R. V. Duncan

Development of ultrasound tomography for breast imaging: technical assessment

Ultrasound imaging is widely used in medicine because of its benign characteristics and real-time capabilities. Physics theory suggests that the application of tomographic techniques may allow ultrasound imaging to reach its full potential as a diagnostic tool allowing it to compete with other tomographic modalities such as x-ray computer tomography, and MRI. This paper describes the construction and use of a prototype tomographic scanner and reports on the feasibility of implementing tomographic theory in practice and the potential of ultrasound (US) tomography in diagnostic imaging. Data were collected with the prototype by scanning two types of phantoms and a cadaveric breast. A specialized suite of algorithms was developed and utilized to construct images of reflectivity and sound speed from the phantom data. The basic results can be summarized as follows. (i) A fast, clinically relevant US tomography scanner can be built using existing technology. (ii) The spatial resolution, deduced from images of reflectivity, is 0.4 mm. The demonstrated 10 cm depth-of-field is superior to that of conventional ultrasound and the image contrast is improved through the reduction of speckle noise and overall lowering of the noise floor. (iii) Images of acoustic properties such as sound speed suggest that it is possible to measure variations in the sound speed of 5 m/s. An apparent correlation with x-ray attenuation suggests that the sound speed can be used to discriminate between various types of soft tissue. (iv) Ultrasound tomography has the potential to improve diagnostic imaging in relation to breast cancer detection.

PdMn and PdFe : New materials for temperature measurement near 2 K

Interest in the critical dynamics of superfluid4He in microgravity conditions has motivated the development of new high resolution thermometry technology for use in space experiments near 2 K. We have developed a magnetic thermometer using dilute magnetic alloys of Mn or Fe dissolved in a pure Pd matrix, similar to previous thermometers used at ultra-low temperatures. These metallic thermometers are easy to fabricate, chemically inert, and can have a low thermal resistance to the stage to be measured. Also, the Curie temperature can be varied by changing the concentration of Fe or Mn, making them available for use in a wide temperature range. The derivative of the magnetic susceptibility was measured for PdMn and PdFe between 1.5 K and 4 K using a SQUID magnetometer. These measurements, as well as preliminary noise and drift measurements, show them to have sub-nK resolution with a drift of less than 10−13K/s.

Investigating the use of multimeters to measure quantized hall resistance standards

A new generation of digital multimeters was used to compare the ratios of the resistances of wire-wound reference resistors and quantized Hall resistances (QHRs). Specifically, a digital multimeter was used to compare the DC voltage ratio of a QHR sample with that of a reference resistor connected in series with the sample. The accuracies are better than 0.1 p.p.m. for ratios as large as 4:1 if the multimeters are calibrated with a Josephson array. >

Ultrasound tomography of breast tissue

The Karmanos Cancer Institute is developing an ultrasound device for measuring and imaging acoustic parameters of human tissue. This paper discusses the experimental results relating to tomographic reconstructions of phantoms and tissue. The specimens were scanned by the prototype scanner at a frequency of 1.5 MHz using 2 microsecond pulses. The receivers and transmitters were positioned along a ring trajectory having a diameter of 20 cm. The ring plane is translated in the vertical direction allowing for 3-D reconstructions from stacked 2-D planes of data. All ultrasound scans were performed at 10 millimeter slice thickness to generate multiple tomographic images. In a previous SPIE paper we presented preliminary results of ultrasound tomographic reconstruction of formalin-fixed breast tissue. We now present new results from data acquired with the scanner. Images were constructed using both reflection-based and transmission based algorithms. The resulting images demonstrate the ability to detect sub-mm features and to measure acoustic properties such as sound speed. Comparison with conventional ultrasound indicates the potential for better margin definition and acoustic characterization of tissue.

