Design and evaluation of a variable aperture collimator for conformal radiotherapy of small animals using a microCT scanner
Design of a variable aperture collimator. A schematic of the collimator is shown in (a), depicting the six trapezoidal blocks of width moving along a hexagonal frame. The collimator aperture size is denoted by . Three of the blocks are fitted with pins at locations that may be acted upon by a driving apparatus in order to adjust the collimator aperture size. A photograph of the manufactured device is shown in (b). The final two-frame assembly generated for use in a microCT scanner is shown schematically from two directions in (c) and (d). The distances from the source to each collimator stage are given as L1 and L2. Other measurements include the source-detector distance and the block thickness . The and axes are oriented as shown.
Adjustment of a variable aperture collimator by a slotted driving plate. As shown in (a), a driving plate is fitted onto each collimator stage so that the pins in three of the blocks fit into its slots. The orientation of the driving plate is quantified by the angle , which is defined such that it varies from 0° (collimator fully closed) to 41° (collimator fully open, as shown). The forces transferred from the driving plate to the blocks are depicted in (b). The applied force may be decomposed into the force along the direction of block motion and the force perpendicular to block motion .
Installation of a variable aperture collimator assembly within a microCT scanner. The gantry of the GE RS120 microCT scanner is shown in (a), including the custom mounting bar between the subject bore and the x-ray source. A head-on view of the installed collimation device is shown in (b), including the motors mounted on the gantry to drive motion of the driving plates and the frame holding the unit together. A tilted view is shown in allowing observation of the mount of the collimator on the gantry bar.
Analytic and experimental evaluation of collimator motion. Plot (a) depicts the aperture size of each collimator stage as a function calculated as a function of driving plate angle. The line represents simulated aperture sizes while the data points show experimentally measured data. Plot (b) shows computed values of the component forces applied to a pin-bearing block along and perpendicular to the direction of block motion as a function of driving plate angle. Laboratory measurements of the torque required to initiate block motion as a function of driving plate angle are shown in plots (c) and (d) for collimator opening and closing, respectively.
Analytic and experimental evaluation of x-ray attenuation profiles produced by the collimator. The two-stage collimator was set at driving plate angles of 0°, 0.7°, 3.7°, 7.4°, and 11.3° (isocenter aperture sizes of 0, 0.3, 1.2, 2.4, and , respectively). The central row depicts the transmission profile predicted by computer simulation, shown in a log scale. The bottom row shows the associated measured transmission profile acquired using the microCT detector.
Determination of collimator alignment. The measured transmission profile acquired with the collimator set at (aperture size ) is shown in (a), with the hexagons fit to the profiles of the upper (dashed) and lower (solid) collimator frames superimposed. A misalignment between the profiles is apparent. This misalignment was corrected by adjustment of the collimator position in and by and , respectively, after which the transmission profile shown in (b) was acquired.
Penumbra measurements acquired from the variable aperture collimator.
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