Using the Total Focusing Method to Improve Phased Array Ultrasonic Imaging

The nondestructive testing (NDT) industry is experiencing an important technological advancement, as total focusing method (TFM) capable inspection devices are making their entry into market. The TFM approach represents a significant step forward for phased array ultrasonic testing (PAUT) technology. However, some PAUT practitioners may still be confused about TFM and its relation to full matrix capture (FMC), as well as the differences between conventional PAUT and TFM/FMC processing. This application note provides a basic understanding of TFM imaging for people who are familiar with PAUT imaging. For conciseness and clarity, aspects related to ultrasound mode conversion are set aside.

Fundamental Differences between Conventional PAUT and TFM

In both PAUT and TFM, a multi-element probe is used to emit pulsed ultrasound waves in the test piece and to record the time-trace of echoes (waveform). These waveforms are then assembled to produce an image of reflectors in the inspected piece. An ultrasound image can be viewed as a stack of sub-images called frames. For instance, a sectorial scan in PAUT is composed of an arrangement of A-scans (amplitude versus time) captured at different angles. Individual A-scans act as frames in the definition of the sectorial scan. The PAUT strategy consists in processing these frames in the fastest way possible, displaying and refreshing the global image in real time.

The fundamental difference between conventional PAUT and the TFM is in the strategy of signal acquisition and frame processing.

Conventional PAUT Imaging

To demonstrate the frame acquisition process in PAUT, an S-scan is used here as an example. The S-scan is composed of individual frames, which correspond to A-scans captured at various angles in the piece. During an acquisition, a group of elements (known as the aperture) fire and record at the same time. A delay is applied to each element to steer the ultrasound beam at the required angle and to focus it at a desired depth in the piece. Each frame is then defined by the refracted angle and the focus depth. So, the total number of frames to acquire is the number of discrete angles composing the global image.

The advantage of PAUT is that it requires a limited amount of acquisitions. The transmitted beams are the result of “physical summation” in the material of individual transmitters’ acoustic amplitude, and reception beams are synthetically obtained from the rapid summation capacity of front-end electronics. Images obtained through PAUT are therefore displayed very fast. The drawback of PAUT is that the frames are focused only at a constant depth. Reflectors located outside of the focal region appear blurry and somewhat larger than an identical reflector appearing in the focal zone.

The total focusing method (TFM) helps solve this resolution problem. The basic concept of TFM is that it displays the amplitude over focal lines at multiple depths, thereby producing a highly resolved image everywhere and not just over a single depth line.

FMC–An Acquisition Strategy

TFM–Image Reconstruction