Target device and use thereof for aligning light beams utilizing cross-hair light beams

A target device is designed for use with a machine for treating a patient with radiation. The device enables accurate determination of an isocenter to which the radiation should be directed, the isocenter being at a point of intersection of at least three composite laser beams. The device enables the laser sets to be adjusted in position and their beams to be adjusted in direction in order to achieve improved accuracy in setting up the isocenter.

FIELD OF THE INVENTION 
The invention relates to a target device for use in aligning a plurality of 
light beams (usually at least three and usually laser beams) that extend 
in different directions towards a common point in space (known as an 
isocenter). The purpose of the procedure is to define this point more 
accurately, and the invention also relates to this procedure. 
BACKGROUND OF THE INVENTION 
Laser emitters are widely used preparatory to radiation treatment of a 
patient for accurately positioning a target area on the patient relative 
to the isocenter of a radiotherapy machine. This use of the target device 
is an important application of the present invention, although it is not 
the only one. The target device can be used in other situations in which 
there is a requirement to align a plurality of beams from lasers or other 
light emitters to cause them to converge at an isocenter. 
In the medical application referred to above, namely when the invention is 
used in conjunction with radiotherapy equipment, there are typically four 
light sources, e.g. laser sets. One laser set is mounted on each of two 
opposite walls of the room in which the equipment is located, and they are 
oriented to direct composite beams horizontally towards each other. These 
are referred to as the lateral laser sets. A third laser set is mounted on 
the ceiling and directs its composite beam downwardly. Ideally, the three 
sets of beams should intersect in space to define the isocenter. The 
fourth laser, called the saggital laser, is mounted on a third wall of the 
room at an elevated location, and directs its single beam towards the 
isocenter at right angles to the two lateral composite beams coming from 
the wall mounted laser sets. 
The composite beam from each of the lateral and ceiling laser sets consists 
of two intersecting portions, each portion being a narrow strip. The two 
strips are perpendicular to each other, one strip being horizontal and the 
other vertical. The two beam portions thus define a cross. In the case of 
the ceiling laser set, the two strips are both horizontal. The single beam 
from the saggital laser consists of only one portion, namely a vertical 
strip. 
The beams are visible to the operator and are designed to meet and display 
a cross on a common target. 
The foregoing procedure is known in the art, but the prior art devices for 
achieving alignment of the beams to define the isocenter have suffered 
from inaccuracies. They have generally relied on use of a pair of 
right-angled wires (crosshairs) that project a shadow image of a cross on 
whatever target is used. A problem when depending on these crosshairs is 
that on some radiotherapy machines they are mounted on a removable tray 
that slides in and out with provision for adjusting its position in the X 
and Y directions (the two horizontal directions). Since the prior art 
laser alignment devices have depended on proper crosshair alignment in the 
initial set up, if the crosshairs are in fact not correctly aligned, the 
lasers will also lack proper alignment. Another problem is that correct 
alignment of the crosshairs depends on the operator's visual dexterity. In 
addition, the alignment checks are performed sequentially, each subsequent 
check requiring a new set up. 
Yet another problem of the prior art alignment techniques resides in the 
fact that, even assuming that all the beams accurately intersect at the 
isocenter, there is no guarantee that they are accurately directed 
relative to each other, which is a requirement of a perfect set up. 
Specifically, the two lateral beams should together define a single 
straight line and the ceiling beam should be at right angles to the 
lateral beams. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to provide apparatus and method 
for overcoming these shortcomings of the prior art, i.e. apparatus and 
method for increasing the accuracy of determining an isocenter in a manner 
requiring a minimum of set up time while avoiding multiple set ups. 
