Patent Publication Number: US-2012033784-A1

Title: X-ray detector and x-ray computer tomography scanner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2010-177905, filed Aug. 6, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an X-ray detector and an X-ray computer tomography scanner. 
     BACKGROUND 
     As is generally known, an X-ray computer tomography (CT) scanner includes an X-ray tube and an X-ray detector and a tomogram of a subject is obtained by irradiating the subject with X rays generated by the X-ray tube, capturing X rays that have passed through the subject by the X-ray detector, and performing processing of a signal thereof. 
     In recent years, multi-slice type X-ray detectors are commercially available on the market. A multi-slice type X-ray detector includes a plurality of detection packs arranged on a substantial arc in a channel direction perpendicular to a body axis direction of the subject and one detection pack includes many detection elements arranged in a matrix form in the channel direction and a slice direction. An electric signal output from each detection pack is converted into a digital signal by a DAS (Data Acquisition System) and a tomogram is generated based on the signal. 
     As detection elements used in an X-ray detector, for example, a detection element composed of a fluorescent substance such as a scintillator that converts X rays into light and a photoelectric conversion element such as a photodiode that converts the light into a charge (electric signal) and a detection element composed of a semiconductor device that directly converts X rays into a charge are known. 
     Detection sensitivity of X rays of such a detection element fluctuates depending on the temperature. Thus, to obtain a tomogram with high precision, it is necessary to stabilize the temperature of each detection element of an X-ray detector to an optimal value during imaging. 
     In view of the above circumstances, the X-ray detector has a function to control the temperature of each detection element. This function has been realized by providing one large heater inside the X-ray detector and controlling the heater. 
     The heater is normally arranged below (opposite side of the X-ray incidence plane) each detection pack so that X rays entering the detection pack are not blocked. In this state, it is necessary to provide the DAS further below the heater or in a side direction of the heater, leading to a larger X-ray detector. 
     The heater controls the temperature of detection elements included in all detection packs at the same time and thus, an AC (alternating current) heater that can be driven at a relatively large capacity is used. X rays emitted from an X-ray tube pass through a human body and thus, the intensity thereof is limited to a minimum necessary level in view of a harmful effect on the health of the subject. Thus, the intensity of X rays entering the X-ray detector is very weak and the amount of charge of a signal output from each detection pack to the DAS is extremely small. Therefore, if an AC heater is used for temperature control of each detection pack, there is the possibility of deterioration of the SN ratio [signal to noise ratio] of a signal output from the detection pack due to noise thereof. 
     Such a problem can be solved by using a DC (direct current) driven heater. However, normally a DC heater is, compared with an AC heater, unfit for large-capacity heating and adequate temperature control capabilities cannot be obtained simply by replacing the heater of a conventionally structured X-ray detector with a DC heater. 
     When the temperature of all detection packs is controlled by one heater, there arises a problem of fluctuations in temperature of each detection pack depending on how heat is conducted from the heater to each detection pack or the like. 
     Therefore, a conventional structure to control the temperature of detection elements of an X-ray detector has many problems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an overall structure of an X-ray CT scanner according to a first embodiment; 
         FIG. 2  is a schematic diagram when an internal structure of the X-ray detector in the embodiment is viewed from a Z-axis direction; 
         FIG. 3  is a perspective view showing a state before a connector board is connected to a detection pack in the embodiment; 
         FIG. 4  is a perspective view when the detection pack and the connector board shown in  FIG. 3  are viewed from an arrow C direction; 
         FIG. 5  is a perspective view of the state in which the detection pack and the connector board shown in  FIG. 3  are mounted; 
         FIG. 6  is a block diagram showing electric circuits and the like of the X-ray detector in the embodiment; 
         FIG. 7  is a flow chart showing a control example of a heater by each heater controller in the embodiment; 
         FIG. 8  is a block diagram showing electric circuits and the like of the X-ray detector in a second embodiment; 
         FIG. 9  is a block diagram showing electric circuits and the like of the X-ray detector in a third embodiment; 
         FIG. 10  is a perspective view showing the state before the connector board is connected to the detection pack in a fourth embodiment; 
         FIG. 11  is a perspective view when the detection pack and the connector board shown in  FIG. 10  are viewed from the arrow C direction; 
         FIG. 12  is a perspective view of the state in which the connector board and the detection pack shown in  FIG. 10  are mounted; 
         FIG. 13  is a perspective view showing the state before the connector board is connected to the detection pack in a fifth embodiment; 
         FIG. 14  is a perspective view when the detection pack and the connector board shown in  FIG. 13  are viewed from the arrow C direction; and 
         FIG. 15  is a perspective view of the state in which the connector board and the detection pack shown in  FIG. 13  are mounted. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an X-ray detector includes a plurality of detection packs and a plurality of heaters. Each of the detection packs includes a plurality of detection elements that detect X rays having passed through a subject and output a detection signal corresponding to the X rays and is arranged along a predetermined direction. Each of the heaters is provided corresponding to each of the detection packs and individually controls the temperature of each of the detection packs. 
