Simultaneous screening for osteoporosis at CT colonography: bone mineral density assessment using MDCT attenuation techniques compared with the DXA reference standard

Abstract

The purpose of this study was to evaluate the utility of lumbar spine attenuation measurement for bone mineral density (BMD) assessment at screening computed tomographic colonography (CTC) using central dual-energy X-ray absorptiometry (DXA) as the reference standard. Two-hundred and fifty-two adults (240 women and 12 men; mean age 58.9 years) underwent CTC screening and central DXA BMD measurement within 2 months (mean interval 25.0 days). The lowest DXA T-score between the spine and hip served as the reference standard, with low BMD defined per World Health Organization as osteoporosis (DXA T-score ≤ -2.5) or osteopenia (DXA T-score between -1.0 and -2.4). Both phantomless quantitative computed tomography (QCT) and simple nonangled region-of-interest (ROI) multi-detector CT (MDCT) attenuation measurements were applied to the T(12) -L(5) levels. The ability to predict osteoporosis and low BMD (osteoporosis or osteopenia) by DXA was assessed. A BMD cut-off of 90 mg/mL at phantomless QCT yielded 100% sensitivity for osteoporosis (29 of 29) and a specificity of 63.8% (143 of 224); 87.2% (96 of 110) below this threshold had low BMD and 49.6% (69 of 139) above this threshold had normal BMD at DXA. At L(1) , a trabecular ROI attenuation cut-off of 160 HU was 100% sensitive for osteoporosis (29 of 29), with a specificity of 46.4% (104 of 224); 83.9% (125 of 149) below this threshold had low BMD and 57.5% (59/103) above had normal BMD at DXA. ROI performance was similar at all individual T(12) -L(5) levels. At ROC analysis, AUC for osteoporosis was 0.888 for phantomless QCT [95% confidence interval (CI) 0.780-0.946] and ranged from 0.825 to 0.853 using trabecular ROIs at single lumbar levels (0.864; 95% CI 0.752-0.930 at multivariate analysis). Supine-prone reproducibility was better with the simple ROI method compared with QCT. It is concluded that both phantomless QCT and simple ROI attenuation measurements of the lumbar spine are effective for BMD screening at CTC with high sensitivity for osteoporosis, as defined by the DXA T-score.

Conflict of interest statement

Conflict of Interest:

Dr. Pickhardt serves as a consultant for Medicsight, Viatronix, Bracco, and Check Cap; and is co-founder of VirtuoCTC. Dr. Summers receives royalties and research support from iCAD, and owns stock in Johnson & Johnson. Binkley is a consultant for Merck, Amgen, Lilly, and Tarsa. All other authors have no conflicts of interest.

Copyright © 2011 American Society for Bone and Mineral Research.

Figures

Figure 1. MDCT-based techniques for assessing bone mineral density at CTC screening

A and B. Phantomless QCT technique: screen capture (A) of the phantomless QCT program and output shows the method of ROI placement for the vertebral body, muscle, and fat, shown here for the T12 level, which is then repeated for the L1–L5 levels. Each plane of measurement (B, yellow lines) is angled to be parallel with the end plates at that level. Note that the QCT-derived T-scores (and Z-scores) are blacked out and were not used in this study. C. Simple ROI technique: standard transverse CTC image (same patient as in A) without oblique angulation shows ROI placement at the L3 level. Unlike QCT assessment, a single ROI placement at an individual level was considered as a primary outcome measure with this “simple” technique. Note: images A and C are from a patient diagnosed with osteoporosis on DXA performed 4 days before CTC (L-spine T-score=−2.8). The QCT value of 69.2 mg/cc (circled in red) and the L3 vertebral attenuation of 98.5 HU are well below the thresholds derived in this study (see Table 1).

