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Technical Developments |
1 From the Division of Thoracic Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, White 270-E, 55 Fruit St, Boston, MA 02114 (M.K.K., S.R., M.M.M., E.F.H., J.O.S., S.L.A.); and GE Medical Systems, Waukesha, Wis (T.L.T.). Received August 11, 2004; revision requested October 14; revision received December 1; accepted January 14. Supported in part by a grant from GE Medical Systems, Waukesha, Wis. Address correspondence to M.K.K. (e-mail: mannudeep_k_kalra{at}yahoo.com).
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ABSTRACT |
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= 0.72). Compared with the fixed tube current technique, z-axis modulation provides acceptable image noise for chest CT, with an 18% and 26% reduction in tube current–time product at 10.0- and 12.5-HU noise indexes, respectively. © RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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To automate selection of tube current to obtain user-specified image noise or mottle at optimum radiation dose, automatic tube current modulation techniques have been introduced (2). These techniques are used to adjust tube current to provide constant image noise on the basis of patient size, attenuation profile, and other scanning parameters, including tube potential, beam pitch, table speed, and rotation time. Automatic tube current modulation techniques may be used to adjust tube current within each section (ie, angular modulation), from section to section in the scanning direction (ie, z-axis modulation), or both (ie, x-, y-, and z-axis or combined modulation) (1–4).
Z-axis modulation is used both in the estimation of attenuation, size, and shape of the area of interest from a single localizer radiograph and in the adjustment of tube current to maintain a constant user-specified quantum noise level (noise index) by using x-ray beam projection data and empirically determined noise prediction coefficients for the reference technique (2.5-mm section thickness at the selected peak tube voltage and 100-mAs tube current–time product for a transverse reconstruction with a standard reconstruction algorithm). This technique provides a noise index to enable selection of the amount of x-ray noise that will be present in the reconstructed images. This technique is used to compute the tube current needed to obtain images with a selected noise index, and an attempt is made for all images to have a similar noise, irrespective of patient size and anatomy. The noise index represents quantitative noise (standard deviation) in the central region of the image obtained from scanning a uniform phantom (5). Instead of selecting the tube current for a given indication, body region, patient size, and age with a fixed tube current technique, selection of noise index with z-axis modulation can help in the acquisition of desired image noise at optimum radiation exposure. A low noise index provides less image noise but results in greater radiation exposure to the patient, whereas a high noise index results in more image noise but is associated with less radiation exposure.
Previous reports have shown low-dose CT scanning of the chest with fixed tube current and angular modulation techniques reduces the tube current–time product associated with CT scanning (6–12). To our knowledge, however, use of z-axis modulation in CT scanning of the chest has not been reported. In addition, the optimal protocol and radiation dose savings for chest CT with z-axis modulation have not been evaluated.
Thus, the purpose of our study was to retrospectively assess the scanning protocol and radiation dose associated with z-axis automatic tube current modulation in multi–detector row chest CT.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionReferences |
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Patients and Scanning Technique
The human research committee of the institutional review board approved the study protocol and waived the informed consent requirement for retrospective evaluation of the records of 53 consecutive patients who underwent follow-up contrast medium–enhanced (iopamidol 320, Isovue; Bracco, Princeton, NJ) CT examination of the chest with a 16–detector row CT scanner (Lightspeed 4.X; GE Healthcare, Waukesha, Wis) and z-axis modulation (Auto mA; GE Healthcare). Our study was compliant with the Health Insurance Portability and Accountability Act. All initial and follow-up CT examinations included in this study were performed in patients who were referred for clinically indicated contrast-enhanced chest CT. These indications included lung cancer, metastatic disease, abnormal findings on chest radiographs, and evaluation of mediastinum, lung hilum, pleural effusion, mesothelioma, and pneumonia.
The mean age of subjects in the study cohort was 54 years (age range, 26–77 years). The study cohort included 28 women (mean age, 52 years; age range, 27–69 years) and 25 men (mean age, 58 years; age range, 26–77 years). The weight of each patient at initial CT scanning with the fixed tube current technique (mean weight, 81.0 kg; weight range, 53.6–113.0 kg) and the weight at subsequent CT scanning with z-axis modulation (mean weight, 81.5 kg; weight range, 54.4–114.3 kg) were recorded.
