Poster Abstracts Tuesday, November 8, 2005 $261

oxy- and deoxy-hemoglobin, this complementary approach allows to analyse neurovascular coupling non-invasively in the human brain. We recorded DC-MEG and trNIRS simultaneously over the left primary motor cortex hand area in healthy subjects during finger movements of the right hand. DC-fields and trNIRS parameters followed closely tile motor task cycles revealing statistically significant differences between periods of finger movements and rest. In subjects with suffident signal-to-noise ratio the analysis of variance of photon time of flight for a 2-layer model demonstrates task-related changes originating from tile deeper layer, i.e., tile cortex. Notably, while onset and relaxation started sinmltaneously trNIRS signals reached 50% of the mmximum level 1-4 sec later than MEG-signals. This approach provides an opportunity to analyse the "hemodynamJc inverse problem" which refers to the challenge of making valid and predse estimates of underlying cortical neuronal activity from measured hemo- dynamic resposes, e.g., in tMRL In addition, tile feasibility to non-invasively monitor cortical low amplitude DC-fields, deoxy- and oxy-Hb simultaneously in humans will allow to scrutinise pathophy- siological stroke concepts, e.g. anoxic depolarisation, periinfarct- depolarisation and spreading depression. Notably fields of even higher magtfftude (up to 2pT) than resolved in this MEG-study have been shown in animal studies.

0636 Quantitation of Regional Brain Iron Content with Magnetic Resonance Imaging

Martin, W 1, Wieler, M a, Gee, M 1. 1University of Alberta, Edmonton, Canada

Background: The distribution of iron in the human brain is hetero- geneous with lffgh concentrations observed primarily in basal ganglia structures. Neurodegenerative disorders such as Parkinson's disease (PD) have been reported to be associated with changes in regional brain iron content, based on direct tissue measurements. The possibility of a non-invasive, semi-quantitative measurement of brain iron with magnetic resonance imaging (MR[) is suggested by the knoval effect of iron on transverse relaxation rates, although trans- verse relaxation is also affected by other sources of magnetic field inhomogeneity, such as that evident at the air-tissue interface in sinuses. Method: We have previously reported a method for measuring brain iron with MRI that mitigates other contributions to changes in transverse relaxation, making the technique most sensitive to iron content, particularly at high field strengths ([Wild et al., Magn Reson Meal 2002; 48:867-876). Tiffs technique was applied on a 3.0 T imaging system in a series of untreated patients with early PD and in age- matched controls. Results: A very close relationship was found between measurements of transverse relaxation rate and literature values for regional brain iron content in basal ganglia structures. The only significant difference between PD patients and controls was in the medial substantia uigra compacta, consistent with increased iron in tiffs structure in early PD. Conclusion: These results support the use of this MRI method at high field strengths for performing in vivo measurements of regional brain iron content, and suggest that nigral iron content may be abnormal in early PD.

0637 Semi-quantitative assess~nent of brain protein synthesis using L-[l-11C]-leucine PET

Muzik, O 1"2, Sundaram, S 1"3, Mu, F 2, Mangner, T 2, Chugani, D 1"2, Chugani, H 12'3. 1Carman and Ann Adams Dept of Pediatrics, Children's Hospital of Ivlichigan PET Center, Wayne State University, Detroit, United States; 2Department of Radiology, Children's Hospital of Michigan, IVayne State University, Detroit, United States; SDepartment of Neurology, Children's Hospital of Michigan, IVayne State University, Detroit, United States

Background: The objective of this study was to derive a stable index of brain protein synthesis based on the unidirectional uptake rate constant (Kcplx) of L-J1-11C]-leucine (LEU) in brain. Method: Fourteen healthy volunteers (6 males, 20-45 years) underwent dyaanffc LEU PET imaging. The tracer LEU (0.35 mCi/kg) was adnffnistered and arterial blood samples were drawll and analyzed for metabolites. The fraction of unmetabolized LEU in plasma was fitted with a sigmoidal function. Time activity curves were fitted with a two- tissue compartmental model consisting of an unidirectional protein pool and a combined pool representing both free and metabolized leucine in cytoplasnc The fraction of leucine originating from exogenous sources was estimated as k2/(k2+k3). Furthermore, the Patlak plot was used to determine the unidirectional uptake rate constant (Kcplx -- (Klk3/(k2 + k3)). Results: Identifiability of individual parameters was excellent (COV < 20%). The fraction of leucine incorporated into protein originating from exogenous amino acids was 0.65 + 0.02 and did not differ between males and females. In contrast, regional Kcplx values in males were in general 30% lower than those in females but following the same rank order ([visual cortex > cerebellum > brainstem > thalamus > frontal cortex > wlffte matter). Kcplx values derived from a Patlak

plot using data between 30 and 60 man pi. were within 10% of these values. Lastly, application of a population-derived blood metabolite correction had only a manor effect on the Kcplx (< 10'%). Conclusion: We conclude that the Kcplx is a stable index of brain protein synthesis rate well-suited for routine application.

0638 In-vivo magnetic lield correlation hnaging of bmnan brain at 3 tesla

Ramani, A 1, Jensen, JH 1 , Kacynski, KR 3, Helpern, JA 1,2,4. 1Department of Radiology, New York University School of Medicine, New York, NY; 2Departments of Psychiatry and Physiology & Neuroscience, New York University School of Medicine; 3Siemens Medical Systems, USA; 4Center for Advanced Brain Imaging; 4The Nathan S. Kline Institute, Orangeburg, New York

Introduction: As water molecules diffuse through a tissue under the influence of a strong uniform applied magnetic field, typically encountered in a MRI scanner, they sample a complex landscape of field inhomogeneities generated by variations in magnetic suscept- ibility. These susceptibility variations arise not only from macroscopic structures, such as air cavities and large veins, but also from micro- scopic structures, such as capillaries and iron-rich cells. The field inhomogeneities experienced by water molecules can be quantified by their magnetic field correlation (MFC). Tiffs is formally defined by Eqn.

K(t2 h) - (B(t2)B(h)} [11

where B(t} is the field shift, relative to the uniform background field, for a particular water molecule at a time t and with the angle brackets indicating an averaging over all the molecules within a given region of interest (1). The MFC can be calculated using an asynmietric spin echo (ASE) sequence (1). In this abstract, we have demonstrated tile feasibility of M F C imaging at 3 Tesla by showing in-vivo MFC images of the human brain and have also correlated M F C values with putative iron concentrations in spedfic brain regions. This technique may provide important information in the assessment of neurological disorders with associated iron abnomlalities, such as Parkinson's disease, Alzheimer's disease, Hallervorden-Spatz syndrome etc. THEORY: Consider an asymmetric SE sequence in wlffch tile 180 ° refocusing pulse is shifted by a time t+ from its standard position (i.e. t,. - 0 corresponds to a conventional SE sequence). If the field inhomogeneities are not too large, then we can write:

, , T E S(t0 S0exp[ 2t~'f K ( . T ) ] [2]