Samir Chowdhury

Modeling spectral distortions in energy resolved photon-counting x-ray detector

Conventional x-ray detectors integrate the photon energy flux, losing individual photon energy information. By contrast, energy resolved photon-counting x-ray detectors (PCXDs) count photons in energy windows, thus retaining some energy information. This provides a number of advantages, including the use of energy information to aid in material discrimination. However, this capability relies on accurately measuring changes in the energy spectrum as the x-ray beam passes through the object. Several effects, including characteristic x-ray effects and charge sharing between detector pixels, result in distortions in the energy spectrum that complicate measuring the attenuating effects of the object on the energy spectrum. Our goal was to investigate and develop models for these effects that would be useful in compensating for them in applications involving spectral analysis. We used a previously developed 6-threshold CdTe-based PCXD to validate the models. Previously with this detector we observed higher than predicted counts at low energies. Characteristic x-rays emitted in the detector can distort the spectrum in a pixel via x-ray escape either out of the detector or into adjacent pixels, giving rise to a count with a reduced energy. The escaped x-rays can also produce reduced-energy counts in adjacent pixels. The second effect, charge sharing, results since x-ray interactions in the detector produce a charge cloud with finite size. If close to a pixel boundary and combined with charge diffusion, reduced-energy counts in both pixels can be produced. In this study, we developed a fast Monte Carlo method for modeling characteristic x-ray effects and an analytic method for modeling charge sharing effects. The models produced energy spectra in good agreement with those measured by the PCXD. These models can be used to improve the performance of energy-based composition estimation and ring correction methods by modeling the spectral distortions present in real detectors.

Dose reduction in molecular breast imaging

Molecular Breast Imaging (MBI) is the imaging of radiolabeled drugs, cells, or nanoparticles for breast cancer detection, diagnosis, and treatment. Screening of broad populations of women for breast cancer with mammography has been augmented by the emergence of breast MRI in screening of women at high risk for breast cancer. Screening MBI may benefit the sub-population of women with dense breast tissue that obscures small tumors in mammography. Dedicated breast imaging equipment is necessary to enable detection of early-stage tumors less than 1 cm in size. Recent progress in the development of these instruments is reviewed. Pixellated CZT for single photon MBI imaging of 99mTc-sestamibi gives high detection sensitivity for early-stage tumors. The use of registered collimators in a near-field geometry gives significantly higher detection efficiency - a factor of 3.6-, which translates into an equivalent dose reduction factor given the same acquisition time. The radiation dose in the current MBI procedure has been reduced to the level of a four-view digital mammography study. In addition to screening of selected sub-populations, reduced MBI dose allows for dual-isotope, treatment planning, and repeated therapy assessment studies in the era of molecular medicine guided by quantitative molecular imaging.

Investigation on a bar detector with 3D position decoding scheme

Simulation studies were performed to find optimum design parameters for a bar detector allowing 3D position decoding of impinging gamma rays. This design incorporates surface treatment, system geometry, photo-sensors, scintillators, and a more robust positioning algorithm. The bar detector simulated consists of a long scintillation crystal, ranged from 5 cm to 20 cm, with a relatively small cross section (4times4 mm2), coupled to two photo sensors, one on each end. Statistical based positioning scheme has been developed to improve positioning accuracy. The method maps event characterization vectors to the associated position based on chi square error. Our simulation shows that a 5cm long bar detector can achieve ~1mm FWHM spatial resolution for a 511 keV gamma photon. The performance improvement of the new positioning scheme over a linear estimator were over 70%. Grounded surface with reflector found to be the most optimum surface condition with respect to the steepest and linear correlation of photo sensor signals. Experimental measurements are in progress to validate the simulation results