Aerogel backed ultrasound transducer

Details Aerogel backed ultrasound transducer Aerogel backed ultrasound transducer

2001-04-13

2002-11-05

Lateef, Marvin M. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

Reexamination Certificate

active

06475151

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to ultrasound transducers, and more specifically to an aerogel backed ultrasound transducer.
BACKGROUND OF THE INVENTION
Generally, ultrasound transducers are used in ultrasound imaging devices for imaging in a wide variety of applications, especially medical diagnosis and treatment. Ultrasound imaging devices typically employ mechanisms to transmit scanning beams of pulsed ultrasound energy and to receive the reflected echoes from each scan. The detected echoes are used to generate an image which can be displayed, for example, on a monitor.
A typical ultrasound transducer comprises an acoustic element which transmits and receives ultrasound waves. The acoustic element may be made of a piezoelectric or piezostrictive material, for example. The acoustic element has a front side from which ultrasonic waves are transmitted and received, and a back side which may be bonded to an acoustic backing layer. An acoustic backing layer dampens the acoustic element to shorten the pulse length, and ringdown and to allow the transmission and reception in one direction. To produce this effect, the acoustic backing layer is typically made of a material having an attenuative nature. Hence, conventional materials used as a backing layer have been dense materials such as tungsten and epoxy.
A significant drawback to using a dense backing layer material is that a large amount of power consumed by the acoustic element is lost in the backing layer rather than being used to transmit ultrasound waves. If 3 dB of the transducer signal is attenuated on the backing material, the equivalent of half the power drawn by the acoustic element is lost. In other words, if the transmission efficiency of the ultrasound transducer is increased by 3 dB, the power needed to drive the transducer can be cut in half for the same signal output.
In order to reduce the amount of power lost in the backing layer, transducers having air backing layers have been used. An air backing layer reflects almost all of the power directed out of the back side of the acoustic element toward the front side of the acoustic element. This occurs because of the large acoustic impedance mismatch between the air and the acoustic element.
There are several significant disadvantages associated with an air back transducer. One is that an air-backed transducer has a longer pulse length than a transducer having a dense backing layer. It is also very difficult to support an acoustic element in air.
Therefore, there is a need for an improved ultrasound transducer which provides effective damping of the acoustic element to reduce pulse length, electrically insulates and supports the ultrasound transducer, and reduces the amount of power lost in the backing layer.
SUMMARY OF THE INVENTION
The present invention provides an ultrasound transducer employing aerogel as a backing material. Aerogels are solids with extremely porous structures. Aerogels are produced by drying wet gels while retaining the spatial structure of the solid which originally contained water or solvent. Aerogels are discussed generally in “Resource Report: Jet Propulsion Laboratory,”
NASA TechBriefs,
Vol. 19, No. 5, May 1995, at 8, 14. The properties and production of aerogels are described in detail in European Patent No. EP 0 640 564 A1 to Gerlach et al. Gerlach et al. suggests aerogels for use as acoustic matching layers on ultrasonic transducers. These and all other references cited herein are expressly incorporated by reference as if fully set forth in their entirety herein.
Aerogels have the lowest known density of all solid materials. Aerogels have densities as low as 0.015 g/cm
3
. Aerogels also have sufficient strength to provide support structure for the acoustic element. In addition, aerogels provide excellent electrical isolation from the rest of the structure.
The ultrasound transducer of the present invention comprises a conventional acoustic element. For instance, the acoustic element may be a piezoelectric or piezostrictive material. An acoustic backing material made of an aerogel material is attached to a back side of the acoustic element.
Before attaching the aerogel backing material to the acoustic element, the aerogel backing material may be coated with a metalized layer so that it is electrically conductive. This allows at least one of the electrical connections to the transducer to be made to the backing material. Otherwise, electrodes must be attached directly to the acoustic element which is a more difficult assembly.
The extremely low density aerogel has a lower acoustic impedance than conventional backing materials, such as tungsten and epoxy, and a lower acoustic impedance than the acoustic element. The acoustic impedance of aerogel approximates the acoustic impedance of air. The mismatch of acoustic impedance between the aerogel backing material and the acoustic element causes ultrasound waves to reflect back towards the front side of the transducer. Therefore, the aerogel backing material provides a transducer with a higher signal output than a transducer employing conventional backing materials. The thickness of the acoustic element is sized such that the reflected ultrasound wave is in phase and additive to the ultrasound wave initially directed toward the front side of the transducer.
The electrical insulating quality of the aerogel provides exceptionally high electrical resistance. The acoustic properties of aerogel isolate the element and increase the transducer's output. Increasing the transducer signal increases signal-to-noise ratio and improves the displayed image.
A matching layer may be attached to the front side of the acoustic element. The matching layer is typically ¼ wavelength thick. The acoustic matching layer can be tuned to shorten the pulse length, yet transmit most of the transducer power through the matching layer. The reduction of the pulse length improves axial resolution for imaging.


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Tsou, Peter, “Jet Propulsion Laboratory,” NASA Tech Briefs,The Digest of New Technology,May 1995, vol. 19, No. 5.

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