Abstract
Both time-of-flight (TOF) and phase-contrast (PC) MRA techniques retrieve a hyperintense signal in the blood vessel from the motion of the spins during image acquisition. In TOF acquisitions, the relatively high signal in the blood vessels arises solely from so-called flow-related enhancement, a phenomenon which is due to the fact that the spins of the blood are maximally aligned with the main magnetic field when they enter an imaged slab or slice. At the same time, the flow-induced dephasing effects are overcome by flow refocusing gradients. The signal amplitude in PC acquisitions is also determined by the inflow phenomenon but the vessel contrast is improved in comparison with TOF examinations by the subtraction of the stationary tissue signals. In practice, a substantial flow-related enhancement is a prerequisite for a good MRA acquisition. However, the rapid succession of RF excitation pulses to which the spins flowing in the imaged volume are submitted gradually decreases their initial signal intensity. The signal amplitude decreases to a steady-state value that is very close to the (steady-state) signal amplitude of the surrounding stationary tissues.
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Marchal, G., Bosmans, H., Wilms, G. (1996). Contrast-Enhanced Magnetic Resonance Angiography. In: Arlart, I.P., Bongartz, G.M., Marchal, G. (eds) Magnetic Resonance Angiography. Medical Radiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-97926-2_8
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DOI: https://doi.org/10.1007/978-3-642-97926-2_8
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