Scanning Slit System Research Articles

Construction and dosimetric characterization of a motorized scanning-slit system for electron FLASH experiments.

Beam scanning is a useful technique for the treatment of large tumors when the primary beam size is limited, which is the case with radiation beams used in FLASH radiotherapy. To optimize beam scanning as a dose delivery method for FLASH radiotherapy, it is necessary to first understand the effects of beam scanning on the FLASH effect. To do so, biological FLASH experiments need to be done using defined beam parameters with beam scanning and compared to the situation without beam scanning. In this regard, we propose implementation of a simple slit scanning system with an electron FLASH beam to obtain a scanned radiation field that closely resembles a static field. A pulsed electron linear accelerator (linac) was used in combination with a scanning slit system in order to simulate a scanned electron beam. Three configurations that produced homogeneous lateral profiles and high enough doses per pulse for FLASH experiments were established. The optimal scanning parameters were found for each configuration by examining the flatness of the obtained lateral dose profiles. Using the optimal scanning parameters, the scanned FLASH beams were dosimetrically characterized and compared to non-scanned open field beam. A final electron FLASH beam scanning configuration was found for a 1mm wide slit at a distance of 350mm from the linac and a 2mm wide slit at distances of 350 and 490mm from the linac. The lateral profiles for these final configurations were found to have a homogeneity that is comparable to the open field profiles. The percentage depth dose (PDD) values found for these final configurations closely matched (by a few percentage) the PDD of the open field beam. Three electron FLASH beam scanning configurations achieved by the motorized slit system were found to produce radiation fields similar to a non-scanned open field electron beam. These final configurations can therefore be used in future biological FLASH experiments to compare to non-scanned beam experiments in order to optimize beam scanning as a technique permitting the treatment of larger tumors with FLASH radiotherapy.

Posterior corneal shape: Comparison of height data from 3 corneal topographers

To compare the ability of 3 clinical corneal topographers to describe the posterior corneal shape. University Medical Center Groningen, the Netherlands. Prospective observational study. Corneas of healthy participants were measured twice with a dual Scheimpflug instrument (Galilei G2), a scanning-slit system (Orbscan IIz), and a single Scheimpflug instrument (Pentacam HR). Height data describing the posterior corneal shape were fit with Zernike polynomials. Mean values with standard deviations (SD), test-retest variability (coefficient of repeatability [CoR]), and interdevice variability were determined for the defocus Z(2,0), astigmatism Z(2,-2) and Z(2,2), and higher-order terms coma Z(3,-1) and Z(3,1), trefoil Z(3,-3) and Z(3,3), and spherical aberration Z(4,0) coefficients for 5.5mm and 8.0mm diameters. For the 5.5mm diameter, CoRs ranged from 0.3 to 4.3μm with the dual Scheimpflug instrument, 1.6 to 5.2μm with the scanning-slit system, and 0.3 to 2.0μm with the singleScheimpflug instrument. The CoR was similar for the Scheimpflug instruments (P=.43) but poorer for the scanning-slit system (P<.001). The CoRs of the Scheimpflug instruments were smaller than the corresponding population SD for defocus, cardinal astigmatism, coma, and spherical aberration. The scanning-slit system failed to provide 8.0mm diameter data. There was a significant bias (interdevice variability) between the Scheimpflug instruments in the higher-order coefficients at both diameters. Repeatability in assessing the posterior corneal shape was generally good for the Scheimpflug instruments but poor for the scanning-slit system. Interdevice variability between the Scheimpflug instruments compromised the interchangeability of higher-order coefficients. For astigmatism, CoR and 95% limitsof agreement of the Scheimpflug instruments typically corresponded to 0.1 diopter per astigmatism term.

Clinical Utility of Combined Placido–Scanning-Slit Midperipheral and Thinnest Point Pachymetry After Corneal Ablation for Myopia

The aim of this study was to assess the random error of the thinnest point and midperipheral pachymetry with a combined Placido-scanning-slit system (Orbscan II) in the intermediate-term follow-up of excimer laser keratorefractive surgery for treating myopia. Sixty-five patients who had undergone aspheric surface ablation for treating myopia with a minimum follow-up of 6 months, and showed no biomicroscopically detectable corneal haze, were subjected to 5 consecutive topographic examinations. All eyes underwent a complete ophthalmic examination before the surgery. The within-subject SD (Sw), repeatability (2.77 × Sw), coefficient of repeatability (2.77 × Sw/mean), and intraclass correlation coefficients of pachymetry at the thinnest point and in each 4 quadrants at a 6-mm diameter were calculated. The patients had a mean age of 31.9 ± 6.2 years, and the average follow-up was for 7.5 ± 2.6 months. The mean ablation depth for myopia was 63.6 ± 23.6 μm. The average pachymetry for the thinnest point and midperipheral superior, inferior, nasal, and temporal locations was 456.9, 602.0, 595.5, 609.7, and 566.1 μm, respectively. The repeatability (and coefficient of repeatability) for the thinnest point and midperipheral superior, inferior, nasal, and temporal locations was 26.5 (5.8%), 37.9 (6.3%), 31.0 (5.2%), 30.5 (5.0%), and 35.4 μm (6.2%), respectively. The intraclass correlation coefficients were 0.96, 0.88, 0.89, 0.97, and 0.91 for each location, respectively. This study provides the repeatability of the thinnest and midperipheral pachymetry using the combined Placido-scanning-slit system in transparent corneas after laser keratorefractive surgery for treatment of myopia. The test-retest reliability provided here will help differentiate real corneal thickness change from measurement noise. For this, only pachymetric changes >6% are likely to be real and therefore useful when evaluating postoperative keratectasia suspects.

