Comparison of pinhole collimator materials based on sensitivity equivalence

Twisted clustered pinhole collimation for improved high-energy preclinical SPECT/PET

Objective.Advanced pinhole collimation geometries optimized for preclinical high-energyɣimaging facilitate applications such asɑandßemitter imaging, simultaneous multi-isotope PET and PET/SPECT, and positron range-free PET. These geometries replace each pinhole with a group of clustered pinholes (CPs) featuring smaller individual pinhole opening angles (POAs), enabling sub-mm resolution imaging up to ∼1 MeV. Further narrowing POAs while retaining field-of-view (FOV) may enhance high-energy imaging but faces geometrical constraints. Here, we detail how the novel twisted CPs (TCPs) address this challenge.Approach.We compared TCP and CP collimator sensitivity at equal system resolution (SR) and SR at matched sensitivity by tuning pinhole diameters for18F (511 keV) and89Zr (909 keV). Additionally, simulated Derenzo phantoms at low activity (LA: 12 MBq ml-1) and high activity (HA: 190 MBq ml-1) levels, along with uniformity images, were compared to assess image resolution and uniformity.Main results.At equal SR, TCP increased average central FOV sensitivity by 15.6% for18F and 29.4% for89Zr compared to CP. Image resolution was comparable, except for89Zr at LA, where TCP resolved 0.80 mm diameter rods compared to 0.90 mm for CP. Image uniformity was equivalent for18F, while for89Zr TCP granted a 10.4% improvement. For collimators with matched sensitivity, TCP improved SR by 6.6% for18F and 17.7% for89Zr while also enhancing image resolution; for18F, rods distinguished were 0.65 mm (CP) and 0.60 mm (TCP) for HA, and 0.70 mm (CP and TCP) for LA. For89Zr, image resolutions were 0.75 mm (CP) and 0.65 mm (TCP) for HA, and 0.90 mm (CP) and 0.80 mm (TCP) for LA. Image uniformity with TCP decreased by 18.3% for18F but improved by 20.1% for89Zr.Significance.This study suggests that the TCP design has potential to improve high-energyɣimaging.

A Simulation Study for Optimal Pinhole Collimator Design in Gamma Camera Systems

Background

The usage of a semiconductor detector with a pinhole collimator can provide high spatial resolution due to its high intrinsic resolution. However, the collimator system has low sensitivity due to the hole's small diameter. Therefore, the optimization between the spatial resolution and sensitivity is critical for determining the image quality in the gamma camera system.

Aims and Objectives

A pinhole collimator was designed and simulated to achieve the desired level of resolution and sensitivity in a gamma camera by utilizing a CdTe semiconductor detector.

Materials and Methods

To conduct this objective, a simulation toolkit based on the Geant4 Application for Tomographic Emission (GATE) was employed. The imaging capabilities of the proposed system were assessed by varying the magnification factor and pinhole diameter to estimate spatial resolution and sensitivity. Moreover, a hot rod phantom was designed to evaluate the system's overall imaging functionality.

Results

Results revealed that an increase in the pinhole diameter was correlated with an increase in sensitivity, while the spatial resolution was decreasing. There were distinct variations in sensitivity and spatial resolution depending on changes in the magnification factor as well. Finally, by analyzing trade-off curves, 1.38±0.081 mm was approximately the optimal pinhole diameter for our proposed system.

Conclusion

The optimum position for a pinhole collimator with a CdTe semiconductor detector was demonstrated.

Development and Evaluation of Image Reconstruction Algorithms for a Novel Desktop SPECT System

OBJECTIVE S

Various iterative reconstruction algorithms in nuclear medicine have been introduced in the last three decades. For each new imaging system, it is wise to select appropriate image reconstruction algorithms and evaluate their performance. In this study, three approaches of image reconstruction were developed for a novel desktop open-gantry SPECT system, PERSPECT, to assess their performance in terms of the quality of the resultant reconstructed images.

METHODS

In the present work, a proposed image reconstruction algorithm for the PERSPECT, referred to as quasi-simultaneous multiplicative algebraic reconstruction technique (qSMART), together with two popular image reconstruction methods, maximum-likelihood expectation-maximization (MLEM) and ordered-subsets EM (OSEM), were implemented and compared. Analytic and Monte Carlo simulations were applied for data acquisition of various phantoms including a micro-Derenzo phantom. All acquired data were reconstructed by the three algorithms using different number of iterations (1-40 ). A thorough set of figures-of-merit was utilized to quantitatively compare the generated images.

