Abstract:
An ultrasonic instrument employs a digital architecture to provide improved stability to sound speed measurements of human bone in vivo. A digitization of the received ultrasonic signal allows numerical analyses to be applied in determining precise arrival time of the waveform. The microprocessor may initiate the ultrasonic signal transmission and detect its arrival and may control the strength of the transmitted signal and the amplification of the received signal to optimize the signal path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of Ser. No. 09/277,481 filed Mar. 26, 1999, which claims benefit to Ser. No. 60/080,158 Mar. 31, 1998; U.S. Pat. No. 6,277,076 
     and is a CIP of Ser. No. 09/094,073 Jun. 9, 1998 U.S. Pat. No. 6,027,449; 
     which is a CIP of Ser. No. 08/795,025 Feb. 4, 1997 Nov. 24, 1998 U.S. Pat. No. 5,840,029; 
     which is a CIP of Ser. No. 08/466,495 Jun. 6, 1995 Feb. 18, 1997 U.S. Pat. No. 5,603,325; 
     which is a CIP of Ser. No. 08/397,027 Mar. 1, 1995 Jan. 16, 1996 U.S. Pat. No. 5,483,965; 
     which is a CIP of Ser. No. 08/072,799 Jun. 4, 1993 Abandoned; 
     which is a CIP of Ser. No. 07/895,494 Jun. 8, 1992 Sep. 6, 1994 U.S. Pat. No. 5,343,863; 
     which is a CIP of Ser. No. 07/772,982 Oct. 7, 1991 Jun. 9, 1992 U.S. Pat. No. 5,119,820; 
     which is a CIP of Ser. No. 07/343,170 Apr. 25, 1989 Oct. 8, 1991 U.S. Pat. No. 5,054,490; 
     which is a CIP of Ser. No. 07/193,295 May 11, 1988 Jun. 5, 1990 U.S. Pat. No. 4,930,511. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices which are used for measuring the density of members, such as bones, and more particularly to devices which utilize ultrasonic acoustic signals to measure the physical properties and integrity of the members. 
     2. Description of the Prior Art 
     The present invention relates to bone measuring instruments and in particular to instruments employing ultrasonic pulses to make measurements of the integrity of human bones in vivo. 
     Osteoporosis or loss of bone mineralization and its cure or prevention are important areas of medical and biological interest. Of particular concern is the loss of trabecular bone, a spongy bone structure forming the interior of vertebrae and other bones. The trabecular bone provides much of the strength of such bones and is disproportionately affected in times of bone mass loss. While it has long been known that the speed of sound through a material will reveal properties of that material, application of this principle for reliable clinically significant bone density measurement has not been easy. 
     An early measurement of sound speed through in vitro bone is described in:  Sonic Measurement of Bone Mass , Clayton Rich et al., a paper delivered at a conference held in Washington, D. C. Mar. 25-27, 1965, NASA publication SP-64. A sinusoidal pulse was timed in its passage from a transmitting transducer through a water bath and excised bone to a receiving transducer. The author noted problems of providing sufficient signal strength in trabecular bone even with the use of an automatic gain control circuit. 
     Sound speed measurement of bone in vivo is described in U.S. Pat. No. 3,847,141 to Hoop issued Nov. 12, 1974. A pulse from a transmitting transducer was propagated through a finger to be received by a receiving transducer; however, the pulse was not timed directly. Rather after filtering, the pulse was used to “retrigger” the transmitting transducer to create a regular series of pulses whose frequency could be determined. This technique is known generally as “sing around”. 
     A doctoral thesis by Langton entitled “The Measurement of Broadband Ultrasonic Attenuation in Cancellous Bone” dated July 1984 describes the measurement of the speed of sound through the os calcis of the heel. The author, however, found that accurate measurement of sound speed was hampered by the difficulty of measuring the beginning of the pulse on an oscilloscope and suggested that the elapsed time between the transmission and reception of a pulse was too short for accurate and repeatable measurements. He proposed that the “sing around” approach of Hoop might be used to correct this latter problem. Langton, observing the considerable frequency distortion of the pulse after passage through the heel, elected to continue his investigation in the area of frequency dependant attenuation rather than sound speed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides significantly improved accuracy in speed of sound measurements of bone in vivo. The present inventors have recognized that prior-art detection systems, using analog threshold detection techniques, were susceptible to timing variations caused by phase and amplitude distortion of the transmitted pulse. Even small variations in detection time of a pulse traveling across the narrow width of the human heel, for example, can produce unacceptable variations in sound speed determination. 
     The present invention reduces variation in detection time by the extensive application of digital control techniques across the entire signal chain. The received signal is digitized to be received by a microprocessor allowing flexible numerical analysis of pulse arrival time tailored to the particular device, frequency range, and region of investigation. Conversion of the received pulse into digital words for receipt by the microprocessor, together with initialization of the transmitted pulse by the microprocessor allows computer-stable timing of the transmission of the pulse. The microprocessor adjusts the transmitted pulse strength and the gain of the receiver amplifier to dynamically optimize signal strength. Digitization of the received pulse also allows a single captured pulse to be used for both the purpose of measuring velocity or time of flight of the ultrasonic signal and in analyzing its attenuation through mathematical techniques such as the fast Fourier transform. 
     In combination these techniques rendered possible clinically accurate speed of sound measurements of bone in vivo. 
