Ultrasonic sensor with starved dilatational modes

Abstract: A method and apparatus to improve the performance of an ultrasonic imaging sensor [11] is disclosed. The key to the performance improvement obtained in the present invention is in the matching of sensor element [10] size to the electrical characteristics of the train of pulses [24,32] used to drive the sensor. The matching causes essentially all the energy provided to the sensor [11] to go into the desired sensor resonances, those in the direction of the sensor's thickness dimension [12]. The matching also minimizes the energy which goes into the undesired dilatational resonance modes [16,18] those in the sensor element's width dimension [14]. The invention discloses the matching of each sensor element [10] so that its maximum dilatational response [54] is essentially at the same frequency as every other element in the array, so that dilatational response is substantially reduced for the array as a whole. The invention also discloses the "fine tuning" of the frequency of the drive pulses, so that the dilatational response is further reduced. The invention therefore permits the fabrication of ultrasonic sensor arrays [11], having high sensitivity, freedom from spurious returns, and low noise signals.

Description: The present invention relates to a method of reducing a dilatational response of an ultrasonic sensor according to the preamble of claim 1 and to an ultrasonic sensor in which dilatational modes are minimized, respectively. The method and the apparatus according to the present invention are to improve the performance and efficiency of ultrasonic sensors which are used for such important tasks as non-invasive medical imaging and non-destructive industrial testing, such as checking the safety of nuclear power plants. Ultrasonic imaging sensors act as both transmitters and receivers of ultrasonic energy. The sensor first acts as a transmitter; emitting ultrasonic energy in a train of high frequency pulses, typically in the range of 2 to 10 Mhz. Then the transmitter is turned off and the sensor acts as a receiver, which listens for returned echoes at the transmitted frequency. A high performance ultrasonic sensor must be sensitive, accurate and have a low level of spurious acoustic responses included noise when excited by a short drive pulse. An acoustic pulse obtained from a short drive pulse gives good axial resolution, but it has a very broad frequency spectrum. The broad frequency spectrum excites spurious acoustic resonances called modes within the sensor. These modes tend to degrade its frequency response and consequently its ability to accurately differentiate closely spaced targets or impedance discontinuities when imaging parts of the human body. A system consisting of an imaging sensor and the associated electronic circuitry must be capable of transmitting the broadest range of frequencies by eliminating or suppressing the spurious modes, so as to enhance its sensitivity to detect the desired targets. Existing ultrasonic sensors are designed to have a particular resonant frequency, which is the frequency at which desired mechanical motion is maximized. At that resonant frequency, the sensor elements are intended to vibrate along a preferred direction fo sound propagation. However, driving the sensor at the desired resonant frequency will cause some of the energy to coupled orthogonally to the desired motion. Such orthogonal motions represent sources of spurious modes in an imaging transducer. For example, if the sensor is in the form of a flat plate, the desired motion, or resonance, is in its thickness dimension. Undesired motions, called dilatational resonance modes, or dilatational modes, occur along the length and width of the plate. The frequency of the modes is inversely proportional to these dimensions. Consequently, if the width is close to the thickness dimension, the spurious dilatational mode will fall close to the pass band of the thickness mode. This is shown in Figure 2 for the element shown in Figure 1. Depending on whether the width is greater than, or less than, the thickness, the spurious modes will fall below or above the main resonance. The length dimension is usually much greater than the width, so that the spurious length mode is of very low frequency. Both situations exist in medical sensors. In an annular array sensor, whose aspect ratio is less than unity, the spurious modes are below the desired response, as shown in Figures 2 through 5. In linear arrays, where the aspect ratio is greater than unity, the spurious modes are above the desired response. In either case, these modes will lengthen the acoustic pulse transmitted into the body, and degrade the axial resolution. Accurate axial resolution translates into an ability to see fine details of tissue structure, and provides the physician a powerful diagnostic tool. A dilatational mode along the width is shown in Figure 1. These undesired motions cause energy to be radiated into, and received from, directions other than that intended. The result is that returns are received from undesired directions, and these unwanted returns constitute noise. This unwanted noise is mixed in with the desired signals, thereby degrading the received signals. The US-A-4,240,003 describes an acoustic imaging transducer comprising an acoustic absorbing backing and a plurality of piezoelectric elements affixed to a surface of the backing for radiating and receiving acoustic waves. Spurious emissions caused by a vibration mode in the acoustic imaging transducer are suppressed by cancelling the net displacement of the center of mass of each piezoelectric element in the transducer array. From the US-A-4,406,967 there is known an ultrasonic probe which contains a piezoelectric element provided at the both sides with a thin layer electrode which is disposed such that at least one of the electrodes is divided into long and small electrode portions to form a unit element. This ultrasonic probe is so constructed that each of the unit elements undergoes no adverse influence from the adjacent unit elements so that it permits a high resolving power without any noise acoustic waves generated. A further