CA1159545A

CA1159545A - Ultrasonic volume measuring system

Info

Publication number: CA1159545A
Application number: CA000364238A
Authority: CA
Inventors: Weston A. Anderson
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1979-11-09
Filing date: 1980-11-07
Publication date: 1983-12-27
Also published as: JPS5675146A; US4341120A; DE3041112A1

Abstract

ULTRASONIC VOLUME MEASURING SYSTEM

ABSTRACT
An ultrasonic imaging system for examining bodily tissue by phasing the actuation of transducers to steer an emitted sound beam in a plurality of radial directions in a fan-shaped sector.
The reflected energy are received from the plurality of radial directions in the sector and the acoustical impedance variations within the bodily tissue is used to provide a reconstructed image of the fan-shaped planar cross-section of bodily tissue examined.
The rotational angle between a reference position and a plurality of selected examining positions of the transducer body is determ-ined and the associated reconstructed images are correlated with determined angles to generate a plurality of cross-sectional images of the three-dimensional tissue portion being examined to enable evaluation of the volume or shape of the three-dimensional tissue portion of interest.

Description

~5~54~i , ~ ~ ~ ~ ~ Description Ultrason~c olomo :IG~'L~ ~L~

Background of the Invention This invention relates generally to apparatus and methodology useful in effecting medical diag-nosis~ and more specifically, relates to systems and methodology utilizing ultrasonic techniques ` for such purposes.
Over the course o the last several decades, ultrasonic technology has played an ever-increasing role in medical diagnostics. Such techniques find applicatian in diagnosis of various medical ailments wherein it is useful to examine internal bodily organs, with the objective o~ locating features or aspects~,o~ such organs which may be indicative of disease, abnormalities and so ~orth.
While early systems of the foregoing type include~-but limited capabilities and display unctions, there have more recently come into use highly sophisticated devices which are capable of providing real time or record-ed displays with ex-cellent detail and good resolution of desired ~ ' ' ' ' ~

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portions of the body being considered.
In a ~ypical such device, and one to which the present invention is directly applicable, the trans-ducer means and imaging system provides a cross-sectional image of an internal body portion with thetransducer elements being scanned to interrogate the corresponding linear section of the body portion.
Such scanning may be effected mechanically by oscil-lating transducer elements through said angular sector or rotating transducer elements to effect a similar result or electronically, by means of a linear array or a phased array wherein the elements, although stationaryr are electronically actuated so - as to produce a sound beam which scans through the interrogated cross-sectional area. In such a linear array system, a plurality of transducer elements are arranged in a side-by-side fashion to extend over a length of perhaps 10 to 15 centimeters. As each transducer element is activated it sends a sound beam directly into the contacting body. Any acous-tical impedance variations within the body will cause some of the sound beam to be reflected back toward the transducer. The same transducer element will intercept the returning sound energy and convert it back to electrical energy. This electrical signal is then processed and may be displayed upon a CRT screen as one line of image data. Next, the adjacent trans-ducer element is pulsed and any reflected signals from it are displayed as an adjacent line of image data on the CRT screen. This process is repeated for each of the remaining transducer elements as a two-dimensional image is built up on the CRT screen.
The final image will contain a number of parallel lines of data representing a rectangular cross-section of acoustical impedance variations within .
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~55~5 the body being examined.
In the phased array a plurality of transducerelements is arranged in compact linear fashion.
Each transducer element is individually connected S to a suitable transmitter and receiver, and the transmitted pulses are so phased as to steer the emitted sound beam in the desired direction. Ad justable delays provided in each receiver channel enhance the reception from the same direction as the transmitted sound beam. By suitably controlling the time of the voltages applied to the transducer elements and by controlling the adjustable delays of the separate receiver channels, the beam can be steered to any desired angle of a fan-shaped sector.
Operation of the phased array is such that a plurality of radial lines defining the fan-shaped sector are successively yenerated, with a relatively high number of such radial lines -- typically of the order of 128 such lines -- being utilized in the course of generatiny the entire sector. The set of such lines is generated over a short period, typi-cally of the order of 1/30th of a second, whereby the corresponding display on the system cathode ray tube (CRT) is a high resolution, substantially real time image of the bodily portion being examined.
This visualization is, in the terminology of the present art, a so-called B-mode display, i~e., one wherein variations of the acoustical impedance of the tissues are translated into brightness varia-tions on the C~T screen.
Details regarding the prior art signal pro-cessing techniques utilized in apparatus of the foregoing type in order to generate the mentioned fan-shaped sector image are set forth in a number of points in the prior art. Reference may useEully be ~1595 ~5 had, for example, to U.S. Patent No. 4,005,382 to William Beaver, entitled 'ISignal Processor for Ultrasonic Imaging", which patent is assigned to - -the assignee of the present application.
It may urther be noted that apparatus of the type to which the present invention is applicable, which apparatus is in substantial accord with the foregoing description, is available commercially from the assignee of the instant application, Varian Associates, Inc. of Palo Alto, California, under Model No. V-3000, which is further described as a "Phased Array Ultrasonograph".
In apparatus of the foregoing type, the linear array of transducer elements is normally carried by a transducer body, which is a hand held or hand-manipulated probe, the longitudinal axis of which is approximately aligned with the plane of the fan-shaped ultrasonic beam, which axis therefore approxi-mately symmetrically divides the included angle of the fan.
In a typical mode of ulitization, the physieian or technician performing the diagnosis places the forward sound beam emitting end of the transducer body in,eontact with the body of the patient~ and angulates the transducer body so as to orient its longitudinal axis at an appropriate position to ob-tain imaging of a desired portion of bodily tissue being examined. It will be appreciated in this con-neetion that the fan-shaped ultrasonic beam is pre-sent in what is substantially a plane, and thus onenormally examines a two-dimensional image (as for example, on the mentioned CRT screen) of the bodily tissue~intersected by the said fan-shapea beam.
It will further be appreeiated, that in many in-stanees the physician or technician is not satisfied ~i5~

