Young Bok Ahn

Methods and evaluations of MRI content-adaptive finite element mesh generation for bioelectromagnetic problems

In studying bioelectromagnetic problems, finite element analysis (FEA) offers several advantages over conventional methods such as the boundary element method. It allows truly volumetric analysis and incorporation of material properties such as anisotropic conductivity. For FEA, mesh generation is the first critical requirement and there exist many different approaches. However, conventional approaches offered by commercial packages and various algorithms do not generate content-adaptive meshes (cMeshes), resulting in numerous nodes and elements in modelling the conducting domain, and thereby increasing computational load and demand. In this work, we present efficient content-adaptive mesh generation schemes for complex biological volumes of MR images. The presented methodology is fully automatic and generates FE meshes that are adaptive to the geometrical contents of MR images, allowing optimal representation of conducting domain for FEA. We have also evaluated the effect of cMeshes on FEA in three dimensions by comparing the forward solutions from various cMesh head models to the solutions from the reference FE head model in which fine and equidistant FEs constitute the model. The results show that there is a significant gain in computation time with minor loss in numerical accuracy. We believe that cMeshes should be useful in the FEA of bioelectromagnetic problems.

Ballistocardiogram artifact removal from EEG signals using adaptive filtering of EOG signals

We estimated ballistocardiogram (BCG) components in EEG signals recorded inside an MRI magnet using the electro-oculogram (EOG) signals recorded simultaneously with the EEG signals. Since the EOG signals are measured near the EEG measuring points, it is thought that the BCG components in the EOG signals resemble the BCG components in the EEG signals. To estimate the BCG components in the EEG signals, we applied the Kalman filter to the EOG and EEG signals recorded inside a 3.0 T MRI magnet. After removing the estimated BCG components from the EEG signals, we extracted the visual-evoked potentials (VEPs) from the BCG-removed EEG signals. To validate the efficacy of Kalman filtering in the BCG artifact removal, we have compared three types of VEPs of eight healthy subjects: one extracted from the raw EEG signals measured outside the magnet and the others extracted from the BCG-removed EEG signals measured inside the magnet. The BCG artifacts have been removed with Kalman filtering as well as with the conventional BCG template subtraction method for the sake of comparison. No significant difference in waveforms, latencies and amplitudes has been found between the two types of VEPs extracted from the two kinds of BCG-removed EEG signals.

A prototype hand-held tri-modal instrument for in vivo ultrasound, photoacoustic, and fluorescence imaging

Multi-modality imaging is beneficial for both preclinical and clinical applications as it enables complementary information from each modality to be obtained in a single procedure. In this paper, we report the design, fabrication, and testing of a novel tri-modal in vivo imaging system to exploit molecular/functional information from fluorescence (FL) and photoacoustic (PA) imaging as well as anatomical information from ultrasound (US) imaging. The same ultrasound transducer was used for both US and PA imaging, bringing the pulsed laser light into a compact probe by fiberoptic bundles. The FL subsystem is independent of the acoustic components but the front end that delivers and collects the light is physically integrated into the same probe. The tri-modal imaging system was implemented to provide each modality image in real time as well as co-registration of the images. The performance of the system was evaluated through phantom and in vivo animal experiments. The results demonstrate that combining the modalities does not significantly compromise the performance of each of the separate US, PA, and FL imaging techniques, while enabling multi-modality registration. The potential applications of this novel approach to multi-modality imaging range from preclinical research to clinical diagnosis, especially in detection/localization and surgical guidance of accessible solid tumors.

Optimal Optical Conditions and Positioning Scheme for an Ultrahigh-Resolution Silicon Drift Detector-Based Gamma Camera

In this study, we optimized the optical conditions and associated positioning scheme for an ultrahighspatial-resolution, solid-state gamma detector. The detector module consisted of an array of seven hexagonal silicon drift detectors (SDDs) packed hexagonally and coupled to a single slab of crystal via a light guide glass. Because the optical behavior and requirements of the detector module and noise characteristics of the SDD sensor are different from those of conventional photomultiplier tube (PMT)-based detectors, the following parameters were studied to determine the optimum condition: scintillator selection, the effect of cooling on signal-to-noise ratio (SNR), the depth dependence of the scintillation light distribution, and optimum shaping time. To that end, a modified, Anger-style positioning algorithm with a denoise scheme was also developed to address the estimation bias (pincushion distortion) caused by the excessively confined light distribution and the leakage current induced by the SDD sensor. The results of this study proved that the positioning algorithm, together with the optimized optical configuration of the detector module, improves the positioning accuracy of the prototype detector. Our results confirmed the ability of the prototype to achieve a spatial resolution of about 0.7mm in full width at half maximum (FWHM) for 122 keV gamma rays under the equivalent noise count (ENC) of 100 (e- rms) per SDD channel. The results also confirmed NaI(Tl) to be a more desirable scintillator for our prototype with an energy resolution performance of about 8%.

