Ultrasound vendors take distinct frequency paths

Some observers of the ultrasound industry point to the proliferationof high-end radiology scanners as a sign of technology convergence.While there are more premium ultrasound systems on the marketnow than five years ago, vendors are boosting the diagnostic powerof their systems in distinctly different ways.

Multiple-frequency imaging exemplifies how ultrasound approachesdiverge. Industry leaders Acuson and ATL have taken unique pathsin this area, for different clinical reasons.

ATL's HDI (high-definition imaging) upgrade to its Ultramark9 ultrasound system, introduced in April (SCAN 4/24/91), offersbroad-bandwidth transducer technology. The first broad-bandwidthtransducer on the UM 9 is the L10-5 linear array. This probe hasfull imaging capabilities over the range of 5 to 10 MHz, accordingto Eugene A. Larson, executive vice president of ATL.

Acuson introduced multihertz imaging with its 128XP upgradein July and extended this capability to more transducers withits XP Xcelerator upgrade last month (see following story). Multihertzprobes switch between two frequencies. Multihertz imaging is distinctfrom broad-bandwidth technology in that two different electronicsignals are sent into the transducer, creating two transmit frequencies,said Clay Larsen, radiology marketing manager for Acuson.

"Companies have been talking about broad bandwidth forquite some time. This is no secret in the marketplace. It is notnew. Acuson has broad-bandwidth transducers, but we have implemented(the technology) in a more useful way," Larsen said.

Signal transmissions in all ultrasound transducers incorporatea bell-curve distribution of frequencies, he said. A 5-MHz transduceris labeled as such when the center frequency in that curve isat 5 MHz. Multihertz probes, however, use transmissions with twodifferent center frequencies.

Ultrasonographers size up their patients before imaging andtry to select a transducer that can be used throughout the entireexam. There is a temptation to choose a lower frequency procedureto assure penetration in patients who may be hard to image. Multihertztechnology allows the sonographer to select a higher frequencytransducer for better resolution and switch to a lower frequencyif better penetration is needed, Larsen said.

Multihertz imaging does have clinical justification, Larsonacknowledged. But this justification may diminish as broad-bandwidthtechnology improves, he said.

Piezoelectric elements--quartz or ceramic transducer crystals--havea natural oscillating frequency according to the thickness atwhich they are cut, he said. Deviation from that natural frequencyresults in a dramatic decline in the transducer's efficiency andin the amount of diagnostic information obtained, Larson said.

THE TERM BANDWIDTH REFERS TO the width of a band of frequencieson the bell curve at the point where the scanner uses the ultrasoundinformation. Ultrasound measurements are typically taken at the6-dB amplitude point, half-way along the bell curve, he said.

The bell curve of a broad-bandwidth transducer is stretchedfurther out. More frequencies are found at the 6-dB level. A 5-MHztransducer with 40% bandwidth will have a bandwidth of 2 MHz,or 40% of the 5-MHz center frequency. In other words, the frequencybell curve stretches from 4 to 6 MHz at the point of acousticmeasurement.

Signal penetration, however, depends on the center transmissionfrequency. Broadening the bandwidth does not improve penetration,said Larsen of Acuson.

"It depends on what you are trying to do. If you are tryingto have one scanhead work in both a better resolution and a betterpenetration mode, then you can certainly do that (by using twosignal transmissions with different center frequencies),"said ATL's Larson.

HDI'S BROAD-BANDWIDTH TECHNOLOGY delivers something different,however. New information is obtained by widening the span of frequenciesmeasured. ATL's digital beamformer is key to the development ofexpanded bandwidth imaging, Larson said.

ATL has replaced the analog delay lines used in other ultrasoundscanners with a digital time delay. The digital front end canaccept returning signals with wider bandwidths. Analog systemshave a 2-MHz maximum bandwidth window, he said.

As long as transducers had fairly narrow bandwidths, this analogwindow was not a constraint. But ATL and other developers havecreated transducers with 80% and higher bandwidth. This is equivalentto 6 MHz of absolute bandwidth for a 7.5-MHz transducer, Larsonnoted.

"If you have 5 MHz worth of bandwidth that you are tryingto put through a 2-MHz window, you lose a little more than halfthe information," he said.

ATL used composite ceramic technology obtained in its acquisitionof Precision Acoustic Devices to broaden the bandwidths on itstransducers. PAD of Fremont, CA, was an independent transducerdeveloper (SCAN 1/17/90).

Although the L10-5 probe has a 62% bandwidth, ATL has reached80% bandwidth on transducers under development. Transducers withbandwidths above 60% are considered broadband. Continued developmentof the technology should result in transducers with bandwidthsas high as 90% in two years, Larson said.

Wider bandwidth is important because different types of tissuerespond to different portions of the frequency spectrum, he said.For instance, fat in the liver has 50% more signal attenuationthan normal liver tissue.

"What we (at ATL) are trying to do is capture the entirespectrum at once and show the attenuation response and the scatteringresponse of all tissue to the full breadth of the spectrum. Ifyou (scan) both (fat and normal liver tissue) at once with thewhole bandwidth, you will get back a different characteristicpattern at different frequencies for the two tissues," Larsonsaid.

Broad-bandwidth imaging thus brings ultrasound closer to theholy grail of tissue characterization, he said.