A basic study for the molecular imaging of dual-energy CT in diagnosing anterior cruciate ligament injury of knee joint

Abstract

Background

Anterior cruciate ligament (ACL) injury is a common disease in clinical practice that seriously affects the daily life of patients.

Purpose

To explore the molecular imaging basis of “diminution sign on dual-energy colour mapping” for the diagnosis of ACL injury by dual-energy computed tomography (DECT).

Material and Methods

The hydroxylysine and hydroxyproline reagents were prepared in different concentrations. The grouping was shown as follows: a simple concentration change group of an amino acid (group 1/2); a mixed solution group with the concentration increasing synchronously (group 3); a mixed solution group with the concentration reverse increasing and decreasing (group 4); and a mixed solution group that fix one amino acid with increasing concentration of the other (group 5/6). The samples were scanned by DECT. The solution CT value and image signal-to-noise ratio were analyzed.

Results

In group 1/2, the brightness of the dual-energy color mapping of each test tube solution and the CT value increased with increasing the concentration of amino acid. In group 6, there was no significant change in the brightness and brilliance of the dual-energy color mapping and the CT value. The remaining three groups showed an increase in the brightness and brilliance of the dual-energy color mapping and the CT value, and this increase was positively associated with the hydroxylysine concentration.

Conclusion

The dual-energy staining of the DECT imaging in “tendon” mode is related to hydroxylysine and hydroxyproline. Moreover, the degree of dual-energy color mapping is positively correlated with the change of CT value.

Keywords

Computed tomography dual-energy anterior cruciate ligament ligament injury hydroxylysine hydroxyproline

Introduction

As a common condition clinically, anterior cruciate ligament (ACL) injury seriously affects the everyday life of patients (1,2). Clinical physical examination is often based on the subjective perception of patients, which may affect the accuracy of the diagnosis. Currently, magnetic resonance imaging (MRI) is the preferred imaging method for ACL injury, with good tissue contrast and high accuracy (3 –5). However, MRI brings about many contraindications. Patients suffering from claustrophobia and those with non-diamagnetic cardiac pacemakers are not suitable candidates for MRI examination (6). A safe and accurate imaging method is needed to serve as a supplement to MRI to meet the special clinical needs.

The dual-energy computed tomography (DECT) technique makes up for the shortcomings of traditional CT in soft tissue imaging (6,7). Relevant research shows that dual-source DECT imaging can not only clearly display the cruciate ligament morphology, alignment, and edge conditions in two and three dimensions, but also allow detailed observation in multiple directions and angles (8). Johnson et al. (9) published an article in European Radiology in 2007, pointing out that collagen differentiation may be related to the densely packed hydroxylysine and hydroxyproline in the side chains of the collagen molecule. Briefly speaking, DECT makes it possible to display tendons and ligaments, makes up for insufficient resolution of soft tissue in traditional CT examination, and allows CT to be applied for the diagnosis of ACL injury. Fickert (10) and Peltola et al. (11) also successively reported the diagnosis of a cruciate ligament injury on piglet animal models and for patients using DECT. Our previous study (12) found a new potential sign of DECT in diagnosing ACL injury, i.e. diminution sign-on dual-energy color mapping, which preliminarily proved the feasibility of this method. In addition, Johnson et al. concluded how “the dense packed hydroxylysine and hydroxyproline in the side chain of collagen molecules in tendon, ligament and other structures have obvious attenuation difference for X-rays with different energies.” However, a detailed basis for such a conclusion has not been provided. Compared with other clinical applications of DECT, few new conclusions were supplemented in subsequent studies on DECT ligament imaging, and most of studies about DECT ligament imaging cited Johnson's theory. As there were no further in-depth research and reports on imaging theory, it remained unclear whether this theoretical basis also applies to the “diminution sign-on dual-energy color mapping.” All in all, the “diminution sign-on dual-energy color mapping” is caused by the decrease and loss of dense-packed hydroxylysine and hydroxyproline components in the collagen molecule side chain of the damaged ligament. However, the weights or roles of these two amino acids in the DECT image remain unclear; besides, it is also not clear whether these two amino acids function together or one amino acid serves as the main amino acid and the other as the auxiliary amino acid. Accordingly, the aim of the present study was to answer the above questions through basic experimental research.

