the invention is based on the appropriate definition of regions of interest ( rois ), as will be explained briefly with reference to fig2 . in a 2 - d x - ray or angiography image 11 containing blood vessels 12 for example , what is understood quite generally by an roi is a user - defined section of the angiography image 11 . as a result of the preceding subtraction of the mask image in the case of dsa sequences the sum of the grayscale values of all pixels lying within the roi in each individual image of the sequence is directly proportional to the mass of the contrast agent that is contained in the 3 - d volume segment v roi defined by the roi . since no depth information at all is in fact contained in a 2 - d x - ray image 11 , present approaches consequently always acquire the 3 - d volume segment v roi which is “ excised ” by the roi defined in 2 - d and which extends over the entire object depth . it is important for the application of the approaches proposed here that an angulation of the angiography system is chosen which has the fewest possible overlays of the tissue region that is to be examined by blood vessels 12 lying spatially in front of or behind said tissue region . the greater the number of such overlays occurring , the more inaccurate will be the estimation of the relative perfusion parameters . defining suitable rois — either for the purpose of before / after comparisons ( e . g . in the case of tumor embolizations ) or for the purpose of left / right comparisons either in the brain or in paired organs such as the kidneys for example — enables results to be computed in relation to the mass ratio of the contrast agent associated with the respective time instant ( in left / right comparisons ) or with the respective two time instants ( in before / after comparisons ) ( and hence of the blood , provided an ideal mixture of blood and contrast agent is assumed ) within the volume segments v roi defined by the rois . meaningful determinations of the relative blood volume naturally demand here that in the case of before / after comparisons the acquisition parameters , such as in particular the angulation of the c - arm and the zoom factor used for example , and the injection protocol remain constant . any changes to the exposure parameters resulting from the automatic dose regulation by the angiography system must be calculated out of the image sequences accordingly in order to allow a meaningful comparison . in the case of left / right comparisons it is of course likewise necessary to choose a suitable injection protocol which prefers no half of the body per se . typically a stationary state is required for determining the blood volume in a tissue region . according to the theory ( see equation ( 5 ) in konstas et al .) the blood volume v in a volume segment can in fact also be calculated from dynamic data as follows : c tissue ( t ) denotes the average contrast agent concentration in the tissue region under examination , while c artery ( t ) denotes the sum of the average contrast agent concentrations in the supplying arteries . in this case the upper integration limit t should be suitably chosen to enable the transported contrast agent bolus to be recorded completely . however , the integration should only include the time in which the contrast agent bolus undertakes a first pass through the tissue so that distortions of the values caused by recirculation of the bolus are avoided . in before / after comparisons with constant injection protocol and constant acquisition parameters as well as in left / right comparisons with appropriately chosen injection protocol it may be assumed for simplicity that the arterial input left and right or , as the case may be , before and after is consistent , such that in the case of before / after comparisons the relation is obtained and in the case of left / right comparisons the analog relation is obtained . it should be noted that a change in the blood flow ( specified in ml / 100 g / min ) in the case of before / after comparisons or a different blood flow left / right in the case of left / right comparisons has no relevance , since the flow has already been eliminated in the course of the derivation of equation ( 1 ). equation ( 1 ) henceforth includes only the time - dependent contrast agent concentrations . taking into account that the concentration of the contrast agent is proportional to the mass of the contrast agent ( concentration = mass / volume ), and assuming that the volume segments being examined are present with at least approximately the same size ( both in before / after and in left / right comparisons ), the proportionality constants ( 1 / volume ) are omitted in the above formulae and the corresponding relative blood volumes can be expressed by means of the contrast agent masses . as already mentioned further above , the contrast agent masses are in turn proportional to the sums of the grayscale values of all pixels lying within the rois ( in each individual image of the sequence ). in the two previous formulae , therefore , the time integrals are placed over the roi - specific time / contrast curves