Abstract:
An acoustic array of independently sensitive detecting elements is produced by locating a pattern of electrodes on a piezoelectric polymer film. Sensitivity of the film and a spaced away supporting structure are used to reduce artifacts caused by acoustic reflection in the supporting structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application Appln. No. 08/795,023, filed Feb. 4, 1997, now U.S. Pat. No. 6,012,779. 
    
    
     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 
     Various devices presently exist which may be used to measure the physical properties and integrity of a member such as a bone. Non-invasive density measuring devices can be used to determine cumulative internal damage caused by micro-crushing and micro-fracturing occurring in the bones of humans or animals such as race horses. Additionally, osteoporosis, or loss of bone mineralization, detection in humans and its cure or prevention are increasingly becoming areas of intense medical and biological interest. As the average age of the human population increases, a greater number of patients are developing complications due to rapid trabecular bone loss. 
     U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuring the density of a bone structure, such as a finger bone or heel bone, to monitor the calcium content thereof. The device includes a pair of opposed spaced ultrasonic transducers which are held within a clamping device clamped on the bone being analyzed. A pulse generator is coupled to one of the transducers to generate an ultrasonic sound wave which is directed through the bone to the other transducer. An electric circuit couples the signals from the receive transducer back to the pulse generator for retriggering the pulse generator in response to those signals. The pulses therefore are produced at a frequency proportional to the transit time that the ultrasonic wave takes to travel through the bone structure, which is directly proportional to the speed of the sound through the bone. The speed of sound through a bone has been found to be proportional to the density of the bone. Thus the frequency at which the pulse generator is retriggered is proportional to the density of the bone. 
     Another device and method for establishing, in vivo the strength of a bone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 to Pratt, Jr. The device includes a launching transducer and a receiving transducer which are connected by a graduated vernier and which determine the speed of sound through the bone to determine its strength. The vernier is used to measure the total transit distance between the surfaces of the two transducers. 
     Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue,  Tissue Characterization With Ultrasound,  CRC publication 2, pp. 207-226) discusses various studies involving attenuation and speed of sound measurements in both cortical and spongy (cancellous or trabecular) bone. The results of these studies reveal a linear relationship between the wet sonic velocity and wet cortical density, and between the dry sonic velocity and the dry cortical density. The transit times of an acoustic signal through a bone member therefore are proportional to the bone density. Langton, et al. (Langton, C. M., Palmer, S. D., and Porter, S. W., (1984), The Measurement of Broad Band Ultrasonic Attenuation in Cancellous Bone,  Eng. Med.,  13, 89-91) published the results of a study of ultrasonic attenuation versus frequency in the os calcis (heel bone) that utilized through transmission techniques. These authors suggested that attenuation differences observed in different subjects were due to changes in the mineral content of the os calcis. They also suggested that low frequency ultrasonic attenuation may be a parameter useful in the diagnosis of osteoporosis or as a predictor of possible fracture risk. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a low cost acoustic transducer array suitable for use in producing bone density images of the human heel or in other similar applications. 
     Specifically, the acoustic transducer array includes a piezoelectric film having multiple electrically independent electrodes positioned over a first surface of the film, each electrode opposed by a second electrode on a second surface of the film opposite the first surface of the film. An acoustic signal passing through a particular location of the film produces a corresponding voltage across one of the first electrodes and second electrode. The film may be a polymer such as polyvinylidene fluoride. 
     Thus it is one object of the invention to provide a simple acoustic transducer array. The electrodes may be applied by low cost metallization techniques providing multiple independent sensors with similar characteristics at low cost. 
     The transducer array may include a contact plate having multiple conductive terminals affixed to a support surface and positioned adjacent to the first surface of the film with different terminals electrically connected to different portions of different first electrodes. 
     Thus it is another object of the invention to provide a simple method of making electrical connections to the array of the present invention. 
     The support surface may be spaced away from the piezoelectric film. Further, the piezoelectric film may be polarized only at the regions near the first electrodes and not at regions between the first electrodes. The conductive terminals may be attached to the first electrodes by means of an acoustically transparent conductor such as metalized mylar. 
     Thus it is another object of the invention to permit the use of an acoustically transparent transducer array while reducing artifacts caused by reflections of acoustic signals off of the supporting structure of the array. By positioning the conductive terminals and support surface away from the regions of sensitivity, using acoustically transparent conductors and decreasing the piezoelectric effect at the point of contact between the terminals and the film, such artifacts are minimized. 
     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; 
     FIG. 11 is a plan view of a foot plate and toe peg used with the embodiment of FIG. 10; 
     FIG. 12 is a cross-sectional detail of the foot plate of FIG. 11 showing the method of attaching the sliding toe peg of the foot plate; 
     FIG. 13 is a block diagram of a system for transporting the acoustic coupling liquid used in the embodiment of FIG. 10; 
     FIG. 14 is a schematic block diagram view of the circuitry of the embodiment of FIG. 10; 
     FIG. 15 is an exploded view of the underside of the foot basin of FIG. 10 showing a c-clamp for holding the opposed ultrasonic transducers in precise alignment and separation; 
     FIG. 16 is a perspective detailed view of the shank of the c-clamp of FIG. 15 showing a lever for moving the separation of the transducers between an open and precisely separated closed position; 
     FIG. 17 is a cross-section of a human heel and ultrasonic transducers of the basin of FIG. 10 showing flexible liquid filled bladders surrounding the transducers and providing a coupling path between the transducers and the heel; 
     FIG. 18 is a plot of the inverse of time of flight (TOF) for two bone conditions and broadband ultrasonic attenuation (BUA) as a function of heel width showing their opposite functional dependencies; 
     FIG. 19 is a plot of bone quality versus bone width as might be obtained from empirical measurement of multiple bone phantoms and as may be used to eliminate bone width effects in the ultrasonic assessment of bone quality; 
     FIG. 20 is an exploded view of the elements of an ultrasonic detector array showing a driving mechanism for improving the resolution of the acquired data and the location of a piezoelectric film detector array above a spatially offset connector; 
     FIG. 21 is a detailed perspective fragmentary view of the piezoelectric film detector with electrodes on its surface as communicating with connector terminals via acoustically transparent conductors; 
     FIG. 22 is a detailed fragmentary view of the piezoelectric film of FIG. 21 showing a method of assembling the acoustically transparent conductors; 
     FIG. 23 is a detailed view of the face of the detector showing its displacement by the driving mechanism of FIG. 20; 
     FIG. 24 is a figure similar to that of FIG. 17 showing use of the detector array to provide focused reception at a point within a patient&#39;s heel; 
     FIG. 25 is a perspective view in phantom of a patient&#39;s heel showing a raster scan pattern of a reception point within the heel to measure volumetric bone density variations within a inner and outer portion of the os calcis; 
     FIG. 26 is a schematic representation of a data cube collected in the scanning shown in FIG. 25 with isodensity lines used to locate a measurement region of interest; 
     FIG. 27 is a flow chart of the operation of the present invention in locating a region of interest uniformly over several patient visits; and 
     FIG. 28 is a perspective view of an embodiment of the invention using a fixed focus transducer array mechanically scanned to provide a plurality of spatially separated measurements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Caliper Embodiment 
     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 the 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  invivo. 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 how 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. 
