The effect of EMAT coil geometry on the Rayleigh wave frequency behaviour

Understanding of optimal signal generation and frequency content for electromagnetic acoustic transducers (EMATs) is key to improving their design and signal to noise ratio. Linear and meander coil designs are fairly well understood, but other designs such as racetrack or focused coils have recently been proposed. Multiple transmission racetrack coil EMATs, with focused and unfocused designs, were constructed. The optimum driving frequency for maximum detected signal was found to range between 1.1 and 1.4 MHz on aluminium for a 1.5 mm width coil. A simple analytical model based on the instantaneous velocity of a wave predicts a maximum signal at 1.44 MHz. Modelling the detection coil as a spatial square wave agrees with this, and predicts a general relation of f =0.761v/L between the optimum frequency f , the wave velocity v, and the coil width L. A time domain model of the detection coil predicts a 1.4-1.5 MHz peak for continuous wave excitation, with a frequency that decreases as the length of the wavepacket is decreased, consistent with the experimental data. Linear coil modelling using the same technique is shown to be consistent with previous work, with improving detection at lower wave frequencies, and signal minima at every integer multiple of the wavelength. Finite Element Analysis (FEA) is used to model the effects of the spatial width of the racetrack generation coil and focused geometry, and no significant difference is found between the focused and the unfocused EMAT response. This highlights the importance of designing the EMAT coil for the correct lift-off and desired frequency of operation. [Abstract copyright: Copyright © 2019 Elsevier B.V. All rights reserved.

Contrast-enhanced magnetomotive ultrasound imaging (CE-MMUS) for colorectal cancer staging : assessment of sensitivity and resolution to detect alterations in tissue stiffness

A key challenge in the treatment of colorectal cancer is identification of the sentinel draining lymph node. Magnetomotive ultrasound, MMUS, has identified lymph nodes in rat models: superparamagnetic iron oxide nanoparticles (SPIONs) accumulated in the lymph are forced to oscillate by an external magnetic field; the resulting axial displacement is recovered allowing structure delineation with potential to indicate alterations in tissue stiffness, but it is limited by small vibration amplitudes. We propose CE-MMUS using SPION loaded microbubbles (SPION-MBs) to enhance sensitivity, reduce toxicity, and offer additional diagnostic or perfusion information. Laser doppler vibrometry measurements was performed on SPION containing tissue mimicking material during magnetic excitation. These measurements show a vibration amplitude of 279 ± 113 μm in a material with Young's modulus of 24.3 ± 2.8 kPa, while the displacements were substantially larger, 426 ± 9 μm, in the softer material, with a Young's modulus of 9.6 ± 0.8 kPa. Magnetic field measurement data was used to calibrate finite element modelling of both MMUS and CE-MMUS. SPION-MBs were shown to be capable of inducing larger tissue displacements under a given magnetic field than SPIONs alone, leading to axial displacements of up to 2.3x larger. A doubling in tissue stiffness (as may occur in cancer) reduces the vibration amplitude. Thus, there is potential for CE-MMUS to achieve improved stiffness sensitivity. Our aim is to define the potential contribution of CE-MMUS in colorectal cancer diagnosis and surgical guidance