To this end, in one aspect the invention provides a target device for use 
in orienting a plurality of composite beams emitted by respective light 
sources, the beams extending in mutually different directions towards an 
isocenter and each beam having a cross-section comprising a pair of strips 
intersecting each other. The device comprises (a) a primary member, e.g. a 
block, having at least one surface inscribed with crosshairs, and (b) a 
secondary member, e.g. an arm mountable on the primary member and 
including at least one plate inscribed with crosshairs and having a hole 
centered on such crosshairs. The secondary member is so mountable on the 
primary member that the plate is positioned along a beam at a location 
spaced towards the source of the beam away from the inscribed surface of 
the primary member whereby this surface receives a central portion of the 
beam that has passed through the hole, and the portion of the plate 
surrounding the hole receives an outer portion of the beam. 
The invention also provides a method of aligning three such composite light 
beams to intersect at an isocenter. The method comprises directing each 
beam onto a plate inscribed with crosshairs while passing a portion of the 
beam through a hole in the plate, which hole is centered on the 
crosshairs. The portion of the beam passing through the hole impinges on a 
surface spaced beyond the plate that is also inscribed with crosshairs. 
The method includes adjusting the location and direction of each beam to 
cause the strips of the beam to coincide with the crosshairs on both the 
plate and the surface beyond the hole in the plate. 
The most likely practical application of the invention resides in a method 
of determining an isocenter for alignment with a target area on a patient 
to be treated with radiation. As before, the isocenter is defined by a 
point of intersection of three composite laser beams. The method includes 
using a device according to the invention by directing each beam from one 
of three locations approximately at right angles to each other onto a 
plate of the secondary member while passing the beam through the hole in a 
plate of the secondary member. The portion of the composite beam that 
passes through the hole impinges on an inscribed surface of the primary 
member spaced beyond the plate. The method includes adjusting the location 
and direction of each composite beam to cause its strips to coincide with 
the crosshairs on both the plate and the inscribed surface beyond. A 
fourth beam having a single strip is directed from a fourth location onto 
inscribed surfaces of both the primary and secondary members. The location 
and direction of the fourth beam are adjusted to cause its strip to 
coincide with inscribed lines on both the primary and secondary members.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows lateral laser sets 10 and 12 mounted by support devices 14 and 
16 on opposite walls 18 and 20 of a room in which radiotherapy equipment 
is to be located. A ceiling laser set 22 is mounted by a support device 24 
on the ceiling 26. All the support devices 14, 16 and 24 are capable of 
adjusting their respective laser sets (and hence their beams) for 
translation in two dimensions--horizontally and vertically in the case of 
the lateral laser sets, and in two horizontal dimensions in the case of 
the ceiling laser set. These support devices also provide for rotational 
adjustment of their laser sets about two axes so that their beams can be 
caused to vary their direction of travel in any desired way. Such support 
devices are known in the art and hence require no further detailed 
description. The saggital laser is not shown in FIG. 1, but will be 
further described below. 
FIG. 2 shows the cross-section of a composite beam 28 from the laser set 
12, this beam 28 consisting of a horizontal strip 30 and a vertical strip 
32. Each of the laser sets 10, 12 and 22 contains two separate lasers, one 
providing the horizontal strip 30 and the other the vertical strip 32. 
Beams 34, 36 from the laser sets 10 and 22 are similar. 
FIG. 3 shows a target device that includes a primary member in the form of 
a cubic block 38 mounted on a support device 40 that provides for 
adjustment of the block 38 relative to a table 42. This adjustment can 
take the form of movement in either or both of the horizontal (X and Y) 
directions. The support device 40 also has provision for levelling the 
block 38 and for rotating the block 38 about a vertical axis in order to 
ensure its correct orientation in space regardless of any inaccuracies of 
the location and orientation of the table 42. 
The block 38 has two cylindrical pins 44 and 46, the former defining a 
vertical axis and the later a horizontal axis. The end faces 48, 50 of the 
pins 44, 46 are inscribed, e.g. etched, with crosshairs 52, 54. The two 
portions of the crosshairs 52 extend at right angles to each other in the 
horizontal plane, while those of the crosshairs 54 extend horizontally and 
vertically. 
