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THE RENAL TRACT 169
A
B
Figure 7.13 (A) Pyonephrosis. Low-level echoes from pus can be seen in the dilated PCS. (Note that absence of
echoes does not exclude pyonephrosis.) (B) A hyperechoic blood clot can be seen within the collecting system of this
dilated kidney.
Like pyonephrosis, low-level echoes may be seen
on ultrasound within the collecting system (Fig.
7.13B). Although ultrasonically it is not possible to
differentiate pyo- from haemohydronephrosis, the
clinical picture can be suggestive of one or the other.
Non-dilated renal obstruction
Obstruction may occasionally be present in the
acute stages before renal dilatation is apparent:
beware—the finding of a non-dilated PCS on ultrasound
does not exclude obstruction in any patient
with symptoms of renal colic.
Spectral Doppler is useful in diagnosing acute,
early renal obstruction, before PCS dilatation develops,
because of the associated increase in blood flow
resistance in the affected kidney (Fig. 7.14). This
causes an increase in the resistance and pulsatility
indices (RI and PI) on the obstructed side, due to a
reduction in diastolic flow. A raised RI in itself is a
non-specific finding, not necessarily indicating
obstruction; it is known to be age-related or can be
associated with extrinsic compression of the kidney
(for example by a fluid collection or mass) or with
some chronic renal diseases or vascular disorders.
This can be overcome by analysing Doppler spectra
from both kidneys and evaluating any difference
between the two sides.
A marked difference in the RI between the kidneys
in a patient with renal colic points towards
obstruction of the kidney with the higher resistance.
12,13 A difference in RI of greater than 6 is
highly suspicious of obstruction in a patient with
renal colic; a reduction in the RI on the affected side
A
B
Figure 7.14 (A) A patient with acute renal colic has a normal-looking, non-dilated RK with a raised resistance index
of 75.8. IVU subsequently confirmed early obstruction of the RK. The resistance index (RI) subsequently returned to
normal following relief of the obstruction. (B) In the same patient, the contralateral, normal kidney has a much lower
resistance index of 67.2.