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Magnetic resonance imaging was developed as an offshoot of nuclear magnetic resonance. The original name for the technology was nuclear magnetic resonance imaging (NMRI), but the term nuclear was dropped because it was thought that, for the general public, it carried negative connotations from its usage in other contexts ( nuclear warfare, nuclear waste, nuclear winter, nuclear meltdown et cetera); patients would associate the examination with radioactivity. Physicists still speak of NMR when discussing non-medical devices operating on the same principles.
First, the spins of the atomic nuclei of the tissue molecules are aligned in a powerful magnetic field. Then, radio frequency pulses are applied in a plane perpendicular to the magnetic field lines so as to cause some of the hydrogen nuclei to gradually change alignment from their upright positions. The frequency of the radio wave pulses used is governed by the Larmor Equation. Magnetic field gradients are then applied in the 3 dimensional planes to allow encoding of the position of the atoms. After this, the radio frequency is turned off and the nuclei go back to their original configuration, but before doing so, their new alignment can be measured by coils wrapped around the patient. These signals are recorded and the resulting data are processed by a computertower of a personal computer. A computer is a device for making calculations or controlling operations that are expressible in numerical or logical terms. While factually accurate, this definition and those found in other dictionaries are so broad that th to generate an image of the tissue. Thus, the examined tissue can be seen with its quite detailed anatomical features. In clinical practice, MRI is used to distinguish pathologic tissue such as a brain tumorA brain tumor is any mass created by an abnormal and uncontrolled growth of cells either found in the brain ( neurons, glial cells, epithelial cells, myelin producing cells etc. or spread ( metastasis) of tumor cells from elsewhere. Brain tumors are usual from normal tissue.
The technique most frequently relies on the relaxation properties of magnetically-excited hydrogenhydrogen helium H Li Full table General Name, Symbol, NumberHydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1 , s Density, Hardness 0. 0899 kg/m3, NA Appearance colorless Atomic properties Atomic weight 1. 00794 amu Atomic radius (ca nuclei in water. The sample is briefly exposed to a burst of radiofrequency energy, which in the presence of a magnetic field, puts the nuclei in an elevated energy state. As the molecules undergo their normal, microscopic tumbling, they shed this energy to their surroundings, in a process referred to as "relaxation." Molecules free to tumble more rapidly relax more rapidly.
T1-weighted MRI scans rely on relaxation in the longitudinal plane, and T2 weighted MRI scans rely on relaxation in the transverse plane. Differences in relaxation rates are the basis of MRI images--for example, the water molecules in blood are free to tumble more rapidly, and hence, relax at a different rate than water molecules in other tissues. Different scan sequences allow different tissue types and pathologies to be highlighted. A contrast agent is sometimes injected in the sample to augment these differences and improve sensitivity.