Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
  • G01N29/04—Analysing solids
  • G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/18—Methods or devices for transmitting, conducting or directing sound
  • G10K11/26—Sound-focusing or directing, e.g. scanning
  • G10K11/34—Sound-focusing or directing, e.g. scanning using electrical steering of transducer arrays, e.g. beam steering
  • G10K11/341—Circuits therefor
  • G10K11/346—Circuits therefor using phase variation

Definitions

• This invention relates to an acoustic variable focal length lens assembly and, more particularly, to such a lens assembly which is suitable for use in an ultrasonic pulse-echo imaging system for imaging a deep structure.
• This beam is normally at least as wide as the physical aperture A, so that the entire cross section of the acoustic lens operates on the ultrasonic wave energy.
• the imaging resolution is directly proportional to the value of the wavelength ⁇ of the ultrasonic wave energy and is inversely proportional to the value of the
• the center electrode alone, provides a beam of ultrasonic wave energy having a width only sufficient to insonify a relatively small central portion of the physical aperture of the acoustic lens.
• Switch means are provided for selectively utilizing the center electrode alone, or, alternatively, utilizing both the center and annular electrodes together.
• the relatively large effective relative aperture (and, hence, a relatively small depth of field) provided by the center and annular electrodes together is useful in providing a relatively high resolution C-scan display of an ultra ⁇ sonic image of a small depth of * insonified structure.
• the relatively small effective relative aperture (and hence the relatively large depth of field) obtained by employing the center electrode alone is useful in providing a somewhat lower resolution B-scan display of an ultrasonic image of a larger depth of insonified structure.
• the present invention is directed to an improvement to provide variable focal length in an acoustic lens assembly comprising first and second spaced acoustic lenses for focusing acoustic wave energy of a predetermined wavelength
• the first acoustic lens has a single relatively short predetermined focal length
• the second acoustic lens is electronically controlled to have any selected one of a predetermined plural number of different preselected discrete focal lengths.
• Each of these discrete focal lengths is substantially larger than the pre ⁇ determined focal length of the first acoustic lens.
• Such an acoustic, variable focal length lens assembly 0 is particularly suitable for use in an ultrasonic pulse-echo imaging system for imaging deep structure insonified with a pulse of ultrasonic wave energy of a predetermined wavelength.
• time-control means responsive to the time of transmission of a pulse of ultrasonic wave energy is g coupled to the second lens for sequentially selecting each of the different discrete focal lengths during the time in which ultrasonic echos are received from successive portions of the insonified deep structure.
• FIGURE 1 is a block diagram of a pulse-echo ultrasonic-imaging display.system incorporating a variable focal length acoustic lens assembly embodying the present 5 invention
• FIGURES 2a and 2b are a cross-section and plan view, respectively, of an embodiment of the multi-electrode transducer portion of the electronic acoustic lens of FIGURE 1;
• FIGURE 3 is a block diagram of an embodiment of the coupling means portion of the electronic acoustic lens of FIGURE 1;
• FIGURES 3a and 3b show a portion of FIGURE 3 and are referred to in the explanation of its operation, 5 and
• FIGURE 4 is a diagram, helpful in explaining the operation of the present invention, showing an example of the respective focal lengths of a variable focal length acoustic lens employing the principles of the present 0 invention .
• FIGURE 1 shows a B-scan ultrasonic imaging system.
• the present invention may be incorporated in an ultrasonic imaging system employing both a B-scan and a C-scan.
• the invention may be incorporated in an ultrasonic imaging system incorporating only C-scan which employs the variable focal length acoustic lens assembly of the present invention to provide a resultant depth of field sufficient to image all points within a deep structure with good focus.
• variable focal length acoustic lens assembly 100 which forms the principal subject matter of the present invention, is utilized to insonify deep structure to be imaged 102 with a focused scanning beam of pulsed ultrasonic wave energy of a predetermined wavelength traveling in ultrasonic wave propagating medium 104.
• variable focal length acoustic lens assembly 100 comprises serially spaced physical acoustic lens 106 and electronic acoustic lens 108.