     More specifically, the present invention provides an ultrasonic densitometer for measurement of the human os calcis in vivo including an ultrasonic signal generator producing a broad band electrical pulse of ultrasonic frequencies and a first ultrasonic transducer connected to the ultrasonic signal generator for producing a corresponding acoustic signal directed along a transmission axis. 
     A second ultrasonic transducer receives the acoustic signals directed along the transmission axis and relays them to an analog to digital converter converting the electrical signals to digital representations. A microprocessor communicating with the ultrasonic signal generator and the analog to digital converter executes a stored program to initiate the transmission of the acoustic pulse and to numerically analyze the digital representation of the received acoustic signal as distorted by the imposition of the human heel between the first and second ultrasonic transducers to measure a time of transmission of the ultrasonic pulse between the first and second transducers. 
     Thus it is one object of the invention to provide improved accuracy in the measurement of time of transmission through the use of numerical analysis which may accommodate for pulse distortion and noise. 
     It is another object of the invention to provide for the digital control by a single microprocessor in both initiation of the transmission of the acoustic pulse and its receipt and analysis such as provides more precise measurement. 
     The ultrasonic signal generator may be a digitally controllable amplifier designed to create a pulsed output. 
     Thus it is another object of the invention to provide control by the microprocessor of the output signal as well as the processing of the received signal. 
     The densitometer may include a digitally controllable automatic gain control circuit connected between the second ultrasonic transducer and the analog to digital converter to receive the electrical signal from this second ultrasonic transducer and receive a control signal from the microprocessor. The microprocessor operating according to its stored program may control the amplification of the electronic signal from the second ultrasonic transducer prior to its receipt by the digital to analog converter. 
     Thus it is another object of the invention to provide precise digital gain control as may be necessary to optimize the sensitivity of the receive transducer and the amplifier circuit to received acoustic signals. 
     It is yet another object of the invention to provide for a conversion of the received signal to a digital form without loss of resolution by controlling the gain to fully use the range of the A to D converter. 
     The microprocessor may further numerically analyze the digital representation of the received acoustic signal as distorted by the human heel to measure a change in shape of the waveform. 
     Thus it is another object of the invention to provide an extremely more accurate machine that provides both a measurement of transit speed of the ultrasonic wave and its attenuation such as may provide alternative views of the bone integrity or which may be combined to provide a more robust measurement of bone integrity. It has been determined that these two measurements supplement each other. 
     The measurement of the time of flight of the pulse and the measurement of pulse shape may be in comparison to previously measured a standard. 
     Thus it is another object of the invention to simplify the comparison of measurements to a standard by use of a microprocessor which may store earlier and later measurements. 
     The densitometer may include a digital display communicating with the microprocessor and the microprocessor may execute a stored program to provide data to that display indicating the physical property of the os calcis of the heel. 
     Thus it is another object of the invention to provide a densitometer having suitable accuracy for a digital display that provides a simple, quantitative and unambiguous measurement of bone integrity that may not be obtained from visual display of waveforms or frequency measurements in a sing around system. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a perspective view of the ultrasound densitometer device constructed in accordance with the present invention; 
     FIG. 2 is a perspective view of an acoustic coupler, two of which are shown in FIG. 1; 
     FIG. 3 is a front view of a transducer face from which acoustic signals are transmitted or by which acoustic signals are received, the face of the other transducer being the mirror image thereof; 
     FIG. 4 is a schematic block diagram view of the circuitry of the ultrasound densitometer device constructed in accordance with the present invention; 
     FIG. 5 illustrates the method of sampling a received waveform used by the circuit of FIG. 4; 
     FIG. 6 is a schematic block diagram view of the circuitry of an alternative embodiment of an ultrasound densitometer constructed in accordance with the present invention; 
     FIG. 7 is a sample of an actual ultrasonic pulse and response from an ultrasonic densitometer according to the present invention; 
     FIG. 8 is a sample plot of relative ultrasound pulse intensity over frequency range; 
     FIG. 9 is a graph in frequency domain illustrating the shift in attenuation versus frequency characteristic of a measured object as compared to a reference; 
     FIG. 10 is a perspective view of an alternative embodiment of the present invention showing a basin for receiving a patient&#39;s foot and having integral opposed ultrasonic transducers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to the drawings, wherein like numbers refer to like parts, FIG. 1 shows a portable ultrasound densitometer  10  for measuring the physical properties and integrity of a member, such as a bone, in vivo. The densitometer  10  as shown in FIG. 1 includes a handle  11  with actuator button  12 . Extending linearly from the handle  11  is a connection rod  13 . The densitometer  10  also includes a fixed arm  15  and an adjustable arm  16 . The fixed arm  15  preferably is formed continuously with the connection rod  13 , and therefore is connected to an end  17  of the connection rod  13 . The adjustable arm  16  is slidably mounted on the connection rod  13  between the handle  11  and a digital display  18  mounted on the rod  13 . The knob  19  may be turned so as to be locked or unlocked to allow the adjustable arm  16  to be slid along the connection rod  13  so that the distance between the arms  15  and  16  may be adjusted. 
