Describe and illustrate how one can use magnetic field gradi

Describe and illustrate how one can use magnetic field gradients in order to encode spatial location information in a MRI signal.

Solution

MRI(magnetic resonance imaging) produces the signals from the tissues of the body within recieve and transmit coils. They single picture of the body but hard to find the different parts seperately. This is possible by the magnetic field gradients. The magnetic field of the scanner is constant but by adding the small fields by changing electric fields. Each scanner has 3 sets of spacial encoding electrical coils, which produces magnetic field in all three directions in x, y ,z directions.

These will change the magnetic field depending on the position of the body. These magnetic fields are weaker than than the scanner magnetic field and vary linearly across the x, y, or z direction. They can even be turned on in combinations to create a linear gradient in any arbitrary direction in space. Precession frequeny of protons also varies according to the magnetic field across space based on the lamor equation.  protons only exchange energy efficiently if the frequency of the energy matches their precession frequency.

Thus by turning the magnetic field gradient the protons in the position experiencing the field slightly differ.  Thus, we get a gradient of precession frequencies along the body that differ. The slice positions are selected by turning the slice gradients and changing the frequency of the excitation pulses which matches the frequency of the required position. Protons not in the site will be in deexicited state since lamor frequency doesnot match Larmor frequency as they will not match the frequency of the pulse, thus they won\'t efficiently receive energy from the pulse.slice-select gradient doesn\'t have to be on during readout since no protons are being excited then - although the readout frequency will be centered at the particular frequency of that slice.