EP0571017A2 - Filtering procedure for an X-ray system and arrangement to carry out such a filtering procedure - Google Patents

Abstract

The basis for this method and arrangement is an X-ray system with an X-ray radiator emitting X-ray quanta and a detector arrangement, supplying at least one measurement signal, for detecting the X-ray quanta which have interacted with an object in an examination area. The method includes the following method steps: a) A measurement is carried out in which a filter is located in the beam path between the X-ray radiator and the examination area. b) A measurement is carried out in which a filter is located in the beam path between the examination area and the detector arrangement, which filter consists of the same material as the filter used in the other measurement. c) The measurement signals obtained in the two measurements are subtractively combined with one another. <IMAGE>

Description

The invention relates to a filter method for an X-ray system and an arrangement for carrying out this filter method. From the journal J. Phys.E., Vol. 18, 1985, pages 354-357, a filter method for an X-ray system is known which irradiates an examination area, the X-ray radiation from the examination area being measured by a detector arrangement. In the known method, a first measurement is carried out with a first filter in the beam path between the X-ray emitter and the examination area and a second measurement with a second filter. The two filters have different absorption edges and are dimensioned such that they have the same absorption or transmission for all X-ray quanta outside the energy range between the absorption edges of the two filters. If the result of the two measurements is subtracted from one another, a difference value results which only depends on the spectral components of the polychromatic X-ray emitter which lie within the energy range between the two absorption edges.

• a) Es wird eine Messung durchgeführt, bei der sich im Strahlengang zwischen dem Röntgenstrahler und dem Untersuchungsbereich ein Filter befindet.
• b) Es wird eine Messung durchgeführt, bei der sich im Strahlengang zwischen dem Untersuchungsbereich und der Detektoranordnung ein Filter befindet, das aus dem gleichen Material besteht wie das bei der anderen Messung benutzte Filter.
• c) Die bei den beiden Messungen erhaltenen Meßsignale werden subtraktiv miteinader kombiniert.

The object of the present invention is to specify another filter method. This object is achieved according to the invention by a filter method for an x-ray system with an x-ray emitter x-ray emitter and a detector arrangement delivering at least one measurement signal for detecting the x-ray quanta which have interacted with an object in an examination area with the following method steps:

• a) A measurement is carried out in which there is a filter in the beam path between the X-ray emitter and the examination area.
• b) A measurement is carried out in which there is a filter in the beam path between the examination area and the detector arrangement, which is made of the same material as the filter used in the other measurement.
• c) The measurement signals obtained in the two measurements are combined with each other subtractively.

So while in the known method with two measurements, filters made of different materials are placed in the beam path between the X-ray source and the examination area, in the case of the invention a filter is placed in the beam path between the X-ray source and the examination area and in the other measurement a filter is placed in the beam path placed between the examination area and the detector arrangement, the filter material being the same in both cases. The same filter can therefore be used for both measurements. However, it is also possible to use two filters made of the same material.

• 1) Bei der elastischen Streustrahlung (Rayleigh-Streuung) ändert sich zwar die Richtung der Röntgenstrahlung, nicht aber ihre Energie.
• 2) Bei der inelastischen (Compton-Streuung) verliert die Röntgenstrahlung bei einer Richtungsänderung Energie. Der Energieverlust hängt von der Größe der Richtungsänderung und von der Energie der Röntgenquanten ab.
• 3) Bei der photoelektronischen Bremsstrahlung wird durch ein mit einem Atom in Wechselwirkung tretendes Röntgenquant ein Elektron vornehmlich aus der K-Schale befreit, wodurch ein Photoelektron (Röntgenquant) entsteht, dessen Energie um den Betrag kleiner ist als die Energie des primären Röntgenquants, der erforderlich ist, um das Elektron aus der K-Schale zu lösen. Dieser Energiebetrag steigt kubisch mit der Ordnungszahl des Atoms im periodischen System an.

