fig1 illustrates the ultrasonic imaging system incorporating the invention . the system includes a transducer array 10 comprised of a plurality of separately driven transducers ( or transducer elements ) 2 , each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 12 . the ultrasonic energy reflected back to transducer array 10 from the object under study is converted to an electrical signal by each receiving transducer 2 and applied separately to a receiver 14 through a set of transmit / receive ( t / r ) switches 16 . transmitter 12 , receiver 14 and switches 16 are operated under control of a digital controller 18 responsive to commands by a human operator . a complete scan is performed by acquiring a series of echoes in which switches 16 are set to their transmit positions , transmitter 12 is gated on momentarily to energize each transducer 2 , switches 16 are then set to their receive positions , and the subsequent echo signals produced by each transducer 2 are applied to receiver 14 . the separate echo signals from each transducer 2 are combined in receiver 14 to produce a single echo signal which is used to produce a line in an image on a display system 20 . transmitter 12 drives transducer array 10 such that the ultrasonic energy produced is directed , or steered , in a beam . to accomplish this , transmitter 12 imparts a time delay t i to the respective pulsed waveforms 24 that are applied to successive transducers 2 . by adjusting the time delays t i appropriately in a conventional manner , the ultrasonic beam can be directed away from axis 26 , which is normal to array 10 , by an angle θ and focused at a fixed range r . a sector scan is performed by progressively changing the time delays t i in successive excitations . the angle θ is thus changed in increments to steer the transmitted beam in a succession of directions . the echo signals produced by each burst of ultrasonic energy reflect from objects located at successive ranges along the ultrasonic beam . the echo signals are sensed separately by each transducer 2 and a sample of the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range . due to the differences in the propagation paths between a reflecting point p and each transducer 2 , however , these echo signals will not be detected simultaneously . receiver 14 amplifies the separate echo signals , imparts the proper time delay to each , and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from point p located at range r along the ultrasonic beam oriented at the angle θ . to simultaneously sum the electrical signals produced by the - echoes impinging on each transducer 2 , time delays are introduced into each one of separate transducer channels of receiver 14 . the beam time delays for reception are the same delays ( t i ) as the transmission delays described above . however , the time delay of each receiver channel continuously changes during reception of the echo to provide dynamic focusing of the received beam at the range r from which the echo signal emanates . under direction of digital controller 18 , receiver 14 provides delays during the scan such that steering of receiver 14 tracks the direction θ of the beam steered by transmitter 12 and samples the echo signals at a succession of ranges r and provides the proper delays and phase shifts to dynamically focus at points p along the beam . thus each emission of an ultrasonic pulse waveform results in acquisition of a series of data points which represent the amount of reflected sound from a corresponding series of points p located along the ultrasonic beam . display system 20 receives the series of data points produced by receiver 14 and converts the data into the desired image . fig2 shows conventional receiver 14 as comprising three sections : a time - gain control section 100 , a receive beamforming section 101 and a midprocessor 102 . time - gain control ( tgc ) section 100 includes a respective amplifier 105 for each of receiver channels 110 and a time - gain control circuit 106 . the input of each amplifier 105 is connected to a respective one of transducers 2 ( shown in fig1 ) to amplify the echo signal which it receives . the amount of amplification provided by amplifiers 105 is controlled by tgc circuit 106 which , in turn , is set by hand operation of potentiometers 108 . the receive beamforming section 101 of receiver 14 includes separate receiver channels 110 , each receiving the analog echo signal from a respective one of amplifiers 105 at an input 111 . each amplified signal is conveyed to a pair of quadrature detectors in the respective receiver channel , where the phases of the mixing reference frequency differ by 90 °. since this reference frequency is made