     Each embodiment will be described below with reference to the drawings. In the description that follows, the same reference numerals are attached to structural elements having substantially the same function and configuration and a duplicate description will be provided only when necessary. 
     First Embodiment 
     First, the first embodiment will be described. 
     [Overall Configuration of the X-Ray CT Scanner] 
       FIG. 1  is a block diagram showing an overall structure of an X-ray CT scanner  1  according to the first embodiment. As shown in  FIG. 1 , the X-ray CT scanner  1  includes a gantry unit A and a console unit B. 
     The gantry unit A acquires projection data (or original data) by irradiating a subject with X rays and detecting X rays that have passed through the subject. There are various type of imaging systems of an X-ray CT system such as a ROTATE/ROTATE type in which an X-ray tube and a two-dimensional detector system integrally rotate around a subject and a STATIONARY/ROTATE type in which many detection elements are arrayed in a ring shape and only the X-ray tube rotates around the subject, and an X-ray CT system of the ROTATE/ROTATE type, which is currently mainstream, is taken as an example. 
     As shown in  FIG. 1 , the gantry unit A includes a fixed unit  11 , a rotating unit  12 , an X-ray tube  13 , an X-ray detector  14 , a data transmission unit  15 , a gantry driving unit  16 , a feeding unit  17 , and a high voltage transformer unit  18 . 
     The central portion of the rotating unit  12  is open together with a cabinet and a subject P placed on a top board of a bed unit is inserted through an opening  19  thereof during imaging. 
     The X-ray tube  13  is a vacuum tube to generate X rays and is provided in the rotating unit  12 . The X-ray detector  14  is used to detect X rays having passed through the subject P and is mounted on the rotating unit  12  in a direction opposite to the X-ray tube  13 . 
     The gantry driving unit  16  rotates the rotating unit  12  around the subject P in the opening  19  at high speed. Accordingly, the X-ray tube  13  and the X-ray detector  14  integrally rotate around the center axis parallel to the body axis direction of the subject P inserted through the opening  19 . 
     Operating power is supplied to the fixed unit  11  from an external power supply such as a commercial AC power supply. The operating power supplied to the fixed unit  11  is transmitted to the rotating unit  12  via the feeding unit  17 . The feeding unit  17  supplies the operating power to each unit of the rotating unit  12 . The high voltage transformer unit  18  includes a high voltage transformer, a filament heating converter, a rectifier, and a high voltage switch and transforms the operating power supplied from the feeding unit  17  into a high voltage, which is supplied to the X-ray tube  13 . 
     Next, the console unit B will be described. The console unit B includes a preprocessing unit  21 , a host controller  22 , a storage unit  23 , a reconstruction unit  24 , an input unit  25 , a display unit  26 , an image processing unit  27 , and data/control bus  28 . 
     The preprocessing unit  21  receives original data from the X-ray detector  14  via the data transmission unit  15  and makes sensitivity corrections and X-ray intensity corrections of the original data. Original data on which preprocessing is performed by the preprocessing unit  21  is called “projection data”. 
     The host controller  22  exercises unified control of various kinds of processing such as imaging processing, data processing, and image processing. 
     The storage unit  23  stores image data such as collected original data, projection data, and CT image data. 
     The reconstruction unit  24  generates reconstruction image data for predetermined slices by performing reconstruction processing on projection data based on predetermined reconstruction parameters (the reconstruction region size, reconstruction matrix size, threshold to extract the region of interest and the like). 
