Abstract:
A method for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources includes the steps of: pulsing all light sources simultaneously; reading the output of each photomultiplier tube; comparing the output of each photomultiplier tube with an expected value; determining whether the output of each photomultiplier tube is within a first specified tolerance; and adjusting each photomultiplier tube if the output of the photomultiplier tube is not within the first specified tolerance.

Description:
FIELD OF INVENTION 
     The present invention relates to a method for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources. 
     BACKGROUND OF THE INVENTION 
     In the human body, increased metabolic activity is associated with an increase in emitted radiation. In the field of nuclear medicine, increased metabolic activity within a patient is detected using a radiation detector such as a scintillation camera. 
     Scintillation cameras are well known in the art, and are used for medical diagnostics. A patient ingests, or inhales or is injected with a small quantity of a radioactive isotope. The radioactive isotope emits photons that are detected by a scintillation medium in the scintillation camera. The scintillation medium is commonly a sodium iodide crystal, BGO or other. The scintillation medium emits a small flash or scintillation of light, in response to stimulating radiation, such as from a patient. The intensity of the scintillation of light is proportional to the energy of the stimulating photon, such as a gamma photon. Note that the relationship between the intensity of the scintillation of light and the gamma photon is not linear. 
     A conventional scintillation camera such as a gamma camera includes a detector which converts into electrical signals gamma rays emitted from a patient after radioisotope has been administered to the patient. The detector includes a scintillator and photomultiplier tubes. The gamma rays are directed to the scintillator which absorbs the radiation and produces, in response, a very small flash of light. An array of photodetectors, which are placed in optical communication with the scintillation crystal, converts these flashes into electrical signals which are subsequently processed. The processing enables the camera to produce an image of the distribution of the radioisotope within the patient. 
     Gamma radiation is emitted in all directions and it is necessary to collimate the radiation before the radiation impinges on the crystal scintillator. This is accomplished by a collimator which is a sheet of absorbing material, usually lead, perforated by relatively narrow channels. The collimator is detachably secured to the detector head, allowing the collimator to be changed to enable the detector head to be used with the different energies of isotope to suit particular characteristics of the patient study. A collimator may vary considerably in weight to match the isotope or study type. 
     Scintillation cameras are used to take four basic types of pictures: spot views, whole body views, partial whole body views, SPECT views, and whole body SPECT views. 
     A spot view is an image of a part of a patient. The area of the spot view is less than or equal to the size of the field of view of the gamma camera. In order to be able to achieve a full range of spot views, a gamma camera must be positionable at any location relative to a patient. 
     One type of whole body view is a series of spot views fitted together such that the whole body of the patient may be viewed at one time. Another type of whole body view is a continuous scan of the whole body of the patient. A partial whole body view is simply a whole body view that covers only part of the body of the patient. In order to be able to achieve a whole body view, a gamma camera must be positionable at any location relative to a patient in an automated sequence of views. 
     The acronym “SPECT” stands for single photon emission computerized tomography. A SPECT view is a series of slice-like images of the patient. The slice-like images are often, but not necessarily, transversely oriented with respect to the patient. Each slice-like image is made up of multiple views taken at different angles around the patient, the data from the various views being combined to form the slice-like image. In order to be able to achieve a SPECT view, a scintillation camera must be rotatable around a patient, with the direction of the detector head of the scintillation camera pointing in a series of known and precise directions such that reprojection of the data can be accurately undertaken. 
     A whole body SPECT view is a series of parallel slice-like transverse images of a patient. Typically, a whole body SPECT view consists of sixty four spaced apart SPECT views. A whole body SPECT view results from the simultaneous generation of whole body and SPECT image data. In order to be able to achieve a whole body SPECT view, a scintillation camera must be rotatable around a patient, with the direction of the detector head of the scintillation camera pointing in a series of known and precise directions such that reprojection of the data can be accurately undertaken. 
