Introduction

We are used to always acquired orthogonal radiographic projections to have a 3-dimensional understanding of the normal anatomy and any pathologic findings. Despite that the radiographic appearance of abnormal findings or normal structures mimicking pathology may still be complicated by superimposed other structures, particularly in the thorax and abdomen. In these cases, adding more radiographic views including opposite lateral projections, horizontal view images, and compression views may be very helpful. A few of these indications and techniques are reviewed. Appropriate radiation protection including lead gowns, thyroid shields and gloves have to be used for acquisition of all these views as they often require a person to be in the room during radiographic acquisition.

Thorax

In our clinic we routinely acquire three thoracic radiographs: both lateral and a ventrodorsal (VD) or dorsoventral (DV) projection. Many smaller pulmonary lesions are not visible if they are not surrounded by well-inflated lung and therefore need to be in the non-dependent side of the thorax to be visible. The VD and DV views can be helpful in determining the laterality and location of a lesion but many lesions are obscured by the spine and sternum, cardiac silhouette and diaphragm and are better visible in the opposite lateral view.

Horizontal view radiographs can provide valuable additional information. Is an increased soft tissue opacity in the cranial thorax caused by loculated or compartmentalized pleural effusion or is there a cranial mediastinal mass? This question often arises in cats with thoracic and pleural space disease. Horizontal beam radiographs with the patient in upright position (suspended by forelimbs) and the beam in a ventrodorsal direction shifts free pleural effusion away from the cranial thorax whereas mediastinal structures remain in place. The same view can be used to caudally displace the diaphragm and in presence of a diaphragmatic hernia, displace abdominal contents away from the cardiac silhouette for better delineation.

Horizontal beam ventrodorsal radiographs with the animal in lateral recumbency are helpful to shift free gas and free fluid in the thorax. Pleural effusion moves to the dependent side of the thorax and the non-dependent lung may be better visible, whereas a small amount of pneumothorax may be easier to detect as it rises to the upper thoracic wall. Pulmonary bulla or mass? This question mostly arises in bullae or cysts that are partially fluid and partially gas filled. In this case a fluid line may be detectable with horizontal beam radiography. In patients with persistent increased opacity in the ventral portion of a lung lobe it can be difficult to determine if this is due to atelectasis or if there is lung pathology such as pneumonia or a mass causing the opacity. Horizontal beam radiography with the lung lobe in question in the non-dependent side of the thorax may resolve that question.

Is there a scapular lesion but it is always superimposed over the thoracic spine and ribs? Not clearly visible in the VD or DV projection? Try positioning the patient in lateral recumbency, laying on the scapula in question. The lower thoracic limb with the possible scapular lesion is held with the elbow at a right angle and, pushed dorsally by the elbow and held in position with a sandbag. The upper thoracic limb is pulled ventrally to create slight rotation of the spine and to project the scapula dorsal to the spine.

Abdomen

Lateral and ventrodorsal views have become the standard of abdominal radiography with some institutions preferring right lateral views, other left lateral views. The left lateral view highlights gas in the pylorus whereas the right lateral view highlights gas in the fundus. To evaluate the pylorus, a ventrodorsal projection is used to put fluid in the fundus and gas in the pyloric antrum. For this reason, three abdominal radiographic projections (left and right lateral and ventrodorsal) are routinely acquired at our institution. Acquiring both lateral projections is also helpful, sometimes essential in diagnosing patients with intestinal obstruction. Redistribution of gas and fluid may highlight foreign bodies or simply clarify that there is an enlarged small intestinal loop suggestive of obstruction.

Free gas can be very difficult to diagnose radiographically if there are only small amounts present in the peritoneal space. Positional radiography can also be used to evaluate for free gas in the abdomen. Horizontal beam projections with the patient on its left side and obtaining a ventrodorsal projection will create a gas-fluid interface and will put the gas in the right lateral abdomen near the pyloric antrum. Since the pylorus is small, the gas accumulation can be seen along the diaphragm and right body wall.

Intestinal contents can be a hindrance when evaluating other structures that may be superimposed. Bladder stones or mineralized gastrointestinal contents? VD views are of limited use as bladder stones tend to be positioned in midline and thus superimposed with the spine. The opposite lateral view can sometimes be useful as bowel loops may shift away from the urinary bladder. But if the superimposition persists, a simple compression view using a wooden spoon or plexiglass paddle over the caudal abdomen can be used to move intestinal loops away from the bladder and clarify the presence of calculi.

Extremities

Many specialized views are described for various musculoskeletal disorders and will not be reviewed here. Horizontal beam projections, however, should be kept in mind for musculoskeletal radiography as well since it often simplifies positioning, or, in case of spinal trauma, does not require repositioning of a potentially unstable spine for a VD projection. Skyline views, oblique projections and stress views should always be considered when initial routine radiographs are not sufficient to determine the problem, particularly in complex joints such as the carpus and tarsus.

References

1.  Avner A, Kirberger RM. Effect of various thoracic radiographic projections on the appearance of selected thoracic viscera. J Small Anim Pract. 2005;46(10):491–498.