A secondary member of the target device takes the form of an arm 56 that 
has a central hole 58 dimensioned to receive the pin 44 as shown in FIG. 
4. The block 38 has slots 60 in its upper surface to hold the arm 56 
firmly in transverse position. At each end of the arm 56 there is a 
depending plate 62 which has a central hole 64 and a pair of crosshairs 66 
inscribed on its inner and outer surfaces, the center of each hole 64 
coinciding with the intersection point that would be defined by the 
crosshairs 66 if the hole were not there. In a similar manner the upper 
surface of the arm 56 has inscribed crosshairs 68 centered on the hole 58. 
With the target device assembled, as shown in FIG. 4, the composite beam 28 
from the laser 12 is directed against the outer surface of one of the 
plates 62. Assuming some initial misalignment, the beam strips 30, 32 will 
not exactly coincide with the crosshairs 66 in the plate 62 surrounding 
the hole 64. A central portion of the beam 28 will pass through the hole 
64 and illuminate the side 72 of the block 38 which is also inscribed with 
crosshairs 70. Again it is assumed in FIG. 4 that the beam strips 30, 32 
fail to coincide with these crosshairs 70. To achieve alignment the 
support device 16 on which the laser 12 is mounted is adjusted until the 
beam strips 30, 32 coincide with both the crosshairs 66 and the crosshairs 
70. This adjustment not only ensures that the beam 28 is accurately 
directed towards the isocenter (the center of the block 38), but also that 
the beam 28 is travelling exactly perpendicularly to the side 72 of the 
block 38. 
A similar alignment procedure is carried out at the other end of the arm 56 
with the composite beam 34 from the laser set 10, the location and 
orientation of the laser set 10 being adjusted by means of its support 
device 14. The arm 56 need not necessarily have two end plates 62, 
although this arrangement provides better balance. If it has only one 
plate 64, it is turned through 180.degree. on the pin 44 to align the 
second lateral beam. 
To align the location and orientation of the ceiling laser set 22, the arm 
56 is removed from the vertical pin 44 and placed on the horizontal pin 46 
so that it extends vertically as shown in FIG. 5. In this vertical 
orientation of the arm 56, the composite beam 36 from the ceiling laser 
set 22 first encounters the plate 62 in the same manner as in FIG. 4, 
while its central portion which passes through the hole 64 encounters the 
end face 48 of the pin 44 for alignment with the crosshairs 52. Since the 
block 38 is so made that the axes of the pins 44, 46 intersect each other 
accurately at right angles at the center of the block, this center becomes 
the isocenter. 
To explain how the target device can be utilized, it will be easiest if it 
is explained in a situation where a new treatment machine has been 
installed, but the laser sets have not yet been mounted at their 
respective locations. 
The target block 38 is placed near the front of the treatment table 42 so 
that the horizontal pin 46 extends past the table edge. See FIG. 6. With 
the treatment machines gantry 74 at 0.degree., a field light in the 
collimator is turned on. The field light projects an area of light onto 
the patients body surface, which geometrically mimics the surface area of 
the treatment beam. The position of this light source (a filament lamp) is 
adjustable to make the light beam symmetrical with respect to the axis of 
rotation of the gantry. The block 38 can then be moved on the table 42 to 
come under a crosshair shadow provided by the field light so that the 
etched lines 76 on the top of the block align with the projected shadow. 
This procedure is conventional. It should be noted that the target arm 56 
has not yet been attached. Once the block has been so aligned, the next 
step is to level it in both the X and Y directions using levelling screws 
associated with the support device 40 and a spirit or digital level. The 
field light can then be turned off. The next step is to slide a tray 78 
with an indicator 80 into treatment rails of the collimator 82 of the 
machine. The indicator 80 is positioned so that it just comes into contact 
with the vertical pin 44 (FIG. 7). The collimator 82 is rotated, which in 
turn rotates the indicator 80 around the pin 44. The block 38 is adjusted 
by fine adjustment X and Y screws in the support device 40 until the 
centre of the pin 44 is in direct line with the centre of the axis of 
rotation of the collimator 82. Next, the horizontal pin 46 is aligned in 
the same manner, except that in this case the gantry is rotated by 
90.degree. and the horizontal pin 46 is centralized with the centre of the 
gantry's axis. To accomplish this it may be necessary to adjust the height 
of the treatment table 42. It may also be necessary to rotate the block 38 
by means of adjustment screws. This block rotation does not affect the 
position of the vertical pin 44 because the rotational axis is centred 
with this vertical pin. When both pins are exactly aligned the exact 
centre of the cubic block 38 becomes the mechanical isocenter. 