     Therefore, in order that the radiation detector be capable of achieving the above four basic views, the support structure for the radiation detector must be capable of positioning the radiation detector in any position relative to the patient. Furthermore, the support structure must be capable of moving the radiation detector relative to the patient in a controlled manner along any path. 
     In order to operate a scintillation camera as described above, the patient should be supported horizontally on a patient support or stretcher. 
     The detector head of the scintillation camera must be able to pass underneath the patient. Therefore, in order for the scintillation camera to generate images from underneath the patient, the patient support must be thin. However, detector heads are generally supported by a pair of arms which extend from a gantry. Thus, the patient support generally must be cantilevered in order for the detector head to be able to pass underneath the patient without contacting any supporting structure associated with the patient support. The design of a cantilevered patient support that is thin enough to work properly with a scintillation camera is exceedingly difficult. Expensive materials and materials that are difficult to work with, such as carbon fibre, are often used in the design of such cantilevered patient supports. 
     A certain design of gantry or support structure for a scintillation camera includes a frame upon which a vertically oriented annular support rotates. Extending out from the rotating support is an elongate support. The elongate generally comprises a pair of arms. The pair of arms generally extends through a corresponding pair of apertures in the rotating support. One end of the pair of arms supports the detector head on one side of the annular support. The other end of the pair of arms supports a counter balance weight. Thus, the elongate support is counterbalanced with a counterweight on the opposite side of the detector head. 
     With such a design of support structure for a scintillation camera, a patient must lie on a horizontally oriented patient support. The patient support must be cantilevered so that the detector head can pass underneath the patient. If the detector head must pass underneath only one end of the patient, such as the patient&#39;s head, the cantilevered portion of the patient support is not long enough to cause serious difficulties in the design of the cantilevered patient support. However, if the camera must be able to pass under the entire length of the patient, the entire patient must be supported by the cantilevered portion of the patient support. As the cantilevered portion of the patient support must be thin so as not to interfere with the generation of images by the scintillation camera, serious design difficulties are encountered. 
     Among the advantages associated with such a design of support structure is that a patient may be partially passed through the orifice defined by the annular support so that the pair of arms need not be as long. However, the patient support must be able to support the patient in this position relative to the annular support, must be accurately positionable relative to the annular support, and must not interfere either with the rotation of the annular support or with the cable which will inevitably extend from the detector head to a nearby computer or other user control. 
     The photomultiplier tubes in a scintillation camera generate electric signals. The signals are processed, and images are created corresponding to the radiation emitted by the patient. 
     The photomultiplier tubes in a scintillation camera must be calibrated from time to time, that is, gain calibration must be performed, to ensure that their output remains constant. 
     Scintillation cameras generally include an light emitting diode for each photomultiplier tube. Typically, to calibrate the photomultiplier tube, the light emitting diode for that particular photomultiplier tube is pulsed, that is, is activated so as to provide a pulse of light. The output of the photomultiplier tube is compared with a known or expected value, such as a previously measured output of the photomultiplier tube. 
     If the output of the photomultiplier tube corresponds to the expected value, within a certain tolerance, the photomultiplier tube likely needs no calibration. 
     If the output of the photomultiplier tube is different from the expected value, that is, outside the tolerance, the photomultiplier tube is probably in need of calibration. However, it is possible that the photomultiplier tube does not need calibration, but rather that it is the output of the light emitting diode that has changed. 
     A prior art method of determining whether the output of the light emitting diode has changed is disclosed in U.S. Pat. No. 5,237,173 to Stark et al. In the disclosed method, the light emitting diode is pulsed. The output of the corresponding photomultiplier tube is then compared with the expected value. The output of the surrounding photomultiplier tubes are then compared with their expected values. If the outputs of all the photomultiplier tubes do not equal their expected values, within an appropriate tolerance, then it is likely that it is the light emitting diode that is malfunctioning. However, if it is only the output of the photomultiplier tube being calibrated that does not correspond to its expected value, then it is likely that it is the photomultiplier tube that is in need of calibration. 
     The above process is repeated for each photomultiplier tube. Accordingly, one disadvantage of this prior art method is that it is slow. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved an improved method for calibrating photomultiplier tubes in a scintillation camera. 
