Sunram 5: An MR Safe Robotic System for Breast Biopsy

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Sunram 5: An MR Safe Robotic System for Breast Biopsy
V. Groenhuis1, F.J. Siepel1, M.K. Welleweerd1, J. Veltman2, S. Stramigioli1,3
1Robotics and Mechatronics, University of Twente, The Netherlands
2Ziekenhuisgroep Twente, Almelo, The Netherlands
3ITMO, Saint Peterburg, Russia
v.groenhuis@utwente.nl
INTRODUCTION
The conventional MRI-guided breast biopsy procedure
is inaccurate due to manual needle insertions and the
necessity of rescanning. Therefore, the procedure would
benefit from an MR safe robotic system to manipulate
the needle precisely towards suspicious lesions under
near-realtime MRI guidance. The main challenge is to
design a system consisting of non-metallic, non-
magnetic and non-conductive materials in order to
eliminate safety hazards and imaging artifacts due to
interference with the MRI’s magnetic field.
Pneumatic stepper motors have been proved an effective
actuation method in several state-of-art robotic
systems[1, 2]. These systems are limited in bandwidth as
5 m long pneumatic tubes are needed between the valve
manifold and the robotic system. This results in a
stepper motor frequency of approximately 10 Hz when
maximum force is necessary. The Stormram 4 needs
approximately one minute to move from one position to
another considering an axis of 640 steps. Higher
velocities are desirable for practical applications[2].
Workspace is important for the breast biopsy robot: a
suspicious lesion may be identified at the medial side or
close to the chest wall. These locations are difficult to
reach in state-of-art systems. Safety is also a concern: in
case of emergency (including loss of pressure or mains
power), the system should be able to eject the needle
from the patient within one second.
This research aims to develop a new robotic system
addressing the aforementioned challenges. The specific
novelties are enhanced speed and accuracy, enlarged
workspace and improved safety.
MATERIALS AND METHODS
Fig. 1 Photo of Sunram 5.
The Sunram 5 robot is shown in Fig. 1. The five degree
of freedom manipulator is actuated by six pneumatic
stepper motors, measures 107 x 72 x 56 mm and has a
mass of 260 g. Two of the motors have a relatively large
step size of 1.7 mm, allowing for high-speed lateral and
needle insertion movements. These motors are coupled
with small-step motors (step size 0.3 mm) to maintain
sub-millimeter accuracy in the same direction.
Each stepper motor consists of two or three pneumatic
cylinders which alternatingly press against a straight or
curved rack to make discrete steps[2]. Besides these six
stepper motors three individual cylinders are also
present to fire the biopsy gun and activate the needle
ejection safety mechanism.
Fig. 2 Needle insertion actuator with safety ejection
mechanism.
Fig. 2 shows a schematic of the three-cylinder needle
insertion stepper motor with integrated safety ejection
mechanism. In normal operation pistons A, B and C are
alternatingly pushed down and up, nudging the rack
(with needle) to the left or right in discrete steps. In case
of emergency all three pistons A, B and C are lifted up
and piston E is pushed to the right by a separate
cylinder. As piston E is connected to the rack holding
the needle, the needle is retracted out of the breast.
The robot is attached to a breast fixation system inspired
by Machnet’s device to immobilize the breast[3]. It can
be manually positioned at a range of orientations
relative to the breast allowing to adapt the compression
direction and biopsy access window to the specific
location of the lesion to be targeted. Fig. 3 shows a
photograph of the entire system during the biopsy
process.
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Fig. 2 Sunram 5 (left) targeting a breast phantom inside the
fixation system (right).
The majority of the Sunram 5 parts was printed using a
high-resolution polyjet printer. Acrylic rods support the
hinges and laser-cut silicone rubber seals make the
cylinders airtight. The removable 14G (2.1 mm) MR
conditional titanium needle is the only metallic
component of the system.
