Factors affecting quality of MRI image.Benefits of circular or square spiral EPI methods - Essay Example

Download file to see previous pages... For example, if a spin is exposed to a magnetic field different from the expected value due to magnetic susceptibility, geometric distortions will be seen from the MRI image (Reinsberg, Doran, Charles-Edwards, and Leach, 2005). In particular, an increased spin produces MRI signals of low intensity, while high magnetic susceptibility areas are seen as areas of total signal loss (Zhou and Gullapalli, 2006). Figure 1 shows an example of metallic susceptibility distortions in the presence of metallic implants. Magnetic susceptibility In magnetic resonance imaging, tissues are temporarily magnetized by the large magnetic field that the imaged subject is exposed to, with the extent of magnetization varying among tissues that differ in magnetic susceptibility. For example, relative to other tissues, air and bone are less susceptible to magnetization. On the other hand, metallic implants have high magnetic susceptibility as compared to the body tissues. These differences in magnetic susceptibility cause field inhomogeneity, especially in tissue boundaries, particularly air-tissue and bone-soft tissue boundaries (Zhou and Gullapalli, 2006). That is why magnetic susceptibility distortions are sometimes seen in such boundaries, like the sinuses (Reinsberg, Doran, Charles-Edwards, and Leach, 2005). 1. Discuss various methods to correct for magnetic susceptibility distortions, while maintaining a good EPI image. ...
a.) Parameters that can optimize EPI image ESP can be decreased through the use of powerful gradients, such as increased gradient slew rate (~20, 000 T/m/s), increased maximum amplitude (350 mT/m), increased rise time rate, increased accuracy, decreased eddy currents, increased voltage, and decreased field strength. In addition, these parameters decrease signal-to-noise ratio. Meanwhile, a decreased ESP results to a smaller total train length time. In fact, it has been estimated that decreasing ESP to 40 ms limits the distortion to only one pixel wide. Similarly, increased data gathering via decreased frequency- and phase encoding steps and ramp sampling, fast analogue to digital converters and large receiver bandwidth decrease the time in which phase errors can accumulate (McMahon, 2012). On the other hand, phase and read directions can be swapped, so that the phase encoding gradient is along the same axis as susceptibility gradients. Although this may not totally prevent distortion, it shows the image in a more presentable and understandable manner (McMahon, 2012; Zhou and Gullapalli, 2006). Figure 2 shows the decrease in magnetic susceptibility upon converting from left-right phase encoding to anterior-posterior phase encoding. Other ways to minimize magnetic susceptibility distortions include decreasing the resolution, use of SE sequence, increased acquisition matrix and proper shimming before image acquisition. It should be noted, however, that heating of nearby tissues can occur when using fast SE acquisition with a high bandwidth to decrease magnetic susceptibility distortions resulting from metallic implants (Zhou and Gullapalli, 2006). b) Chain of consequences after increasing the receiver bandwidth Receiver bandwidth is a ...Download file to see next pagesRead More