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Neuroradiology |
1 From the Institutes of Clinical Radiology (B.B.E.W., R.T.H., K.H., M.F.R.) and Neuroradiology (R.B.), Klinikum Grosshadern, University of Munich, Marchioninistr 15, D-81377 Munich, Germany; and Center for Statistical Sciences, Brown University, Providence, RI (B.S., J.D.B.). Received April 7, 2003; revision requested June 24; final revision received September 17; accepted October 21. Address correspondence to B.B.E.W. (e-mail: b.ertl-wagner@t-online.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Thirty patients were investigated with a standardized CT protocol, with three groups of 10 consecutive patients examined at 80, 120, and 140 kV, respectively. Three blinded readers independently evaluated images and graded image quality parameters, diagnostic confidence, and vascular delineation of intracranial arteries and veins. Vascular CT attenuation values, CT dose indices, and dose length products were assessed quantitatively. For data analysis, a Kruskal-Wallis nonparametric rank F test was used to identify trends and variables that required modeling attention. A proportional odds multinomial regression model was then fit with generalized estimating equations to account for the correlated nature of the data.
RESULTS: Image quality was rated higher with higher kilovoltage settings (P < .001). The severity of imaging artifacts was higher with lower kilovoltage settings (P < .001), while the subjectively rated vessel contrast was lower in the 80-kV group than in the 120-kV group and the 140-kV group (P < .05). Diagnostic confidence was higher in the 120-kV group and 140-kV group (P < .005). Vascular delineation was higher with higher kilovoltage settings for most arterial and venous structures. Differences were more significant for structures in close topographic proximity to bone and for subsegmental arteries and were less significant and, in parts, not significant for the main arterial branches and the large venous sinus. Attenuation values were higher with lower kilovoltage settings (P < .05). The mean dose length product could be reduced from 594 mGy · cm in the 140-kV group to 152 mGy · cm in the 80-kV group.
CONCLUSION: This multireader study of image quality and vessel delineation with cranial multi–detector row CT angiography at various kilovoltage settings demonstrated a superiority of higher voltages with most pronounced effects for vessels adjacent to bone and subsegmental arteries.
© RSNA, 2004
Index terms: Brain, CT, 17.1211 • Computed tomography (CT), image quality, 17.1211 • Computed tomography (CT), multi–detector row, 17.1211
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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In spite of pronounced technical advances, little is known about the optimal imaging parameters for multi–detector row CT angiography of the cerebrovascular system. The aims of optimization of imaging parameters should be minimization of the radiation dose and maximization of tube use, while simultaneously preserving optimal image quality.
As early as the middle 1970s, various authors described the strong energy dependence of iodine and iodinated contrast media (6–8). In 1980, Keller et al (9) published the results of a study that used phantoms in the assessment of the contrast-to-noise ratio for various kilovoltage settings. A skull and water phantom with a plastic tube filled with 2% potassium iodine was used, and kilovoltage settings of 80–140 kV were tested. The optimum kilovoltage setting with the highest contrast-to-noise ratio at the lowest radiation dose was 80 kV.
In a more recent article, the measurement of the regional cerebral blood flow with CT perfusion analysis was compared at 80 and 120 kV, while the tube current was kept constant at 200 mAs. Both the contrast material enhancement and the contrast material between gray and white matter was significantly increased at 80 kV, while the mean radiation dose could be lowered by a factor of 2.8 (10). It is still standard practice, however, to apply higher kilovoltage settings both for contrast media–enhanced cranial CT and for CT angiography of the craniocervical vascular system.