to examine cross-sections alone; his interest may reside in determining the shape or especially the volume of certain bodily organs or voids. For ex-ample, in a number of instances concerned with cardiac studies, it is highly desirable to know the volume of a heart chamber, as for example, of the left ventricle. In other types of situations, for example, in certain conditions of pregnancy, it is highly désirable to be apprised of the fetal volume.
Prior methodology using ultrasonic imaging apparatus of the foregoing type has not, however, been adequate to enable the desired volume measure-ments or shape determinations. Thus, it will be evident that wher the transducer body is merely freely manipulated by hand, there is no known de-terminative relation between transducer body angu-larity and the resulting image; and under such cir-cumstances the examining physician or technician can at most, effect highly qualitative evaluations, i.e., essentially such evaluator is compelled to observe the imaging screen while simultaneously changing the angle of the transducer body, without, however, hav-ing any exact information on the actual angularity.
While in some instances, apparatus of the fore-going type has been equipped with complex positioning arms for the transducer body, which enable rather precise manipulation of same, these highly bulky arrangements have different objectives than measuring volumes. In particular they are merely intended to orient the transducer body axis to enable one specific two-dimensional view. Furthermore, such arms interfere with ease of transducer manipulation by the physician or technician.

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~"~ 595L'~5 In accordance with the foregoing, it may be regarded as an object of the present invention to provide an improvement system in evaluating the volume or shape of a three-dimensional for an ultrasonic imaging system~ which enables use of the said portion S of the bodily tissue being examined.
According to the invention there is provided in an ultrasonic imaging system of the type adapted for examining bodily tissue or the like; said system including a transducer means interrogating said bodily tissue to be examined with an ultrasonic beam which is swept over an internal fan shaped sector of said tissue to be examined; means for determining from the reflected ultrasonic energy resulting from said beam the variations in acoustical impedance of said bodily tissue over the area of said sector and providing therefrom a reconstructed image of a cross-section of bodily tissue corresponding to said sector; said transducer means including a housing body, said body including a longitudinal axis approximately aligned with the plane of said fan-shaped sector and approximately symmetrically dividing the included angle of said fan, and said body being orientable with respect to the bodily tissue being examined to enable said fanshaped sector to intersect and thereby effect imaging of a desired cross~section of said tissue; the improvement enabling evaluation of the volume or shape of a three-dimensional portion of the tissue being examined, comprising: means for determining the rotational angle between a reference position and a plurality of selected examining positions of said transducer body, upon said transducer body being rotated from said reference to said selected positions while at least one point of said longitudinal axis remains substantially spatially fixed, said means for determining comprising a shaft encoder means mounted to said body for providing a signal indicative .~ .

