Method and apparatus for displaying acoustic signal transit times

In a method and apparatus for displaying at least one acoustic propagation property that is present within a body part suitable for diagnosis by acoustic irradiation, ultrasound transmission signals are transmitted along scan lines into the body part by an ultrasound transducer arrangement. Ultrasound signals that have passed through the body part are acquired as transmitted sound signals with a further ultrasound transducer arrangement arranged opposite the first ultrasound transducer arrangement. Parameter values are calculated from the transmitted sound signals that characterize the acoustic propagation property along the scan lines. The parameter values are portrayed on a display dependent on the corresponding scan lines.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a method for displaying acoustic 
signal transit times that occur within a body part suitable for obtaining 
information therefrom by acoustic irradiation, of the type wherein 
ultrasound transmission signals are transmitted into the body part along 
scan lines by an ultrasound transducer arrangement, the ultrasound signals 
that have passed through the body part are acquired as transmitted 
acoustic signals with a further ultrasound transducer arrangement which is 
disposed opposite the ultrasound transducer arrangement, and wherein in 
acoustical signal times occurring along the scan lines are calculated from 
respective transmission points in time of the ultrasound signals and 
corresponding reception points of time of the transmitted sound signals. 
The invention is likewise directed to an apparatus for implementing the 
method of the type having an ultrasound transducer arrangement for 
transmitting ultrasound transmission signals along scan lines into the 
body part, a further ultrasound transducer arrangement arranged opposite 
the transmitting ultrasound transducer arrangement for acquiring 
ultrasound signals that have passed through the body part as transmitted 
sound signals, a processing unit connected to the two ultrasound 
transducer arrangements for identifying acoustic signal transit times from 
the transmitted sound signals, and a display for visual presenting the 
results. 
2. Description of the Prior Art and Related Subject Matter 
In ultrasound diagnostics, body regions or body parts are scanned with 
ultrasound pulses for producing anatomical tomograms. The image data are 
thereby acquired from echo signals or reflection signals that are 
triggered at boundaries of adjoining regions respectively having different 
acoustic characteristic impedances inside the body. The characteristic 
impedance is the product of the density of, and the speed of sound in the 
tissue types. The locus coordinates for the source of the echo signals in 
the tomogram on the display is obtained from the chronological spacing of 
the echoes from the transmitted acoustic pulse and from the propagation 
direction of the acoustic beam, i.e. from the position of the scan line in 
the sectioned plane. The transmitted acoustic beam is focused as sharply 
as circumstances permit. The chronological spacing is calculated in ranges 
or distances using an average speed of sound, amounting to 1540 m/s for 
soft biological tissue. The boundary surfaces of different acoustic 
impedances that, for example, represent organ boundaries, vessel 
boundaries or fine internal structures, are reproduced with relatively 
good geometrical precision in the tomograms obtained in this way, one form 
thereof being referred to as B-images. 
No acoustic propagation properties that are present inside the body part 
itself, or inside the examination region, can be derived from this type of 
image presentation. Such acoustic propagation properties, for example, are 
the speed of sound or the acoustic absorption. 
Knowledge of the acoustic propagation properties, however, is of 
significance for clinical diagnosis. In many instances, for example, in 
solid tumors the speed of sound is higher than in healthy tissue. These 
slight differences in the speed of sound cannot be recognized in the 
normal image presentation. When, for example, different acoustic 
propagation speeds are found within a scanned section plane, this normally 
leads to a slight geometrical distortion in the ultrasound tomogram that 
cannot be recognized. No conclusion about the speed of sound along the 
individual scan lines can be derived from the tomogram. 
There are, however, known image presentation methods that, for example, are 
disclosed in European Application 0 097 917 or U.S. Pat. No. 4,075,883 
that can directly display the geometrical distribution of such parameters. 
The acoustic signal transit time and/or the absorption are thereby 
measured in an acoustic irradiation method. After acoustically irradiating 
the examination region from a suitable number of directions, the local 
distribution of the speed of sound or of the absorption is calculated from 
the measured values with methods similar to those known from computed 
tomography. These methods are extremely time-consuming and employ 
complicated algorithms. They have not been able to find acceptance in the 
practical clinical routine essentially for two reasons. 
