A literature review of magnetic resonance imaging sequence advancements in visualizing functional neurosurgery targets

OBJECTIVE: Historically, preoperative planning for functional neurosurgery has depended on the indirect localization of target brain structures using visible anatomical landmarks. However, recent technological advances in neuroimaging have permitted marked improvements in MRI-based direct target visualization, allowing for refinement of "first-pass" targeting. The authors reviewed studies relating to direct MRI visualization of the most common functional neurosurgery targets (subthalamic nucleus, globus pallidus, and thalamus) and summarize sequence specifications for the various approaches described in this literature. METHODS: The peer-reviewed literature on MRI visualization of the subthalamic nucleus, globus pallidus, and thalamus was obtained by searching MEDLINE. Publications examining direct MRI visualization of these deep brain stimulation targets were included for review. RESULTS: A variety of specialized sequences and postprocessing methods for enhanced MRI visualization are in current use. These include susceptibility-based techniques such as quantitative susceptibility mapping, which exploit the amount of tissue iron in target structures, and white matter attenuated inversion recovery, which suppresses the signal from white matter to improve the distinction between gray matter nuclei. However, evidence confirming the superiority of these sequences over indirect targeting with respect to clinical outcome is sparse. Future targeting may utilize information about functional and structural networks, necessitating the use of resting-state functional MRI and diffusion-weighted imaging. CONCLUSIONS: Specialized MRI sequences have enabled considerable improvement in the visualization of common deep brain stimulation targets. With further validation of their ability to improve clinical outcomes and advances in imaging techniques, direct visualization of targets may play an increasingly important role in preoperative planning

The whole-brain pattern of magnetic susceptibility perturbations in Parkinson's disease

Although iron-mediated oxidative stress has been proposed as a potential pathomechanism in Parkinson's disease, the global distribution of iron accumulation in Parkinson's disease has not yet been elucidated. This study used a new magnetic resonance imaging contrast, quantitative susceptibility mapping, and state-of-the-art methods to map for the first time the whole-brain landscape of magnetostatic alterations as a surrogate for iron level changes in n = 25 patients with idiopathic Parkinson's disease versus n = 50 matched controls. In addition to whole-brain analysis, a regional study including sub-segmentation of the substantia nigra into dorsal and ventral regions and qualitative assessment of susceptibility maps in single subjects were also performed. The most remarkable basal ganglia effect was an apparent magnetic susceptibility increase-consistent with iron deposition-in the dorsal substantia nigra, though an effect was also observed in ventral regions. Increased bulk susceptibility, additionally, was detected in rostral pontine areas and in a cortical pattern tightly concordant with known Parkinson's disease distributions of α-synuclein pathology. In contrast, the normally iron-rich cerebellar dentate nucleus returned a susceptibility reduction suggesting decreased iron content. These results are in agreement with previous post-mortem studies in which iron content was evaluated in specific regions of interest; however, extensive neocortical and cerebellar changes constitute a far more complex pattern of iron dysregulation than was anticipated. Such findings also stand in stark contrast to the lack of statistically significant group change using conventional magnetic resonance imaging methods namely voxel-based morphometry, cortical thickness analysis, subcortical volumetry and tract-based diffusion tensor analysis; confirming the potential of whole-brain quantitative susceptibility mapping as an in vivo biomarker in Parkinson's disease

Measuring Brain Iron In Healthy Controls And Accessing Venous Abnormalities In Parkinson\u27s Disease Using Mri

Iron plays a key role in a number of brain cellular processes. However, abnormal brain iron regulation correlates with a number of neurodegenerative diseases such as Parkinson\u27s disease, multiple sclerosis and so forth. To further elucidate the involvement of iron in neurodegenerative disorders, a robust in vivo quantitative non-invasive assessment of normal brain iron deposition over time is essential. A variety of magnetic resonance imaging (MRI) methods have been used to quantify iron both in vivo and in vitro non-invasively. We first created the baseline of phase value and ∆R2^* as a function of age in seven gray matter nuclei. We also studied how the high iron region in the structures change over time, which is a more sensitive way to study the brain iron deposition. Quantitative susceptibility mapping (QSM) is another approch we used to study normal brain iron deposition as a further step since susceptibility is a tissue property that is free from geometry effect and imaging parameters. Except in the thalamus, iron increases over time in the gray matter nuclei. We found iron content decreases as a function of age in thalamus according to the susceptibility measurements. These measured susceptibility values are also compared with the regional iron concentrations taken from the histochemical post mortem study published by Hallgren and Sourander to establish an in vivo quantitative conversion factor between magnetic susceptibility and iron concentration. The correlation between the magnetic susceptibility and iron concentration found in this thesis is 1.03±0.03 ppb per µg iron/g wet tissue. Idiopathic Parkinson\u27s disease (IPD) remains one of those neurodegenerative diseases where the cause remains unknown. Many clinically diagnosed cases of IPD are associated with cerebrovascular disease and white matter hyperintensities (WMH). In this thesis, we also include a study to investigate the presence of transverse sinus and extracranial venous abnormalities in IPD patients and their relationship with brain WMH. As a result of this preliminary study, we conclude that a major fraction of IPD patients appear to have abnormal venous anatomy and flow on the left side of the brain and neck and that the flow abnormalities appear to correlate with WMH volume

A comparison of phase imaging and quantitative susceptibility mapping in the imaging of multiple sclerosis lesions at ultrahigh field

