Rationale and Objectives
Traditionally, maximum gallbladder wall thickness is measured at a single point on ultrasonography. The purpose of this work was to develop an automated technique to measure the thickness of the gallbladder wall over the entire gallbladder surface using computer tomography (CT).
Materials and Methods
Subjects who had (5-mm) thick and thin (2.5-mm) reconstruction through the abdomen were selected from a research database. Their volumetric computed tomographic images were acquired using a multidetector GE Medical Systems LightSpeed 16 scanner at 120 kVp, ≈250 mAs, with standard filter reconstruction algorithm and segmented in three dimensions. Two segmentation boundaries were obtained, an inner and an outer boundary of the gallbladder wall. The thickness of the wall was quantified by computing the distance between the boundaries over the entire volume using Laplace’s equation from mathematical physics. The distance between the surfaces is found by computing normalized gradients that form a vector field, representing tangent vectors along field lines connecting both boundaries. The Laplacian technique was compared with the well-known Euclidean distance transformation (EDT) technique that provides a three-dimensional Euclidean distance mapping between the two extracted surfaces.
Results
The technique was tested on 10 subjects who had thin- and thick-section computed tomographic datasets reconstructed from a single scan. The mean thickness for the thick- and thin-section CT using Laplace was 3.18 and 2.93 mm, respectively. The smooth transition between surfaces resulting from the Laplace technique resulted in a coefficient of variation that was less than 1% compared to EDT.
Conclusions
EDT technique is very sensitive to imperfect segmentations, resulting in higher variation compared to the Laplacian technique. The smooth transition between surfaces makes the Laplacian technique more robust compared to EDT for the measurement of CT gallbladder thickness.
Thickening of the gallbladder wall is a relatively frequent finding in diagnostic imaging studies. Historically, a thick-walled gallbladder has been regarded as evidence of primary gallbladder disease, and it is a well-known feature of many gallbladder-related diseases. The gallbladder wall is usually perceptible as a thin attenuating rim of soft tissue as shown in Figure 1 . Although its thickness depends upon the degree of gallbladder distention, 3 mm is regarded as the upper limit of normal thickening ( ).
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Figure 1
Gallbladder wall shown by arrow.
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Figure 2
Example of a computed tomographic scan with superimposed segmentation of the gallbladder wall. Although the wall thickness indicated at points A and B appears nearly the same, subsequent analysis reveals that A is thicker than B due to the three-dimensional nature of the gallbladder. It is also worth noting that even though two patients can have a similar maximum diameter, the overall wall thickness may be clearly different.
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Methods
Tissue Segmentation and Preparation of Boundaries
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CT Wall Thickness Calculation
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Experimental Testing
Data acquisition
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Analysis
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Results
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Table 1
Coefficient of Variation in Percentage for Different Methods of Thickness Measurement
EDT1 EDT2 Laplace Thick section CT 4.05 3.78 2.54 Thin section CT 3.55 3.11 2.09
CT, computed tomography; EDT, Euclidean distance transformation.
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Discussion
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Conclusion
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