Simulation of bone in ultrasonic assessment apparatus

Abstract: A simulation device for simulating bone in an ultrasonic assessment apparatus comprises an electrical low-pass filter (20). The filter (20) attenuates the ultrasonic signal of the apparatus to simulate the attenuating effect of bone on the signal transmitted by a transmitting transducer (1) of the apparatus.

Description: This invention relates to a simulation device, to a method for simulating bone in an ultrasonic assessment apparatus, and to such an apparatus. There is an increasing interest in the use of ultrasonic velocity and attenuation (Broadband Ultrasonic Attenuation, BUA) measurements in human bone, particularly the human calcaneum (heel) to identify and monitor osteoporotic changes. A known ultrasonic assessment apparatus is illustrated diagrammatically in Figure 1 of the accompanying drawings. The apparatus comprises a primary ultrasonic transmitting transducer 1, a primary ultrasonic receiving transducer 2 and a vessel 3 containing a suitable liquid 8, typically water, through which the transmitted ultrasonic signal 9 can propagate. An electrical signal generator 4 is connected to supply a short voltage pulse e.g. 600V, 1µs , to the input of the transmitting transducer 1. The output 10 from the receiving transducer is fed to an analyser 11 comprising a digitising unit 5 and a processor 6 programmed to perform a Fast Fourier Transform (FFT) on the digitised data. The processor 6 is also programmed to analyse the resultant frequency spectrum to produce a plot of attenuation as a function of frequency which may then be printed out using a plotter 7 or the like. The slope of the attenuation/frequency plot expressed in dBMHz -1 is known as the BUA value and provides an indication of bone condition. In use, the patient's heel 12 is placed in the vessel 3, as shown, the equipment operates and the frequency spectrum is measured. In this way, the attenuation of the transmitted ultrasonic pulse due to absorption by the patient's heel 12 can be measured over a desired frequency range. Typically, measurements are made over the frequency range from 0.2MHz to 0.6MHz. For the human heel, the slope of a plot of attenuation against frequency may lie in a range from about 20dBMHz -1 (a BUA value of 20) to 120dBMHz -1 (a BUA value of 120), depending on bone condition, with a typical attenuation of about 10dB at 0.2MHz. In an alternative, "dry", design of ultrasonic assessment apparatus as shown in the applicants prior US patent number 5452722, the ultrasonic transmitting and receiving transducers 1, 2 are positioned directly against the patient's heel using soft coupling pads to accommodate variations of heel shape. In order to compensate for any frequency dependency of the transducer response the frequency spectra obtained are initially corrected by subtracting a reference spectrum derived from a reference material, such as de-gassed water. In the case of an assessment apparatus comprising a water-filled vessel, the reference spectrum may be updated regularly. In order to monitor the reliability and repeatability of the measurements it is necessary to provide a standard (a so-called quality assurance phantom) which simulates the attenuation properties of bone. However, such quality assurance phantoms tend to be unreliable, especially when they are used to simulate the attenuating effect of cancellous bone. Furthermore it is very difficult to accurately simulate bone properties using a physical substitute. Epoxy-resin models of CAD heel structures have been produced using stereo-lithography techniques. Models produced in this way have a highly repeatable structure; nevertheless, they are found to exhibit an inter and intra phantom precision (i.e. a coefficient of variation) of about 4%, a value which is considered to be too high. In an alternative design, bone structure is simulated in a model by gelatine pellets randomly dispersed in epoxy resin. However, this design has the drawback that there is very little control over the internal structure of the model. Whereas the aforementioned designs rely upon structural means to simulate the ultrasonic attenuation properties of bone, the present invention, in contrast, uses electrical means. United States Patent No. 3847141 discloses a device for measuring the density of a bone structure so as to monitor the calcium content thereof. If pair of opposed spaced ultrasonic transducers are held within a clamping apparatus closely adjacent the bone being analysed. An electric circuit, including an amplifier and a band pass filter, couples signals from the receiver transducer back to a pulse generator for retriggering the pulse generator at a frequency proportional to the duration that the ultrasonic wave takes to travel through the bone structure being examined. According to one aspect of the invention there is provided an ultrasonic assessment apparatus including ultrasonic transmitting (1) and receiving (2) transducers between which in normal use a patient's body part (12) to be assessed may be located, the apparatus further including a simulation device (20, 33) which is selectively introducible in the absence of a patient's body part into the operation of the apparatus in order to simulate the effect of bone in the ultrasonic assessment apparatus and which is removable from the operation of the apparatus when the apparatus is in normal use, characterised in that the simulation device includes electrical filter means for simulating the attenuating effect of bone on an ultrasonic signal transmitted by the transmitting transducer. For