RESULTS

OSEM depicted reconstructed images of higher (or matching) quality in comparison to qSMART. MLEM also reached nearly similar quality as OSEM but at higher number of iterations. The graph of data discrepancy revealed that the ranking of the three approaches in terms of convergence speed is as qSMART, OSEM, and MLEM. Furthermore, bias-versus-noise curves indicated that optimal bias-noise results were achieved using OSEM.

CONCLUSION

The results showed that although qSMART can be applied for image reconstruction if being halted in the early iterations (up to 5), the best achievable quality of images is obtained using the OSEM.

Experimental Study of an Easily Controlled Ultra-High-Resolution Pixel-Matched Parallel-Hole Collimator with a Small Cadmium Zinc Telluride Pixelated Gamma Camera System

The aim of this study was to develop an easily controlled, ultra-high-resolution, tungsten parallel-hole collimator based on a small pixelated gamma camera system. A small cadmium zinc telluride (CZT) pixelated semiconductor detector (eValuator-2500 detector [eV product, Saxonburg, PA]) was evaluated. This detector is composed of an array of 51.2 × 0.8 × 3-mm³ individual CZT crystal elements. The ultra-high-resolution, pixel-matched, parallel-hole collimators consisted of six layers, with the same between the hole and pixel size. The basic characteristics of the imaging system, such as sensitivity and spatial resolution, was measured using a ⁵⁷Co point source. The measured averages of sensitivity and spatial resolution varied depending on the septal heights of the ultra-high-resolution parallel-hole collimator and source-to-collimator distances. When the 30-mm septal height was at 1-cm source-to-collimator distance, the spatial resolution was approximately 0.85 mm. Using 5-mm septal height, over 0.3 cps/kBq sensitivity was achieved. One advantage of our system is the use of stacked collimators that can select the best combination of system sensitivity and spatial resolution. Our results demonstrated that the developed CZT-pixelated gamma camera system using an ultra-high-resolution parallel-hole collimator of various collimator geometric designs has potential as an effective instrument.

Design and evaluation of an adaptive multipinhole collimator for high-performance clinical and preclinical imaging

OBJECTIVE

Pinhole single-photon emission computed tomography provides superior trade-off between resolution and detection efficiency as compared with conventional parallel-hole collimators for imaging small objects. This study aims to design and evaluate an optimized adaptive multipinhole (MPH) collimator for improved clinical myocardial perfusion single-photon emission computed tomography imaging (MPI) and preclinical small-animal imaging (SAI) of rats based on a clinical scanner.

METHODS

The target resolution and field of view was set to be 1/20 cm for MPI and 0.15/5 cm for SAI, respectively. We determined the design parameters by maximizing the detection efficiency based on system constraints. Point source simulations using Geant4 Application for Emission Tomography were performed for different collimator-to-center of field of view distances to assess the detection efficiency and resolution trade-off. The XCAT phantom with Tc-99m sestamibi distribution and the four-dimensional mouse whole-body phantom with Tc-99m methylene diphosphonate distribution were used to generate noise-free and noisy projections using a three-dimensional analytical MPH projector. Projections were reconstructed using a three-dimensional MPH ordered-subset expectation maximization algorithm. Noise and bias were assessed on the reconstructed images for different collimators.

RESULTS

The design parameters are (i) 14 pinholes with 3.42 mm aperture size, 14.5 cm collimator-to-detector distance for MPI; (ii) six pinholes with an aperture size of 0.94 mm, 21.2 cm collimator-to-detector distance for SAI. For MPI, the projected full width at half maximum values were 10.68 and 8.19 mm for low energy high resolution (LEHR) and MPH, respectively, whereas MPH had double detection efficiency. For SAI, the projected full width at half maximum values for LEHR and MPH were 4.93 and 1.20 mm, respectively, whereas the detection efficiency of MPH showed 17.5% improvement as compared with LEHR. The noise-bias trade-off improved for MPH as compared with LEHR for both MPI and SAI. The proposed collimator will have adjustable collimator-to-detector distances - that is, 14.5 cm for MPI and 21.2 cm for SAI.

CONCLUSION

The new collimator yields substantial improvement in image quality as compared with current MPI using LEHR with extra capability for SAI, bridging the clinical and preclinical imaging based on the same platform.

