using a laboratory setup illustrated in fig1 a tooth under examination was scanned by means of an x - y table 1 under a transducer 3 mounted to a drill arm 2 . the x - y table was controlled by a computer 4 via stepper motors 5 attached to the table &# 39 ; s micrometer screws . in addition , between the stepper motors 5 and the computer 4 , there was attached a separate controller 6 , which served as a buffer amplifier for the stepper motors 5 . the transducer 3 was a focused 5 mhz ultrasound transducer . normally , the ultrasound transducer 3 comprises both a transmitter and a receiver section . currently , most transducers are made of a ceramic piezoelectric material . a needle probe 7 was attached to the metal foil of a miniature buzzer 8 with the help of contact cement . the miniature buzzer 8 was driven by an oscillator 9 , which had an oscillating frequency adjustable in the range of 200 ... 600 hz . the input power of the oscillator 9 was about 4 w , however , the produced output intensity level remained appreciably lower than in commercially available vibrators . a conventional laboratory power supply was used as a power supply 10 . required energy pulses to the transducer were generated by a pulser 11 , which also served as a receiver and amplifier for the received echo signals . the received echo pulse was gated by a gating unit 12 with such a timing as to allow only the echo from the area under examination to pass through the gate . a filter 13 was used for extraction of the vibration frequency of the needle 7 by band pass filtering from the echo pulse , and a rectifier 14 provided the vibration level signal by rectification . the detected signal level was measured by a voltage meter 15 , integrated to the system and controlled by the computer 4 . the waveforms were monitored by an oscilloscope 16 . in addition , the computer 4 supervised the entire measurement session and collected measurement data onto a diskette . the sweep pattern over the tooth under examination was organized in a 25 by 40 cell matrix , in which the cell size was 0 . 5 × 0 . 5 mm 2 . the recorded voltage levels were stored on diskettes . each measurement session over one examination area took about 45 minutes . the examined object was a swine jawbone . the measurements were performed using the following basic settings : the distance of the transducer 3 from the needle probe 7 was approx . 20 mm , the transducer drive signal frequency was 5 mhz , the delay of the needle echo in water was approx . 25 us , and the needle vibrating frequency was approx . 500 hz . the tooth under examination was immersed in water , and the rear teeth ( molars ) were selected for examination . the bath temperature was approx . 20 ° c . data stored on a diskette was analyzed using a basic - language program . input data for the program was taken from the high levels of signal voltage values caused by the needle vibration as well as the voltage levels caused by the tooth &# 39 ; s vibration where detectable from the noise floor . picture generation onto the screen took about one minute . the slowness of program execution was mainly related to the programming language used . successful results in examinations were obviously also attained by wetting the tooth under examination and placing a rubber bladder between the tooth and the transducer . illustrated in fig2 is the measured matrix in approx . 12 - fold enlargement . thus , the matrix cell in the figure has dimensions of approx . 6 × 6 mm 2 . the matrix represents one longitudinally sectioned slice of the tooth . each matrix cell corresponds to one measurement result , which in practice is related to one measured dc voltage value . the needle probe 7 is visible in the middle of the figure as a horizontal bar - like area with the darkest rasterization . the jawbone is to be found at the right side of the figure . thus , the location of the needle probe is easily definable with the help of the arrangement in accordance with the invention . further , the identification of the tooth profile against the background is possible by averaging several subsequently taken pictures . the illustration is enhanced with border lines that define the pulp cavity 30 and the outer surface of the tooth 17 . the quality of measurement results was not essentially influenced by the operating frequency of the modulating needle . by contrast , the needle operating frequency had a significant effect on the measurement speed . a higher frequency resulted in a correspondingly faster measurement . additionally , the disturbance level was related to the needle operating frequency . namely , problems arose from the difficulty of finding such a vibrating frequency , at which the interaction of the pulser &# 39 ; s pulse rate ( approx . 5 . 5 khz ) with the power line ( 50 hz ) disturbances would be at a minimum . a proper frequency range was found at 200 ... 800 hz . in commercial applications the pulse rate should be increased to about 20 khz so that the needle vibrating frequency could be about 1 khz . in this manner , the measurement speed would be sufficiently fast , and furthermore , filtering of the useful signal would be easy to realize . according to fig3 a commercially produced measurement apparatus comprises a transducer unit 18 and an analyzer unit 19 , which further comprises an analog section 20 , a data retrieval section 21 , a data processing section 22 , and a picture processing section 23 . in addition , the system includes a vibrator ( not shown ), which is a compressed - air operated pulp treatment device conventionally used by dentists . the transducer unit 18 includes a linear array ultrasound transducer as well as an actuator mechanism for moving the transducer . this arrangement replaces the x - y table used in the system during laboratory experiments . the transducer unit 18 also includes the electronic circuits required for amplification of transducer signals and control of the actuator mechanism . the design of the transducer unit 18 resembles an electric toothbrush , which is pushed against the tooth during the examination . the transducer unit 18 is cabled to the analyzer unit 19 . the