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 f 10 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.

Optimization of in vivo Cherenkov imaging dosimetry via spectral choices for ambient background lights and filtering

“…The camera was triggered off the linear accelerator (LINAC) pulses to image during irradiation (6MV x-ray with 100 source-to-surface distance). Figure 1(a) shows an anthropomorphic tissue equivalent phantom 26 to represent a breast treatment. A flat tissue phantom composed of the same material was also imaged at isocenter to quantify SNR and Cherenkov to room light background ratio (I CH ∕I BKG ).…”

“…1,24 Studies focused on selecting and evaluating the appropriate intensified cameras were conducted to further improve the image quality. 25,26 Nonetheless, the contribution of the ambient light detected in each image can still contribute to increased noise (e.g., photon noise), reduced image quality, and artifacts. Although in external beam therapy, the time structure can be utilized for background subtraction, and this is not possible with continuous irradiation sources such as those in brachytherapy or proton PBS systems.…”

Estimation of diffuse Cherenkov optical emission from external beam radiation build-up in tissue

“…The image intensifier in the camera used a red-sensitive photocathode, as described by Alexander et al. 20 The camera remained fixed to the ceiling for each imaging acquisition, and the room lights were turned off the minimize external noise.…”

“…For each image acquisition, all Cherenkov frames were summed into a cumulative image 6 and flat-field corrected. 20 The radiant Cherenkov emission was then determined by taking the mean pixel intensity of a rectangular region-of-interest (ROI) at the center of each irradiated phantom. The slopes of the PDD curves were approximated by applying a linear least squares regression between 0 and 0.8 cm using MATLAB’s curve fitting tool.…”

Visual Isocenter Position Enhanced Review (VIPER): a Cherenkov imaging‐based solution for MR‐linac daily QA

“…Each plan was delivered and imaged using a custom C‐Dose camera (DoseOptics LLC, Lebanon, NH, USA) and the accompanying C‐Dose Research software. Similar commercially available C‐Dose cameras have been characterized in‐depth in a previous study 13 . Cumulative, background‐subtracted Cherenkov images, as well as cumulative background images, were acquired at 17 Hz.…”