Investigation of synthetic aperture methods in ultrasound surface imaging using elementary surface types

Abstract

Synthetic aperture imaging methods have been employed widely in recent research in non-destructive testing (NDT), but uptake has been more limited in medical ultrasound imaging. Typically offering superior focussing power over more traditional phased array methods, these techniques have been employed in NDT applications to locate and characterise small defects within large samples, but have rarely been used to image surfaces. A desire to ultimately employ ultrasonic surface imaging for bone surface geometry measurement prior to surgical intervention motivates this research, and results are presented for initial laboratory trials of a surface reconstruction technique based on global thresholding of ultrasonic 3D point cloud data. In this study, representative geometry artefacts were imaged in the laboratory using two synthetic aperture techniques; the Total Focusing Method (TFM) and the Synthetic Aperture Focusing Technique (SAFT) employing full and narrow synthetic apertures, respectively. Three high precision metallic samples of known geometries (cuboid, sphere and cylinder) which featured a range of elementary surface primitives were imaged using a 5MHz, 128 element 1D phased array employing both SAFT and TFM approaches. The array was manipulated around the samples using a precision robotic positioning system, allowing for repeatable ultrasound derived 3D surface point clouds to be created. A global thresholding technique was then developed that allowed the extraction of the surface profiles, and these were compared with the known geometry samples to provide a quantitative measure of error of 3D surface reconstruction. The mean errors achieved with optimised SAFT imaging for the cuboidal, spherical and cylindrical samples were 1.3mm, 2.9mm and 2.0mm respectively, while those for TFM imaging were 3.7mm, 3.0mm and 3.1mm, respectively. These results were contrary to expectations given the higher information content associated with the TFM images. However, it was established that the reduced error associated with the SAFT technique was associated with significant reductions in side lobe levels of approximately 24dB in comparison to TFM imaging, although this came at the expense of reduced resolution and coverage.

Probe Standoff Optimization Method for Phased Array Ultrasonic TFM Imaging of Curved Parts

“…This approach enables an efficient imaging of the part and removes the need for wedges customized to the surface shape [ 11 , 12 ]. Additionally, the adaptive ultrasonic phased array inspection methods require no prior knowledge of the surface profile of the specimen under inspection [ 13 , 14 ]. On this approach, ultrasound transmission and imaging strategies had their capabilities demonstrated in the literature: the use of plane waves imaging methods [ 15 , 16 ], the real-time Dynamic Depth Full Focusing [ 17 ], the use of Virtual Source Apertures [ 18 ] and the Total Focusing Method (TFM) [ 3 ].…”

Plane Wave Imaging Techniques for Immersion Testing of Components With Nonplanar Surfaces

“…The benefit of the immersion approach is that it allows inspection of complex profile components without requiring prior knowledge of their geometries [17], [18]. Instead, those geometries are measured directly from the acquired ultrasonic data through post-processing with a surface reconstruction technique [19], [20]. Hence, once the geometry of a specimen is known (shapes and positions of the front and back surfaces), the same phased array data can be used to form multiple images inside of it [21].…”

Surface reconstruction accuracy using ultrasonic arrays: Application to non-destructive testing

“…To date there is a lack of published literature exploring the influence of the surface geometry on the 44 accuracy of surface reconstructions and internal feature imaging. The recent works of Kerr et al 45 investigated the accuracy of surface reconstructions of 3D metal samples (sphere, cuboid and 46 cylinder) and a more complex human femur bone surface [8,9]. The aim of the present study is to 47 build on such work and elucidate the relationship between an object's surface geometry and the 48 resulting ability to accurately image within it, which is of importance for NDT inspections as a 49…”

“…Focussing Method (TFM) [12,13] or Synthetic Aperture Focusing Technique (SAFT) [8,9,14]. Even 60 minor surface profile errors (less than a fraction of the acoustic wavelength) can result in significant 61 loss of image quality through loss of coherence [15].…”

“…should be noted however that other imaging algorithms (which may have lower spatial resolution) 69 are able to resolve surface geometries with high accuracy [8,9]. 70…”