COMPARISON OF TECHNIQUES FOR MEASURING X-RAY MICROFOCUS DIMENSIONS

A Note on the Measurement of Focal Spot Size Utilizing Star Test Patterns at Low Magnification Factors.

The genetically significant dose (GSD) from beam therapy in 1971 was estimated, based on a nationwide survey carried out for a 30 consecutive day period in June and July 1971. From 1480 hospitals which had therapy units, 348 hospitals were selected at random for the survey. Sixty-eight per cent of the questionnaires were answered. The contribution to the GSD from beam therapy in Japan was estimated by using the results of experiments on scattered and leakage radiation, designed to allow a reasonable projection of the dose; percentage depth dose tables; and determination of the expected number of children from vital statistics. The GSD estimated was 0.25, 0.70 and 0.03 mrad/person/yr for therapies with 60Co units, conventional X-ray units and high-energy accelerators, respectively.

1407_60 An analytic model of cumulative beam breakup has been developed which is applicable to both low-velocity ion and high-energy electron linear accelerators. The model includes arbitrary velocity, acceleration, focusing, initial conditions, beam-cavity resonances, and variable cavity geometry and spacing along the accelerator. Both the steady-state and transient displacements of the beam bunches are calculated. As examples, the solution is applied to hypothetical high-current ion accelerators comprised of superconducting coaxial half-wave resonators. The steady-state transverse motion of particles comprising a longitudinal halo between bunches is also calculated, and effects of nonzero bunch length and arbitrary bunch shape are investigated. The role of focusing in controlling beam breakup is quantified for these cases.

Mammography x-ray tube focal spot sizes are routinely measured during acceptance testing and annual performance audits. The National Electrical Manufactures Association (NEMA) recommends the slit camera for this purpose. Investigated were the effect of slit rotational misalignment, tilt misalignment, image film density, film and screen-film image receptors, microscope magnification and reticule accuracy, and observer variation on slit camera focal spot measurements. Our results indicate that small rotational misalignment (< 5 degrees) and tilt misalignment (< 3 degrees) introduce insignificant error. Measured focal spot size increased slightly with image optical density, indicating that for consistent results the image optical density variations should be minimized. Also desirable for accurate field measurements is a high power microscope (25-50x) and a reticule with divisions of < or = 0.02 mm. Screen-film imaging consistently resulted in a slightly smaller measured focal spot size than direct film. The greatest source of error was due to observer variation. Of interest is that reader variability showed a consistent pattern and variation between two measurements by the same observer was much smaller than between observer variation, suggesting that standardized criteria should be established and a method of reader training developed. The length of the focal spot is defined at a reference axis angle specified by the mammography unit manufacturer. Presented is a tabulation of the focal spot geometry and reference axis angles for the majority of mammography units currently and recently marketed in North America.

In this latest book of Dr. Egan two aspects of mammography, not stressed in his first book in 1964, are presented in detail. The first is a detailed discussion of the methods used in the physical evaluation of the mammographic technique. Experiments are presented demonstrating the failure of conventional x-ray units, designed to operate at or above 50 kilovolt peak, to deliver adequate milliamperage and a satisfactory wave form at low kilovolt peak range. The effects on film quality of smaller focal spots, shorter object-film distances, and off-focus radiation are also presented. The four most important factors affecting roentgenographic detail are motion, object-film distance, columnation, and film type and processing, and Egan presents appropriate tests for evaluating each of these factors. The second aspect and the most important new information in this volume is a detailed discussion of the correlated clinical, radiographic, and pathologic approach to the study of breast

An isotopic radiation source 125I was investigated for use in soft tissue radiaography, with particular attention to possible use in mammography for mass screening. Film sensitivities, exposure rates, absorbed doeses, contrast and resolution were determined, and compared to conventional x-ray units. It was found that contrast and resolution were comparable to 30 kVp x-rays (W anode) but that surface absorbed dose was reduced by a factor of two with 125I. These results experimentally verified the advantage to be obtained from monoenergetic radiations, which had been predicted from theoretical considerations by others. Duration of exposure was estimated to be between 4 and 12 sec with a 40 Ci source. The 60-day half life of 125I may necessitate its use in conjunction with an image intensification system or electron radiographic techniques, in order to preclude source replacement at inconveniently short intervals.

