( t) ASL Modelling and Quantification. David Thomas. Overview of talk. Brief review of ASL. ASL CBF quantification model. ASL CBF quantification model

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1 verview of talk AL Modelling and Quantification David homas CL nstitute of Neurology Queen quare, London, K d.thomas@ucl.ac.uk Brief review of AL Descrie the 2 main AL quantification models model General kinetic model Pros and cons of multi- and single- Quantification methods using single- Assumptions of the models Consensus approach to AL quantification pcal Brief review of AL Laelled - Control ignal intensity in these images proportional to CBF Q: How do we convert these images into quantitative maps (ml/g/min)? image slices xchange of water etween lood and tissue (freely diffusile) AL CBF quantification model B L D utflowing venous lood (M v ) Brain tissue (M ) nflowing arterial lood (M a ) AL CBF quantification model AL CBF quantification model xchange of water etween lood and tissue (freely diffusile) ingle (well-mixed) compartment utflowing venous lood (M v ) Brain tissue (M ) nflowing arterial lood (M a ) Modification of Bloch equation (Detre et al MRM 992) dm M M ( t) = + f M a ( t) f M v ( t) dt Relaxation term olve to give: f λ M app Change in caused y arterial inflow control laelled M = control 2M Change in caused y venous outflow where λ is lood:rain partition coefficient f = + app λ

2 PAL CBF quantification model FAR technique (Kim and Kwong et al MRM 995) AL CBF general kinetic model PAL laeling of a proximal sla of arterial lood (PAR) image slices image slices nsr ssr Laeled mage Control mage Apparent recovery dependent on state and inflow rate of lood magnetization t hink of this as the inflow of a kinetic tracer ( arterial lood water) AL CBF general kinetic model AL CBF general kinetic model Arterial nput Function (AF) Retention of the tracer in the voxel Residue function issue voxel M a (t) PAL (p)cal.5.5 Delivery of the tracer to the voxel Arterial nput Function (M a ) Need to know what these functions are ime after laeling pulse (s) M a (t) = when t < olus arrival time (Δt) M a (t) = when t > Δt + olus duration (τ) ime since eginning of laeling pulse (s) M a (t) = 2M a.α.exp(-t/ ) for PAL M a (t) = 2M a.α for (p)cal α = inversion efficiency AL CBF general kinetic model Residue function (Res) racer reduces due to relaxation of lael m(t) = exp(-t/ ) racer lost due to venous outflow r(t) = exp(-f/λ.t) Res = m(t).r(t) m(t) r(t) m(t).r(t) AL CBF general kinetic model quation for general kinetic model (t) = CBF x [AF(t) * Res(t)] X.5 = t (s) t (s) t (s) t (s) 2

3 AL CBF general kinetic model AL CBF general kinetic model quation for general kinetic model quation for general kinetic model (t) = CBF x [AF(t) * Res(t)] (t) = CBF x [AF(t) * Res(t)] t (s) t (s) AL CBF general kinetic model Parameters of the general kinetic model olution for general kinetic model ensitivity to lood flow Parameters of the general kinetic model Parameters of the general kinetic model ensitivity to olus width ensitivity to olus arrival time 3

4 Quantification of CBF using AL data Quantification of CBF using AL data Acquire AL images over a range of inflow times Acquire AL images over a range of inflow times Fit the data to the general kinetic model Fit the data to the general kinetic model Fitted parameters: CBF, Δt, τ Fitted parameters: CBF, Δt, τ ther parameters needed: (lood ) Ma (equilirium lood signal) issue α (inversion efficiency) λ (lood:rain partition coeff) ther parameters needed: inglevalue - assumed (lood ) Ma (equilirium lood signal) Measured separate issue scans α (inversion efficiency) ingle value - assumed λ (lood:rain coeff) ingle valuepartition - assumed Pros of multi- acquisition Pros of multi- acquisition Allows fitting of AL data to kinetic model Allows measurement of other haemodynamic parameters (e.g. olus arrival time Δt) as well as CBF Allows fitting of AL data to kinetic model Allows measurement of other haemodynamic parameters (e.g. olus arrival time Δt) as well as CBF Controls M patients Cons of multi- acquisition Prolem for single quantification Requires the acquisition of a series of images for single measurement Low temporal resolution Not suitale for dynamic acquisitions e.g. fmr Can have poor measurement efficiency if sampling s where NR is low e.g. < Δt o, can we quantify using a single? 4

5 Prolem for single quantification Δt and CBF oth have strong influence on olution for single CAL quantification se a post-laeling delay (Alsop and Detre JCBFM 996) High CBF, long Δt Alsop and Detre JCBFM 996 Low CBF, short Δt Acquire Acquire agging pulse = 3s; = s olution for single CAL quantification se a post-laeling delay (Alsop and Detre JCBFM 996) Minimizes the dependence on Δt Also reduces the contriution of intravascular signal B lose NR due to extra decay PAR Dependence of PAL on transit time image slices (ms) (ms) Dealing with PAL transit time: QP Assumptions of the standard models nstantaneous exchange / single compartment utflowing venous lood (M v ) Δ Low slice saturated op slice acquire Brain tissue (M ) Δ Δ nflowing arterial lood (M a ) 5

6 Assumptions of the standard models nstantaneous exchange / single compartment Multi-compartment models e.g. see Parkes JMR 25 for review xchange V lood signal (M a, a, CBV a ) B L D utflowing venous lood (M v ) Brain tissue (M, ) nflowing arterial lood (M a ) Assumptions of the standard models No dispersion of the olus nclude dispersion as another parameter,2 ag as close as possile to the tissue tationarity of CBF during measurement Another reason to keep scan duration to a minimum Hrae and Lewis JMR 24; 2 Gallichan and Jezzard MRM 28 tandardization of AL Due to the variety of AL pulse sequences and quantification models, need for standardization MRM Perfusion tudy Group C Action: AL nitiative in Dementia (AD) Consensus Pulished in White Paper in MRM pcal with post-laeling delay Fixed in acquisition PLD =.8 / 2 s τ =.8 s ummary AL provides a non-invasive method for the asolute quantification of CBF (ml/g/min) wo main quantification models model General kinetic model AL can e acquired as multi- or single- Multi- more roust ut time-consuming ingle- etter suited to dynamic imaging ut relies on assumptions for CBF estimatation Constants Measured D:.2/mrm.2597 seful References Detre et al MRM 23:37-45 (992) Alsop and Detre JCBFM 6: (996) Buxton et al MRM 4: (998) Wong et al MRM 39:72-79 (998) Golay et al op Magn Reson maging 5: 27 (24) JMR 22(6) perfusion review edition (25) C AD AL White Paper D:.2/mrm