~5~5'~5 of said rotational angle; means for correlating the associated reconstructed images with the said determined angles, ~hereby to generate a plurality of cross-sectional images o~ said three-dimensional tissue portion being examined, wherein the imaged tissue cross-sections include at least one common point, thereby to enable evaluation of the volume or shape of the said three dimensional tissue por~ion; and means for supporting said transducer body for rotation about said longitudinal axis.

In one embodiment of the invention, the transducer body longitudinal axis is held in a fixed position, and the transducer body rotated about the said axis to successi;re angles at which the cross-sectional views are to be studied. A shaft encoder indicates the successive angular positions as the transducer body is rotated, with the angular position data from the encoder being provided to suitable logic. The images associated with the several angular positions are displayed and light pen or ` 25 ~0 :~5S~5~5 other means are used to define the outline of the portion of th~ image which is of interest at each of the successive cross-sections, thereby to enable area determinations. By standard computational methods, the volume of the said portion being examined may then be determined, from the areas which are indicated for each of the plural cross-sections.
In a second embodiment of the invention, the forward sound-emitting end of the transducer body is maintained in contact with the surface oE the human body being examined, the transducer body being rota~
ted about the forward point of its longitudinal axis to successive angles, to thereby produce the indi-cated successive planar cross-sections. An electri-cal contact point at the upper portion of the trans-ducer body, may in this instance be maintained in contact with a resistive path forming part of a bridge circuit, to thereby provide a measure of the transducer angle.
A further embodiment of the invention is the same as the second embodiment but in this instance, - al shaft encoder is utilized to indicate angularity, with the shaft projection of such encoder having a weighted element secured thereto to hold the encoder shaft projection at a reference position correspond-ing to a vertical orientation of the said weight means. Differing degrees of anyularity are thus measured by rotational displacement of the trans-ducer body from the vertical.

Brief Description of the Drawin~s The invention is diagrarnmatically illustrated,by way of example, in the drawings appended hereto, in which:

~5~S45 , g FIG. 1 is a schematic diagram ~f a first embodi-ment of a system in accordance with the present in-vention, wherein the transducer body and immediately contiguous elements are shown in elevation;
FIG. 2 is a transverse cross-sectional view of the transducer body of FIG. 1, taken along the line

2-2 of FIG. l;
FIG. 3 is a diagrammatic and schematic view similar to FIG. 1, but depicting a second embodiment of the invention, FIG. 4 is a side elevational view of a trans-ducer body and associated elements of FIG. 3;
FIG. 5 ls a side elevational view of a trans-ducer and associated elements, forming part of a third embodiment of the invention;
FIG. 6 is a side elevational view of the appar-atus portions depicted in FIG. 5, and illustrating functioning of same as the transducer-body is angu-lated;
FIG. 7 is a side elevational view similar to FIG. 6, but depicting another embodiment of the in-vention, in which further modes of angulation of the transducer are considered;
FIG. 8 is a side elevational view of a trans-ducer body an~ its sector scan pattern in which said sector format is developed by the mechanical scanning of transducer elements.

Description of Preferred Embodiments In FIGS. 1 and 2 herein, a system 10 in accor-dance with the present invention is set orth. The imaging system 10, with the excepti.on of the speciEic elements shown in the Figure and now to be discussed, generally conforms to the ultrasonic imaging systems of the prior art which have been discussed in the ~L~5~

"Background" portion of this specification, and which are exemplif ied in one commercial embodiment by the aorementioned Varian Associates Model V-3000 "Phased Array Ultrasonograph". Such system thus includes a S linearly arranged array of side-by-side transducers, which array is mounted in known manner toward the lower end 8 of a transducer-carrying body 12, which in many systems of the prior art is manually manipu-lated and typically held in good acoustic contact with the skin 14 of a human subject whose underlying tissues 16 are sought to be examined. In practice, a coupling gel is commonly used to form a good acousti~
cal coupling between the transducer surface and the skin of the subject.
These tissues 16 can, of course, include various internal organs and the like. For example, in a typical case the skin 14 may overlie the chest or abdominal cavity, and it is the heart or other in-ternal organs which are desired to be examined -- by manipulation of the transducer body 12, which enables the generally fan-shaped ultrasonic beam 18 to be - projected into the cavity or other tissue portions where an examination is desired. Again, as pre-viously discussed, the linear array of transducers is phased in operation to cause the emitted sound pulses to be directed along line or radial elements in the fan beam 18, with the reception from the transducer elements being appropriately actuated as to enhance reception from the desired directions, so that the fan beam is effectively composed of a series of radial lines along which information îs obtained.
Additional discussion of this type of apparatus, and additional illustration of same, may be found in such articles as "Cardio-Vascular Diagnosis with Real Time Ultrasound Imaging", by O. T. Vonn Ramm, et al., .

