Arterial input function
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Arterial input function (AIF) is commonly defined as the concentration of the contrast medium in an artery measured over time by placing a region of interest.
Use in MRI and CT
It is important to be precise that on MRI the estimation of the concentration is obtained indirectly from the induced changes in the relaxation times 1.
The arterial input function is graphically described by a time-signal intensity curve on MRI or a time-attenuation curve on CT. These curves are characterized by a baseline (before the arrival of the bolus), a sharp rise to the "peak" (the maximum concentration of contrast medium) followed by a slow decrease towards the baseline 2.
Selection of the region of interest is critical for the correct estimation of Arterial input function and usually involves the choice of a major artery (eg. middle cerebral artery in brain 3 or aorta in abdomen 4).
In neuroimaging, the determination and deconvolution of arterial input function curves are essential in both dynamic susceptibility MRI and CT perfusion to obtain parameters such as cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), time-to-peak (TTP) and time-to-maximum (Tmax).
On the other hand in dynamic contrast-enhanced MRI, it is possible to calculate other perfusion parameters such as Ktrans (used in neuroimaging and abdominal imaging) by deconvoluting arterial input function curves.
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