Carl Meggs

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.

Intraoperative Ultrasound-Guided Resection of Gliomas: A Meta-Analysis and Review of the Literature

BACKGROUND Image-guided surgery has become standard practice during surgical resection, using preoperative magnetic resonance imaging. Intraoperative ultrasound (IoUS) has attracted interest because of its perceived safety, portability, and real-time imaging. This report is a meta-analysis of intraoperative ultrasound in gliomas. METHODS Critical literature review and meta-analyses, using the MEDLINE/PubMed service. The list of references in each article was double-checked for any missing references. We included all studies that reported the use of ultrasound to guide glioma-surgery. The meta-analyses were conducted according to statistical heterogeneity between the studies using Open MetaAnalyst Software. If there was no heterogeneity, fixed effects model was used for meta-analysis; otherwise, a random effect model was used. Statistical heterogeneity was explored by χ(2) and inconsistency (I(2)) statistics; an I(2) value of 50% or more represented substantial heterogeneity. RESULTS A wide search yielded 19,109 studies that might be relevant, of which 4819 were ultrasound in neurosurgery; 756 studies used ultrasound in cranial surgery, of which 24 studies used intraoperative ultrasound to guide surgical resection and 74 studies used it to guide biopsy. Fifteen studies fulfilled our stringent inclusion criteria, giving a total of 739 patients. The estimated average gross total resection rate was 77%. Furthermore, the relationship between extent of surgical resection and study population was not linear. Gross total resection was more likely under IoUS when the lesion was solitary and subcortical, with no history of surgery or radiotherapy. IoUS image quality, sensitivity, specificity, and positive and negative predictive values deteriorated as surgical resection proceeded. CONCLUSION IoUS-guided surgical resection of gliomas is a useful tool for guiding the resection and for improving the extent of resection. IoUS can be used in conjunction with other complementary technologies that can improve anatomic orientation during surgery. Real-time imaging, improved image quality, small probe sizes, repeatability, portability, and relatively low cost make IoUS a realistic, cost-effective tool that complements any existing tools in any neurosurgical operating environment.

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.

Net-shape ceramic manufacturing as an aid to realize ultrasonic transducers for high-resolution medical imaging

High frequency ultrasonic transducers are in demand for medical imaging procedures requiring high spatial resolution. However, cost-effective fabrication for frequencies above approximately 20 MHz is challenging. One of the problems is the need for a thin layer of piezoelectric material. This is difficult because typical thick and thin film processes produce material which is too thin and lapping bulk materials is expensive and inefficient. In this paper, net shape ceramic processing is reported as an alternative. This can be achieved with viscous polymer processing followed by calendering of the green state material to produce thin sheets which can be dried, sintered and then cut to shape. Although such thin specimens are fragile, the addition of a supportive acoustic backing material allows straightforward processing into the final ultrasonic transducer. Here, piezoceramic made with TRS600FG material is reported, finished to thicknesses of 50 µm and 110 µm. The behaviour of these samples has been found to be similar to bulk material, for example with a thickness mode coupling coefficient, kT, of 0.52 and relative permittivity, eR S , of 1540. Prototype ultrasonic transducers with element diameters of a few millimetres have been made and operated at frequencies approaching 50 MHz. Testing has been performed underwater and the successful results suggest that net shape ceramic manufacturing is compatible with the fabrication of high frequency ultrasonic transducers.

Investigation of Using Free-Standing Thick-Film Piezoelectric Energy Harvesters to Develop Wideband Devices

This paper is concerned with the wideband behavior of single-frequency and multi-frequency free-standing thick-film piezoelectric energy harvesters. The energy harvesting devices have been fabricated and brief fabrication information is provided. The individual harvesters have been combined with either symmetric or asymmetric tip masses, with some being connected together to form a harvester array. Testing has been undertaken using harmonic vibrations with a wide range of frequencies and accelerations, and also using a random machinery vibration, and data have been recorded in terms of un-rectified and rectified open-circuit voltage, output power with matched resistive loads, peak-to-peak tip displacement, and even charging rates of capacitors. As a general result, the individual harvesters with asymmetric tip masses have vibrated nonlinearly below and in the vicinity of the resonant frequencies. An individual harvester vibrating at the resonant frequency with 0.5 g acceleration has been able to charge a 1000 μF capacitor to 1 V within 12 min and to 1.5 V within 30 min. Also, the harvester array has exhibited a wideband response, where an open-circuit voltage of above 0.8 V has been provided within a certain range of frequencies. Finally, the harvester array has successfully charged a capacitor on a vibrating test sieve shaker, proving the feasibility of the proposed device in real applications.

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.

P3K-5 Passive Materials for High Frequency Ultrasound Components

This study reports the design of materials suitable for backing in piezocomposite devices specifically operating at frequencies above 30MHz. Tungsten loaded epoxy backing samples were made with varying volume fractions with l-5 mum tungsten powder and RX771/HY1300 epoxy. Higher volume fractions were achieved with high shear milling exploiting a new material fabrication technology using PVB polymer, without the need for pressing. Acoustic measurements were carried out at 36MHz to characterise the samples for use in both single element transducers and arrays. The variation in longitudinal and shear wave velocities and acoustic impedance follow the Devaney scattering model with the acoustic impedance ranging from 3-15MRayls. The results show that this material is suitable for use as backing in high frequency piezocomposite devices and acoustic properties can be tailored by adjusting the volume fraction of tungsten.

Micro-moulded randomised piezocomposites for high frequency ultrasound imaging

1-3 piezocomposites that can operate at over 30 MHz are in demand to improve spatial resolution on a sub-millimetre level for biomedical ultrasound imaging applications. However, the fabrication of such materials remains a challenge as ultrafine dimensions are required in conventional composite designs. We have previously reported the design of randomised composites to eliminate coupling to lateral modes and to relax overall dimensional constraints. In this work, practical fabrication of composites based on these designs has been realised by using a novel micro-moulding approach combining gel casting with soft lithography. Impedance spectra of randomised composites confirm their advantage for suppressing spurious modes. Pulse-echo responses of two completed transducers operating at 30 and 70 MHz have demonstrated their functional performance.

Functional characterisation of high frequency arrays based on micro-moulded 1–3 piezocomposites

A clinical need exists for high frequency ultrasound arrays that can provide improved image quality compared to the single-element transducers currently used in real-time high resolution imaging systems. Miniature arrays based on fine-scale piezocomposites are required for sufficiently sensitive imaging systems. In this paper we report fabrication and functional characterization of prototype linear arrays suitable for high frequency imaging. Array electrodes have been patterned photolithographically on the surface of micro-moulded 1–3 piezocomposites with processes than can be scaled for linear arrays operating at 100 MHz. Functional testing of arrays with 50 µm and 15 µm pitch demonstrates feasibility of this approach.

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.