EP0276437A1 - X-ray source - Google Patents

Abstract

A crystal (4) is provided to channel electrons or positrons of precisely defined energy. The emission of photons takes place on the basis of the effect that periodically accelerated charged particles emit electromagnetic radiation. The emitted photons are tuned to X-ray energies of desired magnitude by setting the electron or positron confinement energy. The generation of the electrons or positrons can be performed in an accelerator (2) via a collimator (3). A filter system (6) can be connected downstream of the crystal. <IMAGE>

Description

The present invention relates to an X-ray source for generating X-rays, which is suitable for medical diagnostic and therapeutic purposes as well as material-physical analyzes and examinations.

It is known to generate X-rays for medical diagnostic purposes in X-ray tubes by braking electrons in the field of the anode. The resulting brake radiation covers a range between 20 keV and 100 keV. The use of a conventional X-ray tube for medical purposes does not appear to be ideal, since only a part of the radiation generated can be used for the desired effect. Because of the continuous radiation spectrum, the irradiated object, be it for testing, analysis, diagnostic imaging or radiation therapy, is not only exposed to the desired energy range, but also to radiation components that are outside the desired range. During the examination, the dose supplied to the object also contains considerable doses of X-rays, which are actually undesirable. These disadvantages are also evident for therapeutic purposes.

For material-physical investigations and analyzes, finely tunable radiation sources in the X-ray range are of great importance in order to find out the type and concentration of the elements to be examined through targeted excitation of the characteristic X-rays. The use of a monochromatically tunable radiation source in the X-ray range is also very important for solid-state analysis, such as structure analysis of gases on metal surfaces or metals on semiconductors.

The invention has for its object to provide an X-ray source that emits monochromatic X-rays of high intensity and with a high degree of polarization.

This object is achieved in that a crystal for channeling electrons or positrons of defined energy is present, the radiation of photons due to the effect that periodically accelerated charged particles emit electromagnetic radiation, and wherein the emitted photons on X-ray energies of the desired size can be adjusted by adjusting the electron or positron injection energy.

In the X-ray source according to the invention, the effect of radiation generation by channeling charged particles in suitable crystals is used. The targeted generation of X-rays takes place in a narrow energy range, which is suitable for medical diagnostic and therapeutic purposes as well as for material-physical analyzes. The medical and technical personnel on the x-ray apparatus are spared, and massive and expensive shielding measures can be dispensed with. The X-ray source according to the invention is particularly suitable in connection with a suitable detector system for the preventive examination of cardiovascular diseases.

• a) Die Energie der Photonen ist durch Veränderung der Energie der auf den Kristall auftreffenden Elektronen (Positronen) leicht variierbar. Die resultierende Strahlung ist viel­seitig verwendbar für medizinisch diagnostiche und thera­peutische Anwendungen sowie Elementanalyse und festkörper­analytische Untersuchungen.
• b) Das Spektrum der Photonen ist sehr engbandig und vermeidet den Nachteil bisheriger konventioneller Röntgenstrahlen­quellen, die ein breitbandiges Spektrum ermittieren.
  
  Als Konsequenz kann der Patient bei medizinisch diagnosti­schen Anwendungen nur mit der für ihn relevanten Strah­lungsenergie bestrahlt werden. Dies führt somit im Gegen­satz zu konventionellen Strahlungsquellen zu einer Dosis­ersparnis.
• c) Die emittierte Strahlung wird gebündelt, d.h. in Vorwärts­richtung als winkelmäßig sehr begrenzte Strahlung emit­tiert. Dies kann in Zusammenhang mit Scanprozeduren be­deutsam sein (Vermeidung mechanischer Blenden und Ver­schlüsse).

In the context of the invention, the injection energy of the charged particles can be adjusted so that monochromatic X-rays are generated in the desired range for medical diagnostic and therapeutic or solid-state analysis purposes. This desired range is approximately between 5 keV and 250 keV. The generation of this monochromatic X-ray radiation is based on the channeling radiation which electrons or positrons emit when passing through crystals under channeling conditions. Taking advantage of this physical effect, three characteristic properties can be used.