     Connected at the end of the fixed arm  15  is a first (left) transducer  21  and at the end of the adjustable arm  16  is a second (right) transducer  21 . As shown in FIGS. 1 and 2, each of the transducers  21  has mounted on it a respective compliant acoustic coupler  23  to acoustically couple the transducer to the object being tested. The acoustic coupler  23  includes a plastic ring  24  and attached pad  26  formed of urethane or other compliant material. FIG. 3 shows a face  28  of the first (left) transducer  21  which is normally hidden behind the compliant pad  26  of the acoustic coupler  23 . The transducer face  28  normally abuts against the inner surface  29  of the pad  26  shown in FIG.  2 . The transducer face  28  shown in FIG. 3 includes an array of twelve transducer elements labeled a-l. The second (right) transducer  21  includes a face  28  which is the mirror image of that shown in FIG.  3 . 
     FIG. 4 generally shows in schematic fashion the electronic circuitry  31  of the densitometer  10 , which is physically contained in the housing of the digital display  18 . 
     An object  32  is placed between the two transducers  21  so that acoustic signals may be transmitted through the object. This object  32  represents a member, such as a bone, or some material with known acoustic properties such as distilled water or a neoprene reference block. As shown in the embodiment illustrated in FIG. 4, the leftmost transducer  21  is a transmit transducer and the rightmost transducer  21  a receive transducer. In fact though, either or both of the transducers  21  may be a transmit and/or receive transducer. The transmit and receive transducers  21  of the circuit of FIG. 4 are connected by element select signals  36  and  37  to a microprocessor  38 . The microprocessor  38  is programmed to determine which one of the respective pairs of transducer elements a through l are to be transmitting and receiving at any one time. This selection is accomplished by the element select signal lines  36  and  37 , which may be either multiple signal lines or a serial data line to transmit the needed selection data to the transducers  21 . The microprocessor  38  is also connected by a data and address bus  40  to the digital display  18 , a digital signal processor  41 , a sampling analog to digital converter  42 , and a set of external timers  43 . The microprocessor  38  has “on board” electrically programmable non-volatile random access memory (NVRAM) and, perhaps as well, conventional RAM memory, and controls the operations of the densitometer  10 . The digital signal processor  41  has “on board” read-only memory (ROM) and performs many of the mathematical functions carried out by the densitometer  10  under the control of the microprocessor  38 . The digital signal processor  41  specifically includes the capability to perform discrete Fourier transforms, as is commercially available in integrated circuit form presently, so as to be able to convert received waveform signals from the time domain to the frequency domain. The microprocessor  38  and digital signal processor  41  are interconnected also by the control signals  45  and  46  so that the microprocessor  38  can maintain control over the operations of the digital signal processor  41  and receive status information back. Together the microprocessor  38  and the digital signal processor  41  control the electrical circuit  31  so that the densitometer  10  can carry out its operations, which will be discussed below. An auditory feedback mechanism  48 , such as an audio speaker, can be connected to the microprocessor  38  through an output signal  49 . 
     The external timer  43  provides a series of clock signals  51  and  52  to the A/D converter  42  to provide time information to the A/D converter  42  so that it will sample at timed intervals electrical signals which it receives ultimately from the transmit transducer, in accordance with the program in the microprocessor  38  and the digital signal processor  41 . The external timer  43  also creates a clock signal  53  connected to an excitation amplifier  55  with digitally controllable gain. Timed pulses are generated by the timer  43  and sent through the signal line  53  to the amplifier  55  to be amplified and directed to the transmit transducer  21  through the signal line  56 . The transmit transducer  21  converts the amplified pulse into an acoustic signal which is transmitted through the object or material  32  to be received by the receive transducer  21  which converts the acoustic signal back to an electrical signal. The electrical signal is directed through output signal  57  to a receiver amplifier  59  which amplifies the electrical signal. 
     The excitation amplifier circuit  55  is preferably a digitally controllable circuit designed to create a pulsed output. The amplification of the pulse can be digitally controlled in steps from one to ninety-nine. In this way, the pulse can be repetitively increased in amplitude under digital control until a received pulse of appropriate amplitude is received at the receiver/amplifier circuit  59 , where the gain is also digitally adjustable. 
     Connected to the receiver amplifier circuit  59  and integral therewith is a digitally controllable automatic gain control circuit which optimizes the sensitivity of the receive transducer  21  and the amplifier circuit  59  to received acoustic signals. The microprocessor  38  is connected to the amplifier circuit and automatic gain control  59  through signal line  60  to regulate the amplification of the amplifier circuit and gain control  59 . The amplified electric signals are directed through lead  61  to the A/D converter  42  which samples those signals at timed intervals. The A/D converter  42  therefore in effect samples the received acoustic signals. As a series of substantially identical acoustic signals are received by the receive transducer  21 , the A/D converter  42  progressively samples an incremental portion of each successive signal waveform. The microprocessor  38  is programmed so that those portions are combined to form a digital composite waveform which is nearly identical to a single waveform. This digitized waveform may be displayed on the digital display  18 , or processed for numerical analysis by the digital signal processor  41 . 
     The densitometer constructed in accordance with FIGS. 1-4 can be operated in one or more of several distinct methods to measure the physical properties of the member, such as integrity or density. The different methods, as described in further detail below, depend both on the software programming the operation of the microprocessor  34  as well as the instructions given to the clinician as to how to use the densitometer. The different methods of use may all be programmed into a single unit, in which case a user-selectable switch may be provided to select the mode of operation, or a given densitometer could be constructed to be dedicated to a single mode of use. In any event, for the method of use of the densitometer to measure the physical properties of a member to be fully understood, it is first necessary to understand the internal operation of the densitometer itself. 
