Topic: 7005April 7, 2012. Qs 1. Discuss the BLIP EPI method presented in Module 1 in detail. How does it work? (7 marks)Echo planar image (EPI) sequences data collection has different ways of comparing with sequences in standard pulse. One of them is phase encoding in EPI which has a constant gradient in phase encoding. The other method is BLIP which is normally applied at every end of read out gradient in short durations. EPI is an imaging method that is fats and flexible. It has good contrast and therefore has a potential of being applied in many clinical situations like brain imaging. EPI does not produce multiple echoes by using RF pulse.
It utilizes quick switching gradients to produce echo train. In EPI the frequency of the encoding gradient moves from positive to negative and ends up creating echoes that are odd and also even (Lu et. al, 2009)In FSE every refocused echo in EPI makes one line in K- space. The gradient either positive or negative read changes the direction of reading the line. BLIP which is a phase makes a new line to be sampled by causing a shift in Ky.
The encoding gradient in blip phase does not change in amplitude. There is only a single excitation and there is no 1800 RF pulses in between echoes. The large phase gradient that occurs before the readout starts is the BLIP. In order to make a shift from one row to another a DKy is required which is a comparison of a small kick. This is the same change that occurs in TR in spin echo sequencing. Every echo is affected by encoding gradients that are next to it, either previous or the one immediately before it (Lufkin, 1989).
Echo planar is superior to conventional in investigations of the human physiological processes in the body. It has an ability of reducing the motion artifacts and the scan time. This is because in EPI a single RF pulse acquires multiple lines of K space. In EPI BLIP is used to encode phases. It is normally applied inb very small amplitudes by putting in signal in K and a new line is recorded.
In EPI, shares in every line of k-space read both positive and negative gradient and this result to changes in direction. BLIP is applied when the read out gradient is at zero. In K space every echo is acquired in a reverse line and before an image is reconstructed an alternative line in K-space must be recorded. The following echoes are affected by BLIP and thus the necessity of a large negative phase gradient for encoding before the start of read out. Due to the fact that the pre-phase gradient is large, the K-space in the signal is kicked to the edge.
In pre phase the read gradient kicks the signal to the right and left side in the K-space while kick signal to in the upper and lower directions. Spin echo EPI excites the signal by 90 and then refocuses the signal by 180 RF. The in EPI flips the spine by not more than 90 RF and the gradient in the prophase must be closer to the RF pulse.