Optimal scanning parameters of lumbar bone density measured by fast kilovoltage-switching dual-energy computed tomography (DECT)

Introduction

Osteoporosis is a systemic osteopathy characterized by decreased bone mass and damaged bone microstructure, leading to increased bone fragility and fracture susceptibility (1). These fractures lead to high morbidity and mortality without timely treatment. Therefore, the early diagnosis and prevention of osteoporosis is very important.

Bone mineral density (BMD) is considered a major indicator for monitoring osteoporosis and estimating fracture risk, therefore, accurate measurement of BMD is very important. Dual-energy X-ray densitometry (DXA) and quantitative computed tomography (QCT) are considered reference standards for measuring BMD (2). DXA is widely used due to its low cost and low radiation dose and provides accurate BMD measurements in vitro (e.g., vertebral body in uniform fluid) (3). However, the measurements have difficulties while meeting vascular calcification, scoliosis, and spinal degeneration which can generate inaccurate diagnoses. In addition, DXA measures the entire vertebral BMD and cannot distinguish cortical bone and cancellous bone (4,5). QCT is a quantitative technique to measure volumetric bone mineral density (vBMD), regardless of bone size and shape, and there is solid evidence supporting its clinical value (6). However, studies have shown that abdominal fat content and spinal marrow fat content can affect the measurement of vertebral BMD by QCT (7-12), especially patients whose BMD <80 mg/cm³. Since QCT was performed by using a compound to make standard phantom of equivalent water and bone, the phantom and the measured part were scanned synchronously, and then transformed into vBMD by linear regression with X-ray attenuation. The absorption and attenuation curve of abdominal fat to X-ray is arcuate, that is to say, the absorption of abdominal fat to X-ray increases with the increase of tube voltage under CT imaging. The more abdominal fat content, the greater the measurement error, the greater the impact on the accuracy of measurement.

The technological innovation of fast kilovoltage (KV)-switching dual-energy computed tomography (DECT) has enabled the accurate measurement of vertebral BMD (13-16). The DECT used the hydroxyapatite water (HAP-H₂O) decomposition method, and the main components of European spine phantom (ESP) vertebrae were equivalent to hydroxyapatite (HAP) and water, so the DECT measurements were relatively less affected by abdominal total adipose tissue (TAT). Cui et al. (17) showed that 0.8 seconds (s)/rotation (rot), 230 mA, and HAP-H₂O measurements of BMD could further ensure the accuracy of measurement. However, their findings did not consider the effects of abdominal fat and ribs on the vertebral body. Therefore, we simulated the abdominal conditions of normal and obese people using the ESP and aimed to explore the optimal scanning parameters of DECT for accurately measuring BMD and to provide suitable DECT scanning conditions for the human body.

Methods

Phantom

The ESP (QRM GmbH, Moehrendorf, Germany) used in this study (18) consists of an epoxy resin and 3 HAP inserts of densities 50 mg/cm³ (osteoporosis), 102 mg/cm³ (osteopenia), and 197 mg/cm³ (normal), labeled as the first (L1), second (L2), and third lumbar vertebrae (L3), respectively. Different sizes of fresh (within 6 hours after slaughter) porcine-isolated fat (without skin) were selected and wrapped around the ESP to simulate the lower TAT of human with different obesity levels. Then, 6 fresh porcine-isolated ribs (from the slaughterhouse) were selected and wrapped around the ESP and fat diagonally on both sides to simulate the upper TAT of people with different obesity levels.

