T1-weighted image

[Inaki4]

I have a couple doubts regarding the Spin-echo sequence applied to acquire a T1-weighted image. I'm gonna assume a short TE to avoid the effect of T2 and I'm going to omit the 180º pulse in the explanation since it is only aimed at removing the effect of T2*. This way I will just focus on the TR parameter, which is the one I don't get for a T1-weighted image.

According to the SE sequence, you apply the 90º pulse and after a TE you measure the echo. The idea is to make the measurement at the instant that you'll get a maximum T1 contrast, like in this image:




But as that image shows, you first apply a 90º pulse to drive the net magnetization to the transverse plane, and after a TR, you apply a second 90º pulse to get the longitudinal component with maximum T1 contrast into the transverse plane, and that is what you measure.

My point is: you need two 90º pulses to be able to measure the longitudinal component at the instant of maximum T1 contrast, but you only measure after the second pulse. This image shows it a little bit:




But according to the SE sequence, you measure the echo periodically after each 90º pulse, not after each two 90º pulses. My questions are

1) According to this, it doesn't make sense to measure after the first 90º pulse right? That way you would measure the whole original longitudinal component (this would make sense for a PD-weighted image).

2) If you repeat this periodically, the first 90º pulse would put the whole longitudinal Mz into the transverse plane. After a TR, you would apply a second 90º pulse to place a portion of the original longitudinal component (the portion with the highest T1 contrast). But TR after the second pulse, the portion of the longitudinal component you would place in the transverse plane is not the same as before, it would be less. And this would go on pulse after pulse. This image shows what I mean:




Basically, what I mean is that the time to recover the longitudinal component depends on the flip angle, right? It wouldn't take the same to recover the longitudinal component after a 90º angle (the longitudinal component must recover from 0) than after an alpha>90º angle (the longitudinal component must recover starting from a negative value). And the second case is what happens after the last 90º pulse I explained before (it is a 90º pulse over a net magnetization that stil isn't pointing upwards since TR is small).

This implies that for each line, you would get different portion of the longitudinal magnetization into the transverse plane, so different contrasts of T1. This would make impossible to compare pixels in different lines in a slice.

I'm sure I am misunderstanding something, but I don't realize what it is.

Hope you I managed to explain myself.

Thanks in advance!!

Iñaki Rabanillo Viloria

[Inaki4]

I think I managed to understand what happens.

First of all, I was assuming that TR wasn't long enough to allow the transverse magnetization to fully vanish, but it is supposed to be designed for that purpose. This means that in my last image, after each TR, there is only longitudinal magnetization.

My first question was:

(11-26-2015 07:46 PM)Inaki4 Wrote:  1) According to this, it doesn't make sense to measure after the first 90º pulse right? That way you would measure the whole original longitudinal component (this would make sense for a PD-weighted image).

The answer is that indeed it doesn't make sense to measure after the first pulse. This is because after the first pulse we bring down the whole longitudinal magnetization, so two tissues which only differ in their T1 relaxation time (same proton density) would be indistinguishable. If we wait for a TR short enough not to allow the longitudinal magnetization to fully recover and then use another 90º pulse to bring it to the XY plane, now it makes sense to measure because since both tissues have different T1, each of them would provide a different measurement, so we will get a T1-weighted image.

I verified this in the book Principles of Magnetic Resonance Imaging: A Signal Processing Perspective by Zhi-Pei Liang and Paul C. Lauterbur, where they explain that this first pulse is called preparatory pulse and it is discarded.

My second question was:

(11-26-2015 07:46 PM)Inaki4 Wrote:  2) If you repeat this periodically, the first 90º pulse would put the whole longitudinal Mz into the transverse plane. After a TR, you would apply a second 90º pulse to place a portion of the original longitudinal component (the portion with the highest T1 contrast). But TR after the second pulse, the portion of the longitudinal component you would place in the transverse plane is not the same as before, it would be less. And this would go on pulse after pulse. This image shows what I mean:

As I mentioned, the TR is assumed long enough to allow the transverse magnetization to vanish. TR after the first 90º pulse, just a portion of the longitudinal magnetization would be recovered. But when we bring it down to the XY plane with a second 90º pulse, the situation is exactly the same that after the first 90º pulse in terms of the longitudinal magnetization recovery. In both cases, it starts to recover from 0, so waiting TR after both pulses would allow to recover the exact same portion of longitudinal magnetization.

What matters for the recovery of the longitudinal magnetization is the Z-component density of spins at the time at which the recovery starts, but the two situations described (inmediately after the first and second 90º RF pulses) differ in the Y-component (if the RF pulse is applied in the X-axis) density of spins.

[AndrewBworth]

Yes, I think you've got it. In NMR we do a thing called "Dummy Scans", where the sequence is run a few times so that you begin at the equilibrium condition. MRI would have a similiar idea, where the first one or two scans wouldn't be recorded. When I first saw the T1 sequence, I thought they should have named it the "Recovery limited" scan.

(11-26-2015 07:46 PM)Inaki4 Wrote:  I have a couple doubts regarding the Spin-echo sequence applied to acquire a T1-weighted image. I'm gonna assume a short TE to avoid the effect of T2 and I'm going to omit the 180º pulse in the explanation since it is only aimed at removing the effect of T2*. This way I will just focus on the TR parameter, which is the one I don't get for a T1-weighted image.

According to the SE sequence, you apply the 90º pulse and after a TE you measure the echo. The idea is to make the measurement at the instant that you'll get a maximum T1 contrast, like in this image:




But as that image shows, you first apply a 90º pulse to drive the net magnetization to the transverse plane, and after a TR, you apply a second 90º pulse to get the longitudinal component with maximum T1 contrast into the transverse plane, and that is what you measure.

My point is: you need two 90º pulses to be able to measure the longitudinal component at the instant of maximum T1 contrast, but you only measure after the second pulse. This image shows it a little bit:




But according to the SE sequence, you measure the echo periodically after each 90º pulse, not after each two 90º pulses. My questions are

1) According to this, it doesn't make sense to measure after the first 90º pulse right? That way you would measure the whole original longitudinal component (this would make sense for a PD-weighted image).

2) If you repeat this periodically, the first 90º pulse would put the whole longitudinal Mz into the transverse plane. After a TR, you would apply a second 90º pulse to place a portion of the original longitudinal component (the portion with the highest T1 contrast). But TR after the second pulse, the portion of the longitudinal component you would place in the transverse plane is not the same as before, it would be less. And this would go on pulse after pulse. This image shows what I mean:




Basically, what I mean is that the time to recover the longitudinal component depends on the flip angle, right? It wouldn't take the same to recover the longitudinal component after a 90º angle (the longitudinal component must recover from 0) than after an alpha>90º angle (the longitudinal component must recover starting from a negative value). And the second case is what happens after the last 90º pulse I explained before (it is a 90º pulse over a net magnetization that stil isn't pointing upwards since TR is small).

This implies that for each line, you would get different portion of the longitudinal magnetization into the transverse plane, so different contrasts of T1. This would make impossible to compare pixels in different lines in a slice.

I'm sure I am misunderstanding something, but I don't realize what it is.

Hope you I managed to explain myself.

Thanks in advance!!

Iñaki Rabanillo Viloria