BASIC RADIOGRAPHY OF THE THORAX

BASIC RADIOGRAPHY OF THE THORAX

Thoracic radiography is an essential diagnostic technique for the diagnosis of intrathoracic and some systemic diseases. It is also one of the most challenging areas of veterinary radiography, regarding radiographic technique and interpretation of the image. To be able to correctly interpret images, it is very important to be familiar with the normal appearance of the radiographs, variations between views, cats and different dog breeds (deep vs barrel-chested) and variations caused by age, body condition and respiratory phase. Don’t forget: a good and consistent radiographic technique with a precise positioning is essential to reduce some potentially complicating factors and make a relevant image, which is easier to evaluate. Poor positioning is one of the most frequent radiographic errors and can require repeated radiographs or lead to missed or incorrect diagnoses.

For the radiographic evaluation of the thorax, the minimum of two orthogonal views should be obtained in order to accurately localize observed changes: one lateral recumbent and either ventrodorsal (VD) for lung pathology or dorsoventral (DV) for cardiac conditions. Frequently, two views are suboptimal, and three projections are advisable – both laterals (right and left) in addition to DV or VD. When an animal is in lateral recumbency, the lower half of the lungs partially collapses, while the upper half remains well aerated, which results in a good contrast between possible pathology and aerated lung. Therefore, the radiological changes in the upper part of the lungs are well visible, but the pathology in the lower part might be overlooked and some diagnoses might be missed if both lateral views are not obtained. Three views are recommended always when looking for metastatic lesions and lung disease and are especially important in case of small lesions.

Positioning can be a matter of clinician’s preference and probably the most important in choosing which views should be taken, is consistency and understanding the differences in appearance between projections. Some features are seen differently in right or left lateral recumbency and in DV or VD view, so it is important to be familiar with the normal appearance of the structures.

Laterolateral views

Right lateral view means, that the animal is lying with its right side down. If only one lateral view is chosen, the right one is mostly preferred, because the cardiac silhouette lies in a more consistent position, the diaphragm covers less of the caudodorsal lung field and enlargement of the sternal lymph node is more easily identified. On the right lateral view, left and right crus of the diaphragm appear parallel, with the right one lying more cranially and caudal vena cava merging with it. On the left lateral view, the cranial lobar pulmonary vessels are easier to recognize and assess. The crura of the diaphragm appear “Y” shaped, with the left crus lying more cranially and the caudal vena cava passing it and merging with the caudal (right) one. The heart has less sternal contact and appears more rounded, the apex may be elevated from the sternum.

For the lateral views, the animal is positioned in right or left lateral recumbency. The forelimbs are pulled cranially to avoid the brachial muscles obscuring the cranial lung lobes, head and neck are gently extended. The spine and sternum should be at the same distance from the tabletop. To achieve that, the sternum (in deep-chested breeds) or spine (barrel-chested breeds) may be supported with a foam wedge. To check the degree of thorax rotation, look at the ribs – costochondral junctions should be at the same level and several pairs of the ribs on the centre of the photo should be superimposed on one another. The centre of the X-ray beam is positioned at the caudal border of the scapula (around the level of the fifth rib) and midway between the sternum and the vertebral bodies. The picture should include the thoracic inlet and the entire diaphragm.

Dorsoventral view (DV)

The DV view is preferred in animals with heart disease and safe for animals in respiratory distress. A dyspnoeic animal should never be placed on its back and may even not tolerate lateral recumbency, so it is better to start with DV view, which might even be the only option.

The cardiac silhouette assumes more constant appearance on DV view, while on the VD it often rotates cranially and to the right, which can result in a false evaluation of its size and shape. The caudal pulmonary vessels and dorsal lung fields are visualized best. A single smooth curve of the diaphragmatic cupula is seen.

For DV positioning, it is important that the thorax is straight, and the sternum and spine are superimposed. The animal is positioned sternally with abducted elbows and the forelimbs extended forward. The scapulae should appear symmetrical. The head and neck are extended and can be supported with a foam pad. Foam wedges may be used to support deep-chested animals. The beam of X-ray is centred in the midline at the level of the caudal scapula.

Ventrodorsal view (VD)

A VD view is useful when lung pathology is suspected (if the animal is not in respiratory distress) and is best to assess the accessory lobe (cardiac silhouette displaces cranially, allowing better visualisation) and the ventral aspect of the lungs. A greater length of the caudal vena cava is seen, than on DV view. Diaphragmatic crura are superimposed over the cupula, giving a three-humped appearance.

The animal is positioned on its back and supported by foam wedges if necessary. The forelimbs are pulled forward. As in DV, the most important is, that the thorax is straight, with spine and sternum superimposed. The X-ray beam is centred in the midline at the level of the mid-sternum.

Thoracic radiographs should be taken at the end of inspiration (or in case of general anaesthesia during manual inflation) when lungs are fully expanded. There are few exceptions, where expiratory radiographs are desirable, because increased intrathoracic pressure or contrast between gas-containing lesions and non-inflated lungs help to visualise the pathology: detection of gas-filled structures, confirmation of collapse of the intrathoracic trachea or mainstem bronchi, detection of hyperinflation and “air trapping” (feline asthma) and detection of a small-volume pneumothorax.

Sedation may help in reducing the stress of the restraint during the procedure. It also minimizes the movement blur by slowing down the respiratory rate. But one must be aware, that general anaesthesia can quickly cause lung atelectasis, which results in increased pulmonary opacity that makes interpretation of lung changes more difficult. Atelectasis can be minimized by inducing anaesthesia when an animal is in sternal recumbency and obtaining DV view first, followed by lateral views. Lungs can also be manually inflated prior to radiographic exposure. For radiography of the cervical part of the trachea, the patient should be extubated.

Exposure factors are selected in a way, that optimizes the radiographic contrast and reduces the potential for movement blur. Select low milliampere second (mAs), which minimizes the exposure time and high kilovoltage (kV) to maximize the grayscale. This lowers the contrast of the image, which is normally high in the thorax because of air-filled lungs, and makes small structures (bronchial walls and vessels) more visible.

LITERATURE:

Holloway A., McConnell F.: BSAVA Manual of Canine and Feline Radiography and Radiology. A Foundation Manual. BSAVA, 2013

Schwarz T., Johnson V.: BSAVA Manual of Canine and Feline Thoracic Imaging. BSAVA, 2008