AbstractID: 9709 Title: Improving the utility of in-room video camera systems for continuous surveillance of patient
motion during radiation treatment
Improving the utility of in-room video camera systems for continuous surveillance of patient motion during radiation treatment
Introduction:
In the present era of IGRT, radiographic, tomographic, ultrasound or optical imaging methods are employed to achieve a new level of
accuracy in patient setup. However, a complementary monitoring system must also be in place to ensure that the patient remains stationary during
treatment after setup. Various systems are available to do so. There are generally two approaches. One involves the use of implanted markers
whose positions can be monitored continuously or periodically using non-ionizing or ionization radiation in combination with an appropriate
detector.
The other relies on surface markers placed on the patient’s torso, or on a reference frame tied to the patient, that can be monitored using
optical or infra-red cameras. Many, if not all, of the real-time on-treatment monitoring systems employ sophisticated hardware and software
technologies that also provide stand-alone capability for patient setup independent of other image guidance methods.
It appears that such
sophisticated, and often more costly, capabilities are not necessary for the increasingly common advanced cone-beam CT image guidance systems,
where the ensurance that the patient remains stationary after setup would suffice.
In this study, we develop a simple video-based system for the purpose of monitoring and tracking patient position during radiation treatment,
but after an initial setup is deemed accurate. The specification of our system is to provide real-time surveillance of the patient position at all times
during a treatment session at a precision compatible to the treatment margins, e.g. better than 2 mm. Most importantly, the method needs to be
applicable even when the treatment room environment changes, such as in the case with couch and gantry positions during non-coplanar treatment.
The Couch Mounted Video Camera System
A standard webcam costing $20 is employed.
The camera is mounted on a detachable rigid post anchored at the end of the treatment couch.
The height and tilt of the camera is such that it views an area measuring 120 cm by 90 cm at an angle of 55 degrees.
Image subtraction and
processing filters are used to detect patient movement in the presence of changing optical, radiation and electronic background. An adjustable
tracking window is preset for processing. Figure 1 includes images obtained from a typical experimental trial, where the subject is asked to lie
supine on the treatment table, and the tracking frame (blue rectangle) is selected to include the chest and abdominal regions of the subject.
Difference and threshold images are then obtained for display and subsequent data analysis. High contrast flat “sticky” markers can be placed on the
patient surface within view of the tracking window for rapid determination of motion. If the average displacement of the markers as determined in
the difference image exceeds the noise threshold value, a visual warning is triggered and displayed on the screen. We have also demonstrated that
our system performs satisfactorily in the presence of environmental disturbances. The results shown in Figure 1A-D are maintained in the presence
of continuous, or “step and treat”, gantry and couch motion.
Phantom Results.
Target displacements of (2, 2, 5) mm for the lead ball phantom and (4,4,7) mm for the flat phantom are readily detected in the lateral,
superior-inferior and vertical directions respectively (Table 1). Investigations to improve the resolution of displacement detection are on-going.
These include optimizing the choice of markers, and the use of more than 1 camera. Most importantly, the system detects these changes in the
presence of environmental disturbances which include large changes in room lighting, and couch and gantry positions. The system fails when the
camera’s view is obstructed.
At present, our analysis method does not consider inadvertent patient motion due to breathing. A displacement threshold larger than the
amplitude of the breathing motion can be used to detect undesirable changes in patient position. A more comprehensive method is being
investigated where the characteristics of the breathing motion can be filtered for the detection of more subtle, but potentially, unacceptable
displacement.
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AbstractID: 9709 Title: Improving the utility of in-room video camera systems for continuous surveillance of patient
motion during radiation treatment
1A
1B.
1C
1D.
Figure 1. A) photograph of subject, with adjustable tracking frame (blue rectangle); B) difference image;
C) threshold image, without filter. Note that only the region within the frame is displayed and evaluated;
D) threshold image, with median filter.
Lateral
Superior/Inferior
Vertical
Lead Ball Phantom
2mm
2mm
5mm
Flat “Sticker” Phantom
4mm
4mm
7mm
Table 1. Threshold values of detected motions by tracking system
Conclusions
For the more than 2 decades, the utility of video cameras systems has only been extended once for the traditional wall-mounted systems [1].
For our study, the simplest of the many recent advances made in video-technologies and image processing have been applied.
promising.
The results are
The cost of our video-monitoring system is almost inconsequential (<$100), and yet it readily complements the advanced cone-beam
CT guidance system which lacks real-time monitoring capabilities to ensure accurate treatment without imparting more radiation dose to the patient.
With further refinement in implementation, such as an efficient method of couch attachment, cable-management, software and user-interface, our
simple system provides a simple but effective approach to real-time patient surveillance that is far superior to the traditional in-room systems.
References
1. Barrett D. Milliken et al. Performance of A Video-Image-Subtraction-Based Patient Positioning System. Int. J. Radiation Oncology Biol.
Phy., Vol. 38, No. 4, pp. 855-866, 1997
2