Matthew D. Williams

Organ and effective doses in newborn patients during helical multislice computed tomography examination

In this study, two computational phantoms of the newborn patient were used to assess individual organ doses and effective doses delivered during head, chest, abdomen, pelvis, and torso examinations using the Siemens SOMATOM Sensation 16 helical multi-slice computed tomography (MSCT) scanner. The stylized phantom used to model the patient anatomy was the revised ORNL newborn phantom by Han et al (2006 Health Phys.90 337). The tomographic phantom used in the study was that developed by Nipper et al (2002 Phys. Med. Biol. 47 3143) as recently revised by Staton et al (2006 Med. Phys. 33 3283). The stylized model was implemented within the MCNP5 radiation transport code, while the tomographic phantom was incorporated within the EGSnrc code. In both codes, the x-ray source was modelled as a fan beam originating from the focal spot at a fan angle of 52 degrees and a focal-spot-to-axis distance of 57 cm. The helical path of the source was explicitly modelled based on variations in collimator setting (12 mm or 24 mm), detector pitch and scan length. Tube potentials of 80, 100 and 120 kVp were considered in this study. Beam profile data were acquired using radiological film measurements on a 16 cm PMMA phantom, which yielded effective beam widths of 14.7 mm and 26.8 mm for collimator settings of 12 mm and 24 mm, respectively. Values of absolute organ absorbed dose were determined via the use of normalization factors defined as the ratio of the CTDI(100) measured in-phantom and that determined by Monte Carlo simulation of the PMMA phantom and ion chamber. Across various technique factors, effective dose differences between the stylized and tomographic phantoms ranged from +2% to +9% for head exams, -4% to -2% for chest exams, +8% to +24% for abdominal exams, -16% to -12% for pelvic exams and -7% to 0% for chest-abdomen-pelvis (CAP) exams. In many cases, however, relatively close agreement in effective dose was accomplished at the expense of compensating errors in individual organ dose. Per cent differences in organ dose between the stylized and tomographic phantoms at 120 kVp and 12 mm collimator setting ranged from -25% (skin) to +164% (muscle) for head exams, -92% (thyroid) to +98% (ovaries) for chest exams, -144% (uterus) to +112% (ovaries) for abdominal exams, -98% (SI wall) to +20% (thymus) for pelvic exams and -60% (extrathoracic airways) to +13% (ovaries) for CAP exams. Better agreement was seen between the two phantom types for organs entirely within the scan field. In these cases, corresponding per cent differences in organ absorbed dose did not vary more than 17%. For all scans, the effective dose was found to range approximately 1-13 mSv across the scan parameters and scan regions. The largest effective dose occurred for CAP scans at 120 kVp.

Aluminum Nitride Ultrasonic Air-Coupled Actuator

A piezoelectric micromachined ultrasonic radiator was developed using aluminum nitride (AlN) for air-coupled applications. A commercially proven CMOS-compatible fabrication process was leveraged to form the devices. The transducer design consists of a radially nonuniform circular composite diaphragm with an integrated layer of AlN between two annular electrodes. Included in the overall system design is a tunable package with back cavity depth control. A multiphysics model was developed that integrates the electrical, mechanical, and acoustic energy domains of the complete system using composite plate mechanics, lumped-element modeling, and linear acoustic theory. Acoustical, mechanical, and electrical characterization of the transducers was conducted, including acoustic far field, laser vibrometry, and electrical impedance measurements. The device characterization results showed poor quantitative match with the system-level model due to large uncertainties in film stress. However, a qualitative comparison between the device behavior and the system model with varying back cavity depth showed promising results. The overall device performance is comparable to those in previous works that utilized alternate piezoelectric films.

Characterization of Aeroacoustic, Silicon Micromachined Microphones for Aircraft Fuselage Arrays

Thispaperdescribesthedesignandcharacterization ofamicromachined microphoneforaircraft fuselagearrays utilized by aeroacousticians to help identify aircraft noise sources and/or assess the effectiveness of noise-reduction technologies. The developed microphone utilizes piezoelectric transduction via an integrated aluminum nitride layer in a thin-film composite diaphragm fabricated using a combination of surface and bulk micromachining. The experimental characterization of several microphones is presented. Measured performance was in line with the Boeing Company specifications for the fuselage array application, including sensitivities of 32:1 � V=Pa to 43:7 � V=Pa,minimumdetectablepressuresaslowas40dB(1Hzbinat1kHz),confirmedbandwidthsupto20kHz, >100 kHz resonant frequencies, and 3% distortion limits between 160 and 172 dB sound pressure level. With this performance, in addition to the small sizes, these microphones are shown to be a viable enabling technology for low-cost, high-resolution fuselage array measurements.

A microelectromechanical systems‐based piezoelectric microphone for aeroacoustic measurements.

