Intensity-based 2-D-3-D registration of cerebral angiograms

We propose a new method for aligning three-dimensional (3-D) magnetic resonance angiography (MRA) with 2-D X-ray digital subtraction angiograms (DSA). Our method is developed from our algorithm to register computed tomography volumes to X-ray images based on intensity matching of digitally reconstructed radiographs (DRRs). To make the DSA and DRR more similar, we transform the MRA images to images of the vasculature and set to zero the contralateral side of the MRA to that imaged with DSA. We initialize the search for a match on a user defined circular region of interest. We have tested six similarity measures using both unsegmented MRA and three segmentation variants of the MRA. Registrations were carried out on images of a physical neuro-vascular phantom and images obtained during four neuro-vascular interventions. The most accurate and robust registrations were obtained using the pattern intensity, gradient difference, and gradient correlation similarity measures, when used in conjunction with the most sophisticated MRA segmentations. Using these measures, 95% of the phantom start positions and 82% of the clinical start positions were successfully registered. The lowest root mean square reprojection errors were 1.3 mm (standard deviation 0.6) for the phantom and 1.5 mm (standard deviation 0.9) for the clinical data sets. Finally, we present a novel method for the comparison of similarity measure performance using a technique borrowed from receiver operator characteristic analysis.     

Protecting File Systems with Transient Authentication

Laptops are vulnerable to theft, greatly increasing the likelihood of exposing sensitive files. Unfortunately, storing data in a cryptographic file system does not fully address this problem. Such systems ask the user to imbue them with long-term authority for decryption, but that authority can be used by anyone who physically possesses the machine. Forcing the user to frequently reestablish his identity is intrusive, encouraging him to disable encryption. This tension between usability and security is eliminated through Transient Authentication, in which a small hardware token continuously authenticates the users presence to the laptop over a short-range, wireless link. Whenever the laptop needs decryption authority, it acquires it from the token; authority is retained only as long as necessary. With careful key management, ZIA imposes an overhead of less than 7% for representative workloads, though some infrequent operations suffer greater overheads. The largest file cache on our hardware can be re-encrypted within five seconds of the users departure, and restored in just over six seconds after detecting the users return. This secures the machine before an attacker can gain physical access, but recovers full performance before a returning user resumes work. Key granularity plays an important role in determining performance; assigning encryption keys on a per-directory basis limits the cost of an exposed key while maintaining acceptable overhead.     

Antiproton-neutron interactions at 100 GeV/c

From an exposure of the 30-inch deuterium filled bubble chamber to a 100 GeV/c antiproton-enriched beam at FNAL, we have calculated the topological cross sections for antiproton-neutron interactions with three or more charged particles in the final state. A comparison of our data with pn data at the same momentum allows us to estimate the annihilation contribution to each multiplicity, and hence calculate the average value, and width, of the annihilation multiplicity distribution. Our results are consistent with those from $\text{p}\text{p}$ data at 100 GeV/c, but depart from the trend of lower-energy $\text{p}\text{p}$ data.     

Off-shell effects in the (p, 2p) reaction

It is shown that, within the limitations of the distorted-wave off-shell impulse approximation, future accurate (p, 2p) experiments near 150 MeV incident energy can be helpful in distinguishing between some of six phenomenological nuclear potentials that are examined. On-shell differences between the Reid soft-core and five different one-boson-exchange potentials are amplified in the (p, 2p) context in such a way that an accurately measurable feature, the peak height ratio for 1p knockout, is sensitive to them.     

Graphical displays of acoustic properties of solids

The presentation of acoustic laws in diagrammatic form has found use in several ways, R12 particularly in determining acoustic quantities from elastic quantities, in choosing materials for line-line matching, and in tracing variations of properties with temperature.     

Pressure effects in a [2+2] photocycloaddition and -reversion

Evidence is presented to support the conclusion that the [2+2] photocycloaddition of acrylonitrile to naphthalene has a late transition state similar in volume to those in concerted thermal cycloadditions.     

Structure of 72,74Se at high spin

Lifetimes of high spin states up to { }=22⁺ in the yrast positive parity bands have been measured to investigate the shape evolution with increasing spin in ^{72, 74}Se. The Q _t values derived from these measurements indicate that prolate shape stabilizes for ⁷²Se, while a triaxial shape develops for ⁷⁴Se at higher spins. Comparison of the observed trend in Q _t with spin for ^{72, 74}Se with that of the corresponding kryptones isotones emphasizes the stability provided by N=38 prolate shell gap even at high rotational frequency.     

Vascular segmentation of phase contrast magnetic resonance angiograms based on statistical mixture modeling and local phase coherence

In this paper, we present an approach to segmenting the brain vasculature in phase contrast magnetic resonance angiography (PC-MRA). According to our prior work, we can describe the overall probability density function of a PC-MRA speed image as either a Maxwell-uniform (MU) or Maxwell-Gaussian-uniform (MGU) mixture model. An automatic mechanism based on Kullback-Leibler divergence is proposed for selecting between the MGU and MU models given a speed image volume. A coherence measure, namely local phase coherence (LPC), which incorporates information about the spatial relationships between neighboring flow vectors, is defined and shown to be more robust to noise than previously described coherence measures. A statistical measure from the speed images and the LPC measure from the phase images are combined in a probabilistic framework, based on the maximum a posteriori method and Markov random fields, to estimate the posterior probabilities of vessel and background for classification. It is shown that segmentation based on both measures gives a more accurate segmentation than using either speed or flow coherence information alone. The proposed method is tested on synthetic, flow phantom and clinical datasets. The results show that the method can segment normal vessels and vascular regions with relatively low flow rate and low signal-to-noise ratio, e.g., aneurysms and veins.     

The characterization of capacitive micromachined ultrasonic transducers in air

Surface micromachined, capacitive ultrasonic transducers have been fabricated using a low thermal budget, CMOS-compatible process. They exhibit interesting properties for transduction in air at frequencies in excess of 1 MHz, when driven from a standard ultrasonic voltage source. Experiments are described using 1 mm square devices in air, operating in both pitch-catch and pulse-echo modes. The dependence on d.c. bias voltage is examined, together with calibration measurements using 1/8 in. microphones. The radiated beam profile, and the farfield directivity pattern, have been measured for both broad bandwidth and one-burst excitation, using a scanned miniature receiver. A 16 element square array is also presented, which has been used to measure the beam cross-sections from a focussed source.