Doubly tuned local coils for MRI and MRS at 1.5 T

Abstract

We describe a doubly tuned radiofrequency (RF) local coil probe designed specifically for performing in vivo image-localized spectroscopy. The probe was designed using principles developed in connection with the counter-rotating-current (CRC) and planar-pair loop gap resonators for magnetic resonance imaging (MRI). The probe design satisfies several criteria useful for in vivo 1H/31P experiments at 1.5 T. First, sensitivity on the low-frequency mode is preserved relative to a singly tuned coil. This result was confirmed by bench-test and in vitro MR experimental data. Second, through principles of intrinsic decoupling the probe is isolated from any externally applied uniform excitation field, which is desirable for in vivo 1H imaging and solvent suppression. Third, the regions of sensitivity of the high- and low-frequency modes of the coil are similar, and therefore spectroscopic volumes of interest identified on an image will reflect the same volumes as those selected during spectroscopy. Finally, interface to the MR system is such that the high- or low-frequency circuits may be selected entirely under software control, with no requirement for changing coils or cables or moving the subject.

Doubly tunable double ring surface coil

“…Unfortunately, it also introduces additional resistance that slightly reduces SNR when compared with the coil without the frequency splitting circuit. The doubly tunable coil produces a more uniform B 1 profile than standard double resonance surface coils (14,15) but still has a better SNR than double resonance volume coils (16 -19).…”

“…The choke inductance L spl is chosen to be approximately 25% of the inductance of main ring L p (15). The tuning capacitance C p and splitting network capacitance C spl required for each ring can be calculated when comparing the reactance in Eq.…”

Progress towards in vivo brain 13C-MRS in mice: Metabolic flux analysis in small tissue volumes

“…Their relatively limited performance in terms of SNR, due to interferences between the loops, became restrictive for high field applications. The same problems affected double-tuned surface coils that became obsolete nowadays [78,79]. In addition, the heteronuclear scalar coupling between 1 H and 13 C is relatively large ( 1 J CH ¼ 120e170 Hz) and the related decoupling power also increases at higher field due to an increased decoupling bandwidth required for the same chemical shift range.…”

The sectorial loop‐gap resonator for 31P NMR of the adult human liver at 1.5 t with surface tissue suppression

“…All lines of magnetic flux must close by passing from one opening to the other. This class of loop-gap resonators has been labeled "controlled return It is apparent that the magnetic field external to the torus can be manipulated in various ways by (1) varying the size of the sector, (2) varying the diameter of the generating cycle, and (3) varying the distance between the generating circle and the axis of rotation. Although not all combinations have been investigated, computer programs described here permit theoretical mapping of the external field with these three variables.…”