Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of its image quality, safety, and low cost. The typical way of implementing ultrasonic imaging is to transmit a pulse at a given frequency and receive its echoes.
Ultrasonic images are typically formed through the use of phased or linear array transducer elements which are capable of transmitting and receiving pressure waves directed into a medium such as the human body. Such transducer elements normally comprise multielement piezoelectric materials, which materials vibrate in response to an applied voltage to produce the desired pressure waves. Regardless of the type of transducer element, these transducer elements may be further assembled into a housing, possibly containing control electronics, the combination of which forms an ultrasonic probe (or transducer).
Transducers (or 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.
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.
In addition, THI provides for clutter suppression from reverberation reduction (e.g., from ribs) due to generation at a distance from the source, clutter suppression from side and grating-lobe reduction, contrast enhancement from the use of higher imaging frequencies (and the frequency dependence of backscatter) and a general IQ improvement from aberration (e.g., multi-path) reduction.
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 fundamental frequency (fo) and receiving reflections at a higher frequency harmonic (e.g., 2fo) to form a high resolution image of an object. By way of example, a wave having a center frequency of 2 MHz can be transmitted into the human body and harmonic frequencies at integer multiples of the fundamental frequency, e.g., 4 MHz and 6 MHz, etc., can be received to form the image.
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. A harmonic signal may be more than 20 dB down (the actual figure depends upon the path length and frequency, the maximum level of nonlinearly generated second harmonic in the tissue is −6 dB) from the fundamental backscatter and therefore wide dynamic range receivers are required. In the near-field, where little harmonic generation has occurred, and in the far-filed 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. In order to improve harmonic imaging the problem of non-uniform harmonic generation needs to be addressed.
This is acutely so with respect to a lack of non-linear second harmonic signal response in the near field. A more continuous or substantial second harmonic return signal component in the near field is desirable.
Several patents have been granted focusing on overcoming the signal-to-noise problem with harmonic imaging. U.S. Pat. No. 5,740,128 to Hossack et al., teaches a transmit element that minimizes the energy transmitted into the body at the range of frequencies where a response generated harmonic is expected as the transmitted energy can not be distinguished from a generated harmonic signal. The techniques revealed by Hossack address the dynamic range between the transmitted or fundamental frequency and the harmonic signal response. Hossack's techniques do not address non-uniform harmonic signal responses in near-field and far-field imaging planes.
U.S. Pat. No. 5,410,516 to Uhlendorf et al., discloses contrast agent imaging along with single pulse excitation techniques such as harmonic imaging. Specifically, Uhlendorf teaches that by choosing a radio-frequency (RF) filter to selectively observe any integer harmonic (2nd, 3rd, etc.), subharmonic (e.g., 1/2 harmonic) or ultraharmonic (e.g., 3/2 harmonic) it is possible to improve the microbubble to tissue ratio. The second harmonic has proven most useful due to the large bubble response at this frequency as compared to higher order integer harmonics, subharmonics or ultraharmonics. The second harmonic also is most practical due to bandwidth limitations on the transducer (i.e., <70% bandwidth, where percent bandwidth is defined as the difference of the high corner frequency −6 dB point from the low corner frequency −6 dB point, divided by the center frequency). However, single pulse excitation techniques together with harmonic imaging suffer from poor microbubble-to-tissue ratio as large tissue integer-harmonic signals mask the signal generated by the contrast agent.
U.S. Pat. No. 5,558,092 to Unger et al., discloses methods and apparatus for performing diagnostic and therapeutic ultrasound simultaneously. Unger introduces a specialized transducer with “diagnostic” elements and “therapeutic” elements. The therapeutic elements are intended to rupture vesicles (microbubbles) containing drugs/genes or other therapeutic materials, while the diagnostic elements are available to monitor results of the rupture events. Unger teaches low frequency high power ultrasonic signals to enhance rupturing of the vesicles for therapeutic reasons. Unger's transducers are complicated, difficult to manufacture, and expensive. The transducers also suffer in performance from a typical phased-array transducer because the full aperture can not be used for imaging as a significant portion of the transducer is dedicated to therapeutic insonification.
U.S. Pat. No. 5,833,613 to Averkiou et at, teaches a multi-pulse transmission signal designed to minimize transmitted noise and to increase the harmonic signal. The technique transmits consecutive pulses with reversed polarities from one another into the body. Reflective addition of the pulses will subtract transmitted second harmonic reflections (undesired) and will cause the generated harmonic waveforms, which return to the transducer in phase, to add coherently thus increasing the signal-to-noise ratio. Like U.S. Pat. No. 5,740,128 to Hossack et al., Averkiou's multi-pulse technique does not address non-uniform harmonic signal responses. Averkiou's multi-pulse technique is susceptible to motion artifacts generated by each subsequent return of the multiple transmission pulses. In addition, Averkiou's multi-pulse technique increases signal-processing overhead, which leads to a decrease in the frame rate for real-time ultrasound diagnostic systems.
