ESR News September 2014
Patient doses from X-ray-based imaging techniques used in osteoporosis
John Damilakis, Professor of Medical Physcics
Many methods for the assessment of vertebral fractures, bone mineral density (BMD) and skeletal status have been developed during recent years. The assessment of vertebral fractures is possible using spinal radiography or computational techniques applied to dual-energy x-ray absorptiometry (DXA) thoracic and lumbar spine images. Moreover, DXA is widely used for ‘areal’ BMD assessment of the lumbar spine, proximal femur and forearm. Volumetric BMD can be measured using CT scanners, a dedicated software package and a calibration phantom, imaged simultaneously with the patient in order to convert the CT numbers into bone mineral density values. This method is known as quantitative CT (QCT). CT can also be used to assess bone structure. Structural analysis of high-resolution CT images can provide important information related to bone status. Specifically, modern multi-detector CT scanners are capable of quantifying characteristics of the trabecular bone network in several skeletal sites such as the radius and vertebrae. Peripheral Quantitative CT (pQCT) devices are dedicated CT scanners optimised for special applications. pQCT can be used to provide information on trabecular bone structure at the distal radius and tibia.
Techniques applied to DXA spine images to assess vertebral fractures are associated with very low radiation doses. Thus, the effective dose from DXA-based vertebral fracture assessment techniques is lower than 0.05 mSv, whereas the dose from a lateral radiograph of the thoracic and lumbar spine is about 10 times higher. Effective doses from DXA for the assessment of BMD are lower than 0.05 mSv. Therefore, DXA can be considered a safe technique for screening osteoporosis. Patient doses from QCT (scout image and 2 slices of 10 mm thickness) range from 0.06 to about 0.3 mSv. The radiation burden, however, from high-resolution CT is considerably higher. Thus, CT protocols used to examine vertebral microstructure provide an effective dose of about 3 mSv. pQCT involves negligible radiation doses (lower than 0.01 mSv) because radiosensitive organs are far from the path of the x-ray beam.
The use of techniques developed for the diagnosis of osteoporosis has increased considerably during recent years. For this reason, the application of radiation protection principles i.e. justification and dose optimisation is necessary. The scanning length of all DXA and CT examinations should be minimal. The patient’s body size should be considered, and the use of proper acquisition protocols is very important, especially for children and adolescents. Although patient radiation doses associated with most x-ray-based imaging techniques used in osteoporosis are low in comparison with those from other x-ray examinations, every facility should take its own action to avoid unnecessary patient exposure to radiation.