Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

382 Small Animal Radiolo g y and Ultrasono graphy
Fig. 3-158 A 7-year-old female English Pointer with
polyuria and polydipsia for 2 weeks. A, The immediate lateral
view of the excretory urogram revealed a normal nephrogram
image. B, The 20-minute view of the excretory urogram
revealed that no collecting structures have become
apparent and that no contrast is seen in the bladder. The density
of the kidney parenchyma is nearly the same as the
immediate view. The nephrogram phase has remained persistent.
Diagnosis: Acute renal failure.
A
B
Functional Aspects of the Nephrogram. A qualitative assessment of renal abnormalities
can be performed by evaluating the nephrogram. 426 Renal perfusion abnormalities,
glomerular dysfunction, intrarenal or extrarenal obstruction, renal tubular necrosis, and
renal or systemic reactions to intravenous contrast administration may alter the opacity
trends of the nephrogram portion of the EU. Standardization of the contrast dosage and
filming sequence is essential to be able to derive functional information from the contrast
study. The time of maximal nephrographic density and the variations in density before and
after maximal opacity are recommended to differentiate the disease processes. Several
abnormal patterns have been described.
A normal nephrographic density that is followed by a progressively increasing density or
by a persistent density has been associated with contrast-induced systemic hypotension, acute
renal obstruction, and contrast-induced renal failure. Poor nephrographic density followed
by decreasing density has been associated with polyuric renal failure or an inadequate contrast
dose. Poor initial density followed by increasing opacity has been associated with acute
extrarenal obstruction, systemic hypotension, and renal ischemia. Poor initial density that
does not decrease may occur with glomerular disease or severe interstitial or tubular disease.
Renal Ultrasonographic Abnormalities
Size. Calipers are part of most ultrasonography machines and can be used to obtain
direct and accurate measurements of the kidneys. All three renal dimensions can be measured
and renal volume can be calculated. 29-31 Renal volumetric dimensions have not
improved the accuracy of ultrasonography in identifying renal abnormalities. The most
useful measurement is the renal length, because it has been repeatedly correlated radiographically
with the length of the second lumbar vertebral body (L2). 22,23,27,28 The possible
caveat to comparing sonographically measured kidneys to the radiographically
measured L2 is that because sonographic measurements are not magnified, they may fall
slightly lower in the established radiographic normal size ranges. Although width and
height have been defined, their utility remains unproved. The ultrasonographic measurements
will not be accurate unless the transducer is positioned perpendicular to the plane
of the kidney. Measurements obtained using the ultrasonography machine calipers actually
may be more accurate than radiographic measurements. Diuresis resulting from administration
of contrast agents, diuretics, or intravenous fluid can cause minimal enlargement of
the kidney, which can be detected and quantitated using the calipers. 32,35,36,477-479 Further,