Synthetic CT Generation from MRI Using Improved DualGAN

Synthetic CT image generation from MRI scan is necessary to create radiotherapy plans without the need of co-registered MRI and CT scans. The chosen baseline adversarial model with cycle consistency permits unpaired image-to-image translation. Perceptual loss function term and coordinate convolutional layer were added to improve the quality of translated images. The proposed architecture was tested on paired MRI-CT dataset, where the synthetic CTs were compared to corresponding original CT images. The MAE between the synthetic CT images and the real CT scans is 61 HU computed inside of the true CTs body shape

Geometric and dosimetric impact of anatomical changes for MR-only radiation therapy for the prostate

PURPOSE: With the move towards magnetic resonance imaging (MRI) as a primary treatment planning modality option for men with prostate cancer, it becomes critical to quantify the potential uncertainties introduced for MR-only planning. This work characterized geometric and dosimetric intra-fractional changes between the prostate, seminal vesicles (SVs), and organs at risk (OARs) in response to bladder filling conditions. MATERIALS AND METHODS: T2-weighted and mDixon sequences (3-4 time points/subject, at 1, 1.5 and 3.0 T with totally 34 evaluable time points) were acquired in nine subjects using a fixed bladder filling protocol (bladder void, 20 oz water consumed pre-imaging, 10 oz mid-session). Using mDixon images, Magnetic Resonance for Calculating Attenuation (MR-CAT) synthetic computed tomography (CT) images were generated by classifying voxels as muscle, adipose, spongy, and compact bone and by assignment of bulk Hounsfield Unit values. Organs including the prostate, SVs, bladder, and rectum were delineated on the T2 images at each time point by one physician. The displacement of the prostate and SVs was assessed based on the shift of the center of mass of the delineated organs from the reference state (fullest bladder). Changes in dose plans at different bladder states were assessed based on volumetric modulated arc radiotherapy (VMAT) plans generated for the reference state. RESULTS: Bladder volume reduction of 70 ± 14% from the final to initial time point (relative to the final volume) was observed in the subject population. In the empty bladder condition, the dose delivered to 95% of the planning target volume (PTV) (D95%) reduced significantly for all cases (11.53 ± 6.00%) likely due to anterior shifts of prostate/SVs relative to full bladder conditions. D15% to the bladder increased consistently in all subjects (42.27 ± 40.52%). Changes in D15% to the rectum were patient-specific, ranging from -23.93% to 22.28% (-0.76 ± 15.30%). CONCLUSIONS: Variations in the bladder and rectal volume can significantly dislocate the prostate and OARs, which can negatively impact the dose delivered to these organs. This warrants proper preparation of patients during treatment and imaging sessions, especially when imaging required longer scan times such as MR protocols

Zur Diffusion adsorbierter Teilchen auf Einkristalloberflächen: Dynamische Untersuchungen mit dem Rastertunnelmikroskop

Subject of the present work is the diffusion of adsorbed particles on single crystal planes, which is investigated by scanning tunneling microscopy (STM) and analyzed quantitatively. By evaluation of the atomic motion of oxygen atoms on Ru(0001) it is demonstated, how the transition from the microscopic to the macroscopic description of diffusion can be realized experimentally. Computer-aided image processing permits the statistical evaluation of long sequences of several thousands of atomic configurations, which were recorded with a fast STM system that allows frame rates of up to 20 frames per second. On the one hand, this enables the measurement of the jump rate of isolated oxygen atoms at room temperature as well as the influence of the mutual O-O interaction on that rate over distances of up to 3 lattice constants; from these data the pair potential can be derived. On the other hand the chemical diffusion coefficients were directly determined at various coverages by analysis of the particle number fluctuations. These are found to be in agreement with the microscopic jump rates under consideration of the mutual O-O interaction. Additionally, variable-temperature measurements on the diffusion of sulfur on Pt(111) with coadsorption of CO are reported. For the diffusion of sulfur an activation energy of 550 meV is found. Coadsorption of CO leaves this activation energy unchanged, however, the prefactor is increased by two orders of magnitude by coadsorption of half a monolayer of CO. Various models for this behavior are discussed

Dynamics of adatom motion under the influence of mutual interactions: O/Ru(0001)

