Celia O'Meara

Qualitative and Quantitative Comparison of PET/CT and PET/MR Imaging in Clinical Practice

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 18F-FDG, 150 MBq of 68Ga-DOTATATE, or 333.8 MBq of 18F-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.

Practical PET Respiratory Motion Correction in Clinical PET/MR

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 (18F-FDG and 68Ga-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 (SUVpeak) and maximum SUV (SUVmax) 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.

Inital experience of imaging cardiac sarcoidosis using hybrid PET-MR - a technologist's case study

Background Cardiac imaging has been identified as a potential use of hybrid PET-MRI. This new technology allows simultaneous Positron Emission Tomography (PET) and MR scanning to occur. This case study features a 34 year old male with known pulmonary sarcoidosis which was diagnosed in 2006 and initially treated with immunosuppressants. He presented with NYHA functional class 2 symptoms. ECHO showed asymmetrical LV anterior and lateral wall hypertrophy with a maximum wall thickness of 18mm in the basal and mid anterior and lateral wall. The patient was referred for a cardiac18F Fluorodeoxyglucose (FDG) PET-MR study to determine the cause of the hypertrophy. The possible differential diagnoses included hypertrophic cardiac myopathy with pulmonary sarcoidosis or cardiac sarcoidosis.

Initial evaluation of a practical PET respiratory motion correction method in clinical simultaneous PET/MRI

Respiratory motion during PET acquisitions can cause image artefacts, with sharpness and tracer quantification adversely affected due to count ‘smearing’. Motion correction by registration of PET gates becomes increasingly difficult with shorter scan times and less counts. The advent of simultaneous PET/MRI scanners allows the use of high spatial resolution MRI to capture motion states during respiration [1, 2]. In this work, we use a respiratory signal derived from the PET list-mode data [3, 4], with no requirement for an external device or MR sequence modifications. Clinical PET data are grouped into 10 respiratory bins based on respiratory signal amplitude derived from the PET list-mode data (Deep breaths outside defined limits are ignored) (Figure ​(Figure1).1). During an extra post-scan 30s PET/MRI acquisition, rapid 2D Gradient Echo MR images are collected and grouped into these 10 respiratory bins. Images in each bin are averaged to form one image per bin (Figure ​(Figure2),2), which are registered to a reference image, forming a patient-specific motion model. Motion estimates from the model are applied directly within the reconstruction of the clinical PET list-mode data using Motion Compensated Image Reconstruction (MCIR) [5], to form one motion-corrected image. On two human subjects (18F-FDG - 1 multiple liver lesions, 1 cardiac) we present PET data motion-corrected with an MRI motion model. Images are assessed visually, with line profiles through ROIs (Figure ​(Figure3),3), and by change of pixel intensity in regions of high activity. Figure 1 Respiratory signal throughout clinical 15 minute PET acquisition, including ‘free breathing’ and ‘breath-hold’ sections. Horizontal lines used to bin the PET data based on signal amplitude are also shown. Deep inhalation ... Figure 2 Binned and averaged MRI slices for 4 out of the 10 bins; ranging from end-expiration to end-inspiration. Figure 3 Uncorrected image (a) with ROI (b), motion-corrected image (c) with ROI (d), line profiles through the liver lesions in both images (e). Image intensity scale is in arbitrary units. In the liver case we see a decrease in tumor ‘smearing’ after MRI model-based correction (Figure ​(Figure3).3). Other areas of high activity in the liver, only marginally visible in the uncorrected image, become apparent in the motion-corrected image. Average intensity increase over the 3 lesions is 11%, while increase in intensity in the cardiac wall in the cardiac patient is 10%.

Hybrid PET/MR metabolic imaging of the reperfused infarct - new biology, future directions

Results The AAR delineated by T2-mapping correlated significantly with both the BARI (R=0.86;P 0.05, Fig 1), with good correlation and low bias (R=0.86, bias 0.75±12.78%, Fig 2). Unexpectedly, we observed reduced FDG uptake in remote normal myocardium associated with microvascular obstruction and angiographic ‘no-reflow’ (TIMI 0) in the infarct related artery. Conclusions

Practical PET respiratory motion correction in clinical simultaneous PET/MR

Respiratory motion during PET acquisition leads to blurring in images and quantification underestimation. We propose a practical, anatomy independent MR-based correction strategy for PET data affected by motion, and show it can improve image quality for both PET acquired simultaneously to the motion-capturing MR, and for PET acquired up to 1 hour earlier during a clinical scan. For our method, a short additional PET/MR sequence is acquired to form a patient-specific MR-based motion model, with a PET-derived respiratory signal and a gradient echo 2D multi-slice sequence. To evaluate our method, uncorrected and motion-corrected images are compared using line profiles, and quantitatively with SUVpeak in avid lesions. We show that clinical PET image quality can be improved using only a short additional PET/MR acquisition with no external respiratory hardware, standard MR sequences and an open registration method.