Radiation detection system including radiation alignment means and isocentrically rotatable detectors

What is claimed is:

1. An assembly for testing the alignment of a radiation source, comprising:

a mounting fixture;

a detector assembly mounted to said mounting fixture, said detector assembly including an isocenter and radiation detection means responsive to radiation, said mounting fixture including means for supporting said detector assembly in such a manner that said detector assembly is rotatable about first and second axes, said first and second axes being arranged such that the isocentricity of the detector assembly is maintained in substantially all rotational positions about each of said axes.

2. An assembly as defined in claim 1 wherein said first axis is substantially perpendicular to said second axis, each of said axes being in a horizontal plane.

3. An assembly as defined in claim 1 including a camera aimed at the isocenter of said detector assembly.

4. An assembly as defined in claim 3 including means for displaying an image detected by said camera.

5. An assembly as defined in claim 1 wherein said radiation detection means include photodiodes.

6. An assembly as defined in claim 1 wherein said radiation detection means include a luminescent screen.

7. An assembly as defined in claim 1 wherein said radiation detection means include means for detecting radiation impinging upon a plurality of discrete points upon said detector assembly, and display means for displaying whether one or more of said discrete points are irradiated.

8. An assembly as defined in claim 1 wherein said radiation detection means include a plurality of discrete sets of radiation detectors, each of said sets being substantially equidistant from said isocenter.

9. An assembly as defined in claim 1 wherein said detector assembly includes means for detecting the location of an edge of a field of radiation generated thereon.

10. An assembly as defined in claim 9 wherein said means for detecting an edge of a field of radiation include a substantially planar surface and a substantially rectangular border marked upon said surface.

11. An assembly as defined in claim 9 wherein said means for detecting an edge of a field of radiation include a plurality of photodetectors.

12. An assembly as defined in claim 9 wherein said detector assembly includes means for detecting whether a target shadow line passes over a preselected line defined upon said detector assembly.

13. An assembly as defined in claim 12 wherein said detector assembly includes means for detecting whether a second target shadow line passes over a second preselected line defined upon said detector assembly.

14. An assembly as defined in claim 9 wherein said detector assembly includes means for detecting whether a first line of radiation passes over a preselected line defined upon said detector assembly.

15. An assembly as defined in claim 14 wherein said detector assembly includes means for detecting whether a second line of radiation passes over a second preselected line defined upon said detector assembly.

16. An assembly as defined in claim 1 including calibration means for determining the rotational position of said detector assembly with respect to at least one of said first and second axes.

17. An assembly as defined in claim 1 including a target defined at the isocenter of said detector assembly.

18. An assembly as defined in claim 17 wherein said target includes a plurality of concentric ellipses.

19. An assembly as defined in claim 1 including means for comparing the intensity of radiation near the edges of a radiation field upon said detector assembly with the intensity of radiation within the field.

20. An assembly as defined in claim 1 wherein said detector assembly includes a substantially planar surface, said surface including markings which define a substantially rectangular border, the isocenter of said detector assembly, and a pair of orthogonal lines.

21. An assembly as defined in claim 1 including means for adjusting the height of said detector assembly.

22. An assembly as defined in claim 1 wherein said detector assembly includes means for detecting whether a target shadow line passes over a preselected line defined upon said detector assembly.

23. An assembly as defined in claim 22 wherein said detector assembly includes means for detecting whether a second target shadow line passes over a second preselected line defined upon said detector assembly.

24. An assembly as defined in claim 1 wherein said detector assembly includes means for detecting whether a first line of radiation passes over a preselected line defined upon said detector assembly.

25. An assembly as defined in claim 24 wherein said detector assembly includes means for detecting whether a second line of radiation passes over a second preselected line defined upon said detector assembly.

26. An assembly as defined in claim 24 wherein said preselected line is defined by a pair of photodetectors.

27. An assembly as defined in claim 24 wherein said preselected line is defined by a line marked upon said detector assembly.

28. An assembly as defined in claim 9 including a target defined upon said detector assembly, said target being positioned at the isocenter thereof.

29. An assembly as defined in claim 28 wherein said target includes a plurality of concentric ellipses.

30. An assembly as defined in claim 1 wherein said mounting fixture includes a rotary stage, a bracket mounted to said rotary stage, said detector assembly being pivotably mounted to said bracket.

