U-net based metal segmentation on projection domain for metal artifact reduction in dental CT

Unlike medical computed tomography (CT), dental CT often suffers from severe metal artifacts stemming from high-density materials employed for dental prostheses. Despite the many metal artifact reduction (MAR) methods available for medical CT, those methods do not sufficiently reduce metal artifacts in dental CT images because MAR performance is often compromised by the enamel layer of teeth, whose X-ray attenuation coefficient is not so different from that of prosthetic materials. We propose a deep learning-based metal segmentation method on the projection domain to improve MAR performance in dental CT. We adopted a simplified U-net for metal segmentation on the projection domain without using any information from the metal-artifacts-corrupted CT images. After training the network with the projection data of five patients, we segmented the metal objects on the projection data of other patients using the trained network parameters. With the segmentation results, we corrected the projection data by applying region filling inside the segmented region. We fused two CT images, one from the corrected projection data and the other from the original raw projection data, and then we forward-projected the fused CT image to get the fused projection data. To get the final corrected projection data, we replaced the metal regions in the original projection data with the ones in the fused projection data. To evaluate the efficacy of the proposed segmentation method on MAR, we compared the MAR performance of the proposed segmentation method with a conventional MAR method based on metal segmentation on the CT image domain. For the MAR performance evaluation, we considered the three primary MAR performance metrics: the relative error (REL), the sum of square difference (SSD), and the normalized absolute difference (NAD). The proposed segmentation method improved MAR performances by around 5.7% for REL, 6.8% for SSD, and 8.2% for NAD. The proposed metal segmentation method on the projection domain showed better MAR performance than the conventional segmentation on the CT image domain. We expect that the proposed segmentation method can improve the performance of the existing MAR methods that are based on metal segmentation on the CT image domain.

A bench-top micro-CT capable of simulating head motions

Computational three-dimensional (3D) models of a dental structure generated from 3D dental computed tomography (CT) images are now widely used in digital dentistry. To generate precise 3D models, high-resolution imaging of the dental structure with a dental CT is required. However, a small head motion of the patient during the dental CT scan could degrade the spatial resolution of CT images to the extent that digital dentistry is no longer possible. A bench-top micro-CT has been built to evaluate the head motion effects on the dental CT images. A micro-CT has been built on an optic table with a micro-focus x-ray source and a flat-panel detector. A rotation stage, placed in between the x-ray source and the detector, is mounted on two-directional goniometers that can rotate the rotation stage in two orthogonal directions while the rotation stage is performing the CT scan. The goniometers can make object motions of an arbitrary waveform to simulate head tilting or head nodding. CT images of a phantom have been taken with and without introducing the motions, and the motion effects on the CT images have been evaluated. Object motions parallel to the detector plane have greater effects on the CT images than those against the detector plane. With the bench-top micro-CT, the motion effects have been visually seen at a tiny rotational motion as small as 0.3°. The bench-top micro-CT can be used to evaluate head motion effects on the dental CT images. The projection data, taken with the motion effects, would be used to develop motion artifact correction methods for a high-resolution dental-CT.

A Tool Box to Evaluate the Phased Array Coil Performance Using Retrospective 3D Coil Modeling

PURPOSE: To efficiently evaluate phased array coil performance using a software tool box with which we can make visual comparison of the sensitivity of every coil element between the real experiment and EM simulation. MATERIALS AND METHODS: We have developed a C++- and MATLAB-based software tool called Phased Array Coil Evaluator (PACE). PACE has the following functions: Building 3D models of the coil elements, importing the FDTD simulation results, and visualizing the coil sensitivity of each coil element on the ordinary Cartesian coordinate and the relative coil position coordinate. To build a 3D model of the phased array coil, we used an electromagnetic 3D tracker in a stylus form. After making the 3D model, we imported the 3D model into the FDTD electromagnetic field simulation tool. RESULTS: An accurate comparison between the coil sensitivity simulation and real experiment on the tool box platform has been made through fine matching of the simulation and real experiment with aids of the 3D tracker. In the simulation and experiment, we used a 36-channel helmet-style phased array coil. At the 3D MRI data acquisition using the spoiled gradient echo sequence, we used the uniform cylindrical phantom that had the same geometry as the one in the FDTD simulation. In the tool box, we can conveniently choose the coil element of interest and we can compare the coil sensitivities element-by-element of the phased array coil. CONCLUSION: We expect the tool box can be greatly used for developing phased array coils of new geometry or for periodic maintenance of phased array coils in a more accurate and consistent manner.

Development of a Korean Standard Structural Brain Template in Cognitive Normals and Patients with Mild Cognitive Impairment and Alzheimer's Disease

PURPOSE: To generate a Korean specific brain template, especially in patients with Alzheimer's disease (AD) by optimizing the voxel-based analysis. MATERIALS AND METHODS: Three-dimensional T1-weighted images were obtained from 123 subjects who were 43 cognitively normal subjects and patients with 44 mild cognitive impairment (MCI) and 36 AD. The template and the corresponding aprior maps were created by using the matched pairs approach with considering differences of age, gender and differential diagnosis (DDX). We measured several characteristics in both our and the MNI templates, including in the ventricle size. Also, the fractions of gray matter and white matter voxels normalized by the total intracranial were evaluated. RESULTS: The high resolution template and the corresponding aprior maps of gray matter, white matter (WM) and CSF were created with the voxel-size of 1 x 1 x 1 mm. Mean distance measures and the ventricle sizes differed between two templates. Our brain template had less gray matter and white matter areas than the MNI template. There were volume differences more in gray matter than in white matter. CONCLUSION: Gray matter and/or white matter integrity studies in populations of Korean elderly and patients with AD are needed to investigate with this template.

