Ultrasonic signal focusing method and apparatus for ultrasonic imaging system

Abstract: An ultrasonic signal focusing method and apparatus is provided, in which an ultrasonic signal reflected from an object is focused at all points from and to which transmission and reception are performed on a scan line. The ultrasonic signal focusing method includes an ultrasonic signal transmission step for moving, focusing and transmission-scanning the transmission scan line in turn, using an array transducer, an ultrasonic signal reception step for receiving all the ultrasonic signals transmitted along the scan lines at the transducer elements which are placed in the location at which reception is enabled, an ultrasonic storing step for storing all the received ultrasonic signals by each scan line, and an ultrasonic focusing step for interpolating an arrival delay amount of the ultrasonic signals stored after arrival at a different point in time and summing the interpolated signal. Thus, all the signals obtained by performing transmission focusing on a number of adjacent scan lines are used to thereby perform reception focusing, to thereby provide the same effect as the case where focal points exists in all the points on the scan line.

Description: The present invention relates to an ultrasonic signal focusing method and apparatus for an ultrasonic imaging system, and more particularly, to an ultrasonic signal focusing method and apparatus for transmission-focusing and/or receipt-focusing of an ultrasonic signal emitted toward an object. In general, an ultrasonic imaging system uses an ultrasonic wave to show the internal sectional structure of an object such as a human body. The ultrasonic imaging system includes an ultrasonic signal focusing device and a imaging device. The ultrasonic signal focusing device emits an ultrasonic signal to an object, and converts the ultrasonic signal reflected from a discontinuous surface of acoustic impedance of the object, into an electrical signal. The imaging device uses the electrical signal to show the internal structure of the object. Examples of ultrasonic imaging systems are found in EP-A-0 293 773 and EP-A-0 56632. In the ultrasonic imaging system, one of crucial factors required for functional improvement is an ultrasonic image resolution. It is steadily under development to improve the resolution. In case of the ultrasonic image, lateral resolution is worse than axial direction resolution. Focusing plays a major role of determining the lateral resolution. To improve the lateral resolution, the ultrasonic imaging system uses, as a probe, an array transducer comprised of a number of transducer elements, and performs transmission focusing and receipt focusing through an electrical signal processing. During performing receipt focusing, it is possible to use dynamic focusing in which the position of a focal point is successively moved from an position close to the array transducer to that far from it, considering the travelling speed of the ultrasonic wave. The dynamic focusing provides a better lateral resolution than the case where the focal point is fixed. A general transmission focusing will be described below with reference to Fig. 1. In Fig. 1, the x-axis is parallel with the ultrasonic transmission plane of an array transducer 10, and the z-axis is perpendicular to the ultrasonic transmission plane thereof. For clarity, a point where a scan line intersects the surface of a particular transducer element in the array transducer 10 is determined as the origin (0, 0), and the transducer element located at the origin is called a "transducer element 0." Here, the scan line connects the center of the ultrasonic transmission plane of the "transducer element 0" with a transmission focal point F in the case where a steering angle is . To make ultrasonic pulses emitted from all transducer elements reach the transmission focal point F at the same time, a transducer element farther from the origin should emit an ultrasonic pulse earlier than that closer to the origin do. For instance, a "transducer element 3" should transmit an ultrasonic pulse earlier by ℓ 3/v than the "transducer element 0" do. Here, "v" is the velocity of the ultrasonic pulse, "r" is the distance from the origin to the focal point F, and "ℓ" is the distance difference between the distance r and the distance r+ℓ from the ultrasonic transmission plane of the transducer element placed at the location which is not the origin to the focal point F. Thus, when the steering angle  by which each of the scan lines indicated as solid lines rotated counterclockwise from the z-axis is used, a transmission delay time t d (x 1 ) for a delay unit corresponding to a "transducer element 1" having a position x 1 is obtained by the following equation (1). t d (x 1 ) = - r 2 + x 2 1 + 2rx 1 sin - r v It is not possible to assign a negative transmission delay time value to the delay units. Therefore, in reality, it is required that all transmission delay time values become positive number by adding a positive value to the transmission delay time values corresponding to all transducer elements. However, for convenience of explanation, it is assumed that it is possible to assign a negative transmission delay time value to the delay unit. Also, an instantaneous time when a "transducer element 0" transmits an ultrasonic pulse is defined as "0." Then, by replacing the r in the equation (1) by vt/2, transmission delay time or reception delay time corresponding to a transducer element having the center position x is expressed as the following equation (2). t d (t,x) = - (vt/2) 2 + x 2 + vtxsin v + t 2 Here, "t" is time taken when the ultrasonic pulse reciprocates from a transducer element to the focal point F. When a transmission delay time of the equation (2) is used for each electrical pulse generated in a pulse generator (not shown), the ultrasonic pulses transmitted toward the focal point F by the transducer elements form ripples as shown in Fig. 2. In Fig. 2, each ripple results