Andreas Goedicke

Simultaneous Reconstruction of Activity and Attenuation for PET/MR

Medical investigations targeting a quantitative analysis of the position emission tomography (PET) images require the incorporation of additional knowledge about the photon attenuation distribution in the patient. Today, energy range adapted attenuation maps derived from computer tomography (CT) scans are used to effectively compensate for image quality degrading effects, such as attenuation and scatter. Replacing CT by magnetic resonance (MR) is considered as the next evolutionary step in the field of hybrid imaging systems. However, unlike CT, MR does not measure the photon attenuation and thus does not provide an easy access to this valuable information. Hence, many research groups currently investigate different technologies for MR-based attenuation correction (MR-AC). Typically, these approaches are based on techniques such as special acquisition sequences (alone or in combination with subsequent image processing), anatomical atlas registration, or pattern recognition techniques using a data base of MR and corresponding CT images. We propose a generic iterative reconstruction approach to simultaneously estimate the local tracer concentration and the attenuation distribution using the segmented MR image as anatomical reference. Instead of applying predefined attenuation values to specific anatomical regions or tissue types, the gamma attenuation at 511 keV is determined from the PET emission data. In particular, our approach uses a maximum-likelihood estimation for the activity and a gradient-ascent based algorithm for the attenuation distribution. The adverse effects of scattered and accidental gamma coincidences on the quantitative accuracy of PET, as well as artifacts caused by the inherent crosstalk between activity and attenuation estimation are efficiently reduced using enhanced decay event localization provided by time-of-flight PET, accurate correction for accidental coincidences, and a reduced number of unknown attenuation coefficients. First results achieved with measured whole body PET data and reference segmentation from CT showed an absolute mean difference of 0.005 cm in the lungs, 0.0009 cm in case of fat, and 0.0015 cm for muscles and blood. The proposed method indicates a robust and reliable alternative to other MR-AC approaches targeting patient specific quantitative analysis in time-of-flight PET/MR.

Analytical model for SPECT detector concepts

Pixellated cadmium-zinc telluride (CZT) detectors, providing higher spatial resolution and energy resolution than current gamma cameras, will improve the image quality of SPECT detector systems. Their performance has to be evaluated in terms of resolution, detector efficiency and image quality in a similar way as has been done earlier for NaI detectors. We have developed an analytical model for spatial resolution and geometric efficiency of collimators specifically for pixellated CZT based detectors. We derive an exact description for a variety of static and rotating detector concepts, use standard performance criteria for detection efficiency, and adapt measures for spatial resolution of pixellated detectors, based on the sampling of the single pixel response function (SPRF). The concept is extended to enable a comparative description of continuous scintillator based SPECT cameras. Tradeoffs among resolution, efficiency, signal-to-noise ratio (SNR) and minimum detectable contrast have been investigated for different detector concepts. Our analysis shows that concepts using rotating collimators suffer from noise accumulation, except for purely hot spot imaging. A finite optimization of current SPECT systems can be done based on this analytical model, and a further increase in image quality will be achieved by the higher spatial resolution and energy resolution of solid-state detectors.

Study-Parameter Impact in Quantitative 90-Yttrium PET Imaging for Radioembolization Treatment Monitoring and Dosimetry

A small positron-generating branch in 90-Yttrium (90Y) decay enables post-therapy dose assessment in liver cancer radioembolization treatment. The aim of this study was to validate clinical 90Y positron emission tomography (PET) quantification, focusing on scanner linearity as well as acquisition and reconstruction parameter impact on scanner calibration. Data from three dedicated phantom studies (activity range: 55.2 MBq-2.1 GBq) carried out on a Philips Gemini TF 16 PET/CT scanner were analyzed after reconstruction with up to 361 parameter configurations. For activities above 200 MBq, scanner linearity could be confirmed with relative error margins <;4%. An acquisition-time-normalized calibration factor of 1.04 MBq·s/CNTS was determined for the employed scanner. Stable activity convergence was found in hot phantom regions with relative differences in summed image intensities between -3.6% and +2.4%. Absolute differences in background noise artifacts between - 79.9% and + 350% were observed. Quantitative accuracy was dominated by subset size selection in the reconstruction. Using adequate segmentation and optimized acquisition parameters, the average activity recovery error induced by the axial scanner sensitivity profile was reduced to +2.4%±3.4% (mean ± standard deviation). We conclude that post-therapy dose assessment in 90Y PET can be improved using adapted parameter setups.

