Introduction

• In recent years computed tomography (CT) Computed tomography has become more readily available in the veterinary field.
• CT uses x-ray to generate cross-sectional images of the body that avoid the superimposition of neighboring structures, which makes it particularly useful when imaging areas of complex anatomy, such as the skull. Furthermore, CT provides excellent contrast resolution, many times superior to that of conventional radiography, and is recognized as superior to all other imaging modalities for imaging bone. In general, however, MRI is preferred over CT for brain imaging due to its superior soft tissue contrast.
• CT has been traditionally carried out with the patient under general anesthesia, due to the requirement to remain immobile for the entire duration of the scan, usually 3-10 minutes depending on the area imaged. With the implementation of helical CT in the 80's, and more so with the newly developed multiple row detectors CT scanners in the late 90's, the time necessary for image acquisition has dramatically decreased. A state-of the-art multislice scanner can acquire images of the whole body of an adult man in < 12 seconds, of the entire thorax in approximately 3 seconds. Even if the cost of such equipment makes it prohibitive in a veterinary practice, single slice helical scanners allow for shorter acquisition times for which patient immobilization may be achieved using heavy sedation alone, rather than general anesthesia. This has the potential to increase the number of animals that can be scanned as well as allowing scanning of animals that are not good candidates for anesthesia.

Uses

Indications

Ear/bullae

• Suspected otitis externa Skin: otitis externa , media Otitis media or interna Otitis interna.
• Suspect nasopharyngeal polyps.
• Pre-operative assessment prior to surgical excision of neoplastic processes.

Oral cavity and masticatory system

• Evaluation of disease.
• Evaluation of oral masses
• Pre-operative assessment prior to surgical excision of neoplastic processes.
• Assessment of the temporomandibular joint and masticatory musculature in patients with problems opening or closing of the mouth.
• Radiation therapy planning Radiotherapy.

Advantages

• The major advantage of CT over conventional radiographs is the higher contrast resolution: CT can discriminate density differences in tissues of 0.25-0.5%, whereas radiography can only differentiate density differences of about 10%.
• Cross-sectional images avoid superimposition of structures in areas of complex anatomy, which is a great advantage for the skull and nose.
• Pre and postprocessing manipulation of the raw CT data is possible, allowing tissues of varying density to be better evaluated.
• Multiplanar image reconstruction and 3D renderings can be configured.
• Initial cost and maintenance of equipment usually less than MRI Magnetic resonance imaging: basic principles Magnetic resonance imaging: brain.
• Imaging times are significantly reduced as compared to MRI, even when axial (conventional) CT is used, and more so with single slice helical (and multiple-row detectors) scanners.
• Refurbished CT equipment can be purchased at affordable prices.
• CT is superior to all other imaging modalities when examining bone.
• Patients with metallic implants/foreign material in the head can usually be imaged (this is generally not possible with MRI).
• With single-slice, helical, and more so with the newest multiple-row detectors scanners, sedation alone could be sufficient for restraint.

Disadvantages

• General anesthesia is typically required General anesthesia: overview.
• Area of interest must fit within the CT gantry (usually not a problem when imaging the canine head).
• Standard CT tables are designed to accommodate the human torso and have weight limitations of 150-200 kg (usually not a problem when imaging dogs).
• Study interpretation may be lengthy due to the large number of images acquired with most CT examinations.
• Ionizing radiation is utilized.
  
  For most intracranial disease MRI is more informative due to better soft tissue contrast.
• High density streak artifacts from metal implants/foreign material may reduce image quality.
• Caudal fossa imaging (cerebellum and brainstem) can be problematic due to beam hardening artifact.

Requirements

Preparation

Procedure

Aftercare

Outcomes

Prognosis

• Not applicable. This is a diagnostic procedure only.

Image interpretation

Ear/bullae

• Normal anatomy  .
• Both the horizontal and vertical components of the ear canal should be air filled and visible throughout their length:
  • The osseous bullae should be thin-walled, air-filled and symmetrical.
  • The small bones of the inner ear are clearly visible.
  • The tympanic membrane and Eustachian tube are not usually visible.
• Pathology:
  • Otitis externa changes associated with the external ear canal:
    • Thickening of the wall of the external ear canal.
    • Obliteration of the external ear canal by a soft tissue opacity.
    • In chronic cases, mineralizations associated with the external ear canal  :
      • Variable severity, but typically smooth and benign-appearing.
  • Otitis media changes associated with the tympanic membrane, tympanic bulla, auditory ossicles:
    • Fluid/soft tissue density in the tympanic bullae .
    • Bulla osteitis:
      • Thickening of the osseous bulla wall.
      • Usually smooth and non-aggressive in appearance.
  • Otitis interna changes associated with the cochlea, internal acoustic meatus, semicircular canal:
    • These structures can be evaluated using high resolution CT (1x1 mm slices).
  • Neoplasia:
    • Focal mass associated with the ear canal, bulla or extending from the surrounding soft tissues.
    • A distinct mass may be see only after administration of IV contrast, as the mass will show contrast enhancement compared to surrounding fluid/exudate which will not enhance.
    • Lysis and aggressive new bone production associated with the bulla is possible.
    • Carcinomas are most common (squamous cell carcinoma Skin: squamous cell carcinoma , adenocarcinoma Adenoma / adenocarcinoma , ceruminous gland carcinoma).

