Improving SNR and B₁ transmit field for an endorectal coil in 7 T MRI and MRS of prostate cancer

Abstract

1Higher magnetic field strengths like 7 T and above are desirable for MR spectroscopy given the increased spectral resolution and signal to noise ratio. At these field strengths, substantial nonuniformities in B 1 1/2 and radiofrequency power deposition become apparent. In this investigation, we propose an improvement on a conventionally used endorectal coil, through the addition of a second element (stripline). Both elements are used as transceivers. In the center of the prostate, approximately 40% signal to noise ratio increase is achieved. In fact, the signal to noise ratio gain obtained with the quadrature configuration locally can be even greater than 40% when compared to the single loop configuration. This is due to the natural asymmetry of the B 1 1/2 fields at high frequencies, which causes destructive and constructive interference patterns. Global specific absorption rate is reduced by almost a factor of 2 as expected. Furthermore, approximately a 4-fold decrease in local specific absorption rate is observed when normalized to the B 1 values in the center of the prostate. Because of the 4-fold local specific absorption rate decrease obtained with the dual channel setup for the same reference B 1 value (20 mT at 3.5 cm depth into the prostate) as compared to the single loop, the transmission power B 1 duty cycle can be increased by a factor 4. Consequently, when using the two-element endorectal coil, the radiofrequency power deposition is significantly reduced and radiofrequency intense sequences with adiabatic pulses can be safely applied at 7 T for 1 H magnetic resonance spectroscopy and MRI in the prostate. Altogether, in vivo 1 H magnetic resonance spectroscopic imaging of prostate cancer with a fully adiabatic sequence operated at a minimum B 1 1 of 20 mT shows insensitivity to the nonuniform transmit field, while remaining within local specific absorption rate guidelines of 10 W/kg. Magn Reson Med 68:311-318, 2012. V C 2011 Wiley Periodicals, Inc.

7 Tesla MRI with a transmit/receive loopless antenna and B1-insensitive selective excitation

“…Compared with the use of internal loop coils for prostate MRI at 7T , the submillimeter dimensions possible with the loopless antenna enable targeted intravascular access under image guidance , not accessible to larger internal coils. Indeed, the 50‐ to 100‐μm resolution achieved here in vitro and/or in vivo are smaller than the best resolution achieved to date at 7T of 0.5 × 0.5 mm 2 , using prostate coils with similar slice thickness .…”

Self-Managed Belief as Part of the “Scientific Method”: Part II—Examples From Published Scientific Work

“…This is typically tested via appropriate phantoms, usually provided by the manufacturer and/or vendor of the scanner, or specifically manufactured in the laboratory [3]. Such tests include not only signal-to-noise ratio (SNR) measurements, but also, particularly in larger, inhomogeneous RF coils, the SNR homogeneity across the sample volume [4][5][6][7][8][9][10][11][12]. Even more challenging would be the QC of hybrid systems like, e.g., PET/MRI [13].…”

Imaging modalities in focal therapy

“…Several studies demonstrated the safety and feasibility of MR spectrocopic imaging at 7T in patients with biopsy proven prostate cancer. 7 T MRI rendered a safe and promising technology [26][27]. Another promising novelty is Correlated Diffusion Imaging (CDI).…”