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Questions and Answers
What is the primary function of pulse sequences in MRI?
Which pulse sequence uses a $180^ imes$ RF pulse to rephase spins?
How does the Fast Spin Echo (FSE) sequence operate?
What happens immediately after the removal of the $90^ imes$ RF pulse?
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What does the term 'echo train length' (ETL) refer to?
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Which mechanism is NOT used for rephasing spins in pulse sequences?
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Which type of contrast can be produced by conventional spin echo pulse sequences?
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What is true about magnetic field gradients in the context of pulse sequences?
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What differentiates the coherent gradient echo from other sequences?
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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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