IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 2, FEBRUARY 2011 443
Pneumatic Actuated Robotic Assistant System for
Aortic Valve Replacement Under MRI Guidance
Ming Li*, Member, IEEE, Ankur Kapoor, Member, IEEE, Dumitru Mazilu, Member, IEEE, and Keith A. Horvath
Abstract—We present a pneumatic actuated robotic assistant
system for transapical aortic valve replacement under MRI guid-
is currently performed manually inside the MRI bore. A robotic
assistance system that integrates an interactive real-time MRI sys-
tem, a robotic arm with a newly developed robotic valve delivery
module, as well as user interfaces for the physician to plan the
procedure and manipulate the robot, would be advantageous for
the procedure. An Innomotion arm with hands-on cooperative in-
terface was used as a device holder. A compact MRI compatible
can be placed close to the volume of interest and requires a sin-
gle image plane was used for image-based robot registration. The
system provides different user interfaces at various stages of the
procedure. We present the development and evaluation of the com-
ponents and the system in ex-vivo experiments.
Index Terms—MRI robot, real-time MRI (rtMRI)-guided inter-
vention, transapical aortic valve replacement.
of the heart. This transapical procedure is performed on the
magnetic resonance imaging (rtMRI) is considered as an ideal
imaging modality for this procedure due to its ability to con-
tinuously evaluate the delivery of the prosthesis and provide
excellent views of valvular and annular anatomy.
MRI scanner is a complicated task due to the limited space and
taneously manipulate multiple tools to delivery the prosthesis.
Our goal is to develop a robotic system to assist in performing
this minimally invasive cardiac procedure under rtMRI guid-
ance. The robot assistance can potentially reduce the cognitive
load on the physician with improved accuracy and repeatability.
Robotics has been used in cardiac surgery in an attempt to
NE of the minimally invasive approaches for aortic valve
replacement is to implant the prosthesis through the apex
Manuscript received May 24 2010; revised August 17, 2010; accepted
October 16, 2010. Date of publication October 28, 2010; date of current version
January 21, 2011. This work was supported through the Intramural Research
Program of the National Heart Lung and Blood Institute and Clinical Center,
National Institutes of Health, Department of Health and Human Services, USA.
Asterisk indicates corresponding author.
*M. Li is with the National Institutes of Health, Bethesda, MD 20892 USA
A. Kapoor, D. Mazilu, and K. A. Horvath are with the National Insti-
tutes of Health, Bethesda, MD 20892 USA (e-mail: email@example.com;
Digital Object Identifier 10.1109/TBME.2010.2089983
surgical robotic system has been successfully used in coronary
aortic valve replacement , and mitral valve repair . How-
ever, the present da Vinci system is not MRI compatible. MRI
for prostate biopsy and brachytherapy –, breast interven-
tion , , interventional spinal procedure , , neu-
rosurgery , , and interventional liver therapy , .
The presence of a strong magnetic field and limited space inside
challenging. Contemporary researchers on medical MRI robot
have primarily focused on percutaneous biopsy, drug injection,
or radiotherapy seed implantation. Improving precision and ac-
curacy, while maintaining compatibility and safety with MRI,
are the prime concerns for these robotic systems.
Our group is currently focusing on developing robotic assis-
tance for aortic valve replacement under MRI guidance ,
. We reported the first generation of the system for implant-
ing balloon-expandable (BE) prosthesis. In this paper, we de-
tail the development and evaluation of the entire pneumatically
actuated robotic assistant system for rtMRI-guided transapical
aortic valve replacement. The contribution of this paper is an in-
complicated cardiac procedure. This system provides coopera-
tive assistance to a physician, while allowing the physician to
retain full control of the procedure. We present a redesigned
robotic module for deploying both self-expanding (SE) and BE
prostheses, and a registration method to correlate the patient
heart, robot, and MRI scanner. We use a compact fiducial that
can be placed close to the volume of interest (VOI) and requires
a single image plane for image registration. While the structure
of the marker is similar to , the image processing and trans-
formation recovery methods presented in this paper are more
suited for MRI inside the heart. In our system, in addition to
image-guided method for robot control, we implemented inter-
active control, which provides direct manipulation assistance.
