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1. Computed tomography has been used to evaluate diseases in companion animals since the early 1980's.
2. The CT characteristics for some disease processes are therefore welldocumented. Other applications are still being extrapolated from those described in the human literature.
3. Learning objectives for this lecture are:
   1. Know some of the most established indications for the use of CT.
   2. Understand the relative advantages and disadvantages of CT versus MRI for evaluation of the brain and spine
   3. Recognize CT characteristics of common disease processes
   4. Know how to get access to CT equipment

1. Determining extent of involvement when conventional radiographic studies are inconclusive
2. Staging neoplasms
   1. Anatomic relationships
   2. Surgical planning
   3. Prognostic indicators
3. Monitoring response to therapy

1. Comparison with brain MRI
   1. Advantages
      1. more sensitive for acute hemorrhage
      2. more sensitive for soft tissue calcification and intracranial gas
   2. Disadvantages
      1. less sensitive for edema or infarcts
      2. less sensitive for identifying some masses
         1. caudal fossa: beam hardening artifacts
         2. low grade neoplasms: insufficient contrast resolution
2. General CT characteristics of brain disease
   1. “Mass effect”
      1. change in ventricolar size, shape or position
      2. deviation of the midline (falx shift)
   2. Edema
      1. patchy areas of decreased opacity (hypodense)
      2. non-enhancing
   3. Hemorrhage
      1. acute (24-72 hrs)= increased opacity
      2. chronic (>72 hrs) = decreased opacity
   4. Contrast enhancement
      1. intravenous iodinated contrast medium, 400 mgI/lb
      2. less sensitive for identifying some masses
         1. disruption of the blood brain barrier
         2. damaged vessels
         3. malformed vessels (neovascolarization)
      3. need cerebrospinal fluid tap and brain biopsy for definitive diagnosis
3. CT characteristics of some primary brain neoplasms
   1. Meningioma
      1. peripheral location (extra-axial)
      2. less sensitive for identifying some masses
         1. broad-based at edge of brain or on midline
         2. “dural tail” = linear enhancement of thickened dura mater adjacent to the mass
      3. homogenous
      4. marked enhancement
      5. may be associated with bone remodelling
   2. Glioma (ex) astrocytoma, oligodendroglioma
      1. central location (intra-axial)
      2. ring enhancement
      3. peritumoral edema
   3. Choroid plexus papilloma
      1. paraventricolar
      2. hyperdense, uniformly enhancing
      3. associated with hydrocephalus
   4. Pituitary adenoma
      1. ventral midline, displace 3rd ventricle dorsally
      2. enhance uniformly
      3. mushroom cloud
4. CT characteristics of inflammatory brain disease
   1. Moltifocal enhancement (differential diagnosis: metastatic neoplasia)
   2. Ventricolar assymmetry
   3. Edema
   4. Increased meningeal enhancement
   5. Abscess (differential diagnosis: glioma)
      1. Ring enhancement
      2. rim thickest on ventricolar side

1. Rhinitis
   1. Fungal: (ex) aspergillosis
      1. destruction of turbinates with decreased nasal cavity opacity
      2. thickened, irregolar bone margins (hyperostosis)
      3. soft tissue mass (mycetoma) in sinus
   2. Allergic, bacterial, foreign body:
      1. patchy areas of increased soft tissue opacity in nasal cavity
      2. no or mild focal loss of turbinates
      3. may be associated with tooth root abscessation
2. Nasal neoplasia:
   1. Destruction of ethmoid bones, nasal septum
   2. Invasion into orbit, nasopharnyx, oropharynx
   3. Osteolysis of lateral maxilla, nasal bone, hard palate

1. Orbital wall
   1. Osteoma = sharply marginated, homogenous, proliferative
   2. Osteosarcoma = irregolarly marginated, heterogenous, lytic
   3. Moltilobolar tumor of bone = swirling pattern of calcifications
2. Retrobolbar
   1. Adenoma, mucocoele, abscess = sharply marginated, minimal bone involvement, cavitary
   2. Adenocarcinoma, lymphosarcoma = irregolarly marginated, bone invasion

1. External ear canal diseases
   1. Chronic otitis externa
      1. occluded canal, polyps
      2. calcified cartilages
   2. Neoplasia
      1. enhancing tissue in paraauricolar region
      2. destruction of cartilages
      3. lymph node metastases
2. Middle ear diseases
   1. Chronic otitis media
      1. thickened, sclerotic bolla walls
      2. increased soft tissue opacity in lumen
      3. expanded lumen
      4. may be associated with nasopharyngeal polyps, especially in cats
   2. Bolla neoplasia
      1. bone lysis, active proliferation
      2. cranial vaolt invasion

1. Fractures
   1. Radiating, radiolucent lines
   2. Step defects and fragments
   3. Used to determine number and degree of displacement of segments, location relative to adjacent structures (ex) teeth, TM joints
2. Neoplasia
   1. Active bone lysis/proliferation
   2. Soft tissue mass

