2005 IEEE Nuclear Science Symposium Conference Record
M07-293
A Simulation Study for SPECT Multi-Pinhole Detector Optimization Byung Jun Min, Yong Choi, Member IEEE, Jinhun Joung, Member IEEE, Nam Yong Lee, Tae Yong Song, Jin Ho Jung, Key Jo Hong Abstract–The aim of this study is to derive optimized parameters for a detector module employing scintillation crystal and multi-pinhole collimator which could be utilized in a variety of SPECT systems. A detector module was simulated to have 100 mm u 100 mm active area with 7 mm thick CsI(Tl) crystal. Monte Carlo simulation studies were performed to determine the optimal number of pinholes using Geant4 Application for Tomographic Emission (GATE). The number of pinholes was varied from 1 to 225, with a 2 mm pinhole diameter and 50 mm focal length. Perpendicular lead septa were employed, between pinholes, to prevent projections from overlapping. Hot and cold rod phantoms were used to evaluate the performance of the proposed system. SPECT images were reconstructed using the OSEM algorithm. Activity distribution was well visualized, and 12 to 6 mm diameter rods could be resolved in the reconstructed images. In this study we have designed a multi-pinhole imaging system providing good resolution and system sensitivity over a large FOV. The detector module, applicable to various SPECT systems, could provide improved performance.
I. INTRODUCTION 1
T
HE multi-pinhole collimator could provide highresolution SPECT imaging with comparable sensitivity compared to the parallel hole collimator. Recently, several systems employing the multi-pinhole collimator have been investigated and demonstrated a superior performance for the multi-pinhole for in vivo imaging of small animals [1-4]. A multi-pinhole collimator which could be utilized with a commercial gamma camera was developed [1]. The pinholes in this collimator were focused on a small field of view (FOV) and generated multiplexed projections through the different pinholes. This system offered a high resolution and system sensitivity. In a similar study, a stationary system dedicated to the imaging of small animals with nonoverlapping projections was proposed [2]. These systems demonstrated that a multi-pinhole collimator could provide an excellent resolution and sensitivity compared to those of single-pinhole and parallel hole collimators. However, due to the small FOV of about 50 mm, the application of the system is limited to small animal imaging. GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG This work was supported by Korea Science & Engineering Foundation (KOSEF), by a grant of the Korea Health 21 R&D Project (02-PJ3-PG6EV06-0002), and by the Siemens Medical Solutions USA, Inc. B. J. Min, Y. Choi, T. Y. Song. J. H. Jung, and K. J. Hong are with the Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, 135-710, Korea (e-mail :
[email protected]) J. Joung is with the Molecular Imaging, Siemens Medical Solutions USA, Inc., Hoffman Estates, IL 60195, USA N. Y. Lee is with the School of Computer Aided Science, Institute of Basic Sciences, Inje University, Kimhae, Kyungnam, 621-749, Korea
0-7803-9221-3/05/$20.00 ©2005 IEEE
A multi-pinhole system used in clinical investigation was introduced with a seven-pinhole collimator [5-6]. It was a planar emission tomography obtained with a limited angle using a standard gamma camera. The collimator comprised of seven-pinhole was used in myocardial perfusion studies. The resolution, however, was poor due to the large pinhole diameter. Therefore, there has been a necessity to develop a multipinhole system having both high resolution and a large FOV. We have conducted this study to provide a new multi-pinhole technique which provides high performance with a large enough FOV for a variety of studies. The aim is to derive an optimized pinhole configuration for a detector module employing scintillation crystal and multi-pinhole collimator which could be utilized in various investigative and clinical studies. II. MATERIALS AND METHODS A. Simulation Tool A Monte Carlo simulation tool, Geant4 application for tomographic emission (GATE) [7], was used to predict the performance, i.e. sensitivity and spatial resolution, of various multi-pinhole collimators. A low energy high resolution (LEHR) parallel hole collimator was also simulated to compare its performance with a multi-pinhole collimator. GATE is based on the well-proved simulation tool GEANT4 [8] and is a user-friendly simulation tool dedicated for SPECT and PET. GATE using a scripting language for interactive control of simulation provides the capability to perform realistic SPECT and PET simulation including: managing time and time-dependent processes, particle physics, detector and source movement, easy geometry modeling, and visualization. A cluster of computers was used to accelerate the simulation of GATE. The cluster was based on Condor and consisted of 6 nodes with 4 XEON 2.6 GHz and 2 Pentium4 3.2 GHz processors. The GATE input script was submitted by the job splitter into the cluster platform. The simulated events were stored in a list mode format (LMF) recording interaction position, deposited energy and the time stamp of each event. B. Detector Module Configuration A detector module was designed to have 100 mm u 100 mm active area and to be applicable to various SPECT systems. The detector module consisted of 100u100u7 mm3 thick CsI(Tl) plate crystal coupled to multi-pinhole collimator. Crystal thickness (7 mm) was selected to provide 95% detection efficiency for 140 keV JTrays.
