Ivo Rausch

Quality control for quantitative multicenter whole-body PET/MR studies: A NEMA image quality phantom study with three current PET/MR systems

PURPOSE Integrated positron emission tomography/magnetic resonance (PET/MR) systems derive the PET attenuation correction (AC) from dedicated MR sequences. While MR-AC performs reasonably well in clinical patient imaging, it may fail for phantom-based quality control (QC). The authors assess the applicability of different protocols for PET QC in multicenter PET/MR imaging. METHODS The National Electrical Manufacturers Association NU 2 2007 image quality phantom was imaged on three combined PET/MR systems: a Philips Ingenuity TF PET/MR, a Siemens Biograph mMR, and a GE SIGNA PET/MR (prototype) system. The phantom was filled according to the EANM FDG-PET/CT guideline 1.0 and scanned for 5 min over 1 bed. Two MR-AC imaging protocols were tested: standard clinical procedures and a dedicated protocol for phantom tests. Depending on the system, the dedicated phantom protocol employs a two-class (water and air) segmentation of the MR data or a CT-based template. Differences in attenuation- and SUV recovery coefficients (RC) are reported. PET/CT-based simulations were performed to simulate the various artifacts seen in the AC maps (μ-map) and their impact on the accuracy of phantom-based QC. RESULTS Clinical MR-AC protocols caused substantial errors and artifacts in the AC maps, resulting in underestimations of the reconstructed PET activity of up to 27%, depending on the PET/MR system. Using dedicated phantom MR-AC protocols, PET bias was reduced to -8%. Mean and max SUV RC met EARL multicenter PET performance specifications for most contrast objects, but only when using the dedicated phantom protocol. Simulations confirmed the bias in experimental data to be caused by incorrect AC maps resulting from the use of clinical MR-AC protocols. CONCLUSIONS Phantom-based quality control of PET/MR systems in a multicenter, multivendor setting may be performed with sufficient accuracy, but only when dedicated phantom acquisition and processing protocols are used for attenuation correction.

Variation of system performance, quality control standards and adherence to international FDG-PET/CT imaging guidelines. A national survey of PET/CT operations in Austria.

AIM To gather information on clinical operations, quality control (QC) standards and adoption of guidelines for FDG-PET/CT imaging in Austrian PET/CT centres. METHODS A written survey composed of 68 questions related to A) PET/CT centre and installation, B) standard protocol parameters for FDG-PET/CT imaging of oncology patients, and C) standard QC procedures was conducted between November and December 2013 among all Austrian PET/CT centres. In addition, a NEMA-NU2 2012 image quality phantom test was performed using standard whole-body imaging settings on all PET/CT systems with a lesion-to-background ratio of 4. Recovery coefficients (RC) were calculated for each lesion and PET/CT system. RESULTS A) 13 PET/CT systems were installed in 12 nuclear medicine departments at public hospitals. B) Average fasting prior to FDG-PET/CT was 7.6 (4-12) h. All sites measured blood glucose levels while using different cut-off levels (64%: 150 mg/dl). Weight-based activity injection was performed at 83% sites with a mean FDG activity of 4.1 MBq/kg. Average FDG uptake time was 55 (45-75) min. All sites employed CT contrast agents (variation from 1%-95% of the patients). All sites reported SUV-max. C) Frequency of QC tests varied significantly and QC phantom measurements revealed significant differences in RCs. CONCLUSION Significant variations in FDG-PET/CT protocol parameters among all Austrian PET/CT users were observed. Subsequently, efforts need to be put in place to further standardize imaging protocols. At a minimum clinical PET/CT operations should ensure compliance with existing guidelines. Further, standardized QC procedures must be followed to improve quantitative accuracy across PET/CT centres.

Influence of PET reconstruction parameters on the TrueX algorithm. A combined phantom and patient study.

