Doctor of Philosophy, Dalhousie University (2010)
Bachelor of Science, University of New Brunswick (2005)
Susceptibility field gradients (SFGs) cause problems for functional magnetic resonance imaging (fMRI) in regions like the orbital frontal lobes, leading to signal loss and image artifacts (signal displacement and "pile-up"). Pulse sequences with spiral-in k-space trajectories are often used when acquiring fMRI in SFG regions such as inferior/medial temporal cortex because it is believed that they have improved signal recovery and decreased signal displacement properties. Previously postulated theories explain differing reasons why spiral-in appears to perform better than spiral-out; however it is clear that multiple mechanisms are occurring in parallel. This study explores differences in spiral-in and spiral-out images using human and phantom empirical data, as well as simulations consistent with the phantom model. Using image simulations, the displacement of signal was characterized using point spread functions (PSFs) and target maps, the latter of which are conceptually inverse PSFs describing which spatial locations contribute signal to a particular voxel. The magnitude of both PSFs and target maps was found to be identical for spiral-out and spiral-in acquisitions, with signal in target maps being displaced from distant regions in both cases. However, differences in the phase of the signal displacement patterns that consequently lead to changes in the intervoxel phase coherence were found to be a significant mechanism explaining differences between the spiral sequences. The results demonstrate that spiral-in trajectories do preserve more total signal in SFG regions than spiral-out; however, spiral-in does not in fact exhibit decreased signal displacement. Given that this signal can be displaced by significant distances, its recovery may not be preferable for all fMRI applications.
View details for DOI 10.1016/j.mri.2012.02.014
View details for Web of Science ID 000305307100002
View details for PubMedID 22503093
This work proposes the use of TurboSPI, a multi-echo single point imaging sequence, for the quantification of labeled cells containing moderate to high concentrations of iron oxide contrast agent. At each k-space location, TurboSPI acquires several hundred time points during a spin echo, permitting reliable relaxation rate mapping of large-R(2)(?) materials. An automatic calibration routine optimizes image quality by promoting coherent alignment of spin and stimulated echoes throughout the multi-echo train, and this calibration is sufficiently robust for in vivo applications. In vitro relaxation rate measurements of SPIO-loaded cervical cancer cells exhibit behavior consistent with theoretical predictions of the static dephasing regime in the spin echo case; the relaxivity measured with TurboSPI was 10.47±2.3 s(-1)/mG, comparable to the theoretical value of 10.78 s(-1)/mG. Similar measurements of micron-sized iron oxide particles (0.96 ?m and 1.63 ?m diameter) show a reduced relaxivity of 8.06±0.68 s(-1)/mG and 7.13±0.31 s(-1)/mG respectively, indicating that the static dephasing criterion was not met. Nonetheless, accurate quantification of such particles is demonstrated up to R(2)(?)=900 s(-1), with a potentially higher upper limit for loaded cells having a more favorable R(2)('):R(2) ratio. Based on the cells used in this study, reliable quantification of cells loaded with 10 pg of iron per cell should be possible up to a density of 27 million cells/mL. Such quantification will be of crucial importance to the development of longitudinal monitoring for cellular therapy and other procedures using iron-labeled cells.
View details for DOI 10.1016/j.jmr.2012.01.017
View details for Web of Science ID 000302338200020
View details for PubMedID 22364896
Functional magnetic resonance imaging (fMRI) in white matter has long been considered controversial. Recently, this viewpoint has been challenged by an emerging body of evidence demonstrating white matter activation in the corpus callosum. The current study aimed to determine whether white matter activation could be detected outside of the corpus callosum, in the internal capsule. Data were acquired from a 4 T MRI using a specialized asymmetric spin echo spiral sequence. A motor task was selected to elicit activation in the posterior limb of the internal capsule.White matter fMRI activation was examined at the individual and group levels. Analyses revealed that activation was present in the posterior limb of the internal capsule in 80% of participants. These results provide further support for white matter fMRI activation.The ability to visualize functionally active tracts has strong implications for the basic scientific study of connectivity and the clinical assessment of white matter disease.
View details for DOI 10.1186/1471-2202-12-56
View details for Web of Science ID 000292841800001
View details for PubMedID 21672250
The idea of fMRI activation in white matter (WM) is controversial. Our recent work has used two different approaches to investigate whether there is evidence for WM fMRI. The first approach used words and faces to elicit interhemispheric transfer activation in the posterior corpus callosum (Sperry task). The second approach used checkerboard stimuli to elicit similar activation in the anterior corpus callosum (Poffenberger task). Using these different tasks, it has been possible to detect WM activation in different regions. In the current study, we report the results of a critical experiment: demonstrating that callosal activation can be experimentally manipulated within the same set of individuals.All subjects completed both the Sperry and Poffenberger tasks. Functional MRI data were acquired at 4T, using an asymmetric spin echo spiral sequence. Data were analyzed with FSL using a model-based approach. Analyses focused on group and individual activations in WM.Corpus callosum activation was elicited for both tasks, with activation varying according to task type. A statistical contrast of the two tasks revealed posterior callosal activation for the Sperry task and anterior callosal activation for the Poffenberger task. The Sperry task showed activation in the isthmus and middle body of the corpus callosum at the group level and in 100% of subjects. The Poffenberger task showed activation in the genu and middle body of the corpus callosum at the group level and in 94% of subjects. The WM activation replicated prior results, with the additional strength of functional mapping within the same group of individuals.
