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Thoracic Imaging |
1 From the Department of Radiology, University Hospital of Vienna, Vienna, Austria. Received February 2, 2004; revision requested April 12; final revision received December 26, 2005; accepted February 2, 2006; final version accepted, February 6. Address correspondence to M.P., University Medical Center Utrecht, Department of Radiology, Heidelberglaan 100, Utrecht NL-3508 GA, the Netherlands (e-mail: m.prokop{at}azu.nl).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Materials and Methods: This retrospective study had institutional review board approval; informed consent was waived. A 100-kVp protocol (volume CT dose index [CTDIvol], 3.4 mGy) was compared with a standard 140-kVp protocol (CTDIvol, 10.4 mGy) in two groups that were each composed of 35 consecutive patients who were suspected of having pulmonary embolism (PE) and scanned with otherwise identical acquisition parameters and contrast material injection protocols. Mean main pulmonary artery enhancement and maximum enhancement in peripheral pulmonary arteries were compared. In a blinded evaluation, the percentages of segmental and subsegmental arteries that were considered analyzable for assessment of PE were determined. Overall image quality and delineation of various anatomic areas were subjectively assessed. Comparison of percentages of analyzable segmental and subsegmental arteries and subjective grading of image quality between the two different protocols were performed with the Mann-Whitney U test.
Results: There were 38 male and 24 female patients (mean age, 61 years; range, 17–86 years) in the final evaluation. There was a significantly higher average CT number in the main pulmonary artery (379 HU ± 95) for the 100-kVp protocol than for the 140-kVp protocol (268 HU ± 63, P < .001, two-sided t test). Maximum CT numbers in peripheral pulmonary arteries at the level of the aortic arch and lung bases, respectively, were 290 HU ± 91 and 279 HU ± 100 for 100 kVp and 185 HU ± 65 and 144 HU ± 63 for 140 kVp (P < .001). Mean percentage of subsegmental arteries considered analyzable per patient was higher for 100 kVp than for 140 kVp (segmental arteries, 92% vs 88%, P = .13; subsegmental arteries, 71% vs 55%, P < .001). Subjective grading of overall image quality and of the delineation of structures in the lungs, mediastinum, and upper abdomen did not significantly differ between protocols.
Conclusion: At reduced radiation exposure, low kilovoltage scanning increases the percentage of central and peripheral pulmonary arteries that can be evaluated with CT angiography without a substantial decrease in image quality.
© RSNA, 2006
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Low kilovoltage scanning has been suggested as a technique to improve contrast enhancement, as well as a technique for radiation dose reduction (9–12). This technique has been applied in angiography and better matches the effective energy of the x-ray beam to the maximum absorption close to the k-edge of iodine. Low kilovoltage techniques substantially increase the relative attenuation (in CT numbers) of contrast material and, thus, vascular enhancement at CT. At the same time, the absolute amount of x-ray attenuation grows as well. As a consequence, increasing body size will disproportionately increase image noise with low kilovoltage techniques compared with high kilovoltage techniques. This is a substantial problem in the abdomen but—because of less-attenuating tissue—much less of a problem in the lungs. The chest is therefore an anatomic region that should be well suited for the use of low kilovoltage techniques.
CT angiography of the pulmonary arteries appears to represent an appropriate model system for the evaluation of low kilovoltage techniques because of a well-circumscribed primary imaging task (detection of PE) and the fact that suboptimum enhancement of the pulmonary arteries directly affects image evaluation. Image quality can be relatively easily quantified by assessing the ability to evaluate small pulmonary arteries. Compared with multidetector CT, single-detector CT suffers much more from partial volume effects that decrease the apparent attenuation of small vessels and make reliable evaluation of small or peripheral vessels much more difficult. In these conditions, low kilovoltage techniques should be particularly advantageous.
The increase in vascular attenuation (signal) at low kilovoltage settings should allow an appropriate increase in image noise without affecting the signal-to-noise ratio compared with higher kilovoltage settings. This should offer the potential for dose reduction with low kilovoltage techniques.
