It is generally well known that the appearance of breast tissue in a mammogram is considerably more complex
in a statistical sense than a simple random Gaussian texture, even when the correlation structure of the Gaussian has
been set to match the power-law power spectrum of mammograms. However there has not been a systematic way to
characterize the extent of departure from a Gaussian process. We address this topic here by proposing a noisy-Laplacian
distribution to model response histograms derived from digital (or digitized) mammograms.
We describe the distribution in terms of the probability density function and cumulative density function, as
well as moments up to fourth order. We also demonstrate the usefulness of the new distribution by fitting it to responses
from digital mammography.
The development of model observers for mimicking human detection strategies has followed from symmetric signals in
simple noise to increasingly complex backgrounds. In this study we implement different model observers for the
complex task of detecting a signal in a 3D image stack. The backgrounds come from real breast tomosynthesis
acquisitions and the signals were simulated and reconstructed within the volume. Two different tasks relevant to the
early detection of breast cancer were considered: detecting an 8 mm mass and detecting a cluster of microcalcifications.
The model observers were calculated using a channelized Hotelling observer (CHO) with dense difference-of-Gaussian
channels, and a modified (Partial prewhitening [PPW]) observer which was adapted to realistic signals which are not
circularly symmetric. The sustained temporal sensitivity function was used to filter the images before applying the
spatial templates. For a frame rate of five frames per second, the only CHO that we calculated performed worse than the
humans in a 4-AFC experiment. The other observers were variations of PPW and outperformed human observers in
every single case. This initial frame rate was a rather low speed and the temporal filtering did not affect the results
compared to a data set with no human temporal effects taken into account. We subsequently investigated two higher
speeds at 5, 15 and 30 frames per second. We observed that for large masses, the two types of model observers
investigated outperformed the human observers and would be suitable with the appropriate addition of internal noise.
However, for microcalcifications both only the PPW observer consistently outperformed the humans. The study
demonstrated the possibility of using a model observer which takes into account the temporal effects of scrolling through
an image stack while being able to effectively detect a range of mass sizes and distributions.
As an important clinical task, evaluating the placement of multiple coronary stents requires fine judgments
of distance between stents. However, making these judgments is limited by low system resolution, noise,
low contrast of the deployed stent, and stent motion during the cardiac cycle. We use task performance as a
figure of merit for optimizing image display parameters. In previous work, we described our simulation
procedure in detail, and also reported results of human observers for a visual task involving discrimination
of 4 gap sizes under various frame rates and number of frames. Here, we report the results of three spatial
model observers (i.e. NPW, NPWE, and PWMF) and two temporal sensitivity functions (i.e. transient and
sustained) for the same task. Under signal known exactly conditions, we find that model observers can be
used to predict human observers in terms of discrimination accuracy by adding internal noise.
The placement of multiple coronary stents requires fine judgments of distance between a deployed stent
and stent/guidewire assembly. The goal of this deployment is to achieve continuous and gapless coverage
between them. However, making these judgments is difficult because of limited system resolution, noise,
relatively low contrast of the deployed stent, and stent motion during the cardiac cycle. In this work, we
extend our previous work by investigating wider range of conditions associated with this task. The present
studies consider number of frames and frame rate separately, and include stabilization of the stents as a way
to quantify the performance effects of stent motion. We find that (1) stabilization reduces the uncertainty
when detecting the gap size; (2) observer performance increases with the number of frames; (3) the effect
of display frame rate is highly dependent on the motion of the target.
Evaluating the placement of multiple coronary stents requires fine judgments of distance between two or more deployed
stents in order to determine if there is continuous coverage without a gap or overlap between the two. These judgments
are made difficult by limited system resolution, noise, relatively low contrast of the deployed stent, and stent motion
during the cardiac cycle. In this work, we assess the effect of frame rate and number of frames used in a sequence on the
detection accuracy of gaps between the stents. Both of these factors can be used to reduce patient dose. We use real X-ray
coronary angiograms as backgrounds along with stents imaged separately with Lucite for similar beam attenuation.
Stents and simulated guidewires are embedded in the angiograms by adding optical densities after scatter subtraction.
Realistic motion is rendered by manually synchronizing the stent densities to vascular features in each image. We find
no significant difference the different frame rates or sequence lengths, indicating potential savings in dose.
Electroabsorption modulators (EAMs) based on the quantum confined Stark effect in multiple quantum wells (MQWs) have advantages for high-speed, low drive voltage, and high extinction ratio applications. In this paper, a traveling- wave electrode structure is proposed to achieve high bandwidths with long devices and lower drive voltages at 1.55 micrometers wavelength. An InGaAsP/InGaAsP MQW traveling-wave electroabsorption modulator (TWEAM) with a bandwidth above 20 GHz is fabricated. A drive voltage of 1.2 V for an extinction ratio of 20-dB is demonstrated. The effects of microwave transmission on the high-speed performance of TWEAMs are discussed. Successful data transmission experiments at 30 Gbit/s show a promising system performance of these devices. By using an integrated tandem TWEAM, pulse data transmission above 100 Gbit/s is achieved.
We analyze the performance of InP/GaAs fused 1.55 micrometers vertical-cavity lasers (VCLs) under analog modulation. Our VCLs employ a strain-compensated InGaAsP/InP multi-quantum well (MQW) active region sandwiched between two AlGaAs/GaAs distributed Bragg reflectors. The first AlGaAs layer of the p-doped top mirror is laterally oxidized for optical and electrical confinement. These devices exhibit the lowest threshold current as well as the highest temperature of continuous-wave operation of any electrically pumped long- wavelength VCL. Two different device designs are investigated and compared. Reduction of the MQW barrier strain and enhancement of the optical index guiding by the oxide layer lead to an improvement of VCL performance. However, parasitic effects limit the modulation bandwidth. Higher order harmonic distortion is measured and simulated using a rate equation model. The model includes a non-linear gain function, gain compression, spontaneous emission and Auger recombination as well as carrier density dependent absorption in the quantum wells which reduces the differential gain. The good agreement between measurement and simulation indicates that electron-photon interaction within the quantum wells dominates the non-linear distortion. Multiple higher order response peaks are measured and reproduced by the model.
Formed in January 1995, WEST is a DARPA-supported consortium investigating technologies for implementing add-drop and cross-connect switches operating at 10 Gbit/s. Using wavelength division multiplexing (WDM), each fiber supports 40 Gbit/s (4 by 10 Gbit/s) aggregate bandwidth for SONET/SDH operation. Consortium members include Rockwell Corporation, Ortel Corporation, UCSB, UCSD, UCLA, and Caltech/JPL.
A numerical program was developed to simulate the performance of fiber optic transmission systems and its accuracy was verified by comparisons between the calculated results and experiment data. Using this program, soliton transmission and dispersion compensation schemes at 40 Gbit/s were compared. It was shown that by using dispersion compensation, linear transmissions over 3000 km with non-return to zero (NRZ) and over 4000 km with return to zero (RZ) data forms are possible. Without special soliton transmission control techniques, single channel soliton transmission has no advantage over dispersion compensation transmission.