UNLABELLED With the increasing use of functional imaging in modern radiotherapy (RT) and the envisaged automated integration of PET into target definition, the need for reliable quantification of PET is growing. Reconstruction algorithms in new PET scanners employ point-spread-function (PSF) based resolution recovery, however, their impact on PET quantification still requires thorough investigation. PATIENTS, MATERIAL, METHODS Measurements were performed on a Siemens PET/CT using an IEC phantom filled with varying activity. Data were reconstructed using the OSEM (Gauss filter) and the PSF TrueX (Gauss and Allpass filter) algorithm with all available products of iterations (i) and subsets (ss). The recovery coeffcient (RC) and threshold defining the real sphere volume were determined for all settings and compared to the clinical standard (4i21ss). PET acquisitions of eight lung patients were reconstructed using all algorithms with 4i21ss. Volume size and tracer uptake were determined with different segmentation methods. RESULTS The threshold for the TrueX was lower (up to 40%) than for the OSEM. The RC for the different algorithms and filters varied. TrueX was more sensitive to permutations of i and ss and only the RC of the OSEM stabilised with increasing number. For patient scans the difference of the volume and activity between TrueX and OSEM could be reduced by applying an adapted threshold and activity correction. CONCLUSION The TrueX algorithm results in excellent diagnostic image quality, however, guidelines for native algorithms have to be extended for PSF based reconstruction methods. For appropriate tumour delineation, for the TrueX a lower threshold than the 42% recommended for the OSEM is necessary. These filter dependent thresholds have to be verified for different scanners prior to using them in multicenter trials.

Can Interim 18F-FDG PET or Diffusion-Weighted MRI Predict End-of-Treatment Outcome in FDG-Avid MALT Lymphoma After Rituximab-Based Therapy?: A Preliminary Study in 15 Patients.

Purpose To determine whether interim 18F-FDG PET or interim diffusion-weighted magnetic resonance imaging (DWI) can predict the end-of-treatment (EOT) outcome after immunotherapy in patients with FDG-avid extranodal marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue (MALT). Materials and Methods Patients with untreated MALT lymphoma prospectively underwent whole-body 18F-FDG PET/CT and DWI before treatment (baseline), and after three cycles (interim) of rituximab-based immunotherapy. Maximum and mean standardized uptake values (SUVmax, SUVmean), and minimum and mean apparent diffusion coefficients (ADCmin, ADCmean), were measured for up to three target lesions per patient. Rates of change between baseline and interim examinations (&Dgr;SUVmax, &Dgr;SUVmean, &Dgr;ADCmin, and &Dgr;ADCmean) were compared, using ANOVAs, between the four end-of-treatment (EOT, after six cycles of immunotherapy) outcomes: complete remission (CR), partial remission (PR), stable disease (SD), or progressive disease (PD). Results Fifteen patients with 25 lesions were included. Lesion-based post hoc tests showed significant differences between CR and PR for &Dgr;SUVmax (P < 0.001), &Dgr;SUVmean (P < 0.001), and &Dgr;ADCmin (P = 0.044), and between CR and SD for &Dgr;SUVmax (P < 0.001), &Dgr;SUVmean (P < 0.001), &Dgr;ADCmin (P = 0.021), and &Dgr;ADCmean (P = 0.022). No lesion showed PD at EOT. Conclusions Both quantitative interim 18F-FDG PET and interim DWI may possibly be useful to predict complete remission at end-of-treatment in MALT lymphoma patients after immunotherapy.

Influence of PET reconstruction parameters on the TrueX algorithm

UNLABELLED With the increasing use of functional imaging in modern radiotherapy (RT) and the envisaged automated integration of PET into target definition, the need for reliable quantification of PET is growing. Reconstruction algorithms in new PET scanners employ point-spread-function (PSF) based resolution recovery, however, their impact on PET quantification still requires thorough investigation. PATIENTS, MATERIAL, METHODS Measurements were performed on a Siemens PET/CT using an IEC phantom filled with varying activity. Data were reconstructed using the OSEM (Gauss filter) and the PSF TrueX (Gauss and Allpass filter) algorithm with all available products of iterations (i) and subsets (ss). The recovery coeffcient (RC) and threshold defining the real sphere volume were determined for all settings and compared to the clinical standard (4i21ss). PET acquisitions of eight lung patients were reconstructed using all algorithms with 4i21ss. Volume size and tracer uptake were determined with different segmentation methods. RESULTS The threshold for the TrueX was lower (up to 40%) than for the OSEM. The RC for the different algorithms and filters varied. TrueX was more sensitive to permutations of i and ss and only the RC of the OSEM stabilised with increasing number. For patient scans the difference of the volume and activity between TrueX and OSEM could be reduced by applying an adapted threshold and activity correction. CONCLUSION The TrueX algorithm results in excellent diagnostic image quality, however, guidelines for native algorithms have to be extended for PSF based reconstruction methods. For appropriate tumour delineation, for the TrueX a lower threshold than the 42% recommended for the OSEM is necessary. These filter dependent thresholds have to be verified for different scanners prior to using them in multicenter trials.

Does Delayed-Time-Point Imaging Improve 18F-FDG-PET in Patients With MALT Lymphoma?: Observations in a Series of 13 Patients.

