Andrew M. Peters

Purpose: This work describes the development of a method to measure the variation of apparent diffusion coefficient (ADC) with diffusion time (Δ) in the brachial plexus, as a potential method of probing microstructure.Methods: Diffusion‐weighted MRI with body signal suppression was used to highlight the nerves from surrounding tissues, and sequence parameters were optimized for sensitivity to change with diffusion time. A porous media‐restricted diffusion model based on the Latour‐Mitra equation was fitted to the diffusion time‐dependent ADC data from the brachial plexus nerves and cord.Results: The ADC was observed to reduce at long diffusion times, confirming that diffusion was restricted in the nerves and cord in healthy subjects. T2 of the nerves was measured to be 80 ± 5 ms, the diffusion coefficient was found to vary from (1.5 ± 0.1) × 10⁻³ mm²/s at a diffusion time of 18.3 ms to (1.0 ± 0.2) × 10⁻³ mm²/s at a diffusion time of 81.3 ms.Conclusion: A novel method of probing restricted diffusion in the brachial plexus was developed. Resulting parameters were comparable with values obtained previously on biological systems. Magn Reson Med 79:789–795, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Purpose: In vivo ³¹P magnetic resonance spectroscopy (MRS) magnetization transfer (MT) provides a direct measure of neuronal activity at the metabolic level. This work aims to use functional ³¹P MRS‐MT to investigate the change in cerebral adenosine triphosphate (ATP) metabolic rates in healthy adults upon repeated visual stimuli.Methods: A magnetization saturation transfer sequence with narrowband selective saturation of γ‐ATP was developed for ³¹P MT experiments at 3 T.Results: Using progressive saturation of γ‐ATP, the intrinsic T₁ relaxation times of phosphocreatine (PCr) and inorganic phosphate (Pi) at 3 T were measured to be 5.1 ± 0.8 s and 3.0 ± 1.4 s, respectively. Using steady‐state saturation of γ‐ATP, a significant 24% ± 14% and 11% ± 7% increase in the forward creatine kinase (CK) pseudo‐first‐order reaction rate constant, k₁, was observed upon visual stimulation in the first and second cycles, respectively, of a paradigm consisting of 10‐minute rest followed by 10‐minute stimulation, with the measured baseline k₁ being 0.35 ± 0.04 s⁻¹. No significant changes in forward ATP synthase reaction rate, PCr/γ‐ATP, Pi/γ‐ATP, and nicotinamide adenine dinucleotide/γ‐ATP ratios, or intracellular pH were detected upon stimulation.Conclusion: This work demonstrates the potential of studying cerebral bioenergetics using functional ³¹P MRS‐MT to determine the change in the forward CK reaction rate at 3 T. Magn Reson Med 79:22–30, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

To develop a method that fits a multipool model to z-spectra acquired from non–steady state sequences, taking into account the effects of variations in T1 or B1 amplitude and the results estimating the parameters for a four-pool model to describe the z-spectrum from the healthy brain. We compared measured spectra with a look-up table (LUT) of possible spectra and investigated the potential advantages of simultaneously considering spectra acquired at different saturation powers (coupled spectra) to provide sensitivity to a range of different physicochemical phenomena. The LUT method provided reproducible results in healthy controls. The average values of the macromolecular pool sizes measured in white matter (WM) and gray matter (GM) of 10 healthy volunteers were 8.9% ± 0.3% (intersubject standard deviation) and 4.4% ± 0.4%, respectively, whereas the average nuclear Overhauser effect pool sizes in WM and GM were 5% ± 0.1% and 3% ± 0.1%, respectively, and average amide proton transfer pool sizes in WM and GM were 0.21% ± 0.03% and 0.20% ± 0.02%, respectively. The proposed method demonstrated increased robustness when compared with existing methods (such as Lorentzian fitting and asymmetry analysis) while yielding fully quantitative results. The method can be adjusted to measure other parameters relevant to the z-spectrum. Magn Reson Med 78:645–655, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Blood vessel segmentation is of great importance in medical diagnostic applications. Filter based methods that make use of Hessian matrices have been found to be very useful for blood vessel segmentation in both 2D and 3D medical images. However, these methods often fail on images that contain high density microvessels and background noise. The errors in the form of missing, undesired broken or incorrectly merged vessels eventually lead to poor segmentation results. In this paper, we present a novel method for 3D vessel segmentation that is also suitable for segmenting microvessels, incorporating the advantages of a line filter and a Hessian-based vessel filter to overcome the problems. The proposed method is shown to be reliable for noisy and inhomogeneous images. Vessels can also be separated based on their scale/thickness so that the method can be used for different medical applications. Furthermore, a quantitative vessel analysis method based on the multifractal analysis is performed on the segmented vasculature and fractal properties are found in all images.

