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'

Production of spin echo by repeated RF pulses. First observed using equal (90°) RF pulses, now commonly used to describe refocusing of transverse magnetization by a 180° RF pulse. By choosing long echo delay times, the spins in a Hahn echo first dephase for a long time, then rephase, which makes the Hahn pulse sequence more susceptible to diffusion effects.

Short name: NC100150, PEG-feron, generic name: Feruglose, preliminary trade name: Clariscanâ„¢
NC100150 injection is the code name for an USPIO (ultrasmall superparamagnetic iron oxide) MRI contrast agent under development. Microvessel permeability depends on functional and morphologic characteristics of cancer vessels and on physicochemical properties of the injected contrast medium molecule.
USPIO particles have a favorable pharmacological and tolerance profile and are being tested clinically of the potential for the quantitative characterization of tumor microvasculature and specifically for measures of the microvessel permeability. Iron-based products take advantage of their large molecular size, which prevents diffusion into body tissues. These agents are disposed of by the liver and spleen as particulate matter.
NC100150 Injection (Nycomed Amersham, Amersham Health ) consists of USPIO particles that are composed of single crystals (4- to 7-nm diameter) and stabilized with a carbohydrate polyethylene glycol (PEG) coat. The iron oxide particles have to be suspended in an isotonic glucose solution. The final diameter of an USPIO particle is approximately 20 nm. Blood pool half-life is more than two hours in humans; the particles are taken up by the mononuclear phagocyte system and distributed mainly to the liver and spleen.
NC100150 would compete with the contrast agents Ferumoxytol from AMAG Pharmaceuticals, Inc. and Vasovistâ„¢ from EPIX Pharmaceuticals, Inc., but at this time the development of NC100150 Injection/Clariscan™ is discontinued.

The principal advantage of MRI at high field is the increase in signal to noise ratio. This can be used to improve anatomic and/or temporal resolution and reduce scan time while preserving image quality. MRI devices for whole body imaging for human use are available up to 3 tesla (3T). Functional MRI (fMRI) and MR spectroscopy (MRS) benefit significantly. In addition, 3T machines have a great utility in applications such as TOF MRA and DTI. Higher field strengths are used for imaging of small parts of the body or scientific animal experiments. Higher contrast may permit reduction of gadolinium doses and, in some cases, earlier detection of disease.
Using high field MRI//MRS, the RF-wavelength and the dimension of the human body complicating the development of MR coils. The absorption of RF power causes heating of the tissue. The energy deposited in the patient's tissues is fourfold higher at 3T than at 1.5T. The specific absorption rate (SAR) induced temperature changes of the human body are the most important safety issue of high field MRI//MRS.
Susceptibility and chemical shift dispersion increase like T1, therefore high field MRI occasionally exhibits imaging artifacts. Most are obvious and easily recognized but some are subtle and mimic diseases. A thorough understanding of these artifacts is important to avoid potential pitfalls. Some imaging techniques or procedures can be utilized to remove or identify artifacts.

See also Diffusion Tensor Imaging.

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