HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Chapman, V. M. Articles by Jaramillo, D. Search for Related Content PubMed PubMed Citation Articles by Chapman, V. M. Articles by Jaramillo, D. Social Bookmarking                      What's this? Hotlight (NEW!) What's Hotlight?

16-MDCT of the Posttraumatic Pediatric Elbow: Optimum Parameters and Associated Radiation Dose

¹ All authors: Massachusetts General Hospital, 7 Norwich Lane, Methuen, MA 01844.

Received September 7, 2004; accepted after revision November 2, 2004.

 
Address correspondence to V. M. Chapman (vchapman{at}partners.org).

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. Our objective was to investigate a low-radiation-dose protocol for 16-MDCT of the posttraumatic pediatric elbow using z-axis automatic tube-current modulation, based on optimum scanning parameters determined in a porcine fracture model, and to report the radiation dose from this technique in nine children with acute elbow trauma.

CONCLUSION. For the posttraumatic pediatric elbow, 16-MDCT using z-axis automatic tube-current modulation was optimal at 100 kVp with a noise index of 20 and a minimum amperage of 25 mA.

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Acute elbow trauma is a common indication for imaging pediatric patients. Radiographic findings for the elbow are abnormal in 33% of such cases, with fractures being the most common finding [1]. Though most fractures are adequately characterized using radiographs, complex fractures or those with a higher potential for complications are frequently evaluated with more advanced imaging, including MRI, arthrography, and CT.

In recent years, MDCT, with its rapid scanning time and 3D postprocessing capabilities, has been widely used in musculoskeletal imaging, particularly in the evaluation of acute skeletal trauma. MDCT has replaced radiography in the evaluation of the face, spine, and pelvis in trauma patients and is being used increasingly in the evaluation of acute extremity trauma [2–5]. However, MDCT scanning is associated with a higher radiation dose than are other radiation-based imaging techniques. Though specific techniques have been developed to reduce the radiation dose associated with MDCT scanning of the chest, abdomen, and pelvis in children [6], similar techniques have not been developed for MDCT of pediatric extremities.

Recent advances in MDCT scanning, including automatic tube-current modulation, have substantially reduced the radiation exposure from MDCT examinations [7]. Automatic tube-current modulation maintains a constant image quality while reducing the radiation dose by automatically adjusting the tube current within the x,y-plane (angular modulation) or the z-axis (z-axis modulation) according to the size and attenuation of the body part being examined, as determined from a single localizer radiograph [8]. Image quality using automatic tube-current modulation is determined by the user-specified noise index, which is approximately equal to the image noise in the center of a scan of a uniform phantom. As the noise index increases, so does the noise in the reconstructed image. Irrespective of patient size and position, automatic tube-current modulation maintains a constant image noise by modulating the amperage within a user-specified range, determined by the minimum and maximum amperage settings.

We report a low-radiation-dose protocol for 16-MDCT of the posttraumatic pediatric elbow using z-axis automatic tube-current modulation, based on optimum scanning parameters determined in a porcine fracture model. Furthermore, we report the radiation dose from this technique in nine children who underwent MDCT for the evaluation of acute elbow trauma.

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Animal Study
The extremity of a 10-week-old Yorkshire pig cadaver was obtained after the animal had been sacrificed as part of a separate study performed by a study collaborator. The limb was disarticulated at the shoulder, and the elbow was hyperextended to create a supracondylar fracture. Our institutional subcommittee on research animal care approved this study.