from the fact that the transducer elements delay one pulse outgoing from the pulse generator by different transmission delay times and transmit the delayed result. During transmission focusing, all ultrasonic pulses reach the focal point F at the same time, and all the ultrasonic pulses having reached the focal point F have the same phase. Thus, an amplitude or intensity of the ultrasonic wave becomes maximized at the focal point F. However, the ultrasonic pulses do not reach the location A or B different from the focal point F at the same time and have a different phase from each other. As a result, the ultrasonic pulses offset each other destructively and the intensity of the ultrasonic wave in the location A or B becomes smaller compared to the focal point F. In this case, as it is distant further toward the lateral direction than the axial direction, the intensity of the ultrasonic pulse becomes much smaller. Resolution is determined as a combinational result of the transmission/reception focusing as described above. Since the focal point should be fixed during transmission focusing, the receipt focusing is ideal. Even in the case of the receipt focusing, the lateral resolution is best in the vicinity of the focal point and becomes comparatively poor in the different positions thereof. The present invention seeks to provide an improved method of obtaining a focused scan line. According to the present invention there is provided a method of obtaining a focused scan line in an ultrasonic imaging system comprising a transducer array which comprises a plurality of transducer elements configured to transmit and receive ultrasonic signals, the method comprising generating a first scan line by transmitting a first set of ultrasonic signals, receiving some or all of the first set of signals reflected from points along the first scan line and generating a second different scan line by transmitting a different second set of ultrasonic signals, receiving some or all of the second set of signals reflected from the points along the first scan line and obtaining a combined signal for each of the points along the first scan line from said reflected signals of the first and second sets. The transmitting of the first set of ultrasonic signals may comprise a first set of transducer elements each emitting an ultrasonic pulse. The transmitting of the second set of ultrasonic signals may comprise a second set of transducer elements each emitting an ultrasonic pulse. At least one transducer element may be common to both sets of transducer elements. The generation of the first and second scan lines may occur in turn. The method may further comprise storing a one of said received signals of said first set of signals at a first predetermined transducer element which has been reflected from one of said points and storing a one of said received signal of said second set of signals at the first predetermined transducer element which has been reflected from said one of said points. The method may further comprise assigning a delay time to said one of said received signals of said second set of signals and adding said delayed signal to said one of said received signals of said first set of signals. The method may further comprise storing received signals received at first and second predetermined transducer elements which have been reflected from one of said points along the first scan line. The method may further comprise calculating signal arrival time delay for said first predetermined transducer element and generating interpolated data in dependence upon said time delay. The calculating of the signal arrival delay time may comprise determining a delay time between receiving a received signal consisting of one of said second set of transmitted signals at said first predetermined transducer element and receiving a reference signal consisting of one of said first set of transmitted signals at said second predetermined transducer element. The calculating of the signal arrival delay time data may comprise determining the signal delay time using: wherein t is the time taken for a signal to return from said one point along the first scan line, X f is a displacement between the second transducer element through which the first scan line passes and a third transducer element through which the second scan line passes, Z f is the distance between the third transducer element and a focal point on said second scan line, X e is a displacement between the first and second transducer elements and v is the speed of ultrasonic signals and adding the delay time to the received signal so as to produce an added signal for reinforcing the reference signal. The method may further comprise generating a further scan line by transmitting a further set of ultrasonic signals, receiving at said first predetermined transducer element a further received signal consisting of one of a further set of transmitted signals reflect from said one point along said first scan line, calculating a further signal arrival delay time data by determining a delay time between receiving the further received signal and the reference signal and adding the further delay time to the further received signal so as to produce a further added signal for reinforcing the reference signal. The method may further comprise summing the added signal and the further added signal to produce a summed signal for the first predetermined transducer element. The method may further comprise determining a focused signal S focused in dependence upon the summed signal for the first predetermined transducer element. The method may further comprise calculating a focused signal S focused comprising determining the focused signal S focused using: wherein A(t, X e , X f ) is an apodization factor. According to the present invention there is also provided apparatus for focusing an ultrasonic signal comprising a transducer array which comprises a plurality of transducer elements configured to transmit and receive ultrasonic signals, means for generating