The field dependence of NMR imaging. II. Arguments concerning an optimal field strength

Abstract

Some of the factors involved in the choice of field strength for NMR imaging are examined. The influences of relaxation times and chemical shift upon image quality and signal-to-noise ratio are highlighted, and power deposition is introduced as a significant factor which may limit the flexibility and information available at higher fields as long as 180 degrees echo pulses continue to be necessary. Chemical-shift imaging is examined and found wanting as a means of coping with chemical-shift artifacts, and the use of multiple echoes (albeit with research) in conjunction with multiple-slice techniques is advocated as representing an efficient data-gathering scheme which can improve image signal-to-noise ratio. With such use, a medium field strength (0.5-1 T) is presented as representing, for general purpose imaging of head and torso, the best current compromise when imaging time is of major importance, with the important caveat that new techniques may always invalidate this conclusion.

Signal‐to‐noise ratio and absorbed power as functions of main magnetic field strength, and definition of “90°” RF pulse for the head in the birdcage coil

“…In agreement with theory and experiment at frequencies up to 64 MHz (1,2), SNR increases at a rate roughly proportional to f at frequencies below 260 MHz when using an excitation pulse that produces center ␣ ϭ /2 or that maximizes A image . Between about 175 MHz and 260 MHz the slope of this line can be seen to decrease slightly because we are calculating SNR for an entire plane, which has a decreasing homogeneity of flip angles with increasing frequency.…”

“…While in a single FID with no frequency-or phase-encoding (as in Eq. [2]) the phase of signal from different locations may result in signal cancellation, in the reconstructed image (where the signal from each voxel has been separated according to position) only the magnitude of the signal in each voxel and the sensitivity of the coil to the location of each voxel are important. A image was calculated as…”

“…Predictions of trends in signal-to-noise ratio (SNR) and specific energy absorption rate (SAR) with increasing static magnetic field (B 0 ) strength based on MR theory, the principle of reciprocity, and analytical RF magnetic field (B 1 ) calculations have been shown to be reasonably accurate at frequencies up to 64 MHz in head-and body-sized samples (1,2). MRI experiments are currently performed at static magnetic field (B 0 ) strengths as high as 8.0 Tesla, where the frequency of the RF magnetic field (B 1 ) for imaging with 1 H is about 340 MHz.…”

“…At these frequencies, significant interaction between the applied B 1 field and human tissues is expected. The effects of this interaction on SNR and the total absorbed power are complicated, and are dependent on the experiment being performed, RF coil type and performance, and even on the specific subject geometry and position in the coil (2,3).…”

Sodium and T1ρ MRI for molecular and diagnostic imaging of articular cartilage

“…Further, the recent proliferation of 7 T whole-body MRI scanners in clinical research centers could have a significant impact on sodium MRI and its potential for clinical use. Since SNR scales as B 7=2 0 (158)(159)(160)(161) and the lack of B 1 penetration and B 0 susceptibility are issues that plague proton imaging, sodium MRI can be particularly advantageous at higher fields. Further, unlike proton T 1 , which increases with field, as the T 1 of sodium is predominantly due to quadrupolar interaction, it may not change appreciably at higher field.…”