Echo Planar Imaging, or EPI, Fast Imaging Techniques (MRI) Essay

Echo Planar Imaging, or EPI, immobile Imaging Techniques (MRI) - Essay ExampleEPI is fast because it uses single excitation of a slice followed by the constant readout through the k-space using GRE pulse rate sequence (Delbeke, Martin, Patton and Sandler, 2002). After the RF excitation MR imaging mostly depends upon the shaping of echo at some point. Spin echo (SE) sequences are the most former MR sequences (in fact in the beginning to imaging) (Westbrook, 2009). EPI is called blip because between echoes strain blip causes transplant in Ky and a new line is sampled. In EPI each gradient refocused echo contributes single line in k-space. The positive and negative read gradients change the direction in which the line is read. In contemporary MR system that are capable of EPI, the fast alter magnetic field linked with the shifting of the magnetic field gradients is capable to produce currents within tissue, which may exceed the nerve depolarization threshold and cause peripher al nerve stimulation (PNS). However, the chance of cardiac muscle stimulation also exists, as a result presenting an artifact (Delbeke, Martin, Patton and Sandler, 2002). From the research on animals it butt end be suggested that stimulation of respiratory system takes place at exposure levels of the order of 3 multiplication that necessary for PNS, while cardiac stimulation requires 80 cadences the PNS threshold. The probability of occurring PNS is mostly in EPI. specially one has to be cautious of 2 situations (a) Whilst sloping slices are utilized and it is probable to have a greater slew rate through adding the contributions from two or three sets of gradient coils. (b) For coronal or segittal EPI where the possible current loops in the torso are greatest when the read gradient is in the head-tool direction. In general, dB/dt is monitored through the s lotner and lead to the likelihood of stimulation (Delbeke, Martin, Patton and Sandler, 2002). Spin Echo EPI The most common ly used sequence is known as swag echo. It is characterized by the initial application of a radio-frequency pulse of 90 degrees, followed by one more in front of 180 degrees, then double the time between these two pulses a signal or echo from stimulated tissue is successively use with several pulse sequences of 90 and 180 degrees, each of which produces an echo which will form the radio jar which provides molecular information. In carefully constructed sequences extra slices are excited while waiting for T1 recovery, so one phase convert step is acquired for several slices during TR (Weishaupt, Koechli and Marincek, 2008). In a spin echo sequence, the phase encoding changes amplitude every TR. This is to give each echo the correct kick to place it on the right line. You can think of it like a soccer ball tied to a piece of fictile. You need a hefty kick to move it to the outer edges of k-space (large phase encoding gradient), and a little kick for a line closer to the center (sm all phase encoding gradient). The spins always return to the centre line (i.e. the elastic in our analogy) because you are re-exciting the spins each TR in a spin echo sequence (Bankman, 2008). (Weishaupt, Koechli and Marincek, 2008). Figure shows spin echo pulse diagram, with the sampling of k space. Reducing the time of the Image For acquiring an image time required is based on the following relation. Tacq = Nacq X Nv X TR Where, Nacq = acquisition number, Ny = Number of steps for phase encoding, and TR= Time period for repetition. Hence in order to reduce time of