a first scan line by transmitting a first set of ultrasonic signals, means for receiving some or all of the first set of signals reflected from points along the first scan line, means for generating a second different scan line by transmitting a second set of ultrasonic signals, means for receiving some or all of the second set of signals reflected from the points along the first scan line and means for obtaining a combined signal for each of the points along the first scan line from said reflected signals. The means for generating the first scan line may comprise a first set of transducer elements each emitting an ultrasonic pulse. The means for generating the second scan line may comprise a second set of transducer elements each emitting an ultrasonic pulse. At least one transducer element may be common to both sets of transducer elements. The first and second scan lines may be generated in turn. The apparatus may further comprise means for storing a one of said received signals of said first set of signals at a first predetermined transducer element which has been reflected from one of said points and means for storing a one of said received signal of said second set of signals at the first predetermined transducer element which has been reflected from said one of said points. The apparatus may further comprise means for assigning a delay time to said one of said received signals of said second set of signals and means for adding said delayed signal to said one of said received signals of said first set of signals. The apparatus may further comprise means for storing received signals received at first and second predetermined transducer elements which have been reflected from one of said points along the first scan line. The apparatus may further comprise means for calculating signal arrival time delay for said first predetermined transducer element and means for generating interpolated data in dependence upon said time delay. The means for calculating the signal arrival delay time may comprise means for determining a delay time between receiving a received signal consisting of one of said second set of transmitted signals at said first predetermined transducer element and receiving a reference signal consisting of one of said first set of transmitted signals at said second predetermined transducer element. The means for calculating the signal arrival delay time data may comprise means for determining the signal delay time using: wherein t is the time taken for a signal to return from said one point along the first scan line, X f is a displacement between the second transducer element through which the first scan line passes and a third transducer element through which the second scan line passes, Z f is the distance between the third transducer element and a focal point on said second scan line, X e is a displacement between the first and second transducer elements and v is the speed of ultrasonic signals and means for adding the delay time to the received signal so as to produce an added signal for reinforcing the reference signal. The apparatus may further comprise means for generating a further scan line by transmitting a further set of ultrasonic signals, means for receiving at said first predetermined transducer element a further received signal consisting of one of a further set of transmitted signals reflect from said one point along said first scan line, means for calculating a further signal arrival delay time data by determining a delay time between receiving the further received signal and the reference signal and means for adding the further delay time to the further received signal so as to produce a further added signal for reinforcing the reference signal. The apparatus may further comprise means for summing the added signal and the further added signal to produce a summed signal for the first predetermined transducer element. The apparatus may further comprise means for determining a focused signal S focused in dependence upon the summed signal for the first predetermined transducer element. The apparatus may comprise means for calculating a focused signal S focused comprising means for determining the focused signal S focused using: wherein A(t, X e , X f ) is an apodization factor. The apparatus may comprise receiver means comprising a plurality of amplifier means, each amplifier means arranged for amplifying a plurality of ultrasonic signals received at a respective transducer element and converting the amplified signals into digital signals, a plurality of intra-channel processor means each for receiving and storing the digital signals, for calculating a respective time delay for each digital signal and for applying said time delays to each stored digital signal so as to reinforce said plurality of ultrasonic signals received at the respective transducer element and reception focusing means for summing outputs of the plurality of infra-channel processing means so as to produced a focused signal. The reception focusing means may comprise apodization calculator means for calculating an apozidation value for each intra-channel processing means, multiplier means for multiplying outputs of each infra-channel processor means bythe respective apozidation value so as to produce a plurality of multiplication results and adder means for adding the multiplication results. Each intra-channel processor means may comprise a plurality of buffer memories for storing digital signals relating to a point on a particular scan line for each scan line, write controller means for outputting write control signals to the buffer memories to store digital signals, delay calculator means for calculating delay times for stored signals, read controller means for outputting read control signals to the buffer memories to allow digital signals stored therein to be read, delay interpolator means for interpolating digital signals read from the buffer memories by delaying signals by delay times calculated by the delay calculator means and apodization means for applying weight values to delayed signals output from the delay interpolator means and adding the weighted values. The apodization means may comprise intra-channel apodization calculator means for calculating an apozidation value for each delayed signals output from the delay interpolator means, multiplier means for multiplying outputs the delay interpolator means by the respective apozidation value so as to produce a plurality of intra-channel multiplication results and adder means for adding the intra-channel multiplication results so as to produce a value for a respective scan line. