Quantitative Imaging of Alpha-Emitting Therapeutic Radiopharmaceuticals / 대한핵의학회잡지

Targeted alpha therapy (TAT) is an active area of drug development as a highly specific and highly potent therapeutic modality that can be applied to many types of late-stage cancers. In order to properly evaluate its safety and efficacy, understanding biokinetics of alpha-emitting radiopharmaceuticals is essential. Quantitative imaging of alpha-emitting radiopharmaceuticals is often possible via imaging of gammas and positrons produced during complex decay chains of these radionuclides. Analysis of the complex decay chains for alpha-emitting radionuclides (Tb-149, At-211, Bi-212 (decayed from Pb-212), Bi-213, Ra-223, Ac- 225, and Th-227) with relevance to imageable signals is attempted in this mini-review article. Gamma camera imaging, single-photon emission computed tomography, positron emission tomography, bremsstrahlung radiation imaging, Cerenkov luminescence imaging, and Compton cameras are briefly discussed as modalities for imaging alpha-emitting radiopharmaceuticals.

An inversion of the conical Radon transform arising in the Compton camera with helical movement

Since the Compton camera was fi rst introduced, various types of conical Radon transforms have been examined. Here, we derive the inversion formula for the conical Radon transform, where the cone of integration moves along a curve in three-dimensional space such as a helix. Along this three-dimensional curve, a detailed inversion formula for helical movement will be treated for Compton imaging in this paper. The inversion formula includes Hilbert transform and Radon transform. For the inversion of Compton imaging with helical movement, it is necessary to invert Hilbert transform with respect to the inner product between the vertex and the central axis of the cone of the Compton camera. However, the inner product function is not monotone. Thus, we should replace the Hilbert transform by the Riemann–Stieltjes integral over a certain monotone function related with the inner product function. We represent the Riemann–Stieltjes integral as a conventional Riemann integral over a countable union of disjoint intervals, whose end points can be computed using the Newton method. For the inversion of Radon transform, three dimensional fi ltered backprojection is used. For the numerical implementation, we analytically compute the Hilbert transform and Radon transform of the characteristic function of fi nite balls. Numerical test is given, when the density function is given by a characteristic function of a ball or three overlapping balls.

Analytic simulator and image generator of multiple-scattering Compton camera for prompt gamma ray imaging

For prompt gamma ray imaging for biomedical applications and environmental radiation monitoring, we propose herein a multiple-scattering Compton camera (MSCC). MSCC consists of three or more semiconductor layers with good energy resolution, and has potential for simultaneous detection and differentiation of multiple radio-isotopes based on the measured energies, as well as three-dimensional (3D) imaging of the radio-isotope distribution. In this study, we developed an analytic simulator and a 3D image generator for a MSCC, including the physical models of the radiation source emission and detection processes that can be utilized for geometry and performance prediction prior to the construction of a real system. The analytic simulator for a MSCC records coincidence detections of successive interactions in multiple detector layers. In the successive interaction processes, the emission direction of the incident gamma ray, the scattering angle, and the changed traveling path after the Compton scattering interaction in each detector, were determined by a conical surface uniform random number generator (RNG), and by a Klein-Nishina RNG. The 3D image generator has two functions: the recovery of the initial source energy spectrum and the 3D spatial distribution of the source. We evaluated the analytic simulator and image generator with two different energetic point radiation sources (Cs-137 and Co-60) and with an MSCC comprising three detector layers. The recovered initial energies of the incident radiations were well differentiated from the generated MSCC events. Correspondingly, we could obtain a multi-tracer image that combined the two differentiated images. The developed analytic simulator in this study emulated the randomness of the detection process of a multiple-scattering Compton camera, including the inherent degradation factors of the detectors, such as the limited spatial and energy resolutions. The Doppler-broadening effect owing to the momentum distribution of electrons in Compton scattering was not considered in the detection process because most interested isotopes for biomedical and environmental applications have high energies that are less sensitive to Doppler broadening. The analytic simulator and image generator for MSCC can be utilized to determine the optimal geometrical parameters, such as the distances between detectors and detector size, thus affecting the imaging performance of the Compton camera prior to the development of a real system.