Material and Methods

Reagents

Reagents used were DL-5-hydroxylysine hydrochloride (cas: 13204-98-3; Sigma Co., H0377-1G) with a solubility of 357.8 mg/mL and L-hydroxyproline (cas: 51-35-4; Ron Co., R000723-20 mg) with a solubility of 50 mg/mL .

Preparation and grouping of solution

The above reagents were respectively prepared into saturated solutions dissolved in ultra-pure water according to their solubility. The treated reagents were then diluted in corresponding proportions with every 25% as a concentration difference to prepare hydroxylysine and hydroxyproline solutions with 25%, 50%, 75%, and 100% concentrations. The solutions were then divided into group 1 and 2 (a pure gradient concentration solution of hydroxylysine or hydroxyproline), group 3 (a mixed solution of the two concentrations increasing synchronously), group 4 (a mixed solution of the two concentrations increasing and decreasing reversely), and group 5 and 6 (a mixture with increasing hydroxylysine or hydroxyproline concentration and the equivalent 50% hydroxyproline or hydroxylysine) (Table 1). The above solution was placed in a small 1.5-mL test tube, and a small 1.5-mL test tube of pure water was set as a blank control.

Table 1.

Grouping of solutions.

Group
Group 1 (1.5 mL per tube, 1341.75 mg A)
	25% A	50% A	75% A	100% A
	89.45 mg/mL A	178.9 mg/mL A	268.35 mg/mL A	357.8 mg/mL A
Group 2 (1.5 mL per tube, 187.5 mg B)
	25% B	50% B	75% B	100% B
	12.5 mg/mL B	25 mg/mL B	37.5 mg/mL B	50 mg/mL B
Group 3 (1.5 mL per tube, 670.875 mg A and 93.75 mg B)
	25% A + 25% B	50% A + 50% B	75% A + 75% B	100% A + 100% B
	89.45 mg/mL A + 12.5 mg/mL B	178.9 mg/mL A + 25 mg/mL B	268.35 mg/mL A + 37.5 mg/mL B	357.8 mg/mL A + 50 mg/mL B
Group 4 (1.5 mL per tube, 670.875 mg A and 93.75 mg B)
	25% A + 100% B	50% A + 75% B	75% A + 50% B	100% A + 25% B
	89.45 mg/mL A + 50 mg/mL B	178.9 mg/mL A + 37.5 mg/mL B	268.35 mg/mL A + 25 mg/mL B	357.8 mg/mL A + 12.5 mg/mL B
Group 5 (1.5 mL per tube, 670.875 mg A and 75 mg B)
	25% A + 50% B	50% A + 50% B	75% A + 50% B	100% A + 50% B
	89.45 mg/mL A + 25 mg/mL B	178.9 mg/mL A + 25 mg/mL B	268.35 mg/mL A + 25 mg/mL B	357.8 mg/mL A + 25 mg/mL B
Group 6 (1.5 mL per tube, 536.7 mg A and 93.75 mg B)
	50% A + 25% B	50% A + 50% B	50% A + 75% B	50% A + 100% B
	178.9 mg/mL A + 12.5 mg/mL B	178.9 mg/mL A + 25 mg/mL B	178.9 mg/mL A + 37.5 mg/mL B	178.9 mg/mL A + 50 mg/mL B

A, hydroxylysine; B, hydroxyproline.

Scanning method

A dual-source CT scanner (Somatom Definition 2012B; Siemens Healthineers, Erlangen, Germany) (Fig. 1) in dual-energy mode was used for scanning. The tube current intensities of bulbs A and B were set as 29 and 123 mAs, respectively. The tube voltages were set at 140 and 80 kV, respectively. The collimation was 20 × 0.6 mm at a pitch of 0.7. The reconstruction slice thickness was 1 mm, the reconstruction increment was 0.7 mm, and the field of view was 250 mm. The kernel was D30s medium smooth convolution with filtered back projection, and the window was Extremity. Each solution group was scanned three times, and a tube of purified water was supplemented as a blank control during scanning.

Image postprocessing

All images were transmitted to the image postprocessing workstation (syngo Multimodality Workplace, VA40; Siemens Healthineers), and dual-energy color mapping was performed using the dedicated dual-energy tendon application. The information was color-coded with a specific color lookup table (“tendon,” which codes tendons and ligaments in shades of yellow to orange). Image reconstruction was conducted along the coronal position of the test tube to show the solution under color mapping (Fig. 2).