Activity quantification combining conjugate-view planar scintigraphies and SPECT/CT data for patient-specific 3-D dosimetry in radionuclide therapy

PurposeThree-dimensional dosimetry based on quantitative SPECT/CT has potential advantages over planar approaches, but may be impractical due to acquisition durations. We combine one SPECT/CT with improved quantification of multiple planar scintigraphies to shorten acquisitions.MethodsA hybrid 2-D/3-D quantification technique is proposed, using SPECT/CT information for robust planar image quantification and creating virtual SPECTs out of conjugate-view planar scintigraphies; these are included in a 3-D absorbed dose calculation. A projection model simulates photon attenuation and scatter as well as camera and collimator effects. Planar and SPECT calibration techniques are described, offering multiple pathways of deriving calibration factors for hybrid quantification. Model, phantom and patient data are used to validate the approach on a per-organ basis, and the similarity of real and virtual SPECTs, and of planar images and virtual SPECT projections, is assessed using linear regression analysis.ResultsOrgan overlap, background activity and organ geometry are accounted for in the algorithm. Hybrid time-activity curves yield the same information as those derived from a conventional SPECT evaluation. Where correct values are known, hybrid quantification errors are less than 16% for all but two compartments (SPECT/CT 23%). Under partial volume effects, hybrid quantification can provide more robust results than SPECT/CT. The mean correlation coefficient of 3-D data is 0.962 (2-D 0.934). As a consequence of good activity quantification performance, good agreement of absorbed dose estimates and dose-volume histograms with reference results is achieved.ConclusionThe proposed activity quantification method for 2-D scintigraphies can speed up SPECT/CT-based 3-D dosimetry without losing accuracy.

Attenuation Corrected Cardiac SPECT Imaging Using Simultaneous Reconstruction and a Priori Information

Attenuation correction in SPECT typically performed using CT or Gadolinium-source based transmission scans, is more and more becoming established practice in many medical applications. For cases where no further anatomical information from a transmission scan is available, different approaches have been proposed in the past, attempting to recover the attenuation information directly from SPECT projection data. However, these methods often result in noticeable image artifacts caused by an inherent cross-dependency between the estimated attenuation and activity distribution.

Contrast medium injection protocol adjusted for body surface area in combined PET/CT

AbstractObjectivesTo evaluate the effect of contrast medium dose adjustment for body surface area (BSA) compared with a fixed-dose protocol in combined positron emission tomography (PET) and computed tomography (CT) (PET/CT).MethodsOne hundred and twenty patients were prospectively included for 18F-2-deoxy-fluor-glucose (18F-FDG)-PET/CT consisting of a non-enhanced and a venous contrast-enhanced CT, both used for PET attenuation correction. The first 60 consecutive patients received a fixed 148-ml contrast medium dose. The second 60 patients received a dose that was based on their calculated BSA. Mean and maximum standardised FDG uptake (SUVmean and SUVmax) and contrast enhancement (HU) were measured at multiple anatomical sites and PET reconstructions were evaluated visually for image quality.ResultsA decrease in the variance of contrast enhancement in the BSA group compared with the fixed-dose group was seen at all anatomical sites. Comparison of tracer uptake SUVmean and SUVmax between the fixed and the BSA group revealed no significant differences at all anatomical sites (all P > 0.05). Comparison of the overall image quality scores between the fixed and the BSA group showed no significant difference (P = 0.753).ConclusionsBSA adjustment results in increased interpatient homogeneity of contrast enhancement without affecting PET values. In combined PET/CT, a BSA adjusted contrast medium protocol should be used preferably.Key Points• Intravenous contrast medium is essential for many applications of PET/CT • Body surface area adjustment of contrast medium helps standardise contrast enhancement • Underdosing or overdosing of contrast medium will be reduced • PET image quality is not influenced • BSA adjusted contrast medium protocol should be used preferably in combined PET/CT

