The navigator technique measures with an additional quick MR prepulse the position, of e.g. the diaphragm before data collecting. Similar respiratory conditions of the patient can be identified and used to synchronize image data acquisition so that respiration induced image blurring is minimized by either respiratory ordered phase encoding or respiratory gating.
The prepulse sequence images a small area perpendicular to the structure, which is moving. The contrast of the interface between the diaphragm and the lung should be high to permit easy automatic detection. After data acquisition, the position of the interface is automatically recorded and imaging data are only accepted when the position of the interface falls within a range of prespecified values.
This technique has the advantage of greater accuracy than other respiratory gating (therefore used for coronary angiography) and has no need for additional sensing MRI equipment, as the MR system itself provides it.

Categories of negative oral contrast agents:
Negative oral contrast media are usually based on superparamagnetic particles and act by inducing local field inhomogeneities, which results in shortening of both T1 and T2 relaxation times. Superparamagnetic contrast agents have predominant T2 weighted effects. Biphasic contrast media are agents that have different signal intensities on different sequences, depending on the concentration at which they are used.
Suitable materials for oral contrast agents should have little or no absorption by the stomach or intestines, complete excretion, no motion or susceptibility artifacts, affordability, and uniform marking of the gastrointestinal tract. Benefits of negative oral contrast agents are the reduction of ghosting artifacts caused by the lack of signal. Superparamagnetic iron oxides produce also in low concentrations a noticeable signal loss; but can generate susceptibility artifacts especially in gradient echo sequences. Perfluorochemicals do not dilute in the bowel because they are not miscible with water.
High cost, poor availability, and limited evaluations of side effects are possible disadvantages.
Negative oral contrast agents are used e.g., in MRCP, where the ingestion of 600-900 ml of SPIO cancels out the signal intensity of the lumen (in addition after the injection of a gadolinium-based contrast medium, the enhancement of the inflammatory tissues is clearer seen), and in MR abdominal imaging of Crohn's disease in combination with mannitol.

Blueberry or pineapple juices are useable for examinations of the pancreas (MRCP, upper abdominal imaging) as cheep contrast agents, because of the content of magnetic substances (e.g. manganese).

See also Ferristene, Ferumoxsil, Oral Magnetic Particles, Gastrointestinal Imaging.

From Toshiba America Medical Systems Inc.;
OPART™ is a 0.35 T superconducting open MR system featuring cryogenless operation. Superconducting speed and performance are combined with the patient access advantages of open MRI design for unmatched clinical versatility. OPART™ features innovative technologies such as digital RF, high speed gradients and optimized RF coils, which support advanced MRI applications.

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.

A gradient system, which changes the readout gradient sinusoidally by connecting a capacitor to the self inductance generated by the gradient coil. Oscillating gradient systems were initially used in the development of EPI.
This electrical oscillating circuit can be driven with minimal power to generate the gradient amplitudes and switching frequencies required for echo planar imaging (EPI).
Disadvantages are that it is not possible to use any arbitrary trapezoidal gradient wave form as can be used in standard MRI. Also, the gradients are inflexible and cannot be used to create other ultrafast sequences and beside, nonlinear sampling of the MR signal is required.