Neurosurg Focus 37 (1):E4, 2014 ©AANS, 2014 Use of computed tomography for assessing bone mineral density JOSEPH J. SCHREIBER, M.D.,1 PAUL A. ANDERSON, M.D.,2 AND WELLINGTON K. HSU, M.D.3 1Department of Orthopaedic Surgery, Hospital for Special Surgery, New York, New York; 2Department of Orthopedics & Rehabilitation, University of Wisconsin, Madison, Wisconsin; and 3Department of Orthopaedic Surgery, Northwestern University Feinberg School of Medicine, Chicago, Illinois Assessing local bone quality on CT scans with Hounsfield unit (HU) quantification is being used with increasing frequency. Correlations between HU and bone mineral density have been established, and normative data have been defined throughout the spine. Recent investigations have explored the utility of HU values in assessing fracture risk, implant stability, and spinal fusion success. The information provided by a simple HU measurement can alert the treating physician to decreased bone quality, which can be useful in both medically and surgically managing these patients. (http://thejns.org/doi/abs/10.3171/2014.5.FOCUS1483) KEY WORDS • Hounsfield unit • osteoporosis • bone mineral density ITH an aging population, osteoporosis is increas- calculated from a region of interest (ROI) on CT scans ingly becoming a public health concern. Recent without any additional cost or radiation exposure. Values estimates suggest that more than 10 million peo- are calculated based on the following formula: HU = ([m Wple are affected in the US, with an additional 44 million - mw]/mw) × 1000, where m is defined as the linear x-ray 11 citizens at risk. The disease is typically characterized by attenuation coefficient of the selected voxel and mw the an age-related reduction in bone strength that predisposes attenuation coefficient of distilled water at room tempera- affected individuals to low-energy fractures. In the spine, ture and pressure. the incidence of osteoporotic vertebral fractures exceeds With the advent of automated exposure control on 1.4 million events annually.13 Osteoporotic fractures are a modern CT scanners, the calibrating phantoms that were significant cause of pain, disability, and decreased quality historically used for quantitative CT are not required.10,19,30 of life, which are associated with substantial health care This technology uses real-time detector feedback and data costs exceeding $17 billion (US dollars) annually.3 from the scout view to adjust tube current and exposure Dual x-ray absorptiometry (DXA) is currently the time. Tissue density and patient body habitus are account- standard for assessing bone mineral density (BMD) and ed for, thereby producing a more homogeneous energy has been correlated with fracture risk and treatment effi- spectrum, shortening radiation exposure time, and result- cacy. 21 While useful for assessing osteopenia or osteopo- ing in more accurate HU values of the targeted tissue. rosis,1 it is not without methodological limitations.37 Important differences in measurements of BMD be- The use of Hounsfield units (HUs) from CT scanning tween DXA and CT should be considered (Table 1). Dual to assess regional BMD of the spine has recently been x-ray absorptiometry is a planar measurement of den- 30 described, with several subsequent studies exploring its sity expressed as grams of mineral per square centime- utility in assessing fracture risk and prognosticating fu- ter scanned (g/cm2), while the values obtained from CT sion success. As described by Pickhardt et al., when CT scans are volumetric (g/mm3). Dual x-ray absorptiometry scans are obtained for other clinical indications, they may includes the posterior elements of the spine, and there- also be used for “opportunistic screening for osteoporo- 27 fore may be inaccurate or not possible in cases of severe sis.” spinal degeneration, scoliosis, or following lumbar sur- Hounsfield unit values are a measurement of the stan- gery. Computed tomography techniques can be limited dardized linear attenuation coefficient of tissue, based on to specific regions of interest, such as the vertebral body a defined scale of 0 for water and -1000 for air. Modern trabeculae, which has been shown to be most predictive radiology imaging software programs allow this to be of fracture.9 One of the disadvantages of using HU values to esti- Abbreviations used in this paper: BMD = bone mineral density; mate bone quality is the lack of clinical support compared DXA = dual x-ray absorptiometry; HU = Hounsfield unit; ROI = with DXA. Bone quality assessment is usually done using region of interest. a DXA T-score, which is defined by the WHO as a value Neurosurg Focus / Volume 37 / July 2014 1 Unauthenticated | Downloaded 09/27/21 12:25 PM UTC J. J. Schreiber, P. A. Anderson, and W. K. Hsu TABLE 1: Comparison of DXA and CT for assessing spinal bone mineral density Parameter DXA CT (HU) radiation dose low high except when used opportunistically cost $200–250 none when used