Junmo An1, Eftychios G. Christoforou2, Karen Chin3, Jeremy Hinojosa3, Dipan J. Shah3, Andrew G. Webb4, and Nikolaos V. Tsekos1
1University of Houston, Houston, TX, United States, 2University of Cyprus, Nicosia, Cyprus, 3Houston Methodist, Houston, TX, United States, 4Leiden University Medical Center, Leiden, Netherlands
Synopsis
Integrated control system of the manipulator and
marker control is important for localization and tracking of multiple optically
detunable MR markers on MR-compatible manipulators. Selecting which markers are
visible on MR images by the motion of the maneuvering portion of the
MR-compatible manipulators allows unambiguous identification of a combination
of markers and simplifies both the data acquisition and the post processing.
This proposed technique can be employed to track multiple marker positions on
interventional devices such as the steerable catheters and the end-effectors of
the MR-compatible manipulator.Introduction
Localization and tracking of interventional tools with
MR markers is among the most important factors in real-time MRI guided procedures.
1
In order to distinguish the markers on the MR images, optically detunable
markers have been proposed to make one or all markers together MR visible per MR
data collection cycle.
2,3 In this study, the integrated control
system of the manipulator and marker control synchronizes the ON/OFF state of the
LED source, controls the manipulator, and triggers the MR scanner; therefore, it
also allows for tracking a combination of markers that correspond to the specific
maneuvering part of the manipulator. Furthermore, we describe a technique for
accurate localization and fast tracking with multiple optically detunable MR
markers that are selectively tuned and detuned by the actuated portion of the MR-compatible
manipulator. This technique allows unambiguous identification of the particular
marker points of the maneuvering manipulator on MR images, and simplifies both
the MR data acquisition and the post processing methods.
Methods
Each MR marker consists of a 3.0 mm outer diameter
inductively coupled solenoid coil, a variable non-magnetic capacitor and a
photoresistor which is optically tuned and detuned; moreover, it is connected
to the marker control module via a 15.0 meter long optical fiber cable. Figure 1 shows our MR-compatible
manipulator arm
4 attached to four optically detunable MR markers.
This robotic arm consists of two rotational degree of freedoms (DoFs) (i.e.,
DoF-1: rotating by an angle ϕ around the X axis, DoF-2: rotating by an angle ψ
around the oblique axis that is orthogonal to X) and one prismatic DoF-3 (i.e.,
inserting the end-effector). Marker #1 and #2 are adequate to calculate its
link length and the rotation angle (ϕ) of DoF-1. In addition, marker #3 and #4
are needed to measure the insertion length (Δ) and the rotation angle (ψ) of the
end-effector. Figure 2 displays the flow diagram of integrated control system
of the manipulator control and marker control. The manipulator control module maneuvers
the robotic arm and determines from manipulator maneuvering (robotStates)
conditions which markers are needed to be switched ON/OFF, markerStates. The
module then sends markerStates to the marker control module. The marker control
module controls the marker visibility, sends TTL pulses to the MR scanner to
trigger data acquisitions, and receives a return TTL pulse after completing data
acquisitions.
Results
A manipulator operation
and MR imaging at 1.5T with the manipulator-driven tracking were performed in various
different manipulator maneuvering (robotStates) states. Figure 3 shows
transverse MR images collected with (a) all markers OFF, (b) all markers ON,
(c-f) the result by sequentially turning ON marker #1, #2, #3 and #4 (TR/TE =
384.4/1.46 ms, flip angle = 1°, bandwidth/pixel = 723 Hz, matrix size = 192 X 134, FOV
= 350 X 244 mm
2, slice thickness = 10 mm). Figure 4 shows sagittal MR images
collected with (a) all markers OFF, (b-e) and collection of four sequential
images of markers #1, #2, #3 and #4 turned ON in turn, which was the result of
control state (markerStates) 1111 being sent by the manipulator control module
to the marker control module (TR/TE = 403.37/1.41 ms, flip angle = 3°, bandwidth/pixel
= 723 Hz, matrix size = 152 X 192, FOV = 336 X 425 mm
2, slice thickness = 10
mm). Figure 5 shows
transverse MR images acquired with marker #3 and #4 with a TurboFLASH sequence
(TR/TE = 403.37/1.41 ms, flip angle = 3°, bandwidth/pixel = 723 Hz, matrix size = 192 X
152, FOV = 192 X 152 mm
2, slice thickness = 10 mm) as ϕ = 0 and the
rotational DoF-2 rotates the end-effector randomly.
Discussion
The described
technique was designed to study accurate localization and fast tracking methods
of the MR-compatible robots using computer-controlled MR markers for MRI-guided
and manipulator-assisted interventions. This technique can be used to track the
motion of multiple marker points on articulated surgical instruments such as
steerable catheters, bendable tools, and the end-effector of the manipulator.
Conclusion
This study demonstrates the
manipulation-driven tracking technique that employs the integrated control
system of the manipulator and marker control to select on-the-fly which MR
markers need to be visible on the MR images to track the maneuvering part of
the MR-compatible manipulator.
Acknowledgements
This study is supported by the National Science
Foundation award (CNS-0932272).References
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