Now that we know about more of the components of a sequence (slice selection etc), we can look further into the anatomy of a sequence.
Step 1 – RF Pulse
An RF pulse is applied to flip the magnetisation into the transverse plane. The RF pulse is usually represented as a ‘sinc’ shape to indicate the envelope of the RF pulse.
Step 2 – Slice Selection (GSS)
The RF pulse must be applied at the same time as a slice-select gradient in order to excite the protons in a particular slice.
The area of the negative lobe of the gradient is equal to half the area of the positive lobe to ensure phase coherence is maintained within the image slice.
Step 3 – Phase Encoding (GPE)
This is applied straight after the RF excitation and slice selection has been completed. The cycle has to be repeated the same number of times as rows in the k-space. Each time, a different phase-encoding gradient is used and a different row is filled. The steps of the phase encoding gradient symbol represent the different strengths of the gradient applied. For simplicity, we have only shown 5 steps although there are typically 256 or 512 depending on the matrix size.
Step 4 – Frequency Encoding (GFE)
The frequency-encoding gradient is applied after each step of the phase encoding gradient and it is during the gradient that the signal is ‘read’ (hence the alternative name of ‘read-out gradient’). From the notes on FE gradient an opposite dephasing gradient is applied first then the rephasing gradient in order to create an echo with a large enough signal.
The negative dephasing gradient is half the area of the rephasing gradient in order to ensure the phase coherence is maximal at the central point of the frequency encoding and, therefore, the acquired signal will be maximal.
Step 5 – Sequence Repetition
After a time (TR) the whole sequence is repeated again with a new RF pulse. The number of times this is repeated depends on the image resolution and the number of phase-encoding steps required.
The number of phase-encoding steps determines the number of lines in k-space and, hence, the resolution (e.g. 512 phase encoding steps gives a resolution of 512 in the y-axis)
The scan time = TR x number of phase-encoding steps x NEX
(NEX = number of signal averages)
We can now go into more detail on different sequences.