Continues from the RF pulse. After a 90°, M is in xy and does not stay there.
Two recoveries run at once. Each has its own time constant.
T1 is how fast Mz grows back toward M0, the equilibrium along B0. After a 90° pulse:
Mz(t) = M0 (1 − exp(−t/T1))
People call it spin-lattice relaxation: the spins dump energy into the surroundings. Fat has a short T1. CSF has a long T1. That difference is later used as contrast. T1 is a time, in milliseconds to seconds, for that tissue.
T2 is how fast the xy component disappears:
Mxy(t) = Mxy(0) exp(−t/T2)
Neighboring spins nudge each other’s fields, so the pack dephases even in a perfect B0. T2 is always shorter than T1. Real coils also see extra dephasing from shims and tissue susceptibility. That faster number is T2*, not T2. Different constant. Leave it for another note.
A T1-weighted image is not a T1 map. Weighted means you picked TR and TE so tissues with different T1 look brighter or darker. A T1 map is a voxel-wise fit of the time constant itself. Do not read a bright pixel as “this voxel’s T1.” Same split for T2-weighted vs T2 map.
Next note: TE/TR contrast is not in this set yet.
中文
接 RF pulse。90° 之後 M 在 xy，它不會待在那裡。
兩種回復同時在跑，各有自己的時間常數。
T1 是 Mz 長回 M0 有多快，M0 是沿 B0 的平衡；90° pulse 之後：
Mz(t) = M0 (1 − exp(−t/T1))
人也叫 spin-lattice relaxation：spin 把能量丟給周圍；脂肪 T1 短，CSF T1 長，這個差後面會當對比。T1 是時間，毫秒到秒，看組織。
T2 是 xy 分量消失有多快：
Mxy(t) = Mxy(0) exp(−t/T2)
旁邊的 spin 會推彼此的場，所以就算 B0 完美，這包也會散相；T2 永遠比 T1 短。真實線圈還會看到 shims 和組織磁化率多出來的散相，那個比較快的數字是 T2*，不是 T2；另一個常數，留給別則筆記。
T1-weighted 影像不是 T1 map；Weighted 的意思是你選了 TR 和 TE，讓 T1 不同的組織看起來比較亮或比較暗。T1 map 是逐 voxel 把時間常數擬合出來，不要把亮像素讀成「這個 voxel 的 T1」；T2-weighted 跟 T2 map 也是同一套區分。
下一則：TE/TR 對比這組還沒寫。