Synopsis

Keywords: Physics & Engineering: Pulse design

Ultra-short or zero echo time (UTE and ZTE) enables MR contrasts that are not possible with conventional methods. In this presentation, you will learn how to achieve an echo time of near zero, the obstacles that must be tackled along the way, and about substances with ultra-short T2. This will set the stage for examples of novel images that cannot be acquired any other way.

Abstract

The shortest possible echo time (TE) for a conventional gradient echo sequence is about a millisecond. This is sufficient to capture signals from most biological tissues because the transverse relaxation time of water is typically much longer. Tissues with very short T₂ are often considered “MRI invisible” as their signal will decay before the echo is acquired.

The TE can be reduced below this boundary by removing the frequency encoding “pre-winder” and starting data acquisition at the center of k-space (UTE)¹, or by exciting the sample while the frequency encoding is active (ZTE)^{2, 3}. This permits the capture of signals with T₂ on the order of 10s of microseconds, making many otherwise invisible substances visible. However, there is no free lunch, and both methods have drawbacks and limitations, relative to gradient and spin-echo sequences, which must be considered.

In this presentation, I will review the theory behind commonly used pulse sequences for UTE and ZTE and how to make them work well. I will focus the discussion on the difficulties associated with reducing the echo time to zero and how to combat these to produce high quality images.

Learning goals

After this presentation you should be able to:
1. Explain how to modify a sequence to substantially reduce the echo-time
2. Explain the difference between UTE and ZTE
3. Evaluate how sequence parameters in UTE and ZTE affect performance and image contrast