Research towards high-repetition rate laser-driven X-ray sources for imaging applications J. G¨ otzfried, 1 A. D¨ opp, 1 M. Gilljohann, 1 H. Ding, 1 S. Schindler, 1 J. Wenz, 1 L. Hehn, 2 F. Pfeiffer, 2, 3 and S. Karsch 1, * 1 Ludwig-Maximilians-Universit¨ at M¨ unchen, Am Coulombwall 1, 85748 Garching, Germany 2 Lehrstuhl f¨ ur Biomedizinische Physik, Physik-Department & Munich School of BioEngineering, Technische Universit¨ at M¨ unchen, 85748 Garching, Germany 3 Institut f¨ ur Diagnostische und Interventionelle Radiographie, Klinikum rechts der Isar, Technische Universit¨at M¨ unchen, 81675 M¨ unchen, Germany Laser wakefield acceleration of electrons represents a basis for several types of novel X-ray sources based on Thomson scattering or betatron radiation. The latter provides a high photon flux and a small source size, both being prerequisites for high-quality X-ray imaging. Furthermore, proof- of-principle experiments have demonstrated its application for tomographic imaging. So far this required several hours of acquisition time for a complete tomographic dataset. Based on improve- ments to the laser system, detectors and reconstruction algorithms, we were able to reduce this time for a full tomographic scan to 3 minutes. In this paper, we discuss these results and give a prospect to future imaging systems. Laser-driven X-ray sources take the middle ground in brilliance and cost between low-cost microfocus X-ray tubes and large-scale synchrotron sources. This applies especially to sources based on laser-wakefield accelera- tion [1], which allow for the production of collimated, femtosecond X-ray beams [2]. In particular, Thomson backscattering and betatron radiation have proven to be the most relevant for applications in the hard X- ray regime [3]. While the emission of radiation in both mechanisms is based on oscillatory motions of relativistic electrons, their oscillation frequencies differ significantly. Thomson backscattering sources rely on electrons oscil- lating in the electromagnetic field of an intense colliding laser pulse, whereas betatron radiation is generated by electrons wiggling transversely in the wake of a highly intense laser pulse traveling through a plasma while be- ing accelerated [2]. Transverse electric fields of several GV/m to TV/m force the accelerated electrons with ini- tial transverse momentum onto oscillating trajectories. This wiggler-like movement leads to a broadband X-ray emission, while the duration of the X-ray pulses is on the order of femtoseconds [4]. Laser-driven sources are there- fore particularly suitable to study ultrafast processes like transitions in the X-ray absorption near edge structures [5]. However, one of the most important medical appli- cations of X-rays remains radiography. Single-shot X-ray imaging has been shown with both Thomson and beta- tron sources [6, 7] and tomographic imaging has been demonstrated using betatron sources [8, 9]. Moreover, the intrinsic small source size of a few microns allows for phase contrast imaging [10, 11]. While previous re- search focused on the potential of this imaging method, the acquisition time for a tomography in these studies has been on the order of several hours [8, 9]. Here we focus on the duration of such scans and report on a successful reduction of this time to a more application-relevant few minute scale. This was achieved by upgrading the experi- mental setup to support 1 Hz repetition rates and making use of advanced reconstruction algorithms. The latter al- lows to acquire a single image per projection angle and perform a consistent reconstruction despite shot-to-shot X-ray flux fluctuations of the source. As a result, the data acquisition time for a centimeter-scale human bone sample was reduced from several hours to 180 seconds [12]. The measurements were performed at the Labora- tory for Extreme Photonics in Garching, Germany. The 800 nm, 27 fs laser pulse was delivered by the ATLAS (Ti:sapphire) laser system. An off-axis parabolic mirror (f/25) focuses the laser pulse containing an energy at the target position of 1.9 ± 0.1 J to a spot size of 30 μm (FWHM intensity), which corresponds to a peak inten- sity of 5.5 × 10 18 W/cm 2 and a peak power of 70 ± 4 TW resulting in a 0 ≈ 1.6. As target a gas cell filled with hydrogen at a density of ∼ 5 × 10 18 cm -3 was used in which a movable piston allowed to adjust its length anywhere between 5 and 15 mm. The cell length was optimized with respect to the X-ray yield exploiting the fact that deep in the electron dephasing regime the electrons perform more betatron oscillations. The optimum X-ray yield was found at a cell length of 11 mm. The accelerated electrons