New Paramagnetic Susceptibility Thermometers for Fundamental Physics Measurements

New paramagnetic susceptibility thermometers have been developed for use in fundamental physics missions in earth orbit. These devices use a SQUID magnetometer to measure the variation in the dc magnetization of a thermometric element that consists of a dilute concentration of manganese in a palladium matrix. Near 2.2 K these new PdMn thermometers have demonstrated a temperature resolution of better than 100 pK/√Hz and a time constant of 50 ms when operated with a 50 K/W thermal resistance to the liquid helium sample. These thermometers have been observed to be remarkably stable, with a drift of less than 10 fK/s. The observed power spectral density of the noise from these thermometers is consistent with separate measurements of the device’s time constant and thermal standoff from the bath. Recently these PdMn materials have been made into thin films and microstructures for use in future studies of quantum liquids, and for possible use in a new class of bolometers and radiometers. These thermometers have been integrated into an experimental cell and thermal isolation network that are adequate to keep stray heats stable to within a few picowatts, with no systematic temperature errors greater than 60 pK, over the course of a planned fundamental physics experiment on Earth orbit.

Decoherence Under a Heat Flux Near the Superfluid Transition in 4He

Measurements of heat transport at the transition from perfect thermal superconductivity to nonlinear heat diffusion in pure 4He provide a very sensitive probe of matter wave coherence. Superfluid heat transport is proportional to the product of the superfluid density and the superfluid velocity, which are both directly related to the superfluid order parameter. From dynamic scaling theory, the correlation length near the superfluid transition provides a measure of the length over which phase fluctuations of the order parameter persist. Our measurements suggest that both the hydrostatic pressure variation within the liquid helium column, together with the heat flux Q, limit the otherwise divergent correlation length near the superfluid transition. Future measurements planned for the microgravity laboratory will provide the fast extensive experimental test of a renormalized, field theoretic description of heat transport near the superfluid transition. It will also provide a conclusive experimental study of the influence of hydrostatic pressure effects and dynamical effects on the correlation length. A new class of microgravity experiments is proposed that will permit measurements to within 10 pK of the superfluid transition temperature, allowing an entirely new class of ultra-accurate scientific investigations to be performed.

Gravitational Effects on Nonlinear Heat Transport Near the Superfluid Transition in 4He

We have recently observed nonlinear heat transport within 30 nK of the superfluid transition temperature using heat flux, Q, in the range 0.1 < Q < 2 erg/(s cm2). While Haussmann and Dohm (HD) accurately predict the initial departure of the thermal conductivity, κ, from the linear response region, κ is greater than expected very close to Tλ. We anticipate that the nature of the thermal conductivitys nonlinearity may depend upon Earths gravity in the low heat flux limit (Q < 0.5 erg/(s cm2)). Comparison of our data to similar data to be taken in a microgravity laboratory will provide an experimental determination of the effect of gravity on nonlinear heat transport near the superfluid transition. The microgravity measurements will also permit the first experimental test of theories that do not consider gravitational effects, such as those by HD.

Sidewall thermometry perturbations to nonlinear heat transport near the λ transition in4He

Abstract The effect of a sidewall thermometry probe on heat transport measurements in liquid 4 He very close to the superfluid transition is simulated numerically. The effective thermal position of the probe changes with closeness of approach to the superfluid transition. The radial heat flux induced by the sidewall probe is calculated for specific probe designs.

Thermal effects on the Josephson series-array voltage standard

Abstract A series-array voltage standard containing 2,076 Josephson junctions has been operated in a liquid helium bath maintained within a refrigerated Dewar over the temperature range 1.56K ≤ T ≤ 4.54K. No systematic variation in the array voltage near 1.018V with temperature was detected over this entire range, indicating that any temperature coefficient of the array voltage must be less than 2×10 -8 V/K. The critical current and gap energy of the weakest junction within the array in the absence of millimeter wave power were measured as a function of temperature over this same range. The noise sensitivity of the array was observed to change abruptly but by a small amount as the helium bath was pumped superfluid. Voltage calibrations were unaffected by the baths superfluid transition.

Adaptive optimal PI controller for high-precision low-temperature experiments

In this paper, we describe an adaptive optimal PI controller for high-precision low-temperature experiments. We used a modified LQR algorithm to obtain the optimal parameters for the PI controller. Since the plant is nonlinear, a gain scheduling method is applied to modify the optimal parameters under different operating conditions. The simulation results show that this controller has good transient response, disturbance rejection ability, and robustness. Furthermore, the controller can accommodate a variety of first-order nonlinear systems. At last, we discuss the design of the optimal PID controller for a class of second order system using the modified LQR algorithm.