ISSN 2398-2977      

Computed tomography

pequis

Synonym(s): CT


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.
Print off the Owner factsheet on Diagnostic imaging to give to your clients.

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) - CTFoot: 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 - CTHead: maxillary sinus cyst 02 - CTHead: maxillary sinusitis - CT.
    • Suspected tooth root abscess or tooth fracture not clearly evident on radiographs Head: M2 root abscess - CTHead: PM4 fracture - CTHead: 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 remodeling 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) - CTHead: 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.
  • Gough S L, Anderson J D C & Dixon J J (2020) Computed tomographic cervical myelography in horses: Technique and findings in 51 clinical cases. J Vet Intern Med 34 (5), 2142-2151 PubMed.
  • Norvall A, Spriet M, Espinosa P et al (2020) Chondrosesamoidean ligament enthesopathy: prevalence and findings in a population of lame horses imaged with positron emission tomography. Equine Vet J WileyOnline.
  • Norvall A, Cota J G, Pusterla N & Cissell D (2020) Ultrasound-guided arthrocentesis of the temporomandibular joint in healthy adult horses is equivalent to blind arthrocentesis. Vet Radiol Ultrasound 61 (3), 346-352 PubMed.
  • Davies T, Skelly C, Puggioni A et al (2020) Standing CT of the equine head: Reducing radiation dose maintains image quality. Vet Radiol Ultrasound 61 (2),137-146 PubMed.
  • Ostrowska J, Lindström L, Tóth T et al (2020) Computed tomography characteristics of equine paranasal sinus cysts. Equine Vet J 52 (4), 538-546 PubMed.
  • Dubois B B, Dixon J J & Witte T H (2019) Assessment of clinical and computed tomographic findings for association with the outcome of intraoral cheek tooth extraction in horses and poniesJAVMA 255 (12), 1369-1376 PubMed.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 WileyOnline.
  • Puchalski S M (2007) Computed tomography in equine practice. Equine Vet Educ 19 (4), 207-209 VetMedResource.

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, USA.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, USA.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, USA.17, 145-157.

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