• a) The energy of the photons can be easily varied by changing the energy of the electrons (positrons) hitting the crystal. The resulting radiation can be used in a variety of ways for medical diagnostic and therapeutic applications, as well as element analysis and solid-state analysis.
• b) The spectrum of the photons is very narrow-band and avoids the disadvantage of previous conventional X-ray sources, which determine a broad-band spectrum.
  
  As a consequence, in medical diagnostic applications, the patient can only be irradiated with the radiation energy relevant to him. In contrast to conventional radiation sources, this leads to dose savings.
• c) The emitted radiation is bundled, ie emitted in the forward direction as radiation which is very limited in terms of angle. This can be important in connection with scanning procedures (avoidance of mechanical shutters and closures).

Another important consideration is based on the fact that the resulting X-ray radiation is polarized. Polarized X-rays have never been used in medical radiology. This gives the opportunity to research these radiation properties. For industrial applications of polarized X-rays, the otherwise customary measures for polarization would be superfluous.

• Fig. 1a, 1b zwei Prinzipdarstellungen zur Erläuterung des Er­findungsgedankens,
• Fig. 2a, 2b zwei Varianten einer Röntgenstrahlenquelle nach der Erfindung,
• Fig. 3a, 3b Röntgendiagnostikanlagen mit Röntgenstrahlenquel­len entsprechend den Fig. 2a und 2b, und
• Fig. 4a, 4b und 4c Kurven zur Erläuterung des Erfindungsge­dankens.

The invention is explained below with reference to the drawing. Show it:

• 1a, 1b two schematic diagrams to explain the concept of the invention,
• 2a, 2b two variants of an X-ray source according to the invention,
• 3a, 3b X-ray diagnostic systems with X-ray sources corresponding to FIGS. 2a and 2b, and
• 4a, 4b and 4c curves for explaining the inventive concept.

The generation of monochromatic X-rays is based on an effect that has been intensively investigated in the field of basic physical research in recent years. The effect is based on the following idea: The spectral intensity distribution of the radiation emitted by a transversely accelerated charge (technical example: synchrotron system) depends crucially on the radius of curvature. If it is possible to replace the macroscopic dimensions that are necessary for the synchrotron with atomic distances, the possibility arises of shifting the photon spectrum towards comparatively higher energies and at the same time towards greater intensities - with otherwise unchanged parameters.

According to a suggestion by AM Kumakhov, Sov. Phys. JETP 45 (1977), page 791, this idea can be realized: electrons (positrons), ie charged particles that "channel" through a crystal, experience a periodically transverse acceleration and emit electromagnetic radiation as a result. The energy of the emitted radiation clearly depends on the energy of the channeling particles. The trajectories of the channeling particles can run through a specific selection of certain crystal symmetry planes or crystal symmetry axes in one plane or relative to an axis. In this case one speaks of planar channeling or axial channeling, as is shown schematically in FIGS. 1a and 1b. In both figures, B denotes the particle path and G denotes a crystal lattice. In both cases len channeling particles perform a bound, periodic movement transversely to an axis of symmetry of the crystal. In crystal physics, this movement is determined by the presence of a transverse crystal field potential. A quantum mechanical description of the channeling movement is necessary for particle energies (electrons or positrons in the MeV range). Solutions to this description are, as is usual in bound quantum mechanical problems, only certain discrete energy eigenvalues that correspond to stable configurations of the transverse components of the trajectories. Radiation transitions with the emission of electromagnetic radiation can take place between these discrete energy levels. As a result of the relativistic increase in mass of the electrons or positrons moving in the longitudinal direction almost at the speed of light (parallel to the crystal symmetry axis), the emitted photons undergo a Doppler shift.

hierbei ist
ϑ die Richtung des ausgesandten Photons relativ zur Teil­chenrichtung,
ℏc das Produkt von Plankschem Wirkungsquantum mal Lichtge­schwindigkeit dividiert durch zwei mal π
V₀ ist die Potentialstärke des gemittelten transversalen Kri­stallfeldpotentiales bei einem harmonischen Potentialansatz, m₀c² ist die Ruheenergie des Elektrons (Positrons).The energy ℏw of the resulting strongly forward-directed radiation depends on the crystal field strengths and on the speed β = v / c with which the electrons (positrons) travel through the crystal.

here is
ϑ the direction of the emitted photon relative to the particle direction,
ℏc the product of Plank's quantum of action times the speed of light divided by two times π
V₀ is the potential strength of the averaged transverse crystal field potential with a harmonic potential approach, m₀c² is the rest energy of the electron (positron).