Image analysis

A region of interest (ROI) was placed in the middle of each test tube where the color mapping was relatively uniform. The CT value and standard deviation (SD) of each ROI were measured. The SD value of the measurement point was defined as noise (N), and the most effective layer that could simultaneously display the color mapping of all test tubes to measure the ROI was selected (Fig. 3). CT value was adopted to evaluate the density difference of test tube solution of each concentration in each group. The image signal-to-noise ratio (SNR) was considered as the objective evaluation standard of image quality, and the SNR of each ROI was calculated according to the formula of SNR = CT value/SD. The average value of the measurement results after three scans was recorded as the final numerical result.

Statistical analysis

SPSS version 23.0 software (IBM Corp., Armonk, NY, USA) was adopted for statistical analysis and data processing. The image data of the standard-dose group and the low-dose group were expressed as mean ± SD, and the comparison was carried out by t-test. The test standard was α = 0.05, and P < 0.05 indicated a statistically significant difference.

Results

Solution situation of pure hydroxylysine concentration change group (group 1)

Group 1 showed bright dual-energy color mapping in each test tube solution in a concentration-dependent manner (from left to right); however, as shown in Fig. 4, the normal purified water control test tube (the leftmost test tube) had practically no dual-energy color mapping. In addition, the CT value of the solution scanned six times exhibited a positive correlation with hydroxylysine concentration (Table 2).

Table 2.

Measurement of CT value of solutions of various concentrations (group 1).

	CT value
	Scan 1	Scan 2	Scan 3
Tube 1	29.9 ± 2.2	31.6 ± 4.0	31.9 ± 3.2
Tube 2	70.8 ± 1.4	72.3 ± 2.2	70.1 ± 2.2
Tube 3	105.4 ± 2.5	106.1 ± 2.5	105.7 ± 2.0
Tube 4	132.8 ± 2.3	134.0 ± 2.2	134.4 ± 2.4

Values are given as mean ± SD.

The solution of simple hydroxyproline concentration change group (group 2)

The dual-energy color mapping of each test tube solution in group 2 was bright, and the gorgeous degree increased with the concentration. Nevertheless, little dual-energy color mapping could be observed in the corresponding control test tube (Fig. 5). In addition, the CT values of the six scanning solutions all presented an increasing trend with the hydroxyproline concentration, the CT values of hydroxyproline solution were close to those of purified water, and the conversion amplitude of CT values of different concentrations was not as good as that of the hydroxylysine solution (Table 3).

Table 3.

Measurement of CT value of each concentration solution (group 2).

	CT value
	Scan 1	Scan 2	Scan 3
Tube 1	5.4 ± 1.9	7.0 ± 2.4	5.1 ± 2.2
Tube 2	6.7 ± 2.3	7.5 ± 2.1	7.7 ± 2.0
Tube 3	7.3 ± 2.5	8.0 ± 2.4	10.6 ± 3.2
Tube 4	9.3 ± 1.5	9.8 ± 3.3	13.3 ± 3.0

Values are given as mean ± SD.

The solution of hydroxylysine and hydroxyproline concentration increasing synchronously group (group 3)

In group 3, the dual-energy color mapping of each test tube solution was bright, and the bright degree raised with the concentration of the two amino acids; while the purified water control test tube showed little dual-energy color mapping (Fig. 6). Moreover, there was a positive association between the CT value of the solution scanned six times and the concentration of the two amino acids (Table 4).

Table 4.

Measurement of CT value of each concentration solution (group 3).

	CT value
	Scan 1	Scan 2	Scan 3
Tube 1	18.1 ± 1.9	16.9 ± 2.9	18.9 ± 2.3
Tube 2	39.5 ± 2.9	33.8 ± 3.3	39.0 ± 3.2
Tube 3	58.3 ± 3.8	59.3 ± 4.3	58.2 ± 3.0
Tube 4	74.1 ± 2.6	74.6 ± 3.6	73.4 ± 4.1

Values are given as mean ± SD.

The solution in the group of reverse increase and decrease of hydroxylysine and hydroxyproline concentration (an increase of hydroxylysine and decrease of hydroxyproline, group 4)

Fig. 7 revealed bright dual-energy color mapping in each test tube solution in group 4 and indicated positive association of the brightness with the concentration of hydroxylysine. As for the control test tube, almost no dual-energy color mapping could be observed (Fig. 7). In addition, the CT value of the solution scanned six times exhibited an increasing trend with the hydroxylysine concentration (Table 5).