MDCT technique—On a 16-MDCT scanner (LightSpeed 16, GE Healthcare), the extremity was scanned in full extension and parallel to the z-axis of the scanner. To ensure optimum application of automatic tube-current modulation, we placed the extremity in the gantry isocenter of the scanner. Each scan included the same coverage: 82.5 mm. At a kilovolt potential of 120, 100, and then 80 kVp, scanning was first performed with a fixed tube current of 200, 100, 50, 25, and then 10 mA to establish the appropriate minimum amperage for later use in clinical automatic tube-current modulation. The remaining parameters were held constant and included a rotation speed of 0.5 sec per rotation, a table speed of 13.75 mm per rotation, a pitch of 1.375, a detector configuration of 16 x 1.25 mm, a slice thickness of 2.5 mm, image spacing of 2.5 mm, full reconstruction mode, and use of a bone reconstruction algorithm. Subsequently, scanning was performed using z-axis automatic tube-current modulation (AutomA, GE Healthcare) with an amperage range of 10 (minimum) to 200 (maximum) mA at a kilovolt potential of 120, 100, and then 80 kVp and a noise index of 20, 30, 40, and then 50. The minimum amperage was set at 10 mA, because it was the lowest the MDCT scanner would allow. The maximum amperage was set at 200 mA, because it was the amperage used in our current departmental protocol for fixed-tube-current MDCT of the elbow in adults. The remaining parameters were again held constant and included a rotation speed of 0.5 sec per rotation, a table speed of 13.75 mm per rotation, a pitch of 1.375, a detector configuration of 16 x 1.25 mm, a reconstructed slice thickness of 2.5 mm, image spacing of 2.5 mm, full reconstruction mode, and use of a bone reconstruction algorithm. Volume CT dose index and dose–length product values were recorded for each scan series.

Image analysis—MDCT images of the extremity were reviewed by two individuals who knew of the presence of the fracture but were unaware of the scanning parameters. MDCT images were assigned a degree of confidence for fracture conspicuity based on the following scale: 0, not evident; 1, poor; 2, good; and 3, excellent. All MDCT images of the extremity were reviewed at a PACS workstation (Impax RS 3000 1K review station, AGFA Technical Imaging Systems) using bone window settings (window level, 500 H; window width, 3,000 H).

Data analysis—Radiation dose and fracture conspicuity data from the fixed-tube-current technique were analyzed to determine the minimum amperage that resulted in both excellent fracture conspicuity, according to both reviewers, and the lowest radiation dose. Radiation dose and fracture conspicuity data from automatic tube-current modulation were examined to determine the combined kilovolt potential and noise index values that resulted in both excellent fracture conspicuity, according to both reviewers, and the lowest radiation dose. These parameters were then applied to the clinical protocol for scanning pediatric patients. Interobserver agreement for grading of fracture conspicuity was determined using the kappa statistic.

The percentage dose reduction of the selected technique was determined by comparison with the current adult-elbow MDCT protocol of our department: fixed-tube-current technique, 120 kVp, 200 mA, a rotation speed of 0.5 sec per rotation, a table speed of 13.75 mm per rotation, a pitch of 1.375, a detector configuration of 16 x 1.25 mm, a reconstructed slice thickness of 2.5 mm, image spacing of 2.5 mm, full reconstruction mode, and use of a bone reconstruction algorithm.

Pediatric Study
We prospectively performed unenhanced MDCT of the elbow on nine pediatric patients (five boys and four girls; age range, 3–13 years; mean, 6.7 years) referred for CT after acute elbow trauma. All patients were scanned in the emergency department or pediatric orthopedics clinic of our hospital within 48 hr of being injured. The study was approved by the institutional review board.

MDCT technique—Patients were scanned on a LightSpeed 16 scanner without the use of sedation or IV contrast medium. Scanning was performed with the patient prone and with the affected arm in a cast, in approximately 90° of flexion, held above the head. Frontal and lateral localizer radiographs were obtained using the minimum technique permitted by the scanner (80 kVp, 10 mA). Coverage included the distal humerus and the proximal radius and ulna for a total of approximately 80 mm. MDCT scan parameters were based on the results of the pig cadaver study and included 100 kVp, z-axis automatic tube-current modulation (noise index, 20; a minimum of 25 mA and a maximum of 200 mA), a rotation speed of 0.5 sec per rotation, a table speed of 13.75 mm per rotation, a beam pitch of 1.375:1, a detector configuration of 16 x 1.25 mm, a reconstructed slice thickness of 2.5 mm, image spacing of 2.5 mm, full reconstruction mode, and use of a bone reconstruction algorithm. Volume CT dose index and dose–length product values were recorded for the MDCT examination of each patient.

