 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The invention relates to a method of positioning a patient arranged on the
top of a patient table, where a first X-ray image is formed with the
central ray extending in a first direction, which image is electronically
stored. The table top is then displaced in a first direction and a second
direction which is perpendicular thereto during a first positioning
operation, the X-ray image and an electronic marker superposed thereon
being shifted relative to one another on a monitor in accordance with the
path of travel. The invention also relates to an apparatus for performing
the method.
For many examinations and treatments in the medical field it is necessary
to bring a given region of a patient in an exactly defined position. For
example, for the crushing of renal calculi the patient must be moved in
space so that the concrement, i.e. the renal calculus, is positioned
exactly in the focus of a shock-wave generator for crushing concrements.
It is known that in apparatus of this kind the location and positioning can
be performed by means of an X-ray fluoroscopy device. The table top is
then displaced under fluoroscopic control from different perspectives
until the image of the renal calculus is situated in a given position
within the fluoroscopic image, for example in the center of the image, the
position being made visible preferably by means of a suitable electronic
marker, for example by means of cross-hairs. It is a drawback that during
such a positioning operation not only the patient but also the operator is
exposed to a comparatively large radiation dose, because fluoroscopy is
continued until the renal calculus is situated in the desired position.
In the method and apparatus described in the preamble, being known from
EP-OS 160 583, the radiation dose is reduced because positioning is
performed by means of a single X-ray image for which the X-rays need only
be briefly switched on. However, accurate positioning can then be realized
only if the concrement happens to be already present in the plane parallel
to the table top in which its desired position is situated. Otherwise
accurate position is not possible, that is to say not in the longitudinal
and the transverse direction of the table top either, because an X-ray
image is a central projection.
SUMMARY OF THE INVENTION
It is the object of the present invention to enable accurate positioning of
a patient using a lower radiation dose.
On the basis of a method of the kind set forth this object is achieved in
accordance with the invention in that from a second perspective a second
image is formed and electronically stored, the table top subsequently
being displaced in a third direction perpendicular to the first two
directions, during a second positioning operation, the displacement in the
third direction being automatically linked to a displacement in the first
and the second direction so that the path of travel in the first and the
second direction is proportional to the product of the path of travel in
the third direction and the path of travel in the first and the second
direction during the first positioning operation, the relationship between
the relative displacement of the marker and the table top displacement
being calculated from the paths of travels after the first exposure, said
displacements being performed as calculated.
Thus, in accordance with the invention two X-ray images are formed from
different perspective and two positioning operations are performed. After
the first positioning operation the region of the patient which is to be
moved to a defined position, for example a renal calculus, is situated on
a straight line which extends through this position and parallel to the
projection direction of the renal calculus. When the paths of travel in
the first and the second direction are coupled to the path of travel in
the third direction so that the paths of travel in the first two
directions are proportional to the product of the path of travel in the
third direction and the paths of travel in the first and the second
direction at the end of the first positioning operation, it can be
achieved that the renal calculus is moved exactly along this straight
line. Because the relative displacement of the marker and the table top
displacement is calculated at the same time, it is ensured that the
electronic marker in the X-ray image coincides with the image of the renal
calculus at the instant at which the renal calculus reaches the desired
position, so that the operator knows that the positioning operation can be
terminated.
It is an advantage of the invention that the patient is already
comparatively near to the desired position after the first positioning
operation. Consequently non-linearities such as may be caused inter alia
by the curvature of the entrance screen of the image intensifier used to
form the X-ray images can have only a limited effect on the accuracy of
positioning. A further advantage consists in that the displacement of the
table top and the electronic marker in the X-ray image can be controlled
via a control unit which is anyhow required for controlling the
displacement of the table top.
The different perspectives in which the X-ray images are formed are
realized in a preferred embodiment of the invention in that the first and
the second image are formed with a central ray extending in different
directions. To this end, the X-ray imaging system must be constructed so
as to be pivotable.
This condition can be dispensed with in a further embodiment of the
invention in that the two X-ray images are formed using the same direction
of the central ray, and in that subsequent to the first positioning
operation and prior to the second X-ray exposure the table top is
displaced in the first or the second direction over a defined distance.
Even though the central beam has the same direction during both exposures,
the concrement will appear in different perspectives because of the
displacement after the first positioning operation. However, the accuracy
of this method is not quite as high as that of the previously described
method.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described in detail hereinafter with reference to the
drawings. Therein:
FIG. 1 shows an X-ray apparatus for carrying out the invention,
FIG. 2 shows a circuit diagram of the table top control system of such an
apparatus, and
FIG. 3 shows the geometrical relationships before and after the positioning
operations.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a lithotriptor apparatus enabling the location of concrements
(renal calculi), their positioning in the focus of a shockwave generator,
and their crushing. The apparatus comprises a shockwave generator 1 which
is capable of generating a shock wave focussed to focus F. The patient 4
to be examined is arranged on a patient table 2 which comprises a table
top 3 which can be displaced in the x-direction (i.e. horizontally with
respect to the plane of drawing of FIG. 1), in the y direction
(perpendicular to the plane of drawing), and in the z direction
(vertically). The table top must be displaced so that the concrement K
within the body of the patient 4 is situated in the focus F of the
shockwave generator 1.