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a conceptual view for explaining the principle of existing art which performs transmission focusing on a focal point F; Fig. 2 shows the state where ultrasonic pulses proceed toward a focal point F in a prior art transmission focusing; Figs. 3 and 5 are conceptual views for explaining ultrasonic signal focusing according to a preferred embodiment of the present invention; Fig. 4 is a view for comparatively explaining the cases where ultrasonic pulses transmitted by a transducer element along scan lines L0 and L2 reach the target point and then return to the transducer element; Fig. 6 shows an ultrasonic signal focusing apparatus according to the present invention; and Fig. 7 shows an intra-channel processing portion in the Fig. 6 apparatus. Fig. 3 is a geometrical view for explaining an ultrasonic signal focusing adopted by the present invention. In Fig. 3, f 0 and f 2 denote focal points, and P1, P1', P2, P3 and P4 denote target points. Also, Z 1 denotes a length of the path until an ultrasonic pulse emitted toward a focal point f 0 along a scan line L0 by a "transducer element 0" reaches a target point P1, Z 2 denotes the length of the path corresponding to a time interval taken until an ultrasonic pulse emitted toward a focal point f 2 along a scan line L2 by a "transducer element 2" reaches the target point P1. Z f denotes the length of the path until an ultrasonic pulse emitted toward a focal point f 2 along a scan line L2 by a "transducer element 2" reaches the focal point f 2 , and X f denotes the distance between the "transducer element 0" and the "transducer element 2." Fig. 4 is a view for explaining ultrasonic pulses transmitted along the scan lines L0 and L2 shown in Fig. 3 and then returning the target point P1. In Fig. 4, the waveform (a) shows the time taken when the ultrasonic pulse transmitted toward the focal point f 0 on the scan line L0 returns to a "transducer element 0" from the target point P1, and the waveform (b) shows the time taken when the ultrasonic pulse transmitted toward the focal point f 2 returns to the "transducer element 0" from the target point P1. A "transducer element -4" through a "transducer element 4" within the section "A" emit the ultrasonic pulses toward the focal point f 0 on the scan line L0 so as to be transmission-focused. A "transducer element -2" through a "transducer element 6" within the section "B" emit the ultrasonic pulses toward the focal point f 2 on the scan line L2 so as to be transmission-focused. The ultrasonic pulse transmitted with respect to the scan line L0 is started from the "transducer element 0" and then reaches the target point P1 after the time taken when travelling the distance Z1 has elapsed. Then, the ultrasonic pulse returns to the "transducer element 0" after the time corresponding to the distance Z1 has elapsed. The ultrasonic pulse transmitted with respect to the scan line L2 is started from the "transducer element 2" and then reaches the target point P1 after the time taken when travelling the distance Z2 has elapsed. Then, the ultrasonic pulse having reached the target point P1 is received at the "transducer element 0" after the time corresponding to the distance Z1 has elapsed. The ultrasonic pulses transmitted toward the focal point f 0 from the "transducer element 0" and the "transducer element 2" reach the focal point f 0 at the same time by transmission focusing. Therefore, the time taken when the ultrasonic pulse transmitted from the "transducer element 0" reaches the point P1 is the same as that when the ultrasonic pulse transmitted from the "transducer element 2" reaches the point P1'. The above relationship is shown in Fig. 3 as a dotted circle having the center of the focal point f 0 . In the case of the two reception ultrasonic pulses received at the "transducer element 0," that is, in the case of the reception ultrasonic pulse obtained from the ultrasonic pulse transmitted toward the transmission focal point f 0 on the scan line L0 and the reception ultrasonic pulse obtained from the ultrasonic pulse transmitted toward the transmission focal point f 2 on the scan line L2, their delay times are adjusted so that the reception ultrasonic pulses reflected from the target point P1 can be added at the same time. By doing so, the signal intensity of the reception ultrasonic pulse which corresponds to the target point P1 is transmitted toward the transmission focal point f 0 and received at the "transducer element 0", is reinforced using the reception ultrasonic pulse which is transmitted toward the transmission focal point f 2 , reflected from the target point P1 and then received at the "transducer element 0." When the signals obtained by performing the transmission focusing with respect to all the scan lines are synthesized, the reception ultrasonic pulses reflected from the final target point are aligned in phase with respect to the envelope and then added thereto. Accordingly, the signal intensity corresponding to the target point becomes larger but the signal intensity corresponding to the remaining points do not become large. Referring to Fig. 3, the delay time necessary for reinforcing the signal intensity will be described below. Considering the "transducer element 0," the "transducer element 2," and the target point P1, the following equation (3) is satisfied. Z 2 = Z f - X 2 f + ( Z f - Z 1 ) 2 Using the relationship of Z 1 =vt/2, the equation (3) can be replaced by the following equation (4). t d ( t , X f ) = ( Z 1 - Z 2 ) v = t 2 - Z f - X 2 f + ( Z f - vt /2) 2 v The equation (4) shows the delay time of the reception ultrasonic pulse received from the target point P1 at the "transducer element 0" with respect to the ultrasonic pulse transmitted toward the focal point f 2 on the scan line L2 from the "transducer element 2." When ultrasonic signals are synthesized after ultrasonic pulses are transmitted and received, there is no need to sum the signals obtained by performing transmission focusing on the adjacent various scan lines at the same ratio therebetween. To improve the lateral resolution, there are various methods, one of which is using weight values. For example, assuming that the weight value of the signal returning from the signal emitted toward the transmission a focal point on the scan line L0 is "1," it is defined that the weight value of the signal returning from the signal emitted toward the transmission focal point on the scan line L2 is "0.7," and the weight value of the signal returning from the signal emitted toward the transmission focal point on the further scan line L3 is "0.5," and then the two weight values are added. This method is called an apodization in a beam forming technique. In this invention, the lateral resolution can be enhanced using apodization appropriately. Untill now, the case where only the "transducer element 0" is received for the scan line L0 has been described. However, all the adjacent transducer elements (e. g. a "transducer element -4" through a "transducer element 4" in Fig. 3) which can be participated in the transmission focusing with respect to the focal point f 0 on the scan line L0 can be contributed in the reinforcement of the signal intensity with respect to all possible target points on the scan line L0. For example, a signal received at the "transducer element 0" after being transmitted along the scan line L0 is defined as a "reference signal". The signal transmitted along the scan line L2 is received by the "transducer element 1" and is then stored. A proper delay time is assigned to the stored signal. The delayed signal is added to the reference signal, in order to reinforce the reference signal obtained from a target point on the scan line L0. It is preferable that the target point may not be fixed, and would be better move on the scan line L0 along with the travelling of the ultrasonic pulse. By doing so, the signals from the object obtained from all the desired target points on the scan line L0 become larger due to constructive or reinforcing interference, but the signals from the other points become smaller due to destructive interference, to thereby improve the resolution. Fig. 5 is a view for explaining the delay time which is used for ultrasonic pulse received at the adjacent "transducer element -1" in order to reinforce the ultrasonic pulse received at the "transducer element 0." For clarity, the transmission scan line L2 passes through the focal point f 2 from the point (X f , 0), and the coordinate of the focal point f 2 is (X f , Z f ). Also, it is defined that a transducer element receiving the ultrasonic pulse is located at the point (X e , 0) and the scan line which can be finally focused is L0 passing through the origin (0,0). Then, to reinforce the ultrasonic pulse received at the "transducer element 0" from the target point P1, the delay time added to the ultrasonic pulse received at the "transducer element -1" is expressed as follows. t d (P1,X f ,Z f ,X e ) = 2Z p v - ( Z t v + r r v ) Here, Z t and r r can be expressed by the following equation (6). Z t = Z f - (Z f - Z p ) 2 + X 2 f , r r = Z 2 p + X 2 e Therefore, the delay time is expressed as the following equation (7). t d = ( P 1, X f , Z f , X e ) = 2 Z p v - [ Z f - ( Z f - Z p ) 2 + X 2 f v + Z 2 p + X 2 e v When P1 is replaced by Z p =vt/2, the following equation (8) is obtained. t d ( t, X f , Z f , X e ) = t - [ Z f - ( Z f - ( vt /2)) 2 + X 2 f v + ( vt /2) 2 + X 2 e v ] Therefore, when the ultrasonic pulse received by the "transducer element -1" located at the point (X e , 0) is s(t, X f , Z f , X e ), the finally resulting signal S focused (t) with respect to the target point P1 is expressed as the following equation (9). Here, A(t, X e , X f ) is the apodization factor as described above, and uses a respectively appropriate function according to t , X e , and X f . denotes a summation for all the transmission scan lines allowing the reception of the ultrasonic pulse with respect to the target point P1, and denotes a summation for all the transducer elements receiving the ultrasonic pulse reflected from the target point P1. In this case, t is also "0" as described above every time when it is transmitted. Also, Xe and X f are added while varying them in an appropriate interval within an appropriate range. To perform the above focusing, even in the case of the finally focused single scan line, signals within a certain range are needed with respect to a number of X e and X f . However, if the position of the finally focused scan line is slowly moved from the previous position, most of the signals can be used without any change. Some signals are nothing but added or a part of the signals is deleted therefrom. Till now, for convenience of explanation, the case where the scan line to be finally focused is fixed has been described. However, the finally focusing scan line may be actually moved, in which case the x-axis of the equations may be moved in parallel therewith. Fig. 6 shows a focusing apparatus for an ultrasonic imaging system according to a preferred embodiment of the present invention. The electrical pulses generated in a transmitter 20 are applied to transducer elements in an array transducer 10, respectively. In this case, each of the electrical pulses is made to have a respectively different time delay according to a transducer element located at a respectively different location, to thereby perform lateral transmission