![]() By applying additional magnetic fields (gradients) that vary linearly over space, specific slices to be imaged can be selected, and an image is obtained by taking the 2-D Fourier transform of the spatial frequencies of the signal ( k-space). ![]() The frequency at which a proton or group of protons in a voxel resonates depends on the strength of the local magnetic field around the proton or group of protons, a stronger field corresponds to a larger energy difference and higher frequency photons. ![]() This appears as a magnetic flux, which yields a changing voltage in the receiver coils to give a signal. The protons will return to the low energy state by the process of spin-lattice relaxation. In response to the force bringing them back to their equilibrium orientation, the protons undergo a rotating motion ( precession), much like a spun wheel under the effect of gravity. A radio frequency pulse is then applied, which can excite protons from parallel to anti-parallel alignment, only the latter are relevant to the rest of the discussion. Most protons align parallel to B 0 as this is a lower energy state. While each individual proton can only have one of two alignments, the collection of protons appear to behave as though they can have any alignment. When inside the magnetic field ( B 0) of the scanner, the magnetic moments of the protons align to be either parallel or anti-parallel to the direction of the field. By varying the parameters of the pulse sequence, different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms therein. Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the signal in space. For this reason, most MRI scans essentially map the location of water and fat in the body. Hydrogen atoms are naturally abundant in people and other biological organisms, particularly in water and fat. In clinical and research MRI, hydrogen atoms are most often used to generate a detectable radio-frequency signal that is received by antennas close to the anatomy being examined. This does not apply on older devices, and details for medical professionals are provided by the device's manufacturer.Ĭertain atomic nuclei are able to absorb and emit radio frequency energy when placed in an external magnetic field. Patients with specific non-ferromagnetic metal implants, cochlear implants, and cardiac pacemakers nowadays may also have an MRI in spite of effects of the strong magnetic fields. Unlike CT and X-ray, MRI uses no ionizing radiation and is, therefore, a safe procedure suitable for diagnosis in children and repeated runs. Contrast agents may be injected intravenously or into a joint to enhance the image and facilitate diagnosis. MRI is a medical imaging technique mostly used in radiology and nuclear medicine in order to investigate the anatomy and physiology of the body, and to detect pathologies including tumors, inflammation, neurological conditions such as stroke, disorders of muscles and joints, and abnormalities in the heart and blood vessels among others. The physics of magnetic resonance imaging ( MRI) concerns fundamental physical considerations of MRI techniques and technological aspects of MRI devices. ( August 2022) ( Learn how and when to remove this template message) Several templates and tools are available to assist in formatting, such as reFill ( documentation) and Citation bot ( documentation). Please consider converting them to full citations to ensure the article remains verifiable and maintains a consistent citation style. This article uses bare URLs, which are uninformative and vulnerable to link rot.
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