identical to that of the transmitted pulses , output signals from low - pass filters in the receiver channel become complex signals ( i and q ) with phases that differ by 90 °. these signals are produced as a stream of digitized output values on an i bus 112 and a q bus 113 . each of these i and q signals represents a demodulated sample of the echo signal at a specific range r . these samples have been delayed such that when they are summed at summing points 114 and 115 with the i and q samples from each of the other receiver channels 110 , the summed signals indicate the magnitude and phase of the echo signal reflected from a point p located at range r on the beam steered at angle θ . mid - processor 102 receives the beam samples from summing points 114 and 115 . the i and q values of each beam sample are signals representing the in - phase and quadrature components of the magnitude of reflected sound from a point ( r , θ ). a detection processor 120 receives the summed i and q values and produces an output signal of magnitude | a |=( i 2 + q 2 ) 1 / 2 . each receiver channel , as shown in fig3 is responsive to a start command , a master clock , a beam angle signal θ and a range signal r from digital controller 18 ( fig1 ) to perform the digital beamforming functions which include : sampling the analog input signal at regular intervals in an analog - to - digital converter 30 ; demodulating the sampled signal in a demodulator 32 ; filtering out the high - frequency sum signals produced by demodulator 32 with low pass filters 34 ; reducing the data rate in decimators 36 ; and time - delaying and phase - adjusting the resulting digital data stream in delay fifos ( i . e ., first - in / first - out registers or memories ) 38 and phase rotator 40 , respectively . the components of the beamformer are controlled by a receive channel control 42a which produces the required clock and control signals in response to commands from digital controller 18 ( fig1 ). demodulator 32 mixes each sample from analog - to - digital converter 30 with a reference signal i that is in - phase and a reference signal q that is in quadrature with the transmitted ultrasonic carrier . low pass filters . 34 are finite impulse response ( fir ) filters tuned to pass the difference frequencies , but block the higher sum frequencies , supplied by demodulator 32 . the output signal of each low pass filter is a stream of digital values representing the i or q component , respectively , of the echo signal . the rate at which the demodulated i and q components of the echo signal are sampled is reduced by decimators 36 . the echo signal represented by the demodulated and decimated digital samples is then delayed by delay fifos 38 and phase - shifted by phase rotator 40 to provide the desired beam steering and beam focusing . the response of an ultrasound imaging system focused in a given direction is peaked for that direction . the response is not zero , however , for other directions . in some ultrasound images , the signal from reflections coming from a given direction is contaminated or obscured by reflections from a bright object ( i . e ., reflector ) in a nearby direction . the ultrasound imaging system in accordance with the present invention incorporates means for adapting the response of the - system to reduce artifacts caused by such interfering reflectors . the dashed line in fig4 shows the response of an ultrasound imaging system as a function of the angle of the reflected signal for a beam which is focused in the direction θ = 0 °. reflected signals originating from angles near θ = 0 ° generate a large response , which decreases for signals originating at other angles . the image formed by the ultrasound system for a given direction θ n is formed from the sum of the reflected signals over all angles , weighted by the system angular response . therefore , a small reflection from the direction of interest θ &# 39 ; can be obscured by a larger reflection from direction θ ≠ θ . this undesirable effect is counteracted , according to the present invention , by modifying the response of the ultrasound imaging system . this modification is achieved by adjusting both the time and phase delays which are applied to the transmitted and receive signals on each transducer element . the delays are adjusted to form a null , or notch , in the direction of the interfering signal , while retaining the large response near the desired focusing direction . the solid line in fig4 represents the response of an ultrasound imaging system focused in the direction θ = 0 ° with a null formed in the direction θ = 22 . 5 °. ( the following parameters , defined hereinafter , were used in conjunction with fig4 : m = 10 , θ 1 = 20 ° and θ 2 = 25 °. the center frequency ƒ of the pulse was 3 . 