H. Hughes

Investigation of dental samples using a 35MHz focussed ultrasound piezocomposite transducer.

Dental erosion and decay are increasingly prevalent but as yet there is no quantitative monitoring tool. Such a tool would allow earlier diagnosis and treatment and ultimately the prevention of more serious disease and pain. Despite ultrasound having been demonstrated as a method of probing the internal structures of teeth more than 40 years ago, development of a clinical tool has been slow. The aim of the study reported here was to investigate the use of a novel high frequency ultrasound transducer and validate it using a known dental technique. A tooth extracted for clinical reasons was sectioned to provide a sample that contained an enamel and dentine layer such that the enamel-dentine junction (EDJ) was of a varying depth. The sample was then submerged in water and a B-scan recorded using a custom-designed piezocomposite ultrasound transducer with a centre frequency of 35 MHz and a -6 dB bandwidth of 24 MHz. The transducer has an axial resolution of 180 microm and a spatial resolution of 110 microm, a significant advance on previous work using lower frequencies. The depth of the EDJ was measured from the resulting data set and compared to measurements from the sequential grinding and imaging (SGI) method. The B-scan showed that the EDJ was of varying depth. Subsequently, the EDJ measurements were found to have a correlation of 0.89 (p<0.01) against the SGI measurements. The results indicate that high frequency ultrasound is capable of measuring enamel thickness to an accuracy of within 10% of the total enamel thickness, whereas currently there is no clinical tool available to measure enamel thickness.

2F-6 Properties and Application-Oriented Performance of High Frequency Piezocomposite Ultrasonic Transducers

Routine high frequency piezocomposite fabrication is now becoming possible. However, specialized characterisation techniques and reference results are absent from the literature and there is an urgent demand for characterisation to support the use of this material in transducers for biomedical diagnosis and non-destructive evaluation. In this paper, we report on a novel procedure for high frequency piezocomposite fabrication and results of intensive characterisation based on electrical impedance spectroscopy and other techniques. 1-3 composite structures have been made with circular PZT pillars with diameter and kerf down to 12 mum and 6 mum respectively, aspect ratio > 3, volume fractions around 40% and active area of around 4 mm using a process which is capable of scalable commercial production. Experimental electrical impedance measurements have proved to be closely matched to one-dimensional models and piezocomposite material performance greatly exceeds that of other materials such as PVDF and LiNbO3. Comprehensive characterization of practical performance has been carried out including both through-transmission and pulse-echo measurements, generating single point results and high resolution 1D and 2D scans to produce 2D and 3D data sets, including tungsten wires test objects to determine axial and lateral resolution and transducer sensitivity. A typical prototype focused transducer is described here, with f- number of 4 and centre frequency of 37 MHz. Performance investigation yielded a through-transmission -3 dB bandwidth of 24 MHz, pulse-echo -6 dB bandwidth of 22 MHz, and lateral and axial resolutions of 250 mum and 150 mum respectively.

5B-2 3D Imaging of Teeth Using High Frequency Ultrasound

It was shown in the late 1960s that the internal structures of teeth could be investigated using ultrasonic pulse echo techniques with 4 MHz contact probes. However, the low frequency limited the resolution of the system and therefore the thickness of dental structures which could be observed. More recent reports have increased frequencies into the region of 10 to 20 MHz. With such frequencies, the resolution in enamel is improved to 0.5 mm. However, the average thickness of enamel in human teeth is around 1.5 mm, implying that even the improved resolution is still inadequate for detailed images and diagnosis. As well as the considerations about the resolution of the system, it has been shown that the attenuation and losses due to acoustic boundaries in tooth structures are detrimental to image reconstruction, with potentially useful information lost or degraded. Therefore, it is essential to have maximum energy transfer into, and back out from, the tooth. The work presented here introduces a novel high frequency focused ultrasound transducer operating at 35 MHz. In order to avoid the natural complexities of the human tooth in the experiment, human incisors were prepared so that only one layer of enamel and dentine were present. The sample was then immersed in distilled water on a translation stage and an x-y raster ultrasound scan was performed. A number of signal processing algorithms were applied to the raw data including correction of distortion and position via correlation and high and low bandpass filtering. The image processing application IMAGEJ was then used to reconstruct a 3D representation and rotation of the processed dataset. The individual A-scans which in turn create the B-scans and 3D images are of a much higher resolution in both the temporal and spatial domain than previously published. The 35 MHz operating frequency gives a resolution of 0.19 mm in the enamel layer, which is at a useful level for the detection of dental caries and more specifically acid erosion. The high frequency also produces a spotsize of 110 mum which allows for accurate localisation in the individual A-scans. The results are believed to be the first known 3D high resolution ultrasound images of the enamel-dentine junction.

