The Concept and Use of Echo Planar Imaging (EPI) - Essay Example

? Echo Planar Imaging, or EPI, Fast Imaging Techniques (MRI) Echo Planar Imaging, or EPI, Fast Imaging Techniques (MRI) Answer 1 The concept of EPI was first presented in 1977 by Mansfield. On the other hand, different technical issues related to EPI have been overcome in the last decade. EPI is a fast, flexible imaging method, with good contrast and resolution, and with many potential applications in clinical imaging, such as functional MRI and rapid entire brain imaging. Though, EPI can be very unforgiving and requires careful parameter selection (Weishaupt, Koechli and Marincek, 2008). EPI is fast because it uses single excitation of a slice followed by the continuous readout through the k-space using GRE pulse sequence (Delbeke, Martin, Patton and Sandler, 2002). After the RF excitation MR imaging mostly depends upon the formation of echo at some point. Spin echo (SE) sequences are the most former MR sequences (in fact earlier to imaging) (Westbrook, 2009). EPI is called blip because between echoes phase blip causes change in Ky and a new line is sampled. In EPI each gradient refocused echo contributes one 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 varying 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 peripheral 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 can be suggested that stimulation of respiratory system takes place at exposure levels of the order of 3 times that necessary for PNS, while cardiac stimulation requires 80 times the PNS threshold. The probability of occurring PNS is mostly in EPI. Particularly 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 scanner and lead to the likelihood of stimulation (Delbeke, Martin, Patton and Sandler, 2002). Spin Echo EPI The most commonly used sequence is known as spin 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 applied with several pulse sequences of 90 and 180 degrees, each of which produces an echo which will form the radio wave which provides molecular information. In carefully constructed sequences extra slices are excited while waiting for T1 recovery, so one phase encoding 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 elastic. 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 (small 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 scan, only a few possible alternatives are there: Decrease TR (the only method of reducing the TR is the reduction in the slice coverage). Reduction in the number of steps for phase encoding (resulting in less spatial resolution). The value of Nacq less than or equal to 1 i.e. Nacq Read More