The purpose of this study was to retrospectively compare a low kilovoltage scanning protocol with a reduced radiation dose with a standard high kilovoltage, moderate-dose protocol for the depiction of central and peripheral pulmonary arteries at single-detector spiral CT.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Patients
We included a total of 70 patients suspected of having acute PE in this analysis. The first group of 35 patients (group A) were the last patients who were scanned with our previous standard 140-kVp protocol, while the second group of 35 patients (group B) were the first 35 patients to be scanned after we changed our CT protocol to a low-dose 100-kVp protocol. The selection was based on consecutive patients for each group. All patients' cases were routine clinical cases attended to during day and night shifts. Both study groups included inpatients and outpatients. Patients were referred to us by the emergency unit (outpatients), the intensive care unit (inpatients), and the department of internal medicine (inpatients). Apart from kilovoltage and tube current–time product settings, identical scanning and contrast agent injection parameters were prescribed. Seven of the spiral CT studies had to be excluded because they were performed with a different contrast injection protocol (ie, a flow rate of 4 rather than 3 mL/sec). One patient did not receive enough contrast material owing to technical problems. As a consequence, 62 CT examinations were included in this comparison study (30 patients in group A underwent scanning with the standard 140-kVp protocol, and 32 patients in group B underwent scanning with the low-dose 100-kVp protocol). In both groups, sex, age, and body weight were obtained from patient records (C.S.).
Image Acquisition
Spiral CT studies were performed by using a single-detector CT scanner (Tomoscan AVE; Philips, Best, the Netherlands). All patients underwent scanning in a caudocranial direction from the level of the diaphragm to the lung apices in a supine position within a single breath hold. The scans were obtained with 3-mm collimation, a table feed of 5 mm (pitch = 1.67), and an increment of 2 mm.
A mechanical injector (Angiomat 3000 CT; Liebel Flarsheim, Hazelwood, Mo) was used for intravenous bolus injection of iodinated contrast material (iopromide, Ultravist; Schering, Berlin, Germany) that had a concentration of 300 mg iodine per milliliter. We injected 140 mL of contrast material at a flow rate of 3 mL/sec and a fixed start delay of 20 seconds (13).
Exposure parameters varied between groups A and B. For the standard protocol in group A, we used 140 kVp and 175 mAs, while patients in group B were scanned with a low-dose, low kilovoltage protocol at 100 kVp and 125 mAs. The tube current–time product value was chosen so that the volume CT dose index (CTDIvol) with protocol B was as close as possible to one-third of the CTDIvol of protocol A. Using data published by ImPACT (http://www.impactscan.org), we determined that the CTDIvol was 10.4 mGy for group A and 3.4 mGy for group B. The scan length was recorded for each patient (C.S.) and amounted to a mean of 21 cm ± 2.5, with a range of 17–25 cm. The mean effective dose calculated for group A by using the ImPACT website was 2.8 mSv for male and 3.1 mSv for female patients. In group B, the effective dose was 0.9 mSv for male and 1.0 mSv for female patients.
Image Evaluation
Images were stored in a picture archiving and communication system (PACS) for clinical interpretation. For this study, images were analyzed at a personal computer–based PACS workstation (Agfa, Mortsel, Belgium) and were evaluated in a cine mode that allowed for interactive mouse-controlled scrolling through the data set. Images of both patient groups were mixed and presented to the observers in random order. All patient and scan data were switched off during viewing. Two chest radiologists (M.P., C.M.S.) with more than 10 years of experience performed consensus interpretation of the CT images.
Measurements of vascular attenuation and image noise were performed in one session (C.S.), and evaluation of vascular enhancement, image quality, and radiologic findings was performed in a separate session (M.P., C.M.S.). We used the following objective and subjective measures.
Quantification of vascular attenuation.—To evaluate attenuation in the central pulmonary arteries, we measured the mean CT number (in Hounsfield units) in the main pulmonary artery by using a region of interest of at least 1 cm2 (range, 1.1–1.7 cm2). We evaluated the attenuation of peripheral pulmonary arteries close to the beginning and the end of each scan in a segmental or subsegmental artery at an apical and a basal section position. For the apical section position, we chose a section in the range between the lower and the upper levels of the aortic arch. Correspondingly, for the basal section position, we chose a section between the inferior pulmonary veins and just above the diaphragm. Care was taken that the section being measured had the fewest breathing and pulsation artifacts within the chosen anatomic range. Because the caliber of the peripheral vessels was too small to reliably set an intraluminal region of interest to determine the mean CT number, we chose the maximum CT number as a proxy for vascular attenuation. This attenuation value was determined by reducing the window width to a one-bit display (width = 1 HU) and then varying the window level until no pulmonary artery cross sections could be seen. The recorded value was the maximum attenuation in any of the peripheral vessels in the selected section.
