I've searched a lot on the web and this may be the best way to learn about MR basic physics. Many thanks to professor Lipton and all the people responsible for takin the decision to upload this series of videos.
NOTICE: If you're watching these videos with the playlist on Autoplay, this is the last video in correct order. The next video in queue is 14 of 56, not 13 of 56!
15:53 "the only thing that we are able to rotate (...) is the component thats parallel to B0" --- why is there no complete 90 degree Magnetization? why arent "partly relaxed" tissues also rotated to 90 degrees??
A good place to play with this: www.drcmr.dk/BlochSimulator/ Choose the "mixed matter" scene - you can do a 90 degree pulse, and another one after a few seconds (make sure t1 and t2 times are set), and you can see signals that weren't initially distinguishable (with the first pulse) become distinguishable.
Hello, can you please help me understand the following about T1 contrast : how does the 2nd 90 degree RF pulse simply "turn" longitudinal magnetization into transverse magnetization? I thought that these were due to 2 different physical phenomena (a minor imbalance between parallel and antiparallel spins for longitudinal component; and phase coherence of all H nuclei spins for transverse component) so I just don't see how you can turn one into the other by adding energy through the RF pulse... This is frustrating to me as the books i use don't really explain it. Thank you
the same way it did the first time. This guy sucked at explaining it though. First of all: An RF pulse does 2 things: It "rotates" the net vector of the protons 90° and it induces phase coherence (all of them point in the same direction while spinning). Now: T2-relaxation is caused by de-phasing of this procession (while their magnetic vectors still point in a 90° angle to B0) and therefore losing transverse magnetisation. T1-relaxation is caused by re-aligning with B0 (and therefore losing the 90° angle) which is a seperate but simultaneous process which takes much longer than T2-decay though. If you apply the 2nd RF-pulse while T1-relaxation has fully occured in some tissues but not in others (called "short TR" or TR ~ T1 or short RF repetition time - all of them meaning the same), only protons which are fully T1-relaxed (are in longitudinal axis again) will be flipped again and produce a transverse signal again (actually this is not exactly true, but for the sake of understanding I'll just leave it there - otherwise we'd have to go into spin changes). If you set a short TE (measure right after the RF), there will be no T2-decay yet and the "main reason" for transverse magnetisation is based on the amount of protons which were already fully relaxed before the RF occured (which is the cas in tissues that have fast T1-relaxation) and therefore have high signal. This is T1-weighted.
What I don’t understand is why is the signal for the T2 decay start at a higher point for a tissue with a faster T1 recovery curve? If more T1 recovery has occurred, wouldn’t there be less transverse magnetization to start?
More protons have recovered -> More protons will be affected by the next 90 RF excitation -> Higher initial signal. The protons that are not recovered are not affected by the next signal as the Dr said at the beginning of the lecture.
Thank you so much Dr. Lipton. Your lectures are really inspiring! I have one question regarding the last point of the view -- manipulating TR. So it is mentioned, with very short TR, the signal gets substantially suppressed because we only have a very limited amount of Mz recovered, the recovered Mz is where we get signal from. I am wondering, with very short TR, it is true that Mz only recovers a little, but M-transverse have not decay a lot; then when we apply the second 90 RF pulse, where does the remaining M-transverse goes?
I guess the remaining Mt would be flipped further beyond 90 degrees (as he mentioned in previous session that if we give RF pulse for a long period of time, the spins will go beyond 90 degrees to 180 270 360 plus ...)
when we are applying a second 90 degree RF pulse, why the transverse component of more T1 recovered (Fat) having greater intensity as compared to other?
this guy started this series well but it starts to be a bit messy now. He completely misses to explain that T2 decay is explained by de-phasing of procession (that was synchronized by the RF-pulse) and therefore losing transverse magnetisation. It has nothing to do with re-aligning with B0 in the longitudinal axis (that is T1-decay). That's a different process.
From Dr. Lipton: "I am not completely sure I understand your question. However, if you are asking why the Mz graph on the top does not go to zero at the vertical black line, that is simply because I did not update the upper graph or discuss what happens to Mz. The discussion was focused on the consequences for Mt and I had not bothered to update the upper graph. Apologies if this was confusing."
Thankssssssssssss So what happens to both longitudinal mangnetization on Mz graph when we give 90º and with 180º pulse with a short TR after we already give the first 90º RF That`s one of my biggest doubt
I've searched a lot on the web and this may be the best way to learn about MR basic physics. Many thanks to professor Lipton and all the people responsible for takin the decision to upload this series of videos.
The essence of MR imaging is the current lecture. Great Work !
GREAT WORK... GREAT TEACHING...HATS OFF TO YOU SIR..
NOTICE: If you're watching these videos with the playlist on Autoplay, this is the last video in correct order. The next video in queue is 14 of 56, not 13 of 56!
Thanks, Professor Lipton. Great explanation and clear elucidation on abstract concepts!