FPGA-Based Interface of Digital DAQ System for Double-Scattering Compton Camera / 대한핵의학회잡지

PURPOSE: The double-scattering Compton camera (DSCC) is a radiation imaging system that can provide both unknown source energy spectra and 3D spatial source distributions. The energies and detection locations measured in coincidence with three CdZnTe (CZT) detectors contribute to reconstructing emission energies and a spatial image based on conical surface integrals. In this study, we developed a digital data acquisition (DAQ) board to support our research into coincidence detection in the DSCC.METHODS: The main components of the digital DAQ board were 12 ADCs and one field programmable gate array (FPGA). The ADCs digitized the analog 96-channel CZTsignals at a sampling rate of 50MHz and transferred the serialized ADC samples and the bit and frame clocks to the FPGA. In order to correctly capture the ADC sample bits in the FPGA, we conducted individual sync calibrations for all the ADC channels to align the bit and frame clocks to the right positions of the ADC sample bits. The FPGA logic design was composed of IDELAYand IDDR components, six shift registers, and bit slip buffer resources.RESULTS: Using a Deskew test pattern, the delay value of the IDELAY component was determined to align the bit clock to the center of each sample bit.We determined the bit slip in the 12-bit ADC sample using an MSB test pattern by checking where the MSB value of one is located in the captured parallel data.CONCLUSION: After sync calibration, we tested the interface between the ADCs and the FPGA with a synthetic analog Gaussian signal. The 96 ADC channels yielded a mean R2 goodness-of-fit value of 0.95 between the Gaussian curve and the captured 12-bit parallel data.

Preliminary Study on Performance Evaluation of a Stacking-structure Compton Camera by Using Compton Imaging Simulator / 의학물리

A Compton camera, which is based on the geometrical interpretation of Compton scattering, is a very promising gamma-ray imaging device considering its several advantages over the conventional gamma-ray imaging devices: high imaging sensitivity, 3-D imaging capability from a fixed position, multi-tracing functionality, and almost no limitation in photon energy. In the present study, a Monte Carlo-based, user-friendly Compton imaging simulator was developed in the form of a graphical user interface (GUI) based on Geant4 and MATLAB (TM). The simulator was tested against the experimental result of the double-scattering Compton camera, which is under development at Hanyang University in Korea. The imaging resolution of the simulated Compton image well agreed with that of the measured image. The imaging sensitivity of the measured data was 2~3 times higher than that of the simulated data, which is due to the fact that the measured data contains the random coincidence events. The performance of a stacking-structure type Compton camera was evaluated by using the simulator. The result shows that the Compton camera shows its highest performance when it uses 4 layers of scatterer detectors.

A Comparative Study of Subset Construction Methods in OSEM Algorithms using Simulated Projection Data of Compton Camera / 핵의학분자영상

PURPOSE: In this study we propose a block-iterative method for reconstructing Compton scattered data. This study shows that the well-known expectation maximization (EM) approach along with its accelerated version based on the ordered subsets principle can be applied to the problem of image reconstruction for Compton camera. This study also compares several methods of constructing subsets for optimal performance of our algorithms. MATERIALS AND METHODS: Three reconstruction algorithms were implemented; simple backprojection (SBP), EM, and ordered subset EM (OSEM). For OSEM, the projection data were grouped into subsets in a predefined order. Three different schemes for choosing nonoverlapping subsets were considered; scatter angle-based subsets, detector position-based subsets, and both scatter angle- and detector position-based subsets. EM and OSEM with 16 subsets were performed with 64 and 4 iterations, respectively. The performance of each algorithm was evaluated in terms of computation time and normalized mean-squared error. RESULTS: Both EM and OSEM clearly outperformed SBP in all aspects of accuracy. The OSEM with 16 subsets and 4 iterations, which is equivalent to the standard EM with 64 iterations, was approximately 14 times faster in computation time than the standard EM. In OSEM, all of the three schemes for choosing subsets yielded similar results in computation time as well as normalized mean-squared error. CONCLUSION: Our results show that the OSEM algorithm, which have proven useful in emission tomography, can also be applied to the problem of image reconstruction for Compton camera. With properly chosen subset construction methods and moderate numbers of subsets, our OSEM algorithm significantly improves the computational efficiency while keeping the original quality of the standard EM reconstruction. The OSEM algorithm with scatter angle- and detector position-based subsets is most available.