TAT area measurement

We wrapped 4 pieces of porcine-isolated fat around ESP separately and scanned by QCT protocol using a 256-section rapid KV-switching DECT scanner (Revolution; GE Healthcare, Waukesha, WI, USA). The scanning parameters were as follows: tube voltage, 120 kV; tube current, 200–370 mA using automatic milliampere technology; tube speed, 0.8 s/rot; and pitch, 0.984:1. The QCT measurement software is calibrated periodically. The reconstructed 1.25 mm thin-slice CT images were transferred to the QCT workstation (Mindways Software Inc., Austin, TX, USA), and the TAT area at L2 central level was measured using “Tissue Composition Analysis” function (Figure 1). The measured areas of 4 porcine-isolated fat were around 140, 240, 340, and 450 cm², respectively. TAT =240 cm² denoted the average TAT in the lower abdomen of normal weight people; TAT =140 cm² + ribs denoted the average TAT in the upper abdomen of normal weight people; TAT =340 cm² + ribs denoted the average TAT in the upper abdomen of obese people; and TAT =450 cm² denoted the average TAT in the lower abdomen of obese people.

Figure 1 QCT measures the abdominal fat at the central level of the L2. The blue part outside the green aperture is the total abdominal fat. QCT, quantitative computed tomography.

Data acquisition and image reconstruction

GE Revolution 256-slice CT was employed to obtain DECT scans of the phantom (the phantom wrapped in fat/fat + ribs). Air correction was performed before each scan. The phantom was placed in the center of the CT bed with a height of 135 cm. A total of 20 groups DECT scans were performed, namely, tube speed 0.5 s/rot with 5 sets of tube currents (200, 240, 280, 320, and 365 mA); tube speed 0.6 s/rot with 5 sets of tube currents (195, 235, 280, 320, and 360 mA); tube speed 0.8 s/rot with 5 sets of tube currents (190, 230, 275, 315, and 355 mA); and tube speed 1.0 s/rot with 5 sets of tube currents (185, 230, 270, 315, and 355 mA). All other acquisition parameters of DECT were applied as follows: gemstone spectral imaging (GSI) mode with rapid KV-switching between 80 and 140 kVp; helical pitch, 0.984:1; reconstruction slice thickness: 2.5 mm. For each group, 10 scans were conducted, with repositioning after each scan to assess the stability of the measurements and calculate the average value for each group. All reconstructed images were transferred to an advanced image processing station (AW4.7, GE Healthcare, IL, USA). The volume CT dose index (CTDIvol) for each scanning condition was recorded.

BMD value measurement

The measurement of HAP was conducted using fast KV-switching DECT HAP-H₂O decomposition technique on the AW4.7 workstation. The HAP value was chosen to represent the BMD. The measurement was completed by 2 physicians independently. The circular region of interest (ROI) with a size of 20×20 mm was placed at the median level of the vertebral body including as much of the bone cancellous as possible, while avoiding the areas with high BMD, for example, bone cortex and vertebral pedicle. The ROI was measured 3 times and the average value was calculated (Figure 2).

Figure 2 Selected fresh pig fat and fat + rib wrapped around the ESP, CT axial diagram. BMD of the lumbar spine was measured, represented by L2. The middle slice (×) was selected on sagittal CT images. ROI (pink ellipse region) was selected from the corresponding axial CT images, and the sizes were all 20×20 mm. ESP, European spine phantom; CT, computed tomography; BMD, bone mineral density; ROI, region of interest.

Statistical analysis

Statistical analysis were performed using SPSS software version 26.0 (IBM Corp., Armonk, NY, USA). The difference between the measured value of ESP L1–3 vertebral body and the actual value was compared by single sample t-test, and the data were in line with normal distribution. Quantitative data were described with x¯±s and P<0.05 was considered statistically significant. Relative error (RE) was used to evaluate the accuracy of the BMD of each ESP vertebra measured by DECT:

RE=measuredvalue−truevaluetruevalue×100%[1]

Root mean square error (RMSE) of BMD measurements in normal and obese people under different scanning conditions was calculated. The accuracy of scanning mode measurements was evaluated by RMSE as it can reflect the accuracy of the measurement very well. The RMSE is the square root of the ratio of the deviation between the measured value and the true value and the number of measurements (N), achieved with the following formula:

RMSE=∑i=1N(yi−y¯i)2N[2]

Results

Measurements under each scanning condition in the upper and lower abdomen for different obesity levels are shown in Tables 1-4, accuracy under each scanning condition in the upper and lower abdomen for different obesity levels are shown in Tables 1-4, and the RMSE of the measurements under each scanning condition in the upper and lower abdomen for different obesity levels are shown in Figure 3.

