- Email: [email protected]
Imaging performance of polycrystalline BaFBr:Eu2+ storage phosphor plates
Materials Science and Engineering B94 (2002) 32 /39 www.elsevier.com/locate/mseb
Imaging performance of polycrystalline BaFBr:Eu2 storage phosphor plates H. Li *, P. Hackenschmied, E. Epelbaum, M. Batentschuk Institute of Material Science VI, University of Erlangen-Nu¨rnberg, Martenstrasse 7, D-91058 Erlangen, Germany Received 6 September 2001; accepted 29 January 2002
Abstract X-ray storage phosphor plates (SPP) initiate a new concept for recording X-ray radiographs, which are increasingly accepted in medical diagnostics and in scientific applications such as structure analysis and materials testing. The present study was undertaken to assess the impact of the median grain size and the fabrication process of BaFBr:Eu2 -based SPP on the image metrics which were characterized by photostimulated luminescence (PSL) sensitivity, modulation transfer function (MTF), Wiener spectrum (WS) and noise equivalent quanta (NEQ). To achieve the highest signal-to-noise ratio (NEQ) which was dependent on X-ray fluence and image spatial frequencies, the grain size has been optimized accordingly. Furthermore, the correlation between the packing structure of grains in the active layer and the granularity noise was investigated. Finally, it was found that for the imaging of Cu /Ka irradiation, which was a softer X-ray quantum energy compared to that used in the medical imaging, variation of the grain size had smaller impact on the MTF and the PSL. # 2002 Elsevier Science B.V. All rights reserved. Keywords: BaFBr:Eu2 ; Storage phosphor; Photostimulated luminescence; Modulation transfer function; Noise power spectrum; Noise equivalent quanta PACS numbers: 42.70; 81.05
1. Introduction X-ray imaging detectors based on storage phosphor are increasingly used in various fields such as medical diagnostics, structure analysis and material testing [1]. Compared to conventional X-ray films and film-screen systems they have several advantages, which are the direct digitization of the image during the readout, the linearity of the photostimulated luminescence (PSL) with X-ray dose over more than six orders of magnitude and the reusability. The image is available almost instantaneous, much less than that required for the wet-chemical development of a film. The physical principle of the storage phosphor systems can be described as follows [1 /3]: during X-
* Corresponding author. Present address: Department of Radiation Oncology, Vanderbilt University, 1301 22nd Ave. South, B-902 TVC, Nashville, TN 37232-5671, USA. Tel.: 1-615-322-5253; fax: 1-615343-0161. E-mail address: [email protected] (H. Li).
ray irradiation the X-ray quanta are absorbed partly or totally in the active layer of the SPP. The SPP is a flexible X-ray sensor in which photostimulable phosphor crystallites (typically BaFBr:Eu2) are uniformly coated on a polymer substrate. Proportionally to the locally deposited X-ray energy, electron /hole-pairs are generated. The electrons can be stored in halide vacancies and form F-centers while holes are stored in the form of Vk centers which can be stabilized by Eu2ions at room temperature. The spatial variation of the storage centers concentration forms latent X-ray image that can be subsequently recovered by scanning the SPP point by point with a focused laser beam (for example, He /Ne). F-center electrons are excited and recombine with trapped holes resulting in the characteristic blue Eu2 luminescence (390 nm). The emitted light is collected by a light guide and detected by a photomultiplier tube. The output signal is amplified, digitized and stored by a computer system. At the completion of the scan the recorded data can be visualized and processed. Residual storage centers, which were not
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 0 6 8 - 5
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
eliminated during the readout, are subsequently bleached with an intense lamp before the next image acquisition. The overall performance of storage phosphor detector system depends on the optimization of both SPP and readout scanner. Although scanner plays a central role in recovering the latent image stored in the SPP and its performance greatly influences the final image quality, it is impossible to acquire a high quality X-ray image without high performance SPP. In recent years, the physical mechanism of photostimulation process and the approaches to increase the conversion efficiency and photostimulability of storage phosphors have been studied extensively [4 /7]. However, the investigation into the fabrication of SPP is very limited. In the course of fabrication several questions are open: what is the optimum grain size of the storage phosphor? which adjustments should be made for the particular application? what is the effect of the processing and the packing structure of grains in the active layer on image properties? It is the intention of this paper to answer these questions. In this work, polycrystalline BaFBr:Eu2 storage phosphor plates (SPP) were fabricated utilizing a wellestablished process. It will be shown how the grain size and the microstructure of the plates influence the image properties. According to a variety of application conditions, such as X-ray fluence, spatial frequency of the image, X-ray photon energy and concrete scanning parameters of the scanner, the grain size has been optimized.
2. Experimental 2.1. Fabrication of SPP The BaFBr:Eu2 powders were prepared by solid reaction method as previously reported [7]. After mixing BaBr2 ×/1H2O and BaF2 together with EuF3 in proper ratios the mixture was firing for 3 h at 850 8C in a reducing atmosphere. The powders were then washed in order to remove residual bromine salts and annealed at 700 8C for 3 h. After milling and screening, powders with appropriate particle size distribution were stored in a desiccator for the further slurry preparation. The grain size distributions were detected by particle analyzer LS100Q (Beckman-Coulter, Krefeld, Germany). BaFBr:Eu2 powder was homogenized together with organic binder and plasticizer in a ball mill. Binder ensure the cohesion of phosphor grains after the solvent is evaporated while plasticizer improve the flexibility. The homogeneous slurry was degassed and spread on a pretreated PET using doctor /blade process. After drying and cutting, samples of 35/8 cm2 format were obtained for further imaging metrics analysis.
