Previously, we have developed semi-selective adiabatic pulses for 1H spectroscopy to uniformly suppress the intense water signal when using a transmitter coil with a inhomogeneous B1 (e.g., a surface coil). This pulse, known as solvent suppressive adiabatic pulse (SSAP), is an adiabatic analog of the well known jump-return sequence which is based on conventional (constant frequency) pulses. An undesirable feature of jump-return, SSAP, and other related binomial pulses is the lack of sharp borders in the frequency response profile. In addition, these pulses are unusable in some preferred sequences for localized spectroscopy such as STEAM and PRESS. In these latter sequences, the water signal is usually suppressed by applying frequency-selective 90~ pulses and then spoiling the magnetization with B0 gradients (a method known as CHESS). CHESS can achieve adequate suppression when using a relatively uniform B1, but without a frequency-selective adiabatic 90l pulse, the performance of CHESS is limited by B1 inhomogeneity. As mentioned above, frequency-selective 180l adiabatic pulses are available, but such pulses are not useful for water suppression in methods such as CHESS which require selective 90l pulse to rotate the magnetization into the transverse plane. This past year, however, we developed a new water suppression method called MEGA which employs selective 180l pulses centered about asymmetric B0 gradient pulses used to dephase the water echoes. The MEGA sequence element can be placed in any pulse sequence which contains a refocusing period (i.e., [-t-180~-t-]). So far, we have implemented MEGA in STEAM and PRESS, and experimental results obtained on phantoms and human brain demonstrate that MEGA can achieve significantly better suppression than CHESS methods. Measurements performed on dilute protein samples using a high resolution spectrometer (500 MHz) demonstrate that MEGA's utility will extend outside the field of in vivo NMR.
Showing the most recent 10 out of 493 publications
