A subject-specific technique for respiratory motion correction in image-guided cardiac catheterisation procedures

We describe a system for respiratory motion correction of MRI-derived roadmaps for use in X-ray guided cardiac catheterisation procedures. The technique uses a subject-specific affine motion model that is quickly constructed from a short pre-procedure MRI scan. We test a dynamic MRI sequence that acquires a small number of high resolution slices, rather than a single low resolution volume. Additionally, we use prior knowledge of the nature of cardiac respiratory motion by constraining the model to use only the dominant modes of motion. During the procedure the motion of the diaphragm is tracked in X-ray fluoroscopy images, allowing the roadmap to be updated using the motion model. X-ray image acquisition is cardiac gated. Validation is performed on four volunteer datasets and three patient datasets. The accuracy of the model in 3D was within 5 mm in 97.6% of volunteer validations. For the patients, 2D accuracy was improved from 5 to 13 mm before applying the model to 2–4 mm afterwards. For the dynamic MRI sequence comparison, the highest errors were found when using the low resolution volume sequence with an unconstrained model

Optical flow-based vascular respiratory motion compensation

This paper develops a new vascular respiratory motion compensation algorithm, Motion-Related Compensation (MRC), to conduct vascular respiratory motion compensation by extrapolating the correlation between invisible vascular and visible non-vascular. Robot-assisted vascular intervention can significantly reduce the radiation exposure of surgeons. In robot-assisted image-guided intervention, blood vessels are constantly moving/deforming due to respiration, and they are invisible in the X-ray images unless contrast agents are injected. The vascular respiratory motion compensation technique predicts 2D vascular roadmaps in live X-ray images. When blood vessels are visible after contrast agents injection, vascular respiratory motion compensation is conducted based on the sparse Lucas-Kanade feature tracker. An MRC model is trained to learn the correlation between vascular and non-vascular motions. During the intervention, the invisible blood vessels are predicted with visible tissues and the trained MRC model. Moreover, a Gaussian-based outlier filter is adopted for refinement. Experiments on in-vivo data sets show that the proposed method can yield vascular respiratory motion compensation in 0.032 sec, with an average error 1.086 mm. Our real-time and accurate vascular respiratory motion compensation approach contributes to modern vascular intervention and surgical robots.Comment: This manuscript has been accepted by IEEE Robotics and Automation Letter

The role of cardiac magnetic resonance imaging in the assessment of right ventricular function in pulmonary hypertension

Pulmonary hypertension (PH) is a rare disease of the pulmonary arteries. It is characterised by vascular proliferation and remodelling resulting in a progressive increase in pulmonary vascular resistance and right ventricular failure. The functional capacity of the right ventricle is the major prognostic determinant in PH, and death usually results from right ventricular failure. Although recent therapeutic advances have improved the short-to-medium term outlook of PH patients, early death due to right ventricular failure remains inevitable in many patients. The imperative role of RV performance in the clinical status and long- term outcome in PH patients is evident. Evaluation of right ventricular function is essential in the management of patients with pulmonary hypertension. Current methods of assessment of PH patients are suboptimal. The right ventricle is difficult to assess due to its position and geometry. Recent developments in imaging techniques, such as cardiac magnetic resonance (CMR) imaging and echocardiography, have improved our understanding of the structure and function of the right ventricle. Assessment of RV function is complex and no single measurement is generally accepted in clinical practice. The experimental work performed in this thesis aimed to improve our understanding of RV function in PH patients and to provide clarity in the role of CMR in the non-invasive assessment and monitoring of pulmonary hypertension patients. A non-invasive measurement of stroke volume would be beneficial to monitor disease progression in pulmonary hypertension patients. Chapter 3 demonstrated that cardiac magnetic resonance imaging provided non-invasive measurements of stroke volume that were as accurate as those obtained by thermodilution measured during right heart catheterisation. Inert gas rebreathing using photoacoustic analysis also provided accurate non-invasive measurements of stroke volume. Chapter 4 compared two patient groups: idiopathic pulmonary arterial hypertension (IPAH) and pulmonary hypertension associated with connective tissue disease (CTDPAH). We clarified that there was no significant differences in RV structure and performance between these two distinct patient groups to account for the poorer prognosis in the CTDPAH group. Treatments for PH are always expensive, sometimes invasive and carry significant side effects. It is imperative that we are able to assess the patient’s response to therapy in a clinically meaningful, accurate and non-invasive manner. The importance of escalating therapy if a patient does not respond to initial treatment has been emphasised in recently published guidelines. Current monitoring techniques have acknowledged limitations and are suboptimal. Chapter 5 presents the results obtained from my contribution to the prospective, longitudinal multinational EURO-MR study. Longitudinal CMR examination identified significant improvements in cardiac function with the introduction of disease-targeted therapy. Baseline and 4 months post therapy CMR scans were compared. It is anticipated that CMR could be a useful monitoring technique for patients with pulmonary hypertension

