Visibility Of Disks Research Articles

SU‐GG‐I‐25: An Image Quality Comparison Study Between XVI and OBI CBCT Systems

Purpose: To evaluate image quality on the X‐ray Volumetric Imager (XVI®, Elekta Medical Systems) and the On‐Board Imager (OBI®, Varian Medical Systems) using a CatPhan® phantom in a single institution setting. Method and Materials: We evaluated four major image quality indices, including high and low contrast resolution, noise, and contrast‐to‐noise (CNR) ratio. For the XVI unit, the four manufacturer‐supplied protocols were measured. For the OBI unit, full bow‐tie and half bow‐tie filters were used in combination with the “full‐fan” and “half‐fan” modes. The mAs was varied for each protocol. High contrast resolution was evaluated using number of line pairs visible per cm and low contrast resolution was evaluated using the visibility of low resolution disks. Noise was computed over circular regions of interest (ROIs) on the uniformity module. CNR was calculated using a polystyrene insert and the background of module CTP404. The images were analyzed using MATLAB. Results: Up to 10 line pairs per cm are visible on OBI for high mAs settings and so are most low contrast discs. 2 line pairs per cm and none of the low contrast discs are visible on XVI. Noise values fall from 35 to 6 on OBI and vary between 10 and 3 for “prostate” protocol on XVI with increasing mAs. On OBI, CNR steadily increased from 4 to 20 with increasing mAs. For the “prostate” protocol on XVI, CNR varies from 10 to 23 and generally increases with mAs. For the four XVI protocols evaluated, mean pixel values in ROIs are found to decrease with mAs. Conclusion: Image quality parameters are generally found to be better on OBI as compared with XVI for clinical protocols. However, this is important to be viewed in the context of the observation that OBI generally delivered higher doses than XVI.Research sponsored by Elekta Inc.

Galaxy mergers with various mass ratios: Properties of remnants

We study galaxy mergers with various mass ratios using N-body simulations, with an emphasis on the unequal-mass mergers in the relatively unexplored range of mass-ratios 4:1-10:1. Our recent work (Bournaud et al. 2004) shows that the above range of mass ratio results in hybrid systems with spiral-like luminosity profiles but with elliptical-like kinematics, as observed in the data analysis for a sample of mergers by Jog & Chitre (2002). In this paper, we study the merger remnants for mass ratios from 1:1 to 10:1 while systematically covering the parameter space. We obtain the morphological and kinematical properties of the remnants, and also discuss the robustness and the visibility of disks in the merger remnants with a random line-of-sight. We show that the mass ratios 1:1-3:1 give rise to elliptical remnants whereas the mass ratios 4.5:1-10:1 produce hybrid systems with mixed properties. We find that the transition between disk-like and elliptical remnants occurs between a narrow mass range of 4.5:1-3:1. The unequal-mass mergers are more likely to occur than the standard equal-mass mergers studied in the literature so far, and we discuss their implications for the evolution of galaxies.

The ultraviolet visibility and quantitative morphology of galactic disks at low and high redshift

We used ultraviolet (200 nm) images of the local spiral galaxies M33, M51, M81, M100, M101 to compute morphological parameters of galactic disks at this wavelength : half-light radius $r_{hl}$, surface brightness distributions, asymmetries ($A$) and concentrations ($C_A$). The visibility and the evolution of the morphological parameters are studied as a function of the redshift. The main results are : local spiral galaxies would be hardly observed and classified if projected at high redshifts (z $\ge$ 1) unless a strong luminosity evolution is assumed. Consequently, the non-detection of large galactic disks cannot be used without caution as a constraint on the evolution of galatic disks. Spiral galaxies observed in ultraviolet appear more irregular since the contribution from the young stellar population becomes predominent. When these galaxies are put in a (log $A$ vs. log $C_A$) diagram, they move to the irregul ar sector defined at visible wavelengths. Moreover, the log $A$ parameter is degenerate and cannot be used for an efficient classification of morphological ultraviolet types. The analysis of high redshift galaxies cannot be carried out in a reliable way so far and a multi-wavelength approach is required if one does not want to misinterpret the data.

Effect of patient support pads on image quality and dose in fluoroscopy

An investigation was performed of the changes in image quality and patient dose as a result of increasing filtration for fluoroscopy performed under automatic brightness control. Filtration was added either at the x-ray tube housing (i.e., scatter-free geometry) or adjacent to a tissue equivalent phantom simulating the patient (i.e., with-scatter geometry). Patient doses were expressed in terms of the total energy imparted to patients simulated by either a 10 cm (i.e., pediatric) or 20 cm (i.e., adult) acrylic phantoms. Changes in image quality were determined by measuring the relative visibility of circular disks in a Leeds Test Object 10 contrast-detail phantom. In the scatter-free geometry, the addition of 4 mm Al filtration reduced the energy imparted by 27% (10 cm phantom) and 20% (20 cm phantom). In the with-scatter geometry, the corresponding reductions in energy imparted were 17% and 9% for the 10 and 20 cm phantoms, respectively. The visibility of low contrast disks generally decreased as the thickness of the added aluminum increased but the location of the added Al (i.e., with-scatter or scatter-free geometry) had no significant effect on the resultant image quality. These results demonstrate that the use of patient support pads with a thickness of approximately 4 mm Al will generally have an adverse impact on fluoroscopic image quality and result in modest reductions (approximately 10%) of adult patient doses.

Enhanced visibility of precipitin disks in radial immunodiffusion measurements

Enhanced visibility of precipitin disks in radial immunodiffusion measurements