Abstract:
An ultrasonic measuring device and method for determining bone density and structure includes an electronic control unit, a positioning unit housing at least a transducer pair and carrying a receiving unit, the unit being adapted to be coupled to a bone segment of the human body and carrying a locating device for ascertaining the position of the bone segment with respect to the unit, and an electronic unit for determining a first waveform representing an ultrasonic signal transmitted through the metaphysis portion of the bony segment and a second waveform representing an ultrasonic signal transmitted through the diaphysis portion, and an electronic processor and display for displaying the first and second waveforms as a measured output.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ultrasonic measuring device for determining bone density and structure. 
     Electronic devices for examining bone tissue and structure are known, which comprise an ultrasonic transducer for feeding pulses into a bone segment for examination (e.g. a finger); a receiver for picking up the pulses that have traveled through the bone segment; and processing circuits for representing the waveform and, given the distance between the transducer and the receiver, calculating the speed of the ultrasonic signal through the bone segment. As the transmission speed of the ultrasonic signal is greatly affected by the characteristics of the bone segment between the transducer and the receiver, and varies alongside a variation in bone structure and density, known devices compare the measured speed value with a reference value to determine a variation in bone structure and density, which normally indicates demineralization of the bone tissue (caused, for example, by osteoporosis). The waveform is also examined by a skilled technician to obtain information, albeit approximate and at times ambiguous, concerning the characteristics of the bone segment. As such, known devices fail to provide for precise analysis closely related to the characteristics of the bone tissue, and interpretation is further complicated in the event the bone segment comprises a distal portion. That is, the distal portion of a bone (FIG. 8) is known to comprise a substantially solid first end portion A (metaphysis) defined by a shell of thin cortical bone containing mainly bone trabeculae; and a more proximal, substantially tubular second portion B (diaphysis) comprising an outer tubular (cortical) portion defining an inner canal containing few bone trabeculae, which, in adults, are reabsorbed to hollow out the canal of the second portion. 
     A known device measuring the above distal portion produces a waveform and calculates the ultrasonic speed of an ultrasonic signal traveling indifferently through the first and second portions, which, as stated, have entirely different structures. As certain bone diseases, however, have a widely differing effect on the first and second portions, a separate analysis of the structural characteristics of the first (metaphysis) and second (diaphysis) portions would be extremely beneficial. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an ultrasonic measuring device for determining bone density and structure, designed to overcome the drawbacks of known devices by, among other things, discriminating between the characteristics of the first and second portion. 
     According to the present invention, there is provided an ultrasonic measuring device for determining bone density and structure, as claimed in claim  1 . 
     The present invention also relates to a method of determining bone density and structure, as claimed in claim  12 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows, schematically, an ultrasonic measuring device for determining bone density and structure in accordance with the teachings of the present invention; 
     FIGS. 2 a ,  2   b ,  2   c  show logic operating block diagrams of the FIG. 1 device; 
     FIG. 3 shows a longitudinal section of an ultrasonic gage of the measuring device according to the present invention; 
     FIG. 4 shows a section of the gage along line IV—IV in FIG. 3; 
     FIG. 5 shows a section of the gage along line V—V in FIG. 3; 
     FIGS. 6 a ,  6   b ,  6   c  show signals acquired by the device according to the present invention; 
     FIG. 7 shows, schematically, a variation of the FIG. 1 measuring device; 
     FIG. 8 shows a view in perspective of a portion of bone tissue. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Number  1  in FIG. 1 indicates as a whole an electronic ultrasonic measuring device for determining bone density and structure, and comprising a central unit  3  housed in a parallelepiped casing (not shown), connected to a positioning gage  5  (shown schematically in FIG.  1 ), and having a keyboard  7  and a video terminal  9 . 
     The central unit comprises a signal generating circuit  11  for generating a periodic signal of given frequency and highly constant amplitude, and which, in the example embodiment shown, generates a periodic pulse signal of 0.8 to 15 MHz frequency, e.g. 1.25 MHz. The output of circuit  11  communicates with the input  13   a  of a demultiplexer circuit  13 , which has two outputs  13   b ,  13   c  communicating respectively with a first and a second piezoelectric transducer  16 ,  17  carried by positioning gage  5  and each for generating, in response to the periodic pulse signal at the input, an ultrasonic signal which is fed to a portion  20  of the human body (shown schematically) placed inside gage  5  (as described later on). Transducers  16  and  17  are separated by a constant distance h, so that the ultrasonic signals generated by them travel along straight parallel propagation paths S 1 , S 2  also separated by distance h, and may generate synchronized ultrasonic signals or ultrasonic pulses at different instants. 