33
2.2. Evaluation of imaging performance The PSL intensity of the powder was measured in an Ulbricht-sphere. A filter combination of UG-11, BG-3 and BG-39 (Schott Glaswerke, Germany) was used to separate the stimulation light from emitted PSL. In order to improve the signal-to-noise ratio of the measurement, lock-in technique was utilized. The image quality evaluation was performed using a compact storage phosphor scanner [3], FASTSCAN, which acquired a 5000/3500 (60 /60 mm2 pixel) image (35 MB) in 25 s. For the X-ray exposure of the SPP a medical X-ray tube (Nanodor II, Siemens AG) with a tungsten anode, 2 mm Al filter and a tube voltage of 75 kVp was utilized. The input photon fluence was estimated as 2/104 photons mm 2 mGy1. All exposures were measured as entrance air kerma using a DALI type 77217 dosimeter with a type 77334 ionization chamber (PTW, Freiburg, Germany) or an ion chamber survey meter model 450 (Victoreen, Cleveland, USA). 2.2.1. Modulation transfer function The modulation transfer function (MTF) is a very important criterion for judging the resolution of a SPP. MTF is defined as the ratio of the contrast in the scanned image to the contrast of the irradiated object which is a function of the spatial frequency. The MTF of the plate was determined from the square-wave-response function of periodic bar pattern: image traces transverse to the periodic bars were selected. These traces were then analyzed and averaged to give the amplitude response at various spatial frequencies up to 5 lp mm 1. The SWRF is determined by normalizing these amplitudes with respect to the contrast of the bar patterns at very low frequencies (0.05 lp mm 1). The MTF can be calculated from the Eq. (1) [8]. p 1 MTF(v) SWRF(v) SWRF(3v) 4 3 1 SWRF(5v) (1) 5
2.2.2. Noise power spectrum (Wiener spectrum) Noise in radiographic images is quantified by measurement of the noise power spectrum (NPS) of the images. The NPS gives a measure of the noise power as a function of spatial frequency. The method used here to measure the NPS was similar to those proposed by Dainty and Shaw [8] from flat field exposures. The noise data were acquired using a measuring slit 1.2 mm long by 60 mm wide. Any obvious trends in the scan direction were removed by fitting a low order polynomial. The Fourier transformation of these noise traces were
34
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
squared and averaged to obtain the one-dimensional NPS after correcting the data for the effects of the scanning aperture. After normalizing by large-area average (measured signal amplitude for a given dose at zero spatial frequency) Wiener spectrum (WS) was obtained. 2.2.3. Noise equivalent quanta The signal-to-noise ratio of the X-ray image can be specified in terms of an apparent number of quanta that would produce the same signal-to-noise ratio using an ideal imaging system, that counts all the input quanta, as in the actual image. This measure, which were derived from MTF and the WS results, gives the image’s output signal-to-noise ratio, expressed in terms of quanta, as a function of spatial frequency. A real system counts only noise equivalent quanta (NEQ) of the input quanta while an ideal system could count all the input quanta. The NEQ spectrum is calculated according to Eq. (2). NEQ(D; v)MTF 2 (v)=WS 2 (D; v)
(2)
3. Results and discussion Fig. 1 shows the PSL intensity of BaFBr:Eu powder as a function of the median grain size. The PSL intensity decreases approximately linearly down to 5 /6 mm. Thereafter, a remarkable deviation from the linearity can be found. Due to intrinsic grainy character of SPP, the impact of the grain size on the imaging performance must be clarified before the manufacturing. Only based on such knowledge rational processing can be established. It is well known that a narrow distribution of the particle size is believed to be always beneficial for the formation of homogenous structure with dense packing. The questions are: firstly, which grain size is required in
Fig. 1. PSL intensity versus median grain size of BaFBr:Eu2 powder.
our case? secondly, how to sustain a narrow distribution of the desired size during slurry preparation? MTF, WS curves (X-ray dose: 12 mGy) for different grain sizes (d50) are demonstrated in Fig. 2. One can see that the reduction of median grain size from 15 to 10 mm leads to considerable enhancement of MTF. Further reduction down to 5 mm results in a minor enhancement of MTF. By contrast, the large-area average PSL of the plates decreases linearly following the PSL properties of the powder (Fig. 1) and thereby the normalized noise power (WS) increases with decreasing grain size, a fact indicative of the luminescence noise (Poisson statistics) dominance at this exposure level if considering the luminescence noise dependence on large-area average and its normalized form by large-area average S0 /Ganx given by Eq. (3): pffiffiffiffiffiffiffiffiffiffiffi Nlum Ganx (3) N˜ 2lum N 2lum =S02 1=(Ganx )
(3?)
where a is the fraction of the absorbed X-ray quanta and G is the gain of the system, which can be described as the average number photoelectrons released in the PMT per absorbed X-ray quantum. nx represents incident X-ray quanta. The luminescence noise is associated with the statistical fluctuations of the stimulated luminescence quanta that are detected by the photomultiplier tube. To understand the mechanism by which the altered grain size leads to variation of MTF and noise behavior, the distribution of readout laser photons in the granular layer has to be evaluated. During the image recovery laser light is scattered at the boundary between the phosphor powder and the organic binder as a result of the mismatch of the refractive index of both materials. The profile of generated photon density distribution is critical to detection sensitivity and spatial resolution. Recently, a computational model based on Monte Carlo algorithm has been established [9] to deal with the complex problem of light spreading that results from multi-scattering and absorption. The photon-density distribution of the exciting laser in the phosphor layer plate can be obtained after following the random walk of each light packet. In addition, the probability of emission of the PSL from the front and rear surfaces can also be simulated. Thereby, the MTF and large-area average sensitivity can be derived. In order to realize the simulation, however, several parameters regarding optical structure of the phosphor plate should be hypothesized. Among these parameters, the mean scattering length is closely correlated to the grain size of the phosphor material. Fig. 3 displays the contour plots of the scattering profiles of the laser photons for scattering lengths of 15 (a) and 5 mm (b). One can see that for decreasing scattering length the slope of the photon density as a
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
35
Fig. 2. MTF and WS curves for SPP with differing median grain sizes. Thickness: 180.092.0 mm, X-ray dose: 12 mGy, X-ray tube (tungsten) voltage: 75 kVp.
