Thank you for the great video, it really helped me a lot, but one question, why there are no rephasing slice selection gradient when the 180 degree RF pulse is applied?
I dont think i have ever commented on a youtube video in my life. But i have to express my thankfulness for your videos and how you always give a recap/refresher to remind us on previous topics. I have my registry soon and these videos have been helping tremendously and can't thank you enough!!!
Wow! The amount of effort I could imagine making this video is commendable. This is a very precise topic. I am really amazed how you have put your explanations into animations. I am really thankful to you for making this series. I am watching this again and again to get my concepts clear. Thank you very much!!!
agree. Cant imagine how much time it takes to make the various animations in your videos. Thank you so much looking forward to the entire MRI series as a radiology resident in the US
Dude! Your program is 10x better than any book, or the MRI schooling I had 15 years ago. It's all due to the actual *moving* diagrams, as well as your brilliant explanations and analogies. Just like some chrome extensions do (i.e. Adblock plus), you need to mention and post a link to submit donations, whilst keeping the actual videos free on RUclips. Unfortunately, I'm guessing that it might be a violation of RUclips's policies, but if not, you absolutely must do that. I'd give ABP donations occasionally over the years when they'd ask, and your material is much, much more deserving. I wish I found your material earlier than several days before my exam. I couldn't find any decent RUclips pages on the subject before that. One, though explaining on a whiteboard well and myself thinking it was the best, didn't even get the basics of parallel vs anti-parallel correct. I just quit looking for alternatives after that. Being that your material is much more recent and thus lesser known at the moment, I hope you pop up as the top search result as soon as possible to help others. Thanks. TL;DR: YOUR MATERIAL MAKES MRI SCHOOLS OBSOLETE!!
How much time you have spent to make these incredible videos. I dont think you are doing this solely for the money. I cant believe how much satisfaction you are gaining by reading the lovely comments. I wish I had such brilliant ability to teach others. I wish you all the good luck in life Micheal. ❤
Hi Doc! Love your videos, you’re a fantastic teacher! @kayk928 asked this question and I had the same question. He said, “why do we not use a rephasing SSG for the 180 degree RF pulse and only for the initial 90 degree pulse?” Do you mind explaining please? Thank you so much in advance!
As he says, the rephasing gradient (SSG) is typically applied after the initial 90º RF pulse to correct dephasing caused by magnetic field inhomogeneities and ensure coherent transverse magnetization. BUT, this transverse magnetization decays faster than we woud like to (T2*, see T2* video), because fast decay means less time to play with the TE and TR (see T1, T2 and PD weighted imaging video), so the 180º pulse is generated to inverse this dephasing, gaining time, and have again net transversal magnetization signal sufficient to be measured. Now, regarding your question, the difference is that after the 180º pulse, no additional rephasing gradient is needed because the 180º pulse itself reverses the phase of the spins. When the 180ª RF is applied, the spins have the same dephase but inverted, (as the T2* video says, the faster spins are now behind, the slower are ahead, and they all eventually catch up and form the spin-echo signal). So here there is no need for a gradient to 'fix' with a rephasing gradient, since the inversion of the phase already makes them all catch up. In other words: Schematic explanation 90º RF pulse --> Mxy occurs but spins within slice have dephase due to in-slice gradient --> rephasing gradient --> all spins are coherently to the same phase --> T2 relaxation is not homogeneous --> spins relax differently and start to dephase (again, for another reason) --> 180º RF pulse to invert this relaxation 'speeds' --> catch up and coherent signal (spin-echo). So the 180º RF already rephases (for another reason), so a rephasing gradient is not needed. I had the same doubt, I hope it is correct and clear! Good luck on your studies :)
Thank you so much for these series! It's helping me A LOT for teaching the MRI subject to my students. I recommend your vids to all my students and coworkers as well ❤
Regarding the rephasing gradient, you, Dr, covered the concept elegantly! Only that the illustration displays a logical error since if we applied a rephasing gradient it should be only be limited to SSG range not to all B0. If we applied rephasing gradient the same way displayed in your presentation (along entire B0), we will face a logical problem since it will cancel the linear variation, having all protons at same precession frequency again. so I recommend editing the red line of rephasing gradient to only fit the SSG not the entire length of B0 What do you think?
Thanks a lot for the lecture, really helped refreshing my knowledge. One thing I find confusing is that your SSG-in-time graph implies that one would have the SSG turned on during the refocussing 180deg pulse. I can't seem to understand what the purpose is of using such configuration, given that the slice of interest was already selected after the 90deg pulse. I get the point of the 180deg pulse but I don't understand why we'd use a gradient here again, could you maybe elaborate on this? Respectully, Wouter.
