ABSTRACT
In this chapter, the physical and biological bases of longitudinal relaxation are described. In white matter, water content, myelin and iron are the dominant factors. The gold standard method to measure T1 is inversion recovery; however, this is slow. Faster methods are Look-Locker (LL) and variable flip angle (VFA). LL samples the recovery of longitudinal magnetisation and measures an effective recovery time called T1*. VFA uses the ratio of two spoilt gradient echoes and can be fast. All methods are vulnerable to B1 errors, and a B1 map is often collected. VFA also requires good spoiling after each RF pulse. An LL acquisition using 30 4 mm slices with in-plane resolution of 1 mm takes under 10 min, giving image noise of 19 ms in white matter. A VFA whole-brain measurement with isotropic 1 mm voxels takes 10 min and gives image noise of 26 ms in white matter. Clinical applications in multiple sclerosis, movement disorders and brain tumours are described. A comprehensive table of normal values at field strengths from 0.2 to 9.4 T is given for a variety of brain tissues.