     The input unit  25  includes a keyboard, various switches, and a mouse and an operator operates the input unit  25  to input various scan conditions such as the slice thickness and the number of slices. 
     The image processing unit  27  performs image processing for the display such as the window conversion and RGB processing on the reconstruction image data generated by the reconstruction unit  24  and outputs the data to the display unit  26 . The image processing unit  27  also generates a tomogram of any section, projection image from any direction, and a so-called pseudo three-dimensional image such as a three-dimensional surface image and outputs such an image to the display unit  26 . The output image data is displayed in the display unit  26  as an X-ray CT image. 
     The data/control bus  28  is a signal wire to transmit/receive various kinds of data, control signals, and address information by connecting each unit. 
     In the description below, the rotation axis of the rotating unit  12  is defined as the Z axis. In a rotating coordinate system around the Z axis, an axis connecting the focal point of the X-ray tube  13  and the center of the detection surface of the X-ray detector  14  and perpendicular to the Z axis is defined as the X axis and an axis perpendicular to the Z axis and the X axis is defined as the Y axis. 
     [X-Ray Detector] 
       FIG. 2  is a schematic diagram showing an internal structure of the X-ray detector  14  when viewed from the Z-axis direction. 
     The X-ray detector  14  is formed in an arc shape around the X-ray tube  13  and includes a collimator unit  101 , K (about 40, for example) detection packs  102  mounted on the collimator unit  101 , a DAS unit  103  provided below each of the detection packs  102 , and an insulation case  104  accommodating the collimator unit  101  and each of the detection packs  102 . 
     The collimator unit  101  has a known structure in which many collimator plates are mounted on a support member in an arc shape around the X-ray tube  13 . 
     The detection packs  102  are one-dimensionally arrayed in the Y axis direction and mounted on the support member of the collimator unit  101 . On the side of the X-ray irradiation surface of the detection packs  102 , many detection elements are arrayed in a matrix shape of M×N (about 64×24, for example) regarding the slice direction (Z axis direction) and the channel direction substantially perpendicular thereto (substantial Y axis direction). 
     The DAS unit  103  includes K DAS boards  105  electrically connected to each of the detection packs  102 . Each of the DAS boards  105  is electrically connected to one detection pack  102  and performs amplification processing and A/D conversion processing on an analog signal (detection signal) output from the connected detection pack  102  when X rays are detected to generate a predetermined digital signal and outputs the signal to the data transmission unit  15 . In  FIG. 2 , only a portion of the K detection packs  102  and DAS boards  105  is illustrated. 
     The insulation case  104  is formed of, for example, a material with high insulation properties such as a resin material and ceramic. An insulation port of a flexible cable  303  (see  FIG. 3 ) to connect each of the detection packs  102  and each of the DAS boards  105  is provided on a surface opposite to the X-ray incidence plane of the insulation case  104  or the side face thereof. 
     In a conventional X-ray detector, for example, one AC heater in a flat long shape is provided below (DAS unit side) each detection pack and the temperature of the insulation case is controlled by the AC heater. 
     [Detection Pack] 
     Next, the detection pack  102  will be described in detail with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a perspective view showing a state before a connector board  301  for connection to the DAS board  105  is connected to the detection pack  102  and  FIG. 4  is a perspective view when the detection pack  102  and the connector board  301  shown in  FIG. 3  are viewed from an arrow C direction. 
     The detection pack  102  in the present embodiment is configured by mounting a flat photodiode board  202  on the top surface of a base board  201  and placing a scintillator block  203  on the top surface of the board  202 . Further, as shown in  FIG. 4 , a pack-side connector  204  (first connector) and a DC driven heater  205  are mounted on the undersurface of the base board  201 . 
     The base board  201  has a flat shape prolonged in the Z axis direction. The width of the photodiode board  202  in the Z axis direction is a little smaller than that of the base board  201  and the width of the scintillator block  203  in the Z axis direction is a little smaller than that of the photodiode board  202 . The base board  201 , the photodiode board  202 , and the scintillator block  203  all have substantially the same width in the Y direction. Therefore, if the base board  201 , the photodiode board  202 , and the scintillator block  203  are mounted by aligning the center of each in the ZY plane, as shown in  FIG. 3 , the side wall in the Y axis direction becomes flush with each other and margin surfaces  201   a ,  201   b  are formed on the top surface of the base board  201 . When mounted on the collimator unit  101 , the margin surfaces  201   a ,  201   b  are brought into close contact with the support member of the collimator unit  101  and fixed by a predetermined method such as screwing. At this point, the side walls in the Y axis direction of the detection packs  102  are brought into close contact so that no gap arises on the X-ray detection surface between the adjacent detection packs  102 . 
