T2 has an irrecoverable decay whereas T2* has a recoverable decay and is always shorter than T2. It is called T2* (T-two-star) (Figure 04-16). This leads to a decay of the observed signal which is faster than T2. Imperfections of the main static magnetic field B₀ (field inhomogeneities). Static and oscillating fields locally induced by neighboring magnetic moments (from other nuclei or unpaired electrons), and This is because two phenomena contribute to the local inhomogeneity experienced by the nuclei: In practice, it is observed that the same sample can show two different T2 relaxation times at the same field strength. It is about 5 for muscle tissue at 0.1 T. Here, the T1/T2 ratio increases rapidly with values of 5-10 covering most tissue types. The T1 value of tissues is usually under 1 second. Zone 2: low mobility with slow molecular motion usually large molecules and ‘bound’ water.Īt low and medium fields, the T2 value is approximately 3 seconds and the T1/T2 ratio is 1 for pure water. Zone 1: high mobility with fast molecular motion usually small molecules and ‘free’ water. So, if we represent T1 and T2 versus the microscopic mobility of the spin system, we will obtain for T1 a curve passing through a minimum, corresponding to the Larmor frequency, and a continuously decreasing curve for T2 (Figure 04-15). This could be due to microscopic susceptibility differences which can induce a T2* effect. Then, while T1 still increases, T2 stays constant (on a plateau) but it might also appear to decrease. With increasing field strength, T2 first increases as does T1. To a large extent, this is the cause of the low or absent signal from solid structures such as compact bone or tendons in medical magnetic resonance imaging. In solids T2 is usually so short that the signal has died out within the first millisecond, whereas in fluids the magnetic resonance signal may last for several seconds. In mobile fluids, T2 is nearly equal to T1, whereas in solids or in slowly tumbling systems (i.e., high-viscocity systems), static-field components induced by neighboring nuclei are operative and T2 becomes significantly shorter than T1. Presence of large molecules, paramagnetic ions and molecules, or other outside interference. Mobility of the observed spin (microviscosity) Resonance frequency (field strength), although for T2 this is less crucial than for T1 at low, medium, and high (but seemingly not ultrahigh) fields T2 is dependent on a number of parameters: This process is characterized by T2, the spin-spin or transverse relaxation. This additional decay of the net magnetization in the x'-y' plane is due to a loss of phase coherence of the microscopic components, which partly results from the slightly different Larmor frequencies induced by small differences in the static magnetic fields at different locations of the samples. The decay of the signal in the x'-y' plane is faster than the decay of the magnetization along the z-axis. Transverse relaxation phenomena induce an increase in dephasing of individual spins, so a progressive decrease of the macroscopic magnetization is observed. However, as time passes, the observed signal starts to decrease as the spins begin to dephase (Figure 04-14). T1: Spin-Lattice Relaxation T1 on the Microscopic Scale Cross Relaxation T1 on the Macroscopic Scale The Partial Saturation Sequence The Inversion Recovery Sequence T2: Spin-Spin Relaxation T2 on the Macroscopic Scale The Spin Echo Sequence Practical Measurements of T1 and T2 In vitro Determination In vivo Determination T1 (T2) Images and Weighted Images Measurements in Medical Diagnosis Rapid Relaxation Estimation Techniques Biomarkersįter a spin system has been excited by an RF pulse, it initially behaves like a coherent system i.e., all microscopic components of the macroscopic magnetization precess in phase (all together) around the direction of the external field.
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