Felis ISSN 2398-2950

Computed tomography (CT)

Contributor(s): Fraser J McConnell, Federica Morandi

Introduction

  • Computed tomography (CT) is a non-invasive imaging modality that utilizes X-rays to generate cross-sectional images of an anatomical area.
  • A typical CT scanner is composed of a gantry (ring-shaped), which houses the X-ray tube and detectors, a table for patient positioning, a generator, which produces high voltage and transmits it to the X-ray tube, and a control desk for the operator.
  • The x-ray tube moves in a circular path within the gantry and around the patient.
  • The patient positioned on the table is advanced into the gantry either continuously or at fixed time intervals.
  • X-rays emitted by the tubes reach the patient and pass through it and are partially absorbed or scattered.
  • X-ray attenuation is measured by detectors which are aligned behind the patient, opposite to the x-ray source.
  • Most images are acquired in a transverse plane.
  • Multiplanar images, such as parasagittal and dorsal, and 3D renderings can be mathematically reconstructed from the raw data.
  • The CT image is generated by the computer after collecting x-ray attenuation data and creating a matrix of values, called Hounsfield units (HU) or CT numbers, depicted in various shades of gray.
  • There are 4096 gray tones which represent different density levels in HUs.
  • The monitor can display a maximum of 256 gray tones though the human eye is able to only discriminate about 20.
  • Water is equal to 0 HU and dry air is equal to 1000 HU.
  • The density levels of almost all soft-tissue organs fall between 10 and 90 HUs.
  • The density level of bone is around 300 HUs.
  • The density level of lung is around 600 HUs.
  • Manipulation of the gray scale can be performed to enhance the appearance of different tissues.
  • When an image is displayed, the grey scale can be adjusted by setting window level and window width.
  • The window width (WW) determines the range of HUs represented on the image, and 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.
  • The window level (WL) selects the center HU of the window width, and should be set as close to the mean density level of the tissue to be examined as possible.
  • For example, a WW=300 and WL=0 indicates that the HU values range from -150 to +150 on the image. If the WL is changed to 200, then the HU values range from +50 to +350 Tympanic bullae: transverse - soft tissue window CT   Tympanic bullae: transverse - bone window CT .

Uses

Brain

Nasal/sinus

  • Indications Computed tomography: nasal chamber:
    • Nasal discharge Nasal discharge.
    • Evaluation of suspected rhinitis Rhinitis and sinusitis Sinusitis (bacterial, fungal or due to foreign body)  Nasal passages 01: CT   Nasal passages 02: CT   Nasopharynx: CT .
    • Evaluation of suspected nasal neoplasia Nasal cavity: neoplasia and differentiation between nasal neoplasia and rhinitis.
    • Pre-operative assessment of complex fractures (multiplanar and 3D reformation.
    • Pre-operative assessment prior to surgical excision of neoplastic processes.

Ear

TMJ

  • Indications:
    • Pain on opening mouth or reluctance to eat/chew.
    • Trauma.
    • Suspected joint subluxation/luxation.
    • Suspected fracture or joint remodeling not seen radiographically.

Other head

  • Indications Computed tomography: head:
    • Trauma.
    • Pre-operative assessment prior to surgical excision of neoplastic processes.
    • Evaluation of tooth disease.
    • Evaluation of orbital disease.

Spine

  • Indications:
    • Trauma.
    • Suspected subluxation/luxation.
    • Fractures Spine: fracture / luxation.
    • Osteomyelitis Osteomyelitis.
    • Intervertebral disk disease or other compressive spinal lesions (CT-myelography).
    • Congenital malformations.
    • Suspected lytic lesions possible primary or metastatic neoplasia.
      In all cases where possible impingement on the spinal cord is of concern, CT-myelography should be considered.

Thoracic

  • Indications:
    • Characterization of thoracic masses not adequately defined via conventional radiography.
    • Pre-operative assessment prior to surgical excision of neoplastic processes of the lungs, mediastinum, ribs, spine or body wall.
    • Metastasis check CT is more sensitive than radiography in the detection of small pulmonary nodules.
    • Evaluation of thrombus formation in the great vessels (with contrast medium).