The optimal imaging parameters for multi–detector row CT angiography of the cerebrovascular system are not clear. Older phantom studies and evaluations of the parenchymal enhancement of the brain or of other organ systems cannot simply be applied to modern multi–detector row CT angiography of the brain. CT technology has come a long way since the 1970s, when the first phantom studies were performed with the CT-1010 scanner (EMI, London, England) (7,9). Filtration processes, radiation spectrum, and beam hardening effects have drastically changed over the years. Although the general relationship of an increased opacity of iodinated contrast media at a lower kilovoltage setting is expected to hold true, the technologic evolution is still likely to have an effect on image quality at various kilovoltage settings. Moreover, the observation of an increased enhancement of the brain parenchyma or of other organ systems may not even be advantageous for CT angiography of the cerebrovascular system.
As there is a very close topographic proximity between the calcified structures of the skull base and the basal cerebral vessels, greater opacification of the vascular systems may hinder image interpretation. Multi–detector row CT now offers improved resolution, enabling the delineation of even small subsegmental vessels, which is of great diagnostic usefulness. To our knowledge, the ability of multi–detector row CT to delineate cerebral vessels at various kilovoltage settings has not been evaluated. Thus, the purpose of our study was to investigate image quality and vascular delineation on multi–detector row CT angiograms of the brain at various kilovoltage settings.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Inclusion criteria were an indication for CT angiography, as stated by both the referring physician and the attending radiologist, and the ability to provide informed consent and comply with the examination. Exclusion criteria consisted of contraindications for iodinated contrast media, such as a known allergy to iodinated contrast media or elevated thyroid function test results.
The first 10 consecutive patients underwent multi–detector row CT angiography with a kilovoltage setting of 140 kV, which used to be standard practice. The next 10 consecutive patients were examined with a reduced kilovoltage setting of 120 kV. The final 10 consecutive patients were examined with a kilovoltage setting of 80 kV. All other imaging parameters remained constant. Intermittent controls of image quality were performed while patient accrual was still active to prevent image quality from falling beyond diagnostically acceptable limits with reduced kilovoltage settings. To reach this end, a radiologist with 6 years of experience in CT angiography (B.B.E.W.) assessed image quality after every five patients were examined and determined whether image quality was sufficient for diagnostic purposes.
Of the 10 patients who underwent CT angiography at 140 kV, four were men and six were women (mean age, 61.6 years; median age, 62 years). Of the 10 patients who underwent CT angiography at 120 kV, six were men and four were women (mean age, 58.4 years; median age, 65.5 years). Of the 10 patients who underwent CT angiography at 80 kV, seven were men and three were women (mean age, 51.5 years; median age, 58.0 years). The sex and age distribution among the groups is provided in Table 1.
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Qualitative image scoring was performed independently by three staff radiologists with experience in CT angiography of the brain (R.B., 11 years; K.H., 5 years; R.T.H., 3 years). All readers were blinded to the kilovoltage setting that was used and the clinical information of the patient. They were first asked to evaluate the vascular delineation on a five-point scale. On this scale, five corresponds to a vessel or vascular segment with optimal delination in its entire length, four to a vessel or vascular segment that can be clearly identified and delineated in more than 75% of its length, three to a vessel or vascular segment that can be clearly identified and delineated in more than 50% but less than 75% of its length, two to a vessel or vascular segment that can be identified but can only be delineated in less than 50% of its length, and one to a vessel or vascular segment that cannot be identified at all. The readers were asked to rate each of the following vascular structures: C1 and C2 segments of the right and left internal carotid arteries; basilar artery; M1, M2, and M3 segments of the right and left middle cerebral artery; A1, A2, and A3 segments of the right and left anterior cerebral artery; P1, P2, and P3 segments of the right and left posterior cerebral artery; anterior and posterior communicating arteries; circle of Willis in its entirety; pericallosal artery; superior sagittal sinus; inferior sagittal sinus; confluence of sinuses; internal cerebral veins; and right and left transverse sinus and cavernous sinus.