     Basin Embodiment 
     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 generally box-shaped mounting case  101  with sloping upper face  102  in which is formed a basin  103 . The basin  103  is sized to receive a human foot and is generally trigonous along a vertical plane aligned with the length of the foot so that when the foot is placed within the basin  103 , the toes of the foot are slightly elevated with respect to the heel of the foot. 
     The transducer arrays  121  are positioned in the case  101  so that they extend into the basin  103  to be on opposite sides of the heel of the foot placed in the basin  103 . When the foot is in position within the basin  103 , the sole of the foot may rest directly on a bottom  104  of the basin  103  with the heel of the foot received within a curved pocket  106  forming a back wall of the basin  103 . As so positioned, the transducer arrays  121  are on either side of the os calcis. It has been demonstrated that placing the transducer approximately 4 centimeters up from the sole and 3.5 centimeters forwardly from the rearward edge of the heel places the transducers in the desired region and focused on the os calcis. The foot may, alternatively, rest on a generally planar foot plate  108  having a contour conforming to the bottom  104  and placed against the bottom  104  between the foot and the bottom  104 . The foot plate  108  holds an upwardly extending toe peg  110  for use in reducing motion of the foot during the measurement process. Referring to FIG. 11, the toe peg  110  is sized to fit between the big toe and the next adjacent toe of a typical human foot and is mounted in a slot  112  so as to be adjustable generally along the length of the foot to accommodate the particular length of the foot. 
     The slot  112  cants inward toward a medial axis  114  of the foot, defined along the foot&#39;s length, as one moves along the slot  112  towards the portion of the foot plate  108  near the heel of the foot. This canting reflects the general relation between foot length and width and allows simple adjustment for both dimensions at once. 
     The toe peg  110  is sized to fit loosely between the toes of the foot without discomfort and does not completely prevent voluntary movement of the foot. Nevertheless, it has been found that the tactile feedback to the patient provided by the toe peg  110  significantly reduces foot movement during operation of the densitometer  100 . Two different foot plates  108 , being mirror images of each other, are used for the left and right foot. 
     Referring to FIG. 12, the toe peg  110  is held to the slot  112  by a fastener  111  having a threaded portion which engages corresponding threads in the toe peg  110 . The head of the threaded fastener  111  engages the slot  112  so as to resist rotation. Thus, the toe peg  110  may be fixed at any position along the length of the slot  112  by simply turning the toe peg  110  slightly about its axis to tighten the threaded fastener  111  against the foot plate  108 . 
     Referring again to FIG. 10, the basin  103  of the densitometer  110  is flanked, on the upper face  102  of the enclosure  101 , by two foot rest areas  116  and  118  on the left and right side respectively. For examination of a patient&#39;s right foot, the patient&#39;s left foot may rest on foot rest area  118  while the patient&#39;s right foot may be placed within basin  103 . Conversely, for examination of the patient&#39;s left foot, the left foot of the patient is placed within basin  103  and the patient&#39;s right foot may rest on foot rest area  116 . The foot rest areas have a slope conforming to that of the upper face  102  and approximately that of bottom  104 . The flanking foot rest areas  116  and  118  allow the densitometer  100  to be used in comfort by a seated patient. 
     When the densitometer  100  is not in use, the basin area  103  is covered with a generally planar cover  120  hinged along the lower edge of the basin  103  to move between a closed position substantially within the plane of the upper face  102  and covering the basin  103 , and an open position with the plane of the cover  120  forming an angle α with the bottom  104  of the basin  103  as held by hinge stops  122 . The angle α is approximately 90° and selected so as to comfortably support the calf of the patient when the patient&#39;s foot is in place within basin  103 . To that end, the upper surface of the cover  120 , when the cover  120  is in the open position, forms a curved trough to receive a typical calf. 
     The support of the patient&#39;s calf provided by the cover  120  has been found to reduce foot motion during operation of the densitometer  100 . 
     Referring now to FIGS. 10 and 12, because the densitometer  100  employs fixed transducers  121 , a coupling liquid is provided in the basin  103  to provide a low loss path for acoustic energy between the transducers  121  and the patient&#39;s foot regardless of the dimensions of the latter. The coupling liquid is preferably water plus a surfactant, the latter which has been found to improve the signal quality and consistency of the reading of the densitometer. The surfactant may be, for example, a commercially available detergent. It will be recognized, however, that other flowable, acoustically conductive media may be used to provide acoustic coupling, and hence, that the term “coupling liquid” should be considered to embrace materials having a viscosity higher than that of water such as, for example, water based slurries and thixotropic gels. 
     For reasons of hygiene, the exhaustion of the surfactant, and possible reduction of signal quality with the collection of impurities in the coupling liquid, it has been determined that the liquid in the basin  103  should be changed in between each use of the densitometer  103 . Changing this liquid is time consuming and ordinarily would require convenient access to a sink or the like, access which is not always available. Failure to change the liquid may have no immediate visible effect, and hence changing the liquid is easy to forget or delay. For this reason, the present embodiment employs an automated liquid handling system linked to the ultrasonic measurement operation through circuitry controlled by microprocessor  38  to be described. 
