Achieving reliable coincidence resolving time measurement of PET detectors using multichannel waveform digitizer based on DRS4 chip

Park, Haewook,Lee, Seungeun,Ko, Guen Bae,Lee, Jae Sung Institute of Physics in association with the Ameri 2018 Physics in medicine & biology Vol.63 No.24

<P>Coincidence resolving time (CRT) is one of the most important physical-performance measures for positron emission tomography (PET), as reconstruction with accurate time-of-flight information enhances the lesion detectability in patient studies. Accordingly, various PET detector designs and high-performance front-end readout circuits have been actively investigated to improve timing performance. The resulting PET detectors are often evaluated using multichannel waveform digitizers for versatile data analysis of the output signals. However, we have found that inappropriate data acquisition (DAQ) using a multichannel waveform digitizer based on the domino-ring-sampler 4 (DRS4) chip can lead to a considerable error when determining CRT. To address this issue, we performed CRT measurements using a pair of Hamamatsu R9800 photomultiplier tube based PET detectors. Then, considering intra- and inter-chip sampling, we employed four different combinations of input channels into the CAEN DT5742B waveform digitizer and obtained 2D CRT maps according to the leading-edge discriminator threshold for assessing each DAQ scheme. The intra-chip CRT measurement exhibited unusual streak patterns in the 2D CRT map and yielded the artificially-low CRT information in PET detector pairs, whereas the inter-chip CRT measurement provided the reliable estimation of timing resolution. Further, we could prevent the high-frequency signal crosstalk among input channels within the DRS4 chip using the inter-chip CRT measurement. We expect that our findings will also be useful for achieving the reliable CRT measurements when using other single-chip-based multichannel waveform digitizers.</P>

Reconstruction of conductivity using the dual-loop method with one injection current in MREIT

Lee, Tae Hwi,Nam, Hyun Soo,Lee, Min Gi,Kim, Yong Jung,Woo, Eung Je,Kwon, Oh In Institute of Physics in association with the Ameri 2010 Physics in medicine & biology Vol.55 No.24

<P>Magnetic resonance electrical impedance tomography (MREIT) is to visualize the internal current density and conductivity of an electrically conductive object. Injecting current through surface electrodes, we measure one component of the induced internal magnetic flux density using an MRI scanner. In order to reconstruct the conductivity distribution inside the imaging object, most algorithms in MREIT have required multiple magnetic flux density data by injecting at least two independent currents. In this paper, we propose a direct method to reconstruct the internal isotropic conductivity with one component of magnetic flux density data by injecting one current into the imaging object through a single pair of surface electrodes. Firstly, the proposed method reconstructs a projected current density which is a uniquely determined current from the measured one-component magnetic flux density. Using a relation between voltage potential and current, based on Kirchhoff's voltage law, the proposed method is designed to use a combination of two loops around each pixel from which to derive an implicit matrix system for determination of the internal conductivity. Results from numerical simulations demonstrate that the proposed algorithm stably determines the conductivity distribution in an imaging slice. We compare the reconstructed internal conductivity distribution using the proposed method with that using a conventional method with agarose gel phantom experiments.</P>

Evaluation of smoothing in an iterative <i>l_p</i>-norm minimization algorithm for surface-based source localization of MEG

Han, Jooman,Kim, June Sic,Chung, Chun Kee,Park, Kwang Suk Institute of Physics in association with the Ameri 2007 Physics in medicine & biology Vol.52 No.16

<P>The imaging of neural sources of magnetoencephalographic data based on distributed source models requires additional constraints on the source distribution in order to overcome ill-posedness and obtain a plausible solution. The minimum <I>l<SUB>p</SUB></I> norm (0 < <I>p</I> ⩽ 1) constraint is known to be appropriate for reconstructing focal sources distributed in several regions. A well-known recursive method for solving the <I>l<SUB>p</SUB></I>-norm minimization problem, for example, is the focal underdetermined system solver (FOCUSS). However, this iterative algorithm tends to give spurious sources when the noise level is high. In this study, we present an algorithm to incorporate a smoothing technique into the FOCUSS algorithm and test different smoothing kernels in a surface-based cortical source space. Simulations with cortical source patches assumed in auditory areas show that the incorporation of the smoothing procedure improves the performance of the FOCUSS algorithm, and that using the geodesic distance for constructing a smoothing kernel is a better choice than using the Euclidean one, particularly when employing a cortical source space. We also apply these methods to a real data set obtained from an auditory experiment and illustrate their applicability to realistic data by presenting the reconstructed source images localized in the superior temporal gyrus.</P>

