LIFE2014GS2-2

2014 年 9 月 24 日－26 日 北海道

双胎間輸血症候群の手術ナビゲーションにおける内視鏡自己位置提示の設計
Workflow-centric design of 3D navigational cues for photocoagulation treatment in twin-to-twintransfusion syndrome○楊 量景, 王 君臣, 安藤 岳洋, 窪田 章宏, 山下 紘正＊, 千葉 敏雄＊, 佐久間 一郎,小林 英津子
L. Yang, J. Wang, T. Ando, A. Kubota, H. Yamashita*, T. Chiba*, I. Sakuma, E. Kobayashi
Graduate School of Engineering, The University of Tokyo, Tokyo, Japan.
*Clinical Research Center, National Center for Child Health and Development, Tokyo, Japan.

Abstract: Twin-to-Twin Transfusion Syndrome (TTTS) due to imbalance circulation between twin fetuses results in highprenatal loss and risk of neurological impairment if not treated before birth. A fetoscope is used to guide laser
photocoagulation of pathological vessels while ultrasonography guides instrument navigation. Mental registration of thetwo sources of guidance coupled with demanding minimally invasive constrains make the procedure challenging.
Therefore, we propose navigational features that provide location awareness of the moving fetoscopic views. Its current
view is graphically illuminated while dimming previous views to provide a dynamic visual cue of the endoscopic trail inthe simultaneously mapped out placental vasculature on a 3D ultrasound model. A workflow-centric philosophy is adopted
to minimize unnecessary burdens to the operating personnel using only fetoscopic and ultrasound imaging without externaltrackers. The navigational cues have been validated through visual inspections. Future study will include usability test of
the navigational cues with expert subjects.
Key Words: Twin-to-Twin Transfusion Syndrome, Fetoscopic Procedures, Laser Photocoagulation
1. BackgroundTwin-to-Twin Transfusion Syndrome (TTTS) occurs in twinsthat share a common placenta suffering from imbalance in blood
circulation due to communicating blood vessel that transfuse
blood from one twin to another. This can lead to 90% prenatal
loss or high risk of neurological impairment if left untreated
before birth.
A common treatment is to perform laser photocoagulation on
the pathological vessels that are responsible for the imbalanceof blood circulation amongst twin fetuses. A fetoscope is used toguide laser photocoagulation on the pathological vessels. Whilefetoscopic procedures reduce the risk of medical complications,they provides limited field-of-view for navigation during
placental imaging. Current practice uses ultrasound fornavigation while the fetoscope inspects vessels on the placenta
surface and guides laser treatment. This involves non intuitive
mental registration of the two different sources of visualizationin addition to the demanding minimally invasive constraint.
2. Objective
To address the challenges posed by the non intuitive
perspective of existing navigation system and contribute to thedevelopment of computer aided surgery for fetoscopicprocedures, this study proposes a workflow centric design ofnavigational features that provide surgeons with location
awareness of the dynamic fetoscopic vision during
photocoagulation treatment for TTTS.
3. Method
Our design approach adopts a workflow centric philosophy inthat it aims to integrate data acquisition, which is required forGS2-2-1

navigational applications into the surgical procedures itself.
Therefore, the navigational module is designed to blend in withthe procedure without having to introduce external systems andadditional tasks for the medical team. This is achieved throughinformation fusion of ultrasonography and endoscopy for arange of navigational purposes including scene construction,tracking, texture mapping, and localization of current field ofview with respect to the intraoperatively constructed map. Thisway of cue augmentation and mapping requires no tracking
information of the freehand fetoscopic camera as it taps on theinformation of existing imaging modalities that is already inplace for the procedure. Fig. 1 is a schematic illustration of thedesign concept that fuses the traditional use of multiple imaging
displays into a single platform that provide navigational view
with photorealistic scene.