This allows the physician to be directly in the loop to adapt the
operative procedure to account for unstructured nature of our
II. CLINICAL BACKGROUND
the aortic annulus and enhances the ability to implant a conven-
tional prosthesis that has known durability and proven success
rate. Because the aortic valve lies in close proximity to the mi-
tral valve and the coronary ostia, the position and orientation of
the implanted prosthesis is critical. Misalignment of the pros-
thesis could result in mitral valve damage or cardiac ischemia.
0018-9294/$26.00 © 2010 IEEE
444 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 2, FEBRUARY 2011
Fig. 1.Placement of a trocar during the preparatory procedure.
Typically, the distance between the aortic annulus and coronary
ostia is about 9 mm, the left and right coronary artery ostia
are 120◦apart in the transaxial view. The prosthesis we used
is a cylindrical shape with three commissures evenly spread at
its circular base . The heights of the commissure and the
lowest portion between commissures are about 20 and 7 mm,
respectively. To deploy the prosthesis at the optimal position,
the lowest portions should be aligned with coronary ostia and
the base should be placed close to the aortic annulus. Clinically,
the tolerance for alignment is 3–4 mm and 5◦.
Use of rtMRI for guidance allows physicians to monitor the
progress of the procedure and also provides the ability to assess
the first set of rtMRI-guided transapical aortic valve replace-
ments in an animal study , . The trocar is first inserted
into the left ventricle. Through a 6-cm chest incision, the peri-
cardium is opened and the apex of the heart is exposed. Two
concentric purse strings are placed around the apex, through
which a 10–12 mm trocar is inserted into the left ventricle .
The physician then inserts and manipulates a long delivery de-
vice through the trocar to implant the prosthesis under rtMRI
guidance. There are several obstacles to the current manual pro-
cedure. Access to the operative field inside the magnet is diffi-
cult for the physician. During the procedure, a surgical assistant
holds a delivery device, on which the physician manipulates.
The physician must manipulate the different components of the
delivery device and other tools, such as balloon catheter or re-
tracting wire, through the delivery device while visualizing the
in-room MRI display simultaneously. In order to properly op-
erate the interventional devices, a coordinated effort between
the physician and the team is critical in the noisy MRI environ-
ment. The use of robotic assistance can alleviate the need of this
level coordination between the physician and the team, provide
dexterous manipulation of the interventional tools inside MRI
scanner, make easy for the physician to monitor, and control
the procedure effectively using the in-room display and robot
The trocar that is inserted into the apex above the diaphragm
mechanical constraints, the relative motion between the aortic
annulus and the apex is limited, and mainly along the long axis.
As a result, robotic assistance without a sophisticated tracking
mechanism is applicable to the transapical valve replacement.
relative 3-D positions. The rendering can be rotated on the in-room display to
match the orientation of patient.
Snapshot of rtMRI shows multiple image planes displayed at their
This situation is unique from other beating heart procedures
such as mitral valve repair or coronary artery bypass.
In a minimally invasive valve replacement, either a BE or a
SE stent is used to support the bioprosthetic valve and anchor it
are slightly different. In a procedure with the BE prosthesis, the
distal end of the loaded delivery device is first placed below the
to the desired position. The inflation of the balloon implant the
prosthesis. In a procedure with the SE prosthesis, the loaded
delivery device is first advanced into the ascending aorta, and
the edge of the inner rod is placed at the aorta annulus level. The
retraction of the sheath makes the crimped prosthesis expand
and affix to the desired position.
III. ROBOTIC ASSISTANT SYSTEM
The robotic assistant system for aortic valve replacement
comprises three major subsystems: the imaging system, the
robotic system, and user interfaces. In this section, we briefly
describe each of these subsystems.