     First transducer  16  is connected to a first ultrasonic receiver  22  carried by gage  5  and facing first transducer  16  along path S 1 ; second transducer  17  is connected to a second ultrasonic receiver  23  carried by gage  5  and facing second transducer  17  along path S 2 ; and the facing respectively emitting and receiving surfaces of transducer  16  and receiver  22  and transducer  17  and receiver  23  are separated by a manually adjustable distance D. More specifically, distance D is measured by a position transducer  24  carried by positioning gage  5  and for generating a signal Dpos proportional to the measured value of distance D. 
     First and second ultrasonic receivers  22 ,  23  communicate respectively at the output with a first and a second input  25   b ,  25   c  of a multiplexer circuit  25 , the output  25   a  of which communicates with the input of a signal amplifier  28 ; and demultiplexer and multiplexer circuits  13  and  25  are controlled synchronously by a remote signal SYNC, so as to connect input  13   a  to output  13   b  and input  25   b  to output  25   a , or to connect input  13   a  to output  13   c  and input  25   c  to output  25   a . The output of signal amplifier  28  is connected to the input of a filter  30  (in particular, a low-pass filter), the output of which is connected to a signal processing circuit  32  (e.g. another filter); and the output of circuit  32  is connected to a first input of a multiplexer circuit  34 , the output of which is connected to an analog-digital converting circuit  36 . Multiplexer circuit  34  also has other inputs, one of which is supplied with position signal Dpos from gage  5 ; and the output of converting circuit  36  communicates with an input of a microprocessor processing and control circuit  40 , which, among other things, controls demultiplexer circuit  13 , multiplexer circuit  25  and multiplexer circuit  34 , communicates with keyboard  7  via an interface circuit  42 , and supplies control signals to a video board  44  driving video terminal  9 . 
     With reference to FIG. 3, number  5  indicates as a whole ultrasonic gage  5 , which comprises a first outer tubular body  50  connected telescopically to a second inner tubular body  51  coaxial with, and sliding with respect to, body  5  along an axis  52 . 
     First tubular body  50  has a first threaded free end  50   a  from which extends a first radial appendix  53  comprising a substantially parallelepiped base portion  53   a  with a threaded through hole  54  engaged by end  50   a , and a straight portion  53   b  extending radially from base portion  53   a . The free end of straight portion  53   b  has a through hole  55  engaged by a parallelepiped container housing first and second piezoelectric transducers  16 ,  17 . 
     Second tubular body  51  projects from body  50  and terminates with an end portion  51   a  supporting a second radial appendix  56  facing first appendix  53 , and which comprises a substantially parallelepiped base portion  56   a  integral with end  51   a , and a straight portion  56   b  extending radially from base portion  56   a . The free end of straight portion  56   b  has a through hole  57  engaged by a cylindrical tubular container housing first and second ultrasonic receivers  22 ,  23 . 
     Second appendix  56  has a pair of through holes  58  extending close to an end edge of hole  57 . More specifically, through holes  58  are located along an axis d 1  perpendicular to the central longitudinal axis d 2  of the appendix, and are symmetrical with axis d 2 . 
     Each hole  58  houses an end portion of a cylindrical rod  60  extending between second and first appendixes  56 ,  53  and parallel to axis  52 . Rods  60  therefore extend parallel to each other, are separated by a distance substantially equal to the distance, measured along axis d 1 , between holes  58 , and define a locating element for correctly positioning (as described later on) a portion of the human body, in particular a finger, with respect to appendixes  53 ,  56 , transducers  16 ,  17 , and receivers  22 ,  23 . The portion of the human body used for measurement purposes may comprise the distal femur, in the case of measurements performed on newborn or premature babies, and the epiphysis of the index finger in the case of measurements for determining diseases relating to rheumatoid arthritis. Whichever the case, the examination region comprises a metaphysis and a diaphysis portion. 
