How Gradient Coils Are Produced | MR System

MRI- Gradient

Gradients 

Gradients or gradient coils are used to vary the magnetic field strength of the extent of the magnetic field. The gradient system consists of three coils that produce a field with changing strength in X,Y,Z directions.The three gradients applied along the X, Y, And Z axes are perpendicular to each other and are used for slice selection, phase encoding, and frequency encoding. In a typical spin-echo imaging slice selection gradient is on for 3ms, the phase encoding gradient for 4ms and frequency encoding (readout) gradient for 8ms. 

How gradient coils are produced? 

Magnetic field strength is proportional to the amount of current passed through the loop of wire, the number of loops in wire, the size of the loop, and how closely the loops are spaced. By changing these factors a coil can be produced, which increases or decreases magnetic field strength in a particular direction. 

Gradient strength (stiffness)is measured in units of G/cm or T/m.This means that the magnetic field strength changes by one Gauss over each centimeter or 10milliTesla over each meter. Stronger gradients (15 or 20 mT/m)allow high-speed and high-resolution imaging. Gradient strength has an effect on slice thickness and FOV that can be used. For the same bandwidth, a weaker gradient will produce a thicker slice and A stronger gradient will produce a thinner slice and smaller FOV. The time is taken by a gradient to reach its maximum amplitude is called rise time. The slew rate is the rate of rising obtained by dividing gradient amplitude by rising time and is measured in T/m/sec.The duty cycle is the time during which the gradient system can be run at maximum power. Gradient coils are located coaxially in the room temperature compartment of the magnet bore. A problem associated with the gradient system is the generation of eddy currents. Eddy current degrades the homogeneity of the magnetic field. It also causes heat production, which results in the evaporation of cryogens. Various methods are used to minimize the eddy current. Apart from localization, gradients are also useful in the gradient-echo sequences. Gradients are used for spoiling or rewinding transverse magnetization. Gradients are used to rephrase the protons when they are going out of phase after the RF pulse is switched off. Thus they eliminate 180 degree pulse and make the gradient echo sequences much faster.