FSE/TSE

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Fast Spin Echo (FSE), also known as Turbo Spin Echo (TSE), is an MRI sequence that modifies the conventional spin echo technique by collecting multiple echoes following a single excitation pulse.

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It dramatically reduces scan time while preserving image contrast.

Mechanism:

A 90° RF excitation pulse is followed by multiple 180° refocusing pulses.
Each echo is acquired at a different phase encoding step within one repetition time (TR), known as the echo train.
The number of echoes collected in one TR is termed the Echo Train Length (ETL).

Key Features:

Feature	Description
Sequence Type	T2-weighted (standard); can also be PD or T1 with parameter adjustment
Echo Train Length (ETL)	Determines the number of echoes/readouts per TR; longer ETL = faster scans
Acquisition Time	Much shorter than conventional Spin Echo
Signal-to-Noise Ratio	Slightly reduced compared to standard SE, but compensated by scan efficiency
Artifact Susceptibility	Reduced motion artifacts, but more blurring at higher ETLs

Advantages:

Shorter acquisition times (important in pediatric, uncooperative, or breath-hold scans)
Better patient tolerance
High-resolution images with strong T2 contrast

Disadvantages:

Image blurring due to long ETLs (especially in T2 FSE)
Not suitable for sequences requiring very sharp delineation of small structures (e.g., inner ear or cranial nerves)
Magnetization transfer and fat saturation effects may be exaggerated