Origin and reduction of motion and f0 artifacts in high resolution T2*-weighted magnetic resonance imaging: Application in Alzheimer's disease patients
Introduction
Many neurodegenerative diseases, such as Alzheimer's, Huntington's and Parkinson's diseases are accompanied by local changes in iron concentration (Pinero and Connor, 2000, Schenck et al., 2006, Stankiewicz et al., 2007). In Alzheimer's disease (AD) recent developments suggest that iron is bound to amyloid beta plaques in the grey matter (GM) (Meadowcroft et al., 2009, Cullen et al., 2006). Tissues with elevated iron concentration generate magnetic field inhomogeneities which can be visualized with MRI using, for example, heavily T2*-weighted sequences. In samples from deceased AD patients it has been shown that the increased concentration of iron leads to significant signal loss on high resolution T2*-weighted images (Meadowcroft et al., 2009, van Rooden et al., 2009). The typical size of amyloid beta plaques ranges from 16 to 150 µm (Benveniste et al., 1999), which makes direct in-vivo visualization of individual plaques within a clinically-acceptable scan time highly unlikely. However, whole body high field (7 T and above) MRI scanners might enable the indirect detection of small changes in iron concentration in vivo due both to the intrinsic higher signal-to-noise, which can be used to increase the resolution, and the increased sensitivity to magnetic field inhomogeneities. Very high resolution (up to 0.2 mm in-plane resolution) T2*-weighted images at 7 T have been reported recently in young healthy volunteers (Zhong et al., 2008, Yao et al., 2009, Duyn et al., 2007, Li et al., 2006, Lee et al., 2010), showing an increased contrast between GM and white matter (WM) regions compared to lower field strengths. However, there are potential problems in applying these types of sequences in patients. Heavily T2*-weighted sequences require a long echo time (TE) to generate sufficient T2* contrast and, because of the high spatial resolution, the scan duration is typically long. As a result of these two factors the images are very sensitive to physiological fluctuations, patient motion and system instabilities, potentially causing blurring, signal cancellation and ghosting. Many of these effects are particularly pronounced at high magnetic field (van Gelderen et al., 2007). Results obtained at our institution in normal healthy volunteers at 7 T are comparable to the high quality anatomical images that have been reported by other authors using a similar sequence (Duyn et al., 2007, Van Gelderen et al., 2007, Li et al., 2006). However, application of the same sequence in AD patients and in subjects with subjective memory complaints (SMC) showed a dramatically reduced image quality.
This study is divided into two parts. First, the source of the image artifacts in AD patients was investigated. Using phantom and healthy volunteer scans the different contributions of translational motion, rotational motion, and time-dependent f0 variations to image artifacts were measured. Secondly, based on the results of the first part (which showed that f0 variations were dominant), a navigator echo based correction technique was designed and implemented to improve the image quality in AD patients and SMC subjects. Qualitative assessment of image quality was achieved using scoring by three observers and quantitative measures of the reduction in ghosting levels were reported.
Section snippets
Materials and methods
Informed consent was obtained from all subjects and this study was approved by the local ethics committee. All experiments were performed on a whole body 7 T system (Philips Healthcare, Best, The Netherlands).
Results
Fig. 1 compares the image quality obtained from a healthy volunteer and an AD patient. It is clear that the image quality from the AD patient is considerably poorer than that obtained in the healthy volunteer. In addition differences in image contrast, ventricular size and brain morphology can be observed which can be attributed to the disease and aging process.
Figs. 2a and b show the estimated time-series of translational and rotational motions measured in an AD patient in the separate study
Discussion
The most important findings of this study were threefold. First the contributions of translational motion and rotation to the amount of artifacts in a T2*-weighted sequence were found to be minimal. Second, the decreased image quality in AD patients and SMC subjects results mainly from fluctuations in f0 during the acquisition. Third, using navigator echoes it is possible to correct for these f0 fluctuations resulting in a substantial increase in image quality.
The amount of artifacts in many AD
Acknowledgments
This research was performed within the framework of CTMM, the Center for Translational Molecular Medicine (www.ctmm.nl), Project LeARN (grant 02N-101).
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