Advanced reconstruction of attenuation maps using SPECT emission data only

Today, attenuation corrected SPECT, typically performed using CT or Gadolinium line source based transmission scans, is more and more becoming standard in many medical applications. Moreover, the information about the material density distribution provided by these scans is key for other artifact compensation approaches in advanced SPECT reconstruction. Major drawbacks of these approaches are the additional patient radiation and hardware/maintenance costs as well as the additional workflow effort, e.g. if the CT scans are not performed on a hybrid scanner. It has been investigated in the past, whether it is possible to recover this structural information solely from the SPECT scan data. However, the investigated methods often result in noticeable image artifacts due to cross-dependences between attenuation and activity distribution estimation. With the simultaneous reconstruction method presented in this paper, we aim to effectively prevent these typical cross-talk artifacts using a-priori known atlas information of a human body. At first, an initial 3D shape model is coarsely registered to the SPECT data using anatomical landmarks and each organ structure within the model is identified with its typical attenuation coefficient. During the iterative reconstruction based on a modified ML-EM scheme, the algorithm simultaneously adapts both, the local activity estimation and the 3D shape model in order to improve the overall consistency between measured and estimated sinogram data. By explicitly avoiding topology modifications resulting in a non-anatomical state, we ensure that the estimated attenuation map remains realistic. Several tests with simulated as well as real patient SPECT data were performed to test the proposed algorithm, which demonstrated reliable convergence behaviour in both cases. Comparing the achieved results with available reference data, an overall good agreement for both cold as well as hot activity regions could be observed (mean deviation: -5.98%).

Simulation-based partial volume correction for dopaminergic PET imaging: Impact of segmentation accuracy

AIM Partial volume correction (PVC) is an essential step for quantitative positron emission tomography (PET). In the present study, PVELab, a freely available software, is evaluated for PVC in (18)F-FDOPA brain-PET, with a special focus on the accuracy degradation introduced by various MR-based segmentation approaches. METHODS Four PVC algorithms (M-PVC; MG-PVC; mMG-PVC; and R-PVC) were analyzed on simulated (18)F-FDOPA brain-PET images. MR image segmentation was carried out using FSL (FMRIB Software Library) and SPM (Statistical Parametric Mapping) packages, including additional adaptation for subcortical regions (SPML). Different PVC and segmentation combinations were compared with respect to deviations in regional activity values and time-activity curves (TACs) of the occipital cortex (OCC), caudate nucleus (CN), and putamen (PUT). Additionally, the PVC impact on the determination of the influx constant (Ki) was assessed. RESULTS Main differences between tissue-maps returned by three segmentation algorithms were found in the subcortical region, especially at PUT. Average misclassification errors in combination with volume reduction was found to be lowest for SPML (PUT < 30%) and highest for FSL (PUT > 70%). Accurate recovery of activity data at OCC is achieved by M-PVC (apparent recovery coefficient varies between 0.99 and 1.10). The other three evaluated PVC algorithms have demonstrated to be more suitable for subcortical regions with MG-PVC and mMG-PVC being less prone to the largest tissue misclassification error simulated in this study. Except for M-PVC, quantification accuracy of Ki for CN and PUT was clearly improved by PVC. CONCLUSIONS The regional activity value of PUT was appreciably overcorrected by most of the PVC approaches employing FSL or SPM segmentation, revealing the importance of accurate MR image segmentation for the presented PVC framework. The selection of a PVC approach should be adapted to the anatomical structure of interest. Caution is recommended in subsequent interpretation of Ki values. The possible different change of activity concentrations due to PVC in both target and reference regions tends to alter the corresponding TACs, introducing bias to Ki determination. The accuracy of quantitative analysis was improved by PVC but at the expense of precision reduction, indicating the potential impropriety of applying the presented framework for group comparison studies.