C-Arm Planning

OsiriX and Horos Course · Vascular Surgery

In this lesson I demonstrate how to use OsiriX to plan the C-arm angles for the intraoperative setting, using the preoperative CT scan. The goal is to find the best angle to open up structures such as the iliac bifurcation (internal vs external iliac) and, when necessary, also adjust the angle to visualize the renal arteries. 1) Tool used: 3D Volume Rendering To calculate angles, I use 3D Volume Rendering. I adjust the volume (crop/resolution) to use less memory when necessary. The initial image corresponds to the anteroposterior (AP) view, like the standard angiography view on the C-arm, with the typical overlaps (e.g., internal iliac over the external iliac). 2) Finding the best angle for the iliac arteries I rotate the volume until the bifurcation “opens up” and the internal iliac is clearly separated from the external iliac. In the video example: Right iliac bifurcation: best visualization at about +38° (range ~35–40°) Left iliac bifurcation: best visualization at about −43° Key point: OsiriX shows the angle in the lower corner (values in degrees). Pay attention to the equipment's reference: on the C-arm, the AP position is usually 0°. In OsiriX, it is important to understand this so you don't confuse it with other references (e.g., reset view). 3) Additional adjustment (obliquity + cranial/caudal) Besides the right/left oblique, sometimes you need to tilt the C-arm cranially/caudally to better “separate” the structures. In the example, I show a case in which: the oblique is around −50° and the superior/inferior angle is around −36° 4) Standardizing the terms with SI and LR (as OsiriX displays them) To avoid confusion between teams and hospitals, I standardize the terms the way OsiriX presents them: Standard AP position: SI = 0° (superior/inferior, craniocaudal) and LR = 0° (left/right, oblique) Cranial oblique: SI greater than 0° Caudal oblique: SI less than 0° Right oblique: LR less than 0° Left oblique: LR greater than 0° In other words: you position the volume in OsiriX the way you want to see it on the C-arm, note the SI and LR values, and take those values into the operating room.