Walter Huda

Binocular three-dimensional perception through stereoscopic generation from rotating images

RATIONALE AND OBJECTIVES The authors evaluated the clinical utility and potential applications of a binocular three-dimensional (3D) image display in diagnostic radiology. MATERIALS AND METHODS Rotating video displays of computed tomographic (CT), magnetic resonance (MR) angiographic, and digital subtraction angiographic (DSA) image data were used to generate stereoscopic image displays with a 3D appearance. Eight physicians viewed and scored eight skeletal and eight vascular-interventional studies with a planar display mode and a cathode ray tube. Each physician then viewed the 3D display of the same data and assessed the change in image findings, as well as any corresponding changes in level of diagnostic confidence. RESULTS Image findings changed in 78 (61%) of the 128 studies after viewing the 3D displays. In 94 (73%) of all 128 studies, the interpreters reported increased confidence in their perception of the findings. Results for the vascular-interventional and skeletal cases were generally very similar. CONCLUSION Binocular 3D stereoscopic displays from rotating images were reported to provide better image conceptualization and a higher degree of confidence in the findings on the images.

Signal-to-noise ratio and radiation dose as a function of photon energy in mammography

This study investigated the signal to noise ratio (SNR) and radiation dose as a function of photon energy in screen-film mammography. An analytical expression was derived for the SNR for monoenergetic photons incident on simulated masses and microcalcifications embedded in uniform slabs of Lucite. The mean dose, D, was determined by dividing the total energy deposited in the Lucite phantom by the corresponding phantom mass. SNR and dose data for different photon energies were normalized to a constant value of energy absorbed by a 34 mg/cm2 Gd2O2S screen. A figure of merit (FOM), defined as SNR2/D, permitted the optimum photon energy to be determined for each imaging task. For microcalcifications, the optimum energy was dependent on the size of the microcalcification, and increased from 19 keV for 100 micrometer to 22 keV for 500 micrometer imaged in a 4 cm Lucite phantom. The optimal photon energy for microcalcifications was 16 - 19 keV for a 2 cm phantom, increasing to 24 - 26 keV for an 8 cm phantom. For simulated masses of all diameters (2 mm to 10 mm) and thickness (0.15 to 0.6 mm), the optimal photon energy was approximately 17 keV for a 2 cm phantom, and increased to approximately 32 keV for an 8 cm phantom.

Detective quantum efficiency of a CsI:Tl scintallator-based scanning slot x-ray detector for digital mammography

An investigation was made of the key determinants of the detective quantum efficiency at zero spatial frequency [DQE(0)] of a CsI:Tl scintillator based scanning slot x- ray detector for digital mammography. The slot x-ray detector was made of a prismatic type thallium activated CsI scintillator (150 micrometer thick) optically coupled to CCDs by fiber optical image guides. Monte Carlo calculations were performed to generate the CsI:Tl scintillator Swank factor on the basis of the energy deposition from pencil beam x-ray sources and light transmission within the CsI:Tl scintillator. A theoretical expression for the detector DQE(0) was obtained which was used to investigate the detector imaging performance as a function of x-ray energy, x-ray exposure, CCD electronic noise level, and optical coupling efficiency of the fiber optic image guide. The Swank factor of the CsI:Tl scintillator was close to unity at x-ray energies below Iodine K-edge (33.2 keV), but decreased to approximately 0.8 at higher x-ray energies up to 40 keV. DQE(0) of the slot x-ray detector was approximately 75% at 15 keV but decreased to approximately 40% at 30 keV. Optimum DQE(0) performance of the slot x-ray detector was generally obtained at a detector x-ray exposure level above approximately 5 to 10 mR and an electronic noise level below approximately 50 electrons rms. A drop in the optical coupling efficiency of the image guide from 1.0 to 0.3 reduced the detector DQE(0) from approximately 75% to approximately 55% in the mammography x-ray energy range. The key finding in this study is that the choice of the x-ray energy has a major impact on the DQE(0) of a CsI:Tl scintillator based slot x-ray detector. Since the x-ray photon energy also affects x-ray tube loading, mean glandular dose and subject contrast, the choices of optimal x-ray spectra from current mammography x-ray tubes require further investigation.

