MRI Frequency Encoding EXPLAINED | MRI Physics Course Lecture 3

[@nofarogdan750]

This is pure gold for medical eng students! thank you!

[@MRIPhysicsEXPLAINED]

Glad it helped and great to hear from my engineering comrades!

[@jasoncai4678]

Looking forward to the next lecture!

[@michaelwinkler8047]

Great work!

[@MRIPhysicsEXPLAINED]

Thank you! Cheers!

[@BrianStJohns]

Lights on is right. For the first time, I feel like I’m actually getting this. I’m going to rewatch all of these several times and take lots of notes.

[@MRIPhysicsEXPLAINED]

Glad they're helping and thanks for watching!

[@yourcuteguy9737]

Really appreciate your awesome video

[@menkiguo7805]

This explains things so well

[@MRIPhysicsEXPLAINED]

Thanks for watching and supporting!

[@Adan_Playz21]

Anxiously waiting sir for your next lecture ( phase encoding ) ......

[@MRIPhysicsEXPLAINED]

Came out yesterday, check it out here! kzbin.info/www/bejne/pHenq4usn6tqnck

[@TAHA-TOUABA-RADIOGRAPHER]

Thank you very much pro

[@philipd.7286]

Okay so question. If we do frequency encoding with a gradient like this. When we go to measure our FT’es signals like at 7:36. We are looking for a difference in amplitude as our contrast. Ideally this difference in amplitude comes from T1 or T2 depending on what sequence we have set up. But here the difference in amplitude is also coming from the fact that we are measuring waves with two different frequencies at the same time point. I.E at 7:36 we are measuring the peak of Fat but not the peak of CSF. So our contrast is slightly influenced by how strong our Gx gradient is.

[@philipd.7286]

Do we just assume the frequency changes induced by our Gx field are small enough to ignore their effects on Amplitude? Is there another way to frequency encode?

[@MRIPhysicsEXPLAINED]

Thanks for the comment and love to see the critical thinking here. I think what you are honing in on is that there is a relationship between frequency and phase, and that this will affect our contrast. You are absolutely right, but it's a little more complicated. The theory unpinning these initial lectures is designed to understand the commonly taught concepts of frequency encoding and phase encoding that you would find in an MRI physics course for medical professionals, and then tie these concepts into how we build an image and image contrast while avoiding the high level math needed to really understand the true image building process (we will cover this in the future)! Highly recommend checking out the next few lectures in the series on phase encoding (kzbin.info/www/bejne/pHenq4usn6tqnck), T2 contrast (kzbin.info/www/bejne/Z53Nq3-gpsyCfcU), T1 contrast (kzbin.info/www/bejne/bqu3dYmOpJxjpc0), and tying it together (kzbin.info/www/bejne/pKm0eHWPi6-oo6c) to get a better grasp of these and will probably answer more of your questions along the way!

[@philipd.7286]

@@MRIPhysicsEXPLAINEDthank you :) They’re next on the list!

[@aurelberger6847]

Thanks for the video, really helpful! But why is B0 in the x-direction at 2:30, shouldn't it be in the z-direction? And why can we make a gradient in B0 to alter the individual larmor frequency of each voxel by applying an additional field in the x-direction (the 2 fields, B0 and additional field, are then orthogonal to each other)?

[@MRIPhysicsEXPLAINED]

Hello and thanks for watching! Great pickup, I think the arrow head on the B0 line is misleading and as you pointed out, suggests that's the direction of the B0 field but what that line is really representing is the fact the B0 field is constant across a slice in the XY plane. Since we're looking directly in the XY plane or down the barrel of the MRI machine, the direction of the B0 field will be along the Z-axis, which is a little hard to draw in 2D so disregard the arrow head in the figure. Hopefully this now answers your second question, where you can see we are adding a gradient field in the X-direction to the constant B0 field across the slice, giving us gradient field with some base value of B0 added everywhere.

[@hangli1622]

@@MRIPhysicsEXPLAINED so the x direction magnetic gradient field actually has its direction parallel to z axis but the magnitude varies along x axis?

[@MRIPhysicsEXPLAINED]

@@hangli1622 x direction field is orthogonal* to the z-axis with magnitude varying online when the gradient coils are turned on

[@hangli1622]

@@MRIPhysicsEXPLAINED if x direction field is orthogonal* to the z-axis, the overall magnetic field is the vector sum of x direction field and z direction field, which is not parallel to the B0(z direction)?

[@MRIPhysicsEXPLAINED]

@@hangli1622 I would think of it less as vector summation and more in terms of scalar quantities. The Larmor frequency is a scalar value, dependent on the magnitude of what magnetic fields the protons see, in our case the magnitude of the B0 field + the magnitude of the gradient field at point X in the slice.

[@AmitJoshi-fu4ir]

Add Fourier transform and K space lecture.

[@MRIPhysicsEXPLAINED]

Right around the corner, stay tuned!

[@eilonwyliao3237]

❤❤❤❤❤❤ very helpful 🎉🎉🎉🎉🎉

[@MRIPhysicsEXPLAINED]

Thanks for the love!

[@EriqHover]

niceee

[@MRIPhysicsEXPLAINED]

Thanks for watching!

[@NinjaSheepa]

nice thanks!

[@MRIPhysicsEXPLAINED]

Thanks for watching and commenting, phase encoding coming soon!

[@karthikraju6286]

Why larmor frequency for csf and fat is different 5:50

[@MRIPhysicsEXPLAINED]

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!

[@javadmahdavi1151]

so, it's clear to me frequency encoding. but what if we have a high-frequency signal on the left side of the image? consider that we have a lower gradient magnetic field on the left side of the image...

[@MRIPhysicsEXPLAINED]

Hmm not sure if I'm following the question... Remember we control what frequencies we will have during signal readout by applying manipulating the magnetic field and thus Larmor frequency with these gradient magnetic fields. So in an idealized situation without any magnetic field inhomogeneities, there shouldn't be a high frequency signal coming from the left side of the image if our lower gradient field is applied there!

[@jaelowkey6113]

You saved me from the fear of my MRI finals😭🙏🏻

[@MRIPhysicsEXPLAINED]

Happy to hear you found these useful!

[@hangli1622]

at 2:20, does the arrow of B0 indicates the direction of primary magnitude field? should it be perpendicular to xy plane?

[@MRIPhysicsEXPLAINED]

No sorry, it should be a simple straight line across the slice at 02:20 without the arrow head. The slice is orthogonal to the primary B0 field and sees a constant B0 value across the slice. But when we apply a gradient field across the slice, it will now vary linearly as shown at 05:48, where the arrowhead is meant to show this field changing from a low to high value moving left to right across the slice, not the direction of the main B0 field. In retrospect this was a poor choice of illustration that has caused a lot of confusion!

[@bromar2014]

Does slice select gradient ONLY happen along the Z axis? And signal frequency encoding only happen on X axis?

[@MRIPhysicsEXPLAINED]

While the slice select gradient only happens along the Z-axis, the frequency encoding and phase encoding gradients are interchangeable and either can be applied on the X or Y axes. Check out the next lecture on phase encoding for more clarification on this: kzbin.info/www/bejne/pHenq4usn6tqnck