Dual-energy dual-source CT with additional spectral filtration can improve the differentiation of non-uric acid renal stones: an ex vivo phantom study

Abstract

Objective: The purpose of this study was to determine the ex vivo ability of dual-energy dual-source CT (DSCT) with additional tin filtration to differentiate among five groups of human renal stone types.

Materials and methods: Forty-three renal stones of 10 types were categorized into five primary groups on the basis of effective atomic numbers, which were calculated as the weighted average of the atomic numbers of constituent atoms. Stones were embedded in porcine kidneys and placed in a 35-cm water phantom. Dual-energy DSCT scans were performed at 80 and 140 kV with and without tin filtration of the 140-kV beam. The CT number ratio, defined as the ratio of the CT number of a given material in the low-energy image to the CT number of the same material in the high-energy image, was calculated on a volumetric voxel-by-voxel basis for each stone. Statistical analysis was performed, and receiver operating characteristic (ROC) curves were plotted to compare the difference in CT number ratio with and without tin filtration, and to measure the discrimination among stone groups.

Results: The CT number ratio of non-uric acid stones increased on average by 0.17 (range, 0.03-0.36) with tin filtration. The CT number ratios for non-uric acid stone groups were not significantly different (p > 0.05) between any of the two adjacent groups without tin filtration. Use of the additional tin filtration on the high-energy x-ray tube significantly improved the separation of non-uric acid stone types by CT number ratio (p < 0.05). The area under the ROC curve increased from 0.78 to 0.84 without fin filtration and to 0.89-0.95 with tin filtration.

Conclusion: Our results showed better separation among different stone types when additional tin filtration was used on dual-energy DSCT. The increased spectral separation allowed a five-group stone classification scheme. Some overlapping between particular stone types still exists, including brushite and calcium oxalate.

Fig. 1

The high (140 kV) and low (80kV) energy spectra simulated for the original factory-installed filtration (a) and with the additional tin filtration (Sn) for the high-energy spectrum (b).

Fig. 1

The high (140 kV) and low (80kV) energy spectra simulated for the original factory-installed filtration (a) and with the additional tin filtration (Sn) for the high-energy spectrum (b).

Fig. 2

Photographs show a porcine kidney that has renal stones sutured inside (a) and the set up of the water phantom (b). Stone-filled kidneys were placed in a 35cm water phantom anterior to a cadaver spine to create realistic scattering and attenuation conditions (b).

Fig. 2

Photographs show a porcine kidney that has renal stones sutured inside (a) and the set up of the water phantom (b). Stone-filled kidneys were placed in a 35cm water phantom anterior to a cadaver spine to create realistic scattering and attenuation conditions (b).

Fig. 3

Graphs shows CT number ratio (CTR) of individual renal stones, sorted in ascending order by CTR. The CTR of non-uric acid stones was always increased using the additional tin filtration. No increase in CTR was observed for uric acid stone types.

Fig. 4

Box-and-whisker plot shows the range (whiskers) and first and third quartiles (boxes) of CT number ratios (CTR) values of five groups of kidney stone types: Group 1: uric acid, uric acid dihydrate and ammonium acid urate; Group 2: cystine; Group 3: struvite; Group 4: calcium oxalate monohydrate, calcium oxalate dihydrate and brushite; Group 5: hydroxyapatite and carbonate apatite. The separation between these groups is shown without (a) and with (b) tin filtration, both at 80/140 kV.

Fig. 4

Box-and-whisker plot shows the range (whiskers) and first and third quartiles (boxes) of CT number ratios (CTR) values of five groups of kidney stone types: Group 1: uric acid, uric acid dihydrate and ammonium acid urate; Group 2: cystine; Group 3: struvite; Group 4: calcium oxalate monohydrate, calcium oxalate dihydrate and brushite; Group 5: hydroxyapatite and carbonate apatite. The separation between these groups is shown without (a) and with (b) tin filtration, both at 80/140 kV.

