Optimization of yttrium-90 PET for simultaneous PET/MR imaging: A phantom study.

PURPOSE: Positron emission tomography (PET) imaging of yttrium-90 in the liver post radioembolization has been shown useful for personalized dosimetry calculations and evaluation of extrahepatic deposition. The purpose of this study was to quantify the benefits of several MR-based data correction approaches offered by using a combined PET/MR system to improve Y-90 PET imaging. In particular, the feasibility of motion and partial volume corrections were investigated in a controlled phantom study. METHODS: The ACR phantom was filled with an initial concentration of 8 GBq of Y-90 solution resulting in a contrast of 10:1 between the hot cylinders and the background. Y-90 PET motion correction through motion estimates from MR navigators was evaluated by using a custom-built motion stage that simulated realistic amplitudes of respiration-induced liver motion. Finally, the feasibility of an MR-based partial volume correction method was evaluated using a wavelet decomposition approach. RESULTS: Motion resulted in a large (∼40%) loss of contrast recovery for the 8 mm cylinder in the phantom, but was corrected for after MR-based motion correction was applied. Partial volume correction improved contrast recovery by 13% for the 8 mm cylinder. CONCLUSIONS: MR-based data correction improves Y-90 PET imaging on simultaneous PET/MR systems. Assessment of these methods must be studied further in the clinical setting.

Attenuation Correction for Magnetic Resonance Coils in Combined PET/MR Imaging: A Review.

With the introduction of clinical PET/magnetic resonance (MR) systems, novel attenuation correction methods are needed, as there are no direct MR methods to measure the attenuation of the objects in the field of view (FOV). A unique challenge for PET/MR attenuation correction is that coils for MR data acquisition are located in the FOV of the PET camera and could induce significant quantitative errors. In this review, current methods and techniques to correct for the attenuation of a variety of coils are summarized and evaluated.

Feasibility of (18)F-Fluorodeoxyglucose radiotracer dose reduction in simultaneous carotid PET/MR imaging.

The purpose of this study was to develop and validate low dose (18)F-FDG-PET acquisition protocols for detection of inflamed carotid plaques specifically for simultaneous PET/MR imaging. The hypothesis was that increasing the duration of the PET acquisition to match that of the MR acquisition might allow for the use of lower levels of the radiotracer, while preserving quantification and image quality. Seven subjects were scanned twice at least one week apart on a simultaneous PET/MR scanner using either the standard clinical dose of (18)F-FDG (373 ± 63 MBq) for 8 minutes or a low dose (93 ± 17 MBq) for 75 minutes. A maximum absolute percent difference of only 4.17% and 7.49% in the left and right carotid TBR was found between the standard dose and four time points of the low dose acquisitions (8, 24, 45, 75 minutes). Only the 8-minute low dose PET data was significantly different in terms of SNR (P = 0.009; % difference = -51%) and qualitative image quality evaluation (P = 0.0005; % difference = -45%). Our preliminary findings indicate that up to 75% reduction of the clinical standard (18)F-FDG dose could be achieved using the proposed acquisition scheme while maintaining accurate quantification and SNR.

Markerless attenuation correction for carotid MRI surface receiver coils in combined PET/MR imaging.

The purpose of the study was to evaluate the effect of attenuation of MR coils on quantitative carotid PET/MR exams. Additionally, an automated attenuation correction method for flexible carotid MR coils was developed and evaluated. The attenuation of the carotid coil was measured by imaging a uniform water phantom injected with 37 MBq of 18F-FDG in a combined PET/MR scanner for 24 min with and without the coil. In the same session, an ultra-short echo time (UTE) image of the coil on top of the phantom was acquired. Using a combination of rigid and non-rigid registration, a CT-based attenuation map was registered to the UTE image of the coil for attenuation and scatter correction. After phantom validation, the effect of the carotid coil attenuation and the attenuation correction method were evaluated in five subjects. Phantom studies indicated that the overall loss of PET counts due to the coil was 6.3% with local region-of-interest (ROI) errors reaching up to 18.8%. Our registration method to correct for attenuation from the coil decreased the global error and local error (ROI) to 0.8% and 3.8%, respectively. The proposed registration method accurately captured the location and shape of the coil with a maximum spatial error of 2.6 mm. Quantitative analysis in human studies correlated with the phantom findings, but was dependent on the size of the ROI used in the analysis. MR coils result in significant error in PET quantification and thus attenuation correction is needed. The proposed strategy provides an operator-free method for attenuation and scatter correction for a flexible MRI carotid surface coil for routine clinical use.

A knowledge-based method for reducing attenuation artefacts caused by cardiac appliances in myocardial PET/CT.

Attenuation artefacts due to implanted cardiac defibrillator leads have previously been shown to adversely impact cardiac PET/CT imaging. In this study, the severity of the problem is characterized, and an image-based method is described which reduces the resulting artefact in PET. Automatic implantable cardioverter defibrillator (AICD) leads cause a moving-metal artefact in the CT sections from which the PET attenuation correction factors (ACFs) are derived. Fluoroscopic cine images were measured to demonstrate that the defibrillator's highly attenuating distal shocking coil moves rhythmically across distances on the order of 1 cm. Rhythmic motion of this magnitude was created in a phantom with a moving defibrillator lead. A CT study of the phantom showed that the artefact contained regions of incorrect, very high CT values and adjacent regions of incorrect, very low CT values. The study also showed that motion made the artefact more severe. A knowledge-based metal artefact reduction method (MAR) is described that reduces the magnitude of the error in the CT images, without use of the corrupted sinograms. The method modifies the corrupted image through a sequence of artefact detection procedures, morphological operations, adjustments of CT values and three-dimensional filtering. The method treats bone the same as metal. The artefact reduction method is shown to run in a few seconds, and is validated by applying it to a series of phantom studies in which reconstructed PET tracer distribution values are wrong by as much as 60% in regions near the CT artefact when MAR is not applied, but the errors are reduced to about 10% of expected values when MAR is applied. MAR changes PET image values by a few per cent in regions not close to the artefact. The changes can be larger in the vicinity of bone. In patient studies, the PET reconstruction without MAR sometimes results in anomalously high values in the infero-septal wall. Clinical performance of MAR is assessed by two physicians' inspection of images generated in 30 patients with and without MAR. Noticeable image differences are judged in 14 of 28 (50%) observations with AICD leads, and significant clinical impact is judged in 2 of 28 (7%) of those observations. A polar map analysis shows significant differences in 10 of 14 (71%) studies with AICD leads, and 0 of 16 (0%) studies without AICD leads. These results show that the MAR method is successful in reducing the magnitude of the metal artefact without incorrectly altering cases without metal artefact. In spite of profound changes to the CT image from the moving metal, the PET ACF in that study was changed by no more than 20%.