Figure 1. MDCT-based techniques for assessing bone mineral density at CTC screening

A and B. Phantomless QCT technique: screen capture (A) of the phantomless QCT program and output shows the method of ROI placement for the vertebral body, muscle, and fat, shown here for the T12 level, which is then repeated for the L1–L5 levels. Each plane of measurement (B, yellow lines) is angled to be parallel with the end plates at that level. Note that the QCT-derived T-scores (and Z-scores) are blacked out and were not used in this study. C. Simple ROI technique: standard transverse CTC image (same patient as in A) without oblique angulation shows ROI placement at the L3 level. Unlike QCT assessment, a single ROI placement at an individual level was considered as a primary outcome measure with this “simple” technique. Note: images A and C are from a patient diagnosed with osteoporosis on DXA performed 4 days before CTC (L-spine T-score=−2.8). The QCT value of 69.2 mg/cc (circled in red) and the L3 vertebral attenuation of 98.5 HU are well below the thresholds derived in this study (see Table 1).

Figure 2. Normal and abnormal L3 vertebral attenuation measurements using the simple ROI method

A. Non-angled transverse CTC image with vertebral ROI at the L3 level shows a trabecular attenuation of 204.5 HU, well above the 145 HU level-specific threshold for 100% osteoporosis sensitivity (Table 1). L-spine DXA T-score was 0.6. B. L3 vertebral ROI shows an attenuation of 80.5 HU in a patient with osteoporosis at DXA (L-spine T-score=−2.9), which is well below the 145 HU threshold.

Figure 3. ROC curves for predicting DXA T-scores for osteopenia/osteoporosis with MDCT attenuation techniques

ROC curves for phantomless QCT and simple ROI MDCT methods for detecting DXA T-scores of −2.5 or lower (A, osteoporosis) and −1.0 or lower (B, low BMD – osteopenia and osteoporosis). AUC values (with 95% CI) are shown for each method and level (MV = best multivariate model). Note how similar the performance is, regardless of specific ROI approach, with sizable overlap of all 95% confidence intervals.

Figure 3. ROC curves for predicting DXA T-scores for osteopenia/osteoporosis with MDCT attenuation techniques

ROC curves for phantomless QCT and simple ROI MDCT methods for detecting DXA T-scores of −2.5 or lower (A, osteoporosis) and −1.0 or lower (B, low BMD – osteopenia and osteoporosis). AUC values (with 95% CI) are shown for each method and level (MV = best multivariate model). Note how similar the performance is, regardless of specific ROI approach, with sizable overlap of all 95% confidence intervals.

Figure 4. Scatterplot of BMD results at QCT compared with L-spine DXA T-scores

Simple scatterplot shows BMD at QCT against L-spine DXA T-score. Patients with osteoporosis, osteopenia, and normal BMD according to central DXA T-score are depicted in purple, orange, and green, respectively. The Pearson product-moment correlation is 0.634.

Figure 5. Reproducibility of MDCT attenuation techniques using supine and prone series as internal controls

Simple and Bland-Altman plots for the phantomless QCT technique (A and B) and the simple ROI technique (C and D) in 45 patients who underwent DXA and CTC within 7 days of each other show slightly better reproducibility for the simple ROI vertebral method. (Note: purple dots = osteoporosis, orange dots = osteopenia, and green dots = normal BMD at DXA according to T-scores).

Figure 6. Inter-observer measurement variability for simple ROI method

Simple (A) and Bland-Altman (B) plots show good reproducibility for trabecular attenuation measurement between the board-certified radiologist and the inexperienced 1st-year medical student (T12-L5ave shown). Note that all cases with osteoporosis according to DXA T-score (purple dots) are within the 95% limits of agreement, without any outliers.

Figure 7. Unsuspected moderate L1 compression deformity in 60-year-old man with abnormally low vertebral attenuation but without osteoporosis by DXA

Sagittal reconstruction of supine CTC series (A) shows a moderate compression fracture at the L1 level. Vertebral attenuation was abnormally low at all levels (B, 108 HU at L3) except for L1 (173 HU, not shown) due to the compression deformity itself. The reported T-scores at central DXA were 0.2 for L1–L4 (C) and −1.1 for the left femoral neck (D). Increased density at L1 level at DXA in C was attributed to degenerative changes.