3.4 Spin Echo Pulse Sequences

Introduction to Spin Echo Sequences

The spin echo (SE) pulse sequence is the fundamental building block of clinical magnetic resonance imaging. Developed by Erwin Hahn in 1950, the spin echo sequence is widely considered the gold standard for image quality and tissue contrast. Its primary advantage is its ability to compensate for static magnetic field inhomogeneities, chemical shift, and magnetic susceptibility effects, resulting in images that are highly robust and free of dephasing artifacts.

Conventional Spin Echo (CSE)

A conventional spin echo sequence consists of two main radiofrequency (RF) pulses: a 90-degree excitation pulse followed by a 180-degree refocusing pulse.

The sequence of events is as follows:

1. A 90-degree RF pulse is applied, tipping the Net Magnetization Vector (NMV) into the transverse plane. Immediately after the pulse, the protons are in phase (coherent).
2. Once the 90-degree pulse is turned off, the protons begin to dephase due to magnetic field inhomogeneities, chemical shift, and tissue susceptibility. This dephasing causes the signal to decay at a rate determined by T2* (T-two star).
3. At a time equal to half of the desired echo time (TE/2), a 180-degree RF refocusing pulse is applied. This pulse flips the precessing spins by 180 degrees in the transverse plane.
4. The 180-degree flip reverses the relative positions of the dephased spins: the spins that were precessing faster (and had moved ahead) are now positioned behind the slower spins. Since they continue to precess at their same speeds, the faster spins catch up to the slower spins.
5. At exactly time TE (twice the TE/2 interval), the spins re-converge and achieve maximum phase coherence. This produces a strong, measurable signal known as a spin echo.

Because the 180-degree RF pulse flips the spins, it cancels out the dephasing caused by static magnetic field inhomogeneities, chemical shift, and susceptibility differences. However, it does not correct for the random dephasing caused by spin-spin interactions (true T2 decay). Therefore, the amplitude of successive spin echoes decays according to the true T2 curve, not the T2* curve.

Fast Spin Echo (FSE) / Turbo Spin Echo (TSE)

In a conventional spin echo sequence, only one line of k-space (one phase-encoding step) is acquired during each repetition time (TR) interval. This makes the sequence very slow, with scan times often exceeding 5 to 10 minutes.

Fast Spin Echo (FSE), also known as Turbo Spin Echo (TSE), is a modification designed to dramatically reduce scan time. Instead of applying a single 180-degree pulse per TR, FSE applies a series of multiple 180-degree refocusing pulses after the initial 90-degree excitation pulse. Each 180-degree pulse in the train refocuses the spins to create a separate echo.

The number of 180-degree pulses (and corresponding echoes) applied in a single TR is known as the Echo Train Length (ETL) or Turbo Factor. Because each echo is phase-encoded with a different amplitude, multiple lines of k-space are filled during a single TR. The scan time is reduced by a factor equal to the ETL:

Scan Time = (TR × Number of Phase Encodings × NEX) / Echo Train Length

For example, if a conventional spin echo sequence takes 8 minutes, running the same scan with an ETL of 8 reduces the scan time to just 1 minute.

Effective Echo Time (TE_eff) and Blurring

In FSE, each echo in the train is acquired at a different echo time (TE) and therefore possesses a different amount of T2 weighting. Early echoes have very little T2 decay (low T2 weighting), while late echoes have significant T2 decay (high T2 weighting).

To control the contrast of the final image, the user selects an effective Echo Time (TE_eff). The scanner assigns the echoes acquired at or near this effective TE to the center lines of k-space. The center of k-space contains the low-spatial-frequency data, which determines the overall contrast and signal-to-noise ratio (SNR) of the image. The outer lines of k-space, which contain high-spatial-frequency data (spatial resolution and edge detail), are filled using echoes acquired at other TEs.

The variation in signal amplitude across the echo train introduces a phenomenon known as image blurring. Because T2 decay occurs continuously throughout the echo train, the later echoes have lower signal intensity, which acts as a low-pass filter on the spatial resolution. Blurring is most pronounced in tissues with a short T2 relaxation time when imaged with long ETLs.

Fast Spin Echo Tradeoffs and Contrast Differences

While FSE is much faster than conventional spin echo, it introduces several distinct image characteristics and physical tradeoffs:

• Bright Fat on T2: Fat appears bright (hyperintense) on T2-weighted FSE images, whereas it appears intermediate-dark on conventional T2-weighted spin echo images. This occurs because the rapid train of 180-degree RF pulses disrupts the spin-spin coupling (known as J-coupling) between adjacent hydrogen nuclei in lipid molecules, preventing normal T2 dephasing.
• Specific Absorption Rate (SAR): The multiple 180-degree pulses deposit a large amount of radiofrequency energy into the patient's tissues. This increases the Specific Absorption Rate (SAR) and can limit the number of slices or the maximum ETL that can be used safely.
• Loss of Magnetic Susceptibility Contrast: FSE is highly efficient at refocusing dephasing, which means it can mask small susceptibility effects, such as small hemorrhages or metal artifacts.

Image Contrast Mechanisms in Spin Echo

Image contrast (T1-weighted, T2-weighted, or Proton Density-weighted) is controlled by adjusting the Repetition Time (TR) and Echo Time (TE):

• TR (Repetition Time): Controls T1 contrast. A short TR (400–700 ms) does not allow tissues to recover their longitudinal magnetization completely, highlighting differences in their T1 recovery rates. A long TR (>2000 ms) allows complete longitudinal recovery, minimizing T1 contrast.
• TE (Echo Time): Controls T2 contrast. A short TE (10–25 ms) minimizes the time allowed for dephasing, minimizing T2 contrast. A long TE (80–120 ms) allows tissues to dephase, highlighting differences in their T2 decay rates.