In the human body, the distribution of water protons is relatively homogeneous resulting in little contrast in proton density weighted (PDw) imaging. To obtain sufficient contrast for detection and characterization of pathological tissue alterations, the MRI sequence is optimized to generate contrast depending on the relaxation behaviour of tissues after spin excitation. The different relaxation properties of tissues and pathologies are; therefore, the basis of clinical MRI.

In the clinical routine, the most commonly applied weightings are T1-weighted (T1w) and T2-weighted (T2w) imaging. The T1 relaxation ("spin-lattice") relaxation describes the recovery of the longitudinal magnetization after spin excitation, the T1 constant is the time period, which is necessary for a tissue to recover 63% of the equilibrium magnetization. Correspondingly, the T2 relaxation ("spin-spin relaxation") characterizes the decay of the transverse magnetization after spin excitation, the T2 constant is the time, after which the transverse magnetization is decayed to 37% of its value directly after excitation. In T1w imaging, tissues with short T1 time are brighter than tissue with long T1 constant (faster recovery of longitudinal magnetization). In T2w imaging, tissues with long T2 constant are brighter than thosee with short T2 (slow decay of the transverse magnetization).

Within the last years, many further contrast mechnisms have been introduced such as T2*, BOLD (blood oxygenation level depdendant contrast), magnetization transfer, diffusion weighted imaging (DWI), perfusion weighted imaging, susceptibility weighted imaging (SWI). The measurement of the relaxation constants is called relaxometry. To increase the contrast in T1w imaging, gadolinium-based contrast-media can be applied, which cause a reduction in the T1 time of the tissues, and therefore higher MRI signal.

Some own contributions
Jungraithmayr W, Chuck N, Frauenfeld T, Weder W, Boss A. Magnetic resonance imaging using ultra-short echo-time sequences in syngeneic mouse lung transplantation. Radiology (in press)

Donati OF, Nanz D, Serra AL, Boss A. Quantitative BOLD response of the renal medulla to hyperoxic challenge at 1.5T and 3.0T. NMR Biomed. 2012 Jan 31. doi: 10.1002/nbm.2781. [Epub ahead of print]

Rossi C, Boss A, Donati OF, Luechinger R, Kollias SS, Valavanis A, Hodler J, Nanz D. Manipulation of cortical gray matter oxygenation by hyperoxic respiratory challenge: field dependence of R(2) * and MR signal response. NMR Biomed. 2012 Feb 6. doi: 10.1002/nbm.2775. [Epub ahead of print]

Boss A, Martirosian P, Jehs M, Dietz K, Rossi C, Claussen CD, Schick F. Influence of oxygen and carbogen breathing on renal oxygenation measured by T2*-weighted imaging at 3.0 T NMR Biomed 22(6):638-45. (2009)

Boss A, Oppitz M, Wehrl H, Rossi C, Feuerstein M, Claussen CD, Drews U, Pichler B, Schick F. Measurement of T1, T2, and Magnetization Transfer Properties During Embryonic Development at 7 Tesla Using the Chicken Model J Magn Reson Imaging 28: 1510-1514 (2008)

Boss A, Schaefer S, Martirosian P, Claussen CD, Schick F, Schaefer J. Magnetic Resonance Imaging of Lung Tissue: Influence of Body Positioning, Breathing and Oxygen Inhalation on signal decay Using Multi-Echo Gradient-Echo Sequences Invest Radiol 43 (6): 433-438 (2008)

Martirosian P, Boss A, Deimling M, Kiefer B, Schraml C, Schwenzer N, Claussen CD, Schick F. Systematic Variation of Off-Resonance repulses for Clinical Magnetization Transfer Contrast Imaging at 0.2, 1.5 and 3.0 Tesla Invest Radiol 43 (1): 16 - 26 (2008)