Table 5.

Measurement of CT value and SD of each concentration solution (group 4).

	CT value
	Scan 1	Scan 2	Scan 3
Tube 1	18.4 ± 2.2	22.5 ± 3.1	21.4 ± 2.1
Tube 2	36.5 ± 4.2	36.0 ± 3.2	39.7 ± 3.0
Tube 3	53.6 ± 4.1	54.8 ± 4.3	55.0 ± 3.2
Tube 4	70.0 ± 5.0	73.5 ± 4.3	69.2 ± 2.5

Values are given as mean ± SD.

Mixed solution group with increasing hydroxylysine concentration and fixed hydroxyproline concentration (group 5)

Bright dual-energy color mapping of each test tube solution could be observed in group 5, and the degree of the brightness was upregulated with the hydroxylysine concentration; while the control sample almost did not show any dual-energy color mapping (Fig. 8). Furthermore, Table 6 indicated a positive correlation of the CT value of the solution after six times of scanning with the concentration of hydroxylysine.

Table 6.

Measurement of CT value of each concentration solution (group 5).

	CT value
	Scan 1	Scan 2	Scan 3
Tube 1	18.6 ± 1.7	16.6 ± 2.2	20.2 ± 2.1
Tube 2	37.4 ± 2.8	37.6 ± 2.2	39.2 ± 1.6
Tube 3	53.9 ± 3.9	54.7 ± 3.4	53.7 ± 4.4
Tube 4	68.9 ± 3.9	68.5 ± 4.6	68.7 ± 6.8

Values are given as mean ± SD.

Mixed solution groups with increasing hydroxyproline concentration and fixed hydroxylysine concentration (group 6)

The brightness of dual-energy color mapping of solution was changed little in each test tube in group 6; similarly, there was almost no dual-energy color mapping in the control test tube (Fig. 9). Besides, despite no longer presenting a trend increased with the hydroxyproline concentration, the CT values of the solution after scanning six times were still stable in an interval (Table 7).

Table 7.

Measurement of CT value of each concentration solution (group 6).

	CT value
	Scan 1	Scan 2	Scan 3
Tube 1	38.8 ± 3.6	38.3 ± 3.3	41.7 ± 2.0
Tube 2	38.0 ± 2.3	36.5 ± 2.9	38.0 ± 3.9
Tube 3	40.6 ± 4.6	38.3 ± 3.2	40.1 ± 3.6
Tube 4	32.2 ± 4.6	32.7 ± 6.1	35.7 ± 3.0

Values are given as mean ± SD.