• Electronic acoustic lens 108 comprises time-control means 110, coupling means 112 and multi-electrode transducer 114.
• physical acoustic lens 106 which is comprised of a solid material (such as polystyrene) having an acoustic index of refraction less than that of a surrounding ultrasonic-wave propagating medium 104 (such as water) , is fixed in position.
• scanning of structure to be imaged 102 with a beam of ultrasonic wave energy is achieved by either moving only multi-electrode transducer 114 or employing auxiliary scanning means, such as Risley prisms (not shown).
• the present invention also contemplates moving both physical acoustic lens 106 and electronic acoustic lens 108 to achieve scanning of structure to be imaged 102 with a beam, of ultrasonic wave energy.
• Time-control means 110 includes a clock for intermittently applying a keying pulse to pulse source 116 of a B-scan ultrasonic imaging system.
• Pulse source 116 includes an R.F. oscillator for applying a pulse of ultrasonic frequency (e.g. 1.5 MHz) through coupling means 112 to multi-electrode transducer 114 in response to each keying pulse from time-control means 110.
• multi-electrode transducer 114 launches a beam of pulsed ultrasonic wave energy having a selected one of a predetermined plural number of preselected discrete focal lengths, each of the discrete focal lengths being substantially larger than the single relatively-short predetermined focal length of physical acoustic lens 106.
• Deep structure 102 in response to the insonification thereof, emits an echo, some of which returns through physical acoustic lens 106 to multi- electrode transducer 114, giving rise,' in turn, to detected electrical signals.
• Coupling means 112 under the control of time-control means 110, couples only those signals which occur during a certain time interval following the insonification of deep structure 102 through imaging electronics 118 to B-scan display 120 of the imaging system. It is essential that the entire depth of structure 102 be within the total depth of field of lens assembly 100 in order to image all of deep structure 102 on B-scan display 120. However, structure 102 has a depth which is greater than the depth of field of any selected single focal length of lens assembly 100.
• coupling means 1.12 sequentially selects, under the control of time-control means 110, each of different discrete focal length- * , of electronic acoustic lens 108, during each of successive portions of the aforesaid certain time interval, to provide an effective resultant depth of field for variable focal length acoustic lens assembly 100 which includes the entire depth of deep structure 102.
• FIGURES 2a, 2b, 3, 3a and 3b In order to more fully understand the manner in which this is accomplished, reference is made to FIGURES 2a, 2b, 3, 3a and 3b.
• multi-electrode transducer 114 comprises a slab of piezoelectric material 200 having its faces covered by a plurality of electrodes.
• common electrode 202 substantially covers one of the two opposite faces of slab 200, -while the other of the two opposite faces of slab 200 is substantially covered by a plurality of separate spatially distributed contiguous electrodes 204, 206, 208, 210, 212 and 214.
• electrode 204 is a center electrode having a circular cross section of a preselected diamter and is surrounded, in turn, by contiguous annular electrodes 206, 208,
• Electrodes 204 . . . 214 substantially cover the entire cross section defined by the outer diamter of outermost electrode 214 (i.e., the inner diamater of each annular electrode is substantially equal to the outer diameter of the electrode that it immediately surrounds) .
• the radial widths are different from each other and are different from the diameter of center electrode 204. The reason for these different radial widths is discussed below.
• Coupling means 112 shown in FIGURE 3, comprises isolation circuit 300, which operates as a transmit-receive switch.
• Circuit 300 permits ultrasonic frequency pulses from pulse source 116 to be forwarded to each of electrodes 204 . . . 214 of transducer 114, but prevents received signals on electrodes 204 . . . 214 from being returned to pulse source 116.
• isolation circuit 300 may comprise a threshold switch (of a type disclosed in the aforesaid application Serial No. 844, 140) consisting of oppositely-poled semiconductor diodes. Such a threshold switch permits each ultrasonic frequency pulse from pulse source 116, which pulses have absolute values of amplitude greater than a semiconductor threshold voltage (e.g. 0.7 volts) , to be coupled in parallel to all of electrodes
• transducer 114 operates to launch a plane wavefront of ultrasonic wave energy in propagating medium 104; i.e., the focal length of the electronic lens 108, in this case, is infinity.
