Things to Avoid When Producing and Evaluating Thoracic Radiographs

Do not obtain thoracic radiographs when the patient is anaesthetised or heavily sedated. Anaesthetised patients will have changes in the lungs (such as atelectasis) and oesophagus (megaoesophagus) that can be mistaken for pathology.

A thoracic radiograph should be obtained on maximum inspiration. This allows for the most contrast between normal and diseased lung. The only exceptions to this rule are patients with tracheal collapse, looking for pneumothorax and air trapping. Both inspiratory and expiratory lateral radiographs should be obtained in patients with tracheal collapse.

When obtaining thoracic radiographs of patients with suspected or confirmed neoplasia it is important to take three views, including a ventrodorsal or dorsoventral and both left and right laterals. Three views are important because the non-dependent (up) lung is best visualised due to the increased air within the pulmonary parenchyma. This results in improved contrast between a pulmonary nodule and aerated lung. The dependent lung is partially atelectatic, which results in decreased contrast between pulmonary nodules and lung parenchyma.

The same principle holds true for the dorsoventral or ventrodorsal radiograph. Nodules in the caudodorsal lung field will be best visualised on a dorsoventral radiograph. When evaluating animals suspected of heartworm disease a dorsoventral radiograph is best for assessing changes of the caudal lobar vessels.

Thoracic Radiograph Artefacts

Pulmonary nodules may be confused with end-on vessels, the costochondral junction and artefacts from the skin surface (nipples, ticks, nodules, etc.). If a pulmonary lesion is seen in only one view it is of questionable significance.

End-on vessels are typically very circular and very opaque for their size. The size of the vessel should be appropriate for its location within the lung field (the vessels are larger in the perihilar region and smaller at the periphery). End-on vessels should overlie linear vascular markings.

By using both views and knowledge of normal anatomy the costochondral junction can be distinguished from metastatic disease.

Nipples or skin nodules are well circumscribed and dense. These can be ruled out because they will be outside the lung on one view. If there is a question whether a skin nodule is mimicking pulmonary disease, a small amount of barium can be placed on the nodule and radiographs repeated.

Skin folds can result in artefacts, which may be confused with pneumothorax or pleural effusion, depending on the view.

Abdomen

The technique for abdominal radiographs will vary depending on the conformation of the patient. In general you should use a moderate kVp and mAs technique. This will give you moderate contrast and latitude (many shades of grey, but contrast between abdominal organs and fat).

To determine the proper exposure evaluate the cranial abdomen to ensure this area is not underexposed. The caudal abdomen should be evaluated for overexposure. In certain animals you may need separate techniques for the cranial and caudal abdomen. The vertebral bodies should be exposed to clearly evaluate the bone, but you should still be able to see the soft tissues of the ventral and lateral body walls.

A minimum of two views is always indicated. This may include either the right or left lateral view and the ventrodorsal view. The routine views should be standardised for the most consistent interpretation.

It is very important that the patients are straight. The ribs should line up over each other in the cranial abdomen on the lateral view. The transverse processes of the lumbar spine should also align in the caudal abdomen and the pelvis should be straight. Foam wedges under the xyphoid are useful for correct positioning for the lateral view in thin patients. Similar to the thorax, the dorsal spinous processes should be centered on the lumbar vertebral bodies. Troughs are useful for positioning for the ventrodorsal view.

The right lateral view is considered best for visualising the spleen and for separation of the kidneys. Also consider which thoracic view is taken (you don't want to have to roll the animal back and forth because you are taking different lateral views of the thorax and abdomen).

Gas within the intestinal tract is highly variable, depending on the view taken. This is particularly important in the stomach. In a left lateral view fluid will fill the fundus and gas will go to the nondependent pylorus. On the right lateral view the fluid will be in the pylorus, while the fundus should contain gas. In a vomiting animal it is probably best to take the left lateral view to assess the pylorus. Gas in other areas of the intestine will also move around when the animal is repositioned.

An abdominal radiograph should include the diaphragm to the coxofemoral joints. Additional more caudal views should be added if disease of the terminal colon, urinary tract, pelvic canal or perineal region is present. For evaluation of the urethra in a male dog a perineal view should be obtained. This is a lateral view with the hindlimbs moved cranially over the caudal abdomen and the centre of the beam positioned at the pelvis.