     Referring to FIG. 13 in the present embodiment, premixed water and surfactant for filling the basin  103  are contained in a removable polypropylene supply tank  124 , whereas exhausted water and surfactant from the basin  103  are received by a similar drain tank  126 . Each tank  124  and  126  contains a manual valve  128  which is opened when the tanks are installed in the densitometer  100  and closed for transporting the tanks to a remote water supply or drain. The supply tank  124  and the drain tank  126  have vents  150 , at their upper edges as they are normally positioned, to allow air to be drawn into or expelled from the interior of the tanks  124  and  126  when they are in their normal position within the densitometer  100  and valves  128  are open. The tanks  124  and  126  hold sufficient water for approximately a day&#39;s use of the densitometer  100  and thus eliminate the need for convenient access to plumbing. 
     The valve  128  of the supply tank  124  connects the tank through flexible tubing to a pump  130  which may pump liquid from the supply tank  124  to a heating chamber  132 . 
     Referring to FIG. 14, the heating chamber  132  incorporates a resistive heating element  164  which is supplied with electrical current through a thermal protection module in thermal contact with the coupling liquid in the heating chamber  132 . The thermal protection module  166  includes a thermostat and a thermal fuse, as will be described below. A thermistor  168 , also in thermal communication with the liquid in the heating chamber, provides a measure of the liquid&#39;s temperature during operation of the densitometer  100 . The heater chamber  132  additionally incorporates an optical level sensor  172 . The level sensor  172  detects the level of liquid in the heating chamber  132  by monitoring changes in the optical properties of a prism system when the prism is immersed in liquid as opposed to being surrounded by air. The operation of the thermistor  168  and the level sensor  172  will be described further below. 
     Referring again to FIG. 13, the heating chamber  132  communicates through an overflow port  134  and flexible tubing to an overflow drain outlet  136 . The overflow outlet  136  is positioned at the bottom of the densitometer  100  removed from its internal electronics. The overflow port  134  is positioned above the normal fill height of the heating chamber  132  as will be described in detail below. 
     The heating chamber  132  also communicates, through its lowermost point, with an electrically actuated fill valve  138  which provides a path, through flexible tubing, to a fill port  140  positioned in the wall of basin  103 . 
     In the opposite wall of the basin  103  is an overflow port  142  which opens into the basin  103  at a point above the normal fill height of the basin  103  and which further communicates, through a T-connector  144 , to the drain tank  126 . 
     A drain  146 , in the bottom  104  of the basin  103 , provides a path to an electronically actuated drain valve  148 . The drain valve  148  operates to allow liquid in the basin  103  to flow through the drain  146  to the T-connector  144  and into the drain tank  126 . The overflow port  142  and drain  146  incorporate screens  152  to prevent debris from clogging the tubing or the drain valve  148  communicating with the drain tank  126 . 
     Referring now to FIGS. 10 and 13, the supply tank  124  and the drain tank  126  are positioned within the case  101  of the densitometer  100  and located at a height with respect to the basin  103  so that liquid will drain from the basin  103  into the drain tank  126  solely under the influence of gravity and so that gravity alone is not sufficient to fill the basin  103  from supply tank  124  when fill valve  138  is open. Further, the heating chamber  132  is positioned above the basin  103  so that once the heating chamber  132  is filled with liquid by pump  130 , the filling of the basin  103  from the heating chamber  132  may be done solely by the influence of gravity. Accordingly, the operation of the densitometer in filling and emptying the basin  103  is simple and extremely quiet. 
     In those situations where plumbing is readily accessible, either or both of the supply and drain tanks  124  and  126  may be bypassed and direct connections made to existing drains or supply lines. Specifically, the pump  130  may be replaced with a valve (not shown) connecting the heating chamber  132  to the water supply line. Conversely, the connection between the T-connector  144  and the drain tank  126  may re-routed to connect the T-connector  144  directly to a drain. 
     Even with the constant refreshing of the coupling liquid in the basin  103  by the liquid handling system of the present invention, the liquid contacting surfaces of the basin  103 , the heating chamber  132 , the valves  138  and  148 , and the connecting tubing are susceptible to bacterial colonization and to encrustation by minerals. The coatings of colonization or encrustation are potentially unhygienic and unattractive. Sufficient build-up of minerals or bacteria may also adversely affect the operation of the densitometer  100  either by restricting liquid flow through the tubing, by interfering with the operation of the valves  138  or  148 , or by adversely affecting the acoustical properties of the transducer array  121 . 
     For this reason, the densitometer  100  is desirably periodically flushed with an antibacterial solution and a weak acid, the latter to remove mineral build-up. These measures are not always effective or may be forgotten, and hence, in the present invention critical water contacting surfaces are treated with a superficial antibacterial material which is also resistant to mineral encrustation. The preferred treatment is the SPI-ARGENT™ surface treatment offered by the Spire Corporation of Bedford Mass. which consists of an ion beam assisted deposition of silver into the treated surfaces. The resulting thin film is bactericidal, fungistatic, biocompatible, and mineral resistant. The properties of being both bactericidal and fungistatic are generally termed infection resistant. 
     This surface treatment is applied to the water contacting surfaces of the basin  103 , the heating chamber  132  and the critical moving components of the valves  138  and  148 . 
     Referring now to FIG. 14, the general arrangement of the electrical components of FIG. 4 is unchanged in the ultrasonic densitometer  100  of FIG. 10 except for the addition of I/O circuitry and circuitry to control the pump  130 , valves  138  and  148 , and heating chamber  132  of the liquid handling system. In particular, microprocessor  38  now communicates through bus  40  with an I/O module  174 , a pump/valve control circuit  160  and a heater control circuit  162 . 
     I/O module  174  provides the ability to connect a standard video display terminal or personal computer to the densitometer  100  for display of information to the user or for subsequent post processing of the data acquired by the densitometer and thus allows an alternative to microprocessor  38  and display  18  for processing and displaying the acquired ultrasound propagation data. 
     The pump/valve control circuit  160  provides electrical signals to the fill valve  138  and the drain valve  148  for opening or closing each valve under the control of the microprocessor  38 . The pump/valve control circuit  160  also provides an electrical signal to the pump  130  to cause the pump to begin pumping water and surfactant from the supply tank  124  under the control of microprocessor  38 , and receives the signal from the level sensor  172  in the heating chamber  132  to aid in the control of the pump  130  and valve  138 . 
     The heater control circuit  162  controls the current received by the resistive heating element  164  and also receives the signal from a thermistor  168  in thermal contact with the heating chamber  132 . A second thermistor  170 , positioned in basin  103  to be thermal contact with the liquid in that basin  103 , is also received by the heater control circuit  162 . 
