Advancements and Breakthroughs in Ultrasound Imaging

Ultrasonic imaging is a powerful diagnostic tool available to medical practitioners, engineers and researchers today. Due to the relative safety, and the non-invasive nature, ultrasonic imaging has become one of the most rapidly advancing technologies. These rapid advances are directly related to the parallel advancements in electronics, computing, and transducer technology together with sophisticated signal processing techniques. This book focuses on state of the art developments in ultrasonic imaging applications and underlying technologies presented by leading practitioners and researchers from many parts of the world

A Scoping Review of Cerebral Doppler Arterial Waveforms in Infants

Cerebral Doppler ultrasound has been an important tool in pediatric diagnostics and prognostics for decades. Although the Doppler spectrum can provide detailed information on cerebral perfusion, the measured spectrum is often reduced to simple numerical parameters. To help pediatric clinicians recognize the visual characteristics of disease-associated Doppler spectra and identify possible areas for future research, a scoping review of primary studies on cerebral Doppler arterial waveforms in infants was performed. A systematic search in three online bibliographic databases yielded 4898 unique records. Among these, 179 studies included cerebral Doppler spectra for at least five infants below 1 y of age. The studies describe variations in the cerebral waveforms related to physiological changes (43%), pathology (62%) and medical interventions (40%). Characteristics were typically reported as resistance index (64%), peak systolic velocity (43%) or end-diastolic velocity (39%). Most studies focused on the anterior (59%) and middle (42%) cerebral arteries. Our review highlights the need for a more standardized terminology to describe cerebral velocity waveforms and for precise definitions of Doppler parameters. We provide a list of reporting variables that may facilitate unambiguous reports. Future studies may gain from combining multiple Doppler parameters to use more of the information encoded in the Doppler spectrum, investigating the full spectrum itself and using the possibilities for long-term monitoring with Doppler ultrasound

Channel characterisation and modelling for transcranial Doppler ultrasound.

The detection of micro-embolic signals (MES) is a mature application of transcranial Doppler (TCD) ultrasound. It involves the identification of abnormally highpitched signals within the arterial waveform as a method of diagnosis and prediction of embolic complications in stroke patients. More recently, algorithms have been developed to help characterise and classify MES using advanced signal processing techniques. These advances aim to improve our understanding of the causes of cereberovascular disease, helping to target the most appropriate interventions and quantifying the risk to patients of further stroke events. However, there are a number of limitations with current TCD systems which reduce their effectiveness. In particular, improvements in our understanding of the scattering effects in TCD ultrasound propagation channels will benefit our ability to develop algorithms that more robustly and reliably identify the consistency and material make-up of MES. This thesis explores TCD propagation channels in three related research areas. Firstly, a method of characterising TCD ultrasound propagation channels is proposed. Isotropic and non-isotropic three dimensional space (3-D) spherical scattering channel models are described in terms of theoretical reference models, simulation models, and sum of sinusoids (SoS) simulators, allowing the statistical properties to be analysed and reported. Secondly, a TCD ultrasound medical blood flow phantom is described. The phantom, designed to replicate blood flow in the middle cerebral arteries (MCA) for TCD ultrasound studies, is discussed in terms of material selection, physical construction and acoustic characteristics, including acoustic velocity, attenuation and backscatter coefficients. Finally, verification analysis is performed on the non-isotropic models against firstly, the blood flow phantom, and secondly, a patient recordings database. This analysis expands on areas of agreement and disagreement before assessing the usefulness of the models and describing their potential to improve signal processing approaches for detection of MES. The proposed non-isotropic channel reference model, simulation model, SoS simulator, and blood flow phantom are expected to contribute to improvements in the design, testing, and performance evaluation of future TCD ultrasound systems

Optical imaging and spectroscopy for the study of the human brain: status report.

This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions

Optical imaging and spectroscopy for the study of the human brain: status report

This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions

Improving the text classification using clustering and a novel HMM to reduce the dimensionality

In text classification problems, the representation of a document has a strong impact on the performance of learning systems. The high dimensionality of the classical structured representations can lead to burdensome computations due to the great size of real-world data. Consequently, there is a need for reducing the quantity of handled information to improve the classification process. In this paper, we propose a method to reduce the dimensionality of a classical text representation based on a clustering technique to group documents, and a previously developed Hidden Markov Model to represent them. We have applied tests with the k-NN and SVM classifiers on the OHSUMED and TREC benchmark text corpora using the proposed dimensionality reduction technique. The experimental results obtained are very satisfactory compared to commonly used techniques like InfoGain and the statistical tests performed demonstrate the suitability of the proposed technique for the preprocessing step in a text classification task.European Union | Ref. FP7/REGPOT-2012-2013.1, 316265 BIOCAPSUniversidade de Vigo | Ref. 12VI0

