The echo time (TE) The echo time (TE) refers to the time between the application of the radiofrequency excitation pulse and the peak of the signal induced in the coil. It is measured in milliseconds. The amount of T2 relaxation is controlled by the TE.

MRI sequence An MRI sequence is a number of radiofrequency pulses and gradients that result in a set of images with particular appearance. Division of MRI sequences This leads to a division of all sequences into proton density (PD) weighted, T1 weighted, T2 weighted, diffusion weighted, flow sensitive and 'miscellaneous'. A number of 'optional add-ons' can also be considered, such as fat or fluid attenuation, or contrast enhanced.

There are many pulse sequences available for MR imaging: Spin echo sequence Inversion recovery sequence FLAIR sequence Gradient echo sequences Diffusion Sequences

Spin-echo pulse sequences Spin-echo pulse sequences are one of the earliest developed and still widely used (in the form of fast spin-echo) of all MRI pulse sequences. The pulse sequence timing can be adjusted to give T1- weighted, proton density, and T2-weighted images. Dual echo and multiecho sequences can be used to obtain both proton density and T2-weighted images simultaneously. The two variables of interest in spin echo sequences are 1. The repetition time (TR) 2. The echo time (TE) All spin echo sequences include a slice selective 90-degree pulse followed by one or more 180 degree refocusing pulses as shown in the diagrams.

Gradient echo sequences (GRE) The gradient echo sequence is the simplest type of MRI sequence. It consists of a series of excitation pulses, each separated by a repetition time TR. Data is acquired at some characteristic time after the application of the excitation pulses and this is defined as the echo time TE. TE is the time between the mid-point of the excitation pulse and the mid-point of the data acquisition Gradient echo sequences (GRE) are an alternative technique to spin echo sequences, differing from it in two principal points: utilization of gradient fields to generate transverse magnetization flip angles of less than 90°

Flip angle The flip angle is usually at or close to 90 degrees for a spin echo sequence but commonly varies over a range of about 10 to 80 degrees with GRE sequences. For the basic GRE sequence FLASH the larger tip angles give more T1 weighting to the image and the smaller tip angle give more T2 or actually T2* weighting to the images.

Inversion recovery pulse sequences Inversion recovery pulse sequences are a type of MRI sequence used to selectively null the signal for certain tissues (e.g. fat or fluid). Inversion recovery can also generate heavily T1- weighted images and was originally developed for this purpose.