For the displacement of the table top in the x direction there is provided
a drive motor Mx which forms part of a control circuit, together with a
position detector Px which measures the position in the x direction. There
is also provided a drive motor Mz for displacing the table top 3 in the z
direction and an associated position detector Pz. The drive motor My and
the position detector Py for the y direction are not shown in FIG. 1.
The locating and the positioning are performed by means of an X-ray imaging
system which comprises an image converter 5 in the form of an image
intensifier and a radiation source in the form of an X-ray source 6. The
radiation beam emitted by the X-ray source 6 is directed onto the entrance
face B of the image intensifier 5. The central ray 7, linking the center
Bc of the image intensifier to the focal spot F, extends vertically in the
initial position. The shockwave generator is moved out of the beam path
during locating or positioning.
The X-ray source 6 and the image intensifier 5 are connected to one another
via a supporting arm 8 which is pivotable about an axis which extends
through the focus F in the y direction. FIG. 1 also shows the image
intensifier 5 in a pivoted position, denoted by broken lines, in which the
central beam 7' intersects the vertical central beam 7 in the focus F.
As appears from FIG. 2, a television camera 9 is connected to the exit of
the image intensifier 5, the video signal of the camera being stored in a
memory 10 after an X-ray exposure. The stored X-ray image is displayed on
a monitor 11. A marker, for example cross-hairs 12, represents the focus F
and is electronically superposed on the X-ray image. When the table top is
displaced, the X-ray image and the cross-hairs 12 are shifted relative to
one another on the monitor. This relative shift can be realized in that
the X-ray image is shifted while the cross-hairs 12 remain stationary.
However, it is more attractive to keep the X-ray image on the monitor
stationary and to shift only the cross-hairs. The cross-hairs 12 thus move
towards the concrement K in the X-ray image, as the concrement K in the
patient moves towards the focus F. Two signals u and v are applied to the
monitor in order to shift the cross-hairs 12 within the X-ray image.
As has already been stated, the drive motors Mx, My and Mz for the
displacement of the table top in the x, y and z directions form control
circuits, in conjunction with the associated position detectors Px, Py and
Pz as well as controllers Rxy and Rz, which circuits determine the
position of the table top. The variables x, y and z for these control
circuits are generated by a control unit 13 which calculates the signals
x, y and z from the signals u and v. Unit 13 also stores the paths of
travel x1 and y1 obtained at the end of the first positioning operation in
the x direction and the y direction. The signals u and v which represent
the coordinates of the cross hairs 12 are supplied by a coordinate
generator 14.
The table top is displaced as follows in order to position the concrement K
in the focus F of the shockwave generator 1.
First of all a first X-ray image is formed during which the X-ray imaging
device 5, 6 occupies the position denoted by non-interrupted lines in FIG.
1. The central ray 7 extends in the vertical direction, perpendicularly to
the table top 3. The X-ray image is stored in the memory 10 and is
displayed on the monitor 11. The table top is then displaced in the x
direction and the y direction, and the cross-hairs 12 are shifted on the
display screen 11 in proportion to the movement of the patient on the
table top.
FIG. 3 shows the geometrical relationships in the X-ray images, be it that
the angle of aperture of the X-ray beam is shown to be substantially
larger than is possible in the device shown in FIG. 1. The position of the
X-ray source during the first X-ray exposure is denoted by the reference
L1. K1 denotes the position of the concrement K prior to the positioning
operation. B1 denotes a point on the plane of screen B of the image
intensifier. The entrance screen is a relatively large area. This area is
represented by a straight line only in the region between the entrance
screen center and the projection K1 of the concrement K1. The distance X1'
between the focus F and the projection K1 of the concrement K1 is measured
on a plane X1 extending perpendicularly to the central ray 7 and through
the focus F. The angle B1 is enclosed by the X-ray beam ray 7 incident on
the concrement K1 and the central ray 7.