In correspondence with the 180-degree pulse, I see another gradient. Does this mean that simultaneously with the 180-degree pulse, a gradient equal to the initial one is applied?
Hi professor！Your video was truly fantastic; your explanation was very clear. I'm really grateful. I have a question I'd like to ask you: since in TE (echo time) the protons in the slice are precessing in phase, making it impossible to determine the exact location of the signal within the slice, why do we need the rephasing gradient step? Wouldn't it be possible to know where the signal is coming from at TE without rephasing? Is the purpose of rephasing to ensure that all the protons in the entire slice precess in phase? If so, what is the objective of doing this?
Hi! Great question. At the end of the RF pulse the protons are in phase, at TE the protons are rapidly dephasing due to T2* decay. They are also dephasing because the gradient we apply in the x direction (frequency encoding gradient) causes spins to precess at different frequencies. These different frequencies allow us to localise signal but we lose transverse signal even quicker as a result of the gradient field. The rephasing gradient allows us to recover some of this lost signal. I go over this in depth during the gradient echo talk (part 1). Check that video out and it should make more sense 🙂
Had I known about you and your channel, Michael, I would've done the ARMRIT exam earlier. I need to imagine all these things in my head for me to learn it and you made it happen. I'm taking my exam in a few weeks. More power to your channel and God Speed! You and your team are truly gifted. Thank you for sharing this to us.
Stupid question, so to adjust the T2 contrast we adjust the TE time, does that mean we adjust when 180pulse is applied since it must be half way between 90 pulse and TE?
Thank you for these amazing videos. Could you please clarify to me the difference between receiver bandwidth and transmit bandwidth? Because I have read in some books that they have different action on the SNR. I'm confused
Pleasure! The term bandwidth means range of frequencies. Transmit bandwidth refers to the range of frequencies that make up the radiofrequency pulse - the larger the transmit bandwidth the thicker the slice selected and therefore higher signal to noise. Receiver bandwidth refers the the range of frequencies across the frequency encoding direction of a slice (it is completely separate process to transmit bandwidth). I cover this concept more in the bandwidth talk of this series - hopefully that video will help 👍🏼
So there will be no table movement in MRI like in CT as a whole...gradient coils will take up the work and plotting signal in K space and acquire the data. Am I right sir??
Good thought. There is table movement in MRI. In theory we could do all the slice selection with the gradient coil and RF pulses. However, the main magnetic field is the most homogenous in the centre of the scanner. Therefore, we will often move the patient (like we do in CT) to ensure the slice we are imaging is near the centre of the magnet.
Was out of town for a week so couldn’t film. Also making all these 3D models and animations from scratch. They will keep coming though, >120 videos filmed this year so far 🙂 hope you found it useful at least
This man deserves an award for how well he explains these topics and how helpful his videos are
exactly!
Thank you for the great video, it really helped me a lot, but one question, why there are no rephasing slice selection gradient when the 180 degree RF pulse is applied?
The 180 degree rf pulse is the rephasing mechanism.
I dont think i have ever commented on a youtube video in my life. But i have to express my thankfulness for your videos and how you always give a recap/refresher to remind us on previous topics. I have my registry soon and these videos have been helping tremendously and can't thank you enough!!!
Thank you for commenting on this one! So glad these videos have been helpful 😊
Wow! The amount of effort I could imagine making this video is commendable. This is a very precise topic. I am really amazed how you have put your explanations into animations. I am really thankful to you for making this series. I am watching this again and again to get my concepts clear. Thank you very much!!!
agree. Cant imagine how much time it takes to make the various animations in your videos. Thank you so much looking forward to the entire MRI series as a radiology resident in the US
Thank you so much! It means the world to me that these videos are helping 🙂 Definitely makes the effort worthwhile 🙏🏻
why do we not use a rephasing SSG for the 180 degree RF pulse and only for the initial 90 degree pulse?
Dude! Your program is 10x better than any book, or the MRI schooling I had 15 years ago. It's all due to the actual *moving* diagrams, as well as your brilliant explanations and analogies.
Just like some chrome extensions do (i.e. Adblock plus), you need to mention and post a link to submit donations, whilst keeping the actual videos free on RUclips. Unfortunately, I'm guessing that it might be a violation of RUclips's policies, but if not, you absolutely must do that. I'd give ABP donations occasionally over the years when they'd ask, and your material is much, much more deserving.