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acceptance angle pinhole plate
lead septa detector
scintillator Fig. 1. Schematic diagram of lead septa with 1 mm lead plate
The schematic diagram of the detector and pinholes are shown in Fig. 1. Various numbers of pinholes, having 2 mm pinhole diameter and 50 mm focal length, were placed in tungsten plates. Pinhole diameter and focal length were selected to have comparable performance to parallel hole collimator. The acceptance angle was controlled by lead septa and focal length. Because multiplexing through different pinholes causes complicated problems in tomographical image reconstruction [1], perpendicular lead septa were employed to acquire non-overlapping projections through different pinholes and to improve reconstructed image quality. C. Optimization of the Multi-Pinhole Collimator In order to determine the optimal number of pinholes, the number of pinholes was varied from 1 to 225 with regular intervals. A 12 mm diameter sphere source and 0.1 mm diameter point source filled with Tc-99m were placed at the center of the FOV to measure sensitivity and resolution, respectively. The source to pinhole distance was 225 mm. The performance of multi-pinhole collimators was compared to LEHR parallel hole collimator (hole size: 1.4 mm, septal thickness: 0.16 mm, hole length: 32.8 mm). Simulation studies were conducted to evaluate the sensitivity and resolution as a function of various numbers of pinholes. D. SPECT Imaging Performance The detector was configured with the parameters providing high performance as demonstrated in the results of this study (see Section III.A), 100u100u7 mm3 CsI(Tl) plate crystal and 81-pinhole array with non-multiplexing. Cylindrical hot and cold rod phantoms filled with Tc-99m were used to investigate the performance of the proposed detector configuration. The phantoms have an outer diameter of 120 mm; the rod diameters in each of the six segments were 12, 10, 8, 7, 6, and 5 mm. Projection data over a 180-degree orbit were acquired using a 225 mm ROR. Fig. 2 illustrates the cross-sectional view of the multi-pinhole detector configuration; the detector module was rotated to acquire 60 projections with 3-degree
increments. detector module
…
phantom
3º
225 mm
G Fig. 2. The cross-sectional view of the multi-pinhole detector configuration
E. Image Reconstruction Images were reconstructed by ordered subset expectation maximization (OSEM) [9]. The unknown activity distribution x was solved by the set of linear equation using the measured projections S
x
j i 1
x
i j
¦a S ¦a t
/P ij
t, j
t
t
t, j
(1)
where at,j̓ and Pjare weight and expected value, respectively. The system matrix (at,j̓) takes into account the effects of pinhole diameter, crystal blurring, and detector geometry. The at,j̓ was obtained by a numerical calculation of the point
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spread function (PSF) of the system using a voxel-driven method [10]. PSF is the detection probability of a gamma ray emitted from a certain object position through a pinhole. The entry of the system matrix can be viewed as the conditional probability: Ps,X = Prob(detected at s _ emitted at X) where s and X are a detection position on the detector and an emission position on a voxel, respectively. Let ) ( p,X ; s ) be the PSF of a gamma ray emitted from X through the pinhole p. The system matrix entry Ps,X satisfies
Ps ,X
1
Xs
G Fig. 3. Simulated tungsten aperture (left) having 81-pinhole arrary placed on regular spacing of 11.1 mm and simulated projection estimated (right) with cylindrical phantom and the multi-pinhole collimator on the left
¦ ³ ³ )( p,X ; s)dsdx p X s
2.0
Sensitivity (cps/PCi)
If the photon is emitted randomly at X with a random direction, Xis parameterized by two angles D and I
X = (cosD cosI, sinIcosI, sinI), 0˺D˺S 0˺I˺S To accelerate the calculation of the emission without violating the probability ranges of D and I, random directions were restricted. Forced detection was used to increase the detection rate by forcing the photon to be directed to the circular target area of the pinhole and the diameter of the circular detection area on the detector. The following pseudo code explains the system matrix generation. For each uniformly distributed voxel X For each pinhole p For each uniformly distributed t in pinhole radius Compute detection s from X and t Add ) ( p, X ; s ) to Ps,X where t is the traversing point in the pinhole.
