Phased array ultrasonics

Principle of operation of phased array (PA). The PA probe consists of many small elements, each of which can be pulsed separately. In the figure the element on the right is pulsed first, and emits a pressure wave that spreads out like a ripple on a pond (largest semicircle). The second to right element is pulsed next, and emits a ripple that is slightly smaller than the first because it was started later. The process continues down the line until all the elements have been pulsed. The multiple waves add up to one single wave front travelling at a set angle. In other words, the beam angle can be set just by programming the pulse timings.

Weld examination by phased array. TOP: The phased array probe emits a series of beams to flood the weld with sound. BOTTOM: The flaw in the weld appears as a red indication on the instrument screen.

Phased array (PA) ultrasonics is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing. Common applications are to to examine the heart noninvasively or to find flaws in manufactured materials such as welds. Single-element (non phased array) probes—known technically as monolithic probes—emit a beam in a fixed direction. To test or interrogate a large volume of material, a conventional probe must generally be physically turned or moved to sweep the beam through the area of interest. In contrast the beam from a phased array probe can be moved electronically, without moving the probe, and can be swept through a wide volume of material at high speed. The beam is controllable because a phased array probe is made up of multiple small elements, each of which can be pulsed individually at a computer-calculated timing. The term phased refers to the timing, and the term array refers to the multiple elements. Phased array ultrasonic testing is based on principles of wave physics that also have applications in fields such as optics and electromagnetic antennae.

Principle of operation

The PA probe consists of many small ultrasonic elements, each of which can be pulsed individually. By varying the timing, for instance by pulsing the elements one by one in sequence along a row, a pattern of constructive interference is set up that results in a beam at a set angle. In other words, the beam can be steered electronically. The beam is swept like a search-light through the tissue or object being examined, and the data from multiple beams are put together to make a visual image showing a slice through the object.

Features of phased array

• The method most commonly used for medical ultrasonography.
• Multiple probe elements produce a steerable, tightly focused, high-resolution beam.
• Produces an image that shows a slice through the object.
• Compared to conventional, single-element ultrasonic inspection systems, PA instruments and probes are more complex and expensive.
• In industry, PA technicians require more experience and training than conventional technicians.

At a construction site, a technician tests a pipeline weld for defects using an ultrasonic phased array instrument. The scanner, which consists of a frame with magnetic wheels, holds the probe in contact with the pipe by a spring. The wet area is the ultrasonic couplant that allows the sound to pass into the pipe wall.

Standards

European Committee for Standardization (CEN)

• prEN 16018, Non destructive testing - Terminology - Terms used in ultrasonic testing with phased arrays
• ISO/WD 13588 - prEN 13588, Non-destructive testing of welds – Ultrasonic testing – Use of (semi-) automated phased array technology

Further reading

External links

• Industrial applications of portable phased array [technical paper]
• Construction Weld Testing Procedures Using Phased Array [technical paper; based on computer modelling]
• Introduction to theory for medical phased array, Martin E. Anderson, Duke University
• Phased array animated simulator [registration required after 5 minutes' use]

Books

• ASME Boiler and Pressure Vessel Code. American Society Of Mechanical Engineers, 2004. Section V — Nondestructive Examination. [See Article 4 — Ultrasonic Examination Methods for Welds. Para E-474 UT-Phased Array Technique]
• ASME Code Case 2235-9, Use of Ultrasonic Examination in Lieu of Radiography — Section I and Section VIII, Divisions 1 and 2. American Society Of Mechanical Engineers, Approval Date: October 11, 2005. [See page 1, footnote 1 on phased array] (PDF here)

See also

• Phased array (general theory and electromagnetic telecommunications).
• Phased array optics