• Coupling means 112 also includes switch means and phase delay circuits for coupling any of selected combinations of electrodes 204 . . . 214 through summer 302 to a common terminal 304 (at the output of summer 302) connected to imaging electronics 118.
• the switch means in FIGURE 3 is functionally shown as comprising triple-throw switch 306, coupling electrode 204 to a first input of summer -302; double-throw switch 308, coupling electrode 206 to a second input of summer 302; double-throw switch 310, coupling electrode 208 to a third input of summer 302; single-throw switch 312, coupling electrode 312 to a fifth input of summer 302, and single-throw switch 312, coupling electrode 214 to a sixth input of summer 302.
• these switch means also include switch control 316 for selectively operating switches 306, 308, 310, 312 and 314 in accordance with time control signals applied thereto from time control 110.
• switch control 316 for selectively operating switches 306, 308, 310, 312 and 314 in accordance with time control signals applied thereto from time control 110.
• the switch means normall would comprise a plurality of normally-closed electronic gates which are selectively opened in any of various predetermined combinations in accordance with applied time-control signals from time control 110.
• the operation of these electronics gates would be functionally equivalent in all respects to the operation of switches 306, 308, 310, 312 and 314, described in detail below.
• Coupling means 112 also includes a plurality of phase delay circuits. Specifically, when double-throw switch 310 is in its upper switch position, any signal appearing on electrode 208 is coupled to the third input of summer 302 without any phase delay. However, when double-throw switch 310 is in its lower switch position, any signal appearing on electrode 208 is coupled to the third input of summer 302 with a phase delay of one-quarter cycle of the ultrasonic frequency, which is provided by phase delay I circuit 318.
• any signal on electrode 206 is coupled to the second input of summer 302 without a phase delay when double-throw switch 308 is in its upper switch position or, alternatively, with a phase delay of one-quarter cycle of the ultrasonic frequency, which is provided by phase delay II circuit 320, when double-throw switch 308 is in its lower switch position.
• any signal on electrode 204 is coupled to the first input of summer 302 without phase delay, when triple-throw switch 306 is in its upper switch position, or with a phase delay of one-quarter cycle of the ultrasonic frequency, which is provided by a phase delay III circuit 322, when triple-throw switch 306 is in its middle switch position, or with a phase delay of one-half cycle of the ultrasonic frequency, which is provided by phase delay IV circuit 324, when triple-throw switch 306 is in its lower switch position.
• the resultant focal length f of a lens assembly consisting of a first lens having a focal length f. in serial spaced relationship with a second lens having a focal length f 2 is given by the following equation:
• a physical imaging lens operates by phase delaying an incident wavefront by an amount which varies continuously as an inverse function of the distance between every point of incidence of the wavefront and the lens axis.
• An electronic lens of the type shown in FIGURES 2a, 2b, 3, 3a and 3b, utilizes a certain finite number of discrete electrodes associated with a discrete finite number of electrical phase delay circuits to simulate the operation of a physical lens. Such an electronic lens would require an infinite number of electrodes placed infinitely close together (which is impossible) in order to exactly duplicate a physical lens.
• a lens in which the phase delay varies inversely as a discontinuous function of off-axis distance still operates to produce an image, without appreciable degradation, so long as the phase delay provided for every point of incidence of the wavefront does not depart from that provided by the continuously varying phase delay of a physical lens by more than one-quarter wavelength of the incident wave (i.e., one-quarter cycle of the ultrasonic frequency) .
• An electronic lens of the type shown in FIGURES 1, 2a, 2b, 3, 3a and 3b is designed in accordance with this Rayleigh condition.
