Multi-pinhole collimator design for small-object imaging with SiliSPECT: a high-resolution SPECT

Performance evaluation of a novel multi-pinhole collimator for dopamine transporter SPECT

There is a tradeoff between spatial resolution and count sensitivity in SPECT with conventional collimators. Multi-pinhole (MPH) collimator technology has potential for concurrent improvement of resolution and sensitivity in clinical SPECT of 'small' organs. This study evaluated a novel MPH collimator specifically designed for dopamine transporter (DAT) SPECT with a triple-head SPECT camera. Count sensitivity was measured with a ^99mTc point source placed on the lattice points of a 1 cm grid covering the whole field-of-view (FOV). Spatial resolution was assessed with a Derenzo type hot rod phantom. An anthropomorphic striatum phantom was scanned with total activity representative of a typical patient scan and different striatum-to-background activity concentration ratios. Recovery of striatum-to-background contrast was assessed by the contrast-recovery-coefficient. Measurements were repeated with double-head SPECT with fan-beam or low-energy-high-resolution-high-sensitivity (LEHRHS) collimators. A patient referred to DAT SPECT because of suspicion of Parkinson's disease was scanned with both LEHRHS and MPH collimators after a single tracer injection. The axial MPH sensitivity profile was approximately symmetrical around its peak, although it was shifted 7 cm towards the patient to simplify positioning. Peak sensitivity of the triple-head MPH system in the center of the FOV was 620 cps MBq^-1 compared to 225 cps MBq^-1 for the double-head fan-beam system. Sensitivity of the MPH system decreased towards the edges of the FOV. The full width of the sensitivity profile at 200 cps MBq^-1 was 21 cm transaxially and 11 cm axially. In MPH SPECT of the Derenzo phantom all rods with ≥ 5 mm diameter were clearly visible. MPH SPECT improved striatal contrast recovery by ≥ 20% compared to fan-beam SPECT. The patient scan demonstrated good image quality of MPH SPECT with almost PET-like delineation of putamen and caudate nucleus. SPECT with dedicated MPH collimators provides considerable improvement of the resolution-sensitivity tradeoff in DAT SPECT compared to SPECT with fan-beam or LEHRHS collimators.

Gamma Radiation Imaging System via Variable and Time-Multiplexed Pinhole Arrays

Biomedical planar imaging using gamma radiation is a very important screening tool for medical diagnostics. Since lens imaging is not available in gamma imaging, the current methods use lead collimator or pinhole techniques to perform imaging. However, due to ineffective utilization of the gamma radiation emitted from the patient’s body and the radioactive dose limit in patients, poor image signal to noise ratio (SNR) and long image capturing time are evident. Furthermore, the resolution is related to the pinhole diameter, thus there is a tradeoff between SNR and resolution. Our objectives are to reduce the radioactive dose given to the patient and to preserve or improve SNR, resolution and capturing time while incorporating three-dimensional capabilities in existing gamma imaging systems. The proposed imaging system is based on super-resolved time-multiplexing methods using both variable and moving pinhole arrays. Simulations were performed both in MATLAB and GEANT4, and gamma single photon emission computed tomography (SPECT) experiments were conducted to support theory and simulations. The proposed method is able to reduce the radioactive dose and image capturing time and to improve SNR and resolution. The results and method enhance the gamma imaging capabilities that exist in current systems, while providing three-dimensional data on the object.

Evaluation of tomographic image quality of extended and conventional parallel hole collimators using maximum likelihood expectation maximization algorithm by Monte Carlo simulations

OBJECTIVE

One of the major problems associated with parallel hole collimators (PCs) is the trade-off between their resolution and sensitivity. To solve this problem, a novel PC - namely, extended parallel hole collimator (EPC) - was proposed, in which particular trapezoidal denticles were increased upon septa on the side of the detector.

MATERIALS AND METHODS

In this study, an EPC was designed and its performance was compared with that of two PCs, PC35 and PC41, with a hole size of 1.5 mm and hole lengths of 35 and 41 mm, respectively. The Monte Carlo method was used to calculate the important parameters such as resolution, sensitivity, scattering, and penetration ratio. A Jaszczak phantom was also simulated to evaluate the resolution and contrast of tomographic images, which were produced by the EPC6, PC35, and PC41 using the Monte Carlo N-particle version 5 code, and tomographic images were reconstructed by using maximum likelihood expectation maximization algorithm.

RESULTS

Sensitivity of the EPC6 was increased by 20.3% in comparison with that of the PC41 at the identical spatial resolution and full-width at tenth of maximum here. Moreover, the penetration and scattering ratio of the EPC6 was 1.2% less than that of the PC41. The simulated phantom images show that the EPC6 increases contrast-resolution and contrast-to-noise ratio compared with those of PC41 and PC35.