Enhanced EMAT techniques for the characterisation of hidden defects

There is an industrial drive for the improved detection of sub-mm sized sur¬face breaking defects using non-destructive evaluation (NDE) methods [1]. Electromagnetic acoustic transducers (EMATs) are a non-contact NDE technique that utilise the generation and detection of Ultrasound using primarily Lorentz force mechanisms [2]. They are relatively safe and inexpensive, however, they suffer from low generation efficiency. The precise industrial drive for this work is improved ultrasonic crack detection of surface defects hidden by a thin metallic paint coating. The majority of standard ultrasonic techniques are not applicable as they require direct contact to the sample surface. Laser techniques, while non-contact, are still impeded by the coating, and eddy current techniques are difficult to implement due to interference from the metallic coating. EMATs are applicable, however their low generation efficiency limits the minimum defect that can be detected. This work presents improved resolution surface wave EMATs using geometric focusing for the detection of sub-mm sized surface breaking defects. Three main design types have been presented: a pseudo-pulse-echo focused meander-line EMAT, a pitch-catch focused racetrack EMAT and a pitch-catch focused linear EMAT. The first two designs have been fully characterised, finding the relations between coil geometry, focal point location and size, and the optimum operation frequencies [3, 4, 5]. Both designs have been used to size the lengths of a set of drilled calibration defects to accuracies of 10.5 and 10.4 mm respectively, and the pitch-catch design has been used to create a calibration curve for defect depth measurements. In addition, both designs have been used to map a pair of real surface breaking cracks in an aluminium billet sample to sub-mm resolution. The pitch-catch design has been used to detect a set of mm-size real thermal fatigue cracks in steel through a 40 — 60 ktm thick metallic paint coating. A four-coil EMAT design based on the pitch-catch focused racetrack EMAT has been built and demonstrated to detect surface breaking defects regardless of their surface orientation. Finally, the meander-line, racetrack, and linear coil design types have been compared based on their signal strength and their performance at lift-off from a sample surface. The meander-line designs have the strongest signal to noise ratios (SNR), with over 40 dB found when in contact with the sample, but the largest SNR loss with increased lift-off, reducing to 0 dB by 0.3 mm lift-off. The linear designs have the weakest SNRs, under 30 dB when in direct contact, but the smallest SNR loss with increased lift-off, dropping to 0 dB by around 1 mm, depending on the frequency of operation. This makes the linear coil designs optimal for situations requiring higher lift-off. Lower frequency designs are shown to perform better with increased lift-off regardless of the coil design, however, lower frequencies have less spatial resolution capabilities. A proposed linear-meander-line phased EMAT design is presented to generated 1 MHz signals but with the improved lift-off capabilities of the linear designs. This proves that surface wave EMATs can be optimised for surface wave detection of sub-mm defects through a metallic paint coating. While pseudo-pulse-echo focused meander-line EMATs are already in exsitence, there was previously no published work on their capabilities and full charaterisation. The other focused designs presented here are new designs in the fiel

Multi-coil focused EMAT for characterisation of surface-breaking defects of arbitrary orientation

Electromagnetic Acoustic Transducers (EMATs) are a useful ultrasonic tool for non-destructive evaluation in harsh environments due to their non-contact capabilities, and their ability to operate through certain coatings. This work presents a new Rayleigh wave EMAT transducer design, employing geometric focusing to improve the signal strength and detection precision of surface breaking defects. The design is robust and versatile, and can be used at frequencies centered around 1 MHz. Two coils are used in transmission mode, which allows the usage of frequency-based measurement of the defect depth. Using a 2 MHz driving signal, a focused beam spot with a width of 1.3±0.25 mm and a focal depth of 3.7±0.25 mm is measured, allowing for defect length measurements with an accuracy of±0.4 mm and detection of defects as small as 0.5 mm depth and 1 mm length. A set of four coils held under one magnet is used to find defects at orientations offset from normal to the ultrasound beam propagation direction. This EMAT has a range which allows detection of defects which propagate at angles from 16° to 170° relative to the propagation direction over the range of 0–180°, and the setup has the potential to be able to detect defects propagating at all angles relative to the wave propagation direction if two coils are alternately employed as generation coils

Lift-off performance of electromagnetic acoustic transducers (EMATs) for surface acoustic wave generation

Electromagnetic acoustic transducers (EMATs) have a behaviour that depends on the distance between the transducer and the surface of the material under test, namely the lift-off. Rayleigh waves generated by EMATs suffer from waveform distortion as the lift-off is increased. This paper describes this distortion numerically and experimentally, focusing on the spatial distribution of the induced currents and hence the ultrasound pulses. This is verified experimentally using an EMAT consisting of a magnet and a single 1.5 mm wide linear coil, showing a decrease in peak frequency for the wideband Rayleigh wave of the order of 100 kHz/mm with lift-off. The behaviour when using EMATs in an array configuration (equivalent to a meander-coil, where the current through neighbouring coils is in opposite directions) is then described. Coil spacing is shown to affect the lift-off behaviour, with the spatial model predicting well the frequency behaviour. The lift-off behaviours of physically spaced coils and those which are pulsed with time delays to give an effective separation are shown to be equivalent for all but the smallest separations; unless very small separations are required to give a high frequency signal, the effective spatial current distribution spread for a single coil counteracts the benefits of reducing the dipole effect. Good performance with lift-off variation is hard to achieve at high frequency, and lift-off must be considered if a particular frequency of operation is required