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Thoracic Imaging |
1 From the Department of Radiology, CT Unit, Hôpital Marie-Lannelongue, 133 Avenue de la Résistance, 92350 Le Plessis-Robinson, France (A.B.S.C., R.H., J.F.P.); Department of Interventional Radiology, Hôpital René Dubos, Cergy-Pontoise, France (H.T.A.); and Department of CT Physics and Applications, Siemens Medical Solutions, Forchheim, Germany (X.C.). From the 2002 RSNA Scientific Assembly. Received February 17, 2003; revision requested May 7; revision received June 25; accepted August 18. Address correspondence to J.F.P. (e-mail: pauljf@ccml.com).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Preliminarily, 90 patients (30 women, 60 men; mean age, 59 years) requiring contrast material–enhanced chest CT were randomly assigned to one of three protocol groups: protocol A, with use of 80 kV and 135 mAs; protocol B, with use of 80 kV and 180 mAs; or the standard protocol, with use of 120 kV and 90 mAs. Contrast material injection protocols were standardized in all groups. Image noise was calculated and plotted against patient weight. Subsequently, another 52 consecutive patients (11 women, 41 men; mean age, 57 years) were assigned to one of the protocols according to their weight: Patients weighing less than 60 kg were assigned to protocol A; patients weighing 60–75 kg, to protocol B; and patients weighing more than 75 kg, to the standard protocol. Two readers evaluated the CT images qualitatively by using a five-point scale. Statistical analyses were performed by using analysis of variance, 
, and Fisher exact tests.
RESULTS: In the preliminary study, the mean image noise values with protocols A (24 HU) and B (20 HU) were significantly higher (P < .001) than that with the standard protocol (12 HU). With protocols A and B, in the patients weighing more than 60 kg and more than 75 kg, respectively, the noise increased exponentially with patient weight. In the subsequent study, qualitative analysis revealed no significant difference between the low-kilovoltage examinations and the standard examination. Compared with use of the standard protocol, use of protocols A and B resulted in the iodine-based contrast material dose being reduced by 54% and 39%, respectively.
CONCLUSION: Weight-adapted low-kilovoltage contrast-enhanced chest CT examinations can be routinely performed with 80 kV. Use of these protocols results in good diagnostic image quality and makes it possible to reduce contrast material use by more than 50%.
© RSNA, 2004
Index terms: Computed tomography (CT), multi–detector row, 60.12112, 60.12114 • Computed tomography (CT), radiation exposure • Dosimetry • Thorax, CT, 60.12112, 60.12114
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Because the thorax is a region of low x-ray attenuation, the radiation dose can be reduced substantially during chest CT because of the high inherent contrast. The results of several studies (2–4) have shown that it is possible to decrease the tube current (ie, millampere second value) without impairing image quality. It has been proposed that the reduction in tube current could be optimized by adapting the radiation dose according to the patient’s weight (5,6). In August 2001, members of the As Low As Reasonably Achievable, or ALARA, conference of the Society for Pediatric Radiology questioned whether it is possible to reduce the dose delivered to children by reducing the kilovoltage (7). However, to our knowledge, the benefits of reducing the kilovoltage at adult chest CT have never been reported.
Our aims were to assess the feasibility of low-kilovoltage (ie, 80-kV) chest CT protocols for adults and to evaluate the effect of such protocols on image quality and iodine-based contrast material dose.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Preliminary Study
This prospective study was conducted in a thoracic and cardiac surgery center (Centre Chirurgical Marie Lannelongue) in two successive steps. The patients were either pre- or postoperative inpatients or nonsurgical outpatients. In the preliminary part of the study, 90 consecutive patients (mean age, 59 years; age range, 20–90 years) requiring chest CT were examined. There were 30 women (mean age, 62 years; age range, 31–90 years) and 60 men (mean age, 59 years; age range, 20–82 years). These patients were randomly assigned to one of the following three protocol groups for contrast material–enhanced chest CT: Protocol A involved the use of 80 kV and 135 mAs; protocol B, the use of 80 kV and 180 mAs; and the standard protocol, the use of 120 kV and 90 mAs. All patients received a standardized intravenous injection of iopamidol (Iopamiron 300, 300 mg of iodine per milliliter; Schering, Berlin, Germany) at 0.8 mL per kilogram of body weight. The injection was performed with a power injector (Medrad, Indianola, Penn) at a rate of 1.0–2.8 mL/sec (mean, 1.8 mL/sec), which was adjusted so that the total injection time lasted 30 seconds for each patient. The start delay before the beginning of the acquisition phase was 25 seconds.
A CT technologist measured contrast enhancement and image noise (defined as the SD of the mean enhancement by using region-of-interest methodology) in a standard 1-cm2 circular region of interest in the ascending aorta, at the level of the right pulmonary artery, at the time of the CT examination. Image noise was the quantitative parameter used to assess image quality. The noise was plotted against the patients’ body weights to analyze the influence of weight on noise with the three protocols.
