Position Mapping, Energy Calibration, and Flood Correction Improve the Performances of Small Gamma Camera Using PSPMT

Transcription

1 Position Mapping, Energy Calibration, and Flood Correction Improve the Performances of Small Gamma Camera Using PSPMT Myung Hwan Jeong, Yong Choi, Member, IEEE, Yong Hyun Chung, Tae Yong Song, Jin Ho Jung, Key Jo Hong, Byung Jun Min, Yearn Seong Choe, Kyung-Han Lee, and Byung-Tae Kim Abstract-- The purpose of this study is to improve the performances of a small gamma camera using position sensitive PMT (PSPMT) by applying position mapping, energy calibration, and flood correction. The small gamma camera consists of a PSPMT coupled with either CsI(Tl) array or NaI(Tl) plate crystals. Flood images were obtained intrinsically in CsI(Tl) array system and with lead hole mask in NaI(Tl) plate system. The position mapping was performed by locating crystal arrays and hole positions in the two systems. The energy calibration was performed using energy discrimination table for each pixel array or for each hole position. The flood correction performed using a uniformity correction table containing the relative efficiency of each image element. The resolution of the CsI(Tl) array system was remained similar before and after corrections. On the other hand, the resolution of the NaI(Tl) plate system was improved about % after correction. The uniformity and linearity improved.9% to.% and. mm to mm, respectively, in the CsI(Tl) array system. The corrections more effectively improve the uniformity and linearity in the NaI(Tl) plate system,.% to 9.% and. mm to mm, respectively. Furthermore, the resolution deterioration observed in the NaI(Tl) plate system at the outer part of FOV, was considerably diminished after the corrections. The results of this study indicate that the correction algorithms considerably improve the performances of a small gamma camera and the performance gain is more prominent in the system employing a plate type crystal. I. INTRODUCTION HERE has been a growing interest in compact and high T resolution small gamma camera which can be used for applications ranging from scintimammography to small animal imaging. Several groups have been working on the development of the small gamma camera using position sensitive photomultiplier tubes (PSPMT) and scintillation crystals []-[]. Images of these small gamma cameras should have high uniformity and linearity. However, if a fixed pulseheight window is used over the entire field of view of a This study was supported by Korea Institute of Science & Technology Evaluation and Planning (KISTEP) and Ministry of Science & Technology through their National Nuclear Technology Program and by a grant of the Korea Health R&D Project (-PJ-PG-EV-), Ministry of Health & Welfare, Republic of Korea. The authors are with the Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, - 7, Korea ( y7choi@samsung.co.kr). camera, different spectra can cause apparent differences in sensitivity in different part of the detector, leading to image nonuniformities []. Furthermore, the PSPMT behaves nonlinear and non-uniform response []. In this reason, the linearity and the uniformity were degraded in small gamma camera system. In order to overcome these problems, several correction methods have been developed. For examples, multiwire readout technique in which, the charge on each anode wire is individually read out and digitized []-[8], and maximumlikelihood position estimation (MLPE) that involves the use of a look-up table (LUT) to map from measured PMT signals to position estimates [9]-[] improved the linearity and spatial resolution. Correction methods using mapped each crystal region []-[] and using the relative efficiency of each image pixel [] were used to improve the linearity and uniformity. Position mapping, energy calibration, and flood correction methods developed by Wojcik et al. []-[] is a practical and efficient method because it is easy to implement in hardware and fast in computation time. The performance characteristics of the correction method before and after corrections, however, were not fully characterized. In addition, these correction algorithms were applied to the array type crystal but were not used for a plate type crystal. Thus, we characterized the performances of a small gamma camera before and after the corrections employing CsI(Tl) array and extended the correction method to a small gamma camera using NaI(Tl) plate. II. MATERIALS AND METHODS A. System Configuration The small gamma camera consists of a general purpose parallel hole collimator, scintillation crystal, PSPMT (Hamamatsu R9), and subtractive resistive readout electronics []. The scintillation crystal employed in this study was either CsI(Tl) array ( mm diameter, pixel size: mm mm mm) or NaI(Tl) plate ( mm diameter, mm thickness). The length of the parallel-hole collimator is mm with hexagonal holes of. mm in diameter and septa thickness of. mm. The 8-X by 8-Y outputs are coupled to preamplifiers and a resistive charge division described in //$. IEEE.

