fig1 illustrates and apparatus suitable for in vivo irradiation of a tissue sample ( e . g . a breast ). the apparatus comprises a penetrating radiation ( in this example x - ray ) beam source 2 that directs a beam of x - ray radiation onto the tissue sample 4 being examined . a series of detectors 6 , 8 , 10 , 12 are arranged below and above the sample 4 to detect both transmitted and scattered x - ray radiation . a more detailed explanation of the source and detector arrangement is given in our co - pending uk patent application filed on the same date as the present application with the title “ penetrating radiation measurements ”. in use , the source and detector arrangement is scanned across the full length of the tissue sample ( e . g . breast ), as indicated by arrow ‘ s ’, whilst the sample is held stationary . the scan is completed in step - wise fashion , with measurements being taken from the detectors at each step . the incident beam can be a slit - form beam having a width ( into the page as illustrated in fig1 ) sufficient to extend across the full width of the sample . alternatively , the beam may be narrower ( e . g . a pencil - form beam ) and be scanned laterally across the sample at each step in the longitudinal direction . in this example the energy ( kv ) of the incident x - ray beam is maintained constant during a scanning process ( possibly having been selected from a number of possible energy levels at the outset depending on the nature of the tissue being examined ). the x - ray dose delivered to the sample can , however , be varied by altering the flux ( ma ) of the x - ray beam and / or by altering the length of time that the apparatus irradiates any particular portion of the sample ( i . e . the duration of each step in the scan ). fig2 to 4 illustrate three alternative schemes for controlling the x - ray dose during a scanning process , in accordance with embodiments of the present invention , that can be used to obtain a balance between overall dose during a scan and the information content of the data collected from the detector measurements . looking first at fig2 , the scan process is started 21 , once the sample is in place . an initial dose level , ‘ low ’ or ‘ high ’, is set 22 ( this may be selected by the operator or always default to ‘ low ’ for instance ) and then a first portion of the tissue sample irradiated ( corresponding to the first scan step ) 23 . as the tissue portion is irradiated , measurements are taken using one of more of the detectors 6 , 8 , 10 , 12 . based on these measurements , the system makes a determination as to whether the measured tissue sample portion properties suggest the portion comprises normal or abnormal tissue 25 . if the tissue portion is determined to be normal , the dose level is maintained or set to ‘ low ’ 26 and the scan proceeds to its next step 28 and the next adjacent portion of the tissue sample is irradiated 23 at the low dose setting . if , on the other hand , it is determined at step 25 that the measurement ( s ) from the tissue portion indicate it is abnormal , the dose level is set or maintained at ‘ high ’ 27 . in this case , when the system proceeds to the next scan step 28 , the next adjacent portion of the tissue is irradiated 23 at the high dose setting . this irradiation cycle continues , with the dose level switching between low and high based on the measured tissue properties until the complete sample has been scanned , i . e . the scan is complete 24 . the scanning process is then stopped 29 . a variation of this process is illustrated in fig3 . as will be appreciated from the description above , in the process illustrated in fig2 it is the tissue portions adjacent abnormal portions that are irradiated at the higher dose level . an abnormal portion identified at a low dose setting is not re - scanned to obtain additional information for that portion . in contrast , the scheme illustrated in fig3 irradiates at a high dose those portions identified by the low dose measurements as abnormal . the high dose measurements are taken before the scan moves on to its next step . thus , the scan starts 31 with a low dose setting 32 and a first tissue sample is irradiated 33 . if this tissue sample is determined to be normal 35 , the scan moves on 38 to irradiate the next adjacent tissue portion in the sample at the low dose setting 32 , 33 . where the tissue portion is determined to be abnormal 35 , however , the system switches to a high dose level 36 without moving on and re - scans ( i . e . continues to irradiate ) the same tissue portion 37 . for instance , where the higher dose is achieved through a longer duration for the scan step , the x - ray beam source and detector arrangement can simply linger at this scan position for a further period of time so that the total duration corresponds to the high dose level . subsequently , the scan then moves on to its next step 38 , the system switches back to a low dose setting 32 and the next adjacent tissue portion in the sample is irradiated 33 . in this illustrated example , the dose level is returned to low at each scan step 32 . it need not be , however . thus , a further variation , not illustrated , is a combination of the fig2 and 3 schemes . where an abnormal tissue portion is identified by a low dose measurement , that portion is irradiated further at the high dose setting and the dose level is then maintained at a high setting until the scan moves on to a position at which the measurement indicates the tissue is normal . only then is the dose level returned to low . the scan continues in this manner until the complete sample has been scanned 34 , 39 . fig4 illustrates another scheme for controlling the dose level during a scanning process . in this example , the complete sample is first scanned at a low dose level and potentially suspect tissue portions noted . the suspect portions are then re - scanned in a second pass at a higher dose level . specifically , once the scan is started 41 , the dose level is set low 42 and the complete sample is scanned step - wise 43 , 45 , 48 until the scan is complete 44 . at each step