Radiology, Vol 170, 39-44, Copyright © 1989 by Radiological Society of North America
Renal calculi after shock wave lithotripsy: US evaluation with an in vitro phantom
PL Choyke, JH Pahira, WJ Davros, E Nilges, AJ Dwyer and SK Mun
Department of Radiology, Georgetown University Medical Center, Washington, DC.
Fragments of renal calculi from patients who had undergone extracorporeal
shock wave lithotripsy (ESWL) were embedded in an in vitro
tissue-equivalent ultrasound (US) phantom to assess (a) the accuracy with
which such fragments could be measured; (b) the influence of stone
composition, stone depth, and transducer frequency; and (c) the effect of
fragment "clumping" on size determinations and acoustic shadowing.
Fragments as small as 0.8 mm in diameter could be detected at 7.5 MHz,
although a practical limitation of 1-2 mm for lower transducer frequencies
(3.5 and 5.0 MHz) was observed. Fragment diameter tended to be
overestimated with US by a mean of 1 mm. Transducers of 5 and 7.5 MHz were
more accurate in determination of size than 3.5-MHz transducers. Struvite
fragments were less echogenic and were therefore measured more accurately,
but smaller struvite fragments were less detectable than other fragment
types. Fragment "clumping" led to significant overestimation of residual
stone size even when small fragments were separated by as much as the
distance equal to the diameter of one fragment. This study demonstrates
that even under the best circumstances the disadvantages of US in assessing
residual stone disease are significant.
