AN ULTRASONIC SYSTEM

Abstract: The errors in phase produced by inaccuracies in the locations of taps on a delay line are compensated for by determining the error of each by transmitting oscillations from a test oscillator through each tap and storing the difference between the actual phase and the intended phase that would result if the tap were perfectly located. When a specific delay is required, the tap having the closest actual delay to that required is selected and a phase correction is made depending upon the known data on the error determined.

Description: This invention is concerned with an ultrasonic system for investigating the internal structure of a body. In certain ultrasound imaging systems, pulses of several sinusoidal pressure waves are successively transmitted by each transducer of an array into a body under examination so as to form a beam along a path in a direction determined by the relative times at which the transducers of the array are excited. Reflection of these pressure waves from targets located in the beam impinge on the individual transducers and cause them to produce corresponding electrical waves. Because of the different distances between a target and each of the transducers, the electrical waves they produce in response to pressure waves reflected from the target are not in phase, so that summing the electrical waves would produce a signal of very low amplitude. If, however, a delay is connected to each transducer that exactly compensates for the differences in time it takes for the reflections to go from the target to each transducer, corresponding sinusoids of the pulse will be cophasal, and a summation of these cophasal waves will produce a strong signal. Because a different delay must be provided for each target range along each beam path, the number of delays required is great and their cost is excessive. U.S. Patent No: 4 140 022 discloses an ultrasonic system comprising an array of transducer elements capable of outputting electrical impulses in response to pressure waves, means coupled to said transducer elements for causing them to periodically transmit bursts of ultrasonic carrier pressure waves in different directions, range focussing means having inputs and outputs, said inputs being connected to said transducer elements' electrical output, a master delay line having a plurality of input taps and an output, the delay between at least some of said taps and the output of said delay line having errors compared with a predetermined value, means for deriving an indication of said errors, and means for respectively connecting the outputs of said range focussing means to selected taps on said master delay line. The Patent describes an ultrasound system in which the cost of providing the delays is considerably reduced. Instead of providing the many delays that are required to cause the pulses of electrical waves produced by the transducers to occur at the same time so that corresponding sinusoids within the pulses are all in phase, fewer delays are provided so that, although the pulses do not arrive at the same time, they do have some overlap. Each transducer is connected to one input of its own mixer and the output of the mixer is connected by a tap selector to a tap on a master delay line. The other input of each mixer is connected to a phase selector that provides the phase of a common local oscillator signal that is required to bring the sinusoids of the intermediate frequency at the outputs of the mixers which occur during the overlapped portions of the pulses into approximate phase coherence at the output of the master delay line. A strong signal is attained when these signals are summed. In order to secure the required degree of phase coherence, however, it is essential that the locations of the taps on the master delay line be such that the delays provided by the taps are accurate i.e. they are within a small fraction of a period of the intermediate frequency wave of the delay they are supposed to provide. The phases of the intermediate frequency is based on the assumption that the locations of all the taps are accurate. Since this is not usually the case, such a system is far from ideal. According to the invention there is provided an ultrasonic system comprising an array of transducer elements capable of outputting electrical impulses in response to pressure waves, means coupled to said transducer means for causing them to periodically transmit bursts of ultrasonic carrier pressure waves in different directions; a master delay line having a plurality of input taps and an output, each tap corresponding to a nominal delay; phase changing range focussing means having inputs and outputs, said inputs being connected to the electrical outputs of said transducer elements, the phases of the outputs of each of said range focussing means being adjusted for attaining phase coherence at the output of said master delay line; tap selecting means for respectively connecting the outputs of said range focussing means to selected taps on said master delay line; characterised in that the system also comprises means