Diasonics upgrade offers solution to dynamic transmit focus problem

Diasonics took a significant step last week to reestablish itsreputation as an innovator in ultrasound. The vendor revealedseveral gray-scale and color enhancements to its Spectra high-endradiology ultrasound system, at least one of which appears tobreak a long-standing technological barrier.

The premium ultrasound segment has become increasingly a neck-and-neckrace to upgrade the image quality and diagnostic power of thesesoftware-driven machines. Through its most recent upgrade, Diasonicshas shown it can tweak and tune with the best of them.

A few industry feathers were ruffled when Diasonics issueda press release announcing a new, proprietary technology, calledconfocal imaging, which enables dynamic focusing of the ultrasoundbeam. Most vendors assumed Diasonics was describing dynamic receivefocus, a technology introduced years ago by Acuson and adoptedby most vendors.

Dynamic receive focus involves the continuous recreation ofan electronic lens within the transducer to optimally accept signalsreturning from various depths in the body. Dynamic transmit focusis more difficult--some would say impossible--to achieve. Diasonicsintroduced both dynamic receive and dynamic transmit focus onits Spectra VST (variable summation technology) scanner upgrade.

"For the first time in medical ultrasound, we have a techniqueby which you can be in transmit focus and in receive focus throughoutthe image," said Omar Ishrak, vice president of product development.

Once an ultrasound beam is transmitted, there is no callingit back. Echoes from different depths up to a maximum penetrationwill bounce back to the transducer for reception and processing.When an attempt is made to create more than one transmit focalzone, signals from previous pulses returning from different depthsin the body will fuzz the picture with artifacts not originatingat the depth of that particular focal zone.

Diasonics was able to create continuous transmit focus in theSpectra VST by accomplishing two things:

• energy of a particular ultrasound beam is concentratedat a specific depth by simultaneously adjusting at least a half-dozentransmission parameters, such as aperture, amount of energy, burstlength and frequency; and

• these beam parameters are readjusted on the fly sothat each beam transmission aims at a different depth and resultsin a minimal spillover of beam signals into other focal zones.

"The problem in the past with dynamic transmit is thatyou get all this interference between the zones. The way we havegotten around it is by reprogramming all of the beam parameters,"said Bruce N. Moore, president of Diasonics' ultrasound division.

This ability to rapidly reprogram the beam transmit parametersis not just a result of software improvements. Spectra's architecturehas a digital real-time controller in its front end that is separatefrom the scanner's main host computer and handles the reprogrammingwork.

"The host software is not controlling the beam parameters.That would be too slow. If you have an architecture that is basedon the host software controlling your front end, you can't implementwhat we have implemented, because you won't have the speed,"Moore told SCAN.

Although Spectra functions with a 48-channel front end originallylicensed from Acoustic Imaging, the controller was designed byDiasonics, he said.

Other advances in Spectra VST include:

• Frequency domain imaging, which the vendor says takesa middle ground in multifrequency imaging between Acuson's multi-Hertzand ATL's broadband high-definition imaging techniques. FDI optimizesfrequencies for particular depths;

• Matched impedance transducers. Diasonics invested inits own transducer development program, creating composite ceramicand epoxy transducers it claims provide impedances that are afactor of three or four lower than conventional transducers. Thisreduces reverberations and clears up near-field haze.

• Color-flow sensitivity was improved through two softwareprograms, parametric clutter elimination and a time-domain signalseparator. These processing developments help reduce tissue artifactsand "flash" problems.

More discussion of Diasonics' VST product will be offered in thenext issue of SCAN.