Item

Abstract

Magnetic resonance spectroscopy (MRS) is a non-invasive and non-ionizing technique
to acquire localized spectra of metabolites in vivo. With increasing static magnetic field
strength B0, the spectral separation of the metabolites and the signal-to-noise ratio
(SNR) of the spectrum increase. Consequently, the number of detectable metabolites
and the spatial specificity are enhanced at ultra-high fields (B0 ≥7 T). At the same time,
the wavelength of the radiofrequency (RF) field is decreased. For proton spectroscopy
at ultra-high fields, the wavelength of the RF field in tissue is smaller than the typical
dimension of a human head. From the perspective of electromagnetic theory, this means
that a quasistatic approximation of Maxwell’s equations is not valid anymore and the
electromagnetic field must be calculated with the full system of coupled partial differ-
ential equations. Therefore, RF coil designs based on the quasistatic approximation,
such as the birdcage coil or loop-only receive arrays, have suboptimal performance at
ultra-high fields.
This PhD project explored the optimization of RF coils for ultra-high field MRS.
The optimization was based on an equivalent surface current distribution surrounding a
human head model. It could be shown, that the equivalent surface current distribution
can be separated into curl- and divergence-free components. The full-wave electromag-
netic field problem was solved by a newly developed dyadic Green’s functions approach.
As a first optimization goal, the SNR was maximized in a spherical- and later in a
realistic human head model. By optimizing the complete set of curl- and divergence-free
surface current components, an upper threshold for the achievable SNR of any receive
array could be calculated; this so-called ultimate intrinsic SNR (UISNR) was studied
at all practically relevant B0 field strengths regarding human head applications. The
UISNR increased superlinearly with B0 in central regions of the human brain. In a next
step, the SNR optimization was done separately for curl- and divergence-free current
components. This yielded a direct performance measure of how close loop-only and
dipole-only receive arrays were able to approach the UISNR in the human head. Based
upon this analysis, field strength specific design guidelines for RF receive arrays were deduced. In conclusion, at ultra-high field strength a combination of loop and dipole
elements is necessary to achieve the best possible SNR at any position in the human
head.
As a second optimization goal, the coupling of multi-channel RF arrays was mini-
mized. For that, a fast analytical model describing the complex mutual coupling between
two surface loops was introduced. To understand and eliminate both electric and mag-
netic coupling between the loops, the influence of the loop geometry and loading by the
sample was systematically examined. For the first time, it was demonstrated that at
400 MHz it is possible to eliminate both, electric and magnetic coupling simultaneously
by proper adjustment of the loop width and overlap. A fully decoupled two channel
prototype array was constructed having superior transmit and receive performance over
a previously used gapped design.