A. Imaging System
The imaging system provides MR sequences for surgical
planning and system registration, as well as rtMRI for inter-
vention. The rtMRI interactive system  consists of an inter-
active user interface, and operating room large screen display,
specialized pulse sequences, and customized image reconstruc-
tion software. With this system, multiple oblique planes can be
imaged and displayed simultaneously at their respective 3-D
locations. High-quality images can be obtained at 5 frames per
second. Fig. 2 shows the 3-D rendering with three image planes
viewing. Image slices can be repositioned and added or omitted
as needed. The MRI tissue contrast can be interactively chan-
neled by toggling saturation pulses off/on to highlight selected
LI et al.: PNEUMATIC ACTUATED ROBOTIC ASSISTANT SYSTEM FOR AORTIC VALVE REPLACEMENT 445
Fig. 3.CAD sketch of the robotic system with patient inside an MRI bore.
ROBOT SYSTEM CHARACTERISTIC
B. Robotic System
The CAD sketch of the robotic system, which operates in the
confined space between the MRI bore and the supine patient is
shown in Fig. 3. The descriptions and functions of all 9 degree
of freedom (DOF) are listed in the Table I. Different manipula-
tion methods, namely, point-to-point, hands-on, and interactive
graphic user interface (GUI) were implemented for communi-
cation between the robot system and the interface system.
The robotic system comprises two components: a 5-DOF
MRI compatible Innomotion arm  (Innomedic, Herxheim,
Germany) was employed to hold the robotic module and move
the valve delivery device on its planned trajectory. The robotic
teractions of the physician with the system.
Diagram showing connections between different subsystems and in-
arm has a remote center of motion (RCM) structure and its
configuration fits into a standard closed MRI scanner.
C. User Interfaces and Workflow
mented different interfaces—cooperative adjustment, operative
plan, and interactive GUI adjustments—to suit the needs at the
different phases. The interactions between the subsystems and
the physician are shown in Fig. 4.
First the patient undergoes an MRI scan (S1) for the physi-
cian to determine the aortic annular diameter, coronary ostial
anatomy, and apical location. In the preparatory phase, after
the patient is intubated and anesthetized, the physician places
the trocar into the heart. The Innomotion robotic arm is then
mounted on the MRI table and adjusted such that its end effec-
tor is close to the trocar port. The robotic VDM with a fiducial
rod attached is mounted on the Innomotion arm. The physician
uses cooperative hands-on interface  to adjust the Innomo-
rod is in place, the user input sensor is detached and the robot
is moved into the bore.
In the preoperative phase, the patient undergoes another MRI
scan (S2) for the physician to plan the trajectory of the delivery
device. At the same time, another MR sequence (S3) is used
for system registration. The Innomotion arm is moved to the
planned trajectory, under image guidance using images from
scan S2. The MRI table is then moved out, and the fiducial rod
is replaced by the delivery device. Thus, direct access to the
aortic annulus is created.
the prosthesis using the VDM via a GUI.
IV. ROBOTIC VALVE DELIVERY MODULE
The robotic module is presumed to be placed inside the MRI
scanner close to the isocenter and operated during imaging.
The presence of a strong magnetic field inside the MRI scanner
demands that the robotic module must be MRI compatible ,
. To maintain image quality and prevent local heating in the
446 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 2, FEBRUARY 2011
Fig. 5.Prototype of robotic VDM, (a) back view and (b) front view.
proximity of the patient, nonconductive plastic materials, such
The robotic module is designed for manipulating a delivery
consists of a 30-cm-long straight plastic inner rod. A hollow
outer sheath protects the loaded prosthesis before it is deployed.
A. Mechanical Design and Implementation
The robotic module comprises two linear joints: the transla-
tion joint “A” and the insertion joint “B,” as well as a rotational
joint “C” (see Fig. 5). The operations of the linear joints and
the rotational joint are independent. Two linear joints can be
independently or simultaneously controlled.
The translation joint (D7) provides linear displacement of the
delivery device (both its inner rod and protecting sheath) along
maticactuators.Thetwo-actuator structureguarantees balanced
motion of a delivery device. The bodies of two linear actuators,
together with a base plate and a top plate form a sliding frame.