     In any of its methods of use, the densitometer is intended to be placed at some point in the process on the member whose properties are being measured. This is done by placing the transducers  21  on opposite sides of the member. To accomplish this, the knob  19  is loosened to allow the adjustable arm  16  to be moved so that the transducers  21  can be placed on opposite sides of the member, such as the heel of a human patient. The outside surfaces of the pads  26  can be placed against the heel of the subject with an ultrasound gel  35  or other coupling material placed between the pads  26  and subject  32  to allow for improved transmission of the acoustic signals between the member  32  and transducers  21 . Once the transducers  21  are properly placed on the member, the knob  19  may be tightened to hold the adjustable arm  16  in place, with the transducers  21  in spaced relation to each other with the member  32  therebetween. The actuator button  12  may then be pressed so that acoustic signals will be transmitted through the member  32  to be received by the receive transducer  21 . The electronic circuit of FIG. 4 receives the electrical signals from the receive transducer  21 , and samples and processes these signals to obtain information on the physical properties and integrity of the member  32  in vivo. The microprocessor  38  is programmed to indicate on the digital display  18  when this information gathering process is complete. Alternatively, the information may be displayed on the digital display  18  when the information gathering process is completed. For example, the transit time of the acoustic signals through the member  32  could simply be displayed on the digital display  18 . 
     Considering in detail the operation of the circuitry of FIG. 4, the general concept is that the circuitry is designed to create an ultrasonic pulse which travels from transmit transducer  21  through the subject  32  and is then received by the receive transducer  21 . The circuitry is designed to both determine the transit time of the pulse through the member  32 , to ascertain the attenuation of the pulse through the member  32 , and to be able to reconstruct a digital representation of the waveform of the pulse after it has passed through the member  32 , so that it may be analyzed to determine the attenuation at selected frequencies. To accomplish all of these objectives, the circuitry of FIG. 4 operates under the control of the microprocessor  38 . The microprocessor  38  selectively selects, through the element select signal lines  36 , a corresponding pair or a group of the elements a through l on the face of each of the transducers  21 . The corresponding elements on each transducer are selected simultaneously while the remaining elements on the face of each transducer are inactive. With a given element, say for example element a selected, the microprocessor then causes the external timer  43  to emit a pulse on signal line  53  to the excitation amplifier circuit  55 . The output of the excitation amplifier  55  travels along signal line  56  to element a of the transmit transducer  21 , which thereupon emits the ultrasonic pulse. The corresponding element a on the receive transducer  21  receives the pulse and presents its output on the signal line  57  to the amplifier circuit  59 . What is desired as an output of the A/D converter  42  is a digital representation of the analog waveform which is the output of the single transducer element which has been selected. 
     Unfortunately, “real time” sampling A/D converters which can operate rapidly enough to sample a waveform at ultrasonic frequencies are relatively expensive. Therefore it is preferred that the A/D converter  42  be an “equivalent time” sampling A/D converter. By “equivalent time” sampling, it is meant that the A/D converter  42  samples the output of the transducer during a narrow time period after any given ultrasonic pulse. The general concept is illustrated in FIG.  5 . The typical waveform of a single pulse received by the receive transducer  21  and imposed on the signal line  57  is indicated by a function “f”. The same pulse is repetitively received as an excitation pulse and is repetitively launched. The received pulse is sampled at a sequence of time periods labeled t 0 -t 10 . In other words, rather than trying to do a real-time analog to digital conversion of the signal f, the signal is sampled during individual fixed time periods t 0 -t 10  after the transmit pulse is imposed, the analog value during each time period is converted to a digital function, and that data is stored. Thus the total analog waveform response can be recreated from the individual digital values created during each time period t, with the overall fidelity of the recreation of the waveform dependent on the number of time periods t which are sampled. The sampling is not accomplished during a single real time pulse from the receive transducer  21 . Instead, a series of pulses are emitted from the transmit transducer  21 . The external timer is constructed to provide signals to the sampling A/D converter  42  along signal lines  51  and  52  such that the analog value sampled at time period t 0  when the first pulse is applied to a given transducer element, then at time t 1  during the second pulse, time t 2  during the third pulse, etc. until all the time periods are sampled. Only after the complete waveform has been sampled for each element is the next element, i.e. element b, selected. The output from the A/D converter  42  is provided both to the microprocessor  38  and to the signal processor  41 . Thus the digital output values representing the complex waveform f of FIG. 5 can be processed by the signal processor  41  after they are compiled for each transducer element. The waveform can then be analyzed for time delay or attenuation for any given frequency component with respect to the characteristic of the transmitted ultrasonic pulse. The process is then repeated for the other elements until all elements have been utilized to transmit a series of pulses sufficient to create digital data representing the waveform which was received at the receive transducer array  21 . It is this data which may then be utilized in a variety of methods for determining the physical properties of the member. Depending on the manner in which the densitometer is being utilized and the data being sought, the appropriate output can be provided from either the microprocessor  38  or the signal processor  41  through the digital display  18 . 