2 detail in []. The data acquisition programs were based on Sparrow s KMAX software[7]. B. Position Mapping Position mapping method was used to correct distorted pixel positions of scintillator. After a raw image was obtained for each pixel position of the CsI(Tl) array, the position mapping table was made by using the raw image. When an event was detected, the event position was defined via the position mapping table. The raw image of the CsI(Tl) array system was acquired using a Na source to obtain better quality image at applied voltage of V because we were unable to resolve the individual crystal elements using 99m Tc source. In the NaI(Tl) plate system, the raw image was obtained using a 99m Tc point source at applied voltage of 9 V which was located at least five times the largest dimension of the useful field of view (UFOV) above the detector. Since each crystal pixel was not separated in NaI(Tl) plate systems, a lead hole mask ( cm cm mm, mm hole diameter, mm pitch) was fabricated to apply to the NaI(Tl) plate system (Figure ). To achieve high resolution, the raw images were acquired by moving the lead mask. mm along the x and y- axes. The obtained raw images were overlapped each other and the position mapping table was generated by using the overlapped raw image. Figure shows the raw images and the map of crystal regions in CsI(Tl) array and NaI(Tl) plate systems. (a) (b) Fig.. Raw images (left column) and position mapping images (right column) obtained with CsI(Tl) array (a) and NaI(Tl) plate (b). Fig.. The basic components of small gamma camera system with a NaI(Tl) plate used for position mapping. C. Energy Calibration The energy calibration was performed using pulse height spectrum of each pixel array or of each hole position. The energy spectrum table was obtained with a general purpose parallel hole collimator using a point source ( mci 99m Tc) which was located cm above the detector. The energy spectra of each crystal array or each hole position from CsI(Tl) array and NaI(Tl) plate have been used to set energy window of each region. Figure shows energy spectra for three different positions of the NaI(Tl) plate system. Fig.. Energy spectra of the each hole in NaI(Tl) plate system mapped by lead hole mask. Those were recorded with 99m Tc point source which was moved from (top) the center to (middle and bottom) the off-center. D. Flood Correction The flood correction was performed using a uniformity correction table containing the relative efficiency of each image element. The uniformity correction table was obtained for about hours with a flood source to minimize the statistical noise. Flood source was sufficiently large to cover //$. IEEE.

3 the entire detector and placed in direct contact with the collimated detector. Flood source consists of a plastic container filled with radioactive solution ( mci 99m Tc) (Figure ). Since the counts of each pixel in raw image were much fewer than the corresponding values of uniformity correction table, the image was multiplied by and then was divided by the obtained uniformity correction table. Figure shows flood correction table image and the corrected image. Fig.. Photograph of uniformity phantom. Fig.. The image of uniformity correction table (left) and the corrected image (right). E. Performance Measurements - Spatial resolution. The spatial resolution was measured using two capillary tubes ( µci 99m Tc) having an inner diameter of. mm. Two-line sources with 8 mm spacing were used to obtain a millimeters-per-pixel calibration. Full width at half maximums (FWHMs) of the profiles of the two-line images were measured when the distance from detector to source varied from mm to mm or the distance from the center to the edge of the detector varied from mm to mm. The images were acquired with % energy window. - Sensitivity. The system sensitivity was measured using a µci 99m Tc point source while the distance from the detector to source varied from mm to mm. Radioactivity decay and background activities were corrected when the sensitivity was acquired. Uniformity correction was not applied and the images were acquired with % energy window. - Linearity. The linearity was measured with parallel-line bar phantom and was expressed as a standard deviation of the line spread function peak separation. After an image of the parallel-line bar phantom was obtained, the linearity was calculated using the line image. - Uniformity. The uniformity was measured using uniformity phantom that was filled with radioactive solution ( µci 99m Tc) (Figure ). To calculate the uniformity of the CsI(Tl) array and NaI(Tl) plate systems, the corrected image was smoothed with Gaussian filter defined by the national electrical manufactures association (NEMA) [8]. III. RESULTS A. Spatial resolution The resolution of the CsI(Tl) array system was remained similar before and after corrections. On the other hand, the resolution of the NaI(Tl) plate system was improved about % after correction (Figure (top)). The resolution of the CsI(Tl) array system was increased from. mm FWHM to.9 mm FWHM at the outer part of FOV. The Figure (bottom) demonstrates that the resolution deterioration observed in the NaI(Tl) plate system at the outer part of FOV was considerably improved from.7 mm FWHM to. mm FWHM after the corrections. System resolution (mm FWHM) System resolution (mm FWHM) 7 7 CsI(Tl) after correction Distance from detector to source(cm) CsI(Tl) after correction Distance from center to source (cm) Fig.. System resolutions measured with varying distance from detector to source (top) and from center to source (bottom). B. Sensitivity //$. IEEE.

