Patent Number: 
Section: description

FIG. 1 shows schematically the first means of the detection apparatus of the invention. These consist of sensor means for sensing the garrma rays. (1) emitted by the source (2), a plurality of means for measuring the gamma-ray flux and means for analyzing the measured flux. The sensor means consist of a collimator (3) which has a scintillating crystal (4) on its rear face. Moreover, the plurality of means for measuring the flux of the gamma rays (1) are in the form of a multi-anode photomultiplier (5) having four anodes. To identify in which direction the radioactive source (2) lies with respect to the center of the detector, the collimator has: a central area (6) comprising a plurality of mutually parallel channels (7) perpendicular to the surface of said collimator; a peripheral area (8) comprising a plurality of divergent channels (9) which are at an angle with the channels (7) of the central area (6) which increases with their distance from said central area. As already mentioned the photomultiplier has four anodes defining four sectorsxe2x80x94North, South, East and Westxe2x80x94with respect to the center of the photomultiplier. Thus, the intensities measured by the various sectors of the photomultiplier are analyzed in the form of four points coded in terms of X and Y with respect to the center of the photomultiplier, thus determining the coordinates of the impacts detected in the four North, South, East and West registers. The apparatus also includes second means (not shown) capable of pin-pointing the radioactive emission source. These second means consist of mechanical means or imparting a movement to the apparatus and means capable of defining the central axis of the apparatus, especially in the form of a light ray. In this example, the detection apparatus is used to detect the sentinel node, labeled with 99 m technetium, especially within the context of breast cancer. During the intervention, the detection apparatus is placed a few centimeters above the region within which the radioactive node probably lies. In a first step, he means for analyzing the intensities measured by the photomultiplier process all of the signals output by the entire sensitive area of the crystal. What is obtained is a number of impacts on the four anodes, these being converted into electrical signals by the photomultiplier. These impacts are distributed within the fourxe2x80x94North, East, South and Westxe2x80x94registers, as shown in FIG. 3. The apparatus is moved by the mechanical means in the direction in which lies that sector whose activity or flux is greater than a minimum background noise threshold measured beforehand. Likewise, if two sectors have an activity greater than the background noise, the apparatus is moved in the direction intermediate between the two sectors until the measured fluxes are almost the same. If three or four sectors have a greater activity, the apparatus then processes only the impacts obtained in the central area of the crystal. To avoid having to reset the registers to zero at each movement of the apparatus, the setting-to-zero may take place at regular intervals, especially every two seconds. As already mentioned, the second step follows on automatically and is restricted to analyzing the central part of the crystal, thus making it possible to center the apparatus more precisely, but more slowly on the hot point, whatever the sources lying within the rest of the scan field. As long as the values of the North, East, South and West registers are unequal, the mechanical movement means are activated for movement in the direction of the sector corresponding to the register containing the maximum value. The four cardinal registers are reset to zero at shorter time intervals than in the initial phase. When the radioactive tissue or organ is in the center of the field, the values contained in the registers are equal to one another, which immobilizes the apparatus and generates a specific signal, for example an audible signal or a flashing light. However, it should be noted that the notion of equality between the register values, must take account of the fact that it results From the temporal accumulation of a number of radioactive disintegrations. These values are therefore subject to statistical fluctuations of the Poissonian type. It is therefore necessary to avoid any movement of the apparatus while it is in the central equilibrium position, and to define a difference threshold between the register below which it is probable that the equilibrium position has been achieved. For this purpose, a fifth register, denoted sigma, contains the sum of the four others. Once equilibrium has been achieved, the central axis of the detector is defined by a light beam of the laser type emitted by the systems (10, 11) shown in FIG. 1. The light beam indicates, on the surface of the skin or on the visible surface of the targeted tissues, the direction of provenance of the source, that is to say the direction in which the radioactive node lies. The system has the advantage of operating in real time, so that the light beam continuously indicates the direction in which the node lies, even in the case of possible displacement of the tissues pressed by the surgeon, for example when he makes an incision in the tissues, seals them, or changes the position of the retractors. As already mentioned, the apparatus of the invention can be applied to the detection of any tissue or organ fixing a radioactive source. This method of detection will be more particularly advantageous for the detection of sentinel nodes, especially within the context of breast cancers or The advantages of the invention are clearly apparent from the description. The ability of the apparatus to precisely locate a radioactive source, so as to minimize the surgical intervention, should in particular be noted.