Corneal thickness, curvature, and elevation readings in normal corneas: Combined Placido–Scheimpflug system versus combined Placido–scanning-slit system

To evaluate agreement in central corneal thickness (CCT), keratometry, and anterior and posterior elevation map measurements in normal corneas between a combined Placido-Scheimpflug system and a combined Placido-scanning-slit elevation topography system. Department of Cataract & Refractive Surgery, Rothschild Foundation, Paris, France. Evaluation of diagnostic test or technology. Measurements were performed with a combined Placido-Scheimpflug system (TMS-5) and a combined Placido-scanning-slit system (Orbscan II). Ultrasound (US) pachymetry was used as the reference for CCT measurements. Bland-Altman plots were used to evaluate agreement between instruments. The mean CCT measurements by US pachymetry, the Placido-Scheimpflug system, and the Placido-scanning-slit system were 556.74 μm ± 42.45 (SD), 543.23 ± 36.73 μm, and 564.45 ± 41.26 μm, respectively. Although the CCT readings were statistically significantly thinner with the Placido-Scheimpflug system than with the other systems, there was high correlation between instruments. Peripheral corneal thickness readings were also thinner with the Placido-Scheimpflug system than with the Placido-scanning-slit system. Keratometry and anterior and posterior best-fit sphere (BFS) measurements were comparable between the 2 optical devices. Anterior and posterior maximum central elevations measured by the 2 instruments were not comparable or strongly correlated. Repeatability after 3 successive measurements was excellent for all parameters except maximum central elevation. Although highly correlated, with corneal thickness readings were not interchangeable between the 2 optical devices. No statistically significant differences in keratometry or BFS measurements were observed between the 2 devices. There were important discrepancies in the maximum central elevation between the 2 topographers. Drs. Gatinel and Saad are consultants to Technolas Perfect Vision. No other author has a financial or proprietary interest in any material or method mentioned.

Orbscan Evaluation of Corneal Power after Myopic PRK: Placido Disc-Based vs Scanning Slit Topography

Purpose To compare Placido disc-based and scanning slit topography in the evaluation of corneal power changes after PRK. Methods The changes in corneal power, evaluated with an Orbscan II utilizing both Placido disc-based and Scanning Slit topography, before and 1, 3 and 6 months after PRK were compared with refractive changes in 109 individuals (149 eyes) who underwent photorefractive keratectomy (PRK) with a sphero equivalent refraction, calculated at the corneal apex, ranging from -0.25 to -12 D (mean -4.58±2.54 D). Results 1,3 and 6 months refractive changes showed a good correlation with changes in BFS (r 2=0.64, r 2= 0.53, r 2= 0.62, respectively); in Sim K's (r 2= 0.77, r 2= 0.8, r 2= 0.78, respectively) in Mean Power 3 mm (r 2= 0.81, r 2= 0.88, r 2= 0.87, respectively) in Mean Power 5 mm (r 2= 0.68, r 2= 0.78, r 2= 0.78, respectively) in Anterior Mean Power 3 mm (r 2= 0.88, r 2= 0.85, r 2= 0.85, respectively) in Anterior Mean Power 5 mm (r 2= 0.75, r 2= 0.7, r 2= 0.75, respectively) in Total Mean Power 3 mm (r 2= 0.75, r 2= 0.71, r 2= 0.79, respectively) in Total Mean Power 5 mm (r 2= 0.75, r 2= 0.71, r 2= 0.78, respectively). However, all differences were statistically significant ( P<0.001). Conclusion The data obtained with the scanning slit system seem to be more reliable, but need correcting factors to improve their precision.

Principle of FT spectrometer based on a lateral effect position sensitive detector and multi channel Fabry–Perot interferometer

The principle of a new type of multi channel Fourier-Transform spectrometer based on a multi channel wedge Fabry–Perot interferometer using a one dimensional lateral effect Position Sensitive Detector and a scanning slit for interferogram readout have been shown. The design of this spectrometer is very compact and the readout electronics very simple. The drawback of using a scanning slit system is minimized by the use of a position sensitive detector where the position is inherently known for each measurement. Experiments show that the position can be resolved with high accuracy and the summation of the two photocurrents gives the interferogram distribution after scanning the slit between the wedge interferometer and the position sensitive detector. The spectral resolution obtained with a 25 mm wedge interferometer together with a 45 mm position sensitive detector and a 25 μm slit is about 5 nm around 600 nm wavelength range, which is close to the theoretically anticipated resolution. The evaluated spectrometer setup show promising results and could be used in applications where compact and low cost spectrometers are required.

