David S. Biller, DVM, DACVR
Professor and Head of Radiology, Kansas State University, College of Veterinary Medicine, Department of Clinical Sciences
Manhattan, KS, USA
Diagnostic radiology is an extremely valuable tool probably the most important in the diagnosis of thoracic disease. It usually reveals more specific information than can be gained from taking a history and performing a physical examination relatively cheap and quickly and providing rapid results. Although we know that all the information gained is important towards developing a list of differential diagnosis.
History and physical examination changes associated with thoracic disease can be ambiguous. Therefore indications for thoracic radiography (noncardiac) can be numerous and varied.
Some indications for thoracic radiographs include:
Understanding the factors involved with the production of a quality radiograph are important, because diagnosis can be limited or made incorrectly by poor quality.
Common quality errors include:
Not including entire thorax on film
Not centering thorax on film
Not taking film on inspiration
Poorly positioned patient
Appropriate technique (mAs, kVp)
Not short enough exposure
Film processing error
Poor environment for evaluating radiograph
The entire thorax (not to mistaken for the entire cat) including thoracic inlet and the entire diaphragm should be on the radiograph. The radiograph should also include the caudal cervical trachea. The lateral radiograph should be centered at the caudal edge of the scapula when the exposure is taken. Make sure the animal is straight. This is best accomplished on the lateral radiograph by having the spine and sternum parallel and equidistant from the table in the lateral view. Padding under the sternum will help accomplish this. Oblique positioning artifact is common and can make the heart appear falsely enlarged and create confusing lung shadows. Proper positioning should also include the extension of forelimbs forward off the chest and parallel to each other. Head in neutral position in lateral view (Flexion of head and neck often causes confusing deviation of thoracic trachea). Do not stretch the animal as this will distort the thorax. Radiographic signs of malpositioning on the lateral radiograph include: lack of superimposition of costochondral junctions, dorsal arches of ribs not at same level on each side, and thoracic vertebral bodies not individually and distinctly visualized. Right lateral recumbency preferred due to a more constant cardiac position. Left lateral recumbency produces a less consistent cardiac position, although it is useful in evaluation of suspected or questionable right sided pulmonary lesions.
Radiographic signs of malpositioning of the VD or DV views include: spine and sternum not superimposed, costal cartilages not symmetrical, dorsal processes of the vertebrae do not appear as oval central opacities superimposed over the vertebral bodies, and contralateral ribs different lengths. Oblique positioning although good for evaluation of extrapleural or thoracic wall lesions (disease) can create confusing artifactual abnormalities such as giving a false appearance of mediastinal shift of the heart or create the appearance of an abnormal heart shape. The dorsoventral (DV) radiograph shows a more constant cardiac position. Pulmonary ventilation is more uniform especially for the caudodorsal lungs and it is less stressful on the respiratory compromised patient. Small very ventral lesions of the right middle and caudal subsegment of the left cranial lung lobe may not be visualized in the DV radiograph. The cardiac position is less constant on the ventrodorsal (VD) radiograph, and appears longer on the VD. The VD radiograph is useful in the evaluation of the heart when small volumes of pleural fluid are present. Fluid falls to dependent areas (paravertebral gutter) and avoids silhouetting (border effacing) the heart.
The most common error associated with poor quality thoracic radiographs is motion of the animal during exposure. This may be due to motion of the animal resisting restraint or from respiration or heart beat (this blurring can cause mis-interpretation). A short exposure time (< 1/30th of a second-1/60th ideal) is the best way to avoid motion. Things that are helpful in decreasing exposure time include: rare earth film, faster intensifying screens, and x-ray machines that allow short exposures (shorter mAs can be achieved by increasing kVp by 15% and decreasing mAs by half).
High kVp, low mAs gives a high latitude / low contrast radiograph (long scale of contrast) which is ideal for the thorax (makes evaluating things like the small pulmonary vessels possible). Use of a grid is important when patient thickness exceeds 10 cm therefore reducing scatter. Assessment of adequate penetration can be made on the VD or DV radiograph by faint visualization of the spine through the cardiac silhouette (or intervertebral disc spaces should be faintly visible through the cardiac shadow), and on the lateral by faint visualization of the dorsal spinous processes of the cranial thoracic vertebrae (or cranial thoracic vertebrae should be moderately underexposed and the mid-thoracic vertebrae should be easily seen). Ribs will also be poorly visualized over the cardiac silhouette. The peripheral lung fields should not be over exposed (high kVp / low mAs technique will help prevent this).
Thoracic radiographs should be taken during peak inspiration with few exceptions (greatest air/tissue ratio and it is the contrast provided by this air/tissue ratio that allows us to visualize intrathoracic structures). The key to making the exposure at full inspiration is to take the radiograph while the animal is breathing in. These differences can equal or mimic those caused by pathology. Expiratory films can sometimes be useful in: detection of dynamic collapse of intrathoracic trachea or bronchi, demonstrate small degree of pneumothorax, and demonstrate air-trapping by the lungs.