Level-set segmentation of pulmonary nodules in megavolt electronic portal images using a CT prior.

PURPOSE: Pulmonary nodules present unique problems during radiation treatment due to nodule position uncertainty that is caused by respiration. The radiation field has to be enlarged to account for nodule motion during treatment. The purpose of this work is to provide a method of locating a pulmonary nodule in a megavolt portal image that can be used to reduce the internal target volume (ITV) during radiation therapy. A reduction in the ITV would result in a decrease in radiation toxicity to healthy tissue. METHODS: Eight patients with nonsmall cell lung cancer were used in this study. CT scans that include the pulmonary nodule were captured with a GE Healthcare LightSpeed RT 16 scanner. Megavolt portal images were acquired with a Varian Trilogy unit equipped with an AS1000 electronic portal imaging device. The nodule localization method uses grayscale morphological filtering and level-set segmentation with a prior. The treatment-time portion of the algorithm is implemented on a graphical processing unit. RESULTS: The method was retrospectively tested on eight cases that include a total of 151 megavolt portal image frames. The method reduced the nodule position uncertainty by an average of 40% for seven out of the eight cases. The treatment phase portion of the method has a subsecond execution time that makes it suitable for near-real-time nodule localization. CONCLUSIONS: A method was developed to localize a pulmonary nodule in a megavolt portal image. The method uses the characteristics of the nodule in a prior CT scan to enhance the nodule in the portal image and to identify the nodule region by level-set segmentation. In a retrospective study, the method reduced the nodule position uncertainty by an average of 40% for seven out of the eight cases studied.

X-ray standing waves: a molecular yardstick for biological membranes.

Structural information on an atomic scale has been obtained for a Langmuir-Blodgett (LB) trilayer system by means of long-period x-ray standing waves. The LB trilayer of zinc and cadmium arachidate was deposited on a layered synthetic microstructure (LSM) consisting of 200 tungsten/silicon layer pairs with a 25 A period. A 30 A thermally induced inward collapse of the zinc atom layer that was initially located in the LB trilayer at 53 A above the LSM surface has been observed. The mean position and width of the zinc atom layer was determined with a precision of +/- 0.3 A.

Limitations to the determination of the optical properties of a thin film by combined ellipsometric and surface plasmon resonance measurements.

We consider the use of the ellipsometric constant Delta and the surface plasmon excitation angle theta of a metal substrate covered by a thin transparent isotropic film for the determination of the refractive index n and thickness d of the film. It is proved, using an expression for Delta(n,d) and theta(n,d) valid to firstorder in d, that it is in general not possible to find a unique n and d from measured values of Delta and theta. Exact calculation of Delta(n,d) and theta(n,d) for the case of a silver substrate covered by an organic thin film and 632.8-nm radiation confirms this result and establishes that the range of d where the proof applies is below ~35 nm. For a value of d of ~40 nm we find that the theoretical limit of uncertainty in n and d when both Delta and theta are known to +/-0.01 degrees is 0.01 and 0.05 nm, respectively.

Long-range surface plasmon electrooptic modulator.

An electrooptic modulator based on electrically varying the degree of coupling of a light beam to a long-range surface plasmon excitation is modeled. A specific embodiment of a modulator is shown to be capable of changing its reflectance from 0.00 to 0.84 on the application of 100 V across a 1-microm thick electrooptic film with a second-order susceptibility of 2 x 10(-7) esu. The affect that the width of the long-range surface plasmon excitation resonance has on the performance of the modulator is considered. Plane-wave calculations show that decreasing the width of the resonance increases the amount of modulation possible for a given voltage change. More realistic calculations which take into account diffraction of a beam of finite width show that a decrease in resonance width can in some cases degrade device performance.