Computed tomography liver volumetry using 3-dimensional image data in living donor liver transplantation: effects of the slice thickness on the volume calculation

Abstract

The purpose of this study was to evaluate the relationship between the slice thickness and the calculated volume in computed tomography (CT) liver volumetry through the comparison of the results from images [including 3-dimensional (3D) images] with various slice thicknesses. Twenty potential adult liver donors (12 men and 8 women) with a mean age of 39 years (range = 24-64 years) underwent CT with a 64-section multidetector row CT scanner after the intravenous injection of a contrast material. Four image sets with slice thicknesses of 0.625, 2.5, 5, and 10 mm were used. First, a program developed in our laboratory for automated liver extraction was applied to the CT images, and the liver boundaries were determined automatically. Then, an abdominal radiologist reviewed all images onto which automatically extracted boundaries had been superimposed and then edited the boundaries on each slice to enhance the accuracy. The liver volumes were determined via the counting of the voxels within the liver boundaries. The mean whole liver volumes estimated with CT were 1322.5 cm(3) from 0.625-mm images, 1313.3 cm(3) from 2.5-mm images, 1310.3 cm(3) from 5-mm images, and 1268.2 cm(3) from 10-mm images. The volumes calculated from 3D (0.625-mm) images were significantly larger than the volumes calculated from thicker images (P < 0.001). The partial liver volumes of right lobes, left lobes, and lateral segments were evaluated in a similar manner. The estimated maximum difference in the calculated volumes of lateral segments was -10.9 cm(3) (-4.63%) between 0.625- and 5-mm images. In conclusion, liver volumes calculated from 2.5-mm-thick or thicker images are significantly smaller than liver volumes calculated from 3D images. If a maximum error of 5% in the calculated graft volume will not have a significant clinical impact, 5-mm-thick images are acceptable for CT volumetry. If the impact is significant, 3D images could be essential.

Fig. 1

Drawing of the spherical model for numerical simulation shows a cross section of a sphere 8 cm in radius at y=0. The y-axis is perpendicular to the plane of the paper. The center of the sphere is located at the origin of the coordinate system. Rectangles show cross sections of CT slices, which are perpendicular to the z-axis, with both thickness and intervals of d mm. The center of the sphere is located at the middle of a slice and the center of the slice plane. For a given slice S, the volume of a part of the sphere within the slice S can be approximated by the volume of a cylinder with a radius of r mm and a height of d mm. The volume of the cylinder was calculated based on three assumptions for the radius: (A) minimum, (B) middle, and (C) maximum. The total volume of the sphere was approximated by the sum of the volumes of these cylinders.

Fig. 2

Liver contours on axial CT images after manual editing show isolation of the liver from surrounding structures. The inferior vena cava, main trunk and bilateral first branches of the portal vein, and major fissures are excluded from the liver region. The contours are drawn on (A) a 0.625-mm thick 3D image and (B) a 10-mm thick image. Partial volume effects are much more prominent on the 10-mm image compared to the 0.625-mm 3D image.

Fig. 2

Liver contours on axial CT images after manual editing show isolation of the liver from surrounding structures. The inferior vena cava, main trunk and bilateral first branches of the portal vein, and major fissures are excluded from the liver region. The contours are drawn on (A) a 0.625-mm thick 3D image and (B) a 10-mm thick image. Partial volume effects are much more prominent on the 10-mm image compared to the 0.625-mm 3D image.

Fig. 3

Scatter plots of estimated whole liver volumes on 0.625-mm versus (A) 2.5-mm, (B) 5-mm, and (C) 10-mm images. Volumes estimated on thicker images were smaller than those estimated on 0.625-mm 3D images. The solid lines represent the line of equality.

Fig. 3

Scatter plots of estimated whole liver volumes on 0.625-mm versus (A) 2.5-mm, (B) 5-mm, and (C) 10-mm images. Volumes estimated on thicker images were smaller than those estimated on 0.625-mm 3D images. The solid lines represent the line of equality.

Fig. 3

Scatter plots of estimated whole liver volumes on 0.625-mm versus (A) 2.5-mm, (B) 5-mm, and (C) 10-mm images. Volumes estimated on thicker images were smaller than those estimated on 0.625-mm 3D images. The solid lines represent the line of equality.

Fig. 4

Plots of estimated whole liver volume differences between 0.625-mm and (A) 2.5-mm, (B) 5-mm, and (C) 10-mm images against their averages. There are no discrepancies in relation to the size of the liver volume measurement. A thinner slice thickness shows a smaller degree of dispersion around the horizontal axis.

Fig. 4

Plots of estimated whole liver volume differences between 0.625-mm and (A) 2.5-mm, (B) 5-mm, and (C) 10-mm images against their averages. There are no discrepancies in relation to the size of the liver volume measurement. A thinner slice thickness shows a smaller degree of dispersion around the horizontal axis.

Fig. 4

Plots of estimated whole liver volume differences between 0.625-mm and (A) 2.5-mm, (B) 5-mm, and (C) 10-mm images against their averages. There are no discrepancies in relation to the size of the liver volume measurement. A thinner slice thickness shows a smaller degree of dispersion around the horizontal axis.

Fig. 5

Scatter plots of estimated whole liver volumes on 0.625-mm images versus standard liver volumes as calculated from body surface area.