Introduction

To date computed tomography (CT) findings have not been studied or been reported on in depth in veterinary trauma patients. In human trauma centres, CT is an integral part of the management of trauma patients. Focussed Assessment with Computed Tomography in Trauma (FACTT) in the admitting emergency room increases patient survival, decreases time in the emergency room (87 to 38 min) and alters the initial treatment plan in 20–34% of cases as compared to radiographs.

Trauma may be penetrating or blunt, with the latter including hit by car (91% of blunt trauma cases) with associated fractures, high rise syndrome, jumping off a moving vehicle, compressive injuries and acceleration/deceleration injuries. The presence of thoracic pathology, as seen on CT as a result of severe blunt trauma in man (Oikonomou et al.) and small animals (Simpson et al. - non-imaging stats) is given below.

CT evaluation and interpretation of thoracic images should be performed according to standard principles including assessing technical faults and artefacts, extrathoracic pathology and intrathoracic pathology. Trauma patients are usually too compromised to administer general anaesthesia. Pain medication and sedation are usually sufficient for imaging when using restraining devices such as bandages, Velcro straps and sand bags. Cats and small dogs are placed in a VetMouseTrap® for CT examinations. Using only sedation often results in some images being compromised, and in particular due to respiratory motion, and repeat CT scans may be indicated in such patients.

Extrathoracic Abdominal Changes

The visible part of the abdomen should be scrutinised for abdominal and retroperitoneal fluid, as well as the presence of free gas and organ displacement.

Extrathoracic Soft Tissue Changes

Subcutaneous gas and soft tissue swelling are readily seen with CT and transverse images cover the whole circumference of the abdomen compared to radiographs, which may miss pathology if limited to lateral or DV/VD locations. In any trauma case, the thoracic and visible abdominal wall should be carefully scrutinized for defects and abnormal content.

Skeletal Changes

The ribs are most commonly involved and may be fractured or subluxated. Diagnosing rib pathology is not always easy. CT is likely to be a more sensitive imaging modality to detect rib fractures than radiographs, and using specialised techniques, such as reconstructed maximum intensity projections (MIP) and volume rendered techniques (VRT), as well as free view, are very helpful. Beware of respiratory motion creating pseudofractures, particularly in patients in lateral recumbency. The dependant ribs do not move much with respiration, whilst the upper ribs may have marked motion and artefactually can mimic rib fractures. Fractures of ribs 1–3 may be associated with cranial mediastinal and brachial plexus pathology, ribs 4–10 with intrathoracic pathology and ribs 9–13 with cranial abdominal pathology. Widened intercostal spaces are indicative of intercostal muscle tearing, as is often seen with bite wounds. Sternal, vertebral, scapula and humeral fractures should be specifically looked and a bone window is advised for this. CT is more sensitive to detect, as well as to define, the extent of the thoracic vertebral fractures and subluxations than radiographs. Trauma related pneumorachis (air in the vertebral canal) has been described in trauma cases and CT will be more sensitive to detect this.

Intrathoracic - Pleural Cavity

Pneumothorax is a common sequel to trauma, and CT is a very sensitive modality to detect small volumes of free thoracic air. Beware of misinterpreting lung edge enhancement (rebound artefact) as a small-volume pneumothorax. Free gas will usually rise to the uppermost body region and use this to aid in distinguishing the edge artefact from true, small-volume pneumothorax. It is crucial to detect a small-volume pneumothorax. Many trauma cases end up in theatre for surgical procedures and if positive pressure manual or ventilator ventilation is done in these patients, terminal tension pneumothorax may result.

Trauma-related pleural cavity haemorrhage is uncommon in our experience. It may take place from the lung, thoracic wall or even herniated liver and spleen or rupture of intercostal arteries secondary to rib fractures. CT is a sensitive tool to detect small volumes of free or trapped thoracic fluid, which will usually be located on the dependant side of the thorax. The HU can be an indication as to cause with reactive effusions being < 15 HU, whole blood 30–45 HU and clotted blood 50–90 HU. Active bleeding scanned post-contrast may have higher attenuation values.

Intrathoracic - Lungs

Pulmonary contusion is the most common trauma-related change in the thorax. Currently, the incidence in our study is about 60%. Contusion occurs as a result of pulmonary compression and tearing with secondary capillary disruption with leakage of blood into the interstitium and alveoli. Contusion usually only becomes visible 4–6 hours post-trauma, when it may only be visible on CT, and progresses to reach maximum lung involvement at 24 hours, when it may be more radiographically visible. In man, clearance of uncomplicated contusion starts at 24–48 hours with complete resolution in 3–14 days. On CT, contusions may be lobar and have a ground glass to consolidated appearance, which may show air bronchograms. The HU of a contused lung in the presence of pneumothorax will be higher than that seen with pneumothorax-associated atelectasis.

Pulmonary laceration and cavitatory lesions are easily seen on CT. The lung may tear or disrupt due to compressive forces, shear injuries, rib penetration or adjacent to previous adhesions. The air-filled cavity, which may also contain blood, is usually surrounded by pulmonary contusion. The cavitatory lesions may take weeks or months to clear.

Intrathoracic - Mediastinal Structures

Early pneumomediastinum is readily detected on CT, which allows more accurate evaluation of gas location than do radiographs. Hypoattenuating gas streaks are seen within the mediastinum and the tracheal and oesophageal walls are readily identified. Haemomediastinum is rare in our experience, but needs to be looked for. Be careful to not misinterpret a normal thymus in young dogs for haemorrhage. Pneumo- and/or haemopericard are rare occurrences, but the cardiac silhouette needs to be carefully evaluated on CT for these changes. Tracheal (and bronchial) rupture should be looked for, particularly in cats.

Diaphragm

Diaphragmatic ruptures are routinely diagnosed on thoracic radiographs. However, if they are accompanied by pleural effusion or are small ruptures, they can be a radiological challenge. CT, and in particular MPR images, have a high sensitivity to detect even small ruptures.

Conclusion

Should CT be available in the practice/institution, it should be the initial thoracic imaging modality of choice in trauma patients due to its rapid acquisition time and greater sensitivity to detect small-volume pneumothorax and other pathology. In our experience, CT detects a lot of thoracic pathology that the admitting clinicians never suspected!

References

1.  Lisciandro GR, et al. Evaluation of a thoracic focussed assessment with sonography for trauma (TFAST) protocol to detect pneumothorax and concurrent thoracic injury in 145 traumatized dogs. J Vet Emerg Crit Care. 2008;18:258–269.

2.  Oikonomou A, Prassopoulus P. CT imaging of blunt chest trauma. Insights Imaging. 2011;2:281–295.

3.  Parry A, Lamb C. Radiology of thoracic trauma in the dog and cat. In Practice. 2012;32:238–246.

4.  Simpson A, et al. Severe blunt trauma in dogs: 235 cases (1997–2003). J Vet Emerg Crit Care. 2009;19:588–602.