Signal-to-noise ratio (MRI)

Signal to noise ratio (SNR) is a generic term which, in radiology, is a measure of true signal (e.g. reflecting actual anatomy) to noise (e.g. random quantum mottle). A lower signal to noise ratio generally results in a grainy appearance to images. 

Each modality has its own source(s) of noise and therefore techniques for signal to noise ratio maximisation are modality specific. 

Radiographic interpretation

Plain radiograph

In radiographs, the larger number of photons absorbed, the greater the SNR, the less noisy the image. The use of high kV and intensifying screens reduce the number of photons and radiation dose to patients, therefore reducing the SNR. Conversely, measures to increase the SNR, such as by increasing the mAs, would increase the patient dose.

CT

Content pendingSNR in CT roughly follows the same principles as those of plain radiographs. It is calculated by comparing the level of a desired signal to the background deviation from normal pixel values. In general, the larger the number of photons absorbed, the greater the SNR.

MRI

The SNR is measured frequently by calculating the difference in signal intensity between the area of interest and the background (usually chosen from the air surrounding the object). In air, any signal present should be noise. The difference between the signal and the background noise is divided by the standard deviation of the signal from the background - an indication of the variability of the background noise. 

SNR is proportional to the volume of the voxel and to the square root of the number of averages and phase steps (assuming constant sized voxels). Since averaging and increasing the phase steps takes time, SNR is related closely to the acquisition time. 

In MRI, the SNR can be improved by:

• volume acquisition as compared to 2D imaging, but imaging time would be increased.
• spin echo sequences as compared to gradient echo.
• decreasing the noise by reducing the bandwidth, using surface coils and  increasing number of excitations
• increasing the signal by decreasing the TE (time to echo) and increasing the TR (time to repeat), slice thickness or field of view

Additionally, SNR can be improved by tweaking scan parameters. Assuming all other factors remain the same, SNR can be improved by:  

• increasing the field of view (FOV)
• decreasing the matrix size
• increasing the slice thickness

Ultrasound

Content pending