MRI: How is the magnetic field gradient used to achieve frequency encoding of the echo?

Q1: Answer the following questions (Approximately half a page) a. What is a magnetic field gradient? b. How is the magnetic field gradient used to achieve frequency encoding of the echo? Q2: Answer the following questions (Approximately half a page) a. Describe the Nyquist theorem b. Calculate the dwell time for a bandwidth of 180,000Hz (+/- 90,000 Hz) c. Calculate the acquisition time of an echo using the dwell time from above for an image matrix of 200×128. d. Calculate the gradient strength (mT/m) required at for this experiment if the FOV = 25cm. (Hint: ?/2? = 42.57 MHz/T) Q3:Answer the following questions (Approximately 3 pages including pulse sequence diagram) a. Draw a pulse program diagram for a basic spin echo pulse sequence, showing all RF pulses, gradients, acquisition of the echo, TR and TE. Do not copy and paste a picture from the web or a book. b. Describe how images with proton density, T1 and T2 contrast are acquired with spin echo pulse sequences with respect to TE and TR. c. Give two examples of how we can make better use of the TR time period with the spin echo pulse sequence. Briefly explain each. Q4:Answer the following questions (Approximately 2 pages including pulse sequence diagram) a. Draw a pulse diagram for a gradient echo pulse sequence showing all RF pulses, gradients, acquisition of the echo, TR and TE. Do not copy and paste a picture from the web or a book. b. Describe how proton density (PD), T1, T2* and T1+T2 contrast are acquired with a gradient echo based imaging sequence in terms of a-pulse, TE, TR and any other additional gradients.