For the forward angle (ϑ ≅ o) the dependence of the radiation energy on the particle injection energy applies.
ℏ ≅ 2 hw₀γ³ / ² with γ = (1-β²) ⁻ ^½ = E / m₀c².

Here E denotes the particle injection energy.

Table 1 shows the relationship between photon energy and particle energy for a crystal Si that has been used frequently in basic research and for the potentially interesting crystal tungsten:

Within the scope of these specifications, X-rays from 0.5 keV to approximately 250 keV are ideally suited for medical diagnostic and therapeutic applications. In particular, the range from 20 keV to 100 keV is particularly interesting for diagnostic applications. The value of 40 keV is typical for standard applications such as thorax applications. In radiation therapy, the range from 10 keV to 30 keV is particularly relevant for tumor application or photon activation therapy. Treatment of deeper tumors requires energies above 30 keV to 250 keV. For elemental analysis X-ray examinations mean that the range between 0.5 keV and approximately 100 keV is important. The numerical values shown here should above all be seen as examples. Of course, new developments in medicine do not exclude an expansion of the areas.

• a) Leichte Einstellbarkeit der Photonenenergie durch Vari­ation der Elektronen(Positronen)-Einschußenergie.
• b) Monochromasie des Röntgenspektrums, das dosissparende An­wendung finden kann. Besonders von Vorteil ist die Anpas­sung der Röntgenenergie an die medizinische Fragestellung (Objektanpassung) und die Möglichkeit der Anpassung der Röntgenstrahlung an das Röntgenkonvertersystem.
• c) Die spektrale Intensitätsverteilung ist den herkömmlichen Röntgenstrahlungsquellen überlegen.

The described generation of channeling radiation distinguishes three characteristic properties:

• a) Easy adjustability of the photon energy by varying the electron (positron) shot energy.
• b) Monochromaticity of the X-ray spectrum, which can be used in a dose-saving manner. The adaptation of the x-ray energy to the medical question (object adaptation) and the possibility of adapting the x-ray radiation to the x-ray converter system are particularly advantageous.
• c) The spectral intensity distribution is superior to conventional X-ray sources.

A system is schematically shown in FIG. 2a, with which X-rays can be generated using the channeling radiation method for medical diagnostic and therapeutic applications and material-physical examinations. This system contains an electron (positron) source 1 for continuous or pulsed operation in combination with the electron (positron) accelerator 2. One of the most important main requirements for the accelerator is the provision of electrons (positrons) of certain energy and certain current strength. A beam guidance system focuses the particle beam S within the so-called critical channeling angle through a collimator 3 on the crystal 4. The crystal 4 is attached to a goniometer in order to enable the specific adjustment of certain crystal symmetry axes. In order to select the emitted X-rays R from the particle beam S, there is a magnet 5 after the crystal 4 which deflected charged particle beam S from the X-ray radiation R emitted in the forward direction and leads to a beam catcher 8. With the help of an additional monochromator system 6, the braking radiation background contained in the photon radiation can be reduced before the radiation reaches the target 7 (for example an X-ray film).