For the z-axis modulation technique assessed in the present study, a noise index is selected with maximum and minimum tube current thresholds. Noise indexes of 10.0 (n = 11), 12.5 (n = 18), and 15.0 (n = 24) HU were used with minimum and maximum tube current thresholds of 75 mA and 380 mA, respectively. Although any noise index from 1.0 to 50.0 HU can be selected for a chest CT examination, a baseline noise index of 10.0 HU was used in the present study on the basis of vendor suggestion. Consequently, to evaluate use of higher noise index for chest CT to reduce associated tube current–time product, noise indexes of 12.5 and 15.0 HU were used, as prior studies have shown statistically significant tube current–time product reduction at these noise indexes in abdominal CT scanning (5,13). The remaining scanning parameters remained constant for all examinations and included 140-kVp tube voltage, 0.938:1 pitch, 16 x 1.25-mm detector configuration (16 detectors with 1.25-mm section thickness), 18.75-mm table feed per gantry rotation, and 0.5–1.0-second gantry rotation time. Images were reconstructed with 5-mm section thickness at a 5-mm interval with a standard reconstruction algorithm and full reconstruction mode. The full reconstruction mode is a vendor-specific feature, which enables acquisition of CT images with nominal section thickness.
To assess the value of z-axis modulation in the reduction of the tube current–time product compared with standard-dose chest CT without compromising image quality, CT images obtained with z-axis modulation were compared with previous images obtained with the same 16–detector row CT scanner according to our department protocols for contrast-enhanced chest CT with fixed tube current (180–300 mA with 0.5–1.0-second tube rotation time). The remaining scanning parameters for examinations performed with fixed tube current were identical to those used with z-axis modulation. The mean interval between examinations performed with the two techniques was 6 months (range, 3–8 months). There was no significant change in patient weight between the examinations (P = .5).
Compared with the fixed tube current technique, z-axis modulation was performed with a wider range of tube currents (ie, 75–380 mA) to determine the complete extent of tube current modulation on the basis of selected noise indexes and patient size (13,14). A minimum tube current of 75 mA was selected on the basis of vendor suggestion, which is intended to avoid an inadvertent decrease in tube current (5). Maximum tube current for the z-axis modulation technique was set at 380 mA, as this was the maximum allowable tube current on the scanner at 140 kVp, and capping of the maximum tube current would have limited the capability to obtain the desired image noise along the scanning direction (z-axis) with z-axis modulation.
Image Analysis
Two subspecialty radiologists (S.L.A. and M.M.M., with 11 and 5 years of experience, respectively) were blinded to the examination techniques, and they independently performed direct side-by-side comparison of CT images acquired with z-axis modulation and fixed tube current at a digital picture archiving system diagnostic workstation (Impax RS 3000 1K review station; Agfa Technical Imaging Systems, Richfield Park, NJ). Blinding was facilitated by removing information pertaining to the scanning parameters and date of the examination from the image display format of the diagnostic workstation.
Both radiologists independently compared the images with constant window width and window level settings to simulate lung window settings (window width, 1500 HU; window level, –600 HU) and soft-tissue window settings (window width, 350 HU; window level, 50 HU). For each patient, images acquired with z-axis modulation and fixed tube current techniques were evaluated at four anatomic levels, which included the glenoid labrum, aorticopulmonary window, left atrium, and dome of the right hemidiaphragm. The level of the glenoid labrum was chosen to assess the effect of z-axis modulation at the level of the shoulders, which has an abrupt attenuation transition (from lower attenuation in the neck and chest to higher attenuation in the shoulder region) and frequent streak artifacts due to the bony pectoral girdle and asymmetric regional anatomy. Images were compared at the level of the top of the right hemidiaphragm to evaluate the effect of z-axis modulation on the depiction of lung parenchyma and mediastinal structures at the site of transition from the thorax to the abdomen. Previous studies have shown increased frequency of streak artifacts with use of the z-axis automatic tube current modulation technique at this location (13–15). Remaining levels, including the aorticopulmonary window and the left atrium, were selected arbitrarily to assess the tube current and image quality with z-axis modulation.