Subsequent Study
On the basis of results of the preliminary study, the next 52 consecutive patients (mean age, 57 years; age range, 21–84 years) were assigned to either the standard protocol or one of the low-kilovoltage chest CT protocols according to their weight. In this part of the study, 11 women (mean age, 46 years; age range, 28–84 years) and 41 men (mean age, 60 years; age range, 21–81 years) were examined. The 15 patients who weighed less than 60 kg were assigned to protocol A (ie, with 80 kV and 135 mAs). The 24 patients who weighed 60–75 kg were assigned to protocol B (ie, with 80 kV and 180 mAs). These 39 patients received 0.8 mL/kg of iopamidol intravenously, as described earlier. The 13 patients who weighed more than 75 kg were assigned to our standard protocol for chest CT—that is, with use of 120 kV, 90 mAs, and 90 mL of iopamidol injected intravenously at 3 mL/sec. The start delay was 25 seconds for all patients.
Two senior radiologists (A.B.S.C., J.F.P.) with 10–12 years of experience, who were unaware of the technical parameters used, independently performed a blinded qualitative analysis of the film hard-copy CT images acquired by using the three protocols. For the low-kilovoltage protocols (ie, protocols A and B), due to brighter contrast, the technologist adapted the mediastinal window settings at imaging as follows: center of 70 HU and width of 530 HU instead of center of 40 HU and width of 400 HU.
The parameters assessed at the subjective CT image readings were overall quality of depiction of the lung and overall quality of depiction of the mediastinum. The images were assessed by using a five-point scale: A score of 1 meant unacceptable; a score of 2, suboptimal; a score of 3, adequate; a score of 4, good; and a score of 5, excellent diagnostic quality. Diagnostic quality was considered to have been achieved when the score was 3 or higher. Images of the lower part of the lung (ie, where the upper part of the abdomen was seen and included views of the liver and the spleen) were not filmed because the amount of noise in the hepatic and splenic parenchyma was great and might have influenced the qualitative evaluation.
CT Imaging
All CT examinations were performed by using a four–detector row scanner (SOMATOM Volume Zoom; Siemens Medical Solutions, Erlangen, Germany). The scanning parameters and average effective radiation doses for the different protocols (according to manufacturer data) are summarized in Table 1.
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test (8). The mean image quality scores assigned by each observer were compared by using analysis of variance. A difference was considered to be significant when the P value was less than .05. For qualitative analysis in the second part of the study, a power analysis was conducted to determine with 90% confidence the minimum sample size necessary to detect a one-point difference in means among the three protocols. Calculations revealed that the required minimum sample size was 38 patients.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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Subsequent Study
In the second part of the study, the age of the patients did not differ significantly among the protocol groups. Because of differences in weight, there were more women in the protocol A group (seven of 15) than in the protocol B group (three of 24) (P = .02) and in the standard protocol group (one of 13) (P = .04). In the subsequent study, all weight-adapted low-kilovoltage CT examinations yielded images of diagnostic quality. Interobserver agreement was good ( = 0.58 with mediastinal window setting, = 0.66 with parenchymal window setting). The image score was always 3 (adequate) or higher for depiction of the mediastinum and the lung parenchyma. The mean scores for depiction of the mediastinum and lung parenchyma were, respectively, 3.7 and 4.2 with protocol A, 3.7 and 3.9 with protocol B, and 4.0 and 4.1 with the standard protocol. Seven (47%) of the 15 CT examinations performed with protocol A and seven (29%) of the 24 examinations performed with protocol B yielded images that were judged to be of excellent diagnostic quality (score, 5) (Fig 3). The mean image quality scores assigned by the two observers are summarized in Table 2.
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The average volumes of contrast material injected were 41 mL with protocol A, 55 mL with protocol B, and 90 mL with the standard protocol. Compared with the average volume of contrast material injected with the standard protocol, the average volume injected was reduced by 54% with protocol A and by 39% with protocol B.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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In a relatively recent study (11), the issue of cancers related to radiation exposure during medical procedures—especially CT examinations—was raised. Thus, physicians, technologists, and manufacturers are now eager to lower exposure levels during CT (12). Owing to the reduced x-ray beam attenuation secondary to lower body mass, lower-weight patients can greatly benefit from radiation dose reductions (13,14). A 4-cm reduction in body diameter causes the x-ray attenuation to decrease by 50% (15). Thus, to minimize the radiation dose, one should take into account the size of the patient when performing CT (9).
The results of several studies of chest CT performed with reduced millampere second settings have shown that relevant information is not lost, even when the tube current—and thus the radiation dose—is reduced by up to 80% (2,4,16).
