T. T. Monajemi

Modeling scintillator-photodiodes as detectors for megavoltage CT

The use of cadmium tungstate (CdWO4) and cesium iodide [CsI(Tl)] scintillation detectors is studied in megavoltage computed tomography (MVCT). A model describing the signal acquired from a scintillation detector has been developed which contains two steps: (1) the calculation of the energy deposited in the crystal due to MeV photons using the EGSnrc Monte Carlo code; and (2) the transport of the optical photons generated in the crystal voxels to photodiodes using the optical Monte Carlo code DETECT2000. The measured detector signals in single CdWO4 and CsI(Tl) scintillation crystals of base 0.275 x 0.8 cm2 and heights 0.4, 1, 1.2, 1.6 and 2 cm were, generally, in good agreement with the signals calculated with the model. A prototype detector array which contains 8 CdWO4 crystals, each 0.275 x 0.8 x 1 cm3, in contact with a 16-element array of photodiodes was built. The measured attenuation of a Cobalt-60 beam as a function of solid water thickness behaves linearly. The frequency dependent modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)] were measured for 1.25 MeV photons (in a Cobalt-60 beam). For 6 MV photons, only the MTF(f) was measured from a linear accelerator, where large pulse-to-pulse fluctuations in the output of the linear accelerator did not allow the measurement of the NPS(f). A two-step Monte Carlo simulation was used to model the detectors MTF(f), NPS(f) and DQE(f). The DQE(0) of the detector array was found to be 26% and 19% for 1.25 MeV and 6 MV photons, respectively. For 1.25 MeV photons, the maximum discrepancies between the measured and modeled MTF(f), relative NPS(f) and the DQE(f) were found to be 1.5%, 1.2%, and 1.9%, respectively. For the 6 MV beam, the maximum discrepancy between the modeled and the measured MTF(f) was found to be 2.5%. The modeling is sufficiently accurate for designing appropriate detectors for MVCT.

A bench-top megavoltage fan-beam CT using CdWO4-photodiode detectors. I. System description and detector characterization

We describe the components of a bench-top megavoltage computed tomography (MVCT) scanner that uses an 80-element detector array consisting of CdWO4 scintillators coupled to photodiodes. Each CdWO4 crystal is 2.75 x 8 x 10 mm3. The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient (R2) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by this detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detectors large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation <0.5%); however, any smaller permanent damage could not be assessed.

Thick, segmented CdWO4-photodiode detector for cone beam megavoltage CT: a Monte Carlo study of system design parameters.

Megavoltage (MV) imaging detectors have been the focus of research by many groups in recent years. We have been working with segmented CdWO4 crystals in contact with photodiodes in our lab. The present study uses both x-ray and optical photon transport Monte Carlo simulations to analyze the effects of scintillation crystal height, septa material, beam divergence, and beam spectrum on the modulation transfer function, MTF(f) and zero frequency detective quantum efficiency, DQE(0), of a theoretical area detector. The theoretical detector is comprised of tall, segmented CdWO4 crystals and two dimensional photodiode arrays with a pitch of 1 mm and a fill factor of 72%. Increasing the crystal height above 10 mm does not result in an improvement in the DQE(0) if the reflection coefficient of the septa is less than 0.8. For a reflection coefficient of 0.975 for the septa, there is a continual gain in the DQE(0) up to 30 mm tall crystals. Similar calculations show that employing a 3.5 MV beam without a flattening filter increases the DQE(0) for 20 mm tall crystals by 9% compared to a typical 6 MV beam with a flattening filter. The severe degradations due to beam divergence on MTF(f) are quantified and suggest the use of focused detectors in MV imaging. It is found that when the effect of optical photons is considered, the presence of divergence can appear as a shift in the location of the input signal as well as loss of spatial resolution.

Performance characterization of a MVCT scanner using multislice thick, segmented cadmium tungstate-photodiode detectors

PURPOSE Megavoltage computed tomography (MVCT) and megavoltage cone beam computed tomography (MVCBCT) can be used for visualizing anatomical structures prior to radiation therapy treatments to assist in patient setup and target localization. These systems are less susceptible to metal artifacts and provide better CT number linearity than conventional CT scanners. However, their contrast is limited by the properties of the megavoltage photons and the low detective quantum efficiency (DQE) of flat panel detector systems currently available. By using higher DQE, thick, segmented cadmium tungstate detectors, the authors can improve the low contrast detectability of a MVCT system. This in turn would permit greater soft tissue visualization for a given radiation dose, allowing MVCT to be used in more clinical situations. METHODS This article describes the evaluation of our prototype system that uses thick, segmented detectors. In order to create images using a dose that would be acceptable for day to day patient imaging, the authors evaluated their system using the low intensity bremsstrahlung component of a 6 MeV electron beam. The system was evaluated for its uniformity, high contrast resolution, low contrast detectability, signal to noise ratio, contrast to noise ratio, and CT number linearity. RESULTS The prototype system was found to have a high contrast spatial resolution of about 5 line pairs per cm, and to be able to visualize a 15 mm 1.5% contrast target with 2 cGy of radiation dose delivered. SNR2 vs radiation dose and mean pixel value vs electron density curves were linear. CONCLUSIONS This prototype system shows a large improvement in low contrast detectability over current MVCBCT systems.

