Tim Trio RF Noise and Spike Check Quality Assurance

RF noise and gradient-induced spike levels are monitored daily. These scans employ the 12-channel RF head coil, and the same Siemens 1.9 Liter doped-water phantom - although technically no sample is required for these measurements. We monitor for and document:

- lack of RF interference from other equipment that could cause image artifacts,
- lack of spiking due to gradient connections.

Noise scans are performed by simply opening the MRI receiver without employing RF or gradient pulses. Spike Check scans employ consistent gradient pulses but no RF. The scans are performed daily and take about 5 minutes. These sequences, which include automated analysis of the data at the scanner immediately after completion of the scan, were developed by our collaborators at the Martinos Center for Biomedical Imaging at Massachusetts General Hospital, and is available from MGH for use here via a C2P agreement involving MGH,  Siemens and Harvard.

The screenshot below shows examples of the noise and spike check analyses provided at the scanner. Shown are a noise summary image (top left); RF noise spectrum (top right); spike check summary image with gradients on as detected by the body coil (lower left) and spike check summary image with gradients off as detected by the head coil (lower right).

The noise scan makes 50 measurements in 10 kHz frequency ranges around the central frequency for our scanner. A noise summary image is generated which shows the mean signal intensity of each RF noise scan in the column direction for all frequency bands displayed as individual rows. Significant signals detected during this process are highlighted as the number of noise "pixels" in the comment below the image. This value is usually zero. The RF spectrum measurement averages repeated scans over a 500 kHz range around the central scanner frequency. The mean power spectrum of readouts from all channels and averages is shown in yellow while the mean power spectrum of all measured readouts (averaged over channels, averages and measurements) is shown in red. (The red display always shows five even-spaced "glitches" due to signal composing.) This is performed with both the head coil and the body coil as the receive coil. In addition to the daily checks, these scans are performed whenever new electrical equipment (visual display aids, audio devices, eye-tracker, etc) are installed in the scanner room to ensure the new equipment does not generate RF noise that could cause image artifacts.

The spike check scans acquire 100 measurements without gradients active, and 100 measurements with the gradient pulsing before signal acquisition. The spike check summary images show signal detected over a 200 kHz frequency range and over the 100 measurements. Again, this is performed with both the head coil and the body coil as the receive coil. The body coil usually picks up some spike signals at the ends of the frequency range - as the body coil is not used for signal reception in functional or structural MRI, this is not a problem but increased numbers of spike signals warrants investigation with the Siemens engineer. The head coil usually registers zero spike signals.

Current plots of RF noise "pixels" and Spike Check counts. Examples for a recent month are shown below.

Summary data from noise and spike-check scans run since December 2011 are available here, with subject lines of "RF Noise run on MRC35380" and "Spike Check run on MRC35380".