The sliding frame can slide on two frame rails. These two frame
rails, together with a connecting plate and a top plate form a
rigid frame. The connecting plate has grooves that match a cor-
responding adaptor on the last joint of the Innomotion robotic
The rotational joint (D6) rotates the delivery device around
its axis. This changes the orientation of the loaded prosthesis. It
is actuated by a linear pneumatic actuator attached on the base
plate of the sliding frame. The linear movement is transmitted
to the rotational movement by a rack-gear transmission.
The insertion joint (D8) moves only the inner rod of the
delivery device, thus advancing the loaded prosthesis out of the
protecting sheath. It is actuated by a linear pneumatic actuator
mounted on a central stage. This central stage can move back
and forth inside the sliding frame.
A semicircular groove embedded on the central stage holds
the inner rod of the delivery device. A U-shape groove installed
on the base rack of the sliding frame holds the sheath of the
delivery device. The semiopen structure of grooves is designed
for easy attachment of the sterilizable delivery device.
The VDM supports deploying both the BE and the SE pros-
thesis. Sole motion of the insertion joint moves only the inner
rod of the delivery device, driving the BE prosthesis out of the
protecting sheath to the desired position. Simultaneously re-
tracting the translation joint and advancing the insertion joint
at the same velocity keeps the inner rod of the delivery device
at its location, and retracts the protecting sheath back to expose
the prosthesis. This simultaneous motion will let the crimped
SE prosthesis expand and affix to the desired position.
B. Actuators, Sensors, and Control
Pneumatic actuators were used in the robotic module. These
cylinders, graphite pistons, brass shafts, and plastic housings
servovalves (GAS Automation GmbH, Germany) were used to
maintain and modulate the air flow between two outputs cor-
responding to two chambers of one cylinder. These pneumatic
valves were located outside the 5-G line. A set of 5-m long
air hoses were used for the pneumatic connections between the
servovalves and the robotic module.
Magnetotranslucent fiber-optical incremental encoders (In-
nomedic, Herxheim, Germany) made of glass were used
to gather positioning information. The optical resolution is
0.002 mm. Optical fibers transferred the signals to optoelec-
tronic conversion circuits located outside the 5-G line.
A 2-GHz Pentium IV computer running Windows XP oper-
ating system with an XMP-SynqNet-PCI hardware interface
(Motion Engineering Inc, St Barbara, CA, USA) was used
for module control. A PIV (proportional position loop integral
and proportional velocity) controller running on the DSP-based
board is used for servoing the position. The control PC was
placed outside of the MR room. It communicated with the elec-
tronic devices that control pneumatic valves and read encoder
signals via the optic network.
To provide accurate position control, we targeted the accu-
racy of the linear joints to be around 1 mm and the accuracy of
the rotation joint as 1◦. In addition to this point-to-point posi-
tion move mode, we implemented a continuous control mode
(velocity mode) to provide an interactive control interface. With
the continuous mode, the physician can start, stop, or resume
the joint motion any time he/she decides, therefore, he/she can
monitor and control the progress during the prosthesis deploy-
ment. In the point-to-point mode, the accuracy of the target po-
sition is the priority. The continuous mode demands a smooth
motion without oscillations during the transmission, i.e., the ve-
locity error during the transmission should be small. It requires
the compromise between the smoothness, velocity, and position
LI et al.: PNEUMATIC ACTUATED ROBOTIC ASSISTANT SYSTEM FOR AORTIC VALVE REPLACEMENT447
Cross-section of fiducial through planes P1 and P2, respectively. (c) and (e)
MRI image corresponding to (b) and (d), respectively.
(a) CAD drawing of fiducial with three elliptical patterns. (b) and (d)
V. SYSTEM REGISTRATION
Coil-based  and image-based registrations ,  are
provides real-time data but is scanner dependent. Image-based
registrations are scanner independent but are not real time. We
noticed that the fiducials used for image-based registrations are
placed at some distance away from the tip of the interventional
tool or the VOI , . In the valve replacement procedure,
a multichannel body coil is used, and the volume that can be
acquired with sufficient image quality is of limited size. Thus,
a fiducial close to the VOI and image isocenter is preferred.
on the table (and thus being able to place the fiducial at the
of inserting the trocar is preformed on the MRI table. In this
section, we present the details of image-based registration using
a compact elliptical fiducial that can be placed in the VOI.
