Radiography of the thorax can be problematical due to difficulties eliminating movement blur resulting from breathing.
High output (high mA capability) X-ray machines enable exposure times to be minimized, reducing the risk of movement blur.
If the machine cannot achieve sufficiently low exposure times, general anesthesia may be required. Under anesthesia respiration can be interrupted by gentle pressure on the rebreathing bag, eliminating movement blur, but the lungs must be held inflated.
Inflation of the lungs is necessary to make small soft tissue opacities more visible and prevent artifacts from atelectasis.
High kV values are preferred for demonstrating the lung fields as this will result in a film of relatively low contrast and high latitude, allowing visualization of a wide range of tissue densities.
A higher kV, along with high mA capability, will also facilitate the use of shorter exposure times.
A secondary radiation grid is required when patient thickness >10 cm.
Close collimation of the primary beam should be practised at all times.
The objective is to produce a radiograph which includes the whole area of interest, is correctly exposed and developed, and is free from movement blur and artefacts.
The film should be clearly marked with the anatomical marker, the patient's identification, the date and the name of the hospital or practice.
Uses
Demonstration of lung pathology .
Assessment of cardiac size and shape .
Confirmation of diaphragmatic herniation .
Examination of esophagus .
Demonstration of pleural space pathology, eg pleural effusion , or pneumothorax .
Detection of fractured ribs , or other rib pathology .
Advantages
Non-invasive, valuable diagnostic tool.
Can be performed under sedation if equipment is adequate.
Can be performed with no chemical restraint if patient is very sick.
Relatively quick and simple where general anesthesia is not required.
Disadvantages
May require general anesthesia. Placing a dyspneic animals in dorsal or lateral recumbency may compromise respiration in some cases. Struggling with a non-compliant, eg undersedated patient may be detrimental to its condition.
Alternative techniques
Ultrasonography may occasionally be an alternative, eg pericardial effusion, pleural effusion , but is ideally used as a supplementary procedure.
Time required Preparation
Dependent upon the method of chemical restraint (GA or sedation).
Procedure
10-15 min or longer, dependent upon skill of radiographer.
Decision taking Criteria for choosing test
Is the examination appropriate?
Can you make the diagnosis without it?
Can it tell you what you need to know?
Will your management be affected by the radiological findings?
Choosing the right projections
Right lateral recumbency
Gives information about lung fields, heart size and shape.
Left lateral recumbency
Both laterals should be performed when looking for subtle changes, eg metastatic deposits in the lungs due to reduced visibility of soft tissue opacities within the lung fields on the side which is compressed by the patient's weight.
Dorsoventral (patient in sternal recumbency)
Gives additional information about lung fields, eg lateralization of a lesion seen on a lateral recumbency film, and particularly about heart size and shape. Results in better inflation of caudodorsal lung fields for increased visualization of pathology in this region, eg pulmonary artery enlargement.
Ventrodorsal
Shows accessory lung lobe and cranial ventral lung fields more clearly (with better inflation). VENTRODORSAL NOT TO BE ATTEMPTED WHEN PLEURAL FLUID SUSPECTED OR SEVERE DYSPNEA PRESENT.
Horizontal beam lateral view
In very dyspneic patients animal it may be difficult to position for standard views.
Standing lateral view will show caudodorsal area (limbs obscure cranial thorax). Small patients may be restrained in a cardboard box.
Other
Adapted projections may occasionally be necessary, for example "lesion orientated obliques" in cases of chest wall masses.
Risk assessment
Suitability for chemical restraint.
Type of chemical restraint: GA or sedation, balancing patient criteria against any limitations of X-ray equipment.
Remove radio-opaque objects, eg collar, lead, harness. Muzzle any patient which does not have a trustworthy temperament as close contact between personnel and patient is involved.
Place the patient in the right or left lateral recumbent position on the X-ray table.
Right lateral is standard, left is supplementary.
Ensure patient is well immobilized, with neck extended to avoid kinking of the trachea, and forelimbs drawn well cranially.
Ensure that spine and sternum are in the same horizontal plane.
It will often be necessary to elevate the sternum with a 15° foam wedge but barrel-chested breeds will not require this.
Center the vertical central ray at the level of the caudal-most point of the scapula, halfway between the head of the rib and the sternum .
Collimate the beam to include the entire extent of the lung fields. Assuming correct centring, cranio-caudal collimation can usually be judged by including the cranial edge of the scapula. The caudal extent of the lung fields will also then be included.
Dorsally, the collimation should normally be well within the skin surface. If this results in cutting off the sternum, then the centring is too far dorsal.
Expose on inspiration (maximum lung inflation) . If patient is anesthetized, expose when lungs are inflated.
Step 2 - Dorsoventral projection
Place the patient in sternal recumbency and immobilize . A positioning trough may be used but this is often unnecessary and sometimes a hindrance, depending on how well the patient complies with sitting on its haunches. If a trough is used for the dorsoventral projection, it should be a little undersized for the patient to facilitate optimum positioning of the legs .
Ensure that the spine and the sternum are in the same vertical plane.
Tape or a small sand bag on the neck is helpful to prevent overlap of cervical tissues on the cranial thorax.
Abduct humeri with elbows flexed to form a broad base of support and prevent the patient from rotating to one side or the other .
Center the beam in the midline at the level of the caudal point of the scapulae .
Collimate to include the full extent of the lung fields.
Expose on inspiration .
Expiratory films may increase visualization of tracheobronchial foreign bodies and small amounts of pleural effusion. Expiratory films to detect very small pneumothoraces are of debatable value. In serial examinations, the same phase of respiration should be used and this will normally be the height of normal inspiration or, under anesthesia, the lungs should be manually held inflated but not over inflated.
Step 3 - Ventrodorsal projection
A positioning trough is usually required, unless the patient is broad-chested and very compliant or anesthetized.
The patient is placed in dorsal recumbency and immobilized.
The forelimbs are secured clear of the lateral and cranial borders of the lung fields.
Ensure that the spine and the sternum are in the same vertical plane .
Center halfway along the sternum by palpation of the cranial and caudal extent of this.
Collimate to include the full extent of the lung fields.
Under anesthesia respiration can be interrupted by gentle pressure on the rebreathing bag, eliminating movement blur, but the lungs must be held inflated.
Demonstration of lung pathology 
Placing a dyspneic animals in dorsal or lateral recumbency may compromise respiration in some cases.