Merge pull request #16 from mmartinbb/mr_physics

theoretical background MR physics

content/MR_Physics.md

@@ -1 +1,69 @@
-# MR Physics
+# MR Physics
+
+
+Magnetic Resonance Imaging (MRI), also known as Magnetic Resonance Tomography or Nuclear Magnetic Resonance Imaging,
+is one of the non-invasive imaging techniques that have superior soft tissue contrasts and potential physiological 
+and functional applications. This type of radiation has not enough energy to remove an electron from an atom but 
+just to excite it to a higher energy state. Since the 1980s, MRI has been a mainstay of non-invasive diagnostic radiology
+because it does not expose the body to radiation. It is frequently used in neuroimaging for the diagnosis and monitoring 
+of diseases, and it has not yet shown any adverse effects from exposure, which is a major benefit over other imaging
+modalities. MRI enables to perform dynamic studies due to it's speed of acquisition. [1][2]
+
+## 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, 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.
+
+
+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 
+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 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 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. 
+
+
+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. 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 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.
+
+
+### MRI Sequences 
+
+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).
+
+There are many sequences that can be used depending on the objective. T
+
+## Multi-echo
+
+
+## Bibliography 
+
+[MRI-powered biomedical devices](https://doi.org/10.1080/13645706.2017.1402188)
+
+[Magnetic resonance imaging](https://doi.org/10.1136/bmj.324.7328.35 )
+
+[nibib](https://www.nibib.nih.gov/science-education/science-topics/magnetic-resonance-imaging-mri)
+
+
+