Abstract
Ultrashort echo time (UTE) MRI has been proposed as a way to produce segmented attenuation maps for PET, as it provides contrast between bone, air, and soft tissue. However, UTE sequences require samples to be acquired during rapidly changing gradient fields, which makes the resulting images prone to eddy current artifacts. In this work it is demonstrated that this can lead to misclassification of tissues in segmented attenuation maps (AC maps) and that these effects can be corrected for by measuring the true k-space trajectories using a magnetic field camera. The k-space trajectories during a dual echo UTE sequence were measured using a dynamic magnetic field camera. UTE images were reconstructed using nominal trajectories and again using the measured trajectories. A numerical phantom was used to demonstrate the effect of reconstructing with incorrect trajectories. Images of an ovine leg phantom were reconstructed and segmented and the resulting attenuation maps were compared to a segmented map derived from a CT scan of the same phantom, using the Dice similarity measure. The feasibility of the proposed method was demonstrated in in vivo cranial imaging in five healthy volunteers. Simulated PET data were generated for one volunteer to show the impact of misclassifications on the PET reconstruction. Images of the numerical phantom exhibited blurring and edge artifacts on the bone-tissue and air-tissue interfaces when nominal k-space trajectories were used, leading to misclassification of soft tissue as bone and misclassification of bone as air. Images of the tissue phantom and the in vivo cranial images exhibited the same artifacts. The artifacts were greatly reduced when the measured trajectories were used. For the tissue phantom, the Dice coefficient for bone in MR relative to CT was 0.616 using the nominal trajectories and 0.814 using the measured trajectories. The Dice coefficients for soft tissue were 0.933 and 0.934 for the nominal and measured cases, respectively. For air the corresponding figures were 0.991 and 0.993. Compared to an unattenuated reference image, the mean error in simulated PET uptake in the brain was 9.16% when AC maps derived from nominal trajectories was used, with errors in the SUV max for simulated lesions in the range of 7.17%-12.19%. Corresponding figures when AC maps derived from measured trajectories were used were 0.34% (mean error) and -0.21% to +1.81% (lesions). Eddy current artifacts in UTE imaging can be corrected for by measuring the true k-space trajectories during a calibration scan and using them in subsequent image reconstructions. This improves the accuracy of segmented PET attenuation maps derived from UTE sequences and subsequent PET reconstruction.
Highlights
The first whole-body hybrid PET-MR scanner has recently been introduced into clinical practice.[1]
As with the tissue phantom and simulations, hyperintense edge artifacts are seen on the air–tissue border in the images reconstructed with nominal trajectories, leading to a layer of bone appearing on the outer edge of the head in the attenuation map
The main improvements were due to the reduction of hyperintense artifacts on the edge of the phantom in the free induction decay (FID) images, which lead to misclassification of soft tissue as bone in the nominal attenuation maps (AC maps) and due to a reduction in blurring on the edge of the phantom that leads to misclassification of air as bone in the nominal attenuation correction (AC) maps
Summary
The first whole-body hybrid PET-MR scanner has recently been introduced into clinical practice.[1]. Assigning attenuation coefficients to bone using MR images is challenging because cortical bone exhibits low proton densities and very fast transverse relaxation rates (T2∗), causing it to appear with low intensities with standard MRI pulse sequences, which do not begin sampling until the signal has decayed substantially.[21]. In this work it is shown that system delays and eddy current effects contribute substantially to errors in UTE derived attenuation maps, by introducing blurring and edge artifacts in the UTE images, especially at the interfaces between bone and soft tissue and between soft tissue and air. It is demonstrated that these artifacts can be corrected for by measuring the true k-space trajectories using a magnetic field camera This leads to improved classification of bone in segmented attenuation maps for use in PET. PET simulations using attenuation maps derived using nominal and measured k-space are presented for one volunteer
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