1. C-13
O-arm Scatter Cloud

2. Introduction The O-arm operates as a mobile cone beam CT.
It has unique features:
Uses a gantry that can be open or close which is very useful in surgery cases.
2D fluoroscopic  images can be reconstructed into 3D images.
3D scans mode can help enhance imaging information (in kyphoplasty procedures, screw position, and orthopedic procedures).
Patient dose and scatter radiation increases.

3. O-arm Image Acquisition
A single rotation (360 degrees) takes 13 seconds.
The beam is pulsed and the total x-ray on time is 3.91 seconds.
192 images are reconstructed with slice thickness of 0.83mm.
3D scan acquisition mode has 4 types of pre-programmed protocols: head, upper torso, lower torso, and leg.  
It has various selections of patient thickness: small, medium, large, and extra large.   With each increase of patient selection, there’s a fixed kVp at 120 and increase in mAs.

4. Objective and Hypothesis
To investigate the scatter cloud produced by an O-arm.
Hypothesis
A uniform scatter cloud is present during O-arm operation.

5. Materials: O-arm The O-arm system
An intraoperative 2D/3D imaging system.
Designed to meet the workflow demands of the surgical environment.
It can be used in variety of procedures including spine, cranial, and orthopedics.

6. Materials: RaySafe Real-Time Dosimeter
RaySafe visualizes X-ray exposure in real-time.
Includes a set of personal radiation dosimeters coupled with a display and software to provide an immediate visual of radiation exposure.
Measurements are simultaneously stored for post-procedure analysis.

7. Materials: Acrylic CT Phantom
The acrylic CT phantom is used for dosimetry measurements.
Two adult abdominal phantoms were utilized.
The two phantoms were placed end to end to more accurately simulate a patient with a higher BMI.

8. Materials: IV Pole
The personal monitoring devices were secured onto an IV pole at different heights.
This was done to measure the dose at different levels of the body.
It was positioned around the O-arm to measure the scatter cloud.

9. Methods
The dosimeters were placed on the IV pole at levels of 2, 3, 4, and 5 feet above the ground.
The IV pole was placed in 6 different locations at two different distances.
Three feet and six feet from the isocenter of the phantom.
Location 7 was only recorded at one distance (six feet) due to the body of the O- arm obstructing IV pole placement. * *
* Symmetry was assumed along the axis of the O-arm

10. Methods Two miscellaneous locations were added.
One behind a lead wall in the corner of the OR (7’8” from isocenter).
Another outside the OR door.

11. Methods The O-arm was set to the XL technique.
O-arm typically used on high BMI patients.
This setting was used to obtain more accurate readings.
120 kVp at 400 mAs was utilized.

12. Findings
The results show between two and four “hot spots” at which the accumulated dose was most intense.
These “hot spots” occurred with greater intensity on the left side of the O-arm.
The most intense readings occurred at the four and five foot levels, and at a distance of three feet from the O-arm.
The lowest values for accumulated dose occurred where the body of the machine shielded the RaySafe dosimeters from the scatter cloud.
The two miscellaneous locations were not included with the graph as they recorded little to no data.

13. Discussion
The observed “hot spots” and variation in intensity based on the height of the RaySafe dosimeters support the hypothesis that the O-arm produces a non-uniform scatter cloud at all levels measured.
The variation in intensity between the three and six foot distances from the O-arm can likely be explained by the inverse-square law.
This was an expected outcome.

14. Discussion
The mirroring of the “hot spots” may be attributed to the radiation source rotating around a single axis as well as some inherent shielding present in the O-arm.
This may also explain the intensity being greater at higher levels.
The O-arm produces a greater amount of scatter than other conventional fluoroscopic techniques, and there is significant variation in the scatter cloud produced.
This information can be valuable for medical workers in determining where to position themselves around the room.

15. References
LANDAUER Real-time Dosimetry Service. (n.d.). Retrieved from Medtronic. (n.d.). Arm - Surgical Imaging Systems. Retrieved from Weir, V., Zhang, J., Fajardo, L., Hsiung, H., & Ritenour, E. (2008). WE-E : Dosimetric Characterization of a Cone Beam O-Arm Imaging System. Medical Physics,35(6Part25), doi: / Software Used: AutoCAD 2019 (Version P ) [Computer software]. (n.d.). Retrieved March 22, 2019, from EZGIF Animated GIF editor and GIF maker. (n.d.). Retrieved March 22, 2019, from Plot.ly. (n.d.). Retrieved March 22, 2019, from Online Graphing Application