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Metallic Prosthesis: Technique to Avoid Increase in CT Radiation Dose with Automatic Tube Current Modulation in a Phantom and Patients¹

¹ From the Division of Abdominal Imaging and Intervention, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, 55 Fruit St, White 270-E, Boston, MA 02114 (T.D., S.M.R.R., M.A.B.); Siemens Medical Solutions, Forchheim, Germany (B.S., C.S., T.F.); and Department of Radiology, Emory University School of Medicine, Atlanta, Ga (S.S., M.K.K.). Supported in part by RSNA R&E Medical Student Departmental Program Grant and University of Vermont, Burlington, Vt. S.M.R.R. supported in part by research fellowship grant from Siemens Medical Solutions. Received September 16, 2004; revision requested November 24; revision received December 6; accepted January 12, 2005. Address correspondence to M.K.K. (e-mail: mkalra{at}emory.edu).

	ABSTRACT

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The institutional review board approved this Health Insurance Portability and Accountability Act–compliant study protocol, with waiver of informed consent. The purpose of the study was to retrospectively evaluate the combined automatic tube current modulation technique in patients with orthopedic metallic prostheses. Five hundred abdominal-pelvic computed tomographic (CT) studies performed with combined modulation technique were reviewed to identify nine patients with metallic prostheses (mean age, 66 years; range, 35–86 years; male-female ratio, 5:4). On the basis of age and transverse abdominal images, these patients were matched with nine others with no metallic prostheses (mean age, 56 years; range, 36–72 years; male-female ratio, 4:5) who were control patients. Images were graded for extent and severity of streak artifacts (grade 1, streak artifact present but not substantially compromising evaluation of adjacent structures; grade 2, streak artifact present and slightly compromising evaluation of adjacent structures; and grade 3, streak artifact present and severely compromising evaluation of adjacent structures). Student t test was performed for statistical analysis. There was no difference in mean effective tube current–time product between study and control patients (P > .49). With automatic tube current modulation, an increase in CT dose caused by metallic prostheses can be successfully avoided.

© RSNA, 2005

	INTRODUCTION

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Radiation-based imaging techniques, which include computed tomography (CT), must aim for diagnostically acceptable image quality at a radiation exposure that is as low as reasonably achievable. Several scanning parameters affect image quality and radiation dose associated with CT scanning; these include tube current, pitch, beam collimation, tube voltage, table speed, and gantry rotation time (1,2). Tube current is the most important of these factors. Automatic tube current modulation is a technique that automatically adjusts the tube current, depending on the size, shape, and density of the scanning region. Prior to the advent of this technology, CT scanning was performed with either a fixed tube current or a manually adjusted tube current, which was based on the weight or regional cross-sectional dimensions of the patient (3,4).

Since automatic tube current modulation adapts tube current, on the basis of the regional density and attenuation profile, data from some researchers (5) indicate that tube current associated with scanning in patients with a metallic prosthesis is higher in the area of the prosthesis; however, there is an overall reduction in radiation dose for the entire scan length in these patients. Results of prior studies also indicated that an increase in the tube current does not lead to a decrease in streak artifacts from prostheses (6). Therefore, a technique was developed to detect the presence of metallic prostheses from the localizer radiograph before actual scanning of the patient and to avoid an increase in tube current by excluding the contribution of the metallic prosthesis to the overall attenuation profile during the calculation of the appropriate tube current to be used. This technique is a standard component of the current combined modulation technique investigated in this study. Thus, the purpose of our study was to retrospectively evaluate the combined automatic tube current modulation technique in patients with orthopedic metallic prostheses.

	Materials and Methods

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Financial Disclosure
Some authors (B.S., C.S., T.F.) are employees of Siemens Medical Solutions. The remaining authors (T.D., M.K.K., S.M.R.R., M.A.B., S.S.), however, had complete access to and control of the data and information.