The invention takes advantage of the fact that X-ray quanta can interact with an object in the examination area in different ways:

• 1) With elastic scattered radiation (Rayleigh scattering) the direction of the X-ray radiation changes, but not its energy.
• 2) With inelastic (Compton scattering), the X-ray radiation loses energy when the direction changes. The energy loss depends on the size of the change of direction and on the energy of the X-ray quanta.
• 3) In the case of photoelectronic brake radiation, an X-ray quantum interacting with an atom primarily frees an electron from the K shell, which creates a photoelectron (X-ray quantum) whose energy is smaller than the energy of the primary X-ray quantum that is required is to release the electron from the K shell. This amount of energy increases cubically with the atomic number in the periodic system.

The method according to the invention makes it possible to separate the components which have arisen as a result of different interactions with the examination area.

A first further development of the invention provides that an essentially monochromatic X-ray emitter is used, that the filter material has an absorption edge at a quantum energy which is slightly lower than the energy of the X-ray quanta emitted by the monochromatic X-ray emitter and that the X-ray quanta from the detector arrangement under one An angle is detected that is greater than the angle at which the energy loss of the X-ray quanta by Compton scattering corresponds exactly to the difference between the energy of the X-ray quanta and the quantum energy at which the filter has its absorption edge. This method allows the scatter cross section to be determined for elastic (coherent) scattered radiation or also for inelastic (incoherent) scattered radiation.

According to another embodiment of the invention, it is provided that an essentially monochiomatic X-ray emitter is used, that the filter material has an absorption edge at a quantum energy which is slightly lower than the energy of the X-ray quanta emitted by the monochromatic X-ray emitter, that the X-ray quanta are below the detector arrangement an angle that is smaller than the angle at which the energy loss of the X-ray quanta by Compton scattering corresponds exactly to the difference between the energy of the X-ray quanta and the energy of the X-ray quanta at which the filter material has an absorption edge and that the quantum energy is energy-resolving is measured. With this configuration, the components due to Compton and Rayleigh scattering can be suppressed, so that only components remain which are generated by photoelectronic braking radiation. It can be used to determine the content of low-order substances, e.g. Determine carbon, oxygen or nitrogen).

Another embodiment of the invention provides that a polychromatic X-ray emitter is used and that the scattered radiation emerging under a certain scattering angle range is measured by the detector arrangement. The measured values obtained after subtractive combination of the measurement signals are only determined by X-ray quanta within a certain energy band; the effect of the other X-ray quanta is eliminated by the subtractive combination.

• Fig. 1 eine Anordnung zur Durchführung des erfindungsgemäßen Filterverfahrens.
• Fig. 2 ein Spektrum, das sich bei einer Ausfährungsform jenseits des Untersuchunbsbereiches ergibt.
• Fig. 3 die Emissionslinien eines für das Verfahren geeigneten monochromatischen Röntgenstrahlers.
• Fig. 4 das Energiespektrum, das sich bei einer anderen Ausführungsform ergibt.
• Fig. 5 ein Bremsstrahlungsspektrum vor und hinter dem Untersuchungsbereich.
• Fig. 6 eine zweite Ausführungsform des erfindungsgemäßen Verfahrens und
• Fig. 7 ein bei der Anordnung nach Fig. 6 verwendbares Filter.

The invention is explained in more detail below with reference to the drawings. Show it:

• Fig. 1 shows an arrangement for performing the filter method according to the invention.
• 2 shows a spectrum which arises in the case of an embodiment beyond the examination area.
• 3 shows the emission lines of a monochromatic X-ray emitter suitable for the method.
• Fig. 4 shows the energy spectrum that results in another embodiment.
• 5 shows a brake radiation spectrum in front of and behind the examination area.
• Fig. 6 shows a second embodiment of the method according to the invention and
• FIG. 7 shows a filter that can be used in the arrangement according to FIG. 6.