The requirements placed on the accelerator 2 are that the beam quality is good and the energy blur is as small as possible. The energy range of the accelerator should be between 10 MeV and 100 MeV; the exact data depend on the desired photon energy and the type of crystal used. The electron (positron) current can be comparatively low in order to still enable high photon flows. On average, currents between 20 µA and 1000 µA must be used. Accelerator 2, shown in Fig. 2a, is a microtron that functions so that the electrons move along circular paths with increasing radii with increasing energy. However, all railroad circles touch at a common point. Suitable microtrons are sold, for example, by Skanditronix with ring diameters between 150 cm to 200 cm. The electron beam is pulsed with pulse lengths of 2 µm to 10 µs and pulse pauses of about 20 ms. The peak current is around 100 mA. Of course, newer developments based on superconducting technology are also well suited.

However, the accelerator 2 can also be arranged in the form of a compact storage ring, as shown in FIG. 2b. Here the electrons from the microtron are fed into the ring along the straight intermediate piece, which works by means of the magnets M1 and M2 in the so-called "race track" principle. This is to ensure that a certain electron current flows at a fixed energy along the ring for a very long time (order of magnitude: one hour), one High-frequency unit H1 replaces loss energy for the electrons, which suffer attenuation losses (eg in the crystal) when circulating. Position-sensitive detectors, which are not shown here, serve as position monitors to maintain the desired beam quality.

As a result of the important boundary condition that particles only channel if they hit within a critical angle φ _cri ∼γ⁻ ^1/2 relative to the crystal symmetry axis, high demands are placed on the particle beam quality. Since each particle beam also has a finite emittance, 3 suitable focusing elements such as magnets or electronic diaphragms are provided as collimators, which ensure that the beam divergence of the particle beam when it hits the crystal is as small as possible. Beam divergences below 0.5 mrad are aimed for.

Due to the mechanism for generating radiation, the time structure of the electron (positron) beam determines the time structure of the electromagnetic radiation emitted. By specifically selecting certain pulse sequences of the particle beam, these can be converted into corresponding photon pulses.

The crystal 4 is located on a goniometer in order to ensure a precise adjustment of the crystal axes relative to the particle beam S. The goniometer should be controlled using a computer if possible. The accuracy of the angle setting should be 0.01 ° for both angles.

Suitable crystals for the generation of channeling radiation are crystals with the highest possible Debye temperature in order to keep thermally excitable lattice vibrations, which lead to line broadening effects, as low as possible. In this aspect, the diamond crystal appears with its well-known Crystal structure and the Si crystal well suited. The latter has the great advantage that it is available in high quality due to the high purity requirements in microelectronics. Heavier systems such as germanium, nickel and gold are also suitable; The high atomic number is particularly noticeable here, which leads to higher crystal field strengths and thus allows the use of low particle energies for comparatively identical photon energies. Tungsten crystals are very cheap due to their very high atomic number. Binary systems such as LiF, MgF₂, BN and BeO, which have already been suitable for basic physical studies, are also suitable for use. The following table 2 shows a rough overview for the production of 50 keV photons with a crystal thickness of 20 µm and an average electron current of 10 µA:

After the goniometer with the crystal system, a deflection magnet 5 must be attached according to FIGS. 2a and 3a in order to select the particle beam from the photon beam emitted in the forward direction.

A filter system 6 (FIGS. 2a, 2b) in the form of a double monochromator serves to improve the monochromatic ratio Δ E / E of the quasi-monochromatic photon line of the channeling radiation. For the above crystals be E / E carries about 0.1. Another important function is the reduction of the accompanying brake radiation background due to dechanneling effects when the electrons pass through the crystal. The reasons for this background are a less than perfect focusing of the beam within the critical channeling angle as well as imperfections or impurities (lattice defects) in the crystal. Technical concepts for the construction of monochromators can be found in Proceedings of the Int. Conf. on X-ray and VUV Synchrotron Radiation Instrumentation, Stanford University, CA, in Nucl. Instr. and Meth. A246 (1986), pages 297 to 309, and 365 to 376, the so-called grazing incidence mirror arrangements being particularly recommended.

The electromagnetic radiation generated in this way strikes the target 7. Any object, living or inanimate, can be placed in front of the target 7. This object can be any object to be tested or analyzed, or a patient who is being X-rayed or being X-rayed. The target 7 can either be an x-ray film or a location-sensitive receiver for x-rays, to name just a few possibilities.

