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.
Inflation of the lungs in this way can make small soft tissue opacities more visible.
A reasonably high kV, along with high mA capability, will facilitate the use of shorter exposure times and avoid excessive image contrast.
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 artifacts.
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 .
Demonstration of mediastinal pathology .
Distal tracheal pathology .
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 animal 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 , but is more often a supplementary procedure.
Time required Preparation
Dependent upon the method of chemical restraint (GA or sedation).
Procedure
10 to 15 minutes, 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.
Ventrodorsal
Shows accessory lung lobe and reveals more of the caudal lobes medially, but heart falls across to right side so not the projection of choice for assessing cardiac outline. VENTRODORSAL NOT TO BE ATTEMPTED WHEN PLEURAL FLUID SUSPECTED OR SEVERE DYSPNEA PRESENT.
Horizontal beam lateral view
In very dyspneic patient 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, eg 'lesion orientated obliques' in cases of chest wall masses. Less diffference between VD and DV view in the cat than the dog.
Risk assessment
Suitability for chemical restraint
Type of chemical restraint: GA or sedation, balancing patient criteria against any limitations of X-ray equipment.
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 degree foam wedge.
Center the vertical central ray approximately 2 cm caudal to 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.
Lengthwise collimation should include the full length of the rib cage.
Dorsally, the collimation should normally be within the skin surface. If this results in cutting off the sternum, then the centering is too far dorsal.
Expose on inspiration .
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.
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 a level 2 cm caudal to the caudal point of the scapulae.
Collimate to include the full extent of the lung fields.
Expose on inspiration. 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 inflated but not over inflated.
Step 3 - Ventrodorsal projection.
A positioning trough is usually required.
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.
Placing a dyspneic animal in dorsal or lateral recumbency may compromise respiration in some cases.