Improved correction for the tissue fraction effect in lung PET/CT imaging

Recently, there has been an increased interest in imaging different pulmonary disorders using PET techniques. Previous work has shown, for static PET/CT, that air content in the lung influences reconstructed image values and that it is vital to correct for this 'tissue fraction effect' (TFE). In this paper, we extend this work to include the blood component and also investigate the TFE in dynamic imaging. CT imaging and PET kinetic modelling are used to determine fractional air and blood voxel volumes in six patients with idiopathic pulmonary fibrosis. These values are used to illustrate best and worst case scenarios when interpreting images without correcting for the TFE. In addition, the fractional volumes were used to determine correction factors for the SUV and the kinetic parameters. These were then applied to the patient images. The kinetic parameters K1 and Ki along with the static parameter SUV were all found to be affected by the TFE with both air and blood providing a significant contribution to the errors. Without corrections, errors range from 34-80% in the best case and 29-96% in the worst case. In the patient data, without correcting for the TFE, regions of high density (fibrosis) appeared to have a higher uptake than lower density (normal appearing tissue), however this was reversed after air and blood correction. The proposed correction methods are vital for quantitative and relative accuracy. Without these corrections, images may be misinterpreted

A hybrid 3-D reconstruction/registration algorithm for correction of head motion in emission tomography

Even with head restraint, small head movements can occur during data acquisition in emission tomography that are sufficiently large to result in detectable artifacts in the final reconstruction. Direct measurement of motion can be cumbersome and difficult to implement, whereas previous attempts to use the measured projection data for correction have been limited to simple translation orthogonal to the projection. A fully three-dimensional (3-D) algorithm is proposed that estimates the patient orientation based on the projection of motion-corrupted data, with incorporation of motion information within subsequent ordered-subset expectation-maximization subiterations. Preliminary studies have been performed using a digital version of the Hoffman brain phantom. Movement was simulated by constructing a mixed set of projections in discrete positions of the phantom. The algorithm determined the phantom orientation that best matched each constructed projection with its corresponding measured projection. In the case of a simulated single movement in 24 of 64 projections, all misaligned projections were correctly identified. Incorporating data at the determined object orientation resulted in a reduction of mean square difference (MSD) between motion-corrected and motion-free reconstructions, compared to the MSD between uncorrected and motion-free reconstructions, by a factor of 1.9

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

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

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

Density variation during respiration affects PET quantitation in the lung

PET quantitation depends on the accuracy of the CT-derived attenuation correction map. In the lung, respiration leads to both positional and density mismatches, causing PET quantitation errors at lung borders but also within the whole lung. The aim of this work is to determine the extent of the associated errors on the measured time activity curves (TACs) and the corresponding kinetic parameter estimates. 5 patients with idiopathic pulmonary fibrosis underwent dynamic 18 F-FDG PET and cine-CT imaging as part of an ongoing study. The cine-CT was amplitude gated using PCA techniques to produce end expiration (EXP), end inspiration (INS) and mid-breathing cycle (MID) gates representative of a short clinical CT acquisition. The ungated PET data were reconstructed with each CT gate and the TACs and kinetic parameters compared. Patient representative XCAT simulations with varying lung density, both with and without motion, were also produced to represent the above study allowing comparison of true to measured results. In all cases, the obtained PET TACs differed with each CT gate. For ROIs internal to the lung, the effect was dominated by changes in density, as opposed to motion. The errors in the TACs varied with time, providing evidence that errors due to attenuation mismatch depend on activity distribution. In the simulations, some kinetic parameters were over- and under-estimated by a factor of 2 in the INS and EXP gates respectively. For the patients, the maximum variation in kinetic parameters was 20%. Our results show that whole lung density changes during the respiratory cycle have a significant impact on PET quantitation. This is especially true of the kinetic parameter estimates as the extent of the error is dependent on tracer distribution which varies with time. It is therefore vital to use matched PET/CT for attenuation correction

Effect of positron range on PET quantification in diseased and normal lungs

The impact of positron range on PET image reconstruction has often been investigated as a blurring effect that can be partly corrected by adding an element to the PET system matrix in the reconstruction, usually based on a Gaussian kernel constructed from the attenuation values. However, the physics involved in PET is more complex. In regions where density does not vary, positron range indeed involves mainly blurring. However, in more heterogeneous media it can cause other effects. This work focuses on positron range in the lungs and its impact on quantification, especially in the case of pathologies such as cancer or pulmonary fibrosis, for which the lungs have localised varying density. Using Monte Carlo simulations, we evaluate the effects of positron range for multiple radionuclides (18F, 15O, 68Ga, 89Zr, 82Rb, 64Cu and 124I) as, for novel radiotracers, the choice of the labelling radionuclide is important. The results demonstrate quantification biases in highly heterogeneous media, where the measured uptake of high-density regions can be increased by the neighbouring radioactivity from regions of lower density, with the effect more noticeable for radionuclides with highenergy positron emission. When the low-density regions are considered to have less radioactive uptake (e.g. due to the presence of air), the effect is less severe

Dynamic imaging and tracer kinetic modeling for emission tomography using rotating detectors

Author name used in this publication: Dagan FengAuthor name used in this publication: Daniel Pak-Kong LunCentre for Multimedia Signal Processing, Department of Electronic and Information EngineeringVersion of RecordPublishe