CT visible internal stone structure, but not Hounsfield unit value, of calcium oxalate monohydrate (COM) calculi predicts lithotripsy fragility in vitro

Abstract

Calcium oxalate monohydrate (COM) stones are often resistant to breakage using shock wave (SW) lithotripsy. It would be useful to identify by computed tomography (CT) those COM stones that are susceptible to SW's. For this study, 47 COM stones (4-10 mm in diameter) were scanned with micro CT to verify composition and also for assessment of heterogeneity (presence of pronounced lobulation, voids, or apatite inclusions) by blinded observers. Stones were then placed in water and scanned using 64-channel helical CT. As with micro CT, heterogeneity was assessed by blinded observers, using high-bone viewing windows. Then stones were broken in a lithotripter (Dornier Doli-50) over 2 mm mesh, and SW's counted. Results showed that classification of stones using micro CT was highly repeatable among observers (kappa = 0.81), and also predictive of stone fragility. Stones graded as homogeneous required 1,874 +/- 821 SW/g for comminution, while stones with visible structure required half as many SW/g, 912 +/- 678. Similarly, when stones were graded by appearance on helical CT, classification was repeatable (kappa = 0.40), and homogeneous stones required more SW's for comminution than did heterogeneous stones (1,702 +/- 993 SW/g, compared to 907 +/- 773). Stone fragility normalized to stone size did not correlate with Hounsfield units (P = 0.85). In conclusion, COM stones of homogeneous structure require almost twice as many SW's to comminute than stones of similar mineral composition that exhibit internal structural features that are visible by CT. This suggests that stone fragility in patients could be predicted using pre-treatment CT imaging. The findings also show that Hounsfield unit values of COM stones did not correlate with stone fragility. Thus, it is stone morphology, rather than X-ray attenuation, which correlates with fragility to SW's in this common stone type.

Figure 1

Examples of COM calculi judged by all observers to be homogeneous (leftmost column) or showing internal structure (other columns). Each column shows one stone, with the topmost image a surface rendering from micro CT, the middle image a slice from micro CT, and the bottom image a slice from helical CT. Grayscale of images was adjusted for maximum viewing detail, and does not accurately reflect x-ray attenuation values.

Figure 2

Fragment of COM stone that did not fall through 2 mm mesh even after 2000 SW’s. Shown is surface rendering of micro CT images (a) and slice through middle of stone (b). The radial shadows through the middle of the slice image are reconstruction artifact, and the brightness at the edge of the stone is due to beam hardening artifact. Careful examination of the serial slices showed no evidence of content other than COM in this fragment.

Figure 3

Number of SW’s-to-last-fragment, normalized to stone weight, for stones judged to be homogeneous in composition/structure (no internal structure) and when stones were seen to have internal structure. a: Micro CT images of stones were used for judging composition/structure. b: Helical CT images were used. Each circle represents a single stone; columns show mean SW count to last fragment for each group. Significance of differences between ‘No internal structure’ and ‘Internal structure’ was tested using Tukey-Kramer HSD test, with data from all three groups included.

Figure 4

Number of SW’s-to-last-fragment for COM stones plotted against the average Hounsfield unit values for each stone, as measured by helical CT. Stone fragility showed no correlation with Hounsfield unit values.