Kidney Stone Structural Analysis By Helical Computed Tomography (CT)

Helical computed tomography has become the radiologic tool of choice in the assessment and
treatment of patients with urinary tract calculi (Hubert et al, 1997; Smith et al, 1999).
However, the full potential of helical CT to differentiate among stone types by structure or
radiodensity has yet to be realized. Most CT scans for stones are used simply to identify
the existence of a stone and give some indication of its size and location. These scans are
viewed using soft tissue windows, in order to look for other possible causes of the
patient's pain, such as appendicitis, gallstones, and colonic diverticulitis. However, soft
tissue windows do not show structure within the kidney stone: stones appear as bright white
objects in these images. The potential for observing structure in stones (using viewing
windows closer to those used to view bone) has not been assessed in clinical studies.
Currently, only the average CT attenuation value of urinary tract calculi has been
investigated as an indicator of stone composition (Nakada et al, 2000; Mostafavi et al,
1998; Kuwahara et al, 1984). The average CT attenuation value has been shown to be useful
for distinguishing some stones (such as uric acid from calcium oxalate) but considerable
overlap in CT attenuation between stone types exist.

Treatment of urinary tract calculi is influenced by many factors including stone location,
size and composition. Shock wave lithotripsy (SWL) is an effective, non-invasive method that
is utilized to treat the majority of renal calculi. However, while some kidney stones are
easily fragmented by SWL, other stones of similar composition are SW-resistant and must be
removed by an invasive method following the failed lithotripsy. In addition, SWL is not
without complications with long-term risks of hypertension and renal insufficiency (Evan et
al, 1998; Willis et al, 1998). Considerable variation in SWL fragility exists within each
major stone composition group that is best explained by stone structural heterogeneity (Saw
et al, 2000). The association of stone structure and SWL fragility is not a new concept as
Dretler and Polykoff (1996), in a retrospective study of calcium oxalate stones, reported
four distinct patterns of stone structure on plain abdominal radiographs. Unfortunately, SWL
fragility was not directly tested with the authors relying on clinical intuition that stones
that on x-ray are smooth and more radiodense (and usually higher calcium oxalate monohydrate
content) tend to be harder to fragment with SWL. Finally, the technology for clinical CT
continues to advance. The latest generation of multidetector helical CT machines have
considerably improved image resolution over single-detector CT technology. These quad-slice
scanners have 4 contiguous, parallel rows of x-ray detectors combined with a higher gantry
rotation speed which increase the speed of data collection by a factor as high as 8 over the
conventional single-slice spiral CT scanners. The evolution from single-slice to
multi-slice scanners does not alter image performance in terms of contrast resolution,
in-plane spatial resolution and radiation dose if irradiating the same volume. However the
benefits of quad-slice spiral CT compared to single-slice spiral CT are significant. The
scans may be performed with thinner CT slices, which means higher spatial resolution along
the longitudinal axis of the patient. The scans can also be performed much faster, which
means improved temporal resolution and less motion artifacts. Thus, the ability to both
predict stone composition from kidney stone CT attenuation values and delineate structural
features necessary to predict stone fragility to lithotripter shock waves-if not now
practical with present technology-will certainly be possible as this technology progresses.