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Daniel Gallichan

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3 papers

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YNIMG Journal 2020 Journal Article

Sharpness in motion corrected quantitative imaging at 7T

Pierre-Louis Bazin
Hannah E. Nijsse
Wietske van der Zwaag
Daniel Gallichan
Anneke Alkemade
Frans M. Vos
Birte U. Forstmann
Matthan W.A. Caan

Sub-millimeter imaging at 7T has opened new possibilities for qualitatively and quantitatively studying brain structure as it evolves throughout the life span. However, subject motion introduces image blurring on the order of magnitude of the spatial resolution and is thus detrimental to image quality. Such motion can be corrected for, but widespread application has not yet been achieved and quantitative evaluation is lacking. This raises a need to quantitatively measure image sharpness throughout the brain. We propose a method to quantify sharpness of brain structures at sub-voxel resolution, and use it to assess to what extent limited motion is related to image sharpness. The method was evaluated in a cohort of 24 healthy volunteers with a wide and uniform age range, aiming to arrive at results that largely generalize to larger populations. Using 3D fat-excited motion navigators, quantitative R 1, R 2 * and Quantitative Susceptibility Maps and T 1-weighted images were retrospectively corrected for motion. Sharpness was quantified in all modalities for selected regions of interest (ROI) by fitting the sigmoidally shaped error function to data within locally homogeneous clusters. A strong, almost linear correlation between motion and sharpness improvement was observed, and motion correction significantly improved sharpness. Overall, the Full Width at Half Maximum reduced from 0. 88 mm to 0. 70 mm after motion correction, equivalent to a 2. 0 times smaller voxel volume. Motion and sharpness were not found to correlate with the age of study participants. We conclude that in our data, motion correction using fat navigators is overall able to restore the measured sharpness to the imaging resolution, irrespective of the amount of motion observed during scanning.

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YNIMG Journal 2018 Journal Article

Diffusion MRI of the human brain at ultra-high field (UHF): A review

Daniel Gallichan

The continued drive towards MRI scanners operating at increasingly higher main magnetic fields is primarily motivated by the maxim that more teslas mean more signal and lead to better images. This promise of increased signal, which cannot easily be achieved in other ways, encourages efforts to overcome the inextricable technical challenges which accompany this endeavor. Unlike for many applications, however, diffusion imaging is not currently able to directly reap these potential signal gains – at the time of writing it seems fair to say that, for matched gradient and RF hardware, the majority of diffusion images acquired at 7T, while comparable in quality to those achievable at 3T, do not demonstrate a clear advantage over what can be obtained at lower field. This does not mean that diffusion imaging at UHF is not a worthwhile pursuit – but more a reflection of the fact that the associated challenges are manifold – and converting the potential of higher field strengths into ‘better’ diffusion imaging is by no means a straightforward task. This article attempts to summarize the specific reasons that make diffusion imaging at UHF more complicated than one might expect, and to highlight the range of developments that have already been made which have enabled diffusion images of excellent quality to be acquired at 7T.

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YNIMG Journal 2007 Journal Article

A calibration method for quantitative BOLD fMRI based on hyperoxia

Peter A. Chiarelli
Daniel P. Bulte
Richard Wise
Daniel Gallichan
Peter Jezzard

The estimation of changes in CMRO2 using functional MRI involves an essential calibration step using a vasoactive agent to induce an isometabolic change in CBF. This calibration procedure is performed most commonly using hypercapnia as the isometabolic stimulus. However, hypercapnia possesses a number of detrimental side effects. Here, a new method is presented using hyperoxia to perform the same calibration step. This procedure requires independent measurement of PaO2, the BOLD signal, and CBF. We demonstrate that this method yields results that are comparable to those derived using other methods. Further, the hyperoxia technique is able to provide an estimate of the calibration constant that has lower overall intersubject and intersession variability compared to the hypercapnia approach.

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