     Because the ultrasonic pulsing and sampling can be performed so rapidly, at least in human terms, the process of creating a sampled ultrasonic received pulse can optionally be repeated several times to reduce noise by signal averaging. If this option is to be implemented, the process of repetitively launching ultrasonic pulses and sampling the received waveform as illustrated in FIG. 5 is repeated one or more times for each element in the array before proceeding to the next element. Then the sampled waveforms thus produced can be digitally averaged to produce a composite waveform that will have a lesser random noise component than any single sampled waveform. The number of repetitions necessary to sufficiently reduce noise can be determined by testing in a fashion known to one skilled in the art. 
     Having thus reviewed the internal operation of the densitometer of FIGS. 1-4, it is now possible to understand the methods of use of the densitometer to measure the physical properties of the member. The first method of use involves measuring transit time of an ultrasonic pulse through a subject and comparing that time to the time an ultrasonic pulse requires to travel an equal distance in a substance of known acoustic properties such as water. To use the densitometer in this procedure, the adjustable arm  16  is adjusted until the member of the subject, such as the heel, is clamped between the transducers  21 . Then the knob  19  is tightened to fix the adjustable arm in place. The actuator button  12  is then pressed to initiate a pulse and measurement. Next the densitometer is removed from the subject while keeping the knob  19  tight so that the distance between the transducers  21  remains the same. The device  10  is then placed about or immersed in a standard material  32  with known acoustic properties, such as by immersion in a bath of distilled water. The actuator button  12  is pressed again so that acoustic signals are transmitted from the transmit transducer  21  through the material  32  to the receive transducer  21 . While it is advantageous to utilize the whole array of elements a through l for the measurement of the member, it may only be necessary to use a single pair of elements for the measurement through the standard assuming only that the standard is homogeneous, unlike the member. The signal profiles received by the two measurements are then analyzed by the microprocessor  38  and the signal processor  41 . This analysis can be directed both to the comparative time of transit of the pulse through the subject as compared to the standard and to the characteristics of the waveform in frequency response and attenuation through the subject as compared to the standard. 
     Thus in this method the densitometer may determine the physical properties and integrity of the member  32  by both or either of two forms of analysis. The densitometer may compare the transit time of the acoustic signals through the member with the transmit time of the acoustic signals through the material of known acoustic properties, and/or the device  10  may compare the attenuation as a function of frequency of the broadband acoustic signals through the member  32  with the attenuation of corresponding specific frequency components of the acoustic signals through the material of known acoustic properties. The “attenuation” of an acoustic signal through a substance is the diminution of the ultrasonic waveform from the propagation through either the subject or the standard. The theory and experiments using both of these methods are presented and discussed in Rossman, P.J., Measurements of Ultrasonic Velocity and Attenuation In The Human Os Calcis and Their Relationships to Photon Absorptiometry Bone Mineral Measurements (1987) (a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science at the University of Wisconsin-Madison). Tests have indicated that there exists a linear relationship between ultrasonic attenuation (measured in decibels) (dB)) at specific frequencies, and those frequencies. The slope (dB/MHz) of the linear relationship, referred to as the broadband ultrasonic attenuation, is dependent upon the physical properties and integrity of the substance being tested. With a bone, the slope of the linear relationship would be dependent upon the bone mineral density. Thus broadband ultrasonic attenuation through a bone is a parameter directly related to the quality of the cancellous bone matrix. 
     The microprocessor  38  may therefore be programmed so that the device determines the physical properties and integrity of the member by comparing either relative transit times and/or relative broadband ultrasonic attenuation through the member and a material of known acoustic properties. When comparing the transit times, the microprocessor  38  may be programmed most simply so that the electronics, having received the acoustic signals after they have been transmitted through the member, determines the “member” transit time of those acoustic signals through the member, and after the acoustic signals have been transmitted through the material of known acoustic properties, determines the “material” transit time of the acoustic signals through the material. These time periods may be measured most simply by counting the number of clock pulses of known frequency emitted by the timer  43  between the time of launching the pulse and the sensing of the received pulse at the A/D converter  42 . The microprocessor  38  then makes a mathematical “time” comparison of the member transit time to the material transit time and then relates that mathematical time comparison to the physical properties and integrity of the member. The mathematical time comparison may be made by either determining a difference between the member transit time and the material transit time, or by determining a ratio between the member transit time and the material transit time. 
     As a second method of using the densitometer, it may also determine the physical properties and integrity of the member  32  by determining and comparing the attenuation of the broadband frequency components of the acoustic signals through the member without reference to a material having known acoustic properties. Using this method, the comparison of velocity to a standard is not necessary and absolute transit time of the pulse need not be calculated since it is attenuation that is measured. In such a mode, it is preferable that the transmit transducer  21  transmits an acoustic signal which has a broad range of frequency components, such as a simple ultrasonic pulse. In any case, the acoustic signal should have at least one specific frequency component. 
     In this attenuation comparison mode, the microprocessor  38  is programmed so that after the receive transducer  21  receives the acoustic signals transmitted through the bone member  32 , it determines the absolute attenuation through the member  32  of the frequency component spectrum of the acoustic signals. It is to facilitate the measurement of attenuation that the excitation amplifier circuit  55  and the receiver amplifier  59  have amplification levels which may be digitally controlled. By successively varying the gain of the amplifiers  55  and  59  on successive pulses, the circuit of FIG. 4 can determine what level of gain is necessary to place the peak of the received waveform at a proper voltage level. This gain is, of course, a function of the level of attenuation of the acoustic pulse during transit through the member  32 . After the receive transducer  21  receives acoustic signals, microprocessor  38  in conjunction with the signal processor  41  determines the absolute attenuation of individual specific frequency components of the received acoustic signal transmitted through the material. The digital signal processor  41  then makes mathematical “attenuation” comparisons of the corresponding individual specific frequency components through the member. A set of mathematical attenuation comparisons between corresponding frequency components may be thereby obtained, one comparison for each frequency component compared. The manner in which the attenuation functions with respect to frequency can thus be derived. The microprocessor  38  and digital signal processor  41  then relate that function to the physical properties and integrity of the member. 
