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STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
CT Applications
Tim Spotswood BVSc, MMedVet (Diag Im)
Diplomate European College of Veterinary Diagnostic Imaging
Computed tomography (CT) is an advanced diagnostic imaging modality that utilizes
x-rays and high-powered computers to construct tomographic (cross-sectional) images of the patient. CT is available at human imaging clinics and hospitals and
specialty veterinary practices in Calgary. CT has the capability of imaging all regions
of the body. Common veterinary applications include the musculoskeletal system,
head (including the nasal cavity, skull, brain), spine, thorax and extra-thoracic structures and abdomen.
CT has two main advantages vs. conventional radiography: the ability to obtain crosssectional images (or tomograms) eliminates superimposition of complex anatomical
structures thereby allowing evaluation of internal structures and anatomy; and exquisite contrast resolution, allowing detection and differentiation of tissue densities over
a very wide range.
CT studies are acquired under sedation or anesthesia to prevent patient motion during image acquisition. The patient is positioned inside the gantry on the CT table. CT
images are obtained by a rotating the x-ray tube located within the gantry (“doughnut”) around the patient, while rows of radiation detectors assimilated the remnant
radiation on the opposite side of the patient. Axial (cross-sectional or transverse) imaging slices are acquired as the tube makes one complete revolution (360°) to obtain
one axial image (slice) while the CT table is stationary. It takes about 1 second to
complete one full revolution. The CT table then advances the patient the predetermined slice interval and the next acquisition takes place. Newer spiral CT scanners
(helical CT) have the capacity to move the patient through the gantry at a continuous
rate while the x-ray tube head rotates simultaneously and continuously around the
patient. Helical CT reduces image acquisition time. Both techniques allow reconstruction of high quality images in body planes other than axial, and allows 3-dimensional
reformatting.
C.A.R.E.’s helical CT machine with a sedated cat positioned within the gantry. The laser
vectors provide reference points for the patient’s positioning relative to the CT table.
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-2-
Standard intravenous iodinated contrast media is often utilized in CT following the
acquisition of standard sequences, and allows visualization of specific anatomic structures; detection of some lesions otherwise not seen; can yield useful information regarding tissue perfusions; and often assists in typing of pathology.
CT is also used for planning radiation therapy, allowing precise locations for therapy
portals.
Skull and Brain
Skull radiographs are challenging to evaluate due to the complex bony anatomy and
superimposition of structures. CT allows precise evaluation of the fine anatomical
bone and soft tissue detail of the skull. CT allows precise determination of the location and extent and severity of skull trauma, neoplastic conditions of the nasal cavity
and bony structures, and diseases of the tympanic bulla and temporomandibular
joints. One of the more common CT procedures in our clinic is the assessment of
retrobulbar masses. Image acquisition is far more rapid and simple than with conventional skull radiography and a nasal CT series typically only takes several minutes to
acquire. Patient positioning is straightforward vs. the multiple and rather challenging
skull views needed to obtain a full series of nasal or skull radiographs. With these advantages, CT has largely replaced skull and nasal radiography in our practice.
Axial (left) and coronal reformatted (right) images of an intracranial arachnoid cyst
(arrows) in a dog.
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-3-
CT has traditionally been used in evaluation of the brain, particularly in acute trauma
cases where rapid acquisition and ultra-short anesthesia or sedation is mandated. CT
is useful for detecting mass lesions such as neoplasms (e.g., pituitary tumors, meningiomas, etc.), abscesses and granulomas, and hematomas.
Transverse (left and middle) and 3D surface shaded (right) images of a canine osteosarcoma involving the facial bones and invading the left nasal cavity, retrobulbar space and
frontal sinus.
Note that CT is less useful in detecting diffuse parenchymal diseases. MRI is vastly
superior for detailed evaluation of brain parenchymal disorders and has become the
gold standard for neuroimaging in human and veterinary medicine. For this reason,
we refer to CT as the “poor man’s MRI” in neuroimaging cases. Importantly, CT and
MRI do not replace a microscope, and cannot reliably determine the type of histopathology present.
Spine
CT is an excellent imaging modality for evaluation of spinal diseases involving the
vertebrae (particularly disc herniation), extradural, and intradural/extramedullary spinal cord lesions. CT is also useful in evaluating the paraspinal soft tissues. Again, MRI
should be considered the standard imaging modality for most spinal cord disease,
particularly intramedullary disease (neoplasia, hemorrhage, edema) and nerve root
tumours. CT is commonly used following myelography to further assess the extent of
intervertebral disc disease or other mass lesions within the spinal canal, and bony lesions resulting from infectious and neoplastic processes. CT is exquisitely sensitive at
detection of iodinated contrast material within the subarachnoid space, requiring
lower volumes of contrast media required for conventional myelography (about a
third of the usual dose). As with any spinal imaging, lesion localization by means of a
meticulous neurological exam is important to allow a detailed evaluation (small slices)
of as small a spinal region as possible.
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-4-
The main advantages of CT over MRI are instant access and availability, rapid image
acquisition, reduced cost to the client and better anesthetic control.
Transverse and sagittal reformatted CT images of lumbosacral junction in a
dog with cauda equina syndrome. Note the marked LS disc protrusion.
Transverse and dorsal reformatted CT images of lumbar disc extrusion in
a dog with hindlimb paresis. The mineralized disc material (arrows) has
caused severe ventrolateral extradural spinal cord compression.