Discussion

The application of DECT in musculoskeletal system imaging mainly includes gout, bone marrow edema, metal artifact removal, and tendon. Compared with the first three clinical applications, the clinical application research of dual-energy tendon is not extensive or in depth (8,13 –15). The principle of dual-energy tendon essentially refers to the identification of collagen, fat, and soft tissue based on Compton scattering and energy dependence of photoelectric absorption effect. For each chemical element, the energy dependence ratio of the absorption cross-section of the two processes is different. Briefly speaking, DECT is sensitive to the chemical composition of objects. In DECT, two original datasets are reconstructed respectively under the voltage energy of two tubes. In addition, each voxel of different energy levels can be assigned two different CT numbers due to the 140-kV and 80-kV spectra. In addition, each CT number pair is represented by a point in a coordinate system defined by CT numbers generated at each specific tube voltage (16 –18). Therefore, the differentiation of different tissues such as collagen and fat is possible in DECT. Since the emergence of the clinical application of tendon, most of the studies only focused on imaging feasibility and proved better soft tissue resolution of DECT imaging than traditional multi-slice spiral CT. On the one hand, DECT imaging can display the shape, course, and edge of the cruciate ligament through the two-dimensional and three-dimensional stereo; on the other hand, DECT imaging also allows multi-directional and multi-angle detailed observation (9,19,20). Still, over the past decade, few studies have focused on the application of DECT imaging in the evaluation of soft tissue injury of the knee joint. Peltola et al. (11) claimed good sensitivity (79%) and high specificity (100%) of DECT as a practical clinical method to evaluate the complete tear of the ACL in patients with acute knee joint injury. Specifically, the inter- and intra-observer proportions of the agreement for ACL tear were excellent or good and the coincidence rate of DECT and MRI in ACL tear was high. However, their study used the GE Discovery CT750 HD (GE Healthcare). They deemed that the collagen-specific color mapping scheme did not seem to be used to evaluate the ACL, posterior cruciate ligament, and popliteal tendon. Gemstone spectral imaging (GSI) with a single energy spectrum of 40–140 keV is usually better than dual-energy bone removal image in evaluating the injury of anterior and posterior cruciate ligament and popliteal tendon. According to some reports, the best soft tissue contrast can be obtained by adjusting the mono-energy spectrum image obtained by the mono-energy spectrum curve, thereby gaining the greatest value for the diagnosis of ligament injury. Glazebrook et al. (21) published a case-control study that included 16 patients with ACL rupture. In their study, a dual-source CT scanner by Siemens was used to scan the participants, and DECT mixed kV image, DECT bone removal image, and DECT tendon-specific color image were applied to diagnose and separate the injury of the ACL. They believed the oblique sagittal position to be the best section for identifying the ACL. Their research results confirmed the high accuracy and consistency of DECT in detecting complete ACL tear in patients with chronic and subacute knee joint trauma. However, they believed that the ligament's color mapping image was not as good as a mixed kV image or dual-energy bone removal image in identifying ACL due to the fan-shaped scattered morphological structure of the ACL. In their papers, there was no color presenting proximally at the tear site, and the potential value of this phenomenon was not understood in diagnosing ACL injury. Fickert et al. (10) carried out in vitro experiments on porcine hind legs. The results showed that the sensitivity and specificity of DECT in diagnosing complete ACL tear were 75% and 68.8%, respectively (the sensitivity and specificity of corresponding MRI were 100% and 75%). In brief, MRI showed higher sensitivity while lower specificity than DECT for a partial tear of the anterior medial bundle of ACL; as for partial laceration of a posterolateral bundle of ACL, DECT was more sensitive but less specific than MRI. Furthermore, MRI had a better positive predictive value (PPV) and negative predictive value (NPV) of a complete ACL tear than DECT. In addition, MRI exhibited better PPV and NPV than DECT when the anterior medial bundle of the ACL was partially torn. Still, DECT had better PPV and NPV than MRI in a partial tear of a posterolateral bundle of the ACL. In their study, dual-source CT was also used for scanning, and pseudo-color coding was carried out on the study ligament in dual-energy “tendon” mode using the corresponding postprocessing workstation. The ligament color mapping was described as a “yellow ribbon,” and the continuity of the color-coded ligament was regarded as the reference standard for diagnosis. The two studies conducted by Glazebrook et al. and Fickert et al. were consistent with the imaging equipment and image postprocessing methods we used in our earlier studies. They also discovered the missing of the dyeing continuity of damaged ligaments. These findings were also consistent with our studies. Nevertheless, the research by Glazebrook et al. did not describe the significance of this phenomenon (the missing of the dyeing continuity of damaged ligaments) as they did not think it had clinical significance. Even in their opinions, this phenomenon affected the normal color-coded of ligaments, thereby reducing the diagnostic efficiency of ligament color mapping. In contrast, the research by Fickert et al. took this phenomenon as one of the criteria for the diagnosis and evaluation of ACL injury. In the present study, the color mapping of ligament injury ligaments was not only the interruption of continuity but also the reduction of staining degree, and we named it “diminution sign-on dual-energy color mapping.”