Energy selective laser-based X-ray source for mammography

X-ray spectra suitable for mammography, created by laser-based x-ray source at laser intensity about 1018 W/cm2, where investigated. The spectra consisted of a continuous bremsstrahlung emission and discrete K(alpha ), K(beta ) lines and have been obtained for Mo, Rh, Ag, In and Sn targets (Z equals 42, 45, 47, 49, and 50) with K(alpha ) emissions at 17.4, 20.2, 22.2, 24.2, and 25.3 keV, respectively. The continuous bremsstrahlung component extended to high energies with no cut-off energy. The shape of the continuous bremsstrahlung spectrum was described by the function E-p, where p (greater than 0) was primarily determined by the temperature of hot electrons produced in laser beam- target interaction. The absolute value of x-ray yield was proportional to the atomic number of the target Z. The intensity of characteristic x-rays was about a factor of five higher than the corresponding bremsstrahlung, and showed weak dependence (increase) when the atomic number of the target increased from Z equals 42 to Z equals 50.

Evaluation of Contrast Enhancement by Digital Equalization in Digital Mammography

Purpose: This study evaluated an algorithm based on a method of contrast enhancement by digital equalization (CEDE). Method: The algorithm was designed to enhance image contrast by employing digital equalization of digital mammograms. The CEDE algorithm was tested using ten mammograms with cancer (13 lesions) taken the University of South Florida data base, together with eight mammograms which only contained benign lesions. Three readers compared the processed images with the original mammograms for lesion conspicuity. A five point ranking scale was employed where a score of 3 corresponded to equal lesion visibility, ranks > 3 corresponded to superior lesion visibility, whereas ranks < 3 corresponded to markedly inferior lesion visibility. Results: The mean observer score for all lesions was always at least equal to that of the original digital mammogram (i.e., 3 or greater), and there was no evidence of any image distortion or other image processing artefacts. The mean rank (± standard deviation) for the 13 malignant lesions was 3.52 ± 0.38. The corresponding rank for the eight benign lesions was 3.33 ± 0.26. These differences were statistically significant in terms of standard error. Conclusion: The CEDE algorithm is capable of significantly enhancing lesion contrast in digital mammograms and our preliminary results indicate that this algorithm merits additional refinement and further (objective) evaluation.

Radiographic technique factors and imaging performance in digital mammography

Simulated mass lesions were superimposed onto an anthropomorphic breast phantom and x-rayed using a small field of view digital mammography system. Digital radiographs were acquired at a range of x-ray tube potential (constant detector exposure), and a range of x-ray tube current-time product values (constant x-ray tube potential). Twelve readers assessed the probability of a simulated mass being present in specified regions of interest, with the resultant data used to determine the area under the receiver operating characteristic curve (Az). The mean Az value obtained at 28 kVp/72 mAs was 0.69. At the same x-ray tube potential, the Az score fell to 0.63 (p less than 0.05) at 32 mAs, whereas the mean Az score of 0.71 was not significantly different for the image acquired at 144 mAs. At a constant detector exposure, reducing the x-ray tube potential to 22 kVp (320 mAs) resulted in a mean Az value of 0.72, whereas increasing the x-ray tube potential to 34 kVp (28 mAs) resulted in a mean Az value of 0.69. For the detection of simulated mass lesions in an anthropomorphic breast phantom, changing the kVP at a constant detector exposure had no significant effect on imaging performance, whereas halving the mAs at a constant kVp reduced the Az value by approximately 10%.

Spatial-frequency-dependent DQE performance of a CsI:Tl-based x-ray detector for digital mammography

Monte Carlo calculations were performed to generate the point spread functions of x-ray photons absorbed in a CsI:Tl x-ray detector at the x-ray energies normally used in mammography (i.e., 20 keV to 50 keV). The corresponding modulation transfer functions [MTF(f)] for the CsI:Tl screen were also computed, taking into account the optical spread of light within the CsI:Tl crystals. The computed MTF(f)s were dominated by scintillation light lateral dispersion within the CsI:Tl screen. For the photon energy range encountered in digital mammography, the MTF(f) was a minimum at an x-ray photon energy just above the k-edge of Iodine (33 keV). Noise propagation theory for a cascaded imaging system was subsequently used to derive a theoretical expression for the detector DQE(f), including the dependence of DQE(f) on the spatial distribution of x-ray photon energy deposition. Detector performance was investigated as a function of x-ray exposure, CCD electronic noise, coupling efficiency of the fiber optical coupler, and the CCD quantum efficiency. Although most of the x-rays are absorbed via the photoelectric effect, the deposited x-ray energy spread within the CsI:Tl screen from the emission of characteristic x-rays can have a marked effect on detector performance, and the DQE(f) was found to decrease rapidly with photon energy just above the Iodine K-edge. X-ray exposure levels to the detector should be greater than or equal to 5 mR with a CCD electronic noise of approximately 20 electrons rms to ensure that DQE(f) performance is not significantly degraded at the spatial frequencies important in digital mammography (i.e., 0 to 10 lp/mm). Light dispersion within the CsI:Tl crystals was the major factor degrading imaging system DQE(f) at higher spatial frequencies. Optical coupling efficiency and CCD quantum efficiency are important system design parameters, which need to be maintained at a relatively high value. An optical coupling efficiency of approximately 0.7, and a CCD quantum efficiency of approximately 0.4, would still permit system DQE values greater than 60% at a spatial frequency of 5 lp/mm.