Three aspects of the robotic system were tested
including speed/accuracy, workspace and safety. The
highest manipulation time from one configuration to
another is based on the range of motion of the different
motors expressed as the number of steps, while the step
sizes determine the theoretical positioning accuracy.
The workspace was evaluated by estimating the volume
percentile of the breast phantom which can be reached
by the biopsy system, taking both the positioning
freedom of the fixation system and the range of motion
of the robot into account. The safety aspect was
evaluated by first inserting a needle in the phantom to a
depth of 50 mm and then activating the safety
mechanism at pressures ranging from 0.1 to 0.4 MPa
while recording the needle movement with a high-speed
camera to measure the needle ejection time.
RESULTS
Speed and accuracy
The six stepper motors have a range of motion varying
from 61 to 167 steps. At a stepping frequency of 10 Hz
the upper bound of the manipulation time from one
position to another is 16.7 seconds. When starting in a
mid-way position and/or taking into account that two
motors can work together in certain directions the
effective maximum manipulation time is reduced to 8.4
seconds.
In the lateral and needle-insertion directions the small-
step actuators yield a step size of 0.3 mm. In vertical
direction two rotational joints have a step size of 0.3°,
which translates to a 0.52 mm displacement at 100 mm
from the rotation axis. Based on these step sizes, sub-
millimeter accuracy can be achieved. Further enhanced
accuracy is possible by coordinating movements of two
motors: in collinear motor pairs, the smallest achievable
step is 0.1 mm which is the greatest common divisor of
its individual step sizes.
Workspace
The orientable fixation system significantly enhances
workspace when the approximate location of the target
lesion is already known. In combination with the high
dexterity of the robotic system, any lesion that is
situated below the fixation system’s top surface is
reachable by the robotic system.
Safety
A minimum pressure of 0.2 MPa is required to
successfully activate the safety mechanism, ejecting the
needle out of the phantom in 1.8 s. At 0.3 MPa, the time
is reduced to 0.53 s, while at 0.4 MPa the needle is
ejected in 0.31 s.
DISCUSSION
Pneumatic stepper motor technology has shown to be an
effective actuation method for the Sunram 5. The six
degrees of freedom combined with a versatile breast
fixation system has potential to target lesions in the
whole breast. Thanks to the dual-speed motors the
manipulation time is under ten seconds, making it an
order of magnitude faster than state-of-art robotic
systems without compromising on accuracy. In case of
emergency the safety mechanism is able to eject the
needle out of the breast within one second at a pressure
of 0.3 MPa or higher. A limitation might be that no
direct position feedback is employed, so a calibration
routine based on MRI or visual feedback may be
necessary prior to the procedure.
The next steps in this research involve quantitative
evaluation of targeting accuracy when performing
biopsies in phantoms under MRI guidance. Needle-
tissue interaction may contribute to deformations of the
needle and/or the breast, potentially introducing
localization errors which need to be assessed and
mitigated. While the Sunram 5 has shown to be a major
advancement over its predecessors, additional work is
required concerning the placement of RF coils inside the
breast fixation system and in sterilization of the device.
REFERENCES
[1] D. Stoianovici et al., “Multi-Imager Compatible, MR
Safe, Remote Center of Motion Needle-Guide Robot,” in
IEEE Transactions on Biomedical Engineering, vol. 65,
no. 1, pp. 165-177, Jan. 2018.
[2] V. Groenhuis et al., “Design and characterization of
Stormram 4: An MRI-compatible robotic system for
breast biopsy,” 2017 IEEE/RSJ International Conference
on Intelligent Robots and Systems (IROS), Vancouver,
BC, 2017, pp. 928-933.
[3] J. Veltman et al., “Magnetic Resonance-Guided Biopsies
and Localizations of the Breast: Initial Experiences Using
an Open Breast Coil and Compatible Intervention
Device,” Investigative Radiology, 2005, vol. 40, no. 1,
pp. 379-384
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