Detection of occult vertebral fractures by quantitative assessment of bone marrow attenuation values at MDCT

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Abstract

Objectives

To determine a cut-off value of Hounsfield attenuation units (HU) at multidetector computed tomography (MDCT) for valid and reliable detection of bone marrow oedema (BME) related to occult vertebral fractures.

Methods

36 patients underwent both MDCT and Magnetic Resonance Imaging (MRI) for evaluation of vertebral fractures of the thoracolumbar spine and were included in this retrospective study. Two readers independently assessed HU values at MDCT in a total of 196 vertebrae. Reliability was assessed by intraclass correlation coefficient and Bland–Altman analysis. For each patient we determined the vertebra with the lowest HU value and calculated the HU-difference to each other vertebral body. HU-differences were subjected to receiver operating characteristic (ROC) curve analysis to determine the diagnostic accuracy for detection of BME as determined by MRI, which served as the reference standard. Results of HU-measurements were compared with standard visual evaluation of MDCT.

Results

HU measurements demonstrated a high interrater reliability (ICC = 0.984). ROC curve analysis (AUC = 0.978) exhibited an ideal cut-off value of 29.6 HU for detection of BME associated with vertebral fractures with an accuracy of 97.4% as compared to 93.4% accuracy of visual evaluation. Particularly, HU-measurements increased the sensitivity for detection of vertebral fractures from 78.0% to 92.7% due to the detection of 7 of 9 occult fractures that were missed by visual evaluation alone.

Conclusions

Assessing bone marrow density by HU measurements using the cut-off of 29.6 HU is a valid and reliable tool for detection of BME related to occult vertebral fractures in MDCT. The introduced technique may allow more accurate treatment decisions and may make further diagnostic work-up with MRI unnecessary.

Introduction

Multidetector computed tomography (MDCT) is regarded as the first imaging technique of choice for the evaluation of vertebral fractures after trauma [1], [2]. MDCT exhibits diagnostic superiority over plain radiographs in inconclusive findings and enables multiplanar reconstructions of vertebral fractures, which is necessary for further therapeutic management [3], [4], [5]. However, as known from other anatomical regions, MDCT may fail in fracture detection if the overlying cortical bone is intact or the degree of trabecular disruption is too small to be directly visualised [6], [7], [8]. Those “occult fractures” are detectable by MRI and/or SPECT/CT, two diagnostic modalities that have been proven to be superior in the visualisation of acute bone trauma [6], [9], [10], [11]. Several comparative studies between MDCT and MRI have demonstrated high numbers of occult vertebral fractures in MDCT, reaching up to 30% [12], [13]. MRI, particularly using a short tau inversion recovery (STIR) sequence, is sensitive in depicting bone marrow oedema as well as subtle fracture lines, which are occult on MDCT.

Detection of occult vertebral fractures is of high clinical relevance as it influences further therapeutic management. Several studies demonstrated the risk of a delayed vertebral collapse in occult fractures, and therefore kyphoplasty or vertebroplasty are recommended as appropriate therapeutic interventions as alternatives to therapy by drugs [14]. Besides strengthening the vertebral body, kyphoplasty and vertebroplasty also result in significant pain relief and functional improvement [15], [16].

Occult fractures are associated with bone marrow oedema, which cannot be depicted visually on MDCT due to the overlying trabecular bone [17]. However, we hypothesised that the bone marrow oedema associated with occult fractures leads to increased Hounsfield attenuation units (HU) at MDCT. Therefore, the aim of our study was to evaluate the bone marrow with objective comparative ROI-based HU measurements to determine a HU cut-off value for valid and reliable detection of bone marrow oedema related to occult vertebral fractures.

Section snippets

Study population

In this retrospective study, we included 36 consecutive patients after spinal trauma (17 female, 19 male; median age, 73.5 years; range 45–90 years) who underwent MRI after MDCT imaging between December 2008 and July 2012 to rule out occult vertebral fractures of the thoracolumbar spine. Delay between MDCT and MRI was between 0 and 7 days (mean, 3 days). Patients were excluded if they suffered from secondary disease such as osseous metastasis or inflammatory osseous diseases. The local

Results

Overall 202 vertebral bodies were imaged in 36 patients. 6 vertebral bodies were excluded from quantitative analysis, one because of vertebral hemangioma, 4 because of vertebral sclerosis due to advanced osteochondrosis and 1 because of total vertebral compression. The remaining 196 vertebrae (97.0%) were further analysed (T9 = 3, T10 = 7; T11 = 17; T12 = 30; L1 = 32; L2 = 32; L3 = 30; L4 = 24 and L5 = 21). The mean area of all 784 ROIs by both readers was 2.8 cm2 ± 0.7 cm2 SD. Investigated levels in patients varied

Discussion

Our study demonstrates that differences in bone marrow density can be used as a valid tool for the detection of post-traumatic bone marrow oedema associated with vertebral fractures.

In correlation with increased amounts of interstitial fluid in the medullary cavity as confirmed by MRI, we observed significantly higher HU values within affected vertebral bodies compared with non-affected levels. However, we observed a large overlap in absolute HU values between affected and unaffected vertebrae,

Conflict of interest

We declare that we have no conflict of interest.

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