At each of the aforementioned four levels, both radiologists independently assessed image noise, diagnostic acceptability, and streak artifacts in mediastinal structures. In addition, the two radiologists also graded depiction of peripheral bronchovascular structures within 2 cm of the parietal pleura at each level. Image noise and diagnostic acceptability in mediastinal and pulmonary structures and depiction of peripheral bronchovascular structures within 2 cm of the parietal pleura were graded on a five-point scale (1, unacceptable; 2, minimally acceptable; 3, acceptable; 4, highly acceptable; 5, excellent). Streak artifacts were graded on a three-point scale (1, streak artifacts absent; 2, streak artifacts present but not interfering with depiction of adjacent structures; 3, streak artifacts present and interfering with depiction of adjacent structures). The image quality parameters and scoring system assessed in the present study were selected on the basis of findings of prior studies (13–16).
The extent of "graininess" or "mottle" on the CT images was the main determinant in the grading of image noise. Image noise was graded as acceptable (score of 3) if average graininess was seen with satisfactory depiction of peripheral bronchovascular structures such as the blood vessels and interface between structures of variable attenuation, unacceptable (score of 1) if graininess interfered with depiction of these structures, and excellent (score of 5) when there was minimal or no appreciable mottle. Diagnostic acceptability was graded on the basis of satisfactory depiction of soft-tissue structures for diagnostic interpretation and degree of image degradation by beam-hardening artifacts (15). Diagnostic acceptability was graded as acceptable, unacceptable, or excellent, respectively, if depiction of soft-tissue structures for diagnostic interpretation and degree of image degradation by beam-hardening artifacts of mediastinal and pulmonary structures were satisfactory, unsatisfactory, or considerably superior.
Tube Current
For each CT examination, tube current values were recorded at all section positions over the course of the whole chest CT examination to determine the average tube current. Tube current–time product for each examination performed with the z-axis modulation technique was calculated by multiplying average tube current and gantry rotation time. Tube current and gantry rotation time were also recorded for examinations performed with the fixed tube current technique. For examinations performed with the fixed tube current technique, tube current–time product values were calculated by multiplying tube current by tube rotation time.
Statistical Analysis
The unpaired t test (Excel; Microsoft, Redmond, Wash) was used to compare weights of patients examined with different noise indexes. Weights of patients with unacceptable image noise scores were compared with weights of patients with acceptable average scores. Multivariate linear regression was performed with statistical software (MedCalc Software, Mariakerke, Belgium) to determine the association between patient weight, mean tube current–time product, and image noise scores in studies performed with z-axis modulation and fixed tube current. An unpaired t test was used to compare weights of patients in whom CT scanning with z-axis modulation decreased the mean tube current–time product with weights of patients in whom CT scanning with z-axis modulation increased the mean tube current–time product. Wilcoxon signed rank test (MedCalc Software) was used to compare image noise, diagnostic acceptability, depiction of small peripheral bronchovascular structures, and streak artifacts in examinations performed with z-axis modulation at different noise indexes and with the fixed tube current technique. The latter test was also used to determine possible differences in image noise between men and women. To minimize the chance occurrence of significant differences due to interdependency of multiple variables (or image quality parameters, which were dependent variables), the Bonferroni adjustment was applied. Accordingly, a P value of .0025 or less indicated a significant difference for comparison of 20 nonparametric variables (five variables each at four levels of the chest in each patient).
Mean and range of difference between average tube current–time product for examinations performed with the z-axis modulation and fixed tube current techniques were also calculated with the Excel (Microsoft) software program. Tube current–time products for the z-axis modulation and fixed tube current techniques were compared with the paired t test. Degree of interobserver concordance was determined with the weighted test and use of statistical software (MedCalc). Interobserver agreement was categorized as poor, fair, moderate, good, or excellent on the basis of coefficient values of less than 0.20, 0.20–0.39, 0.40–0.59, 0.60–0.79, and more than 0.80, respectively (3).
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Results |
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In the study cohort, patient weight had a weak negative correlation with image noise scores in examinations performed with the fixed tube current technique (r = –0.30, P = .04) and a weak positive correlation in examinations performed with the z-axis modulation technique (r = 0.36, P = .01). A strong positive correlation between patient weight and image noise scores was noted in patients who underwent CT scanning with a 10-HU noise index (r = 0.79, P = .006), but no significant correlation was noted in patients who underwent scanning with a 12.5- or 15.0-HU noise index (r = 0.08–0.1, P > .5).
There was no significant difference in image noise between men and women (P = .5). There was good interobserver agreement between the two readers ( = 0.72, P < .05).