function of radius or depth becomes greater, indicating that a decrease of scattering length can confine the scattering behavior of laser photons to a limited volume. Consequently, a resolution enhancement can be achieved, as verified by experimentally determined MTF curves. On the contrary, since less storage centers can be stimulated or bleached, the average PSL intensity becomes smaller. Meanwhile, increased scattering events of emitted PSL photon in the layer results in less PSL photons being able to reach the surface where the PMT is located. This process is dominated by mean scattering length of both laser photons and PSL photons, which correlates closely to median grain size. As shown in Fig. 2, however, further decreasing grain size from 10 to 5 mm leads to minor enhancement of the resolution, in disagreement with our simulated results by which this reduction results in sufficient elevation of MTF. Therefore, we also measured the MTF using another scanner that features higher resolution through
improved scan optics and higher sampling rate (100 samples mm1). Unfortunately, although apparent is the elevation of MTF’s compared to those obtained from FASTSCAN, still no variation of MTF between both plates can be distinguished. Several aspects may account for this disagreement. First, the simulation program scan the information with much smaller sampling interval (about 1 /2 mm) compared with 10 mm here experimentally used. The purpose is to speculate the resolution limit that can be achieved. Such high sampling rate cannot be acceptable due to enormous acquisition time. Second, due to the inhomogeneous distribution of active center (europium ion in the case of BaFBr:Eu2) variation of PSL capacity of individual phosphor crystallite imposes additional noise on the measurement, which deteriorates the MTF behavior. Third, noise from other sources, such as electronics, scan optics, X-ray tube, also complicates the MTF determination. It suggests that although the MTF can
Fig. 3. Simulated contour plots of the logarithm of the density profile of the readout laser photons for varying mean scattering length: (a) 15; and (b) 5 mm.
36
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
be improved apparently through reducing grain size down to 5 mm according to simulation results, in practice it seems no use to decrease the grain size so further. Moreover, when the median grain size is lower than 5 mm the remarkable degradation of the PSL intensity may occur as evidenced by the deviation from the linearity of the IPSL versus median grain size curve (Fig. 1). This deviation may be attributed to the significant role of surface states in this grain size range which leads to more prominent non-radiative electron / hole recombination. Because NEQ is a more fundamental performance quantity expressing the signal-to-noise ratio and its dependence on spatial frequency, the impact of grain size on imaging quality can be clarified by comparing the NEQ values. As shown in Fig. 4, at low spatial frequency (1 lp mm 1) 15 mm-plate is superior to 10 and 5 mm plates due to its higher PSL large-area average. At high frequency level (4 lp mm 1) the 10 mm-plate exhibits much higher signal-to-noise ratio than that of 15 mm-plate thanks to its elevated MTF (see Eq. (2)). Since no more resolution gain and inferior PSL response at zero frequency, 5 mm-plate shows worse NEQ in the whole frequency range than that of 10 mm-plate. Taken together, at this exposure level (12 mGy), a typical X-ray fluence for mammography, 10 mm-plate features the best performance, although its NEQ is slightly lower than that of 15 mm-plate in the low frequency range. However, at higher exposure level, for example, 150 mGy, a typical dose for dental radiography, a rather different picture was obtained. As shown in Fig. 5, at low spatial frequency the plate made of 5 mm powder shows similar noise level as those of 10 and 15 mm plates in spite of its inferior average PSL. This results from its less structure noise due to denser packing of grains and consequent more perfect morphology, which is verified through the scanning electron micrographs (SEM). It indicates that at high X-ray fluence structure noise of plate becomes increasingly dominant. It should be noted that the structure noise effect in the measurement data
becomes less evident at the high frequencies because of the overlap of neighboring pixels. Consequently, 5 mm-plate exhibits the best NEQ at 1.0 lp mm 1. Therefore, for imaging of discrete objects, fine powder as 5 mm should be used. However, at the high frequency (4 lp mm 1), due to more prominent luminescence noise 5 mm-plate shows higher noise level than 10 and 15 mm plates. Taking the effect of MTF into account (Fig. 2), 10 mm-plate exhibits the best performance for tiny object imaging. From above evaluations, it can be concluded that variation of phosphor grain size results in different imaging performance. Conversely, the grain size and its distribution have to be kept unaltered during slurry processing. The concern is that the slurry processing should be efficient with the aim of breaking up particle agglomerates. The question is what occurs if all agglomerates are broken during the slurry process? Unfortunately, the grain size analysis of the slurry is very complicated. Therefore, the grain size distribution of the BaFBr:Eu storage phosphor by milling in ball mill were analyzed and the results were extended to the milling and mixing stage at the slurry preparation. As shown in Fig. 6, the milling up to 6 h reduces the coarse grain portion without altering the grain size diagram form, indicative of the decomposition of agglomerates. After 6 h milling, however, a noticeable increase of fine particles portion can be observed. With the further increase of processing time (for instance 16 h) a striking bimodal distribution of primary particles occurred. This may be explained by the fact that the tetragonal matlockite BaFBr grains are easy cleavable perpendicular to c -axis. Moreover, the remarkable increase of very-fine grains strongly deteriorates the PSL intensity (Fig. 1) which has to be avoided. Consequently, we strived for a narrow distribution of phosphor grains with desired d50 prior to slurry preparation. Further, an optimal slurry process has been established, which assured the breakdown of agglomerates in the slurry and promised
Fig. 4. NEQ’s as a function of median grain size at varying spatial frequency. (a) 1; and (b) 4 lp mm 1. X-ray data as in Fig. 2.
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
37
Fig. 5. WS and NEQ data for SPP with differing median grain size, measured under X-ray dose of 150 mGy.
minor effect on the narrow size distribution of initial phosphors. To take best advantage of the appropriate grain size with high X-ray conversion efficiency, great efforts have been made towards the optimal dense and homogeneous packing of grains in the active layer. BaFBr phosphor plate casting process involves a large number of variables which can greatly affect the macro- and microstructure and consequent properties of the final product. Here we prefer to present the impact of the packing structure of powder on the imaging performance.
