Angiography Techniques for High-field MR

Angiography is a medical imaging technique used for visualising the human blood vessels. In MR angiography (MRA), three important techniques are:

- Contrast enhanced (CE) MRA
- Phase-contrast (PC) MRA
- Time-of-flight (TOF) MRA

CE-MRA uses contrast agents to selectively enhance the signal of blood in the vasculature. As the contrast is only present at high concentrations of the contrast agent (i.e., directly after injection), timing of the CE-MRA data acquisition is very important. The other techniques are completely non-invasive and allow higher resolution because they do not depend on special acquisition timing.
In PC-MRA the phase of the MRI signal is proportional to velocity through the use of special magnetic field gradients. Thus both imaging of the vessels as well as quantitative measurements of the blood flow velocity are possible.
TOF-MRA generates contrast between flowing blood and stationary tissue by exploiting the contrast between unsaturated fresh blood and the surrounding saturated tissue. The inflowing blood carries a higher magnetisation and hence produces higher signal than the stationary tissue.
The challenge with 7T is the high energy deposition inside the patient during the acquisition of the MRA data. To minimize the energy deposition special excitation pulses are developed using the so-called VERSE algorithm. With this algorithm those parts of the radiofrequency excitation pulses are stretched (and thus reduced in power) which are responsible for the majority of the energy deposition – this also requires changing the amplitudes of the field gradients used to encode the slice during RF excitation.
In this research project the VERSE algorithm is optimized to minimize the acoustic noise during the gradient activity. VERSE-optimized RF pulses are then incorporated into standard TOF-MRA pulse sequences to increase the blood vessel contrast. In patients with high-grade gliomas TOF-MRA is then utilized to visualize the intra-tumoral blood vessel architecture with the aim to detect vascular changes after tumor therapy.