IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 5, OCTOBER 2002 2415

High-Speed X-ray Imaging Camera forTime-Resolved Diffraction Studies

Sameer V. Tipnis, V. V. Nagarkar, V. Gaysinskiy, S. R. Miller, and I. Shestakova

Abstract—We report here on a high-speed X-ray imagingcamera, specifically developed for time resolved diffractionstudies using synchrotron and laboratory X-ray sources. Thiscamera is capable of acquiring six X-ray images at speeds of up to2300 frames per second (f/s). The system is based on a modifiedarchitecture charge coupled device (CCD) optically coupled toa fiber-optic taper via an image intensifier. The front end of thetaper is coupled to a specially designed microstructured CsI(Tl)scintillator screen capable of providing high light output, veryhigh-detection efficiency, and excellent spatial resolution. Inaddition to the time resolved diffraction studies, this detector willbe extremely valuable in applications such as dynamic imagingof small animals, X-ray microtomography, and materials scienceapplications. This paper discusses the design and performancecharacterization of the imaging system. Additionally, we presentsome preliminary high-speed X-ray imaging data obtained usinglaboratory X-ray sources.

Index Terms—High-speed X-ray imaging, image sensors, struc-tured CsI, X-ray diffraction.

I. INTRODUCTION

X -RAY diffraction (XRD) has been widely used for probingstructures of macromolecules toward the development of

new drugs as well as to gain information of various biologicalprocesses. Time-resolved diffraction using X-rays from high-in-tensity synchrotron sources is one of the most important toolsused for understanding dynamic processes such as those in-volved in muscle contraction. Synchrotron sources have alsoplayed a pivotal role in the microtomography of bones and teeth,studies of pathological calcifications, and in materials scienceapplications such as polymer processing where time-resolveddata are required.

With the availability of new and improved photon sourcessuch as the advanced photon source (APS) at Argonne NationalLaboratory, the main obstacle in time resolved XRD experi-ments is the lack of suitable fast X-ray imaging systems [1]. Forexample, the time-resolved study of X-ray diffraction duringmuscle fiber contraction requires a system that can providemultiple images on a millisecond time scale, with excellentsignal-to-noise ratio (SNR) and good spatial resolution. Thisis a key requirement, since the most critical and biologicallyimportant changes take place within the first 10 to 20 ms.

At present, there are a number of different techniques avail-able for high-speed imaging, all of which are based on the same

Manuscript received November 2, 2001; revised May 14, 2002. This workwas supported in part by the U.S. National Institutes of Health under Grant 2R44 AR44775-02.

The authors are with Radiation Monitoring Devices, Inc., Watertown, MA02472 USA (e-mail: stipnis@rmdinc.com).

Digital Object Identifier 10.1109/TNS.2002.803878

principle—to expose the film [or equivalent, like a image plateor a charged coupled device (CCD) array] to the radiation forvery short time [2]–[4]. However, none of these can record im-ages in rapid succession or a “burst” mode. The Cinemax imageintensifier [5], which runs at up to 10 000 frames per second(f/s) has overcome this problem. DRS Hadland Ltd., has de-veloped several ballistic imaging systems, streak cameras, andframing cameras based on CCD sensors that can provide re-quired time resolution of a microsecond or faster [6]. However,these systems are primarily designed for optical imaging andoffer a tradeoff between pixel resolution, number of images cap-tured, and the maximum operable speed.

The Imacon 540 X-ray streak camera, which is beingdeveloped by Hadland Ltd., Tring, U.K., is expected to offer10-ps time resolution, however the pixel resolution is limitedto 576 385. Another approach for high speed X-ray imagingis to use an X-ray generator with multiple anodes (eight anodesfor example) and multiple large area detectors (X-ray films,imaging plates, or large area amorphous silicon (a-Si:H) TFTbased arrays with converters) corresponding to the number ofanodes. Each of the anodes is fired sequentially (1s apart)for a short duration (50 ns) and the detectors collect one imageeach [7]. While this is a valid approach for fast imaging, eachdetector produces a different projection of the object, themaximum number of images captured depends on number ofanodes in the X-ray unit, and the total cost of the system issignificantly high if digital technology is used.

However, currently there does not exist an imaging systemspecifically designed for use in structural biology. At present,the most practical and cost-effective design for relativelyfast (readout times of few seconds) digital X-ray imaging indiffraction experiments is based on a CCD detector coupled to ascintillating phosphor converter screen [8]. The performance ofthese detectors is generally limited by two factors. First, at fastreadout speeds in excess of 10pixels/s, current CCD systemsexhibit significant read noise (100 e) making them usefulonly in applications where signal strengths are high. Second,the low conversion yields and longer decay times of existingphosphor screens result in images with low SNR and blurring.

