Auto-masked 2D/3D image registration and its validation with clinical cone-beam computed tomography

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Image Processing

Background:

• Image-guided radiotherapy (IGRT) is crucial for minimizing setup errors between planned and actual patient positioning.
• Accurate patient alignment is essential for effective radiation dose delivery and minimizing off-target exposure.
• Current IGRT methods, such as cone-beam CT (CBCT), involve volumetric data acquisition, which can increase treatment time and radiation dose.

Purpose of the Study:

• To develop and validate a 2D/3D image registration approach for precise patient alignment in radiotherapy.
• To evaluate the accuracy and reliability of the 2D/3D method compared to standard 3D/3D CBCT-to-CT alignment.
• To assess the feasibility of using X-ray images and CT data for rigid alignment of pelvic anatomy.

Main Methods:

• A novel 2D/3D registration algorithm was developed, rigidly aligning computed tomography (CT) data with orthogonal X-ray images.
• The algorithm maximizes pixel intensity agreement between X-rays and CT-derived radiographs, using rank correlation as the intensity metric.
• Regions of interest were selected using 3D segmentations from pre-planning CT data, and validation was performed on 80 pelvic CBCT datasets.

Main Results:

• The 2D/3D registration achieved high accuracy, with root mean square errors <0.18° for rotations and <0.73 mm for translations.
• A mean target registration error of <2 mm in the lesser pelvis was achieved in 94.1% of over 8400 experiments.
• The 2D/3D method demonstrated comparable accuracy to standard 3D/3D CBCT-to-CT alignments for bony anatomy.

Conclusions:

• The developed 2D/3D registration method provides a viable and accurate alternative to CBCT-based alignment for pelvic radiotherapy.
• This approach is suitable when rigid alignment of bony anatomy is sufficient and volumetric data acquisition is not required.
• The method's accuracy is well within acceptable limits for many radiotherapy prescriptions, offering potential benefits in terms of speed and radiation dose.