Tube Current
Regardless of the selected noise index, mean tube current–time product with z-axis modulation had strong linear correlation with patient weight (r = 0.71, P = .02), whereas no significant correlation was noted between mean tube current–time product with fixed tube current and patient weight (r = 0.06, P = .12). Significant positive correlations were also noted between patient weight and mean tube current–time product with noise indexes of 10.0 (r = 0.67, P = .035), 12.5 (r = 0.73, P = .001), and 15.0 HU (r = 0.67, P = .003). With the z-axis modulation technique, mean (± standard error of mean) tube current–time product for patients examined with noise indexes of 10.0, 12.5, and 15.0 HU were 135.6 mAs ± 17.7, 133.2 mAs ± 11.4 and 101.8 mAs ± 9.7, respectively. Corresponding mean tube current–time product values in prior CT examinations performed with the fixed tube current technique were 165.5 mAs ± 8.0, 179.5 mAs ± 7.3, and 164.6 mAs ± 6.7, respectively. Compared with the fixed tube current technique, there was 18% (135.6 vs 165.5 mAs), 26% (133.2 vs 179.5 mAs) and 38% (101.8 vs 164.6 mAs) reduction in mean tube current–time product at noise indexes of 10.0, 12.5, and 15.0 HU, respectively. Mean tube current–time products at noise indexes of 10.0, 12.5, and 15.0 HU were significantly lower than mean tube current–time products for the 53 examinations performed with the fixed tube current technique (169 mAs, P < .01).
Z-axis modulation decreased mean tube current–time product in 43 patients (81%), with a mean reduction of 80.8 mAs ± 5.4 (94.9 vs 175.8 mAs, 46% mean reduction; range, 2.5–142.0 mAs) when compared with the fixed tube current technique. In 10 patients (19%), z-axis modulation resulted in a 21% increase in the mean tube current–time product (167.8 vs 138.7 mAs; standard error of mean, 8.2; range, 2.4–84.0 mAs). Patients in whom the mean tube current–time product increased with z-axis modulation weighed significantly more than patients in whom the mean tube current–time product decreased with z-axis modulation (93.6 kg ± 11.2 vs 78.2 kg ± 5.2) (P = .01). Patients who experienced an increase in the mean tube current–time product with the z-axis modulation technique when compared with the fixed tube current technique were examined with a noise index of 10.0 (n = 8) and 12.5 HU (n = 2).
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Discussion |
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In addition, our study shows that z-axis modulation with noise indexes of 10.0, 12.5, or 15.0 HU provides chest CT images of acceptable quality with 18%, 26%, and 38% reduction in mean tube current–time product, respectively, when compared with the fixed tube current technique. Noise indexes of 10.0 and 12.5 HU seem to be more appropriate for routine chest CT examinations, as all examinations with greater-than-acceptable image noise and less-than-acceptable diagnostic acceptability and streak artifacts were performed with a noise index of 15.0 HU. This finding is in agreement with increased quantitative image noise reported with use of a noise index of 15.0 HU in abdominal-pelvic CT examinations (13). Contrary to prior reports, which address the application of the z-axis modulation technique in abdominal-pelvic CT scanning, we did not find any substantial difference between weights of patients with acceptable and unacceptable image noise with the z-axis modulation technique (5,13,14). Although this disagreement may be explained by the small number of patients in our study cohort (n = 53) compared with the number of patients in an earlier study (n = 153), considerably different diagnostic needs of chest CT, better inherent contrast—especially with the lung window settings, and lower overall x-ray beam attenuation causing little change in image noise in patients with different weights also may have contributed to this finding (13–18). Also, higher noise may be more acceptable in the chest than in the abdomen (1). Prior studies of chest CT and angular modulation have reported changes in quantitative noise from a 26% decrease to a 36% increase, without any increase in image noise (17).
Interestingly, a negative correlation between patient weight and image noise in examinations performed with fixed tube current suggests that lighter patients are often examined with a higher tube current–time product, whereas heavier patients are often examined with a lower tube current–time product. In addition, we noted a significant positive correlation between patient weight and image noise in examinations performed with a noise index of 10.0 HU. Lower image noise in heavier patients examined with z-axis modulation may be explained by an appropriate increase in tube current with the z-axis modulation technique compared with the fixed tube current technique. Greater image noise in lighter patients with the z-axis modulation technique has also been reported in prior abdominal CT studies (13).
