Hip Imaging in Children With Cerebral Palsy Estimation and Intrapatient Comparison of Patient-Specific Radiation Doses of Low-Dose CT and Radiography

Objectives: Hip displacement is the second most common orthopedic problem affecting children with cerebral palsy (CP). Routine radiographic hip surveillance typically involves an anteroposterior (AP) pelvis radiograph. Unfortunately, this imaging protocol is limited by its projectional technique and the positioning challenges in children with CP. Alternatively, hip low-dose computed tomography (LDCT) has been advocated as a more accurate strategy for imaging surveillance as it provides biofidelic details of the hip that is independent of patient positioning. However, the tradeoff is the (presumed) higher radiation dose to the patient. The goal of this study is to estimate patient-specific radiation doses of hip LDCTs and AP pelvis radiographs in CP patients, and perform an intrapatient dose comparison. Materials and Methods: A search of our imaging database was performed to identify children with CP who underwent hip LDCT and AP pelvis radiograph within 6 months of each other. The LDCTs were performed using weightadjusted kVp and tube current modulation, whereas the radiographs were obtained with age-/size-adjusted kVp/mAs. The patient-specific organ and effective doses for LDCTwere estimated by matching the patients to a nonreference pediatric phantom library from the National Cancer Institute Dosimetry System for Computed Tomography database with Monte Carlo-based dosimetry. The patientspecific organ and effective doses for radiograph were estimated using the National Cancer Institute Dosimetry System for Radiography and Fluoroscopy with Monte Carlo-based dose calculation. Dose conversion k-factors of dose area product for radiography and dose length product for LDCTwere adapted, and the estimation results were compared with patient-specific dosimetry. Results: Our study cohort consisted of 70 paired imaging studies from 67 children (age, 9.1 +/- 3.3 years). The patient-specific and dose length product-based effective doses for LDCT were 0.42 +/- 0.21 mSv and 0.59 +/- 0.28 mSv, respectively. The patient-specific and dose area product-based effective doses for radiography were 0.14 +/- 0.09 mSv and 0.08 +/- 0.06 mSv, respectively. Conclusions: The radiation dose for a hip LDCT is similar to 4 times higher than pelvis radiograph, but it is still very low and poses minimal risk to the patient.