The work presented here is a continuation of the devel-opment of a high-speed, X-ray imaging camera previouslyreported in this journal [9]. The work is aimed at developinga CCD system capable of acquiring X-ray imaging data ona millisecond time scale, which is needed for time resolvedX-ray diffraction studies of muscle contraction. The detectorconsists of a microstructured CsI(Tl) scintillator coupled to amodified architecture, fast frame CCD via fiber-optic taper.The system offers a better combination of spatial resolution,

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2416 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 5, OCTOBER 2002

Fig. 1. The high-speed X-ray imaging system. A high light outputmicrostrucutred CsI(Tl) scintillator screen is coupled to CCD chip via a 4 : 1fiber-optic taper.

detection efficiency, wide dynamic range, and a millisecondacquisition time. In addition to the aforementioned application,the detector can also be used in a variety of other areas such asdynamic imaging of small animals, X-ray microtomography,materials science applications, and nondestructive testing.

II. I MAGING SYSTEM

The system is based on a modified architecture CCD speciallydesigned to acquire images at high speeds with sufficient pixelresolution. To increase the active imaging area, this CCD is op-tically bonded to a 4 : 1 demagnification ratio fiber-optic taper.With this, the imaging area at the front face of the taper is about5.7 cm 2.9 cm, to which a microstructured CsI(Tl) scintillatorscreen, specially designed and developed at Radiation Moni-toring Devices, Inc. (RMD), is coupled. A 250m (10 mil) thickberyllium window on the front face of the taper ensures an ef-fective, light tight enclosure for the scintillator screen. The lossof optical signal resulting from the use of a fiber-optic taper ispartially compensated by a high quality image intensifier with amaximum gain on the order of 4000. With an intensifier gain ofunity, we estimate that 40 X-rays of 8 keV are required beforea signal can be detected by the system. Note that the system de-sign does not include a cooling mechanism. Cooling the CCDchip reduces only the dark noise, which in this case is negligiblysmall as compared to the read noise. This feature of the readnoise dominating the dark noise, is common to all high-speedCCD imaging arrays.

Fig. 1 shows a photograph of the system with the frontberyllium window and the housing for the taper removed. Thewindow and the housing were refitted during the experiments(see Fig. 5).

A. The Scintillator Screen

Research related to the growth of micro-structured CsI(Tl)scintillator screens for various X-ray imaging applications suchas structural biology, medical imaging, and nondestructivetesting has been carried out at RMD for the past several years[9]–[14]. The scintillator screen consists of 50m film ofCsI(Tl) deposited on a 2-mm-thick fiber-optic substrate. Themicrostructure of the film, with 3–5 m diameter columns ofCsI(Tl) allows the suppression of the lateral spread of scintilla-

tion light, thus preserving the resolution. The screen providesbetween 98% and 92% attenuation of 8 to 12 keV X-rays.

In addition to its high detection efficiency, the structuredCsI(Tl) screens provide high light output and excellent spatialresolution. An important consideration in the choice of ascintillator used for fast-frame X-ray imaging is its decay timeand afterglow. The scintillation decay time characteristics ofthe RMD CsI(Tl) screens have been studied and reported earlier[9]. It was seen that the RMD screens were well suited for thistype of imaging.

B. The Image Intensifier

In order to compensate for the loss of light signal in the de-magnifying taper, an image intensifier has been incorporatedinto the system. One end of the intensifier is coupled to thetaper, while the other end is coupled to the CCD via a 1 : 1 fiber-optic. The intensifier is a Gen 3 25-mm tube (Model F9860,ITT Industries, Night Vision), which consists of a high efficiencyGaAs photocathode bonded to a specially designed fiber-opticfaceplate, a microchannel plate (MCP) current amplifier, anda phosphor screen with a high-performance fiber-optic twister.The photocathode is very sensitive to low levels of visible andinfrared light in the wavelength region of 500–900 nm. Theelectronic image provided by the photocathode is proximity-fo-cused on the MCP and amplified without distortion. The MCPoutput electron image is then proximity-focused onto a greenP46 type phosphor screen (300-ns decay time) that is depositedon the fiber-optic. The power supply also provides bright sourceprotection to protect the photocathode during exposure to highlevels of light, along with external gain control.