4 The sensitivity of the CsI(Tl) and NaI(Tl) systems were remained similar before and after corrections at the center part of FOV. On the other hand, the sensitivity of the NaI(Tl) system was considerably increased after corrections from. cps/µci to.8 cps/µci at the outer part of FOV (Figure 7 (bottom)). System sensitivity (cps/uci) System sensitivity (cps/uci) C si(tl) after correction Distance from detector to source (cm) N ai(tl) after correction Distance from center to source (cm) Fig. 7. System sensitivities measured with varying distance from detector to source (top) and from center to source (bottom). Fig. 8. Images of parallel-line bar phantom without (left) and with (right) the corrections obtained with the CsI(Tl) array system. Fig. 9. Line source images without (left) and with (right) the corrections obtained with the NaI(Tl) plate system. D. Uniformity The uniformities of the CsI(Tl) array and NaI(Tl) plate systems were summarized in Table. The result showed the improved uniformity after correction in both systems. TABLE UNIFORMITY OF CsI(Tl) ARRAY AND NaI(Tl) PLATE SYSTEMS Uniformity CsI(Tl) array NaI(Tl) plate Before Integral.9%.% Correction Differential.%.7% After Integral.% 9.% Correction Differential 8.%.% C. Linearity The linearities of the CsI(Tl) array and NaI(Tl) plate systems mesured before and after corrections were summarized in table. The result showed the improved linearity after correction in both of the systems. Figures 8 and 9 show images of parallel-line bar phantom. TABLE LINEARITYES OF CsI(Tl) ARRAY AND NaI(Tl) PLATE SYSTEM Linearity CsI(Tl) array NaI(Tl) plate Center. mm. mm Before. mm Correction. mm. mm off-center After Correction Center mm mm. mm off-center mm mm Fig.. Flood source images without (left) and with (right) the corrections obtained with the CsI(Tl) array system. Fig.. Flood source images without (left) and with (right) the corrections obtained with the NaI(Tl) plate system //$. IEEE.

5 IV. DISCUSSION AND CONCLUSION We have characterized the performances of position mapping, energy calibration and flood correction method in a small gamma camera using CsI(Tl) array and PSPMT. The method was extended to NaI(Tl) plate system. In order to evaluate the performances of the correction methods in both CsI(Tl) array and NaI(Tl) plate systems, resolution, sensitivity, linearity and uniformity were assessed before and after the corrections. The resolution of NaI(Tl) plate system was considerably improved about % after the correction. The improvement of resolution in CsI(Tl) array system is less prominent, because the enegy spectrum of each pixel of the CsI(Tl) array scintillator were more homogeneous than one of NaI(Tl) plate scintillator. The energy spectra which were defined by the hole positions of the lead mask were different from each other in the photopeaks in the NaI(Tl) plate system. In this reason, a photopeak of event was not exactly entered into the fixed energy window when the source was moved from the center to the edge of the detector. Therefore, the resolution of the NaI(Tl) plate system was deteriorated. This problem was corrected by using the energy calibration method and the resolution of the NaI(Tl) plate system was improved. Since the resolution of the CsI(Tl) array system was differently obtained according to source places at either between the pixels or a pixel, its resolution was fluctuated when the source was moved from the center to the edge of detector. Because of the variation of the energy spectra, the sensitivity of the NaI(Tl) plate system also was improved from. cps/µci to. cps/µci after correction at the edge of the detector, though the correction method did not affect the sensitivity at the center of the detector. The advantages of using the proposed plate scintillator system with correction algorithms are spatial resolution improvement, more uniform spatial resolution performance, and linearity and uniformity improvement. The advantages of using the array scintillator system with correction methods are linearity and uniformity improvement. Although the resolution of the NaI(Tl) plate system was slightly warse than the CsI(Tl) array system, ( NaI(Tl) plate:. mm, CsI(Tl) array:. mm ), other system performances were better in the NaI(Tl) plate system. The results of this study indicate that the correction algorithms considerably improve the performances of a small gamma camera and the performance gain is more prominent in the system employing a plate type crystal. [] T. Y. Song, Y. Choi, Y. H. Chung, J. H. Jung, Y. S. Choe, K. H. Lee, S. E. Kim, and B. T. 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