Lithotripsy system using ultrasound to locate calculi

A calculi locator is provided having at least one ultrasound transducers in combination with a shock wave generator. The ultrasound transducer is rotatable about the longitudinal axis of a reflector of the shock wave generator. The shock wave generator includes a target focal point. The transducer is used to assure positional coincidence of the true location of the calculi and of the target focal point. Alternatively, either a mathematical correction unit is used or iterative measurements are used to correct for refraction errors in the location of the imaged calculi.

The invention of this Application is concerned with lithotripsy equipment 
for extra-corporeal fragmentation of renal calculi, and more particularly 
with imaging systems used for locating calculi in cooperation with 
lithotripsy type shock wave generators. 
BACKGROUND OF THE INVENTION 
One of the many ailments of mankind is the formation of calculi, that is 
stones or concrements, within the body. More common places for the 
development of such concrements are in the renal system and the 
gallbladder. Some of these stones pass through the renal system without 
causing any adverse affects noticed by the person having the stones. Other 
stones lodge within the kidney, the bladders or passageways between 
kidney, the bladder and the exterior of the body causing pain or impairing 
the operation of organs; then the removal of the calculi becomes 
imperative. 
Originally the only way of removing these stones was through invasive 
surgery. With the advent of improved imaging techniques it became possible 
to utilize percutaneous techniques for the removal of many of the calculi. 
The percutaneous techniques are also invasive and require the insertion of 
means such as a nephrotomy catheter to accomplish the stone removal. The 
catheters are equipped with devices to grasp the calculi or ultrasonic 
generators for fragmenting the calculi by vibrating while contigious 
thereto. 
More recently, extra corporeal shock wave lithotripsy has been applied to 
fragment and thereby eliminate renal calculi. 
A lithotripsy system now being used has a pair of substantially 
orthogonally positioned X-ray generators designed to exactly locate the 
stone to be eliminated. The patient is moved three-dimensionally in a bath 
until cross-hairs on each of two images displayed by the system are 
juxtaposed to the stone. The cross-hairs are each at a line projection of 
the focal point of the shock wave generator. The patient is manipulated 
until the cross-hairs in each of the images are superimposed on the image 
of the stone; at that time the shock wave generator is triggered and the 
shock waves focused to shatter the stone down to sizes where the fragments 
will be washed out of the body through normal body functions. 
The imaging systems used presently for the calculi locating function leave 
room for improvement. Among other things it is desirable to reduce the 
exposure of the patient to X-rays. The patient presently is exposed to 
X-rays, originally to ascertain that stones are indeed causing problems 
and that they can be eliminated by lithotripsy. 
X-rays are used to align the stone with the focal point of the shock wave 
generator and X-rays are used subsequent to the application of the shock 
waves to assure that the stones have been shattered into fragments which 
can be disposed of by normal functions of the body. Thus, the procedure 
requires a relatively large accumulated dose of X-rays. It would be 
advantageous to lower the amount of X-ray radiation required in the 
lithotripsy process. To this end ultrasound has been tried in the past. 
Additionally ultrasound radiation is more capable of imaging gallbladder 
stones than X-ray radiation. However, the prior art attempts at imaging 
with ultrasound for locating calculi to be fragmented by shock waves have 
encountered difficulties. 
In the prior attempts at using ultrasound as the medium for locating the 
calculi, different acoustic windows were used by the ultrasound 
transducers and by the shock wave generator. The prior systems also failed 
to make any effective correction for the distortions of the ultrasound 
signals and the shock waves due to refraction caused by the differences of 
the transmission velocities of the ultrasound signals and the shock waves 
in the different media. 
In addition in the past no provisions were made for enabling a single 
ultrasound transducer to image a plurality of planes for assuring accurate 
location of the calculi. 
Similarly, no provisions were made for varying the field of view of the 
individual transducers to provide for the different views required when 
searching for calculi and when centering the discovered calculi at the 
cross-hairs. 
Accordingly it is an object of the present invention to overcome the 
shortcomings of the prior art ultrasound imaging systems used in calculi 
location and to thereby improve on the presently available calculi 
locating imaging systems used in cooperation with extra corporeal shock 
wave lithotripsy. 
BRIEF DESCRIPTION OF THE INVENTION 
Accordingly a lithotripsy system including a calculi locator is provided 
for use in cooperation with shock wave generators having spark terminals 
at a source focal point of a reflector with a target focal point spaced 
apart therefrom, said lithotripsy system comprising: 
shock wave generator means for generating shock waves at a first focal 
point, 
a target focal point, 
means for causing said shock waves to focus on said target focal point, 
means for imaging the calculi within patients, said imaging means 
comprising: 
ultrasound scanning means for viewing said calculi to control the 
positioning of the calculi at the target focal point, 
means for moving said patient to enable positioning the calculi at the 
target focal point, and 
means for overcoming positioning errors caused by velocity difference in 
the various media traversed by the ultrasound signals in viewing the 
calculi in relation to velocity differences in the various media traversed 
by the shock waves when travelling from the first focal point to the 
target focal point for fragmenting the calculi. 
A feature of the invention includes using ultrasound signal transducer 
means for imaging and combining the ultrasound transducer means with the 
shock wave generator means so that the focal points of an ultrasound 
transducer means and the shock wave generator means coincide. The system 
includes means for acquiring views of different planes through the 
coinciding focal points with the transducer means. 
An alternative or related feature of the invention comprises means for 
computing and correcting the positioning errors caused by the differences 
between signal velocity and shock wave velocity travelling through 
different media; such as water and tissue. 
A related feature of the invention comprises means for rotating said 
ultrasound transducer about the axis of the shock wave generator to 
provide various views of the calculi, while maintaining the coincidence of 
the target focal point and the transducer image cross-hair. In one 
embodiment the locations of the calculi obtained with the transducer in 
different positions are averaged to correct for positioning errors caused 
by refraction. 
Another feature of the invention is the ability to image gallstones without 
the necessity of using dyes or the like. 
Still a further feature of the invention comprises utilization of a pair of 
ultrasound transducers to speed the calculi locating process. The pair of 
ultrasonic transducers are arranged so that the cross-hair in the images 
generated by data from each of the transducers is truly on the target 
focal point of the shock wave generator. The planes imaged by each of the 
transducers are preferably orthogonal or at least at angles with respect 
to each other. Locating with each of the transducers individually 
overcomes positioning errors without the necessity of rotating the single 
transducer about the shock wave generator.

GENERAL DESCRIPTION 
In the illustration of FIG. 1 a lithotripsy system 11 is indicated in 
combined cross sectional and block diagram form. The illustrated system 
comprises shock wave generator 12 which includes a bath 13 in which the 
patient 14 is partially submerged during the treatment. A hydraulic or 
mechanical system for supporting and moving the patient in the X, Y and Z 
directions is indicated by a bed 15. The axes of movement XYZ are shown 
generally at 16. The patient 14 is shown with a stone 17 located in a body 
organ such as a bladder, kidney or gallbladder indicated generally at 18. 
The shock wave generator 12 has a pair of spark terminals, for example, 
indicated at 21. The location of the spark terminals is at a source focal 
point in a partial ellipsoid 22. The target focal point of the shock waves 
generated at the source focal point and reflected by the walls of the 
ellipsoid is at the location of the stone 17. The patient 13 is moved by 
movement of the bed 15 until the stone 17 is at the target focal point 
location. 
It should be understood that while spark terminals are shown in the partial 
ellipsoid, the principal idea is that shock waves are generated by any 
means and focused by any suitable reflector at a target focal point so 
that the stone 17 may be set at the target focal point. 
Means are provided for imaging the calculi, more particularly ultrasound 
transducer 23 is shown. The transducer in one embodiment is a sector 
scanning type of focused transducer. The bed 15 is moved until the calculi 
is at the transducer focal point with the transducer focal point 
coincident with the target focal point of the shock wave generator 12 at 
some time during its scan of an arc. The echoes received from the stone 
and from the body in general are processed in the normal manner of 
processing ultrasonic echo signals in image processor 24 to provide an 
image shown on the display means 25. The image includes an image of the 
calculi 17 located in the organ 18 of the patient 14. 
Means are provided for overcoming the calculi location errors normally 
incurred with ultrasound waves due to refraction; i.e., in general to 
differences in the velocity of the ultrasound signals and the shock waves 
when travelling through the different media on the way to the calculi. The 
unique means include computer means indicated at block 26. 
In the embodiment of FIG. 1 the transducer 23 is shown removed from being 
at the "window" location of the shock waves generated by the spark 
terminals 21. The computer means 26 provides a numerical or computational 
correction to overcome the calculi location errors. The correction 
afforded by unit 26 will be explained in greater detail hereinafter. 
An additional feature and another way of aiding in overcoming the calculi 
location errors as shown in FIG. 1 is the capability to rotate the 
transducer 23 about the axis 38 of the shock wave generator 12. More 
particularly means are provided for rotating the transducer about the 
longitudinal axis of the ellipsoidal reflector of the shock wave generator 
12. These means are indicated as the gear 36 shown at the top of column 37 
fixedly attached to the shock wave generator 12. A motor 27 controlled 
through switch means 28 and coupled to gear 36 through gear 29 rotates 
shock wave generator 12 about the axis 38 responsive to and under control 
of the central processing and control unit (CPU) 30 of the lithotripsy 
system 11. 
The rotation of the shock wave generator 12 causes the transducer 23 to 
rotate about the axis of rotation 38. The bed 15 is moved, if necessary, 
when the transducer stops at different positions to maintain the calculi 
at the transducer's focal point which is coincident with the focal point 
of the shock wave generator, i.e. to maintain the cross-hairs on the 
calculi. After movement of the bed the transducer is returned to the prior 
stone locating position to check that the cross hair is still maintained 
at the calculi. Alternatively, the amount of movement of the bed at 
different positions of the transducer 23 which is the measured differences 
in the position of the imaged calculi are averaged to overcome or reduce 
calculi location errors and the patient is moved to the calculated 
corrected position. 