75 mhz . the transducer array consisted of 128 elements equally spaced at one - half wavelength of the center frequency . r &# 39 ; was made very large to simulate a reflector far from the transducers .) in operation , the first step is to select m control angles . this selection is under operator control . using the display screen , the operator selects a pair of angles , θ 1 and θ 2 , and a depth r &# 39 ;, which approximates the location of the interfering signal . the operator also selects the number of control angles m , m being typically in the range of 4 to 10 . in accordance with the present invention , digital controller 18 , shown in fig1 includes therein a beam - nulling processor ( shown in fig5 ). after selection of m , the beam - nulling processor calculates a set of m evenly - spaced control angles θ 1 , θ 2 , . . . , θ m which span the selected region . where n = 1 , 2 , . . . , n and m = 1 , 2 , . . . , m . in eqs . ( 1a ) and ( 1b ), ƒ is the center frequency of the transmitted ultrasound pulse , and x n is the position of the n - th transducer element . there are a total of n transducer elements . t ( x n , r , θ ) is the time delay applied to the received signal arriving at the transducer element located at x n for a beam focused in the direction θ and at depth r . t ( x n , r &# 39 ;, θ m &# 39 ;) is the time delay calculated for the same transducer element for a beam focused in the direction of the control angle θ m &# 39 ; and at the control depth r &# 39 ;. finally , a n is the amplitude weight applied to the signal from the n - n - th element as part of the normal beamforming summation . next , the beam - nulling processor solves the following 2m equations for the 2m quantities α m and β m , where m and 1 = 1 , 2 , . . . , m : ## equ1 ## finally , given the quantities α m and β m , the beamnulling processor calculates the n time delay corrections , δt 1 , δt 2 , . . . , δt n , using the equation ## equ2 ## the beam - nulling processor retrieves from memory the unmodified parameters for a given beam direction and depth which control the time delays applied to the signals transmitted on each transducer element and also the parameters controlling the time delays applied to the received signals from each transducer element . the n time delay corrections δt n are used to modify these parameters , and the modified parameters are then used for the next transmit - receive cycle for this beam direction . the modified parameters need to be calculated for every beam direction in the image ( which is apparent since eq . ( 1 ) depends upon the beam direction ). as shown in fig3 the total time delay applied to the receive signal is applied partly as a true time delay through the delayed sample clock supplied to analog - to - digital converter 30 and through the digital delay generated in fifos 38 , and partly as a phase rotation through phase rotator 40 . at receive channel control 42a , the total time delay to be applied to the signal for a given element and a given beam direction is calculated and this total time delay is divided into a true time delay and a phase rotation . when the null - generation feature of the invention is active , the correction δt n is added to the total time delay for element n before the calculated total time delay is divided into the true time delay and phase rotation . the same correction δt n is subtracted from the time delay calculated for the transmitted signal for element n for the given beam . in this way a null is formed in the desired direction for both the transmitted beam and the received beam response . fig5 is a block diagram of a beam - nulling processor 44 and a receive and transmit channel control 42 comprising receive channel control 42a and a transmit channel control 42b , all of which are part of digital controller 18 shown in fig1 . beamnulling processor 44 receives from the digital controller operator - selected parameters m , θ 1 , θ 2 and r &# 39 ;, as well as the beam parameters θ and r . processor 44 also receives the total time delay t ( x n , r , θ ) for the n - th channel from receive and transmit channel control 42 . from these parameters , the beam - nulling processor calculates the time delay correction δt n which is sent to the receive and transmit channel control where this value is used to adjust the transmit and receive delays for the n - th channel . those skilled in the art of digital beam forming will be able to derive eqs . ( 1a ), ( 1b ), ( 2a ), ( 2b ) and ( 3 ) from the details of analysis set forth hereinafter . consider the point - spread function for a beamformer with n elements : ## equ3 ## in