Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
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, Moscow, 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
5919-31 V. 4 (p.1 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
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 introduction 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 number 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 considered:
(a) Low energy electron beam (~5 MeV) from a linac is injected in a synchrotron and then accelerated to the top energy
35 or 50 MeV in ~1 ms. After that the beam is kept at the top energy for ~ 4 ms, when laser radiation interacts with it. In
the next stage the electron beam is decelerated down to the energy ~ 5 MeV (or less), ejected from the synchrotron at
this low energy and directed to the beam dump.
5919-31 V. 4 (p.2 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
(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.
Beam
dump
KICK
e
INF
e
5; 35; 50 MeV LINAC
IP
R>99.9%
X-rays
RFC
R>99.9%
t
T = 2 µs
τ
< 40 ps
R>99.9% Pockels
cell 1
P 1 R>99.9%
Pockels
cell 2
Tc = 10 ns
1 ms
<1 ms 1.5-2 ms
λ1
τ
λ2
λ2
P2
t
λ1
< 40 ps
T=2µ
s
T=2µ
s
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
5919-31 V. 4 (p.3 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
this allows utilization of well-adjustable optics, which practically 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
laser–electron 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 applied 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.
Modp
M1
Modn
D
M2
P
AM
CCp
CCn
F
OD
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.
5919-31 V. 4 (p.4 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
Temporal behavior of the EOM voltages for PFB and NFB is presented in Fig. 4. The EOM was controlled directly by
the photocurrent, generated in specially designed high-voltage sub-nanosecond semiconductor structures.
light pulses
PhEn
CCn
Modp
M1
Temporal behavior of the
voltage on the modulator
Modp , generated by
positive feedback
electronic control circuit
CCp.
Modn
Temporal behavior of the voltage on
the modulator Modn , generated by
negative feedback electronic control
circuit CCn .
Time scale: 5 ns
CCp
PhEp
light pulses
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 / τ ) 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.
5919-31 V. 4 (p.5 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
8
6
1
4
2
1,0
2
Photocurrent
V o lta g e , U o
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
0
0,0
0 ,6
0 ,2
3
0 ,0
L a se r p u lse s
1000
1500
Time, ps
2000
0 ,4
L as er p u lse
T ra n s m issio n
0 ,8
5
0,5
1 ,0
6
0 ,5
1
0 ,0
860
880
900
T im e , p s
920
4
0
0 ,0
0 ,5
1 ,0
1 ,5
2 ,0
T im e n o r m aliz e d to T r
Fig.6. Dynamics of a laser controlled by EOM with saw-tooth-shaped NFB and PFB temporally shifted
voltages. (1) NFB control voltage and (2) PFB control voltage, Uo – modulator half wavelength voltage, (3)
modulator transmission, (4) laser intensity, (5) photocurrent, (6) laser pulse (16 ps duration).
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.
5919-31 V. 4 (p.6 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
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 laser 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.
REFERENCES
1.
2.
3.
4.
5.
6.
K. Nieman, R.-J.M. van Geuns, P. Wielopolski, P.M.T. Pattynama, P.J. de Feyter, Noninvasive Coronary Imaging
in the New Millenium: “A Comparison of Computed Tomography and Magnetic Resonance Technique”, Reviews
in Cardiovascular Medicine, v.3, No 2, 273-84, 2002.
S. Achenbach, W.G. Daniel, “Noninvasive Coronary Angiography – An Acceptable Alternative?”, N. Engl. J. Med,
v. 345, No 26, 1909-1910, 2001.
E. Rubenstein, R. Hofstadter, H.D. Zeman, A.C. Thompson, J.N. Otis, G.S. Brown, J.C. Jiacomini, H.J. Gordon,
R.S. Kernoff, D.C. Harrison, W. Thomlinson, “Transvenous coronary angiography in humans using synchrotron
radiation”, Proc. Natl Acad. Sci. USA, 83: 9724-9728, 1986.
T. Dill, W.-R. Dix, C.W.Hamm, M. Jung, W. Kupper, M. Lohmann, B. Reime, R. Ventura, “Intravenous Coronary
Angiography: Experience in 276 patients”, Synchrotron Radiation News, v.11 No 2, 12-20, 1998.
