Marieke van Rosmalen

General introduction and thesis outline 13 1 signal weakens due to two simultaneous relaxation processes that cause a loss of coherence of the spin system. The NMR signal decreases (loss of transverse magnetization or dephasing) with a time constant called the transverse relaxation time (T2, Figure 1.1D ). Concurrently, but much slower, the vector relaxes towards its equilibrium position (recovery of longitudinal magnetization) parallel to the magnetic field: this time constant is called the spin-lattice relaxation time (T1, Figure 1.1E ). Figure 1.1 Principles of MRI physics All protons in body tissue spin on their own axes (A). After placing the patient in a static magnetic field, i.e. the MRI scanner, the resulting magnetization of all protons align parallel (B) to the magnetic field (B 0 ). The protons rotate around B 0 at the Larmor frequency and the average of many protons produces the net magnetization. Then, a radiofrequency pulse (RF pulse) is emitted from the scanner which creates a magnetic field perpendicular to B 0 and the net magnetization moves away from the z axis (C). As soon as the RF pulse is switched off, the protons begin to relax back to their equilibrium. The two main features of relaxation are dephasing of the spins or loss of transverse magnetization (T2 relaxation, D) and realignment along the z axis (T1 relaxation) as an umbrella closing up (E). Every type of body tissue has its own T1 and T2 relaxation times which results in different contrasts in the images. Adjustments in the MRI software enables the scanner to generate T1- or T2-weighted images of the tissues of interest. T1- and T2-weighted MRI provides qualitative information on the anatomical structures of interest. The generation of a T1-weighted image or a T2-weighted image depends on the set echo time (TE) and repetition time (TR) of the MRI sequence. T1-weighted images tend to have a short TE and a short TR. In T1-weighted images tissues that have a slow magnetization