Conventional mitral valve replacement requires a median sternotomy and cardio-pulmonary bypass with aortic crossclamping
and is associated with significant mortality and morbidity which could be reduced by performing the procedure
off-pump. Replacing the mitral valve in the closed, off-pump, beating heart requires extensive development and
validation of surgical and imaging techniques. Image guidance systems and surgical access for off-pump mitral valve
replacement have been previously developed, allowing the prosthetic valve to be safely introduced into the left atrium
and inserted into the mitral annulus. The major remaining challenge is to design a method of securely anchoring the
prosthetic valve inside the beating heart. The development of anchoring techniques has been hampered by the expense
and difficulty in conducting large animal studies. In this paper, we demonstrate how prosthetic valve anchoring may be
evaluated in a dynamic phantom. The phantom provides a consistent testing environment where pressure measurements
and Doppler ultrasound can be used to monitor and assess the valve anchoring procedures, detecting pararvalvular leak
when valve anchoring is inadequate. Minimally invasive anchoring techniques may be directly compared to the current
gold standard of valves sutured under direct vision, providing a useful tool for the validation of new surgical
instruments.
Surgical procedures often have the unfortunate side-effect of causing the patient significant trauma while accessing
the target site. Indeed, in some cases the trauma inflicted on the patient during access to the target greatly
exceeds that caused by performing the therapy. Heart disease has traditionally been treated surgically using
open chest techniques with the patient being placed "on pump" - i.e. their circulation being maintained by
a cardio-pulmonary bypass or "heart-lung" machine. Recently, techniques have been developed for performing
minimally invasive interventions on the heart, obviating the formerly invasive procedures. These new approaches
rely on pre-operative images, combined with real-time images acquired during the procedure. Our approach
is to register intra-operative images to the patient, and use a navigation system that combines intra-operative
ultrasound with virtual models of instrumentation that has been introduced into the chamber through the heart
wall. This paper illustrates the problems associated with traditional ultrasound guidance, and reviews the state
of the art in real-time 3D cardiac ultrasound technology. In addition, it discusses the implementation of an image-guided
intervention platform that integrates real-time ultrasound with a virtual reality environment, bringing
together the pre-operative anatomy derived from MRI or CT, representations of tracked instrumentation inside
the heart chamber, and the intra-operatively acquired ultrasound images.
KEYWORDS: Fluoroscopy, Image registration, Ultrasonography, Heart, Calibration, Visualization, Computed tomography, 3D modeling, In vivo imaging, Detection and tracking algorithms
This study assesses the accuracy of a new transesophageal (TEE) ultrasound (US) fluoroscopy registration
technique designed to guide percutaneous aortic valve replacement. In this minimally invasive procedure, a
valve is inserted into the aortic annulus via a catheter. Navigation and positioning of the valve is guided
primarily by intra-operative fluoroscopy. Poor anatomical visualization of the aortic root region can result in
incorrect positioning, leading to heart valve embolization, obstruction of the coronary ostia and acute kidney
injury. The use of TEE US images to augment intra-operative fluoroscopy provides significant improvements to
image-guidance.
Registration is achieved using an image-based TEE probe tracking technique and US calibration. TEE probe
tracking is accomplished using a single-perspective pose estimation algorithm. Pose estimation from a single
image allows registration to be achieved using only images collected in standard OR workflow. Accuracy of this
registration technique is assessed using three models: a point target phantom, a cadaveric porcine heart with
implanted fiducials, and in-vivo porcine images. Results demonstrate that registration can be achieved with
an RMS error of less than 1.5mm, which is within the clinical accuracy requirements of 5mm. US-fluoroscopy
registration based on single-perspective pose estimation demonstrates promise as a method for providing guidance
to percutaneous aortic valve replacement procedures. Future work will focus on real-time implementation and a
visualization system that can be used in the operating room.
Off-pump, intracardiac, beating heart surgery has the potential to improve patient outcomes by eliminating the need for
cardiopulmonary bypass and aortic cross clamping but it requires extensive image guidance as well as the development
of specialized instrumentation. Previously, developments in image guidance and instrumentation were validated on
either a static phantom or in vivo through porcine models. This paper describes the design and development of a surgical
phantom for simulating off-pump mitral valve replacement inside the closed beating heart. The phantom allows surgical
access to the mitral annulus while mimicking the pressure inside the beating heart. An image guidance system using
tracked ultrasound, magnetic instrument tracking and preoperative models previously developed for off-pump mitral
valve replacement is applied to the phantom. Pressure measurements and ultrasound images confirm the phantom closely
mimics conditions inside the beating heart.
