The present invention generally relates to ultrasonic transducers, and more specifically to ultrasonic array transducers capable of transmitting and receiving ultrasonic pulses at two different frequencies.
Ultrasonic imaging technology has become an important tool for examining the internal structure of living organisms. In the diagnosis of various medical conditions, ultrasonic imaging is often useful to examine soft tissues within the body to show the structural detail of internal tissues and fluid flow. An important application of ultrasonic imaging is in the detection and identification of various internal structural abnormalities, such as cysts, tumors, abscesses, mineral deposits, blood vessel obstructions, and anatomical defects without physically penetrating the skin.
Ultrasonic images are formed by producing very short pulses of ultrasound using an electro-acoustic transducer, sending the pulses through the body, and measuring the properties (e.g., amplitude and phase) of the echoes from tissues within the body. Focused ultrasound pulses, referred to as xe2x80x9cultrasound beamsxe2x80x9d, are targeted to specific tissue regions of interest in the body. Typically, an ultrasound beam is focused at small lateral and depth intervals within the body to improve spatial resolution. Echoes are received by the ultrasound transducer and processed to generate an image of the tissue or object in a region of interest. The resulting image is usually referred to as a B-scan image.
The echoes from soft tissues and from contrast agents, such as various microbubbles, consist of ultrasound signals at the transmitted frequency (the fundamental frequency) as well as signals at various multiples of the transmitted frequency (harmonics). Apart from the fundamental frequency, the strongest harmonic signal is generally at the second harmonic or twice the fundamental frequency.
Ultrasonic beams are subject to random scattering and distortion as they travel through soft tissues, particularly where there are acoustic interfaces such as between muscle and fat. Collectively referred to as tissue aberrations, these tend to degrade the clarity of the B-scan image. However, harmonic echoes generally exhibit less distortion and diffraction than echoes at the fundamental frequency. Thus, an ultrasound image constructed out of harmonic echoes is often sharper, less hazy, and less distorted.
Recently, harmonic ultrasound imaging has come into widespread use, particularly in viewing deep abdominal organs and the heart. In large patients with thick aberrating layers of fat and muscle, or gastric air pockets, harmonic imaging has been found to provide diagnostically superior ultrasonic images of the liver, kidneys, stomach, uterus, ovaries, and other abdominal organs. Because the heart is surrounded by the lungs which contain aberrating pockets of air, harmonic ultrasound imaging frequently provides clearer images of the cardiac chambers and valves.
Conventional ultrasonic transducers consist of electro-acoustic elements of a particular resonant center frequency. Because lower frequency ultrasonic signals are more penetrating and higher frequency ultrasonic signals enable higher resolution, the choice of a transducer""s center frequency is an optimization trade-off between penetration and resolution, depending on the clinical application. Thus, a transducer intended for abdominal use has a lower center frequency of 2.5-5.0 MHz to achieve deep penetration to 18-25 centimeters at lower resolution, while a transducer intended for breast imaging has a higher center frequency of 7-14 MHz to achieve a resolution of 0.2-0.5 mm at reduced penetration. These transducers perform harmonic imaging by having a wide bandwidth such that the transmitted pulses are at the low end of this bandwidth and the harmonic echoes are received at the high end of the same bandwidth. Because a conventional transducer used for harmonic imaging neither transmits nor receives signals at its center frequency, it does not perform efficiently in either transmit or receive. Moreover, transmitting far from a wideband transducer""s center frequency introduces undesirable harmonic distortion. These facts limit both the transmitted signal quality and the quality of the resulting harmonic image. They also limit the electro-acoustic efficiency of the transducer, and, hence, penetration.
The present invention, an ultrasonic array transducer for harmonic imaging, consists of alternating elements of different center frequenciesxe2x80x94fundamental and harmonicxe2x80x94for transmit and harmonic receive, respectively. Lower-frequency elements are optimally matched to the transmitted fundamental frequency, and higher-frequency elements are optimally matched to harmonic echoes. The transmit elements can also be used for receiving echoes at the fundamental frequency. Thus, this transducer has the novelty of being able to optimally receive echoes at both the fundamental and harmonic frequencies simultaneously. Alternatively, the transducer design may be modified to optimally transmit at a fundamental frequency and receive at a subharmonic frequency (half the fundamental frequency).
An ultrasonic array transducer is described for performing harmonic imaging. The transducer consists of alternating elements with center frequency at a fundamental frequency and center frequency at twice the fundamental frequency. The former elements are used to transmit and receive at the fundamental frequency, while the latter elements are used to receive harmonic echoes. Center-to-center element spacing is constrained to less than one quarter of the fundamental wavelength.
A method is further described for step-by-step fabrication of said dual-frequency transducer. Alternating elements are milled to half height from the back side before addition of electrical contacts and the acoustic-damping backing layer. Surface layers are then laid on the front side in the usual manner.
A method and system are further described for operating said ultrasonic transducer. In one embodiment of the present invention, the low-frequency elements are used for transmitting and the high-frequency elements are used for receiving. This has the added benefit of eliminating electronic noise generated by transmit-receive switching.
In a preferred embodiment of the present invention, the low-frequency elements are used for both transmitting and receiving at the fundamental frequency, and the high-frequency elements are used for receiving harmonics echoes. This system has the added benefit of being able to generate fundamental, harmonic, compound, and difference images in real time.
In each embodiment, the high-frequency elements may also be used for transmitting (or transmitting and receiving) at a fundamental frequency and the low-frequency elements may be used for receiving subharmonic echoes.