Equis ISSN 2398-2977

Computed tomography

Synonym(s): CT

Contributor(s): Rachel Murray, Valerie Samii

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.
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Uses

Musculoskeletal imaging

  • Indications:
    • Pain or localized lameness without definitive diagnosis reached via other diagnostic imaging techniques.
    • Suspected soft tissue injuries of the distal extremity not accessible to sonographic evaluation   Foot: straight sesamoidean desmopathy (normal) - CT    Foot: straight sesamoidean desmopathy (abnormal) - CT  .
    • Suspected subchondral, cortical or cancellous bone injury or anomaly not clearly demonstrated radiographically or scintigraphically   Distal phalanx: cyst - CT  .
    • Suspected articular cartilage abnormality or subclinical joint pathology.
    • Pre-operative assessment of complex fractures with 3D reformation.
    •  Bone density evaluation (stress remodeling).

Nasal/sinus/dental imaging

  • Indications:
    • Nasal discharge.
    • Fluid accumulation within the sinuses of unknown cause or origin seen radiographically   Head: maxillary sinus cyst 01 - CT      Head: maxillary sinus cyst 02 - CT    Head: maxillary sinusitis - CT  .
    • Suspected tooth root abscess or tooth fracture not clearly evident on radiographs   Head: M2 root abscess - CT      Head: PM4 fracture - CT    Head: patent infundibulum infection - CT  .
    • Suspected fractures not clearly evident on radiographs   Head: nasal fractures - CT  .
    • Soft tissue masses of the ethmoid turbinates and sinuses not clearly seen on radiographs   Head: ethmoid hematoma - CT  .
    • Nasolacrimal duct atresia or acquired occlusion.
    • Pre-operative assessment of complex fractures with 3D reformation.
    • Pre-operative assessment prior to surgical excision of neoplastic processes.

Guttural pouch/stylohyoid imaging

  • Indications:
    • Swollen throatlatch area without clear definition of soft tissue structures on survey radiographs.
    • Suspected empyema, mycoses, or hemorrhage of the guttural pouches.
    •  Head shaking.
  • Suspected fracture or proliferative remodelling of the stylohyoid bones not seen radiographically   Head: stylohyoid fracture - CT  .

Temporomandibular joint imaging

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

Brain imaging

  • Indications:
    • Seizures.
    • Central nervous system abnormalities.
    • Suspected congenital anomalies.
    • Suspected pituitary-dependent hyperadrenocorticism.
    • Trauma   Head: fracture (pneumocranium) - CT      Head: intracranial hemorrhage - CT  .

Other head imaging

  • Indications:
    • Trauma.
    • Suspected basisphenoid-basioccipital fractures not evident radiographically.
    • Suspected atlanto-occipital and atlantoaxial malformations.
    • Head shaking.
    • Otitis interna/media/externa.
    • Suspected melanomas   Head: melanomas - CT  .

Spine imaging

  • Indications:
    • Trauma.
    • Suspected subluxation/luxation.
    • Fractures.
    • Osteomyelitis   Spine: sacral process abscess - CT  .
    • Cervical stenotic myelopathy.
    • Congenital malformations.

Thoracic imaging (foals, miniatures and small ponies only)

  • Indications:
    • Suspected mediastinal, lung or pleural cavity pathology not seen via other diagnostic imaging modalities.

Abdominal imaging (foals, miniatures and small ponies only)

  • Indications:
    • Suspected organ, peritoneal or retroperitoneal pathology not seen via other diagnostic imaging modalities.

Advantages

  • The major advantage of CT over conventional radiographs is the improved contrast and spatial resolution.
  • 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.
  • Refurbished CT equipment can be purchased at affordable prices.
  • CT is superior to all other imaging modalities when examining bone.

Disadvantages

  • General anesthesia is required   Anesthesia: general - overview  .
  • Area of interest must fit within the CT gantry.
  • In adult horses, only the limbs distal to/including the carpus and tarsus, head, and cranial cervical spine may be imaged.
  • Area of interest must be well defined prior to imaging to avoid patient repositioning and excessive anesthesia times.
  • Standard CT tables are designed to accommodate the human torso and have weight limitations of 150-200 kg.
  • Construction of a larger table with greater weight capacity that couples with the existing manufacturers table is necessary for imaging large, adult equine patients.
  • 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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Prognosis

