Arterial Spin Labelling: Non-invasive measurement of perfusion Michael A. Chappell [email protected] www.ibme.ox.ac.uk/QuBIc Institute of Biomedical Engineering & Wellcome Centre for Integrative Neuroimaging University of Oxford. Perfusion • Perfusion is a measurement of delivery of blood to capillary bed ➡ Related to nutrient delivery to cells and waste removal. ➡ Altered by task activity. ➡ Changes in disease. • Quantity of blood delivered per unit of tissue per unit of time ➡ ml blood / 100g tissue / min ➡ (Dimensions of [T]-1) ➡ Grey matter ‘magic’ number: 60 ml/100g/min • Cerebral Blood Flow (CBF) is a misleading name! • To image perfusion we need a tracer ➡ ASL uses blood-water as an endogenous tracer. Arterial Spin Labelling : M.A. Chappell Perfusion Why use ASL? • 50 ➡ A direct measure of perfusion changes - physiological response. 40 ➡ (Potentially) fully quantitative - possible to 30 calculate absolute perfusion. ml/100g/min ➡ Good for low frequency or ‘one-off’ 20 designs. ➡ Large ‘effect size’. • What are the challenges? ➡ SNR ➡ Temporal sampling - TR and the need for label and control scans. Arterial Spin Labelling : M.A. Chappell Perfusion • ASL is not BOLD! ➡ CBF change is a component of the BOLD signal. ➡ ASL can make absolute measurements under different conditions. ➡ You DONT need a interleaved design with ASL. FMRI Experiments ➡ ‘Rest’ and ‘task’ don’t even need to be in the same session. Stimulus • ASL and BOLD can be combined A B A B A ➡ Dual (mutli-) echo ASL/BOLD (baseline) (stimulus) (baseline) (stimulus) (baseline) e.g. flashing 0 1 2 3 4 5 chequerboard time (TRs) • Simple paradigm design:Arterial Spin Labelling : M.A. Chappell - stimulus vs baseline - constant stimulus “intensity” - constant block lengths - many repetitions: ABABA • Need baseline (rest) condition to measure change !"#$% !"#$%&"'"()*+,-.,/0#1.2"3"2*4&556*7&58*9%--.,(#*+:5;.*-<.*=",2&.*53*>"&&"#*8"-<*!.##.&0?(256.6*+,-.,"%&*@1"(*A%:.&"()* !"#$%&'(')*+,,-.'/01"%+,'2'3"%44+,,-.5.'%67'8+9+:';+<<%:7-! 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