the avoidance of doubt, the term "electrical" includes "electronic" apparatus. A simulation device according to the invention has the advantage that the simulated attenuation is determined by the electrical properties of the electrical filter means, and is not dependent on factors such as the physical structure and composition of a model which, as already explained, may reduce the accuracy of the measurements being made. The ultrasonic signal, or an electrical signal indicative of or relating to the ultrasonic signal, is preferably passed through the electrical or electronic means, where the signal's character, properties or form are altered by the electrical or electronic means in a similar or identical way to the way in which they would have been altered had the signal passed through a particular bone or bone structure. The electrical properties of the electrical means preferably mimic the attenuating properties of the particular bone structure being simulated. In other words, the assessment apparatus may produce the same response when the simulation device is being used as it would if the particular bone being simulated was in place in the assessment apparatus. The filter means may be a band-pass filter and is preferably a low-pass filter. It may have a frequency response tailored to simulate the attenuating effect of a chosen bone condition. The filter means may have a fixed frequency response to simulate the attenuating effect of a particular bone condition. In this case, the filter means may be a low pass filter having a predetermined slope in the roll-off region of the frequency response. Alternatively, the filter means may have an adjustable frequency response so as to be capable of simulating the attenuating effects of a range of different bones and/or bone conditions. In other words the simulation device may include adjustment means for adjusting the performance of the filter means such that the filter means may simulate different attenuation effects. In this case, the electrical filter means may be a band or low-pass filter having an adjustable slope in the roll-off region of the frequency response. The electrical filter means may have an input, an output, a plurality of filter stages and adjustable means (e.g. switch means) for selectively connecting one, and/or an arrangement (e.g. serial) of two or more, of said filter stages between the input and the output whereby to adjust the frequency response of the filter means. The filter stages may all have the same frequency response. In this way, an operator may calibrate or check the ultrasonic assessment apparatus for a number of different bones and/or bone conditions. The electrical filter means may have a slope in the roll-off region of the frequency response which is adjustable over a predetermined range e.g. from about 20dBMHz -1 to about 120dBMHz -1 . The simulation device may also include means for delaying a received signal. The delay means may take the form of an electrical delay line, and the delay line may provide an adjustable delay in order to simulate different propagation velocities of an ultrasonic signal through bone. Typically the ultrasonic assessment apparatus includes a primary ultrasonic transmitting transducer and a primary ultrasonic receiving transducer. In one preferred embodiment of the invention, the simulation device is connectable to the output of the ultrasonic receiving transducer. In this case, the receiving transducer receives the transmitted ultrasonic signal and the simulation device attenuates the received signal in order to simulate the attenuating effect of bone on the transmitted ultrasonic signal. In another preferred embodiment of the invention, the simulation device is placed between the transmitting and receiving transducers of the ultrasonic assessment apparatus in the position normally occupied by a bone under test. In this embodiment, the simulation device comprises secondary ultrasonic receiving and transmitting transducers for respectively receiving an ultrasonic signal from and transmitting an ultrasonic signal to the transmitting and receiving transducers of the ultrasonic assessment apparatus, and the electrical filter means is connected between the secondary receiving and transmitting transducers of the simulation device, whereby to simulate the attenuating effect of bone on the ultrasonic signal transmitted by the transmitting transducer of the ultrasonic assessment apparatus. The aforementioned delay could alternatively be implemented by means of a physical delay in the form of a low velocity material (for example silicone rubber) attached to one or both of the secondary transducers. The filter means may be implemented in any suitable manner, e.g. using analogue or digital circuitry or a computer controlled filter, or any combination of these as appropriate. The character of the primary ultrasonic transmitting and/or receiving transducers may vary depending on the particular ultrasonic assessment apparatus in question. In particular the transducers may be either of the focused or unfocused type. The secondary transducers may therefore be chosen to complement the primary transducers of the assessment apparatus, and in some embodiments the secondary transducers may be removable and/or interchangeable to allow different secondary transducers to be used with different types of ultrasonic assessment apparatus. In accordance with a further aspect of the invention there is provided a method for calibrating an ultrasonic assessment apparatus as defined in claim 13, the apparatus including using electrical filter means to simulate the attenuating effect of bone on an ultrasonic signal transmitted by the transmitting transducer. According to a yet further aspect of the invention there is provided an ultrasonic assessment apparatus incorporating a simulation device as defined herein in accordance with the present invention. The assessment apparatus may include ultrasonic transmitting and receiving transducers and, possibly, means for locating the transducers against or adjacent a patient's body part e.g. heel. The assessment apparatus may be adapted for ultrasonic assessment of any particular bone and may include means for locating the body part containing that bone in the desired position on the apparatus. The invention also provides for use of electrical or electronic filter means for simulating bone in an ultrasonic assessment apparatus. Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates diagrammatically a known ultrasonic assessment apparatus; Figure 2 shows a simulation device according to an embodiment of the invention; Figure 3 shows a simulation device according to a second embodiment of the invention; Figure 4 illustrates the frequency responses of two low-pass filters simulating high and low BUA values; Figure 5 shows a circuit diagram of an analogue simulation device according to the invention; Figure 6 shows different plots of attenuation as a function of frequency derived using the simulation device of Figure 5, and Figure 7 shows a schematic diagram of a digital simulation device according to the invention. Referring to Figure 2, the simulation device comprises a low-pass filter 20 connected to the output of the ultrasonic receiving transducer 2 of the ultrasonic assessment apparatus (generally indicated 25). The transmitted ultrasonic signal 21 propagates through e.g. water 8 contained in vessel 3 of the apparatus and the received signal 22 is directly received by the receiving transducer 2. The low-pass filter 20 attenuates the output signal from the receiving transducer 2 (corresponding to signal 22) in order to simulate the attenuating effect of bone on the ultrasonic signal 23 transmitted by the transmitting transducer 1. The signal is then processed by analyser 11. In the embodiment shown in Figure 3, the simulation device 30 (shown schematically) is placed within vessel 3 between the transmitting and receiving transducers 1, 2 of the ultrasonic assessment apparatus. The device 30 comprises secondary ultrasonic transmitting and receiving transducers 31, 32 and circuitry 33 interconnecting the secondary transducers. Circuitry 33 includes a filter (e.g. low-pass) and may also include an amplifier and/or delay circuitry. The secondary transducers may contact the respective primary transducers (as shown) or alternatively may be coupled to the transducers using an intermediary material, or even a fluid such as water (as in figure 2). The secondary receiving transducer 32 receives an ultrasonic signal 35 transmitted by transducer 1, and the low-pass filter in circuitry 33 attenuates the output signal from the secondary receiving transducer 32. The attenuated signal 36 is then transmitted by the secondary transmitting transducer 31 and is received by the receiving transducer 2 of the assessment apparatus. An amplifier, typically having a gain of about 50 dB, may be provided to compensate for transfer attenuation caused by the use of the secondary transducers 31, 32. As in the case of the simulation device described with reference to Figure 2, the low-pass filter in circuitry 33 is effective to simulate the attenuating effect of bone on the ultrasonic signal transmitted by the transmitting transducer 1. To this end, the frequency responses of the low-pass filters 20, 33 are tailored to simulate a desired attenuating effect. Curve (a) in Figure 4 illustrates the frequency response of a low-pass filter suitable for simulating bone having a relatively low attenuating effect. In this case, the slope in the roll-off region of the frequency response may typically be in the region of 20dBMHz -1 , simulating a low BUA value (i.e. 20). By way of contrast, curve (c) in Figure 4 illustrates the frequency response of a low-pass filter suitable for simulating bone having a relatively high attenuating effect. In this case, the slope in the roll-off region of the frequency response may typically be in the region of 100dBMHz -1 , simulating a high BUA value (i.e. 100). Curves (a) and (c) correspond to the lines (a) and (c) respectively in the graph of figure 6. In general, the design of the electrical filter will be optimised to give a required frequency response commensurate with the attenuating effect of a chosen bone or bone condition. Figure 5 shows, by way of example, a simulation device comprising three, serially-connected, filter stages 40, 41, 42. The first filter stage 40 is connected to the device input (I/P) via a buffer stage 43 which keeps the source impedance to the filter stages low - at about 4.5 KΩ in this embodiment. Filter stages 40 and 41 are second-order filter stages, whereas filter stage 42 is a single order stage. Each stage could be of any order required. In this embodiment, the filter stages are designed so that each order has a slope in the roll-off region of the frequency response of about 20dBMHz -1 , giving an overall slope of about 100dBMHz -1 . It will be appreciated that the simulation device could have fewer than or more than three filter stages, and fewer than or more than five orders. In order to minimize passband ripple, the Q of each filter stage is set at about 0.7, giving a gradual roll-off extending well into the