     Referring now to FIGS. 13 and 14, during operation of the densitometer  100  and prior to the first patient, the basin  103  will be empty, the supply tank  124  will be filled and contain a known volume of water and surfactant, and the drain tanks  126  will be empty. Both manual valves  128  will be open to allow flow into or out of the respective tanks  124  and  126  and the electrically actuated fill valve  138  and drain valve  148  will be closed. 
     Under control of microprocessor  38 , the pump/valve control circuit  160  provides current to the pump  130  which pumps water and surfactant upward into heating chamber  132  until a signal is received from level sensor  172 . When the heating chamber  132  is filled to the proper level as indicated by level sensor  172 , the signal from level sensor  172  to pump/valve control circuit  160  causes the pump  130  to be turned off. At this time, a predetermined volume of liquid is contained in heating chamber  132  which translates to the proper volume needed to fill basin  103  for measurement. 
     Under command of microprocessor  38 , the heater control circuit  162  provides a current through thermal protection module  166  to resistive heating element  164 . The temperature of the liquid in the heating chamber  132  is monitored by thermistor  168  and heating continues until the liquid is brought to a temperature of approximately 39° C. The thermistor and a thermal fuse (not shown) of the thermal protection module  166  provide additional protection against overheating of the liquid. The thermistor opens at 50° C. and resets automatically as it cools and the thermal fuse opens at 66° C. but does not reset and must be replaced. The opening of either the thermistor or the thermal fuse interrupts current to the resistive heating element  164 . 
     When the liquid in the heating chamber  132  is brought to the correct temperature, fill valve  138  is opened by microprocessor  38 , through pump/valve control circuit  160 , and liquid flows under the influence of gravity into the basin  103  at the proper temperature. The control of the temperature of the liquid serves to insure the comfort of the patient whose foot may be in the basin  103  and to decrease any temperature effects on the sound transmission of the water and surfactant. 
     Once the heated liquid has been transferred from the heating chamber  132  to the basin  103 , the fill valve  138  is closed and the pump  130  is reactivated to refill the heating chamber  132 . Thus, fresh liquid for the next measurement may be heated during the present measurement to eliminate any waiting between subsequent measurements. 
     With liquid in place within the basin  103 , the measurement of the os calcis by the densitometer  100  may begin. In this respect, the operation of the ultrasonic densitometer of FIG. 10 is similar to that of the embodiment 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 homogenous standard, i.e. water. In the densitometer  100  of FIG. 10, since the distance between the transducers  121  is fixed, the reference pulse through the homogenous standard material, which is simply the liquid in basin  103 , may be conducted before or after a measurement pulse through a live member is performed. In fact, because the temperature of the liquid in the basin  103  is held steady by the temperature control mechanism as described, the standard transmit time measurement can be made once for the instrument and thereafter only measurement pulses need be transmitted. 
     Preferably, the standard transit time measurement is stored as a number in the memory of microprocessor  38  during the initial calibration of the unit at the place of manufacture or during subsequent recalibrations. During the calibration of the densitometer  100 , the signal from the thermistor  170  is used to produce a transit time corrected for the temperature of the liquid according to well known functional relations linking the speed of sound in water to water temperature. It is this corrected transit time that is stored in the memory associated with microprocessor  38  as a stored standard reference. 
     The transit time of the measurement pulses is compared to the stored standard reference transit times through the coupling liquid to give an indication of the integrity of the member just measured. Thus, one may dispense with the reference pulse entirely. Empirical tests have determined that by proper selection of a standard reference value stored in the memory of microprocessor  38  and by holding the liquid in the basin within a temperature range as provided by the heating chamber  132 , no reference pulse need be launched or measured. 
     Using this variation, a mathematical comparison 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= A ( SOS - B )+ C ( BUA - D )  (1) 
       
     
     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 by dividing a standard value for the member width by the actual 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. 
     BUA is broadband ultrasonic attenuation, as described in greater detail above. The constants A, B, C, and D offset and scale the influence of the BUA measurement relative to the SOS measurement to provide a more effective predictor of bone density. These constants may be determined empirically and may be selected for the particular machine to provide numbers compatible with dual photon absorptiometry devices and to reduce bone width effects. 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 and accurate measurement, mechanical placement may be acceptable for clinical utility, in which case only single transducer elements are required. 
     Upon completion of the measurement, the drain valve  148  is opened by microprocessor  38  through pump/valve control circuitry  160 , and the liquid in the basin  103  is drained through “T”  144  to the drain tank  126 . At the beginning of the next measurement, the drain valve  148  is closed and liquid is again transferred from the heating chamber  132  as has been described. 
     With repeated fillings and drainings of the basin  103 , the level of liquid in the fill tank  124  decreases with a corresponding increase in the level of the liquid in the drain tank  126 . The height of the liquid in each tank  124  and  126  may be tracked by a conventional level sensor such as a mechanical float or a capacitive type level sensor. 
     Preferably no additional level sensor is employed. The volume of liquid for each use of the densitometer  100  is known and defined by the fill level of the heating chamber  132 . The microprocessor  38  may therefore track the level of liquid remaining in the supply tank  124  by counting the number of times the basin  103  is filled to provide a signal to the user, via the display  18  or a remote video display terminal (not shown), indicating that the tanks  124  and  125  need to be refilled and drained respectively. This signal to the user is based on the number of times the basin  103  is filled and a calculation of the relative volumes of the heating chamber  132  and supply tank  124 . 
     After completion of the use of the densitometer  100  for a period of time, the densitometer may be stored. In a storage mode, after both the supply tank  124  and drain tank  126  have been manually emptied, the microprocessor  38  instructs the pump/valve control circuit  160  to open both the fill valve  138  and the drain valve  148  and to run the pump  130 . The drain valve  138  is opened slightly before the pump  130  is actuated to prevent the rush of air from causing liquid to flow out of the overflow port  134 . 
     Referring now to FIGS. 10 and 15, the transducers  121  are inserted into the basin  103  through tubular sleeves  180  extending outward from the walls of the basin  103  at the curved pocket along an axes  212  of the opposed transducers  121 . The tubular sleeves  180  define a circular bore in which the transducers  121  may be positioned. Each transducer  121  seals the sleeve  180  by compression of o-ring  182  positioned on the inner surface of the sleeve  180 . 