Fig. 5

Receiver Operating Characteristic (ROC) analysis of the stone composition differentiation using dual-source DECT with and without additional tin filtration. Four ROC curves are illustrated to indicate the ability of DECT to differentiate five stone groups: Group 1: uric acid, uric acid dihydrate and ammonium acid urate; Group 2: cystine; Group 3: struvite; Group 4: calcium oxalate monohydrate, calcium oxalate dihydrate and brushite; Group 5: hydroxyapatite and carbonate apatite. The ROC curves with tin filtration (b) show larger area under the curve than without tin filtration (a), which suggests a better method to differentiate the five stone groups.

Fig. 5

Receiver Operating Characteristic (ROC) analysis of the stone composition differentiation using dual-source DECT with and without additional tin filtration. Four ROC curves are illustrated to indicate the ability of DECT to differentiate five stone groups: Group 1: uric acid, uric acid dihydrate and ammonium acid urate; Group 2: cystine; Group 3: struvite; Group 4: calcium oxalate monohydrate, calcium oxalate dihydrate and brushite; Group 5: hydroxyapatite and carbonate apatite. The ROC curves with tin filtration (b) show larger area under the curve than without tin filtration (a), which suggests a better method to differentiate the five stone groups.

Fig. 6

Stone composition characterization with color coding. The commercially-available uric acid vs. non-uric acid differentiation algorithm with blue-red color coding is shown in the left panel (a); red: uric acid, blue: non-uric acid. Using dual-source DECT with additional tin filtration, it was possible to further characterize the same stones into one of five groups (b). These five groups and their representing colors are: Group 1 in red: uric acid, uric acid dihydrate and ammonium acid urate, Group 2 in yellow: cystine; Group 3 in green: struvite; Group 4 in turquoise: calcium oxalate monohydrate, calcium oxalate dihydrate and brushite; Group 5 in purple: hydroxyapatite and carbonate apatite.

Fig. 6

Stone composition characterization with color coding. The commercially-available uric acid vs. non-uric acid differentiation algorithm with blue-red color coding is shown in the left panel (a); red: uric acid, blue: non-uric acid. Using dual-source DECT with additional tin filtration, it was possible to further characterize the same stones into one of five groups (b). These five groups and their representing colors are: Group 1 in red: uric acid, uric acid dihydrate and ammonium acid urate, Group 2 in yellow: cystine; Group 3 in green: struvite; Group 4 in turquoise: calcium oxalate monohydrate, calcium oxalate dihydrate and brushite; Group 5 in purple: hydroxyapatite and carbonate apatite.

Fig. 7

A clinical example demonstrating the feasibility of using dual-source DECT with tin filtration to differentiate non-UA stone types. Using commercially software (“Kidney Stone”, Syngo CT Workplace, Siemens Healthcare), the stones were coded in blue (a) indicating a non-UA stone type. With dual-source DECT and additional tin filtration on the high energy tube, it was possible to characterize these stones into Group-4, which was defined to include three stone types: calcium oxalate mono- or dihydrate or brushite. Group-4 is coded in turquoise (b). The stones were subsequently removed from the patient with percutaneous nephrolithotomy and were confirmed to be pure calcium oxalate dihydrate by both Fourier transform infrared spectroscopy and micro-CT.

Fig. 7

A clinical example demonstrating the feasibility of using dual-source DECT with tin filtration to differentiate non-UA stone types. Using commercially software (“Kidney Stone”, Syngo CT Workplace, Siemens Healthcare), the stones were coded in blue (a) indicating a non-UA stone type. With dual-source DECT and additional tin filtration on the high energy tube, it was possible to characterize these stones into Group-4, which was defined to include three stone types: calcium oxalate mono- or dihydrate or brushite. Group-4 is coded in turquoise (b). The stones were subsequently removed from the patient with percutaneous nephrolithotomy and were confirmed to be pure calcium oxalate dihydrate by both Fourier transform infrared spectroscopy and micro-CT.