MAGNETIC-RESONANCE-IMAGING ON A LOW-FIELD SYSTEM (0.2 T) - COMPARISONWITH A 1.0-T SYSTEM

Aim of the study: Using phantom studies, a dedicated low-field MR syst em with 0.2 T and a whole-body MR-scanner with 1.0 T were compared. Me thods: A spin-echo sequence was performed on the 0.2-T MR unit using t he knee coil and on the 1.0-T MR unit with the head coil. In a water-f illed phantom, signal-to-noise ratios (SNR) were calculated and contra st measurements on gels with well-defined relaxation times were obtain ed and compared to nominal relaxation times. Measurements of T1 and T2 relaxation times on the low-field system were compared to the 1.0-T u nit. As a parameter for geometrical image quality, magnetic field dist ortions were calculated. In theory, influence of field strength and an d receiver bandwidth on the minimal echo time and on chemical shift ar tifacts were calculated. Results: The SNR was 63.2 on the 0.2-T and 17 9.6 on the 1.0-T MR unit (difference factor 3, against a theoretical d ifference of 5-16). Relaxation times on the low-field system were sign ificantly (around 40-50 %) lower. Measurements of contrast were simila r on both systems. On the low-field system geometrical distortions of several pixels were recorded. The minimal echo time on the low-field s ystem was 21.3 ms with the low receiver bandwidth and 4.3 ms on the 1. 0-T MR unit. The amount of chemical shift artefacts was the same on bo th systems.Discussion: On low-field MR systems SNR is markedly improve d by small read-out gradients and a low receiver bandwidth (factor 3 v s 5-16 in comparison with the 1.0-T MR unit). However, an optimal homo geneous magnetic field is required, since inhomogeneities may create s evere geometrical distortion.