Collimation Method Research Articles

An alternate line erasure and readout (ALER) method for implementing slot-scan imaging technique with a flat-panel detector-initial experiences

This paper describes and demonstrates an electronic collimation method, referred to as the alternate line erasure and readout (ALER) technique, for implementing slot-scan digital radiography technique with an amorphous silicon (a-Si) thin-film transistor (TFT) array based flat-panel detector. An amorphus selenium (a-Se) flat-panel detector was modified to implement the ALER technique for slot-scan imaging. A stepping-motor driven fore-collimator was mounted in front of an X-ray tube to generate a scanning X-ray fan beam. The scanning speed and magnification were adjusted to synchronize the fan beam motion with the image line readout rate. The image lines on the leading and trailing edges of the fan beam were tracked and alternately reset and read out, respectively. The former operation resulted in the erasure of the scatter signals accumulated in the leading edge image line prior to the arrival of the fan beam. The latter operation resulted in the acquisition of fan beam exposure data integrated in the trailing edge image line right after the fan beam passed. To demonstrate the scatter rejection capability of this technique, an anthropomorphic chest phantom was placed in PA position and scanned at a speed of 576 lines (8.0 cm)/s at 117 kVp and 32 mA. A tungsten bar is placed at the entrance side of the chest phantom to measure the scatter-to-primary ratio (SPR), scatter reduction factor (SRF), and contrast-to-noise ratio degradation factor (CNRDF) in the slot-scan images to evaluate the effectiveness of scatter rejection and the resultant improvement of image quality. SPR and CNRDF in the open-field images were also measured and used as the reference for comparison. A scatter reduction by 86.4 to 95.4% across lower lung and heart regions has been observed with slot-scan imaging. The CNRs have been found to be improved by a factor of 2 in the mediastinum areas over the open-field image as well.

Magnetic collimation and metal foil filtering for electron range and fluence modulation.

We investigated the use of magnetically collimated electron beams together with metal filters for electron fluence and range modulation. A longitudinal magnetic field collimation method was developed to reduce skin dose and to improve the electron beam penumbra. Thin metal foils were used to adjust the energies of magnetically collimated electrons. The effects for different types of foils such as Al, Be, Cu, Pb, and Ti were studied using Monte Carlo calculations. An empirical pencil beam dose calculation model was developed to calculate electron dose distributions under magnetic collimation and foil modulation. An optimization method was developed to produce conformal dose distributions for simulated targets such as a horseshoe-shaped target. Our results show that it is possible to produce an electron depth dose enhancement peak using similar techniques of producing a spread-out Bragg peak. In conclusion, our study demonstrates new aspects of using magnetic collimation and foil filtration for producing fluence and range modulated electron dose distributions.

Demonstration of a collimated in situ method for determining depth distributions using γ-ray spectrometry

In situ γ-ray spectrometry uses a portable detector to quantify radionuclides in materials. The main shortcoming of in situ γ-ray spectrometry has been its inability to determine radionuclide depth distributions. Novel collimator designs were paired with a commercial in situ γ-ray spectrometry system to overcome this limitation for large area sources. Positioned with their axes normal to the material surface, the cylindrically symmetric collimators limited the detection of unattenuated γ-rays from a selected range of polar angles (measured off the detector axis). Although this approach does not alleviate the need for some knowledge of the γ-ray attenuation characteristics of the materials being measured, the collimation method presented in this paper represents an absolute method that determines the depth distribution as a histogram, while other in situ methods require a priori knowledge of the depth distribution shape. Other advantages over previous in situ methods are that this method does not require multiple γ-ray emission energies, provides the capability for characterizing complex depth distributions. The disadvantages are that collimation adds weight to the in situ system and increases the counting time by reducing the detector's field of view. Using a high-purity germanium spectrometry system, collimated in situ measurements of large area sources buried in attenuating materials were performed in laboratory settings and demonstrated the method's capability to yield accurate depth information.

Analytical expressions for field astigmatism in decentered two mirror telescopes and application to the collimation of the ESO VLT

We derive formulae for all parameters defining the field astigmatism of misaligned two mirror telescopes with arbitrary geometries and with stop positions anywhere on the line connecting the vertices of the two mirrors. The formulae show explicitly the dependence of the field astigmatism on the fundamental design parameters and characteristics of the telescope and on the stop position. Special attention is given to the particular case where such a schiefspiegler has been corrected for coma at the field center. In addition, we study the effects of the practical definition that the center of the field is the center of the adapter. Following a recent paper by McLeod, where the field dependence of astigmatism is used to collimate a Ritchey-Chretien telescope with the stop at the primary mirror, we apply our formulae to the Cassegrain focus of the ESO Very Large Telescope (VLT), where the stop is at the secondary mirror and the telescope is only corrected for spherical aberration. We present measurements of the field astigmatism and discuss the accuracy of the collimation method.

