In vivo human retinal and choroidal vasculature visualization using differential phase contrast swept source optical coherence tomography at 1060 nm

A differential phase contrast (DPC) method is validated for in vivo human retinal and choroidal vasculature visualization using high-speed swept-source optical coherence tomography (SS-OCT) at 1060 nm. The vasculature was identified as regions of motion by creating differential phase variance (DPV) tomograms: multiple B-scans were collected of individual slices through the retina and the variance of the phase differences was calculated. DPV captured the small vessels and the meshwork of capillaries associated with the inner retina in en face images over 4 mm^2 in a normal subject. En face DPV images were capable of capturing the microvasculature and regions of motion through the inner retina and choroid

Differential intensity contrast swept source optical coherence tomography for human retinal vasculature visualization

We demonstrate an intensity-based motion sensitive method, called differential logarithmic intensity variance (DLOGIV), for 3D microvasculature imaging and foveal avascular zone (FAZ) visualization in the in vivo human retina using swept source optical coherence tomog. (SS-OCT) at 1060 nm. A motion sensitive SS-OCT system was developed operating at 50,000 A-lines/s with 5.9 μm axial resoln., and used to collect 3D images over 4 mm^2 in a normal subject eye. Multiple B-scans were acquired at each individual slice through the retina and the variance of differences of logarithmic intensities as well as the differential phase variances (DPV) was calcd. to identify regions of motion (microvasculature). En face DLOGIV image were capable of capturing the microvasculature through depth with an equal performance compared to the DPV

Acceleration of Subtractive Non-contrast-enhanced Magnetic Resonance Angiography

Although contrast-enhanced magnetic resonance angiography (CE-MRA) is widely established as a clinical examination for the diagnosis of human vascular diseases, non-contrast-enhanced MRA (NCE-MRA) techniques have drawn increasing attention in recent years. NCE-MRA is based on the intrinsic physical properties of blood and does not require the injection of any exogenous contrast agents. Subtractive NCE-MRA is a class of techniques that acquires two image sets with different vascular signal intensity, which are later subtracted to generate angiograms. The long acquisition time is an important drawback of NCE-MRA techniques, which not only limits the clinical acceptance of these techniques but also renders them sensitive to artefacts from patient motion. Another problem for subtractive NCE-MRA is the unwanted residual background signal caused by different static background signal levels on the two raw image sets. This thesis aims at improving subtractive NCE-MRA techniques by addressing both these limitations, with a particular focus on three-dimensional (3D) femoral artery fresh blood imaging (FBI). The structure of the thesis is as follows: Chapter 1 describes the anatomy and physiology of the vascular system, including the characteristics of arteries and veins, and the MR properties and flow characteristics of blood. These characteristics are the foundation of NCE-MRA technique development. Chapter 2 introduces commonly used diagnostic angiographic methods, particularly CE-MRA and NCE-MRA. Current NCE-MRA techniques are reviewed and categorised into different types. Their principles, implementations and limitations are summarised. Chapter 3 describes imaging acceleration theories including compressed sensing (CS), parallel imaging (PI) and partial Fourier (PF). The Split Bregman algorithm is described as an efficient CS reconstruction method. The SPIRiT reconstruction for PI and homodyne detection for PF are also introduced and combined with Split Bregman to form the basis of the reconstruction strategy for undersampled MR datasets. Four image quality metrics are presented for evaluating the quality of reconstructed images. In Chapter 4, an intensity correction method is proposed to improve background suppression for subtractive NCE-MRA techniques. Residual signals of background tissues are removed by performing a weighted subtraction, in which the weighting factor is obtained by a robust regression method. Image sparsity can also be increased and thereby potentially benefit CS reconstruction in the following chapters. Chapter 5 investigates the optimal k-space sampling patterns for the 3D accelerated femoral artery FBI sequence. A variable density Poisson-disk with a fully sampled centre region and missing partial Fourier fractions is employed for k-space undersampling in the ky-kz plane. Several key parameters in sampling pattern design, such as partial Fourier sampling ratios, fully sampled centre region size and density decay factor, are evaluated and optimised. Chapter 6 introduces several reconstruction strategies for accelerated subtractive NCE-MRA. A new reconstruction method, k-space subtraction with phase and intensity correction (KSPIC), is developed. By performing subtraction in k-space, KSPIC can exploit the sparsity of subtracted angiogram data and potentially improve the reconstruction performance. A phase correction procedure is used to restore the polarity of negative signals caused by subtraction. The intensity correction method proposed in Chapter 4 is also incorporated in KSPIC as it improves background suppression and thereby sparsity. The highly accelerated technique can be used not only to reduce the acquisition time, but also to enable imaging with increased resolution with no time penalty. A time-efficient high-resolution FBI technique is proposed in Chapter 7. By employing KSPIC and modifying the flow-compensation/spoiled gradients, the image matrix size can be increased from 256×256 to up to 512×512 without prolonging the acquisition time. Chapter 8 summarises the overall achievements and limitations of this thesis, as well as outlines potential future research directions.Cambridge Trust China Scholarship Council Addenbrooke’s Charitable Trust National Institute of Health Research, Cambridge Biomedical Research Cente

