(PARACEST) The alteration of the proton density or total water signal changes contrast and can be detected by the MRI scanner. Paramagnetic chemical exchange saturation transfer contrast agents are based upon the magnetization transfer mechanism.
Lanthanide ion complexes formed with tetra-amide based ligands display unusually slow water exchange kinetics and this feature may be used to alter image contrast by applying a selective presaturation pulse in an imaging sequence. This results in chemical exchange saturation transfer (CEST) from the lanthanide-bound water to bulk water thereby altering image contrast.
Chemical Exchange Saturation Transfer (CEST) agents are a class of contrast agents that could potentially revolutionize the MRI field because of their improved sensitivity and can have a great impact on functional magnetic resonance imaging.

(2D TOF MRA) This form of MR angiography is based on the acquisition of multiple, short-TR, gradient echo single slice images. 2D TOF MRA is the preferred technique for visualizing slow flow, how for example it happens in veins. 2D TOF MRA consists of multiple sequentially-acquired single slices, therefore the saturation effects are minimized.

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 range of diagnostics and imaging systems of Siemens Medical Systems covers ultrasound, nuclear medicine, angiography, magnetic resonance, computer tomography and patient monitoring. Siemens is one of the three leading MRI manufacturers, which together account for approximately 80 percent of the MRI machines installed worldwide. Siemens currently offers the Allegra 3T MRI, which is for head scanning only, but the company will also be launching the Trio MRI, a 3T whole body scanner.
Siemens has formed partnerships with more than ten research institutions and private practitioners to define a comprehensive MRI examination and compare MR to currently established cardiovascular modalities, thereby defining optimal diagnosis and treatment.

MRI Scanners:

0.2T to 1.0T:
1.5T:
3.0T to 7.0T:
Hybrid Scanners:
Mobile Solutions:

Magnetic resonance imaging (MRI) is based on the magnetic resonance phenomenon, and is used for medical diagnostic imaging since ca. 1977 (see also MRI History).
The first developed MRI devices were constructed as long narrow tunnels. In the meantime the magnets became shorter and wider. In addition to this short bore magnet design, open MRI machines were created. MRI machines with open design have commonly either horizontal or vertical opposite installed magnets and obtain more space and air around the patient during the MRI test.
The basic hardware components of all MRI systems are the magnet, producing a stable and very intense magnetic field, the gradient coils, creating a variable field and radio frequency (RF) coils which are used to transmit energy and to encode spatial positioning. A computer controls the MRI scanning operation and processes the information.
The range of used field strengths for medical imaging is from 0.15 to 3 T. The open MRI magnets have usually field strength in the range 0.2 Tesla to 0.35 Tesla. The higher field MRI devices are commonly solenoid with short bore superconducting magnets, which provide homogeneous fields of high stability.
There are this different types of magnets:
The majority of superconductive magnets are based on niobium-titanium (NbTi) alloys, which are very reliable and require extremely uniform fields and extreme stability over time, but require a liquid helium cryogenic system to keep the conductors at approximately 4.2 Kelvin (-268.8° Celsius). To maintain this temperature the magnet is enclosed and cooled by a cryogen containing liquid helium (sometimes also nitrogen).
The gradient coils are required to produce a linear variation in field along one direction, and to have high efficiency, low inductance and low resistance, in order to minimize the current requirements and heat deposition. A Maxwell coil usually produces linear variation in field along the z-axis; in the other two axes it is best done using a saddle coil, such as the Golay coil.
The radio frequency coils used to excite the nuclei fall into two main categories; surface coils and volume coils. The essential element for spatial encoding, the gradient coil sub-system of the MRI scanner is responsible for the encoding of specialized contrast such as flow information, diffusion information, and modulation of magnetization for spatial tagging.
An analog to digital converter turns the nuclear magnetic resonance signal to a digital signal. The digital signal is then sent to an image processor for Fourier transformation and the image of the MRI scan is displayed on a monitor.

For Ultrasound Imaging (USI) see Ultrasound Machine at Medical-Ultrasound-Imaging.com.

See also the related poll results: 'In 2010 your scanner will probably work with a field strength of' and 'Most outages of your scanning system are caused by failure of'