Fig. 1 Workflow-centric design concept
In this work, visual cues are proposed to facilitate location
awareness in the vasculature map through highlighting of active
field-of-view. The current view of the moving fetoscope isgraphically illuminated while dimming previous views toLIFE2014

provide a dynamic visual cue of the endoscopic trail in thesimultaneously mapped out placental vasculature on a 3D
ultrasound model. This can be achieved by a straightforward
linear combination of the intensity values of the overlappingimaged region. The common plane where the images are aligned
on is subsequently projected to the ultrasound constructed 3D
scene. Fig. 2 features the overview of the mapping framework.

Based on observations, the moving trail of illuminatedfreehand endoscopic view provides useful visual cue on
instantaneous location as well as a navigational map for
manipulating the fetoscope during placental imaging. We areable to maintain the target registration error within 4 mm for an
overlay of 1/3 the size of a placenta phantom, which has a roundchorionic planar surface and about 18 cm in its longest
dimension. Preliminary visual inspection on ex vivo monkey
placenta also shows smooth mapping for more than 100 framesof image overlay.
5. Discussion
Visual observation of the navigational cues achieved throughthe workflow-centric design approach suggested that the vast
vasculature on the placenta surface can be visualized in a single
integrated navigational platform featuring the topological scene,
updated endoscopic view, and the relative position of the latterwith respect to the former all at the same time.
It is important to note, however, that the navigational cues do
not directly enhance distinctiveness of the blood vessel. It
provides only navigational guidance, which indirectly aid
visualization of the vast vasculature. Nevertheless, because of
its self-contained nature, the workflow-centric design approachallows incorporation of existing and developing technologies invisual enhancement of the blood vessels through contrast agents[2] or hyperspectral imaging [3] readily.
Future studies will include usability test on expert subjects toquantify the target registration error of the navigational cues.

Fig. 2 Overview of the mapping framework
This untracked approach is discussed in a previous work [1]with greater details. In this paper, the relevant procedures will
be explained concisely to ensure a self-contained discussion.
Firstly, registration between the alignment plane and the scene
geometry is done through a one-time ultrasound-based
initialization, where the scene is segmented and endoscope
localized. The spatial relation obtained from registration is used
to map the photorealistic color map of the endoscopic views tothe patches on the 3D scene to provide the mapping of theendoscopic trail and cue of the current view.
In addition, the proposed system also provides a wide
mapped view of the placenta vasculature allowing surgeon tosee regions that have been imaged without having to revisit
them repetitively as in the case of narrow field-of-view
fetoscopy hence addressing limitation in mental capacity forvisualizing the vast placental vasculature with online updatingof endoscopic view in the navigational map.
4. Results
The navigational cues have been validated through visualinspections. Fig 3. shows a 2D topological view with thegraphically illuminated current view as a navigational cue.

2014 年 9 月 24 日－26 日 北海道

6. Conclusion
In this work, we introduced navigational cues that enhance
topological and location awareness for fetoscopic procedures
where application of computer aided surgery technology has
been lagging compared to other field of interventional medicine.
By adopting a workflow-centric philosophy in the developmentof the navigational feature that minimizes procedural
disruptions and unnecessary burdens to the operating personnel
through a tracker-less approach, we hope to bridge the researchgap between the developments of computer aided surgery andits applications for rare disease treatment like the TTTS.
Reference
(1)

(2)

(3)

L. Yang, J. Wang, E. Kobayashi, et al., "Image mapping ofuntracked free-hand endoscopic views to an ultrasoundimage-constructed 3D placenta model," Int J Med Robot
Comp (early access).
K. Harada, M. Miwa, T. Fukuyo, et al., "ICG fluorescenceendoscope for visualization of the placental vascular
network" Minim Invasive Ther Allied Tech, pp. 3-7, 2009.
A. Kubota, L. Yang, J. Wang, et al., "Contrast enhancement
between vasculature and placenta using narrow band
images for TTTS surgery," 28th International Congressand Exhibition of Computer Assisted Radiology andSurgery, Fukoka, Japan, 2014.

Acknowledgement
Supported by JSPS KAKENHI (Grant 20345268) and Grant forFig. 3

Translational Systems Biology and Medicine Initiative (TSBMI).

Graphically illuminated current viewGS2-2-2