     Although the transducers  121  fit tightly within the sleeves  180 , their separation and alignment are determined not by the sleeves  180  but by an independent C-brace  184  comprising a first and second opposed arm  186  separated by a shank  188 . A transducers  121  is attached to one end of each of the arms  186 , the other ends of the arms  186  fitting against the shank  188 . 
     The arms  186  are generally rectangular blocks transversely bored to receive the cylindrically shaped transducers  121  at one end and to hold them along axis  212 . The other ends of the arms  186  provide planar faces for abutting the opposite ends of the block like shank  188 , the abutting serving to hold the arms  186  opposed and parallel to each other. 
     Although the angles of the arms  186  with respect to the shank  188  are determined by the abutment of the planar faces of the arms  186  and the ends of the shank  188 , alignment of the arms  186  with respect to the shank  188  is provided by dowel tubes  190  extending outward from each end of the shank  188  to fit tightly within corresponding bores in the first and second arm  186 . 
     Cap screws  194  received in counterbored holes in the arms  186  pass through the arms  186 , the dowel tubes  190  are received by threaded holes in the shank  188  to hold the arm  186  firmly attached to the shank  188 . The dowel tubes  190  and surfaces between the arms  186  and shank  188  serve to provide extremely precise alignment and angulation of the transducers  121 , and yet a joint that may be separated to permit removal of the transducers  121  from the densitometer  10  for replacement or repair. 
     Transducers  121  are matched and fitted to the arms  186  in a controlled factory environment to provide the necessary acoustic signal strength and reception. In the field, the shank  188  may be separated from one or both arms  186  by loosening of the cap screws  194  so as to allow the transducers  121  extending inward from the arms  186  to be fit within the sleeves  180 . Proper alignment and angulation of the transducers is then assured by reattaching the arm or arms  186  removed from the shank  188  to the shank  188  to be tightened thereto by the cap screws  194 . Thus, the alignment of the transducers is not dependent on the alignment of the sleeves  180  which may be molded of plastic and thus be of relatively low precision. Nor must alignment be tested while the transducers are in the sleeves  180  attached to the basin  103  but may be checked in a central controlled environment. 
     Flexible Bladder Embodiment 
     Referring now to FIGS. 16 and 17, in yet another embodiment of the present invention, the opposed transducers  121  are fitted with annular collars  200  which in turn are attached to flexible bladders  202  extending inward to the basin  103 , each bladder  202  containing a liquid or semi-liquid coupling “gel”  204 . 
     The bladders  202  serve to contain the gel about the face of the transducers  121  and conform to the left and right sides of a patient&#39;s heel  207 , respectively, to provide a path between the transducers  121  and the soft tissue and bone of the heel  207  without intervening air. The bladder  202  further prevents the coupling material from direct contact with the heel to permit selection of the coupling gel  204  from a broader range of materials. 
     Compression of the bladders  202  against the heel  207 , so as to provide the necessary coupling, is provided by a telescoping shank  181  shown in FIG.  16 . In this alternative embodiment of the C-brace  184  of FIG. 15, the shank  188 ′ has been cut into two portions  206  and  208  slidably connected together by dowel pins  210  to provide necessary motion of the transducers  121  inward along their axis to compress the bladders  202  against the heel  207 . One end of each dowel pin  210  is press fit within bores in the shank  188 ′ parallel to the axis  212  of the opposed transducers in portion  206 . The other ends of the dowel pins  210  slide within larger bores in portion  208  so that portions  208  and  206  may slide toward and away from each other parallel to the axis  212 . With such motion, the attached arms  186  move towards and away from each other adjusting the separation of the transducers  121  between an open position for insertion of the heel  207  and a closed position of known separation and orientation where portions  208  and  206  abut. 
     Control of the separation is provided by means of cam pins  214  protruding from portions  206  and  208  on the side away from the extension of the arms  186  and generally perpendicular to the axis  212 . These pins  214  are received by spiral shaped slots in a cam disk  217  fitting over the cam pins  214 . The disk includes radially extending lever  218  whose motion rotates the disk causing the cam pins  214  within the slots  215  to be moved together or apart depending on motion of lever  218 . 
     Thus, the transducers  121  may be moved apart together with the bladders  202  for insertion of the heel  207  into the basin  103 . Once the heel is in place, motion of the lever  218  closes the transducers  121  to a predetermined fixed separation compressing the bladders  202  snugly against the sides of the heel  207 . The elasticity of the bladder filled with coupling gel  204  provides an expanding force against the heel  207  to closely conform the surface of the bladder  202  to the heel  207 . 
     Cancellation of Heel Width Variations 
     Referring to FIGS. 17 and 18, generally the thicker the calcaneus  216  of the heel  207 , the greater the attenuation of an acoustic signal passing through the heel  207  between transducers  121 . Correspondingly, with greater attenuation, the slope of attenuation as a function of frequency, generally termed broadband ultrasonic attenuation (BUA) also increases as shown generally in FIG. 18 by plot  209 . This assumes generally that the coupling medium  204  is of low or essentially constant attenuation as a function of frequency. Greater BUA is generally correlated to higher bone quality. 
     For constant heel thickness, lower TOF (faster sound speed) corresponds generally to higher bone quality. The time of flight (TOF) of an acoustic pulse between the transducers  121  will be proportional to the time of flight of the acoustic pulse through regions A of FIG. 17 comprising the path length through coupling gel  204 , regions B comprising the path length through soft tissue of the heel  207  surrounding the calcaneus  216 , and region C comprising the path length through the heel bone or calcaneus  216 . Thus,              TOF   =         1     V   A          A     +       1     V   B          B     +       1     V   C          C               (   2   )                                
     where V A , V B , and V C  are the average speed of sound through the coupling gel, soft tissue and bone respectively and A, B, C are the path lengths through these same materials. Provided that the separation between the transducers  121  is a constant value K. then time of flight will equal:              TOF   =         1     V   A            (     K   -   C   -   B     )       +       1     V   B          B     +       1     V   C          C               (   3   )                                
     The change in time of flight as a function the thickness of the bone C (the derivative of TOF with respect to C) will thus generally be equal to:          1     V   C       -       1     V   A       .                            
