the whole repeatation of the RF- Grad(phase)---TE sequence is done only to derive the rate of the phase change from the isocenter and when we get that information about the phase change from the isocenter the fourier transformation redistribute the each individual signal according to the phase change having from the isocenter. The individual signal will have different amplitude which will detect the intensity of the image(not the K space which is time zone as k space is different than actual image.....K space is the acquired data over time and the image is derived from it). The individual signal with the rate of phase change will have different amplitude and can be brighter in the periphery of the image or lower in the center of the image.
For 3d FT, wouldn’t the bandwidth of RF for the repeat RF(identical to the slice selection RF) need to change because range of frequencies divided by change in distance is the slope of the gradient? If you keep the same RF bandwidth but your goal was to change the slope, then the distance sampled in the direction of the slice sampling would also need to change accordingly? Thanks
When it is scanned a 2nd time, was the gradient magnetic field on for twice the (or extra) time, or twice the magnitude? I'm going to assume it is twice the time, because according to a previous class, for twice the magnitude, the thickness will change with the same RF frequency tolerance. Did I understand this correctly? Thanks!
This is making sense to me, so by leaving Gsi on longer in each scan, you are essentially allowing the precessions at each Z location within the slice thickness to go more "out of phase", which is captured in the signal amplitude. This "out of phase" difference is later revealed through FT.
Excellent course and lectures! I have a question. Once you enact the phase encoding gradient and then sample at TE with the frequency encoding gradient does not the sample/slice/slab have discreet spin identifiers at each identifiable location along each column and row? IE: Row 1 has a specific phase, but frequency identification changes along the row. Row 2 would have a different phase identity than row 1 (or any other row for that matter) and the frequency identification changes along the row. This would be the same for the entire sample. My question is why do we need to repeat the phase encoding process when it seems as though enough information is present after doing it one time. Thanks for your time and excellent teaching skills Dr. Lipton!
+Curt Sagraves - Dr. Lipton responds: Thanks for your question. This is something many people struggle with. First, your premises are correct; the reason that a single application of the phase encode gradient is not sufficient is NOT because phase does not vary across this dimension each time we apply the gradient. It is because we need a way to detect the phase difference. This is done with the Fourier transform, which requires multiple samples of the MR signal that differ in the way the signal is encoded along the dimension we are trying to discriminate.
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One of the best MRI physics lecture series. Thank you a lot Dr Lipton.
Thank you very much Dr. Lipton! This is the best lectures on MRI out there.
I nearly done half of course. let s say MRİ youtube course. Perfect. Thanks for Dr.Michael Lipton and also who recorded it.
the whole repeatation of the RF- Grad(phase)---TE sequence is done only to derive the rate of the phase change from the isocenter and when we get that information about the phase change from the isocenter the fourier transformation redistribute the each individual signal according to the phase change having from the isocenter. The individual signal will have different amplitude which will detect the intensity of the image(not the K space which is time zone as k space is different than actual image.....K space is the acquired data over time and the image is derived from it). The individual signal with the rate of phase change will have different amplitude and can be brighter in the periphery of the image or lower in the center of the image.
For 3d FT, wouldn’t the bandwidth of RF for the repeat RF(identical to the slice selection RF) need to change because range of frequencies divided by change in distance is the slope of the gradient? If you keep the same RF bandwidth but your goal was to change the slope, then the distance sampled in the direction of the slice sampling would also need to change accordingly? Thanks
When it is scanned a 2nd time, was the gradient magnetic field on for twice the (or extra) time, or twice the magnitude? I'm going to assume it is twice the time, because according to a previous class, for twice the magnitude, the thickness will change with the same RF frequency tolerance. Did I understand this correctly? Thanks!
This is making sense to me, so by leaving Gsi on longer in each scan, you are essentially allowing the precessions at each Z location within the slice thickness to go more "out of phase", which is captured in the signal amplitude. This "out of phase" difference is later revealed through FT.
Excellent course and lectures! I have a question. Once you enact the phase encoding gradient and then sample at TE with the frequency encoding gradient does not the sample/slice/slab have discreet spin identifiers at each identifiable location along each column and row? IE: Row 1 has a specific phase, but frequency identification changes along the row. Row 2 would have a different phase identity than row 1 (or any other row for that matter) and the frequency identification changes along the row. This would be the same for the entire sample. My question is why do we need to repeat the phase encoding process when it seems as though enough information is present after doing it one time. Thanks for your time and excellent teaching skills Dr. Lipton!
+Curt Sagraves - Dr. Lipton responds:
Thanks for your question. This is something many people struggle with. First, your premises are correct; the reason that a single application of the phase encode gradient is not sufficient is NOT because phase does not vary across this dimension each time we apply the gradient. It is because we need a way to detect the phase difference. This is done with the Fourier transform, which requires multiple samples of the MR signal that differ in the way the signal is encoded along the dimension we are trying to discriminate.
very helpful! thanks