Chang-Lae Lee

Radiation dose estimation using preclinical imaging with 124I-metaiodobenzylguanidine (MIBG) PET.

PURPOSE A pretherapyI124-metaiodobenzylguanidine (MIBG) positron emission tomography (PET)/computed tomography (CT) provides a potential method to estimate radiation dose to normal organs, as well as tumors prior to I131-MIBG treatment of neuroblastoma or pheochromocytoma. The aim of this work was to estimate human-equivalent internal radiation dose of I124-MIBG using PET/CT data in a murine xenograft model. METHODS Athymic mice subcutaneously implanted with NB1691 cells that express high levels of human norepinephrine transporter(n=4) were imaged using small animal microPET/CT over 96 h (approximate imaging time points: 0.5, 2, 24, 52, and 96 h) after intravenous administration of 3.07-4.84 MBq of I124-MIBG via tail vein. The tumors did not accumulate I124-MIBG to a detectable level. All four animals were considered as control and organ radiation dosimetry was performed. Volumes of interest were drawn on the coregistered CT images for thyroid, heart, lung, liver, kidney, and bladder, and transferred to PET images to obtain pharmacokinetic data. Based on tabulated organ mass distributions for both mice and adult male human, preclinical pharmacokinetic data were extrapolated to their human-equivalent values. Radiation dose estimations for different age groups were performed using the OLINDA|EXM software with modified tissue weighting factors in the recent International Commission on Radiological Protection (ICRP) Publication 103. RESULTS The mean effective dose fromI124-MIBG using weighting factors from ICRP 103 to the adult male was estimated at 0.25 mSv/MBq. In different age groups, effective doses using values from ICRP 103 were estimated as follows: Adult female: 0.34, 15-yr-old: 0.39 mSv/MBq, 10-yr-old: 0.58 mSv/MBq, 5-yr-old: 1.03 mSv/MBq, 1-yr-old: 1.92 mSv/MBq, and newborn: 3.75 mSv/MBq. For comparison, the reported effective dose equivalent of I124-NaI for adult male (25% thyroid uptake, MIRD Dose Estimate Report No. 5) was 6.5 mSv/MBq. CONCLUSIONS The authors estimated human-equivalent internal radiation dose ofI124-MIBG using preclinical imaging data. As a reference, the effective dose estimation showed that I124-MIBG would deliver less radiation dose than I124-NaI, a radiotracer already being used in patients with thyroid cancer.

Dosimetry in small-animal CT using Monte Carlo simulations

Small-animal computed tomography (micro-CT) imaging devices are increasingly being used in biological research. While investigators are mainly interested in high-contrast, low-noise, and high-resolution anatomical images, relatively large radiation doses are required, and there is also growing concern over the radiological risk from preclinical experiments. This study was conducted to determine the radiation dose in a mouse model for dosimetric estimates using the GEANT4 application for tomographic emission simulations (GATE) and to extend its techniques to various small-animal CT applications. Radiation dose simulations were performed with the same parameters as those for the measured micro-CT data, using the MOBY phantom, a pencil ion chamber and an electrometer with a CT detector. For physical validation of radiation dose, absorbed dose of brain and liver in mouse were evaluated to compare simulated results with physically measured data using thermoluminescent dosimeters (TLDs). The mean difference between simulated and measured data was less than 2.9% at 50 kVp X-ray source. The absorbed doses of 37 brain tissues and major organs of the mouse were evaluated according to kVp changes. The absorbed dose over all of the measurements in the brain (37 types of tissues) consistently increased and ranged from 42.4 to 104.0 mGy. Among the brain tissues, the absorbed dose of the hypothalamus (157.8–414.30 mGy) was the highest for the beams at 50–80 kVp, and that of the corpus callosum (11.2–26.6 mGy) was the lowest. These results can be used as a dosimetric database to control mouse doses and preclinical targeted radiotherapy experiments. In addition, to accurately calculate the mouse-absorbed dose, the X-ray spectrum, detector alignment, and uncertainty in the elemental composition of the simulated materials must be accurately modeled.