     Referring now to FIG. 18, if the velocity of sound through the coupling medium  204  is greater than that through the bone being measured          (         V   A     &gt;     V   C       ,       or                   1     V   C         &gt;     1     V   A           )     ;                          
     then the functional relationship of TOF to heel width will be one of increasing as the heel becomes wider (indicated at plot  213  showing values of 1/TOF). On the other hand, if the velocity of sound through the coupling medium  204  is less than that through the bone being measured          (         V   C     &gt;     V   A       ,       but                   1     V   A         &gt;     1     V   C           )     ,                          
     then the functional relationship of TOF to heel width will be one of decreasing as the heel becomes wider (indicated at plot  211  showing values of 1/TOF). 
     A combined bone health figure may be obtained by combining BUA and 1/TOF measurements (1/TOF because BUA increases but TOF decreases with healthier bone). Further, if (1) the conditions of ultrasonic propagation are adjusted so that the slope of 1/TOF with heel width is opposite in sign to the slope of BUA with heel width (i.e., V A &gt;V C ) and (2) the BUA and 1/TOF measurements are weighted with respect to each other so that the opposite slopes of the BUA and 1/TOF are equal, then the algebraic combination of the BUA and TOF, through addition for example, will produce a bone quality measurement substantially independent of heel width for a range of bone qualities. 
     This can be intuitively understood by noting that as the heel gets wider, it displaces some of the coupling gel  204  from between the heel  207  and each transducer  121 , and by displacing material that conducts sound slower than the bone being measured increasing the total speed with which the sound is conducted. 
     Note that a similar effect may be obtained by proper scaling and combination of BUA and TOF by multiplication and that other functions of attenuation and TOF could be used taking advantage of their functional independence and their functional dependence in part on heel width. 
     Referring now to FIG. 19, generally BUA and TOF are functionally related to both bone quality and bone width. It should be possible, therefore, to solve the equations governing these relationships for bone quality alone and thus to eliminate the effect of the common variable of heel width. With such an approach, the variable of heel width is eliminated not just for a portion but through the entire range of bone measurement provided that the coupling medium is different from the bone being measured so that there will be a width effect in both BUA and TOF measurements. 
     Approximations of the algebraic relationships describing the functional dependence of BUA and TOF on bone quality and bone width, can be obtained through the construction of a set of bone phantoms of different widths and bone qualities when using a particular coupling gel. Generally, for each value of BUA or TOF the data will describe a curve  222  linking that value with different combinations of bone quality and bone width. This data may be placed in a look-up table in the memory of the microprocessor of the densitometer as has been previously described. 
     After BUA and TOF values are determined, the data of the look-up table (comprising many bone quality and bone width pairs for each of the determined BUA and TOF values) are scanned to find a bone quality and width data pair for the BUA value matching a bone quality and width data pair for the TOF value. This is equivalent to finding the intersection of the two curves  222  associated with the measured BUA and TOF values. The matching bone quality values of the data base will give a bone quality having little or no bone width influence. This value may be displayed to the clinician. It is noted that the previously described technique of summing weighted values of BUA and 1/TOF is but a specialized form of this process of algebraic solution. 
     Alternatively, a matching bone width value can be identified, being the width of the measured heel, and used to correct either of the BUA or TOF values for display to the clinician in circumstances where BUA or TOF values are preferred for diagnosis. 
     This ability to cancel out heel width effects will work only for bone qualities where the relationship between the coupling gel  204  and the calcaneus  216  are such as to provide a functional dependence on heel width. Cancellation will not occur, for example, if the density of the calcaneus  216  being measured is substantially equal to the sound speed of the coupling gel  204  and thus where displacement of the coupling gel by similar bone will have no net effect on time of flight. Thus the coupling gel must be properly selected. In this case, materials having higher sound speed may be selected for the coupling material. The difference between the coupling gel and the bone being measured will influence the accuracy of the cancellation of heel width effects. 
     Moderating this desire to improve heel width effects is the importance of keeping the coupling gel  204  close to the acoustic properties of the soft tissue of the heel  207  both to prevent reflection by impedance mismatch and to prevent variations in the thickness of the soft tissue in regions B from adding additional uncertainty to the measurement. The coupling medium of water provides good matching to the soft tissue of the heel  207  and has a sound velocity very close to bone and some osteoporotic conditions. Weighting of the attenuation and propagation time may be made for water. 
     Although the preferred embodiment of the invention contemplates display of a bone quality value or corrected TOF or BUA values, it will be recognized that the same effect might be had by displaying uncorrected BUA or TOF values on a chart and establishing a threshold for healthy or weak bone based on the corrections determined as above. 
     Ultrasonic Densitometer with Scannable Focus 
     Referring now to FIG. 20, a receiving transducer array  300 , similar to array  21  described with respect to FIG. 1, may be positioned adjacent to the heel of a patient (not shown) to receive an ultrasonic wave  410  along axis  304 . The receiving transducer array  300  includes a piezoelectric sheet  302  of substantially square outline positioned normal to the transmission axis  304  and is divided into transducer elements  400  as will be described, each which receives a different portion of the ultrasonic wave  410  after passage through the heel. 
     The piezoelectric sheet  302  may be constructed of polyvinylidene fluoride and has a front face  306  covered with a grid of interconnected square electrodes  308  deposited on the front face by vacuum metallization. These square electrodes  308  are arranged at the interstices of a rectangular grid to fall in rectilinear rows and columns. Referring also to FIG. 23, each square electrode  308  is spaced from its neighboring electrodes  308  by approximately its width. These square electrodes  308  are connected together by metallized traces (not shown) and to a common voltage reference by means of a lead  310 . 
     In the manufacture of the piezoelectric sheet  302 , the polyvinylidene sheet is polarized to create its piezoelectric properties by heating and cooling the sheet in the presence of a polarizing electrical field according to methods generally understood in the art. In the present invention, this polarizing field is applied only to the area under the square electrodes  308  so that only this material is piezoelectric and the material between square electrodes  308  has reduced or no piezoelectric properties. As will be understood below, this selective polarization of the piezoelectric sheet  302  provides improved spatial selectivity in distinguishing between acoustic signals received at different areas on the piezoelectric sheet. 
     Referring now to FIG. 22, opposite each electrode  308  on the back side of the piezoelectric sheet  302  furthest from the source of the ultrasonic wave  410  is a second electrode  312  having substantially the same dimensions as the square electrodes  308  and aligned with corresponding square electrodes  308  along transmission axis  304 . 