Image analysis—MDCT images of all nine children were reviewed by two individuals, in consensus, and graded as adequate or inadequate for interpretation. The images were reviewed at the AGFA Impax RS 3000 1K station using bone window settings (window level, 500 H; window width, 3,000 H).

Data analysis—The proportion of studies that the reviewers graded as adequate for interpretation was determined. The range of values and the average value for volume CT dose index and dose–length product were determined for the elbow MDCT studies.

Localizer radiographs from all nine MDCT scans were examined in retrospect to identify patient positions that resulted in a radiation dose more than 50% greater than that associated with the optimum automatic tube-current modulation observed in the porcine fracture model.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Animal Study
The degree of confidence and radiation dose data from MDCT of the pig cadaver extremity are shown in Tables 1 and 2. Interobserver agreement for grading of fracture conspicuity using a fixed tube current was very good, with a weighted kappa value of 0.84. Interobserver agreement for grading of fracture conspicuity using automatic tube-current modulation was good, with a weighted kappa value of 0.71.

Using a fixed tube current, the lowest radiation dose to which both reviewers assigned an excellent degree of confidence for fracture conspicuity occurred at 100 kVp and 25 mA (volume CT dose index, 0.57 mGy; dose–length product, 5.67 mGy·cm). Using automatic tube-current modulation, the lowest radiation dose to which both reviewers assigned an excellent degree of confidence for fracture conspicuity occurred at 100 kVp and a noise index of 20 (volume CT dose index, 1.45 mGy; dose–length product, 14.49 mGy·cm).

Compared with the fixed-tube-current technique using 120 kVp and 200 mA, automatic tube-current modulation using 100 kVp and a noise index of 20 resulted in an 81% decrease in radiation dose. Of note, though the minimum amperage for automatic tube-current modulation was set at 10 mA, the lowest amperage associated with the scan was 25 mA. Illustrative images of the porcine extremity fracture on MDCT images using various scanning parameters are shown in Figure 1A, 1B, 1C, 1D.

View larger version (108K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —Fractured extremity of Yorkshire pig cadaver imaged with MDCT using z-axis automated tube-current modulation at 100 kVp and various noise index settings. Fracture conspicuity for two fracture lines from minimally displaced fracture of distal humerus (arrows, A) is excellent at noise indexes of 20 (A) and 30 (B).

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —Fractured extremity of Yorkshire pig cadaver imaged with MDCT using z-axis automated tube-current modulation at 100 kVp and various noise index settings. Fracture conspicuity for two fracture lines from minimally displaced fracture of distal humerus (arrows, A) is excellent at noise indexes of 20 (A) and 30 (B).

View larger version (125K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C —Fractured extremity of Yorkshire pig cadaver imaged with MDCT using z-axis automated tube-current modulation at 100 kVp and various noise index settings. With increasing noise index, fracture lines are less distinct, with fracture conspicuity regarded as good at noise index of 40 (C) and poor at noise index of 50 (D).

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1D —Fractured extremity of Yorkshire pig cadaver imaged with MDCT using z-axis automated tube-current modulation at 100 kVp and various noise index settings. With increasing noise index, fracture lines are less distinct, with fracture conspicuity regarded as good at noise index of 40 (C) and poor at noise index of 50 (D).

View larger version (136K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A —13-year-old boy with elbow pain after a fall. MDCT of elbow was performed for preoperative planning. Frontal (A) and lateral (B) radiographs of elbow show displaced fracture of lateral humeral condyle and comminuted fracture of proximal ulna but are limited by overlying splint.

