"Proton CT can provide the image of WEL directly," explained first author Sodai Tanaka, a PhD student at the University of Tokyo (performing research at Hiroshima University). "A key characteristic of our detection system is the simple measurement system, which enables data acquisition of residual proton energy through an irradiated target within a short time, increasing the possibility of realization."

The pCT system

The proposed pCT imaging system is based on a 20 x 20 x 5 cm plastic scintillator, made from almost water-equivalent material, and a CCD camera. During imaging, the proton beam passes through the object being imaged, losing energy as it travels through. The protons are then stopped by the scintillator, which, in this system design, is thick enough to encompass the range of a 70 MeV proton beam.

The CCD camera photographs the generated scintillation light, integrated along the beam direction, and the measured light intensity is then converted to proton beam range using a light-to-range conversion table. Tanaka and colleagues prepared this table in advance of the pCT data acquisition, by irradiating polyethylene objects of various thicknesses.

In a pCT image, WEL is represented by the pixel value. By measuring the proton beam energy before and after the beam penetrates the object, the imaging system creates the sum of the WEL factor along the proton beam path. A pCT image showing WEL distribution can then be reconstructed using a standard method such as filtered back projection (FBP).

Imaging studies

The researchers tested the pCT system using a 70 MeV proton beam, collimated to a field size of 10 x 10 cm, at the National Institute of Radiological Sciences (NIRS). They first acquired pCT images of water in a 24 mm diameter acrylic cylinder with a wall thickness of 2 mm. The 2D projection image of scintillation light detected using the pCT system clearly differentiated the container and the water from the surrounding air.

WEL values calculated from the pCT image were higher than expected for the container and lower for the surrounding air, slightly blurring boundaries in the image. This effect was attributed to multiple Coulomb scattering of the proton beam in the object, which decreased the number of protons near the boundary on the object side, and increased the number on the air side.

The researchers note that multiple Coulomb scattering is an unavoidable physical phenomenon. To correct for this in future, they plan to estimate the level of scattering from acquired pCT and xCT images and use these data as feedback to correct the pCT images.

To evaluate the achievable spatial resolution in pCT images of complex shaped objects, the researchers reconstructed an image of a 2 cm plastic kangaroo. The pCT image clearly showed a thin structure of approximately 1 mm, an inner cavity and a thin skin of approximately 1 mm. Comparison with an xCT image showed similar spatial resolution between the two.

Finally, to determine pixel values of materials other than water, the researchers reconstructed images of air, 99.5% ethanol and 40% dipotassium hydrogen phosphate aqueous solution in the acrylic container. Evaluating the pCT image pixel values by comparing them with the xCT-to-WEL conversion table revealed a maximum error of 8.8%, seen for the dipotassium hydrogen phosphate solution.

The authors suggest that errors in pixel values may arise due to the preparation method of the light-to-range conversion table, as well as the related dependence of the pixel values on object size. When an object is small, the total amount of light generated in the scintillator increases, which leads to overestimation of intensity in the object due to light spread and blurring.

Next, the researchers plan to investigate 200 MeV protons, using a thicker (about 63 mm) bismuth germanate (BGO) scintillator. "We are improving the detection system for high-energy pCT," said Tanaka.

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