diff --git a/content/MR_Physics.md b/content/MR_Physics.md index 25313e0..2886fb8 100644 --- a/content/MR_Physics.md +++ b/content/MR_Physics.md @@ -12,49 +12,49 @@ modalities. MRI enables to perform dynamic studies due to it's speed of acquisit ## Basic Physics Any atomic nucleous with an odd numer of nucleons has spin different from zero and so, a magnetic moment (magnetic dipole). -In the body, we can find several atoms with magnetic moment such as H, P, C, F, Na. Around 60% of the human body is made up -of water that contains hydrogen, which is also present in proteins and lipids. For this reason, hydrogen is very sensitive -to magnetic fields and MRI takes advantage of this phenomenom. +In the body, we can find several atoms with magnetic moment such as H, P, C, F, Na, which are sensitive to magnetic resonance. +Around 60% of the human body is made up of water that contains hydrogen, which is also present in proteins and lipids. +For this reason, hydrogen is the most widely used in MRI. - atoms with magnetic dipole MRI bore contains a powerful magnet which generates an uniform magnetic field B0. Patiens are introduced in this magnetic field -and hydrogen atoms align to the magnetic field. According to Larmour's law,a magnetic dipole inside a magnetic field +and hydrogen atoms align to the magnetic field. According to Larmour's law, a magnetic dipole inside a magnetic field precesses (spins) arround the magnetic field with a frequency proportional to the magnetic field strength. Hence, hydrogen atoms -precess arround the magnetic field generated by the MR. +precess arround the magnetic field generated by the MR with a frequency (Larmour frequency) that follows the equation: w = γ B0 +![](https://www.frontiersin.org/files/Articles/427144/frym-07-00023-HTML-r2/image_m/figure-2.jpg) This precession can be parallel or antiparallel to B0. In the body the number of atoms that precess parallel is different to -the ones that precess antiparallel inducing an small magnetic field which is proportional to B0 and also depends on the density -od hydrogen nuclei. To sum up, static magnetic field (B0) induces a slight magnetization of tissues. +the ones that precess antiparallel producing an small magnetic field which is proportional to B0 and also depends on the density +of hydrogen nuclei. So, the static magnetic field (B0) induces a slight magnetization of tissues. ----- (induced magnetization inside the bore) - -(Mx, My, Mz) Then, a radiofrequency pulse is emitted perpendicular to B0 with the same frequency that the spin precession frequency.Hydrogen atoms -abrosrb energy and spin out of equilibrium.Then, when the RF dissapears, the magnetic momentum gradually goes back to te minimum +abrosrb energy and spin out of equilibrium. Longitudinal magnetization (Mz) of protons in a parallel direction to B0 decreases, and a +transverse magnetization (Mx, My) appears. + +Then, when the RF dissapears, the magnetic momentum gradually goes back to te minimum energy position (magnetic relaxation) while releasing energy. This emited signals are measured into the k-space which is an array -of numbers representing spatial frequencies in the MR image. Each k-space point contains spatial frequency and phase information -about every pixel in the final image. Fourier transformed is performed to the k-space to obtain the final image. By varying the +of numbers representing spatial frequencies in the MR image. (Each k-space point contains spatial frequency and phase information +about every pixel in the final image). Fourier transforme is performed to the k-space to obtain the final image. By varying the sequence of RF pulses applied & collected, different types of images are created. --TR --TE -The angle the magnetization is shifted, is a parameter to be defined -(Flip angle) and depends on the intensity and duration of the RF pulse. ### MRI Sequences -The most common sequences are T1-weighted and T2-weighted images. In neuroimaging, T1-weighted images are commonly used in anatomical -related studies, they are based in longitudinal relaxation and are produced with short TR and TE. -(explicar longitudinal relaxation My, T1).(...) +It's important to understand the meaning of **repetition time (TR)** and **echo time (TE)** in order to comprehend the main +MRI sequences. Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal and +the interval between subsequent pulse sequences delivered to the same slice is known as the repetition time (TR). +The most common sequences are T1-weighted and T2-weighted images. In neuroimaging, **T1-weighted** images are commonly used in anatomical +related studies, they are based on the study of the relaxation of the nuclei in the longitudinal component (Mz) of the magnetization +vector and are produced with short TR and TE.**T2-weighted** images are produced with longer TR and TE. They are based on study of the +variations of the component on the transverse plane of the magnetization during the relaxation, known as transverse relaxation (Mxy). -T2-weighted images are produced with longer TR and TE. They are based on study of the variations of the component on the transverse -plane of the magnetization during the relaxation, known as transverse relaxation (Mxy). +There are many sequences that can be used depending on the objective. T +## Multi-echo ## Bibliography