having summarized various aspects of the present invention , reference will now be made in detail to the description of the invention as illustrated in the drawings . while the invention will be described in connection with these drawings , there is no intent to limit it to the embodiment or embodiments disclosed therein . on the contrary , the intent is to cover all alternatives , modifications and equivalents included within the spirit and scope of the invention as defined by the appended claims . turning now to the drawings , wherein like referenced numerals designate corresponding parts throughout the drawings , reference is made to fig1 which illustrates a block diagram of an ultrasound imaging system capable of implementing the use of combined transmit pulses to introduce second harmonic energy into a patient along with a fundamental frequency transmit signal . that is , varying amounts of second harmonic may be added to the transmit beam on a per element basis , thus creating a transmit beam which is independent of the non - linearly generated second harmonic to create a more continuous transmit harmonic beam . in this regard , an ultrasound imaging system 100 may comprise a transducer 102 , a rf switch 104 , a transmitter 106 , a system controller 108 , an analog to digital converter ( adc ) 110 , a time gain control amplifier 112 , a beamformer 114 , a filter 116 , a signal processor 118 , a video processor 120 , and a display 122 . the transducer 102 may be electrically coupled to rf switch 104 . the rf switch 104 may be configured as shown with a transmit input coupled from the transmitter 106 and a transducer port electrically coupled to the transducer 102 . the output of rf switch 104 may be electrically coupled to an adc 110 before further processing by the time gain control amplifier 112 . the time gain control amplifier 112 may be coupled to a beamformer 114 . the beamformer 114 may be coupled to the filter 116 . the filter 116 may be further coupled to a signal processor 118 before further processing in the video processor 120 . the video processor 120 may then be configured to supply an input signal to a display 122 . the system controller 108 may be coupled to the transmitter 106 , the adc 110 , the filter 116 , and both the signal processor 118 and the video processor 120 to provide necessary timing signals to each of the various devices . as will be appreciated by persons having ordinary skill in the art , the system controller 108 can include one or more processors , computers , and other hardware and software components for coordinating the overall operation of the ultrasonic imaging system 100 . in addition , it will be appreciated that the system controller 108 may include software which comprises an ordered listing of executable instructions for implementing logical ftnctions , which can be embodied in any computer - readable medium for use by or in connection with an instruction execution system , apparatus , or device , such as a computer - based system , processor - containing system , or other system that can fetch the instructions from the instruction execution system , apparatus , or device and execute the instructions . the computer readable medium can be , for instance , an electronic , magnetic , optical , electromagnetic , infrared , or semiconductor system , apparatus , device , or propagation medium . the rf switch 104 isolates the transmitter 106 of the ultrasound imaging system 100 from the ultrasonic response receiving and processing sections comprising the remaining elements illustrated in fig1 . the system architecture illustrated in fig1 provides an electronic transmit signal generated within the transmitter 106 that is converted to one or more ultrasonic pressure waves herein illustrated by ultrasound lines 115 . when the ultrasound lines 115 encounter a tissue layer 113 that is receptive to ultrasound insonification the multiple transmit events or ultrasound lines 115 penetrate the tissue 113 . as long as the magnitude of the multiple ultrasound lines 115 exceeds the attenuation affects of the tissue 113 , the multiple ultrasound lines 115 will reach an internal target 121 . those skilled in the art will appreciate that tissue boundaries or intersections between tissues with different ultrasonic impedances will develop ultrasonic responses at harmonics of the fundamental frequency of the multiple ultrasound lines 115 . as further illustrated in fig1 such harmonic responses may be depicted by ultrasonic reflections 117 . it will be further appreciated that tissue insonified with ultrasonic waves develops harmonic responses because the compressional portion of the insonified waveforms travels faster than the rarefactional portions . the different rates of travel of the compressional and the rarefactional portions of the waveforms causes the wave to distort producing a harmonic signal which is reflected or scattered back through the various tissue boundaries . it is significant to note that while fig1 illustrates only a second harmonic response to the incident multiple ultrasound lines 115 impinging the internal target 121 within the tissue layer 113 , other harmonic responses may also observed . for example , nonlinear propagation of broadband signals may produce subharmonics in a