A feature-based registration was developed to align biplane and tracked ultrasound images of the aortic root with
a preoperative CT volume. In transcatheter aortic valve replacement, a prosthetic valve is inserted into the aortic
annulus via a catheter. Poor anatomical visualization of the aortic root region can result in incorrect positioning,
leading to significant morbidity and mortality. Registration of pre-operative CT to transesophageal ultrasound
and fluoroscopy images is a major step towards providing augmented image guidance for this procedure. The
proposed registration approach uses an iterative closest point algorithm to register a surface mesh generated from
CT to 3D US points reconstructed from a single biplane US acquisition, or multiple tracked US images. The use
of a single simultaneous acquisition biplane image eliminates reconstruction error introduced by cardiac gating
and TEE probe tracking, creating potential for real-time intra-operative registration. A simple initialization
procedure is used to minimize changes to operating room workflow. The algorithm is tested on images acquired
from excised porcine hearts. Results demonstrate a clinically acceptable accuracy of 2.6mm and 5mm for tracked
US to CT and biplane US to CT registration respectively.
KEYWORDS: Heart, 3D modeling, Surgery, 3D image processing, Image segmentation, Virtual reality, Visualization, Magnetic resonance imaging, Image registration, In vivo imaging
As part of an ongoing theme in our laboratory on reducing morbidity during minimally-invasive intracardiac
procedures, we developed a computer-assisted intervention system that provides safe access inside the beating
heart and sufficient visualization to deliver therapy to intracardiac targets while maintaining the efficacy of the
procedure. Integrating pre-operative information, 2D trans-esophageal ultrasound for real-time intra-operative
imaging, and surgical tool tracking using the NDI Aurora magnetic tracking system in an augmented virtual
environment, our system allows the surgeons to navigate instruments inside the heart in spite of the lack of
direct target visualization. This work focuses on further enhancing intracardiac visualization and navigation by
supplying the surgeons with detailed 3D dynamic cardiac models constructed from high-resolution pre-operative
MR data and overlaid onto the intra-operative imaging environment. Here we report our experience during an in
vivo porcine study. A feature-based registration technique previously explored and validated in our laboratory
was employed for the pre-operative to intra-operative mapping. This registration method is suitable for in
vivo interventional applications as it involves the selection of easily identifiable landmarks, while ensuring a good
alignment of the pre-operative and intra-operative surgical targets. The resulting augmented reality environment
fuses the pre-operative cardiac model with the intra-operative real-time US images with approximately 5 mm
accuracy for structures located in the vicinity of the valvular region. Therefore, we strongly believe that our
augmented virtual environment significantly enhances intracardiac navigation of surgical instruments, while on-target
detailed manipulations are performed under real-time US guidance.
Minimally invasive techniques for use inside the beating heart, such as mitral valve replacement and septal defect
repair, are the focus of this work. Traditional techniques for these procedures require an open chest approach
and a cardiopulmonary bypass machine. New techniques using port access and a combined surgical guidance tool
that includes an overlaid two-dimensional ultrasound image in a virtual reality environment are being developed.
To test this technique, a cardiac phantom was developed to simulate the anatomy. The phantom consists of an
acrylic box filled with a 7% glycerol solution with ultrasound properties similar to human tissue. Plate inserts
mounted in the box simulate the physical anatomy. An accuracy assessment was completed to evaluate the
performance of the system.
Using the cardiac phantom, a 2mm diameter glass toroid was attached to a vertical plate as the target
location. An elastic material was placed between the target and plate to simulate the target lying on a soft tissue
structure. The target was measured using an independent measurement system and was represented as a sphere
in the virtual reality system. The goal was to test the ability of a user to probe the target using three guidance
methods: (i) 2D ultrasound only, (ii) virtual reality only and (iii) ultrasound enhanced virtual reality. Three
users attempted the task three times each for each method. An independent measurement system was used
to validate the measurement. The ultrasound imaging alone was poor in locating the target (5.42 mm RMS)
while the other methods proved to be significantly better (1.02 mm RMS and 1.47 mm RMS respectively). The
ultrasound enhancement is expected to be more useful in a dynamic environment where the system registration may be disturbed.
Clinical research has been rapidly evolving towards the development of less invasive surgical procedures. We
recently embarked on a project to improve intracardiac beating heart interventions. Our novel approach employs
new surgical technologies and support from image-guidance via pre-operative and intra-operative imaging (i.e.
two-dimensional echocardiography) to substitute for direct vision. Our goal was to develop a versatile system
that allowed for safe cardiac port access, and provide sufficient image-guidance with the aid of a virtual reality
environment to substitute for the absence of direct vision, while delivering quality therapy to the target. Specific targets included the repair and replacement of heart valves and the repair of septal defects. The ultimate
objective was to duplicate the success rate of conventional open-heart surgery, but to do so via a small incision,
and to evaluate the efficacy of the procedure as it is performed. This paper describes the software and hardware
components, along with the methodology for performing mitral valve replacement as one example of this
approach, using ultrasound and virtual tool models to position and fasten the valve in place.
A spectropolarimetric reflectometer is used to measure the monostatic bidirectional reflectance distribution function (mBRDF) and the complete Mueller matrix of a number of urban type materials over a broad spectral region. Derivative features from these measurements are computed and stochastic models of each material are constructed. The models are then used to generate data for separability studies by implementation of a kernel-based linear discriminant classification
technique. The purpose for this study, selection of materials, measurements process, analysis techniques, and concluding results are all presented.
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