     Holes  58  are so located that, when a finger is placed in the gage with a portion contacting both rods  60 , an end bone portion of the finger is centered between transducers  16 ,  17  and receivers  22 ,  23 . 
     Gage  5  also comprises a device  61  for manually adjusting the distance D between transducers  16 ,  17  and receivers  22 ,  23 . 
     Device  61  comprises a rectangular blade  62  having a first free end  62   a  fixed stably to base portion  56   a  by means of screws, and a second free end  62   b  housed inside a rectangular groove  64  formed in base portion  53   a . Blade  62  is slightly arc-shaped and presses on the bottom portion (not shown) of groove  64 . 
     Blade  62  also comprises a toothed mid portion  62   c ; and gage  5  comprises a tube  65  carried by body  50  and extending along an axis  66  parallel to axis  52 . More specifically, tube  65  comprises a first end portion  65   a  carried by an end body  67  integral with a second end  50   b  of tubular body  50 ; an intermediate portion engaging a through hole  68  formed in appendix  53  and coaxial with axis  66 ; and a second end portion  65   b  adjacent to portion  51   a  of body  51 , and which terminates with an opening closed by a plug  69  with a hole, and also engages a through hole  70  formed in appendix  56  and coaxial with axis  66 . 
     Tube  65  houses the cables supplying the energizing signal to transducers  16 ,  17 , and the cables supplying the output signal to receivers  22 ,  23 . 
     Transducer  24  is defined by a linear potentiometer (not shown) housed inside, and for determining the relative position of, tubular bodies  50 ,  51  to measure distance D and generate position signal Dpos. 
     An elastic element  80  (shown schematically) is interposed between bodies  50  and  51  to retain body  51  inside body  50  and push appendixes  53 ,  56  into contact with each other. The elastic force exerted by elastic element  80  may be adjustable. 
     The general operation of device  1  will now be described with reference to the FIG. 2 a  block diagram, which shows a series of operating steps controlled by microprocessor circuit  40 . 
     To begin with, a block  90  enquires—the enquiry being displayed on video terminal  9 —whether device  1  is to be calibrated or a measuring session performed. 
     If calibration is selected, block  90  goes on to a block  91 , which starts the device calibration procedures in known manner. Conversely, block  90  goes on to a block  92 , which performs a series of bone density and structure measuring operations. Once the device is calibrated, block  91  also goes on to block  92 . 
     To perform the measurements in block  92 , appendixes  53  and  56  are parted manually using device  61  in opposition to elastic element  80 . 
     A first phalanx of one finger is then placed between transducers  16 ,  17  and receivers  22 ,  23 , with the back of the phalanx contacting both rods  60 . In this position, the axis of the finger is roughly perpendicular to axis  52  of the gage and to propagation paths S 1  and S 2 , and the distal metaphysis of the phalanx is located between transducers  16 ,  17  and receivers  22 ,  23 . 
     The distal portion of the phalanx is known to comprise a substantially solid first end portion A (metaphysis) defined by a shell of thin cortical bone containing mainly bone trabeculae; and a more proximal, substantially tubular second portion B (diaphysis) comprising an outer tubular (cortical) portion defining an inner canal containing few bone trabeculae. In adults, the canal of portion B is known to be hollowed out by reabsorption of the bone trabeculae, and portion A is also hollowed out later, though never completely. 
     In the position described above, propagation path S 1  therefore extends through first portion A, and propagation path S 2  through second portion B adjacent to portion A, i.e. the ultrasonic signal produced by transducer  16  travels mainly through the bone portion rich in trabeculae, while the ultrasonic signal produced by transducer  17  travels mainly through the inner canal surrounded by the cortical portion. 
     When device  61  is released, appendixes  53  and  56  are pushed by the elastic element on to opposite lateral portions of the finger, with transducers  16 ,  17  and receivers  22 ,  23  on either side of the finger. In this position, the distal portion (and hence the bone tissue) of the finger is positioned stably with respect to gage  5  and prevented from moving laterally by appendixes  53 ,  56  pressing on either side of the finger; gage  5  is prevented by rods  60  from sliding downwards towards the palm side of the hand, and is prevented by the condyles from sliding outwards; and transducers  16 ,  17  and receivers  22 ,  23  are positioned in parallel facing planes. 
