Physical Principles week 5 Spin echo

Spin echo is the foundation for other pulse sequences

Key components:

The 2 differnt RF pulses are 90-degree excitation (fid) and 180-degree rephasing
The 3 different gradients include frequency encoding, phase encoding, slice select- xyz
all 5 of these are the different pulses that make up the pulse sequence

Universal parameters: TR, TE, phase matrix, frequency matrix, field of view, slice thickness—can be applied to any pulse sequence that is run

Conventional spin echo sequences are rarely used now due to the availability of faster alternatives—due to the fat spin echo sequence
Fast spin echo sequences have largely replaced conventional ones but come with disadvantages that can be managed-—contrast issues
Vendor-specific terminology exists; the course will use vendor-neutral terms aligned with ARRT standards.
Pulse Sequence Definition:
A series of RF pulses, gradient applications, and intervening time periods.
Alternative definition: A carefully coordinated and timed sequence of events to generate a particular type of image contrast.
Spin echo: A 90-degree excitation pulse followed by a 180-degree rephasing pulse to produce an echo, commonly used for accurate contrast.
Most pulse sequences are either spin echoes or gradient echoes
Spin echo uses a 180-degree rephasing pulse

90-degree excitation pulse followed by a 180-degree rephasing pulse.
Advantages:
- Good image quality.
- Versatility (T1, T2, proton density weighting; applicable to various body parts).
- True T2 weighting (important for contrast accuracy).
- Available on all systems because it's the default.
- Gold standard for image contrast and weighting.
Standard spin echo sequences: T1 weighted, T2 weighted, and post-contrast.

Addresses the challenge of scan time reduction while preserving contrast, resolution, and SNR
Time equation: $Time = TR : x : Phase Matrix : x : NSA$
Goal: Reduce scan time without altering TR, phase matrix, or NSA.
Method: Uses a 90-degree excitation pulse followed by multiple 180-degree rephasing pulses within one TR period to produce multiple spin echoes

In conventional spin echo, after the echo, there's significant dead time until the next excitation pulse.
Fast spin echo fills some of this dead time by applying more 180-degree RF pulses.
The number of phase encodings remains the same to maintain resolution.
More than one phase encoding is performed per TR, reducing scan time.
Collecting multiple echoes within the same TR period speeds up the scan
Analogy: Buying multiple items per store trip reduces the total number of trips needed
With a fast spin echo there are fewer total TR time periods.
We are not altering TR, phase matrix, or NSA, but we are decreasing total scan time
Collecting more than one echo in a TR time period is called the echo train.
ETL (Echo Train Length) is the number of 180-degree rephasing pulses in one TR, representing the number of collected echoes
Echo spacing is the time between echoes; some machines allow manual adjustment.
A longer ETL results in a faster sequence because there are fewer total TR time periods.
Relationship: ETL and scan time have an inverse relationship.
ETL is a parameter specific to fast spin echo, unlike the universal parameters

Each 180-degree rephasing pulse creates a new spin echo, and each echo needs its own phase encoding
For every new echo, all steps except the excitation pulse are repeated: 180 RF, slice select gradient, phase encoding gradient, and frequency encoding gradient
Each echo still needs its own unique slope of the phase encoding gradient
A new time equation for fast spin echo: $Time = (TR Imes Phase Matrix Imes NSA) / ETL$
Several phase encodings are performed during each TR because every time we collect a new echo, we need a new phase encoding strength
Several lines of K-space are filled each TR, and the scan time becomes reduced. The higher the ETL, the faster the escape
The phase encoding gradient slope must change for every echo collected
The process is the same as conventional spin echo, except multiple lines in K-space are filled in the same TR time period

Each echo will have a different TE; TE controls T2 contrast
Echoes at the beginning of the echo train have shorter TE values and less T2 contrast
Echoes at the end of the train have higher TE values and more T2 contrast
Solution: Phase reordering

The technologist selects the effective TE (EFFT), which dictates the contrast of the final image.
The effective TE is always in the middle of the echo train.
Protons are relaxing out of the transverse plane, and as they relax out of the transverse plane, you're losing signal
This ensures all echoes are as close to the selected TE value as possible in order to have the best contrast
The echoes are added at the beginning and ending of the echo train to keep those echoes as close to the effective TE as possible

The system orders phase encoding so that those that produce the most signal, the shallowest ones, are used on echoes produced by the 180 pulses nearest to the effective TE
Stronger phase encoding gives us weaker echoes
So a shallow gradient is used so echoes are put at the center of K-space
The result is an image mostly made from data acquired from the correct TE
All these contrast issues are from TE, only T2 contrast issues

180-degree RF pulses reduce spin-spin interactions in fat and increase T2 decay time, causing fat to remain bright on T2 weighted images
T2 weighted sequences should be bright fluid and dark fat; with fast spin echo, fat can be bright
Addressed by using fat saturation (fat sat) sequences which uses an extra RF pulse to make fat dark prior to scanning
Not always used; if fat doesn't interfere with pathology, it may be omitted to preserve SNR

A very long echo train of a fast spin echo can sometimes result in image blurring
Caused by the echoes' spacing/distance to one another; either the ETL is too high, or the echo spacing is too low
Corrected by reducing the echo train length.