In Fig. 1, 1 denotes an X-ray emitter which emits monochromatic X-ray radiation; the X-ray quanta emitted by the radiator 1 thus all have essentially the same energy. A diaphragm 2 provided with a central bore allows only a needle beam (pencil beam) 3 to pass through the x-ray beam emitted by the x-ray emitter 1. The needle beam 3 passes through the central opening in a further diaphragm plate 4. The two diaphragm plates 2 and 4 delimit an examination area in the direction perpendicular to the needle beam 3, in which an examination object 7 is located. The X-ray quanta in the needle beam 3 interact with the examination object 7 and produce, among other things, elastic and inelastic scattered radiation. The scattered radiation, which is generated in the examination object 7 between a minimum angle β 1 and a maximum angle β 2, can be formed through an annular opening 8 in the aperture 4, which is concentric with the needle beam 3 reach an annular detector 9 therethrough. The detector signal is amplified by an integrating amplifier 10 and converted into a digital data word by an analog-digital converter. This data word is proportional to the number of X-ray quanta registered by the annular detector 9 during an integration interval or a measuring time and is independent of the energy of the X-ray quanta.

The data word can be stored in a memory 12 and further processed in an arithmetic logic unit (ALU 13). The units 10-13 are controlled by a control unit 14. The units 12-14 can be part of a microprocessor.

The implementation of a measuring method is explained below with the aid of the arrangement outlined in FIG. 1. A first measurement is carried out first. In this first measurement, there is a filter 5 in the beam path between the monochromatic x-ray emitter 1 and the examination area 7, which has an absorption edge at a quantum energy E _k that is slightly lower than the energy of the x-ray emitter emitted by the x-ray emitter 1.

Dabei ist E_p die Energie des Röntgenquants vor dem Streuproßeß, E_s die Energie des Röntgenquants, nach dem Streuprozeß c eine Konstante und β der Winkel, den die Bahn des gestreuten Röntgenquants mit der Richtung des Nadelstrahls 3 einschließt.2 shows the energy spectrum (ie the intensity of the X-ray radiation as a function of the energy of the X-ray quanta). A line E _p and a component E _s with lower energy can be seen in the spectrum. The line E _p is created by elastic scattering, in which the X-ray quanta are known to lose no energy. The energy E _p is therefore also the energy of the X-ray quanta emitted by the X-ray emitter 1. Component E _s is created by Compton scattering. In this inelastic scattering process, the X-ray quanta lose energy according to the relationship

${\text{1 / E}}_{\text{s}}{\text{- 1 / E}}_{\text{p}}\text{= c (1-cosβ) (1)}$

E _{p is} the energy of the X-ray quantum before the scattering process, E _{s is} the energy of the X-ray quantum, after the scattering process c is a constant and β is the angle that the path of the scattered X-ray quantum includes with the direction of the needle beam 3.

Equation (1) assumes that the electrons are stationary. In reality, however, the electrons move. This leads to a widening of the Compton line (Compton shift). In this case, equation (1) describes the Energy of the Compton peak. The width of the Compton peak is small for scattering under a small scattering angle.

The broadening of the component E _s compared to the component E _{p also} results from the fact that X-ray quanta can reach the detector ring 9 at different scattering angles. If it is ensured that scattered radiation can only reach the detector arrangement at a defined scattering angle, there is approximately a line for component E _s . This can be achieved, for example, by using a primary beam with the shape of a cone shell instead of a needle-shaped primary beam and forming the diaphragm 4 by collimator bodies which are concentric with the axis of symmetry of the cone shell, as described in DE-OS 40 34 602.

The filter 5 shown in FIG. 1 consists of a material with an absorption edge at a quantum energy E _k which is slightly smaller than the energy of the X-ray quanta emitted by the X-ray emitter but greater than the energy E _{s of} the X-ray quanta influenced by the scattering process. 2, the course of the transmission of this filter as a function of the energy of the X-ray quanta is indicated schematically by a dashed curve F. The transmission increases monotonously up to the absorption edge, then jumps to a low value and then rises again. The transmission of the filter 5 for the energy of the primary radiation is denoted by T _p , and the (larger) transmission of the filter for the energy E _s is denoted by T _s . By using the filter 5 in the area between the X-ray source and the examination area, the spectral components E _s and E _{p are} reduced to the same extent - in accordance with the transmission factor T _p .

At the end of the measurement period, the analog-digital converter 11 delivers a signal which is proportional to the time integral over the intensity.