purpose of the data retrieval section 21 is to control the measurement and to gather measurement results from the analog section 20 to the ram memory . the data processing section 22 fetches measurement results from the memory of the data retrieval section 21 and performs required operations for pattern recognition . the picture processing section 23 generates the image files and controls the crt or other similar output device . the system is capable of achieving a picture generation and update rate of at least 1 picture / 10 s . illustrated in fig4 is one measurement channel of the analyzer unit 19 serving one transducer . the total number of channels equals the number of transducers in the linear array . the data retrieval section 21 controls the operation of the actual transducer unit 18 and the analog section 20 . the data retrieval section 21 gives required control information to the timer 22 and controls the gain of the preamplifier a 2 as well as the length of windowing delays . the timer 22 delivers control pulses u 0 to pulser 24 and issues the start moment of the windowing delay to a delay section 25 . the programmable delay section 25 generates a required control signal u 8 of the gating window to an analog switch 26 . the pulser 24 delivers control pulses u 1 to the transducer 27 at a level matched for the ultrasound transducer . the ultrasound transducer 27 is of the pulse echo type . the amplifier a 1 amplifies the echo signal u 1 by approx . 20 db in order to obtain a sufficiently high signal u 2 to be transferred by the cable to the analyzer unit 19 . voltage level of an echo signal u 2 amplified by a controllable amplifier a 2 is adjusted to a proper level for am detection . an am detector 28 generates a signal u 4 which is the envelope signal of an amplified echo signal u 3 . an analog switch 26 gates for further processing an echo signal u 5 , which is the echo received from the desired depth to be examined . a bandpass filter 29 is used to separate a modulating vibrating frequency u 6 from the echo signal u 5 . an amplitude signal u 7 of the vibrating frequency is detected by rectification from the filtered vibration frequency signal u 5 by a rectifier 31 . the rectified signal u 7 is converted by an a / d converter 32 into an appropriate format for digital processing . illustrated in fig5 are the waveforms over one vibrating cycle at different points of the exemplifying block diagram shown fig4 . prior to the illustrated cycle , a sufficiently high number of vibrating cycles are measured so that the voltage level of signal u 7 has already received its final value . one division of the diagram corresponds to approx . 10 us so that the modulation frequency is about 2 khz . in accordance with fig6 a needle 7 of a root channel broach 33 is inserted into the pulp of a tooth 17 . the broach needle 33 is operated by compressed air , which is led to the broach instrument 33 via hoses 34 . the ultrasound transducer unit 35 proper consists of a linear array transducer 36 , which comprises of , e . g ., 8 separate transducers . the linear array transducer 36 is mounted within an array body 37 whose dimensions are : depth a approx . 16 mm , length b approx . 20mm , and height c approx . 20 mm . the array transducer 36 is movable within the body 37 about its longitudinal axis with the help of a motor 39 mounted to supporting arm of the ultrasound transducer unit 35 and an actuator mechanism 38 mounted integral with the transducer body 37 . between the transducer body 37 and tooth 17 is inserted a bladder 42 , filled with , e . g ., water . close to the actuator motor 39 in the handle part of the ultrasound unit 35 is mounted an electronics unit 40 , which serves for the control of the motor 39 and reprocessing of signals . the measurement signals are routed via a cable 41 for further processing . in accordance with fig7 the thickness w of the detected tissue slice is determined by the size of the gating window . with a narrower window , a thinner slice is analyzed , which consequently offers an improved depth resolution by narrowing the gating window . hence , the depth location of the slice is determined by the timed shifting of the window . consequently , the further the gating window is shifted on the time scale from the send instant of the pulse , the deeper the slice under examination is shifted along the depth axis . by virtue of the modulation generated by the vibration of the needle 7 , the echo signals obtained from the array transducer 36 that is i . e ., the echoes received from the needle and other vibrated objects present a signal level significantly higher than any other signals received from other areas within the gated signal window . the operating parameters of an apparatus according to the invention may be varied as follows : vibrator &# 39 ; s vibrating frequency approx . 100 ... 1000 hz , appropriate ultrasound frequency approx . 3 ... 15 mhz , and echo signal amplification approx . 20 ... 60 db . the method in accordance with the invention can be used in addition to the medical applications , also for examining such physical composite structures , in which hard material is combined with pliabe material . replacing the vibration of the measured object , the transducer itself may alternatively be vibrated at a low frequency . an essential characteristic of an embodiment in accordance with the invention is the relative movement between the measured object and the transducer at a low frequency .
the invention concerns an ultrasound based measurement method and an apparatus for examining dense tissues in particular . according to the method , the measured object is subjected to a high - frequency ultrasound signal , and echoes returning from the object are converted into electric signals . according to the invention , either the object or the ultrasound transducer is vibrated at a low - frequency and the level of the low - frequency vibrating signal is detected from the echo signal , whereby interface locations of the vibrating object can be determined from the maximum values of the detected low - frequency signal in relation to the transmitted high - frequency pulse .