31. An assembly for testing the alignment of a radiation therapy machine, comprising:

a mounting fixture;

a detector assembly mounted to said mounting fixture, said detector assembly including a substantially planar surface;

a border, a pair of orthogonal lines, and a target marked upon said substantially planar surface; and

means for supporting said detector assembly upon said mounting fixture in such a manner that said detector assembly is rotatable about first and second axes, said first and second axes being arranged such that the isocentricity of the detector assembly is maintained in substantially all rotational positions about each of said axes, the isocenter of said detector assembly being defined by said target.

32. An assembly as defined in claim 31 including a camera aimed at said substantially planar surface of said detector assembly.

33. An assembly as described in claim 32 including a monitor connected to said camera.

34. An assembly as described in claim 33 wherein said substantially planar surface includes a phosphorescent screen.

35. An assembly as described in claim 31 including means for adjusting the height of said detector assembly.

36. An assembly as described in claim 31 wherein each of said orthogonal lines is aligned with said target.

37. An assembly as described in claim 31 wherein said target includes at least one ellipse.

38. A method of checking the alignment of a radiation head rotatably mounted to a gantry, said radiation head including means for generating a target shadow, comprising the steps of:

providing a detector assembly and means for rotating said detector assembly about at least two substantially perpendicular axes while maintaining the isocentricity of said detector assembly, said detector assembly including a target at the isocenter thereof;

positioning said detector assembly in a first rotational position in opposing relation t said radiation head while said gantry is in a first rotational position;

causing said radiation head to generate a target shadow such that said target shadow appears upon said target;

rotating said gantry a selected number of degrees about a horizontal axis to a second rotational position;

rotating said detector assembly said selected number of degrees about an axis to a second rotational position such that said detector assembly remains in opposing relation to said radiation head; and

observing the position of said target shadow with respect to said target in said second rotational position.

39. A method as defined in claim 38 wherein said target includes a plurality of substantially concentric ellipses, including the step of orienting said detector assembly at a non-perpendicular angle with respect to said radiation head.

40. A method as defined in claim 38 including the step of rotating said radiation head about a second axis orthogonal to said horizontal axis, and observing the position of said target shadow with respect to said target subsequent to rotating said radiation head.

41. A method of determining the edge of a non-ionizing light or an ionizing radiation field generated by a radiation therapy machine, comprising:

providing a detector assembly including a substantially flat, luminescent surface and a border marked upon said surface;

causing said radiation therapy machine to sequentially generate non-ionizing light and ionizing radiation fields upon said substantially flat surface; and

observing the peripheral edges of the respective fields and the luminescence of said surface with respect to the border.

42. A method as described in claim 41 wherein said border is substantially trapezoidal.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to the alignment of radiation instruments such as therapeutic machines and simulators used for cancer treatment.

2. Brief Description of the Prior Art

Various types of radiation equipment require that alignment be checked prior to actual use to insure the radiation will be directed precisely to the target and not elsewhere. Therapeutic machines such as linear accelerators and cobalt treatment machines are two types of such equipment.

Radiation therapy machines used in the radiation oncology departments of hospitals generally include a radiation head mounted to a rotatable gantry. Radiation should be directed by the head to the same point, the isocenter, regardless of the rotational position of the gantry or collimator.

In addition to including means for generating ionizing radiation, the radiation head generally includes an ordinary light source for generating a non-ionizing light beam upon the patient prior to therapy. The head may further include means for generating a target shadow also known as a "cross hair", which becomes visible upon the patient when the light source is actuated. The cross hair is used as one step in insuring that the radiation, such as x-rays, gamma rays or electrons, is directed to a properly positioned patient.

Since the physician or technician must assume that the ordinary light beam is directed at the same point as the subsequently applied therapeutic radiation, it is important that this, in fact, be the case. The conventional method of establishing light/radiation coincidence is to use x-ray film. The film, in its envelope, is first punctured with a needle on the borderline of the light field. It is then subsequently exposed to the radiation. The degree of overlap between the hole marks on the film and the radiation edge indicate the coincidence between these two fields. This technique has several significant drawbacks, namely the subjective marking of the light field and the length of time necessary to process the film. Scanning equipment is also available for scanning the light and radiation fields and determining the coincidence between the two fields and their widths.

A plurality of lasers are also conventionally used to properly position a patient. The lasers are oriented such that each of the beams eminating therefrom intersects each other at the machine isocenter. The beams impinge upon markings upon a patient to insure the patient is positioned to receive radiation from the radiation head only in a specific area. U.S. Pat. Nos. 4,123,660 and 4,223,227 disclose instruments for aligning lasers which include mirrors for detecting any divergences from the main beams.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an instrument which is capable of detecting whether the alignment of a beam of radiation is correct in various rotational positions of a gantry and a collimator.