First, there are only few body parts, such as mammaries and gonads that are 
suitable for acoustic irradiation. Second, measuring errors as a 
consequence of refractions occur in ultrasound computed tomography that 
appear as image unsharpnesses in the computer tomogram. 
An acoustic irradiation method of the type initially cited is known from 
the article by Y. Hayakawa, entitled "Mass Screening of Breast Cancer by 
Ultrasound Transmission Technique-Theoretical Considerations", Proceedings 
of the Fifth Symposium on Ultrasound Electronics, Tokyo 1984, which 
appeared in the Japanese Journal of Applied Physics, Vol. 24 (1985), 
Suppl. 24-1, pp. 82-83. A method suitable for mass screening of the breast 
is proposed therein wherein average speed of sound is measured and after 
subtraction of, for example, the speed of sound of water, the result is 
displayed. In practice, however, this method has not proven satisfactory. 
German patent application P 43 09 596.8 (published after the priority date 
of the present application) discloses that regions having different speeds 
of sound can be recognized in the ultrasound image when a boundary surface 
having a precisely known geometrical decision is located behind the 
structure to be diagnosed. Changes in transit time can then be recognized 
at this "reference surface" in the form of deviations from the "normal" 
geometrical appearance of this boundary surface in the displayed image. 
The reference surface can be produced, for example, by attaching a planar 
reflector plate following (in the direction of sound propagation) the body 
part which is acoustically irradiated. A possible disadvantage of this 
technique is that only echo signals can be evaluated which have passed 
through the examination region in the forward and return directions which 
may result in the generation of stronger reflection echoes than arise from 
the reference surface. Moreover, this reference echo can be completely 
absent from or imperceptible in the image as a consequence of acoustic 
absorption, particularly because of the double acoustic propagation path 
and the acoustic scatter of the reflected part in the examination region. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method and an 
apparatus for obtaining an image of body part or body region by 
acoustically irradiating the body part or region wherein sound propagation 
properties of the body part or body region influenced by tumorous tissue 
can be calculated within a relatively short examination time and can be 
acquired with great precision. 
In a method in accordance with the principles of the present invention, 
this object is achieved by displaying the acoustic signal transit times in 
combination with displayed marks that are arranged on image lines 
corresponding to the scan lines; with the spacings of the marks from the 
origins of the image lines correspond to the acoustic signal transit 
times. 
In an apparatus for implementation of the method, this object is achieved 
by connecting a display monitor, on which the invention display is 
presented, to a processing unit for causing the acoustic signal transit 
times to be displayed dependent on the scan lines. 
An advantage of the inventive method and apparatus is that the body part 
must be completely scanned only once. By contrast to known methods, a body 
part capable of acoustic irradiation such as, for example, a mammary, can 
be completely examined in only a few seconds. The outlay is thereby 
comparatively low. Clinical diagnosis of solid tumors is thus enabled 
because the speed of sound in such tumors is usually higher than the speed 
of sound in healthy tissue. This causes an attenuation of the acoustic 
signal transit time of acoustic pulses that have passed through tumorous 
or tumor-affected tissue. Deviations of the acoustic signal transit times 
can be displayed especially well. Given an examination region having an 
identical, average speed of sound throughout, the position of the marks 
reproduces the geometrical arrangement of the ultrasound receiver. 
Deviations in the presentation of the marks from the geometrical 
arrangement of the ultrasound receiver provide indications that a deviant 
speed of sound is present in the examination region traversed by the 
ultrasound signal. Regions having a higher speed of sound shift the mark 
toward the transmitting ultrasound transducer arrangement; regions having 
a lower speed of sound shift it away from the ultrasound transducer 
arrangement. 
