Quantifying the Area at Risk in Reperfused ST-Segment-Elevation Myocardial Infarction Patients Using Hybrid Cardiac Positron Emission Tomography-Magnetic Resonance Imaging.

BACKGROUND: Hybrid positron emission tomography and magnetic resonance allows the advantages of magnetic resonance in tissue characterizing the myocardium to be combined with the unique metabolic insights of positron emission tomography. We hypothesized that the area of reduced myocardial glucose uptake would closely match the area at risk delineated by T2 mapping in ST-segment-elevation myocardial infarction patients. METHODS AND RESULTS: Hybrid positron emission tomography and magnetic resonance using (18)F-fluorodeoxyglucose (FDG) for glucose uptake was performed in 21 ST-segment-elevation myocardial infarction patients at a median of 5 days. Follow-up scans were performed in a subset of patients 12 months later. The area of reduced FDG uptake was significantly larger than the infarct size quantified by late gadolinium enhancement (37.2±11.6% versus 22.3±11.7%; P<0.001) and closely matched the area at risk by T2 mapping (37.2±11.6% versus 36.3±12.2%; P=0.10, R=0.98, bias 0.9±4.4%). On the follow-up scans, the area of reduced FDG uptake was significantly smaller in size when compared with the acute scans (19.5 [6.3%-31.8%] versus 44.0 [21.3%-55.3%]; P=0.002) and closely correlated with the areas of late gadolinium enhancement (R 0.98) with a small bias of 2.0±5.6%. An FDG uptake of ≥45% on the acute scans could predict viable myocardium on the follow-up scan. Both transmural extent of late gadolinium enhancement and FDG uptake on the acute scan performed equally well to predict segmental wall motion recovery. CONCLUSIONS: Hybrid positron emission tomography and magnetic resonance in the reperfused ST-segment-elevation myocardial infarction patients showed reduced myocardial glucose uptake within the area at risk and closely matched the area at risk delineated by T2 mapping. FDG uptake, as well as transmural extent of late gadolinium enhancement, acutely can identify viable myocardial segments.

Practical PET Respiratory Motion Correction in Clinical PET/MR.

UNLABELLED: Respiratory motion during PET acquisition may lead to blurring in resulting images and underestimation of uptake parameters. The advent of integrated PET/MR scanners allows us to exploit the integration of modalities, using high spatial resolution and high-contrast MR images to monitor and correct PET images degraded by motion. We proposed a practical, anatomy-independent MR-based correction strategy for PET data affected by respiratory motion and showed that it can improve image quality both for PET acquired simultaneously to the motion-capturing MR and for PET acquired up to 1 h earlier during a clinical scan. METHODS: To estimate the respiratory motion, our method needs only an extra 1-min dynamic MR scan, acquired at the end of the clinical PET/MR protocol. A respiratory signal was extracted directly from the PET list-mode data. This signal was used to gate the PET data and to construct a motion model built from the dynamic MR data. The estimated motion was then incorporated into the PET image reconstruction to obtain a single motion-corrected PET image. We evaluated our method in 2 steps. The PET-derived respiratory signal was compared with an MR measure of diaphragmatic displacement via a pencil-beam navigator. The motion-corrected images were compared with uncorrected images with visual inspection, line profiles, and standardized uptake value (SUV) in focally avid lesions. RESULTS: We showed a strong correlation between the PET-derived and MR-derived respiratory signals for 9 patients, with a mean correlation of 0.89. We then showed 4 clinical case study examples ((18)F-FDG and (68)Ga-DOTATATE) using the motion-correction technique, demonstrating improvements in image sharpness and reduction of respiratory artifacts in scans containing pancreatic, liver, and lung lesions as well as cardiac scans. The mean increase in peak SUV (SUV(peak)) and maximum SUV (SUV(max)) in a patient with 4 pancreatic lesions was 23.1% and 34.5% in PET acquired simultaneously with motion-capturing MR, and 17.6% and 24.7% in PET acquired 50 min before as part of the clinical scan. CONCLUSION: We showed that a respiratory signal can be obtained from raw PET data and that the clinical PET image quality can be improved using only a short additional PET/MR acquisition. Our method does not need external respiratory hardware or modification of the normal clinical MR sequences.

Non-small-cell lung cancer resectability: diagnostic value of PET/MR.

