MRI Image Weighting and Contrast PDF

Image weighting and contrast Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation  T1 recovery.  T2 decay.  Relaxation in different tissues.  Image contrast.  Contrast mechanisms.  T1 contrast.  T2 contrast T1 Recovery ( spine-lattice energy transfer) The time...

Image weighting and contrast Hayder Jasim Taher PhD of Medical Imaging Outline of my presentation  T1 recovery.  T2 decay.  Relaxation in different tissues.  Image contrast.  Contrast mechanisms.  T1 contrast.  T2 contrast T1 Recovery ( spine-lattice energy transfer) The time is takes for 63% of longitudinal magnetization to recover by Caused in a tissue nuclei giving up their energy to the hydrogen surrounding environment or molecular lattice.  T1 Recovery Time The time is takes for 63% of longitudinal magnetization to recover in a tissue T1 Recovery Time Typical T1 recovery times of brain tissue at 1 T.  Relaxation in different tissues T1 recovery and T2 decay are exponential processes with time constants T1 recovery time and T2 decay time, which represent the time it takes for 63% of the total magnetization to recover in the longitudinal plane due to spin–lattice energy transfer (T1 recovery time), or lost in the transverse plane via spin–spin relaxation (T2 decay time). Generally, the two extremes of contrast in MRI are fat and water. Relaxation in different tissues Fat and water Fat And Water Fat molecules Water molecules contain atoms of contain two hydrogen arranged hydrogen atoms with carbon and arranged with one oxygen. They consist oxygen atom (H2O). of large molecules Its molecules are called lipids that are spaced apart, and closely packed their molecular together and whose tumbling rate is molecular motion relatively fast. or tumbling rate is relatively slow.  T1 recovery in fat and water T1 recovery in fat T1 recovery in water Short Long T1 recovery occurs due to hydrogen T1 recovery occurs due to hydrogen nuclei giving nuclei giving up their energy to the up energy acquired from the RF excitation pulse surrounding molecular lattice. Fat has a to the surrounding lattice. Water has a high low inherent energy and easily absorbs inherent energy and does not easily absorb energy into its lattice from hydrogen energy into its lattice from hydrogen nuclei. In water, molecular mobility is high, resulting in nuclei. The slow molecular tumbling in less efficient T1 recovery because the fat allows the T1 recovery process to be molecular tumbling rate does not match relatively rapid because the the Larmor frequency and does not allow molecular tumbling rate matches efficient energy exchange from hydrogen  T2 decay Time of 63% of the transverse magnetisation to decay due to dephasing. T2* decay T2* decay  T2 decay time Time of 63% of the transverse magnetisation to decay due to dephasing. T2 decay time  T2 decay in fat and water T2 decay in fat T2 decay in water Short Long T2 decay occurs because the magnetic fields of hydrogen T2 decay in water is less efficient than in fat, as the nuclei interact with each other. This process is efficient in molecules are spaced apart, and spin–spin interactions hydrogen in fat, as the molecules are packed closely are less likely to occur. In addition, magnetic moments of together, and therefore spin–spin interactions are more hydrogen nuclei in water precess much faster than likely to occur. It also occurs because magnetic moments molecular tumbling. As a result, magnetic moments of of hydrogen nuclei in fat precess at a similar frequency to hydrogen nuclei dephase slowly, and there is a gradual, molecular tumbling. As a result, magnetic moments rather than rapid, loss of coherent transverse dephase quickly, and there is a rapid loss of coherent magnetization. transverse magnetization. The T2 decay time of water is therefore long  Image contrast The factors that effect image contrast in diagnostic imaging 1. Intrinsic 2.Extrinic contrast contrast parameters Parameters  T1 recovery time  TR  T2 decay time  TE  Proton density (PD)  Flip angle  Flow  TI  Apparent diffusion  Turbo factor/ echo train coefficient (ADC) length  B value.  Contrast mechanisms High Low signal signal Tissue has a large transvers Tissue has a small transvers components of in-phase components of in-phase magnetization at time TE magnetization at time TE Large signal amplitude is Small signal amplitude is received by the coil received by the coil Bright area on the Dark area on the image image  T1 contrast - As there is more longitudinal magnetisation in T1 time of fat is shorter than fat before the RF pulse there is more transverse magnetisation in fat after RF pulse. of water - TR : It must be shorter than the Fat therefore has a high signal and is T1 times of both fat and water , hyperintanse on a T1 contrast image - Water therefore has a low signal and appear Because of that neither fat or relatively hypointanse on a T1 contrast image water has sufficient time to  T1 contrast - If TR is too long : both the vactors in fat and water return to B0 and fully recover their longitudinal magnetization T1 contrast  T2 weighting or T2 contrast The T2 times of Fat is shorter than The that of water magnitude of trans magnetization of water large Therefor water has a high signal and hyperintense Therefor Fat has a low signal and hypointense on a T2 contrast image  T2 weighting or T2 contrast TE : It must be long enough to give both fat and water time to dephase Image contrast definitions Thank