Target audience

MRI scientists and clinicians interested in understanding the basics of ASL data acquisition.

OUTCOME/Objectives

The objective of this talk is to introduce the methodology of ASL data acquisition and analysis. Upon attendance, the audience should have a basic understanding of the technique and be able to choose the most adequate pulse sequence for a particular clinical application and the most appropriate acquisition parameters.

Perfusion refers to the delivery of oxygen and nutrients to tissue by means of blood flow. Arterial spin labeling (ASL) (Williams et al. 1992) is a MRI technique that allows quantification of blood flow in physiological units of mL· min^-1·100 g^-1, in brain (Haller et al. 2016) and other tissues (Odudu et al. 2018). ASL is a non-invasive technique that does not require the use of any exogenous contrast agent. It uses electromagnetically labeled arterial blood water as endogenous tracer. Perfusion weighted images are obtained by subtraction of labeled images (“label”) from images acquired with control labeling (“control”). The ASL pulse sequence has two different elements: the labeling pulse and the image readout, separated by a time interval to allow the labeled blood to enter the imaged slices. Several strategies for labeling have been proposed: pulsed, continuous and pseudo-continuous (Alsop et al. 2015). Pulsed ASL (PASL) uses a 180º pulse to nearly instantaneously invert the magnetization of blood in a slab proximal to the imaged tissue. Continuous ASL (CASL) uses a long RF pulse in combination with a gradient to achieve velocity-driven adiabatic inversion of the blood flowing through an inversion plane. Pseudo-continuous ASL (PCASL) is a pulsed approximation to CASL, where the long RF pulse is broken in multiple short RF pulses played sequentially (Dai et al. 2008). In all these different approaches, perfusion weighted images are obtained by subtraction of labeled from control images, acquired using a control labeling pulse that does not invert the blood magnetization but compensates for the off-resonance effects induced by the labeling pulse. As opposed to BOLD, ASL contrast is not based on susceptibility, so a T2* weighted imaging sequence is not required or desirable. The use of spin-echo based sequences allows ASL measurements to be performed in regions of high static field inhomogeneities. 3D imaging sequences facilitate the use of background suppression, a technique that suppresses the static tissue signal to reduce noise from motion and other system instabilities. The translation of ASL from research to clinical application has been hampered by the large number of possible implementations, which have made difficult the harmonization and standardization of acquisition protocols. In recent years, an effort has been made within the ASL community to develop technical recommendations for clinical translation of ASL which should serve to accelerate this process (Alsop et al. 2015; Nery et al. 2020).

Acknowledgements

No acknowledgement found.

References

Alsop, D. C., Detre, J. A., et al. (2015). Perfusion MRI for clinical applications: a consensus of the ISMRM perfusion study group and the European consortium for ASL in dementia. Magn Reson Med 73:102-116. Dai, W., Garcia D., et al. (2008). Continuous flow-driven inversion for arterial spin labeling using pulsed radio frequency and gradient fields. Magn Reson Med 60(6): 1488-97. Haller, S., Zaharchuck, G., et al. (2016). Arterial spin labeling perfusion of the brain: emerging clinical applications. Radiology 281(2):337-356. Nery, F., Buchanan C. E., et al. (2020). Consensus-based technical recommendations for clinical traslation of renal ASL MRI. Magn Reson Mater Phy 33(1):141-161. Odudu, A., Nery, F., et al. (2018). Arterial spin labelling MRI to measure renal perfusion: a systematic review and statement paper. Nephrol Dial Transplant 33(suppl_2):ii15-ii21. Williams, D. S., Detre, J. A. et al. (1992). Magnetic resonance imaging of perfusion using spin inversion of arterial water.[erratum appears in Proc Natl Acad Sci U S A 1992 May 1;89(9):4220]. Proc Natl Acad Sci U S A 89(1): 212-6.