ll reversible. Apparent diffusion coefficient (ADC) value, which is dependent on the material being imaged, then can be determined to show the amount of water movement. Because molecules diffuse in three dimensions, diffusion weighted images can be obtained along the X, Y, and Z planes. These images then can be averaged to produce a more accurate measurement. In each of the three planes, two strong dephasing and rephasing gradient pulses are added to a standard MRI pulse sequence at varying degrees of strength. The bright areas show the volume of the brain that may be damaged. Strong magnetic field gradients are used to separate fast and slow moving water molecules. The strength and duration of these gradient pulses are much stronger than those used with standard imaging sequences. The degree of diffusion weighting depends on the region of interest as well as the time between the diffusion gradients. Transverse magnetization is the vector component of the magnetic movement in the XY plane. While the water molecules spin in the strong magnetic field gradients, a phase shift of their transverse magnetization may occur. These phase shifts are comparable to the stationary spins. They are spread out because of the random movement of the water molecules, eventually resulting in signal attenuation.The ADC value and the intensity of the magnetic field gradients directly affect attenuation. Low ADC values indicate less water movement. With less water movement, less dephasing occurs while the diffusion gradients are resulting in a higher signal intensity. To numerically calculate the ADC value, at least two original diffusion weighted images (raw image data) must be obtained from at least two planes. Next, signal intensities of specified regions must be obtained. In ADC mapping, the T1 and T2 relaxation times do not effect signal intensity. However, the ADC value directly affects the signal intensity on the diffusion weighted im...
