One or more bound planets per Milky Way star from microlensing observations.

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity or transit methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17-30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing. These planets are at least as numerous as the stars in the Milky Way. Here we report a statistical analysis of microlensing data (gathered in 2002-07) that reveals the fraction of bound planets 0.5-10 AU (Sun-Earth distance) from their stars. We find that 17(+6)(-9)% of stars host Jupiter-mass planets (0.3-10 M(J), where M(J) = 318 M(⊕) and M(⊕) is Earth's mass). Cool Neptunes (10-30 M(⊕)) and super-Earths (5-10 M(⊕)) are even more common: their respective abundances per star are 52(+22)(-29)% and 62(+35)(-37)%. We conclude that stars are orbited by planets as a rule, rather than the exception.

Energy-based scatter corrections for scintillation camera images of iodine-131.

UNLABELLED: The use of high-dose 131I antibody therapy requires accurate measurement of normal tissue uptake to optimize the therapeutic dose. One of the factors limiting the accuracy of such measurements is scatter and collimator septal penetration. This study evaluated two classes of energy-based scatter corrections for quantitative 131I imaging: window-based and spectrum-fitting. METHODS: The window-based approaches estimate scatter from data in two or three energy windows placed on either side of the 364-keV photopeak using empirical weighting factors. A set of images from spheres in an elliptical phantom were used to evaluate each of the window-based corrections. The spectrum-fitting technique estimates detected scatter at each pixel by fitting the observed energy spectrum with a function that models the photopeak and scatter, and which incorporates the response function of the camera. This technique was evaluated using a set of Rollo phantom images. RESULTS: All of the window-based methods performed significantly better than a single photopeak window (338-389 keV), but the weighting factors were found to depend on the object being imaged. For images contaminated with scatter, the spectrum-fitting method significantly improved quantitation over photopeak windowing. Little difference, however, between any of the methods was observed for images containing small amounts of scatter. CONCLUSION: Most clinical 131I imaging protocols will benefit from qualitative and quantitative improvements provided by the spectrum-fitting scatter correction. The technique offers the practical advantage that it does not require phantom-based calibrations. Finally, our results suggest that septal penetration and scatter in the collimator and other detector-head components are important sources of error in quantitative 131I images.

Use of a head dome system to compare i.v. methacholine-induced bronchoconstriction in conscious vs anesthetized rhesus monkeys.

The use of a newly developed head dome system has allowed measurement of pulmonary function in conscious monkeys. Such information is often desired, so that pharmacological or toxicological effects of administered compounds can be measured in the absence of effects from anesthetic agents. The current study was conducted to gain experience with this method and to allow the determination of the effects of sodium pentobarbital anesthesia (30 mg kg-1 i.v.) on the bronchoconstriction seen during i.v. infusion of methacholine in rhesus monkeys. Bronchoconstriction was measured as changes in respiratory resistance using a Buxco LS20 pulmonary mechanics computer. Four male rhesus monkeys (4.2-5.1 kg) were used. For the anesthetized exposures, the animals were intubated with a 4.0-mm cuffed endotracheal tube attached to a size 'O' Fleisch pneumotachograph. For the conscious exposures, the animals sat in restraining chairs with a custom-built head dome attached to the same pneumotachograph. In both cases, transthoracic pressure was monitored with an intrapleural catheter. Each monkey was infused with methacholine in stepwise doses, while anesthetized and conscious, until a 75% increase in respiratory resistance was seen. The ED50 values of 0.134 and 0.180 mg ml-1 methacholine were not significantly different in anesthetized vs conscious monkeys, respectively.

Post therapy imaging in high dose I-131 radioimmunotherapy patients.

The biodistribution of a trace-labeled I-131 antibody is used to predict the biodistribution of a high dose I-131 antibody for therapy. Internal radiation dose estimates derived from the trace-labeled antibody have been used to determine the I-131 doses in a phase I escalating dose therapy trial for hematologic malignancy. To confirm the hypothesis that the distribution of a trace- and high-dose labeled antibodies are similar, both trace (7-11 mCi, 259-407 MBq) and high-dose (100-800 mCi, 3700-29600 MBq) I-131 radiolabeled antibody infusion were imaged in 12 patients who were treated for leukemia or lymphoma. With specialized imaging techniques using lead attenuation sheets, clearance data from organs were obtained from serial gamma camera images. Biological clearance half times of I-131 from both trace and therapy level doses were in agreement. An exception was a patient who developed human antimouse antibody before therapy, and subsequently had rapid clearance of the therapy dose. The method was feasible, yielded reproducible results, and provided critical data for relating therapy toxicity to radiation absorbed dose estimates.

A method for imaging therapeutic doses of iodine-131 with a clinical gamma camera.

Imaging therapeutic doses of 131I-labeled monoclonal antibody would provide valuable biodistribution data for dosimetry, but gamma cameras are unable to accurately handle the corresponding high counting rate. To image patients undergoing radioimmunotherapy, we attached 1.6- to 6.4-mm-thick Pb sheets to the front face of a high-energy parallel-hole collimator. With this method, we were able to acquire planar images of up to 700 mCi of radiolabeled antibody 1 hr after infusion. Monte Carlo simulations indicated that less than 7% of the events counted in the photopeak window were due to 364-keV photons that scattered in the Pb attenuator. Measurements indicated that the Pb sheets degraded system resolution by no more than 13%. A quantitative comparison of trace and therapy biodistribution data from planar images of the same patient was made using corrections for Pb sheet attenuation and camera deadtime.

Evaluation of a clinical scintillation camera with pulse tail extrapolation electronics.

The performance of a new scintillation camera, designed for high event rate capability, was evaluated. The system consisted of a 400 mm field-of-view Nal(T1) camera with 61 photomultiplier tubes and modified General Electric Starport electronics. A significant feature of the system was circuitry for performing pulse tail extrapolation and separation of individual pulses involved in pulse pile-up events. System deadtime, flood field uniformity, energy resolution, linearity, spatial resolution, and bar phantom image quality were evaluated for count rates up to 200 kcps in a 20% photopeak window. Our results indicate that this camera design does not compromise image quality at normal clinical count rates and at higher event rates can provide better image quality and increased sensitivity over many Anger cameras currently employed in nuclear medicine.