The diffusive motion of O atoms adsorbed on Ru(0001) was observed on the atomic scale by scanning tunneling microscopy with temporal resolution of Z˙10 ms at room temperature. From statistical analysis of the changes of the atom configurations in sequences of more than 1000 images, mean residence times of oxygen atoms in the neighborhood of other oxygen atoms were extracted. The residence times vary by more than 1 order of magnitude over distances of three lattice constants, indicating the necessity of including the lateral interaction in modeling collective phenomena such as surface diffusion or reactions. [S0031-9007(99)09081-X

Fat-constrained 18F-FDG PET reconstruction in hybrid PET/MR imaging

Fusion of information from PET and MR imaging can increase the diagnostic value of both modalities. This work sought to improve (18)F FDG PET image quality by using MR Dixon fat-constrained images to constrain PET image reconstruction to low-fat regions, with the working hypothesis that fatty tissue metabolism is low in glucose consumption

A novel and rapid approach to estimate patient-specific distortions based on mDIXON MRI

While MRI-only radiation treatment planning (RTP) is becoming more widespread, a robust clinical solution for patient-specific distortion corrections is not available. This work explores B 0 mapping based on mDIXON imaging, often performed for MR-only RTP, as an alternative to separate dual-acquisition gradient-recalled echo imaging, with the overarching goal of developing an efficient and robust approach for patient-specific distortion correction. Initial benchmarking was conducted by scanning a phantom and generating B 0 field maps with two approaches: (1) conventional B 0 mapping and (2) experimental mDIXON imaging. Distortion maps were derived from the field maps and compared. The head and neck regions, including brain, of ten healthy volunteers were then evaluated at 1.5 T and 3 T. Distortion maps were again compared between approaches, using difference maps and histogram analysis. Overall, conventional B 0 mapping was well approximated by mDIXON imaging: The distortions of 95% of the voxels in the phantom estimated by mDIXON and conventional B 0 mapping differed by  \u3c0.02 mm (1.5 T) and  \u3c0.04 mm (3 T), while the 95-percentiles of the distortions estimated by conventional B 0 mapping were  \u3c0.06 mm (1.5 T) and  \u3c0.12 mm (3 T). In head and neck the distortions of 99% of the voxels were within  ±0.2 mm at 1.5 T for both approaches and within  ±0.4 mm and  ±0.5 mm at 3 T for mDIXON imaging and conventional B 0 mapping, respectively. The majority of differences in vivo were confined to regions with high spatial variation of the B 0 field, mostly around internal air cavities. For 1.5 T, the mDIXON imaging-based correction alone reduced the 95-percentile of distortions from 0.15 mm to 0.03 mm and within the brain from 0.06 mm to 0.02 mm. Slightly lower reductions were observed at 3 T. In conclusion, mDIXON imaging closely approximated conventional B 0 mapping for patient-specific distortion assessment. Estimates in the brain were in good agreement, and slight differences were observed near air/tissue interfaces in the head and neck. Overall, mDIXON imaging-based B 0 field maps may be advantageous for rapid patient-specific distortion correction without additional imaging

Per-organ assessment of subject-induced susceptibility distortion for MR-only male pelvis treatment planning

BACKGROUND: Patient-specific distortions, particularly near tissue/air interfaces, require assessment for magnetic resonance (MR) only radiation treatment planning (RTP). However, patients are dynamic due to changes in physiological status during imaging sessions. This work investigated changes in subject-induced susceptibility distortions to pelvic organs at different bladder states to support pelvis MR-only RTP. METHODS: Pelvises of 9 healthy male volunteers were imaged at 1.0 Tesla (T), 1.5 T, and 3.0 T. Subject-induced susceptibility distortion field maps were generated using a dual-echo gradient-recalled echo (GRE) sequence with B RESULTS: The time between the initial and final B0 maps was 42.6 ± 13.9 (range: 13.2-62.1) minutes with minimal change in magnet central frequency (0.02 ± 0.05 mm (range: - 0.06 - 0.12 mm)). Subject-induced susceptibility distortion across all bladder states, field strengths, and subjects was relatively small (1.4-1.9% of all voxels in the prostate and seminal vesicles were distorted \u3e 0.5 mm). In the bladder, no voxels exhibited distortions \u3e 1 mm. An extreme case acquired at 3.0 T with a large volume of rectal air yielded 27.4-34.6% of voxels within the CTVs had susceptibility-induced distortions \u3e 0.5 mm across all time points. CONCLUSIONS: Our work suggests that subject-induced susceptibility distortions caused by bladder/rectal conditions are generally small and subject-dependent. Local changes may be non-negligible within the CTV, thus proper management of filling status is warranted. Future work evaluating the impact of multiple models to accommodate for extreme status changes may be advantageous