MRI signal is the xy component of the magnetization. The magnitude is the length of the xy component, while the phase is the direction of the xy component. Let's take gray matter and CSF as an example. When you apply an excitation RF and wait the same amount of time (TE) to acquire the signals, CSF will have a higher signal magnitude than GM because of slower T2 relaxation. However, the directions of the magnetization (phase) of GM and CSF should be the same because the spins were tipped to the same direction by the same RF and have the same angular velocity due to same B0. So, ideally, you should observe intensity variation in magnitude but not in phase. However, there are always imperfections, including B0 offset, inaccuracy of readout time, susceptibility, etc..., which cause the intensity variation in phase. Some MR technologies create intensity variation in phase on purpose, such as phase contrast MRA which allows flowing spins accumulate phases by applying a bipolar gradients along the direction of the flow. This is an interesting topic, and hope my response is helpful for you.
@@chenyangzhao247 thank you for your answer. I have many questions in image reconstruction that well be very nice if u can answer me :) 1- why an FFT shift needs to be carried out on k-space before and after the iFFT is applied. What is the purpose of the FFT shift? 2- Where are low-frequency components located in k-space? 3-What happens if you only apply the FFT shift before, but not after performing the iFFT on the k-space
How? Like …. How!! Even if you programmed a custom pulse, how did you adjust the tempo and the rhythm of the melody!! It’s so creative and cool as hell.
MRI is a perfect instrument bcz the timing is accurate to microseconds, you just need break the norm a little for programming custom sequence (e.g. not fixed TR)
Really nice presentation! Thank you a lot especially for the good examples and visalizations. I found it very helpful to see what happens in the spatial space when the k-space is blacked out at different degrees! All the best
Best of luck to you guys, hopefully ur lab reopens soon! Is it bad for an mri to sit that long and not be used? Like does the helium boil off faster or slower when it’s not in use vs when it’s used every day?