     The base board  201  contains a temperature sensor  206  (see  FIG. 6 ) to detect the temperature of a detection element group. 
     The scintillator block  203  is configured by arraying scintillators converting X rays into visible light in an array form (M×N). The photodiode board  202  has photodiodes as photoelectric conversion elements formed in an array form (M×N) so as to correspond to the scintillators. Thus, one detection element is configured by one scintillator of a scintillator block and one photodiode corresponding thereto. 
     The heater  205  has a substantially flat shape with an opening in a joint portion of the pack-side connector  204  and the base board  201  and operates by receiving the supply of DC from the side of the DAS board  105  via the pack-side connector  204 . The heater  205  is a small heater with widths in the Z axis direction and the Y axis direction not exceeding those in the Z axis direction and the Y axis direction of the base board  201  respectively. 
     The pack-side connector  204  has an output terminal group to output an analog signal from each detection element, a connection terminal for the heater  205 , and an output terminal for the temperature sensor  206 . A DAS-side connector  302  (second connector) provided on the flat connector board  301  to connect each of the terminals of the pack-side connector  204  is mounted on the pack-side connector  204 . The wide flexible cable  303  (communication cable) extending from the DAS board  105  is connected to the connector board  301 . The flexible cable  303  is a bundle of a signal wire (M×N) to transmit an analog signal from each detection element to the DAS board  105 , a power supply line to supply power from the DAS board  105  to the heater  205 , and a signal wire to transmit output from the temperature sensor  206  to the DAS board  105 . As the pack-side connector  204 , the DAS-side connector  302 , and the flexible cable  303 , for example, standard products having as many terminals and signal wires as the number obtained by adding the number of the power supply line for the heater  205  and the signal wire for the temperature sensor to the number of signal wires (M×N) to transmit the analog signal or more may be used. 
     A perspective view of the state in which the connector board  301  is mounted on the detection pack  102  is shown in  FIG. 5 . If the DAS-side connector  302  is mounted on the pack-side connector  204  in this manner, each detection element, the heater  205 , and the temperature sensor  206  are each connected to the DAS board  105  electrically. 
     All of the K detection packs  102  have the configuration described by using  FIGS. 3 to 5 . 
     [Electric Circuit and Heater Control of the X-Ray Detector] 
       FIG. 6  is a block diagram showing electric circuits and the like of the X-ray detector  14 . 
     When X rays emitted from the X-ray tube  13  enter the X-ray detector  14 , unnecessary scattered X rays are eliminated by the collimator unit  101 . Each scintillator of the scintillator block  203  emits light after receiving X rays after scattered X rays being eliminated and each photodiode provided on the photodiode board  202  outputs an electric signal (analog signal) through photoelectric conversion after receiving visible light from the scintillator. Thus, the analog signal output from each detection element and a signal indicating the temperature detected by the temperature sensor  206  are sent to the DAS board  105  connected to each of the detection packs  102  via the pack-side connector  204 , the DAS-side connector  302 , the connector board  301 , and the flexible cable  303 . 
     Each of the DAS board  105  in the present embodiment is provided with a heater controller  401 . Each of the heater controllers  401  is realized by a control circuit of each of the DAS boards  105  and controls the heater  205  of the detection pack  102  connected to each of the DAS boards  105  by fluctuating the current value supplied to the heater  205 . 
       FIG. 7  shows a control example of the heater  205  by each of the heater controllers  401 . The processing shown here is performed independently by the heater controller  401  of each of the DAS boards  105 . 
     In this control example, first the heater controller  401  detects a temperature T of the detection pack  102  connected to the heater controller  401  based on a signal output from the temperature sensor  206  (step S 1 ). 