Quantification of image noise.—Image noise was objectively quantified by measuring the standard deviation of CT numbers in a homogeneous region of interest (size, >1 cm2; range, 1.1–1.7 cm2) that was free of motion or contrast material–induced artifacts and was located in the main pulmonary artery.
Analysis of segmental and subsegmental arteries.—Evaluation was based on transverse images only. We used a CT angiography window setting (width, 450 HU; level, 100 HU) and a lung window setting (width, 1500 HU; level, –500 HU) for this analysis (14).
We used the standard nomenclature outlined by Boyden (15) and Jackson and Huber (16) to identify segmental arteries. This nomenclature assigns 10 segmental arteries on the right and nine segmental arteries on the left. The medial and anterior segments of the left lower lobe originating from a common trunk were considered as one segmental branch, and the apical and the posterior segmental arteries of the left upper lobe were considered as two different branches. Thus, a total of 19 segmental arteries could be expected per patient if no anatomic variant was present. For the subsegmental arteries, we refrained from using the standard nomenclature because anatomic variations are quite common at this level (17). Instead, we determined the number of subsegmental arteries individually for each segment.
Identification of the segmental and subsegmental bronchoarterial bundle in lung window settings was used to differentiate peripheral arteries from veins. To be rated as analyzable, an artery had to be displayed in the CT angiography window setting with a level of contrast enhancement that was considered high enough to enable identification or ruling out of a pulmonary embolus. Segmental or subsegmental vessels that contained emboli according to the criteria defined by Remy-Jardin and coauthors (18) were thus rated as analyzable. Segmental and subsegmental vessels that showed no intravascular filling defects were only counted as analyzable if they could be depicted from their origin to the next dichotomous branching. If arteries were considered nonanalyzable, other reasons apart from lack of contrast enhancement were noted, including cardiac and respiratory motion artifacts, presence of anatomic variants, and pathologic abnormalities. For each patient, we then calculated the percentage of analyzable segmental and subsegmental arteries.
In addition, other abnormalities such as pleural effusion, mediastinal abnormalities, and pulmonary consolidations were recorded.
Subjective evaluation of image quality.—Image quality for making a diagnosis other than PE was subjectively scored by using a five-point scale ranging from 1 to 5 in which a score of 1 corresponded to bad image quality that precluded making a diagnosis; a score of 2, to low image quality that degraded confidence in making the diagnosis; a score of 3, to moderate image quality that was still considered sufficient for diagnosis; a score of 4, to good image quality; and a score of 5, to excellent image quality that enabled excellent differentiation of even small structures (Table). Image quality was assessed separately for selected anatomic areas such as the mediastinum, the lungs, the main pulmonary artery itself at the level of the bifurcation, the liver parenchyma, and the extrahepatic upper abdomen as displayed on the most caudal section acquired. Overall image quality was assessed by calculating the average of four of the five quality scores per data set (excluding the upper abdomen). For this subjective evaluation of image quality, the observers could individually adjust the soft-tissue window settings to compensate for increased attenuation within the pulmonary arteries at low kilovoltage settings.
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Statistical Analysis
Statistical analysis was performed with commercially available software (SPSS, version 11.5; SPSS, Chicago, Ill). P < .05 was considered to indicate a statistically significant difference.
To rule out significant differences in the distribution of age and body mass index in the two study groups, we used the Student t test after testing the normal distribution of data with the Kolmogorov-Smirnov test. The significance of differences in the sex distribution and the presence of PE in the two study groups was evaluated by using a 
2 test.
A two-sided t test was applied to determine whether pulmonary arterial enhancement showed significant differences between the two protocols. This was separately tested for the CT numbers determined in the central pulmonary arteries (main pulmonary artery) and those determined in the peripheral arteries (subsegmental level) located at the lung apex and at the lung base.
