The design and use of a dual-frequency surface coil providing proton images for improved localization in ^{3 1} P spectroscopy of small lesions

Abstract

Nuclear magnetic resonance (NMR) spectroscopy of small lesions is restricted by the difficulties of localizing the surface coil with respect to the lesion and the problem of ensuring that signal is only obtained from the lesion and not from surrounding tissue. A double-tuned coil has been developed that permits NMR proton images to be obtained from a region of interest, prior to carrying out 31P spectroscopy of the same region with the same coil, without the need for further adjustment. The coil provides a means of accurately localizing the region from which the 31P signal is obtained, whilst offering a means of accurately applying 31P signal localization methods, and the possibility of making corrections for the nonuniform sensitivity of a given surface coil. The coil makes use of two parallel resonant circuits, with independent rf connections, but sharing a common coil. Simulated shorted and open circuit lambda /4 cables are used, respectively, to open circuit each circuit at the resonant frequency of the other circuit and ensure that the simulated lambda /4 line is short circuited for each circuit at the circuit's resonant frequency. At 63.6 MHz, the Q of the coil was 190 unloaded and 90 loaded, and at 25.7 MHz the Q was 210 unloaded and 140 loaded, for a 4-cm-diam coil. The coil has been used to obtain proton images and 31P spectra. A circuit employing only one input was also developed.

Practical design of a 4 Tesla double-tuned RF surface coil for interleaved 1H and 23Na MRI of rat brain

“…Over the past 20 years, several designs of double-tuned RF surface coils have been reported for low field applications [18][19][20][21][22][23][24]. In the following, only double-tuned surface coil designs capable of operation at high field (>3T) are considered.…”

“…However, it requires additional measurement time for coil exchange (usually a precious commodity in in vivo experiments), it introduces potential errors due to re-positioning within the MRI scanner (requires a posteriori co-registration of the images), and it does not allow the use of interleaved 1 H and 23 Na pulse sequences. The second method requires a double-tuned RF coil ( 1 H and 23 Na), which is usually made either as a volume [8][9][10][11][12][13][14][15][16][17] or surface [18][19][20][21][22][23][24] RF coil. This design allows interleaved 1 H and 23 Na images to be obtained over essentially the same volume of interest (VOI), without the need for repositioning the sample.…”

Doubly tuned local coils for MRI and MRS at 1.5 T

“…These experiments dictate the use of either singly tuned radiofrequency (RF) coils which are repositioned over the sample for different nuclei or RF coils tuned to multiple frequencies. Doubly tuned NMR receiver coils have been described for in vitro (5, 6 ) and recently for unlocalized in vivo MRS by several investigators (7)(8)(9)(10). However, these RF probe designs have in general compromised sensitivity on the low-frequency resonance mode in order to obtain more than one resonance.…”

A low-lossy double tuned resonator for concurrent /sup 1/H and /sup 31/P nuclear magnetic resonance

“…1 [2]. The relation between the input impedance, &, and the load ZL, for a quarterwave, lossless resonator with a characteristic impedar;ce ZO, is expressed by :…”

“…The use of double tuned resonators allows one to get more efficient and flexible use of an NMR imager/spectrometer [l], [2], [3] . In the lH imaging mode, decoupling of the double tuned surface coil with respect to the body coil, while this one is transmitting the RF burst, is needed.…”