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Published in Mara Cercignani, Nicholas G. Dowell, Paul S. Tofts, Quantitative MRI of the Brain: Principles of Physical Measurement, 2018
Claudine J. Gauthier, Audrey P. Fan
The most established quantitative, functional MRI alternative to BOLD is measurement of cerebral blood flow (CBF) using a class of methods called arterial spin labelling (ASL) (Detre et al., 2012). In most ASL techniques, inflowing blood is magnetically tagged at the level of the carotid arteries with radiofrequency (RF) inversion. Once the spins are inverted, for the ‘tag’ image, there is a prescribed post-label delay (PLD) time to allow the tagged inflowing blood to reach the imaging volume. An image of the brain is acquired, in which the tagged (inverted) blood has a lower signal intensity than the blood that was already in the imaging volume (the static signal). For the subsequent (‘control’) image, the procedure is repeated without the tag, so that all the blood has the same signal intensity. By subtracting the control and tag images, we can isolate the flow-dependent component of the signal from inflowing spins tagged at the level of the neck (Detre et al., 2012; Williams et al., 1992). Rapid acquisition of a separate image with fully relaxed longitudinal magnetisation allows calibration of perfusion images into CBF with units of ml/100 g/min (Figure 15.1).
Biomedical Imaging Magnetic Resonance Imaging
Published in Lawrence S. Chan, William C. Tang, Engineering-Medicine, 2019
In a technique that relies on an endogenous tracer, the magnetization of arterial blood spins is labeled by an inversion RF pulse, causing a signal reduction when the labeled blood enters the imaging location. This method is known as arterial spin labeling (ASL) which can produce quantitative or semi-quantitative maps of cerebral blood flow and other parameters (Alsop et al. 2015). ASL can be implemented in a number of ways depending on the type of the labeling pulse, the imaging sequence employed, the strategies to compensate for magnetization transfer effects, and other factors. Despite the variations, a consensus has been developed for consistent use of ASL techniques (Alsop et al. 2015).
Activity Images
Published in Hector J. Rabal, Roberto A. Braga, Dynamic Laser Speckle and Applications, 2018
One of fMRI techniques used to measure tissue perfusion in vivo is based on the use of an injected contrast agent that changes the magnetic susceptibility of blood and thereby the MR signal, which is repeatedly measured during passage of medicine. The other relies on arterial spin labeling (ASL), where arterial blood water is tagged using radiofrequency pulses that alter the magnetization of the blood before it enters into the tissue of interest. The amount of labeling is measured and compared to a control recording obtained without spin labeling. Both alternatives are expensive and the former is invasive.
A Functional BCI Model by the P2731 working group: Physiology
Published in Brain-Computer Interfaces, 2021
Ali Hossaini, Davide Valeriani, Chang S. Nam, Raffaele Ferrante, Mufti Mahmud
The primary modality within fMRI is BOLD (Blood Oxygen Level Dependent) MRI. As with light, oxyhemoglobin and deoxyhemoglobin and other components of tissue, respond differently to magnetic fields, and these changes can be tracked by radio pulses. fMRI exploits this trait to pinpoint changes in metabolism that accompany up-regulated neural activity. A complementary technique is arterial spin labeling (ASL) MRI which measures cerebral blood flow (CBF). However, ASL’s requirements, which include an injected contrast agent, have limited its use [55].