The inventive method also has advantages over the aforementioned "reference 
plate method". The signal to be interpreted, i.e. the ultrasound signal 
that is passed through the body part, can be acquired with extremely high 
precision because this signal is significantly stronger than a reference 
echo selected in a B-image. The reasons for this are, first, the 
transmitted ultrasound signal in the inventive method and apparatus must 
traverse the body part only once and experiences a correspondingly low 
attenuation due to absorption, and second, the additional attenuation of 
the acoustic pulse due to an incomplete reflection (reflection 
attenuation) at the reference surface is eliminated. An important 
advantage is also in that there are no disturbing signals before the 
arrival of the transmitted sound signal. Noise signals due to multiple 
reflections in the body part or examination region always appear following 
the principal pulse. By comparison, a whole series of reflection echoes 
that may be strong under certain circumstances and that make the 
interpretation of the reference echo more difficult can be present 
preceding the reference echo in a presentation of the reference plate 
method similar to the B-image. As a consequence of the two-fold 
attenuation of the acoustic pulse due to the forward and return path, this 
reference echo can be completely absent under certain circumstances. In 
summary, the measuring precision is improved and the susceptibility to 
artifacts is reduced in the inventive method compared to known methods for 
identifying and displaying an acoustic propagation property. 
An allocation to the anatomy of the examined body part is possible on the 
basis of display of the acoustic signal transit time dependent on the scan 
lines. 
In an embodiment of the method and apparatus the scan lines lie in the 
sectional plane. Conventional and commercially available diagnostic 
ultrasound apparatus can thus be utilized with little additional outlay 
for identifying the acoustic signal transit times. 
In another embodiment, the scan lines are arranged parallel to one another. 
This achieves optimally short acoustic irradiation paths through the body 
part under examination as well as permitting the acoustic signal transit 
time to be allocated to the scan lines with uniformly good resolution. 
In an embodiment another acoustic absorption occurring along the scan lines 
is calculated from the amplitudes of the transmitted sound signals and is 
displayed as a parameter value of a further signal propagation property. 
It is known from the literature that solid tumors often have an increased 
acoustic absorption. A second diagnostic criterion is thus established. 
The combination of the two criteria of acoustic signal transit time and 
absorption enhances the reliability of the diagnosis. 
In an embodiment further echo signals generated in the body part by the 
ultrasound transmission signals are received with the transmitting 
ultrasound transducer arrangement and a tomogram is produced from the echo 
signals and is additionally displayed on the display monitor, with the 
acoustic propagation properties and the tomogram being correlated with one 
another by means of the common or shared scan lines. A presentation that 
is especially easy for an examining person to interpret is achieved due to 
the common display of the acoustic propagation properties and the 
ultrasound tomogram with the common or shared scan lines. Additionally, 
the reliability of the diagnosis is enhanced further because a tissue 
region containing a tumor can be localized as warranted in the tomogram as 
well as from the acoustic propagation properties displayed and 
correspondingly allocated thereto. 
In another embodiment, examination time can be saved by calculating the 
tomogram and the parameter values from the same acoustic pulses 
transmitted by the ultrasound transducer arrangement. 
In an embodiment of the inventive apparatus reception surface of the 
further ultrasound transducer arrangement is fashioned for 
location-independent reception of the transmitted sound signals along all 
scan lines. The allocation of the parameter values to the anatomy is 
defined from the respective position of the scan system and from the 
momentary scan direction. The ultrasound receiver only supplies the 
measured value of the transmitted sound signal. Identifying the location 
of the reception is not necessary. 
In an especially simple and economic embodiment further ultrasound 
transducer arrangement is a piezoelectric foil. Dependent on the size of 
the surface, such foils can have a high capacitance. In order to prevent 
the received signal value from becoming too small due to this high 
capacitance, the foil can be structured in the form of strips. Each strip 
is then electrically connected to its own reception amplifier or to its 
own inputs of a common amplifier for further-processing the output signals 
produced by the respective strips.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Acoustic propagation properties within a body part 2 suitable for diagnosis 
using acoustic irradiation, for example a female breast, can be calculated 
by means of an ultrasound scan with the apparatus shown in a perspective 
view in FIG. 1. During the examination, the body part 2 is fixed between 
two parallel compression plates 4 and 6, similar to x-ray mammography 
examinations. At least one compression plate, the compression plate 6 in 
the example of FIG. 1, is displaceable parallel to the compression plate 
4. In addition to fixing the body part 2, a uniform layering of the body 
part 2 is thus achieved for the ultrasound examination. 
In addition to the body part 2 fixing, a good acoustic coupling between the 
body part 2 and the compression plates 4 and 6 must be produced. This can 
be assured by using known coupling gels or by using an elastic container 
filled with a coupling fluid (not shown) that is compressed together with 
the body part 2. 