A further measurement is then carried out, in which - as indicated by arrows - the filter 5 is moved out of the beam path and a filter 6 into the beam path between the examination area 7 and the detector arrangement 9. The filter 6 must consist of the same material as the filter 5 and can have the same thickness. In the latter case, one could get by with a filter that is above the examination area for one measurement and for the another measurement is arranged below the examination area. - The filter 6 does not affect the scattered components E _p and E _s in the same way. The component E _p is weakened by the filter 6 to the same extent as by the filter 5. On the other hand, the component E _{s is} weakened less because T _{s is} greater than T _p . The measurement time available for this measurement corresponds to the measurement time for the previous measurement.

After the two measurements, the difference between the signals obtained in the measurements can be formed. Since in the two measurements the component E _{p is} damped to the same extent by the filters 5 and 6, the difference between the measurement signals depends only on the component E _s , which is caused by Compton scattering. The difference signal is therefore a measure of the Compton scatter.

If a filter made of the same material as the filter 6 is used in the beam path between the examination area and the detector arrangement, but with a greater thickness by the factor T _s / T _p , then the component E _s experiences the same attenuation in both measurements, while the component E _p is suppressed more during this second measurement. Therefore, if the difference between the measurement signals is formed in the two measurements, the difference signal is independent of E _s and a measure of the elastic scattered radiation. The same result can also be achieved if a filter made of the same material and the same thickness is used in the beam path between the examination area 7 and the detector arrangement 9 as the filter 5 and the intensity of the needle beam 3 or the measurement time by a factor of T. _s / T _p increased.

${\text{S2 = I}}_{\text{p}}{\text{· (T}}_{\text{p}}{\text{A}}_{\text{e}}{\text{+ T}}_{\text{s}}{\text{· A}}_{\text{i}}\text{) (3)}$

Dabei sind A_e und A_i Faktoren, die Streuquerschnitten für elastischen (Rayleigh-) bzw. inelastische (Compton-) Streustrahlung proportional sind und I_p die Intensität im Nadelstrahl 3. Aus den Gleichungen 2 und 3 lassen sich die Streuquerschnitte wie folgt ableiten:

${\text{I}}_{\text{p}}{\text{· A}}_{\text{i}}{\text{= (S2 - S1)/(T}}_{\text{s}}{\text{- T}}_{\text{p}}\text{) (4)}$

${\text{I}}_{\text{p}}{\text{· A}}_{\text{e}}{\text{= (S1 · T}}_{\text{s}}{\text{- S2 · T}}_{\text{p}}{\text{)/(T}}_{\text{s}}{\text{· T}}_{\text{p}}{\text{- T}}_{\text{p}}\text{²) (5)}$

Gleichung 5 zeigt, daß man den Querschnitt A_e für die elastische Streustrahlung auch ermitteln kann, ohne die Filterdicke, die Meßzeit oder die Intensität I_p zu verändern. Allerdings darf man die subtraktive Kombination der Signale S1 und S2 nicht unmittelbar durch Differenzbildung realisieren, sondern durch eine Linearkombination, bei der die Differenz der gewichteten Meßsignale gebildet wird.A modification of the arrangement according to FIG. 1 allows the calculation of the scattering cross section of a volume element for elastic and / or non-elastic scatter radiation. For this purpose, a diaphragm arrangement must be arranged between the detector arrangement 9 and the examination area 7, through which the detector arrangement can only "see" a volume element on the needle beam 3 of the examination region 7. (In this case, it is expedient if the object 7 can be displaced relative to the other components of the arrangement - or vice versa - not only perpendicularly to the needle beam 3, but also in the direction of the needle beam 3, so that each volume element within the body 7 is examined if necessary can be). The following then applies to the measurement signals S1 and S2 obtained in the two measurements:

${\text{S1 = I}}_{\text{p}}{\text{· T}}_{\text{p}}{\text{· (A}}_{\text{e}}{\text{+ A}}_{\text{i}}\text{) (2)}$

${\text{S2 = I}}_{\text{p}}{\text{· (T}}_{\text{p}}{\text{A}}_{\text{e}}{\text{+ T}}_{\text{s}}{\text{· A}}_{\text{i}}\text{) (3)}$

A _e and A _{i are} factors which are proportional to the scattering cross sections for elastic (Rayleigh) and inelastic (Compton) scattering radiation and I _p the intensity in the needle beam 3. The equation 2 and 3 can be used to derive the scattering cross sections as follows:

