Camera selection for real-time in vivo radiation treatment verification systems using Cherenkov imaging

Abstract

Purpose: To identify achievable camera performance and hardware needs in a clinical Cherenkov imaging system for real-time, in vivo monitoring of the surface beam profile on patients, as novel visual information, documentation, and possible treatment verification for clinicians.

Methods: Complementary metal-oxide-semiconductor (CMOS), charge-coupled device (CCD), intensified charge-coupled device (ICCD), and electron multiplying-intensified charge coupled device (EM-ICCD) cameras were investigated to determine Cherenkov imaging performance in a clinical radiotherapy setting, with one emphasis on the maximum supportable frame rate. Where possible, the image intensifier was synchronized using a pulse signal from the Linac in order to image with room lighting conditions comparable to patient treatment scenarios. A solid water phantom irradiated with a 6 MV photon beam was imaged by the cameras to evaluate the maximum frame rate for adequate Cherenkov detection. Adequate detection was defined as an average electron count in the background-subtracted Cherenkov image region of interest in excess of 0.5% (327 counts) of the 16-bit maximum electron count value. Additionally, an ICCD and an EM-ICCD were each used clinically to image two patients undergoing whole-breast radiotherapy to compare clinical advantages and limitations of each system.

Results: Intensifier-coupled cameras were required for imaging Cherenkov emission on the phantom surface with ambient room lighting; standalone CMOS and CCD cameras were not viable. The EM-ICCD was able to collect images from a single Linac pulse delivering less than 0.05 cGy of dose at 30 frames/s (fps) and pixel resolution of 512 × 512, compared to an ICCD which was limited to 4.7 fps at 1024 × 1024 resolution. An intensifier with higher quantum efficiency at the entrance photocathode in the red wavelengths [30% quantum efficiency (QE) vs previous 19%] promises at least 8.6 fps at a resolution of 1024 × 1024 and lower monetary cost than the EM-ICCD.

Conclusions: The ICCD with an intensifier better optimized for red wavelengths was found to provide the best potential for real-time display (at least 8.6 fps) of radiation dose on the skin during treatment at a resolution of 1024 × 1024.

FIG. 1.

(a) A top–down view of experimental setup. The gantry was set to 90°, and the radiation beam was incident on the flat solid water phantom positioned vertically on the treatment table. The cameras were placed so as to image the surface of the phantom at an angle without being obstructed by the gantry. (b) For reference, a white light image of the phantom taken on the CCD camera while in position.

FIG. 2.

Processed Cherenkov images collected using the CMOS [(a)–(c), top row on the same intensity scale] and CCD [(d)–(f), bottom row on the same intensity scale] cameras with varying exposure times. Room lights were off, since there is no mechanism of fast triggering and gating available with these two camera options. Note the Cherenkov signal was low relative to the background room light. At long exposure times with room lights off, a modest contrast is seen. Their utility is feasible, but only at low frame rates (below 1 fps), with noticeable background in the image even after background subtraction.

FIG. 3.

Cherenkov images captured using three types of intensified cameras with room lights on are shown. The HRf ICCD (d)–(f) outperformed the Unigen 2 ICCD (a)–(c) because of overall higher intensifier quantum efficiency. However, best performance was demonstrated from a high-gain electron-multiplied CCD (g), which allowed for single-shot imaging, where a high-signal Cherenkov image can be captured from a single radiation pulse from the Linac (<0.05 cGy of dose); this image is a constructed from a temporal median filter of multiple frames following the same image processing techniques as with the ICCDs. Minor reshaping was implemented to visually negate the effect of small viewpoint angle changes between systems, but was not used in any quantitative analyses. The Cherenkov spectrum plotted in the graph shown in (h) was generated using geant4-based simulations of a 6 MV x-ray beam irradiating a light-skinned tissue volume. The QE curves were plotted from data supplied by the camera manufacturer.

FIG. 4.

Quantitative analysis of a 250 × 150 pixel region shown in (a) for the ICCDs within the incident beam looks at the square of the signal to noise ratio in the region (b) and average Cherenkov intensity in the region (c) versus acquisition frame rate. The yellow vertical line in graph (c) highlights Unigen 2 performance at 4.7 fps, which is the basis for performance comparison (above 0.5% of the bit depth or 327 counts). Intensity values that fall in the orange region of the chart qualify as adequate signal under this defined metric.

FIG. 5.

In vivo Cherenkov images captured during whole-breast irradiation of two patients. The left column shows images captured on the EM-ICCD (a) and (c) and the right column shows images from the Unigen 2 ICCD (b) and (d). All images are self normalized. Images (a) and (b) are presented using an intensity colormap, and research is being done to correlate relative intensity to surface dose. Images (c) and (d) are the same processed images as (a) and (b), respectively, only shown in grayscale, where it is easier to visually distinguish the appearance of the blood vessels.

FIG. 6.

Decision flow chart for camera selection in Cherenkov imaging. Since clinical imaging requires both fast frame rates and ambient room light for patient safety, an intensified, time-gated solution is required. Other cameras can be used for quality assurance applications with room lights off, over longer acquisition intervals.