     Positioning the finger as described above provides, for each measurement, for correctly positioning portions A and B of the bone tissue with respect to transducers  16 ,  17 . 
     With reference to FIG. 2 a , block  92  comprises a block  100 , which provides for automatically acquiring distance D between transducers  16 ,  17  and receivers  22 ,  23 . More specifically, the (analog) signal generated by potentiometer  24  is supplied, via multiplexer  34 , to converter  36 , which supplies microprocessor  40  with the digital value of distance D, which may be displayed on video  9  and used later for calculating other parameters. 
     Block  100  is followed by a first measuring block  120 , which acquires and displays on video  9  the waveform of the ultrasonic signal received by receiver  22 . 
     More specifically, block  120  of microprocessor circuit  40  sets demultiplexer  13  and multiplexer  25  to a first position in which input  13   a  is connected to output  13   b , and input  25   b  to output  25   a ; the alternating signal supplied by circuit  11  to transducer  16  produces a stream of ultrasonic waves along path S 1  through portion A (the trabecular portion) of the bone to receiver  22 ; and the signal generated by receiver  22  is voltage-amplified by amplifier  28 , filtered by filter  30 , possibly processed by circuit  32 , digitized by converter  36 , and supplied to microprocessor circuit  40 . 
     Microprocessor circuit  40  effects (in known manner) a cartesian reconstruction (FIG. 6 a ) of the waveform FO 1  of the ultrasonic signal received by receiver  22 , wherein the X axis represents a time scale and the Y axis an amplitude scale, with time and amplitude values increasing outwards of the origin. 
     The cartesian representation of waveform FO 1  of the signal through distal metaphysis portion A has been found by the inventors to comprise a first portion I (shown enclosed in a rectangle) in turn comprising a number of successive peaks Ptr (normally three or four); a second portion II (shown enclosed in a rectangle) adjacent to the first portion and in turn comprising a small number of peaks of substantially negligible amplitude; and a third portion III (shown enclosed in a rectangle) adjacent to second portion II and in turn comprising a large number of peaks generally of greater amplitude than those of first portion I. 
     The first portion I of waveform FO 1  is assumed by the inventors to relate to the signal portion through the trabecular portion of the bone, and second portion II of the waveform to the signal portion through the cortical portion of the bone. As the received signal has traveled through portion A mainly comprising bone trabeculae, first portion I comprises a large amount of energy (significant peaks Ptr); second portion II comprises very little energy (almost negligible peaks); and third portion III mainly comprises noise caused by bouncing and reflection of the energizing signal. Waveform FO 1  as illustrated is memorized (digitized) by block  120  in a buffer memory  41  communicating with microprocessor circuit  40 . 
     First measuring block  120  is followed by a second measuring block  130 , which acquires and displays on video  9  the waveform FO 2  (FIG. 6 b ) of the ultrasonic signal received by receiver  23 . 
     More specifically, microprocessor circuit  40  sets demultiplexer  13  and multiplexer  25  to a second position in which input  13   a  is connected to output  13   c , and input  25   c  to output  25   a ; the alternating signal supplied by circuit  11  to transducer  17  produces a stream of ultrasonic waves along path S 2  through portion B (the cortical portion and canal) of the bone to receiver  23 ; and the signal generated by receiver  23  is voltage-amplified by amplifier  28 , filtered by filter  30 , possibly processed by circuit  32 , digitized by converter  36 , and supplied to microprocessor circuit  40 . 
     Microprocessor circuit  40  effects (in known manner) a cartesian reconstruction of the waveform of the ultrasonic signal received by receiver  23 , as described above for the signal received by receiver  22 . 
     The cartesian representation (FIG. 6 b ) of waveform FO 2  of the signal through diaphysis portion B has been found by the inventors to comprise a first portion I′ (shown enclosed in a rectangle) in turn comprising a small number of successive peaks of negligible amplitude; a second portion II′ (shown enclosed in a rectangle) adjacent to the first portion and in turn comprising a number of peaks Pco; and a third portion III′ (shown enclosed in a rectangle) adjacent to second portion II′ and in turn comprising a large number of peaks generally of greater amplitude than those of the second portion. 