in the numerator and in the denominator in each case . the general case of the calculation of the change in relative blood volume is explained in more detail with reference to fig3 to 6 . for the purpose of determining relative perfusion data according to the invention a perfusion measurement device 10 is provided in the system control unit 7 , as shown in fig1 . as output of the calculated perfusion data this also effects an insertion for example as a numeric value characteristic of the roi into the image on a display of the traffic - light monitor array 9 . fig3 shows a first time / contrast curve 13 i / t before the treatment and fig4 shows a second time / contrast curve 14 i / t after the treatment . the area auc ( area under the curve ) under the overall curves 13 and 14 is formed by the time integrals . their ratio expresses a change in relative blood volume . in order to calculate the change in relative blood volume the areas under the overall curves 13 and 14 can now be put into the ratio auc after / auc before . for simplicity the calculation of the integrals according to the examples explained with reference to fig3 and 4 can be dispensed with here and in each case the maximum of the associated time / contrast curve can be used instead , as is shown with reference to fig5 and 6 ( in this regard see also fig2 in konstas et al . “ theoretic basis and technical implementations of ct perfusion in acute ischemic stroke , part 1 : theoretic basis ”, ajnr am . j . neuroradiol . 30 , 2009 , pages 662 to 668 ). this simplification is based on the assumption that there are plateau - like maxima of the time / contrast curves at which a saturated state of the contrast agent concentration can be assumed . the advantage of this simplification consists in the fact that it is not necessary to integrate over a relatively long time period and therefore overlay effects caused by the contrast agent flow in draining veins , which could of course also be visible in the projection image , are avoided . however , this simplifying estimation of the relative blood volume requires a greater amount of contrast agent to be administered in order to achieve the stationary state , which is not always desirable or feasible . according to the invention the calculation can now be simplified in that , as shown in fig5 and 6 , the slopes 15 and the maxima 16 of the first simplified time / contrast curve before the treatment and the second simplified time / contrast curve after the treatment are assumed to be straight lines . the maximum intensity 17 i max , v before the treatment and the maximum intensity 18 i max , n after the treatment can then be ascertained in a simple manner . in order to calculate the simplified change in relative blood volume the two maximum intensities are now put into the ratio i max , after / i max , before . in the case of a tumor embolization the tumor can be characterized in the two dsa sequences ( pre - and post - treatment ( before / after )) by means of an roi in each case and then the ratio of the time integrals over the two time / contrast curves determined . according to the above consideration their quotient represents the ratio of the blood volumes before and after the intervention . ideally , no more contrast agent at all accumulates in the tumor after the embolization , thus yielding the ratio v after / v before ˜ 0 as result . as already mentioned , a suitable angulation must be chosen for an examination of said type to ensure that no large blood vessels run through the volume segment defined by means of the roi , since these would distort the result . the relative blood flow can be determined in a comparable way to the determining of the relative blood volume . in this case the so - called “ maximum slope method ” can be used , see equation ( 10 ) in konstas et al . in spite of simplifying assumptions this method is also employed in ct for the purpose of measuring the blood flow . this method provides a simple computing rule for determining the flow f which is assumed as constant over time : here , m ( t ) denotes the mass of contrast agent contained in the tissue volume under examination at the time instant t , and c artery ( t ) denotes the contrast agent concentration in the supplying artery at the time instant t . for the sake of simplicity it is assumed that no venous outflow takes place during the examination time period and that precisely one artery supplies the examined tissue volume . according to this relation the flow f can therefore be determined by dividing the maximum rise of the mass of contrast agent in the tissue by the maximum contrast agent concentration in the supplying artery . the general case of the calculation of the change in relative blood flow is explained in more detail with reference to fig7 and 8 . in this case fig7 shows a first time / contrast curve 19 before the treatment . a first maximum slope 20 is applied to the ascending branch of said first time / contrast curve 19 . fig8 shows a second time / contrast curve 21 after the treatment , to the ascending branch of which a second maximum slope 22 is applied . as also