Method for measuring the focal spot size of an x-ray tube using a coded aperture mask and a digital detector
- Dipartimento di Scienze Fisiche, Universita di Napoli Federico II, Complesso Universitario Monte S. Angelo Via Cinthia, I-80126 Napoli, Italy and Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Napoli, I-80126 Napoli (Italy)
Purpose: 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. Methods: 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. Results: 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 mmx0.105 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 mmx0.104 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 deg. in in-plane rotations and 1 deg. 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. Conclusions: 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.
- OSTI ID:
- 22096976
- Journal Information:
- Medical Physics, Vol. 38, Issue 4; Other Information: (c) 2011 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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LOW-DOSE FLUORORADIOGRAPHY
Related Subjects
ALGORITHMS
BIOMEDICAL RADIOGRAPHY
CALIBRATION
CAMERAS
COLLIMATORS
COMPARATIVE EVALUATIONS
COMPUTERIZED TOMOGRAPHY
EFFICIENCY
IMAGES
KEV RANGE 10-100
MAMMARY GLANDS
PHOTODETECTORS
PHOTONS
RADIATION SOURCES
RESOLUTION
RESPIRATORS
SEMICONDUCTOR DETECTORS
SILICON
TUBES
X RADIATION
X-RAY DETECTION
X-RAY TUBES