are deflected onto a scintil- lating screen by a 0.8 m long 0.85 T dipole magnet of an electron spectrometer. We achieved an electron beam charge of 736 ± 51 pC with a pointing fluctuation of 1.1 ± 0.1 mrad. The X-rays are detected by a scintil- lator based camera located 4.35 m behind the gas cell exit. An array of aluminum filters of different thick- nesses ranging from 5 μm to 610 μm can be inserted into the X-ray beam. The different transmission coeffi- cients for each filter allow the determination of the X-ray spectrum by iteratively optimizing its calculated filter transmissions to the ones measured [13]. The light shield and the Kapton vacuum window (cf. Fig. 1) are opaque for low energetic X-rays (> 50 % transmission for ener- arXiv:1803.05415v1 [physics.med-ph] 14 Mar 2018
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arXiv:1803.05415v1 [physics.med-ph] 14 Mar 2018based on Thomson scattering or betatron radiation. The latter provides a high photon ux and a small source size, both being prerequisites
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Research towards high-repetition rate laser-driven X-ray sources for imagingapplications
J. Gotzfried,1 A. Dopp,1 M. Gilljohann,1 H. Ding,1 S. Schindler,1 J. Wenz,1 L. Hehn,2 F. Pfeiffer,2, 3 and S. Karsch1, ∗
1Ludwig-Maximilians-Universitat Munchen, Am Coulombwall 1, 85748 Garching, Germany2Lehrstuhl fur Biomedizinische Physik, Physik-Department & Munich School of BioEngineering,
Klinikum rechts der Isar, Technische Universitat Munchen, 81675 Munchen, Germany
Laser wakefield acceleration of electrons represents a basis for several types of novel X-ray sourcesbased on Thomson scattering or betatron radiation. The latter provides a high photon flux anda small source size, both being prerequisites for high-quality X-ray imaging. Furthermore, proof-of-principle experiments have demonstrated its application for tomographic imaging. So far thisrequired several hours of acquisition time for a complete tomographic dataset. Based on improve-ments to the laser system, detectors and reconstruction algorithms, we were able to reduce this timefor a full tomographic scan to 3 minutes. In this paper, we discuss these results and give a prospectto future imaging systems.
Laser-driven X-ray sources take the middle ground inbrilliance and cost between low-cost microfocus X-raytubes and large-scale synchrotron sources. This appliesespecially to sources based on laser-wakefield accelera-tion [1], which allow for the production of collimated,femtosecond X-ray beams [2]. In particular, Thomsonbackscattering and betatron radiation have proven tobe the most relevant for applications in the hard X-ray regime [3]. While the emission of radiation in bothmechanisms is based on oscillatory motions of relativisticelectrons, their oscillation frequencies differ significantly.Thomson backscattering sources rely on electrons oscil-lating in the electromagnetic field of an intense collidinglaser pulse, whereas betatron radiation is generated byelectrons wiggling transversely in the wake of a highlyintense laser pulse traveling through a plasma while be-ing accelerated [2]. Transverse electric fields of severalGV/m to TV/m force the accelerated electrons with ini-tial transverse momentum onto oscillating trajectories.This wiggler-like movement leads to a broadband X-rayemission, while the duration of the X-ray pulses is on theorder of femtoseconds [4]. Laser-driven sources are there-fore particularly suitable to study ultrafast processes liketransitions in the X-ray absorption near edge structures[5]. However, one of the most important medical appli-cations of X-rays remains radiography. Single-shot X-rayimaging has been shown with both Thomson and beta-tron sources [6, 7] and tomographic imaging has beendemonstrated using betatron sources [8, 9]. Moreover,the intrinsic small source size of a few microns allowsfor phase contrast imaging [10, 11]. While previous re-search focused on the potential of this imaging method,the acquisition time for a tomography in these studies hasbeen on the order of several hours [8, 9]. Here we focuson the duration of such scans and report on a successfulreduction of this time to a more application-relevant fewminute scale. This was achieved by upgrading the experi-mental setup to support 1 Hz repetition rates and making
use of advanced reconstruction algorithms. The latter al-lows to acquire a single image per projection angle andperform a consistent reconstruction despite shot-to-shotX-ray flux fluctuations of the source. As a result, thedata acquisition time for a centimeter-scale human bonesample was reduced from several hours to 180 seconds[12].