• an electronic lens comprising a transducer having n electrically separate concentric electrodes on a face therepf exhibits a focal length f 2 and complies with the Rayleigh condition for an incident wavefront of ultrasonic energy having a wavelength ⁇ , if the respective outer radii r, . . . r of the respective n electrodes conform with the following equations: - 10 -
• each electrode 20 must have a sufficient number of contiguous concentric electrodes on a face thereof; each electrode having an appropriate radial width, to permit each different set of radii for each different focal length to be achieved by selectively connecting at least some of the electrodes in
• each group consisting of one or more electrodes connected in parallel. This results in t radial width of some of the electrodes being relatively small. The actual size of the radial widths of the electrodes depends on the respective sizes of the require
• equations 2-1 . . . 2-n show tha the respective sizes of r, . . . r vary directly as a function of the size of the focal length f,-
• the present invention in accordance with equation 1, combines a in - variable relatively-long focal length electronic acoustic lens with a fixed relatively-short focal length physical acoustic lens to achieve a variable relatively-short focal length acoustic lens assembly. Further, the value n, and hence the total number of required electrodes, is reduced, because the relative aperture of the variable relatively- long focal length electronic acoustic lens alone becomes small with respect to the desired relative aperture of the variable relatively-short focal length acoustic lens assembly.
• variable focal length acoustic lens assembly 100 is designed to have a selectable focal length of 150 mm, 200 mm, or 250 mm, with a substantially constant relative aperture of one-third.
• physical acoustic lens 106 has a fixed focal length of 250 mm and a physical aperture which at least as large as the maximum physical aperture of electronic acoustic lens 108, so that the physical aperture exhibited by electronic acoustic lens 108 determines the physical aperture of variable focal length acoustic lens assembly 100.
• equation 1 may be employed to determine the different relatively long focal lengths required of electronic acoustic lens 108. Specifically, with a 250 mm focal length physical lens 106, the respective focal lengths of electronic lens 108 must be
• lens assembly 100 since the physical aperture of lens assembly 100 is determined by the physical aperture of electronic lens 108, electronic lens 108 must have respective physical apertures of 50 mm, 66.7 mm, or 83.3 mm for the respective focal lengths of 150 mm, 200 mm, or 250 mm of lens assembly 100, in order for lens assembly 100 to always have a relative aperture of substantially one-third.
• switch control 316 places switches 306 . . . 314 into a selected one of three given combinations of switch settings, each different combinatio corresponding to a different one of the three assumed foca lengths (and corresponding physical aperture) of electroni lens 108.
• FIGURE 3 shows a first given combination of settingsof switches 306 . . . 314, which selects a focal length of 375 mm for electronic lens 108.
• single-throw switches 314 and 312 are open, so that signals at electrodes 214 and 212 are not coupled to summer 302.
• Double-throw switch 310 is in its upper switch position, so that signals at electrode 208 are coupled directly to summer 302.
• Double-throw switch 308 is in its lower switch position, so that signals at electrode 206 are coupled to summer 302 through phase- " delay II circuit 320 with a phase delay of one-quarter cycle of the ultrasonic frequency.
• Triple-throw switch 306 is in its lower switch position, so that signals at electrode 204 are coupled through phase delay IV circuit 324 to summer 302 with a phase delay of one-half cycle of the ultrasonic frequency.
• Table 2 shows the assumed conditions for this first switch combination.
• the respective outer radii 13.69, 19.36 and 23.72 mm ' (shown in Table 2) correspond
• FIGURE 3a shows a second given combination of settings of switches 306 . . .314, which selects a focal length of 1000 mm for electronic lens 108.
• single-throw switch 314 is open, so that signals at electrode 214 are not coupled to summer 302.
• Single-throw switch 312 is closed, so that signals at electrode 212 are coupled directly to summer 302.
• Double-throw switch 310 is in its lower switch position, so that signals at electrode 208 are coupled to summer 302 through phase delay I circuit 318 with a phase delay of one-quarter cycle of the ultrasonic frequency.
• Double- throw switch 308 is in its lower switch position, so that signals at electrode 206 are coupled to summer 306 through phase delay II circuit 320 with a phase delay of one-quarte cycle of the ultrasonic frequency.
