Inflow refreshment is a means of achieving bright blood signal, especially for angiographic purposes. The basic mechanism relies on the physical displacement of blood, which quite literally introduces fresh blood signal between slice selective radiofrequency (RF) pulses. The means of applying this in two-dimensional (2D) and 3D imaging will be discussed.

In standard imaging, the signal of interest is usually not the signal produced by an isolated RF pulse, but the signal resulting from a train of RF pulses, that is, under conditions of dynamic equilibrium. Following each RF excitation, the magnetization vector, M₀, relaxes back to alignment with B₀ by the T1 relaxation process. The strength of the transverse signal following a train of RF pulses depends on the imaging parameters and the intrinsic properties of the body:

The optimum equilibrium signal for static tissue is achieved when the Ernst angle is applied. Using a larger flip angle results in a lower equilibrium signal, that is, a degree of signal suppression is achieved. This feature is exploited in 2D time of flight (TOF) or inflow refreshment imaging, to preferentially suppress static tissue.

Flowing blood that continuously refreshes the imaging plane does not achieve the equilibrium signal reached by the static tissue, and therefore appears as a relatively bright signal (see Fig. 19-1). This is the basis of the inflow refreshment phenomenon:

This so-called TOF mechanism relies on blood spins physically refreshing the imaging plane and exiting the plane rapidly, and thereby avoids becoming suppressed in the manner of static tissue signal.

Flow refreshment is incomplete if the flowing blood has a long residency time in the imaged slice. This occurs primarily under the following three conditions:

Under these conditions blood clears the plane slowly, and therefore experiences multiple RF pulses, serving to reduce the signal contribution from flow.

In the 2D TOF acquisition, each slice is fully acquired before advancing to the next slice location. In this way, extended coverage of the body is achieved by performing multiple slices, sufficient to cover the region of interest. In general, the slices are oriented orthogonal to the major direction of flow. Rapid through-plane flow generally appears bright, whereas in-plane flow or slow through-plane flow appears darker or even isointense with the static tissue (see Fig. 19-2). Signal saturation is particularly problematic for the following:

In-plane signal loss is known to be problematic for certain body regions, such as the carotid arteries where they undergo an excursion perpendicular to the main axis of the neck. However, for regions where blood flow is sufficient to refresh each slice, contrast can be very good (see Fig. 19-3). In the abdomen, respiration and peristaltic motion can result in some residual “static” tissue signal, which may obscure arteries when vessel rendering on the basis of signal brightness is used.