${\text{I.}}_{\text{p}}{\text{· A}}_{\text{i}}{\text{= (S2 - S1) / (T}}_{\text{s}}{\text{- T}}_{\text{p}}\text{) (4)}$

${\text{I.}}_{\text{p}}{\text{· A}}_{\text{e}}{\text{= (S1 * T}}_{\text{s}}{\text{- S2 · T}}_{\text{p}}{\text{) / (T}}_{\text{s}}{\text{· T}}_{\text{p}}{\text{- T}}_{\text{p}}\text{²) (5)}$

Equation 5 shows that the cross section A _e for the elastic scattered radiation can also be determined without changing the filter thickness, the measuring time or the intensity I _p . However, the subtractive combination of the signals S1 and S2 must not be realized directly by forming the difference, but by a linear combination in which the difference between the weighted measurement signals is formed.

As FIG. 2 clearly shows, it is a prerequisite for the separation of the components E _s and E _p that the filter has an absorption edge at a quantum energy E _k which is below E _p and above E _s . For this to be the case, the energy loss E _p - E _{s of} an X-ray quantum in a Compton scattering process must be sufficiently large. According to equation 1, the energy loss E _p -E _{s increases} with the scattering angle. At a certain scattering angle, the energy loss corresponds exactly to the difference between the energy E _p and the quantum energy E _k at the absorption edge. The scattering angles at which the detector arrangement 9 detects the scattered X-ray quanta must therefore be larger than this scattering angle in order for elastically scattered X-ray quanta and inelastically scattered quanta to be separated from one another by a Comptom process.

A monochromatic X-ray radiation could basically be generated using a radionuclide. However, these radiation sources are of low intensity. An X-ray emitter has a much higher intensity, initially generating a polychromatic X-ray radiation, which is converted into quasi-monochromatic fluorescence radiation in a target. Such X-ray emitters are known from EP-OS 292 055 (PHD 87-098 EP) or from DE-OS 40 17 002. 3 shows the emission spectrum of such an X-ray emitter with a target made of tantalum. The spectrum of such a radiator is composed of four K-lines α2, α1, β1 and β2 (increasing energy in the order); all other tantalum fluorescence lines, not shown in FIG. 3, have a wide range underlying energy. The K _α1 line has an energy of 57.532 keV, while the K _β1 line is approximately 7.5 keV higher. In connection with such an X-ray source, a filter made of erbium with an absorption edge at a quantum energy E _k of 57.485 keV is favorable, which lies above the K _α2 line and below the K _∝2 line and below the K _α1 line.

Equations 2 and 3 apply to each of the four lines. However, if the emission line and the line created after scattering are both both above or both below the K absorption edge of the filter, T _p and T _{s are} practically identical, and the contributions of these lines to that resulting after the subtractive combination of signals S1 and S2 Signals cancel out. The K _α2 line, and especially the line resulting from Compton scattering, lies below the absorption edge E _{k of} the erbium filter. The K _β1 and K _β2 lines and the lines resulting therefrom by scattering lie above the absorption edge, as long as the energy loss in the scattering processes is less than 7.5 keV or the scattering angle is less than 90 ^o . Only the K _α1 line makes a contribution because its energy lies above the absorption edge, while the line resulting from this by Compton scattering lies below the absorption edge if the scattering angle is at least 7 ^o .

With slight modifications, it is possible with the arrangement according to FIG. 1 to measure the photoelectronic brake radiation generated by the needle beam independently of the scattered radiation generated by Compton or Rayleigh scattering. For this purpose, the detector ring 9 and the diaphragm 4 or collimator arrangement arranged between this detector ring and the examination area must be designed in such a way that the detector ring from the examination area can only receive radiation at an angle which is greater than 0 ^° and smaller than that scattering angle at which is the energy loss due to Compton scattering in the range of the difference between the energy of the monochromatic radiation source 1 and the quantum energy at which the filter 5 has an absorption edge; In the aforementioned combination of a tantalum fluorescent radiation source and an erbium this angle 7 is ^o. In this case, not only the X-ray quanta influenced by elastic scattering, but also the X-ray quanta caused by Compton scattering have an energy which lies above the absorption edge of the filter 5 or 6. After subtracting the measurement signals (which correspond to the Filter 5 or filter 6 in the beam path), therefore, the influence of these scatter signals is canceled.