This paper describes the packaging and characterization of micromachined piezoelectric microphones designed as aircraft fuselage instrumentation for full‐scale noise characterization flight tests. An important requirement for such microphones is a high maximum sound pressure level (SPL) >150 dB coupled with noise floor <45 dB SPL (narrow bin) and flat frequency response in the audio band (20 Hz–20 kHz). In contrast to many past micromachined piezoelectric microphones that use PZT and ZnO for piezoelectric transduction, aluminum nitride (AlN) was used due to inherent advantages in dielectric loss and signal‐to‐noise ratio. The microphone structure includes a 500–900‐μ m diameter circular diaphragm composed of a structural layer and an annular AlN/metal film stack on a silicon substrate. Promising microphone die were selected via laser vibrometer measurements followed by packaging of the microphones and associated electronics in individual 1/4 in. tubular housings. Characterization of the packaged microphon...

The Nonlinear Behavior of a Post-Buckled Circular Plate

The axisymmetric post-buckling behavior of a circular plate with initial in-plane compression loading is investigated. The von Karman plate equations are solved numerically for a clamped plate and solutions are presented for a range of transverse and in-plane loadings. These solutions can be used to predict the post-buckled behavior of microscale diaphragms. A tradeoff between sensitivity and linearity of post-buckled plates is also discussed.

An aeroacoustic microelectromechanical systems microphone phased array.

Phased microphone arrays are useful tools for noise source localization using a process known as beam‐forming. In scale‐model wind tunnel experiments, the frequency range of interest can extend as high as 90 kHz. In both open and closed wall wind tunnels, microphones with high dynamic range are required to sense large turbulent pressure fluctuations from the open jet shear layer and the tunnel wall boundary layer. Microphones that meet the frequency and dynamic range demands of such experiments are readily available but expensive. When considering the high‐sensor counts typically needed for phased array measurements, total sensor cost can be a limiting factor. The presentation will discuss a proof‐of‐concept phased array consisting of 25 piezoelectric microelectromechanical systems (MEMS) microphones arranged in a log‐spiral pattern on a single printed circuit board. The microphones were designed in‐house and have a dynamic range from 40–160‐dB SPL and possess a resonant frequency greater than 100 kHz. A ...

P2F-2 Vibration of Post-Buckled Homogeneous Circular Plates

The dynamic behavior of an axisymmetric post- buckled circular plate with initial in-plane compression loading is investigated. The static von Karman plate equations are solved numerically for clamped boundary conditions. The static solution is presented for a range of transverse and in-plane loads. Lumped element modeling is used to calculate the mass and compliance of the plate from results of the static solution. The resonant frequency, sensitivity, and maximum linear transverse pressure are calculated for a variety of in-plane loads. These solutions can be used to predict the post-buckled behavior of micromachined plates.

Hearing protection fit-test system pilot study: Results and recommendations for the hearing conservation program

Since the inception of hearing protection device fit-test systems, long-term training benefits have been scarcely investigated. A hearing protection fit-test system was evaluated for performance and training benefits in a pilot study among U.S. Air Force members randomly sampled from those in the hearing conservation program. Subjects were randomly assigned to control or intervention. Both groups received an initial and a 6-month follow-up appointment. At the initial appointment, the control group received standard training for hearing protection, while the intervention group was fit-tested, trained, and fit-tested again. At the follow-up appointment, both groups received fit-testing. Results of the intervention group’s initial appointment will be examined and the two groups’ follow-up fit-test results will be compared. These findings will be discussed in relation to effects on training and retention. Methods for conducting this study and a descriptive analysis of the subject population will also be revie...

Composite Circular Plates With Residual Tensile Stress Undergoing Large Deflections

Many micromachined electroacoustic devices use thin plates in conjunction with electrical components to measure acoustic signals. Composite layers are needed for electrical passivation, moisture barriers, etc. The layers often contain residual stresses introduced during the fabrication process. Accurate models of the composite plate mechanics are crucial for predicting and optimizing device performance. In this paper, the von Karman plate theory is implemented for a transversely isotropic, axisymmetric plate with in-plane tensile stress and uniform transverse pressure loading. A numerical solution of the coupled force-displacement nonlinear differential equations is found using an iterative technique. The results are verified using finite element analysis. This paper contains a study of the effects of tensile residual stresses on the displacement field and examines the transition between linear and nonlinear behavior. The results demonstrate that stress stiffening in the composite plate delays the onset of nonlinear deflections and decreases the mechanical sensitivity. In addition, under high stress the plate behavior transitions to that of a membrane and becomes insensitive to the composite nature of the plate. The results suggest a tradeoff between mechanical sensitivity and linearity. [DOI: 10.1115/1.4005534]

Micromachined Aluminum Nitride Microphone Technology Development

This paper describes the motivation, packaging, and preliminary experimental results for a new passive microelectromechanical systems (MEMS)-based aeroacoustic microphone. The passive technology utilized — piezoelectric transduction, with aluminum nitride serving as the piezoelectric material — together with the small size make this microphone ideal for deployment as fuselage instrumentation in full-scale flight tests. Preliminary results show that the microphone performance characteristics exceed specifications provided by project sponsor Boeing, with a noise floor 20 kHz, and sensitivities from 32.1μV/Pa to 43.7μV/Pa.