1.5 T vs 3.0 T

Comparing 1.5T vs 3T MRI systems identifies in a number of differences:

• increased signal to noise ratio (SNR)
• increased spatial resolution
• increased temporal resolution
• increased specific absorption rate (SAR)
• increased acoustic noise

Signal to noise ratio

Theoretically, signal is proportional to the square of the static field strength (B(B₀) whereas noise increases linearly. This implies that in a perfect system SNR of a 3.0T system would be twice as good as at 1.5T. In reality, due to increase in susceptibility effects in most tissues, the actual improvement is only in the 30-60% range (instead of 100%). With this increased SNR, the spatial resolution and/or acquisition time can be improved depending on which is more important for the particular case.

Specific absorption rate (SAR)

SAR is defined as the amount of radiofrequency energy (joules) deposited in tissues (kg). The limit set by the FDA is an amount which results in increase of 1 degree centigrade in any tissue ². SAR is proportional to the static field (B0(B₀) to the power of 2, meaning a 3.0T system deposits 4 times as much energy within tissue as a 1.5T system. Additionally SAR is proportional to:

• static field strength (B₀) squared
• pulse duration and length
• pulse number
• slice number
• flip angle

The dependence of SAR on flip angle results in standard spin echo sequences, which have 90 degree flip angles, depositing a relatively large amount of energy. As a result there is increased use of gradient echo sequences, which use smaller flip angles. Unfortunately these sequence image T2* and not T2, and are therefore more susceptible to local field artifacts. These problems have largely been overcome with modern units.

Acoustic noise

Rapid gradient switching leads to an increase in the intensity of the acoustic noise, which requires better insulation of both the unit itself and the room.