The National Electrical Manufacturers Association (NEMA) has published three separate standards pertaining to MRI systems, which were produced by the Medical Imaging & Technology Alliance (MITA), a division of NEMA.
The first standard—MS 6-2008 Determination of Signal-to-Noise Ratio (SNR) and Image Uniformity for Single-Channel Non-Volume Coils in Diagnostic MRI—defines a method for evaluating single-channel non-volume special purpose radio-frequency coils for use with MRIs. System head and body coils, as well as single-channel volume specialty coils, are excluded, as are coils that require multiple receiver channels for operation.
The second standard—MS 8-2008 Characterization of the Specific Absorption Rate (SAR) for MRI Systems—describes calorimetric and pulse-energy methods for the determination of SARs in either a phantom or a patient. Either method may be chosen. The results from either method may then be extrapolated to other scan parameters and even to other waveforms.
The third standard—MS 9-2008 Characterization of Phased Array Coils for Diagnostic MRIs—describes a standard means for measuring and reporting SNR and uniformity of signal intensity in images acquired with phased array coils. The quantities are helpful in evaluating the impact of system changes on performance or in demonstrating effectiveness for Federal Drug Administration applications, according to the association.
NEMA said that manufacturers are permitted to use the standards for the determination of system performance specifications. The standardization of performance specifications benefits the prospective equipment purchaser, and the parameters supplied with each NEMA measurement serve as a guide to those factors that can influence the measurement. The standards also can serve as reference procedures for acceptance testing and periodic quality assurance.