This whole playlist has been AMAZING!!! Each one covers the given topic better than any other videos I've seen AND does it in half the time. You are the best :)
Started MRI program and have been struggling to understand the textbook physics until came upon your videos. Now I get it! Thank you so much for your teaching method, it works!
😂in between the video, i get out off face, but after apply 180 degree magnetic field, finally catch up a little! Thank you for your efforts! Highly appreciated sir!
As a nuc med tech training into mri I really appreciate these videos. I realize the reason mri physics have eluded me is because no one really ever explained it very well. Thanks for these.
Thank you for being amazing as always, this was well explained ! I had a little bit of confusion when you said that because of magnetic field inhomogeneity the spins are going to precess with different frequencies and thats why T2* are generated. but since we have different frequencies how do we send an RF pulse that will match these changing frequencies due to inhomogeneity and knock them out of the Z-axis to the x,y axis in the first place???
Thanks so much for your great exaplanation and videos. When 180 pulse applied , the slower spin will be trailing behind the faster spin right. How they catch up ? Couldnot understand that concept
The green is decaying faster then the blue. When you apply the 180 degree pulse, flipping them 180 degree, shouldn't the green be above the blue (instead of blue above the green)?
I had a question! At the diagram at 15:50, looking at TE2 --> is this approximately when we have lost 63% of the signal and basically why we need to time our echo appropriately to get a good image? Also!! Thank you so much for these videos! I'm an MRT(R) from canada, a new graduate looking to take MRI in 2024, so these videos are a big help!! If you need any help vetting questions or writing new ones for your x-ray bank, let me know and I'd love to give back!
Another question for you! At 15:00-15:50... you said this basically that we flip to 90 degrees and turn off the RF pulse and immediately sample. this is where I'm confused.... I thought the TE was when we... flip to 90 ... (wait) ... flip to 180 ... (wait) = TE. and sample here! so am I understanding this right? basically this explains the contrast differences between different tissues.... flip to 90 ... (wait 1 second) ... flip to 180 ... (wait 1 second) TE. and sample here! = small differences / all white flip to 90 ... (wait 2 seconds) ... flip to 180 ... (wait 2 seconds) TE. and sample here! = appropriate differences lets say at 37% transverse mag left. flip to 90 ... (wait 5 seconds ) ... flip to 180 ... (wait 5 seconds) TE. and sample here! = small differences / all black, no signal. in all of these examples we found the T2 values of the tissues souly based on the spin-spin interactions because we applied an RF pulse, waited some time (A), applied a second 180 RF pulse and waited that exact same time(A) and sampled the data. am i understanding this correctly?
Practically when do you apply the 180° flip for a T2 weighted image? T2 relaxation is 63% of transverse signal loss. Do we then flip it when the T2* relaxation signal is at 37%? Is there an automated mechanism that does that because the fewest radiology technicians can react within milliseconds. Also the time T2 is different for different tissues. If the flip was done as I figured which of the 37% signal times(T2 of muscle, fat, CSF) would we take for the T2* flip?
Why is it that a 180 degree RF pulse rephrases all the moments, but the transverse magnetization hits a ceiling at the T2 decay curve? If it is only an issue of how long the RF pulse is on for, then why would having it on for the duration of a 90 degree pulse restore full transverse magnetization and break the ceiling of the T2 decay curve. Something quantum? As always, thank you!
Hi 👋 Not quite sure what you're asking exactly. The 180 degree pulse will only cause rephasing of spins if a previous 90 degree pulse has been applied prior. The first 90 pulse flips the spins in to the transverse plane. The spins will then decay at T2* - some will dephase faster than others depending on tissue type. If we then apply a 180 degree pulse the spins that dephased faster will now be lagging behind the spins that had slower T2* decay. This allows the faster spins to catch up with the slower spins at the TE when the spins will be in 90 transverse plane. If we were to only apply a 180 degree pulse as our first RF pulse the spins would flip a full 180 degrees and have no transverse magnetisation. They will decay at a rate of T1 (ie. regain longitudinal magnetisation) but have to T2 decay because there is no transverse magnetisation in the sample. This is actually a key feature of inversion recovery sequences which we will cover in two weeks time. I may be completely off what you were asking! Feel free to rephrase your question if I haven't answered it! Hope you're doing well!
If you're asking why can spins with a 180 RF pulse 'break through' the 90 degree angle (because surely once they resonate at 90 degrees and we continued to apply an RF pulse they wouldn't continue to flip to angles above 90) then yes you are right, this is due to a quantum property. It has to do with energy states of the spins (at resting state with just the main magnetic field spins are in two energy states - lying parallel [low energy] and antiparallel [high energy]). In the quantum world the spins can exist in both states until measured. Continuing the RF pulse for a longer period of time adds energy into this system and allows the a slight majority of spins to now exist in the antiparallel state (higher energy), therefore they are flipping past the 90 degrees. I hope I'm making some sense, very difficult to explain in text 😅
At 12:36, your explanation does not make sense. I mean both fast and slow spins are experiencing the same changes in their phase, so nothing changes and the leading one will remain the leading again. Could you please explain a bit ?
I mean i think i figured that out. I had the same issue in understanding this. The answer is - you are applying the 180* flip in Z axis (in ZY plane) not in XY plane. If the 180* RF pulse acted in XY plane - then it would happen as you say - nothing would change - the leading one would still be leading. But when it acts in ZY plane - then they flip making like a mirror reflection along the X axis - so they switch and the leading one is now lagging and can catch up
I think that the video shows something that doesn’t really makes sense. In the longitudinal plane the net magnetisation vector is not processing because the individual atoms’ spins are out of phase. If somehow they were IN phase the Net magnetisation vector would have been processing in the exact way you show at the Course 3 11:15 minute. So, I m thinking that when the B1 magnetic field is applied, it turns the net magnetisation vector at the transverse plane, But now, we have every atom processing IN phase and this leads to the movement of the now vertical Net magnetisation vector. This movement I think should be the same with the one you show at the course 3 11:15 as a mentioned before and not as the rotational movement you show at the video. Showing the NET vector changing direction (becoming continuously parallel and anti parallel with the B1) doesn’t make any sense to me. I’m a medical student and I’m trying to figure it out so please, answer if you can. Thank you, your videos help too much!
for those visual learners, here is a simulation for 180 degree flip ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-GDElT6Tz7_Q.html notice that the fastest becomes the latest after the flip