     Referring to FIGS. 20 and 21, a connector board  318  of areal dimension substantially equal to the piezoelectric sheet  302  has, extending from its front surface, a number of conductive pins  320  corresponding to the pads  316  in number and location. The pins  320  are stake-type terminals mounted to an epoxy glass printed circuit board  322  of a type well known to those of ordinary skill in the art. Each conductive pin  320  is connected directly to a preamplifer and then by means of printed circuit traces to a multiplexer  325  to a reduced number of control and data lines  324  which may be connected to the microprocessor  38  of the densitometer through an A to D converter  42  described previously with respect to FIG.  1  and as is well understood in the art. The preamplifers allow grounding of those electrodes  312  not active during scanning to reduce cross-talk between electrodes  312 . 
     As shown in FIG. 21, the pins  320  of the connector board  318  are electrically connected to electrodes  312  on the back surface of the piezoelectric sheet  302  by means of a strip of thin (0.0005″) mylar  316  having conductive fingers  314  on its surfaces. The conductive fingers  314  on the front and rear surfaces of the mylar strip  316  are in electrical communication through a plated-through hole  313  in the mylar  316  connecting the fingers  314 . 
     Each conductive pin  320  is attached to a conductive finger  314  at one edge of the mylar strip  316  at the rear of the mylar strip  316  (according to the direction of the acoustic wave) by means of an anisotropically conductive adhesive film  315  providing electrical conduction only along its thinnest dimension, thus from pin  320  to finger  314  but not between fingers  314  or pins  320 . Anisotropically conductive film suitable for this purpose is commercially available from 3M corporation of Minnesota under the trade name of 3M Z-Axis Adhesive Film. 
     The other end of each plated finger  314  on the front of the mylar strip  316  is then connected to an electrode  312  by a second layer of anisotropically conductive adhesive film  317 . The mylar strip  316  flexes to allows the pins  320  to be spaced away from the electrode  312  to reduce reflections off the pins  320  such as may cause spurious signals at the piezoelectric sheet  302 . The mylar strip  316  and conductive fingers  314  are essentially transparent to the acoustic wave. 
     Referring to FIG. 22, the mylar strips  316  and adhesive film  315  and  317  allow rapid assembly of the transducer  300 . A single layer of conductive film  317  (not shown in FIG. 22) may be applied over the entire rear surface of the piezoelectric sheet  302  and electrodes  312 . Next a plurality of overlapping mylar strips  316  may be laid down upon this surface, each mylar strip  316  extending laterally across the piezoelectric sheet  302  with transversely extending conductive fingers  314  for each electrode  312  of one row of conductive electrode  312 . The overlapping of the mylar strips  316  ensures that only a front edge of each strip  316  adheres to the piezoelectric sheet  302 . Guide holes  319  in the laterally extreme edges of the mylar strips  316  fit into pins in a jig (not shown) to ensure alignment of fingers  314  with electrodes  312 . 
     Next, a second layer of the anisotropically conductive adhesive film  315  is placed on the rear surfaces of the overlapping mylar strips  316  and the conductive pins  320  pressed down on this film  315 , aligned with the other ends of the conductive fingers  314  to attach to their respective fingers  314 . The conductive pins  320  are then raised and fixed in spaced apart relationship with the piezoelectric sheet  302 , the mylar strips  316  flexing to accommodate this displacement. 
     The ultrasonic wave  410  passing through portions of the piezoelectric sheet  302  between electrodes  308  and  312  may thereby be measured at a number of points over the surface of the piezoelectric sheet by the electric signals generated and collected by electrodes  308  and  312  according to multiplexing methods well known in the art. Each electrode pair  308  and  312  provides an independent signal of the acoustic energy passing through the area of the piezoelectric sheet  302  embraced by the electrode pair. 
     A protective frame  325  encloses the piezoelectric sheet  302  and connector board  318  protecting them from direct contact with water of the basin  103  shown in FIGS. 10 and 15 into which the receiving transducer array  300  may be placed. The frame  325  holds on its front face an acoustically transparent and flexible material  326  such as a Teflon film so that the ultrasonic wave  410  may pass into the frame to reach the piezoelectric sheet  302 . 
     The above described array may be used either to receive or transmit acoustic waves and is not limited to use in the medical area but may provide an inexpensive and rugged industrial acoustic array useful for a variety of purposes including industrial ultrasonic imaging and the construction of high frequency synthetic aperture microphones. 
     Positioned behind the frame  325  is an electric motor  328  driving a central gear  330  about an axis aligned with transmission axis  304  and approximately centered within the frame  325 . The central gear  330  in turn engages two diagonally opposed planet gears  332  also turning about axes aligned with the transmission axis. Each planet gear  332  has a rod  334  extending forwardly from a front face of the planet gear  332  but offset from the planet gear&#39;s axis to move in an orbit  336  thereabout. The orbit  336  has a diameter approximately equal to the spacing between electrodes  308 . 
     The rods  334  engage corresponding sockets  338  on the back side of the frame  325  at its opposed corners. Thus activation of the motor  328  causes the piezoelectric sheet  302  and connector board  318  to follow the orbit  336  while maintaining the rows and columns of detector elements  400  in horizontal and vertical alignment, respectively. 
     Referring now to FIG. 23, a sampling of the signals from the detector elements  400  may be made at four points  342  in the orbit  336  at which each electrode  308  is first at a starting position, and then is moved half the inter-electrode spacing upward, leftward, or upward and leftward. The effect of this motion of the detector elements  400  is to double the spatial resolution of the received acoustic signals without increasing the amount of wiring or the number of detector elements  400 . The sampling of acoustic energy at each of the points  342  is stored in the memory of the microprocessor and can be independently processed to derive attenuation, BUA or time of flight measurements or a combination of these measurements. These measurements are then converted to an intensity value of an image so that each pixel of the image has an intensity value proportional to the measured parameter. A clinician viewing the image thus obtains not merely an image of the bone, but an image that indicates bone quality at its various points. 
     A transmitting ultrasonic transducer  408  is positioned opposite the receiving transducer array  300  from the heel  207  and produces a generally planar ultrasonic wave  410  passing into the heel. Generally, the acoustic signal received by each transducer element  400  will have arrived from many points of the heel. 
