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These are good questions, and as always with MRI physics there are layers of ever finer details you will find the more specific you try to understand it. When we talk about precession in MRI physics, what we're really referring to is the precession of the net magnetic vector. Yes, the individual protons precess about their axis when put into a magnetic field. You don't even need an MRI machine, we're in a magnetic field right now on Earth. But does this generate signal in our MRI machine? So we need to get these individual precessing protons aligned, and with enough energy from our magnetic field, we can cause more to align with the field than the random orientations caused by the background kinetic energy. This is what builds our net magnetic vector, and it doesn't necessarily mean all the individual protons are in phase with each other, but there is a net magnetic vector due to their alignment. The RF pulse then tips all of these aligned protons off the B0 axis, and they precess together at the larmor frequency, generating our signal. It is the dephasing of the orientation that causes our signal loss. Check out this article for more information: www.mriquestions.com/how-does-b1-tip-m.html

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I’m just saying this as a fan so that you can improve your channel

Ha I will be the first to admit my animations are rudimentary and not the best, and this is a very difficult subject so not quite sure how to present this in a different way but the key point is if we're able to invert the spins with a strong enough RF pulse, the receiver coil will see a reversal of the spins that causes rephasing of our spins and signal. The screw example at the end is the most tangible example of this I could think of. This is just to get a grasp on what's going on before proceeding to the pulse sequences, starting with the spin-echo sequence ruclips.net/video/vK6PeCPpOLY/видео.html where we flush out this idea out a little more so maybe check out that video and see if it helps further clear up these concepts.

This is because we have changed the magnetic field across the x-axis so that the CSF voxel see a different field strength than the fat voxel, and remember that the Larmor frequency is directly related to the magnetic field strength for each voxel. Hope this helps!

Glad it was helpful!

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Hope it helped and thanks for commenting!

Thank you! Cheers!

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Thank you!

Thanks for commenting!

Such a nice comment, thanks so much and glad you are finding the videos helpful!

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The frequency localizing gradient is not a shift but an encoding of different frequencies along the x-axis. Since we create this gradient magnetic field, we know which frequencies should be where spatially along the frequency encoding axis, so we when we break our raw signal down into frequencies via the Fourier Transform, we get this information. Check out the dedicated frequency encoding lecture here for further info if you haven't already: ruclips.net/video/DYj1SLNppQM/видео.html

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Right around the corner, stay tuned!

Glad to hear it was helpful and thanks for commenting!

😊

Hello, thanks for commenting and great question! While the individual protons will have some baseline spin like all atoms and particles, they are not precessing all together as one. We need to impart them with some angular energy with our RF pulse to get them precessing as one unit together. Hope this helps!

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Me too 😂

😂 It's been a lingering around in the back of my mind somewhere, but first have to wrap up this diffusion weighted imaging series! Next up, DTI

😂

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Will definitely put it on my radar, thanks for watching and commenting! Nearing the final stages of the next lecture on DTI, should be a good one so stay tuned!

Thank you! Hopefully can bring a little entertainment to an otherwise dry subject!

In general, the chosen TR time will affect T1 image contrast, and the chosen TE time will affect T2 image contrast and these values will remain constant for that sequence. For most sequences, these parameters are all ready chosen and optimized so the technologist simply picks the appropriate "protocol" to run. However, you can certainly manually go in and change the values of TE and TR, albeit this will change the contrast of the picture and you'll likely get an angry call from your local radiologist :)

My pleasure!

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Thank you!

You got it my friend! With those math skills you might want to get into this teaching hobby as well!

Wow thank you so much!! Your support really means a lot, working on the next lecture now as we speak so stay tuned!

Thanks for commenting! I actually did a deep dive on this 63% and did a little segment on it in the Q&A video here, check if out if you'd like to learn more ruclips.net/video/3Zjm1wuFo6M/видео.html

@@MRIPhysicsEXPLAINED Neat!

Awesome to hear it helped, thanks for commenting!

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More are in the works!

Glad it helps and thanks for the comment!

Thanks for the comment. This (should have been) already corrected with a caption that pops up during this segment saying the band is analog TV and into the FM range.

Thank you! More videos are in the works!

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Thank you!

Hello, check out the lectures on slice selection ruclips.net/video/v8jW8K1y-KE/видео.html and frequency encoding ruclips.net/video/DYj1SLNppQM/видео.html where we go in-depth on how to localize the signal in the Z and X dimensions in you haven't already. If you still have questions let me know!

@@MRIPhysicsEXPLAINED I'm confused about why we don't get the same problem at 7:55 in the Z-X plane. I can see that using frequency gradient in the X and Y axes causes the same frequencies to repeat and thus we can't resolve along (x+y=c). But why do we not get the same problem when using frequency gradient in both the Z and X axes? How are we still able to resolve voxels along those diagonals?

The beautiful thing about the slice select gradient technique is that we don't have to resolve the signal in the z direction, the gradient is turned on along the z-axis so that when we apply our RF pulse and tune it to the region of the body of interest along the z-axis, we know all the signal we receive is coming from that slice alone. We simply just then turn off the z-axis gradient, the protons in the slice fall back into precession at the Larmor frequency, and the problem then turns simply into localization along the x and y axes for which we do the frequency encoding and phase encoding techniques. Now in 3D MRI acquisition we do replace this slice select gradient technique with a true image encoding gradient along the z-axis, but that will require a little extra knowledge to understand which we'll cover in future lectures 🙂

Thank you for the comment and for keeping these amazing machines running! We've got a whole set of advanced lectures coming that will get into some engineering topics so stayed tuned!

Hello, thank you for sharing!

Much thanks!! Check out the channel this weekend for a NEW lecture!

That is a good question! "The echo-planar imaging (EPI) technique, proposed by Mansfield, overcomes this limitation by determining a complete two-dimensional image matrix in a single experiment. As the name implies, the method uses signal echoes to simultaneously acquire signal from the whole two-dimensional (planar) imaging slice."

@@MRIPhysicsEXPLAINED Thank you very much!