Optimizing brain imaging protocols at high field strength (3.0T)

Appreciating how doubling field strength impacts many imaging parameters can help you optimize your technique, enabling you to fully exploit the capabilitites of your 3.0T system. This application tip covers imaging strategies for 3.0T.

Apart from the gain in SNR, the higher field strength also changes the MR tissue contrast parameters: T1 is increased by about 30%, while T2 and particularly T2* are decreased. Protocols are optimized accordingly.

Because signal-to-noise ratio (SNR) scales linearly with field strength, the intrinsic SNR on a 3.0T system is twice as high as that of a 1.5T system.

Scan faster by reducing NSA

SNR can be traded for faster scanning by reducing the NSA (and scan time) by a factor of four, while maintaining the same SNR as on 1.5T. The extra SNR could also be traded for higher spatial resolution in two ways:

• Use a higher matrix to increase the in-plane resolution.
• Use thinner slices to increase the through-plane resolution.

The higher sensitivity for T2* effects enables higher sensitivity in perfusion and BOLD imaging.

However, in EPI the effects of higher susceptibility are more pronounced. To reduce susceptibility artifacts, the following measures can be taken:

• Use the largest bandwidth (minimum water-fat shift).

• Reduce the echo-train length by using multishot imaging techniques.

• Use the correct fold-over direction in EPI imaging (AP in an axial scan) with the correct fat shift direction to move the artifacts outside the brain (fat shift direction Anterior in case of axial images).

Using the SENSE Head coil is particularly beneficial in DWI: SENSE factor 2 shortens the echo train length in single-shot EPI, so susceptibility effects are significantly reduced and TE can be shorter.

At higher field strengths, the maximum allowed SAR will be reached more rapidly. As on all field strengths, there are several ways to reduce the SAR: increase TR, reduce the number of slices, reduce the TSE echo train length or reduce the flip angle.

With the 3.0T systems, the scan parameter, "B1 mode," - the amplitude (┴T/m) of the RF pulses [also known as the B1 transmit field] - can be manually controlled, to reduce the SAR of a sequence.

Use a lower B1 amplitude to reduce the SAR. Lowering the B1 amplitude may influence the minimum echo spacing or the shortest TR.

T2 relaxation times are shorter at higher field strength.

Optimize T2 weighting by adjusting TE or technique:

T1 relaxation times are longer at higher field strength.

Optimize T1-weighted multislice (T)SE in the following ways:

In inflow MRA, you can achieve better contrast between vessels and background tissue than at lower field strengths because the longer T1 relaxation times enable better background suppression. By using a transmit/receive Head coil all inflowing blood is fresh, yielding maximum signal from flowing blood.