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-5-
Thorax
CT is the gold standard for pulmonary and pleural disease imaging in human medicine. Thoracic CT is also now commonplace in veterinary medicine. CT is more sensitive than conventional radiography for detection of pulmonary nodules and subtle
pulmonary infiltration. Pulmonary and pleural masses can be precisely located, and
tissue sampling by CT guided aspiration or biopsy may be applied. For these reasons,
CT is very useful for surgical planning of thoracic disease. CT is also helpful in further differentiating types of pulmonary infiltration and consolidation, particularly if
concurrent pleural effusion is present. Pulmonary perfusion can also be assessed
(with non-selective CT angiography) and challenging conditions such as lung lobe
torsion and pulmonary thromboembolism can be diagnosed. CT is an excellent imaging modality for evaluation of mediastinal disease, including cranial mediastinal
masses, differentiating esophageal pathology, and assessment of hilar lymph nodes.
Thoracic CT requires meticulous anesthetic technique for best image quality: respiratory movement must be managed and the lungs must be properly inflated to prevent
atelectasis that may mimic pathology.
Thoracic CT showing metastatic disease
not detected on plain film radiography.
Note the multiple pulmonary nodules
(arrows).
Thoracic CT showing an
oesophageal mass (arrows).
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-6-
Abdomen
Although ultrasound is the “work horse” of abdominal imaging in veterinary medicine, CT is can provide invaluable additional information in many cases. The field of
view is limited in abdominal ultrasound, necessitating that the sonographer compiles
a virtual 3-D picture from a composite set of multiplanar images. This can be very
challenging in some cases: where a large abdominal mass is present and the extent
and sometimes origin is not clear; if the pathological process is complex and anatomical structures are distorted; and if ultrasound artifact is excessive (particularly
gassy abdomens). CT allows a more global assessment of the abdomen without sacrificing contrast resolution and anatomical detail that limit conventional abdominal radiography. The images can also be reformatted in any imaging plane. CT may be able
to detect metastatic disease (particularly in the liver and spleen when intravenous iodinated contrast material is used) not seen during an ultrasound examination. Nonselective positive contrast CT has been shown to be an excellent imaging modality for
detection of portosystemic shunts, and is considered the optimal imaging technique
over sonography and portography for PSS in some centres. Probably one of the
commonest indications of abdominal CT in our clinic is the assessment of caudal
vena cava invasion/thrombus formation by adrenal gland masses for surgical planning. Conventional excretory urography is often a finicky and time-consuming procedure, particularly for the assessment of ectopic ureters extending into the pelvic cavity. We have largely replaced this procedure with CT for evaluating ectopic ureters in
our clinic.
Acute pancreatitis (asterisk) with regional
peritonitis in a cat.
Post-contrast image of a right adrenal mass
compressing the caudal vena cava (asterisk),
and invading and distorting the right kidney
(RK).
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-7-
Musculoskeletal System
CT provides exquisite bony detail, and bony lesions are often identified on CT examinations that were simply not detectable on radiographs. CT is capable of detecting
early discospondylitis well before radiographic signs are present. The extent of bony
neoplasms is better assessed with CT than conventional radiographs, which assists
with treatment planning. CT is a very sensitive image modality for assessment of canine elbow dysplasia, far superior to radiography. CT is particularly useful for assessing fragmented medial coronoid process disease, and is routinely utilized in our clinic
for this purpose. The canine shoulder, carpus, stifle, and tarsus have all been studied
using CT. Other clinical applications include complex fractures, suspected osteochondral lesions, various forms of arthritis, joint neoplasia, bicipital tenosynovitis, etc.
CT is very useful in evaluation of soft tissue masses of the extremities. A good example is the assessment of infiltrative lipomas, which may be clearly differentiated from
the surrounding muscles; liposarcomas can also be assessed, which have a wispy appearance and densities intermediate between lipomas and soft tissues masses. CT has
been shown to be superior in most cases for detection of wooden or plant matter
(grass awn) foreign body migration over ultrasonography and MRI. MRI’s superior
contrast resolution is generally considered better for evaluating soft tissues, especially
joints (menisci, cruciate and collateral ligaments, and articular cartilage).
Transverse plane CT images through the level of the medial coronoid
process of the left and right elbow of a 9 month old Labrador retriever.
Note the fissure in the left MCP, and the fragmentation of the right.
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca
STANDARDS OF C.A.R.E.
Clinical Case Management Newsletter
Issue Twenty-Two
August 2009
-8-
CT at the C.A.R.E. Centre
At C.A.R.E., all CT scans are performed by the diagnostic imaging specialty technologist and interpreted immediately by the radiologist. CT is also available after
hours on an emergency basis. In general, CT is performed as an in-patient procedure
at C.A.R.E. once the animal is admitted into the hospital though a primary clinical
service (i.e. surgery, oncology, ophthalmology or internal medicine). The rationale is
that these patients require anesthesia or sedation. Also, once the pathology is defined,
many cases require additional diagnostic procedure in order to establish a definitive
diagnosis directly after the scan, which will be typically performed under the same
anesthetic. For instance, most nasal cases will undergo rhinoscopy and nasal biopsy
immediately after their nasal CT; most spinal cases will immediately go to surgery;
neuro-imaging cases often require a CSF tap, and many oncology cases will undergo
biopsy.
For cases that may require CT as part of the diagnostic work-up, we suggest that
these be referred through the appropriate primary clinical service. If in doubt, please
contact the radiologist directly to discuss the case.
Contact Tim Spotswood, C.A.R.E. Centre Referral Team Member
at [email protected]
The Calgary Animal Referral & Emergency Centre Animal Hospital
www.carecentre.ca