A. Fiducial and MRI sequences
The fiducial rod [see Fig. 6(a)] comprises three elliptical
grooves filled with diluted gadolinium (0.01 mol/L). The fidu-
cial rod is 50-mm long and 11 mm in diameter. The diameter of
the groove is 2.6 mm.
TrueFISP_IR, fast imaging with steady-state precession with
inverse recovery pulse using standard body coil. A good choice
of parameter of inversion time will suppress both blood and
myocardium signal. The parameters of the sequence we used
for acquiring a bright signal of the fiducial is as follows: TR =
800 ms; TE = 2 ms; TI = 706 ms; flip angle = 50◦; slice
thickness = 1.05 mm; FOV = 188 × 287; and matrix = 126 ×
192. The image plane is chosen to pass through or nearly pass
through the center line of the fiducial such that the image plane
intersects with the three ellipses to obtain six bright fiducial
points, as shown in Fig. 6.
B. Image Processing
A straightforward method to determine these image points
uses intensity-weighted centroids of threshold-filtered image.
points with background noise. (c) Recovering fiducial points on the end slabs.
Image processing results for fiducal localization. “+”: Output of least-
Due to the sizes of the fiducial image points, which are only
4–5 pixels, and the noise from the surrounding environment,
localization error could be relatively large. We propose a new
method to increase the accuracy of localization. We assume that
each of these fiducial image points has a Gaussian intensity dis-
tribution, and use least-squares fitting to obtain the parameters
of the Gaussian that match each of the six fiducial points
size = 5 × 5, σ = 0.65) and gi(xi,yi,Pi) are the six Gaussian
intensity distribution, with mode (xi,yi) and parameters Pi.
The starting point for the nonlinear least-squares problem is
obtained from the intensity-weighted centroids of a threshold-
We have observed that the least-squares fitting improves ro-
bustness especially in cases when the image points are non-
circular due to partial volume effect and noises in the region
of interest [see Fig. 7(a) and (b)]. The aforementioned MRI
sequence provides a series of ten slabs with 1.05-mm spacing
for each scan. The central slabs are close to the centerline of
the fiducial, while the slabs toward two ends meet the edges of
the fiducial. The image points on the central four to six slabs are
circular and easily localized, while, those on the end slabs are
elongated and more difficult to localize [see Fig. 7(c)]. On an
average, the least-squares fitting rectifies the location by 0.33 ±
0.42 mm (max. 6.32 mm, n = 534 points in 89 images).
gi(xi,yi,Pi) − Ismooth
where, Ismoothis a Gaussian smoothed intensity image (kernel
C. Transformation Recovery
The next step is to determine the transformation between the
fiducial frame and the image frame based on these image points
is defined at the center of the middle ellipse; z-axis is the cen-
terline of the fiducial; xy-plane is perpendicular to z-axis. Let
(RΠ,TΠ) be the transformation of the plane patch Π in the
fiducial frame. We first find the plane patch Π that can best
represent the image plane, and then use point-to-point registra-
tion to recover the transformation. The algorithm is described
448 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 2, FEBRUARY 2011
1) We first prematch the image plane to the plane patch Π.
We defined a local frame Fcin the orthogonal coordinate
system of the eigenvectors centered at the center of mass
of the six image points Pi. The z-axis is parallel to the
eigenvector with the largest eigenvalue, and the x-axis
is parallel to the eigenvector with the smallest nonzero
eigenvalue, y-axis is perpendicular to xz-plane. We then
transform six image points into the frame Fc, and obtain
2) Let Pf,ibe the points of the intersection of plane Π with
the fiducial in the fiducial frame. pf,iare the projection of
Pf,ion the plane Π. We try to find the plane patch Π such
that pf,ibest match pc,i, which are the projection of Pc,i
on the image plane. We can form a constrained nonlinear
objective. A good guess of the start point for the nonlinear
optimization problem is based on the assumption that the
image plane passes through the center line of the fiducial.