Impact of edema and seed movement on the dosimetry of prostate seed implants.

This article summarizes current knowledge concerning the characterization of prostatic edema and intra-prostatic seed movement as these relate to dosimetry of permanent prostate implants, and reports the initial application to clinical data of a new edema model used in calculating pre- and post-implant dose distributions. Published edema magnitude and half-life parameters span a broad range depending on implant technique and measurement uncertainty, hence clinically applicable values should be determined locally. Observed intra-prostatic seed movements appear to be associated with particular aspects of implant technique and could be minimized by technique modification. Using an extended AAPM TG-43 formalism incorporating the new edema model, relative dose error RE associated with neglecting edema was calculated for three I-125 seed implants (18.9 cc, 37.6 cc, 60.2 cc) performed at our center. Pre- and post-plan RE average values and ranges in a 50 × 50 × 50 mm3 calculation volume were similar at ~2% and ~0–3.5%, respectively, for all three implants; however, the spatial distribution of RE varied for different seed configurations. Post-plan values of D90 and V100 for prostate were reduced by ~2% and ~1%, respectively. In cases where RE is not clinically negligible as a consequence of large edema magnitude and / or use of Pd-103 seeds, the dose calculation method demonstrated here can be applied to account for edema explicitly and there by improve the accuracy of clinical dose estimates.

TH‐C‐303A‐06: Performance Characterization of a MVCT Scanner Using Multi‐Slice Thick, Segmented Cadmium Tungstate‐Photodiode Detectors

Purpose: To evaluate the performance of a MVCT system based on multi‐slice, thick, and segmented cadmium tungstate‐photodiode detectors.Method and Materials: Our MVCTs detector is a 2D array of 1mm × 1mm (pitch) × 10mm (thickness) cadmium tungstate crystals separated by a septa paint of reflectivity higher than 0.975. The scintillators are mounted on ten 16 × 16 element photodiode arrays (SCA‐CA256ES, Semicoa, Costa Mesa, CA). The total array size is 320 detectors along the arc by 16 detectors along the slice thickness direction. The radius of curvature of detector arc is 92.5cm. In our system, the source and detectors remain stationary while the object being imaged is placed on a precision rotating stage. Due to the high dose per pulse provided by our clinical linac, it is not possible to image a test object with a dose of < 50cGy in a 6MV beam. In order to obtain low dose images, we used the small amount of Bremsstrahlung radiation produced in the scattering foils in 6 MeV electron beam after removing the electrons from the beam by placing 4cm of solid water in the beam.Results: Our system demonstrates a uniformity index of 0.4% at 1.9cGy. The noise standard deviation is around 2% at 1.9cGy. CT number linearity (R2= 0.9982) and low contrast resolution (15 mm object with 1.5% contrast at 2 cGy) are superior to published evaluations of commercially available tomotherapy MVCT. The spatial resolution is about 4lp/cm which is limited mainly by the diffused Bremsstrahlung source. Conclusion: Thick segmented cadmium tungstate detectors offer significantly better low contrast resolution per unit dose at MV energy than commercially available gas filled or flat panel detectors. This work demonstrates the feasibility of creating a fully functional MVCT system using this technology.

TU‐FF‐A3‐01: X‐Ray and Optical Monte Carlo Study of Thick, Segmented Scintillators for MV Imaging

Purpose: To study the imagingcharacteristics of thick, segmented, 2‐D CdWO4 crystal‐photodiode detectors as a function of crystal height, septa material and optical reflectivity, x‐ray beam spectrum and beam divergence using a two‐step Monte Carlo approach involving both x‐ray photon transport at megavoltage (MV) energies and the optical photon transport in scintillator and photodiodes.Method and Materials: We have studied the spatial frequency dependent detective quantum efficiency (DQE) of thick, segmented, 2‐D CdWO4 crystals in contact with silicon photodiode arrays. The energy deposited into the 3‐D voxels (1 × 1 × 1 mm3, septa thickness = 0.15 mm, fill factor = 72%) of the detector for each of the 6 and 3.5 MV x‐ray photons in a normally incident pencil beam was calculated using the DOSXYZnrc user code of the EGSnrc Monte Carol system. The isotropically emitted optical photons in each voxel were calculated using the average CdWO4 optical yield and transported to the photodiode array using DETECT2000 optical Monte Carlo code. A 10° beam divergence angle was also simulated. The detector DQE was calculated using the spatial distribution of optical photons.Results: The DQE increases with the crystal height only if the reflectivity of the septa material is high (0.975). For poor reflectivity (0.65 and 0.8), the increase in the DQE of the taller crystals to MV photons is seriously offset (from 42% to less than 20% for 3 cm tall crystals) by the decreased probability of detecting optical photons. Similarly, the increase in DQE due to the lower energy photons is obtained if the high reflectivity of septa material is maintained for the detector. Beam divergence in thick crystals also reduces the DQE. Conclusion: High reflectivity of the septa in thick, segmented scintillation detectors is very important to achieve high DQE.