The aim of this study was to optimise a pulse sequence for high‐resolution imaging sensitive to the effects of conventional macromolecular magnetisation transfer (MT^m) and nuclear Overhauser enhancement (NOE), and to use it to investigate variations in these parameters across the cerebral cortex. A high‐spatial‐resolution magnetisation transfer‐prepared turbo field echo (MT‐TFE) sequence was designed to have high sensitivity to MT^m and NOE effects, whilst being robust to B₀ and B₁ inhomogeneities, and producing a good point spread function across the cortex. This was achieved by optimising the saturation and imaging components of the sequence using simulations based on the Bloch equations, including exchange and an image simulator. This was used to study variations in these parameters across the cortex. Using the sequence designed to be sensitive to NOE and MT^m, a variation in signals corresponding to a variation in MT^m and NOE across the cortex, consistent with a reduction in myelination from the white matter surface to the pial surface of the cortex, was observed. In regions in which the stria was visible on T₂*‐weighted images, it could also be detected in signals sensitive to MT^m and NOE. There was greater variation in signals sensitive to NOE, suggesting that the NOE signal is more sensitive to myelination. A sequence has been designed to image variations in MT^m and NOE at high spatial resolution and has been used to investigate variations in contrast in these parameters across the cortex. Copyright © 2013 John Wiley & Sons, Ltd.

Purpose: To determine if tissue magnetic susceptibility is a more direct marker of tissue iron content than other MR markers of iron. This study presents the first quantitative, in vivo measurements of the susceptibility of the substantia nigra in patients with Parkinson's disease.Materials and Methods: Nine patients and 11 controls were studied at 7 Tesla. Susceptibility maps were created by inverting the filtered phase maps associated with T2* weighted images.Results: On average, patients showed an increase in susceptibility of the pars compacta compared with controls, which correlates with the predicted increase in brain iron in Parkinson's disease. A rostral–caudal gradient in susceptibility was also observed in controls and patients.Conclusion: Susceptibility mapping may provide a new tool for studying the development of Parkinson's disease. J. Magn. Reson. Imaging 2012;35:48‐55.

The transverse relaxation rate (R₂) of fresh human blood has been investigated at high and ultrahigh field, to characterize the R₂ dependency on blood sample oxygenation, hematocrit, and Carr‐Purcell Meiboom‐Gill sequence inter‐echo spacing. Data were fitted to chemical exchange and diffusion models to assess their performance at different field strengths. The diffusion model gave a slightly superior fit at both field strengths, but the difference is unlikely to be relevant for the signal to noise ratio achieved in most in vivo experiments. Fitted model parameters were similar to those found in literature. Magn Reson Med, 2010.

This paper presents methods of measuring the longitudinal relaxation time using inversion recovery turbo spin echo (IR-TSE) and magnetization-prepared rapid gradient echo (MPRAGE) sequences, comparing and optimizing these sequences, reporting T ₁ values for water protons measured from brain tissue at 1.5, 3, and 7T. T ₁ was measured in cortical grey matter and white matter using the IR-TSE, MPRAGE, and inversion recovery echo planar imaging (IR-EPI) pulse sequences.