The measurements were performed at the Labora-tory for Extreme Photonics in Garching, Germany. The800 nm, 27 fs laser pulse was delivered by the ATLAS(Ti:sapphire) laser system. An off-axis parabolic mirror(f/25) focuses the laser pulse containing an energy at thetarget position of 1.9 ± 0.1 J to a spot size of 30 µm(FWHM intensity), which corresponds to a peak inten-sity of 5.5× 1018W/cm2 and a peak power of 70± 4 TWresulting in a0 ≈ 1.6.
As target a gas cell filled with hydrogen at a densityof ∼ 5× 1018 cm−3 was used in which a movable pistonallowed to adjust its length anywhere between 5 and 15mm. The cell length was optimized with respect to theX-ray yield exploiting the fact that deep in the electrondephasing regime the electrons perform more betatronoscillations. The optimum X-ray yield was found at acell length of 11 mm.
The accelerated electrons are deflected onto a scintil-lating screen by a 0.8 m long 0.85 T dipole magnet of anelectron spectrometer. We achieved an electron beamcharge of 736 ± 51 pC with a pointing fluctuation of1.1 ± 0.1 mrad. The X-rays are detected by a scintil-lator based camera located 4.35 m behind the gas cellexit. An array of aluminum filters of different thick-nesses ranging from 5 µm to 610 µm can be insertedinto the X-ray beam. The different transmission coeffi-cients for each filter allow the determination of the X-rayspectrum by iteratively optimizing its calculated filtertransmissions to the ones measured [13]. The light shieldand the Kapton vacuum window (cf. Fig. 1) are opaquefor low energetic X-rays (> 50 % transmission for ener-
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FIG. 1. Experimental Setup for a tomography of a human bone sample at 1 Hz repetition rate. The laser pulse is focused into agas cell where it ionizes hydrogen gas and drives a plasma wave. The resulting charge separation generates large electromagneticfields. These accelerate electrons longitudinally while wiggling them transversely, which leads to betatron X-ray emission. Thebone sample is protected from the laser light by a 15 µm-thick aluminum foil which is passed by the electrons and X-rays. Theelectrons are deflected and analyzed in a magnetic spectrometer whereas the X-rays hit a scintillator which is imaged by a CCD.The camera itself is shielded by another 15 µm thick aluminum foil. The geometric distances result in an image magnificationof the bone sample of ∼ 4.4 − 7.3 on the camera.
gies above 7 keV) and therefore define a cut-off energybelow which the spectrum cannot be reliably retrieved.Based on the spectral measurements, a time averagedflux up to (1.6± 0.35)× 109 photons/msr/s (behind thebone sample and light shield) at 1 Hz repetition rate iscalculated. Comparing our retrieved spectrum to a syn-chrotron equivalent above the cut-off threshold of 7 keVgives a critical energy of Ecrit = 13.5 ± 0.95 keV (cf.Fig. 2). The size of the X-ray beam at the position of theCCD is larger than the sensor. Two dimensional Gaus-sian fits to the CCD images of the least divergent shotsprovide a lower estimate for the X-ray beam divergenceof 12× 6 mrad2 (s.d.).