For the first positioning operation succeeding the first X-ray exposure it
is assumed that the concrement is situated in a plane which contains the
focus F and which extends perpendicularly with respect to the central ray
7, and that positioning is performed so that, if the concrement were
actually situated in the plane containing the focus F and extending
perpendicularly with respect to the central ray, the concrement would be
positioned in the focus F at the end of the positioning operation. The
patient is thus displaced over the distance x1' in the x direction and
accordingly in the y direction over a distance y1' which is not shown in
FIG. 3. However, because the concrement K1 is situated in a plane which is
situated at the distance z2 from the plane x1, the displacement in the
x-direction (and the y-direction) is too large, so that the concrement
will not be situated on the central ray 7 at the end of the first
positioning operation. The position of the concrement at the end of the
first positioning operation and prior to the second X-ray exposure is
denoted by the reference K2 in FIG. 3. The distance between K1 and K2
amounts to distance x1'.
In order to check that the concrement occupies the desired position at the
end of the first exposure, the cross-hairs 12 in the X-ray image on the
monitor 11 are shifted by the signals u and v simultaneously with and in
proportion to the displacement of the table top. The following
relationship then exists between x and y and u and v:
x=a.multidot.c.multidot.u/d (1)
y=a.multidot.c.multidot.v/d (2)
Therein, a is the distance between F and L1, d is the distance between the
radiation source L1 and the entrance plane of entrance screen B of the
image intensifier, and c is a proportionality factor, which represents the
quotient between 1) of the distance between a point B' on the entrance
plane of the image center B.sub.c of the intensifier 5 and the image
intensifier entrance and 2) the value of the signal u or v which is
required for making the cross-hairs 12 coincide with the image of this
point on the monitor. In other words, c is a correction factor for
centering the cross hair positions on the intensifier entrance at point
B.sub.c. Assuming ray 7 is at the center B.sub.c of the intensifier 5,
then C has a value B1'/.mu. for the x component of point B' or B1"/v,
where B1" (not shown in the Figure) represents the y component of point B'
from the center B.sub.c.
In accordance with the equations (1) and (2) the control unit 13 thus forms
the signals x and y from the signals u and v supplied by the coordinate
generator 14. When the concrement has reached the position denoted by the
reference K2 in FIG. 3, the concrement K and the cross-hairs 12 coincide
on the monitor, even though the concrement K2 is not situated on the
central ray 7 as appears from FIG. 3. The values x1' and y1' which are
supplied by the position detectors Px and Py at the end of the positioning
operation are stored in the control unit 13.
As appears from FIG. 3, at the end of the first positioning operation the
concrement K2 is situated at a distance x2 from the focus F in the x
direction and at the distance z2 from focus F in the z direction. Because
the connecting line 7'" between K2 and F has the same direction as the
connecting line between L1 and K1, the angle between the connecting lines
F-K2 and the central ray corresponds to the angle .beta.1; .beta.1 is
.beta.1=arctan(x1'/a) (3)
For a point on the connecting line F-K2 the equations
x=z.multidot.x'/a (4)
and
y=z.multidot.y1'/a (5)
are applicable.
In accordance with the equations (4) and (5) the concrement is shifted
along a straight line 7'" through the focus F when the coordinates x, y
and z relate as defined by the equations (4) and (5). Thus, it is only
necessary to recognize when the concrement F is situated exactly in the
focus, so that the shift can be terminated.
This is realized by means of a second X-ray image for which the X-ray
imaging device is pivoted through an angle b of, for example 40.degree..
Angle b must be larger than half the angle of aperture in order to ensure
that the radiation beam completely covers the entrance planes of the image
intensifier. The second X-ray image is stored in the digital image memory
10 and is displayed on the monitor 11, u and v being preset so that the
cross-hairs 12 again occupy their original position (image center).
Subsequently, the second positioning operation is performed, the table top
being displaced in all three directions until the cross-hairs 12 and the
image of the concrement on the monitor coincide.
It can be demonstrated that the cross-hairs on the monitor coincide with
the image of the concrement and that at the same time the concrement
reaches the focus when the following conditions are satisfied:
1. The values x and y are modified in dependence on z in accordance with
the equations (4) and (5). This means that x and y are also modified
relatively to one another in a well-defined ratio (x1'/y1'). The resultant
shifts for the x direction for the second operation and the y direction
must be made in addition to the shifts already performed during the first
positioning operation. Consequently, during the second positioning
operation the values x1' and y1' must be added to the values calculated
according to the equations (4) and (5), for the first operation
respectively, in order to calculate the desired values for the x position
and the y position for the second positioning operation.
2. The same relationship exists between v and u as between y and x, i.e.
the following equation holds good:
v=u.multidot.y1'/x1' (6)
3. The variables z and u, determining the table top displacement (x, y, z)
in accordance with the equations (4) and (5) and the shift of the
cross-hairs (u, v) in accordance with the equation (6), relate in
accordance with the following equation:
z=u.multidot.A/(B+u.multidot.D) (7)
Therein, A, B and D are constants which are determined by the imaging
device, i.e. by the values a, c and d, the pivot angle b and the angle
.beta.1, during the first X-ray exposure in accordance with the equations
A=a.multidot.c.multidot.cos(.beta.1) (8)
B=d.multidot.sin(b+.beta.1) (9)
D=c.multidot.cos(b+.beta.1) (10)
The pivot angle b is either known in advance when the X-ray imaging system
5, 6 is pivoted each time to an accurately defined angular position, or is
measured by means of a suitable angle detector which is coupled to the
supporting arm 8. The angle .beta.1 results from the value x1' measured
and stored after the first exposure in accordance with the equation (3).
