Byung Jun Min

Application of Cerenkov radiation generated in plastic optical fibers for therapeutic photon beam dosimetry

Abstract. A Cerenkov fiber-optic dosimeter (CFOD) is fabricated using plastic optical fibers to measure Cerenkov radiation induced by a therapeutic photon beam. We measured the Cerenkov radiation generated in optical fibers in various irradiation conditions to evaluate the usability of Cerenkov radiation for a photon beam therapy dosimetry. As a results, the spectral peak of Cerenkov radiation was measured at a wavelength of 515 nm, and the intensity of Cerenkov radiation increased linearly with increasing irradiated length of the optical fiber. Also, the intensity peak of Cerenkov radiation was measured in the irradiation angle range of 30 to 40 deg. In the results of Monte Carlo N-particle transport code simulations, the relationship between fluxes of electrons over Cerenkov threshold energy and energy deposition of a 6 MV photon beam had a nearly linear trend. Finally, percentage depth doses for the 6 MV photon beam could be obtained using the CFOD and the results were compared with those of an ionization chamber. Here, the mean dose difference was about 0.6%. It is anticipated that the novel and simple CFOD can be effectively used for measuring depth doses in radiotherapy dosimetry.

Secondary radiation doses of intensity-modulated radiotherapy and proton beam therapy in patients with lung and liver cancer

PURPOSE To compare the secondary radiation doses following intensity-modulated radiotherapy (IMRT) and proton beam therapy (PBT) in patients with lung and liver cancer. METHODS AND MATERIALS IMRT and PBT were planned for three lung cancer and three liver cancer patients. The treatment beams were delivered to phantoms and the corresponding secondary doses during irradiation were measured at various points 20-50 cm from the beam isocenter using ion chamber and CR-39 detectors for IMRT and PBT, respectively. RESULTS The secondary dose per Gy (i.e., a treatment dose of 1Gy) from PBT for lung and liver cancer, measured 20-50 cm from the isocenter, ranged from 0.17 to 0.086 mGy. The secondary dose per Gy from IMRT, however, ranged between 5.8 and 1.0 mGy, indicating that PBT is associated with a smaller dose of secondary radiation than IMRT. The internal neutron dose per Gy from PBT for lung and liver cancer, 20-50 cm from the isocenter, ranged from 0.03 to 0.008 mGy. CONCLUSIONS The secondary dose from PBT is less than or compatible to the secondary dose from conventional IMRT. The internal neutron dose generated by the interaction between protons and body material is generally much less than the external neutron dose from the treatment head.

Performance evaluation of a PET detector consisting of an LYSO array coupled to a 4 × 4 array of large-size GAPD for MR compatible imaging

We examined a PET detector consisting of an LYSO array coupled to a 4 4 array of large-size Geiger-mode avalanche photodiode (GAPD). The GAPD coupled to 3 mm 3 mm 20 mm LYSO pixel crystal has been investigated for possible use as an MR-compatible PET photosensor. Primary characteristics of a PET detector, such as energy resolution and coincidence timing resolution were measured. Gain variation, count uniformity, and count estimation error of 4 4 array of LYSO-GAPD were measured to evaluate the performance parameters relevant for PET imaging. The energy resolution and coincidence timing resolution with 511 keV gamma rays were 18.5 0.7% and 1.6 ns, respectively. The gain variation, count uniformity for all 16 channels were 1.3:1 and 1.3:1, respectively. The count estimation error between adjacent channels measured with an LYSO connected to a GAPD pixel was negligible (0.24 0.04%). Long-term stability results show that there was no significant change in the photopeak position, energy resolution and count rate for 20 days. Cable lengths up to 300 cm, used between the GAPD and preamplifier, did not affect photopeak position and energy resolution. The performance of the LYSO-GAPD detector inside the MRI exhibited no significant change compared to that measured outside the MRI. The MR images acquired with and without the operating LYSO-GAPD detector located on top of the RF coil showed no considerable degradation in image quality. These results demonstrate the feasibility of using the LYSO-GAPD detector as PET photosensors, which could be used for MR compatible PET development.