Figure 3 RMSE of BMD measurements under different scanning conditions in normal and obese populations. When simulating the abdomen of normal weight people, L4 level (fat area 240 mm², without rib influence condition), 0.8 s/rot, RMSE less than 275 mA under 190 mA condition (190 mA: L1: 3.46, L2: 2.50, L3: 2.13; 275 mA: L1: 3.81, L2: 4.07, L3: 4.76); L1–2 level (fat area 140 mm² with rib influence condition), 0.8 s/rot, small RMSE for L1 and 2 at 190 mA condition (190 mA: L1: 0.69, L2: 1.19). When simulating the abdomen of an obese population, at the L4 level (fat area 450 mm², without rib influence condition), the RMSE at 0.8 s/rot, 315 mA condition was small (315 mA: L1: 2.13, L2: 1.21, L3: 1.66); at the L1–2 level (fat area 340 mm², with rib influence condition), RMSE at 0.8 s/rot, 315 mA condition was less than those at 355 mA (315 mA: L1: 1.52, L2: 0.99, L3: 5.23; 355 mA: L1: 1.78, L2: 2.41, L3: 8.55). RMSE, root-mean-square error; BMD, bone mineral density.

Combining the measurement results and accuracy, RMSE, and radiation dose, the following results were obtained. When simulating the upper abdominal condition (fat area 140 cm² with rib influence) in normal weight people, there was no statistically significant difference in BMD measurements of each vertebra for 0.8 s/rot paired with different tube currents (P>0.05), the RE was the smallest for 0.8 s/rot, 190 mA condition (L1: −0.41%, L2: −0.34%, L3: −1.54%), and the RE was the smallest for 190 mA condition had the smallest RMSE for L1 and 2 vertebral BMD measurements (190 mA: L1: 0.69, L2: 1.19); when simulating the lower abdominal condition of a normal-weight population (240 cm² of fat area, no rib influence), there was no statistically significant difference between 190 and 275 mA conditions for measurements taken in the 0.8 s/rot, 190 and 275 mA conditions (0.8 s/rot, 190 mA, L1: P=0.17, L2: P=0.06, L3: P=0.78; 275 mA, L1: P=0.12, L2: P=0.69, L3: P=0.28), and 190 mA condition RE was less than that of 275 mA (190 mA: L1: 5.27%, L2: 2.44%, L3: −0.24%; 270 mA: L1: 6.71%, L2: 2.5%, L3: 1.73%), and the RMSE in the 190 mA condition was also less than that in the 275 mA condition (190 mA: L1: 3.46, L2: 2.50, L3: 2.13; 275 mA: L1: 3.81, L2: 4.07, L3: 4.76).

The upper abdomen condition of the simulated obese population (fat area 340 cm², affected by ribs), and there was no statistical difference between the measurement results under 0.8 s/rot, 315 and 355 mA conditions (0.8 s/rot, 315 mA, L1: P=0.11, L2: P=0.22, L3: P=0.02; 355 mA, L1: P=0.22, L2: P=0.63, L3: P=0.16), and the RE under 315 mA condition was less than 355 mA (315 mA: L1: 2.71%, L2: −0.76%, L3: −2.61%; 355 mA: L1: 2.79%, L2: 0.87%, L3: −3.66%), and RMSE under 315 mA conditions was also lower than 355 mA (315 mA: L1: 1.52, L2: 0.99, L3: 5.23; 355 mA: L1: 1.78, L2: 2.41, L3: 8.55); at the level of the lower abdomen of the simulated obese population (fat area 450 cm², no rib influence), there was no statistical difference in the measurement results under 0.8 s/rot and 315 mA conditions (L1: P=0.06, L2: P=0.05, L3: P=0.46). RE at 315 mA was small (L1: 3.75%, L2: −1.06%, L3: 0.42%), and RMSE at 315 mA was also small (L1: 2.13, L2: 1.21, L3: 1.66). With the increase of tube current, the CTDIvol value of the same tube speed increased (Figure 4).

