Biomedical subjects

William M D Wright

Publications and source records attributed to William M D Wright.

4 recordsLinked to original sources

PubMed2006

A forward model and conjugate gradient inversion technique for low-frequency ultrasonic imaging.

Emerging methods of hyperthermia cancer treatment require noninvasive temperature monitoring, and ultrasonic techniques show promise in this regard. Various tomographic algorithms are available that reconstruct sound speed or contrast profiles, which can be related to temperature distribution. The requirement of a high enough frequency for adequate spatial resolution and a low enough frequency for adequate tissue penetration is a difficult compromise. In this study, the feasibility of using low frequency ultrasound for imaging and temperature monitoring was investigated. The transient probing wave field had a bandwidth spanning the frequency range 2.5-320.5 kHz. The results from a forward model which computed the propagation and scattering of low-frequency acoustic pressure and velocity wave fields were used to compare three imaging methods formulated within the Born approximation, representing two main types of reconstruction. The first uses Fourier techniques to reconstruct sound-speed profiles from projection or Radon data based on optical ray theory, seen as an asymptotical limit for comparison. The second uses backpropagation and conjugate gradient inversion methods based on acoustical wave theory. The results show that the accuracy in localization was 2.5 mm or better when using low frequencies and the conjugate gradient inversion scheme, which could be used for temperature monitoring.

Acoustics↗

PubMed2005

Air-coupled through-transmission fan-beam tomography using divergent capacitive ultrasonic transducers.

Abstracttrasonic transducers (CUTs) with curved backplates was used to acquire signals through regions of air containing solid objects, air flow, and temperature fields. Fan-beam datasets were collected and used in a tomographic reconstruction algorithm to produce cross-sectional images of the area under interrogation. In the case of the solid objects, occluded rays from the projections were accounted for using a compensation algorithm and a priori knowledge of the object. A rebinning routine was used to pick out parallel ray sets from the fan-beam data. The effects of further reducing the number of datasets also were investigated, and, in the case of imaging solid objects, characteristic Gibbs phenomena were seen in the reconstructions as expected. However, when imaging temperature and flow fields, the aliasing artefacts were not seen, but the reconstructed values decreased with the size of dataset used. The effect of changing the kernel filter function also was investigated, with the different filters giving the best compromise between image noise, reconstruction accuracy, and amount of data required in each scenario.

Algorithms↗

PubMed2003

Simultaneous reconstruction of flow and temperature cross-sections in gases using acoustic tomography.

This paper describes the use of air-coupled ultrasonic tomography for the simultaneous measurement of flow and temperature variations in gases. Air-coupled ultrasonic transducers were used to collect through-transmission data from a heated gas jet. A transducer pair was scanned in two-dimensional sections at an angle to the jet, and travel time and amplitude data recorded along various paths in counter-propagating directions. Parallel-beam tomographic reconstruction techniques allowed images to be formed of variations in either temperature or flow velocity. Results have been obtained using heated jets, where it has been shown that it is possible to separate the two variables successfully.

Acoustics↗

PubMed2002

Ultrasonic imaging in air using fan-beam tomography and electrostatic transducers.

Air-coupled ultrasonic capacitance transducers operating at frequencies of up to 1 MHz have been employed in a fan-beam configuration for the cross-sectional tomographic imaging of temperature fields and flow fields in air, and the location of solid objects. Separate transmitter and receiver transducers were manufactured using thin polymer dielectric membranes and polished metal backplates, and used to acquire through-transmission data. The fan-beam reconstruction was developed in LabVIEW using a re-bin routine combined with a filtered backprojection algorithm and a difference technique to generate the cross-sectional images. The system was first used to reconstruct images showing the locations of solid objects positioned within the scanned region through interpretation of the arrival time of the transmitted ultrasound. The technique was then extended to image the temperature fields produced in air above a small heat source and the flow field produced by a nozzle connected to a regulated compressed air source. Reconstructed temperatures were within 4% of the measured background air temperature and 9% of the air temperature measured above the heat source. Reconstructed images of the flow field above a small nozzle were also presented, showing that the horizontal component of the flow velocity could be resolved using this method.

Journal Article↗