equation ( 4 ), τ n = t n &# 39 ;- t n ; t n &# 39 ;= t ( x n , r &# 39 ;, θ &# 39 ;); t n = t ( x n r , θ ); ω = 2πƒ with ƒ equal to the center frequency ; x n is the location of the n - th element ; and an is the weight of the n - th element . t n is the time delay applied to the n - th element to focus the beamformer in a given direction ( r , θ ); t n &# 39 ; is the actual arrival time at the n - th element of a signal reflected from an object at ( r &# 39 ;, θ &# 39 ;); and . o slashed . n = 2πƒδt n , where δt n is the correction to the time delay t n which will form a null in the desired direction . furthermore , it can be shown that ## equ4 ## at a large distance from the transducer , the time delay across the channels varies linearly across the array . a change in frequency then corresponds to a corresponding change in slope of the time delay , leading to a shift in the focusing direction . in other words , a shift in frequency is equivalent to a shift in the null direction . this means that the null which is calculated for the center frequency will be smeared out ( and made shallower ) when applied to a broadband signal . however , this smearing can be compensated for by originally choosing a wide null . hence from eq . ( 4 ): ## equ5 ## where we denote the unperturbed point spread function as g 0 ( r , θ ). the unperturbed point spread function is given by : ## equ6 ## with weights a n for side - lobe tailoring . c =[ a . sub . 1 e . sup . jωπ1 , a . sub . n e . sup . jωπ2 , . . . , a . sub . n e . sup . jωπn ], ( n × 1 ) vector ( 8 ) c . sub . m =[ c . sub . 1m , c . sub . 2m , c . sub . 3m , . . . , c . sub . nm ], ( n × 1 ) m vector . ( 9 ) the real and imaginary parts of the above vector , denoted by r m and i m , have components represented by eqs . ( 1a ) and ( 1b ). φ =[. o slashed .. sub . 1 , . o slashed .. sub . 2 , . o slashed .. sub . 3 , . . . , . o slashed .. sub . n ], ( n × 1 ) vector . ( 10 ) to reduce artifacts in the ultrasound image , broad nulls are formed at the central frequency with m control angles spaced a predetermined angle apart . at the locations of the nulls it is required that evaluation of eq . ( 11 ) at the m null control angles results in the following equation : ## equ7 ## where τ nm are computed for element location at x n at control points θ m . separating real parts and imaginary parts , using the vector definitions , eq . ( 12 ) reduces to : for m = 1 , . . . , m . ii represents a unit vector and the parentheses represent the inner product . a quadratic functional obtained from the inner product of a phase vector ψ in the neighborhood of . o slashed . corresponds to minimization of the square of the norm of . o slashed ., incorporating constraints with the help of lagrange &# 39 ; s multipliers . ## equ9 ## where results in ## equ11 ## with the identity δt n =. o slashed . n / 2πƒ , eq . ( 17 ) is the same as eq . ( 3 ). this result of eq . ( 17 ) is similar to that described by steyskal in &# 34 ; simple method for phase nulling by phase perturbation &# 34 ; ieee transactions on antennas and propagation , vol . ap - 31 , no . 1 , january 1983 , pp . 163 - 166 , where it was assumed that the g ( θ ) is real . the present invention is not based on such an assumption . the solution disclosed by chakraborty et al ., &# 34 ; beam shaping using nonlinear phase distribution in a uniformly spaced array &# 34 ;, ieee transactions on antennas and propagation , vol . ap - 30 , no . 5 , september 1982 , pp . 1031 - 1035 , was obtained using the orthogonality property as well as matrix identities based on the positive definiteness of the matrices as given by bellman , &# 34 ; introduction to matrix analysis &# 34 ;, 2nd ed ., 1970 mcgraw hill book company , new york . in the present case , lagrange &# 39 ; s method gives the general solution in an easy manner and at the same time has the property of making the norm of the phase vector stationary . to obtain ( α m and β m , eq . ( 17 ) is substituted into eqs . ( 13 ) and ( 14 ) to arrive at : ## equ12 ## where 1 = 1 , 2 , . . . , m . equations ( 18 ) and ( 19 ), in vector form , are the same as eqs . ( 2a ) and ( 2b ). it is seen from eqs . ( 18 ) and ( 19 ) as well as eqs . ( 2a ) and ( 2b ) that the problem only requires solution of a system having 2m equations , where m can be much less than n , the number of elements in the main array . this reduces the complexity of the solution and can be easily implemented in real time . while only certain preferred features of the invention have been illustrated and described , many modifications and changes will occur to those skilled in the art . it is , therefore , to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention .