Direct and fast measurement of CT beam filter profiles with simultaneous geometrical calibration

Abstract

Purpose To accurately measure the beam filter profiles from a variety of CT scanner models and to provide reference data for Monte Carlo simulations of CT scanners. Methods This study proposed a new method to measure CT beam filter profiles using a linear-array x-ray detector (X-Scan 0.8f3–512, Detection Technology Inc, Finland) under gantry rotation mode. A robust geometrical calibration approach was developed to determine key geometrical parameters, by considering the x-ray focal spot location relative to the linear-array detector and the gantry’s angular increment at each acquisition point. CT beam intensity profiles were synthesized from continuously measured data during a 10° gantry rotation range, with calibrated detector response and system geometry information. Relative transmission profiles of nineteen sets of beam filters were then derived for nine different CT scanner models from three different manufacturers. Equivalent aluminum thickness profiles of these beam filters were determined by analytical calculation using the Spektr Matlab software package to match the measured transmission profiles. Three experiments were performed to validate the accuracy of the geometrical calibration, detector response modeling, and the derived equivalent aluminum thickness profiles. Results The beam intensity profiles measured from gantry rotational mode showed very good agreement with those measured with gantry stationary mode, with a maximal difference of 3%. The equivalent aluminum thickness determined by this proposed method agreed well with what was measured by an ion chamber, with a mean difference of 0.4%. The determined HVL profiles matched well with data from a previous study (max difference of 4.7%). Conclusions An accurate and robust method to directly measure profiles from a broad list of beam filters and CT scanner models was developed, implemented, and validated. Useful reference data was provided for future research on CT system modeling.

A method to estimate transmission profiles of bow‐tie filters using rotating tube measurements

“…For instance, if the TF(x) was too high, the SLRI associated with Ω x would also be too high and this can produce a value of TF(x + 1) that is too low, as indicated by Eq. (18). The resulted TFs of these two points (jump and drop) may cancel out in the next point, TF (x + 2).…”

“…The resulted TF will be affected even if the relative output changes by a few percent [see Eq. (18)]. This effect becomes amplified as the deviation from 360°tube rotation increases.…”

“…Their method relied on radiochromic film and a custom‐made aperture apparatus, which was designed to help sample the dose across the BTF. In another approach proposed by Yang et al., a solid‐state linear detector was used in which the angular dependence was calibrated and utilized to synthesize BTF transmission profile from a single gantry rotation. In earlier work, Liu group presented a measurement‐based method that also relied on a solid‐state detector array but with the tube parked in stationary position .…”

1-point RANSAC for circular motion estimation in computed tomography (CT)

“…An alternative approach to improving the result of tomographic reconstruction is the so-called online calibration. Methods of online calibration allow to evaluate and correct geometric errors after or directly during the tomographic experiment [16,17,18,19,20]. It can be seen that there is a gradual transition from methods with an a priori known position and shape of the calibration object (for example, [18], 2014) to methods with the simultaneous reconstruction and and geometric error correction ( [17], 2017).…”

Determination and verification of the x-ray spectrum of a CT scanner

“…In addition, Yang et al proposed a new method to measure CT filter profiles using a linear array solid-state detector for rotating x-ray sources. 35 This approach utilizes both accurate geometric calibration and high angular resolution.…”