     The first portion I′ of waveform FO 2  is assumed by the inventors to relate to the signal portion through the trabecular portion of the bone, and second portion II′ of the waveform to the signal portion through the cortical portion of the bone. As the received signal has traveled through portion B mainly comprising a cortical portion, second portion II′ comprises a large amount of energy (significant peaks Pco); first portion I′ comprises very little energy (almost negligible peaks); and third portion III′ mainly comprises noise caused by bouncing and reflection of the energizing signal. Waveform FO 2  as illustrated is memorized by block  130  in buffer memory  41 . 
     Block  130  is followed by a block  140 , which provides for displaying the acquired, memorized waveforms FO 1  and FO 2 . More specifically, block  140  may: 
     display waveforms FO 1  and FO 2  alternately; 
     display both waveforms FO 1  and FO 2  separately in two different portions of video  9 ; 
     display both waveforms FO 1  and FO 2  superimposed in different colours in the same portion of video  9  and using the same reference system. 
     Block  140  therefore provides the user of device  1  with precise information concerning the examined bone portion, by displaying the waveforms of two widely differing adjacent portions (A and B). The presence of a characteristic, easily identifiable portion (I) in waveform FO 1  provides information concerning the structure of the trabecular bone tissue portion; while the presence of a characteristic, easily identifiable portion (II′) in waveform FO 2  provides information concerning the characteristics of the cortical bone portion. 
     Before terminating the analysis, the present invention also provides for performing a series of automatic operations on waveforms FO 1  and FO 2  (block  150  following block  140 ). 
     Block  150  comprises a first block  200 , which subjects waveform FO 1  to a characteristic-pattern recognition process to define a window F 1  (e.g. a rectangle) enclosing first portion I of waveform FO 1 , and to determine the time limits tI 1  and tI 2  of window F 1  (defined as the points at which the window intersects the time axis). The pattern recognition process may be performed in known manner by determining, in first waveform FO 1 , the first group of adjacent peaks having, on either side, signal portions of a given substantially zero amplitude. 
     Block  200  is followed by a block  210 , which subjects waveform FO 2  to a characteristic-pattern recognition process to define a window F 2  (e.g. a rectangle) enclosing second portion II′ of waveform FO 2 , and to determine the time limits tII 1  and tII 2  of window F 2  (defined as the points at which the window intersects the time axis). The pattern recognition process may be performed in known manner by determining, in second waveform FO 2 , the first group of adjacent peaks having, on one side, signal portions of a given substantially lower amplitude, and, on the other side, signal portions of a higher amplitude. 
     Block  210  is followed by a block  220 , which compares windows F 1  and F 2  to automatically check the measurements are correct. More specifically, if first window F 1  is substantially adjacent to second window F 2  in a cartesian system having the same origin as the cartesian systems of waveforms FO 1  and FO 2 , i.e. if tI 2  is substantially equal to tII 1 , the measurement is considered correct, and block  220  goes on to a block  230 . Conversely, block  220  goes on to a block  240 , which displays a repeat-measurement message on the video, and then goes back to block  92  to perform another measuring cycle. 
     Block  230  performs a series of operations on the portions of waveforms FO 1 , FO 2  in windows F 1  and/or F 2  to obtain information concerning bone density and structure, and whereby block  230  calculates (relative to windows F 1  and F 2 ): 
     the energy E of the signal within the window, by calculating the integral of the waveform portion in the window; 
     the number of peaks Np of the waveform portion in the window; 
     the width W of the window, measured along the X axis; 
     the peak-envelope slope SLP of the signal portion in the window; 
     the peak sharpness SH of the waveform portion in the window; 
     the maximum-peak value Vpm of the waveform portion in the window. 
     The values calculated in block  230  may be combined to obtain one or more indications of the bone tissue condition. 
     FIG. 2 c  shows a series of “clean-up” operations of waveforms FO 1  and FO 2 , performed for example to improve the effectiveness of block  150 , and which may conveniently be performed prior to the block  150  waveform analysis. 
     More specifically, the waveform “clean-up” operations comprise a first block  300 , which retrieves digitized waveform FO 1  from memory  41 , and is followed by a block  310 , which retrieves from memory  41  digitized waveform FO 2 . 
