1. (4)ELECTRONIC SUPPORT SYSTEM
provide voltage and current for all parts of the MRI system, such as;
Gradient coils
Cooling system
Magnet
Computer
(5)COMPUTER AND DISPLAY COMPONENT
The computer processes information from all parts of the MRI system the operator's console containing the computer controls and the display monitor .

2. MRI Physics Excitation
Magnetic resonance imaging (MRI) has become accepted as a powerful imaging modality
MRI uses the magnetic properties of the hydrogen atom to produce Images.
The nucleus of the hydrogen atom is a single proton.
Being a spinning, charged particle. It has magnetic properties may be thought of as a small bar magnet With north and south poles
The first step in MRI is the application of a strong. external magnetic field For this
purpose. the patient is placed Within a large magnet, either a permanent or superconductive magnet

3. The hydrogen atoms Within the patient align in a direction either parallel or anti parallel to the strong external field.
A greater proportion aligns in the parallel direction. so that the net vector of their alignment. And therefore the net magnetic vector, will be in the direction of the external field
Though aligned in a strong magnetic field, the hydrogen nuclei do not lie motionless. Each nucleus spins around the line of the field in a motion known as precession

4. The frequency of precession is an inherent property of the hydrogen atom in a given magnetic field and is known as the Larmor frequency.
The Larmor frequency therefore changes in proportion to magnetic field strength
It is of the order of 10 MHz (megahertz), a frequency In the same part of the electromagnetic spectrum as radio waves.
A second magnetic field is now applied al right angles to the original external field. This second magnetic field is applied at the same frequency as the larmor frequency and is known as the frequency pulse (RF pulse)
A second magnetic coil, the RF coil . Applies the RF pulse. The RF pulse causes the net magnetization vector of the hydrogen atoms to turn towards the transverse plane, i.e. a plane al light angles to the direction of the original.

5. strong external field (Fig. 5. 4). as such
strong external field (Fig. 5.4). as such. the RF pulse adds energy to the system.
Following cessation of the RF pulse, the extra energy is dissipated to the surrounding chemical lattice in a process known as T1 relaxation. In addition, the RF pulse brings the precessing protons Into phase. i.e. their spins are now In synchrony
The process of dephasing, which occurs due to tiny in homogeneities
in the nuclear magnetic environment, is known as T2 relaxation,

6. The component of the net magnetization vector in the transverse plane Induces a current in magnetic coils known radiofrequency, or RF receiver coils.
This current is known as the MR signal and is the basis for formation of an Image.
Note that the MR signal can be produced only when the precession of the spinning protons is in phase.
Complex computer analysis of the MRI signal from the RF- receiver coils is used to produce an MR image

7. Relaxation times
Relaxation is the process whereby nuclear magnetization returns to its resting state (the XY axis reverts back to the Z axis) following a disturbance by an RF pulse.
T1 relaxation time (longitudinal or spin-lattice relaxation)
T2 relaxation time (transverse or spin-spin relaxation)

8. T1 Relaxation Time
The T1 relaxation time is defined as the time needed to achieve 63% of the original longitudinal magnetization (in Z axis B0)
The T1 relaxation time, also known as the spin-lattice relaxation time, is a measure of how quickly the net magnetization vector (NMV) recovers to its ground state in the direction of B0.
The return of excited nuclei ( hydrogen protons) from the high energy state to the low energy or ground state is associated with loss of energy to the surrounding nuclei. Nuclear magnetic resonance was originally used to examine solids in the form of lattices, hence the name "spin-lattice" relaxation.

9. T1 relaxation is an exponential process as shown in the figure to the right. The length of the net magnetization vector for a spin echo sequence is given by the following equation:
Where Mt is the magnetization at time = t, the time after the 90o pulse, Mmax is the maximum magnetization at full recovery.
At a time = one T1, the signal will recover to 63% of its initial value after the RF pulse has been applied. After two T1 times, the magnetization is at 86% of its original length. Three T1 times gives 95%. Spins are considered completely relaxed after 3-5 T1 times.
Another term that you may hear is the T1 relaxation rate. This is merely the reciprocal of the T1 time (1/T1). T1 relaxation is fastest when the motion of the nucleus (rotations and translations or "tumbling rate") matches that of the Larmor frequency. As a result, T1 relaxation is dependent on the main magnetic field strength that specifies the Larmor frequency. Higher magnetic fields are associated with longer T1 times.
T1 weighted images can be obtained using an inversion recovery sequence or by setting short TR (<750ms) and TE (<40ms) values in conventional spin echo sequences. In gradient echo sequences, T1WI can be obtained by using flip angles over 50o and setting the TE to less than 15 ms.

11. Fat has a short relaxation time as compared to water because it can more easily transfer its received energy to its surroundings.