I wish I found your material earlier than several days before my exam. I couldn't find any decent RUclips pages on the subject before that. One, though explaining on a whiteboard well and myself thinking it was the best, didn't even get the basics of parallel vs anti-parallel correct. I just quit looking for alternatives after that.
Being that your material is much more recent and thus lesser known at the moment, I hope you pop up as the top search result as soon as possible to help others. Thanks.
TL;DR: YOUR MATERIAL MAKES MRI SCHOOLS OBSOLETE!!
The quality of these lectures are amazing. Every series on RUclips has pluses and minuses of course but this wins the “all around” prize.
How much time you have spent to make these incredible videos. I dont think you are doing this solely for the money. I cant believe how much satisfaction you are gaining by reading the lovely comments. I wish I had such brilliant ability to teach others. I wish you all the good luck in life Micheal. ❤
Hi Doc! Love your videos, you’re a fantastic teacher! @kayk928 asked this question and I had the same question. He said, “why do we not use a rephasing SSG for the 180 degree RF pulse and only for the initial 90 degree pulse?” Do you mind explaining please? Thank you so much in advance!
As he says, the rephasing gradient (SSG) is typically applied after the initial 90º RF pulse to correct dephasing caused by magnetic field inhomogeneities and ensure coherent transverse magnetization. BUT, this transverse magnetization decays faster than we woud like to (T2*, see T2* video), because fast decay means less time to play with the TE and TR (see T1, T2 and PD weighted imaging video), so the 180º pulse is generated to inverse this dephasing, gaining time, and have again net transversal magnetization signal sufficient to be measured. Now, regarding your question, the difference is that after the 180º pulse, no additional rephasing gradient is needed because the 180º pulse itself reverses the phase of the spins. When the 180ª RF is applied, the spins have the same dephase but inverted, (as the T2* video says, the faster spins are now behind, the slower are ahead, and they all eventually catch up and form the spin-echo signal). So here there is no need for a gradient to 'fix' with a rephasing gradient, since the inversion of the phase already makes them all catch up. In other words:
Schematic explanation
90º RF pulse --> Mxy occurs but spins within slice have dephase due to in-slice gradient --> rephasing gradient --> all spins are coherently to the same phase --> T2 relaxation is not homogeneous --> spins relax differently and start to dephase (again, for another reason) --> 180º RF pulse to invert this relaxation 'speeds' --> catch up and coherent signal (spin-echo). So the 180º RF already rephases (for another reason), so a rephasing gradient is not needed.
I had the same doubt, I hope it is correct and clear!
Good luck on your studies :)
Doc. How do you explain these things so well as tho you we present during the manufacturing of the MRI machine. You're so good wow
a MRI lover who loves to teach...thanks for great explanations ❣
You are so welcome!
Thank you so much for these series! It's helping me A LOT for teaching the MRI subject to my students. I recommend your vids to all my students and coworkers as well ❤
Regarding the rephasing gradient,
you, Dr, covered the concept elegantly! Only that the illustration displays a logical error since if we applied a rephasing gradient it should be only be limited to SSG range not to all B0.
If we applied rephasing gradient the same way displayed in your presentation (along entire B0), we will face a logical problem since it will cancel the linear variation, having all protons at same precession frequency again.
so I recommend editing the red line of rephasing gradient to only fit the SSG not the entire length of B0
What do you think?
Excellent work! You finally made physics easy to understand !
I'm so glad! Thank you!
Thanks a lot for the lecture, really helped refreshing my knowledge. One thing I find confusing is that your SSG-in-time graph implies that one would have the SSG turned on during the refocussing 180deg pulse. I can't seem to understand what the purpose is of using such configuration, given that the slice of interest was already selected after the 90deg pulse. I get the point of the 180deg pulse but I don't understand why we'd use a gradient here again, could you maybe elaborate on this? Respectully, Wouter.
Thank you for wonderful lecture..I have a one question ? First slice selection gradient are applied after 90 pulse or before 90 degree pulse
In correspondence with the 180-degree pulse, I see another gradient. Does this mean that simultaneously with the 180-degree pulse, a gradient equal to the initial one is applied?
Why there is no rephasing pulse after 180 degree pulse
Hi professor！Your video was truly fantastic; your explanation was very clear. I'm really grateful. I have a question I'd like to ask you: since in TE (echo time) the protons in the slice are precessing in phase, making it impossible to determine the exact location of the signal within the slice, why do we need the rephasing gradient step? Wouldn't it be possible to know where the signal is coming from at TE without rephasing? Is the purpose of rephasing to ensure that all the protons in the entire slice precess in phase? If so, what is the objective of doing this?
