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10 Chapter 1 Introduction Functional neuroimaging usually adds diagnostic value when structural imaging result is equivocal. MR based measurements such as perfusion imaging, MR spec- troscopy, diffusion weighted imaging (DWI), and functional MRI (fMRI), are more sensitive to the functional decline preceding the structural changes, and show po- tential for markers for early diagnosis. Molecular imaging using emission tomogra- phies such as positron emission tomography (SPECT) and single-photon emission computed tomography (SPECT) are also used to assess the biology underlying the neurodegeneration process. In current clinical practices, structural MRI and PET are most commonly de- ployed. In this thesis, the data dealt with are mainly the structural MR brain images, therefore we will only introduce the brief background of structural MRI and its application in the AD studies. 1.1.3.1 MRI The physical principle underlying the MRI is nuclear magnetic resonance, which describes the dynamics and manipulates the behavior of nuclear spins or other magnetic systems possessing angular momentum in an external magnetic field (for the physical background of the nuclear resonance, see Slichter, 1990; Levitt, 2008). In MRI, the predominant effect giving rises to the MR signal comes from the proton of hydrogen atom with 1 spin. The spin moments aligned either in parallel or anti- 2 parallel to the external field B0 split into two distinct energy levels, conforming to the Boltzmann distribution at thermal equilibrium, with more spinors in the same direction of B0. The net effect of the spin excess is the magnetization that can be detected. The phenomenon of Larmor precession in classical electromagnetism is retained in quantum mechanics. By applying an additional oscillating radiofrequency (rf) magnetic field tuned to the Larmor frequency of the proton, the magnetization vector can be tipped away from its original longitudinal direction parallel to B0 by π, such that the magnetization rotates in the transverse plane perpendicular to 2
Chapter 1 Introduction 11 B0. The rotation of the transverse magnetization induces voltage in the receiving coil which turns into electric signal. Because of the spin-lattice interaction, the rf signal decays as the magnetization tend to re-align to the direction of the static field B0. The decay is characterized by the spin-lattice relaxation time T1. The interacting nuclei of different spin state lead to the spin-spin relaxation, in which the individual spins fan out, or dephase, and the net transverse component of magnetization decreases at a rate controlled by the spin-spin relaxation time T2. In practice the magnetization undergoes T ′ 2 relaxation due to the inhomogeneities in the external magnetic field. The overall T ∗ 2 decay is combined effect of T2 and T ′ 2. The values of T1 and T2 depend on the physical property of the material, which varies among the population according to the tissue type and condition. The suppression caused by field inhomogeneities can be avoided by an additional flip of spins by π using rf pulses, creating an ‘echo’ of the signal. By tuning the interval TR over which the recovered longitudinal magnetization is periodically tipped to the transverse plane by the rf pulse, and the echo time TE when the echo is detected, the result images displays different forms of contrast. A T1 weighted image acquired with TR ≤ T1, and TE relatively short to T2 highlights the differences in T1 within the sample, while the T2 weighted image produced by a setting of relatively long T1 and TE T2 enhances the contrasts between materials with different T2. With relatively long TR and short TE, the spin density or proton density weighted image displays only the density of spin without contrast in T1 or T2. In imaging experiments, a spatial gradient in the magnetic field is used to encode the spins in the sample with varying frequency and phase at different location. The signal acquired is thus in the k-space of wave vector, which is a Fourier transform of the spatial information. An inverse Fourier transform can be applied to reconstruct the spatial content from the reciprocal domain of k-space, forming the image of the sample. A thourough treatment of the subject from its physical
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Chapter 6 Conclusions 139 Given the
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144 LIST OF PUBLICATIONS using dive
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146 BIBLIOGRAPHY Alzheimer, A. (191
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148 BIBLIOGRAPHY Baillard, C., Hell
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150 BIBLIOGRAPHY of post mortem mag
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152 BIBLIOGRAPHY B. (2008). Three-d
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154 BIBLIOGRAPHY Cootes, T. F., Tay
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156 BIBLIOGRAPHY head in MR images
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158 BIBLIOGRAPHY Evans, A. C., Coll
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BIBLIOGRAPHY 161 Ghanei, A., Soltan
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BIBLIOGRAPHY 163 Heckemann, R. A.,
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BIBLIOGRAPHY 165 R. C. (2003). MRI
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BIBLIOGRAPHY 167 Konrad, C., Ukas,
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BIBLIOGRAPHY 169 Mahony, R. and Man
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BIBLIOGRAPHY 171 disease, mild cogn
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BIBLIOGRAPHY 173 Petersen, R. C., S
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BIBLIOGRAPHY 175 Rorden, C. and Bre
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BIBLIOGRAPHY 179 Barillot, C., Hayn
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BIBLIOGRAPHY 181 Tzourio-Mazoyer, N
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BIBLIOGRAPHY 183 Vos, F. M., de Bru
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BIBLIOGRAPHY 185 Woolrich, M. W., J
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