Arterial input function

Function describing the concentration of a tracer in the blood over time

Arterial Input Function

The arterial input function (AIF) is a critical concept in medical imaging and pharmacokinetics, particularly in the context of dynamic contrast-enhanced imaging and positron emission tomography (PET). It describes the concentration of a tracer or contrast agent in the blood plasma as a function of time after its administration. The AIF is essential for quantifying physiological parameters such as blood flow, blood volume, and tissue perfusion.

Example of an arterial input function

Importance in Medical Imaging

In medical imaging, the AIF is used to interpret the distribution and kinetics of contrast agents or radiotracers within the body. It serves as a reference for understanding how these substances move from the bloodstream into tissues, which is crucial for diagnosing and monitoring various diseases, including cancer, cardiovascular disease, and neurological disorders.

Measurement Techniques

The AIF can be measured using several techniques, depending on the imaging modality:

• Magnetic Resonance Imaging (MRI): In dynamic contrast-enhanced MRI (DCE-MRI), the AIF is typically measured by acquiring rapid sequences of images after the injection of a gadolinium-based contrast agent. The concentration of the contrast agent in the blood is inferred from changes in signal intensity.

• Computed Tomography (CT): In dynamic contrast-enhanced CT (DCE-CT), the AIF is determined by measuring the attenuation of X-rays as they pass through the blood, which changes with the concentration of iodinated contrast agents.

• Positron Emission Tomography (PET): In PET, the AIF is measured by detecting the gamma rays emitted by a radiotracer, such as fluorodeoxyglucose (FDG), as it decays in the blood.

Challenges in Determination

Accurate determination of the AIF is challenging due to several factors:

• Partial Volume Effects: These occur when the blood vessel is smaller than the spatial resolution of the imaging modality, leading to underestimation of the tracer concentration.

• Patient Motion: Movement of the patient during image acquisition can distort the AIF measurement.

• Temporal Resolution: High temporal resolution is required to capture rapid changes in tracer concentration, which can be limited by the imaging technology.

Applications

The AIF is used in various applications, including:

• Quantitative Perfusion Imaging: By modeling the AIF, clinicians can calculate perfusion parameters such as cerebral blood flow and myocardial blood flow.

• Tumor Characterization: In oncology, the AIF helps in assessing tumor vascularity and response to therapy by quantifying parameters like Ktrans (volume transfer constant) and ve (extravascular extracellular volume fraction).

• Pharmacokinetic Modeling: The AIF is a key input in pharmacokinetic models that describe the distribution and elimination of drugs in the body.

Related Pages

• Dynamic contrast-enhanced imaging
• Positron emission tomography
• Pharmacokinetics
• Cerebral blood flow
• Myocardial perfusion imaging