Application of Computed Tomography Scan (CT Scan) in Veterinary Practices-
Computed tomography (CT) is an imaging modality that utilizes x-rays and powerful computers to construct cross-sectional images of patient. It can be thought of as combining the cross-sectional properties of ultrasound with the conventional radiography. CT has been available in human medicine since the mid-1970’s and is rapidly becoming an important imaging tool in veterinary medicine. It is currently available at many university hospitals and specialty veterinary practices worldwide.
Historically, CT has been used primarily to image the brain. Its use has now evolved to study literally all regions of the body. Common applications include the nasal cavity, skull, brain, spine, thorax and extra-thoracic structures, abdominal organs, and the musculoskeletal system.
Computerized axial transverse scanning was first announced in April 1972 by G.N. Hounsfield. The basic concept was quite simple: A thin cross-section of the head, a tomographic slice, was examined from multiple angles with a pencil-like x-ray beam. Tomography is imaging that depicts a cut, slice or section of the body free of superimposition by overlying structures. Hounsfield and a physicist from Tufts University, Alan Cormack, shared the Nobel Prize in physics in 1982 for their work on CT. Let’s review regular radiographs and compare them to CT. When imaging the abdomen with conventional radiographs, the image created directly on the film is relatively low in contrast. The image is not as clear as one might expect because of superimposition of all the anatomic structures within the abdomen. Scatter radiation further degrades the visibility of image detail. One of the main advantages of CT over conventional radiography is the ability to eliminate superimposition. A CT scan results in transverse or axial images. Transverse images are those perpendicular to the long axis of the body. Collimation is required during CT scanning for precisely the same reasons that it is required in conventional radiography. Proper collimation reduces patient dose and restricts the volume of tissue irradiated. More importantly, it enhances image contrast by limiting scatter radiation. CT uses x-rays and computer processing to create cross sectional (transverse) slices of internal structures. CT images are not only clear but can isolate a specific internal region. Each CT slice is formatted from multiple x-ray exposures captured as the scan completes a 360 degree rotation. Transmitted x-ray energy is recorded by detectors positioned opposite the patient. The x-ray energy is converted to an electric signal and sent to the CT computer for processing. The CT computer translates the electronic single to numeric (digital) information, which in turn is used to display images on the computer monitor.
Spiral CT permits rapid acquisition of a volume of data, thus producing high quality two and 3-dimensional images with very short scanning times. Within the spiral CT scanner, the patient is advanced through the gantry as it is continuously rotated so that the movement of the x-ray tube around the patient simulates the threads of the screw. The advantages of spiral CT include 1) no motion artifact, resulting in improved lesion detection, 2) reduced partial volume artifact due to reconstructing smaller intervals, 3) optimized intravenous contrast obtained during peak enhancement, 4) reduced scanning time, and 5) multiplanar images which result in higher quality reconstruction, because there are no gaps in data. These advantages allow CT to compete with the resolution and versatility of MRI. The rapid acquisition of spiral CT images is especially useful for angiographic procedures, because dynamic scanning can be performed during maximum contrast medium opacification without the use of selective catheterization. Time to peak enhancement can be measured for an individual during a test run consisting of a dynamic scan centered over the area of interest after bolus injection of contrast medium. One clinical application of spiral CT angiography is the diagnosis of pulmonary thromboembolism. Spiral CT can non-invasively identify PTE by detecting filling defects within the contrast filled arteries.
Multislice CT (MSCT) allows for simultaneous acquisition of 4, 8, and 16 slices respectively. With this, combined with a reduction in scanner rotation time, (to less than 0.5 seconds) imaging time can be accelerated by a factor of 8 to 32. Vessels with very small diameters are clearly visualized. There is improved spatial resolution along the length of the body allowing for high quality secondary reconstructions or 3-D visualization techniques.