     Shown in FIG. 7 is a sample broadband ultrasonic pulse and a typical received waveform. To achieve an ultrasonic signal that is very broad in the frequency domain, i.e., a broadband transmitted signal, an electronic pulse such as indicated at  70  is applied to the selected ultrasonic transducer in the transmit array  21  which then resonates with a broadband ultrasonic emission. The received signal, such as indicated at  72  in FIG. 7 in a time domain signal plot, is then processed by discrete Fourier transform analysis so that it is converted to the frequency domain. Shown in FIG. 8 is a pair of plots of sample received signals, in frequency domain plots, showing the shift in received signal intensity as a function of frequency between a reference object and a plug of neoprene placed in the instrument. FIG. 9 illustrates a similar comparison, with FIG. 8 using relative attenuation in the vertical dimension and FIG. 9 using power of the received signal using a similar reference material. Both representations illustrate the difference in relative intensities as a function of frequency illustrating bow broadband ultrasonic attenuation varies from object to object. The actual value calculated, broadband ultrasonic attenuation, is calculated by first comparing the received signal against the reference signal, then performing the discrete Fourier transform to convert to frequency domain, then performing a linear regression of the difference in attenuation slope to derive broadband ultrasonic attenuation. 
     The mathematics of the discrete Fourier transform are such that another parameter related to bone member density may be calculated in addition to, or in substitution for, broadband attenuation (sometimes referred to as “attenuation” or “BUA” below). When the discrete Fourier transform is performed on the time-domain signal, the solution for each point includes a real member component and an imaginary member component. The values graphed in FIGS. 8 and 9 are the amplitude of the received pulse as determined from this discrete Fourier transform by taking the square root of the sum of the squares of the real component and the imaginary component. The phase angle of the change in phase of the ultrasonic pulse as it passed through the member can be calculated by taking the arctangent of the ratio of the imaginary to the real components. This phase angle value is also calculated to bone member density. 
     The microprocessor  38  may also be programmed so that the densitometer simultaneously performs both functions, i.e. determines both transit time and absolute attenuation of the transmitted acoustic signals, first through the member and then through the material with known acoustic properties. The densitometer may then both derive the broadband ultrasonic attenuation function and make a mathematical time comparison of the member transit time to the material transit time. The microprocessor  38  and digital signal processor  41  then relate both the time comparison along with the attenuation function to the physical properties and integrity, or density of the member  32 . 
     In yet another possible mode of operation, the microprocessor  38  may be programmed so that the densitometer  10  operates in a mode whereby the need for calculating either the relative transit time or the attenuation of the acoustic signals through a material of known acoustic properties is eliminated. In order to operate in such a mode, the microprocessor  38  would include a database of normal absolute transit times which are based upon such factors as the age, height, weight, race or the sex of the individual being tested as well as the distance between the transducers or the thickness or size of the member. This database of normal transit times can be stored in the non-volatile memory or could be stored in other media. When testing an individual in this mode, the relevant factors for the individual are placed into the microprocessor  38  to select the pertinent normal transit time based on those factors. The transducers  21  are placed on the bone member being tested as described above. When the actuator button  12  is pressed, the acoustic signals are-transmitted through the member  32 . The receive transducer  21  receives those signals after they have been transmitted through the member, and the electronics  31  then determine the “member” transit time of the acoustic signals through the member. The microprocessor  38  and digital signal processor  41  then make a mathematical comparison of the measured member transit time to the selected database normal transit time, and relate the mathematical time comparison to the physical properties and integrity, or density of the member, which is displayed. 
     As an alternative output of the densitometer of the present invention, the digital display  18  could also include a display corresponding to the pattern of the array of elements on the face of the transducer  21  as seen in FIG.  3 . This display could then display, for each element a through l, a gray scale image proportional to the parameter, i.e. transit time or attenuation, being measured. This image may provide a visual indication to an experienced clinician as to the physical properties of the member present in the patient. 
     Shown in FIG. 6 is a circuit schematic for an alternative embodiment of an ultrasonic densitometer constructed in accordance with the present invention. In the circuit of FIG. 6, parts having similar structure and function to their corresponding parts in FIG. 4 are indicated with similar reference numerals. 
     The embodiment of FIG. 6 is intended to function with only a single transducer array  21  which functions both as the transmit and the receive transducer array. An optional reflecting surface  64  may be placed on the opposite side of the member  32  from the transducer array  21 . A digitally controlled multiple pole switch  66 , preferably an electronic switch rather than a mechanical one, connects the input to and output from the elements of the transducer array  21  selectively either to the excitation amplifier  55  or to the controllable gain receiver/amplifier circuit  59 . The switch  66  is connected by a switch control line  68  to an output of the microprocessor  38 . 
