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Proposal of a Compact Repetitive Dichromatic X-ray Generator with Millisecond Duty Cycle for Medical Applications
M.V. Gorbunkova), V.G.Tunkin b),E.G. Bessonov a), R.M. Fechtchenko a), I.A. Artyukov a), Yu.V. Shabalin a), P.V. Kostryukov b), Yu.Ya. Maslova a), A.V. Poseryaev c), V.I. Shvedunov c), A.V. Vinogradov a), A.A. Mikhailichenko d), B.S. Ishkhanov c)
a)

P.N. Lebedev Physical Institute, 119991 Russia, Moscow Leninskii Prospect 53 Physics Department of Moscow State University, 119899 Russia, Mosco w, Vorobyevy Gory c) Nuclear Physics Institute of Moscow State University, 119899 Russia, Moscow, Vorobyevy Gory d) Cornell University, LEPP, Dryden Rd., Ithaca, NY 14850 *Corresponding author: vinograd@sci.lebedev.ru
b)

ABSTRACT
Main practical applications of X-rays lie in the important for the society fields of medical imaging, custom, transport inspection and security. Scientific applications besides of fundamental research include material sciences, biomicroscopy, and protein crystallography. Two types of X -ray sources dominate now: conventional tubes and electron accelerators equipped with insertion devices. The first are relatively cheap, robust, and compact but have low brightness and poorly controlled photon spectrum. The second generate low divergent beams with orders of magnitude higher brightness and well-controlled and tunable spectrum, but are very expensive and large in scale. So accelerator based Xray sources are mainly still used for scientific applications and X-ray tubes ­ in commercial equipment. The latter motivated by the importance for the society made an impressive progress during last decades mostly due to the fast developments of radiation detectors, computers and software used for image acquisition and processing. At the same time many important problems cannot be solved without radical improvement of the parameters of the X -ray beam that in commercial devices is still provided by conventional X -ray tubes. Therefore there is a quest now for a compact and relatively cheap source to generate X-ray beam with parameters and controllability approaching synchrotron radiation. Rapid developments of lasers and particle accelerators resulted in implementation of laser plasma X-ray sources and free electron lasers for various experiments requiring high intensity, shrt duration and monochromatic X-ray radiation. Further progress towards practical application is expected from the combination of laser and particle accelerator in a single unit for effic ient X-ray generation.

1. BASICS AND APPROACH
Thirty years development of laboratory X-ray lasers allowed to reach ~0.1 keV photon energy of coherent X-ray beams in a repetitive mode. Further scaling shows feasibility of ~0.3 keV coherent radiation in such type of devices. However their average power is still insufficient for many practical applications including medicine and inspection. Much higher power is expected to obtain in future free electron lasers. However according to existing projects their size and cost will prevent wide spreading of this kind of machines. In this project a compact repetitive dichromatic X -ray source (see Fig. 1a) based on novel laser and electron accelerator systems is proposed for medical applications. X-rays originate from Thomson scattering of counter propagating laser and electron beams. Such a "laser-accelerator" approach is very flexible in providing an X-ray beam with properties required by numerous medical applications. As a typical example, requiring a high power X -ray beam, coronary angiography is considered here, which is the leading method of imaging of coronary arteries. More than one million coronary angiography diagnostic procedures per year are applied in the US to evaluate the patient's conditions and choose the best heart treatment strategy. The method is not completely safe, however. Prior to X-ray exposure a portion of iodine contrast agent is injected with a catheter (inserted through a groin or an arm) directly into the coronary artery of interest. Therefore there is a risk of blood vessel damage and high radiation dose exposure for patient and doctor. To avoid the risk and make coronary artery imaging a

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routine screening procedure, several alternative noninvasive approaches have being developed [1], [2]. The most promising one uses the dichromatic synchrotron radiation with subsequent subtraction of images obtained with X-ray photon energies on two sides of iodine K-absorption edge. The first human test [3] and further development [4], [5] indicated that synchrotron radiation dichromatography provides better temporal and spatial resolution than all other noninvasive procedures and the patients accept it very well. However, the large scale and high cost of facilities generating synchrotron radiation (SR) prevent introducti on of this method into wide clinical practice (see [4] and [6]).

Fig.1a: The spectrum of dichromatic Thomson scattering x-ray generator as compared with the one of conventional X-ray source (solid line).

Fig.1b: Iodine absorption spectrum (solid line) and the spectrum of dichromatic Thomson scattering x-ray generator.

The X-ray generator based on Thomson scattering of laser beams on electron bunches can be compact and inexpensive as compared to synchrotron radiation sources. There is presently a n umber of laser -electron gamma and X-ray sources that are in operation or in the design stage [7]-[10]. They are designed for experiments in nuclear physics, generation of femtosecond X-ray pulses, medical imaging, protein crystallography etc. However, their average power, spectrum and time structure are still far from what is needed for K-absorption edge imaging of cardiac blood vessels.

