Impact of Point-Spread Function Modeling on PET Image Quality in Integrated PET/MR Hybrid Imaging.

UNLABELLED: The aim of this study was to systematically assess the quantitative and qualitative impact of including point-spread function (PSF) modeling into the process of iterative PET image reconstruction in integrated PET/MR imaging. METHODS: All measurements were performed on an integrated whole-body PET/MR system. Three substudies were performed: an (18)F-filled Jaszczak phantom was measured, and the impact of including PSF modeling in ordinary Poisson ordered-subset expectation maximization reconstruction on quantitative accuracy and image noise was evaluated for a range of radial phantom positions, iteration numbers, and postreconstruction smoothing settings; 5 representative datasets from a patient population (total n = 20, all oncologic (18)F-FDG PET/MR) were selected, and the impact of PSF on lesion activity concentration and image noise for various iteration numbers and postsmoothing settings was evaluated; and for all 20 patients, the influence of PSF modeling was investigated on visual image quality and number of detected lesions, both assessed by clinical experts. Additionally, the influence on objective metrics such as changes in SUVmean, SUVpeak, SUVmax, and lesion volume was assessed using the manufacturer-recommended reconstruction settings. RESULTS: In the phantom study, PSF modeling significantly improved activity recovery and reduced the image noise at all radial positions. This effect was measurable only at a high number of iterations (>10 iterations, 21 subsets). In the patient study, again, PSF increased the detected activity in the patient's lesions at concurrently reduced image noise. Contrary to the phantom results, the effect was notable already at a lower number of iterations (>1 iteration, 21 subsets). Lastly, for all 20 patients, when PSF and no-PSF reconstructions were compared, an identical number of congruent lesions was found. The overall image quality of the PSF reconstructions was rated better when compared with no-PSF data. The SUVs of the detected lesions with PSF were substantially increased in the range of 6%-75%, 5%-131%, and 5%-148% for SUVmean, SUVpeak, and SUVmax, respectively. A regression analysis showed that the relative increase in SUVmean/peak/max decreases with increasing lesion size, whereas it increases with the distance from the center of the PET field of view. CONCLUSION: In whole-body PET/MR hybrid imaging, PSF-based PET reconstructions can improve activity recovery and image noise, especially at lateral positions of the PET field of view. This has been demonstrated quantitatively in phantom experiments as well as in patient imaging, for which additionally an improvement of image quality could be observed.

NEMA image quality phantom measurements and attenuation correction in integrated PET/MR hybrid imaging.

BACKGROUND: In integrated PET/MR hybrid imaging the evaluation of PET performance characteristics according to the NEMA standard NU 2-2007 is challenging because of incomplete MR-based attenuation correction (AC) for phantom imaging. In this study, a strategy for CT-based AC of the NEMA image quality (IQ) phantom is assessed. The method is systematically evaluated in NEMA IQ phantom measurements on an integrated PET/MR system. METHODS: NEMA IQ measurements were performed on the integrated 3.0 Tesla PET/MR hybrid system (Biograph mMR, Siemens Healthcare). AC of the NEMA IQ phantom was realized by an MR-based and by a CT-based method. The suggested CT-based AC uses a template µ-map of the NEMA IQ phantom and a phantom holder for exact repositioning of the phantom on the systems patient table. The PET image quality parameters contrast recovery, background variability, and signal-to-noise ratio (SNR) were determined and compared for both phantom AC methods. Reconstruction parameters of an iterative 3D OP-OSEM reconstruction were optimized for highest lesion SNR in NEMA IQ phantom imaging. RESULTS: Using a CT-based NEMA IQ phantom µ-map on the PET/MR system is straightforward and allowed performing accurate NEMA IQ measurements on the hybrid system. MR-based AC was determined to be insufficient for PET quantification in the tested NEMA IQ phantom because only photon attenuation caused by the MR-visible phantom filling but not the phantom housing is considered. Using the suggested CT-based AC, the highest SNR in this phantom experiment for small lesions (<= 13 mm) was obtained with 3 iterations, 21 subsets and 4 mm Gaussian filtering. CONCLUSION: This study suggests CT-based AC for the NEMA IQ phantom when performing PET NEMA IQ measurements on an integrated PET/MR hybrid system. The superiority of CT-based AC for this phantom is demonstrated by comparison to measurements using MR-based AC. Furthermore, optimized PET image reconstruction parameters are provided for the highest lesion SNR in NEMA IQ phantom measurements.