     The invention relates to a method and apparatus for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources. The method includes the steps of: pulsing all light sources simultaneously; reading the output of each photomultiplier tube; comparing the output of each photomultiplier tube with an expected value; determining whether the output of each photomultiplier tube is within a first specified tolerance; and adjusting each photomultiplier tube if the output of the photomultiplier tube is not within the first specified tolerance. 
     An embodiment of the invention also relates to a method and apparatus for calibrating photomultiplier tubes in a scintillation camera having a plurality of light sources. The method includes the steps of: pulsing all light sources simultaneously; reading the output of each photomultiplier tube; summing the outputs of the photomultiplier tubes; comparing the sum of the outputs of the photomultiplier tubes with an expected sum; determining whether the sum of the outputs of the photomultiplier tubes is within a second specified tolerance; adjusting the light sources if necessary, if the sum of the outputs of the photomultiplier tubes is not within the second specified tolerance, and repeating the above steps; comparing the output of each photomultiplier tube with an expected value; determining whether the output of each photomultiplier tube is within a first specified tolerance; adjusting each photomultiplier tube if the output of the photomultiplier tube is not within the first specified tolerance; comparing the output of each photomultiplier tube with the sum of the outputs of the photomultiplier tubes divided by the number of photomultiplier tubes; determining whether the output of each photomultiplier tube is within a third specified tolerance; and adjusting each photomultiplier tube if the photomultiplier tube is not within the third specified tolerance. 
     The invention also relates to a scintillation camera for obtaining a distribution image of incident gamma rays from a subject, the camera having a scintillator for emitting flashes of light due to incident gamma rays, a plurality of photomultiplier tubes optically coupled with said scintillator for converting the light flashes into respective electric signals which are individually detectable. The scintillation camera includes: a plurality of pulsible light sources, the plurality of light sources being simultaneously pulsible; pulsing means for pulsing the light sources simultaneously; and gain calibration means for comparing an output of each photomultiplier tube to an expected value, and carrying out an effective gain adjustment for each photomultiplier tube. 
     Other advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of preferred embodiments in conjunction with the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a scintillation camera including a detached patient support in accordance with the invention; 
     FIG. 2 is a perspective view of the guide of a scintillation camera; 
     FIG. 3 is a front elevation view of a scintillation camera; 
     FIG. 4 is a side elevation view of a scintillation camera; 
     FIG. 5 is a side elevation view of a scintillation camera; 
     FIG. 6 is a front elevation view of a scintillation camera; 
     FIG. 7 is a top plan view of a scintillation camera; 
     FIG. 8 is a perspective view of the scintillation camera of FIG. 1, including the detached patient support and engaged patient support, with the stretcher removed; 
     FIG. 9 is a side view of a portion of the patient support apparatus; 
     FIG. 10 is a perspective view of the positioner; 
     FIG. 11 is a side elevation view of the positioner; 
     FIG. 12 is a front elevation view of the positioner; 
     FIG. 13 is a plan view of an array of photomultiplier tubes and light sources in accordance with the present invention; 
     FIG. 14 is a flowchart of a first embodiment of the invention; 
     FIG. 15 is a flowchart of the second embodiment of the invention; and 
     FIG. 16 is a flowchart of the third embodiment of the invention. 
    
    
     Similar references are used in different figures to denote similar components. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 to  12 , a nuclear camera  5  is supported and positioned relative to a patient by a support structure  10 . Nuclear cameras are heavy, usually weighing approximately three to four thousand pounds. Thus, the support structure  10  should be strong and stable in order to be able to position the camera  5  safely and accurately. The support structure  10  includes a base  15 , an annular support  20 , an elongate support  25 , and a guide  30 . 