     Subsequently, the heater controller  401  sets a current value I to be supplied to the heater  205  of the detection pack  102  connected to the heater controller  401  based on a detection temperature T (step S 2 ). While the current value I is set by aiming for a temperature at which the detection pack  102  can obtain satisfactory X-ray detection characteristics as a target value in this processing, various methods can be adopted as a concrete setting method. For example, a table associating the current value I to be set with the detection temperature T may be stored in a memory in the DAS board  105  so that the current value I is set by referring to the table. Also, the current value I may be set by substituting the detection temperature T into a predetermined formula. Alternatively, if the detection temperature T falls below the target value, the current value I may be set to a value higher than the current value currently supplied to the heater  205  by a predetermined value and if the detection temperature T exceeds the target value, the current value I may be set to a value lower than the current value currently supplied to the heater  205  by a predetermined value or to zero. When the current value I is set by using a table or formula as described above, the concrete value of the current value I for the detection temperature T may be determined in consideration of theoretically or experimentally derived relationships between the detection temperature T and the current value I. 
     If the current value I is set in this manner, the heater controller  401  supplies the current of the current value I to the heater  205  of the detection pack  102  connected to the heater controller  401  (step S 3 ). The processing in steps S 1  to S 3  is repeated in a predetermined period while the X-ray CT scanner  1  is in a standby state of imaging. 
     As described above, the small heater  205  is mounted on each of the detection packs  102  in the present embodiment and the temperature of each of the detection packs  102  is controlled by the heater  205 . If such a configuration is adopted, compared with a case when a large heater is provided below each detection pack in the past, the space inside the X-ray detector  14  can be saved. 
     Moreover, by providing the heater  205  for each of the detection packs  102  in this manner, adequate temperature control capabilities can be obtained even if a DC heater is adopted as the heater  205 . Therefore, the temperature of each of the detection packs  102  can be controlled without using a large-output AC-driven heater and noise from the heater  205  or a power supply line to the heater  205  will not be entrapped into an analog signal output from each of the detection packs  102 . 
     As a result of prevention of noise from the heater  205  or a power supply line to the heater  205  from being entrapped, the signal wire to transmit the analog signal from each of the detection packs  102  to each of the DAS boards  105  and the power supply line of the heater  205  can be bundled so that the DAS board  105  can be made a power supply source of the heater  205 . Therefore, there is no need to separate the line connecting each of the detection packs  102  and each of the DAS boards  105  into a plurality of lines or to provide a control circuit dedicated to the heater  205  inside the X-ray detector  14 . This also contributes to space saving inside the X-ray detector  14 . 
     Because the temperature of each of the detection packs  102  is individually controlled by each of the heaters  205 , fluctuations in temperature of each of the detection packs  102  can be corrected more easily. 
     Second Embodiment 
     Next, the second embodiment will be described. 
     The present embodiment is different from the first embodiment in that the heater  205  of each of the detection packs  102  is controlled by one heater controller in a unified manner, instead of controlling the heater  205  of each of the detection packs  102  individually by providing the heater controller  401  for each of the DAS boards  105 . The reference numerals are attached to the same locations as those in the first embodiment and a description thereof will not be repeated. 
       FIG. 8  is a block diagram showing electric circuits and the like of the X-ray detector  14  in the present embodiment. 
     As shown in  FIG. 8 , the X-ray detector  14  is provided with a heater controller  501  electrically connected to each of the DAS boards  105 . The configurations of the detection pack  102  and the connector board  301  are the same as those in the first embodiment. 
     With such a configuration, the heater controller  501  controls each of the heaters  205  basically in the same flow as that of the control example shown in  FIG. 7 . First, the heater controller  501  detects temperatures T 1  to T K  of each of the detection packs  102  based on a signal output from each of the temperature sensors  206  (step S 1 ). 
     Subsequently, the heater controller  501  sets current values I 1  to I K  to be supplied to each of the heaters  205  based on the detection temperatures T 1  to T K  (step S 2 ). The current value I x  (1≧x≧K) is the current value to be supplied to the heater  205  provided in the detection pack  102  in which the temperature T x  (1≧x≧K) is detected. In the present embodiment, after heating by each of the heaters  205  is started, the current values I 1  to I K  are set so that the detection temperatures T 1  to T K  become substantially uniform, even before each of the detection temperatures T 1  to T K  is stabilized at a target value. Various methods can be adopted as a concrete setting method. For example, as described in the first embodiment, each of the current values I 1  to I K  is set so that each of the detection temperatures T 1  to T K  approaches a target value and then, these current values I 1  to I K  are corrected so that fluctuations of the detection temperatures T 1  to T K  are rectified. In these corrections, for example, the current value supplied to the heater  205  of the detection pack  102  whose detection temperature is relatively low is slightly increased and the current value supplied to the heater  205  of the detection pack  102  whose detection temperature is relatively high is slightly decreased. 
