The absolute signal-to-noise ratio in MRI acceptance testing

The absolute signal-to-noise ratio (SNR) measurable from images is defined and related to the theoretical fundamental SNR for MRI. Absolute SNR for quadrature head coils was obtained by both measurement and analysis of published data for MRI systems in the range 0.5-1.5 T. Mean absolute SNR for 22 MRI systems was 1.59 +/- 0.25 x 10(4) Hz(1/2) ml(-1) T-1 and did not vary significantly with the static field strength or gradient strength and signal bandwidth. This can be related to the fundamental SNR calculated as 2.36 x 10(4) Hz(1/2) ml(-1) T-1 through an effective noise figure with an average value of 3.4 dB for the 22 systems. Use of this methodology is discussed in relation to acceptance testing and performance assessment.