WO1984001435A1

WO1984001435A1 - Ultrasonic imaging system with cross correlation

Info

Publication number: WO1984001435A1
Application number: PCT/US1983/001469
Authority: WO
Inventors: Philip Shepard Green; James Fred Havlice; John Franklin Holzemer
Original assignee: Stanford Res Inst Int
Priority date: 1982-09-29
Filing date: 1983-09-22
Publication date: 1984-04-12
Also published as: GB8412805D0; NL8320317A; GB2137752A; DE3390245T1; IT8349072A0; IT1168792B; EP0120074A1; JPS59501760A

Abstract

An ultrasonic imaging system (11) for soft tissue includes an array (13) of transducer elements (15-1 through 15-21) for receiving ultrasonic signals reflected from within the medium being examined. A plurality of signal delay lines (22-1 through 22-21) are connected to receive the ultrasonic signals from each of the transducer elements (15-1 through 15-21) of the array (13). A correlation processor (26) is also connected to receive the ultrasonic signals from each transducer element (15-1 through 15-21) in the array (13) for determining differences in delay for each signal from inhomogeneities within the tissue, either through use of a plurality of cross-correlation values for each signal or by identifying a peak amplitude for each signal and comparing a temporal position of the peak amplitudes for each signal, and for adjusting a length of delay of at least one of the plurality of delay lines (22-1 through 22-21) based on the delay differences. As a result of the adjustment, the ultrasonic signals are provided as substantially in phase outputs from each of the plurality of delay lines (22 through 22-21).

Description

Ultrasonic Imaging System With Cross Correlation

CROSS-REFERENCE TO RELATED APPLICATIONS

This application, a concurrently filed, commonly assigned application by James L. Buxton entitled ''VARIABLE DELAY MEMORY SYSTEM" and a concurrently filed commonly assigned application by David A. Wilson, James L. Buxton, Philip S. Green, Donald J. Burch, John F. Holzemer and S. David Ramsey, Jr. entitled "ULTRASONIC IMAGING SYSTEM WITH CORRECTION FOR VELOCITY INHOMOGENEITY AND MULTIPATH INTERFERENCE, USING AN ULTRASONIC IMAGING ARRAY", are directed to related inventions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved ultrasonic imaging system with image enhancement. More particularly, it relates to such an ultrasonic imaging system in which images obtained with the system are improved by correction for erroneous variations in ultrasonic signals reflected from within the object being examined, due to inhomogeneities within the object. Most especially, it relates to such a system which corrects for variations in time delays of signals reflected from within soft tissue being examined with the system. 2. Description of the Prior Art The use of ultrasonic waves in apparatus for the examination of solid objects is now a well known and comparatively well developed art. In such apparatus, an array of ultrasonic transducer elements is used to transmit ultrasonic waves into the object, and reflections of the waves from within the object are used to define geometry and related characteristics of the object's interior. Such ultrasonic imaging apparatus has been found to be particularly useful in medical applications as a non-invasive diagnostic tool. The state of the art in such medical applications has been reviewed, for example, by Havlice and Taenzer, "Medical Ultrasonic Imaging," Proceedings of the IEEE, Volume 67, No. 4, April 19, 1979, pages 620 to 641.

As pointed out in the Havlice and Taenzer article, one type of medical ultrasonic imaging apparatus is of the phased array type, in which all of the ultrasonic transducer elements in the array are activated simultaneously, but different length-of-delay lines are used to direct the ultrasonic waves in a sector scan and sometimes to focus the ultrasonic wave to a particular depth in the sector field of vision.

Specific examples of prior art ultrasonic imaging apparatus employing different length-of-delay lines include U.S. Patents 3,919,024; 3,936,791; 4,058,003; 4,155,259; and 4,180,790. However, the different length-of-delay lines employed in the prior art are for the purpose of steering the ultrasonic beam and focusing the ultrasonic beam by coherent summation of signals reflected from different depths within an object being examined. In addition, it is further known in the art that ultrasonic signals will pass through different materials at different velocities. For example, Chivers, R.C., "The Scattering of Ultrasound from Human Tissues - Some Theoreti cal Models," Ultrasound Med. Bio., Volume 3, pages 1-13 (1977) , discloses the scattering effect of inhomogeneous tissue on ultrasonic signals due to velocity differences in the materials making up the inhomogeneous tissue. Further recognition of such velocity differences and their effect on ultrasonic images is given by Pierce, G. et al, "The Effects of Tissue Velocity Changes on Acounstical Interfaces", J. Ultrasound Med. Volume 1, pages 185-187 (1982). Some attempts have also been made in the prior art to correct for image distortion produced by such velocity differences. Greenleaf et al, "Measurement of Spatial