An important consideration of the z-axis modulation technique that is emphasized in the present study is the potential increase in the mean tube current–time product in certain patients when compared with the fixed tube current technique, with no substantial change in image noise. Although patients in whom the mean tube current–time product increased with z-axis modulation were considerably heavier, this finding must alert physicians and radiologists to the possibility of using a higher tube current with the z-axis modulation technique in heavier patients. While this increase in mean tube current–time product may be necessary to maintain image noise in heavier patients, such an increase may be avoided by selecting a lower maximum tube current threshold similar to a tube current value that would have been used with the fixed tube current technique in the same patient. Alternatively, if higher noise can be tolerated in images, the noise index must be increased to avoid an increase in radiation dose. It is also important to remember that changes in other scanning parameters, including beam pitch, detector configuration, and table speed, change tube current and radiation dose associated with the z-axis modulation technique. Thus, it is important to remember that mean tube current–time product savings reported in our study may not be reproducible in a different set of patients with different examination parameters, such as detector configuration, table speed, and beam pitch.
There are limitations in our study. We did not assess tube current modulation techniques that have been made available by other manufacturers (eg, Siemens Medical Solutions, Erlangen, Germany; Toshiba Medical, Tokyo, Japan) of multi–detector row CT scanners. Another limitation of our study is that the concept of noise index is vendor-specific, and it cannot be used on scanners produced by other manufacturers. The physical basis of the z-axis modulation technique, however, is identical for all scanners, regardless of vendor. We did not perform a power analysis to determine sample size for the number of patients for each noise index. The small sample size of 53 patients is a limitation of our study. In addition, noise index values of less than 10.0 HU were not assessed, as image noise at this noise index was acceptable, and a further decrease in the noise index would have increased the tube current–time product associated with z-axis modulation. Also, we did not evaluate the effect of a noise index greater than 15.0 HU because of an increase in image noise and streak artifacts in some patients. Although tube current–time product would have decreased at greater noise indexes, an increase in image noise also would have decreased diagnostic acceptability of the images. In addition, we evaluated only patient weight, which is the most commonly used anthropomorphic indicator, in the present study, as prior studies have shown no significant difference between linear associations of tube current used with z-axis modulation with patient weights and regional cross-sectional diameters (P > .05) (13).
A limitation of our study could be that all chest CT examinations were performed with a 140-kVp tube voltage, whereas a 120-kVp tube voltage is used in many institutions. A 35% reduction in tube current–time product can be achieved by reducing tube voltage from 140 kVp to 120 kVp if all other scanning parameters are kept constant. As z-axis modulation is used to adjust tube current, taking into account other selected scanning parameters—including tube potential, results of the present study will be applicable at 120-kVp tube voltage, albeit at higher tube currents than those used with 140-kVp tube voltage to maintain constant image quality (5). Alternatively, radiologists can set a limit to the maximum tube current selected with z-axis modulation to avoid an inadvertent increase in radiation dose. Such a limit can be set at a tube current value that would have been used to perform CT scanning with the fixed tube current technique.
Our study shows that z-axis modulation can be used to obtain user-specified image noise with reduced tube current–time product in most patients undergoing chest CT. However, future studies must be performed to refine protocols for the z-axis modulation technique for specific applications of chest CT. Similarly, the appropriate noise index for CT angiography in the evaluation of pulmonary embolism and aortic abnormalities must be determined, as higher image noise may also be acceptable in these situations because of an increase in contrast.
In conclusion, z-axis modulation appears to provide acceptable image noise and diagnostic acceptability for multi–detector row chest CT, with substantial tube current–time product reduction at noise indexes of 10.0 (18%) and 12.5 (26%) HU compared with the fixed tube current technique. On the basis of our findings, noise indexes of 10.0 and 12.5 HU are more appropriate than a noise index of 15.0 HU for routine chest CT; however, future studies need to be performed to more accurately define the extent of radiation dose reduction with the z-axis modulation technique.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, M.K.K., J.O.S., S.L.A.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, M.K.K.; clinical studies, M.K.K., S.R., S.L.A.; experimental studies, M.K.K., T.L.T.; statistical analysis, M.K.K., E.F.H.; and manuscript editing, M.K.K., S.R., M.M.M., J.O.S., S.L.A.
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