Phased Array Imaging Scans
A-Scan Data
Any ultrasonic instrument typically records two fundamental parameters of an echo: how large it is (amplitude), and where it occurs in time with respect to a zero point (pulse transit time). Transit time in turn is usually correlated to reflector depth or distance, based on the sound velocity of the test material and the simple relationship
Distance = velocity x time
The most basic presentation of ultrasonic waveform data is in the form of an A-scan, or waveform display, in which echo amplitude and transit time are plotted on a simple grid with the vertical axis representing amplitude and the horizontal axis representing time. The example below shows a version with a rectified waveform; unrectified RF displays are also used. The red bar on the screen is a gate that selects a portion of the wave train for analysis, typically measurement of echo amplitude and/or depth.
Single Value B-Scans
Another way of presenting this information is as a single value B-scan. A single value B-scan is commonly used with conventional flaw detectors and corrosion thickness gauges to plot the depth of reflectors with respect to their linear position. The thickness is plotted as a function of time or position while the transducer is scanned along the part to provide its depth profile. Correlating ultrasonic data with the actual transducer position enables a proportional view to be plotted and enables the ability to correlate and track data to specific areas of the part being inspected. This position tracking is typically done using electromechanical devices known as encoders. These encoders are used in fixtures that are either manually scanned or in automated systems that move the transducer by a programmable motor-controlled scanner. In either case, the encoder records the location of each data acquisition point with respect to a desired user-defined scan pattern and index resolution.
In the case below, the B-scan shows two deep reflectors and one shallower reflector, corresponding to the positions of the side-drilled holes (SDHs) in the test block.
C-Scan Mapping
Another presentation option is a C-scan, a two-dimensional presentation of data displayed as a top or planar view of a test piece, similar in its graphic perspective to an X-ray image, where color represents the gated signal amplitude or depth at each point in the test piece mapped to its position. Planar images can be generated on flat parts by tracking data to the X-Y position or on cylindrical parts by tracking axial and angular position. For conventional ultrasound, a mechanical scanner with encoders is used to track the transducer's coordinates to the desired index resolution. The following images conceptually show C-scans of a reference block made with a conventional immersion scanning system using a focused immersion transducer.
Phased Array C-Scan
A C-scan from a phased array system is very similar to the one from the conventional probe seen above. With phased array systems, however, the probe is typically moved physically along one axis while the beam electronically scans along the other according to the focal law sequence. Signal amplitude or depth data is collected within gated region of interest just as in conventional C-scans. In the case of phased arrays, data is plotted with each focal law progression, using the programmed beam aperture.
Below is an actual scan of the same test block showed in the previous section using an encoded 5 MHz, 64-element linear array probe with a straight wedge or shoe. Each focal law uses 16 elements to form the aperture, and at each pulsing, the starting element increments by one. This results in forty-nine data points that are plotted (horizontally in the image below) across the transducer’s 37 mm (1.5 in.) length. As the transducer is moved in a straight line forward, a planar C-scan view emerges. Encoders are normally used when precise geometrical correspondence of the scan image to the part must be maintained, although nonencoded manual scans can also provide useful information in many cases.
While the graphic resolution is not fully equivalent to the conventional C-scan because of the larger effective beam size, there are other considerations. The phased array system is field portable, which the conventional system is not, and costs about one-third the price. Additionally, the phased array image was made in a few seconds, while the conventional immersion scan took several minutes. Real-time generation of the C-scan is shown at the bottom.
Cross-Sectional B-Scan
A cross-sectional B-scan provides a detailed end view of a test piece along a single axis. This provides more information than the single value B-scan presented earlier. Instead of plotting just a single measured value from within a gated region, the whole A-scan waveform is digitized at each transducer location. Successive A-scans are plotted over elapsed time or actual encoded transducer position to draw pure cross-sections of the scanned line. This enables visualization of both near and far surface reflectors within the sample. With this technique, the full waveform data is often stored at each location and may be recalled from the image for further evaluation or verification.
To accomplish this, each digitized point of the wave form is plotted so that color representing signal amplitude appears at the proper depth.
Succesive A-scans are digitized, related to color and "stacked" at user defined intervals (elapsed time or position) to form a true new cross-sectional image.
Phased Array Linear Scans
A phased array system uses electronic scanning along the length of a linear array probe to create a cross-sectional profile without moving the transducer. As each focal law is sequenced, the associated A-scan is digitized and plotted. Successive apertures are “stacked” creating a live cross-sectional view. An animated representation of this sequence using a 16-element linear probe is shown below.
In practice, this electronic sweeping is done in real time so a live cross section can be continually viewed as the transducer is physically moved. Below is a real-time image using a 64-element linear phased array probe.
This highlight phrase references the animation below (using a 16-element probe), but then the paragraph below talks about a "real-time image using a 64-element probe" and references again an image below, but I'm confused where that image is because there seems to only be one image.
It is also possible to scan at a fixed angle across elements. As discussed later this is very useful for automated weld inspection. Using a 64-element linear phased array probe with a wedge, shear waves can be generated at a user defined angle (often 45, 60, or 70 degrees). With aperture sequencing through the length of the probe, full volumetric weld data can be collected without the need for physically increasing distance to the weld center line while scanning. This enables single pass inspection along the weld length.
Phased Array Sectorial Scans
Of all imaging modes discussed so far, the sectorial scan is unique to phased array equipment. In a linear scan, all focal laws employ a fixed angle with sequencing apertures. Sectorial scans, on the other hand, use fixed apertures and steer through a sequence of angles.
Two main forms are typically used. The most familiar, very common in medical imaging, uses a zero-degree interface wedge or shoe to steer longitudinal waves at relatively low angles, creating a pie-shaped image showing laminar and slightly angled defects.
The second format employs an angled plastic wedge to increase the incident beam angle for the generation of shear waves, most commonly in the refracted-angle range of 30 to 70 degrees. This technique is similar to conventional angle beam inspection, except that the beam sweeps through a range of angles rather than a just single fixed angle determined by a wedge. As with the linear scan, the image presentation is a cross-sectional picture of the inspected area of the test piece.
The actual image generation works on the same stacked A-scan principle that was discussed in the context of the linear scans introduced in the previous section. The end user defines the angle start, end, and step resolution to generate the sectorial image. Notice that the aperture remains constant, with each defined angle generating a corresponding beam with characteristics defined by aperture, frequency, damping and the like. The waveform response from each angle (focal law) is digitized and plotted related to color at the appropriate corresponding angle, building a cross-sectional image.
In actuality, the sectorial scan is produced in real time, offering continuous dynamic imaging with the probe’s movement. This is very useful for defect visualization and increases probability of detection, especially with respect to randomly oriented defects, because many inspection angles can be used at once.