Time-sharing collimation method for measuring the beating force of an insect

A time-sharing collimation (TSC) method is developed to measure the beating force of an insect. In this method, a rod with a square cross section is used as a sensor block to transform the force variation to tilt angle variation, and an acousto-optic deflector is used as a diffractive scanner to measure these angle variations in two directions. We first calibrate the force variation with respect to the tilt angle variation and find that the uncertainty in force measurement for the TSC system is 8.8×10-4 N. We then apply the method to measure the dynamic angle variations and the beating force of a bumblebee. ©

Scanning beam collimation method for measuring dynamic angle variations using an acousto‐optic deflector

A scanning beam collimation (SBC) method is developed for measuring dynamic angle variations in which an acousto-optic deflector is used as a diffractive scanner to compensate the angle variation in real time, and a position sensor is used to detect the position of the beam that is reflected from the reflector (i.e., object) to be measured. The angle variation can be measured at high speed due to the very high scanning speed available in the acousto-optic deflector. There was good agreement between the angle variation measured using the SBC method and that using a conventional electronic autocollimator. The repeatability, resolution, and linearity of the scanning beam collimation method were determined using a piezo translator to generate tilt angle; the results show that high accuracy also can be achieved in dynamic angle variation measurement. The SBC method was applied to measure the tilt angle variation caused by the beating force of a dragonfly attached to one end of a gauge block. The results confirmed the effectiveness of this new method.

Generation of arbitrary intensity profiles by combining the scanning beam with dynamic multileaf collimation

An algorithm, which combines the scanning beam with dynamic collimation to generate any arbitrary intensity profile, is presented. The desired intensity profile is assumed to be piecewise linear. The dynamic collimation method used is the "sliding window." The algorithm can be used either for a given scanning beam profile or to simultaneously determine the scanning beam profile and the leaf motions required to generate the desired intensity profile, which minimize the total treatment time. The limitations imposed by the physics of an elementary beam are taken into account. The algorithm is an iterative one, with typical calculation times being of the order of a few milliseconds.

True scattering coefficients of turbid matter measured by early-time gating

The true scattering coefficients of turbid matter have been determined by use of picosecond time-resolved imaging. The scattering coefficients measured by the conventional cw collimation method were found to be smaller than those obtained from the early-time-sliced ballistic photons.

Collimation method that uses spiral gratings and Talbot interferometry

The tested light is projected onto an arrangement composed of two special spiral gratings, in which the second spiral grating is positioned at the Talbot image of the first one. From the shapes of the resultant moiré fringes, the quality of collimation of the tested light can be easily checked.

Collimation method using fourier imaging and moire techniques

Two techniques are proposed for beam collimation with the Talbot interferometer. One is called the mismatch method in which two gratings with slightly different periods are used. In the other method (called the grating shift method), one of two gratings of the same period is shifted in its plane in the direction perpendicular to the grating lines.

Identification of the elements 102 and 104 by means of the collimation method

Identification of the elements 102 and 104 by means of the collimation method

透過型顕微干渉法による微小球面の形状測定

An application of transmitted-type microinterferometry on the form measurement of small spherical parts is investigated.Replicas of the spherical parts to be measured, made of methyl-and ethyl-metaacrylate on a plane mirror, are immersed in oil and observed on an interference microscope with a low magnification. By using proper oil, Newton's rings can be observed over required area on the spherical part irrespective of surface finish.By measuring the radii of curvature of five steel balls and their repricas with microscopic collimation method (Guild's mirror method), it is confirmed that the replicas have satisfactory reproducibility and stability.The equation of a sphere representing the form of each specimen is derived by measuring the positions of interference fringes and by applying Deming's method of least squares and mean radius of curvature and form errors are evaluated.As an example of practical application, the tip forms of the standard hammers for Shore hardness test are measured by this method. The result is compared with the measurement by microscopic collimation and it is revealed that the radius of about 1 mm can be measured with the precision of about 3 μm in standard deviation if the specimen has a good sphericity.