Advances in Clinical Molecular Imaging Instrumentation

In this article, we describe recent developments in the design of both single-photon emission computed tomography (SPECT) and positron emission tomography (PET) instrumentation that have led to the current range of superior performance instruments. The adoption of solid-state technology for either complete detectors [e.g., cadmium zinc telluride (CZT)] or read-out systems that replace photomultiplier tubes [avalanche photodiodes (APD) or silicon photomultipliers (SiPM)] provide the advantage of compact technology, enabling flexible system design. In SPECT, CZT is well suited to multi-radionuclide and kinetic studies. For PET, SiPM technology provides MR compatibility and superior time-of-flight resolution, resulting in improved signal-to-noise ratio. Similar SiPM technology has also been used in the construction of the first SPECT insert for clinical brain SPECT/MRI

A novel approach to image reconstruction and calibration for a multi-slit-slat SPECT system

In the context of the development of a simultaneous SPECT/MRI system, we have previously proposed a multi-minislit-slat (MSS) collimator, with multiple sections of short slits in order to improve the angular sampling. The data can be reconstructed using a 3D reconstruction algorithm that models the collimator geometry. One drawback, however, is that the projection data obtained with this collimator are difficult to interpret visually. Also, calibration can be problematic, as each mini-slit only covers part of the object FoV. We have therefore developed an algorithm for transforming the MSS projection data into the traditional sinogram format. These sinograms consist of multiple thin tilted lines with gaps in between due to the lack of detector rotation in this system. The data can be reconstructed using standard parallel-beam algorithms, taking into account the fact that there are data missing. We have shown with simulations and measurements that the algorithm can transform complex data, consisting of multiple rough broken line segments, into simple sine-curves. This algorithm can be useful for interpreting the acquired MSS data, reconstructing images, and calibrating the system

Shielding requirements of a SPECT insert for installation in a PET/MRI system

The objective of this work is to evaluate the shielding requirements of a SPECT insert for installation in the Siemens Biograph mMR in order to perform simultaneous SPECT/MR imaging of the human brain. We intend to use the radionuclides 99mTc, 123I and 111In. The main photopeaks of these radionuclides have the following energies: 140.5, 159.0, 171.3 and 245.4 keV. There is also about ∼3% of emission probability of high energy gamma photons for 123I in the range of 248-784 keV. The main constraints to the design of the gamma shielding are the presence of high energy photons, the weight, the MR compatibility and the PET LSO crystals intrinsic activity. We used GATE to simulate a SPECT acquisition, defining an MRI system with LSO crystals, a partial SPECT ring and a NEMA phantom. We also defined a lead (Pb) base plate (BP) to simulate the support system and three Pb shielding volumes with variable thickness: front and end (FE), back (B), and lateral (L) shield. These volumes reduce interference from out-of-field activity, LSO intrinsic activity and edge effects, respectively. We performed 4 sets of simulations, with variable FE, variable B, variable L and variable BP thickness, respectively, with a NEMA phantom filled with 185 MBq of 123I or 111In. For all simulations, we compared the different energy spectra and count-distribution plots. Results show that a Pb shielding configuration with a thickness of 6 mm-F, 2 mm-E, 3 mm-B, and 5 mm-L is appropriate for the insert. For 123I there is still a high contribution from high energy photons, as the amount of shielding is limited by weight, however this contribution is likely to be overestimated in the simulations as compared to practice. The effect of the LSO intrinsic activity is negligible at the energies of interest

Length functions on currents and applications to dynamics and counting

The aim of this (mostly expository) article is twofold. We first explore a variety of length functions on the space of currents, and we survey recent work regarding applications of length functions to counting problems. Secondly, we use length functions to provide a proof of a folklore theorem which states that pseudo-Anosov homeomorphisms of closed hyperbolic surfaces act on the space of projective geodesic currents with uniform north-south dynamics.Comment: 35pp, 2 figures, comments welcome! Second version: minor corrections. To appear as a chapter in the forthcoming book "In the tradition of Thurston" edited by V. Alberge, K. Ohshika and A. Papadopoulo

A New Concept for a Low-Dose Stationary Tomographic Molecular Breast Imaging Camera Using 3D Position Sensitive CZT Detectors

