Monte Carlo‐derived ^99mTc uptake quantification with commercial planar MBI: Absolute tumor activity

Abstract

Background: Molecular breast imaging (MBI) of 99m Tc-sestamibi is an emerging adjunct qualitative tool in the detection and diagnosis of breast cancer. Purpose: This work outlines the development and performance evaluation of a methodology to absolutely quantify tumor 99m Tc activity uptake using a commercially available dual-headed MBI system by implementing corrections for background, scatter, attenuation, and detector characteristics. Methods: A validated Monte Carlo application of a commercial MBI system was used to simulate over 7000 unique acquisitions of spherical and ellipsoidal tumors in breast tissue. Tumor absolute activity was calculated following background, scatter, and attenuation corrections of tumor region of interest counts. The methodology was first optimized using a set of high-uptake spherical tumors, and its accuracy and precision was then assessed in a set of spherical tumors with clinical uptake conditions. Finally, the performance of the activity methodology was evaluated under various bias and uncertainty conditions to better characterize the technique under expected clinical measurement conditions. Results: In a test set of images with clinically relevant contrast and noise conditions, the mean ± standard deviation relative error in total tumor activity was 0.5% ± 6.5% (n = 2363) under ideal measurement conditions. Allowing for variability in tumor and background contours and in estimated tumor depths, the expected accuracy of the methodology in clinical practice was 0.5% ± 11.1% (n = 2363), with minimal loss of accuracy for ellipsoidal tumors. Conclusions: Planar MBI photopeak images acquired with standard-of -care protocols can be used to accurately quantify absolute tumor 99m Tc activity with an accuracy and precision of 0.5% ± 11.1%. The reported precision was based on a comprehensive evaluation of random errors and systematic biases.

Monte Carlo‐derived 99mTc uptake quantification with commercial planar MBI: Tumor and breast activity concentrations

“…15 Our work demonstrates the feasibility of extending these BPU measurements to also include absolute normal tissue uptake estimates. Additional investigations will be needed to first properly implement this technique in patient images, where parenchymal uptake is not homogeneous and background ROIs throughout the same breast can differ in average pixel counts by up to 35%, 6 and to then determine correlation, if any, between absolute normal tissue activity concentrations to BPU classes and breast cancer risk.…”

“…The simulated data set used to compare tumor activity metrics in this study consisted of both spherical and ellipsoid tumors in "hot" breast tissue. All simulations consisted of 10-min acquisitions of clinically-observed breast compressed thicknesses (b z = 4, 6, 8, 10, 12 cm), normal tissue uptake concentrations (C b = 10, 20, 60 nCi/cm 3 ), tumor diameters (d = 1.0, 1.5, 2.0, 2.5, 3.0 cm), 6 In all acquisitions, the detector heads were placed immediately adjacent to the simulated breast (Figure 1). Ellipsoid tumors were only simulated within the b z = 8 cm breast.…”

“…These thresholds were found to occur at the lower end of clinical values measured at our institution and were implemented to remove simulation images beyond the known capabilities of both the commercial system and the proposed tumor total activity quantification technique. 6 Each acquisition was additionally stratified into 1 of 6 sub-groups for additional analysis based on the tumor's eccentricity (Δd max ) and depth (t z ). The three tumor eccentricity categories, none, low, and high, were defined respectively as tumors with Δd max of 0 cm (i.e., spheres), of 0.5 or 1.0 cm, and of 1.5 or 2.0 cm.…”

“…The first two estimates are calculated using either of the two single detector images and a set of tumor and background ROIs to calculate tumor activity following background, scatter, and attenuation corrections. 6 The third estimate is as a simple geometric mean of both detector tumor ROI counts that includes background and attenuation corrections. The final estimate is calculated following a modified geometric mean approach proposed by Bache and Kappadath which incorporates a dual-energy window scatter correction and a volumetric background correction factor.…”

“…Two tumor activity estimates, A near t and A far t , were calculated following a proposed methodology incorporating normal tissue, scatter, background, and detector response corrections but applied separately to each of the two acquired images. 6 For each acquisition, the near and far detectors were first identified by estimating the tumor relative depth using the total breast thickness (b z ) and the ratio of the background-corrected tumor ROI total counts in the photopeak energy windows images of both detectors 7 :…”