Optical imaging and spectroscopy for the study of the human brain: status report

This report is the second part of a comprehensive two-part series aimed at reviewing an extensive and diverse toolkit of novel methods to explore brain health and function. While the first report focused on neurophotonic tools mostly applicable to animal studies, here, we highlight optical spectroscopy and imaging methods relevant to noninvasive human brain studies. We outline current state-of-the-art technologies and software advances, explore the most recent impact of these technologies on neuroscience and clinical applications, identify the areas where innovation is needed, and provide an outlook for the future directions. Keywords: DCS; NIRS; diffuse optics; functional neuroscience; optical imaging; optical spectroscop

Assessment of White Matter Hyperintensity, Cerebral Blood Flow, and Cerebral Oxygenation in Older Subjects Stratified by Cerebrovascular Risk

Objective: Cerebrovascular disease (CVD) is the fifth most common cause of mortality in the United States. Diagnosis of CVD at an early stage is critical for optimal intervention designed to prevent ongoing and future brain injury. CVD is commonly associated with abnormalities of the cerebral microvasculature leading to tissue dysfunction, neuronal injury and death, and resultant clinical symptoms, which in turn, further impacts cerebral autoregulation (CA). This series of studies aims to test the hypothesis that white matter hyperintensities (WMH) and cerebral hemodynamics (quantified by magnetic resonance imaging (MRI) and an by innovative hybrid near-infrared diffuse optical instrument) can be used as biomarkers to distinguish cognitively healthy older subjects with high or low risk for developing CVD. Methods: Using functional MRI, WMH and cerebral blood flow (CBF) were quantified in 26 cognitively healthy older subjects (age: 77.8 ± 6.8 years). In a follow-up study, significant variability in WMH quantification methodology was addressed, with sources of variability identified in selecting image center of gravity, software compatibility, thresholding techniques, and manual editing procedures. Accordingly, post-acquisition processing methods were optimized to develop a standardized protocol with less than 0.5% inter-rater variance. Using a novel laboratory-made hybrid near-infrared spectroscopy/diffuse correlation spectroscopy (NIRS/DCS) and a finger plethysmograph, low-frequency oscillations (LFOs) of CBF, cerebral oxygenation, and main arterial pressure (MAP) were simultaneously measured before, during, and after 70° head-up-tilting (HUT). Gains (associated with CAs) to magnify LFOs were determined by transfer function analyses with MAP as the input and cerebral hemodynamic parameters as the outputs. In a follow-up study, a fast software correlator for DCS and a parallel detection technique for NIRS/DCS were adapted to improve the sampling rate of hybrid optical measurements. In addition, a new DCS probe was developed to measure CBF at the occipital lobe, which represents a novel application of the NIRS/DCS technique. Results: MRI measurements demonstrate that deep WMH (dWMH) and periventricular WMH (pWMH) volumetric measures are associated with reduced regional cortical CBF in patients at high-risk of CVD. Moreover, CBF in white matter (WM) was reduced in regions demonstrating both pWMH and dWMHs. NIRS/DCS optical measurements demonstrate that at resting baseline, LFO gains in the high-risk group were relatively lower compared to the low-risk group. The lower baseline gains in the high-risk group may be attributed to compensatory mechanisms that allow the maintenance of a stronger steady-state CA. However, HUT resulted in smaller gain reductions in the high-risk group compared to the low-risk group, suggesting weaker dynamic CA in association with increased CVD risks. A noteworthy finding in these experiments was that CVD risk more strongly influenced CBF than cerebral oxygenation. Conclusions: Regional WMH volumes, cortical and WM CBF values, and LFO gains of cerebral hemodynamics demonstrate specific associations with CA and may serve as important potential biomarkers for early diagnosis of CVD. The high spatial resolution, large penetration depth, and variety of imaging-sequences afforded by MRI make it an appealing imaging modality for evaluation of CVD, although MRI is costly, time-limited, and requires transfer of subjects from bed to imaging facility. In contrast, low-cost, portable, mobile diffuse optical technologies provide a complementary alternative for early screening of CVD, that can further allow continuous monitoring of disease attenuation or progression at the subject’s bedside. Thus, development of both methodologies is essential for progress in our future understanding of CVD as a major contributor to the morbidity and mortality associated with CVD today