In this case, (RΠ,TΠ) can be simplified as (Rz(φ),0),
where φ is the rotation angle around the z-axis of the
fiducial frame. We rotate the plane patch Π around the
center line Rz(φ) and determine φ such that it minimizes
3) We now have corresponding point pairs, P∗
the fiducial frame and the image frame, respectively. The
transformationfTican be computed by point-to-point
slab are sufficient to recover the transformation, however,
since the MRI sequence provides a series of ten slabs,
we have more information to improve the accuracy of the
identifiable, bright intensity points corresponding to the
pairs resulting in 24–36 pairs. Thus, the transformation
fTican be solved in a least-squares sense using all these
f,iare the points in the fiducial frame that minimize the
f,iand Pi, in
We evaluated the components of our system and the overall
system. A summary of these results as well as the overall error
in placement of the prothesis is shown in Table II.
A. Valve Delivery Module
We tested the prototype of the robotic VDM that is shown in
Fig. 5. The air pressure for actuating the pneumatic pistons was
75 psi (520 kPa). The maximum force load of the translation
joint was 34 N. The maximum torque of the rotation joint was
We evaluated the accuracy of the robotic module under the
point-to-point mode. With a maximum operational velocity of
10 mm/s, the accuracy of the linear joints was 0.19 ± 0.14 mm.
SUMMARY OF COMPONENT AND SYSTEM EVALUATIONS
With a maximum operational velocity of 5◦per second, the
rotation joint was 0.46◦± 0.27◦.
We also evaluated the smoothness, speed, and error of two
linear joints under the continuous mode. We moved the transla-
tion and insertion joints of the robot in the continuous mode and
recorded their position and velocity using the encoders (reso-
lution 2 μm). In this mode, the maximum position error of the
translation joint is less than 0.3 mm and its maximum velocity
error is less than 0.2 mm/s. The maximum position error of the
insertion joint is less than 0.5 mm and its maximum velocity
error is less than 0.2 mm/s. Simultaneous motion of the linear
joints, for the SE prosthesis deployment, had a maximum posi-
tion error of less than 0.5 mm, and maximum velocity error was
less than 0.5 mm/s.
B. MRI Compatibility
We evaluated the MR compatibility of the entire robotic sys-
tem with a 1.5T Siemens Espree scanner. A 16-cm cylindrical
cession (SSFP) sequence was used with following scanning pa-
bandwidth = 1000 Hz/pixel; flip angle = 45◦; slice thickness =
4.5 mm; FOV = 340 × 283 mm; and matrix = 192 × 129.
This protocol is similar to the one we used in MR scanning for
the cardiac intervention. The imaging series were taken with:
running during imaging. In the latter case, the robotic module
was mounted on the Innomotion arm, its distal end was at a
distance of 150 mm from the center of the image area.
The presence and motion of the robotic system inside the
scanner was found to have no noticeable disturbance in the
image. The observed SNR loss was 8.2% for the entire robotic
system placed in the scanner and in motion. The SNR was
calculated using the definition as the mean value of the 24 cm2
LI et al.: PNEUMATIC ACTUATED ROBOTIC ASSISTANT SYSTEM FOR AORTIC VALVE REPLACEMENT449
Fig. 8. Setup for system level evaluation on a phantom.
area at the center of the image divided by the standard deviation
of the 24 cm2area in the lower right corner of the image.
C. Registration Accuracy
The purpose of the registration in our application is to com-
mand the robotic arm to adjust the delivery device to the pre-
planned trajectory. The path of the preplanned trajectory passes
tion robot. Therefore, the rotation accuracy is more important
in our case.
