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diffusion

diffusion Water compries approximately 70% of the body. Water molecules are in constant random motion called diffusion. Diffusion is a physical property of molecules referring to their ability to move randomly in relation to their thermal energy. Diffusion weighted imaging has made its greatest impact on the imaging evaluation of ischemic stroke. It is a new and efficient method that detects acute ischemic stroke much earlier than conventional T2-weighted MRI and computed tomography. DWI is able to distinguish the location and extent of the ischemic area within one hour following onset of a stroke. By comparison, conventional MRI and CT are sensitive in detecting hemorrhage from infarction and brain ischemia, but not until several hours after onset. Diffusion-weighted imaging detects the slowing of water molecules that occurs during the early stages of a stroke. It measures the net movement of water molecules in the body due to thermal energy (Brownian motion). During a stroke, changes take place in the intra and extracelluar volume. In ischemic areas, a shift in water balance occurs due to a change in the sodium/potassium pump. As a result, sodium and water tend to accumulate in ischemic cells. This accumulation causes the movement of water molecules to slow and the cells to swell, resulting in cytotoxic edema. At a later stage (subacute ischemia), the diffusion coefficient increases well above its normal value. This is associated with vasogenic edema, in which the motion of bulk water takes place. The ischemic area shows up dramatically as a bright area, demonstrating decreased water mobility. One can see the extension of the damaged area with time. Diffusion imaging thus offers the unique opportunity to address noninvasively fundamental issues regarding the response of the brain tissue to CVA at different stages, with potentially important clinical applications. Early detection of CVA, at a stage when tissue damage is still 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 images. The DWI protocol can be used with a variety of MRI pulse sequences, including fast spin echo single shot, high speed STEAM (stimulated echo acquisition mode) imaging, incoherent gradient echo, magnetization prepared rapid gradient echo, echo-planar imaging and navigator echo. However, the pulse sequences must generate signals fairly quickly to detect areas of early ischemia. For this reason, echo-planar imaging is the most frequently used sequence. It requires a higher gradient strength specification. A contrast agent usually is not used with the scanning pulse sequences because water self-diffusion serves as a source of contrast on the MR images. Additional techniques include modified fast spin echo (high speed STEAM) and gradient echo sequence (MP-RAGE, or magnetization prepared rapid gradient echo). The navigator echo can be added to any pulse sequence to decrease motion artifact. The MRI pulse sequences most commonly used during DWI are listed below. Standard fast spin echo produces images similar in appearance to conventional spin echo sequences. However, standard fast spin echo includes extra phase encoding gradients that follow each 180 degree pulse. This type of imaging generates MR signals and measures all of the raw image data (k-space) in a single acquisition. As a result, this sequence has an inherent decrease in the signal-to-noise-ratio (SNR) compared with conventional spin echo. The high-speed STEAM technique is a modified fast spin echo sequence consisting of transverse magnetization, a vector component in the xy plane, that refocuses after the initial RF pulse similar to conventional spin echo. Unlike conventional spin echo, high speed STEAM temporarily stores transverse magnetization in the z plane. A “stimulated” echo is produced when the transverse magnetization is restored and refocused. When using this sequence as a fast scanning technique, small flip angle pulses are used to read out a series of stimulated echoes that are separately phase encoded. The high-speed STEAM technique reduces motion artifact without using strong gradients. In fact, high-speed STEAM generally is considered an alternative technique, its main advantage being that it can be used with MRI systems that do not have stronger gradients. However, high-speed STEAM results in lowered SNR and significant T1 relaxation compared to echo-planar, spin echo and gradient echo imaging. Incoherent Gradient Echo (Gradient Spoiled) (MPGR, FLASH, STAGE, GRE, Field Echo, Short) In the gradient echo, the dephasing effects of the pulsed magnetic field gradients are eliminated. This scanning sequence is similar to the standard spin echo sequence without the 180 pulse. Instead, this method uses a smaller flip angle for excitation. In the gradient echo pulse, a bipolar pair of gradients of opposite polarity but equal amplitude are placed in front of the signal acquisition. Gradient echo imaging is much faster than standard spin echo imaging, although there are more pronounced magnetic susceptibility artifacts on gradient echo sequences compared with standard spin echo sequences. Magnetization prepared rapid gradient echo (Turbo FLASH 3-D MP-RAGE). Magnetization prepared rapid gradient echo (MP-RAGE) is an ultrafast technique that uses a combination of small flip angle RF pulses and a centric phase-encoding scheme that minimizes the effect of T1 relazation on image contrast. Data can be acquired in about on second, reducing the SNR and motion artifact. However, if relaxation is not corrected during data acquisition, blurring and edge enhancement may result. Echo-planar imaging (EPI) is a technique that uses gradient echoes or combines standard spin echo with a train of gradient echoes. It requires hardware modification to the MRI system, including high-performance gradients and ultrafast computers to produce images. High performance gradients are necessary to produce the rapid on-off action needed. The Ultrafast computer is required for complex signal data processing and image reconstruction. Echo planar imaging can be performed as a single shot, during which all data is gathered and processed in a single acquisiton, or as a “multishot” where data is gathered in several individual segments. By virtue of its ultrafast capabilities that eliminate motion artifacts by freezing out macroscopic motion, echo planar imaging is the preferred scanning pulse sequence for DWI. However, gradient induced eddy current effects can occur during echo planar imaging. Eddy currents are the small electrical currents that are generated when the gradients are switched on and off rapidly. If uncompensated, the eddy current effect will cause a decrease in signal and spatial resolution. There also may be a severe chemical shift artifact, a condition that arises when protons from different molecules process at slightly different frequencies. The distinguishing characteristic of EPI is that the spatial frequency domain gets filled after a single RF excitation, producing an entire image. Navigator echo is a standard spin echo sequence that can be used with any of the above pulse sequences previously described to decrease motion artifact. The navigator echo method was modified to correct phase errors from rotation and translation motions in the diffusion-encoding interval. It uses an additional nonphase encoded echo that is acquired before the imaging echo. It is very effective in improving image quality and eliminating macroscopic motion artifacts in high-resolution diffusion weighted images without the addition of expensive hardware and excessive restraints. However, it does not allow full coverage of the entire brain, and it increases the amount of time to perform the DWI exam. Diffusion-weighted imaging is an effective method for detecting the early stages of acute ischemic stroke. Detection during the first hour following a stroke is important in helping to determine whether brain tissue deprived of oxygen can be saved. During DWI, raw image data is acquired, then post-processed with ADC mapping to provide more accurate results of actual tissue damage. The accurate mapping of tissue damage allows clinicians to develop and evaluate new therapies to treat, reverse or lessen the effect of an early stroke. Bibliography:

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