     In the operation of the circuit of FIG. 6, it functions in most respects like the circuit of FIG. 4, so only the differences need be discussed. During the launching of an ultrasonic pulse, the microprocessor  38  causes a signal to appear on the switch control line  68  to cause the switch  66  to connect the output of the excitation amplifier  55  to the selected element in the transducer array  21 . Following completion of the launching of the pulse, the microprocessor  38  changes the signal on the switch control line  68  to operate the switch  66  to connect the selected element or elements as an input to the amplifier  59 . Meanwhile, the pulse propagates through the member  32 . As the pulse transits through the member, reflective pulses will be generated as the pulse crosses interfaces of differing materials in the member and, in particular, as the pulse exits the member into the air at the opposite side of the member. If the transition from the member to air does not produce a sufficient reflective pulse, the reflecting surface  64  can be placed against the opposite side of the member to provide an enhanced reflected pulse. 
     The embodiment of FIG. 6 can thus be used to analyze the physical properties and integrity of a member using only one transducer  21 . All of the methods described above for such measurements may be used equally effectively with this version of the device. The transit time of the pulse through the member can be measured simply by measuring the time period until receipt of the reflected pulse, and then simply dividing by two. This time period can be compared to the transit time, over a similar distance, through a standard medium such as water. The time period for receipt of the reflected pulse could also be simply compared to standard values for age, sex, etc. Attenuation measurements to detect differential frequency measurement can be directly made on the reflected pulse. If no reflecting surface  64  is used, and it is desired to determine absolute transit time, the thickness of the member or sample can be measured. 
     The use of the multi-element ultrasonic transducer array for the transducers  21 , as illustrated in FIG. 3, enables another advantageous feature of the instrument of FIGS. 1-9. In using prior art densitometers, it was often necessary to precisely position the instrument relative to the body member of the patient being measured to have useful results. The difficulty arises because of heterogeneities in the bone mass and structure of actual body members. A measurement taken at one location of density may be significantly different from a measurement taken close by. Therefore prior art instruments fixed the body member precisely so that the measurement could be taken at the precise location each time. 
     The use of the ultrasonic transducer array obviates the need for this precise positioning. Using the instrument of FIGS. 1-9, the instrument performs a pulse and response, performs the discrete Fourier transform, and generates a value for broadband ultrasonic attenuation for each pair of transducer elements a through l. Then the microprocessor  38  analyzes the resulting array of bone ultrasonic density measurements to reproducibly identify the same region of interest each time. In other words, since the physical array of transducers is large enough to reliably cover at least the one common region of interest each time, the measurement is localized at the same locus each time by electrically selecting the proper location for the measurement from among the locations measured by the array. The instrument of FIGS. 1-9 is conveniently used by measuring the density of the os calcis as measured through the heel of a human patient. When used in this location, it has been found that a region of interest in the os calcis can be located reliably and repeatedly based on the comparisons of broadband ultrasonic attenuation at the points in the array. The region of interest in the os calcis is identified as a local or relative minimum in broadband ultrasonic attenuation and/or velocity closely adjacent the region of highest attenuation values in the body member. Thus repetitive measurements of the broadband ultrasonic attenuation value at this same region of interest can be reproducibly taken even though the densitometer instrument  10  is only generally positioned at the same location for each successive measurement. 
     This technique of using a multiple element array to avoid position criticality is applicable to other techniques other than the determination of broadband ultrasonic attenuation as described here. The concept of using an array and comparing the array of results to determine measurement locus would be equally applicable to measurements taken of member-density based on speed of sound transit time, other measurements of attenuation or on the calculation of phase angle discussed above. The use of such a multiple-element array, with automated selection of one element in the region of interest, can also be applied to other measurement techniques useful for generating parameters related to bone member density, such as measuring speed changes in the transmitted pulse such as suggested in U.S. Pat. No. 4,361,154 to Pratt, or measuring the frequency of a “sing-around” self-triggering pulse as suggested in U.S. Pat. No. 3,847,141 to Hoop. The concept which permits the position independence feature is that of an array of measurements generating an array of data points from which a region of interest is selected by a reproducible criterion or several criteria. The number of elements in the array also clearly can be varied with a larger number of elements resulting in a greater accuracy in identifying the same region of interest. 
     In this way, the ultrasound densitometer of the present invention provides a device capable of rapid and efficient determination of the physical properties of a member in vivo without the use of radiation. Because the densitometer is constructed to operate under the control of the microprocessor  38 , it can be programmed to operate in one of several modes, as discussed above. This allows both for flexibility to clinical goals as well as efficient use of the device. 
     Shown in FIG. 10 is another variation on an ultrasonic densitometer constructed in accordance with the present invention. In the densitometer  100  of FIG. 10, there are two ultrasonic transducer arrays  121 , which are generally similar to the ultrasonic transducer arrays  21  of the embodiment of FIG. 1, except that the transducer arrays  21  are fixed in position rather than movable. 
     The densitometer  100  includes a relatively large mounting case  101 , which in this embodiment is rectangular in the top of which is formed a depression or basin  103 . The basin as in  103  is elongated and has a generally triangular cross-sectional shape sized so as to receive a human foot therein. The transducer arrays  121  are positioned in the case  101  so that they extend into the base in  103  to be on the opposite sides of the heel of a foot placed in the basin  103 . The general arrangement of the electrical components of FIG. 4 is unchanged in the ultrasonic densitometer  100  of FIG. 10, except that because the transducer arrays  12  are fixed, they do not contact the opposite sides of the heel of the patient but instead are located spaced away from it. 