PURPOSE: To assess the diagnostic performance of PET/MR in patients with non-small-cell lung cancer. METHODS: Fifty consecutive consenting patients who underwent routine (18)F-FDG PET/CT for potentially radically treatable lung cancer following a staging CT scan were recruited for PET/MR imaging on the same day. Two experienced readers, unaware of the results with the other modalities, interpreted the PET/MR images independently. Discordances were resolved in consensus. PET/MR TNM staging was compared to surgical staging from thoracotomy as the reference standard in 33 patients. In the remaining 17 nonsurgical patients, TNM was determined based on histology from biopsy, imaging results (CT and PET/CT) and follow-up. ROC curve analysis was used to assess accuracy, sensitivity and specificity of the PET/MR in assessing the surgical resectability of primary tumour. The kappa statistic was used to assess interobserver agreement in the PET/MR TNM staging. Two different readers, without knowledge of the PET/MR findings, subsequently separately reviewed the PET/CT images for TNM staging. The generalized kappa statistic was used to determine intermodality agreement between PET/CT and PET/MR for TNM staging. RESULTS: ROC curve analysis showed that PET/MR had a specificity of 92.3 % and a sensitivity of 97.3 % in the determination of resectability with an AUC of 0.95. Interobserver agreement in PET/MR reading ranged from substantial to perfect between the two readers (Cohen's kappa 0.646 - 1) for T stage, N stage and M stage. Intermodality agreement between PET/CT and PET/MR ranged from substantial to almost perfect for T stage, N stage and M stage (Cohen's kappa 0.627 - 0.823). CONCLUSION: In lung cancer patients PET/MR appears to be a robust technique for preoperative staging.

A comparison of CT- and MR-based attenuation correction in neurological PET.

PURPOSE: To assess the quantitative accuracy of current MR attenuation correction (AC) methods in neurological PET, in comparison to data derived using CT AC. METHODS: This retrospective study included 25 patients who were referred for a neurological FDG PET examination and were imaged sequentially by PET/CT and simultaneous PET/MR. Differences between activity concentrations derived using Dixon and ultrashort echo time (UTE) MR-based AC and those derived from CT AC were compared using volume of interest and voxel-based approaches. The same comparisons were also made using PET data represented as SUV ratios (SUVr) using grey matter cerebellum as the reference region. RESULTS: Extensive and statistically significant regional underestimations of activity concentrations were found with both Dixon AC (P < 0.001) and UTE AC (P < 0.001) in all brain regions when compared to CT AC. The greatest differences were found in the cortical grey matter (Dixon AC 21.3%, UTE AC 15.7%) and cerebellum (Dixon AC 19.8%, UTE AC 17.3%). The underestimation using UTE AC was significantly less than with Dixon AC (P < 0.001) in most regions. Voxel-based comparisons showed that all cortical grey matter and cerebellum uptake was underestimated with Dixon AC compared to CT AC. Using UTE AC the extent and significance of these differences were reduced. Inaccuracies in cerebellar activity concentrations led to a mixture of predominantly cortical underestimation and subcortical overestimation in SUVr PET data for both MR AC methodologies. CONCLUSION: MR-based AC results in significant underestimation of activity concentrations throughout the brain, which makes the use of SUVr data difficult. These effects limit the quantitative accuracy of neurological PET/MR.

Qualitative and quantitative comparison of PET/CT and PET/MR imaging in clinical practice.

UNLABELLED: The aim of this study was to prospectively compare whole-body PET/MR imaging and PET/CT, qualitatively and quantitatively, in oncologic patients and assess the confidence and degree of inter- and intraobserver agreement in anatomic lesion localization. METHODS: Fifty patients referred for staging with known cancers underwent PET/CT with low-dose CT for attenuation correction immediately followed by PET/MR imaging with 2-point Dixon attenuation correction. PET/CT scans were obtained according to standard protocols (56 ± 20 min after injection of an average 367 MBq of (18)F-FDG, 150 MBq of (68)Ga-DOTATATE, or 333.8 MBq of (18)F-fluoro-ethyl-choline; 2.5 min/bed position). PET/MR was performed with 5 min/bed position. Three dual-accredited nuclear medicine physicians/radiologists identified the lesions and assigned each to an exact anatomic location. The image quality, alignment, and confidence in anatomic localization of lesions were scored on a scale of 1-3 for PET/CT and PET/MR imaging. Quantitative analysis was performed by comparing the standardized uptake values. Intraclass correlation coefficients and the Wilcoxon signed-rank test were used to assess intra- and interobserver agreement in image quality, alignment, and confidence in lesion localization for the 2 modalities. RESULTS: Two hundred twenty-seven tracer-avid lesions were identified in 50 patients. Of these, 225 were correctly identified on PET/CT and 227 on PET/MR imaging by all 3 observers. The confidence in anatomic localization improved by 5.1% when using PET/MR imaging, compared with PET/CT. The mean percentage interobserver agreement was 96% for PET/CT and 99% for PET/MR imaging, and intraobserver agreement in lesion localization across the 2 modalities was 93%. There was 10% (5/50 patients) improvement in local staging with PET/MR imaging, compared with PET/CT. CONCLUSION: In this first study, we show the effectiveness of whole-body PET/MR imaging in oncology. There is no statistically significant difference between PET/MR imaging and PET/CT in respect of confidence and degree of inter- and intraobserver agreement in anatomic lesion localization. The PET data on both modalities were similar; however, the observed superior soft-tissue resolution of MR imaging in head and neck, pelvis, and colorectal cancers and of CT in lung and mediastinal nodal disease points to future tailored use in these locations.