in the case of the determining of the relative blood volume from 2 - d angiography data , suitable rois should be defined in 2 - d at a suitable angulation of the c - arm , which rois then again characterize 3 - d volume segments that extend over the entire object depth . on the assumption that the arterial inflow left / right or before / after is the same , the relative blood flow can be approximated as follows in the case of left / right comparisons according to the formula this means that — owing to the direct proportionality of contrast agent mass and the attenuation along the x - ray beams — the quotients from the maximum slopes 20 and 22 of the time / contrast curves 19 and 21 must be formed in order to obtain the corresponding estimations of the relative blood flow . thus , as was already the case in the determining of the relative blood volumes , the “ trick ” consists in determining the relative flows ( left / right and / or after / before ), since then the proportionality constants , which are not known due to the absence of depth information , are omitted from the formation of the quotients . the simplified case of the calculation of the change in relative blood flow is explained in more detail below with reference to fig9 and 10 . instead of the maximum slopes 20 and 22 that were explained with reference to fig7 and 8 , alternative parameters for determining blood flow can also be chosen if a specific model of the time / contrast curves i / t is assumed for simplicity . in this simplified model it is assumed that a first time / contrast curve 23 rises linearly until saturation is reached , as revealed in fig9 and 10 . it is easy to show that the slope 24 of the first simplified time / contrast curve 23 in this rise phase is proportional to two other parameters . the first parameter is the intensity value i ′ v at a time instant t ′, which must chosen such that it lies before the maximum contrast is reached . the second parameter is the first integral 25 ( area under the curve ( auc )) of the first time / contrast curve 23 up to the time instant t ′. the same also applies to the after case shown in fig1 , in which a linear rise of a second simplified time / contrast curve 26 until saturation is reached is likewise assumed . here too it holds that the slope 27 of the second simplified time / contrast curve 26 in this rise phase is proportional to the intensity value i ′ n at the time instant t ′. the second integral 28 of the second simplified time / contrast curve 26 up to the time instant t ′ can also be drawn upon again here as the second parameter . since these two parameters are proportional to the maximum slope , they can likewise be used for calculating the relative flow by formation of the quotients of the values before and after a treatment ( or , of course , also referred to a left / right comparison ). the change in relative blood flow can therefore be calculated in a simplified manner as follows : where m is the maximum slope and auc is the area under the time / contrast curve i / t . it is important to bear in mind that this simplifying assumption of a linear rise together with the associated simplified estimation of the relative blood flow has nothing to do with the above - explained assumption of a stationary state which leads to a simplified estimation of the relative blood volume . the invention relates to an imaging method for calculating and deriving relative perfusion data , such as blood volume or blood flow for example , from 2 - d angiography data , for example 2 - d dsa sequences . to clarify : per se this perfusion data represents absolute values ( e . g . where ct perfusion is concerned ). in the case of a 2 - d image series this restriction to relative perfusion data must be applied , since no depth information at all is available . by waiving the requirement for absolute data and considering relative data by quotient formation it is possible to dispense with the depth information , which , of course , is not contained in the 2 - d image sequences . put more precisely , this dispenses with knowledge of the proportionality constant which relates the mass of the contrast agent along an x - ray beam to the concentration of the contrast agent along said x - ray beam .
a method for calculating perfusion data , such as blood volume or blood flow from 2 - d angiography data or dsa sequences , is proposed . an angiography scene is recorded using specific acquisition parameters to generate the 2 - d angiography data or dsa sequences with administration of contrast agent based on a multiplicity of individual angiography images . a region of interest is defined suitable for comparison purposes . the volume segments are defined by the region of interest . the time / contrast curve is determined in the volume segments . perfusion data for calculating the relative perfusion data is ascertained . the perfusion data is compared and the relative perfusion data is calculated . the calculated relative perfusion data is not specified in terms of absolute physical quantities , but is provided simply as ratios , such as left / right or before / after .