Distribution of Refractive Index in Tissues by Ultrasonic Computer Assisted Tomography," Ultrasound Med. Bio., Volume 3, pages 327-339 (1978) and Greenleaf et al, "Refractive Index by Reconstruction: Use to Improve Compound B-Scan Resolution," in Acoustical Holography 7, L. W. Kessler, ed. (Plenum Press, New York, 1977), disclose the use of computed tomographic reconstruction algorithms to correct for time delay errors, but their techniques require a large data base and lengthy computation, and are therefore not useful in realtime imaging. Phillips et al, "Sampled

Aperture Techniques Applied to B-Mode Echoencephalography," in Acoustical Holography 6, N. Booth ed. (Plenum Press, New York, 1975) , disclose the use of phase variation correction and single value cross-correlation to correct for time delay errors produced by skull bone aberrations, but the techniques there described are suitable only for certain types of time delay errors and are therefore not applicable for correcting time delay errors produced by soft tissue inhomogeneities.

The use of a segmented, annular transducer array for ultrasonic imaging apparatus is also disclosed in U.S. Patent 4,270,546, for characterizing a preferred direction of fibrous tissue inside a biological structure, but with no suggestion of utilizing such an array structure to compensate for time delay errors.

Thus, while the art of ultrasonic imaging is a well developed one, there remains a further need for improved techniques of correction for time delay errors, particularly such errors produced by inhomogeneities in soft tissue.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an ultrasonic imaging apparatus for examining soft tissue or other inhomogeneous medium which is capable of correcting for different signal delays in ultrasonic signals reflected from the soft tissue or other medium,which different delays are produced by widely varying inhomogeneities in the tissue or other medium.

It is another object of the invention to provide such an ultrasonic imaging apparatus in which the corrections for the different signal delays are provided on a realtime basis.

It is a further object of the invention to provide such an ultrasonic imaging apparatus in which dynamic focusing of the ultrasonic examining beam may be employed.

The attainment of these and related objects may be achieved through use of the novel ultrasonic imaging apparatus herein disclosed. The ultrasonic imaging system of this invention includes an array of transducer elements for receiving ultrasonic signals reflected from within soft tissue or another medium being examined. There are a plurality of signal delay means, with one of the plurality of delay means being connected to receive the ultrasonic signals from each of the transducer elements in the array. A means is also connected to receive the ultrasonic signals from each transducer element in the array for determining differences in the array for each signal resulting from inhomogeneities within the medium through use of a plurality of cross-correlation values for each signal. This means also adjusts a length of delay of at least one of the plurality of delay means based on the delay differences so that the ultrasonic signals are provided as substantially in phase outputs from each of the plurality of delay means. In another form of the invention, the delay difference determining means identifies a peak amplitude for each signal and compares a temporal position of the peak amplitudes to determine the delay difference.

In a preferred form of the invention, transducer elements of the array are substantially isolated electrically and acoustically from one another. The plurality of delay lines are connected to supply the output simultaneous ultrasonic signals to a summing means. The array of transducer elements is a segmented annular array.

In the use of the ultrasonic imaging apparatus of this invention, the differences in time delays of the reflected ultrasonic signals due to soft tissue inhomgeneities may be calculated and the time delay of the

appropriate delay lines adjusted to make the output ultrasonic signals from the delay lines substantially in phase with respect to each other on a realtime basis. As a result, ultrasonic imaging may be employed in situations where it was previously not practical and additional information may be obtained from ultrasonic images due to improved image quality over that obtained previously.

The attainment of the foregoing and related objects, advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention, taken together with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view of a portion of the invention in use.

Figure 2 is a block diagram of an ultrasonic imaging apparatus in accordance with the invention.

Figure 3 is a plan view of a transducer element array for use with the apparatus of Figure 2.

Figure 4 is a more detailed block diagram of a portion of the apparatus shown in Figure 2.

Figure 5 is a more detailed block diagram of a portion of the apparatus shown in Figures 2 and 4.

