top of page

FREQUENTLY ASKED QUESTIONS

Frequently Asked Questions: Text

QUESTION #1

In every (or most) other books, there's a sentence "lower-energy state is slightly favored, so that there are more protons spinning parallel than anti-parallel" - why is it different in this video (more anti-parallel than parallel)?

QUESTION #2

Specifically, in the context of GRE EPI sequences in fMRI I have often wondered about the following: If EPI images can be acquired in as little as 2 ms (as you mention in the course), then why do we do whole brain imaging at TRs of around 2 seconds? In other words, why is not possible to acquire a whole brain with 30 slices in 60 ms using EPI? Given these 2 s TRs, it seems that we need to wait around 50 ms after the acquisition of each slice before we can acquire the next slice. I am not sure why we need to do this. Is it just to make sure that the signal is completely gone before we initiate the next RF pulse and measure the signal from the next slice? So the delay is there to prevent mixing signals due to the t2-star decay of the signal?  

QUESTION #3

When discussing gradients inherent in the system, I know along the z-axis the force is moving in opposite directions having 0 impact on isocenter. A question is posed about the other gradients. Am I right in thinking the gradients along the x- and y- axis are causing  a force in opposite directions as well, with the same result that isocenter is not affected?

QUESTION #4

Looking at several sources, it seems T2 is the time it takes to drop down to 37% (which means losing 63%) of what your initial magnetization was. The video says we lose 37% (which would imply we have 63% of our initial leftover). Also, just for completeness, an earlier video in this series states the net magnetization vector (NMV) of spins due to external magnetic field (B0) is in the anti-parallel direction. Every other source I've looked at has said the NMV is actually in the parallel direction. 

QUESTION #5

Do the non-Z direction gradient magnetic fields alter the orientation of B0, even to a tiny extent?

QUESTION #6

How are the k-space data (real part, imaginary part, amplitude, frequency and phase) related to the RF signal (amplitude, frequency and phase) response coming out of the tissues?   

QUESTION #7

I came across a practice question that said that decreasing the receiver bandwidth increases imaging time (in a similar way NEX, TR and # of phase encoding steps does; it was not referring to sampling time). Is this true and if so can you please explain it?  

QUESTION #8

In the course, you say that there are slightly more spins antiparallel to the static magnetic field when there is complete longitudinal relaxation. I have seen elsewhere that it is the parallel orientation that has the lower energy state and is therefore the orientation of the few extra spins. Can you please clarify?

QUESTION #9

My impression is that transverse precession causes signals in coils, and those signals somehow get translated to k-space. Is there a dataset out there of raw coil readings?

QUESTION #10

When you give a 90-degree pulse, wouldn’t both longitudinal vectors come to 0?

QUESTION #11

In regards to in-phase and out-phase imaging: with this 180 RF pulse, will water and fat proton transverse magnetization rephrase instead? Will Dixon’s Method be used only in multiple gradient echo (MGE) imaging?

QUESTION #12

Let's assume we are acquiring images for, let's say, a head and we start a multi-spin echo sequence to acquire a T2 contrast weighted series of images with 10 TE's within each TR. After the sequence is finished we may end up with 12 axial images of the brain generated by that particular sequence. All of the images look the same, as in each one looks to be similarly T2 weighted. Each TE within the TR of that particular sequence acquires information with different contrast as answered above. I suppose the answer may be that the 10 TE's are just averaged out each time for each slice. Is this the right way of understanding this? So we are, indeed, limited on how many TE's we can acquire within a TR, not only because of time, but because of the average weighting we are trying to acquire, right? Too many TE's on either side of a point may dilute and/or overpower a particular optimum T2 contrast weight? I hope I'm at least slightly coherent with my thoughts on this. I suppose this kind of thought process would apply to any (or most) multi- TE sequence, yes?    

QUESTION #13

Once you enact the phase encoding gradient and then sample at TE with the frequency encoding gradient, does the sample/slice/slab have discrete spin identifiers at each identifiable location along each column and row? I.E.: Row 1 has a specific phase, but frequency identification changes along the row. Row 2 would have 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. Why do we need to repeat the phase encoding process when it seems as though enough information is present after doing it one time?

QUESTION #14

Explain the color coding on the type of fMRI Activation images that you showed as the visual cortex example.

QUESTION #15

In context of the inversion recovery section, you talked about the timing of the RF pulse application. My question from the graph shown below (similar to one you had drawn during discussion) is:  In terms of signal intensity, what would the points/tissues 1,2 and 3 look like in a grayscale image? If 2 looks completely black and the 1 shows a decent longitudinal recovery and eventually a greater transverse magnetization after the flip, I was not sure what 3 would look like when apparently it has the same magnitude as 1 but in the opposite direction!

QUESTION #16

What kind of changes can we expect in an MRI image if we replace nonmagnetic capacitors on surface coil with magnetic capacitors?

QUESTION #17

I've read there are multiple k-spaces for each series, meaning that if you're scanning 30 slices you have 30 k-spaces, but my colleague said all the information from a sequence goes into only one K-space

Frequently Asked Questions: FAQ
bottom of page