Bone Abstracts

Assessment of metaphyseal bone growth in children by peripheral quantitative computed tomography (pQCT) is limited by use of a thin (2.3 mm) 2D slice to represent the ~20 mm metaphyseal region within which there is substantial local variation in bone properties (Marjanovic et al., 2009). In addition, scan positioning is performed by manual visual growth plate identification from which a region located proximal to the growth plate at 4% of tibia length is scanned. An image analysis protocol was developed by the Children’s Hospital of Philadelphia (CHOP) to assess whole-bone quantitative computed tomography (QCT) of the whole metaphysis using QCT-Pro (Mindways Software). Image properties are used to define the metaphysis region of interest (ROI); from metaphyseal onset defined from peak BMD in medullary bone profiles along limb length, to the proximal end of the metaphysis defined as the point where trabecular BMD reaches 0 g/mm. We hypothesise that use of a larger ROI, and scan site selection by a more objective reference placement method will lead to improved precision in tracking longitudinal bone change in growing children. Twenty children with neurofibromatosis type I (NF1) had QCT scans of their whole tibia using Mindways phantom and software and at 4% distal-proximal tibia length by pQCT (Stratec XCT2000). Changes were assessed using tibial bone mineral content (BMC), cross-sectional area (CSA) and trabecular bone mineral density (BMD) using QCT CHOP protocol and pQCT standard analysis between baseline (age 8.0±1.2 years) and follow-up (age 11.4±1.1 years). Results of the two methods correlated highly (all r>0.7 and P<0.001), although there was a mean offset in values attributable to the 0.06 g/cm density offset employed in the pQCT scanner. Bland-Altman plots showed good agreement with no measurement bias. In conclusion, CHOP assessment may offer higher precision for metaphyseal bone change assessment, with good agreement with low-dose 2D-pQCT. QCT scan speed (<1 min for whole tibia) and the ability to assess bone properties along the whole bone length may offer advantages over established methods, and is more feasible in children with disabilities such as Duchenne Muscular Dystrophy and cerebral palsy.

Disclosure: Dr Keenan Brown is a employee of Mindways, in addition to holding stock in the company.

Reference: Marjanovic EJ, Ward KA, & Adams JE. The impact of accurate positioning on measurements made by peripheral QCT in the distal radius. Osteoporosis International 2009 20 1207–1214.