1. Study Subjects
The study included 457 females and 404 males, aged 22 to 79 years (mean age, 51.4 ± 16.6 years), who underwent chest CT combined with QCT examination in our hospital from January 2020 to June 2022. They were divided into six age groups: 20–29-year, 30–39-year, 40–49-year, 50–59-year, 60–69 year, and 70–79 year. Exclusion criteria include (1) patients with thoracolumbar fractures, tumor, infections or surgery, (2) patients with bone metabolic diseases, (3) patient with endocrine diseases or medication history affecting bone metabolism. Finally, a total of 861 participants were included in the study (Fig. 1). Each age group consisted of 92 (46 males, 46 females), 170 (95 males, 75 females), 130 (62 males, 68 females), 148 (70 males, 78 females), 202 (76 males, 126 females), and 119 (55 males, 64 females) participants, respectively.
In accordance with the Declaration of Helsinki, this study is approved by the Ethics Committee of Tianjin Hospital (Approved Text No.: 2021 医伦审084). This study is a retrospective study, so it did not require patient informed consent.
Figure #1. Flow chart of subject enrollment and exclusion
2. Imaging Examinations
Chest CT scans were performed on a CT750 scanner (GE Healthcare, Milwaukee, WI, USA). Prior to scanning, calibration was done using a QCT calibration phantom. The patient was placed in a supine position, and the scan range extended from 2 cm above the upper margin of the first rib to the lower margin of the third lumbar vertebral body. The scan parameters were as follows: field of view 50 cm, reconstructed field of view 38 cm, tube voltage 120 kV, tube current 200–300 mA, and slice thickness 1.25 mm. Images were transferred to the QCT workstation (Mindways QCT PRO V6.1, USA).
3. Data Measurement
Measurements were conducted by two experienced radiologists (MXH, with 12 years of experience in musculoskeletal imaging diagnosis; WZ, with 33 years of experience in musculoskeletal imaging diagnosis), and the results were averaged.
1) BMD Measurement
Using the "New 3D Spine Examination Analysis" in QCT PRO software, the vBMD of the trabecular bone in the L1, L2, and L3 vertebrae was measured separately, and the average vBMD of L1–L3 was taken as the BMD of the vertebra. By adjusting the axial, coronal, and sagittal images simultaneously, the region of interest (ROI) was placed in the center of the vertebra being measured. The ROI had a height of 9 mm and a distance of more than 3 mm from the cortical bone boundary to avoid partial volume effects of the cortical bone, and the ROI was positioned away from areas of bone sclerosis and the posterior vertebral venous plexus (Fig. 2).
Figure # 2
Measurement of BMD of L1 by QCT PRO software. Figure 2A shows that on the axial position, ROI (yellow ellipse) is located in the center of the L1 vertebra; Fig. 2B and Fig. 2C show the sagittal and coronal positions of ROI, respectively. ROI is located in the center of the vertebral body, avoiding the osteosclerosis area of the vertebral body and the posterior vertebral venous plexus.
2) Abdominal Adipose Tissue Measurement
Using the "New QCT slice range pick" on the QCT workstation, subcutaneous adipose tissue and visceral adipose tissue at the level of the third lumbar vertebra were semi-automatically measured (Fig. 3). SAT was defined as the area of fat between the skin and the outer edge of the abdominal and back muscles at the level of the L3 vertebra. VAT was defined as all intra-abdominal fat tissue regions at the level of the L3 vertebra, including the rectus abdominis, external oblique, quadratus lumborum, and enclosed area below the outer edge of the lumbar muscle.
Figure # 3: Measurement of abdominal fat content at the L3 vertebra level by QCT PRO software. Total adipose tissue: all blue areas; Subcutaneous fat content: blue area outside green circle (shown by →); Visceral fat content: Blue area in green circle (shown by ⭐).
3) Measurement of Psoas Muscle Fat Content
At the level of the L3 vertebra, the fat content of the psoas major was measured using the "New QCT slice range pick" method. An ROI was placed on each side of the psoas major muscle as centrally as possible without exceeding the muscle contour (Fig. 4). The measured values were recorded as the left and right psoas major fat content (Fpsoas-L, Fpsoas-R), and their average was recorded as the psoas muscle fat content (Fp).
Figure # 4
Measurement of psoas fat content at the level of the L3 vertebra by QCT PRO software. 4A and 4B show that on the axial position, the ROI of the left and right psoas major muscles were selected at the L3 vertebra level respectively. The ROI is located in the center of the psoas and does not extend beyond the periphery of the psoas.
4. Statistical Methods
Statistical analyses were conducted using SPSS 26.0 (IBM Corp., USA). Independent-sample t-tests were used to compare the differences in BMD, SAT, VAT, and Fp between males and females within the same age group. One-way analysis of variance (ANOVA) and least-square difference tests were employed to compare the intergroup differences in BMD, SAT, VAT, and Fp among different age groups for both males and females. Pearson correlation analysis was utilized to examine the relationship between BMD and the fat content of abdominal fat as well as the psoas major muscle fat in males and females. P < 0.05 was considered statistically significant.