Felis ISSN 2398-2950

Computed tomography: head

Synonym(s): CT: head, CT: ear/bullae, CT: oral cavity and masticatory system

Contributor(s): Fraser J McConnell, Federica Morandi, Tobias Schwartz

Introduction

  • In recent years computed tomography (CT) Computed tomography (CT) 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

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.
  • 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

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Preparation

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Procedure

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Aftercare

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Outcomes

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Prognosis

  • Not applicable. This is a diagnostic procedure only.

Image interpretation

Ear/bullae

  • Normal anatomy Tympanic bullae: transverse - bone window CT  Tympanic bullae: transverse - soft tissue window CT .
  • 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 Tympanic bullae: (MCT ear canal) transverse - bone window CT  Tympanic bullae: (MCT ear canal) transverse - soft tissue window CT .
    • A thin bony septum divides the bullae into a large ventromedial compartment and smaller dorsolateral compartment.
    • 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 (0.5 x 0.7 mm slices).
    • Neoplasia Nasal cavity: 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, adenocarcinoma, ceruminous gland carcinoma).

Oral cavity and masticatory system

  • Normal anatomy:
    • Dental formula:
      • Deciduous: I 3/3 C 1/1 PM 3/2 M 0/0.
      • Permanent: I 3/3 C 1/1 PM 3/2 M 1/1.
    • 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 cats.
        • Variable degrees of resorption and demineralization of the alveolar bone, tooth loss.
        • Erosion of dental neck and root resorption.
      • 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 maxillary teeth involved.
      • Neoplasia of dental origin Teeth: tumor:
        • 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 Mouth: 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, osteosarcoma , 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:
          • Common in cats with presumed mandibular trauma.
          • 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.
    • Neoplasia:
      • Benign:
        • Osteoma: round, smooth borders, very dense.
        • Occasionally seen in cats. 
        • 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

  • Recent references from PubMed and VetMedResource.
  • Shanaman M, Seiler G, Holt D E (2012) Prevalence of clinical and subclinical middle ear disease in cats undergoing computed tomographic scans of the head. Vet Radiol Ultrasound 53 (1), 76-79 PubMed.
  • King A M, Weinrauch S A, Doust R et al (2007) Comparison of ultrasonography, radiography and a single computed tomography slice for fluid identification within the feline tympanic bulla. Vet J 173 (3), 638-644 PubMed.
  • Detweiler D A, Johnson L R, Kass P H et al (2006) Computed tomographic evidence of bulla effusion in cats with sinonasal disease: 2001-2004JVIM 20 (5), 1080-1084 PubMed.
  • 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.
  • Gotthelf L N (2004) Diagnosis and treatment of otitis media in dogs and cats. Vet Clin North Am Small Anim Pract 34 (2), 469-487 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.
  • 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.
  • Schwarz T, Weller R, Dickie A M et al (2002) Imaging of the canine and feline temporomandibular joint: A review. Veterinary Radiology and Ultrasound 43 (2), 85-97 PubMed.


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