Comparison of topogram-based body size indices for CT dose consideration and scan protocol optimization.

PURPOSE

To retrospectively compare different topogram-based patient body size indices and to determine the optimal topogram-based body size index as a basis for body computed tomography (CT) dose consideration and scan protocol optimization.

METHODS

Forty-three routine thorax and abdomen CT scans are studied retrospectively, with patient ages ranging from 18 to 67 yr. The individual patient's water-equivalent diameter (D(w)) of the scanned body region is computed from CT DICOM images as the "gold standard," after first converting from Hounsfield units values to μa values, where μ is the normalized tissue attenuation coefficient and a is the area per pixel. Four topogram-based body size indices [average diameter (D), girth (G), topogram projection area (E(topo)), and improved topogram projection area (E(topo)('))] are computed and correlated with D(w) using linear regression analysis. Specifically, D is calculated by averaging the coronal and sagittal diameters; G is computed by modeling the patient's cross-section as an ellipse; E(topo) is the product of the mean topogram pixel value and the width of the scanned body region; and (E(topo)(')) incorporates E(topo) with correction of patient miscentering and water attenuation coefficient. The accuracy of these four approaches for estimation of D(w) is assessed using linear regression models. Results are given in terms of 95% confidence intervals (CIs).

RESULTS

Regression analysis results in four different linear models. The standard error (95% CI) for estimation of D(w) from D and G was ±2.8 and ±3.1 cm, respectively (p = 0.297). The standard error for estimation of D(w) from E(topo) was significantly less than that from D (±2.1 cm, p < 0.01). The standard error for estimation of D(w) from (E(topo)(')) was ±1.3 cm, significantly less than that from E(topo) (p < 0.01).

CONCLUSIONS

Among the four topogram-based patient body indices, (E(topo)(')) is the most accurate for estimation of individual x-ray attenuation of the scanned body region. Thus, (E(topo)(')) is an optimal topogram-based patient body size index that is relevant for determining the proper CT dose level for individual patients.