Digital tomosynthesis based digital volume correlation: A clinically viable noninvasive method for direct measurement of intravertebral displacements using images of the human spine under physiological load

Abstract

Purpose We have developed a clinically viable method for measurement of direct, patient‐specific intravertebral displacements using a novel digital tomosynthesis based digital volume correlation technique. These displacements may be used to calculate vertebral stiffness under loads induced by a patient's body weight; this is particularly significant because, among biomechanical variables, stiffness is the strongest correlate of bone strength. In this proof of concept study, we assessed the feasibility of the method through a preliminary evaluation of the accuracy and precision of the method, identification of a range of physiological load levels for which displacements are measurable, assessment of the relationship of measured displacements with microcomputed tomography based standards, and demonstration of the in vivo application of the technique. Methods Five cadaveric T11 vertebrae were allocated to three groups in order to study (a) the optimization of digital volume correlation algorithm input parameters, (b) accuracy and precision of the method and the ability to measure displacements at a range of physiological load levels, and (c) the correlation between displacements measured using tomosynthesis based digital volume correlation vs. high resolution microcomputed tomography based digital volume correlation and large scale finite element models. Tomosynthesis images of one patient (Female, 60 yr old) were used to calculate displacement maps, and in turn stiffness, using images acquired in both standing and standing‐with‐weight (8 kg) configurations. Results We found that displacements were accurate (2.28 µm total error) and measurable at physiological load levels (above 267 N) with a linear response to applied load. Calculated stiffness among three tested vertebral bodies was within an acceptable range relative to reported values for vertebral stiffness (5651‐13260 N/mm). Displacements were in good qualitative and quantitative agreement with both microcomputed tomography based finite element (r2 = 0.762, P < 0.001) and digital volume correlation (r2 = 0.799, P < 0.001) solutions. For one patient tested twice, once standing and once holding weights, results demonstrated excellent qualitative reproducibility of displacement distributions with superior endplate displacements increasing by 22% with added weight. Conclusions The results of this work collectively suggest the feasibility of the method for in vivo measurement of intravertebral displacements and stiffness in humans. These findings suggest that digital volume correlation using digital tomosynthesis imaging may be useful in understanding the mechanical response of bone to disease and may further enhance our ability to assess fracture risk and treatment efficacy for the spine.

A novel approach to evaluate the effects of artificial bone focal lesion on the three-dimensional strain distributions within the vertebral body

“…Finally, this approach was performed using high resolution microCT scans, not applicable in clinics. A similar approach could be applied in the future to clinical CT images [54], but higher measurement uncertainty should be expected [31].…”

Vertebral stiffness measured via tomosynthesis-based digital volume correlation is strongly correlated with reference values from micro-CT-based DVC

“…DTS-DVC imaging of vertebral bodies was performed in a custom radiolucent loading chamber that was designed to accommodate imaging in both DTS and μCT systems as previously described in detail [37]. Briefly, vertebral endplates were potted using a rigid filler material (Bondo Corp, Atlanta, GA) to ensure flat boundary conditions, and the entire load frame was placed in the imaging system (Shimadzu Sonialvision Safire II), while the scanner was kept in the vertical position.…”

“…Specimens were then loaded using weight plates to a fixed load level approximating upper body weight (445 N generated by a stack of five 20 lb [9 kg] weight plates) and rescanned, with 10 minutes wait between loading and imaging to allow for displacement to stabilize. Clinical protocols for adult DTS examinations of the spine were followed during acquisition [37]. Acquisition time for each DTS scan was less than three seconds.…”

“…DTS images were reconstructed at 0.28/0.28/1 mm voxel spacing (corresponding to superior-inferior/lateral-medial/anterior-posterior directions) and resampled to isotropic voxel size (0.28 mm), registered using a 3D region at the inferior endplate (MIPAV, NIH, Bethesda, MD), and scaled to match for the mode and standard deviation of gray level distributions. Displacements were measured at every other voxel along a grid of reference points using the methods and DVC software described extensively previously in previous work [29,37]. Briefly, DVC performs full-grayscale correlations between spherical test regions (radius=4 mm) in nonloaded and loaded volumes, using the nonloaded volume as the reference grid.…”

“…Originally described for microcomputed tomography (μCT) applications [30], DVC remained a powerful tool for laboratory research [31][32][33][34][35][36] with virtually no clinical applicability to the spine due to lack of clinical imaging modalities suitable for spinal DVC measurements. We have recently described a DVC approach for measuring vertebral displacements based on digital tomosynthesis (DTS) in a proof of concept study [37]. In that study, we have shown in vitro and in vivo representative cases to support feasibility of using a clinical DTS imaging modality for DVC analysis of human vertebrae.…”