3a and 3b show the overall system and function of the components corresponding to FIGS. 2a and 2b. A control unit 9 contains a number of control modules which are electrically connected to the various components of the system. For example, module 10 supplies the electrical supply to accelerator 2. Module 11 supplies the magnets and focusing elements electrically, modules 12, 13 are responsible for the goniometer control, crystal monochomator control and vacuum system 15. In the case of FIG. 3b, module 11 also supplies the high-frequency unit H1 with electrical energy. The systems on which FIGS. 3a, 3b are based are dimensioned such that they can be accommodated comfortably in a hospital or an industrial laboratory. The vacuum system 15 consisting of a pump system and vacuum Safe supply lines and housing structures ensure a sufficient vacuum in the area of the electron beam.

The relatively high directional concentration of the channeling radiation is suitable for guiding the beam in scan mode without the use of aperture systems, as is common with conventional X-ray tubes. The position of the highly focused or directed beam is precisely known, in contrast to the broad beam profile in conventional X-ray emitters.

The emitted X-rays from a conventional X-ray tube have a wide, continuous energy spectrum. As shown in Fig. 4a, one sees a continuum of frequencies together with the so-called characteristic lines at certain frequencies or energies. A similarly wide continuous energy spectrum without characteristic lines, as shown in FIG. 4b, provides the spectral intensity distribution of a conventional synchrotron. In contrast, the X-rays generated with channeling show a narrow frequency band. The typical intensity distribution shown in FIG. 4c is very narrow for the discrete lines; typically less than 10% without the use of filtering measures. Thus, a patient is largely only exposed to the desired spectral intensity. In FIGS. 4a, 4b, 4c, the number of photons per second is to be plotted on the ordinate and the X-ray energy in keV is to be plotted on the abscissa.

With the help of the easily adjustable energy of the photons, it is possible to carry out so-called multi-spectral methods in radiology. With a suitable choice of crystals, it is possible to simultaneously generate quasi-monochromatic X-rays with 40 keV to 50 keV or 90 keV to 100 keV. These different energies enable the simultaneous display of bone and soft tissue despite their different absorption properties. The feasibility of the K-edge subtraction method in angiography, in which images below and be made above the energy of the K absorption edge of the contrast medium in order to increase the contrast of the objects to be recorded with a minimum of radiation dose.

This can be achieved in particular by choosing a BN or BeO crystal by generating two quasi-monochromatic X-ray lines, each above or below the K-edge energy of the contrast agent iodine (E _K ≈ 3 keV) by special adjustment of the electron energy.

Subtracting the two lines after radiating the object and detecting it in a detector (e.g. NaJ (Th) or Si detector) results in a large signal difference in the case of iodine, and practically a vanishing signal difference in the case of soft tissues and bones if in front of the object the intensities of the two lines have been chosen to be the same.

The aspect of the easy adjustability of the X-ray energy is advantageous, since one and the same radiation source can be used in diagnostics and therapy.

Another important possibility is the generation of polarized radiation. There are a number of medical, industrial and trace analysis applications in which polarized radiation is important.

Trace elements can be detected in vivo and in vitro due to their characteristic X-rays. In vivo measurements become even more sensitive when polarized X-rays are used, since scattered radiation that arises in thick samples can then be eliminated. The qualitative and quantitative detection of various elements in the body can be carried out by suitable adjustment of the X-ray energies. For example, elements such as arsenic, mercury, cadmium and other heavy metals can be detected using an excitation source.

The X-rays that can be generated are highly forward-focused, narrow-band, monochromatic, suitable for scanning applications, and polarized. All standard applications of X-ray diagnostics, e.g. in the thoracic area, in mammography, pediatrics etc. can be done. The applicability of K-edge subtraction angiography is particularly important in angiography. In cancer research, certain elements such as Iodine, which preferentially accumulates in certain cancer cells, can be selectively irradiated through the targeted adjustability of the photon energy.

