Radiation Apparatus With Distance Mapper for Dose Control

The present invention relates to apparatus for delivering a specific therapeutic dose of x-rays or the like to a target, using optical distance and surface profile measuring equipment.

In U.S. Pat. No. 4,403,337, Kleinman discloses a medical x-ray machine using acoustic pulse-echo ranging in order to automatically determine and set the amount of current and voltage supplied to an x-ray source, as well as the exposure time, based on the thickness of the patient part to be imaged. The ranging system includes a sonic transducer at a known distance from a receptor, and a circuit which determines the travel time of a sonic signal from the transducer to the patient and back to the receptor. The thickness of the patient part can be determined from the actual travel time compared to the time when no patient is present.

In U.S. Pat. No. 4,212,534, Bodlaj describes a device for contact-free measuring of the distance of an object surface from a reference plane having a laser light source producing a beam of light, a light deflector causing the beam to repeatedly scan over the surface of an object and a photodetector. The device measures the interval of time for travel of the beam for at least two specific directions of the beam, and between one of the two directions and a direction at which the detector responds, and uses these intervals to determine the distance.

X-ray or gamma ray apparatus generally produce a divergent beam whose intensity decreases with distance according to an inverse square relationship. Accordingly, in order to deliver a precise dose to a medical patient in order to form a good image on a film plate or to irradiate a tumor without unduly exposing the patient to excess radiation, the precise distance of the patient from the x-ray source must be found. Other factors related to distance which are used in making a precise dose calculation include the thickness of a body part to receive x-rays and the skin surface profile of the area to be irradiated. Previous measurement techniques such as using rulers and calipers to determine distance and thickness, and acoustic measurements, are usually adequate for forming viewable x-ray images on a photographic plate but are not sufficiently accurate for delivering a precise dose to a tumor in a localized area.

It is an object of the present invention to produce an x-ray or gamma ray apparatus with a distance measuring system capable of more accurately determining the precise radiation dose to be delivered to a target area surface of a patient.

The above objects have been met with an optical target profiling system used prior to the use of a radiation treatment process. The target profiler maps a surface, such as a portion of a patient's body so that radiation dose to a portion of the body can be carefully controlled. By creating a topographic map or a series of profiles, the contours of a surface region of the body may be taken into account. Now, three dimensional information is available regarding the target, rather than the one dimensional information of the prior art. The entire apparatus includes an x-ray or isotopic source for producing an x-ray or gamma ray beam directed so as to deliver a dose of radiation to an area of a target surface, an optical distance and surface profile measuring device, and means responsive to the measured distance from the x-ray or isotopic source to the target surface for adjusting the dose of radiation produced by the source. The laser distance and profile measuring device includes a laser or other beam source emitting a light beam and also includes a photodetector, both in known spatial relationship to the source. Mirrors or other means for directing the light beam to a sequence of spots on the target area and for directing light scattered and reflected from the target to the photodetector are provided. Knowing the geometry of the system, including for example, the angles of the directing mirrors, one can compute the distance from the x-ray source to each spot on the target area and produce a surface profile map. The profile map alone is very valuable where distance is measured by another technique or approximated, but profile plus distance is even more valuable. One embodiment rotates a mirror at a known speed. Sensors determine when the mirror is in a known orientation, while the photodetector senses when the light from the target is properly directed to the detector. The time lag between the two times is used to determine the angle of the mirror for computation purposes. Once the surface profile is known, the radiation beam may be attenuated to deliver a desired dose to the target body.

FIG. 1 is a top plan view of an x-ray apparatus of the present invention.

FIG. 2 is a schematic diagram illustrating the triangulation geometry for the apparatus in FIG. 1.

FIG. 3 is a profile map corresponding to a front plan view of a target surface to receive x-rays from the apparatus in FIG. 1.

With reference to FIG. 1, an x-ray or other ionizing radiation source 11 contained in a shielding box 13 produces a beam 12 directed through an aperture 14 in shielding box 13 to a target 15. Beam 12 is a divergent beam and delivers a dose of radiation to a surface area 17 of target 15 according to the inverse square of the distance from the source to the surface 17. Radiation source 11 is typically an electrically powered x-ray tube. Alternatively, source 11 may be a radioactive isotope producing gamma rays. The dose of x-rays delivered to target 15 by an x-ray tube can be adjusted by adjusting the voltage or current or both which energizes the x-ray tube. The dose may also be adjusted by varying the attenuation provided by x-ray or gamma ray attenuating filters 41 in the path of beam 12. Typically target 15 is a patient undergoing medical diagnosis or treatment. However, the x-ray apparatus may also be used for other types of targets.

In order to administer a precise dose of x-rays to a target area, it is necessary to measure the distance from the x-ray source 11 to the target area surface 17. Since a target with many surface features has varying distances to the x-ray source depending on the point being used for measurement, it is preferable that a topographic may be developed of the area of the target surface to receive x-rays. Attenuator 41 need not be a flat plate, and can be a custom built part. It is known in the art that one can make an attenuator with a controlled thickness or attenuation profile, such that the target gets a uniform dose or to adjust the dose locally as required in order to compensate for tissue behavior, using profile information obtained from distance measurements. The present invention includes an optical distance and surface profile mapping device 19, which computes the required distances for various points on the target surface.

