Peripheral nerve block treatments are ubiquitous in hospitals and pain clinics worldwide. State of the art
techniques use ultrasound (US) guidance and/or electrical stimulation to verify needle tip location. However,
problems such as needle-US beam alignment, poor echogenicity of block needles and US beam thickness can
make it difficult for the anesthetist to know the exact needle tip location. Inaccurate therapy delivery raises
obvious safety and efficacy issues. We have developed and evaluated a needle guidance system that makes use
of a magnetic tracking system (MTS) to provide an augmented reality (AR) guidance platform to accurately
localize the needle tip as well as its projected trajectory. Five anesthetists and five novices performed simulated
nerve block deliveries in a polyvinyl alcohol phantom to compare needle guidance under US alone to US placed in
our AR environment. Our phantom study demonstrated a decrease in targeting attempts, decrease in contacting
of critical structures, and an increase in accuracy of 0.68 mm compared to 1.34mm RMS in US guidance alone.
Currently, the MTS uses 18 and 21 gauge hypodermic needles with a 5 degree of freedom sensor located at the
needle tip. These needles can only be sterilized using an ethylene oxide process. In the interest of providing
clinicians with a simple and efficient guidance system, we also evaluated attaching the sensor at the needle hub as
a simple clip-on device. To do this, we simultaneously performed a needle bending study to assess the reliability
of a hub-based sensor.
Trans-esophageal echocardiography (TEE) is a standard component of patient monitoring during most cardiac
surgeries. In recent years magnetic tracking systems (MTS) have become sufficiently robust to function effectively
in appropriately structured operating room environments. The ability to track a conventional multiplanar 2D
TEE transducer in 3D space offers incredible potential by greatly expanding the cumulative field of view of cardiac
anatomy beyond the limited field of view provided by 2D and 3D TEE technology. However, there is currently
no TEE probe manufactured with MTS technology embedded in the transducer, which means sensors must be
attached to the outer surface of the TEE. This leads to potential safety issues for patients, as well as potential
damage to the sensor during procedures. This paper presents a standard 2D TEE probe fully integrated with
MTS technology. The system is evaluated in an environment free of magnetic and electromagnetic disturbances,
as well as a clinical operating room in the presence of a da Vinci robotic system. Our first integrated TEE
device is currently being used in animal studies for virtual reality-enhanced ultrasound guidance of intracardiac
surgeries, while the "second generation" TEE is in use in a clinical operating room as part of a project to
measure perioperative heart shift and optimal port placement for robotic cardiac surgery. We demonstrate
excellent system accuracy for both applications.
In image-guide surgery, optimizing the accuracy in localizing the surgical tools within the virtual reality environment
or 3D image is vitally important, significant effort has been spent reducing the measurement errors at the
point of interest or target. This target registration error (TRE) is often defined by a root-mean-square statistic
which reduces the vector data to a single term that can be minimized. However, lost in the data reduction is the
directionality of the error which, can be modelled using a 3D covariance matrix. Recently, we developed a set
of expressions that modeled the TRE statistics for point-based registrations as a function of the fiducial marker
geometry, target location and the fiducial localizer error (FLE). Unfortunately, these expressions are only as good
as the definition of the FLE. In order to close the gap, we have subsequently developed a closed form expression
that estimates the FLE as a function of the estimated fiducial registration error (FRE, the error between the
measured fiducials and the best fit locations of those fiducials). The FRE covariance matrix is estimated using a
sliding window technique and used as input into the closed form expression to estimate the FLE. The estimated
FLE can then used to estimate the TRE which, can be given to the surgeon to permit the procedure to be
designed such that the errors associated with the point-based registrations are minimized.
Ultrasound is garnering significant interest as an imaging modality for surgical guidance, due to its affordability,
real-time temporal resolution and ease of integration into the operating room. Minimally-invasive intracardiac
surgery performed on the beating-heart prevents direct vision of the surgical target, and procedures such as
mitral valve replacement and atrial septal defect closure would benefit from intraoperative ultrasound imaging.
We propose that placing 4D ultrasound within an augmented reality environment, along with a patient-specific
cardiac model and virtual representations of tracked surgical tools, will create a visually intuitive platform with
sufficient image information to safely and accurately repair tissue within the beating heart. However, the quality
of the imaging parameters, spatial calibration, temporal calibration and ECG-gating must be well characterized
before any 4D ultrasound system can be used clinically to guide the treatment of moving structures. In this paper,
we describe a comprehensive accuracy assessment framework that can be used to evaluate the performance of 4D
ultrasound systems while imaging moving targets. We image a dynamic phantom that is comprised of a simple
robot and a tracked phantom to which point-source, distance and spherical objects of known construction can be
attached. We also follow our protocol to evaluate 4D ultrasound images generated in real-time by reconstructing
ECG-gated 2D ultrasound images acquired from a tracked multiplanar transesophageal probe. Likewise, our
evaluation framework allows any type of 4D ultrasound to be quantitatively assessed.