     The base  15  includes a frame  35 . The frame  35  includes twelve lengths of square steel tubing welded together in the shape of a rectangular parallelepiped. The frame  35  has a front square section  37  and a rear square section  38 . In the illustrated embodiment, the frame  35  is approximately five feet wide, five feet high, and two feet deep (e.g. approximately 1.5 m wide, 1.5 m high and 0.6 m deep). The frame  35  also includes eight triangular corner braces  40  welded to the front square section  37 , that is, each corner of the front square  37  has two corner braces  40 , one towards the front of the front square section  37 , and one towards the rear of the front square section  37 . In the illustrated embodiment, the corner braces  40  are in the shape of equilateral right angle triangles. 
     Attached to the underside of the face  35  are two horizontal legs  45 . Attached to each leg  45  are two feet  50 . An alternative to the use of feet  50  is to attach the base  15  to a floor by way of bolts set into the floor. The legs  45  extend beyond the frame  35  so as to position the feet  50  wider apart to increase the stability of the base  15 . The feet  50  are adjustable so that the base  15  may be leveled. Thus constructed, the base  15  is strong, stable, rigid, and capable of supporting heavy loads. 
     The annular support  20  is vertically oriented, having an inner surface  55  defining an orifice  60 , an outer surface  65 , a front surface  70 , and a rear surface  75 . The annular support  20  is constructed of a ductile iron casting capable of supporting heavy loads. In the illustrated embodiment, the annular support  20  has an outside diameter of about fifty two inches (e.g., about 1.3 m). The annular support  20  is supported by upper rollers  80  and lower rollers  85  which are mounted on the base  15 . The upper rollers  80  and lower rollers  85  roll on the outer surface  65 , thus enabling the annular support  20  to rotate relative to the base  15  in the plane defined by the annular support  20  (e.g., alternatively referred to as a first plane). Each of the upper rollers  80  and lower rollers  85  are mounted onto a pair of corner braces  40  by way of axles with deep groove bearings. The bearings should be low friction and be able to withstand heavy loads. The axles of the upper rollers  80  are radially adjustable relative to the annular support  20 , so that the normal force exerted by the upper rollers  80  on the outer surface  65  is adjustable. The curved surfaces of the upper rollers  80  and lower rollers  85  (i.e. the surfaces that contact the outer surface  65 ) should be tough so as to be able to withstand the pressures exerted by the annular support  20 , and should have a fairly high coefficient of friction so as to roll consistently relative to the annular support  20 . 
     Attached to each pair of corner braces  40  is a stabilizing arm  90  oriented perpendicularly to the plane of the annular support  20 . A pair of small stabilizing rollers  95  are mounted onto each stabilizing arm  90 . Each pair of stabilizing rollers  95  is positioned such that one stabilizing roller  95  on the front surface  70 , and the other stabilizing roller  95  rolls on the rear surface  75 . The stabilizing rollers  95  maintain the annular support  20  in the vertical plane. 
     The elongate support  25  includes a pair of support arms  100 , each of which extends through an aperture in the annular support  20 . The nuclear camera  5  is rotatably attached to one end of the pair of support arms  100 , such that the nuclear camera  5  faces the front surface  70 . A counter weight  105  is attached to the other end of the pair of support arms  100 , such that the counterweight  105  faces the rear surface  75 . 
     The counter weight  105  includes a pair of parallel counter weight members  110 , each of which is pivotally attached to one of the support arms  100 . A first weight  115  is attached to one end of the pair of counter weight members  110 , and a second weight  120  is attached to the other end of the pair of counter weight members  110 . A pair of counter weight links  121  connect the counter weight members  110  to the annular support  20 . Each counter weight link  121  is pivotally attached at one end to its corresponding counter weight member  110 . Each counter weight link  121  is pivotally attached at its other end to a counter weight bracket  122  which is rigidly attached to the annular support  20 . The counter weight links  121  are attached to the counterweight members  110  and counter weight brackets  122  using bolts and tapered roller bearings. Each counter weight link  121  is pivotable relative to the annular support  20  in a plane perpendicular to and fixed relative to the annular support  20 . 