The Mann-Whitney U test was used to test the significance of differences for the percentage of analyzable segmental and subsegmental pulmonary arteries and the significance of differences for the measured image noise between the two scan protocols. The Mann-Whitney U test was also used to compare the subjective grading of image quality in the five anatomic areas and the overall image quality score.
The 2 test was applied to assess whether the distribution of the subjective grading of respiratory motion artifacts, image noise, and the degree of contrast enhancement was significantly different for the two protocols.
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RESULTS |
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2 test). There was also no statistically significant difference for the distribution of body mass index (mean body mass index in group A was 27.8 kg/m2 ± 4.7 [range, 24.2–38 kg/m2], while mean body mass index in group B was 27.4 kg/m2 ± 6.1 [range, 20.4–43 kg/m2]) between the two study groups (P = .371, t test).
Two patients in group A and six patients in group B were referred from the emergency unit for evaluation of PE, whereas only one patient in group A was referred from the intensive care unit. The remaining patients were inpatients who were suspected of having acute PE and who were referred to our department from internal medicine departments.
Quantification of Vascular Enhancement
Both central pulmonary arteries (measured in the main pulmonary artery) and peripheral arteries showed significantly higher attenuation with the 100-kVp protocol than with the standard 140-kVp protocol (Figs 1–3). All differences were significant at a level of P < .001. The average attenuation in the main pulmonary artery was 268 HU ± 63 (95% confidence interval [CI]: 245, 290 HU) in group A and 379 HU ± 95 (95% CI: 346, 412 HU) in group B. In the peripheral pulmonary arteries at the level of the aortic arch, the maximum attenuation was 185 HU ± 65 (95% CI: 162, 208 HU) for group A and 290 HU ± 91 (95% CI: 259, 322 HU) for group B. In peripheral pulmonary arteries at the lung base, the maximum attenuation was 144 HU ± 63 (95% CI: 122, 166 HU) for group A and 279 HU ± 100 (95% CI: 245, 313 HU) for group B.
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Analysis of Segmental and Subsegmental Arteries
A total of 1215 segmental arteries (group A: 587; group B: 628) were analyzed. One patient in group A had undergone right lobectomy. In group B, one patient had undergone left upper lobe resection and one patient had undergone left pneumonectomy. Accessory segmental arteries in the left lower lobe were present in two patients in group A. The percentage of segmental arteries that were considered to have sufficient quality for assessment of PE did not significantly differ between group A (mean, 88% ± 13; median, 94.7%; with 50% within the range of 84%–95%) and group B (mean, 92% ± 11; median, 95%; with 50% within the range of 90%–100%) (P = .13, Mann-Whitney U test) (Fig 4).
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Subjective Grading of Image Quality
Subjective scores for image quality were lower for the 100-kVp images than for the 140-kVp images, but the difference between the average scores did not reach significance (P = .117). Differences in subjective scoring did not reach significance for any of the five anatomic areas (main pulmonary artery: P = .053; lungs: P = .089; mediastinum: P = .696; liver parenchyma: P = .236; extrahepatic upper abdomen: P = .549; 2 test) (Fig 5). None of the scans was considered to have such low image quality that it would interfere with diagnosis (grades 1 or 2) for the anatomic regions evaluated, with the exclusion of the abdomen. Five of the 32 scans in group B and two of the 30 scans in group A were rated as having a moderate image quality (grade 3) that was still considered sufficient for the diagnosis of other diseases besides PE. Four of 32 scans in group B were rated to be of insufficient quality for evaluating abdominal abnormalities (grade 1 or 2) compared with only one of the 30 scans in group A.
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2 test). There was no significant difference for the subjective grading of image noise (mean score, 3.4 vs 3.9; P = .230) or of motion artifacts (mean score, 3.2 vs 3.1; P = .653) for the 140- and 100-kVp images, respectively. Major motion artifacts due to breathing or pulsation (grades 4 and 5) were seen in 20% of the patients in group A (140 kVp) and in 16% of the patients in group B (100 kVp). There was no significantly different distribution between the two study groups regarding the grading of image noise and motion artifacts (P = .176 and P = .653, respectively, 2 test).