Tissue microvascular flow and oxygenation in critically ill patients

PhDThe use of fluid resuscitation and vasoactive agents to optimise global haemodynamics has been demonstrated to improve outcomes in patients undergoing major surgery and in early sepsis. Whether changes in global haemodynamics result in similar improvements in the microcirculation in critically ill patients remains unclear. The aim of this thesis was to investigate the changes in tissue microvascular flow and oxygenation that occur in patients undergoing major surgery and in those with sepsis, and specifically how haemodynamic therapies may affect these changes. The first part of this thesis investigates the treatment pathway of the high risk surgical patient. Analysis of two large health databases was performed and confirmed the existence of a high risk sub-population within the local surgical population. Only about a third of these high-risk patients were admitted to a critical care unit at any stage during their hospital admission. An observational trial was performed examining the relationship between global oxygen delivery, microvascular flow and tissue oxygenation in 25 surgical patients receiving usual care. Data including global haemodynamics, sublingual and cutaneous microvascular flow, and tissue oxygenation were collected before, and for eight hours after surgery. Abnormalities in sublingual microvascular flow were found to be associated with worse outcomes. 4 A randomised controlled study investigating the effects of two goal directed haemodynamic therapy (GDHT) algorithms on tissue microvascular flow and oxygenation compared to central venous pressure guided fluid therapy in 135 perioperative patients was performed. For eight hours after surgery, intravenous fluid therapy was guided by measurements of central venous pressure (CVP group) or stroke volume (SV group). In a third group stroke volume guided fluid therapy was combined with dopexamine (SV & DPX group). In the SV & DPX group, increased global oxygen delivery was associated with improved sublingual and cutaneous microvascular flow. Microvascular flow remained constant in the SV group but deteriorated in the CVP group. Cutaneous PtO2 improved only in the SV & DPX group. There were no differences in complication rates between groups. The importance of derangements in microvascular flow in patients with established sepsis is well recognized. However, little data is available to describe microvascular changes in early sepsis. Observational data were collected in 16 healthy volunteers and within six hours of presentation in 48 patients with sepsis and severe sepsis. Sublingual microvascular flow was impaired in patients with sepsis and severe sepsis compared to healthy volunteers. Greater alterations in flow were seen with increasing severity of illness. The dose-related effects of vasopressor therapy on microvascular flow and tissue oxygenation in sepsis have not been previously fully investigated. The effects of increasing doses of noradrenaline, targeted to achieve successively greater mean arterial pressures, on microvascular flow and tissue oxygenation in 16 patients with septic shock were investigated. Increasing doses of noradrenaline were associated with improvements in 5 global oxygen delivery, cutaneous PtO2 and cutaneous microvascular red blood cell flux. No changes in sublingual microvascular flow were identified. This thesis confirms the existence of a large sub-population of high risk surgical patients. It demonstrates that abnormal microvascular flow in the perioperative period may be associated with poor outcomes. The use of flow guided fluid therapy alongside low dose dopexamine infusion is shown to improve global haemodynamics, microvascular flow and tissue oxygenation in perioperative patients. Microvascular abnormalities are shown to occur in the earliest stages of sepsis with increasing severity of disease being associated with greater changes. Increasing doses of noradrenaline were found to improve global haemodynamics, cutaneous microvascular flow and cutaneous tissue oxygenation in septic shock. Further work is required to investigate the effects of haemodynamic therapies on microvascular flow and organ dysfunction in critically ill patients and the use of the microcirculation as a resuscitation endpoint

Track 1: Surgery, anaesthesia and intensive care

Surgery, anaesthesia and intensive care