Introduction:
- Computed tomography has been used to evaluate diseases in companion animals since the early 1980's.
- The CT characteristics for some disease processes are therefore welldocumented. Other applications are still being extrapolated from those described in the human literature.
- Learning objectives for this lecture are:
- Know some of the most established indications for the use of CT.
- Understand the relative advantages and disadvantages of CT versus MRI for evaluation of the brain and spine
- Recognize CT characteristics of common disease processes
- Know how to get access to CT equipment
General indications for CT in veterinary medicine
- Determining extent of involvement when conventional radiographic studies are inconclusive
- Staging neoplasms
- Anatomic relationships
- Surgical planning
- Prognostic indicators
- Monitoring response to therapy
Brain CT
- Comparison with brain MRI
- Advantages
- more sensitive for acute hemorrhage
- more sensitive for soft tissue calcification and intracranial gas
- Disadvantages
- less sensitive for edema or infarcts
- less sensitive for identifying some masses
- caudal fossa: beam hardening artifacts
- low grade neoplasms: insufficient contrast resolution
- General CT characteristics of brain disease
- “Mass effect”
- change in ventricolar size, shape or position
- deviation of the midline (falx shift)
- Edema
- patchy areas of decreased opacity (hypodense)
- non-enhancing
- Hemorrhage
- acute (24-72 hrs)= increased opacity
- chronic (>72 hrs) = decreased opacity
- Contrast enhancement
- intravenous iodinated contrast medium, 400 mgI/lb
- less sensitive for identifying some masses
- disruption of the blood brain barrier
- damaged vessels
- malformed vessels (neovascolarization)
- need cerebrospinal fluid tap and brain biopsy for definitive diagnosis
- CT characteristics of some primary brain neoplasms
- Meningioma
- peripheral location (extra-axial)
- less sensitive for identifying some masses
- broad-based at edge of brain or on midline
- “dural tail” = linear enhancement of thickened dura mater adjacent to the mass
- homogenous
- marked enhancement
- may be associated with bone remodelling
- Glioma (ex) astrocytoma, oligodendroglioma
- central location (intra-axial)
- ring enhancement
- peritumoral edema
- Choroid plexus papilloma
- paraventricolar
- hyperdense, uniformly enhancing
- associated with hydrocephalus
- Pituitary adenoma
- ventral midline, displace 3rd ventricle dorsally
- enhance uniformly
- mushroom cloud
- CT characteristics of inflammatory brain disease
- Moltifocal enhancement (differential diagnosis: metastatic neoplasia)
- Ventricolar assymmetry
- Edema
- Increased meningeal enhancement
- Abscess (differential diagnosis: glioma)
- Ring enhancement
- rim thickest on ventricolar side
Nasal CT
- Rhinitis
- Fungal: (ex) aspergillosis
- destruction of turbinates with decreased nasal cavity opacity
- thickened, irregolar bone margins (hyperostosis)
- soft tissue mass (mycetoma) in sinus
- Allergic, bacterial, foreign body:
- patchy areas of increased soft tissue opacity in nasal cavity
- no or mild focal loss of turbinates
- may be associated with tooth root abscessation
- Nasal neoplasia:
- Destruction of ethmoid bones, nasal septum
- Invasion into orbit, nasopharnyx, oropharynx
- Osteolysis of lateral maxilla, nasal bone, hard palate
Orbit CT
- Orbital wall
- Osteoma = sharply marginated, homogenous, proliferative
- Osteosarcoma = irregolarly marginated, heterogenous, lytic
- Moltilobolar tumor of bone = swirling pattern of calcifications
- Retrobolbar
- Adenoma, mucocoele, abscess = sharply marginated, minimal bone involvement, cavitary
- Adenocarcinoma, lymphosarcoma = irregolarly marginated, bone invasion
Bolla CT
- External ear canal diseases
- Chronic otitis externa
- occluded canal, polyps
- calcified cartilages
- Neoplasia
- enhancing tissue in paraauricolar region
- destruction of cartilages
- lymph node metastases
- Middle ear diseases
- Chronic otitis media
- thickened, sclerotic bolla walls
- increased soft tissue opacity in lumen
- expanded lumen
- may be associated with nasopharyngeal polyps, especially in cats
- Bolla neoplasia
- bone lysis, active proliferation
- cranial vaolt invasion
Craniofacial CT
- Fractures
- Radiating, radiolucent lines
- Step defects and fragments
- Used to determine number and degree of displacement of segments, location relative to adjacent structures (ex) teeth, TM joints
- Neoplasia
- Active bone lysis/proliferation
- Soft tissue mass
Spine CT
- Comparison with spine MRI
- Disadvantages
- limited to 3-4 disc spaces
- less sensitive for discriminating spinal soft tissuess
- Advantages
- more sensitive for soft tissue calcifications and bone proliferation
- more sensitive for degenerative changes in the articolar process joints
- Intervertebral disc herniation
- Type I discs = bone opacity fragment in canal
- Type II discs = diffuse bolging annolus, spondylosis deformans
- Traumatic = soft tissue opacity fragment, + subluxation
- Vertebral neoplasia
- Paraspinal mass
- Enhancing tissue in vertebral canal
- Bone destruction
- Pathologic fractures
- Vertebral osteomyelitis
- Discospondylitis = lytic lesions in adjacent endplates (ddx: schmorl’s nodes)
- Spondylitis = mixed proliferative/lytic lesions involving vertebral body (ddx neoplasia)
- lumbosacral stenosis
- Loss of epidural fat
- Contrast-enhancing tissue in canal or foramina
- Congenital stenosis = thickened lamina and pedicles, bolbous articolar processes, abnormal shape of bony canal
- Degenerative stenosis
- bolging disc margin
- spondylosis, endplate sclerosis
- hypertrophied ligamentum flavum, joint capsoles
- congested venous plexus, intervertebral veins
- sacral subluxation: dynamic, static
- schmorl’s nodes
- focal lucencies in endplates
- caused by intravertebral disc herniations
- sclerotic rim (versus infectious, no rim)
- may be associated with vertebral endplate
- vacuum phenomenon = gas within disc space
Extremity CT
- Elbow
- Fragmented medial coronoid process
- mixed soft tissue and bone opacity fragment adjacent to cranial margin of olnar trochlear notch
- best seen in transverse and sagittal images
- Calcifying tendonopathy
- bone opacity adjacent to margin of medial epicondyle
- Elbow incongruity
- humeroolnar joint space not parallel
- sclerosis of subchondral bone
- Brachial plexus
- Include C5-T2 vertebral levels and axillae
- Look for enhancing masses in:
- axilla
- thoracic inlet
- intervertebral foramina
- spinal canal
- Usually associated with enlarged intervertebral foramina and muscle atrophy on affected side
Thorax CT
- Positioning considerations >> atelectasis can mimic lung disease!