The device 19 includes a laser or other beam source 21 emitting a light beam 23 and a photodetector 25. Laser 21 and photodetector 25 are in a known spatial relationship with respect to x-ray source 11. A first mirror 27 in a path of light beams 23 directs light beam 23 from laser 21 towards a sequence of spots 29 on target surface 17. First mirror 27 is turnable about at least one axis, as indicated by arrows X in order to place spot 29 of laser light 23 where required on target surface 17. Preferably, first mirror 27 may be turned about two axes, as indicated by arrows X and arrow Y. By swinging first mirror 27 in two directions, an area can be scanned and the contour of the area can be generated. Alternatively, mirror 27 may be replaced by a pair of mirrors turnable about respective orthogonal axes. Laser light 23 after having been redirected by first mirror 27 onto path 28 to spots 29 is scattered by target surface 17, and the scattered light along the path 30 is directed by a second mirror 31 to photodetector 25. A lens 33 focuses the light from mirror 31 onto photodetector 25, and a color filter 35 blocks unwanted light of colors other than that of laser 21.

A motor 37 rotates second mirror 31 at a known and preferably constant speed. As mirror 31 rotates, a sensor 39 detects when mirror 31 reaches a predetermined orientation. Sensor 39 then generates a start signal. Then as mirror 31 continues to rotate, it reaches an orientation where light scattered from point 29 on target surface 17 reaches photodetector 25. Photodetector 25 generates a signal on receiving light from spot 29. The orientation of mirror 31 when detection by photodetector 15 occurs is determined by the time lag between the start signal from sensor 39 and the detection signal from photodetector 25, together with the known speed of rotation of mirror 31. Photodetector 25 is preferably a differential sensing split area detector. This allows sensing of the exact moment at which the scanning mirror 31 is positioned to reflect the laser spot from the target surface 17 into the midpoint of photodetector 25. Midpoint sensing is preferred because temperature, laser power, ambient light, and other parameters affect both halves of the detector equally, therefore cause no differential signal at the midpoint. A phase-locked loop may be used to synchronize the speed of the time base used in determining the angular orientation of mirror 31 to match the motor speed of motor 37. This allows precise prediction of the angular position of the scanning mirror 31 without the use of an expensive precision motor shaft position encoder. Precision motor shaft position encoders may also be used. The angular orientation of first mirror 27 is known from a shaft angle encoder, galvanometer or other means.

With reference to FIG. 2, the distance from the x-ray source to each of the sequence of spots on target surface 17 may be computed from the known geometry of the apparatus, such as by triangulation using the angles of first and second mirrors 27 and 31. For example, the path of laser beam 23 from laser 21 to first mirror 27 defines a baseline represented in FIG. 2 by the base 23 of the triangle. The orientation of first mirror 27 is characterized by an angle .alpha. between the baseline and a path 28 between first mirror 27 and the desired spot 29 on the target surface. Likewise the orientation of second mirror 31 is characterized by an angle .beta. between the baseline and a path 30 between the desired spot 29 on the target surface and second mirror 31. Mirror 31 and the photodetector are located proximate to the baseline 23 to minimize any errors in the calculation of the desired distances. Knowing the distance d between first and second mirrors 27 and 31 along baseline 23 and knowing the characteristic angles .alpha. and .beta. mirrors 27 and 31 the height h, which represents the distance between the baseline 23 and the desired spot 29 on the target surface, may be found according to the formula: ##EQU1## since the laser source photodetector and mirrors of the distance measuring apparatus 19 are in a known relationship to the x-ray source, the distance x between the x-ray source and a baseline 23 is known. Accordingly, the distance D between the x-ray source and each of the sequence of desired spots 29 on the target surface is D=x+d[cot .alpha.+cot.beta.].sup.-. This process may be repeated for each of the sequence of points on the target surface.

The triangulation example just given is not the only way to compute the desired distance measurements. The geometrical relationship between the laser or other beam source, the scanning mirrors, target and detector can be quite complex. However, once the geometry is known the distance can be computed from a derived formula based on that geometry using measurements of predetermined parameters. Since the geometry is fixed for a particular apparatus, the geometric formula for computing the desired distance need be derived only once.

A surface profile map may be formed from the computed distances for the sequence of points on the target surface. If the first mirror 27 is turnable about one axis the locus of points represents a line on the target surface whereas areawise coverage of the target surface is provided when first mirror 27 is scannable about two axes, or when multiple scanning mirrors are used.

With reference to FIG. 3, a surface profile map represented by a series of equidistant curves 45 is seen overlaid over a front view of a target 15, here a patient undergoing medical treatment. The target surface is in this instance the front torso of patient 15. A three-dimensional map of the target is made with a series of scans across the target. Each curve 45 represents a plurality of points 29 calculated to have the same distance from the x-ray source. This three-dimensional map combined with the distances are used to adjust the x-ray dose which the patient 15 receives. The patient 15 may for example be receiving x-rays for the treatment of a tumor located in a specific organ of his or her body, and the treatment may require a specified minimum dose of x-rays directed to that organ while at the same time the rest of the body 15 should not receive an excess amount of x-rays. Knowing the amount of tissue between the skin surface and the target organ and knowing the attenuation factors for that intervening tissue, the amount of x-rays can be fine tuned so that the target organ receives the precise dose required. As already noted above, the attenuator 41 in FIG. 1 need not be a flat plate but can have a controlled three-dimensional profile custom made for the particular patient. The surface profile map is used to make this custom attenuator. Since some patients may require several doses over a period of weeks or months, the particular attenuator can be saved and brought out again for the next treatment dose.

The x-ray apparatus of the present invention allows precise dosage calculations to be made based upon an optical beam base distance and surface profile mapping device which is estimated to be accurate to within one millimeter within the area to be covered by the x-ray beam.