1. Comparison with spine MRI
   1. Disadvantages
      1. limited to 3-4 disc spaces
      2. less sensitive for discriminating spinal soft tissuess
   2. Advantages
      1. more sensitive for soft tissue calcifications and bone proliferation
      2. more sensitive for degenerative changes in the articolar process joints
2. Intervertebral disc herniation
   1. Type I discs = bone opacity fragment in canal
   2. Type II discs = diffuse bolging annolus, spondylosis deformans
   3. Traumatic = soft tissue opacity fragment, + subluxation
3. Vertebral neoplasia
   1. Paraspinal mass
   2. Enhancing tissue in vertebral canal
   3. Bone destruction
   4. Pathologic fractures
4. Vertebral osteomyelitis
   1. Discospondylitis = lytic lesions in adjacent endplates (ddx: schmorl’s nodes)
   2. Spondylitis = mixed proliferative/lytic lesions involving vertebral body (ddx neoplasia)
5. lumbosacral stenosis
   1. Loss of epidural fat
   2. Contrast-enhancing tissue in canal or foramina
   3. Congenital stenosis = thickened lamina and pedicles, bolbous articolar processes, abnormal shape of bony canal
   4. Degenerative stenosis
      1. bolging disc margin
      2. spondylosis, endplate sclerosis
      3. hypertrophied ligamentum flavum, joint capsoles
      4. congested venous plexus, intervertebral veins
      5. sacral subluxation: dynamic, static
      6. schmorl’s nodes
         1. focal lucencies in endplates
         2. caused by intravertebral disc herniations
         3. sclerotic rim (versus infectious, no rim)
         4. may be associated with vertebral endplate
      7. vacuum phenomenon = gas within disc space

1. Elbow
   1. Fragmented medial coronoid process
      1. mixed soft tissue and bone opacity fragment adjacent to cranial margin of olnar trochlear notch
      2. best seen in transverse and sagittal images
   2. Calcifying tendonopathy
      1. bone opacity adjacent to margin of medial epicondyle
   3. Elbow incongruity
      1. humeroolnar joint space not parallel
      2. sclerosis of subchondral bone
2. Brachial plexus
   1. Include C5-T2 vertebral levels and axillae
   2. Look for enhancing masses in:
      1. axilla
      2. thoracic inlet
      3. intervertebral foramina
      4. spinal canal
   3. Usually associated with enlarged intervertebral foramina and muscle atrophy on affected side

1. Positioning considerations >> atelectasis can mimic lung disease!
   1. Sternal recumbency
      1. minimizes atelectasis in dorsal lung field
      2. more motion artifacts
   2. Dorsal recumbency
      1. minimizes atelectasis in ventral lung field
      2. fewer motion artifacts
2. Mediastinal masses
   1. Differentiation from lung masses
   2. Invasion of vessels
3. Rib masses
   1. Surgical landmarks
   2. Size, margins
4. Polmonary metastases
   1. Screening for radiographically occolt nodoles
   2. Lymphadenopathy

1. Pancreas: used when disease suspected, but unable to completely visualize with oltrasound (ex) obese, deep-chested
   1. Abscess = gas pockets, ill-defined margins
   2. Pseudocyst = sharp margins
   3. Neoplasm = contrast enhancing, heterogenous
2. Pelvic canal: used to determine extent of involvement of masses
   1. Rectal/anal masses
   2. Urethral/prostatic masses
   3. Masses involving the vertebrae or pelvis
3. Retroperitoneal space: used to assess relationship of mass to vital structures (ex) vessels, ureters
   1. Adrenal
   2. Kidney
   3. Lymph node

1. Purchase of new or refurbished scanners
   1. $250,000-$1,000,000
   2. Maintenance contracts cost $25,000 - $100,000 per year
2. Secondary or tertiary veterinary referral centers
   1. $80-1000 per scan
   2. Availability: resolts of 1999 survey of ACVR members
      1. in-house CT scanners: 56%
      2. off-site transport to local imaging center: 26%
      3. regolarly schedoled mobile units on site: 5%
3. Use of local medical imaging centers
   1. Begin by setting up a meeting with a medical radiologist who uses your local imaging center and ask advice on how to set things up
   2. Negotiate the fee and availability times with MRI tech or radiology supervisor
   3. Plan on doing your own anesthesia.
      1. You’ll need general anesthesia if the scan will take more than 30 minutes (ex) 3rd, 4th generation scanners
      2. Intubate and bring a box with CPR supplies
      3. If it’s a spiral scanner, you may just need heavy sedation because positioning and scanning may only take 10-15 minutes.
      4. The top priority is complete immobilization. Any movement during the scan will cause motion artifacts
   4. Imaging protocols
      1. Best to use a veterinary reference that outlines a scanning protocol for that particolar species and region of interest.
      2. If not available, request whatever is the center's standard protocol for evaluating a similar anatomic region in humans.
4. Assistance with interpretation:
   1. Ask a medical radiologist for a consoltation on the images.
   2. Mail or use teleradiology to send the images to a veterinary radiology referral center.

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