focusing. The transducer elements emit a respective ultrasonic pulse according to the voltage of the electrical pulses. The ultrasonic pulses are transmitted into an object and then reflected from a point in the object. Subsequently, the reflected ultrasonic pulses are incident to the transducer elements in the array transducer 10 and converted into electrical signals. A receiver 40 which includes a number of amplifiers, amplifies the ultrasonic pulses output from the transducer elements appropriately and converts the amplified results into digital signals. Each intra-channel processor 50 receives the digital signal from each amplifier of the receiver 40, stores it separately every time when it is transmitted, and performs an operation corresponding to the addition of X f term in the above equation (9). A reception focusing portion 60 includes multipliers 62, an adder 64 and an intra-channel apodization value calculator 66. Each of the multipliers 62 multiplies the data output from the corresponding intra-channel processor 50 by an apodization value supplied from the intra-channel apodization value calculator 66, and outputs the multiplication result to the adder 64. The adder 64 adds the data supplied from the multipliers 62 and supplies the resultant final focused value to a display 70. The display 70 displays final focused values as a focused ultrasonic signal, on a monitor 80. Fig. 7 shows a detailed structure of the intra-channel processor 50 shown in Fig. 6. The intra-channel processor 50 includes a plurality of buffer memories 51. Each buffer memory 51 receives radio frequency (RF) data which is applied from the receiver 40 and is converted into digital signal from the received ultrasonic pulse, and stores radio frequency (RF) data in sequence, under the control of a corresponding write controller 53. Each buffer memory 51 supplies the stored data to a delay interpolator 52 under the control of a read controller 54. The read controllers 54 are configured to correspond to each of the plurality of buffer memories 51. Each read controller 54 reads the data stored in a corresponding buffer memory 51, according to a delay amount applied from a focusing delay amount calculator 55. Each buffer memory 51 includes a plurality of static random access memory (SRAM) therein, in which one SRAM records the RF data therein and another SRAM outputs the previously recorded RF data, alternately. Therefore, there is not interference in writing and reading operations. During reading, data is not changed. The focusing delay amount calculator 55 calculate delay times using the above equation (8) in order to focus the data stored in the buffer memory 51. The process adopting the equation (8) is substantially the same as one of the existing digital focusing methods. For example, the focusing delay time expressed in the equation (8) is divided by a sampling interval which is used for the analog-digital conversion in the receiver 40, to be separated into an integer portion and a fraction portion. The focusing delay amount calculator 55 calculates a read address from data of the integer portion and outputs the calculated read address to the read controller 54 corresponding the read address. The focusing delay amount calculator 55 supplies data of the fraction portion to a corresponding delay interpolator 52. Each read controller 54 reads the data from a corresponding buffer memory 51 using the corresponding read address and outputs the read data to a corresponding delay interpolator 52. Each delay interpolator 52 time-delays and interpolates the data output from the corresponding buffer memory 51 according to the fraction data applied from the focusing delay amount calculator 55, to thereby generate the time delayed and interpolated data. To do so, the delay interpolator 52 determines several data corresponding to the fraction data output from the focusing delay amount calculator 55, by using the data supplied from the corresponding buffer memory 51 and interpolates the determined data. As a result, the delay interpolator 52 can delay the data output from the buffer memory 51 by an accurate interval of time according to the delay amount from the focusing delay amount calculator 55. The data generated by the delay interpolator 52 is output to the apodization portion 90. Each multiplier 92 in the apodization portion 90 multiplies the data supplied from the corresponding delay interpolator 52 by the corresponding apodization value calculated in the intra-channel apodization value calculator 96. All the processes described above are performed at the portion corresponding to each transmission scan line at the same time and the generated interpolated data passes through a corresponding multiplier 92 and is supplied to the adder 94. The adder 94 adds the signals output from the multipliers 92 and outputs the finally focused signal. Until now, for convenience of explanation in the present invention, only the case where =0 in the linear array transducer, that is, there is no beam steering, has been described. However, the same method can actually be applied to all the focusing operations in principle. For example, the above-described method can be applied to a convex array, a steering, and a 2-dimensional array. Further, according to the present invention, it is possible to locate a focal point shown in Fig. 2 at a behind position of a array transducer, so that ultrasonic pluses emitted from the array transducer form ripples having a form of divergence. In the present invention, transmission is performed while a focal point is changed in turn. Every time when transmission is performed, reception is performed by all the transducer elements and received data are stored, respectively, and then focusing using the stored signal is performed. In this manner, all the signals obtained by performing transmission focusing on a number of adjacent scan lines are used, to thereby provide the same effect as the case where focal points exist all the points on the scan line to be received.

