Madhuri Kaul

The Atacama Cosmology Telescope: a measurement of the cosmic microwave background power spectrum at 148 and 218 GHz from the 2008 southern survey

We present measurements of the cosmic microwave background (CMB) power spectrum made by the Atacama Cosmology Telescope at 148 GHz and 218 GHz, as well as the cross-frequency spectrum between the two channels. Our results clearly show the second through the seventh acoustic peaks in the CMB power spectrum. The measurements of these higher-order peaks provide an additional test of the ΛCDM cosmological model. At l>3000, we detect power in excess of the primary anisotropy spectrum of the CMB. At lower multipoles 500 < l < 3000, we find evidence for gravitational lensing of the CMB in the power spectrum at the 2.8σ level. We also detect a low level of Galactic dust in our maps, which demonstrates that we can recover known faint, diffuse signals.

The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey

We report on extragalactic sources detected in a 455 deg2 map of the southern sky made with data at a frequency of 148 GHz from the Atacama Cosmology Telescope (ACT) 2008 observing season. We provide a catalog of 157 sources with flux densities spanning two orders of magnitude: from 15 mJy to 1500 mJy. Comparison to other catalogs shows that 98% of the ACT detections correspond to sources detected at lower radio frequencies. Three of the sources appear to be associated with the brightest cluster galaxies of low-redshift X-ray-selected galaxy clusters. Estimates of the radio to millimeter-wave spectral indices and differential counts of the sources further bolster the hypothesis that they are nearly all radio sources, and that their emission is not dominated by re-emission from warm dust. In a bright (>50 mJy) 148 GHz selected sample with complete cross-identifications from the Australia Telescope 20 GHz survey, we observe an average steepening of the spectra between 5, 20, and 148 GHz with median spectral indices of α5-20 = –0.07 ± 0.06, α20-148 = –0.39 ± 0.04, and α5-148 = –0.20 ± 0.03. When the measured spectral indices are taken into account, the 148 GHz differential source counts are consistent with previous measurements at 30 GHz in the context of a source count model dominated by radio sources. Extrapolating with an appropriately rescaled model for the radio source counts, the Poisson contribution to the spatial power spectrum from synchrotron-dominated sources with flux density less than 20 mJy is C Sync = (2.8 ± 0.3) × 10–6μK2.

The Atacama Cosmology Telescope: Calibration with the Wilkinson Microwave Anisotropy Probe using cross-correlations

We present a new calibration method based on cross-correlations with the Wilkinson Microwave Anisotropy Probe (WMAP) and apply it to data from the Atacama Cosmology Telescope (ACT). ACTs observing strategy and map-making procedure allows an unbiased reconstruction of the modes in the maps over a wide range of multipoles. By directly matching the ACT maps to WMAP observations in the multipole range of 400 < l < 1000, we determine the absolute calibration with an uncertainty of 2% in temperature. The precise measurement of the calibration error directly impacts the uncertainties in the cosmological parameters estimated from the ACT power spectra. We also present a combined map based on ACT and WMAP data that has a high signal-to-noise ratio over a wide range of multipoles.We present a new calibration method based on cross-correlations with WMAP and apply it to data from the Atacama Cosmology Telescope (ACT). ACTs observing strategy and map making procedure allows an unbiased reconstruction of the modes in the maps over a wide range of multipoles. By directly matching the ACT maps to WMAP observations in the multipole range of 400 < ell < 1000, we determine the absolute calibration with an uncertainty of 2% in temperature. The precise measurement of the calibration error directly impacts the uncertainties in the cosmological parameters estimated from the ACT power spectra. We also present a combined map based on ACT and WMAP data that has high signal-to-noise over a wide range of multipoles.

