# An adjustable-length Dipole Using Forced-Current Excitation For 7T MR

Jiaming Cui, Ivan E. Dimitrov, Sergey Cheshkov, Minyu Gu, Craig R. Malloy, Steven M. Wright

Research output: Contribution to journalArticle

### Abstract

Ultra-high field imaging of the body and spine is challenging due to the large field-of-view (FOV) required. It is especially difficult on the RF transmission side due to its requirement on both the length and the depth of the <formula><tex>${\text{B}_1}^{+}$</tex></formula> field. One solution is to use a long dipole to provide continuous current distribution. The drawback is the natural falloff of the <formula><tex>$\text{B}_1$</tex></formula> field towards the ends of the long dipole, therefore the <formula><tex>${\text{B}_1}^{+}$</tex></formula> per unit square root of maximum specific absorption rate (<formula><tex>${\text{B}_1}^{+}$</tex></formula>/<formula><tex>$\surd \text{SAR}_\text{max}$</tex></formula>) performance is particularly poor towards the edge. In this study, a segmented element design using forced-current excitation and a switching circuit is presented. The design provides long FOV when desired and allows flexible FOV switching and power distribution without additional power amplifiers. Different element types and element configurations were explored and a segmented dipole design was chosen as the best design. The segmented dipole was implemented and tested on the bench and with a phantom on a 7T whole body scanner. The switchable mode dipole enabled a large FOV or improved <formula><tex>${\text{B}_1}^{+}$</tex></formula>/<formula><tex>$\surd \text{SAR}_\text{max}$</tex></formula> efficiency in a smaller FOV as needed.

Original language English (US) IEEE Transactions on Biomedical Engineering https://doi.org/10.1109/TBME.2017.2788864 Accepted/In press - Dec 30 2017

### Keywords

• 7T
• body coil
• Current distribution
• dipole
• Feeds
• forced-current excitation
• High field MRI
• Phantoms