Fig. 6. Grain size distribution of BaFBr:Eu2 powder as a function of ball milling time.
As shown in the scanning electron graphs in Fig. 7, the plate made under optimized process (process 1) exhibits nearly perfect particle packing, whereas the other plate that underwent non-optimized process (process 2) shows striking packing defects, which prove to be deleterious to the imaging performance. Fig. 8a compares the measured MTF curves for these two plates. In the low frequency range there is little difference. But for the frequencies greater than 2 lp mm 1 the variation is evident. The curve for process 1 plate is considerably higher and consequent has better resolution. This results from more intense scattering of readout laser in the process 2 plate due to loose packing of phosphor grains, which reduces image sharpness. Fig. 8b compares the WS profiles of the two plates at X-ray dose level of 150 mGy. In the low frequency range it can be clearly seen that process 2 plate gives more noise. Further, as we know, quantum noise which corresponds to statistically fluctuations of the X-ray quanta incident at the plate surface is dependent on spatial frequency given by, pffiffiffiffiffiffiffi Nq G anx MTF(v) (4) Assuming other noise components (for example, luminescence noise and readout noise from scanner) constant, the WS curves of process 2 plate should fall off more rapidly as a result of inferior MTF behavior. However, the fact is that the WS still sustain the same
38
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
Fig. 7. Scanning electron photomicrographs of BaFBr:Eu2 SPP: (a) process 1; and (b) process 2.
levels as those of process 1 plate. This indicates that the process 2 plate has a higher white noise level, which is assigned to structure or granularity noise. Granularity noise is related to the grainy structure of the phosphor plates, which results from inhomogeneities of the image receptor corresponding to local variations in the efficiency of converting the incident quantum energy into luminescence, caused by fluctuations in X-ray absorption, by differences in the propagation of laser and luminescence light in the powder layer, by varying PSL property of individual particles. All previously presented data were acquired under the conditions of a medical X-ray tube. It is well known that X-ray energy greatly influences the penetration depth of X-ray radiation, which, in turn, results in a different profile of storage center distribution. Therefore, this variable has to be investigated if SPP will be used for other applications. We present here the measurement using Cu /Ka (E / 8 keV) irradiation which is most common for structure
diffraction analysis. The attenuation coefficient of BaFBr plate for this X-ray energy is 630.0 cm 1 corresponding to a penetration depth of 16.0 mm, which is quite different from the previous measurement condition. In that case, the mean X-ray energy is estimated to be 35 keV with a penetration length of 231.0 mm. We investigated how this variation influences the choice of desired grain size. Due to technique difficulty only MTF and large-area average PSL data are here presented. The thickness of all evaluated plates (Fig. 9) is 180.09/ 2.0 mm. It is remarkable that the variation of grain size leads to much smaller changes in the resolution than the case discussed previously. In addition, the reduction amplitude of large-area average PSL is also much lower. It is not a surprise because most of the information is stored near the surface. It implies that, also verified by simulation results [9], for this low X-ray energy the change of mean scattering length, i.e. median grain size brings about little impact on the resolution. If remarkable resolution enhancement is desired, other methods
Fig. 8. MTF and WS curves of SPP demonstrated in Fig. 7 under X-ray exposure dose of 150 mGy.
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
39
Fig. 9. MTF and large-area average PSL as a function of median grain size measured using Cu /Ka irradiation.
should be used, for example, a thickness value comparable to 60.0 mm, as indicated by simulation results. In that case, 15 mm grain size may be chosen since it provides highest large-area average PSL.
4. Conclusions The investigation of the correlation ‘structure/image properties’ in the BaFBr:Eu SPP shows that decreasing median grain size led to enhancement of MTF and reduction of granularity noise, whereas a deterioration of PSL. For imaging low X-ray fluence a median grain size of d50 /10 mm provided the best image property in the sense of signal-to-noise ratio (NEQ). In the case of high X-ray fluence, for imaging discrete objects the plate made of 5 mm powder demonstrated superior NEQ. However, for spatial frequencies greater than 2 lp mm 1 the plate made of 10 mm powder would still appear to be preferable. Furthermore, it was demonstrated that inhomogeneous structure with loose packing of phosphor powder featured excess granularity noise and degraded MTF. Finally, it was found that varying grain size played a minor impact on the MTF for the imaging of Cu /Ka irradiation where the variation of PSL was also smaller.
Acknowledgements We would like to acknowledge the financial support of the Bayerischer Forschungsverbund fu¨r Oberfla¨chentechnik (FOROB, project II.1). We are also grateful to Professor A. Winnacker for valuable discussions and comments on the manuscript.
References [1] A. Winnacker, Phys. Med. IX 2 /3 (1993) 95 /101. [2] M. Thoms, H. von Seggern, A. Winnacker, Phys. Rev. B44 (17) (1991) 9240 /9247. [3] M. Thoms, H. Burzlaff, A. Kinne, J. Lange, H. von Seggern, R. Spengler, A. Winnacker, Mater. Sci. Forum 107 (1) (1995) 228 / 231. [4] J.M. Spaeth, T.H. Hangleiter, F.K. Koschnick, T.H. Pawlik, Radiat. Eff. Defects Solids 135 (1995) 1 /10. [5] R.J. Klee, J. Phys. D: Appl. Phys. 28 (1995) 2529 /2533. [6] A. Harrison, M.T. Harrison, G.P. Keogh, R.H. Templer, Phys. Rev. B (1996) 5039 /5042. [7] P. Hackenschmied, H. Li, E. Epelbaum, R. Fasbender, M. Batentschuk, A. Winnacker, Radiat. Meas. 33 (2001) 669 /674. [8] J.C. Dainty, R. Shaw, Image Science, Academic Press, New York, 1974. [9] R. Fasbender, H. Li, A. Winnacker, Monte-Carlo simulations of the image properties of storage phosphor plates, to be published elsewhere.