In accordance with the above conditions, u and v can no longer be
independently selected for the second positioning operation. Consequently,
for the second positioning operation v is not directly present via the
generator 14 but is calculated from the value u by the control unit 13 in
accordance with the equation (6). This different formation of v during the
two positioning operations is diagrammatically represented by a switching
unit 15 which connects one control input of the monitor to the output of
the generator 14 during the first positioning operation and connects this
input to an output of the control unit 13 during the second positioning
operation. From the value u the control unit 13 also calculates the value
z in accordance with the equation (7) or the values x and y.
The calculation can be step-wise performed during the second positioning
operation so that in each position the associated values of u, v, z, x,
and y are calcuated for the next shifting step of the cross-hairs and the
table top. This implies that the control unit 13 must perform these
calculations at a corresponding speed. However, it is in principle also
possible to calculate and store the associated pairs of values for all
steps already after the first positioning operation, and to fetch the
paris of values stored for the relevant position during the second
positioning operation. However, this necessitates a correspondingly large
storage capacity of the control unit 13.
Instead of calculating the value z from u it is also possible to calculate
the value u from z. In accordance with the equation (7) this is possible
only by way of a digital iteration method, but the execution of such a
method is not problematic when use is jade of a suitable constructed
control unit (microprocessor). In this case the operator presets the table
top displacement in the z-direction via a suitably constructed control
unit and all other values (x, y, u and v) are derived therefrom so that
the operator can thus directly determine the speed of displacement.
For the example whose geometry is shown in FIG. 3 it was assumed that the
concrement was situated below the plane X1 defined by the focus F. In such
a case the concrement is imaged to the left of the cross-hairs in one
X-ray image and to the right of the cross-hairs 12 in the other X-ray
image. Consequently, the sign of u is reversed during the positioning
operations. However, if the concrement were situated above F, i.,e.
between the parallel planes defined by the image intensifier entrance
plane through B and plane X1 through the focus F, the concrement would be
imaged on the same side of the cross-hairs also after the second X-ray
exposure. In this case u should be varied in the same direction during the
two positioning operations. On the other hand, in the latter case z should
ve varied in the opposite direction in comparison with the situation where
the concrement is situated underneath F. The sign of the displacement of
the table top in the z direction, therefore, can be unambiguously derived
from the variation of u during the first positioning operation (or from
the sign of x1) and the sign of u during the second positioning operation.
So as not to endanger the patient during the pivoting of the image
intensifier in the position for the second X-ray exposure, the distance
between the image intensifier 5 and the focus F can be increased by
shifting the image intensifier in the longitudinal direction of the
supporting arm 8. In such a case the constants A, B and D in the equation
(7) must be adapted to the changed geometry.
The operation of the control unit 13 is digital; the D/A or A/D converters
which are possibly required and via which this unit cooperates with the
other units are not shown in FIG. 2. Preferably, the control unit is
realized by means of a microprocessor which controls the positioning
operations in accordance with a suitable program. In that case the
changing of the generating of the signal v during the second positioning
operation, performed by a switching device 15 in FIG. 2, can be realized
on the basis of software. The microprocessor may also perform the function
of the controllers Rxy and Rz.
The method described above was performed so that the X-ray imaging system
was pivoted through a given angle after the first X-ray exposure, so that
different relative orientations occur during the two X-ray exposures.
Considering that the concrement is also imaged from different orientations
because of the patient displacement prior to the second X-ray exposure, it
will be apparent that the pivoting of the X-ray imaging system is not
absolutely necessary per se in order to obtain different relative
orientations.
Therefore, the second X-ray exposure can also be performed with the same
direction of the central ray, i.e. the X-ray imaging system need not be
pivotable. In order to obtain different orientations, when the central ray
already happens to pass through the concrement during the first exposure,
the table top must be additionally displaced over a defined distance
relative to the central ray, or vice versa, after the first positioning
operation and before the second X-ray exposure. This additional
displacement must be neutralized again during the second positioning
operation (after the second exposure). The simple relationships in
accordance with the equations (1) and (2) again hold good. However,
because the differences in the orientations are smaller than when the
X-ray imaging system is pivoted, the positioning accuracy that can be
achieved by means of this embodiment is lower.
* * * * *
|
|
 |
|
 |
Description |
|