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~i~595~5 which article appears in Volume 6 of Acoustical Holography, Edited by Newell Booth, Plenum Press, New York, 1975.
Although the present FIG. l embodiment makes S use of a phased array transducer means, it will be appreciated that the fan-shaped sector scan could also be created by a mechanically scanned transducer, as mentioned above in the Background of the Invention.
An alternative embodiment in which such a mechanically scanned fan-shaped sector sound beam format is de-veloped by means of rotating transducer elements will be described below in connection with FIG. 8. Indeed, the benefits of the invention are also applicable to the simplest static imaging system in which no fan-lS shaped s~ctor format is developed and to other cross-sectional imaging systems such as the linear array system which forms a rectangular-shaped sound beam format.
Pursuant to the usual operation of systems of the FIG. 1 type, the acoustical signal data proceeds via a connecting cable 20 to system electronics 22, which generates the appropriate signals for the transducer and appropriately processes the return signals from the transducers, as known in the art, and may provide data to a display mean.s 24~ The latter typically comprises a CRT where visual display - is desired, although other possibilities are known where permanent records or the like are required.
Again, it is emphasized that as thus far discussed, all such elements are well-known in the prior art.
In accordance with the present invention, a shaft encoder 24 is secured at the rearward end 27 of transducer body 12, with its projecting and ro-tatable shaft 26 being aligned with the longitudinal 35 axis 28 of body 12. The shaft axis 26 is rigidly , .

~15~5~

secured to the rearward end 27 of body 12, in con-sequence of which, upon rotation of body 12 about axis 28, the shaft 26 is made to rotate with same.
Shaft encoder 24 may be a standard 'loff-the-shelf'l type of device, numerous types of which areavailable and well known in the electronics and re-lated art. Suitable devices of this type are, for example, available under the trademark "Ro~aswitch", from Disc Instruments, Inc. of Costa Mesa, Cali-fornia. As is known in the pertinent art, encoderdevices of this type can permit a determination of the precise direction and degree of rotation of the associated shaft, and thus provide an output signal, in the present instance via a line 30, which is specifically indicative of the angular position of the rotated shaft with respect to a reference pOSition.
In the present system 10 it is seen that the lower or forward end 8 of transducer body 12, is surrounded by an annular collar 32, with an inter-vening bearing 34. Collar 32, in turn, is rigidly connected via a member 36 to the body 33 of encoder 24. Preferably, the lower or orward end of the collar 32 is provided with a ring 40 having high friction characteristics, such as rubber or the like.
In consequence of the foregoing arrangement, it will be clear that when transducer body 12 is placed with its forward sound projecting end 8 in good acoustic contact with the skin 14, the collar 32 in-cluding the friction ring 40, will similarly bearagainst the skin surface. In conse~uence, the oper-ator may now proceed to effect rotation of body 12 about its longitudinal axis 28. The outer cage-like portion defined by collar 32, member 36, an~ the 35 rigidly-connected encoder body 33 remain, however, , :.

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stationary as rotation of body 12 is effected, with consequent turning of shaft 26.
By virtue of this arrangement, it will be clear _ that the projected fan beam 18, which initially may be oriented as shown in FIG. 1 and at FIG. 2, may be rotated through successive angular positions -- as suggested at 18A, 18B, and 18C in the transverse cross-sectional orientation, perpendicular to axis - - - - 28, of FIG. 2. As this operation is effectedl how-ever, the angular position data is provided via line 30 to system electronics 22. Simultaneously, the display at 24, which receives its input through line 44 from system electronics 22, changes with the specific rotational angle of body 12. Such angle may further, if desired, be provided directly to the display means whereby the operator may be apprised of the exact rotational angle at which the view is seen.
With the aid of the foregoing, it may be direct-ly apprecia-ted how a volume of tissue imaged at dis-play 24 may be examined and evaluated. In partic-ular, the operator rotates transducer body 12 while simultaneously observing display 24 which may indi-cate the angle associated with the ~isplay on view.
For example, such displays may be effected at each success~ve 1 position.
While this arrangement enables the operator to make direct visual estimates of the shape or volume of thë tissue portion being examined, a more precise scheme is enabled. In particular, a light pen means 46 or other device may be provided, which means is utilized to determine areas at each successive angular position selected in accordance with a pre-- set program, as for example, each 10. The operator thus stops the rotation of body 12, e.g., lP, 20, 30, etc~, and using the light pen means 46 may .

