This paper describes numerical and visual evaluations of compressed sensing MRI (CS-MRI) using 2D Cartesian sampling by numerical simulation. The BrainWeb MRI Data Base was used for test images. 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 the numerical evaluation. Sampling ratio was 50%. Reconstruction was performed by minimizing the L1 norm of a transformed image using wavelet transform and total variation, subject to data fidelity constraints. The conjugate gradient method was used in the minimization of the object function. In the absence of noise, the root mean square error (RMSE) of T1WI was in the range of 2.99 to 3.57; that of the anatomical region of interests (ROIs) was in the range of 1.77 to 8.53; those of T2WI were 4.72 to 5.65 and 3.28 to 5.54; and those of PDWI were 1.91 to 2.36 and 1.32 to 2.09. Visual evaluation was performed by three radiologists on the basis of three categories: artifact, anatomical structure, tissue contrast. CS image quality was nearly equal to that of the original image, although a few artifacts were visible. If the noise level was assumed to be 30 dB or less, T1-CS image and PD-CS images were not significantly degraded compared to noise-free images.
Two-dimensional radial MRI using compressed sensing (2D radial CS) enables incoherence sampling in k space unlike conventional Cartesian MRI, however 2D radial CS has not been sufficiently investigated. Numerical and visual evaluations of 2D radial CS were performed in this paper. Three brain anatomical ROIs (white matter, gray matter, cerebrospinal fluid) of a T1-weigthted image (T1WI), a T2-weighted image (T2WI) and a proton density-weighted image (PDWI) were used for the numerical evaluation. The Brainweb MRI Data Base was used for test images. Projection of 80 spokes with linear sampling of 256 pixels was used. Reconstruction was performed by minimizing the L1 norm of a transformed image using wavelet transform and spatial finite-differences (total variation), subject to data fidelity constraint. In the absence of noise, the root mean square error (RMSE) of T1WI was in the range of 3.75 to 5.05; that of the anatomical region of interests (ROIs) was in the range of 1.54 to 10.24; those of T2WI were 8.75 to 11.65 and 4.31 to 6.99; and those of PDWI were 3.44 to 4.46 and 1.34 to 3.09. Visual evaluation was performed by three radiologists on the basis of three categories: artifact, anatomical structure, and tissue contrast. Average percent scores of the visual evaluation were 96% for T1WI, 74–81% for T2WI, and 81–89% for PDWI.
Evaluation of dosimetric impact of the interplay effect between multi-leaf collimator (MLC) movement and tumor respiratory motion during volumetric modulated arc therapy (VMAT) delivery using polymer gel dosimeter was taken as an example in this article. An excellent gas barrier PAN (polyacrylonitrile) bottle filled with polyacrylamide-based gel dosimeter contained magnesium chloride as a sensitizer (iPAGAT dosimeter) was set to the QUASAR™ respiratory motion phantom (Modus), and was moved with motion amplitudes (peak-to-peak amplitude) of 1 and 2 cm with a 4 second period during VMAT delivery by the Novalis Tx linear accelerator (Varian/BrainLAB). Two spherical GTVs with 2 cm diameter and two PTVs were defined considering the respiratory motion and setup uncertainties. Three-dimensional (3D) dose distribution in iPAGAT dosimeter was read out by the 3T MRI system, and was evaluated by the dose profiles, gamma analysis and the dose-volume histogram (DVH) using in-house developed software. As a result, interplay effect was negligible since dose coverage of GTV was sufficient during VMAT delivery with simulated respiratory motion.
High-dose-rate (HDR) brachytherapy is performed with the remote after-loading system (RALS) to transport an Ir-192 source directly to inside or near the tumor. Quality assurance (QA) of equipment should be performed at sufficient frequency to ensuring safety and quality of HDR brachytherapy treatment. Polymer gel dosimeters have been attracting attention in recent years as a QA tools of HDR brachytherapy, because they can measure the three-dimensional steep dose gradients around HDR sources. In this paper, we introduce our preliminary results using VIPET polymer gel dosimeters for Ir-192 HDR brachytherapy dosimetry.
Gel dosimeters are a three-dimensional imaging tool for dose distribution induced by radiations. They can be used for accuracy check of Monte Carlo simulation in particle therapy. An application was reviewed in this article. An inhomogeneous biological sample placing a gel dosimeter behind it was irradiated by carbon beam. The recorded dose distribution in the gel dosimeter reflected the inhomogeneity of the biological sample. Monte Carlo simulation was conducted by reconstructing the biological sample from its CT image. The accuracy of the particle transport by Monte Carlo simulation was checked by comparing the dose distribution in the gel dosimeter between simulation and experiment.
A three-dimensional dosimetry method is strongly required in the dose distribution measurement of a patient QA of a heavy ion therapy. Nanocomposite Fricke gel dosimeters are the most possible candidate for this purpose. Experimental dose distribution measurements were carried out using a scanning irradiation port of Gunma University Heavy Ion Medical Center. The result showed no significant LET dependence and indicated a possibility for a precise dosimetry of a heavy ion therapy. It also indicated the importance of three-dimensional dosimetry in the commissioning process of the treatment accelerator.
We have proposed a novel polymer gel dosimeter containing of 2-hydroxyethyl methacrylate (HEMA), nonaethylene glycol dimethacrylate (9G), and tetrakis (hydroxymethyl) phosphonium chloride (THPC) with radiation-crosslinked hydroxypropyl cellulose (HPC) gel sheet. The transparent sheet-type dosimeters became white and cloudy by irradiation with gamma-rays and heavy ions such as He ions (150 MeV/u), C ions (290 MeV/u), Fe ions (500 MeV/u). The cloudiness increased with increasing dose. The cloudiness distribution with the sheet-type dosimeter was obtained by using a flatbed scanner to evaluate the dose distribution. Recently, we prepared a three-dimensional dosimeter by putting the gel sheets on top of another in the glass vessel. Three-dimensional dose distribution of the dosimeter irradiated with C ions was evaluated by the reconstruction of the data of each layer.
At present, the development of the accelerator-based irradiation system for boron neutron capture therapy (BNCT) is energetically performed by various groups in the world. Especially in Japan, BNCT using various accelerator-based irradiation systems may be carried out at plural facilities in the near future. Thus, it is the time when BNCT is shifting from a special particle therapy to a general therapy, now. In order to promote this shift, not only the development and improvement for the irradiation system but also the preparation and improvement in the physical engineering and medical physics, such as dosimetry system, etc., is important. Recently, as part of the improvement in the dosimetry method for BNCT, the estimation method of three-dimensional dose distribution using gel detector is focused. In this paper, the principle of BNCT, especially for dose deposition, is introduced, and the studies for gel detector in BNCT are introduced referring to the proceedings of the international symposium for BNCT.