View details for DOI 10.1016/j.neuroimage.2010.07.028
View details for Web of Science ID 000283825000003
View details for PubMedID 20643213
Recently, functional magnetic resonance imaging (fMRI) activation has been detected in white matter, despite the widely-held belief that fMRI activation is restricted to gray matter. The objective of the current study was to determine whether the regions of white matter fMRI activation were structurally connected to the functional network in gray matter. To do this, we used fMRI-guided tractography to evaluate whether tracts connecting regions of gray matter fMRI activation were co-localized with white matter fMRI activation. An established interhemispheric transfer task was employed to elicit activation in the corpus callosum. Diffusion tensor imaging (DTI) tractography was used to determine the existence of tracts that connected regions of gray matter fMRI activation to regions of activation in the corpus callosum. Corpus callosum activation was detected in the majority of participants. While there was individual variability in the location of corpus callosum activation, activation was commonly observed in the callosal mid-body, isthmus/splenium, or both. Despite the variability, gray matter fMRI-guided tractography identified tracts that were co-localized with corpus callosum fMRI activation in all instances. In addition, callosal activation had tracts to bilateral gray matter fMRI activation for 7/8 participants. The results confirmed that the activated regions of the corpus callosum were structurally connected to the functional network of gray matter regions involved in the task. These findings are an important step towards establishing the functional significance of white matter fMRI, and provide the foundation for future work combining white matter fMRI and DTI tractography to study brain connectivity.
View details for DOI 10.1016/j.neuroimage.2009.12.102
View details for Web of Science ID 000274948400029
View details for PubMedID 20053383
Functional MRI (fMRI) is of limited use in areas such as the orbitofrontal and inferior temporal lobes due to the presence of local susceptibility-induced field gradients (SFGs), which result in severe image artifacts. Several techniques have been developed to reduce these artifacts, the most common being the dual-echo spiral sequences (spiral-in/out and spiral-in/in). In this study, a new multiple spiral acquisition technique was developed, in which the later spiral acquisitions are acquired asymmetrically with the peak of a spin-echo causing increased R(2)-weighting but matched R(2)'-weighting. This sequence, called asymmetric spin-echo (ASE) spiral, has demonstrated significant improvements in minimizing the signal loss and increasing the image quality as well as optimal blood-oxygen-level-dependent (BOLD)-weighting. The ASE spiral is compared to conventional spiral-out using both signal-to-noise ratio (SNR) and whole brain fMRI activation volumes from a breath-hold task acquired at 4 Tesla. The ASE dual spiral has exhibited SNR increases of up to 300% in areas where strong SFGs are present. As a result, the ASE spiral is highly efficient for recovering lost activation in areas of SFGs, as demonstrated by a 16% increase in the total number of activated voxels over the whole brain. Post spin-echo ASE spiral images have decreasing SNR due to R(2) signal losses, however the increase in R(2)-weighting leads to a higher percentage of signal changes producing ASE spiral images with equivalent contrast-to-noise ratio (CNR) for each echo. The use of this sequence allows for recovery of BOLD activation in areas of SFG without sacrificing the CNR over the whole brain.
View details for DOI 10.1002/nbm.1380
View details for Web of Science ID 000268286500010
View details for PubMedID 19382109
The majority of functional magnetic resonance imaging (fMRI) studies restrict their focus to gray matter regions because this tissue is highly perfused relative to white matter. However, an increasing number of studies are reporting fMRI activation in white matter. The current study had two objectives: 1) to evaluate whether it is possible to detect white matter fMRI activation and 2) to determine whether certain MRI contrast mechanisms are more sensitive to white matter activation (i.e., BOLD contrast- versus T(2)-weighting). Data were acquired from a 4 T MRI using an asymmetric spin echo spiral sequence (ASE spiral). This technique collected three images with equal BOLD contrast weighting and increasing T(2)-weighting. An interhemispheric transfer task was used to elicit activation in the corpus callosum. White matter fMRI activation was examined for the averaged ASE spiral data and for each image separately. Callosal activation was present in all subjects as well as in the group analysis. Analyses revealed that increasing T(2) contrast improved sensitivity as measured by percent signal change. The results suggest that it is possible to detect white matter activation in fMRI and that ASE spiral showed increasing sensitivity to this activation as a function of T(2)-weighting. The findings provide further support for the investigation of white matter fMRI.
View details for DOI 10.1016/j.neuroimage.2008.11.005
View details for Web of Science ID 000263862900011
View details for PubMedID 19084071