Purpose To determine whether in patients with extranodal marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue lymphoma (MALT), delayed–time-point 2-18F-fluoro-2-deoxy-d-glucose-positron emission tomography (18F-FDG-PET) performs better than standard–time-point 18F-FDG-PET. Materials and Methods Patients with untreated histologically verified MALT lymphoma, who were undergoing pretherapeutic 18F-FDG-PET/computed tomography (CT) and consecutive 18F-FDG-PET/magnetic resonance imaging (MRI), using a single 18F-FDG injection, in the course of a larger-scale prospective trial, were included. Region-based sensitivity and specificity, and patient-based sensitivity of the respective 18F-FDG-PET scans at time points 1 (45–60 minutes after tracer injection, TP1) and 2 (100–150 minutes after tracer injection, TP2), relative to the reference standard, were calculated. Lesion-to-liver and lesion-to-blood SUVmax (maximum standardized uptake values) ratios were also assessed. Results 18F-FDG-PET at TP1 was true positive in 15 o f 23 involved regions, and 18F-FDG-PET at TP2 was true-positive in 20 of 23 involved regions; no false-positive regions were noted. Accordingly, region-based sensitivities and specificities were 65.2% (confidence interval [CI], 45.73%–84.67%) and 100% (CI, 100%-100%) for 18F-FDG-PET at TP1; and 87.0% (CI, 73.26%–100%) and 100% (CI, 100%-100%) for 18F-FDG-PET at TP2, respectively. FDG-PET at TP1 detected lymphoma in at least one nodal or extranodal region in 7 of 13 patients, and 18F-FDG-PET at TP2 in 10 of 13 patients; accordingly, patient-based sensitivity was 53.8% (CI, 26.7%–80.9%) for 18F-FDG-PET at TP1, and 76.9% (CI, 54.0%–99.8%) for 18F-FDG-PET at TP2. Lesion-to-liver and lesion-to-blood maximum standardized uptake value ratios were significantly lower at TP1 (ratios, 1.05 ± 0.40 and 1.52 ± 0.62) than at TP2 (ratios, 1.67 ± 0.74 and 2.56 ± 1.10; P = 0.003 and P = 0.001). Conclusions Delayed–time-point imaging may improve 18F-FDG-PET in MALT lymphoma.

Variation of system performance, quality control standards and adherence to international FDG-PET/CT imaging guidelines

AIM To gather information on clinical operations, quality control (QC) standards and adoption of guidelines for FDG-PET/CT imaging in Austrian PET/CT centres. METHODS A written survey composed of 68 questions related to A) PET/CT centre and installation, B) standard protocol parameters for FDG-PET/CT imaging of oncology patients, and C) standard QC procedures was conducted between November and December 2013 among all Austrian PET/CT centres. In addition, a NEMA-NU2 2012 image quality phantom test was performed using standard whole-body imaging settings on all PET/CT systems with a lesion-to-background ratio of 4. Recovery coefficients (RC) were calculated for each lesion and PET/CT system. RESULTS A) 13 PET/CT systems were installed in 12 nuclear medicine departments at public hospitals. B) Average fasting prior to FDG-PET/CT was 7.6 (4-12) h. All sites measured blood glucose levels while using different cut-off levels (64%: 150 mg/dl). Weight-based activity injection was performed at 83% sites with a mean FDG activity of 4.1 MBq/kg. Average FDG uptake time was 55 (45-75) min. All sites employed CT contrast agents (variation from 1%-95% of the patients). All sites reported SUV-max. C) Frequency of QC tests varied significantly and QC phantom measurements revealed significant differences in RCs. CONCLUSION Significant variations in FDG-PET/CT protocol parameters among all Austrian PET/CT users were observed. Subsequently, efforts need to be put in place to further standardize imaging protocols. At a minimum clinical PET/CT operations should ensure compliance with existing guidelines. Further, standardized QC procedures must be followed to improve quantitative accuracy across PET/CT centres.

Technical and instrumentational foundations of PET/MRI

This paper highlights the origins of combined positron emission tomography (PET) and magnetic resonance imaging (MRI) whole-body systems that were first introduced for applications in humans in 2010. This text first covers basic aspects of each imaging modality before describing the technical and methodological challenges of combining PET and MRI within a single system. After several years of development, combined and even fully-integrated PET/MRI systems have become available and made their way into the clinic. This multi-modality imaging system lends itself to the advanced exploration of diseases to support personalized medicine in a long run. To that extent, this paper provides an introduction to PET/MRI methodology and important technical solutions.

