Ultrasound is superior to computed tomography for assessment of medullary nephrocalcinosis in hypoparathyroidism

Abstract

Context: Nephrocalcinosis is a complication of hypoparathyroidism and other metabolic disorders. Imaging modalities include ultrasonography (US) and computed tomography (CT). Few studies have compared these modalities, and standard clinical practice is not defined.

Objective: The objective of the study was to determine the preferred method for assessing nephrocalcinosis.

Design: The design of the study was a retrospective, blinded analysis.

Setting: The study was conducted at a clinical research center.

Patients: Twenty-two hypoparathyroid subjects and 7 controls participated in the study.

Interventions: Contemporaneous renal US and CT images were reviewed in triplicate by 4 blinded radiologists. Nephrocalcinosis was classified using a 0-3 scale with 0 meaning no nephrocalcinosis and 3 meaning severe nephrocalcinosis.

Main outcome measures: Intraobserver, interobserver, and interdevice agreements were measured.

Results: Intraobserver agreement was high, with an overall weighted kappa of 0.83 for CT and 0.89 for US. Interobserver agreement was similar between modalities, with kappas of 0.74 for US and 0.70 for CT. Only moderate agreement was found between US and CT scores, with an intermodality kappa of 0.47 and 60% concordance. Of discordant pairs, 81% had higher US scores and only 19% had higher CT scores. Of nephrocalcinosis seen on US and not CT, 45%, 46%, and 9% were grades 1, 2, and 3, respectively. Overall, US scores were higher than CT with a cumulative odds ratio (95% confidence interval) of 5.97 (2.60, 13.75) (P < .01). In controls, 100% of US ratings were 0, and 95% of CT ratings were 0.

Conclusions: US is superior to CT for assessment of mild to moderate nephrocalcinosis in patients with hypoparathyroidism. This finding, in combination with its low cost, lack of radiation, and portability, defines US as the preferred modality for assessment of nephrocalcinosis.

Figure 1.

Representative images for nephrocalcinosis grading system. Panels A–D are US images corresponding to grades 0, 1, 2, and 3, respectively. Panels E–H are CT images corresponding to grades 0, 1, 2, and 3 respectively. The arrow in panel F points to a single punctate calcification in the medullary periods typical of grade 1 nephrocalcinosis.

Figure 2.

Density plot showing relative frequencies of paired observations of US and CT scores, expressed as percentages. The cells along the diagonal show the relative frequency of the scores in which both devices are in perfect agreement. The cells above the diagonal have a higher density than the cells below the diagonal, illustrating the tendency of the US to score individual scans higher than CT.