Method and apparatus for the characterization of tissue or other structure

A method and apparatus are provided for determining correlation length of body tissue, metals, crystals, or other materials, substances and the like having an ordered internal stucture (hereinafter structures) and for utilizing such correlation length to test, classify or otherwise characterize the structure. The structure is scanned using standard ultrasonic scanning technology and the output from such scan is processed to determine correlation length in one dimension (length), two dimensions (area), three dimensions (volume). A fourth dimension of time may also be used with one or more of the other three dimensions in the correlation length determination.

FIELD OF THE INVENTION 
This invention relates to nondestructive testing and classification and 
more particularly to a method and apparatus for utilizing the output 
signals from an ultrasonic scanner to determine the correlation length of 
a tissue or other material or structure to be tested or classified, such 
correlation length being a generally unique characteristic which may be 
utilized to classify a structure and thereby to define certain 
characteristics thereof. 
BACKGROUND OF THE INVENTION 
There are a number of fields in which improved noninvasive and 
nondestructive testing and characterization techniques are required. In 
the field of medicine, various noninvasive testing techniques currently 
exist, such as ultrasonic scanning and x-rays, which produce images of 
bone or tissue. Such images are useful for detecting abnormalities in size 
or shape of an organ, for detecting blockages or other abnormalities in 
arteries or for detecting fractures in bones. However, such images 
generally do not reveal chemical or other structural changes in tissue or 
bone which may be useful in the early diagnosis of various medical 
problems. For example, a change in bone structure might be indicative of 
the onset of osteoporosis, while changes in the organization of certain 
cells within an organ might be an early indication of cancer. Other organ 
diseases, including those of the heart, lung, liver and kidney, might also 
result in organizational changes in all or a portion of the affected organ 
which, if detected, could be used for diagnostic purposes. Further, since 
a noninvasive procedure such as ultrasonic scanning which does not involve 
any known risk or substantial discomfort to the patient may be performed 
at frequent intervals, such a technique could also be utilized to 
determine the effectiveness of various treatment regimens so that such 
regimens may be adjusted to meet the needs of the patient. 
Similarly, with the aging infastructure in the United States and other 
countries, aging airline fleets and the like, there is an increasing need 
for a capability to nondestructively test structures, composite materials 
and equipment before problems develop. Again, various techniques are 
currently available which can detect cracks, breaks, bends or similar 
abnormalities which would turn up when an image of a structure, including 
hidden structures, is produced. However, problems in such structures can 
also arise as a result of oxidation, structural fatigue, or other 
compositional or structural changes which may occur as a result of time, 
use, shocks or corrosion which might not show up in an image of the 
structure. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of this invention, it has been found that 
correlation length is a unique characteristic of materials, substances, 
etc. having ordered internal structures such as those within human tissue 
or various crystals. The structure may be scanned using standard 
ultrasonic scanning equipment and technology. By scanning, for example, 
tissue which is known to be healthy norms can be established for various 
types of tissue. Similarly, norms can be established for other structures 
such as crystals. With such norms established for normal and/or abnormal 
structures, it is possible by scanning the structure to be categorized 
with an ultrasonic scanner, and processing the output from the ultrasonic 
scanner to determine the correlation length for the structure, to 
determine if the structure is normal or abnormal. Such determination may 
be a simple binary determination wherein a threshold is established, with 
readings on one side of the threshold being considered normal and readings 
on the other side of the threshold abnormal. More detailed determinations 
may be made from the varying correlation lengths of the structures. 
The correlation lengths of human tissue may also change with age. Similar 
changes may also occur in other cellular structures. Thus, correlation 
length may also be usable, by establishing suitable norms, for dating 
unknown items or for other similar purposes. 
Correlation length is obtained by mathematically manipulating a covariance 
function of the ultrasonic scanner output. It is preferable that the auto 
covariance be utilized for this purpose. The covariance function, and thus 
the correlation length, may be determined in a single dimension as length, 
in two dimensions as an area, in three dimensions as a volume, or in one 
or more of the three dimensions plus time. For one dimension, the 
determination is made on a single ultrasonic beam. For two dimensions, the 
determination is made on a succession of ultrasonic beams as the beam is 
swept across the tissue or other structure. For three dimensions, in 
addition to the beam sweep, the scanner may also be moved in known ways in 
a direction perpendicular to the sweep to provide three dimensional 
information. By providing successive sweeps of the same line, area or 
volume, changes with time can also be noted, thus adding a potential 
fourth dimension. An equation for determining correlation length in 
general is presented in the material to follow as is a specific circuit 
for implementing the determination of correlation length. 