Comparison of In Vivo Confocal Microscopy of Human Cornea by White Light Scanning Slit and Laser Scanning Systems

To compare in vivo corneal imaging by scanning slit white light and laser confocal microscopy systems. Twenty healthy individuals and 10 patients with corneal dystrophies including Fuchs, granular, Map-Dot-Fingerprint dystrophies, and amiodarone-induced keratopathy were examined with the ConfoScan 3 (Nidek Technologies) scanning slit white light confocal microscope equipped with x40 front lens (Zeiss) and the Rostock Cornea Module (RCM) for HRT II (Heidelberg Engineering) laser confocal microscopy system equipped with Olympus x60 front lens. The endothelial cell density counts were performed and results were compared. For additional validation of endothelial cells density results, separate counts were carried out using the specular microscope SP-1000 (Topcon). The healthy and pathologically changed corneal structures are imaged in a similar manner by both systems. The differences in quality of acquired images in contrast and brightness between the systems are debatable, although the laser system was more efficient for epithelium imaging and white light scanning slit was better for evaluation of endothelium. Some of the examinations completed with the laser system, which requires applanating its front lens to the cornea, imaged dark striae in posterior stroma and Descemet membrane folds, resembling those observed in corneal pathologies, whereas most were probably induced by the pressure applied to the cornea during examination. All the results of cell density (for patients and for subjects with no ophthalmic disease) counts performed with the RCM (laser system) were higher than those with the ConfoScan 3 (scanning slit system) by 36.7% +/- 11.9% (SD) and 30.2% +/- 11.3% higher than SP-1000 (specular). The difference in cell counts between the RCM and other methods was increasing at higher cell densities. The morphologic findings in examinations performed with the laser confocal microscopy system are generally comparable to white light scanning slit confocal microscopy. Direct applanation of the front lens of the laser system to the cornea may generate certain changes in confocal microscopy outcomes, including imaging of Descemet membrane folds and dark lines in stroma, which should be differentiated from pathologic alterations. Comparison of the endothelium cell density counts obtained with the 2 systems should be made cautiously because significant differences might occur--RCM configured with the Olympus x60 front lens was found to overestimate the results compared with both CS3 and SP-1000 microscopes, with the range of disparity increasing for higher cell densities.

TH-C-I-611-06: MTF(f), NPS(f) and DQE(f) of An X-Ray Imaging System Based On a Photon Counting MCP Detector

Purpose: To evaluate inherent MTF(f), NPS(f) and DQE(f) of a prototype x-ray imaging system with photon counting microchannel plate (MCP) detector. Method and Materials: The MCP detector was used in edge illumination mode as a linear array x-ray detector in scanning slit image acquisition geometry. The detector FOV was 60 mm. MCPs with 63 mm length, 25 μm channel diameter and 5 μm channel wall thickness were used. The slit width was 0.25 mm. Delay line position encoding electronics was used. Images were acquired using an x-ray tube with 0.4 mm nominal focal spot size at 40 and 90 kVp using an SID of 50 cm and a magnification 1.1. MTF was measured using edge spread function method. NPS was calculated using flat field corrected flood images. DQE was calculated from MTF, NPS and using both measured and Monte Carlo calculated QE. Results: For 25 μm channel MCP, 50% MTF was achieved at 6 lp/mm and 4 lp/mm for 40 and 90 kVp tube voltages, respectively. The highest visualized frequency was 11 lp/mm at 40 kVp. NPS was shown to be frequency independent. QE measured at 90 kVp was 56% and 70% for 25 μm and 5 μm channel MCPs, respectively. Calculated QE was 64%, 86% and 92% for 25, 5 and 2 μm channel MCPs, at 90 kVp. The above QE values represent the DQE(0). The MTF(f) curves normalized by QE represent DQE(f) due to frequency independent nature of the NPS. Conclusion: MCP detector has shown adequate MTF(f) and DQE(f) for potential application in medical imaging. This is in addition to its inherently high count rate and scatter rejection feature of the scanning slit system. Combination of these capabilities in the photon counting system would allow for significant decrease in patient dose.

Scanning slit chest radiography: a practical and efficient scatter control design.

To design a chest radiography unit that eliminates scatter. A scanning slit radiography unit that also incorporates a conventional grid was designed. The scanning slit geometry defined a 10-mm-wide beam at the image receptor. With a typical heavy-duty radiographic tube, the proposed system was capable of delivering an entrance skin exposure of up to 32 microC/kg for a 1-second scan. Monte Carlo simulations indicated that the scatter-to-primary ratio (S/P) imaged with the scanning slit system alone was 0.125 for a 35 x 43-cm, 23-cm-thick Lucite phantom imaged at 140 kVp. When the system was coupled with 4:1, 8:1, and 12:1 grids, S/P decreased to 0.063, 0.055, and 0.054, respectively. A scanning slit system coupled with a conventional grid is a practical and highly efficient method of controlling scatter and will result in minimal soft-tissue and bone measurement errors in dual-energy chest radiography.