P3Q-1 Ultra Precision Grinding in the Fabrication of High Frequency Piezocomposite Ultrasonic Transducers

High frequency ultrasonic transducers are needed for high spatial resolution measurements in applications such as medical diagnosis and nondestructive testing. However, cost-effective fabrication of high performance transducers with frequencies above 20 MHz is challenging because of the need for a thin layer of active material. Piezocomposites are the material of choice in such transducers at lower frequencies, but current fabrication methods cannot easily achieve sufficiently thin active layers. Commercially, piezocomposite is usually finished to thickness by grinding, providing surface finish acceptable for most applications. However, conventional grinding is insufficiently precise for high frequency operation and is subject to undesirable intra-process variation. The most widely used alternative is precision lapping and polishing, but this is slow and therefore expensive. In the work reported here, an alternative process of ultra precision grinding was studied, using the Loadpoint PicoAce machine operating in the ductile machining mode. To determine the capabilities of this machine for piezocomposite processing, 1-3 connectivity material was fabricated using both a standard commercial process and a novel approach based on viscous polymer processing. Unsupported layers of piezocomposite of thickness much less than 100 mum have been achieved with surface roughnesses of less than 1 mun and minimal discontinuity between the ceramic and polymer phases. These results suggest that ultra precision grinding may have a role to play in practical implementation of high frequency piezocomposites for ultrasonic transducers

Fundamental performance characterisation of high frequency piezocomposites made with net-shape viscous polymer processing for medical ultrasound transducers

High frequency ultrasound (HFUS) transducers are in demand for medical diagnoses requiring high spatial resolution. Compared with bulk piezoceramics, conventional crystals, and piezopolymers, piezoceramic-polymer composites have highly desirable properties such as improved piezoelectric coupling and acoustic matching to tissue. However, for 30 MHz operation, a typical 1-3 piezocomposite is approximately only 50 mum thick, requiring ceramic pillar widths of around 15 mum or less. Fabrication is thus challenging with dice-and-fill technology. This may be overcome using micromoulding of ceramic paste produced by viscous polymer processing (VPP). This brings a need for material characterisation to support the new approach but conventional techniques cannot be used due to the small structures involved. This paper therefore reports characterisation using electrical impedance spectroscopy followed by data fitting to a theoretical model based on the 1D piezoelectric wave equation and homogenized piezocomposite model of Smith and Auld. Results are presented from multiple VPP and HFUS piezocomposites spanning a development program of several years. The piezocomposites investigated showed effective piezoelectric properties in the following ranges: c33: 1.76-6.77 (times 1010 Nm2, e33: 2.04-8.50 (Cm-1), eR S: 70.6-460, d33: 7.82-12.7 (times 10-11 mV-1), h33: 2.01-2.09 (times 109 Vm-1) and kT: 0.45-0.51. These data confirm that the VPP fabrication method has good potential for HFUS piezocomposites.

Comparison of Wax and Wax-free Mounting of Irregular Piezocomposite Materials for Thinning for High-frequency Medical Devices

Ultrasound is used in more than 20% of biomedical imaging scans. Several clinical applications would benefit significantly from the higher spatial resolution offered by higher frequency operation than is presently common. A key issue in the development of ultrasound imaging arrays to operate at frequencies above 30 MHz is the need for photolithographic patterning of array electrodes. To achieve this directly on the surface of the piezoceramic-polymer composite material requires planar, parallel and smooth surfaces. An investigation of the surface finishing of piezocomposite material by mechanical lapping and/polishing has recently demonstrated that excellent surface flatness can be obtained. However, the use of wax mounting during the surface processing causes problems because of the required temperatures and the need to remove wax from the very fragile substrates after finishing. Conventional tape mounting cannot withstand the high lateral forces generated by the irregularly shaped samples presently produced by the composite development process. Wax-free mounting has been developed as an alternative, utilising a porous ceramic vacuum chuck incorporated within otherwise standard mechanical lapping/polishing equipment. High frequency array elements have been successfully fabricated on composite surfaces and good electrode edge definition and electrical contact to the composite have been obtained. It is expected that the use of the wax-free equipment and techniques will reduce the eventual cost and increase the yield of such components when they reach production.