2. PRINCIPAL SCHEME OF X-RAY GENERATOR. PHOTON FLUX AND BEAM TIME STRUCTURE
To meet the requirements of noninvasive dichromatic coronary angiography, we need an X-ray signal with specific energy and time structure repeating 30 times per second. Each X-ray pulse consists of two quasi-monochromatic halfpulses, peaked on short and long wavelength sides of the iodine K -absorption edge ­ 33.17 keV (see Fig. 1b). The time interval between pulses allows readout for imaging system. The total exposure time shorter than 4 ms is required to exclude blurring caused by biological motion [11]. The photon flux necessary for on -line imaging is ~2·1014 photon/sec [6]. We are proposing a new concept of compact Thomson scattering X-ray generator. The idea is schematically illustrated in Fig 2. X-ray generator contains acceleration systems, laser systems having two different wavelengths, optical circulator and laser-electron interaction chamber. An accelerator system consists of the linear accelerator, synchrotron or compact storage ring, electron beam transport lines and a beam dump. The linear accelerator injects a single bunch with ~1 nC charge into the synchrotron or storage ring with 30 Hz repetition rate. The laser-electron beam interaction occurs in the straight section of this synchrotron or storage ring. Used electron bunch, having energy spread and emittance increased, is ejected from the storage ring or synchrotron with the same 30 Hz rate.

3. ACCELERATOR SYSTEM
Three versions of the accelerator system will be (a) Low energy electron beam (~5 MeV) from a 35 or 50 MeV in ~1 ms. After that the beam is k the next stage the electron beam is decelerated this low energy and directed to the beam dump. considered: linac is injected in a synchrotron and then accelerated to the top energy ept at the top energy for ~ 4 ms, when laser radiation interacts with it. In down to the energy ~ 5 MeV (or less), ejected from the synchrotron at

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(b) The first version, but the energy of the linear accelerator and injection energy equal the top energy of the synchrotron 35 or 50 MeV. The ejection energy is 5 MeV or less. (c) Electron beam with 35 or 50 MeV from a linac is injected into a compact storage ring (having average radius ~0.5 m), kept there at fixed energy for ~ 5 ms, and then ejected from the ring at the same energy and directed to the beam dump. In all versions the repetition rate is 30 Hz.

Fig. 2: Dichromatic X-ray generator with millisecond duty cycle.

The main advantages of the system, which uses the synchrotron, are: low electron energy at the beam dump for the radiation safety and use of low energy inexpensive linac. The main advantage of the storage ring is the possibility to use less expensive magnets with fixed magnetic field (having DC power supplies or even using permanent magnets). In turn

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this allows utilization of well-adjustable optics, which practi cally excludes the beam losses inside the storage ring and, hence, brings the risk of patient/doctor irradiation to its minimum. All losses and radiation occur in the beam dump, which can be screened well. The final choice of accelerator system will be made later. The accelerator or damping ring is equipped with interaction chamber, located in the straight section. This chamber serves also as a part of optical circulator based on Pockels cell and a ring-type high Q resonator. Two successive specially formed trains of picosecond laser pulses arrive into the optical circulator (Fig. 2). The resulting laser flux at the laser­electron interaction point is also shown. The X -ray flux generated via Thomson scattering at every laser shot has the same time structure.

4. OPTICAL SOURCE OF X-RAY GENERATOR
For generation of the pulse trains desired we are proposing and planning to fabricate special laser system based on master oscillator and amplifiers. A new type of picosecond laser will be tested for optical source of x-ray generator in diode and lamp pumped versions. The laser is controlled by negative and positive feedbacks [12] with high -speed optoelectronics [13] and specially designed electro-optical modulator (EOM) capable to govern time structure of the generated radiation with sub-nanosecond accuracy. No saturable absorber or other nonlinear elements is needed for picosecond pulse generation in such a laser. The electro-optical control system can be used for active mode locking (AML) as well. Low-voltage modulators based on the transverse Pockels effect, which are usually made according to a bisectional thermally compensated design, offer an elegant solution for production of the needed laser radiation time structure. When the varying control voltages are a pplied to the sections of such a modulator, the phase shifts are added, i.e., the control voltages are summed in fact. Therefore a single intracavity modulator can be used for AML, Qswitching, external positive and negative feedbacks (PFB, NFB) [14] -[16]. For picosecond pulse generation, temporal behavior of losses introduced by modulator should have short spikes in transmission of several hundreds picoseconds duration. The voltage pulses formed in the NFB circuit should be sawtooth-shaped with a steep sub-nanosecond front and a decay time equal to ~ (1-2.5)Tr, where Tr is a laser cavity roundtrip time. The voltage pulses formed in the PFB circuit may be saw-tooth-shaped with a steep sub-nanosecond front and a decay time equal to ~ (0.5-1.5)Tr, as well as short pulses whose duration is determined by the photocurrent response of the high-voltage sub-nanosecond semiconductor structures. The version with short pulses PFB control experimentally realizes in lamp pumped Nd:YAG laser is schematically shown in Fig. 3.