Imaging performance of polycrystalline BaFBr:Eu2 storage phosphor plates H. Li *, P. Hackenschmied, E. Epelbaum, M. Batentschuk Institute of Material Science VI, University of Erlangen-Nu¨rnberg, Martenstrasse 7, D-91058 Erlangen, Germany Received 6 September 2001; accepted 29 January 2002
Abstract X-ray storage phosphor plates (SPP) initiate a new concept for recording X-ray radiographs, which are increasingly accepted in medical diagnostics and in scientific applications such as structure analysis and materials testing. The present study was undertaken to assess the impact of the median grain size and the fabrication process of BaFBr:Eu2 -based SPP on the image metrics which were characterized by photostimulated luminescence (PSL) sensitivity, modulation transfer function (MTF), Wiener spectrum (WS) and noise equivalent quanta (NEQ). To achieve the highest signal-to-noise ratio (NEQ) which was dependent on X-ray fluence and image spatial frequencies, the grain size has been optimized accordingly. Furthermore, the correlation between the packing structure of grains in the active layer and the granularity noise was investigated. Finally, it was found that for the imaging of Cu /Ka irradiation, which was a softer X-ray quantum energy compared to that used in the medical imaging, variation of the grain size had smaller impact on the MTF and the PSL. # 2002 Elsevier Science B.V. All rights reserved. Keywords: BaFBr:Eu2 ; Storage phosphor; Photostimulated luminescence; Modulation transfer function; Noise power spectrum; Noise equivalent quanta PACS numbers: 42.70; 81.05
1. Introduction X-ray imaging detectors based on storage phosphor are increasingly used in various fields such as medical diagnostics, structure analysis and material testing [1]. Compared to conventional X-ray films and film-screen systems they have several advantages, which are the direct digitization of the image during the readout, the linearity of the photostimulated luminescence (PSL) with X-ray dose over more than six orders of magnitude and the reusability. The image is available almost instantaneous, much less than that required for the wet-chemical development of a film. The physical principle of the storage phosphor systems can be described as follows [1 /3]: during X-
* Corresponding author. Present address: Department of Radiation Oncology, Vanderbilt University, 1301 22nd Ave. South, B-902 TVC, Nashville, TN 37232-5671, USA. Tel.: 1-615-322-5253; fax: 1-615343-0161. E-mail address: [email protected] (H. Li).
ray irradiation the X-ray quanta are absorbed partly or totally in the active layer of the SPP. The SPP is a flexible X-ray sensor in which photostimulable phosphor crystallites (typically BaFBr:Eu2) are uniformly coated on a polymer substrate. Proportionally to the locally deposited X-ray energy, electron /hole-pairs are generated. The electrons can be stored in halide vacancies and form F-centers while holes are stored in the form of Vk centers which can be stabilized by Eu2ions at room temperature. The spatial variation of the storage centers concentration forms latent X-ray image that can be subsequently recovered by scanning the SPP point by point with a focused laser beam (for example, He /Ne). F-center electrons are excited and recombine with trapped holes resulting in the characteristic blue Eu2 luminescence (390 nm). The emitted light is collected by a light guide and detected by a photomultiplier tube. The output signal is amplified, digitized and stored by a computer system. At the completion of the scan the recorded data can be visualized and processed. Residual storage centers, which were not
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 0 6 8 - 5
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
eliminated during the readout, are subsequently bleached with an intense lamp before the next image acquisition. The overall performance of storage phosphor detector system depends on the optimization of both SPP and readout scanner. Although scanner plays a central role in recovering the latent image stored in the SPP and its performance greatly influences the final image quality, it is impossible to acquire a high quality X-ray image without high performance SPP. In recent years, the physical mechanism of photostimulation process and the approaches to increase the conversion efficiency and photostimulability of storage phosphors have been studied extensively [4 /7]. However, the investigation into the fabrication of SPP is very limited. In the course of fabrication several questions are open: what is the optimum grain size of the storage phosphor? which adjustments should be made for the particular application? what is the effect of the processing and the packing structure of grains in the active layer on image properties? It is the intention of this paper to answer these questions. In this work, polycrystalline BaFBr:Eu2 storage phosphor plates (SPP) were fabricated utilizing a wellestablished process. It will be shown how the grain size and the microstructure of the plates influence the image properties. According to a variety of application conditions, such as X-ray fluence, spatial frequency of the image, X-ray photon energy and concrete scanning parameters of the scanner, the grain size has been optimized.
2. Experimental 2.1. Fabrication of SPP The BaFBr:Eu2 powders were prepared by solid reaction method as previously reported [7]. After mixing BaBr2 ×/1H2O and BaF2 together with EuF3 in proper ratios the mixture was firing for 3 h at 850 8C in a reducing atmosphere. The powders were then washed in order to remove residual bromine salts and annealed at 700 8C for 3 h. After milling and screening, powders with appropriate particle size distribution were stored in a desiccator for the further slurry preparation. The grain size distributions were detected by particle analyzer LS100Q (Beckman-Coulter, Krefeld, Germany). BaFBr:Eu2 powder was homogenized together with organic binder and plasticizer in a ball mill. Binder ensure the cohesion of phosphor grains after the solvent is evaporated while plasticizer improve the flexibility. The homogeneous slurry was degassed and spread on a pretreated PET using doctor /blade process. After drying and cutting, samples of 35/8 cm2 format were obtained for further imaging metrics analysis.