An ultrasound applicator 10 is arranged at that side of the compression 
plate 6 facing away from the body part 2 and is acoustically applied 
thereto. The ultrasound applicator 10 has a linear array 12 of transducer 
elements 14 arranged side-by-side. A linear scan of the body part 2 to be 
examined can be implemented with this ultrasound transducer arrangement 
along parallel scan lines 13 lying in a section plane. For that purpose, 
different groups of transducer elements 14 are activated in a known way. 
The ultrasound applicator 10 is connected to an ultrasound processor 15 
wherein known electronic units and a display 16 are accommodated. The 
basic operation of such an ultrasound processor (with the exception of the 
modifications discussed herein) has been described in many references; for 
example, Erich Krestel, "Bildgebende Systeme fur die medizinische 
Diagnostik", Second Revised and Expanded Edition, 1988, Chapter 11, 
"Sonographie", pp. 557-591. 
In order to permit the body part 2 under examination to be completely 
scanned in a number of different section planes, the ultrasound applicator 
10 is arranged in the compression plate 6 so as to be displaceable 
transversely relative to the scan direction of the linear scan, as 
illustrated by an arrow 17. To that end, guide channels 18 are provided in 
the compression plate 6, with corresponding projections 20 at the 
applicator housing being guided therein. The position of the ultrasound 
applicator 10 can be varied, for example, using a stepping motor (not 
shown). The position of the ultrasound applicator 10 can be identified 
from the control signals for the stepping motor. If, however, the position 
is manually varied, the position must be acquired with a position sensor 
whose position signal is evaluated for identifying the position of the 
scan plane. 
Since the acoustic pulses transmitted by the transducer array 12 must pass 
through the compression plate 6 to the body part 2, the compression plate 
6 is constructed of a material having good acoustic conduction. Moreover, 
the material is selected such that only slight acoustic reflections occur 
at the boundary surfaces. Acrylic glass, for example, is a suitable 
material. 
In another embodiment that, however, is not shown in detail here, in a 
transducer mosaic is integrated into that side of the compression plate 6 
facing toward the body part 2, this transducer mosaic residing in direct 
contact with the body part 2 during the examination and not having any 
mechanically moving parts. Scanning along the scan lines 13 and the 
selection of the individual section planes are controlled and implemented 
completely electronically. 
Of course, a scanning with a single transducer can also be implemented, the 
position thereof in the surface having to be acquired or predetermined. 
For the reception of the acoustic pulses that have propagated through the 
body part 2, a large-area piezoelectric foil 22 is arranged on that 
surface of the compression plate 4 facing toward the body part 2. For 
example, a PVD foil can be utilized as the piezoelectric foil. In order to 
avoid excessively high capacitance values of the foil 22 caused by the 
foil 22 having a large-area expanse, the foil 22 can also be structured 
striplike. Each foil element 24 is then connected to its own pre-amplifier 
in the ultrasound processor 15. 
For evaluating the transmitted sound signals received by the piezoelectric 
foil 22, the ultrasound processor 15 includes an evaluation unit 26 that 
acquires the acoustic signal transit times as well as the amplitudes of 
the transmitted sound signals and allocates them to the scan lines 13 for 
display on the display 16, as set forth in greater detail below. 
FIG. 2 shows the chronological curve of the transmitted sound signals from 
which the parameter values associated with acoustic propagation properties 
of the body part Z along the scan lines 13 are calculated. For example, 
the signal curves a, b and c of three scan lines 13 from which the 
acoustic signal transit times are calculated first are shown. Proceeding 
from a transmitted ultrasound signal 30, a counter is started in every 
instance and is in turn stopped by the received transmitted sound signal 
32. The counter readings corresponding to the acoustic signal transit 
times t.sub.1, t.sub.2 and t.sub.3 are entered into in a memory together 
with an allocation to the corresponding scan lines. 
It should be noted that the signal shapes shown in FIG. 2 are only intended 
to characterize the envelopes of the transmission signal and reception 
signal; the actual ultrasound signals are composed of high-frequency 
pulses in the megahertz range that are a few cycle periods long. 
