Phase vs freq encoding

[univox_high_flyer]

Hi,

I was wondering why phase encoding requires many gradient strengths while frequency encoding only one. Why not the other way round?
Can't a single phase encoding gradient provide a unique phase for all the spins in the sample?
Not sure if I'm missing something obvious here.

Thanks!

[AndrewBworth]

Yes it can. I've been thinking about this a bit, but I haven't sat down to do the math, there is probably a good reason for it. But if anyone would like to go ahead here it is:

To assign isotropic voxels a unique frequency in one shot, your gradient strengths would be, in whatever order makes the most sense - Gz:Gy:Gx = FOVz:FOVy:FOVx. Now the resolution of your Fourier transform is going to be 1/N*dt or 1/T one over total sampling time. Strongest gradient I have ever heard of is 300 mT/m, I think that is the record. Lastly your signal strength due to gradient dephasing is going to be I think
sinc(gammaH*Gz*FOVz/2) * t)sinc(gammaH*Gy*FOVy/2 * t)*sinc(gammaH*Gx*FOVx/2 * t).
Keep in mind T2 and T2* and see what is feasible.

(11-04-2015 07:32 AM)univox_high_flyer Wrote:  Hi,

I was wondering why phase encoding requires many gradient strengths while frequency encoding only one. Why not the other way round?
Can't a single phase encoding gradient provide a unique phase for all the spins in the sample?
Not sure if I'm missing something obvious here.

Thanks!

[AndrewBworth]

Woops I mean your gradient strength will have to be Gz:Gy:Gx = Fovz*Fovy*Fovx : Fovy*Fovx : Fovx

[AndrewBworth]

(11-04-2015 07:32 AM)univox_high_flyer Wrote:  Hi,

I was wondering why phase encoding requires many gradient strengths while frequency encoding only one. Why not the other way round?
Can't a single phase encoding gradient provide a unique phase for all the spins in the sample?
Not sure if I'm missing something obvious here.

Thanks!

Oh geez, sorry. Yes, in THEORY one gradient set can provide unique frequencies to every voxel. But unfortunately, all the different frequencies rapidly add up to zero (Dephasing), so you can't acquire this signal for very long. However - this is a problem worth thinking hard about and looking into - It will teach you a lot about MRI.

[Elad P]

RE: Phase vs freq encoding

I think I saw an official question/answer in MRIQuestions exactly to what you are asking.
My understand is that it is true that if you only use one gradient for phase and one gradient for frequency each voxel will have a unique freq,phase pair "id". But you can't extract from the data the phase for a specific voxel (you get a sum of all phases).

Look up the question.

[Carenasage]

Are the phase encoding strengths related to the football shape or rectangle ? Does the lines that are thicker or thinner define it ?