Figure 6 is a plot of distance versus delay, useful for understanding operation of the apparatus in Figures 2-5. DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, more particularly to Figure 1, there is shown an ultrasonic transducer element array 10 as used in the invention and a lens 12 for focusing ultrasonic signals from the transducer element array 10 to a focal point 14. Although the lens 12 may be made of transparent plastic, it should be recognized that such focusing can be accomplished with a curved transducer or solely electronically as well. As shown in Figure 1, when used to examine tissue, ultrasonic signals from transducer array 10 are transmitted into tissue 16 and are reflected from focal point 14 along path A, which passes through muscle 18 and fat region 20. Path B is entirely through muscle 18.

As is known in the art, the velocity of such ultrasonic signals through various tissues varies significantly. Table I shows typical published velocities for various types of tissue. As can be seen from the table, the velocities for blood and muscle are similar, while the velocity in fat is 6-10% less.

It thus can be seen that the block of fat 20 surrounded by muscle 18 will produce a time delay difference between paths A and B of

where L is the path length in the fat 20. For example,

5mm of fat 20 (which would only be 10% of the path length for a 5cm deep artery) will cause a delay error of 0.32 microseconds, using average velocity values for fat and muscle. This represents a phase shift of more than 360º at a signal frequency of 4MHz, and will result in severe defocusing of the ultrasonic beam. In accordance with this invention, variable lengths of delay lines can be used to correct image distortion produced by such time delay differences.

Figure 2 shows a sector scan ultrasonic imaging apparatus 11 in accordance with the invention. The apparatus 11 includes an array 13 of ultrasonic transducers 15-1 through 15-21. The transducer elements 15-1 through 15-21 are each connected to a time gain controlled amplifier 17-1 through 17-21 by lines 19-1 through 19-21. Amplifiers 17-1 through 17-21 have their outputs connected by lines 21-1 through 21-21 to delay lines 22-1 through 22-21. The outputs of amplifiers 17-1 through 17-21 are also connected by lines 24-1 through 24-21 to correlation processor 26, the nature of which will be explained in more detail below in connection with Figure 5. The correlation processor 26 is also connected to delay lines 22-1 through 22-21 by lines 27-1 through 27-21. Output lines 28-1 through 28-21 respectively connect the delay lines 22-1 through 22-21 to a summing circuit 30. Output line 32 of the summing circuit 30 is connected to a detector circuit 34. Output line 36 from the detector circuit 34 is connected to a low pass filter (LPF) circuit 38. Output line 40 from the LPF circuit is connected to scan converter circuitry 42. A mechanical scanning system 44, which oscillates the transducer array 13 to produce the sector scan, is also connected to the scan converter circuitry 42 by line 46. The scan converter circuitry 42 converts angular coordinates of the mechanical scan produced by the mechanical system 44 to conventional X-Y coordinates for display 45, which is connected to the scan converter circuit 42 by line 47. Display 45 is of the conventional raster scan type. Front panel controls 48 are connected to ultrasonic transmitters 50 by line 52. The ultrasonic transmitters 50 are respectively connected to the transducer elements 15-1 through 15-21 by lines 54-1 through 54-21, transmit/receive switching circuits 53-1 through 53-21, and lines 55-1 through 55-21. The transmit/receive switching circuits 53-1 through 53-21 prevent overload of the receivers during transmission. The front panel controls 48 are also connected to the time gain controlled amplifiers 17-1 through 17-21 by lines 56-1 through 56-21. In addition, the front panel controls 48 are connected to the correlation processor 26 by line 58, and to the scan converter circuit 42 by line 60.

In operation of the system shown in Figure 2, ultrasonic signals supplied by transmitters 50 are transmitted into soft tissue to be examined with the apparatus by transducers 15-1 through 15-21. These signals are reflected from within the soft tissue being examined and back toward the transducers 15-1 through 15-21. As a result of inhomogeneities within the soft tissue the reflected signals received at the transducers 15-1 through 15-21 are delayed for one or more of the transducers. The signals supplied on lines 19-1 through 19-21 to amplifiers 17-1 through 17-21 are therefore not in phase with respect to one another. The amplified signals are supplied on lines 21-1 through 21-21 and 24-1 through 24-21, respectively, to delay lines 22-1 through 22-21 and the correlation processor 26. As a result of the cross-correlation operations performed by the correlation processor 26, to be explained in more detail below in connection with Figure 5, appropriate signals are supplied on lines 27-1 through 27-21 to the delay lines 22-1 through 22-21 to cause adjustments of the times of delay in certain of the delay lines 22-21 as needed so that the output signals from the delay lines as supplied on lines 28-1 through 28-21 are brought back into phase with respect to one another. The now in phase signals are added in summing circuit 30 and supplied through detector circuit 34 and LPF circuit 38 to scam converter circuit 42. Scan converter circuit 42 utilizes the sector scanning information supplied by mechanical scanning system 44 on line 46 and the summed simultaneous signals to generate the ultrasonic image on display 45.

