Time-of-flight (TOF) effects refer to signal variations resulting from the motion of protons flowing into or out of an imaging volume during a given pulse sequence. In both spin-echo and gradient-echo imaging, inflow of spins results in increased signal; this phenomenon is known as flow-related enhancement. Conversely, outflow of spins may result in decreased signal intensity, a phenomenon known as high-velocity signal loss, also called "washout".
[left] TR >> T1 so there is complete recovery of Mz back to Mo before the 2nd RF-pulse.
[right] TR is shorter and Mz does not fully recover before next RF-pulse is applied. After a few pulses a new steady-state longitudinal magnetization (Mss) is reached, with Mss < Mo.
High-velocity signal loss. High-velocity signal loss, also known as "washout", is a time-of-flight effect that results in decreased intravascular signal. This phenomenon occurs especially in spin-echo imaging and is typically seen in arteries and larger veins that are flowing perpendicular plane of imaging. Unlike flow-related enhancement, high-velocity signal loss can occur anywhere in the imaging slab and is not confined to the end slices.
High-velocity signal loss occurs when spins flow out of slice before they are stimulated by both 90° and 180° pulses. It is therefore most prominent on long TE spin-echo images, since long echo times provide a greater opportunity for spins to flow out of the slice between the 90° and 180° pulses.
Advanced Discussion (show/hide)»
Historically, flow-related enhancement was sometimes called "paradoxical" enhancement, but this term has largely fallen into disuse.
The reason high-velocity signal loss does not occur on GRE imaging is that the refocusing mechanism (gradient reversal) is not slice selective.
Axel L. Blood flow effects in magnetic resonance imaging. AJR Am J Roentgenol 1984; 143:1157-1166.
Bradley WG Jr, Waluch V. Blood flow: magnetic resonance imaging. Radiology 1985; 154:443-450. Bradley WG Jr, Waluch V, Lai K-S, et al. The appearance of rapidly flowing blood on magnetic resonance images. AJR Am J Roentgenol 1084; 143:1167-1174.
Wehrli FW. Time-of-flight effects in MR imaging of flow. Magn Reson Med 1990; 14:187-193.
Wait a minute! If a partial flip angle pulse tips less magnetization into the transverse plane than a 90° pulse does, how can the signal be higher?