Review of SPECT collimator selection, optimization, and fabrication for clinical and preclinical imaging

In single photon emission computed tomography, the choice of the collimator has a major impact on the sensitivity and resolution of the system. Traditional parallel-hole and fan-beam collimators used in clinical practice, for example, have a relatively poor sensitivity and subcentimeter spatial resolution, while in small-animal imaging, pinhole collimators are used to obtain submillimeter resolution and multiple pinholes are often combined to increase sensitivity. This paper reviews methods for production, sensitivity maximization, and task-based optimization of collimation for both clinical and preclinical imaging applications. New opportunities for improved collimation are now arising primarily because of (i) new collimator-production techniques and (ii) detectors with improved intrinsic spatial resolution that have recently become available. These new technologies are expected to impact the design of collimators in the future. The authors also discuss concepts like septal penetration, high-resolution applications, multiplexing, sampling completeness, and adaptive systems, and the authors conclude with an example of an optimization study for a parallel-hole, fan-beam, cone-beam, and multiple-pinhole collimator for different applications.

Advances in pinhole and multi-pinhole collimators for single photon emission computed tomography imaging

The collimator in single photon emission computed tomography (SPECT), is an important part of the imaging chain. One of the most important collimators that used in research, preclinical study, small animal, and organ imaging is the pinhole collimator. Pinhole collimator can improve the tradeoff between sensitivity and resolution in comparison with conventional parallel-hole collimator and facilities diagnosis. However, a major problem with pinhole collimator is a small field of view (FOV). Multi-pinhole collimator has been investigated in order to increase the sensitivity and FOV with a preserved spatial resolution. The geometry of pinhole and multi-pinhole collimators is a critical factor in the image quality and plays a key role in SPECT imaging. The issue of the material and geometry for pinhole and multi-pinhole collimators have been a controversial and much disputed subject within the field of SPECT imaging. On the other hand, recent developments in collimator optimization have heightened the need for appropriate reconstruction algorithms for pinhole SPECT imaging. Therefore, iterative reconstruction algorithms were introduced to minimize the undesirable effect on image quality. Current researches have focused on geometry and configuration of pinhole and multi-pinhole collimation rather than reconstruction algorithm. The lofthole and multi-lofthole collimator are samples of novel designs. The purpose of this paper is to provide a review on recent researches in the pinhole and multi-pinhole collimators for SPECT imaging.

Rapid additive manufacturing of MR compatible multipinhole collimators with selective laser melting of tungsten powder

PURPOSE

The construction of complex collimators with a high number of oblique pinholes is very labor intensive, expensive or is sometimes impossible with the current available techniques (drilling, milling or electric discharge machining). All these techniques are subtractive: one starts from solid plates and the material at the position of the pinholes is removed. The authors used a novel technique for collimator construction, called metal additive manufacturing. This process starts with a solid piece of tungsten on which a first layer of tungsten powder is melted. Each subsequent layer is then melted on the previous layer. This melting is done by selective laser melting at the locations where the CAD design file defines solid material.

METHODS

A complex collimator with 20 loftholes with 500 μm diameter pinhole opening was designed and produced (16 mm thick and 70 × 52 mm(2) transverse size). The density was determined, the production accuracy was measured (GOM ATOS II Triple Scan, Nikon AZ100M microscope, Olympus IMT200 microscope). Point source measurements were done by mounting the collimator on a SPECT detector. Because there is increasing interest in dual-modality SPECT-MR imaging, the collimator was also positioned in a 7T MRI scanner (Bruker Pharmascan). A uniform phantom was acquired using T1, T2, and T2* sequences to check for artifacts or distortion of the phantom images due to the collimator presence. Additionally, three tungsten sample pieces (250, 500, and 750 μm thick) were produced. The density, attenuation (140 keV beam), and uniformity (GE eXplore Locus SP micro-CT) of these samples were measured.

RESULTS

The density of the collimator was equal to 17.31 ± 0.10 g∕cm(3) (89.92% of pure tungsten). The production accuracy ranges from -260 to +650 μm. The aperture positions have a mean deviation of 5 μm, the maximum deviation was 174 μm and the minimum deviation was -122 μm. The mean aperture diameter is 464 ± 19 μm. The calculated and measured sensitivity and resolution of point sources at different positions in the field-of-view agree well. The measured and expected attenuation of the three sample pieces are in a good agreement. There was no influence of the 7T magnetic field on the collimator (which is paramagnetic) and minimal distortion was noticed on the MR scan of the uniform phantom.

CONCLUSIONS

Additive manufacturing is a very promising technique for the production of complex multipinhole collimators and may also be used for producing other complex collimators. The cost of this technique is only related to the amount of powder needed and the time it takes to have the collimator built. The timeframe from design to collimator production is significantly reduced.