Figure 4 CTDIvol values under different energy spectrum scanning conditions. CTDIvol, computed tomography dose index.

Discussion

Currently, QCT and DXA are commonly used to measure BMD. DXA has been widely used in many regions. However, it measures areal (2-dimensional) BMD (aBMD; g/cm²), whereas QCT measurements focus on vBMD (mg/cm³) which avoids measurement inaccuracies caused by overlapping projections (19). CT is also a commonly used imaging method for the diagnosis of osteoporosis. In practical work, QCT is mostly carried out simultaneously with clinical routine CT scans without additional radiation dose and scan time. Recently, DECT has become progressively more widespread in clinical application. DECT has the unique ability to differentiate a variety of material types, for example, HAP, based on differential X-ray attenuation at different photon energies. Li et al. (20) demonstrated that both QCT and DECT can accurately measure BMD while scanning ESP. Cui et al. indicated that the combination of 0.8 s/rot tube speed and 230 mA tube current are the optimal DECT scanning parameters, and the basic material pairs (HAP-water) can further ensure the accuracy while measuring BMD. They also showed that DECT can accurately measure vertebral BMD. However, the effect of adipose tissue on the BMD measurement was ignored in this study. In our previous study, we demonstrated that abdominal adipose tissue has a certain effect on the BMD measurement (21). A greater TAT content results in a greater BMD error, thus the accuracy of BMD is limited. Javed et al. (22) used DXA to measure the BMD of a bovine femur and found that as the fat layer around the femur increased, the BMD also gradually increased, which suggests that the absorption of photons by fat gives a spuriously high reading of BMD. Yu et al. (11) used DXA and QCT scans of ESP wrapped in different thicknesses of fat layers and found that increasing the thickness of the fat resulted in decreased QCT BMD measurements, forming measurement errors, but with less and more uniform measurement errors compared to DXA BMD measurements. DECT is a scanning method that uses high and low energy switching to measure the BMD of the vertebral body by means of material separation technique. DECT makes the composition of the vertebral body in the body membrane be completely represented by HAP and water, and therefore not susceptible to abdominal fat and other external factors. Ye et al. (23) showed that the RMSE measured by both DECT and QCT was positively correlated with the TAT and further indicated that a greater TAT area led to a greater RMSE which suppressed the accuracy of BMD. Therefore, the effect of abdominal fat content should be considered while measuring vertebral BMD using DECT and QCT in patients with severe osteoporosis in clinical practice. Therefore, this study simulated the lower abdominal by wrapping fat around the ESP. Moreover, our preclinical observation indicated that scanning range should include 4–8 ribs while measuring L1 and L2 vertebral BMD and the study showed that ribs had no significant effect on the accuracy of vertebral BMD using DECT (24). Therefore, in this study, 6 fresh porcine-isolated ribs (from the slaughterhouse) were selected and wrapped around the ESP and fat diagonally on both sides to simulate the upper TAT of people with different obesity levels. These studies suggest that the optimal scanning parameters of energy spectrum CT, which were studied by phantom as 0.8 s/rot and 230 mA, and the influence of fat and rib on BMD measurement is not taken into account, the optimal scan parameters will change. Therefore, we simulated various upper and lower abdominal phantoms and explored the optimal scanning parameters to guarantee the accuracy of lumbar spine BMD measurement.

