Characteristic of Magnetic Resonance Imaging
The basic components of all MRI system can be
divided into four major components which are the magnet, the gradient coils,
radio frequency (RF) coil and computer. Refer to the diagram below for understanding the
components.
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| The basic component of MRI scanner (medicalradiation.com) |
i)Magnet
The function of magnet is producing a stable and
very intense magnetic field. However, it consists of several types of magnets
that can be used in magnetic field which is superconducting magnet, resistive
magnet and permanent magnet. Most of MRI system used superconducting magnets
because of its capability in generating a stronger and stable magnetic field
compared to the other two types which is resistive and permanent magnet that
can generate very low and limited of field strengths.
ii) Gradient Coil
The function of gradient coils are used to produce
deliberate variations in the main magnetic field. Magnetic field is orderly or
alike over the region of the patient’s body when the MRI system is in resting
state. However, during the imaging processing the field must be distorted with
gradient. A gradient is a change in field strength from one point to another in
the patient’s body. Gradient coils are contained within the magnet assembly.
During an imaging procedure the gradients, the sound or noise are produced that
are come from the turning on and off the magnet for many times.
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| The position of RF coils and patient. (medicalradition.com) |
The radio frequency (RF) is used as the
communication link; as for both transmitting and receiving signals with the
patient’s body in producing an image. The RF coils are located within the
magnet assembly and closed to the patient’s body. There are different coil
designs for different parts of human bodies. But the three basic types are
body, head and surface coils (Figure 1). The surface coils are used to receive
signals from almost small part of human bodies to produce a better image
quality compared with the body and head coils. However, the surface coil can be
in the pattern of single coils or in a variety of several coils which consists
of its own receiver circuit. However, transmitter and receiver are connected in
this RF coil. The RF transmitter generated RF energy, which is used to the
coils and then transmitted to the patient’s body. The energy is generated as a
series of discrete RF pulses. The transmitter should be able in producing
approximately high power outputs on the order of several thousand watt.
However, the transmitter consists of certain components such as RF modulators
and power amplifiers but it is considered as a unit that produces pulses of RF
energy for a better understanding. Next, the receiver. When a short period
after a sequence of RF pulses is transmitted to the patient’s body, the resonating
tissue will respond by returning an RF signal. This RF signals are then picked
up by the coils and the receiver will process it. After that, the signal will
be converted into a digital form and send to the computer.
iv) Computer
A digital computer is a fundamental element in MRI
system. The production and display of an MRI image is a series of several
specific steps that have to be managed and controlled by the computer. There
are some steps or process before a computer can display the images which is
acquisition control, image reconstruction, image storage and retrieval, and
lastly is viewing control and post processing.
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