All modern CT scanners operate at 120 or 140 kV for routine standard imaging because these kilovoltage settings result in good image quality without excessive tube load. The scanning parameters recommended by manufacturers are designed for adult patients with average weights, and the adjustment of parameters—especially that of radiation doses for individual patients—is left up to the user of the scanning equipment. Recently, with use of computer and phantom stimulations, it was established that using 80 kV instead of 120 kV can result in the radiation dose used in pediatric contrast examinations being reduced by one half without affecting image quality if the signal-to-noise ratio is used as a criterion (15).
Reducing the kilovoltage from 120 to 80 kV leads to a 65% decrease in radiation dose at a constant tube current setting because the radiation dose varies with the square of the kilovoltage. This reduction correlates with increased image noise and potentially with decreased image quality (14). Thus, we compensated for this kilovoltage reduction by increasing the tube current by 50% with protocol A and by 100% with protocol B, while keeping all other parameters constant. According to the manufacturer’s data, the calculated average radiation dose could be reduced by 56% with protocol A and by 41% with protocol B, as compared with the mean dose delivered with the standard protocol.
In our study, we evaluated the feasibility of low-kilovoltage chest CT. In the preliminary part of the study, evaluation of the image noise with the standard protocol enabled us to choose a noise threshold of 20 HU as the criterion of acceptable image quality. This threshold was subsequently used to determine acceptable weight limits for the low-kilovoltage protocols.
The second phase of our study involved the qualitative evaluation of images obtained by using weight-adapted CT protocols. The lung parenchyma was adequately visualized at the low-kilovoltage examinations. All images were of diagnostic quality, and at a number of low-radiation-dose CT examinations, the quality was excellent. In addition, with use of 80 kV, higher vascular contrast was appreciable at image analysis.
In the clinical situations encountered in a thoracic surgical center, performing low-kilovoltage CT seems to be an adequate way to minimize radiation dose, especially in patients who require several follow-up examinations.
A second advantage of performing low-kilovoltage CT is that it enhances the iodine-induced contrast and thus reduces the amount of iodine-based contrast material needed to image lower-weight patients, because the attenuation of iodine-based contrast material, owing to iodine’s high relative atomic number (Z = 53), increases with reduced x-ray energy (17). The volume of iodine-based contrast material injected during the low-kilovoltage CT examinations in the present study was adapted in consideration of this information. Results of the first part of our study showed that the vascular enhancement achieved with the low-kilovoltage protocols (ie, protocols A and B) was up to 65% higher than that achieved with the standard protocol, which involved the use of 120 kV and 0.8 mL/kg of contrast material. Consequently, the volume of contrast material needed (50–60 mL for patients weighing 60–75 kg) was substantially reduced compared with the 90 mL previously used for chest CT at our institution. The minimum injected volume was as low as 30 mL, which was administered to a 38-kg patient.
The lower volume of intravenous contrast material required for low-kilovoltage chest CT is a direct benefit to the patient in terms of renal protection. Furthermore, the lower injection speed (maximum, 2 mL/sec) reduces the risk of contrast material extravasation. The use of lower kilovoltages also has an economic effect because the injected volume of iodine-based contrast material can be lowered substantially, without causing image degradation, to yield substantial financial savings. In total, there was a 45% reduction in contrast material use in our study population. This means that the packaging of contrast material also might need to be adapted accordingly in the future.
There were some limitations to this study. We did not evaluate the image quality of thin-section chest CT scans, which are used to diagnose diseases of the interstitium. Also, the use of weight-adapted low-kilovoltage protocols for specific applications such as pulmonary embolism screening has not been studied. We believe, however, that CT for evaluation of possible pulmonary embolism or aortic dissection performed with 80-kV protocols may be beneficial for patients weighing less than 75 kg because higher intravascular contrast is expected with a low kilovoltage.
The accuracy of diagnoses of parenchymal diseases was not assessed in this study, even though it has been reported that low-tube-current CT protocols involving the use of just 10%–20% of the radiation delivered with standard-dose CT can enable reliable measurement of the soft-tissue attenuation of pulmonary nodules with a diameter of more than 5 mm and the detection of smaller nodules equally well in many cases (16). The body mass index (or thorax circumference) might be more accurate than the body weight in determining scanning parameters; however, the applicability of this measurement in routine clinical use also needs to be considered.
In conclusion, the use of weight-adapted low-kilovoltage CT results in substantial enhancement of the iodine-induced contrast, even with a markedly reduced radiation dose, which facilitates the added patient benefit of decreased contrast material use. However, to maintain diagnostic image quality, the milliampere second settings need to be adapted to the patient’s weight. This weight-adapted low-kilovoltage chest CT protocol is now routinely used in our chest and cardiac surgery center.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS DISCUSSIONREFERENCES |
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