Abstract
Positron emission tomography (PET) is a highly sensitive medical imaging technique commonly used to detect and assess tumor lesions. Magnetic resonance imaging (MRI) provides high resolution anatomical images with different contrasts and a range of additional information important for cancer diagnosis. Recently, simultaneous PET-MR systems have been released with the promise to provide complementary information from both modalities in a single examination. Due to long scan times, subject nonrigid bulk motion, i.e., changes of the patient's position on the scanner table leading to nonrigid changes of the patient's anatomy, during data acquisition can negatively impair image quality and tracer uptake quantification. A 3D MR-acquisition scheme is proposed to detect and correct for nonrigid bulk motion in simultaneously acquired PET-MR data. A respiratory navigated three dimensional (3D) MR-acquisition with Radial Phase Encoding (RPE) is used to obtain T1- and T2-weighted data with an isotropic resolution of 1.5 mm. Healthy volunteers are asked to move the abdomen two to three times during data acquisition resulting in overall 19 movements at arbitrary time points. The acquisition scheme is used to retrospectively reconstruct dynamic 3D MR images with different temporal resolutions. Nonrigid bulk motion is detected and corrected in this image data. A simultaneous PET acquisition is simulated and the effect of motion correction is assessed on image quality and standardized uptake values (SUV) for lesions with different diameters. Six respiratory gated 3D data sets with T1- and T2-weighted contrast have been obtained in healthy volunteers. All bulk motion shifts have successfully been detected and motion fields describing the transformation between the different motion states could be obtained with an accuracy of 1.71 ± 0.29 mm. The PET simulation showed errors of up to 67% in measured SUV due to bulk motion which could be reduced to less than 10% with the proposed motion compensation approach. A MR acquisition scheme which yields both high resolution 3D anatomical data and highly accurate nonrigid motion information without an increase in scan time is presented. The proposed method leads to a strong improvement in both MR and PET image quality and ensures an accurate assessment of tracer uptake.
Highlights
Positron emission tomography (PET) is a commonly used tool to detect and quantify FDG uptake in tumors.[1,2] Recently, hybrid PET-magnetic resonance (MR) scanners have been introduced to combine the excellent sensitivity of PET with high-resolution multicontrast MR images
Other approaches require dedicated Magnetic resonance imaging (MRI) sequences which are optimized for motion estimation but do not yield high resolution anatomical data required for medical diagnosis and strongly restrict information available from MR. To overcome these limitations we present the use of a 3D high-resolution Radial Phase Encoding (RPE) MR acquisition scheme, which allows for the reconstruction of images with different temporal resolutions from the same acquired data.[20,21]
The images used for motion estimation are reconstructed from a variable number of radial phase encoding lines depending on the duration of each bulk motion state [Fig. 4(d)]
Summary
Positron emission tomography (PET) is a commonly used tool to detect and quantify FDG uptake in tumors.[1,2] Recently, hybrid PET-magnetic resonance (MR) scanners have been introduced to combine the excellent sensitivity of PET with high-resolution multicontrast MR images. Sequential PETMR systems consist of PET and MR components which are spatially separated to avoid any mutual interference allowing for a coregistered PET-MR acquisition.[3] For simultaneous PET-MR systems, the PET and MR gantry are integrated such that PET and MR cover the same field of view. After synchronizing the clocks which control data acquisition of PET and MR, these systems yield simultaneously obtained image information.[4,5] The advantage of simultaneous PET-MR systems is the potential to speed up the acquisition of PET-MR data and providing additional information about motion from MRI
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