the present invention will now be described more specifically with reference to the following embodiments . it is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only ; it is not intended to be exhaustive or to be limited to the precise form disclosed . for the sector imaging using a one - dimensional array according to the present invention , the received focusing delay of a channel can be represented as t rx = x   sin   θ c + 1 r × x 2  cos 2  θ 2  c , where x is the distance between the channel and the array center , c is the sound velocity , θ is the steering angle , and r is range of the focal point . note that the first term is independent of the range r and is also known as the steering term . the second term is known as the focusing term and is a function of r . for the dynamic focusing , the system focuses along the entire range of interest . therefore , t rx needs to be constantly updated at every range , every channel and every steering angle . let φ = x 2 cos 2 θ . since the other term ( i . e ., − x sin θ / c ) is independent of r and can be specified at the beginning of each beam , the total delays at a range r can then be calculated given the value of φ . consequently , given the initial parameters ( including − x sin θ / c and φ ), the delay controller can effectively perform dynamic focusing at every range r . for the three - dimensional imaging using a two - dimensional array , the delay can be derived in a similar fashion . t rx = -  ( x   tan   α + y   tan   β ) c  ( 1 + tan 2  α + tan 2  β ) 1 / 2 + 1 r × ( x 2 + y 2 ) + ( x   tan   β - y   tan   α ) 2 2  c  ( 1 + tan 2  α + tan 2  β ) , where ( x , y ) is the coordinate of a particular channel with the array center at position ( 0 , 0 ); α and β are the steering angles along the lateral direction and the elevational direction , respectively . fig1 shows the coordinates . similar to the one - dimensional case , the formula is also divided into a term independent of range r and a term with r at the denominator . in other words , the following term can be defined : φ = ( x 2 + y 2 ) + ( x   tan   β - y   tan   α ) 2 1 + tan 2  α + tan 2  β . note that the d ) value of channel ( x , y ) is identical to that of channel (− x , − y ). therefore , for an n - by - n two - dimensional array , the number of delay control units can be reduced to n 2 / 2 . fig2 shows the block diagram based on direct implementation of the dynamic focusing . at the beginning of each beam , the initial parameters described previously are sent to each channel . the delay unit in each channel then has to calculate the overall focusing delay at each range . for an n - by - n two - dimensional array with maximum π / 4 steering angle in both α and β directions , the number of beams is 2n 2 assuming the nyquist spatial sampling . consequently , the number of initial parameters , which needs to be loaded into the receive beamformer at the beginning of each beam , is 4n 4 ( i . e ., n 2 channels times 2n 2 beams times 2 parameters ). at a typical n ( e . g ., 64 ), this requires a significant amount of data communication . the purpose of this invention is to devise an efficient method such that the total number of φ can be dramatically reduced . the simplification scheme keeps the first term unchanged but uses the same φ for adjacent channels . in other words , the proposed scheme makes certain approximation to the focusing term but maintains the same steering accuracy . suppose d is approximated by φ ′, i . e ., where δ is the approximation error . then the approximated total delay t rx ′ becomes : t rx ′ = t rx + δ 2  rc , where t rx is the original delay . the last term is a delay error which may cause the image quality degradation . nonetheless , the delay error decreases as range r increases . consequently , the image quality degradation occurs only at shallower depths . the φ values of neighboring channels are approximated by a single value . in other words , the two - dimensional array is divided into sub - arrays . determination of the sub - array locations is critical in minimizing the focusing error . one approach is to partition the array into concentric rings . as an example , the number of channels an in a ring can be determined as the following δ n ≦{ square root over (( n − 0 . 5 ) 2 + 16 ( n − 0 . 5 ) ε 1 g × f / number )}−( n − 0 . 