Claims: A method of obtaining a focused scan line in an ultrasonic imaging system comprising a transducer array (10) which comprises a plurality of transducer elements (transducer elements -5,..., -1,0,1,..., 7) configured to transmit and receive ultrasonic signals, the method comprising: generating a first scan line (L0) by transmitting a first set of ultrasonic signals; receiving some or all of the first set of signals reflected from points (P1, P2) along the first scan line; characterised by generating a second different scan line (L2) by transmitting a different second set of ultrasonic signals; receiving some or all of the second set of signals reflected from the points along the first scan line; and obtaining a combined signal for each of the points along the first scan line from said reflected signals of the first and second sets. A method according to claim 1, wherein the transmitting of the first set of ultrasonic signals comprises a first set of transducer elements (transducer elements -4,..., 0,..., 4) each emitting an ultrasonic pulse. A method according to claim 2, wherein the transmitting of the second set of ultrasonic signals comprises a second set of transducer elements (transducer elements -2, -1,0,... , 6) each emitting an ultrasonic pulse. A method according to claim 3, wherein at least one transducer element is common to both sets of transducer elements. A method according to any preceding claim, wherein the generation of the first and second scan lines occurs in turn. A method according to any preceding claim, further comprising storing a one of said received signals of said first set of signals at a first predetermined transducer element (transducer element 0) which has been reflected from one (P1) of said points (P1, P2) and storing a one of said received signal of said second set of signals at the first predetermined transducer element (transducer element 0) which has been reflected from said one (P1) of said points (P1, P2). A method according to claim 6, further comprising assigning a delay time to said one of said received signals of said second set of signals and adding said delayed signal to said one of said received signals of said first set of signals. A method according to any preceding claim, further comprising storing received signals received at first and second predetermined transducer elements (transducer element -1, transducer element 0) which have been reflected from one (P1) of said points (P1, P2) along the first scan line (L0). A method according to claim 8, further comprising calculating signal arrival time delay (t d ) for said first predetermined transducer element (transducer element -1) and generating interpolated data in dependence upon said time delay. A method according to claim 9, wherein the calculating of the signal arrival delay time (t d ) comprises determining a delay time between receiving a received signal consisting of one of said second set of transmitted signals at said first predetermined transducer element (transducer element -1) and receiving a reference signal consisting of one of said first set of transmitted signals at said second predetermined transducer element (transducer element 0). A method according to claim 10, wherein the calculating of the signal arrival delay time data (t d ) comprises: determining the signal delay time using: wherein t is the time taken for a signal to return from said one point (P1) along the first scan line (L0), X f is a displacement between the second transducer element (transducer element 0) through which the first scan line (L0) passes and a third transducer element (transducer element 2) through which the second scan line (L2) passes, Z f is the distance between the third transducer element and a focal point (f 2 ) on said second scan line, X e is a displacement between the first and second transducer elements and v is the speed of ultrasonic signals and adding the delay time to the received signal so as to produce an added signal for reinforcing the reference signal. A method according to claim 10, further comprising: generating a further scan line by transmitting a further set of ultrasonic signals; receiving at said first predetermined transducer element (transducer element -1) a further received signal consisting of one of a further set of transmitted signals reflect from said one point (P1) along said first scan line (L0); calculating a further signal arrival delay time data by determining a delay time between receiving the further received signal and the reference signal and adding the further delay time to the further received signal so as to produce a further added signal for reinforcing the reference signal. A method according to claim 11, further comprising summing the added signal and the further added signal to produce a summed signal for the first predetermined transducer element. A method according to claim13, further comprising determining a focused signal S focused in dependence upon the summed signal for the first predetermined transducer element. A method according to claim 12, comprising calculating a focused signal S focused comprising; determining the focused signal S focused using: wherein A(t, X e , X f ) is an apodization factor. Apparatus for focusing an ultrasonic signal comprising: a transducer array (10) which comprises a plurality of transducer elements (transducer elements -5,..., -1, 0, 1,..., 7) configured to transmit and receive ultrasonic signals; means for generating a first scan line (L0) by transmitting a first set of ultrasonic signals; means for receiving some or all of the first set of signals reflected from points (P1, P2) along the first scan line; characterised by means for generating a second different scan line (L2) by transmitting a second set of ultrasonic signals; means for receiving some or all of the second set of signals reflected from the points along the first scan line; and means for obtaining a combined signal for each of the points along the first scan line from said reflected signals. Apparatus according to claim 16, wherein the means for generating the first scan line (L0) comprises a first set of transducer elements (transducer elements -4,..., 0,..., 4) each emitting an ultrasonic pulse. Apparatus according to claim 17, wherein the means for generating the second scan line (L2) comprises a second set of transducer elements (transducer elements -2, -1, 0,..., 6) each emitting an ultrasonic pulse. Apparatus according to claim 18, wherein at least one transducer element is common to both sets of transducer elements. Apparatus according to any one of claims 16 to 19, wherein the first and second scan lines (L0, L2) are generated in turn. Apparatus according to any one of claims 16 to 