Building Deformation Error Histogram and Quality Assurance of Deformable Image Registration
S B Park*, H Kim, M Yao, R Ellis, M Machtay, J W Sohn, Case Western Reserve University, Radiation Oncology Department, Cleveland, OHSU-E-J-87 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: To quantify the systematic error of a Deformable Image Registration (DIR) system and establish Quality Assurance (QA) procedure.
Methods: To address the shortfall of landmark approach which it is only available at the significant visible feature points, we adapted a Deformation Vector Map (DVM) comparison approach. We used two CT image sets (R and T image sets) taken for the same patient at different time and generated a DVM, which includes the DIR systematic error. The DVM was calculated using fine-tuned B-Spline DIR and L-BFGS optimizer. By utilizing this DVM we generated R' image set to eliminate the systematic error in DVM,. Thus, we have truth data set, R' and T image sets, and the truth DVM.
To test a DIR system, we use R' and T image sets to a DIR system. We compare the test DVM to the truth DVM. If there is no systematic error, they should be identical. We built Deformation Error Histogram (DEH) for quantitative analysis.
The test registration was performed with an in-house B-Spline DIR system using a stochastic gradient descent optimizer.
Our example data set was generated with a head and neck patient case. We also tested CT to CBCT deformable registration.
Results: We found skin regions which interface with the air has relatively larger errors. Also mobile joints such as shoulders had larger errors.
Average error for ROIs were as follows; CTV: 0.4mm, Brain stem: 1.4mm, Shoulders: 1.6mm, and Normal tissues: 0.7mm.
Conclusions: We succeeded to build DEH approach to quantify the DVM uncertainty. Our data sets are available for testing other systems in our web page. Utilizing DEH, users can decide how much systematic error they would accept. DEH and our data can be a tool for an AAPM task group to compose a DIR system QA guideline.