W.-R. Dix, W.Kupper, T. Dill, C.W. Hamm, H. Job, M. Lohmann, B. Reime, R. Ventura, “Comparison of
intravenous coronary angiography using synchrotron radiation with selective coronary angiography”, J. of
Synchrotron Radiation, v. 10, No 3, 219-227, 2003.
M.Ando and C.Uyama (Eds.), Medical application of synchrotron radiation Springer-Verlag, 1998.
5919-31 V. 4 (p.7 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes:
Please verify that (1) all pages are present, (2) all figures are acceptable, (3) all fonts and special characters are correct, and (4) all text and figures fit within the
margin lines shown on this review document. Return to your MySPIE ToDo list and approve or disapprove this submission.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
W. J. Brown, S. G. Anderson, C. P. J. Barty, S. M. Betts, R. Booth, J. K. Crane, R. R. Cross, D. N. Fittinghoff, D. J.
Gibson, F. V. Hartemann, E. P. Hartouni, J. Kuba, G. P. Le Sage, D. R. Slaughter, A. M. Tremaine, A. J. Wootton,
P. T. Springer and J. B. Rosenzweig, “Experimental characterization of an ultra fast Thomson scattering X-ray
source with three-dimensional time and frequency-domain analysis”, Physical Review Special Topics Accelerators and Beams, volume 7, 060702, 1- 12, 2004.
M.Venturini, R.Warnock, R.Ruth, J.A.Ellison, "Coherent Synchrotron Radiation and Bunch Stability in a Compact
Storage Ring", Phys. Rev. Special Topics - Accelerators and Beams, 8, 014202, 1-15, 2005
Ch.X. Tang, W.H. Huang, H.B. Chen, C. Cheng, Y. Cheng, Q. Du, T.B. Du, Y.Ch. Du, X.Z. He, J.F. Hua, G.
Huang, Y.Ch. Ge, Y.Zh. Lin, B. Xia, M.J Xu, X.D. Yuan, Sh.X. Zheng, “Researches of Thomson Scattering X-ray
Source at Tsinghua University”, Proceedings of the 2004 FEL Conference, 622-624.
F.Carroll, “Tunable, Monochromatic X-Rays: An Enabling Technology for Molecular/Cellular Imaging and
Therapy”, Journal of Cellular Biochemistry 90:502–508, 2003.
A.M. Babunashvili, V.A.Ivanov, S.A.Biryukov, “Stenting of Coronary Arteries”, ACB, Moscow, 2001.
Gorbunkov M.V. “Method of ultra short light pulses generation“. Patent RF No. 2056684 (priority date
29.10.1993).
M.V. Gorbunkov, Yu.V. Shabalin. “Method of laser radiation stabilization” Patent RF No. 2163412 (priority date
22.07.1999).
M.V. Gorbunkov, Yu.V. Shabalin. “Two-Loop Feedback Controlled Laser: New Possibilities For Ultrashort Pulses
Generation And High-Level Stabilization.” Proc. SPIE, Vol. 4751, p. 463 (2002).
M.V. Gorbunkov, V.B. Morozov, A.N. Olenin , L.S. Telegin, V.G. Tunkin, Yu.V. Shabalin, D.V. Yakovlev, “Laser
with intracavity control of radiation.“ Patent RF No. 2240635 (priority date 20.08.2003).
M.V. Gorbunkov, A.V. Konyashkin, P.V. Kostryukov, V.B. Morozov, A.N. Olenin, V.A. Rusov, L.S. Telegin,
V.G. Tunkin, Yu.V. Shabalin, D.V. Yakovlev “Pulsed-diode-pumped, all-solid-state, electro-optically controlled
picosecond Nd: YAG lasers.“ Quantum Electron, 35, (1), 2005, p.2.
5919-31 V. 4 (p.8 of 8) / Color: No / Format: A4 / Date: 8/10/2005 6:40:35 AM
SPIE USE: ____ DB Check, ____ Prod Check, Notes: