Infographics / MRI Physics

T1 recovery & T2 decay relaxation curves

Two graphs of magnetisation against time after a 90 degree pulse. Left: longitudinal magnetisation Mz recovering as an inverted exponential toward M0, with T1 defined as the time to reach 63 percent of M0; fat recovers quickly (short T1) and water or CSF recovers slowly (long T1). Right: transverse magnetisation Mxy decaying exponentially toward zero, with T2 defined as the time to fall to 37 percent of M0; fat decays quickly (short T2) and water or CSF decays slowly (long T2). For a given tissue T1 is longer than T2.

T1 longitudinal recovery and T2 transverse decay curves Two graphs of magnetisation against time after a 90 degree pulse. Left: longitudinal magnetisation Mz recovering as an inverted exponential toward M0, with T1 defined as the time to reach 63 percent of M0; fat recovers quickly (short T1) and water or CSF recovers slowly (long T1). Right: transverse magnetisation Mxy decaying exponentially toward zero, with T2 defined as the time to fall to 37 percent of M0; fat decays quickly (short T2) and water or CSF decays slowly (long T2). For a given tissue T1 is longer than T2. Longitudinal (T1) recovery Mz = M0(1 − e^(−t/T1)) — rises toward M0 M0 0 longitudinal (Mz) time after 90° pulse → 63% M0 fat water / CSF Transverse (T2) decay Mxy = M0 · e^(−t/T2) — falls toward 0 M0 0 transverse (Mxy) time after 90° pulse → 37% M0 fat water / CSF T1 = time for Mz to recover to 63% of M0; T2 = time for Mxy to fall to 37% of M0 (i.e. 63% lost). Fat: short T1 (~250 ms) and short T2 (~80 ms). Water / CSF: long T1 and long T2 (~2–3 s at 1 T). For a given tissue T1 ≥ T2 — in most tissues T1 ≫ T2, so recovery is much slower than decay.
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