Optimization of multiply acquired magnetic flux density <i>B_z</i> using ICNE-Multiecho train in MREIT

Nam, Hyun Soo,Kwon, Oh In Institute of Physics in association with the Ameri 2010 Physics in medicine & biology Vol.55 No.9

<P>The aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the electrical properties, conductivity or current density of an object by injection of current. Recently, the prolonged data acquisition time when using the injected current nonlinear encoding (ICNE) method has been advantageous for measurement of magnetic flux density data, <I>Bz</I>, for MREIT in the signal-to-noise ratio (SNR). However, the ICNE method results in undesirable side artifacts, such as blurring, chemical shift and phase artifacts, due to the long data acquisition under an inhomogeneous static field. In this paper, we apply the ICNE method to a gradient and spin echo (GRASE) multi-echo train pulse sequence in order to provide the multiple <I>k</I>-space lines during a single RF pulse period. We analyze the SNR of the measured multiple <I>B<SUB>z</SUB></I> data using the proposed ICNE-Multiecho MR pulse sequence. By determining a weighting factor for <I>B<SUB>z</SUB></I> data in each of the echoes, an optimized inversion formula for the magnetic flux density data is proposed for the ICNE-Multiecho MR sequence. Using the ICNE-Multiecho method, the quality of the measured magnetic flux density is considerably increased by the injection of a long current through the echo train length and by optimization of the voxel-by-voxel noise level of the <I>B<SUB>z</SUB></I> value. Agarose-gel phantom experiments have demonstrated fewer artifacts and a better SNR using the ICNE-Multiecho method. Experimenting with the brain of an anesthetized dog, we collected valuable echoes by taking into account the noise level of each of the echoes and determined <I>B<SUB>z</SUB></I> data by determining optimized weighting factors for the multiply acquired magnetic flux density data.</P>

A technique to consider mismatches between fMRI and EEG/MEG sources for fMRI-constrained EEG/MEG source imaging: a preliminary simulation study

Im, Chang-Hwan,Lee, Soo Yeol Institute of Physics in association with the Ameri 2006 Physics in medicine & biology Vol.51 No.23

<P>fMRI-constrained EEG/MEG source imaging can be a powerful tool in studying human brain functions with enhanced spatial and temporal resolutions. Recent studies on the combination of fMRI and EEG/MEG have suggested that fMRI prior information could be readily implemented by simply imposing different weighting factors to cortical sources overlapping with the fMRI activations. It has been also reported, however, that such a hard constraint may cause severe distortions or elimination of meaningful EEG/MEG sources when there are distinct mismatches between the fMRI activations and the EEG/MEG sources. If one wants to obtain the actual EEG/MEG source locations and uses the fMRI prior information as just an auxiliary tool to enhance focality of the distributed EEG/MEG sources, it is reasonable to weaken the strength of fMRI constraint when severe mismatches between fMRI and EEG/MEG sources are observed. The present study suggests an efficient technique to automatically adjust the strength of fMRI constraint according to the mismatch level. The use of the proposed technique rarely affects the results of conventional fMRI-constrained EEG/MEG source imaging if no major mismatch between the two modalities is detected; while the new results become similar to those of typical EEG/MEG source imaging without fMRI constraint if the mismatch level is significant. A preliminary simulation study using realistic EEG signals demonstrated that the proposed technique can be a promising tool to selectively apply fMRI prior information to EEG/MEG source imaging.</P>

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

Jung, Jin Ho,Choi, Yong,Hong, Key Jo,Min, Byung Jun,Choi, Joon Young,Choe, Yearn Seong,Lee, Kyung-Han,Kim, Byung-Tae Institute of Physics in association with the Ameri 2009 Physics in medicine & biology Vol.54 No.14