Computed tomography systems are known to produce a picture of the three-dimensional electron density distribution in medicine or industrial non-destructive testing. These systems require the transmission of X-rays from different angles through the examination object. If this is realized by rotating the X-ray tube around the examination object, there are longer times per scan. By means of the invention, the Compton-scattered X-rays can be measured in the examination object after excitation with scan beams of certain set monochromatic energies. This makes it possible to reconstruct the three-dimensional distribution of the electron density in the examination object without moving the patient or the radiation source. The use of different energies also allows imaging tailored to the different body tissues, as is known from standard radiography.

A system for generating quasi-monochromatic X-rays of variable energy can work as follows: With electrons in the energy range between 10 MeV and 60 MeV that channel through an 18 µm thick diamond crystal in (110) mode, quasi-monochromatic photons between 20 keV and 130 keV can be generated . With an average electron current of I = 20 µA and a beam difference of 0.5 mrad FWHM at the crystal, the photon rate would be move between 1 to 3 × 10¹⁰ quanta / s (see table 3). The radiation is strongly bundled forward under the solid angle Δ Ω = π / γ².

With Si crystals, mean electron currents of at least 100 µA can be used without the crystal becoming unusable due to the thermally induced lattice defect formation and the radiation spectrum becoming unstable over time. When using tungsten crystals, even higher currents can be used. With the beam guidance system of the electron accelerators in question that can be implemented today, beam divergences with 0.2 mrad FWHM can be achieved. These would increase the photon fluxes in Table 4 by 1 to 2 orders of magnitude.

Angiographic applications, which are supposed to represent the coronary arteries or blood vessels with the aid of peripheral contrast agent injection with the aid of the two-spectra subtraction, would be a significant advance since they represent a simplification of the examination (risk reduction due to elimination of the catheter guidance procedure). This would also result in an increase in patient throughput. As described in US Pat. No. 4,432,370, a K-edge subtraction method is proposed which makes the monochromatic X-radiation required for this available with the aid of a synchrotron radiation source and a subsequent crystal double monochromator arrangement. This synchrotron radiation source can be replaced by the technically far simpler and cheaper channeling radiation source, which must be matched by the choice of the electron energies in such a way that the radiation energies matched to the contrast medium can be adjusted. By choosing a BN or BeO crystal, the two X-ray lines above and below 3 keV (contrast medium iodine) can be generated simultaneously. This eliminates the time-consuming process of energy variation necessary for the synchrotron. For the representation of thinner vessels with about 0.5 mm φ, a photon flux of about 10 quanta / cm²s is required in the monochromatic line at 31 keV. This value results from the required image dose of approximately 200 µR for lumen display with 0.5 mm φ with a signal-to-noise ratio of more than 100. Since the subtraction image is to be created within one cardiac cycle, the exposure time must be approximately 15 ms at most.

With a channeling radiation source and a tungsten crystal, a photon flux of 10 11 quanta / cm 2 s can be generated on a 30 keV X-ray line if the electron energy is approximately γ = 30 and the average current is 100 μA. This flow of photons is of the desired order. At a crystal detector distance of 2 m, the radiation illuminates a partial area of Δ F = 130 cm². This is still too small to represent the entire coronary system satisfactorily. The desired, larger diagnostically relevant area can, however, be detected by extensive scanning.

Claims (4)

1. X-ray source, characterized in that a crystal (4) for channeling electrons or positrons of defined energy is present, the radiation of photons due to the effect that periodically accelerated charged particles emit electromagnetic radiation, and wherein the emitted photons on X-ray energies desired size can be adjusted by setting the electron or positron injection energy. 2. X-ray source according to claim 1, characterized in that for generating an electron or positron current of the desired energy, a system with an accelerator (2) is provided, in which the electrons or positrons revolve in a closed cycle in which the channeling crystal is introduced. 3. X-ray source according to claim 1 or 2, characterized in that the injection energy is set so that X-rays are generated in the energy range between 0.5 keV and 250 keV. 4. X-ray source according to one of claims 1 to 3, characterized by the application in an imaging X-ray device, for example in the embodiment of an X-ray fan beam scanner.

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