MRI Pulse Sequences and Tissue Contrast

Learning Outcomes

• Define time to repeat (TR) and time to echo (TE) of radio frequency pulse sequences.

• Explain the use of RF pulse sequences for the measurement of proton density, T1-weighted, and T2-weighted signals.

• Describe the variation of spin-spin and spin-lattice relaxation times between different tissues.

• Explain the use of RF pulse sequences for creating different tissue contrasts.

Tissue Contrast and Relaxation Times

• The goal is to use differences in T1 and T2 relaxation times to create tissue contrast in MRI.

• Here's a table of approximate T1 and T2 relaxation times for various tissues at 1 Tesla:

  • Fat: T1 = $250$ ms, T2 = $80$ ms

  • Liver: T1 = $450$ ms, T2 = $40$ ms

  • Kidney: T1 = $550$ ms, T2 = $60$ ms

  • Muscle: T1 = $800$ ms, T2 = $45$ ms

  • White matter: T1 = $650$ ms, T2 = $90$ ms

  • Grey matter: T1 = $800$ ms, T2 = $100$ ms

  • Cerebrospinal Fluid (CSF): T1 = $2000$ ms, T2 = $150$ ms

  • Water: T1 = $3000$ ms, T2 = $3000$ ms

  • Bone, teeth: T1 = Very long, T2 = Very short

• Generally, T1 > T2 for most tissues, except for water where $T1 = T2$.

• Grey matter contains 10-15% more water than white matter.

• CSF is cerebrospinal fluid.

Measuring T1 and T2

• Recall that the decay of transverse magnetization ($B_T$) during spin-spin relaxation is due to both intrinsic and extrinsic effects.

• Extrinsic effects cause fast decay of $B_T$ and do not reflect tissue properties.

• To measure T2 accurately, the extrinsic effects are suppressed using a spin-echo technique.

Spin-Echo Technique

• A 180° RF pulse is applied after the initial 90° pulse that created $B_T$. This forces dephasing proton spins to rephase.

• The true amplitude of the decaying $B_T$ signal is recovered as an 'echo'.

• TE (Time to Echo): The time between the 90° pulse and the receipt of the echo signal. Varying TE allows probing of the T2 decay.

Measuring T2

• T2 is measured by detecting the decay of induced radio signals (echoes) using radio frequency coils/antennae.

Measuring T1

• T1 cannot be measured directly like T2; it requires a different RF pulse sequence.

• TR (Time to Repeat): The time between successive 90° RF pulses.

• If TR is shorter than the T1 of the tissue, the magnitude of $B_T$ is inhibited (reduced) after the subsequent 90° pulse.

• Long TR (longer than T1) allows full recovery of longitudinal magnetization ($B_L$).

• Short TR (shorter than T1) results in reduced $B<em>T$ because $B</em>L$ doesn't fully recover.

Pulse Timing Parameters

• TE and TR are used together to measure T1 and T2 decays.

• TE probes $B_T$ decay.

• TR probes $B_L$ recovery.

• Together, TE and TR determine the signal amplitude of the echo, influencing image contrast.

T1-Weighted Images (T1WI)

• Short T1 = Larger signal = Bright in T1WI.

• Longer T1 = Less signal = Dark in T1WI.

• Shorter T1 means larger $B<em>L$ recovered, leading to a larger $B</em>T$ detected during the next 90° pulse.

• To maximize T1 contrast, a mid TR is used to capture the greatest difference in recovery between tissues with different T1 values.

• To minimize T2 contrast contribution, a short TE is used.

T2-Weighted Images (T2WI)

• $B_T$ decays after the 90° RF pulse.

• If two tissue types start to decay at the same time, the one with the shorter T2 decays faster.

• Short T2 = Less signal = Dark in T2WI.

• Long T2 = Stronger signal = Bright in T2WI.

• To maximize T2 contrast, a mid TE is used (but not so long that the signal is negligible).

• To minimize T1 contrast contribution, a long TR is used.

Proton Density (PD) Weighted Images

• PD-weighted images measure the number of H-1 protons in an area by measuring signal strength instead of T1 or T2 decay times.

• To get a PD-weighted image, minimize both T1 and T2 contrast contributions.

• T1 is minimized with a long TR (large signal and small T1 contrast).

• T2 is minimized with a short TE (large signal and small T2 contrast).

• A large $B<em>T$ (for large signal detection) is achieved by having a large amount of $B</em>L$ recovered and less dephasing of proton spins.

Pulse Timing Summary

• T1-weighted: short TE, mid TR

• T2-weighted: mid TE, long TR

• PD-weighted: short TE, long TR

• Pulse timing determines MRI contrast!

How to Read MRI Images

T1-weighted

• Provides good contrast between gray matter (dark gray) and white matter (lighter gray) tissues.

• CSF is void of signal (black).

• T1 (ms):

  • White matter: $650$

  • Grey matter: $800$

  • CSF: $2000$

• Short T1 = larger signal = bright in image

• Longer T1 = less signal = dark in image

T2-weighted

• CSF appears bright.

• Some T2 sequences demonstrate additional contrast between gray matter (lighter gray) and white matter (darker gray).

• T2 (ms):

  • White matter: $90$

  • Grey matter: $100$

  • CSF: $150$

• Short T2 = less signal = dark in image

• Long T2 = stronger signal = bright in image

PD-weighted

• Provides good contrast between gray (bright) and white (darker gray) matter, with little contrast between brain and CSF.

MRI as Diagnostic Tool

• MRI can be used to visualize alterations of brain structure, e.g., in older individuals with white matter disease.