• Triple-throw switch 306 is in its middle switch position, so that signals at electrode 204 are coupled through phase delay III circuit 322 to summer 302 with a phase delay of one-quarter cycle of the ultrasonic frequency.
• Table 3 shows the assumed conditions for this second switch combination.
• the respective outer radii 22.36 and 31.62 mm (shown in Table 3) correspond to the respective values r. and r 2 of equations 2-1. . .2-n, when a focal length of 1000 mm is substituted for f 2 and 1 mm is substituted for ⁇ .
• the actual physical aperture of electronic lens 108 in its second switch position is 63.24 mm (i.e., double the outermost radius of 31.62 mm). This is substantially equal to the design criteria physical aperture of 66.7 mm.
• Second Switch Combination Lens Assembly 100 - Focal Length 200 mm
• FIGURE 3b shows third given combination of settings of switches 306. . . 314, which selects a focal length of infinity for electronic lens 108.
• single-throw switches 314 and 312 are closed, so that signals at electrodes 214 and 212 are coupled directly to summer 302.
• Double-throw switches 310 and 308 are in their upper switch positions, so that signals at electrodes 208 and 206 are coupled directly to summer 302.
• Triple-throw switch 306 is in its upper switch position, so that signals at electrode 204 are coupled directly to summer 302.
• Table 4 shows the assumed conditions for this third switch combination. Because in this third switch combination all of electrodes 204 . . . 214 are directly coupled to summer 302, a plane wavefront of received signals is detected by electronic lens 108 (i.e., electronic lens 108, in this third combination, has a focal length f j of infinity) . Therefore, in this third combination, all of electrodes 204 . . . 214 are effectively connected in parallel and operate electrically as though they were one single circular cross section electrode having a radius of 41.67 mm (equal to the outer radius of outermost electrode 214).
• the outer radius of 41.67 mm of electrode 214 is selected to provide electronic lens 108 with a physical aperture of 83.34 mm (i.e., double the outermost radius of 41.67 mm), that is required to provide lens assembly 100 with the desired relative aperture of one- third.
• the front surface 400 of deep structure 102 which is to be imaged, is situated 125 mm from the effective principal plane 402 of acoustic lens assembly 100, while rear surface 404 of deep structure 102 is situated 275 mm from principal plane 402.
• the total depth of deep structure 102 is 150 mm.
• time control 110 operates the switch means to maintain summer 302 decoupled from transducer 114 until a first given time at which echoes originating at front surface 400 are received by transducer 114. At this first given time, time control 110 operates the switch means to their first given switch combination so that acoustic lens assembly 100 exhibits a focal length of 150 mm. This first given switch combination is maintained until a second given time at which echoes originating 175 mm from principal plane 402 are received by transducer 114. At this second given time, time control 110 operates the switch means to their second given switch combination, so that acoustic lens assembly 100 exhibits a focal length of 200 mm.
• This second given switch combination is maintained until a third given time at which echoes originating 225 mm from principal plane 402 are received by transducer 114.
• time control 110 operates the switch means to their third switch combination, so that lens assembly 100 exhibits a focal length of 250 mm.
• Time control 110 maintains the switch mean in their third switch combination until echoes originating 11 inches from principal plane 402 are received by transducer 114.
• isolation circuit 300 may include appropriate phase delay circuits to provide electronic lens 108 with a desired focal length other than infinity (in accordance with equations 2-1 . . . 2-n) .

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• 1978
  • 1978-12-05 JP JP50024078A patent/JPS55500006A/ja active Pending
  • 1978-12-05 GB GB7927203A patent/GB2036317B/en not_active Expired
  • 1978-12-05 WO PCT/US1978/000186 patent/WO1979000373A1/en unknown
• 1979
  • 1979-07-03 EP EP19790900067 patent/EP0007310A4/en not_active Withdrawn
• 1980
  • 1980-11-18 FR FR8024851A patent/FR2476360A1/fr not_active Withdrawn

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