Age- and sex-adjusted CT-based reference values for temporal muscle thickness, cross-sectional area and radiodensity.

Muscle mass has been traditionally assessed by measuring paraspinal muscle areas at the level of the third lumbar vertebra on computed tomography (CT). Neurological or neurosurgical patients seldom undergo CT scans of the lumbar region. Instead, temporal muscle thickness (TMT), cross-sectional area (TMA) and radiodensity measured from head CT scans are readily available measures of muscle mass and quality in these patient cohorts. The purpose of this retrospective study was to establish CT-based reference values for TMT, TMA and radiodensity for each decade of age from 0 to 100 years normalized by age and sex, and to define cut-off values for subjects at risk for sarcopenia as defined by the European Working Group on Sarcopenia in Older People (EWGSOP). Subjects diagnosed with a concussion at the Oulu University Hospital between January 2014 and December 2022 (n = 9254) were identified to obtain a reference population. Subjects with significant pre-existing co-morbidities were excluded. TMT, TMA and radiodensity were measured, measurement reliability was quantified, and sex-adjusted reference values were calculated for each age decade. Quantile regression was used to model age-related changes in muscle morphomics. A total of 500 subjects [250 (50.0%) males] with a mean age of 49.2 ± 27.9 years were evaluated. Inter- and intra-observer reliability was almost perfect for TMT and TMA, and substantial-to-almost perfect for radiodensity. The mean TMT, TMA and radiodensity were 5.2 ± 1.9 mm, 284 ± 159 mm and 44.6 ± 17.7HU, respectively. The cut-off values for reduced TMT, TMA and radiodensity for males/females using the European Working Group on Sarcopenia in Older People compliant criteria were ≤ 4.09 mm/≤3.44 mm, ≤ 166 mm/≤156 mm, and ≤ 35.5HU/≤35.2HU, respectively. We described a standardized CT-based TMT and TMA measurement protocol practical for clinical use with almost perfect reliability. Using the protocol, we produced quantile regression models for the detection of reduced TMT, TMA and radiodensity at the lowest 5th, 10th, 20th, 30th, 40th and 50th percentiles as well as the EWGSOP compliant criteria cut-off values for reduced muscle mass to facilitate generalizable radiological sarcopenia research.