prior art document, EP-A-219 919, describes an ultrasonic sensor comprising an array of piezoelectric transducers having a thickness dimension and the whidth dimension, the ratio of said whidth dimension to said thickness dimension being such that a dilatational response occurs at a specific frequency, and comprising an emitter and a receiver for a train of drive pulses. The development of an effective method of suppressing the undesirable dilatational modes would constitute a major technological advance in the technology of ultrasonic imaging. The improved performance that would result from such an innovation would substantially improve the performance of ultrasonic imaging equipment used for medical imaging and for important industrial applications. Accordingly, the present invention provides a method of reducing a dilatational response of an ultrasonic sensor comprising the features of claim 1 and an ultrasonic sensor comprising the features of claim 8, respectively. SUMMARY OF THE INVENTION The Ultrasonic Sensor with Starved Dilatational Modes disclosed and claimed in this patent application overcomes the problem of undesired dilatational modes. The keys to the success of this invention are: (a) designing each of the sensor elements in the array to have a particular specified ratio between their thickness and their width, the sensor's "aspect ratio", and (b) using a pulse train having a precise number of pulses to drive the ultrasonic sensor. The number of pulses in the pulse train is chosen to match the sensor's aspect ratio. When the optimum number of pulses is used in the pulse train, very little energy is available at the dilatational modes' frequencies, and the dilatational modes of the sensor are significantly reduced. The energy can be further "fine tuned" by adjusted by small variations in the frequency of the pulses. The Applicant's Ultrasonic Sensor with Starved Dilatational Modes provides much better imaging performance than existing ultrasonic sensors. This innovative method and apparatus provides a powerful tool that will enable medical personnel, industrial technicians, and other users of ultrasonic sensors to obtain detailed, noise-free ultrasonic imagery with ease and convenience. An appreciation of other aims and objectives of the present invention and a more complete and comprehensive understanding of this invention may be achieved by studying the following description of a preferred embodiment and by referring to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross sectional view of an individual sensor element [10] which is part of an annular array sensor [11]. Figures 2 through 5 show how the electrical input impedance of a typical individual sensor element varies as a function of frequency. Figure 2 refers to a sensor element having an aspect ratio (thickness dimension divided by width dimension) of 0.90. Figures 3, 4, and 5 have aspect ratios of 0.80, 0.60, and 0.40, respectively. Figure 6 shows the frequency content of a single-pulse train, and Figure 7 shows the receiver response of an annular array sensor, when such a single pulse train is transmitted, and received from a target located in a water tank. Figures 8 and 9 show the same information as Figures 6 and 7, respectively, for a two-pulse train. Similarly Figures 10 and 11 show this information for a three pulse train, and Figures 12 and 13 show the same information for a four-pulse train. Figure 14 shows the frequency of maximum dilatational response for each of the individual elements of an existing 12 element annular array in which no particular effort has been made to optimize the dilatational response frequencies. In this case, the resonant frequency of each element is somewhat different from that of each other element. Figure 15 shows how the frequency of dilatational response could be optimized, so that the frequency of maximum dilatational response would be essentially identical for each of the elements in the annular array. A similar optimization could be performed for the elements of a linear array. Figures 16 and 17 show the frequency content of pulse trains containing 2.5 and 3.5 pulses, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENT As indicated in Figure 1, the cross section through an individual element [10] of an ultrasonic annular array [11] is generally rectangular in shape, having a thickness dimension [12] which is less than its width dimension [14]. It is desired that ultrasonic waves generated in the transducer radiate in the direction of the thickness dimension [12], i.e. normal to the width dimension, [14], and that no energy be radiated in the direction of the width dimension [14]. Energy radiated in the direction of the width dimension [14] is due to dilatational modes. The present invention reduces the energy radiated in the width dimension [14] by a combination of design techniques. First of all, each element [10] is proportioned so that it will have a well defined resonance in the thickness dimension [12] and a well defined resonance in the width dimension [14], The resonance in the the thickness dimension [12] is the the desired frequency, and the resonance in the width dimension [14] is the dilatational frequency, f dil . Figure 2 indicates how the f dil [16] is close to the desired frequency [17] when the aspect ratio is 0.90, i.e. when the width is only slightly greater than the thickness. Figure 3 indicates how, for an element having an aspect ratio of 0.80, the f dil [16] moves away from the desired frequency [17]. Figure 4 indicates how the f dil [16] moves still further away from the desired frequency [17] as the aspect ratio decreases to 0.60. Figure 5 shows how the f dil [16] moves even further from the desired frequency [17] for an aspect ratio of 0.40, i.e. when the width dimension [12] is 2.5 times the thickness dimension [12]. Secondly, each of the elements [10] is proportioned so that they all have essentially the same dilatational frequency f dil . Figure 14 indicates that, for an existing 12 element annular array, the dilatational frequencies [50] of each element [10] are only slightly different from each other, even though no attempt has been made to adjust the frequencies to be