passband. In order to simulate the broadband ultrasonic attenuation of bone, the turnover frequency of each filter stage is preferably set at about 0.2MHz, the lowest frequency at which bone begins to have a significant attenuating effect on ultrasound. In this particular embodiment, the actual turnover frequencies of the first and second order filter stages are 0.185 MHZ and 0.183 MHZ respectively, so that, like bone, the level of attenuation will be about 10dB at 0.2MHz. In this embodiment, the filter has an adjustable control 44 for selectively connecting a desired number of the stages between the device input (I/P) and the device output (O/P). Thus, if only the first stage 10 is selected (giving two orders) the resultant slope in the roll-off region of the frequency response will be about 40dBMHz -1 , if the first and second stages 40, 41 are selected (giving four orders) the resultant slope will be about 80dBMHz -1 , whereas if all three stages 40, 41, 42 are selected (giving five orders) the resultant slope will be about 100dBMZ -1 . Figure 6 shows actual plots of attenuation as a function of frequency in the roll-off region derived using the simulation device described with reference to Figure 5. The terminals of the adjustable control 44 are labelled a, b and c. These terminals correspond to the three lines a, b, and c on the graph of figure 6. In other words if the adjustable control 44 is set to position a then the attenuation response of line a in figure 6 will be achieved. Similarly for terminals b and c. A simulation device incorporating an adjustable filter of the kind described is beneficial in that a single device has the capability to simulate the different attenuating effects produced by a range of different bone conditions - hitherto, it has been necessary to simulate different bone conditions using different phantoms. The velocity of ultrasound through bone is typically in the range from 1475 to 1675 msec -1 . However, when the simulation device is in use the propagation speed will be much higher. A delay line 45 may be included in order to compensate for this difference. In this way, the assessment apparatus will respond to a simulated input signal as though it were a genuine signal. The propagation speed through bone is dependent upon bone condition. Accordingly, the simulation device may have an adjustable delay line giving the device the capability to simulate a range of different velocities of ultrasound in bone as well as a range of different attenuation levels. Figure 7 shows a schematic diagram of a digital implementation of a simulation device according to the present invention. An analogue implementation (such as that shown in Figure 5) may lead to problems with one or more of: flexibility, size, accuracy or available delay. By implementing the simulation device using digital components some or all of these problems may be eliminated. In the embodiment of Figure 7 the digital electronic phantom consists of a receiving transducer (A) which transmits a received signal to an analogue to digital converter (ADC) (70). The ADC is connected to a digital delay circuit (72) which is usable to slow the signal down to the speed associated with an ultrasound signal. The signal is then passed to a digital low pass filter (74) which simulates the operation of the ultrasound signal passing through a bone. The resulting signal is then passed through a digital analogue converter (DAC) (76) and amplified using amplifier (78). Finally, the signal is output using receiving transducer B. The digital embodiment of Figure 7 corresponds broadly to the analogue embodiment of Figure 3, in that the phantom is located between the transducers of the ultrasonic assessment apparatus and includes a pair of secondary transducers. Alternatively, a digital embodiment may be devised which corresponds to the analogue embodiment of Figure 2 i.e. where the necessary circuitry is located after the receiving transducer of the ultrasonic assessment apparatus. The low pass filter may be implemented using an appropriate digital software program and the entire device (including the ADC, low pass filter, delay and DAC) may be implemented using a microprocessor. Preferably the ultrasonic assessment apparatus should provide the following characteristics: Ultrasonic signal of approximately 4 µsec is produced by a voltage pulse of 600V with a width of 1 µsec. Measurements are to be recorded over a frequency range of 0.2 to 0.6 MHZ. Simulate attenuation characteristics of bone ranging from 20 to 120 dBMHz -1 . The cut off frequency is to be assumed to be 0.18MHz. The phantom produces a constant attenuation of 10 dB at 0.2 MHZ. A delay of 25 µsec is implemented to simulate the propagation of ultrasound through bone. The phantom provides reproducibility and precision. An appropriate filter of any type may be designed, but in one embodiment a third order digital Butterworth low pass filter was designed with a cut off frequency of 0.18 MHZ. It has a third order real zero at z = -1 a first order real pole with a radius 0.22353 and angle 0° and a second order complex conjugate pair with radius 0.61402 and angle ±61.48°. This gave a recurrence formula of: y[n]= 1.22804cos(61 48°)y[n-1]-(0.61402) 2 y[n-2]+v[n]+2v[n-1]+v[n-2] where, v(n)=0.22353v[n-1]+x[n]+x[n-1] The above embodiments of the present invention have been described by way of example only and various alternative features or modifications from what has been specifically described and illustrated can be made within the scope of the invention, as will be readily apparent to a person skilled in the art.