It is another object of the invention to provide an instrument capable of detecting whether two beams are coincident or not.

A still further object of the invention is to provide an instrument capable of checking the optical distance indicator of a radiotherapy machine.

A still further object of the invention is to provide an instrument which is capable of determining whether the alignment of a plurality of laser beams is correct.

A still further object of the invention is to provide an instrument capable of all of the above functions.

In accordance with these and other objects of the invention, an instrument is provided which includes a mounting fixture, a detector assembly mounted to the mounting fixture, the detector assembly including radiation detection means, first means for rotating the detector assembly about a first axis with respect to the mounting fixture, second means for rotating the detector assembly about a second axis with respect to the mounting fixture, the first and second rotating means maintaining the isocentricity of the detector assembly in substantially all rotational positions. The first and second axes are perpendicular to each other. A camera or the like may be aimed at the isocenter to provide an enlarged image of the detector assembly, which may be displayed on a monitor.

A detector assembly in accordance with the invention preferably includes a substantially planar top surface. The detection means include a plurality of radiation detectors, such as photodiodes, or a phosphorescent screen. The radiation detectors are preferably arranged in a staggered configuration a selected distance from the isocenter. This allows the radiation levels near the "edge" of a radiation or light field to be detected. The relative positions of two different fields can be compared by noting whether the radiation levels of each drop off at the same points. Coincident beams should result in radiation levels which drop off in substantially the same positions near the "edges" of the respective fields.

The detector assembly in accordance with the invention also preferably includes a top surface upon which a target shadow or cross hair can be observed. The isocenter may be marked by a plurality of concentric circles of ellipses. Ellipses are preferred as they appear as circles when the top surface of the assembly is viewed at an angle.

The detector assembly may also include a second set or sets of staggered radiation detectors. At least two such sets are provided, the two sets forming substantially a right angle with the isocenter. If the cross hair passes through certain of the detectors in each set, it is properly aligned. The alignment of other light sources, such as lasers, can also be determined by noting which detectors in the sets are actuated.

If a phosphorescent screen or the like is employed as the detector assembly, it is provided with a border, a pair of intersecting lines, and one or more circles or ellipses marked upon the surface thereof. The border is preferably trapezoidal and corresponds with the edges of the light field generated by the radiation equipment on the detector plane. The pair of lines intersect at the isocenter, and the centers of the circles or ellipses are at the isocenter. Such markings may be provided on non-phosphorescent detector assemblies as well. Since the operator is provided with an enlarged view of the surface of the detector assembly, he can easily observe any misalignment of the radiation equipment or associated lasers and make the proper adjustments.

Methods of determining light/radiation field coincidence, laser alignment, and cross hair alignment are also provided by the invention. Each of the methods can be performed with an apparatus as described above.

The light field/radiation field coincidence test may be performed by providing a detector assembly including means for detecting radiation intensity at a plurality of points within a selected area, directing a light beam at the detector assembly such that a light field is defined upon the detector assembly, the light field including an area of relatively high intensity and a border about this area of declining intensity, the detecting means detecting the declining intensity of at least part of the border. The light beam is then discontinued, and a radiation beam directed towards the detector assembly. The radiation beam causes a radiation field to be defined upon the detector assembly, the radiation field including an area of relatively high intensity and a border about the area of declining intensity. The detecting means detects the radiation intensity of at least part of the border. The outputs of the detecting means in response to the light and radiation beams are compared to determine whether the borders of the respective fields are substantially coincident.

The cross hair test is conducted by directing a light beam at the detector assembly, causing a target shadow in the form of a cross hair to be defined upon the detector assembly, and detecting the light radiation intensity upon the detector assembly at a plurality of points thereon adjacent to the cross hair or intersecting the cross hair.

Laser alignment may be conducted by directing a laser beam towards a detector assembly, and detecting whether the beam crosses selected detectors within the detector assembly.

The isocentricity of the gantry and collimator may be determined by observing the position of the cross hair with respect to a marked isocenter of the detector assembly. The gantry and detector assembly may be rotated in the same direction and the same number of degrees to determine whether the center of the cross hair remains within an acceptable distance of the isocenter of the detector assembly. The collimator is tested by rotating it with respect to the detector assembly and observing the center of the cross hair with respect to the isocenter of the detector assembly. By magnifying the user's view of the surface of the detector assembly through the use of a videocamera and monitor or the like, the relative positions of the cross hair and isocenter can easily be observed.