Radiotracer dose reduction in integrated PET/MR: implications from national electrical manufacturers association phantom studies.

UNLABELLED: With the replacement of ionizing CT by MR imaging, integrated PET/MR in selected clinical applications may reduce the overall patient radiation dose when compared with PET/CT. Further potential for radiotracer dose reduction, while maintaining PET image quality (IQ) in integrated PET/MR, may be achieved by increasing the PET acquisition duration to match the longer time needed for MR data acquisition. To systematically verify this hypothesis under controlled conditions, this dose-reduction study was performed using a standardized phantom following the National Electrical Manufacturers Association (NEMA) IQ protocol. METHODS: All measurements were performed on an integrated PET/MR whole-body hybrid system. The NEMA IQ phantom was filled with water and a total activity of 50.35 MBq of (18)F-FDG. The sphere-to-background activity ratio was 8:1. Multiple PET data blocks of 20-min acquisition time were acquired in list-mode format and were started periodically at multiples of the (18)F-FDG half-lives. Different sinograms (2, 4, 8, and 16 min in duration) were reconstructed. Attenuation correction of the filled NEMA phantom was performed using a CT-based attenuation map template. The attenuation-corrected PET images were then quantitatively evaluated following the NEMA IQ protocol, investigating contrast recovery, background variability, and signal-to-noise ratio. Image groups with half the activity and twice the acquisition time were evaluated. For better statistics, the experiment was repeated 3 times. RESULTS: Contrast recovery, background variability, and signal-to-noise ratio remained almost constant over 3 half-life periods when the decreasing radiotracer activity (100%, 50%, 25%, and 12.5%) was compensated by increasing acquisition time (2, 4, 8, and 16 min). The variation of contrast recovery over 3 half-life periods was small (-6% to +7%), with a mean variation of 2%, compared with the reference setting (100%, 2 min). The signal-to-noise ratio of the hot spheres showed only minor variations over 3 half-life periods (5%). Image readers could not distinguish subjective IQ between the different PET acquisition setups. CONCLUSION: An approach to reduce the injected radiotracer activity in integrated PET/MR imaging, while maintaining PET IQ, was presented and verified under idealized experimental conditions. This experiment may serve as a basis for further clinical PET/MR studies using reduced radiotracer dose as compared with conventional PET/CT studies.

Quantitative (90)Y image reconstruction in PET.

PURPOSE: Positron emission tomography (PET) imaging is increasingly used to confirm localization of (90)Y microspheres in the treatment of liver cancer. The aim of this work was to evaluate the quantification of (90)Y PET data on a current generation time-of-flight extended axial field-of-view PET∕CT camera. METHODS: The International Electrotechnical Commission (IEC) body phantom was used to image six spheres of varying diameters containing a high concentration of (90)Y solution in a lower concentration background. Multiple PET studies were acquired of the phantom over a number of days during decay. The effect of reconstruction parameters in OSEM was evaluated both qualitatively and quantitatively. Expected values of total phantom activity, hot-sphere, and background concentration were compared to measured values from the reconstructed data as well as misplaced events in a cold insert. The partial volume effect was measured and the effects of time-of-flight during reconstruction on hot contrast recovery and background variability were evaluated according to NEMA-NU2-2007 protocol, and compared to that for (18)F. The method was applied to a patient study following radioembolization to estimate actual implanted radioactivity. RESULTS: Increasing the number of OSEM iterations visually deteriorated image data and resulted in a larger overall difference of hot concentration measures when considering both count high and count poor data. The average difference between measured and true total activity and background concentration was found to be +5% and +5%, respectively. Measured hot-sphere concentration was linear across all datasets, and while estimated to be within error of expected values, was consistently underestimated by an average of 23%, 12%, and 8%, when using a CT-derived, 50% threshold-derived, and 70% threshold-derived volume of interest, respectively. Partial volume effects were evident in all but the largest sphere, following an expected relationship between object size and recovery coefficient, inferior to that of (18)F. Time-of-flight improved contrast of hot-spheres but resulted in a deterioration of background variability, following a similar trend to that seen with (18)F. The patient data estimated a total implanted activity of 1643 MBq, compared to the intended dose of 1780 MBq, with a difference most likely due to residual and error in the initial dose calibration. CONCLUSIONS: Quantitative (90)Y PET with a state-of-the-art PET∕CT scanner with time-of-flight and standard corrections for photon interactions demonstrates consistent and acceptable measures of total activity and radionuclide concentration across a range of realistic count statistics. The method is suitable for measuring the radioactivity delivered at the time of (90)Y therapy with the potential for absorbed dose calculation.