CT is particularly valuable for imaging intracranial lesions as well as those involving the nose and sinuses, middle ear, and the periorbital region. In these areas, CT is superior to either standard radiography or ultrasound. Although MRI is considered the optimum technique for central nervous system diagnosis, good quality CT can provide adequate diagnostic information for many brain lesions. Intracranial masses and fluid-filled cavities usually are easily demonstrated with CT. Vascular lesions are usually not as well-defined, but intravascular contrast media aid in this regard. Image interpretation involves standard principles of radiography including an evaluation of size, shape, location, and density of visible structures. The ventricular system, tentorium cerebelli, and midline falx cerebri are readily visible on CT scans. The remaining brain tissue is relatively uniform and homogeneous. Alteration in ventricular size, shape, or position, deviation of midline, and calcification of masses are easily seen on survey images and are important signs of mass lesions. Peripheral edema may be seen as decreased density surrounding the mass. Areas of acute hemorrhage may appear as radiodense due to presence of hemoglobin. As clot resorption occurs, the radiographic density decreases until it may appear radiolucent and appear similar to edema. Intravascular contrast agents usually enhance masses, demonstrating vascular alterations and areas of disruption of the blood brain barrier. Certain canine brain tumors may have distinguishing features on CT images based on location and pattern of contrast enhancement. Meningiomas typically are peripherally located and homogeneously enhance. Astrocytomas and gliomas typically show peripheral enhancement with central lucency. Choroid plexus masses are often relatively dense and enhance uniformly. Pituitary tumors may be recognized by their location and typically show uniform contrast enhancement.
Other regions of the head besides the brain are also well imaged with CT. CT is the preferred diagnostic imaging method for evaluating the nose, sinuses, and periorbital areas, including cases with possible invasion of the cranial vault. If there is destruction of major skull bones, 3-D reconstruction can be dramatic. Reformatted images in the sagittal and dorsal planes often are very useful in evaluating CT scans of the head. CT can also aid in the diagnosis of nasopharyngeal stenosis. The bony tympanic bullae, internal acoustic meatus, semicircular canals, and ossicles are all well visualized by CT enabling accurate diagnosis of otitis media and interna. Chronic otitis externa can have mineralization which is exquisitely visualized with CT. Contrast enhancement allows differentiation between ceruminous debris and masses/ thickening in the horizontal ear canal.
In intervertebral disc protrusion or herniation, CT may provide valuable additional information following standard radiographic examination. It is essential that the lesion be localized by neurologic examination and standard radiography prior to performing CT. CT should be used to obtain additional information regarding a specific spinal lesion at a specific site, not as a fishing expedition over a long segment of the spine. With the exception of lesions in the caudal lumbar and sacral region, a myelogram with standard radiographs should be performed first, followed immediately by CT, if indicated. Thus, the CT is a myelographic study. Good quality CT images demonstrate very thin contrast columns not visible on standard radiographs. CT can be used to demonstrate lateralization of a lesion. Spinal cord compression is more accurately assessed and usually appears more dramatic on CT as compared with standard radiographs. For the lumbosacral area, evaluation of bone remodeling, evidence of cauda equina compression by either soft tissue or bone remodeling within the spinal canal, and comparison of size and density of intervertebral foramina, both between right and left sides at the same space and between different intervertebral disc spaces is possible.
Cortical bone detail cannot be seen with MRI no matter what pulse sequence is used. Therefore CT is the method of choice for imaging ulnar coronoid processes. Its’ superior resolution over standard radiography for this purpose cannot be overstated. Radiographic evaluation of joint diseases with CT uses the same principals as standard radiographs. Radiographic changes include subchondral bone lucency and sclerosis, fracture lines, osteophytosis, soft tissue swelling, and soft tissue mineralization. The advantage of CT is that superb detail allows accurate evaluation of lesions seen minimally, if at all, on standard radiographs. CT allows visualization of both the secondary arthritic changes typically seen radiographically, but also clearly shows the fragmentation of the coronoid process that can be difficult to visualize with certainty with radiographic examination. CT arthrography has been studied and can be helpful in diagnosing partial cranial cruciate tears. Complex fractures and their association to the previous anatomic structure can be depicted with CT. The ability to reconstruct images in three dimension can help visualize the pathologic abnormality and help plan for the best therapeutic remedies. CT performed on patient’s with pelvic trauma identified soft tissue lesions not seen by radiographic examination, including hemorrhage and muscular trauma. In most cases, CT also identified bone lesions not seen radiographically.
Brachial plexus tumors are a cause of forelimb lameness and can be identified with CT. Special patient positioning and large fields of view allow optimum visualization of these tumors which are often outside of the spinal canal.
CT is underused for thoracic examinations in veterinary medicine. Evaluation of pulmonary and mediastinal masses, including lymphadenopathy and evaluation of spine and rib involvement with thoracic masses are examples of candidates for CT scanning. CT is considered the most sensitive method for detection of human pulmonary metastases. CT evaluation of vaccine associated sarcomas has proven to determine the full extent of disease, often altering recommendations based on the CT images, including preoperative radiation therapy when surgery alone was deemed inadequate to affect local tumor control. Alterations in the surgical approach were also made due to CT findings.
