Objective Evaluation of Cerebral Venous Sinus Attenuation on Plain CT Brain and Detecting Anemia. Noticing the “Unnoticed”
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.323896
Source of Support: None, Conflict of Interest: None
Keywords: Computed tomography, hematocrit, hemoglobin, Hounsfield unit, superior sagittal sinusKey Messages: Objective attenuation values of dural sinuses on plain CT can be positively correlated with Hb and HCT values, detecting anemia with 100% specificity considering the cut off of 35.5 HU
Non-contrast computed tomography (NCCT) of the head is still a chosen imaging modality in an emergency setting for patients presenting with various neurological symptoms. It gains popularity due to its wide availability and cost-effectiveness. We routinely notice that there is attenuation variability in the dural venous sinuses. Many a time, there is misperception regarding sinus attenuation which is confused with either thrombosis or anemia especially in an acute setting where no immediate laboratory investigations are available. Objective measurement of venous sinus would be more helpful rather than just visual perception. There had been suggested correlation of the hematocrit (HCT)/hemoglobin (Hb) values and the CT attenuation of the intravascular blood in the cerebral venous sinuses, however, there is limited evidence to establish a potential association between the two. The role of CT as an effective means of diagnosing intracranial hemorrhage has been studied which helps diagnose many intracranial pathologies. However, CT attenuation values being utilized for diagnosing or correlating hemoconcentration-related pathologies such as anemia, polycythemia, and thrombosis have been less explored., The ability to confidently categorize anemia, thrombosis, or polycythemia from the CT attenuation value of the sinuses might influence patient management while the laboratory investigations are awaited. There is no substantial data providing a linear relationship between the HCT/Hb values and sinus attenuation on CT. Thus, our main objective was to determine the objective correlation between CT attenuation of the cerebral venous sinus and Hb as well as HCT values. The secondary objective was to detect anemia from measuring venous sinus attenuation.
It is an exploratory study conducted in a tertiary care hospital from January 2015 to December 2015.
Inclusion criteria: All patients who underwent plain CT scan with no structural abnormalities and had HB and HCT levels obtained within 24 h of the scan.
Exclusion criteria: Contrast received intravenously within 24 h of the scan, trauma, hemorrhage, intracranial surgery or any other intracranial pathology, and artifacts obscuring the normal visualization of venous confluence.
The above criteria led to the inclusion of 200 patients, who had undergone plain CT of the head and whose laboratory records were also evaluated.
All the patients had a plain CT scan on Philips brilliance 64-slice CT scanner according to the institutional protocol with scan parameters varying slightly as 80–150 mA, 80–120 kV, slice thickness 5 mm (reconstruction images slice thickness—1 mm) pitch, 0.675.
As a routine protocol, calibration of Tube and detectors was being done in Fast Cals as a mandatory procedure which was necessary for obtaining accurate reproducible quantitative analysis.
Moreover, calibration was also done at the time of replacement of detector, tube, and collimator using:
The images were reviewed and after identification of venous sinus confluence (torcular Herophili) and inferior aspect of the superior sagittal sinus (SSS), an axial section was selected which showed minimal beam hardening artifact. For measuring the CT attenuation, the best-fit circle (region of interest) was placed in the venous sinus confluence or inferior aspect of SSS as shown in [Figure 1]. The average attenuation of the sinus was measured and recorded in all the patients using computer software for each scan. Hounsfield unit (HU) calibration was cross-checked by performing Air and Water phantom scanning.
Hb and HCT values from the complete blood count (CBC) results were obtained from the clinical records of each patient from the hospital database. Patients who had the results of CBC done within 24 h before or after plain CT were included in this study. In patients, whose records showed multiple CBC results done within that 24 h period, the results with the shortest time interval from scan time were recorded.
Descriptive statistics were used to describe the data, frequency, and percentage for categorical variables and mean values with standard deviations for quantitative variables. Two-tailed unpaired t-test was used to test the difference between two independent samples. Correlation and regression analyses were used to assess the correlation between two quantitative variables. The analysis was carried out via STATA 14.
Out of the 200 study participants, there were 106 males and 94 females. The mean (SD) age was 41.8 (17.5) years (range: 1–80 years).
A correlation matrix [Table 1] represents the correlation coefficient for all individual variables. There was a significant correlation observed between Hb-HU and HCT-HU. Age was not correlated with Hb, HCT, and HU.
There was no statistically significant difference in age between males and females. But, Hb, HCT, and HU showed a significant difference between these groups [Table 2] and [Table 3].
The simple linear regression model revealed that HU (P value < 0.001) was significantly correlated with Hb [Figure 2] and a regression model was, Hb = 2.1 + 0.2 × HU. Similarly, HU (P value < 0.001) was significantly correlated with HCT [Figure 3] and regression model was HCT = 6.2 + 0.7 × HU
There were 33 anemic patients out of 200. The mean age was 43.6 years (range 1–80) and mean Hb value of 9.4 g (range 3.2–10.9) [Table 4].
Receiver-operating characteristics (ROC) curve analysis of HU shows an area under the curve 0.85 with HU cutoff 35.5 with the specificity of 100% for the detection of anemia.