5 ), where n is the inner radius of the ring , ε 1 is a pre - specified fractional phase error ( i . e . delay error times the carrier frequency over 2π ); and g is a gain factor defined as g = exp  [ γ  ( n n 0 - 1 ) ] , where n 0 is the radius of the first ( smallest ) ring , and γ is a parameter for finely tuning the array partition . in general , g can be any function of n over n 0 that increases monotonically . this is due to the fact that as more and more channels become active , the delay error can also increase without significantly affecting the image quality because r also increases . after determining the radius n , each ring needs to be further partitioned into smaller zones . the angular span of each zone can be defined ξ = m × δ   n n , where m is a pre - specified aspect ratio . through this procedure , the array partition can be made . channels inside the same zone use the same φ ′ value and thus , the same delay control unit . the common φ ′ value can be defined as the mean of all the φ values inside the same zone . a typical segmentation pattern is shown in fig3 . let ε 1 be 0 . 05 , n 0 be 4 , γ be 0 . 15 , m be 3 and f / number be 2 , the 64 - by - 64 array can be controlled by using only 227 control units . note that the total number of zones is 454 due to the symmetry in φ . also note that an ε 1 of 0 . 05 means that the maximum focusing phase error is 18 degrees ( i . e ., 0 . 05 * 360 ). 100581 the range of values of φ is limited since many values are either the same or only slightly different from each other . the range of φ that is used by all beams at a particular range is shown in the top panel of fig4 . because larger φ values occur at outside channels , larger quantization error of φ can be tolerated in this case since these channels do not become active until a relatively large r is reached . in other words , non - uniform quantization of φ can be used . it can be shown that the number of quantization levels l using non - uniform quantization can be obtained as l = n 8  2 × ɛ 2 × f / number , where ε 2 is an error constraint , and the quantized value of φ is φ i = 8 ×( ε 2 × λ × f 1 number ) 2 ×( 2 i 2 − 2 i + 1 ), i ∈[ 1 , l ], where λ is wavelentgh of the carrier . if a uniform quantizer is used in the same situation , at least l 2 levels need to be used to obtain the same level of delay accuracy . let n be 64 , ε 2 be 0 . 1 ; carrier frequency be 4 mhz and f / number be 2 , there are only 28 qunatization levels . among these , the 28th φ is never used if the spatial quantization in fig4 is used . therefore , 27 levels are adequate . for a given φ ′, there is an error exists at all ranges . further improvement can be achieved by properly choosing a reference range . a modified delay to t rx ″ can be expressed as the following t rx ″  ( r ) = t rx ′  ( r ) -  δ 2  r 0  c = t rx  ( r ) + δ 2  rc -  δ 2  r 0  c where r 0 is the reference range . in other words , t rx ″ is equal to the ideal value t rx at a range of interest r 0 . generally speaking , delay error would become q - times of the original error at r with q  ( r ) ≡  ( δ 2  rc -  δ 2  r 0  c ) / δ 2  rc  =  r 0 - r  r 0 by properly selecting r 0 , q can be less than 1 , meaning errors are further suppressed . simulations have been done to demonstrate efficacy of the invention . the simulations are based on the angular spectrum method . fig5 and fig6 show four beam patterns at 15 mm and 30 mm respectively . each figure shows the projection of the beam plots along the x ( upper ) and y ( lower ) directions , wherein the first plot assumes ideal delays with no approximation ; the second plot employs the grouping technique shown in fig3 ; the third plot utilizes the non - uniform quantization scheme shown in fig4 ; and fourth plot combines both techniques . in the case of 15 mm , sidelobes are elevated due to quantization errors . nonetheless , the mainlobe width is unchanged and the increase in sidelobe levels is minimal . considering that 15 mm is shallow for most clinical situations , the degradation is negligible . at 30 mm , the four beam patterns are virtually identical as shown in fig6 . simulated sound fields at non - zero steering angles also have similar results . a block diagram of the proposed architecture for the invention is shown in fig7 in which a delay table and a multiplexer are used to implement dynamic focusing control . in addition , the connections between the look - up - table and the multiplexer shown in fig8 reveal that not all connections are used . thus , the size of the multiplexer can be further reduced . note that each φ corresponds to a delay change pattern . the delay change pattern can be a single bit data stream with 1 representing delay change and 0 representing no change . comparison between the invention and conventional approaches is summarized in table 1 . based on the design procedures outlined above , different segmentation schemes can be implemented . the invention can be extended to any field with array focusing in the near - field , not necessarily restricted in the medical ultrasound . while the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments , it is to be understood that the invention needs not be limited to the disclosed embodiments . on the contrary , it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures .