DOCTOR OF MEDICAL SCIENCE only initially, as speed of data acquisition and repeatability is impor- tant determinants (Raichle 2000). Thus, shorter-lived rCBF PET tracers such as oxygen-15 labelled water (H 15O) are preferable. Human brain mapping under 2 METHODOLOGY: rCBF MEASURED BY PET increasing cognitive complexity 15O-LABELLED WATER AS A rCBF PET TRACER Accurately quantifiable rCBF measures using both tomographic and using regional cerebral blood non-tomographic techniques have had great importance in the un- derstanding of the pathophysiology of the brain, e.g. in cerebrovas- flow measurements and positron cular disease, particularly acute ischemic stroke. Thus, much inter- est has been directed at characterizing the “penumbra”, a zone of emission tomography non-functioning, but viable, neural tissue with a rCBF of around 0.20 ml•min-1•g-1 surrounding the irreversibly damaged ischemic Ian Law core (Astrup et al. 1981; Baron 1999; Lassen 1990; Trojaborg and Boysen 1973). The penumbra may progress to infarct in hours to days, but is potentially salvageable if reperfused, e.g. by thrombo- This review has been accepted as a thesis together with six previously pub- lytic therapy, within this time window with important implications lished papers, by the University of Copenhagen, February 5, and defended for long-term clinical outcome. on August 31, 2007. The most widely applied rCBF PET tracer both for quantification Research Centre for brain and blood vessels, Akita, Japan, and clinical Physio- and in activation studies is H 15O. This is because of the short half- logic/Nuclear Medical Department, Rigshospitalet, Copenhagen, Denmark. 2 life (123 sec) that allows sequential studies, the ease of production Correspondence: PET & Cyklotron enheden, Afsnit 3982, Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen, Denmark. and administration, and low toxicity. The first published paper with H 15O as a quantitative rCBF PET tracer was in 1969, where Ter-Pog- E-mail: ilaw2pet.rh.dk 2 ossian and co-workers employed intracarotid injections of the tracer Official opponents: Albert Gjedde and Martin Lauritzen. mixed with blood using washout kinetics (Ter-Pogossian et al. 1969). Presently, quantification is most likely to be based on an adaptation Dan Med Bull 2007;54:289-305 of the Kety tissue autoradiographic method (ARG) (Kety 1951; Kety 1960). A freely diffusible, biologically inert radiotracer is injected in- AIM travenously, followed by a measurable progressive accumulation of 15 The aim of this thesis was to implement and evaluate methods to regional counts in the brain. However, H2 O is not a fully ideal rCBF perform human brain mapping studies of behaviours of increasing tracer, primarily due to it’s limited diffusion across the blood-brain- cognitive complexities using positron emission tomography (PET) barrier (Bolwig and Lassen 1975; Eichling et al. 1974). The water ex- and regional cerebral blood flow (rCBF) measures, and to under- traction has been measured to be between 87% (Friis et al. 1980) and stand the underlying limitations and advantages of existing methods 94% (Paulson et al. 1977). As ARG assumes diffusion equilibrium revealed through the introduction of a new PET rCBF tracer, Car- between tissue and venous blood, rCBF will be progressively under- 10 bon-10-labelled carbon dioxide ( CO2), and a new two-tissue com- estimated at high rCBF values (Herscovitch et al. 1983; Herscovitch partment kinetic model for the quantification of rCBF using oxy- et al. 1987; Pawlik et al. 1993; Raichle et al. 1983). Better flow tracer 15 15 gen-15 labelled water (H2 O). characteristics can be obtained with the lipophilic O-labelled n-bu- tanol. However, this tracer is metabolized and, thus, not biochem- INTRODUCTION ically inert. Moreover, it has a more complicated radiochemical syn- The process of assigning a physiological parameter signifying some thesis that could limit total number of scans feasible, and there are aspect of brain function to a spatial representation of the brain de- uncertainties about the exact brain blood partition coefficient in hu- fines the term “functional neuroimaging”. The overall medically rele- mans (Pawlik et al. 1993). Although, the quantitative differences be- vant aim is to attach physiological validity to brain-behaviour models tween the two tracers are in the order of 5-10 ml•min-1•g-1 in grey at the systems level, and provide a space where physiological predic- matter, direct comparisons have not shown appreciable performance tions and hypothesis can be made in conversation with these models differences in human brain mapping studies (Quarles et al. 1993). furthering the understanding of the intact and diseased brain. The neuroimaging techniques, primarily PET and functional magnetic TRACER KINETICS – THE AUTORADIOGRAPHIC ONE-TISSUE 15 resonance imaging (fMRI), enable documentation with very accurate COMPARTMENT MODEL USING H2 O spatial localization of the neurobiological organization in the human In the ARG the regional counts are summated over a given time pe- brain of often very complex behavioural manifestations and their dy- riod and represented by a single static PET scan, R. This scan can be namic interactions in vivo. The physiological basis for functional converted to represent the distribution of regional tissue concentra- neuroimaging using PET is the coupling between regional cerebral tions via scanner cross calibration. The model explains R by the glucose metabolism (rCMRglc), rCBF and regional neural activity rCBF, often denoted as f, and the radioactivity concentration of the (Villringer and Dirnagl 1995). The exact mechanisms are still de- arterial input function over time, Ca(t’). The final formulation of the bated and a thorough discussion lies outside the scope of this thesis. one-tissue compartment model gives: From its onset the study and use of rCBF had a particular geo- t t graphical affinity to Copenhagen and the southern Swedish towns of 2 2 t Malmø and Lund. It was here that professors Niels A. Lassen and R = ∫ R(t) dt = f ∫ ∫ Ca (t’)exp [–(f/p)(t – t’)] dt’ dt eq. (1) t t 0 David Ingvar in the 1960’ies established methods that quantified 1 1 rCBF using intracarotid injections of Xenon-133 (Høedt-Rasmus- where R(t) refers to instantaneous tissue concentration at time t, sen 1967) and directly demonstrated in normal man its modulation and R to the accumulation integral from scan start, t1, to scan end, with mental activity (Ingvar and Risberg 1965; Lassen et al. 1978; t2. The averaged brain tissue-to-blood partition coefficient of water, Roland and Larsen 1976; Roland et al. 1977). They were for at least a p, is fixed. A look-up table can be calculated for conversion of local decade almost alone among neuroscientist worldwide in embracing tissue counts, R, and the corresponding f or rCBF (Alpert et al. 1984; this principle (Raichle 2000). Functional brain mapping using rCM- Kanno et al. 1987; Kanno and Lassen 1979; Reivich et al. 1969). It is Rglc PET was applied early on (Mazziotta et al. 1981a), however essential that the arterial input function is corrected for delay and DANISH MEDICAL BULLETIN VOL.54NO. 4/NOVEMBER 2007 289 deconvolved with the dispersion function (Iida et al. 1986). The as- arterial input function were developed (Iida et al. 1986), and a more sumptions are that p and rCBF are homogenous in a region of inter- accurate parallel two-tissue compartment model was proposed est, and that rCBF is constant within the period of measurement. (Gambhir et al. 1987). The concept of the perfusable tissue fraction, α, that denotes the net fractional mass of tissue in a ROI that is ca- LIMITATIONS OF THE AUTORADIOGRAPHIC ONE-TISSUE pable of exchanging water, was intended to exclude non-perfused COMPARTMENT MODEL spaces, such as CSF, and to address the systematic rCBF underesti- Already in the initial implementation of the model it was recognized mation in the autoradiographic method introduced by the PVE that there would be a problem with tissue inhomogeneity (Hersco- (Iida et al. 1989) (Figure 1). The perfusable tissue fraction was inte- vitch et al. 1983), which simply adheres to the inability to correctly grated into early models based on intra-arterial injections of Xenon- 15 quantify the regional radioactivity distribution using methods of 133 (Høedt-Rasmussen 1967) or H2 O (Ter-Pogossian et al. 1969), limited spatial resolution, such as PET (Hoffman et al. 1979; Mazzi- external stationary sodium iodide detectors and a two-tissue com- otta et al. 1981b). As PET scanner resolution is finite, the counts in a partment clearance model that, however, being non-tomographic region of interest (ROI) is likely to represent a volume-weighted av- did not allow estimation of absolute functional volumes. 15 erage of contributions from different tissue segments, e.g. grey mat- In our implementation using H2 O and PET (eq. 2) we validated ter, white matter, oedematous, and neoplastic tissues, each with dif- four different models, two one-tissue compartment models with ferent rCBF’s and partition coefficients. Some of these counts arise and without a vascular term and two two-tissue compartment from crosstalk from neighbouring regions with “spill-out” of counts models with a white matter, or more correctly “slow” component, from high activity structures, usually grey matter, and “spill-in” to flow that was either fixed at 0.20 ml•min-1•g-1 or variable. The aim low activity structures, usually white matter and cerebral spinal fluid was to segregate, based on kinetic behavior, the tissue compartments (CSF). This phenomenon is termed the partial volume effect (PVE) in a ROI, and assign a perfusion value and a functional volume to (Figure 1). In other words there is a confounding influence of the each compartment (Figure 2). The models were applied to the size of a structure on the values quantified. This can be particularly macaque monkey (IV) and human (V).