Contrast agents are of considerable value when evaluating abdominal organs. Gastrointestinal contrast is helpful to identify and differentiate the gastrointestinal tract from other structures, particularly in the cranial abdomen. The usefulness of CT in evaluating canine adrenal masses is well established. CT has proven to be an excellent technique for obtaining samples of abnormal tissues. Fluoroscopy, ultrasonography, and computed tomography can all be used to allow direct placement of a needle into a specific site both for diagnostic as well as therapeutic purposes. However with fluoroscopy, demonstration of lesions in two planes is required for accurate needle placement and resolution is, at times, suboptimal. Gas, bone, or surface irregularities may obscure deep-seated lesions when ultrasonography is employed. CT-guided biopsy has been recommended for use in humans when lesion visibility or accessibility by other imaging modalities is unsatisfactory. Via virtue of its high contrast resolution, excellent display of anatomic topography, and disregard for overlying gas or bone, CT may offer superior visualization of lesions, the needle pathway, and tip of the needle as well as surrounding structures. In humans, CT is often selected over fluoroscopy for biopsy guidance of thoracic lesions that are small, subpleural, ill-defined, or juxta vascular. Biopsies of the pelvis, retroperitoneum, adrenals, pancreas, and small orbital masses are often preferentially performed with CT rather than ultrasound guidance.
Direct injection of contrast media into a popliteal lymph node is an easy technique for identification of thoracic ducts, and this technique can be applied to the diagnosis of diseases associated with chest lymphatic drainage. Single and dual phase computed tomographic angiography has been used to evaluate the portal vein, its tributaries, and intrahepatic branches. CT angiography is a fast, minimally invasive procedure that will image all portal tributaries and branches as they fill with contrast medium during a single peripheral venous injection. CT angiography is able to diagnose single and multiple portosystemic shunts, located both within and outside of the liver. Trans-splenic CT portography can also be performed. In one study, the information gained from CT portography resulted in a decrease in surgical time necessary compared with similar surgeries performed without angiographic information.
CT SCANNER USES IN VETERINARY MEDICINE
Computed tomography (CT) technology is very important when it comes to in-depth, high-resolution diagnostic animal imaging, including for domestic animals such as dogs and cats of virtually any size.
Veterinary CTs are widely used to diagnose abnormalities in the brain, nose, ear, and musculoskeletal system. Compared to other radiography and ultrasound techniques, CT also provides increased levels of information that technicians and veterinarians can use to provide better diagnostic accuracy and confidence, as well as veterinary care.
Other standard uses of CT in veterinary medicine include:
- Diagnosing lung, nasal, or ear diseases in animals
- Screening lungs for metastatic cancer
- Diagnosing abdominal and orthopedic conditions
- Increasing tissue differentiation when iodine-based contrast media is injected
- Guiding tissue sampling of deep structures within the thorax
- Conducting oncology or trauma cases in animals
Additionally, CT scanning is an optimal solution when treating pets and other animals that have metal implants, and are therefore unable to be imaged with MRI.
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Computed tomography (CT) was introduced into clinical practice in Britain in the early 1970s, and was developed initially as a brain scanner but other applications were quickly realized. The applications in veterinary medicine are continually being explored, and accessible CT imaging centers are emerging. In many cases, CT imaging can provide valuable information that cannot be obtained with ultrasound and radiography. It is important for veterinarians to know the indications for CT imaging so this modality can be incorporated into a case when financially feasible.
Advantages of computed tomography
Conventional radiographs depict a three dimensional object as a two dimensional image. Their main limitation is that overlying tissues are superimposed on the image. Computed tomography overcomes this problem by scanning thin slices of the body with a narrow x-ray beam which rotates around the body. Another limitation of the conventional radiograph is its inability to distinguish between two tissues with similar density, such as soft tissue and fluid. Computed tomography can differentiate between tissues of similar density because of the narrow x ray beam and the use of “windowing” .
How it works
The information acquired by CT is stored on a computer as digital raw data and an image can be displayed on a video monitor or printed on to x ray film. The image is made up of a matrix of thousands of tiny squares or pixels (65000 pixels in a conventional image). Each pixel has a CT number (measured in Hounsfield units) attributed to it. The CT number is a measure of how much of the initial x-ray beam is absorbed by the tissues at each point in the body. This varies according to the density of the tissues. The denser the tissue is, the higher the CT number, ranging from -1000 HU (air) to 1000 HU (bone). Soft tissues average 0 CT units.