In the example of FIG. 2, the acoustic signal transit time t.sub.2 in the 
diagram b is shorter than the acoustic signal transit times t.sub.1 and 
t.sub.3 of diagrams a or, respectively, c. This means that tissue having a 
higher speed of sound therein is located in the acoustic propagation path 
of the corresponding scan line 13 than is the case in the two other scan 
lines 13. 
For calculating the absorption along the scan lines 13, the amplitudes of 
the envelopes of the transmitted sound signals 32 are evaluated. In FIG. 
2, the transmitted sound signals 32 in diagram b have a lower amplitude 
than the transmitted sound signals 32 of diagrams a and c because of a 
higher absorption. 
FIG. 3 shows the graphic display of the information of a section plane 
acquired from the ultrasound scan along parallel scan lines 13 in 
accordance with the invention. The graphic illustration includes an 
ultrasound tomogram 38 wherein discontinuous changes of the acoustic 
impedance within the body part 2 have been rendered visible as 
light-colored locations in a known way. A region 40 that has a higher 
speed of sound therein and a higher absorption than the surrounding tissue 
is also present in the section plane shown with the tomogram 38. These 
acoustic propagation properties deviating from the average parameter 
values can be an indication of a tumor. The region 40 may only be slightly 
visible in the ultrasound tomogram 38 under certain circumstances or may 
be overlaid with artifacts and noise, so that it cannot be immediately 
recognized in the ultrasound tomogram 38 by itself. 
In addition to the tomogram 38, the graphic display in FIG. 3 includes the 
acoustic signal transit time along the individual scan lines 13 by means 
of marks 42 allocated to the scan lines 13. For example, the marks 42 can 
appear as colored or as bright spots on the display 16, similar to 
boundaries of different acoustic impedances in the ultrasound tomogram 38. 
The illustration of the marks 42 is selected such that the distance of the 
marks 42 from the origin 43 of an image line which, for example, 
corresponds to the scan line, i.e. to the location of the transducer array 
14, represents the criterion of the acoustic signal transit time. The 
actual distance of the marks 42 from one another is actually tighter than 
shown in FIG. 3, so that the individual marks 42 are smeared to form a 
line 44. Without the presence variations in the speed of sound, the line 
44 would simulate the geometry of the ultrasound transducer arrangement 
22. Given speed of sound differences in the body part 2, this displayed 
presentation is distorted and deviates from the profile of the ultrasound 
transducer arrangement 22. In a manner corresponding to the display of the 
acoustic signal transit times, the absorptions along the scan lines 13 are 
additionally shown with marks 46 that are likewise smeared to form a line 
48 because of the density of the scan lines 13. 
As an example, FIG. 4 shows what is here a perspective illustration of the 
marks 42 for the acoustic signal transit times, these having been 
calculated from all scanned section planes. Given different speeds of 
sound in the body part 2, portrayal resembling a mountain whose height 50 
characterizes regions with higher speeds of sound arises. An indication of 
tumors present in the body part 2 can thus be supplied, their exact 
position being possibly recognizable in the associated tomogram 38. 
FIG. 5 shows an illustration of the acoustic signal transit times in a 
different manner from the display shown in FIG. 4. The acoustic signal 
transit times in FIG. 5 are displayed as contour lines 52, with regions 
between two contour lines being identified additionally if desired by 
coloring in the manner of illustrations of altitude regions in maps. As 
already discussed in connection with the illustration of FIG. 4, the 
section plane having higher speeds of sound therein, i.e. shorter acoustic 
signal transit times, can also be marked and selected. 
The two illustrations of the acoustic signal transit times of FIGS. 4 and 5 
can likewise be selected for overview presentations of the acoustic 
absorption, whereby the illustration of the acoustic signal transit time 
and the illustration of the acoustic absorption can ensue simultaneously 
on the display 16. 
The illustrations of FIGS. 4 and 5 are especially advantageous for 
screening examinations to which ultrasound tomograms 38 are only allocated 
when suspicious regions having increased speed of sound therein or 
increased acoustic absorption have been recognized. The ultrasound 
tomograms 38 can already have been prepared in the ultrasound scanning for 
identifying the acoustic propagation properties and can be retrieved as 
needed from a memory. 
Although modifications and changes may be suggested by those skilled in the 
art, it is the intention of the inventors to embody within the patent 
warranted hereon all changes and modifications as reasonably and properly 
come within the scope of their contribution to the art.