Multi‐frequency improved constant amplitude pulses for broadband inversion

Abstract

Optimization of constant amplitude broadband inversion pulses for maximum inversion over a range of multiple fields yields more regular pulses with better tolerance to B(1) errors than those optimized for a single field. These multi-frequency improved constant amplitude (MICA) pulses as constructed for (13)C broadband inversion give best results for HMQC and especially HSQC type experiments. Most of the advantages of MICA pulses versus other inversion pulses in these experiments can be attributed to their relatively short durations. Linearly truncated versions of these pulses do not offer any advantage over MICA pulses for these applications. MICA inversion pulses can also be used for decoupling and a (13)C decoupling example gives twice the decoupling bandwidth as the GARP1 sequence at the same power level with no appreciable increase in decoupling sidebands.

Constant amplitude broadband refocusing pulses from numerical optimization

“…[10] Typically, very few of the initial data sets approached the lowest achievable residual for each type of pulse, and most attempts ended in false minima well short of best residuals. It must then be concluded that not only is it unlikely that any of these solutions are the global best solutions for their cases, but in fact, they almost certainly are not global best solutions.…”

“…These results also compare fairly well with the best constant amplitude inversion pulses reported previously. [8][9][10] The phase profiles determined for the three types of best 60-kHz bandwidth pulses are shown in Fig. 1.…”

“…[10] All pulses were designed around a field frequency of 18 kHz and bandwidths of 40, 60 and 80 KHz to be useful for high-field broadband 13 C NMR experiments. One hundred twenty-eight-step 90x pulses were created to give 256-step refocusing pulses.…”

“…[1][2][3][4][5][6][7][8][9][10] Single pulse refocusing solutions with bandwidths to 20 kHz have been successful, [11,12] but current high field broadband refocusing is generally performed with pairs of adiabatic or constant amplitude broadband inversion pulses as this method outperforms existing single pulses at the higher required bandwidths. [13,14] The primary source of difficulty with creating refocusing pulses in the case of numerical optimization solutions is defining the target function for optimization.…”

“…The initial success of the method for creating refocusing pulses was previously demonstrated. [10,15] The goal of this work is to explicitly explore the use of a numerical optimization process to establish the parameters needed for minimal duration of 90x excitation pulses to provide broadband 180-degree refocusing pulses at power levels and in bandwidths appropriate for high-field 13 C NMR applications.…”