The optimal scanning parameters in our study relate to the changes of tube current and rotation speed. We found that two key parameters of CT scanning, namely, tube current and rotation speed, were related to the radiation dose. Radiation dose increases with tube current, and the longer the tube is rotated around, the more information is collected and the higher the radiation dose. In this study, we simulated upper and lower abdominal conditions in normal and obese people and scanned the phantom using the machine’s own tube current and rotational speed.

When simulating the upper abdomen of normal weight people, taking into account the two factors of measurement value and RE, it can be seen that the measurement results of 0.8 s/rot with different tube currents are also more accurate. In terms of the optimal dose of radiation, 0.8 s/rot, 190 mA can accurately measure vBMD; in the same way, when simulating the lower abdomen of normal-weight people, 0.6 s/rot, 280 mA RE is lower than 0.8 s/rot, 190 mA, but the radiation dose of 0.8 s/rot, 190 mA is lower, so 0.8 s/rot, 190 mA can be chosen to accurately measure vBMD. In clinical practice, low radiation dose scanning without affecting the accuracy of bone density has become the focus of clinical promotion, under the premise of meeting the needs of image quality in clinical diagnosis, reducing the radiation dose of the recipient, or in the case of the radiation dose cannot be lowered, to maximize the diagnostic imaging information is the direction of clinical efforts. Therefore, we need to reduce the radiation dose while meeting the diagnostic requirements.

The increase of fat area will affect the bone density measurement results, because due to the large area of fat, the absorption attenuation curve of abdominal fat to X-ray shows a bow-back upward behavior, that is, under the condition of CT imaging, the absorption of fat to X-ray is increased with the increase of tube voltage. Ribs are bone structures with a high capacity to absorb X-rays, and thus may affect the measurement results. In contrast, DECT is a scanning method that utilizes high and low energy switching to measure the bone density of the vertebral body using a substance separation technique (12). Meanwhile, energy spectral CT puts the composition within the somatic vertebrae is completely represented by HAP and water, and therefore is less susceptible to external factors such as the rib cage. This study shows that the difference in BMD measured with low mA is statistically significant in the case of 0.8 s/rot, the accuracy and precision of BMD values measured by low mA tube current is low, but the stability is poor, and it is necessary to increase the tube current to more accurately measure the lumbar spine BMD; therefore, for the obese population, the above approach is used for comparison when simulating the conditions of the obese population’s upper and lower abdomen, and the selection of 0.8 s/rot, 315 mA can measure vertebral BMD more accurately.

In this study, the precision of energy-spectrum CT was worse than that of QCT, suggesting that current energy-spectrum CT measurements of BMD are slightly less reproducible. The CTDIvol of the GSI partial-scan protocol was higher than that of QCT. The CTDIvol of the DECT was higher than that of QCT; the 0.8 s/rot 190 mA scan protocol, with a lower radiation than that of conventional QCT, reduced the radiation dose without compromising the accuracy of BMD. Energy spectrum CT usually has a high CTDIvol, so it is recommended to use bone density as one of the post-processing methods of clinical energy spectrum CT scanning, not to scan alone.

There are some limitations to this study. First, it was performed using only DECT, and the optimal conditions obtained in this study may not be applicable to other CTs with an energy spectrum. Secondly, it considered the effect of abdominal fat on the measurement of lumbar spine bone density, but the composition of ESP is different from the actual composition of the human vertebrae; therefore, further validation is needed in additional studies. Finally, it did not elucidate the effects of tube rotation speed and tube current on the measurement of BMD by ESP CT.

Conclusions

The results of this study suggest that the use of appropriate scanning parameters allows DECT to correctly measure BMD when the BMD of the material to be measured is within the appropriate range. Taking the radiation dose into account, the condition of 0.8 s/rot 190 mA is recommended for BMD measurement in the case of people with normal weight, and 0.8 s/rot 315 mA is recommended for BMD measurement in the case of obese people.

Acknowledgments

Funding: None.

Footnote

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-23-1741/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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