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Understanding MRI Pulse Sequences: Spin Echo, T1 and T2 Weighting, Slides of Medical Genetics

Acharya Nagarjuna UniversityMedical Genetics

An in-depth explanation of mri pulse sequences, focusing on spin echo, t1 and t2 weighting. It discusses the role of pulse sequences in highlighting differences among spin density, t1, and t2 relaxation time constants of tissues, leading to the high contrast sensitivity of mr images. The document also covers the three major pulse sequences - spin echo, inversion recovery, and gradient recalled echo - and their significance in mr imaging.

Typology: Slides

2012/2013

Uploaded on 02/13/2013

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Pulse Sequences
•Emphasizing the differences among spin
density, T1, and T2 relaxation time
constants of the tissues is the key to the
exquisite contrast sensitivity of MR images.
–Tailoring the pulse sequences—that is, the
timing, order, polarity, and repetition
frequency of the RF pulses and applied
magnetic field gradients—makes the emitted
signals dependent on T1, T2 or spin density
relaxation characteristics.
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Download Understanding MRI Pulse Sequences: Spin Echo, T1 and T2 Weighting and more Slides Medical Genetics in PDF only on Docsity!

Pulse Sequences

  • Emphasizing the differences among spin density, T1, and T2 relaxation time constants of the tissues is the key to the exquisite contrast sensitivity of MR images. - Tailoring the pulse sequences—that is, the timing, order, polarity, and repetition fre quency of the RF pulses and applied magnetic field gradients—makes the emitted signals dependent on T1, T2 or spin density relaxation characteristics.
  • MR relies on three major pulse sequences:
    • spin echo,
    • inversion recovery, and
    • gradient recalled echo.

SPIN ECHO

  • Spin echo describes the excitation of the magnetized protons in a sample with an RF pulse and production of the FID, followed by a second RF pulse to produce an echo. - Timing between the RU pulses allows separation of the initial FID and the echo and the ability to adjust tissue contrast.
  • After a time delay of TE/2, where TE is the time of echo, a 180-degree RF pulse is applied, which inverts the spin system and induces a rephasing of the transverse magnetization. - The spins are rephased and produce a measurable signal at a time equal to the time of echo (TE).
  • This sequence is depicted in the rotating frame.
  • The racers start running at the 90-degree pulse, but quickly their tight grouping at the starting line spreads out (dephases) as they run at different speeds.
  • After a short period, the runners are spread out along the track, with the fastest runners in front and the slower ones in the rear. - At this time (TE/2), a 180-degree pulse is applied and the runners all instantly reverse their direction, but they keep running at the same speed as before.
  • Even in a field of runners in which each runs at a markedly different speed from the others, they all will recross the starting line at exactly TE. - The MR signal is at a maximum (i.e., the peak of the FID envelope) as the runners are all in phase when they cross the starting line.
  • They can rim off in the other direction, and after another time interval of TE/2 reverse their direction and run back to the starting line. - Again, after a second TE period, they will all cross the starting line (and the FID signal will be at its third peak), then head off in the other direction.
  • The maximal echo amplitude depends on the T2 constant and not on T2*, which is the decay constant that includes magnetic field inhomogeneities. - Of course all MR signals depend on the proton density of the tissue sample, as well.
  • Just before and after the peak amplitude of the echo (centered at time TE) digital sampling and acquisition of the signal occurs.
  • Multiple echoes generated by 180-degree pulses after the initial excitation allow the determination of the “true T2” of the sample.
  • Signal amplitude is measured at several points in time, and an exponential curve is fit to this measured data