The foregoing and other objects, features and advantages of the invention 
will be apparent from the following more particular description of a 
preferred embodiment of the invention as illustrated in the accompanying 
drawings:

DETAILED DESCRIPTION 
FIG. 1 illustrates a system 10 which may be utilized to characterize a 
structure 12 by use of correlation length. The system may also be utilized 
to display correlation lengths in one, two or three dimensions in a 
predetermined format and to make certain determinations from the computed 
correlation lengths for structure 12 to, for example, identify the 
structure or determine if any abnormalities exist in the structure. The 
system ma also be utilized for localizing abnormalities in a given 
structure and may, in some instances, also be useful in dating a 
structure. 
The structure 12 may, for example, be a human or animal organ such as a 
heart, liver, kidney, or the like, some other tissue structure of a body, 
or some type of crystal or other material such as a metal or other 
material being used in a construction or manufacturing application. 
The structure 12 is scanned by a standard ultrasonic scan head 14 which 
may, for example, be an electronic phased array scan head or a 
mechanically rotating scan head. Signals to drive scan head 14 to project 
an ultrasonic beam 16 at structure 12 are obtained from a standard 
ultrasonic scanner system 18 and echoes picked up by head 14 from 
structure 12 are applied to scanner system 18. 
In a standard system, the output signal "g" from the ultrasonic scanner 18 
is utilized to drive a scan converter 20 which, in turn, causes an image 
of the structure 12 being scanned to appear on a display 22. Display 22 
would typically be a standard cathode ray tube display monitor. In 
accordance with the teachings of this invention, an additional device 24 
is inserted in the system between scanner 18 and scan converter 20 to make 
the correlation length (S) determination for the structure. While in the 
discussion to follow it will be assumed that device 24 is implemented in 
hardware as shown in FIG. 4, and this may be the fastest implementation, 
it is to be understood that the determination of correlation length "S" 
could also be done by programming a processor used elsewhere in the system 
to perform this function, may be a mircroprocessor used solely for this 
function, or may be a processor operating with a microcode ROM. The last 
implementation may be the preferred embodiment, effecting a reasonable 
compromise between cost and speed of operation. 
The correlation length determined by element 24 may be applied to scan 
converter 20, causing a display of correlation length in an appropriate 
format to appear on display 22. The format for the display may, for 
example, be graphic, as shown in FIG. 3, may be alphanumeric, or may be 
indicated as a change in brightness, color or the like for the structure 
under test or a part thereof, depending on the length at the given point. 
The correlation output from element 24 may also be applied as one input to 
a processor 26, the other input to this processor being from a correlation 
length storage device 28. Processor 26 may be a general purpose 
microprocessor, some type of special purpose circuit which may receive an 
input over line 30 indicating a type of structure being scanned or a 
processor used for other functions which is also suitably programmed to 
perform this function. Store 28 may contain normal correlation lengths or 
normal correlation length ranges for various structures, for example, for 
various human organs where the system is being utilized for medical 
diagnosis, and may also contain indications of various types of 
abnormalities which may be indicated for a particular type of tissue with 
different correlation lengths or length ranges. Alternatively, a threshold 
value may be stored for a given tissue or organ with correlation lengths 
on one side of the threshold being considered normal and correlation 
lengths on the other side of the threshold being considered abnormal for a 
given type of tissue. When the system is being used for other 
applications, similar information might be stored for other appropriate 
types of structures. 
Processor 26 retrieves appropriate correlation length information for the 
structure 12 under test from store 28, compares current inputs from 
element 24 against the retrieved correlation length values, and generates 
an appropriate output to indicate whether the structure under analysis is 
normal or abnormal and, under proper circumstances, the nature of the 
abnormality. Abnormality may exist for the entire structure under 
analysis, for example, for a liver or kidney, or may be localized in a 
particular region of the organ as, for example, with cancer. 
The output from processor 26 is applied to scan converter 20 and may be 
utilized to cause a display either in addition to or instead of the 
display caused by the output from element 24. Again, the information may 
be conveyed in graphic or alphanumeric form or may be reflected by 
changing the color, intensity or the like on the image of the structure 
being generated on display 22 to reflect a particular abnormality and 
possibly to reflect the region of the organ or the like where abnormality 
exists. 
FIG. 2 illustrates the scanning of the structure 12, FIG. 2 basically being 
an image 12' of the structure as it might appear on display 22. Scan head 
14 is adapted to successively generate a plurality of ultrasonic scanning 
beams which are illustrated as the beams 16' in FIG. 2. As illustrated for 
the beam 16A, each line has a plurality of points 32 thereon at which 
correlation length determinations may be made. For example, there may be 
100 to 200 scan lines 16' during a given scan with approximately 500 
points 32 at which correlation length determinations are made for each of 
the lines 16'. 