Abdominal

  • Indications:
    • Evaluation of peritoneal, retroperitoneal and visceral pathology not adequately depicted by other imaging modalities.
    • Pre-operative assessment prior to surgical excision of neoplastic processes of the abdomen, pelvic region or body wall when other imaging modalities are not sufficient.
    • Characterization of portosystemic shunts (rare in cats compared to dogs).
    • Evaluation of pancreatitis Pancreas: disease - overview.
    • Evaluation of abdominal trauma when ultrasonography is not sufficient.
    • Evaluation of thrombus formation in the great vessels (saddle thrombi) or vascular invasion by abdominal masses (with contrast medium) when ultrasonography is not sufficient.
    • Screening prior to renal transplant.

Musculoskeletal

  • Indications:
    • Pre-operative assessment of complex fractures (multiplanar and 3D reformation).
    • Bone density evaluation.
    • Pre-operative assessment prior to surgical excision of neoplastic processes - especially when evaluating possible bone invasion from a soft tissue neoplasm.

Advantages

  • The major advantage of CT over conventional radiographs is the higher contrast resolution: CT can discriminate density differences of 0.25-0.5%, radiography of only about 10%.
  • Cross-sectional images avoid superimposition of structures in areas of complex anatomy such as the skull and nose.
  • Structures that cannot be evaluated via other imaging techniques may be readily imaged with CT.
  • Structure and function may be evaluated concurrently in some cases.
  • 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: the newest, multiple-row detectors helical scanners allow imaging of the entire human thorax in as little as 3 seconds.
  • Refurbished CT equipment can be purchased at affordable prices.
  • CT is superior to all other imaging modalities when examining bone.

Disadvantages

  • General anesthesia General anesthesia: overview is typically required.
  • With single slice helical CT, and even more so with the newest, multiple-row detectors helical scanners, sedation Sedation or sedative protocol alone can be sufficient.
  • The area of interest must fit within the CT gantry this is typically not a problem when imaging domestic cats; it may become an issue with large wild felids.
  • Standard CT tables are designed to accommodate the human torso and have weight limitations of 150-200 kg.
  • 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 this is especially true if a multislice detector is used.
  • 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