1.0 LEHR Pinhole
0.5 0.0 0
50
100
150
200
250
Number of Pinholes Fig. 4. Planar sensitivity of the multi-pinhole collimators and LEHR parallel hole collimator TABLE I COMPARISON OF SPATIAL RESOLUTION ESTIMATED WITH MULTI-PINHOLE AND LEHR PARALLEL HOLE COLLIMATOR
III. RESULTS A. Optimization of the Multi-Pinhole Collimator Fig. 3 illustrates the tungsten plate having 81-pinhole array with 13q acceptance angle. There is no multiplexing through different pinholes due to lead septa; there are 81 zones clearly observed in the projection. Fig. 4 illustrates planar sensitivity of multi-pinhole collimator plotted as a function of pinhole numbers and LEHR parallel hole collimator. As the number of pinholes increased sensitivity was improved and reached a plateau at about 90 pinholes. Table I shows the spatial resolutions estimated with multipinhole collimators at 64, 81, and 100 pinholes which have comparable sensitivity compared to LEHR parallel hole collimator.
1.5
Number of Pinholes
64
81
100
LEHR
Resolution (mm FWHM)
9.3
9.0
9.6
10.9
The data shown in Fig. 4 and Table I indicate that the 81pinhole collimator has better resolution and comparable sensitivity compared to the LEHR collimator. B. SPECT Imaging Performance Fig. 5 shows hot and cold rod phantoms and reconstructed images using the multi-pinhole collimator. Images were reconstructed using the OSEM algorithm with 10 iterations and 6 subsets. Hot rods from 12 to 6 mm diameter and cold rods from 12 to 7 mm diameter could be resolved in the reconstructed images.
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detector module designed in this study could provide improved performance and could be applicable to a variety of SPECT systems. REFERENCES [1]
GG
G
[2]
[3]
[4]
[5]
Fig. 5. Resolution phantoms (top) and reconstructed images using multipinhole (bottom): hot rod (left) and cold rod (right) phantom
[6]
IV. DISCUSSION AND CONCLUSION
[7] [8]
In this study, we have proposed a multi-pinhole detector module having better resolution and comparable sensitivity compared to the LEHR parallel hole collimator. 7 mm thick CsI(Tl) crystal was selected to provide 95% detection efficiency for 140 keV J-ray. This crystal is equivalent to 10 mm thick NaI(Tl) crystal which is most commonly used in clinical systems. The effects of various numbers of pinholes on system performance were evaluated by Monte Carlo simulation, GATE. Employing the multi-pinhole system, compared to the single-pinhole system, allowed us to increase the system sensitivity with negligible degradation of spatial resolution. Lead septa preventing overlapping through the different pinholes made a dead space on the crystal. As a result the system sensitivity was non-linear to the number of pinholes. 81-pinhole configuration had a good resolution and comparable sensitivity compared to the LEHR parallel collimator. Planar sensitivity and resolution was 1.9 cps/PCi and 9.0 mm FWHM, respectively. The optimized multi-pinhole detector module was evaluated by tomography images of resolution phantoms. Using the proposed detector module, excellent quality images could be reconstructed over a large FOV. The results suggest that a multi-pinhole system could provide improved performance and can be utilized in a variety of SPECT systems [11-12]. Multi-pinhole images were reconstructed using OSEM. A system matrix was obtained by numerical calculation of PSF for OSEM. The forced detection method was used to accelerate the system matrix generation. Although PSF can be measured by experiment [13], it would be very time consuming to measure and susceptible to measurement errors. The numerical method is more flexible and faster than an experimental measurement. We have designed a multi-pinhole imaging system providing good resolution and system sensitivity. The
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