  • Not applicable. This is a diagnostic procedure only.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Dakin S G et al (2014) Technical set-up and radiation exposure for standing computed tomography of the equine head. Equine Vet Educ 26 (4), 208-215 VetMedResource
  • Raes E et al (2014) Computed tomographic features of lesions detected in horses with tarsal lameness. Equine Vet J 46 (2), 189-193 PubMed.
  • van Hamel S E et al (2014) Contrast-enhanced computed tomographic evaluation of the deep digital flexor tendon in the equine foot compared to macroscopic and histological findings in 23 limbs. Equine Vet J 46 (3), 300-305 PubMed.
  • Barba M & Lepage O M (2013) Diagnostic utility of computed tomography imaging in foals: 10 cases (2008-2010). Equine Vet Educ 25 (1), 29-38 PubMed.
  • Judy C E (2013) Cross-sectional imaging: More ways to look at a problem. Equine Vet Educ 25 (12), 624-627 VetMedResource
  • Claerhoudt S et al (2012) Differences in the morphology of distal border synovial invaginations of the distal sesamoid bone in the horse as evaluated by computed tomography compared with radiography. Equine Vet J 44 (6), 679-683 PubMed.
  • Vallance S A, Bell R J W, Spriet M, Kass P H & Puchalski S M (2012) Comparisons of computed tomography, contrast enhanced computed tomography and standing low-field magnetic resonance imaging in horses with lameness localised to the foot. Part 1: Anatomic visualisation scores. Equine Vet J 44 (1), 51-56 PubMed.
  • Avella C S & Perkins J D (2011) Computed tomography in the investigation of trauma to the ventral cranium. Equine Vet Educ 23 (7), 333-338 VetMedResource.
  • Beccati F et al (2011) Comminuted basilar skull fracture in a colt: Use of computed tomography to aid the diagnosis. Equine Vet Educ 23 (7), 327-332 VetMedResource.
  • Coleman M C, Chaffin M K, Arnold C E & Young B D (2011) The use of computed tomography in the diagnosis of an ectopic ureter in a Quarter Horse filly. Equine Vet Educ 23 (12), 597-602 VetMedResource.
  • Nagy A & Dyson S (2011) Anatomical, magnetic resonance imaging and histological findings in the accessory ligament of the deep digital flexor tendon of forelimbs in nonlame horses. Equine Vet J 43 (3), 309-316 PubMed.
  • Powell S (2011) Investigation of pelvic problems in horses. In Pract 33 (10), 518-524 VetMedResource.
  • Perrin R A R et al (2011) The use of computed tomography to assist orthopaedic surgery in 86 horses (2002-2010). Equine Vet Educ 23 (6), 306-313 VetMedResource.
  • Lacombe V A, Sogaro-Robinson C & Reed S M (2010) Diagnostic utility of computed tomography imaging in equine intracranial conditions. Equine Vet J 42 (5), 393-399 PubMed.
  • Naylor R J et al (2010) Histopathology and computed tomography of age-associated degeneration of the equine temporohyoid joint. Equine Vet J 42 (5), 425-430 PubMed.
  • Poulin Braim A E et al (2010) Computed tomography of proximal metatarsal and concurrent third tarsal bone fractures in a Thoroughbred racehorse. Equine Vet Educ 22 (6), 290-295 VetMedResource.
  • Powell S E (2010) Use of multi-detector computed tomographic angiography in the diagnosis of a parapharyngeal aneurysm in a 6-week-old foal. Equine Vet J 42 (3), 270-273 PubMed.
  • Kinns J & Pease A (2009) Computed tomography in the evaluation of the equine head. Equine Vet Educ 21 (6), 291-294 VetMedResource.
  • Veraa S, Voorhout G & Klein W R (2009) Computed tomography of the upper cheek teeth in horses with infundibular changes and apical infection. Equine Vet J 41 (9), 872-876 PubMed.
  • Veraa S, Dijkman R, Klein W R & van den Belt A J M (2009) Computed tomography in the diagnosis of malignant sinonasal tumours in three horses. Equine Vet Educ 21 (6), 284-288 VetMedResource.
  • Windley Z, Weller R, Tremaine W H & Perkins J D (2009) Two-and three-dimensional computed tomographic anatomy of the enamel, infundibulae and pulp of 126 equine cheek teeth. Part 2: Findings in teeth with macroscopic occlusal or computed tomographic lesions. Equine Vet J 41 (5), 441-447 PubMed.
  • Windley Z, Weller R, Tremaine W H & Perkins J D (2009) Two-and three-dimensional computed tomographic anatomy of the enamel, infundibulae and pulp of 126 equine cheek teeth. Part 1: Findings in teeth without macroscopic occlusal or computed tomographic lesions. Equine Vet J 41 (5), 433-440 PubMed.
  • Del Chicca F, Kuemmerle J M, Ossent P, Nitzl D & Fuerst A (2008) Use of computed tomography to evaluate a fracture associated with a subchondral pedal bone cyst in a horse. Equine Vet Educ 20 (9), 515-519 VetMedResource.
  • Desbrosse F G et al (2008) A technique for computed tomography of the foot in the standing horse. Equine Vet Educ 20 (2), 93-98 VetMedResource.