     The guide  30  attaches the elongate support  25  to the annular support  20 , and controls the position of the elongate support  25 , and hence the scintillation camera  5 , relative to the annular support  20 . A pair of brackets  125  is rigidly attached to the annular support  20 . A pair of rigid links  130  is pivotally attached at support arm pivot points  135  to the support arms I 100 . The pair of links  130  is also pivotally attached at bracket pivot points  140  to the brackets  125 . At the support and pivot points  135  and bracket pivot points  140  are tapered roller bearings mounted with bolts. Each link  130  is pivotable relative to the annular support  20  in a plane perpendicular to and fixed relative to the annular support  20 . Thus, as the annular support  20  rotates relative to the base  15 , the respective planes in which each link  130  and each support arm  100  can move remain fixed relative to the annular support  20 . 
     A pair of linear tracks  145  are rigidly attached to the front surface  70  of the annular support  20 . The tracks  145  are oriented such that they are parallel to the respective planes in which each link  130  and each support arm  100  can move. A pair of rigid sliding arms  150  (not shown in FIG. 1) include camera ends  155  and straight ends  160 . Each camera end  155  is pivotally attached to one of the support arms  100  at the point of attachment of the scintillation camera  5 . Each straight end  160  includes a pair of spaced apart cam followers or guides  165  slidable within the corresponding track  145 . Thus, movement of the scintillation camera  5  relative to the annular support  20  (i.e. we are not concerned, at this point, with rotational movement of the scintillation camera  5  relative to the elongate support  25 ) is linear and parallel to the plane of the annular support  20 . Note that if the camera ends  155  were pivotally attached to the support arms  100  between the nuclear camera  5  and the annular support  20 , the movement of the nuclear camera  5  relative to the annular support  20  would not be linear. 
     Movement of the scintillation camera  5  relative to the annular support  20  is effected by an actuator  170 . The actuator  170  includes a fixed end  175  pivotally attached to the annular support  20 , and a movable end  180  pivotally attached to the elongate support  25 . The actuator  170  is extendable and retractable, and is thus able to move the elongate support  25  relative to the annular support  20 . 
     Movement of the annular support  20  relative to the base  15  is effected by a drive unit  185 . The drive unit  185  includes a quarter horsepower permanent magnet DC motor and a gearbox to reduce the speed of the output shaft of the drive unit  185 . Alternatively, other types of motors could be used, such as hydraulic or pneumatic motors. The output shaft of the drive unit  185  is coupled, by means of a toothed timing belt  195  and two pulley wheels  200 , to the axle of a drive roller  190 , which is simply one of the lower rollers  85 , thus driving the drive roller  190 . Power is then transferred from the drive roller  190  to the annular support  20  by friction between the drive roller  190  and the outer surface  65  of the annular support  20 . 
     The support structure  10  of the illustrated embodiment is designed to operate with an apparatus for supporting and positioning a patient, such apparatus including a detached patient support  205 , an engaged patient support  210 , and a cylinder  245 . 
     The detached patient support  205  includes rigid patient frame  215  supported by four casters  220 . Mounted near the top of the patient frame  215  are first support wheels  225  for supporting a stretcher  227  upon which a patient is lying. Two parallel, spaced apart side rails  230  are rigidly attached to the patient frame  215 . The first support wheels  225  and the side rails  230  are arranged to enable the stretcher  227  to roll lengthwise on the detached patient support  205 . Thus, if the patient support  205  faces the front surface  70  such that the patient support is central and perpendicular relative to the annular support  20 , the stretcher  227  is movable on the first patient support wheels  225  substantially along the axis of the annular support  20 . A gear box and motor unit  237  driving at least one of the first patient support wheels  225  moves the stretcher  227  as described. A 0.125 horsepower permanent magnet DC motor has been found to be adequate. 
     The detached patient support  205  can be used both for transporting a patient to and from the scintillation camera  5  and support structure  10  therefor, and for supporting and positioning a patient relative to the base  15  during operation of the scintillation camera  5  and support structure  10 . To ensure that the detached patient support  205  remains stationary during operation of the scintillation camera  5 , four stabilizers  233  can be lowered. Thus lowered, the stabilizers  233  ensure that the detached patient support remains stationary relative to the floor. 