A single scan line 16 (or 16'), with correlation length being determined at 
each point 32 therealong, provides a one dimensional indication of 
correlation length for the structure 12. Determining correlation length 
using all the scan lines 16 gives a two dimensional correlation area for 
the structure. Moving scan head 14 o scan successive sections of structure 
12 provides a three dimensional indication of correlation volume. The 
fourth dimension of time may be added by repeating a one, two or three 
dimensional scan at successive time intervals. The values (S) determined 
at each point 32 may be stored and used to generate a correlation length 
indication for such point, or the values at each point may be summed or 
otherwise combined in a predetermine way to obtain a correlation length 
value (or values) which may be used for comparison or other purposes. 
FIG. 3 illustrates what correlation length S represents. FIG. 3 is a graph 
of correlation length (S) versus range (distance) along the scan line 16A 
shown in FIG. 2. The value of (S) goes u in regions of the scanned 
structure where the scale of organization in the structure is measurably 
larger; the value of (S) goes down in those regions where the scale of 
organization is measurably smaller. These changes in S along any one scan 
line, and in particular along line 16A as illustrated portray the 
variation in the measurable scale of organization within the scanned 
structure. The lower limits on the measurable scale of organization are 
dictated by the wavelength of the ultrasonic beam being utilized and to 
some extent by the focus of the beam. Thus, to the extent it is desired to 
measure correlation length with finer resolution on scale of organization, 
a higher frequency, shorter wavelength ultrasonic scanning beam must be 
utilized. Such measurable variations in correlation length and the 
localization of these variations within the field of view provided by all 
the scan lines, 16, can render in image or numeric form indications of 
defects, abnormalities, variations in composition, in strain, in density, 
in velocity of propogation, in velocity of motion, in elasticity and in 
the order within the scanned structure. 
FIG. 4 shows a circuit which might be utilized as the circuit 24 to 
generate correlation length at each sample point 32. The circuit of FIG. 4 
basically implements the following equation for correlation length S: 
##EQU1## 
In the above equation C(.DELTA..chi.), represents the covariance function, 
this function being the autocorrelation function for the preferred 
embodiment. 
In particular, the input to the circuit 40 is the g.sub.1 signal output 
from ultrasonic scanner 18. This signal is fed to a finite impulse 
response filter 42. Filter 42 consists of a sampler or tapped analog delay 
line 44 which stores the values for a predetermined number of successive 
sample points 32, and a summing circuit 46 to which the values stored in 
delay line 44 are applied. 
The summing circuit 46 also functions to divide the summed value by the 
number of inputs so that the output from circuit 42 on line 48 is the 
average value (g.sub.1) of the g.sub.1 values stored in delay line 44. A 
variable tap is used to obtain a value g.sub.2 on line 50 which is a 
delayed version of g.sub.1. The delay can vary from zero to the maximum 
delay with the exact value of this delay not being critical. 
The moving estimate of value on line 48 and the delayed value on line 50 
are applied as the two inputs to a difference circuit 52 which functions 
to subtract the value on line 48 from the value on line 50. From the 
equations previously provided, it is seen that the output on line 54 from 
difference circuit 52 is the value "f". The f signal on line 54 is applied 
through two channels. The first channel includes a finite impulse response 
filter 56 which functions to integrate the f signal. The output from 
circuit 56 is then applied as both inputs to a multiplier circuit 58. The 
resulting output on line 60 is a taper weighted version of the 
.intg.C(.DELTA..chi.)d.DELTA..chi. function of the prior equations and is 
written here as .intg.C.sub.w (.DELTA.t)d.DELTA.t. This value is applied 
as the numerator input to divider circuit 62. 
The signal on line 54 is also applied as both inputs to a multiplier 
circuit 64 which functions to perform an f.sup.2 function. The f.sup.2 
output from circuit 64 is applied to finite impulse response filter 66 
which effectively integrates the output from circuit 64, resulting in a 
signal on line 68 which is equal to the C(o) function of the prior 
equations and is written here as C.sub.e (o) The signal on line 68 is 
applied as the denominator input t divider circuit 62. The output from 
divider circuit 62 gives a measure for correlation length (S). 
The value (S) for a known structure may, for example, be stored in store 28 
while the value (S) for unknown structures may be applied to processor 26 
for analysis and classification. 
A system has thus been provided which permits tissue, crystals or other 
structures or materials to be determined and categorized by determining 
their correlation length and utilizing such correlation length to effect 
the desired characterization and differentiation. 
While the invention has been particularly shown and described above with 
reference to a preferred embodiment, it is apparent that the foregoing 
other changes in form and detail may be made therein by one skilled in the 
art while still remaining within the spirit and scope of the invention.