Development of a dual modality imaging system: a combined gamma camera and optical imager

Several groups have been reporting on the development of dual modality gamma camera/optical imaging system, a useful tool for understanding biological processes in experimental animals. While the previously reported dual modality imaging instrumentation employed separated gamma camera and optical imager, we designed a new detector using a PSPMT that is capable of imaging both optical photons and gamma rays for combined gamma camera and optical imager. The proposed combined system consists of parallel-hole collimator, array type crystal and position sensitive (PS) PMT. The top surface of collimator and array crystals left open to allow optical photons to reach to the PSPMT. The detector was designed to be operated in a light tight box. Pulse height spectrum and planar images of Tc-99m phantom and mini-Derenzo phantom were obtained to evaluate the gamma imaging performance. Pulse height spectra and planar images of black mask having various hole sizes with green LED were obtained to estimate the optical imaging performance. A mouse phantom containing optical and gamma ray source was imaged to assess the imaging capability of the system. Sensitivity, energy resolution and spatial resolution of the gamma camera image acquired with Tc-99m were 1.1 cps/kBq, 23% and 2.1 mm, respectively. Rod sizes from 4.8 to 2.4 in the mini-Derenzo phantom were clearly resolved. The spatial resolution of the optical image acquired with the LED was 3.5 mm. Each hole location of the mask was accurately delineated. Gamma ray and optical photon images of the mouse phantom were successfully obtained using the single detector. The preliminary results indicated that both optical photon and gamma ray imaging is feasible using a detector based on a PSPMT proposed in this study.

A Compact SPECT/CT System for Small Animal Imaging

A dual-modality compact SPECT/CT system for small animal imaging was developed. The SPECT system consisted of a pinhole collimator and continuous NaI(Tl) scintillation crystal coupled to a PSPMT. The CT system consisted of a microfocus X-ray tube and a CMOS flat-panel detector. The SPECT system was mounted perpendicular to the X-ray system. Individual projections of the SPECT and the CT were acquired by rotating the animal on a vertical axis in front of the detectors. The SPECT and CT images were reconstructed using OSEM and Feldkamps cone-beam algorithms, respectively. Mouse and rat SPECT images demonstrated detailed activity distribution at the expected structures. A CT image obtained with 40 kVp and 0.5 mA presented high-resolution anatomic details. Fused SPECT/CT images demonstrated good agreement between the CT images and the corresponding uptake of the radiotracer. The SPECT/CT system developed in this study provides high-quality dual-modality images and could be useful to obtain functional images with high resolution morphology information

Development of PET using 4 × 4 array of large size Geiger-mode avalanche photodiode

Geiger-mode avalanche photodiode (GAPD) has been demonstrated to be a high performance PET sensor because of high gain, fast response, low excess noise, low bias voltage operation and magnetic field insensitivity. The purpose of this study is to develop a PET for human brain imaging using 4 × 4 array of large size GAPD. PET detector modules were designed and built to develop a prototype PET. The PET consisted of 72 detector modules arranged in a ring with an inner diameter of 330 mm. The LYSO arrays consisted of 4 × 4 array of 3 × 3 × 20 mm3 pixels, which were 1-to-1 coupled to 4 × 4 arrays of 9 mm2 GAPD pixels (SensL, Ireland). The GAPDs were tiled together using flip chip technology on glass and operated at a bias voltage of 32 V for a gain of 3.5 × 106. The signals of the each module were amplified by a 16 channel preamplifier circuit with differential outputs and then sent to a position decoder circuit (PDC), which readout digital address and analog pulse of the one interacted channel from 64 signals of 4 preamplifier boards. The PDC output signals were fed into FPGA-embedded DAQ boards. The analog signal was sampled with 100 MHz, and arrival time and energy of the digitized signal were calculated and stored. The coincidence data were sorted and reconstructed by standard filtered back projection. The energy and time resolution of LYSO-GAPD block detector for 511-keV was 20.4% and 2.4 ns, respectively. The developed PDC could accurately provide the interacted PET signal and reduce the number of the readout channels of PET detector modules based on array type GAPD. The rods down to a diameter of 3.5 mm were resolved in hot-rod phantom image acquired with the brain PET which is similar to the image obtained by Monte Carlo simulation. Activity distribution pattern between white and gray matter in Hoffman brain phantom was well imaged. These results demonstrate that high performance PET could be developed using the GAPD-based PET detectors, analog and digital signal processing methods designed in this work. The prototype brain PET will be tested in a clinical 3T MRI to construct a combined PET-MRI.