The radiation field size can be checked by activating the radiation apparatus and observing the field generated upon the surface of the detector assembly with respect to a border marked upon this surface. Such observation is preferably conducted outside the room where the radiation equipment is located by viewing the surface of the detector assembly on a monitor or the like which is located outside the room.

Laser alignment may be checked by observing the lines illuminated by the lasers at least one axis marked upon the detector assembly and passing through the isocenter thereof. If the illuminated lines pass through the isocenter and are substantially collinear with the marked axis or axes, they are properly aligned.

The individual tests described above may be selected on the basis of the type of detector assembly employed. If radiation detectors are incorporated within the detector assembly which transmit signals upon the detection of radiation, one set of tests may be employed. If the detector assembly is of the type including a phosphorescent screen and a border, a pair of intersecting axes, one or more centrally positioned circles marked on the surface of the screen, a different set of tests are employed. The detector assembly in either event is preferably rotatable about a pair of perpendicular axes and isocentric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a first embodiment of the invention for checking the alignment of a radiation therapy machine;

FIG. 2 is a rear perspective view of a detector assembly and support thereof;

FIG. 3 is a front perspective view thereof;

FIG. 4 is a side elevation view of a portion of a rotary stage and associated support bracket;

FIG. 5 is a top schematic plan view of the top surface of the detector assembly;

FIGS. 6a-6c are schematic diagrams of preamplifier circuits associated with the edge and laser detectors of the detector assembly;

FIG. 7 is a top schematic plan view of the top surface of the display plate;

FIG. 8 is a schematic circuit diagram of a laser detection circuit;

FIG. 9 is a truth table indicating which light emitting diodes are illuminated upon illumination of a set of three photodetectors;

FIGS. 10A-10B are circuit diagrams of a laser detection circuit;

FIG. 11 is a table illustrating which light emitting diodes are illuminated during a cross hair test;

FIGS. 12A-12L are circuit diagrams illustrating an edge detection circuit;

FIG. 13 is a schematical illustration of a radiation field and photodetectors positioned near the edge of the field;

FIG. 14 is a schematical circuit diagram illustrating a priority circuit and logic used in determining radiation field size;

FIG. 15 is a truth table illustrating which light emitting diodes are illuminated upon illumination of various sets of photodetectors;

FIG. 16 is a side elevation view of an alternative embodiment of the invention; and

FIG. 17 is a top plan view of a phosphorescent screen and surface indicia used as a detector assembly.

DETAILED DESCRIPTION OF THE INVENTION

A system 10 for determining whether a radiation therapy machine or the like and other associated devices are properly aligned is shown in FIG. 1. The system includes the following basic components: a rotary stage 12, a detector assembly 14 pivotably mounted to the rotary stage, a video camera 16, a monitor 18, and a display panel 20. The detector assembly is isocentric, and the camera is aimed at the isocenter thereof.

The rotary stage 12 and camera 16 are both movably mounted to a support rail 22 having a cylindrical base and four projecting flanges. The camera shown in FIG. 1 is secured to a vertical support 24 including a clamp-like base 26 which engages the support rail. (Alternatively, as shown in FIGS. 2-3, the detector assembly 14 may be mounted to this support 24). The rotary stage is mounted to a tower 28 which includes a similar, clamp-like base 29. By loosening the clamp-like bases of the camera support and/or the fixture, the operator may slide either structure along the rail. The rotary stage 12 may be moved vertically by means of a translational stage 30 supported by the tower. Three legs 32 are clamped to the support rail 22. The foot portions of the legs engage the horizontal upper surface 34 of the table 36 which is later used to support a patient.

The detector assembly 14 is positioned directly beneath the collimator 38 of a radiation therapy machine 40 such as a linear accelerator. The collimator 38 is mounted to a rotatable gantry 42 so that radiation may be directed towards a patient from a number of radial directions. The gantry is rotatable about a horizontal axis which runs parallel to the longitudinal axis of the support rail 22, and is referred to hereafter as the y axis. The x axis is perpendicular to the y axis in the horizontal plane, and the z axis is normal to the horizontal plane. The collimator 38 is rotatable about the z axis and emanates the light and radiation fields along this axis. Axes y and z intersect at the isocenter. All of the motions of the accelerator 40 are accordingly isocentric if aligned properly. The table 36 for supporting the patient is also rotatable isocentrically.

A plurality of laser sources 44 are positioned about the table. Each laser directs a beam of light towards either a patient positioned upon the table 36 or the detector assembly 14, whichever is in the light path. In either case, an illuminated line is formed on the target by each laser beam. Each of these lines should cross the isocenter.