opportunistically ROI includes cortex, trabecular bone, pedicles, posterior elements any specific ROI units planar (g/cm2) volumetric (g/mm3) acceptance gold standard novel contraindications lumbar surgery, severe spinal degeneration, scoliosis none corresponding to the number of standard deviations that edges, osseous abnormalities, and voids such as vascular an individual’s BMD differs from that of a young, healthy channels. The software calculates the average HU in the 1 control as follows: T-score = (BMD(patient) - BMD(reference))/ ROI for each image. An average of the 3 measurements SD(reference), where BMD(reference) and SD(reference) are bone determines the HU for individual vertebral levels. Others mineral density and its standard deviation of healthy con- have found that a single midbody ROI is as reliable as the trols in their 20s, respectively. average of 3 measurements and have used an automated It is the T-score, rather than the BMD or HU, that is software program to generate values.26,27 used to both diagnose and measure treatment efficacy, as precise parameters based on HUs not yet readily avail- Study Cohorts able. T-score evaluations are somewhat limited in clinical All studies had institutional review board exemp- utility, as the majority of patients who sustain fragility tions. Several patient cohorts were used to answer specific fractures are not in the osteoporotic range.14 clinical questions. Twenty-five consecutive patients who The purpose of this paper is to review the reliabil- underwent CT scanning to evaluate possible spinal in- ity and validity of the techniques used to estimate bone jury and who had recently undergone DXA were used to health using CT HU measurements. We will also demon- evaluate reliability and correlations. Eighty patients who strate examples of their practical use to identify patients were evaluated for trauma were used to measure norma- at risk for osteoporosis and how these values could be tive values.30 Twenty patients with adjacent-segment frac- used for surgical planning in cases of trauma, degenera- tures after spinal fusion along with 20 matched nonfrac- tion, and deformity. ture controls were used to assess the association between HU and adjacent-segment fracture.22 The CT scans of 28 Methods patients with a combined 52 levels of lumbar interbody fusion were used to assess the association between HU Technique for HU Assessment of the Spine and fusion success. Schreiber et al. described a simple technique for Reliability of HU Measurements assessing regional BMD of the spine that was corre- lated with results of DXA scans as well as compressive The intra- and interobserver reliability of HU mea- strength in osseous models.30 Using standard picture ar- surements was performed using an interclass correlation chiving and communication system software, an ROI is coefficient. Two observers independently measured HU drawn at 3 points on axial images that are obtained as at 3 locations of 4 vertebrae in 25 subjects. parallel to the endplates as possible: just inferior to the superior endplate, midvertebral body, and just superior Normative Data to the inferior endplate (Fig. 1). The ROI is drawn en- Normative data stratified by sex and decade of life capsulating only cancellous bone and avoiding cortical were determined by Schreiber et al.30 based on CT scans FIG. 1. Computed tomography scans illustrating the technique for calculating vertebral BMD with HUs. A: Sagittal slice of the L-3 vertebral body demonstrating axial planes of interest. B–D: Axial images showing HU values generated by the imaging software program. The mean value of B–D is used for calculating vertebral body bone density. 2 Neurosurg Focus / Volume 37 / July 2014 Unauthenticated | Downloaded 09/27/21 12:25 PM UTC Use of computed tomography for assessing bone mineral density from 80 consecutively presenting trauma patients grouped by decade from 10–19 years to 80–89 years, with 10 pa- tients in each decade. These data were compared with other published normative data.22 Correlation of HU and DXA Twenty-five patients undergoing lumbar CT and DXA within 6 months of each other were identified, and the BMD, T-score, and HU value were measured from L-1 to L-4.30 These patients had a mean age of 71.5 years with a range of 33–87 years. There were 18 female and 7 male patients. Correlation of T-score and BMD from DXA to HU measurement was performed using the Pear- son correlation coefficient. Each patient was grouped into normal (T-score ≥ -1.0), osteopenia (T-score -1.0 to FIG. 2. No differences were observed between HU values obtained -2.5), and osteoporotic (T-score ≤ -2.5). The HU thresh- at different locations within lumbar vertebral bodies. old values for each bone health state were reported as the mean and 95% confidence intervals. decrease in HU with age (Fig. 3 right). The mean HU in males ranged from 270 in young adults to 85 in those in HU Value and Fracture Risk their 9th decade.