Fourier Telescopy (FT) is an active imaging method which interferes spatially diverse, frequency-encoded laser beams
on a distant target, and records a time history of the reflected intensity measured by a single photodetector on a large
receiver. FT has been studied extensively for imaging Geostationary objects, using high-energy pulsed lasers to project
triplets of laser beams, by gradually stepping over time through the multitude of u,v-plane baselines required for accurate
object reconstruction. Phase closure among the received triplets plays a key role in canceling out random atmospheric
phase errors between laser beams. A new method has been devised to apply FT to rapidly moving targets, such as LEO
space objects. In order to implement the thousands of baselines in a short engagement time, approximately 20
continuous-wave laser beams are simultaneously broadcast, and the baseline configurations are rapidly changed through
a dynamic optical element. In order to eliminate unknown atmospheric errors, a new type of global phase closure has
been developed, which allows image reconstruction from the time history of measured total reflected intensity,
originating from the complex 20-beam interference patterns. In this paper, we summarize the new FT LEO method, and
give a detailed derivation of the phase closure and image reconstruction algorithms that will lead to ultra-high resolution
images of fast-moving space objects.
Software has been developed for the SAINT program that simulates the operation of a Fourier Telescopy imaging system that could potentially be used to create images of a satellite in low earth orbit. Fourier telescopy uses multiple beams that illuminate the target with a fringe pattern that sweeps across it due to frequency differences between beams. In this way the target spatial frequency components are encoded in the temporal signal that is reflected from the target. The software simulates the propagation effects and target interaction effects that would be present in a real system. This enables the creation of a simulated received signal as a function of time. A particular problem was accurately modeling the appearance of the target as its aspect changes during a rapid transit over the transmitter and receiver. A novel reconstructor has been developed that compensates for atmospheric phase fluctuations affecting the large number of beams transmitted simultaneously (~10). The reconstructor solves for hundreds of image Fourier components simultaneously, permitting rapid reconstruction of the image.
Coherent light of one color will form laser speckle upon reflection from a rough object. This laser speckle provides information about the shape of an object. Further information can be obtained if two colors are used, and if the colors are sufficiently close in wavelength that the interference is also measurable. The two speckle patterns and the interference can be shown to provide the minimum information sufficient to form a band-limited image of the object using a root-matching technique described herein. This root-matching technique is performed in the far-field or object plane. This technique is relatively slow and sensitive to noise, and so is supplemented with a technique that minimizes error in the pupil plane. A hybrid technique that combines the two approaches is shown to reproduce images effectively with reasonable computation time.
This paper describes a novel area detector for direct conversion and readout of the x-ray energy that eliminates multiple conversions and coupling stages which degrade performance. The pixel array and readout electronics are fabricated on the same piece of silicon. The detector consists of a uniform layer (approximately 300 micrometers) of amorphous selenium alloy vapor-deposited on an electronic readout array fabricated using conventional complementary metal oxide semiconductor (CMOS). The CMOS array features 66 micrometer pixels in a 1024 X 832 array providing a 5.5 X 6.75 cm image area. Each pixel has active circuitry including signal amplification, pixel selection and reset, while peripheral circuitry on one end of the array provides shift registers, sample and hold and multiplexing. The CMOS readout array was fabricated at a standard facility on a 10-cm diameter silicon wafer using 2 micrometer CMOS process. Fifteen separate image sensors were assembled for evaluation in a 3 X 5 format to provide a 20 X 27 cm composite field of view. Missing data between sensors is recovered by acquiring three sub-exposures, between which the array is translated diagonally approximately 2 mm. Total exposure time for an average breast is less than one second. Conversion efficiency was found to be approximately 120 electrons per absorbed x-ray (19 keV average). Electronic readout noise was measured to be 2.4 ADU corresponding to approximately 500 electrons. Detective quantum efficiency was found to be 0.65 at low spatial frequency (0.25 lp/mm) and at 0.2 at high spatial frequency (8 lp/mm) for x-ray fluence ranging from 5 - 35 mR. Images of an ACR phantom show visualization of all of the fibers, specks and masses when displayed with a linear lookup table on a high-resolution monitor. These studies demonstrated that there is a slight but measurable image retention evident as 'ghost' images. The two most effective means to reduce this effect are flushing the sensors with infrared light or x-rays between exposures and reversing the applied voltage on the selenium layer. A number of improvements designed to increase sensitivity and reduce noise also have been identified and are being implemented. Sample images were acquired from four volunteer human subjects at exposure factors identical to their film-screen mammograms. The results suggest that the detector performance is suitable for further clinical investigation.
Tungsten (W) target x-rays tubes are being studied for use in digital mammography to improve x-ray flux, reduce noise and increase tube heat capacity. A parametric model was developed for digital mammography to evaluate optimization of x-ray spectra for a particular sensor. The model computes spectra and mean glandular doses (MGD) for combinations of W target, beam filters, kVp, breast type and thickness. Two figures of merit were defined: (signal/noise)2/MGD and spectral quantum efficiency; these were computed as a means to approach optimization of object contrast. The model is derived from a combination of classic equations, XCOM from NBS, and published data. X-ray spectra were calculated and measured for filters of Al, Sn, Rh, Mo and Ag on a Eureka tube. (Signal/noise)2/MGD was measured for a filtered W target tube and a digital camera employing CsI scintillator optically coupled to a CCD for which the detective quantum efficiency (DQE) was known. A 3-mm thick acrylic disk was imaged on thickness of 3-8 cm of acrylic and the results were compared to the predictions of the model. The relative error between predicted and measured spectra was +/- 2 percent from 24 to 34 kVp. Calculated MGD as a function of breast thickness, half-value layer and beam filter compares very well to published data. Best performance was found for the following combinations: Mo filter with 30 mm breast, Ag filter with 45 mm, Sn filter for 60 mm, and Al filter for 75 mm thick breast. The parametric model agrees well with measurement and provides a means to explore optimum combinations of kVp and beam filter. For a particular detector, this data may be used with the DQE to estimate total system signal-to-noise ratio for a particular imaging task.
Digital imaging offers major advantages over conventional film radiology, especially with respect to image quality, the speed with which the images can be viewed, the ability to perform image processing, and the potential for computer aided diagnosis. A typical mammographic image requires 10 million pixels of data, assuming 50 micrometers square pixels. Currently, there are not single sensor that can satisfy these specifications. One approach to acquiring full-breast digital images utilizes multiple sub-images from two 1024 by 1024 pixel charge coupled devices. This paper describes how the full-breast image is obtained by translating the sensor apparatus and 'stitching' the sub-images together. Radiologist desire seamless full-breast images, so a 'blending' process was developed to prevent visible seams in the full-breast image. Also, flaws in the detection system are removed by image processing techniques. FInally, the process of enhancing an image for film printing is described.
The resolution achievable in imaging objects in space from ground-based telescopes is limited by atmospheric turbulence. If enough naturally occurring illumination is available then speckle imaging techniques can be used to recover the original object phase using short exposure images. Analogous techniques exist for recovering the phase of a laser illuminated object from measurements of either the incoherent Fourier modulus or coherent Fourier modulus. In both cases many exposures are required to accumulate sufficient statistics. In the case of coherent illumination lack of a priori information concerning the object makes image reconstruction very difficult. In this paper we discuss one approach to circumventing these difficulties, in which multiple modulated laser beams are broadcast off of an object and the relative phase between the beams is measured at a simple light-bucket receiver. The original object phase is recovered from the phase differences using an iterative reconstructor.