The identification of the vulnerable carotid plaque and haemodynamic compromise of the brain in carotid artery stenosis

Introduction: Carotid stenosis plays a large role in the aetiology of ischaemic stroke. The main mechanism of carotid stenosis causing stroke is the forming of thrombus and consequently embolus formation. Another mechanism is the compromise in haemodynamics: reduced blood flow distal from the stenosis causing hypoperfusion of the brain. This work investigates the current prevalence of carotid stenosis in ischaemic stroke. It also explores the role of transcranial Doppler (TCD) and brain perfusion imaging with magnetic resonance imaging in patients with carotid stenosis. Methods: The current prevalence of carotid stenosis was assessed in a comprehensive Central London hyper-acute stroke unit and a systematic review with meta-regression analysis was conducted on the prevalence of carotid stenosis. Patient individual risk factors and morphological characteristics of the carotid plaque were associated with the presence of micro-embolic signals on TCD. The perfusion of the brain was assessed in patients with carotid stenosis and those who underwent carotid endarterectomy (CEA). Results: The prevalence of carotid stenosis 350% in the local stroke unit was 19.0%, including 7.9% with symptomatic stenosis. The pooled prevalence estimate of carotid stenosis, described in 37 studies in the literature, was 16.0% and has not declined over time. Intraplaque haemorrhage was associated with a higher risk of future stroke by detection of micro-embolic signals on TCD. Haemodynamic factors played a great role in stroke, especially in patients with stenosis 370%. Cerebral perfusion improved significantly in patients who underwent CEA, especially in those who initially had 370% stenosis. Conclusion: Morphology of the plaque, more than the degree of stenosis, is an important predictive feature of the unstable carotid plaque, whilst the degree of stenosis is more relevant to the hypoperfused brain. There is evidence for a synergic role of embolism and haemodynamic compromise as a mechanism of ischaemic stroke in carotid stenosis

Carotid Ultrasound for Stroke Prediction

Introduction: The aims of this thesis were to identify if carotid endarterectomy was cost-effective and affordable in the United Kingdom and secondly to explore the potential of contrast enhanced ultrasound and plaque texture analysis for risk stratification in asymptomatic patients with carotid atherosclerosis. Methods: A cost-utility analysis based on results from the Asymptomatic Carotid Surgery Trial was performed using a Markov transition state model. Three cross-sectional studies of symptomatic and asymptomatic individuals with 50-99% carotid stenosis were performed for late phase and dynamic phase contrast enhanced ultrasound, followed by plaque texture analysis. Results: There was a high probability of surgical endarterectomy lying under the £20-30, 000 per quality adjusted life year National Institute for Health and Clinical Excellence acceptability threshold in the United Kingdom. In men under 75 years of age, the cost per quality adjusted life year gained was lower and in women there was improved effectiveness with reduced long-term costs. Late phase contrast enhanced ultrasound imaging of carotid atherosclerosis suffered from a tissue suppression artefact which limited its ability to image microbubble retention. Quantification of plaque perfusion using low mechanical index imaging demonstrated a pseudoenhancement phenomenon from non-linear propagation, which artificially increased far wall intensity, again limiting its use for quantification of plaque perfusion. Semi-quantitative grading of plaque perfusion revealed no significant difference in generalised plaque perfusion between symptomatic and asymptomatic individuals, however detection of ulceration using dynamic contrast enhanced ultrasound showed a trend towards an association with symptomatic status. Type II plaque showed a significant independent association with symptomatic status. Conclusion: Carotid endarterectomy is likely to be cost-effective in those under 75 years of age, particularly women. However, without further selection, the upfront costs and high number needed to treat with endarterectomy limit its potential as a large scale strategy. Improvements in non-linear pulse sequencing are required before quantitative contrast enhanced ultrasound can reliably be used for functional imaging of carotid atherosclerosis. Qualitative assessment of plaque perfusion is unlikely to gain widespread use due to its high subjectivity. However assessment of plaque type and to a lesser extent imaging of ulceration using contrast enhanced ultrasound are promising and reproducible imaging biomarkers for further study. Validation of these markers with histology and then prospective study of individuals with these plaque phenotypes is proposed. In the future individuals with a recent transient ischaemic attack and moderate (50-69%) stenosis may prove to be an ideal group for risk stratification