in the low dose scan a determination is made as to whether the portion of the tissue sample being irradiated is normal or abnormal 45 . where a portion is determined to be abnormal its position in the scan is stored 46 , 47 . once the low dose scan is complete 44 , assuming one or more abnormal tissue portions have been detected 49 , the x - ray beam source and detector arrangement return to its starting position 50 . otherwise , if there were no abnormal tissue portions detected , the scan stops 56 . in the case where abnormal portions have been detected , the dose level is set to high 51 and the x - ray beam source and detector arrangement is moved 52 to the first stored scan position 47 corresponding to the location of an identified abnormal portion of the tissue sample . this portion is irradiated at the high dose level and further measurements collected 53 . if there are further abnormal portions that have been identified , the x - ray beam source and detector arrangement is moved to the next scan position corresponding to the location of an abnormal tissue portion 55 and this portion is irradiated at the high dose level 53 . once all of the identified abnormal portions have been re - scanned 54 , the scanning process is stopped 56 . in all of the processes described above , the dose level can be kept low unless a suspect area of tissue is identified in the sample , only such suspect areas ( and / or areas adjacent to them ) being irradiated at a higher dose . this potentially minimises the total dose during a scan , whilst ensuring that the measurements taken from the suspect tissue areas have the best possible information content to enable a more accurate determination of the abnormal tissue characteristics and a better subsequent diagnosis . typically the sample will be continuously irradiated throughout the scan . alternatively , the x - ray beam can be arranged to be incident on the sample in an intermittent fashion , so that there are periods between steps in the scan during which the beam is not incident on the sample . this may have the effect of further reducing the dose absorbed by the sample in any one scan . in addition ( and in some cases even as an alternative ) to varying a dose level , it may be desirable to vary other system parameters to increase the data content of measurements obtained from suspect ( e . g . abnormal ) tissue sample portions , to minimise the necessary data processing capacity and / or to minimise dose . for example , a broad , slit - type beam may be used to irradiate a sample at a low dose setting to initially determine areas of abnormal tissue , or the complete sample may be irradiated at once ( e . g . using a conventional x - ray transmission measurement technique as in mammography ). however , to minimise the dose during irradiation at a high dose level , it may be desirable to use a more focussed beam , e . g . a pencil beam , directed only at the area of interest . in some cases , it may be desirable to restrict the number of detectors used at a low dose setting in order to limit the data that is processed to make an initial determination as to whether a tissue portion is normal or abnormal . when examining suspect areas ( whether at a high dose or not ), however , it will generally be desirable to use more detectors , taking a greater variety of measurements ( in accordance , for example , with the approach described in co - pending pct patent application numbers pct / gb04 / 005185 and pct / gb05 / 001573 ) to maximise the information content of the measured tissue data . so , for example , considering the exemplary apparatus of fig1 , it might be possible to detect suspect areas using data from only transmission measurements from detector 6 , or another single detector or perhaps a combination of two detectors . once suspect areas have been identified , data from the whole arrangement of detectors can then be used to extract further information about the tissue characteristics . another measure that might be usefully adopted is to provide variable geometry detectors so that a detector can be optimised based , for example , on the particular tissue characteristic , type or property of interest and / or so that a single detector can be used to take a variety of measurement , e . g . at different scatter angles . looking at fig1 , for example , detectors 10 are arranged to be variable angle so that , assuming appropriate collimation , they can be used to detect scattered radiation at multiple selected angles . it will be appreciated that description above is given by way of example and various modifications , omissions or additions to that which has been specifically described can be made without departing from the invention . for instance , whilst the embodiments have been illustrated with reference to two dose level states , high and low , it is possible to apply the same principles using three or more dose levels or two , three or more other system states that provide alternative or additional data for analysis . also , whilst the scanning of the beam across the sample has been described above as a step - wise process , this may be a continuous motion along the sample for all or part of the scan . for instance , the scan may proceed in a continuous fashion until a region of suspect tissue is detected , at which point the scan may slow or even stop to collect additional data and / or to carry out further measurements .
a method for irradiating a biological tissue sample is provided , the method comprising : irradiating a portion of a biological tissue sample with a penetrating radiation beam for a first exposure period ; subsequently irradiating the same or an adjacent biological tissue portion with a penetrating radiation beam for a second exposure period ; the radiation dose incident on the tissue sample during the second exposure period being higher than the dose during the first exposure period . also provided is an apparatus operative in accordance with the method . the method and apparatus have particular application in the characterisation of body tissue by x - ray diffraction , both in vitro and in vivo .