Contributors: Alison Dickie, Fraser J McConnell

 Species: Feline   |   Classification: Techniques

Introduction Requirements Preparation Procedure Aftercare Outcomes Further Reading

Introduction

  • The basic radiographic projections used to image the head have been described Radiography: skull (basic) Radiology: skull.
  • The complex anatomy of the head 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. Specifically angled projections can be difficult to achieve in cats due to their round head conformation.
  • The production of good quality radiographs using appropriate film-screen combinations with careful radiographic and processing techniques is also vital to aid interpretation.
  • The head 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 cat.
  • 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, neoplasia.
  • Identification of bony involvement in association with other conditions, eg otitis media  Otitis media, soft tissue 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: basic principles and CT Computed tomography (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  Head: normal - ventrodorsal radiograph .
  • 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 cassette 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.
Caudorostral projection with horizontal beam
  • Allows differentiation of fluid from soft tissue masses in frontal sinuses (fluid-gas interface) Head: frontal sinuses - caudorostral radiogaph .
Rostrocaudal open mouth projection
  • Allows visualization of the tympanic bullae.
  • Projects these structures ventral to the temporal bone.
  • Both tympanic bullae projected onto a single film allowing comparison.
  • Careful positioning required ensuring symmetry Head: normal - rostrocaudal radiograph .
Rostro 10° ventral-dorsol caudal oblique projection
  • Allows visualization of the tympanic bullae freeprojected from head and neck tissues Head: right middle and external ear tumor - rostro10°ventral-dorsocaudal oblique radiograph .
  • Easier symmetrical positioning of tympanic bullae compared to rostrocaudal open mouth view.
Oblique projections with rotation around the long axis of the head 
  • Used to visualize individual feline temporomandibular joints and tympanic bullae, maxillae and mandibles Head: mandibular fracture and TMJ luxation - oblique radiograph .
    Requires general anesthesia. 
  • 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.
  • Structure close to the cassette is freeprojected ventrally to other skull bones and thus seen best.
  • Some views are taken with an open mouth position (maxilla and mandible).
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.
  • Reqiures small flexible cassettes or dental occlusal films.

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 General anesthesia: overview .

Requirements

Personnel

Other involvement

  • Radiographer, veterinary nurse, technician or veterinarian to produce radiographs.
  • Radiologist or veterinarian 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

  • Cat 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 plane is vertical and the interpupillary line is horizontal.
  • 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 (nose)

  • Cat is positioned as for a ventrodorsal skull view.
  • Tie or tape endotracheal tube to mandible.
  • Pass tape round mandibular canine teeth and open mouth fully.
  • Place cassette beneath cats head.
  • Direct beam at 10° in a caudoventral direction.
  • Center on the midline of the palate at the level of the 3rd upper premolar.

Step 3 - Caudorostral projection with horizontal beam (frontal sinuses)

  • Position cat in ventral recumbency.
  • Position head horizontally on block, nose slightly upward Radiography: skull caudorostral 01 .
  • Position radiographic cassette vertically in front of cats nose.
  • Position X-ray tube behind cats hindlegs with proper distance Radiography: skull caudorostral 02 .

Step 4 - Rostrocaudal open mouth projection (tympanic bullae)

  • Cat 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 sagittal 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.
  • Secure to the lower jaw using tape.
  • Remove endotracheal tube or tape to mandible.
  • Center on the base of the tongue.
  • Symmetry is important so advisable to process film and check positioning is adequate before moving cat.

Step 5 - Rostro10° ventral-dorsocaudal oblique projection (tympanic bullae)

  • Put patient in dorsal recumbency, extend head 10° from vertical position of the mandible, mouth closed.
  • Maintain head position with foam wedges and tape.
  • Place cassette beneath head.
  • Center beam 1 cm ventral to mout  Radiography: skull rostro10°ventral-dorsocaudal .

Step 6 -  Left 20° Ventral-RightDorsal Oblique view / Right 20° Ventral-LeftDorsal Oblique view (TMJ & tympanic bulla)

  • Put cat in lateral recumbency, mouth closed.
  • Place cassette beneath head.
  • Center beam at area of TMJ.
  • Rotate beam 20° towards ventral. Alternatively rotate the head 20° dorsally around its median axis.
  • If the head is rotated maintain head and body position with foam wedges.

Step 7 - Left 45°Ventral-RightDorsal Oblique projection / Right 45°Ventral-LeftDorsal Oblique projection with open mouth (maxilla)

  • Put cat in lateral recumbency, mouth open.
  • Place cassette beneath head.
  • Center beam at nasal cavity.
  • Rotate beam 45° towards ventral. Alternatively rotate the head 45° dorsally around its median axis.
  • If the head is rotated maintain head and body position with foam wedges.
  • Maintain open mouth position with radiolucent gag or manually with radiolucent tape.

Step 8 - Left 45° Dorsal-RightVentral Oblique projection /  Right 45° Dorsal-LeftVentral Oblique projection with open mouth (mandible)

  • Put cat in lateral recumbency, mouth open.
  • Place cassette beneath head.
  • Center beam at mandibles.
  • Rotate beam 45° towards dorsal. Alternatively rotate the head 45° ventrally around its median axis.
  • If the head is rotated maintain head and body position with foam wedges.
  • Maintain open mouth position with radiolucent gag or manually with radiolucent tape.

Step 9 - Dorsoventral intra-oral projection (incisors)

  • Cat 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 10 - Ventrodorsal intra-oral projections

  • Cat is positioned as for a ventrodorsal skull view.
  • Endotracheal tube is tied to maxilla.
  • Mandible view - corner of cassette placed as far caudally in mouth as possible between endotracheal tube and 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 11 - Lateral intra-oral dental projections

  • Cat 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.

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.
  • Contrast radiographic procedures to further investigate soft tissue structures associated with the head.
  • Dacryocystorhinography allows visualization and evaluation of the nasolacrimal duct.
  • Fistulography permits investigation of the depth and origin of draining wounds or fistulous tracts.

Further Reading

Publications

Refereed papers