All readers were also asked to rate their certainty of diagnosis on a five-point scale. On this scale, five corresponds to a full and confident certainty of diagnosis based on the results of CT angiography alone; four to a good certainty of diagnosis based on the results of CT angiography alone—however, additional imaging would increase the certainty of diagnosis; three to an adequate certainty of diagnosis based on the results of CT angiography alone—however, additional imaging would be desirable; two to a marginal certainty of diagnosis based on the results of CT angiography alone—however, additional imaging would be required to establish the diagnosis; and one to a situation in which the diagnosis is uncertain based on the results of CT angiography alone. The readers were also asked to note if they would request an additional imaging study to improve their certainty of diagnosis.
Final diagnoses were established with a combination of the full clinical history of the patient, clinical follow-up, and a consensus reading by two radiologists. These radiologists were not involved in the scoring as readers and were familiar with the full clinical history and follow-up of the patient. They were asked to review all images and establish a final diagnosis in consensus. Consensus was achieved among the radiologists in all instances, which rendered the involvement of a third radiologist unnecessary.
Each reader was additionally asked to rate overall image quality on a five-point scale. On this scale, five corresponds to excellent image quality, four to good image quality, three to adequate image quality, two to marginally acceptable image quality, and one to unacceptable image quality. Moreover, each reader was asked to subjectively evaluate vessel contrast on a five-point scale. On this scale, five corresponds to excellent vessel contrast, four to good vessel contrast, three to adequate vessel contrast, two to marginally acceptable vessel contrast, and one to unacceptable vessel contrast.
The readers were also asked to assess image artifacts on a five-point scale. On this scale, five corresponds to complete absence of imaging artifacts; four to mild artifacts not interfering with diagnostic decision making; three to moderate artifacts slightly interfering with diagnostic decision making; two to pronounced artifacts interfering with diagnostic decision making—however, it is still possible to arrive at a diagnosis; and one to a situation in which artifacts completely hinder diagnostic decision making. Every reader assessed a total of 38 qualitative data points (27 data points for arterial structures, seven for venous structures, and four for image quality parameters) for each patient.
In addition, a quantitative image analysis was performed. Regions of interest (ROIs) were placed in consensus by two radiologists in the following vessels: C1 segment of the right and left internal carotid arteries, basilar artery, M1 segment of the right and left middle cerebral artery, superior sagittal sinus, confluence of sinuses, and right and left transverse sinus. Moreover, an additional ROI with an approximate area of 2 cm2 (mean ± SD, 1.98 cm2 ± 0.05; range, 1.91–2.07 cm2) was placed in the white matter of the left side of the centrum semiovale and in the background noise. Attenuation values were measured in Hounsfield units and assessed for each ROI.
Additionally, the CT dose index and the dose length product were assessed for each examination. The effective dose (Doseeff) was estimated from the dose length product according to the method of Hidajat et al (11) with the following equation: Doseeff = dose length product x 0.0023.
Statistical Analysis
Comparisons of vascular delineation and image quality parameters across the three kilovoltage settings, as graded by each of three blinded readers on a five-point scale, were performed in a two-stage process. First, data were approximated as being continuous, and an average grade was produced for each patient at each kilovoltage setting by calculating the average score of the three readers. The Kruskal-Wallis nonparametric rank F test was then used to test the null hypothesis that the mean grade was the same across the three kilovoltage settings. An advantage of the Kruskal-Wallis nonparametric rank F test is that no distributional assumptions are required for the data. Furthermore, the slightly conservative nature of the test allows us to narrow the large number of vessels and image parameters down to those likely exhibiting true differences among the three kilovoltage settings.
This first stage was used to determine which vessels exhibited significant differences in vessel delineation and which image quality parameters exhibited significant differences in quality among the three kilovoltage settings. In the second stage, the significant differences mentioned previously were analyzed more closely with a proportional odds multinomial regression model for ordinal data. All observations for each study corresponded to the three readers and were used in fitting the model. Generalized estimating equations were used to adjust all standard errors for the correlation existing between grades given by different readers for the same study. The key advantage of the proportional odds multinomial regression model is that it allows one to take advantage of the ordinal nature of the data; wherein, we are modeling cumulative logits of the probability of a response while adjusting for explanatory variables. The assumption made is that the influence of explanatory variables, which in this case are the kilovoltage settings, is independent of the cut point for the cumulative logit. For example, when comparing one kilovoltage setting with another, the odds ratio for the probability of a grade of five versus the probability of a grade of less than five is the same as the odds ratio for the probability of a grade of more than or equal to three versus the probability of a grade of less than three.
