Patent Application: US-95374001-A

Abstract:
an ultrasonic transducer , method , and system are disclosed for performing ultrasonic harmonic imaging in a medium or a living body . the ultrasonic transducer consists of a linear array of alternating long and short elements . a first set of transducer elements is for transmitting and receiving at a fundamental frequency , and a second set of transducer elements is for receiving second harmonic or subharmonic echoes , each set operating at their respective center frequencies . this dual - frequency ultrasonic transducer is coupled to an ultrasound system wherein transmit beamforming is done at the fundamental frequency , and receive beamforming is done at the second harmonic or subharmonic frequency . when receive beamforming at the fundamental frequency is added , the method enables parallel fundamental , harmonic , compound , and difference imaging . these methods may be utilized to improve ultrasonic harmonic imaging of hard - to - image patients by optimizing the transmission of fundamental - frequency ultrasound beams and the receiving of second harmonic or subharmonic echoes , while minimizing harmonic distortion and signal losses .

Description:
a dual - frequency ultrasonic array transducer for harmonic imaging , shown in fig1 a , comprises an array 100 of individual piezoelectric elements coupled to an ultrasound scanner . the piezoelectric elements consist of long elements 102 whose center frequency is the fundamental transmit frequency , and short elements 104 whose center frequency is twice the fundamental frequency ( second harmonic ). a transmit pulse 140 at the fundamental frequency is applied to each long element . a transmit beamformer appropriately delays pulses applied across the electronic aperture so as to focus the acoustic beam 130 emitted by the array of long elements . acoustic beam 130 is transmitted at the fundamental frequency and is reflected by target 120 , which may be living tissue in a body or an inanimate structure within a medium which allows ultrasound signals to pass and be reflected . a reflected echo 132 contains pulses at the fundamental frequency , pulses at the second harmonic , and pulses at other frequencies . the long transducer elements 102 optimally receive acoustic signals at the fundamental frequency which is their center frequency . the short transducer elements 104 optimally receive acoustic signals at the second harmonic frequency which is equal to their center frequency . all transducer elements convert the received acoustic signals into electrical pulses . in particular , the short transducer elements 104 preferentially convert second harmonic echoes into electrical pulses 142 at the second harmonic frequency . this method of harmonic signal transduction is advantageous over conventional ultrasound transducers because the usually weaker second harmonic echoes are received by the short transducer elements at their center frequency where electro - acoustic conversion efficiency is maximal , hence where transducer sensitivity is greatest . similarly , fundamental - frequency pulses are transmitted by the long transducer elements at their center frequency , ensuring acoustic beams without contamination by non - fundamental frequencies and minimal transducer heating . this is in contrast to a conventional ultrasound transducer used in harmonic imaging . the wide bandwidth 202 of a conventional wideband transducer is illustrated in fig2 . the center frequency f t of the conventional transducer is chosen such that the fundamental frequency f 0 to be transmitted is within its bandwidth ( defined in this illustration as − 3 db down from the peak at the center frequency ) at the low - frequency end , and the second harmonic frequency 2f 0 to be received is also within its bandwidth at the high - frequency end . at f 0 and 2f 0 the transducer is not only less efficient than at its center frequency , but also distorts frequency and phase modulations in the signals by asymmetrically suppressing the lower sideband of the fundamental - frequency signal and the upper sideband of the second harmonic signal . the present invention solves this problem by transmitting fundamental - frequency pulses at the center of the bandwidth 204 of the long elements and receiving second harmonic signals at the center of the bandwidth 206 of the short elements . the transmitted signal itself may be of narrow bandwidth 208 , thus , utilizing only a fraction of the available bandwidth of the transmit elements . an ultrasonic transducer for subharmonic imaging , shown in fig1 b , comprises an array 101 of individual piezoelectric elements coupled to an ultrasound scanner . the piezoelectric elements consist of short elements 103 whose center frequency is the fundamental transmit frequency , and long elements 105 whose center frequency is half the