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Ultrasonic diagnostic imaging with blended tissue harmonic signalsRelated Patent Categories: Surgery, Diagnostic Testing, Detecting Nuclear, Electromagnetic, Or Ultrasonic Radiation, Ultrasonic, Anatomic Image Produced By Reflective Scanning, Electronic Array ScanningUltrasonic diagnostic imaging with blended tissue harmonic signals description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070149879, Ultrasonic diagnostic imaging with blended tissue harmonic signals. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This is a divisional application of U.S. patent application Ser. No. 08/943,546, filed Oct. 3, 1997 and entitled "ULTRASONIC DIAGNOSTIC IMAGING OF RESPONSE FREQUENCY DIFFERING FROM TRANSMIT FREQUENCY" which claims the benefit of U.S. Provisional Application No. 60/032,771 filed Nov. 26, 1996. [0002] This invention relates to ultrasonic diagnosis and imaging of the body and, in particular, to new methods and apparatus for ultrasonically imaging with a response frequency which differs from the transmitted frequency. [0003] Ultrasonic diagnostic imaging systems have been used to image the body with the enhancement of ultrasonic contrast agents. Contrast agents are substances which are biocompatible and exhibit uniquely chosen acoustic properties which return readily identifiable echo signals in response to insonification. Contrast agents can have several properties which enables them to enhance an ultrasonic image. One is the nonlinear characteristics of many contrast agents. Agents have been produced which, when insonified by an ultrasonic wave at one frequency, will exhibit resonance modes which return energy at other frequencies, in particular, harmonic frequencies. A harmonic contrast agent, when insonified at a fundamental frequency, will return echoes at the second, third, fourth, and higher harmonics of that frequency. [0004] It has been known for some time that tissue and fluids also have inherent nonlinear properties. Tissue and fluids will, even in the absence of a contrast agent, develop and return their own non-fundamental frequency echo response signals, including signals at harmonics of the fundamental. Muir and Carstensen explored these properties of water beginning in 1980, and Starritt et al. looked at these properties in human calf muscle and excised bovine liver. [0005] While these non-fundamental frequency echo components of tissue and fluids are generally not as great in amplitude as the harmonic components returned by harmonic contrast agents, they do exhibit a number of characteristics which may be advantageously used in ultrasonic imaging. One of us (M. Averkiou) has done extensive research into these properties in work described in his doctoral dissertation. In this exposition and other research, the present inventors have seen that the main lobe of a harmonic beam is narrower than that of its fundamental, which they have found has implications for clutter reduction when imaging through narrow orifices such as the ribs. They have seen that the sidelobe levels of a harmonic beam are lower than the corresponding sidelobe levels of the fundamental beam, which they have found has implications for off-axis clutter reduction. They have also seen that harmonic returns from the near field are also relatively less than returning energy at the fundamental frequency, which they have found has implications for near field clutter rejection. As will be seen, these properties may be exploited in the methods and constructed embodiments of the present invention. [0006] In accordance with the principles of the present invention, an ultrasonic imaging system and method are provided for imaging tissue and fluids from response frequencies which differ from the transmitted frequency, in particular echoes returned from the tissue or fluids at a harmonic of a transmitted fundamental frequency. The imaging system comprises a means for transmitting an ultrasonic wave at a fundamental frequency, means for receiving echoes at a harmonic frequency, and an image processor for producing an ultrasonic image from the harmonic frequency echoes. [0007] In a preferred embodiment of the present invention the transmitting and receiving means comprise a single ultrasonic probe. In accordance with a further aspect of the present invention, the probe utilizes a broadband ultrasonic transducer for both transmission and reception. [0008] In accordance with yet another aspect of the present invention, partially decorrelated components of received harmonic echoes are produced and utilized to remove artifacts from the harmonic image, providing clearly defined images of tissue boundaries such as that of the endocardium. In a preferred embodiment the partially decorrelated components are produced by processing the harmonic echoes through different passbands. [0009] The methods of the present invention include the use of harmonic echoes to reduce near-field or multipath clutter in an ultrasonic image, such as that produced when imaging through a narrow acoustic window such as the ribs. In accordance with yet a further aspect of the present invention, harmonic and fundamental echoes are blended in a common image to reduce clutter, image at appreciable depths, and overcome the effects of depth-dependent attenuation. [0010] In the drawings: [0011] FIG. 1 illustrates in block diagram form an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention; [0012] FIGS. 