In spite of a difference in the final viewpoints, the images, descriptions, or data displayed by the above studies highly suggest the potential ability of “diminution sign-on dual-energy color mapping” to diagnose an ACL injury. However, the reason for the loss of dual-energy color mapping, i.e. the decrease of dual-energy dyeing, remains unclear. Johnson et al. (9) regarded the principle of DECT ligament imaging as obvious attenuation differences of dense-packed hydroxylysine and hydroxyproline in the side chain of collagen molecules in ligament components under X-rays with different energies. Therefore, the tissue characteristic image reflecting the histochemical composition can be obtained using the difference of CT value changes corresponding to soft tissues under different dual-energy X-rays. Almost all subsequent DECT ligament imaging studies have agreed with the above view, without any other disputes. Accordingly, we made an educated guess that the decrease of dual-energetic ligament color mapping is due to the loss of dense-packed hydroxylysine and hydroxyproline components in the side chain of collagen molecule after a ligament injury. The analysis results of our research data were shown as follows: (i) the degree of dual-energy color mapping varied with the hydroxylysine or hydroxyproline concentration (accompanied by a positive correlation relationship), while purified water as a blank control had almost no color coding on account of the deletion of hydroxylysine or hydroxyproline; (ii) when hydroxylysine and hydroxyproline were mixed according to the same concentration change trend, the degree of dual-energy color mapping was also positively correlated with the changes of these two amino acids, i.e. when the two amino acids were mixed according to the increasing concentration, the degree of dual-energy color mapping also showed an increasing trend; (iii) when the concentration of hydroxyproline in the mixed solution was constant and the concentration of hydroxylysine was increased, an increasing trend could be observed in the degree of dual-energy color mapping; (iv) when the concentration of hydroxyproline in the mixed solution was increased while the concentration of hydroxylysine was unchanged, there was no obvious change in the degree of dual-energy color mapping; (v) when the concentration of two amino acids in the mixed solution changed in reverse direction, the change trend of dual-energy color mapping degree was consistent with the change of hydroxylysine concentration; and (vi) the change of the concentration of the two amino acids was also accompanied by the change of CT value, and its change trend was consistent with the trend of dual-energy color mapping. We believe the above findings can explain the following points: (i) color mapping (i.e. dual-energy staining) in “tendon” mode after DECT imaging is related to hydroxylysine and hydroxyproline, which is consistent with the research by Johnson et al. In addition, DECT ligament imaging is based on the obvious attenuation difference between dense-packed hydroxylysine and hydroxyproline in collagen molecular side chains in ligament components to X-rays with different energies. Further, the tissue characteristic image that can reflect histochemical components can be acquired by using the difference of CT value changes corresponding to soft tissues under X-rays with different energies; (ii) the change of the concentration of hydroxyproline and/or hydroxylysine affects the degree of dual-energy color mapping. When the concentration of the two amino acids up-regulates, the degree of dual-energy color mapping also correspondingly increases. We believe that this discovery can confirm the molecular basis of “diminution sign-on dual-energy color mapping.” In short, ligament injury destroys the integrity of the ligament, reduces the content of these two amino acids in ligament collagen to some degrees, and then causes “diminution sign-on dual-energy color mapping” after DECT imaging; (iii) the roles (or weights) of the two amino acids in dual-energy color mapping are different. Hydroxylysine has a major role in dual-energy color mapping, followed by hydroxyproline; the degree of dual-energy color mapping depends more on the change of hydroxylysine content; and (iv) there is a positive association between dual-energy color mapping and CT value. Specifically, the change of dual-energy color mapping degree is accompanied by the change of CT value, i.e. after ligament injury, the degree of dual-energy color mapping decreases, and the CT value of the damaged ligament also decreases. The above outcome is consistent with some previous research results.

The present study has some limitations. First, the size of the sample was small. Briefly, the amino acid solution was added in a small 1.5-mL test tube for scanning and postprocessing analysis; the small test tube was more vulnerable to interference from other substances such as air outside the test tube, thereby affecting the accuracy of image postprocessing analysis. If a large test tube is used, the analysis results may be more convincing. Second, two in vitro pure amino acid reagent solutions were used as research objects to deduce their situation in vivo. However, some other components may also have a positive or negative effect on these two amino acids because hydroxylysine and hydroxyproline are not the only components in ligament collagen. All in all, further studies need to be performed to address the above limitations.

In conclusion, the color mapping (dual-energy staining) of the DECT imaging in “tendon” mode is related to hydroxylysine and hydroxyproline. The changes of the concentration of hydroxylysine and/or hydroxyproline affect the degree of dual-energy color mapping. In other words, there is a positive correlation between the hydroxylysine and/or hydroxyproline concentration and dual-energy color mapping degree. Furthermore, the two amino acids play different roles in dual-energy color mapping. Hydroxylysine plays the main role in dual-energy color mapping, while hydroxyproline plays the minor role. The degree of dual-energy color mapping depends more on the change of hydroxylysine content. The change of dual-energy color mapping degree is accompanied by the change of CT value, and they are also positively correlated.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Rui Bai

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