     The engaged patient support  210  includes second support wheels  235 . The second support wheels  235  are positioned such that the stretcher  227  rolled along the first support wheels  225  can roll onto the second support wheels  235  until the stretcher  227  is either fully or partially supported by the second support wheels  235 . The engaged patient support  210  also includes four transverse wheels  240 . 
     The cylinder  245  is rigidly mounted to the annular support  20 . The cylinder  245  is aligned with the orifice  60  of the annular support  20  such that the cylinder is coaxial with the annular support  20 . The cylinder  245  includes a smooth inner surface  246  upon which rest the transverse wheels  240  of the engaged patient support  210 . Thus, the arrangement is such that the patient remains stationary substantially along the axis of the annular support  20  as the annular support  20  rotates relative to the base  15 , regardless of whether the board or stretcher is supported by the first support wheels  225 , the second support wheels  235 , or both. 
     The engaged patient support  210  also includes a stabilizer  250 . The stabilizer  250  includes outside wheels  255  to maintain the engaged patient support  210  horizontal, that is, to stop the engaged patient support from tipping relative to the cylinder  245 . The outside wheels  255  roll on the outside surface  243  of the cylinder  245 . The stabilizer  250  also includes end wheels  256  to prevent the engaged patient support  210  from moving in a direction parallel to the axis of the cylinder  245 . The end wheels  256  roll on the ends  244  of the cylinder  245 . 
     FIGS. 10,  11 , and  12  illustrate a preferred embodiment of the present invention. A detector head  305  of the nuclear camera  5  is supported between the two support arms  100  by a positioner  320 . The detector head  305  is supported between the two support arms  100  by a positioner  320 . The detector head  305  includes a casing  310  in which is contained a scintillation crystal and photomultiplier tubes. Attached to the underside of the casing  310  is a collimator plate  315 . The collimator plate  315  is made of lead perforated by narrow channels, and includes a collimator support  325  extending from the two edges of the collimator plate adjacent the support arms  100 . The collimator plate  315  is attached to the casing  310  by way of bolts  311 . By removing the bolts  311 , the collimator  315  can be removed from the casing  31  arid replaced by another collimator plate  315 . A particular design and weight of collimator is selected depending on the isotope being used or the type of study being conducted. Thus, the collimator plate  315  must be changed from time to time. Since the collimator plates  315  vary considerably in weight from one to another, the location of the center of gravity of the detector head  305  is dependent upon the weight of the collimator plate  315  attached to the casing  310 . Since the angle of the detector head  305  relative to the patient must be adjusted by an operator of the nuclear camera  5 , the detector head  305  must be rotatable relative to the arms  100 . If the center of gravity of the detector head  305  is positioned approximately on the axis of rotation of the detector head relative to the support arms  100 , then the detector head  305  will be balanced, and the angle of the detector head  305  relative to the support arms  100  will be adjustable by hand. However, changing the collimator plates moves the center of gravity of the detector head. Since collimator plates  315  are so heavy it becomes inconvenient or impossible to adjust the angle of the detector head  305  by hand. The positioner  320  enables the operator to adjust the position of the center of gravity of the detector head  305  to be approximately aligned with the point of rotation of the detector head  305 , which passes through support arms  100 . 
     The positioner  310  attaches the detector head  305  to the support arms  100  and includes a pair of rigid elongate detector head links  330  for aligning the centre of gravity of the detector head  305  relative to the support arms  100 . Each detector head link  330  is rotatable relative to the support arms  310  in a plane substantially parallel to its adjacent support arm  310 . Each detector head link  330  includes an arm end  335  rotatably attached to the adjacent support arm  100  by way of an arm axle  340 . Each detector head link  330  also includes a head end  345  rotatably attached to the detector head  305  by way of a head axle  350 . 