Imaging of Thermally Ablated Tissue Using Ultrasonic Elastography

High intensity focused ultrasound (HIFU) is a promising noninvasive technique that thermally ablates tumors lying deep in the tissue, but the extent of thermal necrosis is difficult to quantify with current B-mode imaging techniques. Utilizing the fact that necrotic tissue is stiffer than normal tissue, we propose a new ultrasound elastographic imaging method to assess the progress of HIFU treatment. The method applies a mechanical compression to tissue being examined by pushing with a transducer, acquires its B-mode images freehand, and computes the phase difference between preand postcompression complex baseband echoes as an indicator of tissue stiffness. Being able to detect small displacements, the proposed method is implemented in real time in a commercial diagnostic ultrasonic scanner. It is capable of producing strain images at a rate of up to 24 frames/s. The experimental results show that the method is a viable technique to monitor the change in stiffness of tissue under treatment with HIFU. Introduction In this paper the progress of HIFU tissue treatment is investigated using an ultrasonic elastography. A lot of research has been done on quantitative ultrasonic imaging using attenuation coefficient, nonlinear parameter, inverse scattering, etc. Despite massive efforts, so far none of these imaging approaches have been a great success. More than a decade ago, an imaging method termed ultrasonic elastography was introduced by Ophir et al. [1] to diagnose cancerous tissues which had been difficult to image with conventional B-mode imaging techniques. The elastography falls into two categories: one is quantitative elastography which determines stress, strain and elasticity distribution of an object being imaged [2,3], and the other is rather qualitative elastography which estimates only strain for practical purposes [4]. Elastography was shown to be a modality that can provide information about the size and location of tissue thermal lesions by Stafford et al. [5]. The measured lesion sizes correlated well with the gross pathologic findings. By varying ultrasound treatment intensity levels and exposure times to obtain lesions of different sizes and performing elastographic imaging, Righetti et al. [6] showed that elastography can accurately depict the size and position of HIFU induced lesions and that it may be a reliable method of estimating and characterizing them. It was shown by Varghese et al. [7] that ultrasound based in vivo elastography can accurately depict the zone of necrosis following radio frequency ablation, providing clinicians with valuable feedback that would decrease local recurrence rates after radio frequency ablative therapy. Doyley et al. [8] reported the result of a preliminary study that lesions in liver tissue produced by focused ultrasound surgery were clearly visible in elastographic images and that the images had a better contrast than other ultrasonic imaging techniques. Kallel et al. [9] demonstrated that reversible changes in tissue elastic properties were induced by low power energy deposition and that they could be detected by elastography. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2042-2047 doi:10.4028/www.scientific.net/KEM.270-273.2042

Unmatched projector/backprojector pair for demultiplexing in multipinhole emission computed tomography

Statistically based iterative algorithms such as maximum likelihood-expectation maximization (ML-EM) are used for image reconstruction in single photon emission computed tomography (SPECT). Unmatched projector/backprojector pairs are sometimes used to accelerate the iteration process in the reconstruction algorithm. In this work, we propose and explore the use of an unmatched projector/backprojector pair for demultiplexing in multipinhole SPECT. Several simulations are conducted to evaluate the performance of the proposed method with uniform, hot-rod, and cold-rod phantoms. The proposed method incorporates an unmatched backprojector to utilize selective multiplexed projection data in reconstruction algorithms, while the projector is modeled as accurately as possible to represent realistic imaging geometry and the physical effects of multipinhole SPECT. The root mean square (rms) error and backprojection speed are evaluated to determine an unmatched backprojector. Our results demonstrate that the proposed method provides high-quality multipinhole SPECT images without multiplexing-related artifacts when a well-chosen unmatched backprojector is used.

Performance Characterization of a Silicon Drift Detector for Gamma Ray Imaging

This study examined the intrinsic performance of silicon drift detector (SDD)-based gamma detectors under a variety of conditions. The prototype detector consisted of an array of seven hexagon-shaped SDDs optically coupled to a single slab of a scintillator. The active area of the SDD sensor was 15.2 mm in diameter, as measured from one vertex to another. The detector unit (SDD array, scintillator and preamplifier circuits) was operated in a cooling chamber with a typical operating temperature of -20°C. Nitrogen gas was supplied to the detector unit to prevent condensation. The drift time was measured using a LED pulse generation device and the longest drift time was measured to be 4.6 μsec from the edge of the sensor. The intrinsic energy resolution with a BBFe source for direct X-ray conversion was 3% at the 5.9 keV peak. For indirect conversion, i.e. photon detection, the energy resolution for CsI(Tl) and Nal(Tl) was 7.9% and 8.2% with a 13 μsec and 2.71 μsec shaping time, respectively. For this indirect conversion measurement, the temperature was set to -20°C and a 1 × 1 × 1 cm3 cube scintillator was coupled directly to the sensor. For the intrinsic spatial resolution measurement with a hole-phantom (3 × 2 mm diameter holes), the x and y directional profiles at a center hole were 2.2 and 2.1 mm in FWHM, respectively. Overall, the intrinsic performance of the SDD prototype is quite promising and advantages of this technology makes it highly feasible for use as a gamma ray detector.

