PET and MRI are exceptionally powerful technologies. By combining these modalities together in a PET/MRI system, we now have the capability to examine a wide range of physiological and pathological processes with both modalities in spatial and temporal register. But without reliable MRI-based AC, the vast potential of PET/MRI is unattainable.
In this chapter, I have attempted to put MRI-based AC in a broader context. I began by exemplifying the importance of quantitative PET/MRI in three medical fields, namely oncology, neurology, and cardiology. Next, a relatively comprehensive picture of the factors that influence quantification in PET was provided, helping the reader to understand how attenuation fits in. I subsequently focused specifically on attenuation: what it is, how it is corrected, and how to obtain the µ-map from emission or transmission data. Ultimately, I discussed MRI-based AC, emphasizing its relationship to existing AC techniques. I went on to discuss some directions for future research, highlighting research covered in the subsequent chapters of this thesis. In particular, the overarching objective of my thesis is to improve quantification in PET and SPECT images that are attenuation corrected with an MRI-based µ-map. I begin by comparing MRI-based AC in PET versus SPECT to see which modality is more quantitatively accurate. I subsequently focus on a particular issue in MRI-based AC: i.e. assigning reliable µ-coefficients to the lungs. I finish by comparing the three approaches to making an MRI-based µ-map, namely segmentation, registration, and mapping, to establish the circumstances that dictate good versus poor performance.
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