What increase in TR is often required when using magnetisation transfer in MRA?

When using magnetisation transfer in Magnetic Resonance Angiography (MRA), an increase in the repetition time (TR) is typically required to enhance the signal from the moving blood in the vessels while suppressing the background tissue signals. The particular increase from 21 mSec to 36 mSec represents a substantial adjustment that allows for improved image contrast and resolution.

Magnetisation transfer techniques work by selectively saturating the protons in non-moving tissue (like muscle or fat), which in turn enhances the signal from the protons in the flowing blood. This necessitates sufficient time (TR) between excitation pulses to capture the desired signals effectively. The choice to extend the TR to this specific range ensures optimal conditions for imaging by balancing the need for contrast against other factors such as scan time and overall imaging quality.

This specific range reflects common practice observed in MRA procedures, where imaging parameters are finely tuned to achieve the best possible results in visualizing vascular structures. The increment in TR allows the sequence to effectively exploit the magnetisation transfer phenomenon while remaining within the practical limits of imaging speed.