Outflowrelated Signal Loss washout effectT2 flow void

When images are obtained with a spin-echo (SE) pulse sequence, blood flowing at a high velocity perpendicular to the imaging plane produces a weaker signal than the surrounding stationary tissue. This phenomenon is caused by the washout of flowing spins from the slice during the imaging process.

Spin-echo techniques are characterized by a sequence of slice-selective 90° and 180° radio frequency (RF) pulses. Only those tissue components that are affected by both pulses can provide an MR signal. Moving material, such as blood in the vessels, flowing through the excited slice at a suffi

Ultralydsscanning Carotis Externa

Fig. 1. Comparison of flow profiles in the external carotid artery (ECA), the internal carotid artery (ICA) and the middle cerebral artery (MCA). There is very pulsatile flow in the ECA. However, in the intracranial vessels, the variation of flow during heart cycle is much less pronounced [Courtesy of Steffi Behnke, MD, Dept. of Neurology, Saarland University Hospitals]

Fig. 1. Comparison of flow profiles in the external carotid artery (ECA), the internal carotid artery (ICA) and the middle cerebral artery (MCA). There is very pulsatile flow in the ECA. However, in the intracranial vessels, the variation of flow during heart cycle is much less pronounced [Courtesy of Steffi Behnke, MD, Dept. of Neurology, Saarland University Hospitals]

High Velocity Signal Loss

ciently high velocity, is affected by only one of these pulses, and therefore does not contribute to the MR signal. This is the so-called flow void (Fig. 2).

The intensity of the vascular signal declines with

• decreasing slice thickness, s,

• increasing flow velocity, v.

If the blood flow velocity is so high that all spins leave the slice between the 90°- and 180°-pulses (v > s/(TE/2)), there will be no signal at all and the vessel will appear dark.

Spins flowing within the imaging plane are not affected by this phenomenon.

Fig. 2. In spin-echo sequences, blood flowing out of the measured slice in the time between 90°- and 180°-ra-diofrequency pulses leads to signal loss

The washout effect is observed only for SE sequences and is most pronounced on T2- weighted imaging because of the long echo times used. With gradient-echo (GRE) techniques, the echo is refo-cused without a 180° pulse simply by reversing the imaging gradients. Since only one RF pulse is needed to form an echo, the washout effect does not occur.

With standard SE sequences the washout effect provides valuable and reliable information about blood flow. The absence of a flow void on T2-weighted imaging should be considered as indicative of very slow flow or even occlusion of the vessel (Fig. 3). On the other hand, occlusion of the vessel can be excluded if a flow void is present.

Basilar Artery Occlusion
Fig. 3. Axial T2-weighted spin-echo image of the pons region. Left. Missing flow void in the basilar artery indicates thrombosis of the vessel. Right After thrombolysis, the vessel (arrow) appears dark due to restored flow

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  • saare
    Can vessel thrombosis appear dark on t2?
    8 years ago

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