<P>Several groups have reported the development of dual modality Gamma camera/optical imagers, which are useful tools for investigating biological processes in experimental animals. While previously reported dual modality imaging instrumentation usually employed a separated gamma camera and optical imager, we designed a detector using a position sensitive photomultiplier tube (PSPMT) that is capable of imaging both gamma rays and optical photons for combined gamma camera and optical imager. The proposed system consists of a parallel-hole collimator, an array-type crystal and a PSPMT. The top surface of the collimator and array crystals is left open to allow optical photons to reach the PSPMT. Pulse height spectra and planar images were obtained using a Tc-99m source and a green LED to estimate gamma and optical imaging performances. When both gamma rays and optical photon signals were detected, the signal interferences caused by each other signal were evaluated. A mouse phantom and an ICR mouse containing a gamma ray and optical photon source were imaged to assess the imaging capabilities of the system. The sensitivity, energy resolution and spatial resolution of the gamma image acquired using Tc-99m were 1.1 cps/kBq, 26% and 2.1 mm, respectively. The spatial resolution of the optical image acquired with an LED was 3.5 mm. Signal-to-signal interference due to the optical photon signal in the gamma pulse height spectrum was negligible. However, the pulse height spectrum of the optical photon signal was found to be affected by the gamma signal, and was obtained between signals generated by gamma rays with a correction using a veto gate. Gamma ray and optical photon images of the mouse phantom and ICR mouse were successfully obtained using the single detector. The experimental results indicated that both optical photon and gamma ray imaging are feasible using a detector based on the proposed PSPMT.</P>

Compact soft x-ray transmission microscopy with sub-50 nm spatial resolution

Kim, Kyong Woo,Kwon, Youngman,Nam, Ki-Yong,Lim, Jong-Hyeok,Kim, Kyu-Gyum,Chon, Kwon Su,Kim, Byoung Hoon,Kim, Dong Eon,Kim, JinGon,Ahn, Byoung Nam,Shin, Hyun Joon,Rah, Seungyu,Kim, Ki-Ho,Chae, Jin Seok Institute of Physics in association with the Ameri 2006 Physics in medicine & biology Vol.51 No.6

<P>In this paper, the development of compact transmission soft x-ray microscopy (XM) with sub-50 nm spatial resolution for biomedical applications is described. The compact transmission soft x-ray microscope operates at λ = 2.88 nm (430 eV) and is based on a tabletop regenerative x-ray source in combination with a tandem ellipsoidal condenser mirror for sample illumination, an objective micro zone plate and a thinned back-illuminated charge coupled device to record an x-ray image. The new, compact x-ray microscope system requires the fabrication of proper x-ray optical devices in order to obtain high-quality images. For an application-oriented microscope, the alignment procedure is fully automated via computer control through a graphic user interface. In imaging studies using our compact XM system, a gold mesh image was obtained with 45 nm resolution at ?580 magnification and 1 min exposure. Images of a biological sample (<I>Coscinodiscus oculoides</I>) were recorded.</P>

Conductivity imaging with low level current injection using transversal <i>J</i>-substitution algorithm in MREIT

Nam, Hyun Soo,Lee, Byung Il,Choi, Jongsung,Park, Chunjae,Kwon, Oh In Institute of Physics in association with the Ameri 2007 Physics in medicine & biology Vol.52 No.22

<P>An aim of magnetic resonance electrical impedance tomography (MREIT) is to visualize the internal current density and conductivity of the electrically imaged object by injecting current through electrodes attached to it. Due to a limited amount of injection current, one of the most important factors in MREIT is how to control the noise contained in the measured magnetic flux density data. This paper describes a new iterative algorithm called the transversal <I>J</I>-substitution algorithm which is robust to measured noise. As a result, the proposed transversal <I>J</I>-substitution algorithm considerably improves the quality of the reconstructed conductivity image under a low injection current. The relation between the reconstructed contrast of conductivity and the measured noise in the magnetic flux density is analyzed. We show that the contrast of first update of the conductivity with a homogeneous initial guess using the proposed algorithm has sufficient distinguishability to detect the anomaly. Results from numerical simulations demonstrate that the transversal <I>J</I>-substitution algorithm is robust to the noise. For practical implementations of MREIT, we tested real experiments in an agarose gel phantom using low current injection with amplitudes 1 mA and 5 mA to reconstruct the interior conductivity distribution.</P>

