Advantages of low field imaging are the small-sized 5 Gauss fringe field and therefore the less static magnetic field exposure for the surrounding area, as well as less contraindications causing lower risks for the MRI safety by implemented metal and magnetic devices and equipment.
Low field systems are sometimes for restricted use, e.g. dedicated extremity scanner or open MRI devices. Open MRI devices equipped with permanent magnets are well-suited for MR guided interventions because these machines combine the lower magnetic fields of this type of magnets and the better patient access of open MRI scanner.
In some cases, the contrast of different tissues is better at lower field strength, depending on their T1 or T2 relaxation times. The disadvantage of the lower signal to noise ratio are a poor resolution and a longer scan time for a good image quality.

See also Claustrophobia, Contraindications and MRI Safety.

See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'

From Siemens Medical Systems;
This open MRI system is a dedicated extremity scanner, which is easy to use and easy to install. Whether you want to use it as a supplement to your whole-body scanner or you want to specialize in orthopedic exams, MAGNETOM Jazz™ offers you a cost effective opportunity to make the most of your capabilities.

From ONI Medical Systems, Inc.;
MSK-Extreme™MRI system is a dedicated high field extremity imaging device, designed to provide orthopedic surgeons and other physicians with detailed diagnostic images of the foot, ankle, knee, hand, wrist and elbow, all with the clinical confidence and advantages derived from high field, whole body MRI units. The light weight (less than 650 kg) of the OrthOne System performs rapid patient studies, is easy to operate, has a patient friendly open environment and can be installed in a practice office or hospital, all at a cost similar to a low field extremity machine.
New features include a more powerful operating system that offers increased scan speed as well as a 160-mm knee coil with higher signal to noise ratio, and the option of a CD burner.

From MagneVu;
The MagneVu 1000 is a compact, robust, and portable, permanent magnet MRI system and operates without special shielding or costly site preparation.
This MRI device utilizes a patented non-homogeneous magnetic field image acquisition method to achieve high performance imaging. The MagneVu 1000 MRI scanner is designed for MRI of the extremities with the current specialty areas in diabetes and rheumatoid arthritis. Easy access is afforded for claustrophobic, pediatric, or limited mobility patients.
In August 1998 FDA marketing clearance and other regulatory approvals have been received.
Until 2008, over 130 devices in the US are in use. Some further developments of MagneVu's extremity scanner are: 'truly Plug n' Play MRI™' and iSiS ( which adds wireless capability to the second generation MV1000-XL).

From Esaote S.p.A.; Esaote introduced the S-SCAN at RSNA in November 2007. The S-SCAN is a dedicated joint and spine MR scanner derived from the company's earlier G-SCAN system. Unlike the G-SCAN, neither the patient table nor the magnet can rotate from horizontal to vertical position. The patient table can only moved manually. Improved electronics, new coils for lumbar and cervical spine, new pulse sequences, a modified version of the magnet poles and gradient coils are used with a new software release in the S-SCAN.
Esaote North America is the exclusive U.S. distributor of this MRI device.