     After the current values I 1  to I K  are set in this manner, the heater controller  501  supplies currents of the current values I 1  to I K  to the heaters  205  of the corresponding detection packs  102  (step S 3 ). The processing in steps S 1  to S 3  is repeated in a predetermined period while the X-ray CT scanner  1  is active. 
     As described above, the heater  205  of each of the detection packs  102  is controlled by the one heater controller  501  in the present embodiment. Even with such a configuration, like in the first embodiment, the space inside the X-ray detector  14  can be saved. Moreover, a DC-driven heater can be adopted as each of the heaters  205  and thus, noise from the heater  205  or a power supply line to the heater  205  will not be entrapped into an analog signal output from each of the detection packs  102 . 
     If fluctuations in detection temperature of each of the temperature sensors  206  arise, each of the heaters  205  can be driven to correct the fluctuations and thus, the temperature of each of the detection packs  102  can be maintained uniform immediately after temperature control of each of the detection packs  102  is started even before each of the detection temperatures T 1  to T K  is stabilized at a target value. Therefore, even when, for example, a sufficient temperature control time cannot be secured before imaging due to an urgent diagnosis, local deterioration of a CT image will not occur. 
     Third Embodiment 
     Next, the third embodiment will be described. 
     The present embodiment is different from the second embodiment in that the temperature sensor  206  is provided in a portion of the detection packs  102 , instead of providing the temperature sensor  206  in all the detection packs  102 . The reference numerals are attached to the same locations as those in the first and second embodiments and a description thereof will not be repeated. 
       FIG. 9  is a block diagram showing electric circuits and the like of the X-ray detector  14  in the present embodiment. 
     As shown in  FIG. 9 , of the three detection packs  102  arranged consecutively, the detection pack  102  positioned in the center is provided with the temperature sensor  206  and the two detection packs  102  adjacent to the detection pack  102  are not provided with the temperature sensor  206 . That is, if, of the K detection packs  102 , the detection pack  102  arranged at one end of the X-ray detector  14  is defined as the first and the detection pack  102  arranged at the other end as the K-th, the temperature sensor  206  is provided in the second, fifth, eighth, eleventh, . . . detection packs  102 . The detection pack  102  provided with the temperature sensor  206  and the two adjacent detection packs  102  are defined as a group below and it is assumed that L such groups are present in the X-ray detector  14 . 
     With the configuration described above, the heater controller  501  controls each of the heaters  205  basically in the same flow as that of the control example shown in  FIG. 7 . First, the heater controller  501  detects temperatures T G1  to T GL  of the detection packs  102  arranged in the center of each group based on a signal output from each of the temperature sensors  206  (step S 1 ). 
     Subsequently, the heater controller  501  sets current values I G1  to I GL  to be supplied to the heater  205  in each group based on the detection temperatures T G1  to T GL  (step S 2 ). The current value I GX  (1≧x≧L) is the current value to be supplied to the heater  205  in the group to which the detection pack  102  in which the temperature T x  (1≧x≧L) is detected belongs. Various methods can be adopted as the setting method of the current values I G1  to I GL  in the processing. For example, as described in the first embodiment, each of the current values I G1  to I GL  may be set so that each of the detection temperatures T G1  to T GL  approaches a target value and further, as described in the second embodiment, each of the current values I G1  to I GL  may be corrected so that fluctuations of the detection temperatures T G1  to T GL  are rectified. 
     After the current values I G1  to I GL  are set in this manner, the heater controller  501  supplies currents of the current values I G1  to I GL  to the heaters  205  in the corresponding groups (step S 3 ). The processing in steps S 1  to S 3  is repeated in a predetermined period while the X-ray CT scanner  1  is active. 
     As described above, the temperature sensor  206  is provided in a portion of the detection packs  102  in the present embodiment and detection temperatures of these temperature sensors  206  are used to control the heater  205  of each of the detection packs  102 . With the configuration described above, there is no need to provide the temperature sensor  206  in all the detection packs  102  and thus, the control configuration is simplified and also manufacturing costs of the X-ray detector  14  can be reduced. 
