Real-time acoustic emission monitoring of powder mass flow rate for directed energy deposition.

In order to ensure a reliable and repeatable additive manufacturing process, the material delivery rate in the directed energy deposition (DED) process requires in situ monitoring and control. This paper demonstrates acoustic emission (AE) sensing as a method of monitoring the flow of powder feedstock in a powder fed DED process. With minimal calibration, this signal closely correlates to the actual mass flow rate. This article describes the fabricated mass flow monitoring system, documents various conditions in which the actual flow rate deviates from its set value, and details situations that highlight the system's utility. While AE mass flow monitoring is not free of concerns, its features make it an attractive measurement technique in the powder-fed DED process. The work presented here highlights the results obtained and illustrates that accurate monitoring of powder flow in real-time regardless of environmental conditions within the build chamber is possible.

Investigating the limit of detectability of a positron emission mammography device: a phantom study.

PURPOSE: A new positron emission mammography (PEM) device (PEM Flex Solo II, Naviscan Inc., San Diego, CA) has recently been introduced and its performance characteristics have been documented. However, no systematic assessment of its limit of detectability has been evaluated. The aim of this work is to investigate the limit of detectability of this new PEM system using a novel, customized breast phantom. METHODS: Two sets of F-18 infused gelatin breast phantoms of varying thicknesses (2, 4, 6, and 8 cm) were constructed with and without (blank) small, shell-less contrast objects (2 mm thick disks) of varying diameters (3-14.5 mm) [volumes: 0.15-3.3 cc] and activity concentration to background ratio (ACR) (2.7-58). For the phantom set with contrast objects, the disks were placed centrally inside the phantoms and both phantom sets were imaged for a period of 10 min on the PEM device. In addition, scans for the 2 and 6 cm phantoms were repeated at different times (0, 90, and 150 min) post phantom construction to evaluate the impact of total activity concentration (count density) on lesion detectability. Each object from each phantom scan was then segmented and placed randomly in a corresponding blank phantom image. The resulting individual images were presented blindly to seven physician observers (two nuclear medicine and five breast imaging radiologists) and scored in a binary fashion (1-correctly identified object, 0-incorrect). The sensitivity, specificity, and accuracy of lesion detectability were calculated and plots of sensitivity versus ACR and lesion diameters for different phantom thicknesses and count density were generated. RESULTS: The overall (mean) detection sensitivity across all variables was 0.68 (95% CI: [0.64, 0.72]) with a corresponding specificity of 0.93 [0.87, 0.98], and diagnostic accuracy of 0.72 [0.70, 0.75]. The smallest detectable object varied strongly as a function of ACR, as sensitivity ranged from 0.36 [0.29, 0.44] for the smallest lesion size (3 mm) to 0.80 [0.75, 0.84] for the largest (14.5 mm). CONCLUSIONS: The detectability performance of this PEM system demonstrated its ability to resolve small objects with low activity concentration ratios which may assist in the identification of early stage breast cancer. The results of this investigation can be used to correlate lesion detectability with tumor size, ACR, count rate, and breast thickness.

Evaluation of the quantitative accuracy of a commercially available positron emission mammography scanner.

PURPOSE: The PEM Flex Solo II (Naviscan, Inc., San Diego, CA) is currently the only commercially available positron emission mammography (PEM) scanner. This scanner does not apply corrections for count rate effects, attenuation, or scatter during image reconstruction, potentially affecting the quantitative accuracy of images. The aim of this work is to measure the overall quantitative accuracy of the PEM Flex system and to determine the individual contributions of error from count rate effects, attenuation, and scatter. METHODS: Gelatin phantoms were designed to simulate breasts of different thicknesses (4-12 cm) with varying uniform background activity concentration (AC) (0.007-0.5 microCi/cc), cysts, and lesions (2:1, 5:1, and 10:1 lesion-to-background ratios). The overall error was calculated from ROI measurements in the phantoms with a clinically relevant background AC (0.065 microCi/cc). The error due to count rate effects was determined by comparing the overall error at multiple background AC to the error at a very low background AC (0.007 microCi/cc) where count rate effects are considered negligible. A point source and cold gelatin phantoms of different thicknesses were used to assess the errors due to attenuation and scatter. The maximum pixel values of the point source in gelatin and in air were compared to determine the effect of attenuation, while scatter was evaluated by comparing the sum of all pixel values in gelatin and in air. RESULTS: The AC in the uniform background was underestimated in phantoms of all thicknesses, with the exception of the 4-cm-thick phantoms in which the measured AC was within +/- 7% of the true value (0.065 microCi/cc). The degree of underestimation in the uniform background increased with the phantom thickness, up to 34% +/- 6% in the 12 cm phantoms. The AC in all lesions was underestimated by more than that measured in the background (22% for the 2:1 lesions in the 4 cm phantom) and this underestimation increased with increasing thickness and lesion-to-background ratio (85% for the 10:1 lesions in the 12 cm phantoms). The error due to count rate effects reduced the measured AC with respect to its true value. This error increased with increasing background AC (23% +/- 6% at 0.065 microCi/cc to 85% +/- 1% at 0.5 microCi/cc for the 4 cm phantoms) and decreased with increasing phantom thickness (23% +/- 6% for the 4-cm-thick phantoms to 7% +/- 7% for the 12-cm-thick phantoms at 0.065 microCi/cc). Attenuation was a substantial source of error and reduced the measured AC by 51% +/- 10%-77% +/- 4% in the 4-12 cm phantoms, respectively. Scatter increased the total signal measured in images by a relatively constant amount (23% +/- 9%) for all thicknesses. CONCLUSIONS: Applying corrections for count rate effects, attenuation, and scatter will be essential for the PEM Flex Solo II to be able to produce quantitatively accurate images.

Soil aggregation and carbon sequestration are tightly correlated with the abundance of arbuscular mycorrhizal fungi: results from long-term field experiments.

We examined the role of arbuscular mycorrhizal fungi (AMF) in ecosystems using soil aggregate stability and C and N storage as representative ecosystem processes. We utilized a wide gradient in AMF abundance, obtained through long-term (17 and 6 years) large-scale field manipulations. Burning and N-fertilization increased soil AMF hyphae, glomalin-related soil protein (GRSP) pools and water-stable macroaggregates while fungicide applications reduced AMF hyphae, GRSP and water-stable macroaggregates. We found that AMF abundance was a surprisingly dominant factor explaining the vast majority of variability in soil aggregation. This experimental field study, involving long-term diverse management practices of native multispecies prairie communities, invariably showed a close positive correlation between AMF hyphal abundance and soil aggregation, and C and N sequestration. This highly significant linear correlation suggests there are serious consequences to the loss of AMF from ecosystems.