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Fig.3. Positive and negative feedback loops controlled picosecond lamp pumped YAG-Nd laser. AM active laser medium; M1, M 2 cavity mirrors; P polarizer; D diaphragm; Mod n, Modp electrooptic modulator sections of negative and positive feedback loops; CCn, CCp positive and negative feedback control circuits; OD positive feedback optical delay line; F neutral density filter.

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Temporal behavior of the EOM voltages for PFB and NFB is presented in Fig. 4. The EOM was controlled dir ectly by the photocurrent, generated in specially designed high -voltage sub-nanosecond semiconductor structures. light pulses

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Fig.4. Temporal behavior of PFB (short pulse version) and NFB saw-tooth-shaped control voltages.

Time dependence of t he laser radiation governed by a combination of NFB and PFB in sub-millisecond and picosecond timescales is presented in Fig. 5a, 5b.

Fig.5a. Train of stabilized picosecond pulses.

Fig.5b. Laser pulse measured by streak-camera and its fit by

exp(- (2 ln 2 t / ) 2 ) pulse shape.

The second version is based on the saw-tooth-shaped PFB control voltage. The proper choice of NFB and PFB delay times makes it possible to realize quite a short EOM transmission spike as a result of the combined action of NFB and PFB. The short spike development in EOM transmission can be presented in this case as follows: the steep front of the PFB voltage increases EOM transmission; the steep front of the NFB voltage lowers it. If the delay times and sensitivities in the PFB and NFB circuits are optimized, a narrow spike in the modulator transmission is formed. For a quite low PFB voltage, such a spike does not prevent the NFB from playing its stabilizing role. As a result, temporal behaviour of the EOM transmission looks similar to the one in the first version, but with shorter spike. Spike shortening is especially pronounced in the case of ideal photo detector with instantaneous photocurrent response. The results of simulation of laser dynamics for the ideal photo detector are shown in Fig.6.

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Fig.7. Scheme of a picosecond diode -pumped laser controlled by a combination of NFB and PFB: AM ­ active medium, M1, M2, M3 ­ high reflector mirrors, M4 ­ output mirror, OD1, OD2 ­ optical delay lines, P ­ polarizer, EOM ­ low-voltage electro-optical modulator, BS ­ beam splitter.

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The final decision in favor of one version of PFB realization in master oscillator of X-ray generator light source will be made after experimental investigation of diode -pumped repetitive picosecond solid-state laser controlled by optoelectronic feedbacks. The scheme of this laser is shown in Fig.7.

5. SUMMARY AND DISCUSSION
The described TXRG meets the requirements mentioned above for noninvasive coronary angiography. Main comparative characteristics of TXRG are as follows: 1. The X-ray beam parameters required can be obtained with a compact accelerators and laser systems that are expected to be substantially cheaper than SR facilities used currently for medical research. 2. The use of a synchrotron solves radiation safety problems by ejecting electron bunches at low energy. 3. The short (~ 1 ms) interaction time between the laser radiation and electron beam leads to the considerable reduction of difficulties associated with emittance degradation due to intrabeam scattering. 4. Laser system with two different, but close wavelengths provides two X-ray pulses for K-absorption edge subtraction imaging in a simple way. 5. The use of two independent lasers makes it possible to compensate bunch emittance degradation, especially important for the second pulse. Thus the ratio of X-ray pulse energies can be optimized for the highest subtraction image contrast. 6. The use of repetitive lasers decreases average laser power by an order of magnitude compared with CW lasers considered in other projects. 7. The frequency of mJ scale picosecond pulses can be doubled with high efficiency and therefore offer a choice between 1st and 2nd laser harmonics and between electron energies (35 or 50 MeV respectively) to use better optical materials and different operational energy. 8. The optical circulator, in contrast to resonance super-cavity, does not require phase matching of picosecond laser pulses and avoids the usage of specially designed servo system for frequency stabilization. 9. Stable 1 ms trains of las er pulses are produced by a new type of repetitive lamp or diode-pumped all solid state ~1 µm-wavelength lasers (Nd or Yb doped crystals) governed by opto-electronical feedbacks. 10. The combined action of PFB and NFB stabilizes the laser radiation and decreas es pulse duration to a few tens picoseconds without saturable absorbers or other nonlinear elements.

ACKNOWLEDGEMENTS
We are grateful to N. A. Borisevich and V. A. Petukhov for fruitful discussions, to D.V. Yakovlev and L. S. Telegin for help in work. The work was partially supported by the Program of Fundamental Research of RAS, subprogram "Laser systems", RFBR grants 05-02-17162 and 05-02-17448a and ISTC project 1794.

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