(MRA) Magnetic resonance angiography is a medical imaging technique to visualize blood filled structures, including arteries, veins and the heart chambers. This MRI technique creates soft tissue contrast between blood vessels and surrounding tissues primarily created by flow, rather than displaying the vessel lumen. There are bright blood and black blood MRA techniques, named according to the appearance of the blood vessels. With this different MRA techniques both, the blood flow and the condition of the blood vessel walls can be seen. Flow effects in MRI can produce a range of artifacts. MRA takes advantage of these artifacts to create predictable image contrast due to the nature of flow.
Technical parameters of the MRA sequence greatly affect the sensitivity of the images to flow with different velocities or directions, turbulent flow and vessel size.
This are the three main types of MRA:
All angiographic techniques differentially enhance vascular MR signal. The names of the bright blood techniques TOF and PCA reflect the physical properties of flowing blood that were exploited to make the vessels appear bright. Contrast enhanced magnetic resonance angiography creates the angiographic effect by using an intravenously administered MR contrast agent to selectively shorten the T1 of blood and thereby cause the vessels to appear bright on T1 weighted images.
MRA images optimally display areas of constant blood flow-velocity, but there are many situations where the flow within a voxel has non-uniform speed or direction. In a diseased vessel these patterns are even more complex. Similar loss of streamline flow occurs at all vessel junctions and stenoses, and in regions of mural thrombosis. It results in a loss of signal, due to the loss of phase coherence between spins in the voxel.
This signal loss, usually only noticeable distal to a stenosis, used to be an obvious characteristic of MRA images. It is minimized by using small voxels and the shortest possible TE. Signal loss from disorganized flow is most noticeable in TOF imaging but also affects the PCA images.
Indications to perform a magnetic resonance angiography (MRA):
Conventional angiography or computerized tomography angiography (CT angiography) may be needed after MRA if a problem (such as an aneurysm) is present or if surgery is being considered.

See also Magnetic Resonance Imaging MRI.

On October 19, 2001, Philips Medical Systems completed an acquisition strategy through its purchase of Marconi Medical Systems.

The History of Marconi Medical Systems
2001 Royal Philips Electronics and Marconi plc announced that Philips has agreed to acquire Marconi Medical Systems for $1.1 billion.
2000 Marconi introduces Infinite Detector Technology for Mx8000 multislice CT scanner, which acquires an unprecedented 16 simultaneous slices with sub-millimeter isotropic accuracy.
1999 At RSNA, Picker International unveils the new Marconi Medical Systems name and corporate vision.
1998 Picker International acquires the Computed Tomography Division of Elscint Ltd, immediately positioning Picker at the forefront of major global CT suppliers.
1986 Picker produces the industry's first 1.0T MR imager.
1981 Picker is sold to General Electric Co. Ltd. of England (GEC). Picker merged with Cambridge Instruments, GEC Medical, and American Optical to form Picker International.
1967 The name changed from Picker X-ray to Picker Corporation. Picker acquired Dunlee.
1946 The Dunlee Corporation started in Chicago by Dunmore Dunk and Zed. J. Atlee to meet demand for quality X-ray tubes and special purpose tubes.
1915 James Picker Company formed in New York City offering sales and service of X-ray equipment, film and accessories.

See also Philips Medical Systems and MRI History.

Sequential line imaging technique utilizing two orthogonal oscillating magnetic field gradients, a SFP pulse sequence, and signal averaging to isolate the NMR spectrometer sensitivity to a desired line in the body.

Nanoparticles may be utilize as a new class of uniform, biodegradable and non-toxic superparamagnetic contrast agents (Fe3O4). The preparation process of these particles is simple, does not involve any toxic material and the yield is close to 100%. The particles are usually of varying sizes from several to several hundred nanometer. They are irregular in shape and highly light-absorbing. They have no magnetic hysteresis at ambient temperatures, which is characteristic of superparamagnetic materials. Each magnetic nanoparticle is composed of a very thin organic nucleus (5-10%) and a thick shell of magnetite.
Different techniques were established for coating these magnetite nanoparticles with several functional and biocompatible polymers. Both the coating and the magnetite production processes are controllable, so that it is possible to prepare particles with a specific size of each particle component as well as particles coated with protein ligands for tissue specific imaging applications.

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.