I have a couple of questions regarding MRI theory; I have read around but am struggling to find and answer and move on in my understanding; as such any help would be much appreciated:
When designing a pulse sequence for T1 contrasts, the TR is chosen to permit signal detection near the point of maximum difference in longitudinal relaxation between the protons in the molecules of interest. However, as I understand it the TR determines the time between RF pulses, not the time between RF pulse and data acquisition (see attached image). Is it not key to measure the magnetization at the TR point before applying the RF pulse i.e. why is the DAQ in the attached image not immediately preceding the second 90 degree pulse?
In a similar vein, in such a pulse sequence where TR < T1, when the second RF pulse is applied will the pulse not lead the net magnetization vector to overshoot the intended 90 degrees to B0?
Question regarding k-space: In my (incorrect) understanding of MRI acquisition, the various field gradients are used to select and determine signal from defined regions in space. First slice selection occurs using Gz. Then a phase encoding gradient Gy selects a 'row' from which a frequency encoding gradient Gx permits localization of the measured signal. However, this conceptualization is clearly wrong as the data is collected in k-space, so the Gy/Gz gradients are used to collect data regarding spatial frequencies. But then how is the signal localized? In a very rudimentary example, say a slice is selected using Gz, the first row ('Y1') in k-space selected with the phase encoded gradient Gy and each column along Y1 filled with signal detected during the application of the frequency encoding gradient Gx. In my understanding, this means only high-frequency data (top row of k-space) from a single row of the XY plane in the slice of interest is collected. As k-space is filled, changes in Gy field strength also moves spatially through the Y axis of the slice; this would mean only partial spatial frequency data is collected for each spatial location in the slice of interest. Clearly my understanding is wrong somewhere...
Thanks a lot for your help.