(MLSI) Variations of sequential line imaging techniques that can be used if selective excitation methods that do not affect adjacent lines are employed. Adjacent lines are imaged while waiting for relaxation of the first line toward equilibrium, which may result in decreased image acquisition time. A different type of MLSI uses simultaneous excitation of two or more lines with different phase encoding followed by suitable decoding.

Rapid echo planar imaging and high-performance MRI gradient systems create fast-switching magnetic fields that can stimulate muscle and nerve tissues produced by either changing the electrical resistance or the potential of the excitation. There are apparently no effects on the conduction of impulses in the nerve fiber up to field strength of 0.1 T. A preliminary study has indicated neurological effects by exposition to a whole body imager at 4.0 T. Theoretical examinations argue that field strengths of 24 T are required to produce a 10% reduction of nerve impulse conduction velocity.
Nerve stimulations during MRI scans can be induced by very rapid changes of the magnetic field. This stimulation may occur for example during diffusion weighted sequences or diffusion tensor imaging and can result in muscle contractions caused by effecting motor nerves. The so-called magnetic phosphenes are attributed to magnetic field variations and may occur in a threshold field change of between 2 and 5 T/s. Phosphenes are stimulations of the optic nerve or the retina, producing a flashing light sensation in the eyes. They seem not to cause any damage in the eye or the nerve.
Varying magnetic fields are also used to stimulate bone-healing in non-unions and pseudarthroses. The reasons why pulsed magnetic fields support bone-healing are not completely understood. The mean threshold levels for various stimulations are 3600 T/s for the heart, 900 T/s for the respiratory system, and 60 T/s for the peripheral nerves.
Guidelines in the United States limit switching rates at a factor of three below the mean threshold for peripheral nerve stimulation. In the event that changes in nerve conductivity happens, the MRI scan parameters should be adjusted to reduce dB/dt for nerve stimulation.

An undesirable background interference or disturbance that affects image quality.
The Noise is commonly characterized by the standard deviation of signal intensity in the image of a uniform object (phantom) in the absence of artifacts. The measured noise may depend on the particular phantom used due to variable effects on the Q of the receiver coil.
Noisy images appear when the SNR-Rate is too low - this is induced by the operator. Image artifacts and RF noise can often be caused by the presence and/or operation of a medical device in the MR environment. There are various noise sources in any electronic system, including Johnson noise, shot noise, thermal noise. Materials produce their own characteristic static magnetic field that can perturb the relationship between position and frequency essential to accurate image reconstruction.
RF noise, which often appears as static on the image, can be caused by a medical device located anywhere in the MR procedure room. RF noise is a result of excessive electromagnetic emissions from the medical device that interfere with the proper operation of the MR scanner. Since the MR procedure room is shielded from extraneous RF fields entering the room (Faraday cage), operation of electromagnetically noisy equipment outside the room does not typically affect the MR scanner.

See Signal to Noise Ratio and Radio Frequency Noise Artifact.

Radiographic low-osmolar nonionic contrast agents have less side effects and fewer nephrotoxicity than ionic, high-osmolar agents. Gadolinium-based MRI contrast agents have a different formulation from iodinated X-ray contrast media, and there is no known cross sensitivity between these two types of contrast agents. Intravenous MRI contrast agents, specifically the gadolinium chelates have a high safety and lack of nephrotoxicity compared with X-ray contrast media.
The used gadolinium chelates differ in following properties: linear (e.g., gadodiamide and gadoversetamide have nonionic linear structures) vs. macrocyclic cores, and ionic vs. nonionic types. The nonionic molecules have lower osmolality and viscosity, which increase digestibility at greater concentrations, and make faster bolus injections conceivable. The macrocyclic molecules (e.g., gadoteridol has a nonionic macrocyclic ring structure) are more stable and show fewer tendencies to dissociate free Gd.

See also ProHance®, Omniscan®, OptiMARK®, Ionic Intravenous Contrast Agents.

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

Open MRI scanners have been developed for people who are anxious or obese or for examination of small parts of the body, such as the extremities (knee, shoulder). In addition, some systems offer imaging in different positions and sequences of movements. The basic technology of an open MRI machine is similar to that of a traditional MRI device. The major difference for the patient is that instead of lying in a narrow tunnel, the imaging table has more space around the body so that the magnet does not completely surround the person being tested.
Types of constructions:
Difficulties with a traditional MRI scan include claustrophobia and patient size or, for health related reasons, patients who are not able to receive this type of diagnostic test. The MRI unit is a limited space, and some patients may be too large to fit in a narrow tunnel. In addition, weight limits can restrict the use of some scanners. The open MRI magnet has become the best option for those patients.
All of the highest resolution MRI scanners are tunnels and tend to accentuate the claustrophobic reaction. While patients may find the open MRI scanners easier to tolerate, some machines use a lower field magnet and generates lower image quality or have longer scan time. The better performance of an advanced open MRI scanner allows good image quality caused by the higher signal to noise ratio with maximum patient comfort.

See also Claustrophobia, MRI scan and Knee MRI.