fundamental frequency ( primary subharmonic ). a transmit pulse 141 at the fundamental frequency is applied to each short element . a transmit beamformer appropriately delays pulses applied across the electronic aperture so as to focus the acoustic beam 131 emitted by the array of long elements . acoustic beam 131 is transmitted at the fundamental frequency and is reflected by target 121 , which may be living tissue in a body or an inanimate structure within a medium which allows ultrasound signals to pass and be reflected . a reflected echo 133 contains pulses at the fundamental frequency , pulses at the primary subharmonic , and pulses at other frequencies . the short transducer elements 103 optimally receive acoustic signals at the fundamental frequency which is their center frequency . the long transducer elements 105 optimally receive acoustic signals at the subharmonic frequency which is equal to their center frequency . all transducer elements convert the received acoustic signals into electrical pulses . in particular , the long transducer elements 105 preferentially convert subharmonic echoes into electrical pulses 143 at the primary subharmonic frequency . a longitudinal cross - section of the dual - frequency ultrasonic array transducer is shown in fig3 a . the drawings are exemplary of a harmonic imaging transducer , but apply equally to the design and fabrication of a subharmonic imaging transducer . long elements 102 and short elements 104 are arranged in alternating positions along the length of the array . the center - to - center spacing of the elements is optimally chosen to be less than or equal to the wavelength of the upper bandwidth limit of the second harmonic ( or higher of the two frequencies of interest ) to ensure adequate acceptance angles for beam steering and near - field beam focusing . a common electrical contact 306 is shown on the anterior surface of the transducer elements , and individual electrical contacts 308 and 310 are shown on the posterior surfaces of the long and short elements respectively . the transducer elements are embedded in a backing layer 320 which provides both acoustic damping and structural mounting . the forward surface of the array is coated with two ( or more ) quarterwave matching layers 322 and 324 whose function is to match the acoustic impedance of the transducer elements to the acoustic lens 326 . the thickness of each matching layer is one quarter of the wavelength of the second harmonic ( or the higher of two frequencies ). two layers grouped together may also effectively serve as a quarterwave matching layer for signals at the fundamental frequency ( or the lower of two frequencies ). ceramic piezoelectric transducers typically have a high acoustic impedance of 15 - 25 mrayls , much higher than that of soft tissues at approximately 1 . 5 mrayls . quarterwave matching layers having intermediate acoustic impedances , reduce acoustic reflectance at the interface between two different materials by reducing the difference in acoustic impedance at that interface . an optional standoff pad 328 puts the skin line a few millimeters away from the transducer elements to eliminate the near - field alternating - line dropout (“ picket fence ”) artifact due to transmitting and / or receiving through every other element in the array . if the center - to - center element spacing and associated element pitch are small enough , the standoff pad may not be necessary . [ 0034 ] fig3 b shows an elevational cross - section of the same transducer . flex circuit 334 is attached to anterior electrical contact 306 . flex circuit 336 is attached to posterior electrical contact 310 ( shown in fig3 a ). flex circuit 332 is attached to posterior electrical contact 308 . a method of fabricating the dual - frequency ultrasonic array transducer is illustrated in fig4 a - 4 f . fig4 a shows a mounting block 412 serving as a substrate to support a ceramic piezoelectric block 410 . the ceramic piezoelectric block is diced into individual long elements 102 separated by kerfs 414 in fig4 b . the kerfs are inter - element spacings that are filled with non - conductive acoustic - damping material . every other element is milled down to half height to form the short elements 104 . electrical connection layers 308 and 310 are deposited on the rear surfaces of all transducer elements , connected to respective flex circuits ( e . g ., 332 ), and then filled in with acoustic - damping backing material 320 as shown in fig4 d . the entire block is inverted and the original mounting block 412 is removed as illustrated in fig4 e . the anterior electrical connection layer 306 , quarterwave matching layers 322 and 324 , acoustic lens 326 , and optional standoff pad 328 are successively deposited or mounted on top of the array of exposed transducer elements . the dual - frequency ultrasonic array