Objective The aim of this study was to compare the use of high-resolution phase and QSM images acquired at ultra-high field in the investigation of multiple sclerosis (MS) lesions with peripheral rings, and to discuss their usefulness for drawing inferences about underlying tissue composition. Materials and methods Thirty-nine Subjects were scanned at 7 T, using 3D T2*-weighted and T1-weighted sequences. Phase images were then unwrapped and filtered, and quantitative susceptibility maps were generated using a thresholded k-space division method. Lesions were compared visually and using a 1D profiling algorithm. Results Lesions displaying peripheral rings in the phase images were identified in 10 of the 39 subjects. Dipolar projections were apparent in the phase images outside of the extent of several of these lesions; however, QSM images showed peripheral rings without such projections. These projections appeared ring-like in a small number of phase images where no ring was observed in QSM. 1D profiles of six well-isolated example lesions showed that QSM contrast corresponds more closely to the magnitude images than phase contrast. Conclusions Phase images contain dipolar projections, which confounds their use in the investigation of tissue composition in MS lesions. Quantitative susceptibility maps correct these projections, providing insight into the composition of MS lesions showing peripheral rings

Magnetic Resonance Imaging of Iron in Early Multiple Sclerosis at 3 Tesla

Multiple sclerosis (MS) is the most common neurological disease in young Canadians, yet its etiology remains obscure. Two possibly related findings in MS are brain iron deposition and the presence of small veins in white matter lesions. This thesis concerns the development and application of 3 Tesla magnetic resonance imaging tools to image iron and veins in early multiple sclerosis. To facilitate measurements of iron concentration as well as production of cerebral venograms, we first optimized multi-echo susceptibility weighted imaging (SWI), using numerical simulations and input from physicians. We validated measurements of R2*, an MRI parameter that scales linearly with iron concentration. Subsequently, we proposed quantification of the caliber of the internal jugular veins (IJVs) from magnetic resonance venograms. IJVs are implicated in the chronic cerebrospinal venous insufficiency model of MS, an increasingly disputed theory that attributes iron deposition in MS to venous abnormalities. We report that the coefficient of variation of measurements of average cross-sectional area of the IJVs is on the order of 7%. We performed quantitative investigations of iron concentration in a cohort of patients at risk of MS diagnosis, compared to healthy controls. We report increased R2* (putative iron) in deep as well as cortical grey matter in patients. We subsequently measured IJV area, finding a trend for reduced total IJV caliber in patients; however, we found no correlation between R2* changes and IJV area. We investigated the ability of multi-echo SWI to detect central veins within white matter hyperintensities (WMHs). We found that patients who converted from clinically isolated syndrome (CIS) to MS had a larger fraction of lesions with central veins compared to patients with non-converted CIS and healthy controls. Moreover, all patients who received a diagnosis of MS within the study window had \u3e40% lesions with central veins at their CIS baselines, suggesting there may be predictive value in this biomarker. The subjects from these last two studies represent a subset of our cohort in an ongoing longitudinal study. Using methodology described herein, we are equipped to further investigate different biomarkers of disease to better understand early pathology in MS

Methodological considerations for neuroimaging in deep brain stimulation of the subthalamic nucleus in Parkinson’s disease patients

Deep brain stimulation (DBS) of the subthalamic nucleus is a neurosurgical intervention for Parkinson’s disease patients who no longer appropriately respond to drug treatments. A small fraction of patients will fail to respond to DBS, develop psychiatric and cognitive side-effects, or incur surgery-related complications such as infections and hemorrhagic events. In these cases, DBS may require recalibration, reimplantation, or removal. These negative responses to treatment can partly be attributed to suboptimal pre-operative planning procedures via direct targeting through low-field and low-resolution magnetic resonance imaging (MRI). One solution for increasing the success and efficacy of DBS is to optimize preoperative planning procedures via sophisticated neuroimaging techniques such as high-resolution MRI and higher field strengths to improve visualization of DBS targets and vasculature. We discuss targeting approaches, MRI acquisition, parameters, and post-acquisition analyses. Additionally, we highlight a number of approaches including the use of ultra-high field (UHF) MRI to overcome limitations of standard settings. There is a trade-off between spatial resolution, motion artifacts, and acquisition time, which could potentially be dissolved through the use of UHF-MRI. Image registration, correction, and post-processing techniques may require combined expertise of traditional radiologists, clinicians, and fundamental researchers. The optimization of pre-operative planning with MRI can therefore be best achieved through direct collaboration between researchers and clinicians

Quantitative susceptibility mapping (QSM) as a means to measure brain iron? A post mortem validation study

AbstractQuantitative susceptibility mapping (QSM) is a novel technique which allows determining the bulk magnetic susceptibility distribution of tissue in vivo from gradient echo magnetic resonance phase images. It is commonly assumed that paramagnetic iron is the predominant source of susceptibility variations in gray matter as many studies have reported a reasonable correlation of magnetic susceptibility with brain iron concentrations in vivo. Instead of performing direct comparisons, however, all these studies used the putative iron concentrations reported in the hallmark study by Hallgren and Sourander (1958) for their analysis. Consequently, the extent to which QSM can serve to reliably assess brain iron levels is not yet fully clear. To provide such information we investigated the relation between bulk tissue magnetic susceptibility and brain iron concentration in unfixed (in situ) post mortem brains of 13 subjects using MRI and inductively coupled plasma mass spectrometry. A strong linear correlation between chemically determined iron concentration and bulk magnetic susceptibility was found in gray matter structures (r=0.84, p<0.001), whereas the correlation coefficient was much lower in white matter (r=0.27, p<0.001). The slope of the overall linear correlation was consistent with theoretical considerations of the magnetism of ferritin supporting that most of the iron in the brain is bound to ferritin proteins. In conclusion, iron is the dominant source of magnetic susceptibility in deep gray matter and can be assessed with QSM. In white matter regions the estimation of iron concentrations by QSM is less accurate and more complex because the counteracting contribution from diamagnetic myelinated neuronal fibers confounds the interpretation