33
2.2. Evaluation of imaging performance The PSL intensity of the powder was measured in an Ulbricht-sphere. A filter combination of UG-11, BG-3 and BG-39 (Schott Glaswerke, Germany) was used to separate the stimulation light from emitted PSL. In order to improve the signal-to-noise ratio of the measurement, lock-in technique was utilized. The image quality evaluation was performed using a compact storage phosphor scanner [3], FASTSCAN, which acquired a 5000/3500 (60 /60 mm2 pixel) image (35 MB) in 25 s. For the X-ray exposure of the SPP a medical X-ray tube (Nanodor II, Siemens AG) with a tungsten anode, 2 mm Al filter and a tube voltage of 75 kVp was utilized. The input photon fluence was estimated as 2/104 photons mm 2 mGy1. All exposures were measured as entrance air kerma using a DALI type 77217 dosimeter with a type 77334 ionization chamber (PTW, Freiburg, Germany) or an ion chamber survey meter model 450 (Victoreen, Cleveland, USA). 2.2.1. Modulation transfer function The modulation transfer function (MTF) is a very important criterion for judging the resolution of a SPP. MTF is defined as the ratio of the contrast in the scanned image to the contrast of the irradiated object which is a function of the spatial frequency. The MTF of the plate was determined from the square-wave-response function of periodic bar pattern: image traces transverse to the periodic bars were selected. These traces were then analyzed and averaged to give the amplitude response at various spatial frequencies up to 5 lp mm 1. The SWRF is determined by normalizing these amplitudes with respect to the contrast of the bar patterns at very low frequencies (0.05 lp mm 1). The MTF can be calculated from the Eq. (1) [8]. p 1 MTF(v) SWRF(v) SWRF(3v) 4 3 1 SWRF(5v) (1) 5
2.2.2. Noise power spectrum (Wiener spectrum) Noise in radiographic images is quantified by measurement of the noise power spectrum (NPS) of the images. The NPS gives a measure of the noise power as a function of spatial frequency. The method used here to measure the NPS was similar to those proposed by Dainty and Shaw [8] from flat field exposures. The noise data were acquired using a measuring slit 1.2 mm long by 60 mm wide. Any obvious trends in the scan direction were removed by fitting a low order polynomial. The Fourier transformation of these noise traces were
34
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
squared and averaged to obtain the one-dimensional NPS after correcting the data for the effects of the scanning aperture. After normalizing by large-area average (measured signal amplitude for a given dose at zero spatial frequency) Wiener spectrum (WS) was obtained. 2.2.3. Noise equivalent quanta The signal-to-noise ratio of the X-ray image can be specified in terms of an apparent number of quanta that would produce the same signal-to-noise ratio using an ideal imaging system, that counts all the input quanta, as in the actual image. This measure, which were derived from MTF and the WS results, gives the image’s output signal-to-noise ratio, expressed in terms of quanta, as a function of spatial frequency. A real system counts only noise equivalent quanta (NEQ) of the input quanta while an ideal system could count all the input quanta. The NEQ spectrum is calculated according to Eq. (2). NEQ(D; v)MTF 2 (v)=WS 2 (D; v)
(2)
3. Results and discussion Fig. 1 shows the PSL intensity of BaFBr:Eu powder as a function of the median grain size. The PSL intensity decreases approximately linearly down to 5 /6 mm. Thereafter, a remarkable deviation from the linearity can be found. Due to intrinsic grainy character of SPP, the impact of the grain size on the imaging performance must be clarified before the manufacturing. Only based on such knowledge rational processing can be established. It is well known that a narrow distribution of the particle size is believed to be always beneficial for the formation of homogenous structure with dense packing. The questions are: firstly, which grain size is required in
Fig. 1. PSL intensity versus median grain size of BaFBr:Eu2 powder.
our case? secondly, how to sustain a narrow distribution of the desired size during slurry preparation? MTF, WS curves (X-ray dose: 12 mGy) for different grain sizes (d50) are demonstrated in Fig. 2. One can see that the reduction of median grain size from 15 to 10 mm leads to considerable enhancement of MTF. Further reduction down to 5 mm results in a minor enhancement of MTF. By contrast, the large-area average PSL of the plates decreases linearly following the PSL properties of the powder (Fig. 1) and thereby the normalized noise power (WS) increases with decreasing grain size, a fact indicative of the luminescence noise (Poisson statistics) dominance at this exposure level if considering the luminescence noise dependence on large-area average and its normalized form by large-area average S0 /Ganx given by Eq. (3): pffiffiffiffiffiffiffiffiffiffiffi Nlum Ganx (3) N˜ 2lum N 2lum =S02 1=(Ganx )
(3?)
where a is the fraction of the absorbed X-ray quanta and G is the gain of the system, which can be described as the average number photoelectrons released in the PMT per absorbed X-ray quantum. nx represents incident X-ray quanta. The luminescence noise is associated with the statistical fluctuations of the stimulated luminescence quanta that are detected by the photomultiplier tube. To understand the mechanism by which the altered grain size leads to variation of MTF and noise behavior, the distribution of readout laser photons in the granular layer has to be evaluated. During the image recovery laser light is scattered at the boundary between the phosphor powder and the organic binder as a result of the mismatch of the refractive index of both materials. The profile of generated photon density distribution is critical to detection sensitivity and spatial resolution. Recently, a computational model based on Monte Carlo algorithm has been established [9] to deal with the complex problem of light spreading that results from multi-scattering and absorption. The photon-density distribution of the exciting laser in the phosphor layer plate can be obtained after following the random walk of each light packet. In addition, the probability of emission of the PSL from the front and rear surfaces can also be simulated. Thereby, the MTF and large-area average sensitivity can be derived. In order to realize the simulation, however, several parameters regarding optical structure of the phosphor plate should be hypothesized. Among these parameters, the mean scattering length is closely correlated to the grain size of the phosphor material. Fig. 3 displays the contour plots of the scattering profiles of the laser photons for scattering lengths of 15 (a) and 5 mm (b). One can see that for decreasing scattering length the slope of the photon density as a
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
35
Fig. 2. MTF and WS curves for SPP with differing median grain sizes. Thickness: 180.092.0 mm, X-ray dose: 12 mGy, X-ray tube (tungsten) voltage: 75 kVp.
