Key variables for interpreting 99mTc-mercaptoacetyltriglycine diuretic scans: development and validation of a predictive model

Abstract

Objective: The purpose of this study was to facilitate interpretation of (99m)Tc-mercaptoacetyltriglycine (MAG3) diuretic scans by identifying key interpretative variables and developing a predictive model for computer-assisted diagnosis.

Materials and methods: Ninety-seven studies were randomly selected from an archived database of MAG3 baseline and furosemide acquisitions and scan interpretations (obstruction, equivocal finding, or no obstruction) derived from a consensus of three experts. Sixty-one studies (120 kidneys) were randomly chosen to build a predictive model for diagnosing or excluding obstruction. The other 36 studies (71 kidneys) composed the validation group. The probability of normal drainage (no obstruction) at the baseline acquisition and the probability of no obstruction, equivocal finding, or obstruction after furosemide administration were determined by logistic regression analysis and proportional odds modeling of MAG3 renographic data.

Results: The single most important baseline variable for excluding obstruction was the ratio of postvoid counts to maximum counts. Renal counts in the last minute of furosemide acquisition divided by the maximum baseline acquisition renal counts and time to half-maximum counts after furosemide administration in a pelvic region of interest were the critical variables for determining obstruction. The area under the receiver operating characteristic curve (AUC) for predicting normal drainage in the validation sample was 0.93 (standard error, 0.02); sensitivity, 85%; specificity, 93%. The AUC for the diagnosis of obstruction after furosemide administration was 0.84 (standard error, 0.06); sensitivity, 82%; specificity, 83%.

Conclusion: A predictive system has been developed that provides a promising computer-assisted diagnosis approach to the interpretation of MAG3 diuretic renal scans; this system has also identified the key variables required for scan interpretation.

Fig. 1. 88-year-old woman with suspected obstruction showing different regions of interest

A, Technetium-99m–mercaptoacetyltriglycine (MAG3) image obtained 2–3 minutes after injection shows whole-kidney region of interest (ROI). B, MAG3 image obtained 20 minutes after injection to generate renogram curve shows parenchymal (cortical) defined ROI representing parenchymal function uncontaminated by pelvic or calyceal retention. C, MAG3 image obtained 2 minutes after furosemide administration shows whole-kidney ROI. D, MAG3 image obtained 2 minutes after furosemide administration shows ROI over retention in pelvis and calyces.

Fig. 2

Receiver operating characteristic plot shows accuracy of probability of normal kidney drainage predicted with logistic model against expert consensus for training sample (n = 120 kidneys) and validation sample (n = 71 kidneys).

Fig. 3. Receiver operating characteristics

A, Plot shows accuracy of probability of interpretation of obstruction versus interpretation of no obstruction or equivocal findings predicted with proportional odds model against expert consensus for training sample (n = 120 kidneys) and validation sample (n = 71 kidneys). B, Plot shows accuracy of probability of interpretation of obstruction or equivocal finding versus no obstruction predicted by proportional odds model against expert consensus for training sample (n = 120 kidneys) and validation sample (n = 71 kidneys).