However, this does not apply to the photoelectronic brake radiation. This radiation arises when X-ray quanta free one electron from the K shell of an atom, creating a photoelectron whose energy is less than the energy of the primary X-ray quantum. The energy difference compared to the generating (primary) X-ray quantum depends on the atomic number. It is e.g. for carbon approx. 284 eV, for nitrogen approx. 400 eV, and for oxygen 532 eV. If it is greater than the energy difference between the quantum energy of the absorption edge and the energy of the monochromatic radiation - which is the case with the tantalum emitter / erbium filter combination - the energy of the photoelectronic brake radiation is below the quantum energy of the absorption edge, so that, as in connection with Fig. 2 has been explained, a separate detection of this radiation is possible.

This modification has particular advantages if the X-ray quanta are measured with energy resolution. A suitable detector 9, e.g. a germanium detector, which generates an impulsive signal when an X-ray quantum is detected, the amplitude of which is proportional to the energy of the X-ray quanta. A pulse height analyzer must be provided behind the amplifier 10, which registers the number of pulses for the different amplitude ranges, the amplitude of which falls within the respective amplitude range. With each measurement, this pulse height analyzer provides a number of numbers that represent the measured energy spectrum, i.e. characterize the intensity as a function of energy.

The results that can be achieved in this way can be understood from FIG. 4, which shows the energy spectrum occurring behind the examination object in the two measurements. A line E _p , which is caused by the energy of the monochromatic emitter and which, for example, corresponds to the K _∝1 line of the tantalum fluorescent radiation, can be seen. The line resulting from Compton scattering at E _s lies below E _p , but above the quantum energy E _{k of} the absorption edge of the filter, which is effective in the two measurements in the beam path in front of and behind the examination area. Below the absorption edge E _{k there} is a continuous spectrum, namely that Photoelectronic brake radiation spectrum. It is assumed that carbon (C), nitrogen (N) and oxygen (O) are present in the investigation area as elements with the lowest atomic number. If an X-ray quantum liberates an electron from the K shell of a carbon atom, a bremsradiation spectrum results, the highest energy of which is below E _k and about 284 eV lower than E _p . The highest energy of the brake radiation spectrum caused by the nitrogen component is approx. 400 eV lower than E _p , while with oxygen the highest energy is approx. 532 eV below E _p .

If more than one of the elements C / N / O is present in the examination area, the energy spectrum has a step-shaped course in its short-wave part. The height of each of the stages is a measure of the proportion of carbon, nitrogen and oxygen. The ratio of the three components to one another can therefore be determined by means of a suitable curve fitting method. Since explosives are known to have a well-defined C / N / O ratio, this method can be used to detect explosives within a wider area of investigation, for example in baggage control.

Figures 5 to 7 serve to explain a method that works with polychromatic X-rays. The curve P shown in solid line in FIG. 5 represents the energy spectrum of such an X-ray emitter which comprises an X-ray tube with a tungsten anode. One recognizes the typical course of a brake radiation spectrum with two intensity peaks in the medium energy range, which are caused by the characteristic radiation of tungsten. The curve shown in dashed lines with S represents the spectrum (to a different scale than the spectrum P) represent the results when X-rays having the energy spectrum P in the examination region at a scattering angle of eg 140 ^o scattered. The radiation scattered at such an angle is essentially caused by Compton scattering processes which, according to equation (1), leads to an increasing energy loss with increasing energy of the X-ray quanta.

If you measure the scattered X-rays and insert a filter with an absorption edge at the quantum energy E _a (this can be, for example, a tungsten filter with an absorption edge at approx. 70 keV) between the examination area and the detector arrangement. then there is a small attenuation for energies of the X-ray quanta below E _a and a large attenuation for energies above E _a .

If a further measurement is carried out and a filter made of the same material is inserted into the beam path between the radiation source and the examination area, the transmission jump caused by the absorption edge is due to the energy loss in the Compton scattering process at the lower energy E _b . Spectral components above E _b have a large attenuation and spectral components below E _b have a low attenuation.