Fully three-dimensional OSEM-based image reconstruction for Compton imaging using optimized ordering schemes

Kim, Soo Mee,Lee, Jae Sung,Lee, Chun Sik,Kim, Chan Hyeong,Lee, Myung Chul,Lee, Dong Soo,Lee, Soo-Jin Institute of Physics in association with the Ameri 2010 Physics in medicine & biology Vol.55 No.17

<P>Although the ordered subset expectation maximization (OSEM) algorithm does not converge to a true maximum likelihood solution, it is known to provide a good solution if the projections that constitute each subset are reasonably balanced. The Compton scattered data can be allocated to subsets using scattering angles (SA) or detected positions (DP) or a combination of the two (AP (angles and positions)). To construct balanced subsets, the data were first arranged using three ordering schemes: the random ordering scheme (ROS), the multilevel ordering scheme (MLS) and the weighted-distance ordering scheme (WDS). The arranged data were then split into <I>J</I> subsets. To compare the three ordering schemes, we calculated the coefficients of variation (CVs) of angular and positional differences between the arranged data and the percentage errors between mathematical phantoms and reconstructed images. All ordering schemes showed an order-of-magnitude acceleration over the standard EM, and their computation times were similar. The SA-based MLS and the DP-based WDS led to the best-balanced subsets (they provided the largest angular and positional differences for SA- and DP-based arrangements, respectively). The WDS exhibited minimum CVs for both the SA- and DP-based arrangements (the deviation in mean angular and positional differences between the ordered subsets was smallest). The combination of AP and WDS yielded the best results with the lowest percentage errors by providing larger and more uniform angular and positional differences for the SA and DP arrangements, and thus, is probably optimal Compton camera reconstruction using OSEM.</P>

Design and simulation of a novel method for determining depth-of-interaction in a PET scintillation crystal array using a single-ended readout by a multi-anode PMT

Ito, Mikiko,Lee, Jae Sung,Park, Min-Jae,Sim, Kwang-Souk,Hong, Seong Jong Institute of Physics in association with the Ameri 2010 Physics in medicine & biology Vol.55 No.13

<P>PET detectors with depth-of-interaction (DOI) encoding capability allow high spatial resolution and high sensitivity to be achieved simultaneously. To obtain DOI information from a mono-layer array of scintillation crystals using a single-ended readout, the authors devised a method based on light spreading within a crystal array and performed Monte Carlo simulations with individual scintillation photon tracking to prove the concept. A scintillation crystal array model was constructed using a grid method. Conventional grids are constructed using comb-shaped reflector strips with rectangular teeth to isolate scintillation crystals optically. However, the authors propose the use of triangularly shaped teeth, such that scintillation photons spread only in the <I>x</I>-direction in the upper halves of crystals and in the <I>y</I>-direction in lower halves. DOI positions can be estimated by considering the extent of two-dimensional light dispersion, which can be determined from the multiple anode outputs of a position-sensitive PMT placed under the crystal array. In the main simulation, a crystal block consisting of a 29 × 29 array of 1.5 mm × 1.5 mm × 20 mm crystals and a multi-anode PMT with 16 × 16 pixels were used. The effects of crystal size and non-uniform PMT output gain were also explored by simulation. The DOI resolution estimated for 1.5 × 1.5 × 20 mm<SUP>3</SUP> crystals was 2.16 mm on average. Although the flood map was depth dependent, each crystal was well identified at all depths when a corner of the crystal array was irradiated with 511 keV gamma rays (peak-to-valley ratio ∼9:1). DOI resolution was better than 3 mm up to a crystal length of 28 mm with a 1.5 × 1.5 mm<SUP>2</SUP> or 2.0 × 2.0 mm<SUP>2</SUP> crystal surface area. The devised light-sharing method allowed excellent DOI resolutions to be obtained without the use of dual-ended readout or multiple crystal arrays.</P>