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Abdominal CT imaging useful in osteoporosis detection
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Routine abdominal CT scans administered for other clinical indications can be used for screening patients for osteoporosis, according to data published in the Annals of Internal Medicine. Researchers report that these findings could prevent the need for other imaging, radiation exposure, cost, equipment or patient time.
In a cross-sectional study of 1,867 adults undergoing CT and DXA (n=2,067 pairs; women, 81%; mean age, 59.2 years) within a 6-month period over a duration of 10 years, Perry J. Pickhardt, MD, from the department of radiology at the University of Wisconsin School of Medicine and Public Health, and colleagues evaluated the CT-derived bone mineral density (BMD) assessment compared with DXA measures for detecting osteoporosis. They used CT scans performed for other clinical indications.
According to data, CT-attenuation values were significantly lower at all vertebral levels for patients with DXA-defined osteoporosis (P<.001).
“An L1 CT-attenuation threshold of 160 Hounsfield units (HU) or less was 90% sensitive and a threshold of 110 HU was more than 90% specific for distinguishing osteoporosis from osteopenia and normal BMD; a threshold of 135 HU resulted in a balanced sensitivity and specificity of approximately 75% for each,” the researchers wrote.
Moreover, the researchers wrote that positive predictive values for osteoporosis were 68% or greater at L1 CT-attenuation thresholds less than 100 HU; negative predictive values were 99% at thresholds greater than 200 HU.
Of the 119 patients with at least one moderate-to-severe vertebral fracture, the researchers found that 62 (52.1%) demonstrated nonosteoporotic T-scores (DXA false-negative results), and the majority of patients (97%) had L1 or mean T12 to L5 vertebral attenuation of 145 HU or less. Similarly, data indicate performance was seen at all vertebral levels. However, intravenous contrast did not affect CT performance, they added.
In an accompanying editorial, Sumit R. Majumdar, MD, MPH, from the University of Alberta and William D. Leslie, MD, MSc, from the University of Manitoba, wrote that future studies will need to replicate the findings by Pickhardt and colleagues.
“Pickhardt and associates have laid all the groundwork needed to justify using conventional CT imaging to detect incidental osteoporosis — it is now up to the rest of us to safely and cost-effectively translate this new knowledge into everyday clinical practice,” they wrote.
For more information:
Majumdar SR. Ann Intern Med. 2013; 158: 630-631.
Pickhardt PJ. Ann Intern Med. 2013; 158: 588-595.
Disclosure: The study was supported by NIH grants. See the study for a full list of disclosures.
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Donald A. Bergman, MD
This study was undertaken to address the problem of "under diagnosis [of osteoporosis] due to under testing." The study looked at CT imaging of the L1 vertebra, which is always included in CT studies of the chest and abdomen. The authors used CT attenuation measurements (the "slowing" of the X-ray as it passes through bone). The amount of L1 of attenuation correlated well with the DXA scores (which are also based on X-ray attenuation.) The authors also found good correlation with L1 attenuation and attenuation of T12 through L5.
The advantage of this approach is that the osteoporosis screening data is always available since L1 is always part of abdominal and chest CT scans. Also the CT attenuation data is easily scalable, so that, depending on the desires of the treating physician, the threshold can be set to improve sensitivity (reduce false negatives) or improve selectivity (reduce false positives). The problem is that it is not clear how this approach will predict fractures, particularly in patients with better attenuation scores (similar to the problem of predicting fracture risk in patients with osteopenic, rather than osteoporotic, DXA scores).
The solution will be to construct a FRAX fracture risk model for CT attenuation scores similar to the one used to improve fracture risk prediction in patients with osteopenic DXA scores (as suggested in the accompanying editorial).
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