In both measurements, the spectral components experience a lower attenuation below E _b and higher attenuation above E _a , although the attenuation effect (with the same filter thickness) is somewhat less on the primary side than on the secondary side. If you compensate for these differences in absorption or transmission by making the filter a little thicker on the primary side or - with the same thickness of the filter - increasing the measurement time accordingly if the filter is inserted on the secondary side, the influence of the spectral components increases below E _b and above E _a essentially when the signals obtained in the two measurements are subtracted from one another. This is not the case only in the area between E _b and E _a . The difference signal therefore corresponds to the signal that would have resulted if the X-ray emitter only had X-ray quanta with an energy between E _b and E _a . The method described thus effects bandpass filtering.

For the described embodiment with a filter of an absorption edge at 69.5 keV and a scattering angle of 140 ^°, the difference results in a band-pass filter, the X-ray quanta with energies in the range of 56 keV to 69.5 keV on the secondary side makes effective, which energy from 69.5 to 91.5 keV on the primary side. When replacing the filter by a tungsten cerium filter having a K-absorption edge at 40.45 keV, an energy band from 35.5 to 40.45 keV obtained at a scattering angle of 140 ^o with this method, on the secondary side or from 40.45 to 47 keV on the primary side. - The width of the energy band, which is effective by this method, depends on the scattering angle and decreases with it. With a scattering angle of 90 ^o, for example, the energy band to be emphasized with a tungsten filter is sufficient from 61.2 keV to 69.5 keV on the secondary side or from 69.5 to 80.44 keV on the primary side.

A device with which this method can be carried out is described below with reference to FIG. 6. The device has a measuring head 15 which is provided with a gap 16 perpendicular to the plane of the drawing in FIG. 6. The gap 16 fades out a fan-shaped beam of rays from the polychromatic beam of rays of an X-ray emitter, not shown, which strikes a rotatable roller 17 with a material that absorbs the X-ray radiation. In the roller there are two 180 ^o mutually offset helical slits provided so that a pencil beam is faded out 18 in each roll position of the fan-shaped radiation beam 17 which is pivoted upon rotation of the roller in a direction perpendicular to the plane of the drawing.

The needle beam 18 passes through an examination object 19 and generates (Compton) scattered radiation therein. The scattered radiation, which is scattered at an angle of approximately 140 ^° with the needle beam, passes through two slots 19 in the measuring head, which are perpendicular to the plane of the drawing and on both sides of the plane defined by the gap 16, and strikes two detector arrangements 20 each consisting of a plurality of detector elements in the measuring head. The detector elements, which extend perpendicular to the plane of the drawing, detect the scattered radiation from different depths of the object because of the slot geometry.

To the extent described so far, the arrangement according to FIG. 6 is known from EP-PS 184 247. In addition, however, a filter arrangement 21 is provided in the beam path between the object 19 and the measuring head 15. With this filter arrangement, four different measurements are carried out for each position of the needle beam 18.

As is apparent from Fig. 7, the 6 shows the filter arrangement in a comparison with FIG. 90 ^o rotated position, the filter assembly comprises a holder 215 for four filter plates 210 .... 213th The two filter plates 210 and 211 are made of tungsten and have the same thickness. The two filter plates 212 and 213 are made of cerium and have the same thickness. There is a space between adjacent filter plates through which X-rays can pass unaffected.

In a first measurement, the filter is positioned in the beam path in such a way that the needle beam 18 can pass between the filter plates 210 and 211 without being weakened. The scattered radiation, on the other hand, strikes the plates 210 and 211 on their way to the slots 19 and is influenced thereby. The filter is then moved laterally so that the filter plate 211 replaces the needle jet 18 in the second measurement. The scattered radiation then reaches the slots 19 unhindered. For the reasons explained in connection with FIG. 5, this second measurement takes somewhat longer than the first measurement. The measured values supplied by each individual element of the detector arrangements 20 for the same position of the needle beam 18 and the two positions of the filter arrangement 21 are subtracted from one another. As explained in connection with FIG. 5, the difference signal is equivalent to a measurement signal that would result if the spectrum of the X-ray emitter were restricted to a specific energy band (E _b -E _a - see FIG. 5).

