title: A gamma-distribution convolution model of99mTc-MIBI thyroid time-activity curves.
abstract: BACKGROUND
  The convolution approach to thyroid time-activity curve (TAC) data fitting with a gamma distribution convolution (GDC) TAC model following bolus intravenous injection is presented and applied to99mTc-MIBI data. The GDC model is a convolution of two gamma distribution functions that simultaneously models the distribution and washout kinetics of the radiotracer. The GDC model was fitted to thyroid region of interest (ROI) TAC data from 1 min per frame99mTc-MIBI image series for 90 min; GDC models were generated for three patients having left and right thyroid lobe and total thyroid ROIs, and were contrasted with washout-only models, i.e., less complete models. GDC model accuracy was tested using 10 Monte Carlo simulations for each clinical ROI.
  
  
  RESULTS
  The nine clinical GDC models, obtained from least counting error of counting, exhibited corrected (for 6 parameters) fit errors ranging from 0.998% to 1.82%. The range of all thyroid mean residence times (MRTs) was 212 to 699 min, which from noise injected simulations of each case had an average coefficient of variation of 0.7% and a not statistically significant accuracy error of 0.5% (p = 0.5, 2-sample paired t test). The slowest MRT value (699 min) was from a single thyroid lobe with a tissue diagnosed parathyroid adenoma also seen on scanning as retained marker. The two total thyroid ROIs without substantial pathology had MRT values of 278 and 350 min overlapping a published99mTc-MIBI thyroid MRT value. One combined value and four unrelated washout-only models were tested and exhibited R-squared values for MRT with the GDC, i.e., a more complete concentration model, ranging from 0.0183 to 0.9395.
  
  
  CONCLUSIONS
  The GDC models had a small enough TAC noise-image misregistration (0.8%) that they have a plausible use as simulations of thyroid activity for querying performance of other models such as washout models, for altered ROI size, noise, administered dose, and image framing rates. Indeed, of the four washout-only models tested, no single model approached the apparent accuracy of the GDC model using only 90 min of data. Ninety minutes is a long gamma-camera acquisition time for a patient, but a short a time for most kinetic models. Consequently, the results should be regarded as preliminary.
authors.affiliation: Department of Medical Imaging, Royal University Hospital, College of Medicine, University of Saskatchewan, 103 Hospital Drive, Saskatoon, SK, S7N 0W8, Canada. carl.wesolowski@gmail.com.|Nuclear Medicine, Department of Radiology, Faculty of Medicine, Memorial University of Newfoundland, 300 Prince Philip Drive, St. John's, NL, A1B 3X7, Canada. carl.wesolowski@gmail.com.
authors.forename: Carl A
authors.identifier: http://orcid.org/0000-0003-0134-9346
authors.initials: CA
authors.lastname: Wesolowski
authors.affiliation: Department of Medical Imaging, Royal University Hospital, College of Medicine, University of Saskatchewan, 103 Hospital Drive, Saskatoon, SK, S7N 0W8, Canada.
authors.forename: Surajith N
authors.initials: SN
authors.lastname: Wanasundara
authors.affiliation: Department of Medical Imaging, Royal University Hospital, College of Medicine, University of Saskatchewan, 103 Hospital Drive, Saskatoon, SK, S7N 0W8, Canada.
authors.forename: Michal J
authors.initials: MJ
authors.lastname: Wesolowski
authors.affiliation: Department of Nuclear Medicine, Hacettepe University Medical School, Sıhhiye, Ankara, 06100, Turkey.
authors.forename: Belkis
authors.initials: B
authors.lastname: Erbas
authors.affiliation: Department of Medical Imaging, Royal University Hospital, College of Medicine, University of Saskatchewan, 103 Hospital Drive, Saskatoon, SK, S7N 0W8, Canada.
authors.forename: Paul S
authors.initials: PS
authors.lastname: Babyn
keywords: 99mTc-hexakis-methoxy-isobutyl-isonitrile; Gamma camera; Gamma distribution convolution; Thyroid; Time-activity curve