Hi! Great question. At the end of the RF pulse the protons are in phase, at TE the protons are rapidly dephasing due to T2* decay. They are also dephasing because the gradient we apply in the x direction (frequency encoding gradient) causes spins to precess at different frequencies. These different frequencies allow us to localise signal but we lose transverse signal even quicker as a result of the gradient field. The rephasing gradient allows us to recover some of this lost signal. I go over this in depth during the gradient echo talk (part 1). Check that video out and it should make more sense 🙂
Thank you so much! I'll check out the gradient echo video. Again, I love your tutorial videos! I really appreciate your efforts@@radiologytutorials
Lucky to be ur first viewer!
I’m the lucky one ☝🏼 thanks for watching!
Had I known about you and your channel, Michael, I would've done the ARMRIT exam earlier. I need to imagine all these things in my head for me to learn it and you made it happen. I'm taking my exam in a few weeks. More power to your channel and God Speed! You and your team are truly gifted. Thank you for sharing this to us.
I'm so glad the videos have been helpful! Best of luck with your exam. Please do let me know how it goes. Really appreciate your kind words 😊
my hero
You have done for Radiology Physics what 3Blue1Brown did for Calculus! Hats off!!!!
Stupid question, so to adjust the T2 contrast we adjust the TE time, does that mean we adjust when 180pulse is applied since it must be half way between 90 pulse and TE?
Great question. Exactly, if it's a spin echo sequence the 180 degree pulse must be moved in proportion to the change in TE.
@@radiologytutorials thank you so much!
Thank you sir. Your concepts are on point. It’ll be better if you can release more videos as we have our exams on 15th of july.
Who is this guy? I can’t believe I’m not only understanding but even enjoying this!
❤❤
❤❤
❤️❤️
Thank you for these amazing videos. Could you please clarify to me the difference between receiver bandwidth and transmit bandwidth? Because I have read in some books that they have different action on the SNR. I'm confused
Pleasure! The term bandwidth means range of frequencies. Transmit bandwidth refers to the range of frequencies that make up the radiofrequency pulse - the larger the transmit bandwidth the thicker the slice selected and therefore higher signal to noise.
Receiver bandwidth refers the the range of frequencies across the frequency encoding direction of a slice (it is completely separate process to transmit bandwidth). I cover this concept more in the bandwidth talk of this series - hopefully that video will help 👍🏼
Thank you.
Wow 🤯 thank you! That’s very kind. Made my week 🙏🏻
Thank you for such tremendous effort. The way of teaching is pretty clear. i have recommended your channel to my friend. (MRI Engineer)
Awesome! Thank you so much for spreading the word 🥳
amazing video, mock exam in 2 weeks.. wont be able to finish revising by reading the books. This helps a lot. Thank youuu!!
Good luck with the studying Audrey! you've got this 🙌
So there will be no table movement in MRI like in CT as a whole...gradient coils will take up the work and plotting signal in K space and acquire the data. Am I right sir??
Good thought. There is table movement in MRI. In theory we could do all the slice selection with the gradient coil and RF pulses. However, the main magnetic field is the most homogenous in the centre of the scanner. Therefore, we will often move the patient (like we do in CT) to ensure the slice we are imaging is near the centre of the magnet.
@@radiologytutorials okay sir...Thank you for your kind reply ☺
Any time. Feel free to ask anything 🙂
Waited alot
Was out of town for a week so couldn’t film. Also making all these 3D models and animations from scratch. They will keep coming though, >120 videos filmed this year so far 🙂 hope you found it useful at least
i don't usually write comments, but this time i have to. you are great! this is so helpful. so thank you!
Wow, thank you. I’m glad it was helpful 🙂
Thank you so much !
Thanks a lot sir Michael, 🎉very much excited for this new video
Hope you found it useful! Thanks Fazal
Thanks a lot sir for conceptual understanding🙏🙏
Always a pleasure Sohail. Thank you!
Thanks Doc
Feels like my tuition fee should hand it to you instead of my uni
😂 love this!
Thank you!
This is wonderful explanation
Thank you. Really appreciate it 🙂
These are all just absolutely fantastic.
Thank you Dalton!
❤❤
Wonderfull, thank you!!
My pleasure! 🙂
Thank you doctor 😘
You're welcome 😊
Thanks alot
Pleasure Merlo 🙂