To image an area of the body in which many of the tissues have a similar density – for example, the mediastinum or abdomen – a narrow range of CT numbers is selected. These can be spread out over the available gray scale so that two tissues with only a little difference in density will be ascribed separate shades and can therefore be differentiated. For example, a window width of 500 is often used for imaging the mediastinum, with a window level of 39. The level refers to the CT number at the center of the selected window. Thus, in this case, all pixels within the range – 211 to 289 will be displayed. Most of the lungs (largely air) will have CT numbers below – 211 and will therefore appear completely black on the final image .
If you want to see all the lungs you need a much wider window and higher level . The value of windowing is that the raw data from a single scan can be displayed in different ways to give useful images of a wide range of tissues.
Indications for computed tomography
Nasal and sinus
CT can usually discriminate between most classes of nasal disease. Bacterial rhinitis will generally have diffuse exudate between turbinates and a small amount of sinus fluid. Fungal rhinitis will have the same findings as rhinitis, but in addition, regions of turbinate lysis and sloughing will be seen. Neoplasia will usually present as a focal, destructive mass. For all forms of nasal disease, CT provides more information than radiography for prognosis and treatment planning, and it also guides biopsy procedures. Because of its greater bone imaging capabilities, CT is preferred over MRI for nasal disease.
In general, CT is very useful to provide the following information about masses:
- What is the origin of the mass?
- Where are the margins of the mass?
- Is the mass invading important structures?
- Is underlying bone destruction present?
- Is surgical resection possible?
If these questions are answered and a biopsy is obtained, an accurate prognosis can be given. Studies have shown that the gross margin of tumors often extends beyond palpable limits. By accurately mapping the margins of a mass, surgery and radiation therapy have a greater chance of success. MRI is also very accurate at depicting mass margins.
Ear Diseases
CT can depict the anatomy of the ear with great detail. The main indication for imaging the ear with CT is to document that there is middle or inner ear involvement and to determine which surgical procedure is indicated (ear canal ablation, bulla osteotomy, etc.). In cases of otitis media, fluid is visible in the osseous bulla. If chronic, the osseous bulla becomes thick and sclerotic. Rarely, tumors of the ear are encountered and are often differentiated from otitis by destruction of the bulla.
Orthopedics
CT is relatively insensitive at detecting soft tissue injuries. Ultrasound and MRI have much greater soft tissue imaging capabilities and are preferred. CT is useful for confirming medial coronoid process fractures in the elbow. These fractures are very difficult to visualize radiographically, but are easily detected with CT.
Head Trauma
For imaging of the head, CT has great advantage over radiographs. Small fractures can be difficult to see on radiographs, but are easy to visualize with CT. This is especially true for fractures involving the TM joints or calvarium. In the first 24 hours after skull trauma, CT can detect subarachnoid and brain hemorrhage. After this time, MRI is more useful for this purpose.
Ureters
CT excretory urography is useful for confirming and localizing the presence of ureteroliths or other causes of ureteral obstruction. It can also be utilized to evaluate for ectopic ureters and is preferred over standard radiographic techniques at this time, though studies need to be done to determine if it has greater accuracy.
Chest
CT of the chest has been revolutionized by two new developments in scanning. These are the ability to scan quickly with the new spiral scanners and the ability to acquire very thin slices (high resolution CT). With spiral CT scanners, the entire chest can be scanned during a single breath. This also means that small intrapulmonary abnormalities such as a pulmonary metastasis will not be missed because of variations in inspiratory effort.
Contrast enhanced CT of the chest is usually used for staging lung cancer and for assessing masses in the mediastinum. High resolution CT is used to image the lung parenchyma. The thin slices (1.5 mm) improve the resolution, making it possible to diagnose and assess the extent of diseases. Pulmonary diseases can be characterized with greater clarity than radiographs, but at this time the added information does not add great value to prioritizing differentials or changing the diagnostic plan.
Abdomen
Ultrasound is the preferred modality to assess the abdomen in animals. As a diagnostic tool, CT provides no advantage for the diagnosis of most conditions (there are a few exceptions). The pelvis inlet is one place in the abdomen that is difficult to visualize with ultrasound, so CT is useful to stage tumors in this area.
Spine
In general, MRI is the preferred modality to assess problems of the spinal chord. With CT imaging, contrast in the subarchnoid space (such as with myelography) is necessary to outline the spinal chord and visualize lesions around the chord. The exception to this generalization is with the lumbosacral region. A large amount of epidural fat around the nerve roots in this region facilitates visualization of lesions at this site without contrast. Tumors involving the vertebral bodies can also be well characterized with CT.