15:53 "the only thing that we are able to rotate (...) is the component thats parallel to B0" --- why is there no complete 90 degree Magnetization? why arent "partly relaxed" tissues also rotated to 90 degrees??
A good place to play with this: www.drcmr.dk/BlochSimulator/
Choose the "mixed matter" scene - you can do a 90 degree pulse, and another one after a few seconds (make sure t1 and t2 times are set), and you can see signals that weren't initially distinguishable (with the first pulse) become distinguishable.
Best teaching ever got on mri...thank you very much sir....
Hello, can you please help me understand the following about T1 contrast : how does the 2nd 90 degree RF pulse simply "turn" longitudinal magnetization into transverse magnetization? I thought that these were due to 2 different physical phenomena (a minor imbalance between parallel and antiparallel spins for longitudinal component; and phase coherence of all H nuclei spins for transverse component) so I just don't see how you can turn one into the other by adding energy through the RF pulse...
This is frustrating to me as the books i use don't really explain it.
Thank you
the same way it did the first time. This guy sucked at explaining it though. First of all: An RF pulse does 2 things: It "rotates" the net vector of the protons 90° and it induces phase coherence (all of them point in the same direction while spinning). Now: T2-relaxation is caused by de-phasing of this procession (while their magnetic vectors still point in a 90° angle to B0) and therefore losing transverse magnetisation. T1-relaxation is caused by re-aligning with B0 (and therefore losing the 90° angle) which is a seperate but simultaneous process which takes much longer than T2-decay though. If you apply the 2nd RF-pulse while T1-relaxation has fully occured in some tissues but not in others (called "short TR" or TR ~ T1 or short RF repetition time - all of them meaning the same), only protons which are fully T1-relaxed (are in longitudinal axis again) will be flipped again and produce a transverse signal again (actually this is not exactly true, but for the sake of understanding I'll just leave it there - otherwise we'd have to go into spin changes). If you set a short TE (measure right after the RF), there will be no T2-decay yet and the "main reason" for transverse magnetisation is based on the amount of protons which were already fully relaxed before the RF occured (which is the cas in tissues that have fast T1-relaxation) and therefore have high signal. This is T1-weighted.
What I don’t understand is why is the signal for the T2 decay start at a higher point for a tissue with a faster T1 recovery curve? If more T1 recovery has occurred, wouldn’t there be less transverse magnetization to start?
More protons have recovered -> More protons will be affected by the next 90 RF excitation -> Higher initial signal. The protons that are not recovered are not affected by the next signal as the Dr said at the beginning of the lecture.
Amazing lecture.. Thank you very much... U have made a great difference...
The best one on T1 weighted Nd T2 weighted contrasts
Thank you so much Dr. Lipton. Your lectures are really inspiring!
I have one question regarding the last point of the view -- manipulating TR. So it is mentioned, with very short TR, the signal gets substantially suppressed because we only have a very limited amount of Mz recovered, the recovered Mz is where we get signal from. I am wondering, with very short TR, it is true that Mz only recovers a little, but M-transverse have not decay a lot; then when we apply the second 90 RF pulse, where does the remaining M-transverse goes?
I guess the remaining Mt would be flipped further beyond 90 degrees (as he mentioned in previous session that if we give RF pulse for a long period of time, the spins will go beyond 90 degrees to 180 270 360 plus ...)
when we are applying a second 90 degree RF pulse, why the transverse component of more T1 recovered (Fat) having greater intensity as compared to other?
because it has less proton density
Can you have multiple TEs per RF pulse? seems like you could almost take a video for an individual slice with a bunch of TE settings.
The greatest explanation ever!
You are a great teacher!
Extremely helpful! Awesome teacher!
I still don not get what n1, n2 and nt are and rappresent
Awesome explanation !!!
:P
perfect explanation
Extremely helpful, thank you very much!
this guy started this series well but it starts to be a bit messy now. He completely misses to explain that T2 decay is explained by de-phasing of procession (that was synchronized by the RF-pulse) and therefore losing transverse magnetisation. It has nothing to do with re-aligning with B0 in the longitudinal axis (that is T1-decay). That's a different process.
For T1 contrast TE should be long not short
T1: Short TE 10-30ms (less T2 info due to short TE) and TR 350-700ms whereas T2: Long TE 100-130ms and long TR 4000-8000ms
someone can explain me ??? when you give a 90º pulse ( min 14- 15 of the video) woludn t both longitutinal vectors came to 0????????
From Dr. Lipton: "I am not completely sure I understand your question. However, if you are asking why the Mz graph on the top does not go to zero at the vertical black line, that is simply because I did not update the upper graph or discuss what happens to Mz. The discussion was focused on the consequences for Mt and I had not bothered to update the upper graph. Apologies if this was confusing."
Thankssssssssssss
So what happens to both longitudinal mangnetization on Mz graph when we give 90º and with 180º pulse with a short TR after we already give the first 90º RF
That`s one of my biggest doubt