PET/MRI for oncological brain imaging: A comparison of standard MR-based attenuation corrections with a novel, model-based approach for the Siemens mMR PET/MR system

The aim of this study was to compare attenuation-correction (AC) approaches for PET/MRI in clinical neurooncology. Methods: Forty-nine PET/MRI brain scans were included: brain tumor studies using 18F-fluoro-ethyl-tyrosine (18F-FET) (n = 31) and 68Ga-DOTANOC (n = 7) and studies of healthy subjects using 18F-FDG (n = 11). For each subject, MR-based AC maps (MR-AC) were acquired using the standard DIXON- and ultrashort echo time (UTE)–based approaches. A third MR-AC was calculated using a model-based, postprocessing approach to account for bone attenuation values (BD, noncommercial prototype software by Siemens Healthcare). As a reference, AC maps were derived from patient-specific CT images (CTref). PET data were reconstructed using standard settings after AC with all 4 AC methods. We report changes in diagnosis for all brain tumor patients and the following relative differences values (RDs [%]), with regards to AC-CTref: for 18F-FET (A)—SUVs as well as volumes of interest (VOIs) defined by a 70% threshold of all segmented lesions and lesion-to-background ratios; for 68Ga-DOTANOC (B)—SUVs as well as VOIs defined by a 50% threshold for all lesions and the pituitary gland; and for 18F-FDG (C)—RD of SUVs of the whole brain and 10 anatomic regions segmented on MR images. Results: For brain tumor imaging (A and B), the standard PET-based diagnosis was not affected by any of the 3 MR-AC methods. For A, the average RDs of SUVmean were −10%, −4%, and −3% and of the VOIs 1%, 2%, and 7% for DIXON, UTE, and BD, respectively. Lesion-to-background ratios for all MR-AC methods were similar to that of CTref. For B, average RDs of SUVmean were −11%, −11%, and −3% and of the VOIs 1%, −4%, and −3%, respectively. In the case of 18F-FDG PET/MRI (C), RDs for the whole brain were −11%, −8%, and −5% for DIXON, UTE, and BD, respectively. Conclusion: The diagnostic reading of PET/MR patients with brain tumors did not change with the chosen AC method. Quantitative accuracy of SUVs was clinically acceptable for UTE- and BD-AC for group A, whereas for group B BD was in accordance with CTref. Nevertheless, for the quantification of individual lesions large deviations to CTref can be observed independent of the MR-AC method used.

Reproducibility of MRI Dixon-based attenuation correction in combined PET/MR with applications for lean body mass estimation

The aim of this study was to assess the reproducibility of standard, Dixon-based attenuation correction (MR-AC) in PET/MR imaging. A further aim was to estimate a patient-specific lean body mass (LBM) from these MR-AC data. Methods: Ten subjects were positioned in a fully integrated PET/MR system, and 3 consecutive multibed acquisitions of the standard MR-AC image data were acquired. For each subject and MR-AC map, the following compartmental volumes were calculated: total body, soft tissue (ST), fat, lung, and intermediate tissue (IT). Intrasubject differences in the total body and subcompartmental volumes (ST, fat, lung, and IT) were assessed by means of coefficients of variation (CVs) calculated across the 3 consecutive measurements and, again, across these measurements but excluding those affected by major artifacts. All subjects underwent a body composition measurement using air displacement plethysmography (ADP) that was used to calculate a reference LBMADP. A second LBM estimate was derived from available MR-AC data using a formula incorporating the respective tissue volumes and densities as well as the subject-specific body weights. A third LBM estimate was obtained from a sex-specific formula (LBMFormula). Pearson correlation was calculated for LBMADP, LBMMR-AC, and LBMFormula. Further, linear regression analysis was performed on LBMMR-AC and LBMADP. Results: The mean CV for all 30 scans was 2.1 ± 1.9% (TB). When missing tissue artifacts were excluded, the CV was reduced to 0.3 ± 0.2%. The mean CVs for the subcompartments before and after exclusion of artifacts were 0.9 ± 1.1% and 0.7 ± 0.7% for the ST, 2.9 ± 4.1% and 1.3 ± 1.0% for fat, and 3.6 ± 3.9% and 1.3 ± 0.7% for the IT, respectively. Correlation was highest for LBMMR-AC and LBMADP (r = 0.99). Linear regression of data excluding artifacts resulted in a scaling factor of 1.06 for LBMMR-AC. Conclusion: LBMMR-AC is shown to correlate well with standard LBM measurements and thus offers routine LBM-based SUV quantification in PET/MR. However, MR-AC images must be controlled for systematic artifacts, including missing tissue and tissue swaps. Efforts to minimize these artifacts could help improve the reproducibility of MR-AC.