chapter 17 Small Animal Soft Tissue

INTRODUCTION

The term soft tissue describes the areas of the body that surround the skeletal structures. Unlike radiography of bone tissue, visualization of soft tissue can be difficult because it involves only slight differences in radiographic density. Production of a soft tissue radiograph that has high contrast between the various adjacent soft structures is almost impossible without the use of contrast media. To achieve the correct contrast, density, and visualization, a number of factors must be considered:

1. To attain a long scale of contrast with good visualization of the internal soft tissue structures, a relatively high kilovoltage and low milliamperage-seconds are used.

2. A grid is necessary for areas of dense tissue to maintain image clarity and radiographic detail.

3. An exposure time of 1/30 second or less is necessary for thorax radiography to minimize motion caused by cardiac and respiratory movement.

4. Proper preparation is necessary for abdominal radiography. The patient should be fasted for 12 to 24 hours and given a cleansing enema at least 1 hour before radiography.

5. Exposure of the thorax and abdomen must be taken during the correct phase of respiration: inspiration for the thorax and expiration for the abdomen.

PHARYNX

Lateral View

The patient is placed in lateral recumbency with the forelimbs pulled in a caudal direction. The head and neck are extended cranially and placed in a true lateral position (Figs. 17-1 and 17-2). A sponge wedge pad placed under the mandible helps eliminate obliquity of the skull and frees the larynx from the mandible to allow better visualization of the laryngeal region. The air passages of the upper respiratory tract act as a negative contrast agent and permit the soft tissue structures of the pharyngeal region to be differentiated. The field of view should include the entire area of the neck between the lateral canthus of the eye and the third cervical vertebral body.

Figure 17-1 Correct positioning for the lateral view of the pharynx.

Figure 17-2 Radiograph of the lateral view of the pharynx.

BEAM CENTER: Over pharynx

MEASUREMENT: At level of base of skull

THORAX

Dorsoventral View

The dorsoventral view of the thorax is preferred for the evaluation of the heart because the heart is closer to the sternum and is in near-normal suspended position within the thorax. Unfortunately, it may be difficult to position larger dogs for the dorsoventral projection because of their deep chests. The dorsoventral view requires great care to ensure that the sternum is superimposed over the vertebral column. If this position is impossible to execute, it may be necessary to attempt a ventrodorsal projection.

The patient is placed in sternal recumbency with the thoracic vertebrae superimposed over the sternum (Figs. 17-3 and 17-4). The forelegs are pulled slightly forward to prevent the elbows from tucking under the thorax. The rear legs are allowed to flex in a natural crouching position. This crouched position may be difficult for the canine patient with hip dysplasia, and it may be necessary to consider the ventrodorsal view. The head is lower and is placed between the two forelimbs. The field of view should include the entire thorax. The rule is “the thorax is inside the rib cage”; if you include all of the ribs, you will radiograph the entire thorax.

Figure 17-3 Correct positioning for the dorsoventral view of the thorax.

Figure 17-4 Radiograph of the dorsoventral view of the thorax.

The exposure must be taken at the peak of inspiration to allow complete radiographic visualization of the lung tissue. The patient’s breathing should be observed several times before making the exposure. This allows the radiographer ample time to make the exposure at the proper phase of respiration.

BEAM CENTER: Over caudal border of scapula

MEASUREMENT: At level of caudal border of scapula

Ventrodorsal View

The ventrodorsal view of the thorax is advocated when a full view of the lung fields is necessary. This projection provides a better view of the accessory lung lobes and caudal mediastinum. Although it is easier to control a patient in ventrodorsal position, this view is contraindicated for patients in obvious respiratory distress. Placing such an animal on its back would be dangerous and could possibly cause further respiratory problems.

The patient is placed in dorsal recumbency with the forelimbs extended cranially (Figs. 17-5 and 17-6). The hind limbs can assume a normal position. Great care must be taken to ensure that the patient is in a true ventrodorsal posture. The sternum must be superimposed over the spine. If rotation is encountered, a V trough or sandbags placed under the pelvic region may be helpful. The field of view should include the entire thorax. The rule is “the thorax is inside the rib cage”; if you include all of the ribs, you will radiograph the entire thorax.