  • Puchalski S M (2008) Principles of digital x-ray imaging: Computed tomography and digital radiography. Equine Vet Educ 20 (2), 99-102 Wiley Online Library.
  • Rodriguez M J et al (2008) Computed tomorgraphic anatomy of the temporomandibular joint in the young horse. Equine Vet J 40 (6), 566-571 PubMed.
  • Stewart A J et al (2007) Computed tomographic diagnosis of a pathological fracture due to rhodococcal osteomyelitis and spinal abscess in a foal. Equine Vet Educ 19 (5), 231-235 VetMedResource.
  • Munsterman A S, Alexander K, Samii V F & Latimer F G (2007) Computed tomography in the diagnosis of septic physitis in two foals. Equine Vet Educ 19 (4), 200-206 VetMedResource.
  • Puchalski S M (2007) Computed tomography in equine practice. Equine Vet Educ 19 (4), 207-209 VetMedResource.
  • Bauer S, Livesey M A, Bjorling D E & Darien B J (2006) Computed tomography assisted surgical correction of persistent right aortic arch in a neonatal foal. Equine Vet Educ 18 (1), 32-36 VetMedResource.
  • Lischer C J et al (2005) Fracture of the paracondylar process in four horses: advantages of CT imaging. Equine Vet J 37 (5), 483-487 PubMed.
  • Wion L, Perkins G, Ainsworth D M, Dykes N L & Divers T J (2001) Use of computerised tomography to diagnose a Rhodococcus equi mediastinal abscess causing severe respiratory distress in a foal. Equine Vet J 33, 523-526 PubMed.
  • Morrow K L, Park R D, Spurgeon T L, Stashak T S & Arceneaux B (2000) Computed tomographic imaging of the equine headVet Radiol Ultrasound 41, 491-497 PubMed.
  • Widmer W R, Buckwalter K A, Fessler J F, Hill M A, VanSickle D C & Ivancevich S (2000) Use of radiography, computed tomography and magnetic resonance imaging for evaluation of navicular syndrome in the horse. Vet Radiol Ultrasound 41, 108-116 PubMed.
  • Riggs C M, Whitehouse G H, Boyde A (1999) Structural variation of the distal condyles of the third metacarpal and third metatarsal bones in the horse. Equine Vet J 31, 130-139 PubMed.
  • Riggs C M, Whitehouse G H & Boyde A (1999) Pathology of the distal condyles of the third metacarpal and third metatarsal bones of the horse. Equine Vet J 31, 140-148 PubMed.
  • Whitton R C, Buckley C, Donovan T, Wales A D & Dennis R (1998) The diagnosis of lameness associated with distal limb pathology in a horse: A comparison of radiography, computed tomography and magnetic resonance imaging. The Vet J 155, 223-229 PubMed.
  • Hanson J A, Seeherman C A, Kirker-Head A & OCallaghan M W (1996) The role of computed tomography in evaluation of subchondral osseous lesions in seven horses with chronic synovitis. Equine Vet J 28, 480-488 PubMed.
  • Barbee D D (1996) Computed tomography (CT): a dip into the future. Equine Vet J 28, 92 PubMed.
  • Tietje S, Becker M & Bockenhoff G (1996) Computed tomographic evaluation of head diseases in the horse: 15 cases. Equine Vet J 28, 98-105 PubMed.
  • Moore B R, Holbrook T C, Stefanacci J D, Reed S M, Tate L P & Menard M C (1992) Contrast-enhanced computed tomography and myelography in six horses with cervical stenotic myelopathy. Equine Vet J 24, 197-202 PubMed.
  • Wisner E R, OBrien T R, Pascoe J R, Koblik P D & Hornof W J (1991) Osteomyelitis of the axial border of the proximal sesamoid bones in seven horses. Equine Vet J 23, 383-389 PubMed.
  • Allen J R, Barbee D D, Boulton C R, Major M D, Crisman M V, Murnane R D (1987) Brain abscess in a horse: Diagnosis by computed tomography and successful surgical treatment. Equine Vet J 19, 552-555 PubMed.
  • OCallaghan M W, Pascoe J R & Tyler W S (1987) Exercise-induced pulmonary haemorrhage in the horse: results of a detailed clinical, post mortem and imaging study. IV. Changes in the bronchial circulation demonstrated by C.T. Scanning and microradiography. Equine Vet J 19, 405-410 PubMed.

Other sources of information

  • Kraft S L & Gavin P (2001)Physical principles and technical considerations for equine computed tomography and magnetic resonance imaging. In:The Veterinary Clinics of North America: Equine Practice. Eds: Kraft S L & Roberts G D. W B Saunders, Philadelphia.17, 115-130.
  • Tucker R L & Farrell E (2001)Computed tomography and magnetic resonance imaging of the equine head.In:The Veterinary Clinics of North America: Equine Practice. Eds: Kraft S L & Roberts G D. W B Saunders, Philadelphia.17,131-144.
  • Tucker R L & Sande R D (2001)Computed tomography and magnetic resonance imaging in equine musculoskeletal conditions.In:The Veterinary Clinics of North America: Equine Practice.Eds: Kraft S L & Roberts G D. W B Saunders, Philadelphia.17, 145-157.
  • OBrien R T & Biller D S (1998)Dental imaging.In:The Veterinary Clinics of North America: Equine Practice. Eds: Gaughan E M & DeBowes R M. W B Saunders, Philadelphia.14, 259-272.


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