Pixelated CZT detectors have been used in a variety of molecular imaging applications for many years. The interplay of gamma camera and collimator geometric design, gantry motion, and image reconstruction determines the image quality and dose-time-FOV trade-offs. In particular, Molecular Breast Imaging (MBI) has been shown to provide excellent diagnostic results in patients with dense breast tissue, but higher than mammography patient dose and long imaging time impede its wide adoption. We propose a new transformative system concept combining the advantages of CZT detectors (superior energy and position resolution and depth of interaction sensing), multi-pinhole collimation and novel image reconstruction to mitigate those drawbacks without compromising diagnostic content. The closely spaced pinholes allow tomographic image reconstruction, improve sensitivity and angular sampling, but result in significant multiplexing. Novel de-multiplexing algorithms have been developed to mitigate the adverse multiplexing artefacts using the DOI. GATE simulations of the new camera demonstrate a potential to reduce the patient dose by at least a factor of 5 in comparison to planar MBI, thus reducing the dose to the level of an average mammography scan. The first prototype has been built at Kromek with 3D position sensitive CZT detectors and is being evaluated using an "activity-painting"setup with a point 57Co source. Initial results demonstrate the expected performance improvement with the use of sub-pixelisation and DOI. The next steps of the development will include accurate evaluation of the image quality and the dose reduction followed by building a larger scale clinical prototype using optimised detector design

Design of an Ultra-low-dose, Stationary, Tomographic Molecular Breast Imaging System

Molecular Breast Imaging (MBI) has been shown to have high sensitivity in detection of cancer, even in patients with dense breasts where conventional mammography has issues. However the technique has limited acceptance due to the relatively high radiation dose and long imaging time. Improved lesion detection can be achieved using tomography, however this normally involves detector motion and complex mechanics. Our goal is to develop a low-dose stationary tomographic MBI system with similar or better sensitivity for lesion detection to conventional planar MBI. The proposed system utilizes state-of-the-art cadmium zinc telluride (CZT) detectors based on 2mm pixels, with sub-pixelization and depth of interaction (DOI) capability, combined with densely packed multi-pinhole collimators. Use of closely-spaced pinholes improves efficiency and angular sampling, but results in significant multiplexing. De-multiplexing algorithms have been developed that take advantage of the DOI acquisition to achieve tomographic reconstruction using two opposing planar detectors which apply mild compression to the breast. Simulation studies of multiple lesions with clinically realistic contrast have been used to demonstrate the feasibility of the design and to characterize the expected performance. Reconstruction without de-multiplexing resulted in significant artefacts. De-multiplexing without DOI had limited success but with DOI resulted in artefact-free images, with good contrast and axial plane definition. Lesion detectability was preserved even with reduction of acquisition time (or radiation dose) by a factor of 4. Further optimization has potential for even greater dose reduction. A prototype system is currently being constructed to validate these findings

Challenges in Optimization of a Stationary Tomographic Molecular Breast Imaging System

A prototype Molecular Breast Imaging (MBI) system is currently under development, motivated by the need of a practical low-dose system for use in patients with dense breast tissue, where conventional mammography is limited. The system is based on dual opposing CZT detector arrays and multi-pinhole collimators which allow for multiplexing in the projection data. We have performed optimization of various design parameters based on either contrast-to-noise ratio (CNR) in the reconstructed images or area under the localization receiver operating characteristics curve (LROC-AUC) obtained using the scan statistic model. The optimizations were based on simulated data, and the parameters investigated were pinhole size and opening angle, pinhole separation and collimator-to-detector separation. The two optimization approaches resulted in similar design parameters, allowing for reconstruction of tomographic images with high CNR and lesion detectability, which can lead to a reduced dose or scan time as compared to planar MBI

Towards Accurate Partial Volume Correction - Perturbation for SPECT Resolution Estimation

The accuracy of quantitative SPECT imaging is limited by the Partial Volume Effect as a result of the relatively poor spatial resolution. There is currently no consensus on the optimal Partial Volume Correction (PVC) algorithm in the application of SPECT oncology imaging. Several promising candidates require information on the reconstructed resolution - usually in the form of the Point Spread Function (PSF). A particular challenge that SPECT poses for PVC is that the resolution is known to vary with position in the field-of-view, as well as with activity distribution and reconstruction method. In this work, we assessed the potential benefit of using perturbation to measure case-specific resolution for PVC. A small point source was used to measure the resolution in phantoms designed to replicate the issues encountered in oncology imaging, including anthropomorphic phantoms which had not previously been examined in perturbation applications. Results demonstrate that, provided that a sufficient number of iterations is used for image reconstruction, perturbation can be used to measure a case-specific PSF. When PVC is applied with this case-specific PSF, quantitative accuracy is improved compared with no correction or applying PVC with an inappropriate PSF