Assessment of Other Diagnoses
Twelve patients in group A and 10 patients in group B were found to have PE. Pleural effusion was seen in 13 patients (eight in group A and five in group B), a lung mass was seen in nine patients (four in group A and five in group B), emphysema was present in four patients (two each in group A and group B), parenchymal opacities were present in six patients (three each in group A and group B), consolidations were present in five patients (one in group A and four in group B), and pleural changes were present in three patients (one in group A and two in group B).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Lowering the kilovoltage is especially attractive for CT angiographic applications because the enhancement by iodinated contrast material increases as the effective energy of the x-ray beam decreases and comes nearer to the maximum absorption close to the k-edge of iodine. Low kilovoltage scanning has been suggested as a technique for improving contrast enhancement (9,11,12). The goal of our study was to assess the effect of a low kilovoltage technique on vascular enhancement and the visualization of small pulmonary arteries at CT angiography of the chest, even with low exposure settings.
In accordance with theory, we found a substantial increase in vascular attenuation with a low kilovoltage technique. This could be confirmed not only for the large central vessels but also for the small peripheral vessels. The difference between the high and low kilovoltage protocols was larger for peripheral arteries than for the pulmonary trunk. This is probably due to the fact that peripheral arteries are subject to partial volume effects between the surrounding lung parenchyma and the vessel lumen, in addition to the differences in enhancement.
Improved visualization of small peripheral pulmonary arteries was achieved even though we used a low-dose protocol that reduced radiation exposure to the patient by a factor of approximately three compared with our previous standard settings.
Although we found no significant impact of the kilovoltage on the visualization of the segmental arteries (92% with 100 kVp vs 88% with 140 kVp), a significant improvement was seen for the delineation of the subsegmental arteries. With 140 kVp, only 55% of subsegmental arteries were rated as analyzable, versus 71% with the 100-kVp protocol. These numbers are in agreement with results of previous studies that involved the use of a similar scan protocol with 3-mm collimation and 5-mm table feed (effective section thickness, 3.6 mm). At 120 kVp, 83%–95% of segmental arteries and 37%–43% of subsegmental arteries were analyzable (5,18).
It has been shown previously that employing thinner sections increases the number of analyzable subsegmental arteries to 65% (5,18). In these studies, 2-mm collimation and a 4-mm table feed were used, corresponding to an effective section thickness of 2.8 mm. The fact that we found a larger number of analyzable subsegmental arteries (71%) with our 100-kVp protocol, although the effective section thickness amounted to 3.6 mm, suggests that the increased enhancement achieved with the lower kilovoltage settings can compensate for partial volume effects caused by thicker collimation and can substantially improve the visibility of subsegmental arteries. This effect should not be limited to single-section scanning, because the effective section thickness of 2.8 mm evaluated by Remy-Jardin et al (5) is in the same range as (and even slightly lower than) the effective section thickness (3.2 mm) achieved with a four–detector row scanner with 4 x 2.5-mm collimation and a 15-mm table feed (19). Such a four–detector row protocol is suggested in dyspneic patients (20) and by some authors as the routine scanning technique (21). The use of thinner sections (eg, 1.25 mm at four–detector row scanning) has been shown to provide sufficient visualization of up to 94% of subsegmental arteries (17,22,23). Nevertheless, combining multisection scanning with a low kilovoltage technique should further improve the visualization of small and even more peripheral arteries. In accordance with results of a multidetector CT study (24), our results showed that kilovoltage settings can be lowered without a significant change in image quality because the signal-to-noise ratio is kept approximately at the same level.
Our study combined the low kilovoltage technique with a substantially reduced radiation exposure. Our goal was to take advantage of the increased vascular enhancement (signal) to allow for an increase in image noise while still maintaining an adequate signal-to-noise ratio within the pulmonary arteries. We found the average CT number in the pulmonary trunk to be 268 HU for the 140-kVp technique and 379 HU for the 100-kVp technique. The average noise was 14 HU for the 140-kVp technique and 22 HU for the 100-kVp technique. The ratio of vascular attenuation to noise can be seen as a proxy of the signal-to-noise ratio and is in a similar range for the low kilovoltage technique (ratio = 17) and the standard technique (ratio = 19).