sense . nonlinear propagation is essentially the intermixing of all the frequency components present in the wave where the frequencies generated are the summation and difference frequencies of the components present in the broadband signal . those ultrasonic reflections 117 of a magnitude that exceeds the attenuation effects from traversing tissue layer 113 may be monitored and converted into an electrical signal by the combination of the rf switch 104 and transducer 102 . the electrical representation of the ultrasonic reflections 117 may be received at the adc 110 where they are converted into a digital signal . the time gain control amplifier 112 coupled to the output of the adc 110 may be configured to adjust amplification in relation to the total time a particular ultrasound line 115 needed to traverse the tissue layer 113 . in this way , response signals from one or more internal targets 121 will be gain corrected so that ultrasonic reflections 117 generated from relatively shallow objects do not overwhelm in magnitude ultrasonic reflections 117 generated from insonified objects further removed from the transducer 102 . the output of the time gain control amplifier 112 may be beamformed , filtered and demodulated via beamformer 114 , filter 116 , and signal processor 118 . the processed response signal may then be forwarded to the video processor 120 . the video version of the response signal may then be forwarded to display 122 where the response signal image may be viewed . it will be further appreciated by those of ordinary skill in the art that the ultrasonic imaging system 100 may be configured to produce one or more images and / or oscilloscopic traces along with other tabulated and or calculated information that would be useful to the operator . an ultrasonic imaging system 100 in accordance with the present invention can produce harmonic response signals in the far - field due to the nonlinear propagation of the lower transmit frequency . transmitted harmonic signal may be used to generate harmonic signal in the near field , temporal or spatial apodization of the transmit signal as well as optimizing the spatial distribution of the harmonic in the near field . transmit apodization may weight a subset of the transducer &# 39 ; s 102 elements more than another subset of elements . the shifting of the fundamental frequency of a transducer 102 in accordance with the present invention to very low frequencies , such as those below 1 . 5 mhz , is illustrated in fig2 a and 2b . the extra bandwidth of single crystal transducers and muts provides an additional degree of freedom for processing harmonic response signals over a wide frequency range . [ 0043 ] fig3 is a schematic diagram , which shows the effective penetration of the combined frequency signal proposed hereby . that is , varying amounts of at least one harmonic of the fundamental frequency is added to the transmit beam on a per element basis , resulting in a transmit beam which is independent of non - linearly generated equivalent harmonics , e . g ., the second harmonic received in the return echo . the harmonic portion of the transmit beam may be focused at a depth which is different than the depth at which the fundamental components are directed . indeed , the focal gain of the transmitted harmonic beam may be optimized such that the degree of field divergence beyond the focal plane ensures that there will be minimal transmitted second harmonic in the focal plane of the fundamental . the result is a more continuous transmit harmonic beam , and a better composite image than can be realized with conventional thi imaging techniques and apparatus . as mentioned above , little harmonic generation takes place in the shallow depths . more particularly , most second harmonic generation takes place at the final axial maximum of the fundamental field . as a result , a thi image may give an impression of a split in image quality between the fundamental axial position where non - linear based second harmonic generation largely takes place , and the near field where nonlinear second ( and higher ) harmonic generation is limited . in principle , any method of this invention , and any system , which implements same , increases the depth of the field of the harmonics , particularly , the second harmonic , by introducing a second , very shallow , strongly “ focused ” harmonic beam . this second focal zone is used for a very short range ( and therefore , time ). the first focal zone is used beyond the shallow depth , and comprises ( but not in all cases is limited to ) the fundamental frequency . because of the strong focus and short range of the first focal zone , the line period may be short with little effect on the final frame rate . in this method , both images are formed from non - linearly generated harmonic , and thus both images benefit from the image enhancements associated with the properties of non - linear generation . while particular embodiments of the invention have been disclosed in detail in the foregoing description and drawings for purposes of example , it will be understood by those skilled in the art that variations and modifications thereof can be made without departing from the spirit and scope of the invention as set forth in the following claims .