Magnetic resonance imaging
Magnetic Resonance Imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.
An MRI scanner is a device in which the patient lies within a large, powerful magnet where the magnetic field is used to align the magnetization of some atomic nuclei in the body, and radio frequency magnetic fields are applied to systematically alter the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body. Magnetic field gradients cause nuclei at different locations to precess at different speeds, which allows spatial information to be recovered using Fourier analysis of the measured signal. By using gradients in different directions, 2D images or 3D volumes can be obtained in any arbitrary orientation.
MRI provides good contrast between the different soft tissues of the body, which makes it especially useful in imaging the brain, muscles, the heart, and cancers compared with other medical imaging techniques such as computed tomography (CT) or X-rays. Unlike CT scans or traditional X-rays, MRI does not use ionizing radiation. [3]
How MRI works
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Main article: Physics of magnetic resonance imaging
MRI machines make use of the fact that body tissue contains lots of water, and hence protons ( 1 H nuclei), which get aligned in a large magnetic field. Each water molecule has two hydrogen nuclei or protons. When a person is inside the powerful magnetic field of the scanner, the average magnetic moment of many protons becomes aligned with the direction of the field. A radio frequency current is briefly turned on, producing a varying electromagnetic field. This electromagnetic field has just the right frequency, known as the resonance frequency, to be absorbed and flip the spin of the protons in the magnetic field. After the electromagnetic field is turned off, the spins of the protons return to thermodynamic equilibrium and the bulk magnetization becomes re-aligned with the static magnetic field. During this relaxation, a radio frequency signal (electromagnetic radiation in the RF range) is generated, which can be measured with receiver coils.
Information about the origin of the signal in 3D space can be learned by applying additional magnetic fields during the scan. These additional magnetic fields can be used to only generate detectable signal from specific locations in the body (spatial excitation) and/or to make magnetization at different spatial locations precess at different frequencies, which enables k- space encoding of spatial information. The 3D images obtained in MRI can be rotated along arbitrary orientations and manipulated by the doctor to be better able to detect tiny changes of structures within the body. These fields, generated by passing electric currents through gradient coils, make the magnetic field strength vary depending on the position within the magnet. Because this makes the frequency of the released radio signal also dependent on its origin in a predictable manner, the distribution of protons in the body can be mathematically recovered from the signal, typically by the use of the inverse Fourier transform.
Protons in different tissues return to their equilibrium state at different relaxation rates. Different tissue variables, including spin density, T 1 and T 2 relaxation times, and flow and spectral shifts can be used to construct images. By changing the settings on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.
MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, muscle, connective tissue and most tumors.
Magnetic field
MRI scans require a magnetic field with two properties, uniform field density and strength. The magnetic field cannot vary more than 1/10,000 of 1% and field strength ranges (depending on the scanner) from 0.2 to 3 teslas in strength in currently clinically used scanners, with research scanners investigating higher field strengths such as seven teslas. The lower field strengths can be achieved with permanent magnets, which are often used in “open” MRI scanners, for claustrophobic patients. [8] Higher field strengths can be achieved only with superconducting magnets. An MRI with a 3.0 tesla strength magnet may be referred to as a “3-T MRI” or “3-tesla MRI”
Since the gradient coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong fields [9] (see also the subsection on acoustic noise).
Contrast agents and implants
MRI contrast agents may be injected intravenously to enhance the appearance of blood vessels, tumors or inflammation. Contrast agents may also be directly injected into a joint in the case of arthrograms: MRI images of joints. Unlike CT, MRI uses no ionizing radiation and is generally a very safe procedure. Nonetheless the strong magnetic fields and radio pulses can affect metal implants, including cochlear implants and cardiac pacemakers. There are many electronically activated devices that have approval from the US FDA to permit MRI procedures in patients
under highly specific MRI conditions (see www.MRIsafety.com). In the case of cochlear implants, the US FDA has approved some implants for MRI compatibility. In the case of cardiac pacemakers, the results can sometimes be lethal so patients with such implants are generally not eligible for MRI.