Phantom Study
An elliptical Plexiglas container (32 x 20 cm in transverse and anterior-posterior diameters) was filled with water and was scanned with a 16-section multi–detector row CT scanner (Somatom Sensation 16; Siemens Medical Solutions, Forchheim, Germany) with a fixed tube current and a combined-modulation technique (CARE Dose4D; Siemens Medical Solutions). The phantom was first scanned with a fixed effective tube current–time product of 200 (effective tube current–time product = [tube current x gantry rotation time]/pitch). Subsequently, the phantom was scanned with the combined-modulation technique with an image quality reference tube current–time product of 200. The remaining scanning parameters were held constant and included 140 kVp, 0.5-second gantry rotation time, 16 x 1.5 mm detector configuration (16 detector data channels of 1.5-mm section thickness each), 24-mm table feed per gantry rotation, soft-tissue reconstruction algorithm (B31 medium), 5-mm reconstructed section thickness, and 5-mm section intervals.

A cobalt-chrome metallic hip endoprosthesis (Omnifit EON; Stryker Howmedical Osteonics, Mahwah, NJ) with a titanium acetabular cup (Trilogy; Zimmer, Warsaw, Ind), which is used for total hip arthroplasty, was then placed in the phantom to determine the effect of the metallic prosthesis on the combined-modulation technique. The prosthesis was attached to the dependent surface of the phantom with adhesive tape. The phantom was placed in the same position in the gantry isocenter and then rescanned by using identical fixed–tube current techniques and combined-modulation techniques. Thus, scanning was performed with fixed–tube current techniques and combined-modulation techniques for the phantom without the metallic prosthesis, and then it was repeated for the phantom with the metallic prosthesis.

Two radiologists (M.A.B., with 10 years of experience, and M.K.K., with 5 years of experience), unaware of the scanning technique, independently compared the extent and severity of streak artifacts on images acquired with fixed–tube current techniques and combined-modulation techniques. All images were reviewed at a digital picture archiving and communication system diagnostic workstation (Impax RS 3000 1K; Agfa Technical Imaging Systems, Ritchfield Park, NJ) at the same window level and window width (40 and 400 HU, respectively). A three-point scale was used to grade the severity of streak artifacts from the metallic prosthesis (grade 1, minimum streak artifacts; grade 2, moderate streak artifacts; and grade 3, severe streak artifacts).

For each series of images, the CT dose index volume and dose-length product values were recorded from the user interface on the scanner. The CT dose index volume is a descriptor of the average dose within a scan volume (relative to a standardized CT phantom), and it is now required to be displayed on the user interface of the CT scanner. In addition, effective tube current–time product values used with the scanner were recorded for all images obtained with the combined-modulation technique.

Patient Study
The human research committee of the institutional review board approved the Health Insurance Portability and Accountability Act–compliant study protocol, with waiver of informed consent. Five hundred consecutive contrast material–enhanced abdominal-pelvic CT examinations, performed from April to September 2004 with the same 16-section multi–detector row CT scanner as was used for the phantom study with the combined-modulation technique, were retrospectively reviewed to identify patients with metallic prostheses (T.D., S.M.R.R.). All examinations were reviewed at a picture archiving and communication system diagnostic workstation. Nine patients (mean age, 66 years; range, 35–86 years; male-female ratio, 5:4) with metallic prostheses served as the study patients. Six patients had a right hip prosthesis, two patients had a left hip prosthesis, and one patient had bilateral hip prostheses. The standard departmental protocol for the combined-modulation technique used for scanning study patients included an image quality reference tube current–time product of 160. The remaining scanning parameters included 0.5-second gantry rotation time, 140 kVp, 16 x 1.5 mm detector configuration, 24-mm table feed per gantry rotation, soft-tissue reconstruction algorithm (B31 medium), 5-mm reconstructed section thickness, and 5-mm section intervals. The cross-sectional transverse diameter was measured at the upper pole of the right kidney for all study patients (mean, 333 mm; range, 290–370 mm).

