Detective quantum efficiency of intensified CMOS cameras for Cherenkov imaging in radiotherapy

Abstract

In this study the metric of detective quantum efficiency (DQE) was applied to Cherenkov imaging systems for the first time, and results were compared for different detector hardware, gain levels and with imaging processing for noise suppression. Intensified complementary metal oxide semiconductor cameras using different image intensifier designs (Gen3 and Gen2+) were used to image Cherenkov emission from a tissue phantom in order to measure the modulation transfer function (MTF) and noise power spectrum (NPS) of the systems. These parameters were used to calculate the DQE for varying acquisition settings and image processing steps. MTF curves indicated that the Gen3 system had superior contrast transfer and spatial resolution than the Gen2+ system, with [Formula: see text] values of 0.52 mm^-1 and 0.31 mm^-1, respectively. With median filtering for noise suppression, these values decreased to 0.50 mm^-1 and 0.26 mm^-1. The maximum NPS values for the Gen3 and Gen2+ systems at high gain were 1.3 × 10⁶ mm² and 9.1 × 10⁴ mm² respectively, representing a 14x decrease in noise power for the Gen2+ system. Both systems exhibited increased NPS intensity with increasing gain, while median filtering lowered the NPS. The DQE of each system increased with increasing gain, and at the maximum gain levels the Gen3 system had a low-frequency DQE of 0.31%, while the Gen2+ system had a value of 1.44%. However, at a higher frequency of 0.4 mm^-1, these values became 0.54% and 0.03%. Filtering improved DQE for the Gen3 system and reduced DQE for the Gen2+ system and had a mix of detrimental and beneficial qualitative effects by decreasing the spatial resolution and sharpness but also substantially lowering noise. This methodology for DQE measurement allowed for quantitative comparison between Cherenkov imaging cameras and improvements to their sensitivity, and yielded the first formal assessment of Cherenkov image formation efficiency.

Figure 1.

(A) Schematic of imaging geometry. (B) Spectral sensitivity and quantum efficiency curves for both the Sb-K-Na-Cs Gen2+ and GaAs-O-Cs Gen3 intensifiers. (C) Color image of the tissue phantom, as well as absorption and reduced scattering coefficients as a function of wavelength.

Figure 2.

Pixel response of both the Gen3 and Gen2+ cameras to variation in incident light with power measured in the emission plane.

Figure 3.

Cherenkov images of a chest phantom taken with both Gen2+ and Gen3 systems with three different levels of processing showing an unfiltered single frame, spatial and temporal median filtered single frame image, and an average image from all the 100 frames.

Figure 4.

MTF curves compared for Gen3 and Gen2+ systems, with and without median filtering. The dotted line represents 10% of the MTF.

Figure 5.

Two dimensional darkfield NPS from raw data: (a) Gen3 system; (b) Gen2+ system.

Figure 6.

Two dimensional flat illumination NPS from raw data: (a) Gen3 system; (b) Gen2+ system.

Figure 7.

NPS from raw data at various CMOS gain settings: (a) Gen3 system; (b) Gen2+ system.

Figure 8.

NPS from raw data at various intensifier gain settings: (a) Gen3 system; (b) Gen2+ system.

Figure 9.

NPS curves compared for Gen3 and Gen2+ systems, with and without median filtering, for the maximum intensifier and CMOS gain settings used.

Figure 10.

DQE from raw data at various CMOS gain settings: (a) Gen3 system; (b) Gen2+ system.

Figure 11.

DQE from raw data at various intensifier gain settings: (a) Gen3 system; (b) Gen2+ system.

Figure 12.

DQE curves compared for Gen3 and Gen2+ systems, with and without median filtering.