Abstract
Purpose
Parallel transmission (PTx) requires knowledge of the inline image produced by each element. However, inline image mapping can be challenging when transmit fields exhibit large dynamic range. This study presents a method to produce high quality relative inline image maps when this is the case.
Theory and Methods
The proposed technique involves the acquisition of spoiled gradient echo (SPGR) images at multiple radiofrequency drive levels for each transmitter. The images are combined using knowledge of the SPGR signal equation using maximum likelihood estimation, yielding an image for each channel whose signal is proportional to the inline image field strength. Relative inline image maps are then obtained by taking image ratios. The method was tested using numerical simulations, phantom imaging, and through in vivo experiments.
Results
The numerical simulations demonstrated that the proposed method can reconstruct relative transmit sensitivities over a wide range of inline image amplitudes and at several SNR levels. The method was validated at 3 Tesla (T) by comparing it with an alternative inline image mapping method, and demonstrated in vivo at 7T.
Conclusion
Relative inline image mapping in the presence of large dynamic range has been demonstrated through numerical simulations, phantom imaging at 3T and experimentally at 7T. The method will enable PTx to be applied in challenging imaging scenarios at ultrahigh field.
Parallel transmission (PTx) requires knowledge of the inline image produced by each element. However, inline image mapping can be challenging when transmit fields exhibit large dynamic range. This study presents a method to produce high quality relative inline image maps when this is the case.
Theory and Methods
The proposed technique involves the acquisition of spoiled gradient echo (SPGR) images at multiple radiofrequency drive levels for each transmitter. The images are combined using knowledge of the SPGR signal equation using maximum likelihood estimation, yielding an image for each channel whose signal is proportional to the inline image field strength. Relative inline image maps are then obtained by taking image ratios. The method was tested using numerical simulations, phantom imaging, and through in vivo experiments.
Results
The numerical simulations demonstrated that the proposed method can reconstruct relative transmit sensitivities over a wide range of inline image amplitudes and at several SNR levels. The method was validated at 3 Tesla (T) by comparing it with an alternative inline image mapping method, and demonstrated in vivo at 7T.
Conclusion
Relative inline image mapping in the presence of large dynamic range has been demonstrated through numerical simulations, phantom imaging at 3T and experimentally at 7T. The method will enable PTx to be applied in challenging imaging scenarios at ultrahigh field.
| Original language | English |
|---|---|
| Journal | Magnetic resonance in medicine |
| DOIs | |
| Publication status | E-pub ahead of print - 26 Aug 2015 |
Keywords
- B1 mapping
- Parallel transmission
- Ultrahigh field MRI