General
  • Jones J C (2000) Three-dimensional computed tomography: user-friendly images. Compend Contin Educ Pract Vet 22 (6), 562-566 PubMed.
  • Tidwell A S, Johnson K L (1994) Computed tomography-guided percutaneous biopsies in the dog and cat: description of the technique and preliminary evaluation in 14 patients. Vet Radiol Ultrasound 35 (6), 445-456 VetMedResource.
Brain
  • Tyson R, Graham J P, Bermingham E et al (2005) Dynamic computed tomography of the normal feline hypophysis cerebri (glandula pituitaria). Vet Radiol Ultrasound 46 (1), 33-38 PubMed.
  • Kawasaki Y, Tsuruta T, Setogawa Y et al (2003) Hydrocephalus with visual deficits in a cat. J Vet Med Sci 65 (12), 1361-1364 PubMed.
  • Kaser-Hotz B, Rohrer C R, Stankeova S et al (2002) Radiotherapy of pituitary tumors in five cats. JSAP 43 (7), 303-307 PubMed.
  • Meij B P, Voorhout G, Van Den Ingh T S et al (2001) Transsphenoidal hypophysectomy for treatment of pituitary-dependent hyperadrenocorticism in 7 cats. Vet Surg 30 (1), 72-86 PubMed.
Nasal/sinus
  • Nöller C, Henninger W, Grönemeyer D H et al (2006) Computed tomography-anatomy of the normal feline nasolacrimal drainage system. Vet Radiol Ultrasound 47 (1), 53-60 PubMed.
  • Whitney B L, Broussard J, Stefanacci J D (2005) Four cats with fungal rhinitis. J Feline Med Surg (1), 53-58 PubMed.
Ear and TMJ
  • Bischoff M G, Kneller S K (2004) Diagnostic imaging of the canine and feline ear. Vet Clin North Am Small Animal Pract 34 (2), 437-458 PubMed.
  • Dickie A M, Doust R, Cromarty L et al (2003) Comparison of ultrasonography, radiography and a single computed tomographic slice for the identification of fluid within the canine tympanic bulla. Res Vet Sci 75 (3), 209-216 PubMed.
  • Garosi L S, Dennis R, Schwarz T (2003) Review of diagnostic imaging of ear diseases in the dog and cat. Vet Radiol Ultrasound 44 (2), 137-146 PubMed.
  • Muilenburg R K, Fry T R (2002) Feline nasopharyngeal polyps. Vet Clin North Am Small Animal Pract 32 (4), 839-849 PubMed.
  • Schwarz T, Weller R, Dickie A M et al (2002) Imaging of the canine and feline temporomandibular joint: a review. Veterinary Radiology and Ultrasound 43 (2), 85-97 PubMed.
Head and neck - others
  • Drost W T, Mattoon J S, Samii V F et al (2004) Computed tomographic densitometry of normal feline thyroid glands. Veterinary Radiology and Ultrasound 45 (2), 112-116 PubMed.
  • Attali-Soussay K, Jegou J P, Clerc B (2001) Retrobulbar tumors in dogs and cats: 25 cases. Vet Ophthalmol (1), 19-27 PubMed.
  • Calia C M, Kirschner S E, Baer K E et al (1994) The use of computed tomography scan for the evaluation of orbital disease in cats and dogs. Veterinary and Comparative Ophtalmology (1), 24-30 VetMedResource.
Spine
  • Jaeger G H, Early P J, Munana K R et al (2004) Lumbosacral disc disease in a cat. Vet Comp Orthop Traumatol 17 (2), 104-106 VetMedResource.
Thorax
  • Prather A B, Berry C R, Thrall D E (2005) Use of radiography in combination with computed tomography for the assessment of noncardiac thoracic disease in the dog and cat. Veterinary Radiology and Ultrasound 46 (2), 114-121 PubMed.
  • Yoon J, Feeney D A, Cronk D E et al (2004) Computed tomographic evaluation of canine and feline mediastinal masses in 14 patients. Veterinary Radiology and Ultrasound 45 (6), 542-546 PubMed.
  • Cipone M, Diana A, Gandini G et al (2003) Use of computed tomography in thoracic diseases of small animals. Veterinary Research Communications 27 (Suppl 1), 381-384 PubMed.
  • Henninger W (2003) Use of computed tomography in the diseased feline thorax. JSAP 44 (2), 56-64 PubMed.
  • Schwarz L A, Tidwell A S (1999) Alternative imaging of the lung. Clin Tech Small Anim Pract 14 (4), 187-206 PubMed.
Abdomen
  • Head L L, Daniel G B, Becker T J et al (2005) Use of computed tomography and radiolabeled leukocytes in a cat with pancreatitis. Veterinary Radiology and Ultrasound 46 (3), 263-266 PubMed.
  • Nakamura M, Chen H M, Momoi Y et al (2005) Clinical application of computed tomography for the diagnosis of feline hepatic lipidosis. J Vet Med Sci 67 (11), 1163-1165 PubMed.
  • Bouma J L, Aronson L R, Keith D G et al (2003) Use of computed tomography renal angiography for screening feline renal transplant donors. Veterinary Radiology and Ultrasound 44 (6), 636-641 PubMed.
  • Head L L, Daniel G B, Tobias K et al (2003) Evaluation of feline pancreas using computed tomography and radiolabeled leukocytes. Veterinary Radiology and Ultrasound 44 (4), 420-428 PubMed.
  • Reichle J K, DiBartola S P, Léveillé R (2002) Renal ultrasonographic and computed tomographic appearance, volume and function of cats with autosomal dominant polycystic kidney disease. Veterinary Radiology and Ultrasound 43 (4), 368-373 PubMed.
  • Yamazoe K, Ohashi F, Kadosawa T et al (1994) Computed tomography on renal masses in dogs and cats. J Vet Medi Sci 56 (4), 813-816 PubMed.
Musculoskeletal
  • Vignoli M, Ohlerth S, Rossi F et al (2004) Computed tomography-guided fin-needle aspiration and tissue-core biopsy of bone lesions in small animals. Veterinary Radiology and Ultrasound 45 (2), 125-130 PubMed.
  • Radlinsky M A (2001) Ancillary diagnostic techniques for the lame patient. Vet Clin North Am Small Animal Pract 31 (1), 181-192 PubMed.

Other sources of information

  • Assheuer J, Sager M (1997) MRI and CT atlas of the dog. Blackwell Science, Berlin; Wien; Oxford. Unfortunately, no specific atlas for the cat is available to this date.
  • Romans L E (1995) Introduction to Computed Tomography. Williams and Wilkins, Media PA.


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