Opportunistic Osteoporosis Screening at Routine Abdominal and Thoracic CT: Normative L1 Trabecular Attenuation Values in More than 20 000 Adults

Abstract

Background Abdominal and thoracic CT provide a valuable opportunity for osteoporosis screening regardless of the clinical indication for imaging. Purpose To establish reference normative ranges for first lumbar vertebra (L1) trabecular attenuation values across all adult ages to measure bone mineral density (BMD) at routine CT. Materials and Methods Reference data were constructed from 20 374 abdominal and/or thoracic CT examinations performed at 120 kV. Data were derived from adults (mean age, 60 years ± 12 [standard deviation]; 56.1% [11 428 of 20 374] women). CT examinations were performed with (n = 4263) or without (n = 16 111) intravenous contrast agent administration for a variety of unrelated clinical indications between 2000 and 2018. L1 Hounsfield unit measurement was obtained either with a customized automated tool (n = 11 270) or manually by individual readers (n = 9104). The effects of patient age, sex, contrast agent, and manual region-of-interest versus fully automated L1 Hounsfield unit measurement were assessed using multivariable logistic regression analysis. Results Mean L1 attenuation decreased linearly with age at a rate of 2.5 HU per year, averaging 226 HU ± 44 for patients younger than 30 years and 89 HU ± 38 for patients 90 years or older. Women had a higher mean L1 attenuation compared with men (P < .008) until menopause, after which both groups had similar values. Administration of intravenous contrast agent resulted in negligible differences in mean L1 attenuation values except in patients younger than 40 years. The fully automated method resulted in measurements that were average 21 HU higher compared with manual measurement (P < .004); at intrapatient subanalysis, this difference was related to the level of transverse measurement used (midvertebra vs off-midline level). Conclusion Normative ranges of L1 vertebra trabecular attenuation were established across all adult ages, and these can serve as a quick reference at routine CT to identify adults with low bone mineral density who are at risk for osteoporosis. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Smith in this issue.

Figure 1:

Examples of age-related bone loss by trabecular L1 attenuation assessment at CT. Transverse (axial) CT scans at the L1 level (top row) in adult patients of varying ages (age and sex denoted at the bottom). Magnified views of the L1 vertebra are shown in soft tissue (second row from top) and bone (third row from top). Standard placement of the region of interest (ROI) for trabecular attenuation measurement and the mean Hounsfield unit value within the ROI are shown in the third row from the top. Sagittal reconstructions with soft tissue and bone windows are shown (bottom row; including sagittal L1 ROI placement for the first patient, left). As is typical, trabecular attenuation values progressively decreased with increasing patient age. The loss of bone mineral density is more apparent on soft tissue images (top two rows). F = female, M = male.

Figure 2:

Example from manual-automated subanalysis. Unenhanced sagittal CT images centered on the lumbar spine from a 66-year-old man in, A, soft tissue and, B, bone windows show the L1 level (arrows). Note the higher band of trabecular density at the midvertebral level. C, Magnified view of L1 shows the level of axial manual region-of-interest (ROI) placement for the subanalysis comparison with the automated technique; the yellow midline placement matches the automated level, whereas the white line reflects the standard manual level. The manual sagittal ROI placement is shown in B. The 20-HU increase observed in this subanalysis with the automated technique can be explained by the different level for ROI placement.

Figure 3:

Plot shows normative reference values for trabecular L1 attenuation values for opportunistic osteoporosis screening. The median and the mean ± standard deviation (SD) values for L1 trabecular attenuation in Hounsfield units are shown for each age group (x-axis). The plot of the mean Hounsfield units (red dots) shows that age-related L1 trabecular bone loss is fairly linear. The normative reference ranges can serve as a quick reference while performing CT examination for other clinical indications. Error bars indicate standard deviations, which are fairly uniform across the age spectrum.

Figure 4:

Variations in mean L1 trabecular attenuation values according to sex, presence of intravenous (IV) contrast agent, and method of measurement (manual vs automated [auto]). Mean L1 trabecular attenuation is higher for women until postmenopausal age, when values for men and women are strikingly similar (top). The effect of intravenous contrast agent is most notable for patients younger than 40 years. For older adults, the effect of intravenous contrast agent may not be clinically relevant for opportunistic bone mineral density screening (middle). The fully automated technique for L1 trabecular attenuation yields higher values compared with manual region-of-interest placement (average, ∼20 HU) (bottom). Age groups are on the x-axis and error bars indicate standard deviations. * P < .01; ** P < .005.