C. The CCD Sensor

The CCD chip consists of 14 m 14 m pixels in a1024 2048 array. However, to accommodate high speeds,only 1024 512 pixels are used as an active imaging area.The rest of the pixels are optically masked to form an on-chipstorage buffer. Since the on-chip data shifting rates can beextremely fast (800 ns/line), this arrangement allows forcapturing six images at the rate of up to 2300 f/s. Apartfrom the burst mode operation, this camera is also capable ofacquiring 1024 512 pixel images up to the speed of 110 f/sin a continuous mode. This mode of operation is particularlyrelevant for other applications such as dynamic studies in smallanimals and nondestructive testing. Table I summarizes theimportant specifications of this system.

III. EXPERIMENTAL MEASUREMENTS

A. Scintillator Screen Response Time

CsI(Tl) has a characteristic decay time of1 s. In order toascertain that CsI(Tl) screens are suitable for high-speed X-rayimaging, the contribution of the scintillation decay “tails” insubsequent time slices was studied. To this end, the scintillationintensity after X-ray excitation was measured up to 9 ms, usinga pulsed X-ray generator (Model XRS-3, Golden Engineering).The generator has a fixed voltage of 300 kVp, producing 25-ns(FWHM) X-rays pulses and 25 pulses/s. The scintillator emis-sion light passes through a monochromator and is detected with

TIPNIS et al.: HIGH-SPEED X-ray IMAGING CAMERA FOR TIME-RESOLVED DIFFRACTION STUDIES 2417

TABLE ISPECIFICATIONS OF THEIMAGING SYSTEM

Fig. 2. Calculated buildup of intensity in the scintillator screen as a functionof time. After 6 ms, when the last image will be acquired in the six-frame modeof camera operation, this contribution is seen to be about 0.0005%.

a fast-response photolimiter tube (PMT), whose signal is readout by a digital storage oscilloscope and then transferred to aPC for analysis.

This signal trace was then used to generate a curve in orderto calculate the build-up of intensity. The measured intensitytrace was added to itself at intervals of 1 ms (which would bethe imaging speed at 1000 f/s). Fig. 2 shows the resultant “sum”trace. The build-up of intensity from prior exposures, to the last(sixth) image was seen to be about 510 , which is ex-tremely small.

We have observed some evidence that the afterglow contri-bution may be a function of the energy of the incident X-rays,and this contribution may diminish as the energy is increased.Other investigators have also documented the effect of X-raydose and thallium concentration on the CsI(Tl) afterglow [15].Further detailed investigation of the effect of incident excitationenergy on the afterglow will be undertaken in the future.

B. Screen Spatial Resolution

The spatial resolution of the CsI(Tl) screen was measured interms of the modulation transfer function [MTF(f)]. The MTF(f)was calculated from the finely sampled line spread function(LSF) measured from a 10m tantalum (Nuclear Associates#07-024) slit using the method described by Fujitaet al. [16].Note that this measurement was done using a different CCD

Fig. 3. Pre-sampled MTF of a 50�m thick RMD CsI(Tl) screen. The screenwas coupled to a cooled CCD via a demagnifying taper, with a resultant effectivepixel size of 57�m.

Fig. 4. Measured signal as a function of integration time for a constant inputintensity.

system, which is routinely used at RMD to measure the MTF ofscreens. The screen was pressure coupled to the front end of a3 : 1 fiber-optic taper whose smaller end is optically bonded to a1024 1024 pixel CCD with 19 m pixels. With the demagni-fying taper, the system has an effective pixel size of 57m, anda Nyquist limit of 8.77 lp/mm. Fig. 3 shows a graph of the mea-sured pre-sampled MTF of a 50m thick RMD CsI(Tl) screen.As can be seen, the screen exhibits excellent spatial resolutionup to 12 lp/mm. Point spread functions (PSF) of CsI(Tl) screensfabricated for crystallography applications have been measuredin earlier work and can be found in [4].

C. System Linearity

For the system linearity measurements, the front-end beryl-lium window was removed, to expose the bare taper to a con-stant intensity red LED (630 nm). Images were acquired at var-ious integration times and the average signal in the image wasrecorded as a function of these times. The results are shown inFig. 4, proving that the system indeed has a linear response overthe entire range of possible integration times.