KEYWORDS: Heart, Endoscopy, In vitro testing, Autoregressive models, Visualization, Augmented reality, Data modeling, Magnetic tracking, Real time imaging, Transducers
Catheter-driven cardiac interventions have emerged in response to the need of reducing invasiveness associated
with the traditional cut-and-sew techniques. Catheter manipulation is traditionally performed under real-time
fluoroscopy imaging, resulting in an overall trade-off of procedure invasiveness for radiation exposure of both
the patient and clinical staff. Our approach to reducing and potentially eliminating the use of flouroscopy
in the operating room entails the use of multi-modality imaging and magnetic tracking technologies, wrapped
together into an augmented reality environment for enhanced intra-procedure visualization and guidance. Here
we performed an in vitro study in which a catheter was guided to specific targets located on the endocardial atrial
surface of a beating heart phantom. "Therapy delivery" was modeled in the context of a blinded procedure,
mimicking a beating heart, intracardiac intervention. The users navigated the tip of a magnetically tracked
Freezor 5 CRYOCATH catheter to the specified targets. Procedure accuracy was determined as the distance
between the tracked catheter tip and the tracked surgical target at the time of contact, and it was assessed under
three different guidance modalities: endoscopic, augmented reality, and ultrasound image guidance. The overall
RMS targeting accuracy achieved under augmented reality guidance averaged to 1.1 mm. This guidance modality
shows significant improvements in both procedure accuracy and duration over ultrasound image guidance alone,
while maintianing an overall targeting accuracy comparable to that achieved under endoscopic guidance.
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.
A 2D ultrasound enhanced virtual reality surgical guidance system has been under development for some time in
our lab. The new surgical guidance platform has been shown to be effective in both the laboratory and clinical
settings, however, the accuracy of the tracked 2D ultrasound has not been investigated in detail in terms of the
applications for which we intend to use it (i.e., mitral valve replacement and atrial septal defect closure). This
work focuses on the development of an accuracy assessment protocol specific to the assessment of the calibration
methods used to determine the rigid transformation between the ultrasound image and the tracked sensor.
Specifically, we test a Z-bar phantom calibration method and a phantomless calibration method and compared
the accuracy of tracking ultrasound images from neuro, transesophageal, intracardiac and laparoscopic ultrasound
transducers. This work provides a fundamental quantitative description of the image-guided accuracy that can
be obtained with this new surgical guidance system.
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.
This paper proposes an assessment protocol that incorporates both hardware and analysis methods for evaluation of
electromagnetic tracker accuracy in different clinical environments. The susceptibility of electromagnetic tracker
measurement accuracy is both highly dependent on nearby ferromagnetic interference sources and non-isotropic. These
inherent limitations combined with the various hardware components and assessment techniques used within different
studies makes the direct comparison of measurement accuracy between studies difficult. This paper presents a multicenter
study to evaluate electromagnetic devices in different clinical environments using a common hardware phantom
and assessment techniques so that results are directly comparable. Measurement accuracy has been shown to be in the
range of 0.79-6.67mm within a 180mm3 sub-volume of the Aurora measurement space in five different clinical
environments.
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.
KEYWORDS: Error analysis, Calibration, Manufacturing, Data modeling, Position sensors, Optical tracking, Accuracy assessment, Sensors, Data conversion, Systems modeling
Highly accurate spatial measurement systems are among the enabling technologies that
have made image-guided surgery possible in modern operating theaters. Assessing the
accuracies of such systems is subject to much ambiguity, though. The underlying
mathematical models that convert raw sensor data into position and orientation
measurements of sufficient accuracy complicate matters by providing measurements
having non-uniform error distributions throughout their measurement volumes.
Users are typically unaware of these issues, as they are usually presented with only
a few specifications based on some "representative" statistics that were themselves
derived using various data reduction methods. As a result, much of the important
underlying information is lost. Further, manufacturers of spatial measurement
systems often choose protocols and statistical measures that emphasize the strengths
of their systems and diminish their limitations. Such protocols often do not reflect
the end users' intended applications very well. Users and integrators thus need to
understand many aspects of spatial metrology in choosing spatial measurement systems
that are appropriate for their intended applications. We examine the issues by
discussing some of the protocols and their statistical measures typically used by
manufacturers. The statistical measures for a given protocol can be affected by many
factors, including the volume size, region of interest, and the amount and type of
data collected. We also discuss how different system configurations can affect the
accuracy. Single-marker and rigid body calibration results are presented, along with
a discussion of some of the various factors that affect their accuracy. Although the
findings presented here were obtained using the NDI Polaris optical tracking systems,
many are applicable to spatial measurement systems in general.
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