Background

Cerebrovascular ischemic events are the third leading cause of death after acute myocardial infarction and cancer and represent the second cause of disability in the Western world [1, 2]. A significant portion of the ischemic events have their cause in the embolization occurred from the carotid artery [3, 4].

In the past years it was thought that the stenosis degree was the leading parameter related to the patient’s risk to develop cerebrovascular events [5] but in the last decade several researches have demonstrated that the plaque morphology and characteristics play a fundamental role in the development of embolic cascade [6, 7]. Therefore identification of carotid artery features that are associated with plaque vulnerability is extremely important in order to correctly stratify the risk of occurrence of cerebrovascular events [8, 9] that’s why several imaging techniques are trying to identify those characteristics [10, 11].

Ultrasonography (US) is nowadays considered the first-line exam [12] but in most of centres Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) are required to confirm or to rule out the presence of pathological stenosis and\or plaque’s characteristics related to the vulnerability [13, 14]. MRI offers, other than the potentiality of quantify the degree of stenosis, the opportunity to identify the presence of intra-plaque-hemorrhage (IPH), lipid-rich necrotic core (LRNC) as well as the status of the fibrous cap [15, 16].

In the assessment of the carotid arteries, CT is a frequently used technique because of the wide availability, the rapid time execution and its potentiality to identify some features related to the plaque vulnerability [17–19]. In the CT assessment of the carotid plaque the analysis of the attenuation value is a fundamental parameter in order to classify the type of the tissue that composes the plaque [20].

The purpose of this paper is to assess the intra-plaque attenuation variability in order to verify the potential reproducibility of HU measurements.

Methods

Study design and patient population

In this retrospective study, 68 consecutive patients (males 42, females 16; median age 64 ± 11 years, age range 44–82 years) that underwent CTA between January 2012 and May 2012 were included. The IRB approval was obtained and because of the retrospective nature of the analysis patient’s consent was waived.

In our Hospital CTA is performed after ultrasonography that is used as exam to rule-out significant atherosclerotic disease of the supra-aortic vessels. In particular CTA is performed when a) carotid sonography shows a pathological stenosis (>50% according to the NASCET criteria [21]) or plaque’s features related to plaque vulnerability (such as ulcers of irregular luminal morphology) b) sonography cannot adequately assess degree of stenosis and plaque’s characteristics because of anatomical conditions.

CTA technique

None of the patients included in this research had any contraindications to iodinated contrast media. CTA was performed using a 16-multi-detector row CT system (Philips Brilliance, Eindhoven, Netherlands). Acquisitions were performed from the aortic arch to the circle of Willis before and after administration of contrast medium. Bolus tracking technique was used in all cases. The ROI trigger threshold was placed into the aortic arch and six seconds after the beginning of the i.v. administration of 80 ml of pre-warmed contrast medium (Ultravist 370; Schering, Berlin, Germany) into median cubital vein (flow rate of 5 ml/s) the monitor scanning began. The trigger threshold was set at + 80 HU above the baseline. Four seconds after having reached the threshold the angiographic phase began. CT technical parameters were: matrix 512×512, field of view 14–19 cm; slice thickness 0.6 mm, interval 0.3 mm, 180–220 mAs; 120–140 kV. C-filter algorithm of reconstruction was applied.

Plaque analysis

The window parameters (center/level) was set at W850:L300 according to Saba et al [21] and in the first phase the observer (XX, 12 years of experience in vascular imaging) analysed the dataset acquired after administration of contrast material. The carotid artery plaque was identified and for each slice 4 different circular or elliptical ROIs were traced. The 4 ROI were exactly the same in each slice and their dimension was variable ≥ 1 and ≤ 2 mm2 in the different slices. When small cluster of calcium were identified, these are avoided in the ROI tracings. We considered 2 main exclusion criteria 1) calcified plaques 2) presence of artefacts (movement or dental artefacts). In the second phase the basal scan were assessed. The observers identified the same slice correspondent to the angiographic phase and the same ROIs (for locations and dimension) used in the angiographic phase were applied in the slices (Fig. 1).

Fig. 1
figure 1

In panel a and b the right carotid plaque of a 61 years old male patient is given. The 4 ROI are visible in the pre contrast (a) and post-contrast scan (b). In panel c and d the left carotid plaque of a 68 years old female patient is given. The 4 ROI are visible in the pre contrast (c) and post-contrast scan (d). The white arrow indicates the lumen of the carotid artery

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At the end of the analysis the ROI were grouped according to their HU value from the lower to the higher values and ROI 1 represents the lower HU values whereas ROI 4 the higher.

Statistical analysis

In this study the normality of each continuous variable group was tested using the Kolmogorov-Smirnov Z test and appropriate tests for Gaussian or non-Gaussian values were selected. For Gaussian values, continuous data were described as the mean value ± SD whereas for non-gaussian values median values were given. Wilcoxon test was applied to test the difference of attenuation of the ROIs traced before and after administration of contrast material whereas the Mann-Whitney test was used to test the differences between the ROI. A p value < 0.05 was regarded to indicate statistical significance association and all values were calculated using a two-tailed significance level. R software (www.r-project.org) was employed for statistical analyses.

