the present invention can be exemplified but not limited by various embodiments as described hereinafter . please refer to fig3 , which is a flow chart of a method for evaluating the influence of penetration effect on crystals according to one embodiment of the present invention . in the present embodiment , the method 2 for evaluating the influence of penetration effect on crystals comprises steps hereinafter . first , in step 20 , a virtual source is provided . the virtual source can be a gamma - ray source or an x - ray source , but not limited thereto . the virtual source in the present embodiment is a gamma point source to generate uniform radioactive rays , which can be simulated by software . such simulation is well known to those with ordinary in the art , and thus description thereof is not presented herein . then , in step 21 , a first absorption result of each crystal in a crystal array is simulated with respect to radioactive rays generated by the virtual source . the crystal array comprises a plurality of crystals arranged therein . each crystal comprises a crystalline material characterized in that a radioactive ray is immediately absorbed once it enters the crystalline material . in this step , the arrangement of crystals in the crystal array is similar to that of the detection device to be substantially calibrated . the first absorption result is simulated by monte carlo simulation , but not limited thereto , as long as utilized simulation is able to simulate the interaction between the radioactive ray and the scintillation crystals . in step 21 , for non - penetration simulation , a virtual material with an attenuation coefficient of 10 8 cm − 1 is set . the virtual material is used as the crystalline material in step 21 . it is noted that the attenuation coefficient is not limited to the setting in the present embodiment . it can provide a quite good result if the attenuation coefficient is designated more than about 50 cm − 1 . however , since the attenuation coefficient influences the absorptivity , it can be designated as high as possible , for example , more than 10 8 cm − 1 . however , the range of this value accepted by simulation software has to be taken into account . the present invention is not limited to the range of this value depending on simulation software . therefore , in fig4 a , the crystalline material comprises a virtual material with an extremely high attenuation coefficient and exhibits a high absorptivity with respect to the gamma ray 91 generated by the virtual source 93 so that the gamma ray is absorbed immediately once it enters the crystals . therefore , the penetration effect of crystals can be eliminated when such crystalline material is used for simulation , as is expressed in equation ( 1 ): wherein n denotes the number of crystals in the crystal array , c i sim1 denotes the first absorption result in step 21 detected by the i th crystal ; a denotes the source activity for simulation ; and f i simother denotes the influence upon the i th crystal caused by effects other than the crystal penetration effect . the effects other than the crystal penetration effect include geometric effects , attenuation effects , etc . based on the physical model of simulation software . then , in step 22 , a second absorption result of each substantial crystal in a substantial crystal array is simulated with respect to the radioactive ray generated by the virtual source . the substantial crystal array comprises a plurality of substantial crystalline materials arranged therein . the arrangement of crystals in the crystal array is similar to that of the detection device to be substantially calibrated . the substantial crystalline materials comprise scintillation crystalline materials such as cerium doped lutetium orthosilicate ( lso ), cerium doped lutetium yttrium orthosilicate ( lyso ) and bismuth germanate ( bgo ). in the present invention , the scintillation crystals are not limited to the above three materials . the crystalline materials for simulation should match the crystals used in the detection device to be substantially calibrated . as shown in fig4 b , since a substantial crystalline material is used for simulation , the crystal penetration effect takes place when a radioactive ray enters the crystal c , penetrates the crystals c and b , and then is detected in the crystal a . therefore , in step 22 , the same source is used with substantial crystalline materials for simulation to acquire the following result , as expressed in equation ( 2 ), which includes the influence caused by the crystal penetration effect . c i sim2 = a × f i simother × f i cp , 1 ≦ i ≦ n ( 2 ) wherein c i sim2 denotes the second absorption result in step 22 detected by the i th crystal ; and f i cp denotes the influence upon the i th crystal caused by effect . similarly , f i simother the crystal penetration denotes the influence upon the i th crystal caused by effects other than the crystal penetration effect . then , in step 23 , the second absorption result is divided by the first absorption result to obtain a crystal penetration effect factor of each substantial crystal in the substantial crystal array . the influence of the penetration effect on the i th crystal is expressed in equation ( 3 ): during an efficiency detection experiment , the results detected by each crystal depend on the following factors , the crystal - level detection efficiency , the source activity , the crystal penetration effect , and at least one physical effect factor . the crystal - level detection efficiency depends on the scintillation efficiency of each scintillation crystal and the properties of each pmt and electronic element . the detection efficiency calibration is to compensate the difference . the at least one physical effect factor can be geometric factors , scattering , positron range , other physical effects other than the penetration effect of crystals and combination of the above . based on the influence of penetration effect on crystals , the present invention further provides a method for evaluating the crystal - level detection efficiency . please refer to fig5 , which is a flow chart of a method for evaluating the crystal - level detection efficiency according to one embodiment of the present invention . the steps 30 to 33 are similar to steps 20 to 23 in fig3 and descriptions thereof are not presented herein . in the flow chart in fig5 , the method further comprises steps herein . in step 34 , a substantial source is provided . the shape , size , position and type of the substantial source are similar to those of the virtual source in step 30 . in the present embodiment , the substantial source is a gamma - ray source , but not limited thereto . then in step 35 , a detection device , of which the detection efficiency is to be crystal - level calibrated , is used to perform an imaging process so as to obtain imaging information c i exp . the crystal array used in the detection device is the substantial crystal array in step 32 . the detection device comprises scintillation detectors , photomultiplier tubes ( pmts ), and electronic circuits that comprise scintillation crystal arrays . the detection device is well known to those with ordinary skill in the art , and thus description thereof is not presented herein . the detection device can be a positron imaging detection device or an x - ray source , a single photon imaging detection device or an x - ray imaging detection device that can detect radioactive rays , but is not limited thereto . then in step 36 , the influence f i other of at least one physical effect is evaluated . the at least one physical effect is one effect other than the penetration effect and is well known to those with ordinary skill in the art , and thus description thereof is not presented herein . it is noted that f i other and f i simother in equations ( 1 ) and ( 2 ) are not the same because not all the physical phenomena can be simulated by simulation software . at last , step 37 is performed to calculate the detection efficiency of the i th crystal in the substantial crystal array , as expressed in equation ( 4 ): eff i = c exp i /( f cp i × f other i ) ( 4 ) in equation ( 4 ), eff i denotes the detection efficiency of the i th crystal . the larger value of eff i indicates that the radioactive ray ( such as gamma ray ) is more easily detected by the crystal . for example , if eff 10 is twice the eff 15 , it indicates that the count of signal detected by the 10 th crystal is twice the count of signal detected by the 15 th crystal . by the result of eff i in equation ( 4 ), the flow chart in fig6 can be proceeded , wherein fig6 is a flow chart of a method for calibrating the detection efficiency in crystal level according to one embodiment of the present invention . steps 40 to 47 in fig6 are similar to steps 30 to 37 in fig5 . however , in fig6 , step 48 is performed to calibrate the detection efficiency of each substantial crystal in the substantial crystal array by equation ( 4 ). please refer to fig7 a and fig7 b for schematic diagrams showing calibrating the detection efficiency in crystal level according to the present invention . label 95 in denotes uniform and parallel gamma rays . assuming that there are five crystals # 1 -# 5 with the same size in a crystal array 50 in a detection device 5 , the detection efficiency of these five crystals are provided in table 1 , wherein eff 1 to eff 5 denote the crystal efficiency , respectively . when the detection device 5 is used to detect a gamma - ray source capable of generating parallel and uniform gamma rays at a far end , the probability for each crystal being hit by gamma rays is identical . since the detection efficiency of each crystal may differ , for example , the results detected by the crystals are shown in table 2 , wherein count 1 to count 5 represent the counts detected by crystals # 1 ˜# 5 . table 2 shows the results when the detection efficiency is not calibrated . however , table 2 may mislead that there are a strong source 96 and a weak source 97 arranged vertically as shown in fig7 b , which results in higher values for the crystal # 1 and the crystal # 5 and lower value for the crystal # 3 . in fact , this is attributed to the difference in detection efficiency between the crystals . accordingly , the original value for each crystal has to be calculated with the detection efficiency eff i by equation ( 4 ) so as to acquire a calibrated result that can truly reflect the condition of the sources as shown in table 3 . in the present embodiment , the calibrated detecting result is acquired by dividing the original value for each crystal by the detection efficiency . it is noted that the present invention is not limited to such division calculation . moreover , the eff i value can be inversed and then multiplied with the original value for each crystal so that the highest part of detection efficiency is reduced and the lowest part is enhanced . therefore , the method for calibrating is not limited to division calculation . the detection efficiency calibration based on eff i can be achieved by hardware such as electronic circuits or software . although the present invention has been disclosed and illustrated with reference to particular embodiments , the principles involved are susceptible for use in numerous other embodiments that will be apparent to persons skilled in the art . the present invention is , therefore , to be limited only as indicated by the scope of the appended claims .