Techniques and Optimization
In this chapter, we discuss the factors that affect image quality of DW-MRI and practical methods to improve the quality of DW-MR images obtained from clinical scanners. We demonstrate how simple phantoms may be used to improve image quality so that high-
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CONTENTS
SUMMARY 2.1
Introduction
2.2
Factors that Affect Image Quality Using EPI DW-MRI Acquisition 20 Image Signal-to-Noise (SNR) 20 Image Artefacts 20
2.2.1 2.2.2
19
2.3 2.3.1
Bulk Diffusion Phantoms 21 Optimizing Image Quality Using a Phantom 21
2.4
Optimizing Image Quality on Volunteer or Patient Studies 24 Fat Suppression 25 Single-Shot Acquisition vs. Multiple Signals Averaging 26
2.4.1 2.4.2 2.5 2.5.1 2.5.2 2.6
Optimization Strategies for the Selection of b-Values 27 Quantitative Data 27 Qualitative Data 28 Summary of Technique Optimization for DW-MRI in the Body 30
References
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In this chapter, we discuss the factors that affect image quality of DW-MRI and practical methods to improve the quality of DW-MR images obtained from clinical scanners. We demonstrate how simple phantoms may be used to improve image quality so that high-quality images can be attained consistently. We review the relative merits of fat-suppression schemes and how these may be applied to different clinical applications. We discuss the optimization of b-values and the number of b-values required for clinical imaging and further illustrate the range of considerations required for these choices. We survey the different data acquisition methods using breath-holding, physiological gating or free breathing, and how the choice of data acquisition method may relate to a particular scanning application. Finally, we provide recommendations to enable the reader to develop their own practical diffusion-weighted imaging protocols in the body.
2.1 Introduction
David J. Collins, MInstP Matthew Blackledge, MSc CR UK – EPSARC Cancer Imaging Centre, Institute of Cancer Research & Royal Marsden Hospital, Downs Road, Sutton SM2 5PT, UK
Over the past decade, DW-MRI has been a great success in neurological imaging, particularly for the assessment of acute cerebral vascular events (Bammer et al. 2001; Marks et al. 2008) and in mapping the anatomical cerebral pathways using DTI (Chen et al. 2001; Bammer et al. 2002). DW-MRI has become practical in extracranial applications following the introduction of parallel imaging techniques in the
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D. J. Collins and M. Blackledge
late 1990s (Pruessmann et al. 1999; Blaimer et al. 2004; Larkman et al. 2007) and the continued improvements in MR hardware. The use of parallel imaging in combination with diffusion-weighted single-shot echo-planar imaging (EPI) has overcome many of the challenges that have limited its earlier implementation in the body. Although using EPI DW-MRI acquisition and enhanced hardware advances have enabled highquality DW-MR images in the body to be acquired within a reasonable time frame, a number of challenges still remain. As is well known, EPI is highly sensitive to static magnetic field inhomogeneity, chemical shift artefacts and eddy currents resulting from the application of the diffusion encoding gradients (Le Bihan et al. 2006). Physiological motions within the body are an additional challenge and the number and range of diffusion weightings (b-values) used withi
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