Effects on Apparent T2 in Multi Pulse Spin Echo1

Effects on Apparent T2 in Multi Pulse Spin Echo1 - Effects...

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Unformatted text preview: Effects on Apparent T2 in Multi Pulse Spin Echo Bhairav B Mehta Introduction The transverse component of the magnetization decreases gradually to zero. This is an exponential decay with the time constant as T 2 . T 2 relaxation time is time constant for spinning protons to lose the phase coherence among the nuclei spinning perpendicular to the main field. The value of T 2 depends on the mobility of the protons which is dependent on tissue property. Thus by knowing the value of T 2 provides us the insight of the content of the voxel. Due to off resonance the apparent value of T 2 decrease it follows a T 2 * decay. To remove the artifacts due off resonance we use spin echo which uses one 90 excitation and one 180 0 refocusing pulse. Then to decrease the scan time they introduced Fast spin echo in which multiple refocusing pulses were used to get multiple lines in k-space. Initially it was interpreted that echoes follow a T 2 decay curve. But then it was observed there was some variation in the decay. In this report I have tried to explain few possible reasons that affect the apparent decay or vary the transverse magnetization. To support the argument I have attached few figures using simulation of Bloch equation in matlab. Dependency of steady state signal on T 1 as well as TR In single pulse spin echo sequence the equation the steady state equation was assumed to be. where is the value of stable magnetization, TR is the repetition time, TE is echo time. The slope of ) against TE gives T 2 . But the actual steady state equation for spin echo is Thus we see that the slope of ) against TE is dependent on T 1 , T 2 , TE and TR, and T 2 cannot be determined without making a separate determination of T 1 . This dependency affects the signal and produces an error. The relative error is given by the equation The exact equation (2) does reduce to equation (1) (i.e. ) under the following condition: i.e. short echo time OR i.e. fully relaxed. Similarly while applying multiple refocusing pulse the signal equation is not only function of T 2 but also T 1 , TR and TE. Let us consider a pulse sequence with two refocusing pulses one at TE 1 /2 and other at (TE1 + TE 2 /2). The steady state signal equation is given as The relative error changes to: If TE 1 =TE 2 =TE/2 the first echo will come at TE/2 and second echo will come at TE. Below are few figures from simulation of the Bloch equation which illustrate the variation in signal by just the varying T 1 and keeping other parameters same. 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0- 1- 0 . 5 0 . 5 1 F i g A t i m e in m s normalised Mz 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 0 . 5 1 F i g B t i m e in m s normalised Mxy Figure 1 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0-1-0 . 5 0 . 50 ....
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Effects on Apparent T2 in Multi Pulse Spin Echo1 - Effects...

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