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'
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