mounted on the MRI table. The fiducial was submerged under
water. The robot was commanded to rotate around RCM, first
to rotate and stop at seven different poses in the craniocaudal
direction and then seven different poses in the axial direction
of the MRI scanner. We recorded the robot joints at each pose
and calculated the relative angular values between any of the
two poses. These values served as the ground truth. We also
scanned the fiducial at each pose and computed the rotation
from the scanner coordinate frame to the fiducial coordinate
We calculated the Euler angles between any of two poses
based on the computedsTf. The rotation errors using single
centermost slab in craniocaudal and axial directions are 0.81◦±
0.74◦and 0.19◦± 0.17◦, respectively. The rotation errors us-
ing all slabs in craniocaudal and axial directions are 0.62◦±
0.50◦and 0.36◦± 0.30 deg, respectively. As a comparison, the
transformation is also computed for the same set of poses using
the marker provided along with the Innomotion robot . This
marker consists of four spherical hollow balls (overall dimen-
sions: 100 × 80 × 20 mm) filled with MR contrast agent rigidly
attached to the last joint of the Innomotion robot. The rotation
errors using these large spherical markers in craniocaudal and
from the VOI. Further, the robot registration using this marker
can only be done before the patient is placed on the table.
D. System Level Evaluation
The motivation of our phantom based experiment (see Fig. 8)
is to test the feasibility of the integrated system from the en-
adjustment of the prosthesis. Second row shows the position adjustment of the
prosthesis. Third row shows the deployment of the SE prosthesis.
Sequence of MR images showing the progress of using our robotic
gineering point of view. A phantom was designed to emulate
the dimensions of the valve replacement situation. It consisted
of a plastic tube with 25-mm diameter, which served as the
aorta. The diameter of the tube is typical size of adult human
aortic root. This was mounted on one side of a 200 × 100 ×
tic membrane located on the opposite side of the tank, served
as the apex. A 12–15-mm trocar was inserted into the spherical
joint. The distance from spherical joint to the end of the plastic
tube was 50 mm, which is the typical distance from the heart
apex to the aorta annulus as measured in clinical scenario. The
trocar insertion point had some compliance due to the mounting
We tested the robotic system for SE prosthesis deployment.
The SE prosthesis requires coordinated motion between two
coupled pneumatic joints, thus making it a more challenging
scenario. We aimed to deploy the SE prosthesis such that its
proximal edge is on the edge of the tube under rtMRI guidance
using robotic system.
Fig. 9 shows the progress of the orientation adjustment and
the position adjustment of the prosthesis, as well as the progress
of the deployment of the SE prosthesis. After the prosthesis was
deployed, we measured the distance between the edge of the
tube and the edge of the SE prosthesis by caliper, and defined it
as the position error. The average of absolute system level error
over seven trials was 1.14 ± 0.33 mm.
To evaluate the performance of the robotic system without
the error associated with rtMRI guidance, we also performed
the valve deployment experiment with direct visualization. The
average distance based on nine trials for a SE prosthesis was
0.8 ± 0.4 mm, representing the cumulative error from robotic
arm, the VDM, and the motion and/or slippage of the prosthesis
during the deployment, but not due to MRI guidance. The most
likely reason for the difference can be attributed to the low
spatial and temporal resolution of rtMRI.
450 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 2, FEBRUARY 2011
VII. DISCUSSION AND CONCLUSION
We developed a robotic assistant system for assisting in
transapicalaorticvalve replacement underrtMRIguidance. Our
system integrates a rtMRI system, a robotic arm with a newly
cian to plan the procedure and manipulate the robot. The pres-
ence and motion of the robotic system inside the MRI scanner
was found to have no noticeable disturbance in the image.
A compact 3-DOF robotic VDM was designed and con-
structed for manipulating and placing both BE and SE prosthe-
sis into a heart inside MRI scanner. We have shown the robotic
module possesses sufficient position and velocity accuracy.
We used a compact fiducial pattern placed in the VOI for
robot registration. This fiducial allows minimal disruption of
the workflow during the preparatory procedure of placing the
trocar and inserting the delivery device using hands-on cooper-
ative interface. The MRI sequence used for imaging the fiducial
successfully suppressed the signal from the blood and the my-
ocardium, enabling suitable localization of gadolinium filled
fiducial. With a new localization and transformation recovery
method, this compact fiducial leads to registration accuracy
comparable to the result by using a larger marker, which can
only be placed 15 cm away from the VOI.
