Changes in SNR
- Increased RF noise from the environment (e.g. loss of integrity of RF screening).
- Loading of the RF coil. This is the primary source of noise in systems of field strength greater than 0.3 T. In normal imaging, the coil load is the patient, and so during QA on test objects it is important to make sure the coil experiences a similar load.
- Improperly tuned RF coil. This will cause a reduction in SNR.
- RF receiver chain faults. A fault in any of the low-pass filters, quadrature phase detection, gain stages, the pre-amp, the decoupling mechanisms or the RF coil itself can cause loss of signal and increase in noise.
- RF transmit system faults. Wrong flip angles can be generated when the calibration of the RF transmit path is adversely affected.
- The ghosting associated with non-uniformity sources clearly affects the SNR.
- Bad choice of pulse sequence parameters. Thin slice widths, small fields of view, short TRs and long TEs all contribute to a reduction in the SNR. It is therefore necessary to ensure that parameters a kept constant during QA tests, and that the tests are optimised in assessing the quality of a system.
- Some manufacturers employ raw data filters to improve the SNR at the expense of resolution. This reconstruction parameter needs to be borne in mind when considering factors which affect the SNR.
Further reading on this topic:
Books: IPEM Report 80 p13-14