After a further displacement of the filter arrangement 21, the cerium filter 212 is penetrated by the needle beam 18 in a third measurement. The scattered radiation, on the other hand, reaches the detector arrangement 20 unhindered through the slots 19. After a renewed displacement of the filter arrangement, the primary beam passes through the space between the two cerium filters 212 and 213 in a fourth measurement, which then filter the scattered radiation before it passes through the slots 19. For each detector element and for each needle beam position, the difference between the signals measured at the third and fourth positions of the filter arrangement is again formed, which results in a difference signal that corresponds to an energy band that is lower than the energy band that results from the difference of the first and the second measurement with the tungsten filters 210 and 211 results.

Object 18 is thus irradiated with two different energies, which is essential for the so-called “dual energy” methods. These methods provide additional information about the examination object 19. With the method according to the invention, such a dual-energy method can be carried out without the spectrum of the X-ray radiation generated by the X-ray emitter having to be changed, for example by switching over the high voltage to that contained in the X-ray emitter X-ray tube is put on. Nor is it necessary to measure the scattered X-ray radiation in an energy-resolved manner in order to carry out the dual-energy method.

As described in an article by Harding & Tischler (Phys Med.Biol, Vol. 31, 477-489, 1986), it is possible with a dual-energy method to separately detect the attenuation by means of Compton scattering and by photoelectric absorption . For this purpose, the two sets of difference signals resulting from the four measurements must be combined with one another in the manner mentioned in the publication.

Claims (7)

Filter method for an x-ray system with an x-ray emitter x-ray emitter and a detector arrangement delivering at least one measurement signal for detecting the x-ray quanta which have interacted with an object in an examination area with the following method steps: a) A measurement is carried out in which there is a filter in the beam path between the X-ray emitter and the examination area. b) A measurement is carried out in which there is a filter in the beam path between the examination area and the detector arrangement, which consists of the same material as the filter used in the other measurement. c) The measurement signals obtained in the two measurements are combined with each other subtractively. Filter method according to claim 1,
characterized in that an essentially monochromatic x-ray emitter is used, that the filter material has an absorption edge at a quantum energy which is slightly lower than the energy of the x-ray quanta emitted by the monochromatic x-ray emitter and that the x-ray quanta are detected by the detector arrangement at an angle that is greater than the angle at which the energy loss of the X-ray quanta due to Compton scattering corresponds exactly to the difference between the energy of the X-ray quanta and the Qunate energy at which the filter has an absorption edge. Filter method according to claim 1,
characterized in that an essentially monochromatic x-ray emitter is used in that the filter material has an absorption edge at a quantum energy which is slightly lower than the energy of the x-ray quanta emitted by the monochromatic x-ray emitter, and that the x-ray quanta can be detected by the detector arrangement at an angle which is smaller than the angle at which the energy loss of the X-ray quanta by Compton scattering corresponds exactly to the difference between the energy of the X-ray quanta and the quantum energy at which the filter material has an absorption edge, and that Energy dissolving is measured. Method according to claim 1,
characterized in that a polychromatic X-ray emitter is used and that the scattered radiation emerging under a certain scattering angle range is measured by the detector arrangement. Filter method according to claim 2 or 3,
characterized in that an x-ray emitter emitting the tantalum fluorescent radiation and an erbium filter are used. Arrangement for performing the filtering method according to claim 1,
characterized by an x-ray system with an x-ray emitter and a detector arrangement for detecting the x-ray quanta which have interacted with an examination object, with filter means for introducing a filter either into the beam path between the x-ray emitter and the examination object or into the beam path between the examination object and the detector and with means for subtractively combining the measurement signals supplied by the detector arrangement. An arrangement according to claim 6, comprising a polychromatic X-ray source and a detector arrangement for detecting at an angle of more than about 90 ^o scattered radiation,
characterized in that a filter arrangement is provided with at least one flat filter which can be displaced in at least two positions perpendicular to the beam path running between the X-ray emitter and the examination area, the filter being penetrated by the primary radiation in one position and the scattered radiation in the other position .

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