New MR sequences in daily practice: susceptibility weighted imaging. A pictorial essay

Background Susceptibility-weighted imaging (SWI) is a relatively new magnetic resonance (MR) technique that exploits the magnetic susceptibility differences of various tissues, such as blood, iron and calcification, as a new source of contrast enhancement. This pictorial review is aimed at illustrating and discussing its main clinical applications. Methods SWI is based on high-resolution, threedimensional (3D), fully velocity-compensated gradientecho sequences using both magnitude and phase images. A phase mask obtained from the MR phase images is multiplied with magnitude images in order to increase the visualisation of the smaller veins and other sources of susceptibility effects, which are displayed at best after postprocessing of the 3D dataset with the minimal intensity projection (minIP) algorithm. Results SWI is very useful in detecting cerebral microbleeds in ageing and occult low-flow vascular malformations, in characterising brain tumours and degenerative diseases of the brain, and in recognizing calcifications in various pathological conditions. The phase images are especially useful in differentiating between paramagnetic susceptibility effects of blood and diamagnetic effects of calcium. SWI can also be used to evaluate changes in iron content in different neurodegenerative disorders. Conclusion SWI is useful in differentiating and characterising diverse brain disorders

Developing novel non-invasive MRI techniques to assess cerebrospinal fluid-interstitial fluid (CSF-ISF) exchange

The pathological cascade of events in Alzheimer’s disease (AD) is initiated decades prior to the onset of symptoms. Despite intensive research, the relative time-course/interaction of these events is yet to be determined. Recent evidence suggests that impairments to brain clearance (facilitated by the compartmental exchange of cerebrospinal-fluid (CSF) with interstitial-fluid (ISF)), contributes to the build-up of amyloid and tau (AD hallmarks). Therefore, abnormalities in CSF-ISF exchange dynamics, may represent an early driver of downstream events. Clinical evaluation of this hypothesis is hampered due to the lack of non-invasive CSF-ISF exchange assessment techniques. In this thesis, the primary aim was to develop a physiologically relevant, non-invasive CSF-ISF exchange assessment technique that would circumvent the limitations associated with current procedures (primarily their invasiveness). Towards this goal, animal studies were conducted to investigate the feasibility of a contrast enhanced-magnetic resonance imaging (CE-MRI) approach as a potential non-invasive CSF-ISF exchange imaging technique. Another aim of this thesis was to investigate whether the proposed MRI platform could detect abnormalities in CSF-ISF exchange, a condition hypothesised to occur in the early stages of AD. As such, pharmacological intervention studies were conducted to alter CSF-ISF exchange dynamics. CE-MRI, in conjunction with high-level image post-processing, demonstrated high sensitivity to physiological CSF-ISF exchange. This novel, non-invasive platform, captured dynamic, whole-brain infiltration of contrast agent from the blood to the CSF and into the parenchyma, via a pathway named ‘VEntricular-Cerebral TranspORt (VECTOR)’. Additionally, the platform detected significant abnormalities in CSF-ISF exchange following pharmacological intervention, demonstrating the potential of VECTOR in the study of the parenchymal accumulation of aberrant proteins. Development of this platform is a breakthrough step towards the clinical assessment of CSF-ISF exchange abnormalities to study its role in disease onset/progression, an approach that may inform understanding of the causal sequence of pathological events that occurs in AD development

Establishment of a reproducible and minimally invasive ischemic stroke model in swine

The need for advances in the management/treatment options for ischemic stroke patients requires that upcoming preclinical research uses animals with more human-like brain characteristics. The porcine brain is considered appropriate, although the presence of the rete mirabile (RM) prevents direct catheterization of the intracranial arteries to produce focal cerebral ischemia. To develop a reproducible minimally invasive porcine stroke model, a guide catheter and guide wire were introduced through the femoral artery until reaching the left RM. Using the pressure cooker technique, Squid-12 embolization material was deposited to fill, overflow, and occlude the left RM, the left internal carotid artery, and left circle of Willis wing up to the origins of the middle cerebral arteries (MCAs), mimicking the occlusion produced in the filament model in rodents. Longitudinal multimodal cerebral MRI was conducted to assess the brain damage and cerebral blood supply. The technique we describe here occluded up to the origins of the MCAs in 7 of 8 swine, inducing early damage 90 minutes after occlusion that later evolved to a large cerebral infarction and producing no mortality during the intervention. This minimally invasive ischemic stroke model in swine produced reproducible infarcts and shows translational features common to human stroke

Focal Spot, Fall/Winter 1996

https://digitalcommons.wustl.edu/focal_spot_archives/1071/thumbnail.jp

Development and assessment of new post-processing methodologies in 3D contrast enhanced MRI

