10 minute read
Lumbar Bone Density Measurements
Using CT and MRI Scans as Alternatives to DEXA
Evaluating and assessing vertebral bone quality prior to spinal surgery is important to increase the long-term success of instrumentation and limit the risk of complications. Subsidence, device migration, and pedicle screw pullout are among the possible complications of instrumentation in patients with poor bone quality.[1-5]
Dual-energy x-ray absorptiometry (DEXA) is a well-established tool for assessing bone mineral density (BMD) and has served as the reference standard for classifying bone as normal, osteopenic, or osteoporotic.[6,7] However, DEXA has shortcomings for spinal applications due to the inclusion of osteophytes and cortical bone in bone measurements, which may lead to falsely elevated BMD results.[8,9] In light of this, there has been recent interest in alternative methods to measure patients’ bone quality using existing imaging techniques: computed tomography (CT) or magnetic resonance imaging (MRI).[3,5,10,11]
Spinal surgery candidates almost universally undergo preoperative CT and/or MRI. As a result, use of these images to assess bone quality may eliminate the need for a DEXA, thus reducing radiation exposure to patients, cost of care, and patient inconvenience. Further research in various spine surgery populations is needed to determine how well CT or MRI correlate with either DEXA or bone quality outcomes like implant subsidence and screw pullout. Population composition, such as age or sex, may affect how well CT or MRI bone quality measurement methodologies perform.[12,13] This is a growing area of research that may provide alternatives to DEXA for future spinal surgery screenings and may yield greater benefits to patients in reducing postoperative complications. In the present article, we introduce recent methodologies that utilize CT and MRI o measure bone quality.
Measuring Bone Quality With CT
Tools in the Picture Archiving and Communication System (PACS) or similar programs are used to evaluate bone quality using CT or MRI. Measuring Hounsfield units (HU) from CT scans can provide a direct assessment of bone quality.
CT scan measurements for bone quality are most often performed by first locating a midsagittal cut through the center of the vertebral body (Figure 1A). Then, on the axial view, a region of interest (ROI) is drawn on the cancellous bone of the vertebral body, avoiding cortical regions (Figure 1B).[14] The measurements should avoid the vascular area of the venous plexus, which is just anterior to the thecal sac.[15] A composite score is calculated by averaging each of the middle section measurements from each vertebral body from L1 to L4.
Measuring Bone Quality With MRI
Bone quality in MRI is measured by calculating the vertebral bone quality (VBQ) score based on signal intensity (SI) of the vertebral bodies.[16] T1-weighted scans are used to measure VBQ based on SI of the images.[17] Using the midsagittal view of the lumbar spine, an ROI is drawn over the center of each vertebral body from L1 to L4 (Figure 2). In addition, an ROI is drawn over the thecal sac immediately posterior to the L3 vertebra to measure the SI of the cerebral spinal fluid (CSF).
Using the equation derived from Ehresman et al, the median of the vertebral measurements is divided by the CSF to yield an MRI lumbar composite value (Equation 1).[18]
VBQ score = (Median SI[L1-L4])/SI[CSF]
Alternate Measurement Methods
A major challenge for current research is that the measurement methodologies are not yet completely standardized. Deliberation on the most efficient or effective methods for measuring bone quality from CT images or MRIs are seldom published. Table 1 (see PDF) displays the cutoff values for these imaging techniques based on current literature.
However, some studies have reported on various aspects of methodology. For example, the comparison between using the sagittal vs axial view of the vertebral body to capture bone quality using CT has been compared in a few studies.12,19,20 These studies have concluded that the sagittal view was more efficient while maintaining the effectiveness of the measurements.
Variations for calculating MRI VBQ have been tested; one study found that the VBQ’s ability to classify bone quality did not considerably change regardless of incorporating the CSF in Equation 1.12 In addition, using means verses medians in the MRI VBQ calculations was compared, and, regardless of method, the correlations did not appreciably change.12 Other proposed changes to MRI methodology have been published in the literature. Huang et al utilized a calculation similar to Equation 1; however, the numerator was simply the SI of the S1 vertebral body, which was measured using the midsagittal view. 21 This equation yielded moderate to strong correlations with DEXA T-scores, with an accuracy of 82% in predictive ability. Other methods are still being explored and produced for both CT and MRI bone quality measurements.
In a study by Roch et al, the VBQ measurements were derived from three versions of MRI images (T1, T2, and Short Tau Inversion Recovery, or STIR) and combined to improve the accuracy of the bone quality assessment.22 While these results are intriguing, completing these imaging techniques, measuring each one, and combining them in a specific equation may be difficult to adopt for uses in the clinical setting.
Study population is an important factor to consider when evaluating CT or MRI bone quality assessments. As discussed by Courtois et al, who found a poor relationship between MRI and DEXA, the findings may not be as sensitive in younger patients undergoing treatment for painful disc degeneration as compared to older patients in whom bone quality may be more compromised.12 While this study population of younger patients with symptomatic disc degeneration had a moderate relationship between CT and DEXA, Kohan et al found a poor correlation between CT and DEXA in an older adult spinal deformity patient population. 23 Research is still underway to understand which measurements are most appropriate for specific populations and applications.
Conclusion
While DEXA remains the gold standard for evaluating BMD prior to spine surgery, there is increased interest in CT- and MRI-based methods. CT and MRI show potential for becoming platforms to estimate bone quality. However, results may vary depending on population composition, such as age or sex. More research should be encouraged to expand knowledge in this field of study.
References
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AUTHORS
Emily C. Courtois, MS
Alexander M. Satin, MD
Mary P. RogersLaVanne, PhD
Peter B. Derman, MD, MBA
From the Texas Back Institute in Plano, Texas.
