CT comes to the Fox Valley

Computed Tomography at FVARC

Computed tomography (CT) has been available at the Fox Valley Animal Referral Center since December 2008. CT is useful for diagnosing disorders of the nasal cavity, middle and inner ear, brain, abdomen, lung, mediastinum, and the musculoskeletal system. There are several major advantages of CT over conventional radiography. First, cross-sectional CT images eliminate superimposition. Second, because of the high-contrast resolution of CT, various soft tissues can be distinguished. Third, data from a CT scan can be viewed as images in multiple planes, greatly enhancing diagnostic capabilities and aiding in therapeutic planning.

Unlike conventional radiography in which a broad x-ray beam is emitted from a stationary tube, CT images are made using a narrow x-ray beam that is emitted from a tube as it revolves 360 degrees around the patient. As the x-ray tube rotates, the patient is moved through the aperture of the scanner on a table in small increments (axial scan) or continuously (helical scan). Opposite to the x-ray tube are electronic detectors that record the intensity of x-rays exiting the patient. A computer reconstructs the data obtained from the multidirectional x-ray beam into cross-sectional images of the patient. Scan time is typically 30 to 60 seconds. Accurate patient positioning is vital to obtain optimal CT images. For this reason, animals are sedated or anesthetized.

CT images represent thin sections or “slices” of the patient, thus multiple images are required to view one structure. Thin slices are used to obtain high resolution images of small, high-contrast structures. Thicker slices are used to distinguish between tissues of lower contrast.

CT images are digital (comprised of pixels). Each pixel in a CT image is assigned a number (Hounsfield Unit [HU] or CT number) which represents the amount of x-ray attenuation by the tissue. HU is a unitless number representing the degree of x-ray attenuation in tissue relative to water. The scale of HU ranges includes -1000 HU for air, 0 HU for water, and +3,000 HU for compact cortical bone. Once a CT scan has been acquired, the range of HU displayed can be adjusted according to the tissue of interest.

CT can distinguish between various soft tissues and fluid types, such as blood versus transudate.

Images are usually acquired in the transverse plane, but data can be reprocessed to produce images in the sagittal, dorsal, coronal, or oblique planes as well as three-dimensional reconstructions. Intravenous contrast medium can be combined with CT to more clearly identify structures. Contrast CT studies include urography, myelography, portography, and angiography.

Head CT

CT provides superior high resolution imaging of fine structures including tooth roots, orbits, nasal turbinates, the nasopharynx, and cribriform plates. CT is particularly well-suited for examination of the nasal cavity because of the high contrast provided by bone and air-containing structures. It is the best imaging modality for diagnosing nasal Aspergillosis, showing turbinate destruction in exceptional detail. CT is ideal prior to rhinoscopy to aid in planning biopsies and treatment.

Brain CT is performed in cases of seizures, neurologic disease localized to the brain or cranial nerves, and head trauma. While MRI provides better soft tissue definition, CT is generally less expensive than MRI and has a shorter scan time, which may enable scanning without anesthesia in critically ill patients.

Thoracic CT

Of major importance, CT has significantly higher sensitivity for detecting pulmonary nodules than radiology. One study reported that nodules as small as1 mm can be detected on CT, while nodules must be 7-9 mm to be reliably detected on radiographs. While there are many indications for thoracic CT, a primary use is to rule out metastasis prior to a surgical procedure. Additionally, by manipulating image display, CT enables detailed assessment of the location and extent of a lesion, aiding in tumor staging, radiation therapy, and surgical planning.

Abdominal CT

Uses in veterinary medicine include but are not limited to the diagnosis of portosystemic shunts, splenic and hepatic neoplasia, adrenal disease, and ectopic ureters.

Musculoskeletal CT

CT is more sensitive than radiography in detecting small fractures because the cross-sectional images eliminate superimposition. It is also useful in the diagnosis of intervertebral disc disease and other orthopedic conditions that are sometimes difficult to appreciate with conventional radiography.

For more information on CT at FVARC, please call (920) 993-9193.