A miniature SPECT using multi-pinhole collimator with vertical septa

We had previously reported a simulation study on a multi-pinhole collimator (MP) having lead vertical septa which could provide improved angular sampling and enlarged imaging field of view (FOV) compared to low energy high resolution parallel-hole collimator (LEHR) using same size detector. The aim of this study was to develop a miniature SPECT to verify the performance of the proposed MP. A detector having 70 mm × 70 mm active area was consisted of a 6 mm thick NaI(Tl) crystal coupled to a 127 mm diameter position-sensitive photomultiplier tube (PSPMT). A 7×7 pinhole collimator with 2 mm diameter pinhole and 40 mm focal length was fabricated to evaluate the performance compared to a typical LEHR. Additionally, a detector having 50 mm × 50 mm active area with 5×5 pinhole collimator and LEHR was investigated to evaluate enlarged imaging FOV. Planar spatial resolution, sensitivity, and resolution hot- and cold-rod phantom images were acquired. Images were reconstructed by the use of a dedicated MLEM algorithm with an unmatched projector/backprojector pair. Spatial resolution and sensitivity were 4.7 mm FWHM and 0.25 cps/μCi at 60 mm distance, respectively. Although the detector size was smaller than the phantom size, MP allowed to image entire phantom images while LEHR provided the images with truncation artifact. The reconstructed images using 60 and 30 projections with both 7×7 pinhole collimator and LEHR showed a similar quality image to the image using 120 projections. However, the reconstructed images with 10 projections using 7×7 pinhole collimator showed better quality than those with LEHR. The reconstructed images using the MP provided high quality images with enlarged imaging FOV even when insufficient angular sampling data were used which would be useful to develop a stationary SPECT.

Optics Optimization for a Solid State Gamma Camera Detector Module

The aim of this study is to optimize optics design parameters for a solid state gamma camera detector module. The design parameter includes surface treatments, crystal size, energy threshold effect, light pipe thickness, and fill factor for sensor packing. The solid state gamma camera detector module proposed in this study was consisted of hexagonally packed 72 silicon drift diodes (SDDs) and a single slap of NaI(Tl) crystal with a light pipe that was optically coupled with SDD sensors. Cramer-Rao (CR) lower bound analysis was conducted to optimize optics parameters. Monte Carlo simulation using DETECT2000 was performed to generate necessary data for the CR bound analysis. The preliminary study showed that white reflector (reflection coefficient = 0.9) on the side surface improved the light collection efficiency (LCE) by 84% compared to that of black paint case (reflection coefficient = 0.1) while the spatial resolution was less sensitive to the side surface treatment. The result also suggested no light pipe for the best spatial resolution performance. However, for the mechanical support and encapsulation of the NaI(Tl) scintillator, 2 mm thick light pipe found to be the candidate for the module. The intrinsic spatial resolution with 2% energy threshold was 0.8 mm in FWHM at the useful field-of-view. The results conclude that the scintillation crystal designed in this study was optimum at 106 mm times 112.6 mm size with white reflector side surface and 2 mm light pipe thickness.

An Investigation to Design High Performance Multi-Pinhole Collimator

The aim of this study is to design a detector module employing multi-pinhole collimator which could be utilized in various SPECT systems. Monte Carlo simulations using Geant4 application for tomographic emission (GATE) were performed to estimate the performance of various multi-pinhole collimators and compared to that of low energy high resolution (LEHR) parallel-hole collimator. The simulated detector module was designed to have 100 mm times 100 mm field of view. In order to determine the optimal number of pinholes, the number of pinholes having 2 mm pinhole diameter and 50 mm focal length was varied from 1 to 225. Perpendicular lead septa were employed to acquire non-overlapping projections and to improve reconstructed image quality. The proposed multi-pinhole detector module was evaluated by the reconstructed images of simulated phantoms. Planar sensitivity and resolution obtained using 81-pinhole collimator were 1.9 cps/muCi and 8.7 mm FWHM at 225 mm distance. 81-pinhole configuration demonstrated better resolution and comparable sensitivity compared to LEHR parallel-hole collimator (2.0 cps/muCi and 10.9 mm FWHM). Multi-pinhole detector module could provide the reconstructed images of 120 mm diameter phantoms without truncation while LEHR parallel-hole detector provided the truncated images. The designed multi-pinhole detector module which could be extended to cover large FOV allows to achieve high quality images even with reduced projection data.