     Even with the configuration in the present embodiment, like in the first embodiment, the space inside the X-ray detector  14  can be saved. Moreover, a DC-driven heater can be adopted as each of the heaters  205  and thus, noise from the heater  205  or a power supply line to the heater  205  will not be entrapped into an analog signal output from each of the detection packs  102 . 
     Fourth Embodiment 
     Next, the fourth embodiment will be described. 
     The present embodiment is different from each of the above embodiments in that the heater is provided on the connector board  301 , instead of in the detection pack  102 . The same reference numerals are attached to the same structural elements as those in each of the above embodiments and a description thereof will not be repeated. 
       FIG. 10  is a perspective view showing a state before the connector board  301  for connection to the DAS board  105  is connected to the detection pack  102  and  FIG. 11  is a perspective view when the detection pack  102  and the connector board  301  shown in  FIG. 10  are viewed from the arrow C direction. 
     As described above, the detection pack  102  is not provided with a heater (see  FIG. 11 ). On the other hand, the connector board  301  is provided with a DC-driven heater  304  bent along an outer edge thereof. Power supply lines  303   a ,  303   b  to supply a current to the heater  304  are provided at both ends of the flexible cable  303 . Then, the power supply line  303   a  is connected to one end of the heater  304  and the power supply line  303   b  is connected to other end thereof. 
     A perspective view of the state in which the connector board  301  configured as described above is mounted on the detection pack  102  is shown in  FIG. 12 . If the connector board  301  is mounted on the detection pack  102 , as shown in  FIG. 12 , the heater  304  faces the detection pack  102  with a gap formed therebetween. In this state, the heater controller  401  in the first embodiment or the heater controller  501  in the second/third embodiment performs the processing shown in  FIG. 7  and when a current is supplied to each of the heaters  304 , heat generated by each of the heaters  304  is transmitted through an air space in the gap to heat each of the detection packs  102 . 
     All of the K detection packs  102  have the configuration described by using  FIGS. 10 to 12 . 
     In the present embodiment, the heater  304  provided in each of the detection packs  102  may be controlled, like in the first embodiment, by the separate heater controllers  401  or, like in the second embodiment, by the one heater controller  501 . Also in the present embodiment, the temperature sensor  206  may be provided, like in the first/second embodiments, in each of the detection packs  102  or, like in the third embodiment, in a portion of the detection packs  102 . 
     As described above, the heater  304  is provided on the connector board  301  mounted on the detection pack  102  in the present embodiment, instead of providing the heater in the detection pack  102 . With the configuration described above, there is no need to provide heater terminals in the pack-side connector  204  and the DAS-side connector  302 . Further, even if an error such as breaking of wire occurs in the heater, the error can be handled by replacing the connector board  301  that is cheaper than a detection pack so that maintenance costs of the X-ray CT scanner  1  can be held down. 
     Fifth Embodiment 
     Next, the fifth embodiment will be described. 
     The present embodiment is different from the fourth embodiment in that a cover member covering the gap between the detection pack  102  and the connector board  301  is provided on the connector board  301 . The same reference numerals are attached to the same structural elements as those in each of the above embodiments and a description thereof will not be repeated. 
       FIG. 13  is a perspective view showing a state before the connector board  301  for connection to the DAS board  105  is connected to the detection pack  102  and  FIG. 14  is a perspective view when the detection pack  102  and the connector board  301  shown in  FIG. 13  are viewed from the arrow C direction. 
     A cover member  305  in a rectangular frame shape is provided on the surface on the side of the connector board  301  on which the DAS-side connector  302  along a circumference thereof. The cover member  305  is formed of, for example, a material with high insulation properties such as a resin material and ceramic and the height thereof substantially matches the width of the gap formed between the detection pack  102  and the connector board  301  when the connector board  301  is mounted on the detection pack  102 . 
