BLIP Echo Planar Imaging Method - Essay Example

Sampling of k-space is different in EPI that enables high quality of images acquired. EPI uses quickly switching gradients to produce its echo train as against use of RF pulses in FSE. EPI frequency encoding gradient oscillates from positive to negative to create odd and even echoes. Each read gradient echo corresponds to one k-space with the positive and negative gradients changing the direction of the line that is read. The changing of direction of the line read results in a phase ‘blip’ causing a shift in Ky and this method of phase encoding is referred to as blip EPI.

Characteristics of EPI: EPI has three distinct characteristics in terms of speed, the variety of contrast and the flexibility in terms of available resolution for imaging small structures like the pituitary gland. EPI offers higher speeds (6 to 10 times) of imaging of short temporal events under motion. It provides with high image quality by collecting more averages with increased slice frequency per TR. The signal to noise ratio of the images is high and the resolution is also high. Its usefulness depends on the MR system used and the efficiency is determined by the ability to perform 1282 or 2562 in a very short time to avoid large off resonance artifacts.

It means that high quality images of moving patients can be obtained by using the speed of EPI. A variety of tissue contrasts (T1, T2 and T2*) are allowed by EPI for image acquisition and small structures are imaged using the flexibility in resolution where the field view can be reduced, the echo train length can be increased or geometric distortions can be reduced to increase the resolution. Also, just half of the data of the image can be gathered that can be further synthesized through conjugation to obtain the remaining data.

Flexibility in contrast can be used to produce an image similar in standards to that of an SE image with the same TE and TR by using an excitation pulse in the front part followed by a 1800 pulse (T2* dephasing) to create the first echo. EPI compared to conventional fast imaging methods: EPI uses the Blip EP method of phase encoding where each phase blip between echoes causes a shift in k-space line. Each k-space line is contributed by the gradient refocused echo and is either read positively or negatively through the gradient change.

The quickly switching gradients that produce the echo train are responsible for the odd and even echoes. When the lines are read, the reversed read lines are reordered before constructing the image. Also, the rf pulse is not used to create multiple echoes as in conventional FSE. Figure 1: (a) Pulse diagram of FSE, 4 echoes and (b) pulse diagram of EPI. Source: (McMahan, 2012). Benefits in EPI: Acquiring single snap shot images is much faster (20-100 ms) using EPI when compared to conventional FSE as gradient echoes take less time as all the encoding steps are obtained after one single excitation pulse is applied.

This is in contrast to the train of rf pulses applied that increases TR. Also, EPI allows flexibility in contrast that enables even small structures to be imaged very fast. Further, slice coverage is increased with EPI when compared to FSE. EPI has fewer rf pulses that result in low specific absorption rate (SAR). As SAR is low, TR is also low, resulting in more slices to be covered in the same TR as in FSE. Sensitivity to resonance effects: A disadvantage in EPI is that it is sensitive to off resonance effects of the echo gradients