Wideband phased-array transducer for uniform harmonic...

Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation

Reexamination Certificate

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Reexamination Certificate

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06425869

ABSTRACT:

FIELD OF THE INVENTION
The present disclosure relates to ultrasonic imaging. More particularly, the invention relates to a system and method that permits uniform harmonic imaging and uniform contrast agent detection and destruction.
BACKGROUND OF THE INVENTION
Tissue Harmonic Background
Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of its image quality, safety, and low cost. Ultrasonic images are typically formed through the use of phased or linear array transducers which are capable of transmitting and receiving pressure waves directed into a medium such as the human body. Such transducers normally comprise multielement piezoelectric materials, which vibrate in response to an applied voltage to produce the desired pressure waves. Piezoelectric transducer elements are typically constructed of lead zirconate titanate (PZT), with a plurality of elements being arranged to form a transducer assembly. A new generation ultrasonic transducer known as a micro-machined ultrasonic transducer (MUT) is also available. MUTs are typically fabricated using semiconductor manufacturing techniques with a number of elements typically formed on a common substrate to form a transducer assembly. Regardless of the type of transducer element, the transducer elements may be further assembled into a housing possibly containing control electronics, the combination of which forms an ultrasonic probe. The ultrasonic probe may include acoustic matching layers between the surface of the various types of elements and the probe body. Ultrasonic probes may then be used along with an ultrasonic transceiver to transmit and receive ultrasonic pressure waves through the various tissues of the body. The various ultrasonic responses may be further processed by an ultrasonic imaging system to display the various structures and tissues of the body.
To obtain high quality images, the ultrasonic probe must be constructed so as to produce specified frequencies of pressure waves. Generally speaking, low frequency pressure waves provide deep penetration into the medium (e.g., the body), but produce poor resolution images due to the length of the transmitted wavelengths. On the other hand, high frequency pressure waves provide high resolution, but with poor penetration. Accordingly, the selection of a transmitting frequency has involved balancing resolution and penetration concerns. Unfortunately, resolution has suffered at the expense of deeper penetration and vice versa. Traditionally, the frequency selection problem has been addressed by selecting the highest imaging frequency (i.e., best resolution) which offers adequate penetration for a given application. For example, in adult cardiac imaging, frequencies in the 2 MHz to 3 MHz range are typically selected in order to penetrate the chest wall. Lower frequencies have not been used due to the lack of sufficient image resolution. Higher frequencies are often used for radiology and vascular applications where fine resolution is required to image small lesions and arteries affected by stenotic obstructions.
Recently, new methods have been studied in an effort to obtain both high resolution and deep penetration. One such method is known as harmonic imaging. Harmonic imaging is grounded on the phenomenon that objects, such as human tissues, develop and return their own non-fundamental frequencies, i.e., harmonics of the fundamental frequency. This phenomenon and increased image processing capabilities of digital technology, make it is possible to excite an object to be imaged by transmitting at a low (and therefore deeply penetrating) fundamental frequency (f
o
) and receiving reflections at a higher frequency harmonic (e.g., 2f
o
) to form a high resolution image of an object. By way of example, a wave having a frequency less than 2 MHz can be transmitted into the human body and one or more harmonic waves having frequencies greater than 3 MHz can be received to form the image. By imaging in this manner, deep penetration can be achieved without a concomitant loss of image resolution.
Transducers have been designed for transmit frequencies in the range of 2 MHz to 3 MHz for sufficient resolution of cardiac valves, endocardial borders and other cardiac structures. Lower transmit frequencies have been used for Doppler but not for B-mode imaging. Heretofore, transmit frequency selection has been determined based on the capabilities of fundamental response imaging which required relatively high fundamental frequencies in order to obtain adequate resolution for diagnostic purposes.
However, in order to achieve the benefits of transmitting at a lower frequency for tissue penetration and receiving a harmonic frequency for improved imaging resolution, broadband transducers are required which can transmit sufficient bandwidth about the fundamental frequency and receive sufficient bandwidth about the harmonic(s). The s4 transducer available with the SONOS™ 5500 an ultrasound imaging system manufactured by and commercially available from Agilent Technologies, U.S.A., has a suitable bandwidth to achieve harmonic imaging with a single transducer thus providing a significant clinical improvement. Furthermore, the combination of the s4 transducer and the SONOS™ 5500 provide multiple imaging parameter choices using a single transducer, thus providing a penetration choice as well as a resolution choice.
However, several problems exist with the current harmonic imaging technology due to the fact that current transducer designs have been based on fundamental imaging and not harmonic imaging. The goal with harmonic imaging is to generate harmonic signals in the body of high enough intensity to be above the noise floor of the system. Theoretically, a harmonic signal will be more than 20 dB down from the fundamental backscatter and therefore wide dynamic range receivers are required. In the near-field, where harmonic responses have not yet formed and in the far-field where attenuation has taken over, it is not uncommon for a harmonic response to be 30-40 dB down from the fundamental backscatter. It is critical that the magnitude of the harmonic signal generated in the body be over both the noise floor of the system and the transmitted second harmonic backscatter. This is difficult to attain across the entire field of view, particularly in the near-field, where harmonics have not had the time to build and in the far-field where attenuation of the signal becomes a problem. In order to improve harmonic imaging the problem of non-uniform harmonic generation needs to be addressed.
Contrast Imaging Background
Harmonic imaging can also be particularly effective when used in conjunction with contrast agents. In contrast agent imaging, gas or fluid filled micro-sphere contrast agents known as microbubbles are typically injected into a medium, normally the bloodstream. Because of their strong nonlinear response characteristics when insonified at particular frequencies, contrast agent resonation can be easily detected by an ultrasound transducer. By using harmonic imaging after introducing contrast agents, medical personnel can significantly enhance imaging capability for diagnosing the health of blood-filled tissues and blood flow dynamics within a patient's circulatory system. For example, contrast agent harmonic imaging is especially effective in detecting myocardial boundaries, assessing microvascular blood flow, and detecting myocardial perfusion.
In addition to today's problems with Tissue Harmonic imaging, there are similar problems associated with the imaging of contrast agents. The power or mechanical index of the incident ultrasonic pressure wave directly affects the contrast agent acoustical response. At lower powers, microbubbles formed by encapsulating one or more gaseous contrast agents with a material forming a shell thereon resonate and emit harmonics of the transmitted frequency. The magnitude of these microbubble harmonics depends on the magnitude of the excitation signal pulse. At higher acoustical powers, microbubbles rupture a

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