Basics of thoracic computed tomography

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Computed tomography (CT) is an advanced diagnostic imaging technique that produces cross-sectional images of the body, providing superior soft tissue detail compared to conventional radiography. This chapter summarizes an overview of the key indications, patient preparation considerations, and technical principles for performing thoracic CT examinations in veterinary patients. CT excels at characterising diseases affecting the thoracic structures, including the lungs, airways, pleural space, mediastinum, and chest wall. The technique offers sensitivities surpassing radiography for identifying pathological processes and assessing their extent and progression. Proper patient positioning and restraint are imperative to achieve diagnostic-quality images. CT can also guide interventional sampling procedures not feasible under radiographic or ultrasonographic guidance alone.

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3.1 Contrast-enhanced CT image of the cranial thorax of an 8-year-old Domestic Longhaired cat with a mineralized sarcoma arising from the caudal aspect of the left scapula. This image is set with a narrow window (level 65 HU, width 187 HU) to emphasize the peripheral contrast enhancement of the mass at the expense of creating increased visibility of image noise and streak artefacts. The lungs cannot be appropriately assessed with this window.
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3.2 Contrast-enhanced CT image of the cranial mediastinum in a 6-year-old Labrador Retriever with an extraskeletal osteosarcoma. The image is set with a narrow window (level 70 HU, width 220 HU). The mass extended from the mid-cervical region to the cranial mediastinum where it deviated and compressed contrast-enhanced blood vessels, the trachea (T) and the oesophagus (O). There is also compression of the cranial lung lobes.
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3.3 High-resolution CT image (1 mm slice width, 1 second rotation time, high spatial algorithm) of the caudodorsal thorax of a 10-year-old Miniature Poodle with a salivary adenocarcinoma. The image is set with a centre in the negative HU range and a wide window (level -744 HU, width 2456 HU). There is a single small lung nodule (arrowhead) in the dorsal aspect of the accessory lung lobe (AL) assessed as a potential metastasis. This nodule was not seen on thoracic radiographs. A post-chemotherapy treatment follow-up CT examination 4 weeks later revealed no growth of the nodule. A = aorta; C = caudal vena cava; O = catheterized oesophagus.
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3.4 Positioning of a sedated dog for a CT examination. The dog has been safely anchored to the table in ventral recumbency with the help of a foam trough, a foam wedge for the head and Velcro bands.
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3.5 (a) The VetCat Trap CT device positioned on the CT table and aligned with the positioning laser lights. (b) A sedated Domestic Shorthaired cat undergoing a CT examination in a VetCat Trap device for cancer restaging. The cat is cushioned with towels and the device is closed with a strap. Intravenous contrast medium is delivered from a power injector via the left tube and oxygen is supplied via the connected tube on the right. (c) High-resolution CT image of the same cat showing the lungs along with parts of the device and cushioning material. (b, courtesy of Misha Jarrett)
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3.6 (a) An anaesthetized Boston Terrier in ventral recumbency and ‘head first’ position on the CT table. Notice the anaesthetic tubing and pulse oximetry cable crossing the gantry, which can create significant artefacts. (b) The same dog in ventral recumbency and ‘tail first’ position. When the thorax is scanned, the anaesthetic equipment will not cross the gantry and artefacts will be avoided. Notice the extended thoracic limbs are positioned outside the scanning area. Electrocardiography clips (not seen here) should be placed well cranial or caudal to the scanning area.
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3.7 The ‘canine muffin’ sign. Occasionally, CT offers completely unexpected new vistas. This 4-year-old Saint Bernard was positioned for a thoracic CT scan in the largest trough available. The CT image of the caudal thorax reveals a muffin-shaped cross-sectional anatomy of the dog, indicating that the trough was not big enough and was potentially restricting respiration.
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3.8 Dorsal plane CT image of an 11-year-old Domestic Shorthaired cat with a cavitated mass in its left caudal lung lobe (arrowhead). The image was reconstructed from a helical CT series with 1 mm slice thickness and a pitch of 1, resulting in an image resolution close to the transverse plane images. The main advantages of this plane are better alignment of the lesion with the bronchial and vascular tree and increased viewing area.
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3.9 (a) Set-up for a Labrador Retriever for CT-assisted fine-needle aspiration of a lung mass. Three spinal needles have been placed with the needle tips in the thoracic wall muscles aligned with the laser light, along the transverse image plane of the patient. (b) Mid-thoracic CT image of a 3-year-old crossbreed dog with tracheobronchial lymphadenopathy, demonstrating the set-up for CT-assisted transpulmonary fine-needle lymph node aspiration. The needle is supported by a box, aligned with the image plane, and advanced subcutaneously (left side of image). A black streak artefact emanates from the needle tip. From this image, the depth and angle of needle advancement can be planned, and aspiration then pursued accordingly. (c) CT image from a recheck scan of the same dog as in (b). There is a small area of pulmonary haemorrhage along the needle track (arrowhead) and a small pneumothorax (not seen here). The dog recovered uneventfully from anaesthesia and a granulomatous lymphadenopathy was diagnosed.
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