Claims: An ultrasonic assessment apparatus including ultrasonic transmitting (1) and receiving (2) transducers between which in normal use a patient's body part (12) to be assessed may be located, the apparatus further including a simulation device (20, 33) which is selectively introducible in the absence of a patient's body part into the operation of the apparatus in order to simulate the effect of bone in the ultrasonic assessment apparatus and which is removable from the operation of the apparatus when the apparatus is in normal use, characterised in that the simulation device (20, 33) includes electrical filter means for simulating the attenuating effect of bone on an ultrasonic signal transmitted by the transmitting transducer (1). Apparatus according to claim 1 wherein in use an ultrasonic signal produced by the transmitting transducer (1), or an electrical signal indicative of or relating to the ultrasonic signal, is passed through the electrical filter means such that the signal's character, properties or form are altered by the electrical filter means in a similar or identical way to the way in which they would have been altered had the signal passed through a particular bone or bone structure. Apparatus according to claim 1 or claim 2 wherein the filter means is a low-pass filter. Apparatus according to claim 3 wherein the filter means has a fixed frequency response having a predetermined slope in the roll-off region of the frequency response to simulate the attenuating effect of a particular bone condition. Apparatus according to claim 3 wherein the filter means includes adjustment means (44) for adjusting the characteristics of the filter means such that the filter means is operable to simulate different attenuation effects of a range of different bones and/or bone conditions. Apparatus according to claim 5 wherein the electrical filter means is a low-pass filter and the adjustment means is usable to adjust the slope in the roll-off region of the frequency response of the filter means. Apparatus according to any one of the above claims including means (72) for delaying a received signal. Apparatus according to claim 7 wherein the delay means is an electrical delay line which is operable to provide an adjustable delay in order to simulate different propagation velocities of an ultrasonic signal through bone. Apparatus according to any one of the above claims wherein the simulation device is connectable to the output of the ultrasonic receiving transducer (2). Apparatus according to any one of claims 1 to 8 wherein the simulation device (20, 33) is locatable between the primary transmitting and the primary receiving transducer of the ultrasonic assessment apparatus in the position normally occupied by a bone under test, and the simulation device (20, 33) includes secondary ultrasonic receiving (32) and transmitting (31) transducers for respectively receiving an ultrasonic signal from and transmitting an ultrasonic signal to the transmitting (1) and receiving (2) transducers respectively, and the electrical filter means is connected between the secondary receiving (32) and transmitting (31) transducers of the simulation device (20, 33). Apparatus according to claim 10 wherein the secondary transducers (31, 32) are removable to allow different secondary transducers to be used. Apparatus according to any one of the above claims including an analyzer (11) for processing the signal of an ultrasonic pulse transmitted from the transmitting transducer (1). A method for calibrating an ultrasonic assessment apparatus, the apparatus including ultrasonic transmitting (1) and receiving (2) transducers between which in normal use a patient's body part (12) to be assessed may be located, and a simulation device (20, 33) including electrical filter means to simulate the attenuating effect of bone on an ultrasonic signal transmitted by the transmitting transducer (1), the method including the steps of (i) introducing the simulation device (20, 33) which is selectively introducible in the absence of a patient's body part into the operation of the apparatus in order to simulate the effect of bone in the ultrasonic assessment apparatus, and (ii) removing the simulation device from the operation of the apparatus when the apparatus is in normal use.

Inventors: LANGTON CHRISTIAN MCDONALD, LANGTON, CHRISTIAN MCDONALD
Applicants: MCCUE PLC
Application year: 1996
Main classification: A61B8
IPC codes: A61B8, G01N29