Brain
As with the spine, MRI is preferred over CT for detecting brain diseases. MR has much greater contrast resolution for soft tissues and has higher sensitivity for detecting smaller neoplastic, inflammatory, and vascular lesions. The tradeoff is a higher expense and longer image acquisition times (~30-40 minutes for MRI vs. 5 minutes for CT). With contrast enhancement, CT can detect most lesions that are larger than 1 in size. CT is very sensitive at detecting pituitary macroadenomas since the pituitary gland does not have a bloodbrain barrier and will readily contrast enhance.
The Basics
The ability to obtain cross-sectional images is advantageous because it allows evaluation of internal structures and anatomy that cannot be seen on conventional radiographs due to superimposition. Because x-rays are used to construct the CT image, veterinarians not familiar with CT can quickly learn to interpret CT images using basic radiology principles. Learning cross-sectional anatomy is the challenging part, since most of us were not trained to view anatomy as slices of tissue. Specialty CT and anatomy textbooks along with a growing list of veterinary CT literature makes this task less daunting.
CT images are acquired by placing the patient within the gantry (opening) of the CT scanner. Some form of restraint, sedation or anesthesia is necessary to prevent patient motion during image acquisition. The body part of interest is initially scanned to produce the “pilot” image (a CT-generated radiograph). From this pilot radiograph, image slices of the region of interest are planned. The thickness of the tissue slice is determined by the operator and varies depending on the part of interest. Generally speaking, the narrower the slice thickness (more properly termed slice collimation), the better the image detail. Thicker samples reduce total image acquisition time, however, by covering a larger area per slice. Often, a large region of interest will be scanned using thick collimation (e.g., 10 mm collimation of the abdomen or thorax); narrower slices can then be obtained for better detail of a smaller specific site of pathology or region of interest. The interval (gaps) between slices (if any) is also determined by the operator. Usually, contiguous images are obtained. It is possible to interleave (overlap) images, resulting in greater image detail during multiplanar reconstruction (e.g., images of the cribriform plate for evaluation of tumor extension) or to simply skip intervals of tissue (e.g., high-resolution pulmonary thoracic CT for diffuse disease).
CT images are obtained by a rotating the x-ray tube head around the patient. The tube makes a complete revolution (360°) to obtain one axial (cross-sectional) 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 ability to move the patient through the gantry at a continuous rate while the x-ray tube head rotates continuously around the patient. Advantages of spiral CT include reduced image acquisition time and the ability to reconstruct higher quality images in body planes other than axial, including 3-dimensional images.
The newest generation CT scanners are called multidetector (multislice) scanners. They have the ability to obtain multiple slices in one revolution (e.g., 4, 8, 12, 16, 32, 64 or more axial images per revolution) that drastically reduces image acquisition time and allows reconstruction of images in any anatomic plane (including phenomenal 3-D reconstructions) without loss of image detail. Multidetector scanners have revolutionized CT imaging in human medicine. Cost aside, they hold tremendous promise for veterinary applications because complete scans may be made with in a matter of a few seconds, perhaps using only sedation!
As mentioned, CT uses x-rays to acquire data. Because of powerful computers and very sensitive x-ray detectors, CT has the ability to detect and differentiate tissue densities over a very wide range, in striking contrast to conventional radiographs. Tissue densities, known as CT numbers or Hounsfield Units (HU), are quantified and have a range of approximately +3000 to-1000. Pure water has an HU of 0, pure air-1000 and the densest dense bone +2000 or more (depending on the CT scanner). The operator has the ability to view images using various “windows” which include the width of the window and the level (centering) of the window. This allows adjustment of the image to maximize detail of a particular area (e.g., lung tissue versus mediastinal structures). Since HU values are quantified linear attenuations of the tissue, once an image is obtained, it can be manipulated by adjusting the windows as well as various reconstruction algorithms from a single acquisition. This is in contrast to MRI, in which each image plane acquisition and each type of imaging sequence must be derived from a new scan, greatly increasing imaging time.
The basic composition of a CT image is the pixel and voxel. Each square in the image matrix (viewing monitor) is called a pixel, representing a 2-D image of a tiny elongated block of tissue volume called a voxel (3-D). Each pixel/voxel can only display one shade of gray. It is the voxel size that determines spatial resolution (the ability to separate two structures of different density).
After the acquisition of standard CT images, it is often beneficial to rescan the patient following intravenous iodinated contrast administration. This allows visualization of specific anatomic structures, detection of some lesions otherwise not seen and can yield useful information regarding the type of pathology present (e.g., walled off abscess, tissue necrosis, neovascularization of some tumors, etc.).
CT is also used in conjunction with radiation therapy. CT images are analyzed by a treatment-planning computer that provides precise locations for therapy portals. This allows much more precise localization of the radiation therapy beam and higher doses to be safely administered.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
