Original Article Muscle and Myotendinous Tissue Properties at the Distal Tibia as Assessed by High-Resolution Peripheral Quantitative Computed Tomography M.C. Erlandson,* ,1 A.K.O. Wong, 2,3 E. Szabo, 2 N. Vilayphiou, 4 M.A. Zulliger, 4 J.D. Adachi, 3 and A.M. Cheung 2 1 College of Kinesiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada; 2 Department of Medicine and Joint Department of Medical Imaging, University Health Network,Toronto, Ontario, Canada; 3 Department of Medicine, Michael G. DeGroote School of Medicine, McMaster University, Hamilton, Ontario, Canada; and 4 SCANCO Medical AG, Bruettisellen, Switzerland Abstract High-resolution peripheral quantitative computed tomography (HR-pQCT) quantifies bone microstruc- ture and density at the distal tibia where there is also a sizable amount of myotendinous (muscle and tendon) tissue (MT); however, there is no method for the quantification of MT. This study aimed (1) to assess the fea- sibility of using HR-pQCT distal tibia scans to estimate MT properties using a custom algorithm, and (2) to determine the relationship between MT properties at the distal tibia and mid-leg muscle density (MD) ob- tained from pQCT. Postmenopausal women from the Hamilton cohort of the Canadian Multicenter Osteo- porosis Study had a single-slice (2.3 ± 0.5 mm) 66% site pQCT scan measuring muscle cross-sectional area (MCSA) and MD.A standard HR-pQCT scan was acquired at the distal tibia. HR-pQCT-derived MT cross- sectional area (MTCSA) and MT density (MTD) were calculated using a custom algorithm in which thresh- olds (34.22–194.32 mg HA/cm 3 ) identified muscle seed volumes and were iteratively expanded. Pearson and Bland-Altman plots were used to assess correlations and systematic differences between pQCT- and HR- pQCT-derived muscle properties.Among 45 women (mean age: 74.6 ± 8.5 years, body mass index: 25.9 ± 4.3 kg/ m 2 ), MTD was moderately correlated with mid-leg MD across the 2 modalities (r = 0.69–0.70, p < 0.01). Bland- Altman analyses revealed no evidence of directional bias for MTD-MD. HR-pQCT and pQCT measures of MTCSA and MCSA were moderately correlated (r = 0.44, p < 0.01). Bland-Altman plots for MTCSA re- vealed that larger MCSAs related to larger discrepancy between the distal and the mid-leg locations.This is the first study to assess the ability of HR-pQCT to measure MT size, density, and morphometry. HR-pQCT- derived MTD was moderately correlated with mid-leg MD from pQCT.This relationship suggests that distal MT may share common properties with muscle throughout the length of the leg. Future studies will assess the value of HR-pQCT-derived MT properties in the context of falls, mobility, and balance. Key Words: Cross-sectional area; HR-pQCT; muscle and myotendinous tissue density; pQCT. Introduction Skeletal muscle plays a key role in activities of daily living and is essential for maintaining posture, balance, and bone health. Sarcopenia, the age-related decline in muscle mass and function, is associated with increased risk of falls, frac- tures, hospitalization, and mortality (1–3). Muscle imaging has the potential to play an important role in understand- ing the etiology of age-related musculoskeletal condi- tions, such as sarcopenia, in addition to examining the efficacy of interventions aimed at improving or maintain- ing musculoskeletal health. Recently, efforts have been made Received 03/2/16; Revised 09/13/16; Accepted 10/31/16. *Address correspondence to: M.C. Erlandson, PhD, College of Kinesiology, University of Saskatchewan, Saskatoon, Canada. E-mail: marta.erlandson@usask.ca Journal of Clinical Densitometry: Assessment & Management of Musculoskeletal Health, vol. 20, no. 2, 226–232, 2017 © Copyright 2016 by The International Society for Clinical Densitometry 1094-6950/20:226–232/$36.00 http://dx.doi.org/10.1016/j.jocd.2016.10.005 226