     Block  310  is followed by a block  320  in which, for each X-axis value “i” in the reference system of waveform FO 1 , the corresponding amplitude value VFO 1 i of waveform FO 1  is determined; for a corresponding X-axis value “i” in the reference system of waveform FO 2 , the corresponding amplitude value VFO 2 i of waveform FO 2  is determined; amplitude value VFO 2 i is subtracted from VFO 1 i, i.e. VFODi=VFO 1 i−VFO 2 i; the resulting value VFODi of the subtraction is assumed, for that particular X-axis point “i”, to represent a new so-called difference waveform FOD; and the above operations are repeated for all the points “i” corresponding to Y-axis values of waveforms FO 1  and FO 2 , so as to subtract waveform FO 2  from waveform FO 1  and generate difference waveform FOD (FIG. 6 c ). 
     Block  320  is followed by a block  330  in which each negative X-axis value of difference waveform FOD is made equal to zero to generate a corrected difference waveform for use in the waveform analysis in block  150 . 
     The reason for the above operations lies in the peaks in portion II of waveform FO 1 , which, though of limited amplitude, may nevertheless interfere with the window-definition operations in block  200 . The above operations, on the other hand, provide for subtracting from the peaks in portion II the peaks in corresponding portion II′ to produce an obviously negative difference signal, which is converted by block  330  into a zero signal to form, in other words, a zero-amplitude portion to the right of the group of peaks Ptr in the difference signal, and so enable better selection of peaks Ptr. The above operations have substantially no effect on the amplitude of peaks Ptr in the difference signal, by portion I of waveform FO 1  corresponding to a portion I′ comprising peaks of very limited amplitude. Similarly, waveform FO 1  may be subtracted from waveform FO 2 , and difference waveform FOD may be subjected to the same processing as in block  330 . 
     The system according to the present invention therefore provides for eliminating the drawbacks typically associated with known systems. 
     The device described, in fact, provides for simultaneously measuring two adjacent bone portions (the distal metaphysis and adjacent proximal diaphysis portion) differing widely as to anatomical structure despite forming part of the same bone portion. Waveforms FO 1  and FO 2  also provide for discriminating between the two portions to obtain separate information relative to the distal metaphysis and the proximal diaphysis portion. Device  1  in fact provides for displaying and processing two different waveforms—FO 1  and FO 2 —with clearly visible portions I and II′, which may be analyzed and compared, even only visually, to obtain various information concerning the characteristics of the distal metaphysis and the proximal diaphysis portion. 
     The automatic operations performed in blocks  200 ,  210 ,  220  also provide for determining correct positioning of transducers  16 ,  17  and correct performance of the measurements, as well as for repeating any unconfirmed measurements. Automatically defining windows enclosing characteristic portions of the waveform enables signal-analysis algorithms (block  230 ) to be applied to characteristic waveform portions to obtain data accurately representing the condition of the bone tissue; and the operations in blocks  300 - 330  assist in accurately defining the windows. 
     Clearly, changes may be made to the device as described and illustrated herein without, however, departing from the scope of the present invention. 
     The device  1   a  shown in FIG. 7 differs from the FIG. 1 device by the output of first ultrasonic receiver  22  communicating directly with the input of a signal amplifier  28   a , the output of which communicates with the input of a filter  30   a  (in particular a low-pass filter) having an output connected to the input of a variable-gain amplifier  28   b ; by the output of second ultrasonic receiver  23  communicating directly with the input of a signal amplifier  29   a , the output of which communicates with the input of a filter  31   a  (in particular a low-pass filter) having an output connected to the input of a variable-gain amplifier  31   b ; and by the outputs of amplifiers  28   b  and  31   b  communicating respectively with a first and a second input  26   a ,  26   b  of a multiplexer circuit  26  having a single output connected to the input of multiplexer circuit  34 . As for the rest, device  1   a  is identical to, and operates in the same way as, device  1  described with reference to FIG. 1, except that the signal generated by each ultrasonic receiver  22 ,  23  is amplified by a specific variable-gain amplifying chain (comprising amplifiers  28   a , 28   b  and  29   a , 31   b  respectively) to enable the amplification factor of waveform FO 1  to be varied independently from that of waveform FO 2 ; and multiplexer circuit  26  provides for selecting one or other of the signals generated by receivers  22 ,  23 .