A probe, not an echo

Researchers at the University of Southern California's Viterbi School of Engineering have successfully demonstrated a novel "High-resolution Ultrasonic Transmission Tomography" (HUTT) system that offers 3D images of soft tissue that are superior to those produced by existing commercial X-ray, ultrasound or MRI units.

Vasilis Marmarelis, a professor of biomedical engineering at the Viterbi School, presented HUTT images of animal organ tissue in San Diego at the 28th International Acoustical Imaging Symposium, held March 20-23.

According to Marmarelis, HUTT offers nearly order-of-magnitude improvement in resolution of structures in soft tissue (i.e., 0.4 mm, compared to 2 mm for the best alternatives). Several other features promise to make the technology a scientific and clinical tool of great power:

*Robust algorithmic tools enable HUTT to differentiate separate types of tissue based on their distinctive "frequency-dependent attenuation" profiles, that should allow clinicians to distinguish malignant lesions from benign growths in a non-invasive and highly reliable manner.

*In addition to improved resolution, the system can locate tissue features with extreme precision in a objective, fixed-coordinate 3D grid, crucial for guiding surgical procedures.

*Scans can be performed in a matter of a few minutes and because they are ultrasonic, they do not use potentially harmful ionizing radiation.

*The system requires a minimum of special pre-scan procedures and appears likely, in clinical use, to be more comfortable for patients than alternatives.

"The HUTT imaging system is a novel and potentially very useful approach to diagnostic ultrasound," said Dr. Phillip W. Ralls, a professor and vice chair in the USC Keck School of Medicine department of radiology. "The potential clinical benefits of the superb images obtained by this completely safe, non-invasive technique are very exciting."

According to Marmarelis, the key features distinguishing HUTT from all previous ultrasound imaging systems is the use of multi-band analysis with sub-millimeter ultrasonic transducers in transmission mode, rather than the commonly used echo mode, to create the 3-D image.

He explains that in traditional hand-held ultrasound systems, sound waves are broadcast into the tissue, and the echoes produce an image of the reflecting interfaces – that is, the sound transmitter and the receiver are both on the same side of the sample.

However, only a tiny fraction of the transmitted sound comes back as echo on soft tissues, while a much larger fraction (about 2000 times bigger) is transmitted through the soft tissue.

Using the sound transmitted through tissue allows the formation of better images with greater clarity and resolution.

A hand-held apparatus cannot objectively locate objects in 3D space (in a fixed-coordinate system), but only allows the user to subjectively observe where an object is in relation to other observable structures. Therefore, it is operator-dependent.

The HUTT system transmits an extremely short ultrasonic pulse (about 250 nanosecond) of 4-12 megahertz frequency (far above human hearing) and picks up the pulse on the other side after it has traveled through the imaged object.

The transmitted pulses come from an array of very small ultrasonic transducers of sub-millimeter dimensions. A parallel array of transducers on the other side receives the pulses after they travel through the imaged tissue.

A sophisticated coding/decoding signal scheme recognizes a small "sweet spot" of the signal coming from the opposite transducer, and only that transducer, and ignores all other pulses transmitted by neighboring transducers.

The transducer is able to distinguish the right signal from the right transducer by using coding that is almost identical to that used by a cell phone to detect signals sent to its number -- and its number only -- from the flood of electronic signals on the air at any given time.

When the transducer captures the signal, it is processed with advanced signal processing algorithms, specially developed by Marmarelis' group, to form the multi-band images. Different kinds of tissue allow slightly more, or slightly less of the pulse through -- the loss is called "attenuation," and varies according to the type of tissue, and the frequency of the pulse.

"Typically the resulting images represent minute variations in relative attenuation over various frequency bands and they define the different sections of the tissue in the image," said Marmarelis.

The two arrays, transmitter and receiver, are mounted on opposite sides of a drum that spins as it rises around the object (which is suspended in water), creating a stack of tomographic image slices which visualization algorithms turn into 3D images.

In the first set of experiments using the HUTT system, the Marmarelis team easily located a set of small metal balls smaller than a millimeter in diameter embedded in agar medium.

Many critical refinements occurred during the five-year process of development, as the team gained proficiency in imaging animal tissue, notably sheep kidneys and bovine liver.

The most critical feature of the HUTT imaging technology is its potential to reliably differentiate types of tissue based on their multi-band signatures caused by their varying attenuation patterns. This promises to allow non-invasive detection of lesions in clinical diagnosis, which represents the "holy grail" of medical imaging.

The team found it possible to identify various anatomical structures within the kidney based on their distinctive attenuation characteristics, so that computerized algorithms could display in color-coded fashion one tissue in red, another in green, and so forth -- thus assisting visualization in 3D.

The technology could also be used to isolate one type of tissue , allowing, for example, all the blood vessel structures to be displayed alone and studied.

"Preliminary results on a sheep kidney show exquisite anatomic and tissue detail," commented radiologist Ralls.

Source : University of Southern California