The null hypothesis of equality of parameter estimates from the proportional odds multinomial regression model among the three kilovoltage settings was assessed with a score test with two degrees of freedom, which resulted from the generalized estimating equations. Also, odds ratios and 95% CIs for each pair wise comparison of kilovoltage settings were then calculated and reported.
Comparisons of the quantitative assessment of vascular opacification, which was measured in Hounsfield units, among the various ROI for the three kilovoltage settings were made with a one-way analysis of variance model, assuming normally distributed error terms. Least square means were calculated for each of the three kilovoltage settings, and P values for formal contrasts of images in the 80-kV group versus images in the 120-kV group, images in the 80-kV group versus images in the 140-kV group, and images in the 120-kV group versus images in the 140-kV group were calculated and reported.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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Image quality parameters were rated in the respective groups (Table 2). The readers graded overall image quality as an average of 4.0 in the 80-kV group, 4.4 in the 120-kV group, and 4.8 in the 140-kV group. Differences were significant between all groups, with a P value of less than .001—as calculated with the Kruskal-Wallis nonparametric rank F test—and a P value of less than .001—as calculated with the proportional odds multinomial regression model with generalized estimating equations. The estimated odds ratios were 5.66 (95% CI: 1.91, 16.80) between the 120-kV group and the 80-kV group, 32.38 (95% CI: 9.95, 105.32) between the 140-kV group and the 80-kV group, and 5.72 (95% CI: 2.57, 12.73) between the 120-kV group and the 140-kV group. For example, these odds ratios indicate that the odds of obtaining a more favorable grade with the 120-kV setting are 5.66 times greater than with the 80-kV setting. Thus, one is more likely to get a more favorable grade of image quality by using the higher kilovoltage setting.
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Vessel contrast was rated by the readers to be an average of 4.4 in the 80-kV group, an average of 4.8 in the 120-kV group, and an average of 4.8 in the 140-kV group. Differences were significant between the 120-kV group and 80-kV group and between the 140-kV group and 80-kV group with a P value of less than .01, as calculated with the Kruskal-Wallis nonparametric rank F test and a P value of less than .05, as calculated with the proportional odds multinomial regression model. Differences were not significant between the 120-kV group and 140-kV group. The estimated odds ratios were 6.69 (95% CI: 2.17, 20.66) between the 120-kV group and the 80-kV group and 6.69 (95% CI: 2.17, 20.66) between the 140-kV group and the 80-kV group.
Readers rated their certainty of diagnosis to be an average of 4.4 in the 80-kV group, an average of 4.5 in the 120-kV group, and an average of 4.9 in the 140-kV group. Differences were significant between the 140-kV group and the 80-kV group and between the 140-kV group and the 120-kV group with a P value of .002, as calculated with the Kruskal-Wallis nonparametric rank F test and a P value of .003, as calculated with the proportional odds multinomial regression model. Differences were not significant between the 120-kV group and the 80-kV group. The estimated odds ratios were 10.17 (95% CI: 3.31, 31.26) between the 140-kV group and the 80-kV group and 6.23 (95% CI: 1.86, 20.87) between the 140-kV group and the 120-kV group. Additional imaging studies were requested by the majority of readers in two patients in the 80-kV group, in three patients in the 120-kV group, and in no patients in the 140-kV group.