Oral cavity and masticatory system

• Normal anatomy:
  • Dental formula:
    • Deciduous: I 3/3 C 1/1 PM 3/3 M 0/0.
    • Permanent: I 3/3 C 1/1 PM 4/4 M 2/3.
  • The carnassial teeth are the mandibular M I and the maxillary PM IV.
  • Temporomandibular joint: Mandibular condyle (mandible) and fossa (zygomatic bone), disk (not visible on CT), retroarticular process of the mandibular fossa).
  • Masticatory muscles: masseter, temporalis, medial and lateral pterygoid muscles.
• Pathology:
  • Tooth pathology:
    • Periodontal disease:
      • Common in older dogs.
      • Variable degrees of resorption and demineralization of the alveolar bone, tooth loss.
    • Periapical abscess   :
      • Focal area of lysis centered around a specific tooth root.
      • Bone lysis can be marked and can affect the maxilla and nasal cavity with secondary rhinitis if upper PM IV is affected.
    • Neoplasia of dental origin:
      • Young animals:
        • Ameloblastoma expansile, lace-like multiloculated lytic lesion .
        • Odontoma very radiodense and containing multiple elements of teeth.
      • Epulides:
        • Fibromatous soft tissue mass of variable size associated with the gingiva.
        • Ossifying similar to fibromatous, but can ossify and has broader base of attachment.
        • Acanthomatous can invade adjacent bone and may appear very similar to malignant soft tissue tumors; can also show mineralizations.
  • Oral neoplasia:
    • Malignant:
      • Soft tissue tumors with secondary bone invasion (melanoma and squamous cell carcinoma most common):
        • Soft tissue mass of various size +/- variable amount of associated bony lysis   .
      • Primary osseous tumors (fibrosarcoma Fibrosarcoma , osteosarcoma Osteosarcoma , chondrosarcoma Chondrosarcoma ):
        • Can show variable amount of new bone proliferation and periosteal reaction as well as lysis .
    • Benign:
      • Less common than malignant:
        • Fibroma, osteoma, chondroma.
        • Tend to show more sclerosis and have well-demarcated margins.
        • May destroy adjacent bone by pressure necrosis.
  • Masticatory system:
    • Temporomandibular joint abnormalities:
      • Luxation of the mandibular condyle:
        • Consequence of trauma.
        • Best seen on sagittal reconstruction.
      • Ankylosis:
        • Ankylosing new bone formation around joints.
        • Erosion of joint surface.
  • Fractures of mandibular ramus and/or zygomatic arch.
    • May restrict mandibular movement (= false ankylosis, best seen on open mouth CT).
    • Dorsal plane and 3D reconstructions most helpful.
  • Developmental diseases:
    • Craniomandibular osteopathy Craniomandibular osteopathy :
    • Bilateral extensive new bone formation along mandible and tympanic bullae.
    • Can lead to false ankylosis of the TMJ.
  • TMJ Dysplasia:
    • Misshapened TMJ joint.
    • Has been associated with open jaw lock
      • CT: Rostral luxation of mandibular condyle and contralateral displacement of coronoid process of the mandible lateral to zygomatic arch.
  • Neoplasia:
    • Benign:
      • Osteoma: round, smooth borders, very dense.
      • May cause false ankylosis.
    • Malignant:
      • Osteosarcoma, fibrosarcoma, multilobular osteochondrosarcoma, other.
      • Can be predominantly osteoproductive or lytic.
      • Multilobular osteochondrosarcoma (dogs) have typical appearance of lobulated bony mass.
  • Masticatory musculature:
    • Neurogenic atrophy: trigeminal nerve root pathology will cause major atrophy of masticatory muscles, easily visible on CT.
    • Myositis: inflammatory early response may cause focal contrast enhancement. Chronic fibrotic changes will cause local muscle atrophy.
    • Neoplasia: sarcomas - rare.

Further Reading

Publications

Refereed papers

Ear/bullae

• Recent references from PubMed and VetMedResource.
• Bischoff M G, Kneller S K (2004) Diagnostic imaging of the canine and feline ear. Vet Clin North Am Small Animal Pract 34 (2), 437-458 PubMed.
• Dickie A M, Doust R, Cromarty L et al (2003) Comparison of ultrasonography, radiography and a single computed tomographic slice for the identification of fluid within the canine tympanic bulla. Res Vet Sci 75 (3), 209-216 PubMed.
• Garosi L S, Dennis R, Schwarz T (2003) Review of diagnostic imaging of ear diseases in the dog and cat. Veterinary Radiology and Ultrasound 44 (2), 137-146 PubMed.
• Ziemer L S, Schwarz T, Sullivan M (2003) Otolithiasis in three dogs. Veterinary Radiology and Ultrasound 44 (1), 28-31 PubMed.
• Russo M, Covelli E M, Meomartino L et al (2002) Computed tomographic anatomy of the middle ear. Veterinary Radiology and Ultrasound 43 (1), 22-26 PubMed.
• Love N E, Kramer R W, Spodnick G J et al (1995) Radiographic and computed tomographic evaluation of otitis media in the dog. Veterinary Radiology and Ultrasound 36 (5), 375-379 VetMedResource.

Oral cavity

• Recent references from PubMed and VetMedResource.
• Kafka U C, Carstens A, Steenkamp G et al (2005) Diagnostic value of MRI and CT for oral masses in dogs. J S Afr Vet Assoc 75 (4), 163-168 PubMed.

Masticatory System

• Recent references from PubMed and VetMedResource.
• Schwarz T, Weller R, Dickie A et al (2002) Imaging of the canine and feline temporomandibular joint: A review. Veterinary Radiology and Ultrasound 43 (2), 85-97 PubMed.