Our results suggest that valuable information pertaining to vertebral BMD assessment is currently going unused at CTC screening, as well as other noncontrast abdominal CT examinations. By applying either phantomless QCT analysis or even single-level ROI measurement to noncontrast CT studies, we can identify individuals at risk for osteoporosis, as well those who are not. We acknowledge that DXA BMD measurement is imperfect for fracture risk prediction, but believe it appropriate to frame all CT-based results in terms of the DXA T-scores, since this represents the necessary reference standard because of the WHO definition of osteoporosis.2
For individuals at risk that fall below our derived CT-based thresholds for 100% sensitivity for osteoporosis, formal DXA evaluation would likely be warranted. This opportunistic approach that effectively piggy-backs lumbar spine assessment at CTC screening requires no additional scanning, radiation, or costs, all of which may limit the use of QCT as a primary screening test for osteoporosis. Furthermore, use of the simple ROI technique (eg, at L1) entails only minimal effort from the radiologist, with a negligible investment of interpretation time.
Osteoporosis represents a major public health issue and there is a growing appreciation of the need for wider screening efforts. The recently revised and expanded recommendations by the USPSTF underscore the need for more screening.16
Although a number of investigators have suggested that QCT may in fact be better than DXA for BMD assessment,17–23
DXA nonetheless remains the defining test for osteoporosis and our work does not seek to change this. T-scores derived from tests other than DXA, such as quantitative CT and ultrasound, cannot be applied to the WHO diagnostic classification and are not currently applicable to FRAX.2
However, by harnessing the attenuation data contained within routine CT scans, we may be able to make a positive impact in terms of “opportunistic” screening, with little or no down side, and more efficient use of existing resources. Body CT is a common procedure in the U.S., with an estimated 25 million scans in adults covering the chest and/or abdomen performed each year.24
Even if just a small fraction of these cases were captured for opportunistic BMD screening, the overall impact could be substantial. Although it is difficult to predict the net effect on the overall number of DXA screening exams, the percentage of cases positive for osteoporosis would likely increase, further improving cost-effectiveness.
We initially sought to compare only the phantomless QCT against the DXA reference standard. Previous studies with phantomless QCT have demonstrated high precision and opened the door for QCT assessment without the use of an external phantom.25, 26
However, our preliminary investigations suggested that simple trabecular ROI measurement of the lumbar spine, without the use of fat and muscle as internal controls, plane angulation, or formal derivation of BMD values or T-scores yielded results that were similar to phantomless QCT. By including this “simple” ROI technique in this work, we have shown that non-angled ROI attenuation measurement at a single lumbar level provides for rapid assessment that is equivalent to the full multi-level phantomless QCT approach in terms of predicting the DXA T-score. In fact, reproducibility of results between the supine and prone series in our study was better
with the simple ROI technique compared with QCT. This may relate to wider variations in fat and muscle attenuation, which might impact the phantomless QCT results. Fully-automated volumetric trabecular bone measurement at CT represents another attractive potential solution. Preliminary research has demonstrated good reproducibility with this non-operator-dependent approach.4
Although single-level vertebral assessment is discouraged at DXA2
and may strike some as a gross oversimplification, this approach is better suited to CT attenuation measurement due to the high level of anatomic detail and ability to directly measure representative regions of trabecular bone. However, due to the known differences in BMD and therefore CT attenuation at different lumbar levels, which are lowest at L3, level-specific thresholds are required. It should also be noted that our results apply only to non-contrast CT as we did not study the effect of IV contrast. It is likely that higher ROI threshold values for osteoporosis would be needed to account for enhancement of the vertebral body, although one small-cohort QCT study did not find a significant difference.27
The impact on phantomless QCT measurement would presumably be more complex due to the differences in contrast enhancement of bone, muscle, and fat.
Until better understood, the use of CT attenuation data for BMD assessment may raise potential management issues when discordant with the DXA results. For example, approximately 15% of adults below the threshold for 100% osteoporosis detection will have a normal
T-score reported at central DXA. Because the CT-based techniques are a direct measure of trabecular bone, whereas DXA is a planar technique confounded by cortical bone and degenerative changes, one could argue that the CT results may better reflect the true BMD status of the patient. Of course, when an unsuspected lumbar compression fracture is identified at CT (or other VFA technique) in a patient without osteoporosis according to their DXA T-score, the fracture finding strongly influences both diagnosis and treatment, as seen with the FRAX fracture risk assessment tool.28
In a significant fraction of cases (8% in the current series), the lumbar T-score will not be reported at DXA, typically due to prominent degenerative changes. This is of course not an issue at CT due to its cross-sectional nature that provides for direct trabecular assessment.
BMD assessment adds to the growing list of serendipitous “extracolonic” screening opportunities at CTC beyond the colorectal evaluation alone, which is of proven value.5, 7, 29
Simultaneous screen detection of unsuspected abdominal aortic aneurysms and extracolonic cancers are recognized benefits of CTC,30, 31
which further enhance its cost-effectiveness.32
Further investigation is needed to assess the potential positive impact that routine osteoporosis screening at CTC might convey. Additional opportunities associated with the extracolonic information available at CTC include measurement of visceral fat, liver attenuation, and abdominal aortic calcification. With respect to musculoskeletal data at CTC beyond the lumbar spine, evaluation of the femoral neck has been largely untouched, as is the case with muscle evaluation for sarcopenia.33, 34
Although much of the attention related to extracolonic evaluation at CTC screening was initially focused on the potential negative impact, perhaps due to lingering concerns from the whole-body CT screening debacle, we anticipate continued investigations into more ways to add value well beyond colorectal cancer screening.
Our study has some limitations. This is a retrospective, single-center study. The thresholds and diagnostic performance measures (sensitivity, specificity, AUC) are study-dependent and require larger cohorts and external validation. The study cohort was relatively young in terms of osteoporosis screening, with a low prevalence of compression fractures. Assessment of an older, more enriched cohort might improve the performance of the CT-based measures. By choosing CT-based thresholds that are 100% sensitive for detecting DXA-defined osteoporosis, the specificity values are quite low, with nearly half of cases below the various thresholds with osteopenic BMD and sometimes even normal. If imperfect sensitivity values were felt to be more appropriate (eg, 80% or 90%), the specificity would improve. However, by using this opportunistic CT data, the number of normal DXA studies would be decreased. The impact of intravenous contrast on trabecular bone measurement at CT was not assessed in this study and further investigation is also warranted. Although this opportunistic strategy of incidental osteoporosis screening at CT would conceivably reduce the number of unnecessary DXA studies, and thus reduce costs, these savings were not assessed formally.
In conclusion, our study shows that both phantomless QCT and simple ROI attenuation measurements of the lumbar spine are effective for BMD screening at non-contrast CT, with high sensitivity for osteoporosis as defined by the DXA T-score. Single-level ROI vertebral attenuation measurement at CTC is reproducible, requires little effort, and adds no radiation or costs but can provide valuable BMD data for osteoporosis screening. This approach could potentially be applied at the L1 level to any non-contrast CT of the chest and/or abdomen, harnessing data that currently goes unused in routine clinical practice. Additional large-scale validation is required before more widespread use in other body CT applications can be advocated, particularly for studies employing IV contrast.