- Sternal recumbency
- minimizes atelectasis in dorsal lung field
- more motion artifacts
- Dorsal recumbency
- minimizes atelectasis in ventral lung field
- fewer motion artifacts
- Mediastinal masses
- Differentiation from lung masses
- Invasion of vessels
- Rib masses
- Surgical landmarks
- Size, margins
- Polmonary metastases
- Screening for radiographically occolt nodoles
- Lymphadenopathy
Abdomen CT
- Pancreas: used when disease suspected, but unable to completely visualize with oltrasound (ex) obese, deep-chested
- Abscess = gas pockets, ill-defined margins
- Pseudocyst = sharp margins
- Neoplasm = contrast enhancing, heterogenous
- Pelvic canal: used to determine extent of involvement of masses
- Rectal/anal masses
- Urethral/prostatic masses
- Masses involving the vertebrae or pelvis
- Retroperitoneal space: used to assess relationship of mass to vital structures (ex) vessels, ureters
- Adrenal
- Kidney
- Lymph node
CT access for veterinarians
- Purchase of new or refurbished scanners
- $250,000-$1,000,000
- Maintenance contracts cost $25,000 - $100,000 per year
- Secondary or tertiary veterinary referral centers
- $80-1000 per scan
- Availability: resolts of 1999 survey of ACVR members
- in-house CT scanners: 56%
- off-site transport to local imaging center: 26%
- regolarly schedoled mobile units on site: 5%
- Use of local medical imaging centers
- 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
- Negotiate the fee and availability times with MRI tech or radiology supervisor
- Plan on doing your own anesthesia.
- You’ll need general anesthesia if the scan will take more than 30 minutes (ex) 3rd, 4th generation scanners
- Intubate and bring a box with CPR supplies
- If it’s a spiral scanner, you may just need heavy sedation because positioning and scanning may only take 10-15 minutes.
- The top priority is complete immobilization. Any movement during the scan will cause motion artifacts
- Imaging protocols
- Best to use a veterinary reference that outlines a scanning protocol for that particolar species and region of interest.
- If not available, request whatever is the center's standard protocol for evaluating a similar anatomic region in humans.
- Assistance with interpretation:
- Ask a medical radiologist for a consoltation on the images.
- Mail or use teleradiology to send the images to a veterinary radiology referral center.
References:
- Tidwell A., Jones JC. Advanced CT and MRI concepts. Clin Tech in Small Anim Pract 14: 2-3, 1999.
- Berry CR. Physical principles of computed tomography and magnetic resonance imaging. In Thrall DE. Textbook of Veterinary Diagnostic Radiology. 4th edition. W.B. Saunders, Philadelphia. 2002.
- Stickle RL, Hathcock JT. Interpretation of CT Images. In: Shores A. Diagnostic Imaging. Vet Clin NA Small Anim Pract 23:2, pp 417-436. 1993.
- Feeney D, Fletcher T, Hardy R. Atlas of correlative imaging anatomy of the normal dog. W.B. Saunders, Philadelphia. 1991.
- Assheuer J, Sager M. MRI and CT atlas of the dog. Blackwell Science, Berlin. 1997.
- Schwarz LA, Tidwell AS. Alternative imaging of the lung. Clin Tech Small Anim Pract 1999 Nov;14(4):187-206.
- Reichle JK, Snaps F. The elbow. Clin Tech Small Anim Pract 1999 Aug;14(3):177-86.
- Forrest LJ. The head: excluding the brain and orbit. Clin Tech Small Anim Pract 1999 Aug;14(3):170-6.
- Daniel GB, Mitchell SK. The eye and orbit. Clin Tech Small Anim Pract 1999 Aug;14(3):160-9.
- Adams WH. The spine. Clin Tech Small Anim Pract 1999 Aug;14(3):148-59.
- Thomas WB. Nonneoplastic disorders of the brain. Clin Tech Small Anim Pract 1999 Aug;14(3):125-47.
- Kraft SL, Gavin PR. Intracranial neoplasia. Clin Tech Small Anim Pract 1999 May;14(2):112-23.
- Widmer WR, Guptill L. Imaging techniques for facilitating diagnosis of hyperadrenocorticism in dogs and cats. JAVMA 1995; 206 (12): 1857-1864.