A simple and improved digital timing method for positron emission tomography

A simple and improved digital timing method has been developed for positron emission tomography (PET). The so-called initial rise interpolation method is based on an important characteristic of gamma signal: a properly pre-amplified and sampled gamma signal pulse can be characterized to arrive with an initial rise from baseline and then to go up with a maximum rise. Pulse arrival time is obtained by calculating the intersection of the initial rise line with the baseline for each gamma signal pulse. In this study, a FPGA-based data acquisition (DAQ) card was used for data acquisition and processing. We measured coincidence timing resolution of two types (fast and slow) of recently developed 3 mm × 3 mm Geiger mode avalanche photodiodes (GAPDs) using 3 different digital timing methods: initial rise interpolation (IRI), digital CFD and maximum rise interpolation (MRI). Furthermore, simulation has been performed to evaluate effects of pulse rise time, pulse amplitude and front-end noise level on timing resolution estimated by the three digital timing methods. Measured results show that, IRI method provided the best timing resolution for both types of GAPDs: 0.7 ns FWHM for fast GAPD and 1.5 ns for slow GAPD (digital CFD: 1.5 ns and 2.2 ns; MRI: 1.8 ns and 2.7 ns). In accordance with measured results, simulation results also show that IRI method provided the best timing resolution. Based on these experimental results, we concluded that the developed simple and improved digital timing method is reliable and useful for the development of high performance PET.

Performance amelioration for small animal SPECT using optimized pinhole collimator and image correction technique

The aim of this study was to improve the performance of pinhole single photon emission computed tomography (SPECT) fabricated with a position sensitive photomultiplier tube using an optimized pinhole collimator and image correction technique. The center-of-rotation was aligned to prevent mechanical shift using a leveling-laser system. Based on the previous results of pinhole aperture simulation and optimization study, a pinhole collimator for the small animal SPECT was fabricated in the range of optimal pinhole diameter and channel height. Previously reported image correction technique of position mapping, energy calibration and flood correction procedures developed for array type scintillator was revised and applied to plate type crystal. Phantom and small animal studies were performed to investigate the system performance. Image corrections, center-of-rotation alignment, and collimator optimization were necessary to obtain high resolution and high quality SPECT images. The SPECT system ameliorated by this study could be utilized in small animal and molecular imaging studies required to provide high spatial resolution with moderate sensitivity.

MR compatible brain PET using tileable GAPD arrays

The aim of this study is to develop a MR compatible PET that is insertable to MRI and allows simultaneous PET and MR imaging of human brain. The brain PET having 72 detector modules arranged in a ring of 330 mm diameter was designed. Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm LYSO crystals coupled to a tileable 4 × 4 array Geigermode avalanche photodiode (GAPD) and designed to locate between RF and gradient coils. Signals of the each module were transferred to preamplifiers using flexible flat cable of 3 m long, and then sent to a position decoder circuit (PDC), which outputs digital address and an analog pulse of the one interacted channel from preamplifier signals. The PDC outputs were fed into FPGA-embedded DAQ boards. The analog signal was digitized, and arrival time and energy of the signal were calculated and stored. All electronics were located outside MR bore to minimize signal interference between PET and MR. Basic performance of the PET components and cross-compatibilities of the PET module and MR were evaluated. Imaging performance of the designed brain PET was investigated using Monte Carlo simulation and experimental measurement. The degradation of PET performance caused by the 3 m long cable and the PDC was negligibly small. No obvious differences of the PET module performance measured inside/outside MR bore were observed. The SNR of various MR sequence phantom images acquired with/without the PET module were also similar. Activity distribution patterns of hot-rod phantoms were well imaged without distortion, and rods down to a diameter of 3.5 mm were resolved in both simulation and experiment. Gray and white matter of the Hoffman brain phantom was also well imaged. Preliminary experimental results demonstrate that MR compatible high quality PET imaging is feasible using the GAPD arrays, electronics, signal processing method and MR insertable PET design schemes developed in this study.

Development of filtering methods for PET signals contaminated by RF pulses for combined PET-MRI

This paper presents the development of filtering methods for positron emission tomography (PET) signals contaminated by radio frequency (RF) pulses for combined PET and clinical 3-T magnetic resonance imaging (MRI). The filtering methods include software, hardware, and hybrid correction methods. In the software correction method, PET signals are assessed, and valid signals are identified based on the characteristics of a typical PET signal using Field-Programmable Gate Array (FPGA)-based programming. The hardware correction method makes use of differential-to-single-ended and low-pass filter circuits for PET analog signals. The hybrid correction method involves the sequential application of both the hardware and software methods. Both valid and contaminated PET signals are measured with an oscilloscope. An evaluation is then made of the performance (energy resolution, photopeak channel, total counts, and coincidence timing resolution) of the PET detector modules with and without various MR sequences (gradient echo, spin echo T1 sequence). For all correction methods, the energy resolution, photopeak position, and coincidence timing resolution with MR sequences are similar (<; 3%) to those without MR sequences. However, the total count of each module depends greatly on the method applied. The hybrid correction method displays an ability to preserve (<; 1%) the total counts of the modules during various MR sequences. The results show that this filtering method, which can reject noise signals and reduce count loss while preserving the valid analog signals of MR sequences, is reliable and useful for the development of simultaneous PET-MRI.