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delineate the perimeter of the tissue portion deemed of interest. Thus, for example r if the left ven-tricle of the heart is being examined and its volurne being ascertained, the operator at each 10 or o-ther -pre-set position, may use the light pen to delineate the outline of the said ventricle for th~t particular cross-section. The image may be maintained at a selected point in the cardiac cycle by suitable - triggering from the R-wave of the electrocardiagram as is well-known in the art. By use of suitable screen coordinates the outline of the area of in-terest may be expressed as a series of digitized coordinates. The signal from the light pen means 46 is thus provided to system electronics 22, and thence to a volume computer 47, whereby by application of standard numerical techniques, the volume o~ the entire void or tissue portion may be ascertained by numerical processing of the successive areas cal- -culated for a full 180 sweep of successive sections through the void or tissue being evaluated.
In FIGS. 3 and 4 herein, a second embodiment of the present invention is set forth. In this in-stance, the basic operation of the transducer and imaging system is similar to that which has been described in connection with FIG. 1 with the trans-ducer body 12 being as previously described. In the prPsent instance, however, the said transducer body is not rotated about its longitudinal axis 28 but rather about a line 52 which is transverse to the longitudinal axis 28, and residiny (during use) approximately adjacent to the surface of the skin.
In this instance, it is thus contemplated that the transducer body 12 will be rotated through angles A
of varying and successive values, in order to again produce successive cross~sections through the under-~, .
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lying tissue as previously discussed. The plane of the fan beam is perpendicular to the plane of the drawing, and is indicated by line 51 when the trans-ducer body 12 is perpendicular to the skin surface, and by line 53 when transducer body 12 has been ro-tated about line 52 by angle A.
In the present instance, instead of utilizing a shaft encoder as heretofore discussed, a semi-cir-cular resistive track 54 is provided, with an elec-trical contact slider point 56 (see FIG. 4) at oneside of transducer body 12 being maintained in con-tact with the resistive track 54. Resistive track 54 is mounted upon an upstanding support 48 with platform 50 attached to the base of support 48.
Platform 50 is held flat against the surface of the skin by handle means 62. Transducer body 12 is ~!
mounted for rotation about line 52 via axle support 59. One end 56A of resistive track 54 is connected via line 55 to one leg of a bridge circuit 58, with the sliding contact point 56 proceeding via line 57 to the other end of the same leg of the bridge cir-cuit. By application of standard bridge circuitry techniques, the degree of angularity A of transducer body 12 may be directly ascertained, as body 12 is rotated about line 52 via axle support 59. Thus once again an angle position data signal is provided from bridge circuit 58 at output 60 which can be provided to system electronics 22 and processed as aforementioned. During rotation of ~he body 12 to its successive angular positions, the apparatus can be conveniently positioned and maintained by handle means 62.
In FIGS. 5 and 6, a yet further embodiment oE
the invention is set forth. In this instance the body 12 is again rotated about a line 63 along and , ' ~s~sL~ -in contact with the skin surface. This line again intersects the longituidinal axis 28 of body 12.
Thus, in the present instance, the transducer body 12 can be rotated to successive angular positions about an axis of rotation coinciding with line 63.
In order to determine the degree of angularity in the embodiment of FIGS. 5 and 6, a further shaft encoder 66 is utilized, which can be of the same -- - - - general type discussed in connection with FIG. 1.
In the present instance, the shaft encoder 66 is secured to one side of transducer body 12. The axis 64 o~ the sha~t 68 of the encoder is parallel to line 6~ The shaft 68 of the encoder has secured there~ l weight 72, which weight will tend to be oriente.~ at its lowermost point, i.e., as the body 12 is angulated, as for example to the position 12' shown in FIG. 6, the weight will tend to remain in its lowermost position, causing the shaft 68 to ro- , tate slightly to maintain such attitude~ Thus, it will be evident that again the rotation of the shaft 68 of encoder 66 will provide a direct indi- .
cation of the angular position of body 12, which provides angle position data via a line 70, pro-ceeding again to system electronics 22, where it may be processed as aforementioned to u.ltimately provide volume computation or the like.
In a further embodiment of the invention, the transducer body is angulated back and forth to ` cause the cross-sectional area being observed to be rotated about line 63 in contact with the skin surface. The line 63 is parallel with encoder shaft 68.
: FIG. 7 il.lustrates a transducer which incor-porates three encoders similar to those illustrated 35 in FIGS. 5 and 6. Encoders 81 and 82 are mounted at .