the same. In Figure 14, it can be seen that the range of f dil [52] is from about 1.18 Mhz, at the center of the array [11] to about 2.18 Mhz at the outer edge of the annular array [11], with most of the element's [10] f dil . being clustered at about 1.6 Mhz. It has been shown experimentally that the f dil of any element can be adjusted to fall at any desired frequency in this range by relatively small adjustments to its dimensions. For example, decreasing the width of a ring shaped element [10] having an f dil of 1.5 Mhz by 10 thousandths of an inch will increase its f dil to 1.9 Mhz. Figure 15 illustrates what would happen if each of the individual elements [10] were individually designed with a carefully adjusted aspect ratio. In this case, f dil [54] of each individual element [10] would be essentially the same. Thirdly, the drive pulses are transmitted as well-defined pulse trains having a specific number of pulses. If necessary, the frequency of the drive pulses will be "fine tuned" to achieve the deepest possible null at the frequency of dilatational resonance. In what follows, pairs of Figures are shown, in which the first Figure illustrates the frequency spectrum of the transmitted pulse train, and the second Figure illustrates the frequency spectrum of the signal received back when that pulse train is transmitted into a standard ultrasonic target. Figure 6 illustrates the transmitted frequency spectrum [26] of a single pulse wave train [24]. Figure 7 shows the dilatational return in the received spectrum [26] below about 2.4 Mhz; the null [28] between the dilatational response and the desired response is not distinct. Figure 8 illustrates the transmitted frequency spectrum [30] of a two pulse wave train [32]. Figure 9 shows that the dilatational response is separated by a deeper null [34] in the received spectrum [26], so that the dilatational response is more distinct. Figure 10 illustrates the frequency spectrum [35] of a three pulse wave train [36]. Figure 9 shows that there are two distinct dilatational responses separated by two nulls [38,40], so that the dilatational response is still further from, and more distinct from, the desired frequency response. Finally, Figure 12 shows the transmitted spectrum [41] of a four pulse wave train [42]. Figure 13 indicates that there are 3 frequency regions in which the dilatational response is received, separated from the desired response by three nulls [44,46,48]. There may also be a fractional number of pulses, as shown in Figures 16 and 17, which show the frequency spectra corresponding to 2.5 [56] and 3.5 [58] pulses, respectively. The whole or fractional number of transmitted pulses [24,32,36,42,56,58] is chosen so that the frequency spectrum of the pulse train has a null at the same frequency as the f dil of the sensor elements. Thus when the array is driven by a train of pulses having the correct number of pulses, essentially all the energy is transmitted at the primary, desired, resonance mode, and essentially no energy is transmitted at the dilatational modes of the sensor. The result is suppression of the undesired dilatational modes.

Claims: A method of reducing a dilatational response of an ultrasonic sensor, comprising a. fabricating at least one element (10) of said sensor (11) with a thickness dimension (12), and a width dimension (14); the ratio of said width dimension (14) to said thickness dimension (12) being such that a dilatational response of said thickness dimension (12) occurs at a specific frequency f dil ; and b. driving at least one element (10) of said ultrasonic sensor (11) with a train of drive pulses; characterized in that the number of pulses in said train is selected so that a frequency spectrum of said train has a null at said frequency f dil . A method of reducing a dilatational response of an ultrasonic sensor [11], as in Claim 1, wherein said element [10] of said ultrasonic sensor [11] is an annular ring. A method of reducing a dilatational response of an ultrasonic sensor [11], as in Claim 1 or 2, wherein said ultrasonic sensor [11] is an annular array sensor. A method of reducing a dilatational response of an ultrasonic sensor [11] as in Claim 3, wherein each ring shaped element [10] of said sensor [11] is fabricated so that said dilatational frequency f dil of each element [10] is substantially identical. A method of reducing a dilatational response of an ultrasonic sensor [11], as in Claim 1, wherein said element [10] of said ultrasonic sensor [11] is a linear rectangular element. A method of reducing a dilatational response of an ultrasonic sensor [11], as in Claim 5, wherein said ultrasonic sensor [11] is a linear array sensor. A method of reducing a dilatational response of an ultrasonic sensor [11], as in Claim 6, wherein each ring shaped element [10] of said sensor [11] is fabricated so that said dilatational frequency f dil of each element [10] is substantially identical. An ultrasonic sensor [11] in which dilatational modes are minimized, characterized by a. a train of drive pulses having at least one selected variable null frequency [16].; and b. at least one element [10] in said ultrasonic sensor [11], the ratio of whose width dimension [14] to thickness dimension [12] is selected so that its frequency of maximum dilatational response, f dil , of said thickness dimension [12] will correspond closely to said null frequency [17]. An ultrasonic sensor [11] as in Claim 8 in which said element [10] of said ultrasonic sensor [11] is a circular ring. An ultrasonic sensor [11] as in Claim 9 in which said ultrasonic sensor [11] is an annular array sensor. An ultrasonic sensor as in one of Claims 8 to 10, characterized in that said element (10) of said ultrasonic sensor (11) is a linear rectangular element. An ultrasonic sensor as in Claim 11, characterized in that said ultrasonic sensor (11) is a linear array sensor. An ultrasonic sensor as in one of Claims 8 to 12, characterized in that said dilatational response f dil is substantially identical (54) for each of said elements (10).

Inventors: DIAS J FLEMING, DIAS, J.FLEMING
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1990
Main classification: G10K11
IPC codes: B06B1