Referring to FIGS. 1-3, the detector assembly 14 is supported by a mounting bracket 46 including a pair of parallel arms 48. The detector assembly 14 is positioned between the bracket arms and pivotably secured thereto by a pair of opposing pins 50. It is rotatable about the arms defined by the pins. The detector assembly may be maintained in a selected rotational position by frictional engagement with the bracket arms or by mechanical locking means.

The mounting bracket 46 is supported by the rotary stage 12. The rotary stage includes a calibrated, rotatable ring 52 to which the back plate 53 of the mounting bracket 46 is secured. The calibrated ring 52 and mounting bracket 46 are accordingly rotatable about an axis which is perpendicular to the axis defined by the pivot pins 50. The position of the center of the detector assembly 14 does not change regardless of the rotational position of the bracket 46 with respect to the rotary stage. In other words, the rotary stage 12 and mounting bracket 46 allow the detector assembly to be rotated about two axes while maintaining its isocentricity.

One or more set screws 54,55 are provided for fine tuning the rotational position of ring 52 locking the ring 52 in any rotational position, respectively. The calibration markings on the ring 52 indicate the rotational position of the detector assembly 14. These markings allow the operator to correlate the rotation of the gantry with the rotation of the detector assembly. A third adjustment screw 56 allows the rotary stage 12 to be moved with respect to a mounting plate 57 and parallel to the y axis.

Referring to FIGS. 3 and 5, the detector assembly 14 includes a plexiglass or other substantially transparent housing 58 including a pair of opposing extensions 59 through which the pivot pins 50 extend. The housing includes a substantially square upper wall defining a flat upper surface 60. The isocenter is defined by the centers of three concentric ellipses 62 marked upon this upper surface. A silicon PIN diode 64 having an active area of 2.71.times.2.71 mm.sup.2 is mounted to the housing beneath the isocenter. It is employed as a reference photodiode.

Four sets 66 of seven similar photodiodes 64, used as "edge" detectors, are arranged at selected radial distances from the isocenter. The photodiodes may be individually mounted to the housing, or parts of an array mounted thereto. Their positions correspond generally with the field sizes of the light and radiation beams which are subsequently directed thereon. In the illustrative embodiment of the invention shown and described herein, the photodiodes 64 in each set 66 are arranged in staggered relationship, the center to center spacings of the stagger being about 0.5 mm. While seven photodiodes are shown in each set, it will be appreciated that a greater or lesser number can be employed. Each of the photodiodes is positioned beneath the transparent upper wall of the housing. It is accordingly important that the upper surface 60 is both clean and substantially free of dust when the detector assembly is employed.

Four opaque, rectangular plates 68 are secured to the upper wall of the housing 58. Each plate includes three slits 70 extending therethrough. The slits are arranged in staggered relationship. The opposing center slits of two opposing plates are positioned on a line 72 provided on the upper surface 60 of the detector assembly 14 and running parallel to or collinear with the axis defined by the pivot pins 50. The other opposing pair of center slits 70 are arranged along a line 74 running perpendicular to the first-mentioned line 72. A photodiode, also referred to herein as a "laser detector", is positioned beneath each of the slits 70.

As shown in FIGS. 6a and 6b, each photodiode 64 (designated as D1-D7 in the figure for the edge detectors, D-right, D-left, and D-middle for the laser detectors) is connected to a transimpedance, FET input, low noise preamplifier, the output of which goes to a fifty pin edge connector designated as J1. The transimpedance preamplifier converts current generated in each diode, due to incident light or radiation, to voltage output. The transfer function of the amplifier is: ##EQU1## where:

s=j.omega.

R.sub.F =Feedback resistor

C.sub.F =Feedback capacitor

C.sub.i =Input capacitance of amplifier and detector (Typ. value, 1 pf).

A=Open loop gain (.apprxeq.300 at 10 KHz)

The band width (BW) is: ##EQU2##

Using the values from FIGS. 6a and 6b, ##EQU3## for edge detector ##EQU4## for laser detector

For a single pole amplifier the rise time is approximated: ##EQU5##

FOR EDGE: t.sub.r =0.35/241*10.sup.3 =1.5 .mu.sec

FOR LASER: t.sub.r =0.35/8*10.sup.3 =44 .mu.sec

The circuit is fast to respond to radiation pulses, which are about 10 .mu.sec wide, (t.sub.r =1.5 .mu.sec<10 .mu.sec) and has good low pass (L.P.) characteristics to reduce noise at the am