Introduction

Bone is a dynamic organ that serves mechanical and homeostatic functions, which alters during aging. This organ has a complex structure that can be studied from the macro-structural level (cortical and trabecular) to the microstructural level (Haversian system). Bone mineral density (BMD) quantification is currently the common clinical tool for the diagnosis of osteoporosis and is performed by dual-energy x-ray absorptiometry (DXA). BMD as a gold standard is not a good marker in many cases. There are cases where BMD is close to normal but with high fracture risk1, 2. Moreover, there are three main components in the cortical bone as mineral, collagen, and water, while BMD only provides information regarding minerals. Our main aim was to assess the relationships between bone features and age, extracted from different sites with available modalities; DXA, QUS, MRI, and MDCT-QCT to see which modality can model the age-related alterations for bone. We explored this relationship with a healthy population of 50 participants.

Methods

Participants: After approval of the human ethics committee and written informed consent, a cross-sectional population of 50 Iranian healthy volunteers (22M.28F, 30-60 years old) were recruited (Table 1).
The population of the two genders was both age and BMI matched. The healthiness of the participants was investigated through four steps: filling a questionnaire, blood test, DXA scan, and eventually the specialist confirmation. Each participant went under four different modalities of bone assessment and different parameters were extracted as shown in Figure 1. The imaging site was selected according to the clinical routine of the imaging modality.
DXA: aBMD for L1-L4 spine, proximal femur, and forearm was measured by DXA (discovery W, Hologic Inc., Walton, MA).The mineral content was also used to see if the participant is in the normal category defined by WHO (aBMD; T score ≥ ^3,4 Ultrasound (QUS): Ultrasound measurements of the calcaneus were performed with an Achilles plus device (GE Medical Systems Lunar, Madison, WI). This modality provided parameters as follows: Speed of Sound (SOS), Broadband Ultrasound (BUA) and Stiffness Index (SI).
Quantitative Computed Tomography (QCT): Multi-Detector Computed Tomography (MDCT) images of the distal tibia (38%) (Brilliance, Philips Medical Systems, Amsterdam, Netherlands) was acquired in Laleh hospital for all participants. The imaging parameters validated ⁵ were as follows: Hybrid iterative reconstruction algorithm (iDose - level 7), 90 kVp, ~30 effective mAs, 3 mm section thickness, 0.15*0.15 mm2 pixel size, 0.8 pitch factor, YC kernel, and 16×0.7 mm nominal collimation. The left leg of the subjects and solid-CIRS phantom (50, 100 and 250 mg CHA/cm3) were scanned, simultaneously. The cortical, trabecular and integral (cortical plus trabecular) volume BMD (vBMD) was calculated. Magnetic Resonance Imaging (MRI): T1 quantification was assessed on the left leg (distal tibia (38%)) at 1.5 T MR scanner (Siemens Healthcare, Erlangen, Germany) with variable time repetition (VTR) ⁶ Statistical Analysis: To test the normality assumption of the parametric tests, Kolmogorov-Smirnov was used. Mean and standard deviation (SD) were provided as descriptive statistics. The correlation between bone features and age were analyzed by Pearson correlation (r) analysis (ᾱ=0.05).

Results

The mean and standard deviation (SD) of the measurements are presented in Table 2. The highest correlation with age were related to free water T1 in cortical bone (r= 0.77), (p<0.001). Moreover, the correlations between SOS and cortical vBMDs with age were (r=-0.35) and (r=-0.40) (p<0.001), respectively.

Discussion and conclussion

Table 2 demonstrated that among QUS-derived features, SOS that measures the elasticity and architecture of bone⁷ showed the highest correlation (-0.348, p<0.05), which could barely be enough to model the age-related alterations. DXA and QCT parameters (measures of BMD) had a low correlation with age. QCT-derived features with the mean correlation of 0.316 (p<0.05) predicted age-related alterations better than DXA-derived features (-0.285, p<0.05) due to the ability to discriminate between trabecular and cortical and measuring vBMD rather than aBMD (being independent of the patient’s size). Cortical bone-free water T1 shows the highest correlation with age. The pulse sequence used for measuring T1 deployed proper TE value; large enough for the bound water signal to decay and small enough for free water to be captured. Therefore, T1 of free water, residing in the pores of the cortical bone is captured which reflects the information of the pore size and volume. During aging, the surface-to-volume ratio of the pores decreases leading to the restricted mobility of the water molecules and increasing the value of their T1 ⁶. There was no correlation between trabecular or integral vBMD measurements and age. This might be due to small trabecular area and relatively poor resolution of MDCT at 38% of the distal tibia length.
The systematic bone alteration with aging, in the bony skeleton, in 50 Iranian healthy population was assessed. The study showed that MRI could be a good modality to be added to the clinical routine for this purpose.

Acknowledgements

No acknowledgement found.