MRI Introduction
A Magnetic Resonance Imaging (MRI) scan used a strong magnetic help and radio wave to produce highly detailed image of tissues and organ with in the body



Tissue Magnetization:
When the patient is placed in the magnetic field the tissue became temporarily magnetized because of the alignment of the proton, as described previously this is a very low level effect that disappears when the patient is removed from the magnetic field. the ability of MRI between two different tissue is based on the fact of different tissue, both normal and pathologic, will become magnetized to different level or will. Change their level of magnetization at different rate

Superconducting:
MRI system most use superconducting magnet the primary advantage is that a superconducting magnet is capable of producing a much stronger and stable magnetic field than the other two type consider below A superconducting magnetic is an electromagnet that operates in a superconducting state

Gradient Coils
When the MRI system is in a resting State and not actually producing an image the magnetic field is quite uniform or homogeneous over the region of the patientís body to produce an image the MRI system must first stimulate hydrogen nuclei in a specific 2D image plane in the body, and then determine the location of those nuclei within that plane as they process back to their static state. These two tasks are accomplished using gradient coils which cause the magnetic field within a localized area to vary linearly as a function of spatial location. As a result, the resonant frequencies of the hydrogen nuclei are spatially dependent within the gradient. Varying the frequency of the excitation pulses controls the area in the body that is to be stimulated. The location of the stimulated nuclei as they process back to their static state can also be determined by using the emitted resonant RF-frequency and phase information.

An MRI system must have x, y, and z gradient coils to produce gradients in three dimensions and thereby create an image slice over any plane within the patient's body. The application of each gradient field and the excitation pulses must be properly sequenced, or timed, to allow the collection of an image data set. By applying a gradient in the z direction, for example, one can change the resonant frequency required to excite a 2D slice in that plane. Therefore, the spatial location of the 2D plane to be imaged is controlled by changing the excitation frequency. After the excitation sequence is complete, another properly applied gradient in the x direction can be used to spatially change the resonant frequency of the nuclei as they return to their static position. The frequency information of this signal can then be used to locate the position of the nuclei in the x direction. Similarly, a gradient field properly applied in the y direction can be used to spatially change the phase of the resonant signals and, hence, be used to detect the location of the nuclei in the y direction. By properly applying gradient and RF-excitation signals in the proper sequence and at the proper frequency, the MRI system maps out a 3-D section of the body.

Transmitter:
The RF transmitter generate the RF energy which is applied to the coils and then transmitted to the patientís body the energy is generated as a series of discrete RF pulse the characteristics of an image are determined by the specific of RF pulse .the transmitter actually consist of several components , such a RF Produlator and power amplifier, but for our purpose here use will consider it's as a unit that produces pulse of RF energy the transmitter must be capable of producing relatively high power output on the order of several thousand Watts

Image Signal Processing:
Both frequency and phase data are collected in what is commonly referred to as the k-space. A two-dimensional Fourier transform of this k-space is computed by a display processor/computer to produce a gray-scale image.