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
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.