for measuring the errors in actual delay with respect to the nominal delays associated with each of the taps of said master delay line and for storing indications of these errors; said phase changing range focussing means also being responsive to the indication of error stored for each tap, so as to compensate the output of the master delay line for the errors in the delays associated with said taps. The system provides a variable phase shift unlike the fixed phase shift in U.S. 4 140 022. This increases the accuracy of the operation of the system. This allows cheaper delay lines to be used than the previously used ideal delay lines, and eliminates the need to use "trimmer" delay lines. The amount of the errors may be determined once-for-all and stored, but is preferably re-assessed at frequent intervals e.g. each time the system is started up. In order that the invention shall be clearly understood, an exemplary embodiment thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is an overall functional block diagram for apparatus incorporating this invention; and Figure 2 is a block diagram illustrating the incorporation of this invention into a system such as described in the aforesaid U.S. Patent 4,140, 022. In Figure 1, an analog signal to be delayed by a precise amount is applied via a finely quantized phase shifting means to Q to a coarsely quantized delay D. The desired delay for the analog signal is applied to a ROM R that is preloaded so as to cause a control C₁ to cause the delay D to introduce into the path of the analog signal the one of its coarsely quantized delays that is closest to the desired delay. The ROM R is preloaded with measured values of each of the coarsely quantized delays, and the difference between the actual quantized delay selected in any given instance and the desired delay is derived by a subtractor S and supplied to an adder A. Also provided to the adder A is a signal indicating the phase shift that would be required for range focussing as described in the aforesaid patent if the delay provided by D is the same as the desired delay. The output of the adder A is applied to a controller C₂ that selects the finely quantized phase provided by the phase- shifting means Q that will cause the effective delay introduced into the signal path by Q and D to be satisfactory even though the delays provided by D contain errors. Figure 2 illustrates one way of incorporating the invention into a pulsed ultrasound system. The normal operation is as follows. A microprocessor 2 informs a RAM 4 of the angle at which an acoustic beam of pulses of sinusoidal pressure waves is to be launched into the body of a patient and, in response to this information, a device 6 respectively provides pulses to transducers contained in a block 8 having a relative timing necessary to cause the acoustic beam to be launched in the desired direction. When reflections of an acoustic pulse arrive at each transducer, it outputs corresponding electrical pulses. The processing of the output of only one of the transducers in th block 8 is now shown and described, but the outputs of the other transducers will be processed in the same manner. The output of a transducer is applied via an amplifier 10 to one input of a mixer M₁, and the output of the mixer M₁ is applied to a tap selector 12 on a master delay line MD. The micro-processor 2 selects the number of the tap closest to the ideal delay and places it in the RAM 4, and the tap selector 12 selects the tap that it is instructed to select by the RAM 4. If the delays thus selected for each transducer were ideal, the sinusoids of the electrical pulses from each transducer would be cophasal at the end of the delay line MD. If an error in phase of 22.5 degrees could be tolerated, the taps would have to be spaced at points separated by one-sixteenth of the period of the waves being delayed. This would be nearly impossible to attain if the outputs of the transducers were applied directly to the taps because the frequency involved would be several megacycles. The number of taps and the expense thereof can be significantly reduced by using a mixer such as M₁ to heterodyne the signals from a transducer to a much lower IF frequency as described in the aforesaid patent. And, as also described in that patent, a further significant reduction can be made in the number of taps by spacing them so that the envelopes of the pulses from each transducer arrive at the end of the delay line with some overlap. The fact that the sinusoids in the pulses would therefore no longer be cophasal is taken care of by adjusting the phase of the IF wave supplied by the mixer M₁. The mixer M₁ is supplied with one of numbered discrete phases of the output of an oscillator O₁. The number of the phase that is closest to an ideal phase is supplied by the microprocessor 2 to the RAM 4 and a phase selector 14 applies this phase to the mixer M₁. This is called focussing. If the taps on the master delay line MD are perfectly located, the only phase error would be due to the fact that the mixer M₁ can only operate at a discrete phase. While the