Segmentation of individual renal cysts from MR images in patients with autosomal dominant polycystic kidney disease

Abstract

Objective: To evaluate the performance of a semi-automated method for the segmentation of individual renal cysts from magnetic resonance (MR) images in patients with autosomal dominant polycystic kidney disease (ADPKD).

Design, setting, participants, & measurements: This semi-automated method was based on a morphologic watershed technique with shape-detection level set for segmentation of renal cysts from MR images. T2-weighted MR image sets of 40 kidneys were selected from 20 patients with mild to moderate renal cyst burden (kidney volume < 1500 ml) in the Consortium for Radiologic Imaging Studies of Polycystic Kidney Disease (CRISP). The performance of the semi-automated method was assessed in terms of two reference metrics in each kidney: the total number of cysts measured by manual counting and the total volume of cysts measured with a region-based thresholding method. The proposed and reference measurements were compared using intraclass correlation coefficient (ICC) and Bland-Altman analysis.

Results: Individual renal cysts were successfully segmented with the semi-automated method in all 20 cases. The total number of cysts in each kidney measured with the two methods correlated well (ICC, 0.99), with a very small relative bias (0.3% increase with the semi-automated method; limits of agreement, 15.2% reduction to 17.2% increase). The total volume of cysts measured using both methods also correlated well (ICC, 1.00), with a small relative bias of <10% (9.0% decrease in the semi-automated method; limits of agreement, 17.1% increase to 43.3% decrease).

Conclusion: This semi-automated method to segment individual renal cysts in ADPKD kidneys provides a quantitative indicator of severity in early and moderate stages of the disease.

Trial registration: ClinicalTrials.gov NCT01039987.

Figure 1.

T2-weighted coronal magnetic resonance images illustrating three groups of cyst-burden severity. (A) In the mild cyst-burden group, cysts are scattered and discretely defined by surrounding renal parenchyma. Most cysts are well separated without touching each other. (B) In the moderate cyst-burden group, the number and volume of cysts are greater than those of the mild cyst-burden group. Most cysts remain discernible and surrounded by renal parenchyma, although some neighboring cysts may share borders. (C) In the severe cyst-burden group, kidney tissue is almost completely replaced by innumerable cysts, with little discernible renal parenchyma. Segmentation of individual cysts by the computer is extremely challenging in this group.

Figure 2.

Magnetic resonance images from a 41-year-old man with autosomal dominant polycystic kidney disease and moderate cyst burden (corresponding to Figure 1B). The images illustrate an intermediate step of (A) manual cyst counting and (B) total cyst volume measurement using a region-based thresholding method. (A) The manual counting method was performed using an in-house cyst-labeling computer program. At the initial slice, a radiologist manually flagged all identified cysts by electronic marker (red marks). At the next slice, the carryover cysts are flagged with blue marks, whereas newly identified cysts are labeled in red marks. After all cysts were manually marked over the entire set of kidney images, the total number of cysts flagged with red marks is automatically computed. (B) On each slice of kidney MR images, a binary signal-intensity map is generated by determining a threshold signal intensity that visually distinguishes the cyst and renal parenchyma regions. In the binary map, cysts that are brighter than renal parenchyma are represented as white regions, whereas the background renal parenchyma is designated as black regions.

Figure 3.

Overview of the semi-automated segmentation processes of individual cysts.

Figure 4.

Individual renal cysts segmented and color-coded using the semi-automated method. (A) Magnetic resonance image from an 18-year-old man with autosomal dominant polycystic kidney disease (ADPKD) in the mild cyst-burden group (corresponding to Figure 1A) and (B) 41-year-old man with ADPKD and moderate cyst burden (corresponding to Figure 1B).

Figure 5.

Comparison of the manual and semi-automated renal cyst counts in 20 patients. (A) Scatter plot of the manual versus semi-automated renal cyst counts in 20 patients. Line shows line of regression fitted using a least-squares method (with coefficient of 0.98; R²=0.98; P<0.001). (B) Bland-Altman plot for the manual versus semi-automated renal cyst counts.

Figure 6.

Comparison of the semi-automated and region-based thresholding measurements of total cyst volume in kidneys. (A) Scatter plot of the semi-automated versus region-based thresholding measurements of total cyst volume in kidneys. Line shows line of regression fitted using a least-squares method (with coefficient of 0.99; R²=1.00; P<0.001). (B) Bland-Altman plot for the semi-automated versus region-based thresholding measurements of total cyst volume in kidneys.