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

Novel X-ray imaging technology enables significant patient dose reduction in interventional cardiology while maintaining diagnostic image quality

Objectives: The purpose of this study was to quantify the reduction in patient radiation dose during coronary angiography (CA) by a new X-ray technology, and to assess its impact on diagnostic image quality. Background: Recently, a novel X-ray imaging technology has become available for interventional cardiology, using advanced image processing and an optimized acquisition chain for radiation dose reduction. Methods: 70 adult patients were randomly assigned to a reference X-ray system or the novel X-ray system. Patient demographics were registered and exposure parameters were recorded for each radiation event. Clinical image quality was assessed for both patient groups. Results: With the same angiographic technique and a comparable patient population, the new imaging technology was associated with a 75% reduction in total kerma-area product (KAP) value (decrease from 47 Gycm(2) to 12 Gycm(2), P<0.001). Clinical image quality showed an equivalent detail and contrast for both imaging systems. On the other hand, the subjective appreciation of noise was more apparent in images of the new image processing system, acquired at lower doses, compared to the reference system. However, the higher noise content did not affect the overall image quality score, which was adequate for diagnosis in both systems. Conclusions: For the first time, we present a new X-ray imaging technology, combining advanced noise reduction algorithms and an optimized acquisition chain, which reduces patient radiation dose in CA drastically (75%), while maintaining diagnostic image quality. Use of this technology may further improve the radiation safety of cardiac angiography and interventions

Integration of anatomical and hemodynamical information in magnetic resonance angiography

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Gadolinium-enhanced magnetic resonance angiography of abdominal aortic aneurysms

AbstractPurpose: The objective of this study was to assess the usefulness of gadolinum-enhanced magnetic resonance angiography (MRA) for defining anatomic features relevant to performing aortic surgery for aneurysmal disease.Methods: Anatomic data defined by MRA, including abdominal aortic aneurysm (AAA) size and character, as well as the status of the celiac, mesenteric, renal, and iliac arteries, were correlated with angiography, ultrasonography, computed tomography, or operative data in 43 patients. Five MRA sequences were obtained in an hour-long examination optimized for aortoiliac, splanchnic, and renal artery imaging at 1.5 T in a body coil. Four of the sequences were performed during or after infusion of gadolinium to improve image quality.Results: MRA correctly defined the maximum aneurysm diameter, as well as its proximal and distal extent in all patients. MRA detected 33 of 35 significant stenoses among 153 splanchnic, renal, or iliac branches examined (sensitivity = 94% and specificity = 98%). MRA did not resolve the degree of aortic branch stenotic disease sufficiently to precisely grade its severity. MRA did not reliably define the status of the inferior mesenteric artery, lumbar arteries or internal iliac arteries. One ruptured AAA and one inflammatory AAA were correctly diagnosed by MRA. No patient had a contrast reaction or contrast-induced renal toxicity related to administration of gadolinium.Conclusion: Gadolinium-enhanced MRA of AAA provides appropriate, essential anatomic information for aortic reconstructive surgery in a 1-hour examination devoid of contrast-related renal toxicity or catheterization-related complications attending conventional arteriography. (J VASC SURG 1995;21:656-69.

Non-contrast Magnetic Resonance Angiography for Evaluation of Peripheral Arterial Disease

Peripheral arterial disease (PAD) is a major cause of morbidity and mortality in the USA with an estimated prevalence of up to 20% in those over 75 years. Vascular disease and kidney impairment frequently coexist; prevalence of moderate to severe renal dysfunction in PAD patients is estimated at 27-36%. Knowledge of location, severity, and extent of PAD is imperative for accurate diagnosis and treatment planning. However, all established imaging modalities that are routinely used for treatment planning are contra-indicated in kidney disease patients. Contrast-enhanced x-ray and CT angiography are unsafe due to exposure to nephrotoxic contrast material and ionizing radiation. Recently, the FDA has also warned against the use of gadolinium-enhanced MRA (Gd-MRA) due to evidence that gadolinium could trigger a life-threatening condition known as nephrogenic systemic fibrosis (NSF) in patients with moderate to severe kidney dysfunction. There is a clinical need to develop vascular imaging techniques that are safe in patients with coexisting PAD and renal insufficiency. The focus of this thesis was the development of a non-contrast alternative to Gd-MRA for imaging of peripheral vessels from renal to pedal arteries with MRI. A new imaging sequence for non-contrast visualization of the abdominal and pelvic arteries was designed, implemented, and validated in a small cohort of PAD patients against Gd-MRA. In addition, an existing fast spin-echo based technique for unenhanced imaging of the lower extremities was optimized for improved performance in a clinical setting