A second U.S. patent to Averkiou et al., U.S. Pat. No. 5,879,303, teaches a method for ultrasonic imaging using reflections from both the fundamental and one or more harmonic signals in the presence of depth dependent ultrasound signal attenuation. Averkiou et al. Determined that harmonics could be created as ultrasound waves passed through tissue and became distorted. The distortion was found to create harmonic signal components imaging with which is referred to in the art as tissue harmonic imaging (THI). Hence, the '303 patent teaches that images may be reconstructed to contain both fundamental, e.g., 3 MHz, and harmonic, 6 MHz, frequency components from a transmit signal which contains only the fundamental transmit frequency.
By transmitting energy only at the fundamental frequency, e.g., 3 MHz, and by removing reflections from the fundamental and using only generated harmonics to create the image, multi-path clutter from undesired structures in the near-field may be removed utilizing the inventive THI concepts set forth in the '303 patent. While the '303 patent discusses a need to reduce multi-path clutter in the near-field, the '303 patent fails to address the need to quickly generate harmonic signal responses in the near-field, where the generated harmonic signals are generally 30 dB down from the fundamental. The '303 patent also fails to address the need for deeper signal penetration in order to generate harmonic signal responses with a suitable energy level in the far-field where the energy is also more than 30 dB down.
U.S. Pat. No. 6,117,082 to Bradley et al., teaches medical diagnostic ultrasound imaging at a fractional harmonic such as f0/2 or 3f0/2, where f0 is the fundamental frequency of the transmit beam. To improve the fractional harmonic imaging, the '082 patent proposes adding a fractional harmonic seed component, for example, at f0/2 or 3 f0/2. That is, the '082 patent teaches that adding a subharmonic seed signal with the fundamental frequency along with the fundamental transmit signal will cause the subharmonic of the fundamental transmit signal to develop more quickly. However, because of the time required to develop tissue harmonics, there is an inconsistency of energies of the different frequencies received back from the same tissue depths.
U.S. Pat. No. 6,283,919 B1, to Roundhill, et al., commonly owned and incorporated by reference herein in its entirety, teaches an ultrasonic diagnostic imaging system and method in the field of tissue harmonic imaging (THI) whereby both fundamental and harmonic components are returned in the echo signal and analyzed. The invention disclosed uses the harmonic echo signal components to reduce near-field or multi-path clutter in the ultrasonic image, such as that produced when imaging through narrow acoustic windows such as the ribs. The invention thereby allows imaging ay appreciable depths and substantially decreases the effects of depth-dependent attenuation.
U.S. Pat. No. 6,312,379 B1 to Bradley et al, discloses an ultrasound imaging method which includes transmitting a pre-distorted at least one of a plurality of waveforms as a function of non-linearity, e.g., a device non-linearity, a wavelength propagation non-linearity, etc. The transmitted waveform may comprise a fundamental spectral component and a harmonic spectral component from the transducer, where an attenuated normalized peak of the harmonic spectral component is reduced at a region spaced from the transducer as compared to the region adjacent the transducer. The transmitted waveform is preferably pre-distorted to include a harmonic spectral peak suppressed by about 4 dB or more at a region of interest spaced from the transducer relative a harmonic spectral peak at a region associated with transmission of a waveform comprising a fundamental spectral component adjacent the transducer.
Pending U.S. patent application Ser. No. 09/802,491, filed Mar. 9, 2001, commonly owned, and incorporated herein by reference in its entirety, discloses an ultrasonic imaging system and method wherein fundamental and harmonic components of the received signal are located in the transmit pass band. Pending U.S. patent application Ser. No. 10/026,997, filed Dec. 19, 2001, commonly owned, and incorporated herein by reference in its entirety, discloses an ultrasonic imaging system and method which uses a small signal at a harmonic frequency for imaging blood vessels. That is, a low energy signal at the fundamental and a harmonic is focused on blood vessels until that time that the contrast agent appears, whereafter the composition of the transmitted signal is changed to insonify the CA.
A fundamental problem associated with the conventional THI is that it does not address focusing of the various frequency components at transmission to maximize return at various depths. That is, the prior art teaches that both the fundamental and harmonic frequencies are focused at the same depth. Because it takes some time for the tissue harmonic to develop, none or little harmonic signal energy is received from shallow depths. More particularly, while the conventional art may teach or suggest transmitting ultrasound energy with a beam including both fundamental and harmonic frequency components, the beam is always focused at the same depth. In such practice, however, the harmonic components, which take a finite time to develop, are found to be limited with respect to the near field. That is, the production of harmonics is a function of propagation path length so that in the near field, little harmonic signal is generated.