While the peak detection of plurality of cross-correlation value approaches of this invention can be used with essentially any array of ultrasonic transducers suitable for ultrasonic imaging applications, a segmented annular array 13 of the type shown in Figure 3 is one especially preferred. In this array, element 15-1 is centrally disposed, with the remaining elements 15-2 through 15-21 being formed from segments of concentric annuli around the first element 15-1. Each of the transducer elements 15-1 through 15-21 are isolated electrically and acoustically from each other.

Figure 4 shows further details of the ultrasonic imaging system 11 of Figure 1. It should be understood that the signal detecting and delay circuitry indicated at 100-1 is supplied for each of the individual transducer segments 15-1 through 15-21 shown in Figures 2 and 3. The transducer segment 15-1 is connected to the time gain controlled amplifier 17-1 through an impedance matching transformer 102-1 by lines 104-1 and 106-1. The output of amplifier 17-1 is connected to an analog-to-digital converter 108-1 by line 110-1. A 25 MHz clock signal on line 112-1 forms a second input to the analog-to-digital converter 108-1. The output of analog-to-digital converter 108-1 is supplied on line 114-1 as an input to a dual port random access memory (RAM) 116-1, which serves as a delay line in the system. An input addressing circuit 118-1 is connected by line 120-1 to the dual port RAM 116-1. The input addressing circuit 118-1 receives the 25 MHz clock signal on line 122-1 and generates an address for storing information on line 114-1 in the delay line memory 116-1. The output of dual port RAM 116-1 is supplied on line 124-1 to summing circuit 30. Comparable signal detection and processing circuits for the other transducer segments 15-2 through 15-21 provide additional inputs on lines 124-2 through 124-21 to summing circuit 30. The output of dual port RAM 116-1 is also supplied on line 126-1 to a buffer memory 128-1. The buffer memory 128-1 is connected by line 130-1 to the correlation processor 26. Additional inputs are supplied to the correlation processor 26 on lines 130-2 through 130-21 from other signal detection and processing circuits associated with the segments 15-2 through 15-21. The output of correlation processor 26 is provided on line 132 to microcomputer 134. The outputs of microcomputer 134 are provided on lines 136-1 through 136-21 to output addressing circuit 138-1 and corresponding output addressing circuits in the signal detection and processing circuits for the other transducer segments 15-2 through 15-21. Output addressing circuit 138-1 provides addresses on line 140-1 to the dual port RAM 116-1, in order to provide the output signals on line 124-1. The summing circuit 30 is connected to detector 34 by line 32. This digital implementation of the delay portion of the signal detection and delay circuits 100-1 provides a low cost and reliably controlled implementation for the delay lines 22-1 through 22-21 of Figure 2. This form of the delay lines is also highly compatible with the correlation processor 26 and the microcomputer 134 which interacts with the correlation processor and the delay lines.

Figure 5 shows details of the correlation processor 26 in Figure 4. Lines 130-1 through 130-21 are connected from buffer memory 128-1 in detection and delay circuitry 100-1 and the corresponding buffer memories in the signal detection and delay circuitry for the other transducer segments 15-2 through 15-21 to fast RAH file cache memories 200-1 through 200-21. A first output address counter 202 is connected by odd number designated lines 204-1 through 204-21 to the corresponding odd number designated cache memories 200-1 through 200-21 to provide control signals for those cache memories. A second output address counter 206 is connected by even number designated lines 204-2 through 204-20 to provide control inputs to the even number designated file cache memories 200-2 through 200-20. Multiply accumulator circuits 208-1 through 208-20 are connected to receive X and Y inputs from adjacent file cache memories 200-1 through 200-21 by lines 210-1 through 210-20 and 212-1 through 212-20, respectively. The multiply accumulator circuits 208-1 through 208-20 supply their products on lines 214-1 through 214-20 to buffer memories 216-1 through 216-20, which also may be implemented with fast RAMs. The buffers 216-1 through 216-20 are connected to compare circuits 218-1 through 218-20 by lines 220-1 through 220-20. The compare circuits 218-1 through 218-20 are interconnected through lines 222-1 through 222-19 so that signals from adjacent buffers 216-1 through 216-20 can be compared. Peak picking logic circuits 224 are connected to each of the compare circuits 218-1 through 218-20 by lines 226. Line 132 provides data inputs from the cross-correlation processing in correlation processor 26 to microcomputer 134 (Figure 4) . With a 100 nanosecond multiply-accumulator time, a 256 bit sample length, a 40 sample time shift and 200 lines the cross-correlation calculation of processor 26 is carried out at a speed of about 205 microseconds. Thus, the cross-correlation calculationsin accordance with this invention may be carried out in the apparatus on a realtime basis.