To achieve a decent resolution, the sample was placed60 cm to 100 cm behind the target which therefore was lo-cated in front of the electron beam spectrometer (in con-trast to [9]). In order to minimize bremsstrahlung back-ground from electrons colliding with the sample mount,the bone is supported by a thin 3D printed acrylic plasticholder [14]. The signal of bremsstrahlung photons fromthe bone sample, holder and aluminum foil is negligiblecompared to the betatron signal. Furthermore, the sam-ple is protected from laser radiation by an aluminum foil,which is wrapped around the rotation stage (and there-fore rotates with the sample).
In preceding experiments [8], a direct detection X-rayCCD camera was employed for imaging purposes. Thistype of detector offers a high resolution (e.g. 13.5 µm inthe case of a Princeton Instruments PIXIS-XO:2048 ),but its sensitivity drops rapidly for radiation beyond10 keV. Furthermore, due to the high pixel number ofthe CCD chip and low-noise readout architecture, thistype of cameras typically has a readout time of severalseconds. This limits the shot frequency to around 0.1 Hz,which significantly increases the time needed for a tomog-raphy.
EX−ray[keV] 5 10 20 30 50 80χ 1
2[mm] 0.02 0.13 1.8 2.7 8.5 19.7
TABLE I. Half-value thickness χ 12
for human bones at differ-
ent energies [15].
In order to minimize the sample illumination - andtherefore the exposure to ionizing radiation - the energyof the X-rays should be chosen such that the correspond-ing half-value thickness is on the order of the sample’sphysical thickness (cf. Table I). This poses a major dif-ficulty for medical imaging with direct-detection CCDssince they are insensitive to radiation transmitted bye.g. human bones. However, the detection efficiency forhigh energetic photons can be drastically increased byconverting the incoming X-rays into photons of visiblewavelengths via scintillators.
For the quick tomography experiments we thereforehave used a scintillator-based CCD. It features an imageintensifier with variable gain. This camera uses a SonyICX285 Progressive Scan 2/3 rectangular CCD chip andsupports frame rates up to 30 fps. In practice, the max-imum shot frequency was limited to 1 Hz due to ourlaboratory data acquisition system. Due to the built infiber-optics taper (50:11), the camera’s effective pixel sizeis 29 µm. The P43 phosphor scintillator offers sensitivityalso for high X-ray energies (up to 100 keV) and thereforemakes this indirect detection method suitable for medicalimaging of thicker samples (cf. Fig. 2).
Figure 3 shows a comparison for the two types of detec-tors at the same angle of the sample and similar X-rayspectrum. As shown in the insets, the direct-detectionCCD has a better spatial resolution. In contrast, thephoton energy range detected by the camera is not welltransmitted by the bone and only limited informationabout its inner structure can be extracted.
3
FIG. 2. Comparison of the two different detection meth-ods (blue) and reconstructed X-ray spectrum at the detector(red). The shaded red area indicates the corresponding rmserror. At X-ray energies relevant for imaging bone samples,i.e. above 20 keV, indirect detection cameras have to be useddue to their much higher sensitivity for high energetic X-rays.
Another approach to reduce the acquisition time formedical applications is to minimize the necessary dataunderlying the tomographic reconstruction. This re-quires advanced reconstruction algorithms capable ofhandling reconstruction artifacts which are prone to ap-pear for small data sets [12]. The research of such al-gorithms has proven a prosperous field over the last twodecades and lead to the development of statistical itera-tive algorithms (SIRs) which are now becoming state ofthe art for computed tomographic technology [16] (seeFig. 4). These algorithms iteratively improve the recon-structed sample and apply weighting factors to emphasizetomograms with better signal to noise ratio. As a proofof principle we performed a quick tomography encom-passing 180 consecutive individual tomograms spanning180◦ at 1◦ step size. The entire data set was taken withinthree minutes at 1 Hz repetition rate [12].