     The positioner  310  also includes a pair of locks  355  for selectively preventing rotation of the detector head  305  relative to the detector head links  330 . Each lock  355  includes the collimator support  325  extending from the detector head  305  from the collimator plate  315 . Each lock  355  also includes a block  360  for supporting the detector head link  330  on the collimator support  325 . Each block  360  includes a pair of pins  365  located either side of the head axle  350 . 
     Referring generally to FIGS. 13 to  16 , there is shown an array of photomultiplier tubes  605  located in the detector head of a scintillation camera. Surrounding each photomultiplier tube are six light emitting diodes  610 . In the present invention, there is, advantageously, more than one light emitting diode per photomultiplier tube. 
     The present invention allows all photomultiplier tubes to be calibrated efficiently and accurately. All light emitting diodes are pulsed simultaneously. The outputs of the photomultiplier tubes are then measured or read. The sum of all the photomultiplier tube outputs is then summed. The sum of all the photomultiplier tube outputs is then compared with an expected value, such as the sum of previously measured outputs of the photomultiplier tubes. 
     If the sum of the outputs of all the photomultiplier tubes is not within the tolerance, then it is likely that one or more of the light emitting diodes is malfunctioning and is in need of repair of replacement. If this is so, then the light emitting diodes are repaired or replaced and all light emitting diodes are again pulsed simultaneously, the outputs of the photomultiplier tubes are measured and then summed. Note that it may be necessary to determine whether it is in fact the light emitting diodes that are malfunctioning or the photomultiplier tubes that are malfunctioning or in need of adjustment. Such techniques are known in the art, and may involve the pulsing of individual light emitting diodes and the measurement and comparison of adjacent photomultiplier tubes. 
     Note that even if the sum of the outputs of all the photomultiplier tubes is not compared to an expected value in order to confirm the integrity of the light emitting diodes, the relevant embodiment includes enough light emitting diodes that the malfunctioning of one or a few light emitting diodes will not render the following steps ineffective in achieving an acceptable calibration of the photomultiplier tubes. In effect, by pulsing a large number of light emitting diodes, one achieves consistent illumination. Also, other methods may be employed to maintain the integrity of the light emitting diodes. 
     Once the sum of the outputs of all the photomultiplier tubes is, within the given tolerance, equal to the expected value of such sum, then the output of each photomultiplier tube is compared to its expected value, such as the value of a previously measured output of the photomultiplier tube measured after all light emitting diodes were pulsed. 
     If the output of a specific photomultiplier tube is, within a certain tolerance, equal to the expected value, then the photomultiplier tube is deemed to need no further calibration. 
     If the output of a specific photomultiplier tube is not within the tolerance, then the photomultiplier tube should be calibrated. 
     Optionally, a further comparison may be made. The output of each photomultiplier tube can be compared to the sum of the outputs of the photomultiplier tubes, divided by the number of photomultiplier tubes. Following such a comparison, a photomultiplier tube may be adjusted with reference to such comparison, or with reference both to such comparison and the previous comparison of the output of the photomultiplier tube to its expected value. 
     Once the photomultiplier tubes have been calibrated, the above process may be repeated if desired, beginning with the step of pulsing all light emitting diodes. 
     Referring specifically to FIG. 14, there is illustrated an embodiment of the method of the present invention. At step  620  Pulse All LEDs, or light emitting diodes, all light emitting diodes are pulsed simultaneously. At step  625  Read All PMT Outputs, or photomultiplier tube outputs, the output generated from each individual photomultiplier tube is read. At step  650  Compare Each PMT Output To Its Expected Value, the output of each individual photomultiplier tube is compared to an expected value for that particular photomultiplier tube, that is, a value that would be expected if the photomultiplier tube needed no calibration. At step  655  Within Tolerance, it is determined for each photomultiplier tube whether or not calibration is necessary, that is, whether the output of each photomultiplier tube deviates unacceptably from its expected value. At step  660 , each photomultiplier tube in need of calibration is calibrated, that is, only those photomultiplier tubes whose outputs were not within the relevant tolerance values. At step  680  Repeat?, the above method is repeated if desired. Alternatively, at step  685  End, the above method is terminated. 
