Radiofrequency
B1
Gradients
Alternatively to coherence selection
by B0 gradients, a series of works
have dealing with the possibility to perform this selection by using radiofrequency
(B1) gradients. Some advantages of such approach are:
- There is no need for recovery times after the
gradient and, therefore,
eddy current effects are avoided.
- There is no need for pre-emphasis.
- The
lineshape is not distorted.
- Insensitive to susceptibility inhomogeneities.
- The gradient is frequency selective.
- The lock system is not perturbed.
- The B1 gradient pulse acts simultaneously for excitation
and for defocusing-refocusing
purposes.
The main difficulty is to produce strong uniform RF field gradients
Theoretical and practical aspects of B1 gradients has recently been
reviewed
(
97PROG101
,
96ENC3938
,
95BOOK
,
and
00ENC1937
).
The following applications have been described:
- Solvent suppression schemes
(
93JMRA239-105
and
94JMRA256-106
).
- 2D COSY
(
93JMRA115-103
and
94JMRA110-108
) and 2D COSY-DQF
(
93JMRA223-105
and
92JMR611-100
) experiments.
- 2D TOCSY experiment
(
98JMR183-133
).
- 2D NOESY experiment
(
94JMRA109-107
,
95JMRB310-109
and
98JMR183-133
).
- 2D HSQC experiment
(
95JMRA229-112
).
- Selective 1D COSY
(
95JMRA278-117
), selective 1D-TOCSY (
98JMR183-133
)
and selective-1D NOESY (
98JMR183-133
) experiments.
- Isotope filtering scheme
(
97JMR80-127
and
97JMR340-125
).
- Molecular diffusion studies
(
89JMR1-81
,
96JCP4405
,
91JMR589-95
,
95JMRB32-106
,
98JMR245-134
, and
99JMR7-141
).
- A DPFGE scheme using RF gradients has been theoretically analyzed and
compared to the original sequence
(
96MRC807
).