Lapis ISSN 2398-2969

Computed tomography

Synonym(s): CT, CAT scan

Contributor(s): Anna Meredith, Livia Benato

Introduction

  • Computed tomography (CT) is a non-invasive imaging modality that can be used to evaluate the anatomy, function and pathology of many structures.
  • Tomographic scanners rotate to divide the object and organize it into spatially consecutive, parallel image sections.
  • Most images are acquired in a transverse plane.
  • Multiplanar images and 3D renderings can be mathematically reconstructed.
  • X-ray attenuation is measured by detectors which are aligned behind the patient, opposite to the x-ray source.
  • The CT computer collects x-ray attenuation data and generates a matrix of values, Hounsfield units (HU), depicted in various shades of gray.
  • There are 4096 gray tones which represent different density levels in HUs.
  • Water is equal to 0 and dry air is equal to 1000 HU.
  • The monitor can display a maximum of 256 gray tones though the human eye is able to only discriminate about 20.
  • Manipulation of the gray scale can be performed to enhance the appearance of different tissues.
  • The density levels of almost all soft-tissue organs lie between 10 and 90 HUs.
  • The density level of bone is around 300 HUs.
  • The density level of lung is around 600 HUs.
  • The window level should be set as close to the mean density level of the tissue to be examined.
  • The window width influences the contrast of the image.
  • The narrower the window, the greater the contrast since the 20 observable gray tones cover a smaller range of densities.

Uses

  • Computed tomography is a very useful technique to investigate not only dental diseases   Skull: abscess 04 - CT   in rabbits, but also pathological and non-pathological abnormalities of the skull   Skull: fluid within left bulla (middle ear) - CT  , bones and chest   Abdomen: liver cysts 01 - CT    Abdomen: liver cysts 02 - CT  , as well as detection of tumors.

Advantages

  • The major advantage of CT over conventional radiographs is the improved contrast and spatial resolution. 
  • Unlike conventional x-rays, CT scanning provides very detailed images of many types of tissue as well as the lungs, bones and blood vessels.
  • Structure and function may be evaluated concurrently in some cases. 
  • CT examination is generally painless, fast and simple.
  • CT is less sensitive to the high respiratory rate of rabbits than magnetic resonance imaging (MRI)   Magnetic resonance imaging  .
  • CT can be performed if the rabbit is presented with a metal medical device, unlike MRI.
  • Pre and postprocessing CT data manipulation is possible, allowing tissues of varying density to be better evaluated. 
  • Multiplanar image reconstruction and 3D renderings can be configured. 
  • Initial cost and maintenance of equipment is not prohibitive as compared to MRI. 
  • Imaging times are significantly reduced as compared to MRI. 
  • Refurbished CT equipment can be purchased at affordable prices. 
  • CT is superior to all other imaging modalities when examining bone.

Disadvantages

  • Rarely available.
  • Expensive.
  • Not suitable for detecting abnormalities of soft tissue.
  • General anesthesia is required   Anesthesia: overview  .
  • There is always a slight chance of excessive exposure of the animal to radiation.
  • The risk of allergic reaction to contrast materials has been reported in the literature in people but not in rabbits. However, it is a side effect that needs to be taken in consideration during the procedure. 
  • The area of interest must be well defined prior to imaging to avoid patient repositioning and excessive anesthesia times. 
  • Study interpretation may be lengthy due to the large number of images acquired with most CT examinations. 
  • Ionizing radiation is utilized.

Requirements

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Preparation

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Procedure

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Aftercare

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references fromPubMedpublished during the last 12 months.
  • Capello V & Cauduro A (2008)Clinical technique: application of computed tomography for diagnosis of dental disease in the rabbit, guinea pig and chinchilla.J Exotic Pet Med17(2), 93-101.
  • Bouchgua M et al (2006)Bone mineral density assessed by computed tomography in an in vivo rabbit model of osteoarthritis.Osteoarthritis & Cartilage15Suppl C, 173-174.
  • Souza M. (2006)Diagnosing a tooth root abscess in a guinea pig using microcomputed tomography imaging.J Exotic Pet Med15(4), 274-277.

Other sources of information

  • Girling S (2002)Mammalian imaging and anatomy.In:BSAVA Manual of Exotic Pets. Eds: Meredith A & Redrobe S. 4th edn. pp 1-12.
  • RadiologyInfo"- a public information website developed and funded by the American College of Radiology (ACR) and the Radiological Society of North America (RSNA) -www.radiologyinfo.org.


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