To evaluate whether the significantly increased image noise influenced diagnostic evaluation of findings other than PE, we performed a subjective assessment of image quality. This subjective assessment of image quality did not reveal significant differences for the two scan protocols, although there was a trend toward reduced quality with the low kilovoltage protocol; this was presumably due to the significantly increased image noise. For the lungs and mediastinum, no scan was rated below 3, a rating corresponding to moderate image quality that was still considered sufficient for diagnosis. Image quality was viewed as inadequate only for the liver in a few patients, a finding that was also seen in one patient with the standard protocol. We do not consider this relevant because contrast material injection for evaluation of the pulmonary arteries is also not optimized for abdominal or liver imaging. To reduce the effect of the increased noise and to compensate for the increased attenuation within the pulmonary vessels, we found increasing the width of the mediastinal window setting to be very helpful for clinical evaluation of the scans, although we did not formally analyze preferred settings for each patient.
Owing to the constraints associated with radiation exposure to the patients, we did not conduct double examinations but instead compared two different study groups. Statistical tests revealed that the two groups did not differ significantly with respect to body mass index, age, sex, the presence of PE, and associated disease. We therefore assume that the observed differences between the two groups of patients reflect the effects of the two different CT protocols rather than other factors.
We believe that our results are also valid for multidetector CT scanning. For this technique as well, an increase in intravascular enhancement can be expected. Because thinner sections are chosen at a constant patient dose, partial volume effects will be reduced but images will suffer from increased image noise. Use of a low kilovoltage technique can be expected to partially compensate for the increased noise by increasing the signal-to-noise ratio. To what degree a dose reduction will be possible with multidetector scanners as well will heavily depend on the detection efficiency of the detector arrays and will require further study.
Our study had the following limitations: We simultaneously changed two variables in our protocol, tube current–time product and kilovoltage, which makes it difficult to separately consider the effect of the two factors on the resulting performance differences. It has to be kept in mind, however, that changing the kilovoltage will always simultaneously affect dose and contrast, even if the tube current–time product values are left unchanged. What is more, the change in dose at a constant tube current–time product will also depend on a variety of other factors, such as prefiltering, scanner type, scanner geometry, and the scanner manufacturer (25). This was why we chose to use a predefined target value for the CTDIvol instead. Because we found a significantly improved delineation of small vessels with lower kilovoltage although we reduced the dose by a factor of three, it can be concluded that the low kilovoltage protocol is well suited for CT angiography of the pulmonary vasculature, irrespective of whether the dose is kept constant or reduced by up to a factor of three relative to our initial dose values.
We also did not document any specific parameters characterizing the cardiac function of the patients. It is true that a reduced cardiac output has a considerable effect on vascular enhancement at CT angiography, but we consider the two study groups sufficiently matched that we are able to exclude cardiac function as having a substantial impact on the study results.
Finally, this study was performed with a single-detector spiral CT scanner. However, on the basis of the physical background of increased attenuation of contrast medium with low kilovoltage, our study setup should also be able to serve as a model for multidetector scanners to improve the existing standard protocols with respect to both reduction of radiation dose and improvement of image quality.
We conclude that it is beneficial to use a low kilovoltage protocol for CT angiography of the pulmonary arteries because vascular enhancement is increased and evaluation of peripheral arteries is improved while at the same time patient radiation exposure can be reduced.
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FOOTNOTES |
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Abbreviations: CI = confidence interval • CTDIvol = volume CT dose index • PE = pulmonary embolism
Authors stated no financial relationship to disclose.
Author contributions: Guarantor of integrity of entire study, C.S.; study concepts, M.P., C.S.; study design, M.P.; literature research, C.S.; clinical studies, C.S., M.P.; data acquisition, C.S.; data analysis/interpretation, C.M.S., M.P.; statistical analysis, M.W.; manuscript preparation and editing, C.S.; manuscript definition of intellectual content, C.S., M.P., C.M.S.; manuscript revision/review, C.M.S., M.P., C.J.H.; manuscript final version approval, M.P., C.S.
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