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right angles to the transducer axis 28 and at right angles to each other thereby providing inforrnation with respect to the vertical of the fan plane and also indicate any angular motion take place within the image plane itself. In addition, angle encoder 84 can be used in cases where the transducer axis 28 is not aligned along the vertical direction to indi-cate any rotation about axis 28. The angular posi-~- - ~~ ~~ ~ ~tions of these three encoders are provided via data lines 88 to system electronics 22 where it may be processed as aforementioned to provide volume com-putation or the like.
FIG. B is illustrative of systems that employ mechanical scanning to develop a sector format in contrast to the above-described systems which used an electronic steered array. In this system, four - transducer elements 91, 92, 93 and 94 are mounted on the periphery of a wheel which is driven by motor 96. As each transducer element is rotated through the desired sector region 98, the trans-ducer is activated by a series of pulses to send out acoustical beams into the tissue within region 98 and echoes therefrom are received by the same transducer and coupled via line 99 to the systems electronics 22. As in the case illustrated in FIG. 5, encoder 66' with shaft 68' and weight 72' will pro-vide a direct indication of the angular position of .. . . _ . . .. . . _ transducer body 12' with respect to the vertical.
Thus; the angular orientation of the scan plane 98-can be determined with respect to the vertical so that as the transducer body 12' is rotated about axis 63', which is parallel to shaft axis 68', the relative orientation o~ these various scan planes is determined. This information is coupled to the system electronics via line 70'.

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~L5~5~5 While the present invention has been particu-larly set forth in terms of specific embodiments ~hereof, it will be understood in view of the in-stant disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope o~ the present teaching. Accordinyly, the invention is to be broadly construed, and limited ~ ~ ~~ --~ ~ only by the scope and spirit of the claims now appended hereto.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: -

1. In an ultrasonic imaging sytem of the type adapted for examining bodily tissue or the like; said system including a transducer means interrogating said bodily tissue to be examined with an ultrasonic beam which is swept over an internal fan shaped sector of said tissue to be examined; means for determining from the reflected ultrasonic energy resulting from said beam the variations in acoustical impedance of said bodily tissue cover the area of said sector and providing therefrom a reconstructed image of a cross-section of bodily tissue corresponding to said sector; said transducer means including a housing body, said body including a longitudinal axis approximately aligned with the plane of said fan-shaped sector and approximately symmetrically dividing the included angle of said fan, and said body being orientable with respect to the bodily tissue being examined to enable said fanshaped sector to intersect and thereby effect imaging of a desired cross-section of said tissue; the improvement enabling evaluation of the volume or shape of a three-dimensional portion of the tissue being examined, comprising:
means for determining the rotational angle between a reference position and a plurality of selected examining positions of said transducer body, upon said transducer body being rotated from said reference to said selected positions while at least one point of said longitudinal axis remains substantially spatially fixed, said means for determining comprising a shaft encoder means mounted to said body for providing a signal indicative of said rotational angle; means for correlating the associated reconstructed images with the said determined angles, whereby to generate a plurality of cross-sectional images of said three-dimensional tissue portion being examined, wherein the imaged tissue cross-sections include at least one common point, thereby to enable evaluation of the volume or shape of the said three dimensional tissue portion; and means for supporting said transducer body for rotation about said longitudinal axis.

2. A system in accordance with claim 1, including means for displaying the plurality of cross-sectional images correspond-ing to said determined angles and said reference position; and means for computing from the areas thereby indicated for each said cross-section, the approximate volume of the said three dimensional portion of interest.

3. A system in accordance with claim 1, in which said transducer means includes at least one transducer element which is mechanically scanned over an arc, whereby said beam is swept over an internal cross-sectional area which is a fan-shaped sector of said tissue being examined.

4. A system in accordance with claim 1, in which said transducer means is a phased array of transducer elements which emits said ultrasound beam and which scans said beam over said internal cross-sectional area in a fan-shaped sector.

5. A system in accordance with claim 1 in which said transducer means is a linear array of transducer elements emitting said ultrasound beam and scanning said beam over a rectangular internal cross-sectional area.