     Referring specifically to FIG. 15, there is illustrated an embodiment of the method of the present invention. At step  620  Pulse All LEDs, or light emitting diodes, all light emitting diodes are pulsed simultaneously. At step  625  Read All PMT Outputs, or photomultiplier tube outputs, the output generated from each individual photomultiplier tube is read. At step  630  Sum Outputs Of PMTs, the outputs of each individual photomultiplier tube are added together. At step  635  Compare Sum Of Outputs with Expected Sum, the sum of the outputs of the photomultiplier tubes is compared with a value that it is expected to be if all light emitting diodes and photomultiplier tubes are functioning properly. At step  640  Within Tolerance?, a determination is made as to whether the sum of the outputs of the photomultiplier tubes is close enough to the value it is expected to be if all light emitting diodes and photomultiplier tubes are functioning properly. At step  645  Adjust LEDs, the light emitting diodes are adjusted, repaired or replaced, if necessary, if the sum of the outputs of the photomultiplier tubes is not within an acceptable tolerance. At step  650  Compare Each PMT Output To Its Expected Value, the output of each individual photomultiplier tube is compared to an expected value for that particular photomultiplier tube, that is, a value that would be expected if the photomultiplier tube needed no calibration. At step  655  Within Tolerance, it is determined for each photomultiplier tube whether or not calibration is necessary, that is, whether the output of each photomultiplier tube deviates unacceptably from its expected value. At step  660 , each photomultiplier tube in need of calibration is calibrated, that is, only those photomultiplier tubes whose outputs were not within the relevant tolerance values. At step  680  Repeat?, the above method is repeated if desired. Alternatively, at step  685  End, the above method is terminated. 
     Referring specifically to FIG. 16, there is illustrated an embodiment of the method of the present invention. At step  620  Pulse All LEDs, or light emitting diodes, all light emitting diodes are pulsed simultaneously. At step  625  Read All PMT Outputs, or photomultiplier tube outputs, the output generated from each individual photomultiplier tube is read. At step  630  Sum Outputs Of PMTs, the outputs of each individual photomultiplier tube are added together. At step  635  Compare Sum Of Outputs with Expected Sum, the sum of the outputs of the photomultiplier tubes is compared with a value that it is expected to be if all light emitting diodes and photomultiplier tubes are finctioning properly. At step  640  Within Tolerance?, a determination is made as to whether the sum of the outputs of the photomultiplier tubes is close enough to the value it is expected to be if all light emitting diodes and photomultiplier tubes are functioning properly. At step  645  Adjust LEDs, the light emitting diodes are adjusted, repaired or replaced, if necessary, if the sum of the outputs of the photomultiplier tubes is not within an acceptable tolerance. At step  650  Compare Each PMT Output To Its Expected Value, the output of each individual photomultiplier tube is compared to an expected value for that particular photomultiplier tube, that is, a value that would be expected if the photomultiplier tube needed no calibration. At step  655  Within Tolerance, it is determined for each photomultiplier tube whether or not calibration is necessary, that is, whether the output of each photomultiplier tube deviates unacceptably from its expected value. At step  660 , each photomultiplier tube in need of calibration is calibrated, that is, only those photomultiplier tubes whose outputs were not within the relevant tolerance values. At step  665  Compare Each PMT Output With The Sum Of Outputs Of PMTs Divided By Number Of PMTs, the sum of the outputs is divided by the number of photomultiplier tubes, and the output of each photomultiplier tube is compared to such value. At step  670  Within Tolerance, it is determined for each photomultiplier tube whether or not calibration is necessary, that is, whether the output of each photomultiplier tube deviates unacceptably from the average value of the photomultiplier tube outputs. At step  675 , each photomultiplier tube in need of calibration is calibrated, that is, only those photomultiplier tubes whose outputs were not within the relevant tolerance values. At step  680  Repeat?, the above method is repeated if desired. Alternatively, at step  685  End, the above method is terminated. 
     Numerous modifications, variations and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.