A system for assuring the rotational fidelity of the transducer is 
provided. More particularly, motor 47 is provided coupled to gear 32 
through drive gear 32'. The motor is actuated responsive to the operation 
of switch 49 to cause the transducer 23 to rotate about its own axis 33 
for assuring that the transducer is rotatively aligned with the focal 
spot. The switch 49 operates responsive to a signal from CPU 30 through 
correction unit 26. If the alignment of the transducer is true, the cross 
hairs remain on calculi 17 during the entire rotation. 
FIG. 2 is a plan view of the spark generator 12. In the embodiment of FIG. 
2 two transducers 23 and 23' are shown integrally located within the shock 
wave generator. In this arrangement there is no need to rotate the 
transducers about the axis of the spark gap shock wave generator. However, 
the transducers may be equipped to be rotated about their own axes, as 
indicated by gears 32 and 32'. The bed 15 is moved until the cross hairs 
in the image generated from data from each of the transducers are directly 
on target. This assures that the target is correctly located and errors 
due to refraction of the sound waves are minimized. When two transducers 
are used either two displays, one multiplexed display or one composite 
display, are shown on monitor 25. 
FIG. 3 helps to explain the correction computations used to overcome the 
calculi location errors occurring because of the differences in the 
velocity of the shock waves and of the ultrasonic signals in the different 
media traversed in travelling to the calculi in the patient. 
The ultrasound transducer 41 projects a series of beams as it scans over an 
arc. In FIG. 3A the arc of the series of beams is represented by the 
central beam 42. The beams cause echoes to be generated by tissue 
boundaries. The calculi generates such an echo. 
The system normally treats the beam and the echo as if they had travelled 
along a straight line. Actually because of the differences between the 
velocity of the sound waves in water and in tissue, the calculi 43 which 
really is at location 43r appears at location 43i. Thus, there is an 
angular error .alpha.2-.alpha.1 and a distance error d1-d2, where: 
.alpha..sub.1, is the angle between the beam 42 and the normal 44 to the 
skin surface 46 at point p, where the beam is incident to the skin 
surface, 
.alpha..sub.2, is the angle between the refracted beam 42' and the normal 
to the skin surface at the point p, 
d1 is the distance from point p to the apparent position of 43i of the 
calculi, and 
d2 is the distance from the point p to the actual position 43r of the 
calculi. 
The angles and the velocities are related by Snell's Law: 
##EQU1## 
where: V1 is the velocity of sound in water, (1500 m/sec) and 
V2 is the average velocity of sound in tissue, (1540 m/sec). 
The distance d2 is easily determined. The time t1 for the echo to return 
from the calculi is known. That time multiplied by the velocity of sound 
in the tissue gives the distance to the calculi along beam 42'. 
Normally the distance between the source and the object using a single 
velocity results in an error along the direction of the vector as well as 
an angular deviation due to Snell's law. During the first few frames of 
the scan the ultrasonic system will detect the position 46 of the skin 
surface. The position of the skin surface stands out because, as is well 
known, it is the first strong echo received. 
FIG. 3B shows an embodiment of equipment for calculating the angle between 
the ultrasound beam and the normal to the skin surface for every imaging 
vector. A curve is fit to the data obtained to indicate the skin surface 
46. Thus FIG. 3B shows a curve fitting means 51. The curve fitting means 
could be any well known means for curve fitting, such as a mean square 
operator using the data obtained at the skin. 
After a curve is fitted, the angle .alpha.1 between the normal to the skin 
and the beams going from the transducer to the patient is obtained in 
means 52. The angle is obtained for each of the beams incident to the 
skin. 
The sin of the angle .alpha.1 is calculated at 53 and divided by the 
velocity value V1 at block 54. The quotient is multiplied by the velocity 
V2 at multiplier 96 which gives a value of sin .alpha.2. The angle 
.alpha.2 is obtained from this value at operator 57. The value of .alpha.2 
enables finding the actual line (i.e. line 42') for each of the beams upon 
which the real calculi is located. 
Similar calculations may be made for correcting the shock wave generator 
target focal point for positional errors due to velocity differences and 
refraction. The corrected transducer focal point is set to coincide with 
the corrected target focal point. 
In practice the lithotripsy equipment comprising the assembly of an 
ultrasonic transducer and a shock wave generator is used to determine the 
location of calculi and to destroy the calculi with shock waves generated 
extracorporeally. The actual location of the calculi is obtained either 
by: rotating one transducer about the axis of the shock wave generator and 
if necessary averaging calculi locations obtained or using two transducers 
without rotation about the shock wave generator axis. 
The actual location of the calculi is shown on the image and the patient is 
moved until the cross hairs on the image coincide with the imaged calculi. 
At which time, the shock wave generator is triggered. After a series of 
shock waves have been generated and transmitted to fragment the calculi, 
the image is again provided to check to see if the calculi has indeed been 
fragmented and to further check and see if any of the fragments need 
further fragmentation. 
While the invention has been described with regard to specific embodiments 
it should be understood that these embodiments are made by way of example 
only and not as a limitation on the scope of the invention, which is 
defined by the appended claims.