RF Transmit Path

[aelster]

I think the answers to your first two questions are already fairly well explained on the following page:
http://mriquestions.com/lp-vs-cp-quadrature.html

Your third question (conversion of RF channel voltages to gray scale) is not so simple. Basically, the voltages recorded represent the MR signal (plus noise) from the entire image which is modulated on top of a carrier wave at the Larmor frequency. This must be "decoded" through Fourier transformation and other methods to sort out which signal is coming from where in the body. The final magnitude of this processed data is typically displayed on a pixel by pixel basis in gray scale from black (low signal) to white (high signal).