     Referring now to FIG. 24, if the transducer elements  400  were focused as indicated by depicted transducer elements  400 ′ to follow a hemisphere  402  having a radius and hence focus at a particular volume element or voxel  404  within the heel, acoustic signals from other voxels could be canceled providing greater selectivity in the measurement. In this focusing of the transducer elements  400 ′, the signals from each of the elements  400 ′ are summed together. Constructive and destructive interference of ultrasonic waves  410  from the heel  207  serve to eliminate acoustic signals not flowing directly from focus volume element  404 . 
     For example as depicted, two acoustic signals  405  and  406  from focus voxel  404  both crest at the location of a transducer element  400 ′ as a result of the equidistance of each transducer element  400 ′ from focus voxel  404 . When the signals from transducer elements  400 ′ are summed, the signal from focus voxel  404  will increase. In contrast, acoustic signals from other voxels not equidistant to transducer elements  400 ′ will tend to cancel each other when summed and thus decrease. 
     The present invention does not curve the transducer elements  400  into a hemisphere but accomplishes the same effect while retaining the transducer element  400  in a planar array by delaying the signals received by the transducer elements  400  as one moves toward the centermost transducer element  400 ″ so as to produce an effective hemispherical array. Like a hemispherical array, the center-most transducer elements  400  appear to receive the acoustic wave a little later than the transducer elements  400  at the edge of the receiving transducer array  300 . By using a phase delay of the signals instead of curving the receiving array  300 , the position of the focus voxel  404  at which the receiving array  300  is focused, may be scanned electrically as will be described. The signals from each of the transducer elements  400  are received by the A/D converter  42  and stored in memory. Phase shifting as described simply involves shifting the point at which one starts reading the stored signals. 
     Adjusting the phase of the acoustic signals received by each of the transducer elements  400  allows the location of the focus voxel  404  from which data is obtained to be scanned through the heel. The phase is simply adjusted so that the effective arrival time of an acoustic signal originating at the desired location is the same for each of the transducer elements  400 . 
     Referring now to FIG. 25, the location of focus voxel  404  may be moved in a first and second raster scan pattern  412  and  414  (as readings are taken over many ultrasonic pulses) to obtain separated planes of data normal to the transmission axis  304 . The first plane of data  412  may, for example, be positioned near the outer edge of the os calcis  216  to measure the cortical bone quality while the second plane  414  may be placed in a centered position in the trabecular bone to obtain a somewhat different reading, both readings providing distinct data about the bone. 
     It will be understood that this same approach of scanning in different planes may be used to obtain a volume of data within the heel  207 , in this case, the focus voxel  404  being moved to points on a three dimensional grid. 
     In another embodiment (not shown) the transmitting ultrasonic transducer may be an array and the phases of the ultrasonic signals transmitted by each of the elements of the array may be phased so as to focus on a particular voxel within the heel. In this case, the receiving array may be a single broad area detector or may also be an array focused on the same voxel for increased selectivity. The focus point of the transmitting and receiving arrays may also be shifted with respect to each other to investigate local sound transfer phenomenon. As before, the focal points of either array may be steered electrically by the microprocessor through a shifting of the phases of the transmitted and received signals. To collect data, each element of the transmit array may be energized individually while all receive elements of the receive array are read. This may be continued until each of the elements of the transmit array have been energized. 
     Alternatively, referring to FIG. 28, the receiving array  300  may be actually formed so that its elements follow along the hemisphere  402  so as to have a fixed focus on focus voxel  404 . Additional circuitry to effect the phase adjustment needed to focus the array is not needed in this case. The receiving array  300  is attached to an X-Y-Z table  600  providing motion in each of three Cartesian axes under the control of the microprocessor via stepper motors  610 . At each different location of the table  600 , data may be collected from focus voxel  404  to establish the data points on the three dimensional grid. The transmitting array  408  may be held stationary or may be moved with the scanning of the receiving array  300  and may be focused as well. 
     Referring now to FIG. 26, such a data volume  415  may include a plurality of data voxels  416  each providing a measured member parameter for the bone or tissue at that point in the heel. A point of minimum bone density  418  may be found within this data volume  415  and used to identify a region of interest  420  which will serve as a standard region for measuring the bone density of the heel. This region may be automatically found after collection of the data volume  415  and only those voxels  416  within the region of interest  420  may be used for a displayed measurement. This automatic location of a region of interest  420  provides a much more precise bone characterization. 
     Acquiring a data volume  415  also provides the opportunity to use the extra data outside the region of interest  420  to ensure that the same region of interest  420  is measured in the patient&#39;s heel over a series of measurements made at different times. The data volume  415  may be stored in memory as a template that may be matched to subsequently acquired data volumes. The region of interest  420  spatially located with respect to the first template, may then be used as the region of interest for the subsequent data volumes aligned with that template to provide more repeatability in the measurement. 
     Referring now to FIG. 27 in such a template system in a first step  500 , a collection of a data volume  415  within the heel is obtained. At decision block  502 , if this is a first measurement of a particular patient, a region of interest  420  is identified at process block  504  from this data, as a predetermined volume centered about a point of minimum bone density  418  as described with respect to FIG.  26 . At process block  506 , the data volume is stored as a template along with the region of interest defined with respect to the data of the template. 
     Referring again to decision block  502  on a subsequent measurement of a patient, the program may proceed to process block  508  and the template previously established may be correlated to a new data volume  415  collected at process block  500 . The correlation process involves shifting the relative locations of the two data volumes to minimize a difference between the values of each data voxel  416  of the data volumes. In most situations, this will accurately align the two data volumes so that corresponding voxels  416  of the two data volumes  415  measure identical points within the patient&#39;s heel. The region of interest  420  associated with the template is then transferred to the new data volume as it has been shifted into alignment with the template so that the identical region of interest may be measured in a patient even if the patient&#39;s foot has taken a different alignment with respect to the transducer array  300  and  408 . This use of the template&#39;s region of interest  420  is indicated by process block  510 . 
     At process block  512 , an index is calculated at the region of interest  420  for the new data volume  415  being typically an average value of a bone parameter such as BUA or time of flight for the voxels  416  within the region of interest  420 . This data is then displayed to the clinician at process block  520  as has been described. 
     It is specifically intended that the present invention not be specifically limited to the embodiments and illustrations contained herein, but embrace all such modified forms thereof as come within the scope of the following claims.