     In the operation of the ultrasonic densitometer  100  of FIG. 10, the basin  103  is filled with water. The patient places his or her foot in the basin  103  with their sole resting on the bottom and their heel resting against the back of the basin  103 . The intent is to position the transducer arrays  121  on either side of the os calcis. For the average adult population, it has been demonstrated that placing the transducers approximately 4 cm up from the sole and 3.5 cm forwardly from the rearward edge of the heel places the transducers in the desired region and focused on the os calcis. 
     The principle of operation of the ultrasonic densitometer of FIG. 10 can be understood as similar to that of FIG. 1, except that the order of pulsing and measurement can be varied. In the apparatus of FIG. 1, the measurement pulse through the member was generally performed before the reference pulse through the homogeneous standard, i.e. water. In the densitometer  100  of FIG. 10, since the distance between the transducers is fixed, the reference pulse through the homogeneous standard material, which is simply the water in the basin  103 , may be conducted either before or after measurement pulse through a live member is performed. In fact, if the temperature of the water in basin  103  is held steady by a temperature control mechanism, the standard transit time measurement can be simply made once for the instrument and thereafter only measurement pulses need be transmitted. The transit times of the measurement pulses then may be correlated to the standard reference transit time through the water to give an indication of the integrity of the member just measured. Through empirical experimentation with the densitometer of FIG. 10, it has been determined that it is also possible if desired to dispense with the reference pulse entirely. This may be done by determining a standard transit time value or, since the distance between the transducers is fixed, a standard transit velocity. By empirical testing, it has been determined that by proper selection of such a standard value, and by holding the water in the basin within a temperature range, no reference pulse need be launched or measured. 
     Using this variation, a mathematical relation of the measured transit time, or transit velocity, must be made to the standard. Since in the interests of accuracy, it is preferred to use both changes in transit time (velocity) and changes in attenuation to evaluate a member in vivo, the following formula has been developed to provide a numerical value indicative of the integrity and mineral density of a bone: 
     
       
         Bone integrity value=(SOS−T) 2  ×(BUA/1000) 
       
     
     In this formula, “SOS” indicates the speed of sound or velocity of the measurement ultrasonic pulse through the member and is expressed in meters per second. The speed of sound (SOS) value is calculated from the measured transit time measured. For an adult human heel, it has been found that assuming a standard human heel width of 40 mm at the point of measurement results in such sufficient and reproducible accuracy that actual measurement of the actual individual heel is not needed. 
     In the above formula, “T” represents a standard minimum value. Two alternative values are possible. One alternative is to set T to the speed of sound value for water, i.e., the reference pulse velocity. This value is about 1500 n/sec for water at 28° C. The principal drawback to this approach is that it has been found surprisingly that some people actually have a density value in their heel that is below that of water. For such persons, using the standard water velocity would make the bone integrity value a negative number. Therefore, another alternative is to use the lowest measured human value as T which in the experience of the investigators here to date is 1475 n/sec. 
     Lastly in the above formula, BUA is broadband ultrasonic attenuation as described in greater detail above. The division of 1000 merely scales the influence of the BUA measurement relative to the SOS measurement which has been determined to be a more effective predictor of bone density. 
     Measured values of SOS range between 1475 and 1650 m/sec. Measured values of BUA range between 30 and 100 dB/MHz. Using a T=1475, these ranges yield values ranging from very small, i.e., 18 up to relatively large, i.e., around 3000. Thus the bone integrity values thus obtained exhibit a wide range and are readily comprehensible. It has been determined again by clinical testing that persons with a bone integrity value of less than 200 have low spinal bone mineral density, that those in the range of 200-400 have marginal spinal bone mineral density and that those having bone integrity values of over 400 have acceptable and high levels of spinal bone mineral density. 
     To verify the accuracy of this approach in predicting spinal bone density, patients were tested using the apparatus of FIG.  10  and also with a dual photon absorptiometry densitometer of accepted standard design. The results of using the ultrasonic densitometer of FIG. 10 have demonstrated that the speed of sound measurement made using this device had a correlation in excess of 0.95 with the measured values of spinal bone density indicating very good consistency with accepted techniques. However, an occasional patient was tested who exhibited an SOS value in the normal range, but who exhibited a BUZ value indicating very poor bone integrity. Accordingly, the bone integrity value was developed to accommodate such deviant results. The value is weighted toward the SOS since that is the principally used reliable predictor value with a secondary factor including BUA to include such individuals. In fact, the power of the SOS factor may also be increased to the third or fourth power as opposed to merely the second power, to increase the importance of the SOS term. Since this method utilizing ultrasonic measurement of the heel is quick and free from radiation, it offers a promising alternative for evaluation of bone integrity. 
     The densitometer  100  may be used with or without an array of ultrasonic transducers in the transducers  121 . In its simplest form, the mechanical alignment of the heel in the device can be provided by the shape and size of the basin  103 . While the use of an array and region-of-interest scanning as described above is most helpful in ensuring a reproducible accurate measurement, mechanical placement may be acceptable for clinical utility in which case only single transducer elements are required. 
     It is specifically intended that the present invention not be specifically limited to the embodiment and illustrations contained herein, but embrace all such modified forms thereof as come within the scope of the following claims.