orientate the skull so the nose points towards the X-ray tube with the hard palate and saggital plane perpendicular to the cassette. May help to place foam wedge behind head.
  • Open the mouth and secure in position with tape around the upper and lower canines. May be useful to tie tape from mandible to sandbag placed on abdomen.
  • Pull the tongue forward and secure to the lower jaw using tape.
  • Remove endotracheal tube or tape to mandible and extend ears laterally.
  • Center on the base of the tongue.
  • Symmetry is important so advisable to process film and check positioning is adequate before moving dog.
  • May be necessary to slightly angle the hard palate away from the vertical depending on the skull type and area under investigation.

Step 4 - Lateral oblique projections (nose-up)

  • The dog is placed in lateral recumbency with the side of interest nearest the cassette.
  • The skull is positioned as for a true lateral view (basic skull) then the nose is tilted upwards. This position is maintained using a foam pad.
  • The angle of the median saggital plane relative to the cassette varies from 10° - 30° for a dolichocephalic or mesaticephalic skull and 20° - 30° for a brachycephalic skull.
  • Center on the lower temporomandibular joint or tympanic bulla and collimate around the area of interest.
  • Both open and closed mouth views should be taken of the temporomandibular joints.
  • Each joint imaged separately so must perform from both sides.
  • Each side of the skull must be projected at the same angle of obliquity to allow comparison.

Step 5 - Dorsoventral oblique projections

  • The dog is placed in lateral recumbency.
  • The frontal sinus and teeth under investigation should be nearest the film while the temporomandibular joint under investigation should be furthest from it.
  • Place the cassette beneath the head.
  • Natural dorsoventral oblique projection (approximately 10°) - head is positioned so the zygomatic arch, mandible and nose are resting on the cassette.
  • 20° dorsoventral oblique projection - dorsal aspect of head is elevated using a foam wedge so the interpupillary line is 20° to the vertical / sagittal plane is 20° to horizontal.
  • Variety of angles can be acheived by progressively elevating the dorsal aspect of the head.
  • 45° dorsoventral oblique view - head can be placed in V-shaped trough and positioned so interpupillary line is at 45° to horizontal.
  • Position is maintained by a sandbag over the neck.
  • Center to the area of interest and collimate accordingly.
  • Each side of the skull must be projected at the same angle of obliquity to allow comparison.

Step 6 - Ventrodorsal 20° oblique projection

  • The dog is placed in lateral recumbency with the tympanic bulla of interest nearest the cassette.
  • Ventral aspect of the skull is elevated using a foam wedge so sagittal plane of head is at 20° to horizontal.
  • Position is maintained by an additional foam wedge beneath the neck and shoulder.
  • Center at the base of the cranium.
  • Each side of the skull must be projected at the same angle of obliquity to allow comparison.

Step 7 - Dorsoventral intra-oral projection (incisors)

  • Dog is positioned as for dorsoventral intra-oral view of nasal chambers (basic skull).
  • Endotracheal tube is tied to mandible.
  • Flexible cassette is placed in mouth on top of tongue and endotracheal tube.
  • Direct beam approximately 20° from the vertical in a rostro-caudal direction.
  • Center on incisors.

Step 8 - Ventrodorsal intra-oral projections

  • Dog is positioned as for a ventrodorsal skull view.
  • Endotracheal tube is tied to maxilla and tongue is pulled rostrally and positioned centrally.
  • Mandible view- corner of cassette placed as far caudally in mouth as possible above endotracheal tube and below tongue so edges reach commisures of lips.
  • Center to middle of mandible.
  • Incisor view -cassette placed in mouth.
  • Direct beam approximately 20° from the vertical in a rostro-caudal direction.
  • Center on midline of chin.

Step 9 - Lateral intra-oral dental projections

  • Dog is positioned in lateral recumbency with side under investigation uppermost.
  • Flexible cassette or dental occlusal film is placed in mouth against medial surface of mandible and held in place using forceps.
  • Center on affected tooth.
  • May be necessary to angle the tube head to keep the film perpendicular to the beam.

Step 10 - Open mouth ventrodorsal 45° oblique projection

  • Position dog with affected side downwards.
  • Place the cassette below the head.
  • Rotate the dog onto its back until the interpupillary line is 45° to the cassette.
  • Place a foam wedge under the mandible to help maintain the position of the head.

    Open mouth using radiolucent block, eg foam wedge, roll of gauze, syringe case. Metal gag can be used as long as doesnt obscure area of interest.
  • Center beam on first molar of lower maxilla.

Open mouth dorsoventral 45° oblique projection

  • Position dog with affected side downwards.
  • Place the cassette below the head.
  • Rotate the dog onto its sternum until the interpupillary line is 45° to the cassette.
  • Place a foam wedge under the maxilla to help maintain the position of the head.
  • Open mouth using radiolucent block.
  • Center beam on first molar of lower mandible.

Aftercare

Outcomes

Reasons for Treatment Failure

  • Poor patient positioning resulting in distortion of the structure or superimposition preventing it being visualized.
  • Inappropriate views selected.
  • Failure to identify left and right.
  • Failure to remove endotracheal tube for some views.
  • Inappropriate exposure factors, film handling or film processing and equipment failure resulting in non-diagnostic films.

Supplementary studies

  • Specialized views to allow further investigation of the teeth including parallel and bisecting angle techniques Dental radiography: overview.
  • Contrast radiographic procedures to further investigate soft tissue structures associated with the head.
  • Canalography allows evaluation of the external ear canal and the integrity of the tympanic membrane to be assessed but is rarely useful.
  • Sialography allows visualization and evaluation of the salivary glands and their associated ducts.
  • Dacryocystorhinography allows visualization and evaluation of the nasolacrimal duct.
  • Sinography permits investigation of the depth and origin of draining wounds or fistulous tracts.

Further Reading

Publications

Refereed papers