Combining Surface Treatments With Shallow Slots to Improve the Spatial Resolution Performance of Continuous, Thick LYSO Detectors for PET

Positron emission tomography (PET) detectors based on continuous scintillation crystals can achieve very good performance and have a number of practical advantages compared to detectors based on a pixelated array of crystals. Our goal is to develop a thick continuous detector with high energy and spatial resolution, along with high γ -photon capture efficiency. We examine the performance of two crystal blocks: a 46 × 46 × 14 mm3 and a 48×48×25 mm3 block of LYSO (Lutetium Yttrium Orthosilicate). Using Maximum Likelihood (ML) positioning based upon the light response function (LRF) in the 14 mm thick crystal, we measure a spatial resolution of 3 mm in the central region of the crystal with degradation near the edges due to reflections off the crystal sides. We also show that we can match the spatial resolution achieved using a 14 mm thick crystal by using a 25 mm thick crystal with slots cut into the gamma entrance surface to narrow the LRF. We also find that we can improve the spatial resolution performance near the detector edges by reducing the reflectivity of the crystal sides, albeit with some loss in energy resolution.

Signal analysis for improved timing resolution with scintillation detectors for TOF PET imaging

Clinical TOF PET systems offer significantly degraded timing performance in comparison to that measured with small crystal geometries of the same scintillators. The usage of long, narrow crystal geometries is a major contributor to the degradation in system timing performance. We explore the effect of increased pixel length on timing performance of LaBr3[Ce], CeBr3 and LYSO detectors. Using fast signal digitization we characterize the response of the detector to 511keV photons interacting at different depths. Systematic shift in time pickoff with interaction depth is shown to degrade the timing performance of long crystals. Collected light, signal shape and signal arrival time are shown to correlate with interaction depth. The depth dependence of detector response shows strong dependence on surface treatment, with finer surface treatment corresponding to reduced depth sensitivity. The correlated dependencies of time pickoff, energy and signal shape offer a method for recovering timing information lost due to interaction depth dispersion. An energy-based correction to the time pickoff is shown to improve detector timing resolution. The improvement in timing performance of thick detectors with single sided readout demonstrates the benefit of inclusion of additional signal information attainable by fast signal digitization.

Design and Performance of a High Spatial Resolution, Time-of-Flight PET Detector

This paper describes the design and performance of a high spatial resolution positron emission tomography (PET) detector with time-of-flight capabilities. With an emphasis on high spatial resolution and sensitivity, we initially evaluated the performance of several 1.5 ×1.5 and 2.0 ×2.0 mm2 and 12-15 mm long LYSO crystals read out by several appropriately sized PMTs. Experiments to evaluate the impact of reflector on detector performance were performed and the final detector consisted of a 32 ×32 array of 1.5 ×1.5 ×15 mm3 LYSO crystals packed with a diffuse reflector and read out by a single Hamamatsu 64 channel multi-anode PMT. Such a design made it compact, modular and offered a cost-effective solution to obtaining excellent energy and timing resolution. To minimize the number of readout signals, a compact front-end readout electronics that summed anode signals along each of the orthogonal directions was also developed. Experimental evaluation of detector performance demonstrates clear discrimination of the crystals within the detector. An average energy resolution (FWHM) of 12.7 ±2.6% and average coincidence timing resolution (FWHM) of 348 ps was measured, demonstrating suitability for use in the development of a high spatial resolution time-of-flight scanner for dedicated breast PET imaging.

Development of a high-resolution and depth-of-interaction capable detector for time-of-flight PET

PET with its quantitative powers is becoming increasingly popular in the clinic. While the detector spatial resolution and sensitivity directly affect its ability, it has been shown that including the time-of-flight information further enhances its powers. Currently numerous approaches are being pursued to improve spatial resolution and timing, but most involve trade-offs. We describe here the development of a high-resolution PET detector with time-of-flight capabilities. The detector design is based on our previously developed pixelated Anger-logic detector where an array of individual crystals is readout by an array of larger photomultiplier tubes (PMTs) coupled to it via a light-guide. Depth-of-interaction (DOI) measurement in this design is accomplished by making use of a dual crystal-layer offset relative to each other. With a target spatial resolution of 1–2 mm, we have carefully evaluated the performance of several 1.5 × 1.5 and 2.0 × 2.0 mm2 and 10–20 mm long LYSO crystals readout by several appropriately sized PMTs. Experiments and simulations were used to investigate the design, and optimize performance of the detector. An experimental prototype using a single 8 × 7 array of 1.5 × 1.5 × 12 mm3 LYSO crystals readout by a 7-PMT array of the Hamamatsu R4124 PMTs was developed. A high-speed waveform sampling data acquisition system based on the DRS4 switched-capacitor that digitizes data at 5 GS/s was also built. Experimental evaluations demonstrate that the detector provides very good timing and also successfully discriminates 1.5 × 1.5 mm2 cross-section scintillation crystals. Bench-top timing measurements with a dual-layer detector demonstrate that relatively good timing can be maintained in a stacked crystal arrangement, suggesting the feasibility for extending the approach to incorporate DOI with this design.

