A method to assess the loss of a dipole antenna for ultra‐high‐field MRI

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In design and manufacture of radiofrequency (RF) coils for MRI, a benchtop method for evaluating the relative sources of RF power loss can be very valuable in comparing different designs and identifying inconsistencies in manufacture. In particular, gaining a sense of power lost in the coil relative to power lost in the sample has been of interest, so that coil loss can be minimized. At current clinical field strengths, the standard workhorse RF coils are surface coil loops and birdcage volume coils. Both designs are resonators for which the quality (Q) factor is defined as 1JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM1/v/2018-01-24T161827Z/r/image-png
In practice, the Q factor of an RF coil is more often defined as JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM2/v/2018-01-24T161827Z/r/image-png , where JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM3/v/2018-01-24T161827Z/r/image-png is the resonant frequency, and JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM4/v/2018-01-24T161827Z/r/image-png is the full width at half maximum of the coil resonance, which can be measured by S12 between a pair of inductive probes loosely coupled to an isolated surface coil 2. The contribution of coil noise to the detected signal is well understood, and is easily characterized by the Q ratio (i.e., the ratio of the Q of the coil in free space divided by the Q when it is placed next to the body). The noise attributed to coil losses is given by 3JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM5/v/2018-01-24T161827Z/r/image-png where rcoil and rsample are the equivalent resistances representing power loss in the coil and the sample, respectively. At low frequencies, because radiation loss is negligible, Equation (2) can also be written as 3JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM6/v/2018-01-24T161827Z/r/image-png
The Q ratio is a widely used metric for predicting the performance of surface coils 4.
Analysis of the ultimate intrinsic signal‐to‐noise ratio (UISNR) demonstrates that, at ultra‐high field (UHF), detectors with uniform loops of current are insufficient to capture all of the available signal‐to‐noise ratio (SNR) 5. As an alternative to loop coils, electric dipole antennas haven been introduced 7, which can provide additional SNR 8, and are used increasingly in UHF MRI 10. Dipole antennas differ from conventional surface coil loops in many ways. The most common dipole antenna consists of a single long conductor with a gap in the middle, usually bilaterally symmetrical, with power fed or signal detected across the gap 18. Dipole antennas are extremely common in telecommunications, and the half‐wavelength self‐resonant dipole antenna is considered one of the most efficient designs for far‐field applications 19. However, for MRI, simulations show that shortening the dipole antenna can provide higher SNR for a particular depth of interest and higher specific absorption rate (SAR) normalized JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM7/v/2018-01-24T161827Z/r/image-png in transmission, compared with a half‐wavelength self‐resonant dipole 13. Many modifications of dipole dimensions or shapes have been proposed by different groups to improve SAR‐normalized JOURNAL/mrim/04.02/01445475-201803000-00057/math_57MM8/v/2018-01-24T161827Z/r/image-png or SNR performance for particular regions of interest 12.
In free space, an electric dipole antenna is a very efficient radiator and radiation resistance is the dominant source of loss 26. In contrast, at low frequency, unloaded surface coil loops act as energy storage devices whose losses are dominated by resistive losses in the coil itself 27. However, past research has shown that when a dipole antenna is well loaded by the sample, radiation loss is significantly diminished 22. Because of the inconsistent contribution of radiation loss in unloaded and loaded cases, it is not possible to assess the body‐noise dominance of a dipole antenna by conventional measurements of Q ratio.
For an electric dipole antenna in free space, several approaches have been proposed to measure the coil loss 28. Wheeler suggests that enclosing the dipole antenna within a conducting cage (a “Wheeler cap”) will eliminate the radiation loss without significantly changing resistive loss 28.

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