CONCLUSION

When compared with PC41 and PC35, EPC6 improved trade-off between resolution and sensitivity, reduced penetrating and scattering ratios, and produced images with higher quality. EPC6 can be used to increase detectability of more details in nuclear medicine images.

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.

Advances in preclinical SPECT instrumentation

Preclinical SPECT of rodents is both in demand and very demanding. The need for high spatial resolution in combination with good sensitivity has given rise to considerable innovation in the areas of detectors, collimation, acquisition geometry, and image reconstruction. Some of the developments described herein are beginning to carry over into clinical imaging as well.

The effects of object activity distribution on multiplexing multi-pinhole SPECT

We aim to study the effects of activity distribution for multiplexing multi-pinhole (MPH) SPECT. Three digital phantoms, including a hot rod, a cold rod and a cold sphere phantom, were used. Different degrees of multiplexing were obtained by (i) adjusting the MPH pattern for the same 4-pinhole collimator (scheme 1) and (ii) increasing the number of pinholes (scheme 2). Noise-free and noisy projections were generated using a 3D analytical MPH projector based on the same acquisition time. Projections were reconstructed using OS-EM without resolution recovery. Normalized mean-square-error (NMSE), noise, image profiles and signal-to-background ratios (SBR) were assessed. For the hot rod phantom, the NMSE-noise trade-offs slightly improves for multiplexing designs in scheme 2. Substantial artifacts were observed and the NMSE-noise trade-offs slightly worsened for multiplexing designs for the cold phantoms. Resolutions slightly degraded for higher degrees of multiplexing (∼39-65%) for the cold rod phantom. For the cold sphere phantom, image profiles showed non-multiplexing designs better emulated the phantom, while ∼20% multiplexing performs similarly as compared to non-multiplexing in SBR. Our results indicate that multiplexing can help for sparse objects but leads to a significant image degradation in non-sparse distributions. Since many tracers are not highly specific, and the gain of detection efficiency by allowing multiplexing is fairly offset by image degradations, multiplexing needs to be kept to a minimum for optimum MPH collimator designs.

An enhanced high-resolution EMCCD-based gamma camera using SiPM side detection

Electron-multiplying charge-coupled devices (EMCCDs) coupled to scintillation crystals can be used for high-resolution imaging of gamma rays in scintillation counting mode. However, the detection of false events as a result of EMCCD noise deteriorates the spatial and energy resolution of these gamma cameras and creates a detrimental background in the reconstructed image. In order to improve the performance of an EMCCD-based gamma camera with a monolithic scintillation crystal, arrays of silicon photon-multipliers (SiPMs) can be mounted on the sides of the crystal to detect escaping scintillation photons, which are otherwise neglected. This will provide a priori knowledge about the correct number and energies of gamma interactions that are to be detected in each CCD frame. This information can be used as an additional detection criterion, e.g. for the rejection of otherwise falsely detected events. The method was tested using a gamma camera based on a back-illuminated EMCCD, coupled to a 3 mm thick continuous CsI:Tl crystal. Twelve SiPMs have been mounted on the sides of the CsI:Tl crystal. When the information of the SiPMs is used to select scintillation events in the EMCCD image, the background level for (99m)Tc is reduced by a factor of 2. Furthermore, the SiPMs enable detection of (125)I scintillations. A hybrid SiPM-/EMCCD-based gamma camera thus offers great potential for applications such as in vivo imaging of gamma emitters.

Evaluation of image reconstruction for mouse brain imaging with synthetic collimation from highly multiplexed SiliSPECT projections

We have performed a theoretical study to explore the potential and limitations of synthetic collimation for SPECT imaging with stacked-detector acquisition (dual magnification). This study will be used to optimize SiliSPECT, a small-animal SPECT for imaging small volumes such as a mouse brain at high sensitivity and resolution. The synthetic collimation enables image reconstruction with a limited number of camera views and in the presence of significant multiplexing. We also developed a new formulation to quantify the multiplexed object sensitivity and investigated how this changes for different acquisition parameters such as number of pinholes and combinations of front and back detector distances for imaging objects as small as the mouse brain. In our theoretical studies, we were not only able to demonstrate better reconstruction results by incorporating two detector magnifications in comparison to either one alone, but also observed an improved image reconstruction by optimizing the detector-collimator distances to change the multiplexing ratio between the front and back detectors.

Molecular imaging and therapy of cancer with radiolabeled nanoparticles

This review summarizes the current state-of-the-art of radiolabeled nanoparticles for molecular imaging and internal radiotherapy applications targeting cancer. With the capacity to provide enormous flexibility, radiolabeled nanoparticles have the potential to profoundly impact disease diagnosis and patient management in the near future. Currently, the major challenges facing the research on radiolabeled nanoparticles are desirable (tumor) targeting efficacy, robust chemistry for both radionuclide encapsulation/incorporation and targeting ligand conjugation, favorable safety profile, as well as certain commercial and regulatory hurdles.