This thesis aims to investigate some of the methods currently used in contrast MR imaging. It specifically focuses on methods that require subtraction of noncontrast enhanced (pre) 3D imaging data sets from contrast-enhanced (post) data, collected within a single imaging session. Current methods assume that there is little or no intra-scan patient motion and thus do not attempt to correct for this. This thesis aims to determine if such motion does exist and if so what methods are best suited to correct it. The thesis begins by describing some of the relevant MR physics and history of contrast enhancement in chapter 1, and expands on this in chapter 2 by focusing on angiographic, and contrast-enhanced techniques. Chapter 2 continues by investigating an MP RAGE subtraction technique for producing venograms, which requires pre and post-contrast data subtraction. Data is collected for 20 patients and the effects of motion correction on the resulting venograms are investigated. Chapter 3 investigates a different type of pre and post-contrast enhanced study where it is used for tumour volume measurement. Examining the effects on tumour volumes measured with and without the realignment correction provides quantitative evidence that realignment is a requirement in this and similar types of study. To enable the significance of segmentation accuracy on realigmnent to be tested a phantom pre and post-contrast data set is developed in chapter 4. Chapter 5 uses this data set to test the effects of differing segmentation accuracies, with respect to the accurately segmented phantom data, on realignment accuracy where the pre and post-contrast data differ by known rotations and translations. This provides information on the effects of contrast enhancement on realignment accuracy, as well as providing information on the required brain segmentation accuracy required to accurately realign these data sets. Chapter 6 expands on this work by testing segmentation accuracy effects on two real patient data sets. The first patient data set differs from the phantom data in terms of its noise characteristics and the second has a space occupying lesion similar to those regularly encountered in the clinical setting. Chapter 7 aims to develop an automatic technique for segmenting, realigning and visualising venographic data using the venography technique described in chapter 2. It uses a histogram and morphological operations to ensure that all of the contrast enhanced data is removed from the data, whilst attempting to segment the brain to an acceptable accuracy. Although this algorithm is specifically designed for venograms visualisation, it would require only a small amount of adjustment enabling it to be applied to the tumour volume measurement technique described in chapter 3. Chapter 8 tests this algorithm using the data collected in chapter 2 and measures its performance in producing satisfactory brain segmentations, which is required for accurate realignment. This would also be required for accurate realignment in tumour volume measurement studies. Chapter 8 also measures the algorithms capabilities in correctly producing visualisation data sets for the purposes of venography. The algorithm has limited success in both brain segmentation and venous visualisation, nevertheless this is encouraging as a first attempt as the algorithm is being applied to real patient data sets reflecting a range of pathological conditions and not only to selected normal data sets. Chapter 8 suggests some modifications that could be applied to the algorithm that might improve its future success. This includes modifying it to become a semi-automated technique. (Abstract shortened by ProQuest.)

Respiratory organ motion in interventional MRI : tracking, guiding and modeling

Respiratory organ motion is one of the major challenges in interventional MRI, particularly in interventions with therapeutic ultrasound in the abdominal region. High-intensity focused ultrasound found an application in interventional MRI for noninvasive treatments of different abnormalities. In order to guide surgical and treatment interventions, organ motion imaging and modeling is commonly required before a treatment start. Accurate tracking of organ motion during various interventional MRI procedures is prerequisite for a successful outcome and safe therapy. In this thesis, an attempt has been made to develop approaches using focused ultrasound which could be used in future clinically for the treatment of abdominal organs, such as the liver and the kidney. Two distinct methods have been presented with its ex vivo and in vivo treatment results. In the first method, an MR-based pencil-beam navigator has been used to track organ motion and provide the motion information for acoustic focal point steering, while in the second approach a hybrid imaging using both ultrasound and magnetic resonance imaging was combined for advanced guiding capabilities. Organ motion modeling and four-dimensional imaging of organ motion is increasingly required before the surgical interventions. However, due to the current safety limitations and hardware restrictions, the MR acquisition of a time-resolved sequence of volumetric images is not possible with high temporal and spatial resolution. A novel multislice acquisition scheme that is based on a two-dimensional navigator, instead of a commonly used pencil-beam navigator, was devised to acquire the data slices and the corresponding navigator simultaneously using a CAIPIRINHA parallel imaging method. The acquisition duration for four-dimensional dataset sampling is reduced compared to the existing approaches, while the image contrast and quality are improved as well. Tracking respiratory organ motion is required in interventional procedures and during MR imaging of moving organs. An MR-based navigator is commonly used, however, it is usually associated with image artifacts, such as signal voids. Spectrally selective navigators can come in handy in cases where the imaging organ is surrounding with an adipose tissue, because it can provide an indirect measure of organ motion. A novel spectrally selective navigator based on a crossed-pair navigator has been developed. Experiments show the advantages of the application of this novel navigator for the volumetric imaging of the liver in vivo, where this navigator was used to gate the gradient-recalled echo sequence

Neurovascular Surgery

This open access book presents the diagnosis, investigation and treatment of neurovascular diseases, and offers expert opinions and advice on avoiding complications in neurovascular surgery. It also covers complication management and post-operative follow-up care. The book is divided in to three parts; the first part discusses common approaches in neurovascular surgery, describing the steps, indications for and limitations of the approach, as well as the associated complications and how to avoid them. The second part addresses surgical treatment based on pathology, taking the different locations of lesions into consideration. The third part focuses on the technological developments that support neurovascular surgery, which may not be available everywhere, but have been included to help vascular surgeon understand the principles. This book is a guide for young neurosurgeons, neurosurgery residents and neurosurgery fellows, as well as for medical students and nurses who are interested in neurosurgery or are associated with this field in any way. It is also a useful teaching aid for senior neurosurgeons