20, further comprising means for storing a one of said received signals of said first set of signals at a first predetermined transducer element (transducer element 0) which has been reflected from one (P1) of said points (P1, P2) and means for storing a one of said received signal of said second set of signals at the first predetermined transducer element (transducer element 0) which has been reflected from said one (P1) of said points (P1, P2). Apparatus according to claim 21, further comprising means for assigning a delay time to said one of said received signals of said second set of signals and means for adding said delayed signal to said one of said received signals of said first set of signals. Apparatus according to anyone of claims 16 to 22 further comprising means for storing received signals received at first and second predetermined transducer elements (transducer element -1, transducer element 0) which have been reflected from one (P1) of said points (P1, P2) along the first scan line (L0). Apparatus according to any one of claims 16 to 23 further comprising means for calculating signal arrival time delay (t d ) for said first predetermined transducer element (transducer element -1) and means for generating interpolated data in dependence upon said time delay. Apparatus according to claim 24, wherein the means for calculating the signal arrival delay time (t d ) comprises means for determining a delay time between receiving a received signal consisting of one of said second set of transmitted signals at said first predetermined transducer element (transducer element -1) and receiving a reference signal consisting of one of said first set of transmitted signals at said second predetermined transducer element (transducer element 0). Apparatus according to claim 25, wherein the means for calculating the signal arrival delay time data (t d ) comprises: means for determining the signal delay time using: wherein t is the time taken for a signal to return from said one point (P1) along the first scan line (L0), X f is a displacement between the second transducer element (transducer element 0) through which the first scan line (L0) passes and a third transducer element (transducer element 2) through which the second scan line (L2) passes, Z f is the distance between the third transducer element and a focal point (f 2 ) on said second scan line, X e is a displacement between the first and second transducer elements and v is the speed of ultrasonic signals and means for adding the delay time to the received signal so as to produce an added signal for reinforcing the reference signal. Apparatus according to claim26, further comprising: means for generating a further scan line by transmitting a further set of ultrasonic signals; means for receiving at said first predetermined transducer element (transducer element -1) a further received signal consisting of one of a further set of transmitted signals reflect from said one point (P1) along said first scan line (L0); means for calculating a further signal arrival delay time data by determining a delay time between receiving the further received signal and the reference signal and means for adding the further delay time to the further received signal so as to produce a further added signal for reinforcing the reference signal. Apparatus according to claim 27, further comprising means for summing the added signal and the further added signal to produce a summed signal for the first predetermined transducer element. Apparatus according to claim 28, further comprising means for determining a focused signal S focused in dependence upon the summed signal for the first predetermined transducer element. Apparatus according to claim 29, comprising means for calculating a focused signal S focused comprising: means for determining the focused signal S focused using: wherein A(t, X e , X f ) is an apodization factor. Apparatus according to claim 16 comprising: receiver means (40) comprising a plurality of amplifier means, each amplifier means arranged for amplifying a plurality of ultrasonic signals received at a respective transducer element (transducer elements -5,..., -1, 0, 1,..., 7) and converting the amplified signals into digital signals; a plurality of intra-channel processor means (50) each for receiving and storing the digital signals, for calculating a respective time delay for each digital signal and for applying said time delays to each stored digital signal so as to reinforce said plurality of ultrasonic signals received at the respective transducer element; and reception focusing means (60) for summing outputs of the plurality of intra-channel processing means so as to produced a focused signal. Apparatus according to claim 31, wherein the reception focusing means (60) comprises: apodization calculator means (66) for calculating an apozidation value for each intra-channel processing means (50); multiplier means (62) for multiplying outputs of each intra-channel processor means (50) by the respective apozidation value so as to produce a plurality of multiplication results; and adder means (64) for adding the multiplication results. Apparatus according to claim 31 or 32, wherein each intra-channel processor means (50) comprises: a plurality of buffer memories (51) for storing digital signals relating to a point (P1) on a particular scan line (L0) for each scan line; write controller means (53) for outputting write control signals to the buffer memories to store digital signals; delay calculator means (55) for calculating delay times for stored signals; read controller means (54) for outputting read control signals to the buffer memories to allow digital signals stored therein to be read; delay interpolator means (52) for interpolating digital signals read from the buffer memories by delaying signals by delay times calculated by the delay calculator means; and apodization means (90) for applying weight values to delayed signals output from the delay interpolator means and adding the weighted values. Apparatus according to claim 31, wherein the apodization means (90) comprises: intra-channel apodization calculator means (96) for calculating an apozidation value for each delayed signals output from the delay interpolator means; multiplier means (92) for multiplying outputs the delay interpolator means by the respective apozidation value so as to produce a plurality of intra-channel multiplication results; and adder means (94) for adding the intra-channel multiplication results so as to produce a value for a respective scan line.

Inventors: BAE MOO HO, BAE, MOO HO
Applicants: MEDISON CO LTD, MEDISON CO., LTD.
Application year: 1998
Main classification: G01S7
IPC codes: A61B8, G01S7, G10K11, G01N29