D. Detective Quantum Efficiency

The detective quantum efficiency (DQE) of the imagingsystem was measured at Brandeis University using a 17 keVcontinuous flux X-ray source. Since the energy of theX-rays was higher than what would normally be used for thetime-resolved studies (which will use 8–10 keV X-rays at

2418 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 5, OCTOBER 2002

Fig. 5. The experimental setup at RMD for fast frame X-ray imaging.

the synchrotron), a thicker, 100m CsI(Tl) screen was usedfor these measurements. The CsI(Tl) screen was pressurecoupled to the taper and the front-end was then made lighttight with the beryllium window in place. The system wasexposed to the X-rays through a 150m diameter hole in atantalum mask. Several sheets of lead were placed around thehole to make sure that X-rays do not penetrate the rest of themask, except for the central aperture. Images of the hole thuscaptured were examined to measure the output signal intensity(ADU). The imaging system was then replaced with a PMTbased counting system, which is routinely used for measuringthe input X-ray signal. From the these two measurements,the system DQE (for a fixed gain) was calculated using theformula: . The system DQE thuscalculated was found to be 32%.

E. X-ray Imaging at RMD

The system performance was characterized in a series ofmeasurements at RMD. During these tests, preliminary X-rayimaging data at several different speeds were acquired. Afull-wave rectified X-ray source, with a copper target was usedduring this evaluation. The source provided X-rays of 8 keV,an energy typically used in structural biology experiments.A 50 m thick CsI(Tl) screen was pressure coupled to thefront end of the fiber-optic taper and the camera front waspositioned at a distance of about 20 cm from the source. Inthis configuration, the X-ray beam illuminated an area ofapproximately 2-cm diameter of the scintillator. Fig. 5 showsthe experimental setup. Note that the beryllium window allowsa light tight pressure coupling of the scintillator screen to thefront end of the taper.

A windup type wristwatch with balancing wheels was stuckto the beryllium window with a thin polymer adhesive tape, andused as the imaging phantom. Images were acquired at severaldifferent speeds from 30 to 500 f/s. Fig. 6 shows three suchimages, one each from the six image sequence, acquired at 120,240, and 500 f/s, with corresponding exposures of 8, 4, and 2 msper image. It should be pointed out that these are raw images,with no flat field correction or any other image processing. Note

(a)

(b)

(c)

Fig. 6. Images of a watch phantom acquired at various speeds (exposure):(a) 120 f/s (8.3 ms); (b) 240 f/s (4.1 ms); (c) 500 f/s (2 ms).

TIPNIS et al.: HIGH-SPEED X-ray IMAGING CAMERA FOR TIME-RESOLVED DIFFRACTION STUDIES 2419

that the laboratory source of the type used for this experiment, isnot the most suitable one for fast-frame imaging due to its lowflux and the variations in the X-ray intensity arising from the120-Hz ac ripple. Nevertheless, these data show that the systemis capable of acquiring images at high frame rates. Although wehave yet not tested the system at a synchrotron source, whichhas a dc X-ray flux, several orders of magnitude higher thanlaboratory sources, such a source will allow us to fully exploitthe speed and usefulness of the system.

Along with the time resolved diffraction studies, this detectorwill be extremely valuable in applications such as dynamicimaging of small animals, X-ray microtomography, and mate-rials science. In the case of dynamic imaging in small animals,or any kind of medical imaging, it will be particularly importantto balance the advantage of using the image intensifier, with thelinearity of the signal intensity as a function of the X-ray expo-sure. It may be necessary to use a low gain in order to achievethis balance. Another application where this system may beused is in perfusion computed tomography (CT) studies. Weintend to explore the feasibility of using this system for suchstudies, by conducting experiments at a medical facility usingstandard perfusion phantoms.

IV. SUMMARY

A high-speed X-ray imaging camera capable of acquiring sixX-ray images at speeds of up to 2300 f/s, has been developed foruse in time resolved diffraction studies at synchrotron sources.CsI(Tl) screens provide good light output and exhibit a responsetime that is adequately fast for high-speed X-ray imaging. Pre-liminary imaging tests with a laboratory source indicate the use-fulness of this camera for future high-speed imaging experi-ments at synchrotron facilities. Currently, we are in the processof planning such time-resolved muscle fiber diffraction experi-ments at the Advanced Photon Source (APS), Argonne NationalLaboratory.

ACKNOWLEDGMENT

The authors would like to thank Drs. C. Brecher andA. Lempicki of ALEM Associates for allowing the use of

the 8-keV X-ray source as well as for useful discussions onthe contribution of afterglow as a function of the incidentX-ray energy. Thanks are also due to Prof. H. Huxley of TheRosenstiel Basic Medical Sciences Research Center, BrandeisUniversity, for the use of the 17-keV continuous X-ray sourceused in the DQE measurements.

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