Korean J Urol.  2015 Aug;56(8):587-593. 10.4111/kju.2015.56.8.587.

Evaluation of low-dose dual energy computed tomography for in vivo assessment of renal/ureteric calculus composition

• Mahalingam H
• Lal A
• Mandal AK
• Singh SK
• Bhattacharyya S
• Khandelwal N

• KMID: 2164410
• DOI: http://doi.org/10.4111/kju.2015.56.8.587

Abstract

PURPOSE
This study aimed to assess the accuracy of low-dose dual-energy computed tomography (DECT) in predicting the composition of urinary calculi.
MATERIALS AND METHODS
A total of 52 patients with urinary calculi were scanned with a 128-slice dual-source DECT scanner by use of a low-dose protocol. Dual-energy (DE) ratio, weighted average Hounsfield unit (HU) of calculi, radiation dose, and image noise levels were recorded. Two radiologists independently rated study quality. Stone composition was assessed after extraction by Fourier transform infrared spectroscopy (FTIRS). Analysis of variance was used to determine if the differences in HU values and DE ratios between the various calculus groups were significant. Threshold cutoff values to classify the calculi into separate groups were identified by receiver operating characteristic curve analysis.
RESULTS
A total of 137 calculi were detected. FTIRS analysis differentiated the calculi into five groups: uric acid (n=17), struvite (n=3), calcium oxalate monohydrate and dihydrate (COM-COD, n=84), calcium oxalate monohydrate (COM, n=28), and carbonate apatite (n=5). The HU value could differentiate only uric acid calculi from calcified calculi (p<0.001). The DE ratio could confidently differentiate uric acid, struvite, calcium oxalate, and carbonate apatite calculi (p<0.001) with cutoff values of 1.12, 1.34, and 1.66, respectively, giving >80% sensitivity and specificity to differentiate them. The DE ratio could not differentiate COM from COM-COD calculi. No study was rated poor in quality by either of the observers. The mean radiation dose was 1.8 mSv.
CONCLUSIONS
Low-dose DECT accurately predicts urinary calculus composition in vivo while simultaneously reducing radiation exposure without compromising study quality.

Keyword

Radiation dosage; Urolithiasis; X-ray computed tomography

MeSH Terms

Adult
Apatites/analysis
Calcium Oxalate/analysis
Female
Humans
Image Interpretation, Computer-Assisted/methods
Kidney Calculi/chemistry/pathology/*radiography
Magnesium Compounds/analysis
Male
Middle Aged
Phosphates/analysis
Prospective Studies
Radiation Dosage
Tomography, X-Ray Computed/methods
Ureteral Calculi/chemistry/pathology/*radiography
Uric Acid/analysis
Waist Circumference
Young Adult
Apatites
Calcium Oxalate
Magnesium Compounds
Phosphates
Uric Acid

Figure

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  Fig. 1 Depiction of region of interest (ROI) placement on a calculus and the information obtained. In this example, 1437.9/902.7/1063.3 represents the Hounsfield unit (HU) values at 80 kV, 140 kV, and the weighted average HU value, respectively. The dual-energy ratio is calculated by dividing the mean HU value at 80 kV by the mean HU value at 140 kV.

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  Fig. 2 Waist circumference distribution of the study group.

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  Fig. 3 Percentages of studies in different waist circumference groups rated as good or average in quality by the two observers.

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  Fig. 4 Flow chart describing the approach to diagnosing calculus composition by use of dual-energy (DE) ratio values.

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