The Larmor precession frequency is the rate of precession of a spin packet under the influence of a magnetic field. The frequency of an RF signal, which will cause a change in the nucleus spin energy level, is given by the Larmor equation. The frequency is determined by the gyro magnetic ratio of atoms and the strength of the magnetic field. The gyromagnetic ratio is different for each nucleus of different atoms.
The stronger the magnetic field, the higher the precessional frequency. If an RF pulse at the Larmor frequency is applied to the nucleus of an atom, the protons will alter their alignment from the direction of the main magnetic field to the direction opposite the main magnetic field. As the proton tries to realign with the main magnetic field, it will emit energy at the Larmor frequency. By varying the magnetic field across the body with a magnetic field gradient, the corresponding variation of the Larmor frequency can be used to encode the position. For protons (hydrogen nuclei), the Larmor frequency is 42.58 MHz/Tesla.

See also Larmor Equation.

MRI of the lumbar spine, with its multiplanar 3 dimensional imaging capability, is currently the preferred modality for establishing a diagnosis. MRI scans and magnetic resonance myelography have many advantages compared with computed tomography and/or X-ray myelography in evaluating the lumbar spine. MR imaging scans large areas of the spine without ionizing radiation, is noninvasive, not affected by bone artifacts, provides vascular imaging capability, and makes use of safer contrast agents (gadolinium chelate).
Due to the high level of tissue contrast resolution, nerves and discs are clearly visible. MRI is excellent for detecting degenerative disease in the spine. Lumbar spine MRI accurately shows disc disease (prolapsed disc or slipped disc), the level at which disc disease occurs, and if a disc is compressing spinal nerves. Lumbar spine MRI depicts soft tissues, including the cauda equina, spinal cord, ligaments, epidural fat, subarachnoid space, and intervertebral discs. Loss of epidural fat on T1 weighted images, loss of cerebrospinal fluid signal around the dural sac on T2 weighted images and degenerative disc disease are common features of lumbar stenosis.

Common indications for MRI of the lumbar spine:
Lumbar spine imaging requires a special spine coil. often used whole spine array coils have the advantage that patients do not need other positioning if also upper parts of the spine should be scanned. Sagittal T1 and T2 weighted FSE sequences are the standard views. With multi angle oblique techniques individually oriented transverse images of each intervertebral disc at different angles can be obtained.

See also the related poll result: 'MRI will have replaced 50% of x-ray exams by'

There are different types of contraindications that would prevent a person from being examined with an MRI scanner. MRI systems use strong magnetic fields that attract any ferromagnetic objects with enormous force. Caused by the potential risk of heating, produced from the radio frequency pulses during the MRI procedure, metallic objects like wires, foreign bodies and other implants needs to be checked for compatibility. High field MRI requires particular safety precautions. In addition, any device or MRI equipment that enters the magnet room has to be MR compatible. MRI examinations are safe and harmless, if these MRI risks are observed and regulations are followed.

Safety concerns in magnetic resonance imaging include:

It is important to remember when working around a superconducting magnet that the magnetic field is always on. Under usual working conditions the field is never turned off. Attention must be paid to keep all ferromagnetic items at an adequate distance from the magnet. Ferromagnetic objects which came accidentally under the influence of these strong magnets can injure or kill individuals in or nearby the magnet, or can seriously damage every hardware, the magnet itself, the cooling system, etc.. See MRI resources Accidents.
The doors leading to a magnet room should be closed at all times except when entering or exiting the room. Every person working in or entering the magnet room or adjacent rooms with a magnetic field has to be instructed about the dangers. This should include the patient, intensive-care staff, and maintenance-, service- and cleaning personnel, etc..
The 5 Gauss limit defines the 'safe' level of static magnetic field exposure. The value of the absorbed dose is fixed by the authorities to avoid heating of the patient's tissue and is defined by the specific absorption rate. Leads or wires that are used in the magnet bore during imaging procedures, should not form large-radius wire loops. Leg-to-leg and leg-to-arm skin contact should be prevented in order to avoid the risk of burning due to the generation of high current loops if the legs or arms are allowed to touch. The patient's skin should not be in contact with the inner bore of the magnet.
The outflow from cryogens like liquid helium is improbable during normal operation and not a real danger for patients.
The safety of MRI contrast agents is tested in drug trials and they have a high compatibility with very few side effects. The variations of the side effects and possible contraindications are similar to X-ray contrast medium, but very rare. In general, an adverse reaction increases with the quantity of the MRI contrast medium and also with the osmolarity of the compound.

See also 5 Gauss Fringe Field, 5 Gauss Line, Cardiac Risks, Cardiac Stent, dB/dt, Legal Requirements, Low Field MRI, Magnetohydrodynamic Effect, MR Compatibility, MR Guided Interventions, Claustrophobia, MRI Risks and Shielding.

The definition of a scan is to form an image or an electronic representation. The MRI scan uses magnetic resonance principles to produce extremely detailed pictures of the body tissue without the need for X-ray exposure or other damaging forms of radiation.
MRI scans show structures of the different tissues in the body. The tissue that has the least hydrogen atoms (e.g., bones) appears dark, while the tissue with many hydrogen atoms (e.g., fat) looks bright. The MRI pictures of the brain show details and abnormal structures (brain MRI), for example, tumors, multiple sclerosis lesions, bleedings, or brain tissue that has suffered lack of oxygen after a stroke. A cardiac MRI scan demonstrates the heart as well as blood vessels (cardiovascular imaging) and is used to detect heart defects with e.g., changes in the thickness and infarctions of the muscles around the heart. With MRI scans, nearly all kind of body parts can be tested, for example the joints like knee and shoulder, lumbar, thoracic and cervical spine, the pelvis including fetal MRI, and the soft parts of the body such as the liver, kidneys, and spleen. The MRI procedure includes three to nine imaging sequences and may take up to one hour.

See also Lumbar Spine MRI, MRI Safety and Open MRI.

From Hitachi Medical Systems America, Inc.;
because of its dependability, the MRP-7000â„¢ remains popular more than a decade after the first U.S. system was shipped. This system maintains a high resale value, what has made it one of the most sought-after scanners on the used MRI equipment market.