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Using the Total Focusing Method to Improve Phased Array Ultrasonic Imaging


Introduction

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

(1)The first element fires in the FMC sequence. (2) All probe elements receive the returning signal. (3) Elementary A-scans stored in full matrix capture.
(4) The second element fires in the FMC sequence. (5) All probe elements receive the returning signal.

TFM–Image Reconstruction

(6) A-scans subjected to delay and sum processing. (7) TFM reconstruction.

If the PAUT acquisition strategy—where each frame requires an acquisition—were used for the total focusing method, the time to generate a TFM image would be increased dramatically. With the TFM, the number of pixels constituting an image is much higher than the number of discrete angles required to generate a S-scan. For example, an S-scan that is swept through 100 angles requires 100 acquisitions, whereas a TFM image of 100 × 100 pixels would require 10,000 acquisitions. To avoid this problem, another acquisition strategy is used, in which frames are calculated in post processing as illustrated in Figure 2. This strategy requires a set of focal laws corresponding to each pixel position, and a set of raw fundamental waveforms called full matrix capture (FMC). With these two elements, fundamental waveforms can be delayed and summed appropriately to synthetically generate ultrasound beams, both in transmission and reception, and focused at each pixel location. The generated image is therefore “focused everywhere”.

The FMC acquires all the waveforms between all individual pairs of probe elements. The full aperture of the probe is generally used as it gives the best focusing result for a given probe design. In this case, the number of acquisitions required to build the FMC is equal to the number of elements of the probe. The FMC contains all the information regarding sound propagation between each element of the probe, including reflections at interfaces and scattering by flaws. Any type of PAUT acquisition can be reconstructed using FMC, including a sectorial scan, plane wave imaging (PWI), dynamic depth focusing (DDF), etc.

Using the FMC acquisition process, the number of acquisitions required to generate an image is about the same as PAUT but storing the individual FMC data sets requires significant storage capacity, transfer bandwidth, and processing power. Depending on the electronics of the device used, obtaining the TFM/FMC results can be slower than PAUT.

Illustration of Differences between PAUT and TFM Images on an Experimental Case

To illustrate differences in PAUT and TFM imaging, a setup is presented in which a linear phased array (PA) probe is used to scan identical side-drilled holes (SDHs) that are vertically distributed in a steel block.

A PAUT S-scan (a) and a TFM image (b) obtained with the same inspection configuration using an OmniScan™ X3 flaw detector are presented below.

In the S-scan, each frame is acquired using a unique focal distance of 20 mm (represented by a red dashed line). SDHs located in the focal area appear with similar amplitude and size. For such a focal distance, the area where the image resolution is optimal is larger compared to a shorter focal distance, which explains this result. SDHs located far from the focal depth appear distorted and with a significantly lower amplitude. Several images with various focal distances are needed for a more uniform sizing of all the SDHs.

In the TFM image (b), ultrasound beams are focused at each pixel. As you can see, each one of the SDHs appears with an optimal resolution, and only one image is required to adequately size SDHs located over a larger depth range. Nevertheless, lateral distortion is observed for SDHs located at the extremities of the electronic focusing capabilities. This distortion is intrinsic to PA imaging and is therefore still present in the TFM image.

Probe conducting FMC scan
PAUT scan compared to TFM image

Summary of the Advantages Provided by TFM/FMC Acquisition

The principal difference between the TFM and PAUT is the nature and the number of frames composing the images.

In PAUT, frames are one-dimension signals or A-scans. Only real-time, front-end electronics summation acts as post processing, and frames are acquired and presented on the fly.

On the other hand, the TFM frames are zero-dimension data points from a focused beam at each pixel’s coordinates. The number of frames to process in the TFM is therefore far more numerous than in PAUT. The FMC acquisition process is required to produce synthetic focalized beams in post processing.

The principal advantage of TFM is that the entire image is displayed with an optimal resolution, compared to an image produced with PAUT, which is only highly resolved in the focal area of the beam. The one constraint worth noting in using the total focusing method is the electronic focusing capacity, a limitation carried over from phased array imaging.

Olympus IMS
Products used for this application

The OmniScan X3 flaw detector is a complete phased array toolbox. Innovative TFM delivers outstanding images that help inspectors identify flaws with confidence while powerful software features and simple workflows help you get to work fast.
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