Table 3 summarizes the statistical analyses of reader ratings of the various intracranial vascular structures. Average reader ratings were higher in the 140-kV group than in the 120-kV group and in the 120-kV group than in the 80-kV group. Differences were significant for the Kruskal-Wallis nonparametric rank F test, with a P value of less than .01 for the following vessels or vascular segments: bilateral transverse sinus; cavernous sinus; confluence of sinuses; basilar artery; bilateral C1 and C2 segments of the carotid artery; left M1, M2, and M3 segments and right M2 and M3 segments of the middle cerebral artery; left P2 and P3 segments and right P1, P2, and P3 segments of the posterior cerebral artery; bilateral A2 and A3 segments of the anterior cerebral artery; anterior and posterior communicating arteries; and the circle of Willis. Differences were not significant and the odds ratio model was not fitted accordingly for the following vascular structures: superior and inferior sagittal sinus, internal cerebral veins, right M1 and left P1 segments of the middle cerebral artery and posterior cerebral artery, A1 segments of the anterior cerebral artery, and the pericallosal artery.
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Moreover, differences between the groups were more pronounced for the smaller vascular structures. Odds ratios tended to be higher for the A3, M3, and P3 segments than for the A2, M2, and P2 segments. Odds ratios also tended to be higher for the A2, M2, and P2 segments than for the A1, M1, and P1 segments (Table 3). Generally, the differences between the groups were less pronounced for the venous structures than for the arterial vascular structures, with the exception of the cavernous sinus.
Figures 1–3 show coronal and sagittal reconstructions of multi–detector row CT angiograms of the brain at kilovoltage settings of 140, 120, and 80 kV, respectively. The vessels in proximity to the skull base and the small subsegmental arteries are discerned less clearly at lower kilovoltage settings.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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As expected, we have found significantly higher quantitative attenuation values with ROI analysis at the lower kilovoltage settings; however, this increase in attenuation did not lead to an increase in image quality or produce improved vessel delineation. Overall image quality was rated to be significantly higher with the higher kilovoltage settings, while the presence of image artifacts was reduced. Certainty of diagnosis also improved when the tube voltage was increased to 140 kV.
Interestingly, the readers subjectively rated the vascular contrast to be better in the 120- and 140-kV groups than in the 80-kV group, even though absolute attenuation values have demonstrated vascular opacification and vessel-to-parenchyma contrast to be significantly higher with the lower kilovoltage setting. This is in accordance with an observation made in a phantom study with different densities of contrast media, in which an optimal attenuation of iodinated contrast medium of 150–200 HU was found, and an increased error of vascular measurements was described for vessel attenuations of less than 100 HU or more than 250 HU (17). Moreover, the attenuation of the surrounding structures (ie, the brain parenchyma or the skull base) is also increased at lower kilovoltage settings, which leads to reduced contrast between the vessel and its surroundings.
Thus, with CT angiography, higher attenuation-to-noise ratios do not appear to inevitably be better—especially when the close proximity of osseous and vascular structures in the intracranial situation are taken into account. Our readers have given lower ratings for the vascular delineation for most intracranial vascular structures at lower kilovoltage settings. These differences were most pronounced for vessels in close topographic proximity to the complex osseous structures of the skull base; therefore, vascular delineation was particularly reduced for the carotid arteries—especially for the C2 segments—and for the cavernous sinus at lower kilovoltage settings. This may be mostly due to an increased beam attenuation both for iodine and for calcium at the lower kilovoltage settings, which obscures the exact boundaries between osseous and vascular structures (16).
Moreover, reader ratings of the delineation of smaller vascular structures such as the A3, M3, and P3 segments of the intracranial arteries were also reduced at lower tube voltages. This is most likely due to increased noise at lower kilovoltage settings. Investigators that previously assessed the delineation of osseous structures at lower kilovoltage settings demonstrated increased noise and reduced delineation of the ossicles of the middle ear at lower tube voltages (18,19). Differences were less pronounced and in parts were not significant for larger vascular structures not directly adjacent to the skull base, such as the large venous sinus and the A1, M1, and P1 segments of the intracranial arteries. Doses were higher with higher kilovoltage settings, though a factor of up to 3.9 was observed between the 80-kV group and the 140-kV group. Thus, it should be differentiated whether the higher dose is really justified by the indication for CT angiography. If a cerebral venous thrombosis or an obstruction of the large intracranial segments of the arterial vessel are to be excluded, CT angiography with 80 kV and a lower radiation dose appears to be sufficient. If the intraosseous portion of the carotid arteries or the cavernous sinus are the ROIs or if small segments of the intracranial arteries are sought, however, a higher radiation dose seems to be justified to reach optimal image quality.