The acquisition rate in these last experiments was lim-ited by the data acquisition system, while both the vac-uum and laser system would have supported shot fre-quencies up to 5 Hz. Beyond this, nowadays 100-TW-class laser systems with 10 Hz repetition rate are com-mercially available. If this could be fully exploited, atomography as presented in this paper would take 18 sec-onds to acquire and a high-resolution tomography with720 projection angles would only need slightly more thana minute - even with current laser technology.
Nevertheless, demands on the photon energy continueto grow as full body CTs or non-destructive testingof thick samples require much higher photon energies,larger X-ray beam diameters and a higher time-averagedflux. Without any new mechanisms in the generationof betatron X-rays, the laser repetition rate must be in-creased in order to gain higher mean brilliances. How-
(b)
(d)
(c)
(a)X-ray CCD
Scintillator
FIG. 3. Comparison of imaging using an X-ray CCD cam-era and a scintillator-based detector. While the X-ray CCDproduces sharper images, showing for instance signs of edgeenhancement, the lower part of the bone sample remains al-most opaque. In contrast, the scintillator camera is more sen-sitive in the > 10 keV regime, clearly showing the trabecularstructure.
ever, this poses a challenging task which current com-mercially available laser systems cannot satisfy: con-ventional Ti:sapphire laser systems have typical aver-age powers of ∼ 50 Watt which is limited by currentcrystal cooling concepts, such that an increased repe-tition rate usually comes at the cost of lower single-pulse energy. With new cooling concepts and/or laserarchitectures, this limit may be overcome in the future.Even though laser-wakefield acceleration at kHz repeti-tion rates has recently been demonstrated using mJ-classlaser systems [17–19], the reduced pulse energy results inelectron beams of lower energy (a few MeV) and negligi-ble betatron emission. For the generation of high ener-getic X-rays with kHz repetition rates, sources based onThomson-backscattering might be a viable alternative,since in this case the X-ray energy follows a favorableE ∝ 4γ2 scaling (e.g. 50 MeV electrons scattered with
4
FIG. 4. Quick tomography of a bone sample and compar-ison of two different reconstruction algorithms [12]. In theleft case filtered back projection (FBP) is used whereas thereconstruction in the right case was done via statistical itera-tive reconstruction (SIR). In contrast to FPB, SIR is capableof handling under-sampling artifacts which are due to the lim-ited number of acquired tomograms.
800 nm light produce 60 keV radiation) [20].
To conclude, we have demonstrated 1 Hz operation ofa laser-driven betatron source for imaging applications.In the near-term, these betatron sources might be fur-ther improved by using controlled-injection schemes [21]and operation at 10 Hz should be possible. At higherrepetition rates it will be easier to reach the requiredX-ray energies for medical tomographies using Thomsonbackscattering sources.
Based on the premise of laser-driven sources, the Mu-nich universities LMU and TUM have established thenew Centre for Advanced Laser Applications (CALA),which hosts several landmark laser installations aimingat both delivering higher laser pulse energies as well asproviding a joule-scale kHz laser system. The formeris met by the ATLAS-3000 laser, one of the few 1 Hzmulti-petawatt laser systems in the world. The PFS-prolaser system in contrast will provide hundreds of mJ oflaser pulse energy at kHz repetition rates. Both lasersystems will drastically increase the average X-ray fluxand therefore constitute another step towards real lifeimaging applications of laser-driven X-ray sources.
ACKNOWLEDGEMENTS
The authors thank F. Schaff and T. Baum (TUM) forproviding bone samples. This work was supported byDFG through the Cluster of Excellence Munich-Centrefor Advanced Photonics (MAP EXC 158), the DFG Got-tfried Wilhelm Leibniz program, TR-18 funding schemes,by EURATOM-IPP and the Max-Planck-Society.
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