The target arm 56 is now installed on top of the block 38 (FIG. 6), and the 
lateral laser sets are positioned on their respective mounting devices. 
They are turned on and beamed onto the target plates 62. The composite 
beams must project onto the crosshairs etched on the target plates 62 as 
well as on the target surfaces 72 of the block. The correct positions of 
the aligned laser sets can now be recorded. 
The target arm 56 is then positioned with the target plates 62 pointing 
upwards (FIG. 8). The sagittal laser 84, which is mounted on the wall 86 
in front of the radiotherapy machine between the walls 18 and 20, is now 
employed. The saggital laser beam is a single strip longitudinally along 
the table 42 directly in line with the isocenter. The purpose of this 
single plane laser is to provide for the fact that, when the gantry is in 
its upright position, (0.degree.) it completely blocks the ceiling laser. 
The saggital laser then becomes the sole longitudinal reference and its 
vertical strip is aligned both with a vertical inscription 73 on the block 
38 and a vertical inscription 81 on an inside surface of an end plate 62 
(see FIG. 4), when the arm 56 is in the orientation shown in FIG. 8. In 
the same manner one of the two lateral lasers is blocked when the gantry 
is in the 90 or 270 degree position. The ceiling laser then becomes the 
sole reference in the Y axis. This is an important reason for the present 
invention. With so many lasers converging to a point it has been difficult 
and time consuming to insure that every laser line is perfectly aligned. 
The final laser set to be aligned is on the ceiling. The gantry must be 
rotated out of the way of the vertical composite beam so that it can 
project onto the target device. The arm 56 is attached in the vertical 
orientation, as mentioned above, and the alignment procedure is carried 
out in the same manner as described with the lateral laser sets. 
After all the laser sets have been correctly aligned, the crosshair wires 
against which the operator initially aligned the target device can be 
checked and repositioned if necessary. This is accomplished by first 
bringing the gantry back to 0.degree., turning on the field light and 
projecting the crosshair shadows onto the target device. The crosshairs 
must project onto the etched lines of the plate 62 as well as those of the 
inscribed surface of the target block beyond the plate 62 to be in correct 
alignment. 
The last check to be performed is gantry sag. This is done with the arm 56 
removed. The gantry is rotated to 0.degree. and the indicator tray 78 is 
put back into the collimator 82 without the indicator 80. A right angled 
pointer 86 is mounted on the indicator arm 88 (FIG. 9). The tip of the 
pointer 86 is positioned at the exact centre of the vertical pin 46. The 
gantry is then rotated to 90.degree.. The pointer 86 will still be on the 
vertical etched line on the pin 46, but will move down from the horizontal 
line. The distance the pointer tip moves is the sag measurement and can be 
read on a scale 90 provided on the end face of the pin 46. The gantry is 
then rotated to 270.degree. and a reading on the scale is taken again. 
FIG. 10 illustrates the exact point 88 on which the block must be centered. 
It must also be properly oriented in the X, Y and Z directions. 
As indicated above, while the preferred form of light source is a laser, 
the invention is also applicable to use with collimated incandescent light 
sources. 
Also, the invention is not limited to the determination of an accurate 
isocenter for a radiotherapy or like machine. The invention can be used 
for the determination of an isocenter defined by a plurality of light 
beams, regardless of the use to which this isocenter is put.