There are some limitations to our study that need to be taken into account. First, we examined different groups of patients with various kilovoltage settings while keeping all other parameters constant. It would have been scientifically desirable to examine the same patient group with all three different kilovoltage settings; however, this seems unethical, since the radiation dose for each patient would be tripled. Thus, a study design was chosen and approved by the local institutional review board in which consecutive patients were assigned to the various protocols. We chose to include consecutive patients and not to randomize them to achieve a reduction in patient dose in the course of the study and to guarantee a prompt intervention, should image quality fall beyond reasonable limits with a lowered dose.
The study was started with the 140-kV setting, which used to be standard practice in the department, and was consecutively lowered to 120 kV and 80 kV, respectively. Intermittent control of image quality was performed by the study group prior to formal data assessment while patient accrual was still active. As a consequence of this consecutive design, we had a rather large disparity in the ages of the three groups, with a younger median age in the 80-kV group as compared with the 120-kV group and the 140-kV group. We would not expect this disparity to be a confounding variable, however, as we would not anticipate younger patients to have poorer vascular delineation. In the case of confounding variables, we would have expected the opposite to happen, in that the group of older patients who underwent imaging with the higher kilovoltage setting would have poorer vascular delineation.
Second, even though the distribution of diagnoses is fairly even among the groups, there is no complete match regarding diagnoses and demographic data. The majority of patients in each group had normal findings. We did not aim to assess diagnostic accuracy, but rather to evaluate image quality parameters and vascular delineation. If diagnostic accuracy was to be assessed, a different study design with a reference standard would need to be chosen, and larger patient numbers would need to be examined in each group. Thus, unnecessarily high patient exposure or insufficient image quality would be risked. Our aim was to assess image quality and vascular delineation with a limited number of patients in a structured multireader study to assist in choosing an optimal tube voltage for intracranial CT angiography with multi–detector row CT.
In conclusion, in our multireader study of image quality and vessel delineation of cranial multi–detector row CT angiography at various kilovoltage settings, we found a superiority of higher voltages with most pronounced effects for vessels adjacent to bone and subsegmental arteries. Smaller or insignificant differences regarding the effect of kilovoltage settings were found for the large venous sinus and for the large segments of the intracranial arteries. For general use, we recommend use of 120 kV or 140 kV for CT angiography of the intracranial vessels. In some patients, a voltage of 80 kV may suffice, though, if only images of the large venous sinus and the large segments of the intracranial arteries are sought. If vascular structures near the skull base or small segments of intracranial arteries are to be assessed, however, a higher kilovoltage setting may be warranted.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, B.B.E.W., R.T.H.; study concepts, B.B.E.W., R.T.H., R.B.; study design, B.B.E.W., B.S., J.D.B.; literature research, B.B.E.W., R.T.H.; clinical studies, B.B.E.W., R.T.H., R.B., K.H.; data acquisition, B.B.E.W., R.T.H., R.B., K.H.; data analysis/interpretation, B.B.E.W., B.S., J.D.G.; statistical analysis, B.S., J.D.B.; manuscript preparation, B.B.E.W.; manuscript definition of intellectual content, B.B.E.W., R.B., J.D.B., M.F.R.; manuscript editing, B.B.E.W.; manuscript revision/review, B.B.E.W., R.B., B.S., J.D.B., M.F.R.; manuscript final version approval, all authors
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