T2的奧妙

T2 is the "transverse" relaxation time. It is a measure of how long transverse magnetization would last in a perfectly uniform external magnetic field (figure7). Alternatively, it is a measure of how long the resonating protons remain coherent or precess (rotate) "in phase" following a 90° RF pulse. T2 decay is due to magnetic interactions that occur between spinning protons. Unlike T1 interactions, T2 interactions do not involve a transfer of energy but only a change in phase, which leads to a loss of coherence.

T2是“橫向”的鬆弛時間。它是衡量橫向磁化量在一個完全均勻的外部磁場中（圖7）會持續多久。或者說，它是衡量共振質子在90 ° 脈衝後保持多久同調或“同相”的進動（旋轉）。T2衰減是由於自旋質子之間的磁相互作用發生。不像T1相互作用，T2的相互作用不涉及能量的轉移，但只有在相位改變，從而導致喪失同調（同相）。

T2 relaxation depends on the presence of static internal fields in the substance. These are generally due to protons on larger molecules. These stationary or slowly fluctuating magnetic fields create local regions of increased or decreased magnetic fields, depending on whether the protons align with or against the main magnetic field (as discussed in Fundamentals of MRI – Part I). Local field non-uniformities cause the protons to precess (rotate) at slightly different frequencies. Thus following the 90° pulse, the protons lose coherence and transverse magnetization is lost. This results in both T2* and T2 relaxation.

T2弛豫取決於物質中內磁場作用下。這些通常是由於較大的分子的質子所造成。這些靜止的或緩慢波動磁場產生微觀區域磁場的增加或減少，取決於質子是否順或逆著主磁場。微觀磁場的非均勻性導致質子進動（旋轉）頻率的略微不同。因此90 °脈衝後，質子失去同相而導致橫向磁化減少。這樣的結果產生T2和T2*鬆弛。

When paramagnetic substances are compartmentalized, they cause rapid loss of coherence and have a short T2* and T2. For example, figure 8 illustrates that the magnetization induced inside a deoxygenated red blood cell is greater than in the plasma outside the red cell because the intracellular deoxyhemoglobin is paramagnetic. This compartmentalization of substances with different degrees of induced magnetization leads to magnetic non-uniformity with shortened T2*, causing the free induction decay (FID) to decay more rapidly. Since gradient echo images are essentially rephased FID images, this also leads to signal loss on gradient echo images. Thus acute and early subacute hemorrhage (containing deoxy and intracellular methemoglobin, respectively) appear dark on T2-weighted gradient echo images. The different magnetic field inside and outside red cells results in rapid dephasing of water protons diffusing across the red cell membrane in an acute hematoma with secondary T2-shortening and loss of signal (as seen in figure 9).

當順磁性物質是條塊分割，導致快速失去同調性和有短T2*和T2。例如， 圖8顯示去氧紅血球內的磁化率大於在血漿中而不是紅血球內的磁化率，因為細胞內的去氧血紅蛋白是順磁性。這種物質因不同程度的磁化率而作的細分使磁場不均勻性而導致T2 *縮短，再導致自由感應衰減（FID）更迅速地衰減。更由於梯度回波影像須使FID聚相，這些細微的磁化率變化導致梯度回波影像訊號損失。因此急性和早期的亞急性出血（分別含去氧和細胞內的高鐵血紅蛋白，）梯度回波T2加權圖像上出現暗。紅血球內部和外部感受到的不同磁場導致在急性血腫造成的水質子在紅血球細胞膜擴散形成的快速去相所造成再次的T2縮短和訊號消失（如圖9）。

As the natural motional frequency of the protons increases, T2 relaxation becomes less and less efficient and T2 prolongs. Rapidly fluctuating motions (such as in liquids) average out so there are no significant internal fields and there is a more uniform internal magnetic environment. The hydration-layer water in brain edema has a shorter T1 than bulk phase water like CSF, yet the motion of the protons in brain edema is not so slow that T2 relaxation is efficient, so T2 remains long. This accounts for the intense appearance of the vasogenic edema associated with brain tumors on T2-weighted MR images (figure 10).

當質子的自然運動頻率增加，T2弛豫變得越來越沒高效率導致T2延長。急劇波動的運動（如液體）平均掉，所以沒有明顯的內場而導致內部磁場更加均勻。腦水腫的水化層水有個比CSF等體相水較短的T1，但腦水腫中的質子的運動並不是如此緩慢，T2弛豫是有效的，所以T2仍然保持很長。這說明了為何T2加權圖像上，腦腫瘤血管性水腫造成高訊號的原因（圖10）。

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s2=s1e(-t/t2) > t2=t ln(s2)/ln(s1) ---t為兩訊號間隔時間