MR Essentials - Echoes, Decay, Relaxation, and Contrast - USA

Welcome to the MR Essentials - Echoes, Decay, Relaxation, and Contrast Online Training course. Throughout this course we will discuss, what happens to magnetization after RF excitation. As the magnetization decreases rather quickly and the MR signal decays, we can create echoes of the lost MR signal for a while. It takes some time before the longitudinal magnetization fully recovers - these processes are known as relaxation.   Upon successful completion of this course, you will be able to: Describe longitudinal and transverse relaxation Identify the difference between T1 and T2 relaxation times Explain how different types of contrasts are obtained Recognize the components of a spin echo sequence Congratulations. You have completed the MR Essentials - Echoes, Decay, Relaxation, and Contrast Online Training. Take time to review the summary of the material covered in this course before moving onto the final assessment.   Describe longitudinal and transverse relaxation Longitudinal Magnetization: the buildup and recovery of the longitudinal magnetization is an exponential process; referred to as spin-lattice relaxation Transverse Magnetization: the decay of transverse magnetization occurs when the rotating spins get out of phase due to unavoidable interactions between the nuclei; referred to as spin-spin relaxation. The induced MR signal deteriorates as transverse magnetization decays   Identify the difference between T1 and T2 relaxation times T1 relaxation: the time constant for longitudinal relaxation that represents the portion of exponential growth, not the time span of total magnetization recovery. After T1, longitudinal magnetization recovers to approximately 63% of its final value T2 relaxation: The time constant for transverse relaxation. After T2, the phase coherence of spins has dropped to approximately 37%   Explain how the different types of contrasts are obtained Proton Density: depends on the differences in the number of hydrogen protons per voxel and is produced by selecting a long TR and short TE T2 contrast: depends on the speed of transverse relaxation and is produced by a long TR and long TE T1 contrast: depends on the speed of longitudinal relaxation and is produced by a short TR and short TE   Recognize the components of a spin echo sequence Spin Echo: the rephasing MR signal that is generated from the application of the 180o pulse 90o Pulse: the initial RF pulse that flips the magnetization in the transverse direction (xy-plane). the 90-degree pulse induces the MR Signal but this transverse magnetization decays quickly over time. 180o Pulse: this pulse brings the out-of-phase spins back into phase Echo Time (TE): the time from the center of the 90-degree pulse to the signal maximum Repetition Time (TR): the time interval between repetitions of the 90-degree pulse Select the link below to view/print your review material before proceeding to the final quiz. The build-up and recovery of the longitudinal magnetization is an exponential process referred to as longitudinal relaxation. T1 is the relaxation time T1 is not the time span of total magnetization recovery, only the time constant for exponential growth Spin-lattice Relaxation     T1 Recovery Learn about T1 time constant. The graph shows how after the time interval T1, the longitudinal magnetization MZ has recovered 63% of its final value. After 5 T1 times, recovery is practically complete. It is important to remember that the T1 constant depends on the tissue type. T1 Constant in Different Tissues Learn about the T1 constant in different tissues. Element HTMLMz   100% 63% 240                   680   809                                                                                  2500   t[ms]Fat                                                        White Matter                                      Gray Matter                                                                                                    CSF   0.2 T 1.0 T 1.5 T Fat 240 Muscle 370 730 863 White matter 388 680 783 Gray Matter 492 809 917 CSF 1,400 2,500 3,000                * CSF: Cerebro Spinal Fluid Sound File Audio ScriptDifferent types of tissue have different relaxation times. These different T1 times are responsible for the sharp image contrast obtained with MR. For example, fat has a short T1 while water has a long T1. [Pause 5 seconds for graph to display] The table shows the T1 value for each tissue is unique and also depends on the strength of the magnetic field. Immediately after the RF-Pulse, the spins are called phase-coherent. They act like one large magnet that rotates in the xy-plane. They can induce a signal in the receiver coil. Transverse magnetization begins to decay as the rotating spins get out of phase due to the interaction between the nuclei (spin-spin relaxation). The MR signal deteriorates when Transverse Relaxation occurs.     T2 Decay Learn about the T2 time constant. After time interval T2, the phase coherence of the spins has dropped to approximately 37%. After 2 T2, it drops to approximately 14 % and after 5 T2, phase coherence has just about disappeared. Short T2: tissue with a “tight” molecular structure Long T2: tissue with a “loose” molecular structure T2 Constant in Different Tissues Learn about the T2 constant in different tissues. Element HTMLWhite MatterCSFGray Matter84     92      1011400                                                                                   2500   t [ms]Mz 100% 63%Fat Tissue T2 [ms] Liver 43 ± 6 Skeletal Muscle 47 ± 6 Heart Muscle 57 ± 9 Kidneys 58 ± 8 Spleen 62 ± 17 Fat 80 ± 36 White Brain Matter 92 ± 20 Gray Brain Matter 101 ± 13 CSF (estimated) 1400 ± 250 Sound File Audio ScriptThe T2 time constant for each tissue is unique, but largely field-independent. This graph illustrates T2 values for some different tissue types. For example, fat has a short T2 and water has a long T2. [Pause 5 seconds for graph to display] The table shows the T2 value for each tissue is unique but not dependent on the value of the magnetic field. T1 : longitudinal relaxation time T2: transverse relaxation time t M In MRI the contrast is determined by : Proton density T1 Relaxation time T2 Relaxation time   Physical parameters (Flow, Diffusion, Susceptibility) Sequence parameters (ex. TR, TE) TR TE short long long short PD T1 T2 The maximum value that the longitudinal magnetization recovers in different tissues depend on proton density   Depends on the differences in the number of hydrogen protons per voxel   Produced by selecting a long TR and short TE Mz            Mxy 500 300 TR [ms] 15 60           90         120 TE [ms] TR long                                            TE short PD contrast   T2 contrast is based on the speed of transverse relaxation   With longer echo times, the curves diverge and the contrast is produced by the T2 relaxation   Produced by selecting a long TR and long TE Mz              Mxy 500 3000 15 60      90          120 TR [ms] TE [ms] TR long                                                             TE long T2 contrast 120 ms 60 ms 90 ms Free Induction Decay (FID) has a shorter effective time constant: T2* The magnetic field inhomogeneities force the spins to go out of phase more quickly. Effects of these magnetic field inhomogeneities include: Static T2* - static differences in the main magnetic field Tissue Specific T2* - susceptibility magnetic field differences Need to eliminate static field dephasing effects and susceptibility losses using Spin Echo 90o T2 FID T2* Apply a 180o RF Pulse to rephase the protons and create a Spin Echo. The time of the echo is called TE The 180o occurs exactly between the 90o excitation pulse and the Spin Echo 90o 180o FID Spin echo TE = 2 τ TE T2 T2* Depends on the speed of longitudinal relaxation   The shorter the T1 time constant, the higher the signal and the brighter the image or contrast.   Produced by a relatively short TR (i.e. 500 ms) and a short TE (i.e. 20 ms) 500 TR [ms] 15               60            90        120 TE [ms] TR short                                                   TE short T1 contrast Mz         Mxy Disclaimer: Please note that the learning material is for training purposes only! For the proper use of the software or hardware, please always use the Operator Manual issued by Siemens Healthineers. This material is to be used as training material only and shall by no means substitute the Operator Manual. Any material used in this training will not be updated on a regular basis and does not necessarily reflect the latest version of the software and hardware available at the time of the training. Please contact your local Siemens sales representative for the most current information. Functionalities described in the material or parts of this functionality may not yet be released for customers and not yet be commercially available in every country. Due to regulatory requirements, the future availability of said functionalities or parts thereof in any specific country is not guaranteed. The Operator Manual shall be used as your main reference, in particular for relevant safety information like warnings and cautions. The reproduction, transmission or distribution of this training or its contents is not permitted without express written authority. Offenders will be liable for damages. All rights, including rights created by patent grant or registration of a utility model or design, are reserved. After the longitudinal magnetization has been tilted in the xy-plane by an alternating field (RF pulse), it immediately rotates at Larmor frequency and induces the MR signal (FID). Transverse Magnetization Mxy decays (shown in red) Longitudinal magnetization Mz recovers (shown in green) This process is known as relaxation. MXY MZ "T2" "T1"