Nine additional patients (mean age, 56 years; range, 36–72 years; male-female ratio, 4:5) with no metallic prosthesis who underwent abdominal-pelvic CT scanning with an identical combined-modulation technique served as control patients. The control group included nine consecutive patients identified in a retrospective review of 77 consecutive contrast-enhanced abdominal-pelvic CT examinations performed in May 2004. All patients without a metallic prosthesis (control patients) were matched for age and cross-sectional transverse dimension at the level of the upper pole of the right kidney with patients who had metallic prostheses (study patients). The average cross-sectional transverse dimension at the level of the upper pole of the right kidney for control patients was 331 mm (range, 281–412 mm).

Two radiologists (M.A.B., with 10 years of experience, and M.K.K., with 5 years of experience), who were unaware of the scanning technique, independently evaluated CT images of the patients with metallic prostheses that were performed by using the combined-modulation technique. Images were graded for the severity and extent of streak artifacts from metallic prostheses at a digital picture archiving and communication system diagnostic workstation with the same window level and window width of 40 and 400 HU, respectively. Severity and extent of streak artifacts from the metallic prostheses were graded on a three-point scale (grade 1, streak artifact present but not substantially compromising the evaluation of adjacent structures; grade 2, streak artifact present and slightly compromising the evaluation of adjacent structures; and grade 3, streak artifact present and severely compromising the evaluation of adjacent structures).

To evaluate the effect of metallic prostheses on radiation dose associated with the combined-modulation technique, effective tube current–time product values used for scanning the patients with metallic prostheses and those without metallic prostheses were recorded at all section positions.

Statistical Analysis
The ² test, which was calculated with a software program (MedCalc Software; MedCalc, Mariakerke, Belgium), was used to compare CT dose index volume and dose-length product values associated with scanning of the phantom with the metallic prosthesis and of the phantom without the metallic prosthesis. Effective tube current–time product values for CT images of the phantom with the metallic prosthesis and for those of the phantom without the metallic prosthesis acquired with the combined-modulation technique were compared by using the paired t test with the software program (MedCalc Software; MedCalc). The Student t test also was used to compare the ages and cross-sectional transverse dimensions of patients in the study and control groups with spreadsheet software (Excel; Microsoft, Redmond, Wash). Qualitative scores for the severity of metallic streak artifacts were compared with the Wilcoxon signed rank test, which was determined with a software program (MedCalc Software; MedCalc). The degree of interobserver agreement was determined with the test by using the software program. The coefficient values for interobserver agreement were considered as slight (0.20), fair (0.21–0.40), moderate (0.41–0.60), substantial (0.61–0.80), or almost perfect (0.81–1.00). The mean values for effective tube current–time product for study and control patients also were compared by using the Student t test. A P value of less than .05 was considered to indicate a statistically significant difference.

	Results

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Phantom Study
Both radiologists noted streak artifacts from the metallic prosthesis on images acquired with combined-modulation and fixed–tube current techniques. There was no significant difference in qualitative assessment of severity and extent of streak artifacts from the metallic prosthesis on CT images acquired with the combined-modulation and the fixed–tube current techniques (P > .99). An almost perfect interobserver agreement between the two radiologists for grading of the streak artifacts from the metallic prosthesis was noted ( coefficient = 1.00, P < .05).

As expected, CT dose index volume and dose-length product values for fixed–tube current CT scanning of the phantom with the metallic prosthesis and of the phantom without the metallic prosthesis were identical to each other (20.4 mGy · cm and 325 mGy · cm, respectively). With the combined-modulation technique, CT dose index volume and dose-length product values for the phantom without the metallic prosthesis were 7.8 mGy · cm and 125 mGy · cm, respectively. Likewise, CT dose index volume and dose-length product for CT scanning of the phantom with the metallic prosthesis, performed by using the combined-modulation technique, were 7.6 mGy · cm and 121 mGy · cm, respectively. The ² test did not reveal a statistically significant difference between CT dose index volume or dose-length product values for scanning of the phantom with the metallic prosthesis and for that of the phantom without the metallic prosthesis performed by using the combined-modulation technique (P = .8). Similarly, there was no statistically significant difference between effective tube current–time product values for the phantom with the metallic prosthesis and those for the phantom without the metallic prosthesis scanned with the combined-modulation technique (P = .2) (Fig 1).