Results

Of the 136 carotid arteries that were imaged with CTA, sixteen carotid arteries were excluded because no evidence of plaque was found. Other 41 carotid arteries were excluded because their plaque was heavily calcified and other 12 because of the presence of artefacts (movements n = 6 and dental artefacts n = 6). The remaining 67 carotid artery plaques were analysed and 192 slices were found where it was possible to trace 4 ROIs with area between 1 and 2 mm2.

In the Table 1 are summarized the results of the attenuation values of the four ROIs in the basal and post-contrast phase. The average value of attenuation before contrast medium was 41.591 HU (SD 8.1) and 54.159 (SD 15.7) after its administration. The average value of the ROI used was 1.532 mm2 (SD 0.163).

Table 1 Summary values of attenuation (measured in HU) of the ROIs before and after administration of contrast medium (with the Standard Deviation)
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By comparing the attenuation values among the four different ROIs (summarized in the Table 2) we found that in the basal scan no statistic differences among the four ROIs was found whereas in the scan performed after contrast material in most of cases there were difference among the ROI values. Standard deviation analysis was also performed (summarized in the Table 3). In the Fig. 2 is given a box-plot that shows the attenuation values of the ROI and the Standard Deviations before and after administration of contrast material.

Table 2 Mann-Whitney analysis of the ROI attenuation values (p values are given)
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Table 3 Mann-Whitney analysis of the Standard Deviation of ROI attenuation values (p values are given)
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Fig. 2
figure 2

The box-plot of the ROIs values obtained before and after contras-medium (a) and the Standard Deviation (b)

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Wilcoxon analysis was used to assess the difference of attenuation before and after administration of contrast material. By comparing the overall attenuation of the ROI of the plaques before and after administration of contrast medium we found a statistically significant difference (p value = 0.001).

Discussion

Attenuation analysis of carotid artery plaque represents an important criterion for the identification of plaque’s vulnerability because some types of tissue (fatty ones) are associated with an increased risk [22] of cerebrovascular events whereas others (calcified tissues) are associated with a reduced risk [23]. In this study our purpose was to assess the intra-plaque attenuation variability in order to verify the potential reproducibility of HU measurements and to assess the polymorphism of the plaque.

Our analysis relies to previous histopathological studies that have been demonstrated that multiple type of tissues are present in the carotid artery plaques [24, 25] and that this features can be visible also in the CT images.

We excluded from the ROI tracings those areas where small calcium deposits or cluster of calcium were visible and we considered as exclusion criteria for a slice the presence of heavily calcified plaques (that determines HU values usually very high up to 800–1000). Nowadays it is extremely important to obtain information that allows to identify and distinguish the fatty plaque (components) from the mixed plaques (components).

Moreover the structure of the carotid artery plaque, when big calcification are present, is characterized by the peripheral location of the calcium components with the lipid components close to the intima layer and it is this part of the plaque that may evolve to rupture and embolization.

By comparing the attenuation values among the four different ROIs the Mann-Whitney test did not show statistical differences among the four ROIs in the basal scan, whereas in the scan performed after contrast material administration in most cases there were difference among ROI values. This is an interesting point because shows 2 different types of information: 1) in the basal scan there are some differences (Table 1) but these are not statistically significant 2) After contrast material the differences between the ROI are statistically significant.

These results indicate that ROI sampling for the HU assessment of the carotid artery plaque in the scan acquired after administration of contrast material can produce heterogeneous results according to the position of the ROI in the plaque and that the values are statistically different in the different part of the plaque. Moreover this heterogeneity is mainly due to the effect of the contrast material, as demonstrated by the fact that in the basal scan these differences are not significant whereas after contrast medium become statistically significant. Therefore, the difference of attenuation identified in the ROI may represent a difference of contrast material acquisition in different areas of the plaque. These results are in line with previously published papers that have demonstrated how carotid artery plaques shows enhancement after administration of contrast material [26–29]. Therefore, the attenuation value of the carotid artery plaque after administration of contrast material seems to represent not only the plaque type but also the amount of contrast material that enters in some areas of the plaques that is mainly due to the neovascularization or to the rupture of the fibrous cap [9, 30]. This approach suggests that for the HU quantification of the tissue type the only pre-contrast CT should be performed and not the one after contrast material. This finding is concordant with a the study results of a paper published by Park et al [12] in 2012 where the authors demonstrated in a small cohort of carotid arteries (n = 43) that is the attenuation value obtained in he pre-contrast scan that is associated with the cerebrovascular symptoms whereas the attenuation values obtained after contrast medium do not show statistically significant association with the cerebrovascular symptoms. Further analysis are needed to assess if the basal scan may be helpful to stratify the risk to develop cerebrovascular events.

We also assessed, using the Wilcoxon analysis, the global variation of attenuation of the plaque’s ROI before and after contrast medium (basal value = 41.591 HU; contrast value = 54.159) and we found a statistically significant difference (p value = 0.001).

Conclusion

In conclusion, results of our study suggest that the ROI sampling performed in the CT dataset acquired after administration of contrast medium show significant degree of heterogeneity and that the ROI have a statistically significant differences. This effect may be driven by a different amount of contrast acquisition in some areas of the carotid artery plaque.