An energy-optimized collimator design for a CZT-based SPECT camera

Abstract

In single photon emission computed tomography, it is a challenging task to maintain reasonable performance using only one specific collimator for radiotracers over a broad spectrum of diagnostic photon energies, since photon scatter and penetration in a collimator differ with the photon energy. Frequent collimator exchanges are inevitable in daily clinical SPECT imaging, which hinders throughput while subjecting the camera to operational errors and damage. Our objective is to design a collimator, which is independent of the photon energy, performs reasonably well for commonly used radiotracers with low- to medium-energy levels of gamma emissions. Using the Geant4 simulation toolkit, we simulated and evaluated a parallel-hole collimator mounted to a CZT detector. With the pixel-geometry-matching collimation, the pitch of the collimator hole was fixed to match the pixel size of the CZT detector throughout this work. Four variables, hole shape, hole length, hole radius/width and the source-to-collimator distance were carefully studied. Scatter and penetration of the collimator, sensitivity and spatial resolution of the system were assessed for four radionuclides including ⁵⁷Co, ^99mTc, ¹²³I and ¹¹¹In, with respect to the aforementioned four variables. An optimal collimator was then decided upon such that it maximized the total relative sensitivity (TRS) for the four considered radionuclides while other performance parameters, such as scatter, penetration and spatial resolution, were benchmarked to prevalent commercial scanners and collimators. Digital phantom studies were also performed to validate the system with the optimal square-hole collimator (23 mm hole length, 1.28 mm hole width, and 0.32 mm septal thickness) in terms of contrast, contrast-to-noise ratio and recovery ratio. This study demonstrates promise of our proposed energy-optimized collimator to be used in a CZT-based gamma camera, with comparable or even better imaging performance versus commercial collimators such as low-energy high resolution (LEHR) and medium energy general purpose (MEGP) collimators.

Introduction

Single photon emission computed tomography (SPECT) radiotracers emit photons over a range of energies and collimator specifications vary with the energy levels of the radionuclides being imaged. Hence multiple collimators are required for imaging such as commercially available low-energy high-resolution (LEHR) and medium-energy general-purpose (MEGP) collimators coupled with conventional NaI(Tl) detectors [1]. This necessitates collimator exchanges between scans, sometimes even multiple times during the same day, and therefore hampers an efficient management of clinical imaging due to the time-consuming procedures. More importantly, low energy collimators generally are quite fragile, and their thin septa can be damaged easily by mechanical abuse (dropping, stacking on sharp objects, etc.).

Although, collimators are crucial for producing high quality tomographic images, they introduce artifacts into images from photons penetrating through the collimators and decreases sensitivity by absorbing/scattering photons. Thus, a careful collimator design with various criteria is essential in a new SPECT system to improve image quality [2]. The aim of this work, subsequently, has been to design a single collimator with improved sensitivity while maintaining comparable resolution over a broad spectrum of diagnostic energy levels.

The collimated gamma rays are traditionally detected by a scintillator coupled with photomultiplier tubes, which is still largely used in in vivo and non-invasive cross-sectional nuclear imaging. A renewed effort is being made to improve the quality of SPECT images by replacing these conventional indirect-conversion detectors with direct-conversion semiconductor detectors with better energy resolution [3], [4]. Recent advances in fabricating solid-state detectors such as cadmium zinc telluride (CZT) [4], [5] together with optimization of collimator designs [6], [7], [8] promise improvement in resolution and sensitivity of the front-end system. To that end, our energy-optimized collimator has been designed for a SPECT scanner equipped with a pair of relatively large-area CZT detectors with a small pixel pitch to operate over a broad range of gamma emission energies without collimator changes.

For a pixelated detector, a straightforward approach for maximizing sensitivity is to match the collimator holes with detector pixels to obtain a pixel-geometry-matching collimator (PGMC) [9], [10]. Detectors combined with PGMCs are expected to be more efficient than non-pixel-matching collimations. The length, size, and shape of collimator holes, however, remain to be optimized to achieve energy independence. Researchers have proven that the extended low-energy general-purpose collimator (ELEGP) is a better choice than the MEGP collimator for mid-energy photons emitted from radiotracers such as ¹¹¹In and ¹²³I [11]. It was also found that the best performance could be obtained from a collimator that allowed a moderate amount of septal scatter and penetration [11], [12]. Improved reconstruction algorithms also help compensate for the serious scattering and penetration [13]. New camera architecture [6] has sought to trade-off spatial resolution and sensitivity with pixelated semiconductor detectors. However, the majority of these papers have focused only on a specific radionuclide. Other efforts have been limited to optimizing collimators for simultaneous or sequential dual-isotope imaging [14], [15]. There has been no initiative so far to actually get rid of the energy dependency of SPECT nuclides on collimators by exploiting the superior energy resolution and high stopping power of CZT or CdTe detectors to avoid collimator changes.

In this paper, we describe the design characteristics of an optimized parallel-hole PGMC for a SPECT gamma camera using a CZT detector based on the Geant4 Monte Carlo simulations. Materials used are introduced with details on the tools adopted for evaluation, followed by the performance of the designed collimator combined with the large-area CZT detector. Our goal is to maximize sensitivity for acceptable values of other indices at low and medium energy gamma emissions from ^99mTc, ⁵⁷Co, ¹²³I and ¹¹¹In radionuclides. Digital phantom studies are performed to validate the optimized energy-optimized collimator.