Crack Detection Using Low-frequency Ultrasonic Waves Caused by Low Directivity and Scattering

In recent years, nondestructive testing techniques for maintenance and checking of materials and structures have been more required with social infrastructure’s aging. Although ultrasonic waves are valid in internal inspection, conventional ultrasonic detection techniques are inadequate for detecting fatigues or closed cracks, because ultrasound waves hardly reflect at them[1]. However, it has been reported that the transmitted wave through a closed crack contains nonlinear ultrasonic waves[2] such as super- and sub- harmonics, and they have been expected to be used for an alternative detecting method. In our previous research, we experimentally examined transmission of ultrasonic waves through contact surfaces of two metal blocks stacked and compressed in order to simulate a closed crack as in ref[3]. The experiments showed that an ultrasonic wave transmitted over a contact solid interface consists of not only the transmitted fundamental frequency component but also much lower frequency spectra[4]. The magnitude of the low-frequency component is a function of the incident angle and expected to use for developing a comprehensive method which allows unskilled engineers to detect cracks. In this paper, we focused on angles for developing a new method by low-frequency components. The objective of this paper is investigating characteristics of low frequency spectra for the relative angle between the transmitting and receiving transducer. We conducted two experiments. The first experiments showed that the ratio of low frequency components changed significantly when the incident angle was near the directivity angle. If there is a crack in a specimen, ultrasonic waves, after scattered at the crack tip, are received from oblique direction. This can cause change in the ratio of low frequency components. In the second experiments, in order to identify the position of the crack tip, we used the dependence of ratio of low frequency components on the incident angle. From the above results, we proposed a new method to identify cracks by using low frequency components and the directivity angle

A novel magnetic resonance imaging postprocessing technique for the assessment of intervertebral disc degeneration-Correlation with histological grading in a rabbit disc degeneration model.

Introduction:Estimation of intervertebral disc degeneration on magnetic resonance imaging (MRI) is challenging. Qualitative schemes used in clinical practice correlate poorly with pain and quantitative techniques have not entered widespread clinical use. Methods:As part of a prior study, 25 New Zealand white rabbits underwent annular puncture to induce disc degeneration in 50 noncontiguous lumbar discs. At 16 weeks, the animals underwent multi-echo T2 MRI scanning and were euthanized. The discs were stained and examined histologically. Quantitative T2 relaxation maps were prepared using the nonlinear least squares method. Decay Variance maps were created using a novel technique of aggregating the deviation in the intensity of each echo signal from the expected intensity based on the previous rate of decay. Results:Decay Variance maps showed a clear and well demarcated nucleus pulposus with a consistent rate of decay (low Decay Variance) in healthy discs that showed progressively more variable decay (higher Decay Variance) with increasing degeneration. Decay Variance maps required significantly less time to generate (1.0 ± 0.0 second) compared with traditional T2 relaxometry maps (5 (±0.9) to 1788.9 (±116) seconds). Histology scores correlated strongly with Decay Variance scores (r = 0.82, P < .01) and weakly with T2 signal intensity (r = 0.32, P < .01) and quantitative T2 relaxometry (r = 0.39, P < .01). Decay Variance had superior sensitivity and specificity for the detection of degenerate discs when compared to T2 signal intensity or Quantitative T2 mapping. Conclusion:Our results show that using a multi-echo T2 MRI sequence, Decay Variance can quantitatively assess disc degeneration more accurately and with less image-processing time than quantitative T2 relaxometry in a rabbit disc puncture model. The technique is a viable candidate for quantitative assessment of disc degeneration on MRI scans. Further validation on human subjects is needed