Mesenchymal stem cells promote proliferation of endogenous neural stem cells and survival of newborn cells in a rat stroke model

Mesenchymal stem cells (MSCs) secrete bioactive factors that exert diverse responses in vivo. In the present study, we explored mechanism how MSCs may lead to higher functional recovery in the animal stroke model. Bone marrow-derived MSCs were transplanted into the brain parenchyma 3 days after induction of stroke by occluding middle cerebral artery for 2 h. Stoke induced proliferation of resident neural stem cells in subventricular zone. However, most of new born cells underwent cell death and had a limited impact on functional recovery after stroke. Transplantation of MSCs enhanced proliferation of endogenous neural stem cells while suppressing the cell death of newly generated cells. Thereby, newborn cells migrated toward ischemic territory and differentiated in ischemic boundaries into doublecortin+ neuroblasts at higher rates in animals with MSCs compared to control group. The present study indicates that therapeutic effects of MSCs are at least partly ascribed to dual functions of MSCs by enhancing endogenous neurogenesis and protecting newborn cells from deleterious environment. The results reinforce the prospects of clinical application using MSCs in the treatment of neurological disorders.

Observations of Oxygen Administration Effects on Visuospatial Cognitive Performance using Time Course Data Analysis of fMRI

PURPOSE: This study attempted to investigate the effects of supply of highly concentrated (30%) oxygen on human ability of visuospatial cognition using time course data analysis of functional Magnetic Resonance Imaging (fMRI). MATERIALS AND METHODS: To select an item set in the visuospatial performance test, two questionnaires with similar difficulty were developed through group testing. A group test was administered to 263 college students. Two types of questionnaire containing 20 questions were developed to measure the ability of visuospatial cognition. Eight college students (right-handed male, average age of 23.5 yrs) were examined for fMRI study. The experiment consisted of two runs of the visuospatial cognition testing, one with 21% level of oxygen and the other with 30% oxygen level. Each run consisted of 4 blocks, each containing control and visuospatial items. Functional brain images were taken from 3T MRI using the single-shot EPI method. Using the subtraction procedure, activated areas in the brain during visuospatial tasks were col-orcoded by t-score. To investigate the time course data in each activated area from brain images, 4 typical regions (cerebellum, occipital lobe, parietal lobe, and frontal lobe) were selected. RESULTS: The average accuracy was 50.63 +/-8.63 and 62.50 +/-9.64 for 21% and 30% oxygen respectively, and a statistically significant difference was found in the accuracy between the two types of oxygen (p<0.05). There were more activation areas observed at the cerebellum, occipital lobe, parietal lobe and frontal lobe with 30% oxygen administration. The rate of increase in the cerebellum, occipital lobe and parietal lobe was 17% and that of the frontal lobe, 50%. Especially, there were increase of intensity of BOLD signal at the parietal lobe with 30% oxygen administration. The increase rate of the left parietal lobe was 1.4% and that of the right parietal lobe, 1.7%. CONCLUSION: It is concluded that while performing visuospatial tasks, high concentrations of oxygen administration make oxygen administration sufficient, thus making neural network activate more, and the ability to perform visuospatial tasks increase.

Observations of Oxygen Administration Effects on Visuospatial Cognitive Performance using Time Course Data Analysis of fMRI

PURPOSE: This study attempted to investigate the effects of supply of highly concentrated (30%) oxygen on human ability of visuospatial cognition using time course data analysis of functional Magnetic Resonance Imaging (fMRI). MATERIALS AND METHODS: To select an item set in the visuospatial performance test, two questionnaires with similar difficulty were developed through group testing. A group test was administered to 263 college students. Two types of questionnaire containing 20 questions were developed to measure the ability of visuospatial cognition. Eight college students (right-handed male, average age of 23.5 yrs) were examined for fMRI study. The experiment consisted of two runs of the visuospatial cognition testing, one with 21% level of oxygen and the other with 30% oxygen level. Each run consisted of 4 blocks, each containing control and visuospatial items. Functional brain images were taken from 3T MRI using the single-shot EPI method. Using the subtraction procedure, activated areas in the brain during visuospatial tasks were col-orcoded by t-score. To investigate the time course data in each activated area from brain images, 4 typical regions (cerebellum, occipital lobe, parietal lobe, and frontal lobe) were selected. RESULTS: The average accuracy was 50.63 +/-8.63 and 62.50 +/-9.64 for 21% and 30% oxygen respectively, and a statistically significant difference was found in the accuracy between the two types of oxygen (p<0.05). There were more activation areas observed at the cerebellum, occipital lobe, parietal lobe and frontal lobe with 30% oxygen administration. The rate of increase in the cerebellum, occipital lobe and parietal lobe was 17% and that of the frontal lobe, 50%. Especially, there were increase of intensity of BOLD signal at the parietal lobe with 30% oxygen administration. The increase rate of the left parietal lobe was 1.4% and that of the right parietal lobe, 1.7%. CONCLUSION: It is concluded that while performing visuospatial tasks, high concentrations of oxygen administration make oxygen administration sufficient, thus making neural network activate more, and the ability to perform visuospatial tasks increase.