Two pulse sequences for NMR imaging
This thesis contains the study of two pulse sequences for NMR imaging. The first sequence is a spin echo based Driven Equilibrium Fourier Transform (DEFT) sequence. In its theoretical form the pulse sequence consists of a -90oX+-180oY+-spin echo- 180oY+ -90oX- RF-pulse train where the latter 90 degree pulse appears at the time of a second spin echo. The special feature of the sequence is the return back to equilibrium of the transverse component of magnetisation by means of the second 90 degree pulse prior to the repetition of the sequence. For relatively short sequence repetition times (TR = 200-400ms) this results in a spin echo signal not only being stronger in signal strength compared to a standard CPMG pulse sequence, but also containing a mixture of T1 and T2 information. A practical DEFT sequence has, however, failed to appear as a commercially available pulse sequence. Finding the steady state solution for the sequence using the Bloch equations reveals one explanation for the lack of commercial interest. The sequence steady state NMR signal becomes proportional to 1/(1 + T1/T2). The fraction T1 over T2 varies little for healthy tissue and the sequence therefore promises poor image contrast. However, tissue pathology alone or tissue pathology containing a contrast agent can cause large changes in T1 or T2 and it has therefore been one of the prime goals for the thesis to produce images of pathological conditions using the DEFT sequence to reveal some of the sequence clinical potential.