Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 51
Fig. 2-19 Graphs showing the normal
values (predicted value ±95%
confidence interval) of the left
atrium (A) and aortic root (B).
(From Bonagura JD, O’Grady MR,
Herring DS: Echocardiography:
principles of interpretation. Vet Clin
North Am 1985; 15:1191.)
A
B
Special transducers (e.g., transesophageal transducers) have been used to obtain highresolution
images of heart base structures. However, these transducers are expensive and
have not yet become common in clinical practice. 65,66
Doppler echocardiography allows determination of the direction and velocity of
blood flow by using information from the Doppler shift effect. This physical principle
results from a shift in frequency of an echo that is induced by a change in position of the
structure that is generating the echo. The shift will be to a higher frequency if the structure
is moving toward the transducer and to a lower frequency if it is moving away from
the transducer. The magnitude of the shift is related to the object’s speed of movement.
This Doppler shift induced by the moving red blood cells can be used to assess hemodynamic
information. Conventionally, flow or movement away from the transducer is displayed
below, and flow toward the transducer is displayed above, the baseline. The
pressure differential or gradient across a valve (∆P [mm Hg]) is estimated by applying
the modified Bernoulli equation to the known velocity of flow (V [m/s]) across a valve
(∆P = 4 V 2 ). 21