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Line graph shows effective tube current–time product used for scanning phantom with prosthesis and phantom without prosthesis by using the combined-modulation technique compared with fixed–tube current technique. Note that there is no increase in effective tube current–time product for scanning the phantom with the prosthesis by using combined-modulation technique. Arrows indicate position of metallic prosthesis.

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 2a. Images show how combined-modulation technique helped avoid an increase in effective tube current–time product in region of the metallic prosthesis. (a) Transverse CT image acquired with combined-modulation technique in 71-year-old man with left hip prosthesis shows streak artifacts (129 effective mAs). (b) Transverse CT image acquired with combined-modulation technique in 72-year-old man without metallic prosthesis (145 effective mAs).

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 2b. Images show how combined-modulation technique helped avoid an increase in effective tube current–time product in region of the metallic prosthesis. (a) Transverse CT image acquired with combined-modulation technique in 71-year-old man with left hip prosthesis shows streak artifacts (129 effective mAs). (b) Transverse CT image acquired with combined-modulation technique in 72-year-old man without metallic prosthesis (145 effective mAs).

	Discussion

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Automatic tube current modulation automatically adjusts the tube current, depending on the size, shape, and attenuation of the region of interest. There are three types of commercially available automatic tube current modulation techniques: angular, z-axis, and combined modulation. The angular-modulation technique (x-axis and y-axis) adapts the tube current for each projection angle to the attenuation of the patient, to minimize x-rays in projection angles that are less significant, for reduction of the overall noise content; for example, anteroposterior or posteroanterior angles are less important compared with lateral projections because they cause less beam attenuation and, hence, are associated with less noise. The z-axis–modulation techniques select an appropriate tube current value for each section position in the scanning direction on the basis of a user-specified image quality value and a single localizer radiograph. The combined-modulation technique (CARE Dose4D; Siemens Medical Solutions) uses both automatic tube current adaptation in the scanning direction (z-axis modulation) and online tube current modulation for each tube rotation (angular modulation).

An attenuation profile along the patient's long axis is first estimated from the localizer radiograph in the direction of the projection with a mathematic algorithm. This attenuation profile contains information about the patient's size, anatomic shape, and attenuation at each position in the z-axis. To obtain transverse images, tube-current settings are calculated on the basis of these attenuation profiles. The correlation between attenuation profile and tube current for each image is defined by an analytic function, which adapts the effective tube current–time product to the patient's size, anatomic shape, and attenuation on the basis of a user-defined image quality reference tube current–time product, to maintain the desired diagnostic image quality along the scanning direction. The technique performs this tube current modulation online during each tube rotation, with tube current values adapting to the changing angular attenuation profiles of the patient.

The image quality reference tube current–time product value is selected according to the diagnostic requirements of the study and the individual preference of the radiologist. For a given scanning protocol, this value reflects the mean effective tube current–time product, which is used for a reference patient defined as a typical adult who weighs 70 to 80 kg (for adult protocols). Since the combined-modulation technique adapts the tube current to the individual patient size on the basis of the image quality reference tube current–time product values, the image quality reference tube current–time product is changed only if an adjustment for image quality is required.

Metallic prostheses generate starburst or streak artifacts that can substantially degrade CT image quality and make it hard for the radiologist to evaluate adjacent structures, which is especially crucial when assessment is required for potential surgical revision. These prostheses block incident x-ray beams, which lead to missing projection data and artifacts on the reconstructed CT images (7). The severity of streak artifacts depends on the composition of the metallic prosthesis used. Cobalt-chrome alloy and stainless steel are known to cause more artifacts than do titanium prostheses (8,9). Researchers in prior studies showed that tube current and tube potential (peak voltage) do not affect streak artifacts from metallic prostheses (6). Therefore, it is important to detect and allow for metallic prostheses when scanning with the automatic tube current modulation technique so that patients do not receive higher radiation doses in those regions with a metallic prosthesis. To the best of our knowledge, no investigators in a peer-reviewed study to date have reported automatic detection of a metallic prosthesis from the localizer radiograph and its exclusion from estimation of tube current used for scanning with automatic tube current modulation techniques.