transducer is connected to the front end of an ultrasound scanner in a manner described in the block diagram of fig5 a . the drawings are exemplary of a harmonic imaging subsystem , but apply equally to the operation of a subharmonic imaging subsystem . in this embodiment , the front - end controller 510 sequences the transmit timing and receive beamforming events to be performed by the front - end circuitry . transmit pulse generators 520 produce precisely - timed pulse sequences for each active channel in the transmit aperture with a delay profile necessary for electronic beam focusing . the transmitter array 522 , driven by timed transmit pulse sequences at the fundamental frequency , sends its output to individual long elements 102 in the transducer . harmonic echoes are received by individual short elements 104 , whose outputs are fed into preamplifiers 504 and analog - to - digital converters 506 . receive aperture switching array 530 selectively passes signals from those channels within the active receive aperture to the receive beamformer 532 . the receive beamformer applies specified delays to each channel in the receive aperture to electronically focus the received signals from a particular focal depth , sums them together , and optionally selects those signals within a specified temporal ( depth ) window from the transmit event . the resulting second harmonic signal is then sent to the scan converter of the ultrasound scanner to be assembled into a viewable image . in an alternative preferred embodiment of the invention , illustrated in fig5 b , transmit - receive ( t / r ) switches 502 are added to the long elements 102 . this enables them to be used for both transmitting and receiving . additional preamplifiers 504 and analog - to - digital converters 506 are connected to the receive side of each t / r switch . the output of these additional channels is fed into a separate receive aperture switching array 540 and receive beamformer 542 for imaging at the fundamental frequency . the resulting summed fundamental - frequency signal is sent to the scan converter independently of the summed second harmonic signal ( described above ). the block diagram of an ultrasound scanner for simultaneous fundamental - frequency and second harmonic imaging is shown in fig6 . again , the drawings are exemplary of a harmonic imaging system , but apply equally to the concept of a subharmonic imaging system . the output of the fundamental - frequency beamformer 542 is fed into one scan converter 644 to assemble the fundamental - frequency image . the output of the second harmonic beamformer 532 is fed into a second scan converter 634 to assemble the second harmonic image . these images may be displayed individually or side - by - side on the viewing screen of the ultrasound scanner . in addition , the two images may be summed together by summing unit 652 to form a dual - frequency compound image . compound imaging in general has been found to be useful in reducing speckle and anisotropic reflection artifacts , and in improving image smoothness . the two images may also be subtracted by subtraction unit 654 to form a difference image . because fundamental - frequency images contain echoes with both linearly - propagated and non - linearly - propagated components , and second harmonic images tend to be comprised primarily of non - linearly - propagated echoes , subtraction imaging may be expected to better visualize the spatial patterns of non - linear propagation and reflection , hence , providing a new parameter for ultrasonic imaging and potential tissue differentiation . in another alternative embodiment of the invention , the short elements may be used for transmitting at a fundamental frequency and the long elements may be used for receiving at one half the fundamental frequency for the purpose of subharmonic imaging . in addition , the short elements may be used for receiving at the fundamental frequency so as to enable simultaneous fundamental - frequency and subharmonic imaging . in the foregoing , an ultrasonic array transducer has been described for transmitting at a fundamental frequency and receiving harmonic echoes from a medium or body , the transducer consisting of alternating elements of two different center frequencies . a method has also been described for fabrication of this type of transducer . a method and system have further been described for displaying fundamental , harmonic , compound , and difference images using this transducer . although the present invention has been described with reference to specific exemplary embodiments , it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in these claims . accordingly , the specification and drawings are to be regarded in an illustrative rather than a restrictive sense . 1 . k . honda . “ multi - 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