     A perspective view of the state in which the connector board  301  configured as described above is mounted on the detection pack  102  is shown in  FIG. 15 . If the connector board  301  is mounted on the detection pack  102 , as shown in  FIG. 15 , the gap formed between the detection pack  102  and the connector board  301  is covered with the cover member  305 . In this state, the heater controller  401  in the first embodiment or the heater controller  501  in the second/third embodiment performs the processing shown in  FIG. 7  and when a current is supplied to each of the heaters  304 , heat generated by each of the heaters  304  is transmitted through an air space in the gap to heat each of the detection packs  102 . Since the gap is covered with the cover member  305 , heat generated by the heater  304  efficiently warms the detection pack  102  without being lost to the surroundings. 
     All of the K detection packs  102  have the configuration described by using  FIGS. 13 to 15 . 
     In the present embodiment, the heater  304  provided in each of the detection packs  102  may be controlled, like in the first embodiment, by the separate heater controllers  401  or, like in the second embodiment, by the one heater controller  501 . Also in the present embodiment, the temperature sensor  206  may be provided, like in the first/second embodiments, in each of the detection packs  102  or, like in the third embodiment, in a portion of the detection packs  102 . 
     In the present embodiment, as described above, the cover member  305  to cover a gap formed between each of the detection packs  102  and each of the connector boards  301  is provided. With such a configuration, the detection pack  102  is efficiently warmed by the heater  304  and thus, the temperature of each of the detection packs  102  is swiftly controlled and also power consumption of the X-ray CT scanner  1  is reduced. 
     According to the configurations disclosed in the first to fifth embodiments, suitable effects such as being able to save space inside the X-ray detector  14 , reducing entrapment of noise from the heater  205  or a power supply line to the heater  205  into an analog signal output from each of the detection packs  102 , and being able to correct fluctuations in temperature of each of the detection packs  102  are achieved. 
     (Modifications) 
     The configurations disclosed in each of the above embodiments can be embodied by appropriately modifying each structural element in the stage of implementation. Concrete modifications are, for example, as follows: 
     (1) In each of the above embodiments, a case when the DAS unit  103  includes as many the DAS boards  105  as the number of the detection packs  102  is illustrated. However, one DAS board  105  may be connected to a plurality of the detection packs  102  to reduce the number of the DAS boards  105 . In such a case, each of the DAS boards  105  may process analog signals output from the plurality of the detection packs  102  connected to the DAS board  105 . Further, if, like in the first embodiment, the heater controller  401  is realized by a control circuit of the DAS board  105 , the heater controller  401  of each of the DAS boards  105  may drive each of the heaters  205  of the plurality of the detection packs  102  connected to the DAS board  105 .
 
(2) In each of the above embodiments, a case when a detection element of the detection pack  102  is composed of scintillators and photodiodes is illustrated. However, the detection element may be configured by other methods such as using a semiconductor device that directly converts X rays into a charge.
 
(3) In the third embodiment, a case when one group is defined by the three detection packs  102  and the detection pack  102  arranged in the center of each group is provided with the temperature sensor  206  is illustrated. However, similar heater control may be exercised by defining the two, four or more detection packs  102  as a group or the temperature sensor  206  in each group may be provided in the detection pack  102  arranged at an end, instead of the detection pack  102  arranged in the center thereof.
 
     Further, only one temperature sensor  206  may be provided for all the detection packs  102  so that the heater  205  of each of the detection packs  102  is controlled based on the detection temperature by the temperature sensor  206 . Even in such a case, the temperature of each of the detection packs  102  can unchangingly be controlled by a DC-driven heater so that a noise reduction effect similar to that in each of the above embodiments can be gained. 
     (4) In the fourth embodiment, a case when the heater  304  bent in a “ ” shape is provided along an outer edge of the connector board  301  is illustrated. However, the heater  304  provided on the connector board  301  may be a heater in a flat shape as shown in the first embodiment or a heater meandering along an outer edge of the connector board  301 .
 
(5) In the fifth embodiment, a case when the cover member  305  is provided on the connector board  301  is illustrated. However, the cover member  305  may be provided on the detection pack  102 , instead of the connector board  301 .
 
(6) In each of the above embodiments, the heaters  205 ,  304  are assumed to be DC driven. However, an AC-driven heater may be adopted as the heaters  205 ,  304 . Even in such a case, it is unchangingly possible to save space in the X-ray detector  14  and the temperature of each of the detection packs  102  can be maintained uniform. Moreover, there is no need to use a large AC heater as in the past and thus, entrapment of noise into an analog signal output from each of the detection packs  102  can also be reduced.
 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.