Advances make laser imaging simpler and three-dimensional

A,bCalibration diagram of the PACTER system (A) and imaging (b) procedures. BT, beam trap; DAQ, data acquisition unit; HWP, half-wave plate; PBS, polarizing beam splitter; ER, ergodic relay. The differences between the two modes are highlighted by the black dotted boxes. vsSchematic of the single-element ultrasonic transducer fabricated on the ER. d1D (t) PACTER signals detected by the ultrasonic transducer at specific times t₀, t₁ And t_NOT. eReconstruction of a 4D (xyzt) image of human palmar vessels from signals in d. Credit: Natural biomedical engineering (2023). DOI: 10.1038/s41551-023-01149-4

There are times when scientific progress takes the form of discoveries of something completely new. Other times, progress comes down to doing something better, faster, or easier. New research from the lab of Caltech’s Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, is the latter.

In the article “Ultrafast longitudinal imaging of hemodynamics via single-shot volumetric photoacoustic tomography with a single-element detector” published in the journal Natural biomedical engineeringWang and postdoctoral researcher Yide Zhang show how they simplified and improved an imaging technique they first announced in 2020.

This technique, a form of photoacoustic imaging technology called PATER (Photoacoustic Topography Through an Ergodic Relay), is a specialty of Wang’s group.

In photoacoustic imaging, laser light is pulsed into tissue where it is absorbed by tissue molecules, causing them to vibrate. Each vibrating molecule serves as a source of ultrasound waves that can be used to image internal structures in a manner similar to how ultrasound imaging is performed.

However, photoacoustic imaging is technologically challenging because it produces all of its imaging information in a single, short burst. To capture this information, early versions of Wang’s photoacoustic imaging technology required arrays of hundreds of sensors (transducers) to be pressed against the surface of the tissue being imaged, making the technology complicated and expensive.

Wang and Zhang reduced the number of transducers required by using a device called an ergodic relay, which slows the speed at which information (in the form of vibrations) flows through a transducer. As explained in a previous article on PATER: “In computing, there are two main ways of transmitting data: serial and parallel. In serial transmission, data is sent in a single stream via a single communications channel. In parallel transmission, multiple pieces of data are sent at the same time via multiple communication channels.

“Both types of communication are roughly analogous to how cash registers might be used in a store. Serial communication would be like having a cash register. Everyone gets in the same line and sees the same cashier Parallel communication would be like having multiple cash registers and a line for each.

“The system designed by Wang with 512 sensors is similar to the store with many cash registers. All sensors operate at the same time, each taking into account part of the data on the ultrasonic vibrations generated by the laser pulse. System vibrations occur in a single, short burst, a single sensor would be overwhelmed if used to try to collect all the data in that short time.

“This is where the ergodic relay comes in. As Wang describes it, an ergodic relay is a kind of chamber around which sound can resonate. As the ultrasonic vibrations pass through the ergodic relay, they extend over time.

“To return to the cash register metaphor, it would be as if another employee assisted the single cashier by telling customers to take a few laps around the store until the cashier was ready to see them, so that the cashier Don’t be overwhelmed.”

The latest version of this technology, called PACTER (Photoacoustic Computed Tomography Through an Ergodic Relay) goes even further, allowing the system to operate using a single transducer which, through the use of software, can collect as much data as 6,400 transducers.

PACTER enhances PATER in two other ways, says Wang, who is also president of Andrew and Peggy Cherng Medical Engineering Leadership and managing director of Medical Engineering.

An improvement is that PACTER can create three-dimensional images, while PATER can only generate 2D images. This was made possible by the development of improved software.

“The transition to 3D imaging significantly increases data requirements. The challenge was to route the extremely increased data through a single transducer,” says Zhang. “Our solution emerged by modifying our approach. Rather than a straightforward, computationally intensive method for reconstructing 3D images from data from a single transducer, we first extended one transducer to thousands of virtual transducers. This idea simplified the process of reconstructing 3D images, aligning more closely with traditional methods of our photoacoustic imaging.

Second, unlike PATER, PACTER does not need to be calibrated each time it is used.

“With PATER, we had to calibrate it every time to use it, which is just not practical. We got rid of that one-time calibration per use,” says Wang.

The calibration was necessary because when the system sent a pulse of laser light into tissue, an “echo” from that pulse bounced back into the transducer, preventing it from detecting direct ultrasound information.

Wang says PACTER gets around this problem by adding something called a delay line to the system. The delay line forces the echo to take a longer physical path back to the transducer so that it arrives after receiving the direct ultrasound information.

“Even though I always said it was possible, I knew it would be a challenge,” Wang says.