Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter One Introduction 9
signal is not strong enough (e.g., the image is too black), the gain setting should be
increased or a lower frequency transducer selected. If the signal is too bright, the gain
should be decreased or a higher-frequency transducer selected. The use of a high-frequency
transducer at high gain settings to compensate for lack of ultrasonographic penetration
produces artifacts that may result in incorrect interpretation. Because air and bone reflect
a large percentage of the ultrasound beam, the direction from which the organ is being
imaged may be altered to avoid imaging through them. This is important when imaging
the heart. The cardiac window is selected because of the absence of lung at the cardiac
notch regions. The patient may be imaged from below to take advantage of lung atelectasis
on the recumbent side. It is also important when imaging the abdomen. The liver may
be imaged through the intercostal spaces if the stomach is full of air and located between
the usual position of the transducer caudal to the costochondral junctions. The liver and
the air-filled bowel may be displaced away from the examination area by gentle pressure on
the abdominal wall using the transducer.
Patient positioning for ultrasonography varies with the examination being performed
and, to some degree, operator preference and dexterity. Many ultrasonographers prefer to
examine the abdomen with the patient in dorsal recumbency. Most of the examination is
performed from the ventral abdominal wall with some areas, such as the liver and gall bladder,
examined through an intercostal space. Some individuals prefer to examine the
abdomen with the patient in lateral recumbency. A satisfactory examination can be
performed from either position. If an animal cannot be restrained in either position, the
examination may be accomplished with the patient standing. For cardiac examination,
most individuals prefer to position the animal in lateral recumbency with the
examination performed from the dependent side. This produces a larger air-free cardiac
window. The examination may be performed from the upper side; however, inflation of the
lung can reduce the size of the cardiac window. For examining most other body areas, the
position that is most comfortable for the operator and the patient should be satisfactory.
SYSTEMATIC EVALUATION FOR RADIOGRAPHIC AND
ULTRASONOGRAPHIC PATHOLOGY
The importance of systematically evaluating a radiograph needs to be emphasized repeatedly.
The tendency to rely on inspiration, first impressions, and “having seen one like this
before” in evaluating radiographs is poor practice that will result in diagnostic errors. It is
difficult to ignore the clinical signs and concentrate solely on the radiographic information,
especially when the clinician and radiographer are one and the same person; however, this
should be attempted. Ideally, the radiographs should first be evaluated without knowledge
or consideration of the animal’s history or clinical signs. Then, the radiographs should be
reevaluated in light of the patient’s history and clinical signs for the purpose of answering
those questions raised by the initial findings. There are several systems for radiographic evaluation,
including (1) the inside-out method, beginning at the center of the radiograph and
observing structures in ever-enlarging concentric circles, (2) the outside-in method, which
is the opposite of the inside-out method, and (3) the inventory method, evaluating each
organ according to a predetermined list of those structures that should be present in any
given area. The method used is a matter of personal preference and training experience;
however, once adopted the method should be used consistently.
Ultrasonographic examinations should proceed in an orderly fashion, with each organ
or area of the animal being evaluated completely. Most organs should be evaluated in at
least two planes. In some areas, such as cardiac ultrasonography, these planes have been
very well defined and should be adhered to strictly so that artifacts that mask or mimic disease
are not created. In other areas, the planes are less well defined and oblique, or off-axis
views may be as valuable as the standard examination planes. Ultrasonography usually is
performed with some specific area of interest, such as evaluation of a mass in the region of
the spleen. This area should be evaluated carefully, but other structures may be important
in determining a final diagnosis and these also should be examined (e.g., presence of focal
abnormalities within the liver in a patient with a splenic mass). The ultrasonographic
examination of specific organs or anatomical regions must be approached from more than