function of radius or depth becomes greater, indicating that a decrease of scattering length can confine the scattering behavior of laser photons to a limited volume. Consequently, a resolution enhancement can be achieved, as verified by experimentally determined MTF curves. On the contrary, since less storage centers can be stimulated or bleached, the average PSL intensity becomes smaller. Meanwhile, increased scattering events of emitted PSL photon in the layer results in less PSL photons being able to reach the surface where the PMT is located. This process is dominated by mean scattering length of both laser photons and PSL photons, which correlates closely to median grain size. As shown in Fig. 2, however, further decreasing grain size from 10 to 5 mm leads to minor enhancement of the resolution, in disagreement with our simulated results by which this reduction results in sufficient elevation of MTF. Therefore, we also measured the MTF using another scanner that features higher resolution through
improved scan optics and higher sampling rate (100 samples mm1). Unfortunately, although apparent is the elevation of MTF’s compared to those obtained from FASTSCAN, still no variation of MTF between both plates can be distinguished. Several aspects may account for this disagreement. First, the simulation program scan the information with much smaller sampling interval (about 1 /2 mm) compared with 10 mm here experimentally used. The purpose is to speculate the resolution limit that can be achieved. Such high sampling rate cannot be acceptable due to enormous acquisition time. Second, due to the inhomogeneous distribution of active center (europium ion in the case of BaFBr:Eu2) variation of PSL capacity of individual phosphor crystallite imposes additional noise on the measurement, which deteriorates the MTF behavior. Third, noise from other sources, such as electronics, scan optics, X-ray tube, also complicates the MTF determination. It suggests that although the MTF can
Fig. 3. Simulated contour plots of the logarithm of the density profile of the readout laser photons for varying mean scattering length: (a) 15; and (b) 5 mm.
36
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
be improved apparently through reducing grain size down to 5 mm according to simulation results, in practice it seems no use to decrease the grain size so further. Moreover, when the median grain size is lower than 5 mm the remarkable degradation of the PSL intensity may occur as evidenced by the deviation from the linearity of the IPSL versus median grain size curve (Fig. 1). This deviation may be attributed to the significant role of surface states in this grain size range which leads to more prominent non-radiative electron / hole recombination. Because NEQ is a more fundamental performance quantity expressing the signal-to-noise ratio and its dependence on spatial frequency, the impact of grain size on imaging quality can be clarified by comparing the NEQ values. As shown in Fig. 4, at low spatial frequency (1 lp mm 1) 15 mm-plate is superior to 10 and 5 mm plates due to its higher PSL large-area average. At high frequency level (4 lp mm 1) the 10 mm-plate exhibits much higher signal-to-noise ratio than that of 15 mm-plate thanks to its elevated MTF (see Eq. (2)). Since no more resolution gain and inferior PSL response at zero frequency, 5 mm-plate shows worse NEQ in the whole frequency range than that of 10 mm-plate. Taken together, at this exposure level (12 mGy), a typical X-ray fluence for mammography, 10 mm-plate features the best performance, although its NEQ is slightly lower than that of 15 mm-plate in the low frequency range. However, at higher exposure level, for example, 150 mGy, a typical dose for dental radiography, a rather different picture was obtained. As shown in Fig. 5, at low spatial frequency the plate made of 5 mm powder shows similar noise level as those of 10 and 15 mm plates in spite of its inferior average PSL. This results from its less structure noise due to denser packing of grains and consequent more perfect morphology, which is verified through the scanning electron micrographs (SEM). It indicates that at high X-ray fluence structure noise of plate becomes increasingly dominant. It should be noted that the structure noise effect in the measurement data
becomes less evident at the high frequencies because of the overlap of neighboring pixels. Consequently, 5 mm-plate exhibits the best NEQ at 1.0 lp mm 1. Therefore, for imaging of discrete objects, fine powder as 5 mm should be used. However, at the high frequency (4 lp mm 1), due to more prominent luminescence noise 5 mm-plate shows higher noise level than 10 and 15 mm plates. Taking the effect of MTF into account (Fig. 2), 10 mm-plate exhibits the best performance for tiny object imaging. From above evaluations, it can be concluded that variation of phosphor grain size results in different imaging performance. Conversely, the grain size and its distribution have to be kept unaltered during slurry processing. The concern is that the slurry processing should be efficient with the aim of breaking up particle agglomerates. The question is what occurs if all agglomerates are broken during the slurry process? Unfortunately, the grain size analysis of the slurry is very complicated. Therefore, the grain size distribution of the BaFBr:Eu storage phosphor by milling in ball mill were analyzed and the results were extended to the milling and mixing stage at the slurry preparation. As shown in Fig. 6, the milling up to 6 h reduces the coarse grain portion without altering the grain size diagram form, indicative of the decomposition of agglomerates. After 6 h milling, however, a noticeable increase of fine particles portion can be observed. With the further increase of processing time (for instance 16 h) a striking bimodal distribution of primary particles occurred. This may be explained by the fact that the tetragonal matlockite BaFBr grains are easy cleavable perpendicular to c -axis. Moreover, the remarkable increase of very-fine grains strongly deteriorates the PSL intensity (Fig. 1) which has to be avoided. Consequently, we strived for a narrow distribution of phosphor grains with desired d50 prior to slurry preparation. Further, an optimal slurry process has been established, which assured the breakdown of agglomerates in the slurry and promised
Fig. 4. NEQ’s as a function of median grain size at varying spatial frequency. (a) 1; and (b) 4 lp mm 1. X-ray data as in Fig. 2.
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
37
Fig. 5. WS and NEQ data for SPP with differing median grain size, measured under X-ray dose of 150 mGy.
minor effect on the narrow size distribution of initial phosphors. To take best advantage of the appropriate grain size with high X-ray conversion efficiency, great efforts have been made towards the optimal dense and homogeneous packing of grains in the active layer. BaFBr phosphor plate casting process involves a large number of variables which can greatly affect the macro- and microstructure and consequent properties of the final product. Here we prefer to present the impact of the packing structure of powder on the imaging performance.