Sci‐PM Sat ‐ 01: Imaging performance of a bench‐top megavoltage CT scanner

The ultimate goal of this project is to create a focused 2D MV detector with high detective quantum efficiency so reasonable low contrast resolution (LCR) at low dose can be obtained in MVCT. As an initial step an 80‐element detector is fabricated by tiling 8‐element CdWO 4 (element size 0.275 × 0.8 × 1 cm3) and photodiode arrays on an arc (radius = 110 cm). A precision rotary stage and its control are added to create a third generation CT scanner. The attenuation of Co 60 and 6 MV beams are measured as a function of solid water thickness, fit to a second order polynomial to correct for spectral hardening artifacts. A calibration procedure was established to remove ring artifacts caused by distinctly asymmetric line spread functions at the ends of 8‐element blocks. Low contrast resolution as a function of dose and object size, the signal to noise ratio (SNR) as a function of dose, and linearity of CT numbers with density were quantified. Throwing away one‐ninth of collected projection angles to reduce the dose per image adversely affects the resolution in 6 MV images; 15 mm targets at 1.5% are visible at 7cGy. Low contrast target of 1.5% at 6 mm is visible in Co 60 at 2cGy. LCR in objects stays approximately constant while dose is reduced from 17 to 2cGy. Contrast decreases with diameter decrease. SNR2 from a uniform phantom increases linearly with dose (R2=0.9977). CT numbers as a function of the density show a linear trend (R2=0.9923).

Po‐Poster ‐ 01: A bench‐top megavoltage CT scanner

The purpose of this project was to design, fabricate and test the data acquisition timing control, precision rotary stage control, and analog data multiplexer unit for a prototype megavoltage computed tomography (MVCT) detector. An 80‐element prototype detector array is made with CdWO 4 (element size 0.275 × 0.8 × 1 cm3) scintillators and photodiodes on an arc (radius of 110 cm). In addition to designing and fabricating an in‐house data acquisition system (front‐end integrators, data multiplexer unit, and timing control), a precision rotary stage and its control are added to create a third generation MVCT scanner.Data acquisition is synchronized with radiation pulses from a linac. Response of detector as a function of dose rate was studied by varying the source to detector distance. A narrow slit beam, at five locations, was used to measure the pre‐sampled MTF. Detector signal in open beam was measured for a number of radiation pulses to use the periodogram method for NPS estimation. Using the measured MTF, NPS, and photon fluence impinging on detector,DQE was calculated. Detector response is linear as a function of dose rate, however shows a non‐linear component while measuring the attenuation by solid water due to the polyenergetic spectrum. Beam‐hardening correction is necessary before image reconstruction. MTF at the Nyquist frequency (0.16 mm−1) is 0.48. Zero‐frequency DQE in 6 MV at 21% is higher than any experimental MVCT detector. The basic performance of the prototype detector is satisfactory for producing reasonable low contrast resolution in MVCT images with low dose.

TU‐C‐I‐609‐10: Preliminary Study of Pixel Pitch and Effect of Divergence On Thick Cadmium Tungstate Detector

Purpose: To theoretically study the effects of pixel pitch and beam divergence on 1 cm thick CdWO 4 crystals for use in megavoltage cone beam computed tomography (MVCBCT). Method and Material: CdWO 4 linear array (80‐elements, each 0.275 × 0.8 × 1 cm3 in fan beam) has been proven to be a good candidate for MVCT in our lab. For this study, DOSXYZnrc Monte Carlo code was used to find the point spread function (PSF) as the distribution of energy deposited in a theoretical flat detector (30 × 30 × 10 mm3, 0.01 × 0.01 × 10 mm3 voxels). A pencil‐beam of 6 MV photons was incident at a range of angles (0°, 5°, 10°, 15°, 20°) with respect to the normal. In each case, the detectorPSF (ignoring optical spread) was convolved with a realistic source function to give the system PSF in the plane of the object. A realistic object to detector magnification of 1.4 was used in the system PSF calculations. The PSF at 0.01 mm pitch was re‐binned into several pitches (0.5, 0.7, 1.0 mm) and Fourier transformed to obtain the system modulation transfer function (MTF). Results: This analysis suggests an optimum pitch of 0.5 mm which cannot be fabricated using CdWO 4 because of the cleavage plane. However, the pre‐sampled detectorPSF is further degraded due to small optical leakage through reflective coating and scintillator‐photodiode interface, and the scattered radiation from the object. So, in practice a pitch of 1 mm may be sufficient. The degradation of the system MTF with the increasing divergence is significant between 0° and 5°, and becomes large at 10°. Conclusion: Practical pitch of 1 mm is not ideal yet maybe sufficient. The use of flat panel photodiode arrays for MVCBCT is precluded due to divergence; instead a focused detector should be used.