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Quality control

To guarantee the best possible image quality, and to detect any malfunctioning, regular quality control is recommended (figure 8.31).
The different parameters in MRI control include: the signal, geometric parameters, NMR, artifacts and spectroscopy.

Signal parameters

The main signal parameter is the signal-to-noise ratio measurement. The measurement is carried out:

• either by selecting regions of interest in void zones (air)
• or in a region of interest corresponding to the subtraction of 2 images of a homogeneous phantom.

The uniformity of the emitting coils must also be evaluated, as heterogeneity of radiofrequency emission could lead to differences in flip angles in the volume, thus modifying contrast. This measurement, given as a percentage, is carried out on a homogeneous phantom, if necessary by sliding the image or increasing the number of acquisitions when the signal-to-noise ratio is low.

Geometric parameters

The aim of these tests is to check the quality of spatial encoding (slice selection, phase and frequency encoding). Useful criteria in this test include:

• Position, thickness and profile of slice
• Spatial linearity and geometric deformation, in the phase and frequency encoding direction
• Spatial resolution

NMR parameters

• Absolute precision and Repeatability of T1 and T2 measurements
• Contrast and contrast-to-noise ratio (figure 8.32) with T1 and T2 weighting

Artifacts

• Phantoms
• Chemical shift and fat signal suppression

MR spectroscopy parameters

• Evaluation of the parameters of B0 homogeneity for a centered volume
• Accuracy of signal amplitude and the signal-to-noise ratio of the principle peaks