Effects of radiation dose and display contrast on low-contrast phantom image visibility

Computed radiography (CR) radiographs were generated of a low contrast phantom with 5 mm diameter disks. The radiation exposure incident on the imaging plate was varied from approximately 0.1 mR to approximately 10 mR, with the phantom images printed to film using a range of display contrast settings. Changing the radiation exposure by two orders of magnitude had only a modest effect on disk detection performance (approximately 20%), and much less than predicted by signal detection theory for the perception of noise limited objects. For images generated at approximately 1 and approximately 10 mR, increasing the display contrast markedly improved the disk detection performance (approximately 50%). There was approximate agreement between the experimental data and the corresponding theoretical predictions for the detection of contrast limited objects. For the contrast detail phantom employed in this study, disk detection was primarily contrast limited, with image noise being relatively unimportant. Lesion detection with an anthropomorphic phantom containing a structured background wold be unlikely to change this conclusion, since noise is expected to be most important for low contrast objects viewed against a uniform background. Contrast enhancement, as opposed to increasing radiation exposure, is therefore the method of choice for improving the detection of 5 mm diameter sized low contrast lesions in CR images.

Wavelet CFAR detector for mass detection in mammography

We introduce a continuous scale wavelet detector for identifying masses in mammograms. Continuous-scale wavelet algorithms have been discussed in the past, however this is the first reported algorithm that uses a scaled version of the same mother wavelet at each scale of analysis. This single mother wavelet property leads to a simpler implementation and a more direct application of detection theory to recognition problems than traditional multiscale analysis. In addition, we show that a continuous-scale search is necessary for computer aided diagnosis of mammography since traditional solutions using dyadic scales either fail to detect some masses or signal too many false alarms. Our novel wavelet detector combines a wavelet formulation with the classical theory of constant false alarm rates detectors. Finally, we show that our algorithm is able to detect masses in actual mammograms that could not be seen using conventional windowing and leveling or other traditional methods of contrast enhancement.

ULTRA-SMALL FOCAL SPOT X-RAY SOURCES FOR HIGH RESOLUTION DIGITAL MAMMOGRAPHY

The laser-based x-ray source utilizes a high-power and coherent light beam for x-rays production. In this device laser beam impinge on a solid target. Initially, a thin layer of solid density cold plasma (with electron temperature Te∼0.5 keV is being created on the surface of the target [1]. In the next stage, energy is transferred from the laser light to free electrons in the solid plasma. As a result, a low density hot plasma is being created above the target with hot electron temperature, Te≈25 keV). Up to 40% of laser light energy can be transferred to hot (suprathermal) electrons. A significant fraction of hot electrons returns to the positively charged space-charge region on the target surface previously created due to emission of suprathermal electrons. Consequently, the hot electrons penetrate the target producing a burst of incoherent x-rays, composed of continuous bremsstrahlung emission and discrete characteristic x-ray emission lines [2], The effective size of the Laser Produced Plasma (LPP) x-ray source and the duration of the x-ray pulse always exceeds the size and the duration of laser light pulse. Nevertheless, it is extremely small (tens of microns) and short (picoseconds). The conversion efficiency of the LPP x-ray source (F x ) defined as the ratio of the energy emitted in the x-ray burst generated by LPP to the energy in the laser light pulse, is approximately proportional to the square root of the laser beam intensity, i.e. to the peak electric field of the laser light, and to the atomic number Z of the target, F x ∝ E peak · Z [3]. It is analogous to the x-ray tube conversion efficiency law, F x ∝ V·Z and similar conversion efficiency can be expected (i.e. below 1%) in both cases.