Diagnostic accuracy of quantitative dual-energy CT-based bone mineral density assessment in comparison to Hounsfield unit measurements using dual x-ray absorptiometry as standard of reference

Highlights

• Material decomposition-based DECT BMD assessment yields high diagnostic accuracy.

• HU measurements show significantly lower diagnostic accuracy compared to DECT material decomposition.

• Focal HU measurements should not be routinely used to diagnose osteoporosis.

Abstract

Purpose

To assess the diagnostic accuracy of phantomless dual-energy computed tomography (DECT)-based volumetric material decomposition to assess bone mineral density (BMD) of the lumbar spine for the detection of osteoporosis compared to Hounsfield unit (HU) measurements with dual x-ray absorptiometry (DXA) as reference standard.

Method

A total of two hundred lumbar vertebrae in 53 patients (28 men, 25 women; mean age, 52 years, range, 23−87 years) who had undergone clinically-indicated third-generation dual-source DECT and DXA within 30 days were retrospectively analyzed. For volumetric BMD assessment, dedicated DECT postprocessing software using material decomposition was applied, which enables color-coded three-dimensional mapping of the trabecular BMD distribution. Manual HU measurements were performed by defining five trabecular regions of interest (ROI) per vertebra as suggested by literature. The DXA T-score served as standard of reference (osteoporosis: T < -2.5). Sensitivity, specificity and the area under the curve (AUC) were primary metrics of diagnostic accuracy.

Results

An optimal patient-based DECT-derived BMD cut-off of 84 mg/cm³ yielded 96 % sensitivity (22/23) and 93 % specificity (28/30) for detecting osteoporosis, while an optimal CT attenuation cut-off of 139 HU showed 65 % sensitivity (15/23) and 93 % specificity (28/30) for the detection of osteoporosis. Overall patient-based AUC were 0.930 (volumetric DECT) and 0.790 (HU analysis) (p < .001). Pearson’s product-moment correlation showed higher correlation between DECT BMD and DXA values (r=0.780) compared to HU and DXA values (r=0.528) (p < .001).

Conclusions

Phantomless volumetric DECT yielded significantly more accurate BMD assessment of the lumbar spine and superior diagnostic accuracy of osteoporosis compared to HU measurements.

Introduction

Osteoporosis is a frequent metabolic bone disorder, characterized by decrease of bone mineral density (BMD) [1]. It is associated with increased fracture risk, deformations and immobility, therefore entailing a substantial economic and social burden [2].

Dual x-ray absorptiometry (DXA) is cost-effective, widely available and represents the current gold-standard for BMD assessment according to the World Health Organization (WHO) [1]. In 1994 the WHO proposed DXA as the preferred osteoporosis screening approach due to high reproducibility and evidence [1]. The WHO recommended using sex-related standard deviation parameters for the diagnosis of osteoporosis due to significant differences between men and women regarding BMD, especially due to the great importance of postmenopausal osteoporosis. In this context, the T score (the comparison with healthy 30-year old individuals of the same sex) represents the gold standard for diagnosing osteoporosis (osteoporotic BMD [T < −2.5], osteopenic BMD [−1.0 ≤ T ≤ −2.5], normal BMD [T < −1.0]. Basis for the establishment of sex-related DXA-based BMD threshold values and the corresponding T score were large studies evaluating BMD for fracture prediction [[3], [4], [5], [6]]. However, various limitations of DXA have been described such as distortion of estimated bone mass values caused by overlying soft tissue, vascular calcifications, bowel contents and degenerative spine changes [7,8]. In addition, DXA only permits areal BMD assessment of the entire vertebral body despite the fact that osteoporosis primarily affects the metabolically active trabecular rather than cortical bone [9,10]. Quantitative computed tomography (QCT) has been established for volumetric BMD assessment of specifically trabecular bone. While some studies showed similar diagnostic accuracy of QCT and DXA for diagnosing osteoporosis [9], other studies demonstrated greater sensitivity of spinal QCT for BMD assessment and fracture prediction compared to DXA [[11], [12], [13]]. However, the application of QCT remains limited by the need for calibration phantoms that do not represent the true composition of trabecular bone and prevent retrospective opportunistic BMD assessment [14,15].

The wide incidence and socioeconomic impact of osteoporosis suggests the usefulness of opportunistic BMD assessment during computed tomography (CT) examinations. Multiple studies investigated trabecular Hounsfield unit (HU) measurements for opportunistic BMD assessment and screening for osteoporosis during regularly performed CT examinations with promising results [[16], [17], [18], [19]]. However, performing a single HU measurement in a single vertebral body to measure BMD, as recently suggested by Jang et al., is likely to give only a partial, over-simplified insight into BMD dynamics and is presumably unfit to rule out or confirm the diagnosis of osteoporosis [19]. Therefore, several studies suggest multilevel vertebral HU assessment for diagnosing osteoporosis [[20], [21], [22]]; the combination with calibration approaches may further improve the diagnostic performance [23]. Nevertheless, the inhomogeneous composition of trabecular bone consisting of bone minerals, collagen matrix, water, bone marrow and adipose tissue is likely to cause inaccuracy of HU analysis in order to assess osteoporosis [14,24,25].

Dual-energy CT (DECT) allows for material differentiation by using energy dependence of the photoelectric effect at different x-ray spectra [26,27]. This technique has provided novel clinically relevant information regarding various musculoskeletal (MSK) applications compared to conventional CT [28]. Initial studies involving early dual-energy CT concepts for BMD assessed were published more than two decades ago [[29], [30], [31], [32]]. Recently, a phantomless DECT postprocessing algorithm which allows for volumetric opportunistic BMD assessment of lumbar trabecular bone has been described [27]. Initial studies have shown promising results in vivo for the detection of osteoporosis compared with DXA and in vitro compared with pull-out-forces in human cadaver vertebra specimens [33,34].

We hypothesized that phantomless volumetric DECT BMD assessment based on material decomposition may yield more accurate BMD values and higher diagnostic accuracy for the detection of osteoporosis compared to HU measurements. Thus, the purpose of this study was to compare the diagnostic accuracy of both approaches using DXA as reference standard for BMD assessment according to the WHO.