reflections of each pulse are being received from different depths, the tap of the master delay line MD is left unchanged; but the phases selected by the phase selector 14 are changed a number of times in order to keep the sinusoids focussed, i.e. reasonably close to a cophasal relationship. From this, it can be seen that any error in tap location will cause an error in the phase of the sinusoids at the end of the master delay line MD. In order to avoid this, the accuracy, of the tap locations has had to be high, but this is expensive. In order to reduce the accuracy required and decrease the expense, the following circuit elements are preferably provided. The output of a test oscillator O₂ is energised by command from the microprocessor 2. The output of O₂ is connected to the input of the amplifier 10 and to one input of a phase detector PD. A mixer M₂ is connected between the end of the master delay line MD and the other input of the phase detector PD. The local oscillator O₁ that was coupled to the phase selector 14 for the mixer M₁ is also coupled to the mixer M₂ so as to make the frequency of the oscillations provided to the phase detector PD from the mixer M₂ the same as the frequency of the test oscillator O₂. The output of the phase detector PD is applied via an A/D device 20 to the microprocessor 2. During the test procedure, the microprocessor 2 sets the phase selector 14 at some predetermined phase and causes the tap selector 12 to connect the output of the mixer M₁ to all of the taps on the master delay line MD in succession. As the phase indication for each tap is received from the A/D device 20, the microprocessor 2 determines the difference between it and the ideal value and outputs to the RAM 4 the number of the phase that is closest to the value that should be used in order to attain the desired degree of phase coherence. These numbers are stored in the RAM 4 and control the phase of the oscillator O₁ that is applied to the mixer M₁. In the event that a selected tap on the master delay line MD is so erroneously located that its delay is in error such that the desired pulse envelope overlap is not attained, the quality of the image would be reduced because an insufficient number of sinusoids would be added together. This would require a change in tap as well as the probable selection of a different phase for the mixer M₁. If the spacing between taps is not greater than one cycle of IF frequency, the accumulation of the phase errors would indicate the number of cycles of error in the delay and could be used by the microprocessor 2 to select a different tap.

Claims: An ultrasonic system comprising an array of transducer elements (8) capable of outputting electrical impulses in response to pressure waves, means (6) coupled to said transducer means for causing them to periodically transmit bursts of ultrasonic carrier pressure waves in different directions; a master delay line having a plurality of input taps and an output, each tap corresponding to a nominal delay; phase changing range focussing means (O₁,M₁,2,4,14) having inputs and outputs, said inputs being connected to the electrical outputs of said transducer elements, the phases of the outputs of each of said range focussing means being adjusted for attaining phase coherence at the output of said master delay line; tap selecting means (12) for respectively connecting the outputs of said range focussing means to selected taps on said master delay line; characterised in that the system also comprises means (O₂, M₂, PD, 20, 4) for measuring the errors in actual delay with respect to the nominal delays associated with each of the taps of said master delay line and for storing indications of these errors; said phase changing range focussing means also being responsive to the indication of error stored for each tap, so as to compensate the output of the master delay line for the errors in the delays associated with said taps. A system as claimed in claim 1 comprising a first mixer (M₁) having first and second inputs and an output, a test oscillator (O₂) coupled to said first input, a second oscillator (O₁), means (14) for adjusting the phase coupled being between said second input of said first mixer and said second oscillator, a second mixer M₂ having two inputs and an output, means for coupling one of said inputs of said second mixer to the output of said master delay line (MD), means for coupling the other input of said second mixer to said second oscillator, phase detecting means (PD) for deriving at an output thereof an indication of the difference in phase between alternating current voltage waves respectively applied to two inputs thereof, means for coupling one of said latter inputs to said test oscillator, means for coupling the other one of said latter inputs to the output of said second mixer, and means (2,4,20) for storing indications of phase differences that appear at the output of said phase detecting means.

Inventors: SAVORD, BERNARD J.
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1987
Main classification: G01S7
IPC codes: G01S7, G01S15, G10K11, A61B8, H03H17