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Questions and Answers
What is the primary function of pulse sequences in MRI?
What is the primary function of pulse sequences in MRI?
- To utilize only $90^ imes$ RF pulses for imaging
- To rephase spins and manage inhomogeneities (correct)
- To create images without any RF pulses
- To minimize the use of magnetic field gradients
Which pulse sequence uses a $180^ imes$ RF pulse to rephase spins?
Which pulse sequence uses a $180^ imes$ RF pulse to rephase spins?
- Gradient echo
- Steady state free precession
- Spin echo (correct)
- Incoherent gradient echo
How does the Fast Spin Echo (FSE) sequence operate?
How does the Fast Spin Echo (FSE) sequence operate?
- It employs a train of $180^ imes$ rephasing pulses. (correct)
- It requires longer TR and TE intervals.
- It produces only one spin echo.
- It uses a single $180^ imes$ RF pulse.
What happens immediately after the removal of the $90^ imes$ RF pulse?
What happens immediately after the removal of the $90^ imes$ RF pulse?
What does the term 'echo train length' (ETL) refer to?
What does the term 'echo train length' (ETL) refer to?
Which mechanism is NOT used for rephasing spins in pulse sequences?
Which mechanism is NOT used for rephasing spins in pulse sequences?
Which type of contrast can be produced by conventional spin echo pulse sequences?
Which type of contrast can be produced by conventional spin echo pulse sequences?
What is true about magnetic field gradients in the context of pulse sequences?
What is true about magnetic field gradients in the context of pulse sequences?
What differentiates the coherent gradient echo from other sequences?
What differentiates the coherent gradient echo from other sequences?
What is primarily accomplished by using intervening time periods TR and TE in pulse sequences?
What is primarily accomplished by using intervening time periods TR and TE in pulse sequences?
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Study Notes
Pulse Sequences
- A series of RF pulses, gradient applications and intervening time periods.
- Selecting the intervening time periods (TR & TE) allows control of image weighting
- Needed because without a refocusing mechanism for spins, signal is insufficient to produce an image (dephasing happens immediately after the RF excitation pulse)
Pulse Sequences: Purposes
- Rephasing spins to remove inhomogeneity effects
- Manipulating TE & TR to produce different types of contrast
Spin Rephasing
- Using a 180∘180^\circ180∘ RF pulse
- Using magnetic field gradients
Pulse Sequences and Rephasing Mechanisms
- Spin Echo sequences use 180∘180^\circ180∘ pulses to rephase spins
- Gradient echo sequences use gradients to rephase spins
- Fast spin echo, inversion recovery, STIR, FLAIR, and steady state free precession are types of pulse sequences.
Conventional Spin Echo Pulse Sequence (T1, T2, Proton Density)
- 90∘90^\circ90∘ excitation pulse followed by a 180∘180^\circ180∘ rephasing pulse followed by an echo
- After the 90∘90^\circ90∘ RF pulse, spins dephase and NMV decays
- A 180∘180^\circ180∘ RF pulse flips the dephased nuclei through 180∘180^\circ180∘
- Faster-precessing nuclei now behind the slow ones, the fast eventually catch up with the slow reforming the NMV. This is called rephasing.
- The signal in the receiver coil is regenerated, this is called a spin echo
- 180∘180^\circ180∘ Rephasing pulses may be applied either once or several times to produce either one or several spin echoes.
Fast or Turbo Spin Echo (FSE/ TSE)
- Much faster version of conventional spin echo
- Employs a train of 180∘180^\circ180∘ rephasing pulses, each producing a spin echo
- This train of spin echoes is called an echo train
- The number of 180∘180^\circ180∘ RF pulses and resultant echoes is called the echo train length (ETL) or turbo factor.
RF Rephasing
- The RF signal is applied to the spin.
- Spin aligns with the magnetic field then is flipped by the 90° RF pulse, dephases and then rephased by the 180° RF pulse.
- An echo is then detected.
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