Feasibility of reducing the dose of MAG3 for renal imaging: A simulation study

Objectives 99mTc mercaptoacetyltriglycine (MAG3) is a widely used radiopharmaceutical for renal imaging; the radiation dose to the patient is low but it could be further decreased, if the administered MAG3 dose could be reduced without loss of diagnostic accuracy. Our goal was to simulate reduced MAG3 dosing to determine the effect on common quantitative variables.

Methods 50 MAG3 baseline scans randomly selected from our archived database of patients with suspected renovascular hypertension, comprised the study population. Patients received an average injected dose (TrueD) of 302 MBq of MAG3 (range 185 to 403) and were imaged for 24 minutes with calculation of a camera based MAG3 clearance (clr), kidney to background ratio (ktb), time to peak (ttp) and time to half peak (tthp). A reduced dose (RedD) of 74 MBq was simulated in all 50 studies by multiplying each image pixel by the ratio TrueD/74. Noise was added as follows: each pixel of value n was substituted with a value randomly selected from a Poisson distribution with mean n. Using the originally defined regions of interest, quantitative variables were re-calculated for each kidney using the simulated dose, and results compared to the original dose by paired t-test. The number of patients with matching clinical result were tabulated for each dose pair, using published 5th and 95th percentile of normal values as limits of normality.

Results Linear correlation coefficients, paired t-test results and clinical matches between TrueD and RedD quantitative variables are summarized below. 29 patients had normal clr in the full dose scans, of which 25 (86%) were normal with RedD. 100% of abnormal patients were detected with RedD. Most cases of disagreement were associated with poor renal function and were likely related to oversensitivity to noise since determination of peak was based on 1-sec intervals.

Conclusions Simulated data shows that the MAG3 clearance and quantitative variables show good agreement when the MAG3 dose is reduced to 74 MBq.