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Collimators for Gamma Ray Imaging
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
A parallel-hole collimator can be regarded as a numerous amount of closely packed long, parallel holes through a slab of highly attenuating material such as lead or tungsten. Traditionally, it is fabricated by sheets of lead foil, folded in half-hexagonal holes precisely stacked together to form a honey-grate structure. Next to folded collimators exist cast collimators based on precise moulding techniques. The latter type of collimator is more precise and also can be fabricated with smaller thickness of the septa t. Besides the septal thickness, the most important design parameters of a parallel-hole collimator are its height h and hole size g (Figure 14.1). A parallel-hole collimator makes direct 1-to-1 projection images of the source on the detector. Therefore, the FOV of a parallel-hole collimator is equal to the size of the detectors, which makes this type of collimator generally applicable for all types of nuclear medicine examinations.
Linac-Based SRS/SBRT Dosimetry
Published in Arash Darafsheh, Radiation Therapy Dosimetry: A Practical Handbook, 2021
Karen Chin Snyder, Ning Wen, Manju Liu
The radiation field can be defined by multiple collimators on a linac, Figure 18.3. The primary collimator is a conical tungsten block that collimates the beam to the largest circular field size. It is located beneath the target before the flattening filter and monitor chamber. The secondary collimator typically consists of mobile jaws. These are made of thick high-Z material used to collimate the beam into square fields. In Elekta and Siemens machines, the secondary collimator consists of the MLC. In Varian machines, the MLCs are tertiary collimators.
Mammography and Interventional Breast Procedures
Published in Raymond Taillefer, Iraj Khalkhali, Alan D. Waxman, Hans J. Biersack, Radionuclide Imaging of the Breast, 2021
Collimator: Collimators regulate the size and the shape of the x-ray beam. Beam collimation is intended to decrease scatter radiation and unnecessary patient exposure. In mammography, rectangular collimation is used to match the shape of the image receptor [19].
Modeling of dose and linear energy transfer homogeneity in cell nuclei exposed to alpha particles under various setup conditions
Published in International Journal of Radiation Biology, 2023
Adrianna Tartas, Mateusz Filipek, Marcin Pietrzak, Andrzej Wojcik, Beata Brzozowska
The use of a collimator was simulated for the bottom-up setup, with a 5 mm thick aluminum collimator placed 1 mm of air above the source and 1 mm of air below the Mylar foil, as shown in Figure 1. The collimator had a dimension of 6 cm × 5 cm and a thickness of 5 mm. The 140 hexagonal holes with a diagonal of 5 mm were separated with 0.25 mm thick wall. The fourth irradiation setup included rotating collimator. To simulate the rotation, the hexagonal shape of the central hole (where the diagonals of the collimator intersected) was divided into six equilateral triangles. The rotation axis was set at 1/3 of the height of one of the triangles (the bottom one), dropped onto the side of the hexagon as it was shown in Figure 2. The collimator was rotated in steps of 5 degrees. The angular speed should be adjusted to the time of irradiation in such a way that the collimator would make a full rotation. If the speed must be higher, the number of rotations should be an integer. Rotating the collimator allowed to minimize the radiation attenuation by the collimator walls.
WAG/RijCmcr rat models for injuries to multiple organs by single high dose ionizing radiation: similarities to nonhuman primates (NHP)
Published in International Journal of Radiation Biology, 2020
Brian L. Fish, Thomas J. MacVittie, Aniko Szabo, John E. Moulder, Meetha Medhora
Nonanesthetized rats were immobilized in a plastic jig and irradiated using a XRAD 320KV orthovoltage X-ray system (Medhora et al. 2014). The X-ray system was operated at 320 kVp and 13 mA with a half value layer of 1.4 mm Cu. The dose rate was 1.73 Gy.min−1. Each rat was confined in a separate chamber in a plastic jig, which allows irradiation of 2–4 rats for simultaneous PA exposure. The chambers were placed on a plane perpendicular to the beam direction with distance from source to the midline of rats set at 61 cm. Collimator jaws were used to define a radiation field of 22.5 × 22.5 cm2 at midline and large enough to cover all chambers with adequate (at least 2 cm) margin. This model was used to measure H-ARS by 30 days after TBI. Morbidity from hematopoietic toxicity was evaluated in 11- to 12-week-old female rats after 7 Gy (n = 41), 7.5 Gy (n = 37) and 8 Gy (n = 25) as well as at 7 Gy (n = 37) for 8-week-old male rats.
Focus small to find big – the microbeam story
Published in International Journal of Radiation Biology, 2018
The micro-collimator was used in early microbeams. It is the most straightforward method and its advantages include easy location of beam and accurate dose delivery. Facilities that employed collimators were able to achieve a beam size of a few microns. However, due to scattering, the biological samples were required to be placed very close to the collimator. The particle scattering also reduced beam energy delivered to biological samples. Collimators of various materials and thicknesses were tested to minimize the scattering and to achieve a better accuracy (Folkard et al. 1997). An alternative collimation system contains a pair of aligned microapertures: the first one defines beam size and the second one placed closer to samples functions as an anti-scattering device for the first aperture. Many laboratories used this system, including JAERI (Takasaki, Japan) (Kobayashi et al. 2009), the INFN-LNL (Legnaro, Italy) (Gerardi, Galeazzi and Cherubini 2005) and Columbia University (New York, USA) (Randers-Pehrson et al. 2001). Figure 2(A) illustrates the two-microaperture collimation system.