Question

A Magnetic Resonance Imaging (MRI) sequence is shown in the figure below, wherein \(T_R\) is the repetition time and \(T_E\) is the echo time. Which one of the following options is the correct name of this MRI sequence?

• A. Spin echo sequence
• B. Echo planar imaging sequence
• C. Inversion recovery sequence
• D. Saturation recovery sequence

Hint:

90-degree pulse plus a 180-degree refocusing pulse at TE/2, with an echo at TE, is the spin echo sequence.

Answer 1

The Correct Option is A

Step 1: Understanding the Question.
The figure shows a repeating RF pulse pattern of \(90^{\circ}\) followed by \(180^{\circ}\), with the data (echo) collected partway between one \(90^{\circ}\) pulse and the next. We need to name this pulse sequence.

Step 2: Key Concept.
In MRI, different pulse sequences are built from different combinations of RF pulses and their timing. A \(90^{\circ}\) pulse tips the net magnetization into the transverse plane, and a \(180^{\circ}\) pulse, applied at time \(T_E/2\) after the \(90^{\circ}\) pulse, refocuses the spins that have dephased due to magnetic field inhomogeneities. This refocusing produces an echo signal at time \(T_E\) after the \(90^{\circ}\) pulse, exactly what the 'Data acquisition' line in the figure marks. This \(90^{\circ}\)-\(180^{\circ}\)-echo pattern, repeated every \(T_R\), is the defining structure of the spin echo sequence.

Step 3: Detailed Explanation.
An inversion recovery sequence would start each cycle with an extra \(180^{\circ}\) inversion pulse before the \(90^{\circ}\) pulse, which is not shown here.
A saturation recovery sequence uses only \(90^{\circ}\) pulses, with no \(180^{\circ}\) refocusing pulse at all, so it cannot explain the \(180^{\circ}\) pulse seen in the figure.
Echo planar imaging collects many echoes rapidly after a single excitation using a train of gradient reversals, not a repeated \(90^{\circ}\)-\(180^{\circ}\) RF pattern for each echo.
The figure's \(90^{\circ}\) excitation pulse, followed by a \(180^{\circ}\) refocusing pulse at half the echo time, followed by data acquisition at the echo time \(T_E\), matches only the spin echo sequence.

Step 4: Final Answer.
This is the spin echo sequence.
\[ \boxed{\text{Spin echo sequence}} \]