We performed numerical and visual evaluation of compressed sensing MRI (CS-MRI) using 3D Cartesian sampling by numerical simulation. Three brain anatomical ROIs (white matter, gray matter, cerebrospinal fluid) of a T1-weighted image (T1WI), a T2-weighted image (T2WI) and a proton density-weighted image (PDWI) were used for numerical evaluation. Sampling ratio of the Cartesian grid was 30%. Reconstruction was performed by the projection onto convex sets (POCS) method with soft thresholding, subject to data fidelity constraints. In the absence of noise, RMSE of 3D-T1WI was 1.50, ant that of the 2D-T1WI of the transverse plane was in the range of 1.06 to 1.54; anatomical ROIs was in the range of 0.75 to 2.80; those of T2WI were 3.20, 2.77 to 3.06, and 1.81 to 4.51; those of PDWI were 1.69, 1.33 to 1.49, and 1.08 to 1.86. Visual evaluation was performed by three radiologists on the basis of three categories: artifact, anatomical structure, and tissue contrast. Average score of the visual evaluation indicated that anatomical structure and tissue contrast of CS images were equal to those of the original image, although a few artifacts were visible. If noise level was assumed to be 20 dB or less, anatomical structure and tissue contrast were not significantly degraded compared to noise-free CS images.
With rapid advances being made in radiotherapy treatment, three-dimensional (3D) dose measurement techniques of great precision are required more than ever before. It is expected that 3D polymer gel dosimeters will satisfy clinical needs for an effective detector that can measure the complex 3D dose distributions. Polymer gel dosimeters are devices that utilize the radiation-induced polymerization reactions of vinyl monomers in a gel to store information about radiation dose. The 3D absorbed dose distribution can be deduced from the resulting polymer distribution using several imaging modalities, such as MRI, X-ray and optical CTs. In this article, the fundamental characteristics of polymer gel dosimeter are reviewed and some challenging keys are also suggested for the widely spread in clinical use.
Radiochromic hydrogel dosimeters are a useful tool for the verification of 3D dose distributions using optical CT scanners which are lower cost and with higher spatial resolution in a limited time than MRI scanners. In this paper, recent development of radiochromic micelle gel and genipin-gelatin gel dosimeters are described. They have the advantage of water equivalency, low or no diffusion property, and lower toxicity. Micelle gels consist of leuco dye, surfactant, radical initiator (halocarbons), and gelling agent. The dose sensitivities are affected by the concentration of each component and the temperature during irradiation. In non-diffusing genipin-gelatin gels, radiation induced bleaching is observed. The dose sensitivity is strongly affected by the concentration of genipin and sulfuric acid and by the blending time with gelatin. Higher dose sensitivity and standardization of the dose evaluation procedure will be the future issue.
Fricke gel dosimeters are based on the aqueous ferrous sulfate solution that has been used as a reliable chemical dosimeter for more than 90 years. The amount of radiation-induced oxidation of ferrous ions to ferric ions could be evaluated three-dimensionally via optical computed tomography (OCT) or magnetic resonance imaging (MRI). Three-dimensional dosimetry is expected to be a useful tool, in particular, for the veriﬁcation of complex radiation dose planning in advanced radiation cancer therapy. Compared with other 3D gel dosimeters, there are several problems such as retention of dose distribution or dose sensitivity; however, they can easily be prepared in a chemistry laboratory. In addition, a unique gel dosimeter was also reported for heavy-ion beam irradiation. In this review, recent papers concerning the Fricke gel dosimeter and its application in 3D dosimetry are summarized.
Three dimensional dose distribution by gel dosimeters is often read out by an MRI scanner, which is readily available in most hospitals. Both the T1 and T2 relaxation rates are used to evaluate the absorbed dose. This paper will describe the principle of the imaging sequences (spin echo, multiple spin echo, and inversion recovery methods) to measure the relaxation rate and the factors to affect the precision (signal noise ratio, temperature during scanning, coil, imaging artefact). Additionally, the magnetization transfer method will be mentioned as an alternative for low-density foam gel dosimeters.
This article describes an optical computed tomography (OCT) for polymer gel dosimetry, focusing on two systems recently constructed by us. The first OCT system, which is categorized as a first-generation system, comprised a single He-Ne laser, photodiode, and mechanical stages for moving and turning gel dosimeter. Projection data per angle are acquired from turned gel dosimeter. In this system, the reconstructed image is obtained using filtered back-projection (FBP) method from projection data. The second OCT system is a 2D-OCT scanner that utilizes a light panel and a camera detector. The dose-response relationship between the optical density and radiation dose reconstructed from the scanning images is comparable to that obtained using magnetic resonance imaging. Although there is much room for improvement in the image artifacts due to reflection and refraction of light and so on, OCT is expected as a modality for the future polymer gel dosimetry as the rapid, high-resolution, highly accurate evaluation tool.
Three-dimensional (3D) radiation dosimeter has received growing interest with the implementation of highly conformal radiotherapy treatments. Optical Computed tomography (CT) scanners have been developed for 3D quantitative measurement of optical attenuation. Malachite green and Leuco Crystal Violet (LCV) dye radiochromic gels were recently developed as radiation dosimeters in combination with optical CT scanners. Malachite green and LCV are colorless dyes and they become green or violet after irradiation, respectively. The dyes mixed with gelatin and surfactant became hardened and can be used as a 3D gel dosimeter. The radiochromic gels dosimeter with optical CT scanners provides many of desired features such as: low cost, easy to fabricate, low toxicity and fast readout. In this article, the fundamental characteristics of radiochromic gel dosimeter with optical CT scanners are reviewed.