Evaluation of a method for projection-based tissue-activity estimation within small volumes of interest

Abstract

A new method of compensating for tissue-fraction and count-spillover effects, which requires tissue segmentation only within a small volume surrounding the primary lesion of interest, was evaluated for SPECT imaging. Tissue-activity concentration estimates are obtained by fitting the measured projection data to a statistical model of the segmented tissue projections. Multiple realizations of two simulated human-torso phantoms, each containing 20 spherical “tumours”, 1.6 cm in diameter, with tumour-to-background ratios of 8:1 and 4:1, were simulated. Estimates of tumour- and background-activity concentration values for homogeneous as well as inhomogeneous tissue activities were compared to standard SUV metrics on the basis of accuracy and precision. For perfectly registered, high-contrast, superficial lesions in a homogeneous background without scatter, the method yielded accurate (<0.4% bias) and precise (<6.1%) recovery of the simulated activity values, significantly outperforming the SUV metrics. Tissue inhomogeneities, greater tumour depth and lower contrast ratios degraded precision (up to 11.7%), but the estimates remained almost unbiased. The method was comparable in accuracy but more precise than a well-established matrix inversion approach, even when errors in tumor size and position were introduced to simulate moderate inaccuracies in segmentation and image registration. Photon scatter in the object did not significantly affect the accuracy or precision of the estimates.

Hybrid Imaging: Instrumentation and Data Processing

“…Other post-reconstruction corrections consist of voxel-based methods, such as the popular Mueller-Gaertner method (MGM) [170], image deconvolution [171], and the "region-based voxel-wise correction" (RBV) [172]. Alternative approaches for PVC operate in sinogram space, where statistical noise is spatially uncorrelated and easier to incorporate in the PVC procedure [173][174][175][176]. Reconstruction-based methods enhance the spatial resolution by using the implicit PVC compensation through system-response modeling [177].…”

Improved quantification for local regions of interest in preclinical PET imaging

“…The activity within the J compartments A j is represented by an average value inside the compartment that can be determined by fitting the measured projection data to the model in Eq. 1, as explained in (Southekal et al 2012). Taking into account that the joint likelihood of measuring an entire projection data set is given by the product of the Poisson probability density function for each measured projection ray, the vector A can be determined by maximizing the log likelihood for the expected value λ i : $$\sum _{j^{\prime }=1}^J{A}_{j^{\prime }}^{(k)}\left[\sum _i\frac{P_{i{j}^{\prime }}{P}_{ij}}{{\lambda }_i^{false(kfalse)}}\right]=\sum _i\frac{P_{ij}\left(n_i-g_{\mathrm{italicout},i}^{false(kfalse)}\right)}{{\lambda }_i^{(k)}}$$ where n i are the measured counts per LOR.…”

“…In this work we implemented and evaluated for PET imaging a local projection algorithm (LPA), that was originally proposed and implemented for SPECT quantification by (Moore et al 2012, Southekal et al 2012) who, at that time, called this the “local VOI” method of compensating for partial-volume and spillover effects. The approach takes advantage of a higher resolution image co-registered with the PET data, which makes possible a high-resolution segmentation of, typically, a few tissues within a volume of interest centered on the lesion that needs to be quantified.…”

“…As mentioned, the evaluation of the LPA method for a multiple-pinhole micro-SPECT system (Moore et al 2012) and for a clinical SPECT system (Southekal et al 2012) was presented in previous work. In this paper we discuss the implementation and evaluation of this method when it is applied, along with the positron range correction described in (Cal-González et al 2011), for high-resolution PET imaging.…”

Partial volume correction for improved PET quantification in 18F-NaF imaging of atherosclerotic plaques

“…Numerous approaches have been proposed for partial volume correction (PVC) in nuclear medicine applications. 12 – 16 These techniques can be divided into two main categories: within-reconstruction and post-reconstruction methods, 12 which include projection-based methods, such as the local projection (LP) method proposed for SPECT 17 , 18 and for pre-clinical PET. 19 …”