We proposed multiple interfaces for the valve replacement
procedure. We believe, that in the engineering of robots for
medical applications, detailed analyses of the functions of the
entire system, i.e., robot, interfaces, and application, taken as
single entity, is arguably more important than the individual
application, separately). Thus, having a combination of more
than one interface such as image guided, console guided, or
from the entire system.
We tested the integrated system with a phantom. The re-
sults show that all subsystem cooperate smoothly to provide
assistance for bioprosthetic aortic valve replacement. The sta-
ble phantom is not an ideal replica of the beating heart. But with
proper anatomical dimension between the aortic annulus and
the apex, it provides a reasonable situation to validate the coor-
dinated working of the different components of the integrated
system before sacrificing animal lives.
car that is inserted into the heart apex anchors the heart. Based
on our experience on the manual procedure, we note that with
the trocar in place, the relative motion of the aortic annulus with
respect to the apex is limited and mainly along the long axis. In-
teractive control, which provides direct manipulation assistance
allows the physician to be directly in the loop to adapt the oper-
ative procedure to account for this motion. The physician, who
performs manual valve replacement evaluated the system in a
phantom study and commented that the use of the system min-
imized the cognitive burden of manipulation of multiple tools
from awkward angles, and the interactive interface kept the first
hand control on the procedure. The performance of using inter-
active interface to control the system in a beating heart shall be
further evaluated in our future work in an animal study.
M. Li and A. Kapoor contributed equally to this work.
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Ming Li (S’03–M’06) received the B.Sc. degree in
biomedical engineering from Shanghai Jiao Tong
in precision machinery engineering from the Univer-
sity of Tokyo, Tokyo, Japan, in 2000, and the Ph.D.
degree in computer science from the Johns Hopkins
University, Baltimore, MD, in 2005.
In 2005, she joined National Institutes of Health,
Bethesda, MD, as a Staff Scientist. She is currently
the bioengineering section Chief of Cardiothoracic
Blood Institute, National Institutes of Health. Her research interests include
medical robotics, surgical navigation, medical imaging, human-machine inter-
action, and system integration.
Ankur Kapoor (S’04–M’07) received the Bachelor
of Engineering degree (with highest honors) in me-
chanical and electrical and electronics from the Birla
Institute of Technology and Science, Pilani, India,
in 2000, and the M.S degree in computer science
and the Ph.D. degree in computer science from the
Johns Hopkins University, Baltimore, MD, in 2006
and 2007, respectively.
Since the fall of 2007, he has been a Research
Fellow at Radiology and Imaging Sciences, Clinical
Center, National Institutes of Health, Bethesda, MD.
His research interests include robot assisted minimally invasive surgery, ar-
chitecure for surgical systems, and image or visually guided interventions and
Dumitru Mazilu (M’01) received the B.Sc. degree
and the Ph.D. degree in mechanical engineering from
University of Craiova, Romania, in 1984 and 1998,
He was a Research Fellow at the Johns Hopkins
University, Baltimore, MD, in 2000. From 2003 to
partment, Johns Hopkins University, Baltimore, MD.
He is currently a Research Scientist at the National
Institutes of Health, Bethesda, MD. His current re-
search interest include the design of medical robotics
and medical devices. He is the author or coauthor of numerous scientific papers
and several national and international patents. During his career he participated
in the design and development of several medical robots and medical devices.
Keith A. Horvath received the B.S. degree in bio-
He finished his training in general and cardiothoracic
surgery at the Brigham and Women’s Hospital.
He joined the cardiac surgical faculty at North-
western University, Chicago, IL, in 1996. In 2004,
he moved to the National Institutes of Health. He is
currently the Chief of the Cardiothoracic Surgery at
SuburbanHospital, as wellastheDirector oftheCar-
diothoracic Surgery Research, National Heart, Lung,
Blood Institute, National Institutes of Health, Bethesda, MD, and an affiliate
with Johns Hopkins Medical Center. His research interests include minimally
invasive cardiac surgery, robotic assisted cardiac surgery, stem cell therapy, and