In addition to adjusting the time delays of signals received from the segments 15-1 through 15-21 of the annular array, the system of Figures 2-5 allows the delay lines 22-1 through 22-21 to be used for dynamic focusing of the ultrasonic signals within tissue being examined. Figure 6 is a plot of delay in nanoseconds versus distance from the transducer array 12 for a 15 cm focus range within the tissue. The lines 300-1 through 300-6 and 302-1 through 302-6 represent the delay curves for each ring of the annular array 12 into which the transducer segments 15-1 through 15-21 are arranged. As shown, the lines 300-1 through 300-6 and 302-1 through 302-6 converge at 304 for a predetermined geometric focus for the annular array 13. In order to provide dynamic focusing varying from that predetermined geometric focus, delays for each of the six annular rings as shown in the curves 300-1 through 300-6 and 302-1 through 302-6 are provided. In addition to such relative time delays for focusing of the ultrasonic signals to different depths within the tissue being examined, the time delay correction for errors induced by Inhomogeneities within the tissue are superimposed on any focusing delays provided for signals from transducer elements 15-1 through 15-21. In typical use for examination of arteries such correction delays would vary between 0.01 and 2 microseconds. It should now be apparent to those skilled in the art that a novel ultrasonic imaging apparatus capable of achieving the stated objects of the invention has been provided. The ultrasonic imaging apparatus compensates for time delay errors produced by inhomogeneities in tissue being examined on a realtime basis. The manner in which the time delay errors are corrected in the system of this invention allows correction of such errors arising from many different kinds and shapes of tissue inhomogeneities, without requiring particular relationships between the delayed signals and other ultrasonic signals.

It should further be apparent to those skilled in the art that various changes in form and details of the invention as shown and described may be made. For example, a linear array could be substituted for the segmented annular array shown. It is intended that such changes be included within the spirit and scope of the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. An ultrasonic imaging system for use with an inhomogeneous medium comprising an array of transducer elements for receiving ultrasonic signals reflected from within the medium being examined, a plurality of signal delay means, one of said plurality of delay means being connected to receive the ultrasonic signals from each of the transducer elements of said array, means also connected to receive the ultrasonic signals from each transducer element in said array for determining differences in delay for each signal from inhomogeneities within the medium through use of a plurality of cross-correlation values for each signal, and for adjusting a length of delay of at least one of said plurality of delay means based on the delay differences so that the ultrasonic signals are provided as substantially in phase outputs from each of said plurality of delay means.

2. The ultrasonic imaging system of Claim 1 in which the transducer elements in said array are substantially isolated electrically and acoustically from one another, and said plurality of delay lines are connected to supply the output in phase ultrasonic signals to a summing means.

3. The ultrasonic imaging system of Claim 1 in which said array of transducer elements is a segmented annular array.

4. An ultrasonic imaging apparatus for use with an inhomogeneous medium comprising an array of transducer elements for receiving ultrasonic signals reflected from within the medium being examined, a plurality of signal delay means, one of said plurality of delay means being connected to receive the ultrasonic signals from each of the transducer elements of said array, means also connected to receive the ultrasonic signals from each transducer element in said array for determining differences in delay for each signal from inhomogeneities within the medium through identifying a peak amplitude for each signal and comparing a temporal position of the peak amplitudes for each signal, and for adjusting a length of delay of at least one of said plurality of delay means based on the delay differences so that the ultrasonic signals are provided as in phase outputs from each of said plurality of delay means.

5. The ultrasonic imaging system of Claim 4 in which the transducer elements are substantially isolated electrically and acoustically from one another, and said plurality of delay lines are connected to supply the output simultaneous ultrasonic signals to a summing means.

6. The ultrasonic imaging system of Claim 4 in which said array of transducer elements is a segmented annular array.