Thick continuous crystal design for PET

Our goal is to develop a thick continuous detector with high spatial resolution (1–2-mm). We will use simulations with experimental measurements to examine performance of two crystal blocks: a 50×50×25mm3 and a 25×25×25mm3 block of LYSO. Based upon the light distribution pattern in the 50×50×25mm3 crystal, calculations show that a minimum spatial resolution of 2mm can be achieved in the central region of the crystal with some degradation near the edges. Monte Carlo simulations modeling Compton scatter and scintillation photon travel paths indicate that with Anger positioning, a spatial resolution of 2mm is achieved in the central region of the crystal but there is a significant degradation near the crystal edge due to light reflections. We show that we can improve the spatial resolution obtained with Anger positioning at the center and the edges of the crystal by simulating 5mm deep slots cut into the entrance surface of the crystal. Using a maximum-likelihood (ML) positioning algorithm, there is some improvement in the spatial resolution in the central region, but a more noticeable improvement at the unslotted crystal edges. We perform simulations to determine the best spatial resolution that can be achieved in the two continuous LYSO crystal blocks. We measure the energy and timing resolution of both crystals as a first step towards measuring the spatial resolution.

Initial imaging results from a high spatial-resolution time-of-flight PET detector designed for dedicated breast imaging

This paper discusses initial imaging results from a high-resolution time-of-flight detector specifically developed for a limited-angle dedicated breast PET scanner. To maintain high spatial-resolution and sensitivity, the detector design consists of 32 × 32 array of 1.5 × 1.5 × 15 mm3 LYSO crystals coupled to a single Hamamatsu H8500 multi-anode photomultiplier tube with a modified high-voltage divider circuit. To minimize the number of readout channels, compact front-end electronics that summed anode-signals along each of the orthogonal directions was also developed. Experimental performance evaluation of a complete detector-module demonstrates excellent energy, timing resolution and clear discrimination of most crystals. An average energy resolution of about 12.7% FWHM and an average coincidence timing resolution of 348 ps for two such detectors was measured. The dedicated breast PET scanner comprises of two parallel detector heads, each 15 cm by 10 cm and comprised of two rows of three detector-modules. We also experimentally evaluated the imaging capability of the scanner design via an experimental benchtop-demonstrator consisting of two fully assembled detector-modules on opposing translational stages. Imaging experiments with a hot lesion phantom that had an 8-mm diameter lesion with 8:1 activity uptake ratio, successfully demonstrate the capability of the system in imaging small lesions in a uniform background.

Characterizing the spatial resolution performance of continuous & thick LYSO crystals

Our goal is to develop a thick continuous detector with high spatial resolution. We will use simulations with experimental measurements to examine performance of two crystal blocks: a 46×46×14mm3 and a 48×48×25mm3 block of LYSO. Based upon the light distribution pattern in the 46×46×14mm3 crystal, we can measure spatial resolution of 3mm in the central region of the crystal with degradation near the edges due to reflections off the crystal sides. We show that we can match the spatial resolution obtained with Maximum Likelihood (ML) positioning of the 14mm thick crystal with that of the 25mm thick crystal using slots cut into the gamma entrance surface of the crystal. Also by modifying the reflectivity of the crystal sides we can improve the spatial resolution performance near the edges with some loss in energy resolution.