The algorithm was incorporated into the automatic tube current modulation technique assessed in our study. The algorithm is used to estimate any abrupt increase in attenuation numbers caused by metallic structures from the localizer radiograph and neglects these numbers for calculation of the optimal tube current settings. Thus, regions with very high attenuation values, such as those with a metallic prosthesis, do not contribute to the estimation of tube current settings in the combined-modulation technique. The algorithm is important for managing radiation dose associated with automatic tube current modulation techniques, as investigators in previous studies have reported no improvement in image quality or reduction in streak artifacts with an increase in tube current for scanning of body regions with metallic prostheses or devices (6).

Our study shows that use of the algorithm with the combined-modulation technique does not increase the tube current and radiation dose when the area with the metallic prosthesis is scanned. Findings of the phantom study corroborated the results of the patient study and indicated that there was no increase in effective tube current–time product when the combined-modulation technique was used for scanning regions with a metallic prosthesis. Moreover, there was no difference in the severity and extent of streak artifacts from the metallic prosthesis between images acquired with combined-modulation techniques and those acquired with fixed–tube current techniques. Therefore, our results with this algorithm are in contradistinction to those of the prior report of increased patient dose in the region of the metallic prosthesis with a prior version of automatic tube current modulation from a different vendor (5). Successful validation of the technique assessed in our study is important, as investigators in other studies have documented that combined-modulation techniques substantially help in the reduction of radiation dose compared with results with the more commonly used fixed–tube current technique (10–12). A 20%–60% dose reduction, depending on the anatomic region and patient habitus, with improved image quality has been reported with the combined-modulation technique assessed in the present study (10,11). Researchers in another study with the combined-modulation technique from a different vendor (3D Auto mA; GE Yokogowa Medical Systems, Tokyo, Japan) also reported dose reductions of 60% in abdominal-pelvic CT examinations (12). Therefore, incorporation of the technique assessed in our study helps in the maintenance of this desirable radiation dose reduction with the combined-modulation technique in the presence of metallic prostheses.

There were limitations in our study. We did not perform a power analysis to determine how many subjects should have been included in the patient and control groups, since there were no published studies in the medical literature in which the researchers looked at the effect of metallic prostheses on the combined-modulation technique. In addition, we did not examine the effect of the composition of metallic prostheses (titanium, cobalt-chrome, or stainless steel) on radiation dose associated with the combined-modulation technique. Findings in all studies were consecutively reviewed, however, and patients with metallic prostheses were selected without regard for the composition of the metallic prosthesis. The phantom used in our study was a simplified model and did not have surrounding structures, such as those that would represent retroperitoneal fat, visceral structures, and vertebrae, to simulate cross-sectional anatomy. The findings of the phantom experiment, however, were confirmed with those of the patient study, which showed that there was no increase in effective tube current–time product or radiation dose with the use of this combined-modulation technique in the presence of metallic prostheses and that image quality was not significantly changed.

In summary, results of our study indicate that, with the combined-modulation technique, we successfully avoided an increase in radiation dose caused by the high attenuation of the metallic prosthesis in both the phantom and patients who underwent CT. Moreover, there was no substantial difference in the extent and severity of streak artifacts on images acquired with this type of combined-modulation or fixed–tube current technique.

	FOOTNOTES

 
² Current address: University of Vermont, Burlington, Vt.

See Materials and Methods for pertinent disclosures

Author contributions: Guarantor of integrity of entire study, M.K.K.; 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., S.M.R.R.; clinical studies, T.D., M.K.K., S.M.R.R., S.S.; experimental studies, T.D., M.K.K., S.M.R.R., B.S., C.S., T.F., S.S.; statistical analysis, M.K.K., S.M.R.R.; and manuscript editing, M.K.K., S.M.R.R., B.S., M.A.B., S.S.

	References

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