Investigation of geometric distortions on magnetic resonance and cone beam computed tomography images used for planning and verification of high-dose rate brachytherapy cervical cancer treatment.
OBJECTIVE
To measure the amount of geometric distortions present in the three-dimensional imaging modalities--cone beam computed tomography (CBCT) and magnetic resonance imaging (MRI)--used at University of California, San Francisco, CA, for gynecologic high dose rate brachytherapy.


METHODS
An MRI- and CT-compatible water phantom with two different sets of support structures was designed and built for this study. The support structures were used to precisely position catheters that were filled with either an MRI contrast agent or a string of radio-opaque markers. The first support structure without anatomy was built to test system-based distortions. A second structure included two types of gynecologic applicators as well as several anatomical structures, including bones and rectum to test object-induced distortions. Images were acquired with CT (for reference), kilovoltage CBCT, and MRI (1.5 T with T1- and T2-weighted images). The difference in catheter positions between the images and the CT images was analyzed.


RESULTS
For CBCT, the mean of the absolute deviations was below 1mm in all directions. The inherent uncertainty in the measurement of distortion was less than 0.5mm. MRI presented mean absolute system-based distortions between 0.6 and 1.1mm in the central region of the image and between 0.7 and 2.3mm in the outer region. Images with the applicator and anatomy in place created mean absolute distortions of 0.4, 0.8, and 0.8mm or less for CBCT, MR-T1, and MR-T2 images, respectively.


CONCLUSIONS
The distortions measured in the presence of applicators are small enough to validate the use of CBCT and 1.5 T MRI for GYN brachytherapy treatment planning and verification.
Department of Radiation Oncology, University of California San Francisco, San Francisco, CA 94143, USA.
Jean-François
JF
Aubry
Joey
J
Cheung
Olivier
O
Morin
Luc
L
Beaulieu
I-Chow
IC
Hsu
Jean
J
Pouliot
D016477:Artifacts; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation / Q000379:methods*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D002583:Uterine Cervical Neoplasms / Q000175:diagnosis* / Q000532:radiotherapy*