Table 1.

Overview of the main macro- and meso-scale imaging techniques.

Macro- and Meso-Scale Imaging Technique	Brief Description of the Technique	Invasiveness	Outcomes	Spatial Resolution	2D or 3D	Advantages	Disadvantages
					In Vitro/In Vivo Application
Radiography	Based on the interaction between a beam of photons (X-rays) directed from a source to a receptor. The atoms of the body prevent, in a percentage dependent on their atomic number, some photons from reaching the receptor, reproducing a “negative” image of the body	No Radiation dose: 40–50 times lower, if compared to computed tomography (CT) scans (e.g., radiographs of the abdomen → 0.25 mGy) [39]	Estimation of density variation (fracture risk prediction) by means of two indexes: Singh index [40] for proximal femur and cortical–medullary index [41] for hand radiographs	0.17 mm/pixel → The size of the monitor screens used in digital radiography is sufficient for 35 × 43 cm2 radiographs to be displayed at a resolution of 2048 × 2560 pixels [42]	2D	Clear identification of distal radius fractures [43]	Difficult detection of hip and spine fractures Insensitive to changes in Bone Mineral Density (BMD)until 20 to 40% of bone mass lost [43]
					In vivo
Dual-energy X-ray Absorptiometry (DXA)	Involves the emission of two X-ray beams with different energy levels, that collide with the body of the patient. Once the absorption of the soft tissue has been subtracted, it is possible to determine the absorption of the beam by the bone and therefore the BMD	No Low radiation dose (0.001–0.003 mGy for L-spine, to 0.004 mGy for total body) [37]	Determination of areal BMD in g/cm2 Calculation of bone mineral content (BMC = BMD × area) Calculation of T-score and Z-score (negative for values under the average BMD), that are numerical indexes for the evaluation of osteoporosis.	1 pixel → ≃ 0.56 × 0.56 mm2. (for a Hologic system) [44]	2D	Ease of use of the equipment Standardization Short examination time [45]	No bone architecture detection (no difference between cortical and trabecular bone) Sampling errors Incorrect evaluation in obese patients [37]
					In vivo
Vertebral Fracture Assessment (VFA)	Special DXA analysis that permits the detection of spinal fractures from a lateral image of the spine	No Lower radiation exposure with respect to spine radiography [46]	Spinal fracture detection [47]	Low spatial resolution	2D	Possibility to add a VFA scan after areal BMD assessment High sensitivity High specificity [48]	Low spatial resolution
					In vivo
Quantitative Computed Tomography (QCT)	X-ray-based technique that measures BMD. It produces cross-sectional images of X-ray absorption coefficient (measured in Hounsfield units) calibrated to water. It is used to evaluate fracture risk primarily at the lumbar spine and at the hip [49]	Medium–high invasiveness Medium–high radiation dose (0.2–0.4 mGy for a spine exam) [50]	True measurement of BMD assessment (areal BMD does not predict if an individual patient will eventually fracture)	100× higher resolution with respect to conventional radiologic imaging [51]	3D (multiple slices are obtained and then reconstructed)	Fracture risk prediction in patients with scoliosis, obesity, etc. without having artificially high BMD values, as in DXA [52] High reproducibility Assessment of cortical and trabecular bone Good accuracy and precision [37]	Relevant radiation dose Low accessibility High cost [53]
					In vivo
Magnetic Resonance Imaging (MRI)	MRIs employ a magnetic field that forces protons in the body to align with that field. When a radiofrequency current is pulsed through the patient, the protons are strained against the pull of the magnetic field. When the radiofrequency field is turned off, the MRI sensors detect the released energy as the protons realign with the magnetic field. The time it takes for the protons to realign, as well as the amount of energy released, changes depending on the environment and the chemical nature of the molecules	No MRI does not use ionizing radiation	Bone fracture detection Parameters: T2* [54] (effective transverse relaxation time) → a function of the density and orientation of the trabeculae [55] R2* → rate constant of the free induction signal (lower with respect to the control in osteoporotic women’s bone marrow [56])	MRI scanners used for medical purposes could reach typical resolutions of around 1.5 × 1.5 × 4 mm3 [57]	3D	Useful in age-related fracture detection (marrow fat increases with age and in osteoporosis, allowing better contrast with the trabecular bone) Investigation of cortical water content [43]	Presence of a magnetic field (risk for patients with pacemakers and all implants containing iron) Noise up to 120 dB Use of contrast agents Claustrophobia side effect
					In vivo