Plain CT of the brain is an important first-line investigation of choice in an acute neurologic setup. It is not only important to identify what is seen on the scan but also the concealed areas like the confluence of venous sinuses. Unless clinically suspected areas often go unnoticed due to their location and varied appearance. We routinely do not measure HU from venous sinuses, and even if we measure, we do not label patients as anemic or polycythemic based on HU values. There are no clear-cut guidelines for the measurement of HCT based on HU values. Patients with higher HU values can be either polycythemic or thrombotic and, therefore, further studies are needed to confirm the diagnosis. There are previous studies that have showed a significant correlation of Hb values with CT attenuation of the blood in aorta. The same can be applied in the intracranial cerebral sinuses where many times patients are referred directly for head CT while laboratory investigations are awaited. There are specific signs for the diagnosis of sinus venous thrombosis, with a reported sensitivity of unenhanced CT of 73%. However, there are interpretation difficulties causing false-positive and false-negative diagnoses affecting patient management. Diagnosing anemia, thrombosis, or polycythemia correlating the CT attenuation values might prove to be fruitful in early patient management especially in an emergency scenario where the mode of varied clinical presentations causes a clinical dilemma. Moreover, excluding them with confidence is also more challenging when these are clinically suspected.
Fanous et al. suggested that due to the complexity of the dural sinus it is difficult to show a strong correlation between CT attenuation and Hb levels and, therefore, if the attenuations are measured at multiple places and at larger areas, then the mean values would be more accurate. We had taken our measurements with a best-fit circle region of interest at the torcular Herophili inferior aspect of superior sagittal sinus which consists of a larger area, and thus, permitting reproducible attenuation measurements.
In the current study, we demonstrate the correlation of objective attenuation values of the sinus to HCT (Hb) and provide a regression equation to predict the Hb and HCT of a patient from the attenuation HU values of the intracerebral venous sinus.
In our study, the attenuation values ranged from 26.6 to 65.78 HU with a mean of 48.9 HU as compared to Lee et al. which showed a range of 24.67–53.67 HU with a mean of 43.28. Whereas, Black et al. showed a range of 32–69 HU.
The substantial association of Hb and HCT with blood attenuation values has been established by various studies,,,, which were also reflected strongly in our study showing a linear relationship between the aforementioned variables. Al-Ryalat further showed a stronger linear correlation with Hb than HCT. However, we found a stronger correlation of HU values with HCT as compared to Hb.
The highest attenuation value observed in our study was 65.78 HU in a male patient aged 18 years with Hb of 18.1 g/dL and HCT of 54.4. Our study had 33 patients having Hb less than 11 g/dL which is considered as anemia according to WHO classification. Considering the cutoff of 35.5 HU, we were able to show the specificity of 100% for the detection of anemia. Patients with anemia showed a mean of 41.3 HU attenuation. The correlation of plain CT scan and anemia has been studied by a few authors utilizing attenuation values in the left ventricle of the heart., Fanous et al. in their study of non-enhanced CT of the brain in superior sagittal sinus gave a cutoff of 35 HU for detection of anemia with a specificity of 100% which was observed in our study as well.
Black DF et al. had suggested that HU greater than 70 is to be considered suspicious for venous sinus thrombosis. Whereas, Buyck suggested a cutoff of 62 HU with sensitivity and specificity of 95%. Considering both the values for our study, we had two patients with attenuation values more than 62 and none with attenuation values more than 66 HU. Thus, we can hypothesis that considering 70 HU as the upper limit will reduce the number of false positives and would strongly predict cerebral venous and sinus thrombosis.
In yet another study of Besachio, HU values more than 65 showed a sensitivity of 84% and specificity of 96% in identifying thrombosis. They also showed that considering a threshold of 69 HU will reduce sensitivity to 64% but specificity increased to 100%.
Our study did not show any statistically significant difference in age between males and females. However, the mean of HB, HCT, and HU showed a significant difference between these groups supporting gender discrepancy suggested by Al-Ryalat et al. We also observed that the mean attenuation value was higher in men (50 HU) when compared to women (47.8 HU).
Al-Ryalat et al. in their study showed a decrease in the HCT and Hb levels with age and increase in baseline attenuation on NCCT due to an age-related increase in blood viscosity. Thus, sinus attenuation in the elderly has to be judiciously considered as it may mask the thrombus in clinically suspected cases and therefore further confirmatory studies would be necessary. In our study, we could not establish such a correlation as the elderly population was very less.
Bruni et al. suggested that an increase in average torcular attenuation value (HU) by 1 increases the Hb level by 1.63 g/L (r=0.463; P < 0.001) for their entire study group. However, the correlation of these values was found to be stronger in male patients than in females. Thus, predictive HU value for anemia was developed only for men in their study. In contrast to their study, we were able to correlate both the genders and develop a regression equation predicting HCT and HB, thus, identifying anemia from CT attenuation values of the intracerebral venous sinuses. Zhou et al., in a recent study, concluded that CT attenuation of difference between the interventricular septum and left ventricle cavity could predict the severity of anemia with HU value 13.5 the sensitivity and specificity for diagnosing severe anemia was good in both the genders (94.7% and 83.6% in male; 82.4% and 84.6% in female).
We measured HU values only once for all our patients. We do not know the hydration status of these patients. Moreover, most of our patients were on an outpatient department basis. This was one of the limitations of the study.
Our study had some limitations as a generalization of the findings may be limited due to the study design and therefore other confounding factors influencing hemoconcentration were overlooked. The data evaluation suggests that the upper limit of normal sinus attenuation is 65 HU. This study was not conducted to assess thrombosis from venous sinus attenuation values. Thus, the predictive value in diagnosing sinus venous thrombosis can only be hypothesized from the present study.
In conclusion, objective attenuation values of dural sinuses on plain CT can be positively correlated with Hb and HCT values, which would help to determine normal as well as pathological entities like anemia. We can predict anemia by considering the cutoff of 35.5 HU in the venous sinus confluence with 100% specificity. Assessing the dural venous sinuses using CT attenuation values can be used by a radiologist to correlate diverse clinical scenarios as and when suspected, thus, helping clinicians in prompt treatment and further plan of management of such patients especially in an acute setting.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]