Dose reduction and image quality optimizations in CT of pediatric and adult patients: phantom studies

Multi-detector computed tomography (MDCT) can be used to easily and rapidly perform numerous acquisitions, possibly leading to a marked increase in the radiation dose to individual patients. Technical options dedicated to automatically adjusting the acquisition parameters according to the patients size are of specific interest in pediatric radiology. A constant tube potential reduction can be achieved for adults and children, while maintaining a constant detector energy fluence. To evaluate radiation dose, the weighted CT dose index (CTDIw) was calculated based on the CT dose index (CTDI) measured using an ion chamber, and image noise and image contrast were measured from a scanned image to evaluate image quality. The dose-weighted contrast-to-noise ratio (CNRD) was calculated from the radiation dose, image noise, and image contrast measured from a scanned image. The noise derivative (ND) is a quality index for dose efficiency. X-ray spectra with tube voltages ranging from 80 to 140 kVp were used to compute the average photon energy. Image contrast and the corresponding contrast-to-noise ratio (CNR) were determined for lesions of soft tissue, muscle, bone, and iodine relative to a uniform water background, as the iodine contrast increases at lower energy (i.e., k-edge of iodine is 33 keV closer to the beam energy) using mixed water-iodine contrast normalization (water 0, iodine 25, 100, 200, and 1000 HU, respectively). The proposed values correspond to high quality images and can be reduced if only high-contrast organs are assessed. The potential benefit of lowering the tube voltage is an improved CNRD, resulting in a lower radiation dose and optimization of image quality. Adjusting the tube potential in abdominal CT would be useful in current pediatric radiography, where the choice of X-ray techniques generally takes into account the size of the patient as well as the need to balance the conflicting requirements of diagnostic image quality and radiation dose optimization.

Development of virtual monochromatic imaging technique with spectral CT based on a photon-counting detector

With the advent of the coherent age the implementation of massive digital signal processors (DSP) co-integrated with high speed AD and DA converters became feasible allowing for the realization of huge flexibility of transponders. Today the implementation of variable transponders is mainly based on variable programming of DSP to support different modulation formats and symbol rates. Modulation formats with high flexibility are required such as pragmatic QAM formats and hybrid modulation formats. Furthermore, we report on an implementable probabilistically shaping technique allowing for adjusting the bitrate. We introduce fundamental characteristics of all modes and describe basic operation principles. The modifications of the operational modes are enabled simply by switching between different formats and symbol rates in the DSP to adjust the transponders spectral efficiency, the bitrate and the maximum transmission distance. A fine granularity in bitrate and in maximum transmission distance can be implemented especially by hybrid formats and by probabilistically shaped formats. Furthermore, latter allow for ~+25% increase of the maximum transmission distance due their operation close to the Shannon limit as a consequence of their 2D Gaussian like signal nature. If the flexibility and programmability of transponders is implemented, it can be utilized to support different strategies for the application. The variability in symbol rate is mainly translated into variability in bitrate and in bandwidth consumption. Contrary the variable spectral efficiency translates into a variation of the maximum transmission reach and of the bitrate. A co-adjustment of both options will lead to a superior flexibility of optical transponders to address all requirements from application, transponder and fiber infrastructure perspective.

Experimental feasibility of multi-material decomposition imaging in small animal SPECT/CT system

The photon counting detectors such as cadmium zinc telluride (CZT) and cadmium telluride (CdTe) have powerful advantages compared to energy integrating detectors. CZT or CdTe can detect individual gamma-ray or x-ray photon with energy discrimination. In recent years, energy-resolving and material decomposition x-ray imaging based on photon counting detectors has attracted attention from biomedical imaging researchers. We evaluated a large-area (20 cm × 20 cm) CZT detector originally built as a radionuclide detector for a small animal SPECT/CT system in combination of a microfocus x-ray source with a general goal of developing a material-decomposition imaging method. In this paper, we present experimental results from a feasibility study of multi-material decomposition imaging using the developed small animal SPECT/CT system. Our small animal SPECT/CT system has the unique capability to arrange detectors and sources flexibly. For the multi-material decomposition scan, the CZT detector and the x-ray tube were re-arranged in line. List-mode data of a phantom containing 7 different materials, with a range of densities and atomic numbers, illuminated by x-ray were acquired. The x-ray exposure conditions were: 50 kVp and 0.5 mA with 1.5-mm Al filtration. The acquired image was corrected for bad and hot pixels, gain, and offset. The attenuation profiles of each material were calculated against the x-ray energy. Measured attenuation profiles of each material were in a good agreement with the reference data of the NIST physics laboratory. In this study, we demonstrated that multi-material decomposition imaging is experimentally feasible using the photon-counting CZT detector and polychromatic x-ray. Since our system allows rotation and a large active area of CZT, we will acquire material-decomposition data tomographically in the near future.