2, 3, 4, and 5 illustrate certain properties of harmonic echoes which may be advantageously applied to ultrasonic imaging applications; and [0013] FIGS. 6 and 7 illustrate passband characteristics used to explain the performance of the embodiment of FIG. 1; [0014] FIG. 8 illustrates typical fundamental and harmonic frequency passbands of an embodiment of the present invention; [0015] FIG. 9 illustrates an FIR filter structure suitable for use in the embodiment of FIG. 1; [0016] FIG. 10 illustrates in block diagram form a portion of a preferred embodiment of the present invention; [0017] FIG. 11 illustrates the operation of the normalization stages of the embodiment of FIG. 10; [0018] FIG. 12 is a block diagram of one of the multiplier accumulators used in the filters of the embodiment of FIG. 10; [0019] FIG. 13 illustrates typical fundamental and harmonic frequency passbands of the embodiment of FIG. 10; [0020] FIG. 14 illustrates the blending of fundamental and harmonic signal components into one ultrasonic image; and [0021] FIG. 15 illustrates the passbands of a time varying filter used in the formation of blended images. [0022] Referring first to FIG. 1, an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention is shown in block diagram form. A central controller 120 commands a transmit frequency control 117 to transmit a desired transmit frequency band. The parameters of the transmit frequency band, f.sub.tr, are coupled to the transmit frequency control 117, which causes the transducer 112 of ultrasonic probe 110 to transmit ultrasonic waves in the fundamental frequency band. In a constructed embodiment a band of frequencies located about a central frequency of 1.67 MHz is transmitted. This is lower than conventional transmitted imaging frequencies, which generally range from 2.5 MHz and above. However, use of a typical transmit frequency of 3 or 5 MHz will produce harmonics at 6 and 10 MHz. Since higher frequencies are more greatly attenuated by passage through the body than lower frequencies, these higher frequency harmonics will experience significant attenuation as they return to the probe. This reduces the depth of penetration and image quality at greater imaging depths, although the harmonic signals, created as they are during the propagation of the transmitted wave through tissue, do not experience the attenuation of a full round trip from the transducer as the fundamental signals do. To overcome this problem, the central transmit frequency in the illustrated embodiment is below 5 MHz, and preferably below 2.5 MHz, thereby producing lower frequency harmonics that are less susceptible to depth dependent attenuation and enabling harmonic imaging at greater depths. A transmitted fundamental frequency of 1.67 MHz will produce second harmonic return signals at 3.34 MHz in the illustrated embodiment. It will be understood, of course, that any ultrasonic frequency may be used, with due consideration of the desired depth of penetration and the sensitivity of the transducer and ultrasound system. [0023] The array transducer 112 of the probe 110 transmits ultrasonic energy and receives echoes returned in response to this transmission. The response characteristic of the transducer can exhibit two passbands, one around the fundamental transmit frequency and another about a harmonic frequency in the received passband. For harmonic imaging, a broadband transducer having a passband encompassing both the transmitted fundamental and received harmonic passbands is preferred. The transducer may be manufactured and tuned to exhibit a response characteristic as shown in FIG. 6, in which the lower hump 60 of the response characteristic is centered about the transmitted fundamental frequency f.sub.t, and the upper hump 62 is centered about the received harmonic frequency f.sub.r of the response passband. The transducer response characteristic of FIG. 7 is preferred, however, as the single dominant characteristic 64 allows the probe to be suitable for both harmonic imaging and conventional broadband imaging. The characteristic 64 encompasses the transmitted fundamental frequency f.sub.t, and also the harmonic receive passband bounded between frequencies f.sub.L and f.sub.c, and centered about frequency f.sub.r. As discussed above, a low fundamental transmit frequency of 1.67 MHz will result in harmonic returning echo signals at a frequency of 3.34 MHz. A response characteristic 64 of approximately 2 MHz would be suitable for these fundamental and harmonic frequencies. [0024] Tissue and cells in the body alter the transmitted fundamental frequency signals during propagation and the returned echoes contain harmonic components of the originally transmitted fundamental frequency. In FIG. 1 these echoes are received by the transducer array 112, coupled through the T/R switch 114 and digitized by analog to digital converters 115. The sampling frequency f.sub.s of the A/D converters 115 is controlled by the central controller. The desired sampling rate dictated by sampling theory is at least twice the highest frequency f.sub.c of the received passband and, for the preceding exemplary frequencies, might be on the order of at least 8 MHz. Sampling rates higher than the minimum requirement are also desirable. Continue reading about Ultrasonic diagnostic imaging with blended tissue harmonic signals... 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