Cross-sectional geometry of the femoral midshaft in baboons is heritable

The goal of this study is to evaluate a new method based on a coded aperture mask combined with a digital x-ray imaging detector for measurements of the focal spot sizes of diagnostic x-ray tubes. Common techniques for focal spot size measurements employ a pinhole camera, a slit camera, or a star resolution pattern. The coded aperture mask is a radiation collimator consisting of a large number of apertures disposed on a predetermined grid in an array, through which the radiation source is imaged onto a digital x-ray detector. The method of the coded mask camera allows one to obtain a one-shot accurate and direct measurement of the two dimensions of the focal spot (like that for a pinhole camera) but at a low tube loading (like that for a slit camera). A large number of small apertures in the coded mask operate as a "multipinhole" with greater efficiency than a single pinhole, but keeping the resolution of a single pinhole. X-ray images result from the multiplexed output on the detector image plane of such a multiple aperture array, and the image of the source is digitally reconstructed with a deconvolution algorithm. Images of the focal spot of a laboratory x-ray tube (W anode: 35-80 kVp; focal spot size of 0.04 mm) were acquired at different geometrical magnifications with two different types of digital detector (a photon counting hybrid silicon pixel detector with 0.055 mm pitch and a flat panel CMOS digital detector with 0.05 mm pitch) using a high resolution coded mask (type no-two-holes-touching modified uniformly redundant array) with 480 0.07 mm apertures, designed for imaging at energies below 35 keV. Measurements with a slit camera were performed for comparison. A test with a pinhole camera and with the coded mask on a computed radiography mammography unit with 0.3 mm focal spot was also carried out. The full width at half maximum focal spot sizes were obtained from the line profiles of the decoded images, showing a focal spot of 0.120 mm x 0.105 mm at 35 kVp and M = 6.1, with a detector entrance exposure as low as 1.82 mR (0.125 mA s tube load). The slit camera indicated a focal spot of 0.112 mm x 0.104 mm at 35 kVp and M = 3.15, with an exposure at the detector of 72 mR. Focal spot measurements with the coded mask could be performed up to 80 kVp. Tolerance to angular misalignment with the reference beam up to 7 degrees in in-plane rotations and 1 degrees deg in out-of-plane rotations was observed. The axial distance of the focal spot from the coded mask could also be determined. It is possible to determine the beam intensity via measurement of the intensity of the decoded image of the focal spot and via a calibration procedure. Coded aperture masks coupled to a digital area detector produce precise determinations of the focal spot of an x-ray tube with reduced tube loading and measurement time, coupled to a large tolerance in the alignment of the mask.

A hole diameter measurement is a common task in mechanical engineering metrology. Inner hole dimension is usually measured with contact gauges touching the inner wall. Another approach is the hole diameter estimation by a contact or optical measurement of the diameter at the top or bottom of the hole. Such approach does not allow for the measurement of the real geometry of the hole inner walls. As the hole diameter decreases, small geometry variations along the hole axis lead to a large relative error between the actual and measured diameter value. We assessed several different optical schemes allowing for an optical measurement of a hole diameter by reflection of light towards the hole inner wall. We selected and tested a reflection optical laser triangulation approach for measurement of side wall of a recess and we analyzed the allowable depth of a bore enabling the hole diameter estimation by scanning of the inner wall along the hole axis.

Deep x-ray lithography in combination with electroforming is capable of producing high precision metal parts in small lot series. This study deals with a high aspect ratio structure with overall dimensions on the order of 10 mm x 7 mm x 1.5 mm, with the smallest line width being 150 {micro}m. The lateral deviation from the design is to be kept to a minimum, preferably below 5 {micro}m. To ensure adequate quality control, a semi-automated metrology technique has been established to measure all parts. While the paper will give a brief overview of all involved techniques, it focuses on the method to measure the top and bottom of the parts and the top of geometries following the process. The instrument used is a View Engineering Voyager V6x12 microscope, which is fully programmable. The microscope allows direct measurement of geometries but also is capable of saving all captured data as point clouds. These point clouds play a central role when evaluating part geometry. After measuring the part, the point cloud is compared to the computer aided design (CAD) contour of the part, using a commercially available software package. The challenge of proper edge lighting on a nickel alloy part is evaluated by varying lighting conditions systematically. Results of two conditions are presented along with a set of optimized parameters. With the introduced set of tools, process flow can be monitored by measuring geometries, e.g. linewidths in every step of the process line. An example for such analysis is given. After delivery of a large batch of parts, extensive numbers of datasets were available allowing the evaluation of the variation of part geometries. Discussed in detail is the deviation from part top to part bottom geometries indicating swelling of the PMMA mold in the electroplating bath.

Lattice structure metamaterials offer a variety of unique and tailorable properties, yet industrial adoption is slowed by manufacturability and inspection-related difficulties. Despite recent advances in laser powder bed fusion additive manufacturing, the sub-millimeter features of lattices are at the edge of process capabilities and suffer from low geometric quality. To better understand their complex process-structure-property (PSP) relationships, octahedron structures were manufactured across a power spectrum, inspected, and mechanically tested. X-ray computed tomography was used to characterize lattice geometry, and demonstrated that lattice strut geometry measures, increased significantly as a function of laser power. Furthermore, lattices are shown to exhibit a direct correlation between laser power and mechanical performance metrics. Performance variations up to 60% are shown as a function of process parameters despite nominally identical geometry. Significant geometry variations are found to be the cause of performance variation, while material properties as measured by microindentation hardness are constant across the studied parameter range. PSP relationships are modeled, and the limitations of these models are explored. It was found that resulting models can predict mechanical performance based on geometric characteristics with R2 values of up to 0.86. Finally, mechanistic causes of observed performance changes are discussed.