An assessment of the impact of incorporating time-of-flight information into clinical PET/CT imaging.

The introduction of fast scintillators with good stopping power for 511-keV photons has renewed interest in time-of-flight (TOF) PET. The ability to measure the difference between the arrival times of a pair of photons originating from positron annihilation improves the image signal-to-noise ratio (SNR). The level of improvement depends upon the extent and distribution of the positron activity and the time resolution of the PET scanner. While specific estimates can be made for phantom imaging, the impact of TOF PET is more difficult to quantify in clinical situations. The results presented here quantify the benefit of TOF in a challenging phantom experiment and then assess both qualitatively and quantitatively the impact of incorporating TOF information into the reconstruction of clinical studies. A clear correlation between patient body mass index and gain in SNR was observed in this study involving 100 oncology patient studies, with a gain due to TOF ranging from 1.1 to 1.8, which is consistent with the 590-ps time resolution of the TOF PET scanner. The visual comparison of TOF and non-TOF images performed by two nuclear medicine physicians confirmed the advantages of incorporating TOF into the reconstruction, advantages that include better definition of small lesions and image details, improved uniformity, and noise reduction.

Impact of time-of-flight on PET tumor detection.

UNLABELLED: Time-of-flight (TOF) PET uses very fast detectors to improve localization of events along coincidence lines-of-response. This information is then utilized to improve the tomographic reconstruction. This work evaluates the effect of TOF upon an observer's performance for detecting and localizing focal warm lesions in noisy PET images. METHODS: An advanced anthropomorphic lesion-detection phantom was scanned 12 times over 3 days on a prototype TOF PET/CT scanner (Siemens Medical Solutions). The phantom was devised to mimic whole-body oncologic (18)F-FDG PET imaging, and a number of spheric lesions (diameters 6-16 mm) were distributed throughout the phantom. The data were reconstructed with the baseline line-of-response ordered-subsets expectation-maximization algorithm, with the baseline algorithm plus point spread function model (PSF), baseline plus TOF, and with both PSF+TOF. The lesion-detection performance of each reconstruction was compared and ranked using localization receiver operating characteristics (LROC) analysis with both human and numeric observers. The phantom results were then subjectively compared to 2 illustrative patient scans reconstructed with PSF and with PSF+TOF. RESULTS: Inclusion of TOF information provides a significant improvement in the area under the LROC curve compared to the baseline algorithm without TOF data (P = 0.002), providing a degree of improvement similar to that obtained with the PSF model. Use of both PSF+TOF together provided a cumulative benefit in lesion-detection performance, significantly outperforming either PSF or TOF alone (P < 0.002). Example patient images reflected the same image characteristics that gave rise to improved performance in the phantom data. CONCLUSION: Time-of-flight PET provides a significant improvement in observer performance for detecting focal warm lesions in a noisy background. These improvements in image quality can be expected to improve performance for the clinical tasks of detecting lesions and staging disease. Further study in a large clinical population is warranted to assess the benefit of TOF for various patient sizes and count levels, and to demonstrate effective performance in the clinical environment.

18FDG-PET imaging in canine lymphoma and cutaneous mast cell tumor.

Positron Emission Tomography (PET) using the glucose analog 2-deoxy-2-[18F]fluoro-D-glucose (18FDG) is a common imaging modality for diagnosis and management of many human malignancies. We evaluated 18FDG-PET in dogs with either multicentric lymphoma (LSA) or cutaneous mast cell tumor (MCT). A prototype large field-of-view PET scanner was used to collect whole-body images in nine dogs with LSA or MCT. Both tumors were characterized by avidity for 18FDG. In dogs with LSA, 18FDG-PET correctly identified involvement of superficial and internal lymph nodes, liver, and spleen. Repeated PET scans after induction chemotherapy demonstrated resolution of abnormal 18FDG uptake within these sites. In dogs with MCT, 18FDG-PET correctly identified MCT metastasis to regional lymph nodes in all dogs in which this was suspected or confirmed with cytology or biopsy before the PET scan. In two dogs, additional sites of mast cell disease were identified with 18FDG-PET that were undetected on physical examination and/or regional lymph node cytology. 18FDG-PET holds promise as a whole-body staging method for canine LSA and MCT.