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.

Magnetic Resonance Imaging Compatibility of the Polymer-based Cochlear Implant

OBJECTIVES: In this study, we compared the magnetic resonance (MR) image artifacts caused by a conventional metal-based cochlear implant and a newly developed liquid crystal polymer (LCP)-based device. METHODS: The metal-based cochlear implant system (Nurobiosys Co.) was attached to side of the head of a subject and the LCP-based device was attached to opposite side. In both devices, alignment magnets were removed for safety. Magnetic resonance imaging (MRI) was performed on a widely used 3.0 T and an ultra-high 7.0 T MRI machine. 3.0 and 7.0 T MR images were acquired using T1- and T2*-weighted gradient echo sequences, respectively. RESULTS: In the 3.0 T images, the metal-based device on the left side generated the significant amount of artifacts. The MR images in the proximity of the metal package were obscured by the artifacts in both axial and sagittal views. On the other hand, the MR images near the LCP-based device were relatively free from the artifacts and clearly showed the brain structures. 7.0 T MR images showed the more severe distortion in the both sides but the metal-based cochlear implant system caused a much larger obscure area than the LCP-based system. CONCLUSION: The novel LCP-based cochlear implant provides a good MRI compatibility beyond present-day cochlear implants. Thus, MR images can be obtained from the subjects even with the implanted LCP-based neural prosthetic systems providing useful diagnostic information. Furthermore, it will be also useful for functional MRI studies of the auditory perception mechanism after cochlear implantations as well as for positron emission tomography-MRI hybrid imaging.

A Case of Gastric and Colonic Submucosal Tumors after the Removal of 51 Anisakis Larvae / 대한내과학회지

A 60-year-old man visited the outpatient clinic complaining of spasmodic abdominal pain. The symptoms had started 4 days previously, within 1 hour of eating sliced raw flatfish (sashimi) and a piece of flatfish gut. Gastroscopy revealed generalized mucosal edema, erythema, multiple erosions, and numerous worms penetrating the gastric mucosa at the greater and lesser curvatures of the middle and lower corpora. Fifty-one worms, 0.5-3 cm in length, were successfully removed with biopsy forceps. Three weeks later, a normal mucosa-covered, 3.5-cm round submucosal tumor (SMT) was found on the greater curvature of the lower corpus and four yellowish, 1-cm SMTs were found on the hepatic flexure of the colon. Histopathological examination revealed marked eosinophilic infiltration in the gastric and colonic mucosa and an eosinophilic abscess in the submucosa. Here, we report a case of eosinophilic granuloma manifesting as SMTs in the stomach and colon after the endoscopic removal of a large number of anisakids.

Clinical Outcomes of Stercoral Colitis / 대한내과학회지

BACKGROUND/AIMS: Stercoral colitis is an inflammatory condition related to increased intraluminal pressure, itself caused by impacted fecal material. Stercoral colitis is a rare condition and has a generally poor prognosis. The aims of this study were to investigate the clinical characteristics and outcomes of stercoral colitis according to management strategy. METHODS: From January 2004 to August 2009, 11 patients were diagnosed with stercoral colitis at our center. The medical records of these individuals were reviewed retrospectively with regard to the clinical characteristics, management strategy, and clinical outcomes. We defined severe stercoral colitis as stercoral colitis complicated by systemic inflammatory response syndrome, sepsis, or septic shock. RESULTS: Eleven patients (three men and eight women) with a mean age of 70+/-8 years were included. Ten patients were elderly with constipation as a predisposing factor. Nine patients had severe stercoral colitis according to out criteria. Of these, five patients underwent surgery, and the other four were treated with a conservative management strategy. One patient (20%) in the surgical group and all patients in the conservative management group (n=4) died. CONCLUSIONS: Stercoral colitis should be considered in elderly patients with predisposing factors and presents as fecal impaction with colonic wall thickening or pericolic fat stranding on CT scan. In patients with severe stercoral colitis, early surgery may be effective in reducing mortality.

Precordial ST-Segment Elevation in Acute Right Ventricular Myocardial Infarction

It is rare to observe ST-segment elevations in the precordial leads that are caused by an occlusion of the right coronary artery and/or its branches. We report here on two cases of acute occlusion of the right coronary artery or its branches that caused acute right ventricular myocardial infarction with ST-segment elevations in the anterior precordial leads. These cases should remind us that the presence of diffuse ST-segment elevations in the precordial leads could be due to acute occlusion of the right coronary artery.