Estimation of the minimum detectable activity of preclinical PET imaging systems with an analytical method

Abstract

Purpose: The traditional figures of merit used in the evaluation of positron emission tomography (PET) systems, including system sensitivity and spatial resolution, do not directly reflect the minimum detectable activity (MDA) performance, despite the fact that it is one of the most important tasks for a PET system. MDA, as a combination of the more traditional PET system parameters, is directly related to lesion detection. However, MDA evaluation is task specific and cannot be done by a single measurement. Therefore, a simple method to evaluate system detectability needs to be developed.

Methods: In this work, an analytical method of MDA estimation was developed, taking into account system sensitivity, spatial resolution, source properties, and noise propagation in image reconstruction by using the Rose criterion and/or the Curie equation as the detection standard. In the implementation, the source background, as well as the intrinsic activity background from the scintillation material of the system, was also taken into consideration. The accuracy of this method was evaluated in two commercially available preclinical PET systems, with phantom experiments that were designed to closely mimic in vivo tumor uptake without introducing finite boundaries between the source and the background.

Results: The lesion contrast-to-noise ratio calculated by the analytical evaluation showed good agreement with that obtained from the experiments. Visual assessment of the reconstructed images at the detection limit (based on analytical evaluation) also was in agreement with the Rose criterion. The MDA performance was quantitatively compared between the two preclinical PET systems and showed different detection limits under different imaging conditions, suggesting that the detection limit of a PET system strongly depends on the lesion properties and acquisition settings.

Conclusions: An analytical method of evaluating the PET system detectability was developed and validated by experiments. Overall, the analytical MDA calculation provides a simple way to evaluate the signal detectability of a PET system and can be used for comparing different systems. It also provides guidelines for designing new PET tomographs as well as optimizing data acquisition protocols.

Figure 1

Schematic drawing of the experimental setup for (a) cases 1 and 2 and (b) case 3.

Figure 2

(a) Point source maximum value (▲ for 1.0 mm source and ▲ for 0.75 mm source), PVE corrected point source maximum value (◆ for 1.0 mm source and ◆ for 0.75 mm source), and background signal obtained from ROI analysis on the reconstructed images. (b) Background noise (fractional standard deviation) obtained from analytical estimation and ROI analysis. The data shown here were acquired with the Inveon system and is for 20 min frames.

Figure 3

CNR obtained from ROI analysis versus CNR obtained by analytical estimation for the Inveon and the Focus220 tomographic systems acquired with both 20 and 40 min frames.

Figure 4

The maximum value of the point source obtained from ROI analysis for (a) the Inveon and (b) the Focus220 systems and the maximum value after applying the PVE correction for (c) the Inveon and (d) the Focus220.

Figure 5

The ROI based CNR versus the analytical based CNR for (a) the Inveon and (b) the Focus220 systems. Sources located at the geometric center (◆), 5 mm off-center (◆), and 10 mm off-center (◆) are shown.

Figure 6

The intrinsic radioactivity of ¹⁷⁶Lu in the LSO scintillators for Focus220 and the Inveon systems at different LLDs. (a) Measured true coincidence count rate. (b) Equivalent total center activity based on the system sensitivity (solid line) and equivalent activity concentration based on the size of the active FOV (dotted line).