Black Blood and Bright Blood
Black blood and bright blood images of the heart (a short-axis view)
The contrast of flowing blood depends on many things, and it's not straightforward (see references below). However, we can make some statements in general which refer to the nature of how SE and GE images are generated. We must consider two effects: high-velocity signal loss, and flow related enhancement (FRE). Both are time-of-flight (TOF) effects.
High-Velocity Signal Loss (occurs in SE)
To explain black blood in SE we must understand high-velocity signal loss (or washout). This effect occurs only in SE, because of the presence of the 180° pulse. The tissues in a slice will receive both the (excitation) 90° pulse and the (refocussing) 180° pulse. Both pulses are slice selective. This is how we produce an echo in SE. But if the blood that was in the slice when the 90° pulse was applied has left the slice when the 180° pulse is applied (at TE/2), there will be no prepared magnetisation to refocus and form an echo from the blood. The blood in the slice at the time of the 180° pulse will not have received the 90° pulse, and so there will be no magnetisation in the transverse plane to refocus to an echo. Remember, in SE, tissues and blood must receive both RF pulses to produce an echo. So when blood flows out of the slice, "black blood" appears on the image (this is the absence of signal). This effect can be partial if only some of the excited blood has left the imaging slice between the two RF pulses.
High-velocity signal loss in SE images. (a) initial state, (b) excitation pulse, (c) excited blood leaves the slice, (d) the excited blood which has left the slice does not receive the slice-selective 180° pulse, (e) no signal is returned from the excited blood (Mxy of the blood has decayed away).
In GE, high-velocity signal loss does not occur. This is because the gradient which forms the echo in GE is not slice selective. Even if excited blood flows out of the slice, wherever the blood ends up, it will generate a signal appropriate to the location of the blood when it received the 90° excitation pulse. This produces the "bright blood" appearance.
No high-velocity signal loss in GE images. (a) initial state, (b) excitation pulse, (c) a measurement of the decaying signal of both the blood and the slice is taken quickly (short TR) before the Mxy decays away.
Flow Related Enhancement (FRE: important in GE)
The presence (in SE) or lack (in GE) of high-velocity signal loss is the main determinant of the difference in signal in flowing blood. However, FRE enhancement contributes to the bright blood effect in GE images.
As we gather data from a slab or slice during a GE imaging pulse sequence, the magnetisation of the tissues in the slice become partially saturated. This means that the longitudinal magnetisation is not allowed to fully recover between each repetition of the pulse sequence. However, blood which is not in the imaging slice has not had any prior RF pulses applied to it. If this blood flows into the slice, then its full longitudinal magnetisation will produce a brighter signal when a portion of it is flipped into the transverse plane (by the excitation pulse) for signal measurement. When multiple slices are scanned in a stack, this occurs most prominently in the first slice of the stack into which the fresh blood flows, and so it is sometimes referred to as an "entry slice" phenomenon. In conventional GE, slices are usually acquired one at a time, and so every slice is an "entry slice".
In SE, fresh blood which moves into a slice can also cause a higher signal (FRE) just like in GE, if that fresh blood has not left the slice by the time the 180° pulse is applied. This may be the case in slow venous flow, or when the direction of flow is not perpendicular to the slice. However, high-velocity signal losses quickly have a greater effect as the speed of blood flow increases. In that case, fresh blood moves in for the 180° pulse as well as the 90° pulse, so there is nothing to refocus in the transverse plane!
If you want to use reminders, remember these hints:
- SE (black blood): blood flows out of the slice and doesn't get the slice selective 180° pulse. This causes high-velocity signal loss.
- GE (bright blood): blood flows out of the slice, but it doesn't matter because the readout gradient is not slice selective. High-velocity signal loss does not occur.