Fig. 6. Grain size distribution of BaFBr:Eu2 powder as a function of ball milling time.
As shown in the scanning electron graphs in Fig. 7, the plate made under optimized process (process 1) exhibits nearly perfect particle packing, whereas the other plate that underwent non-optimized process (process 2) shows striking packing defects, which prove to be deleterious to the imaging performance. Fig. 8a compares the measured MTF curves for these two plates. In the low frequency range there is little difference. But for the frequencies greater than 2 lp mm 1 the variation is evident. The curve for process 1 plate is considerably higher and consequent has better resolution. This results from more intense scattering of readout laser in the process 2 plate due to loose packing of phosphor grains, which reduces image sharpness. Fig. 8b compares the WS profiles of the two plates at X-ray dose level of 150 mGy. In the low frequency range it can be clearly seen that process 2 plate gives more noise. Further, as we know, quantum noise which corresponds to statistically fluctuations of the X-ray quanta incident at the plate surface is dependent on spatial frequency given by, pffiffiffiffiffiffiffi Nq G anx MTF(v) (4) Assuming other noise components (for example, luminescence noise and readout noise from scanner) constant, the WS curves of process 2 plate should fall off more rapidly as a result of inferior MTF behavior. However, the fact is that the WS still sustain the same
38
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
Fig. 7. Scanning electron photomicrographs of BaFBr:Eu2 SPP: (a) process 1; and (b) process 2.
levels as those of process 1 plate. This indicates that the process 2 plate has a higher white noise level, which is assigned to structure or granularity noise. Granularity noise is related to the grainy structure of the phosphor plates, which results from inhomogeneities of the image receptor corresponding to local variations in the efficiency of converting the incident quantum energy into luminescence, caused by fluctuations in X-ray absorption, by differences in the propagation of laser and luminescence light in the powder layer, by varying PSL property of individual particles. All previously presented data were acquired under the conditions of a medical X-ray tube. It is well known that X-ray energy greatly influences the penetration depth of X-ray radiation, which, in turn, results in a different profile of storage center distribution. Therefore, this variable has to be investigated if SPP will be used for other applications. We present here the measurement using Cu /Ka (E / 8 keV) irradiation which is most common for structure
diffraction analysis. The attenuation coefficient of BaFBr plate for this X-ray energy is 630.0 cm 1 corresponding to a penetration depth of 16.0 mm, which is quite different from the previous measurement condition. In that case, the mean X-ray energy is estimated to be 35 keV with a penetration length of 231.0 mm. We investigated how this variation influences the choice of desired grain size. Due to technique difficulty only MTF and large-area average PSL data are here presented. The thickness of all evaluated plates (Fig. 9) is 180.09/ 2.0 mm. It is remarkable that the variation of grain size leads to much smaller changes in the resolution than the case discussed previously. In addition, the reduction amplitude of large-area average PSL is also much lower. It is not a surprise because most of the information is stored near the surface. It implies that, also verified by simulation results [9], for this low X-ray energy the change of mean scattering length, i.e. median grain size brings about little impact on the resolution. If remarkable resolution enhancement is desired, other methods
Fig. 8. MTF and WS curves of SPP demonstrated in Fig. 7 under X-ray exposure dose of 150 mGy.
H. Li et al. / Materials Science and Engineering B94 (2002) 32 /39
39
Fig. 9. MTF and large-area average PSL as a function of median grain size measured using Cu /Ka irradiation.
should be used, for example, a thickness value comparable to 60.0 mm, as indicated by simulation results. In that case, 15 mm grain size may be chosen since it provides highest large-area average PSL.
4. Conclusions The investigation of the correlation ‘structure/image properties’ in the BaFBr:Eu SPP shows that decreasing median grain size led to enhancement of MTF and reduction of granularity noise, whereas a deterioration of PSL. For imaging low X-ray fluence a median grain size of d50 /10 mm provided the best image property in the sense of signal-to-noise ratio (NEQ). In the case of high X-ray fluence, for imaging discrete objects the plate made of 5 mm powder demonstrated superior NEQ. However, for spatial frequencies greater than 2 lp mm 1 the plate made of 10 mm powder would still appear to be preferable. Furthermore, it was demonstrated that inhomogeneous structure with loose packing of phosphor powder featured excess granularity noise and degraded MTF. Finally, it was found that varying grain size played a minor impact on the MTF for the imaging of Cu /Ka irradiation where the variation of PSL was also smaller.
Acknowledgements We would like to acknowledge the financial support of the Bayerischer Forschungsverbund fu¨r Oberfla¨chentechnik (FOROB, project II.1). We are also grateful to Professor A. Winnacker for valuable discussions and comments on the manuscript.
References [1] A. Winnacker, Phys. Med. IX 2 /3 (1993) 95 /101. [2] M. Thoms, H. von Seggern, A. Winnacker, Phys. Rev. B44 (17) (1991) 9240 /9247. [3] M. Thoms, H. Burzlaff, A. Kinne, J. Lange, H. von Seggern, R. Spengler, A. Winnacker, Mater. Sci. Forum 107 (1) (1995) 228 / 231. [4] J.M. Spaeth, T.H. Hangleiter, F.K. Koschnick, T.H. Pawlik, Radiat. Eff. Defects Solids 135 (1995) 1 /10. [5] R.J. Klee, J. Phys. D: Appl. Phys. 28 (1995) 2529 /2533. [6] A. Harrison, M.T. Harrison, G.P. Keogh, R.H. Templer, Phys. Rev. B (1996) 5039 /5042. [7] P. Hackenschmied, H. Li, E. Epelbaum, R. Fasbender, M. Batentschuk, A. Winnacker, Radiat. Meas. 33 (2001) 669 /674. [8] J.C. Dainty, R. Shaw, Image Science, Academic Press, New York, 1974. [9] R. Fasbender, H. Li, A. Winnacker, Monte-Carlo simulations of the image properties of storage phosphor plates, to be published elsewhere.