View larger version (130K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B —13-year-old boy with elbow pain after a fall. MDCT of elbow was performed for preoperative planning. Frontal (A) and lateral (B) radiographs of elbow show displaced fracture of lateral humeral condyle and comminuted fracture of proximal ulna but are limited by overlying splint.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C —13-year-old boy with elbow pain after a fall. MDCT of elbow was performed for preoperative planning. Axial (C) and coronal (D) MDCT images show degree of displacement of lateral condyle fracture fragment (arrow) and comminuted proximal ulnar fracture (arrowheads, C).

View larger version (98K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2D —13-year-old boy with elbow pain after a fall. MDCT of elbow was performed for preoperative planning. Axial (C) and coronal (D) MDCT images show degree of displacement of lateral condyle fracture fragment (arrow) and comminuted proximal ulnar fracture (arrowheads, C).

View larger version (102K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2E —13-year-old boy with elbow pain after a fall. MDCT of elbow was performed for preoperative planning. Three-dimensional image shows fractures of lateral condyle and proximal ulna and displacement and relationship of fracture fragments to donor sites and one another.

Review of the localizer radiographs of these patients revealed that one patient had increased attenuation and resulting increased radiation dose associated with positioning the axis of the forearm along the x,y-plane. Three patients had increased attenuation and increased radiation dose because of positioning of the elbow below the level of the skull vertex. Localizer radiographs of three patients with variations in elbow position are shown in Figure 3A, 3B, 3C.

View larger version (107K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A —Variation in radiation dose as function of patient position, shown on scout images from MDCT of elbow in three children evaluated for acute elbow trauma. 6-year-old girl is correctly positioned with elbow superior to vertex of skull and forearm angled to x,y-plane, with resulting volume CT dose index of 0.98 mGy and dose–length product of 9.73 mGy·cm.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B —Variation in radiation dose as function of patient position, shown on scout images from MDCT of elbow in three children evaluated for acute elbow trauma. 11-year-old girl is incorrectly positioned with forearm in x,y-plane, with resulting increased attenuation and volume CT dose index of 4.49 mGy and dose–length product of 37.04 mGy·cm.

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C —Variation in radiation dose as function of patient position, shown on scout images from MDCT of elbow in three children evaluated for acute elbow trauma. 4-year-old boy is incorrectly positioned with skull in plane with elbow, with resulting increased attenuation and volume CT dose index of 7.38 mGy and dose–length product of 55 mGy·cm.

Top Abstract Introduction Subjects and Methods Results Discussion References

We chose to use automatic tube-current modulation to avoid increased beam attenuation and artifacts from nonideal patient positioning. Though a fixed-tube-current technique using a lower kilovolt potential or amperage results in a constant and predictable reduction in the radiation dose associated with MDCT, the image quality varies inversely with the attenuation of the body part being examined. In fact, the fixed-tube-current technique using 100 kVp and 25 mA for scanning of the porcine fracture resulted in a lower radiation dose than did automatic tube-current modulation; however, four of the nine subsequent pediatric elbow MDCT studies were associated with increased attenuation as a result of positioning, and this increase would have resulted in reduced image quality (i.e., increased noise) using a fixed tube current.

We believe it is most important to ensure diagnostic-quality images while minimizing the radiation dose and that this is best achieved using automatic tube-current modulation.

The utility of automatic tube-current modulation and the importance of proper elbow positioning are reflected in the wide range of volume CT dose index and dose–length product values for the nine patients scanned. Angling the axis of the forearm to the x,y-plane and positioning the elbow superior to the vertex of the skull results in the lowest radiation dose, and failure to do so results in increased attenuation and, consequently, increased radiation dose. Though the maximum radiation dose observed in the current study was substantially greater than the average, the results of the porcine fracture study indicated that the radiation dose in these cases was less than would have resulted from our previous departmental protocol for the fixed-tube-current technique.

To ensure the minimum radiation dose using automatic tube-current modulation, one may assess the estimated volume CT dose index and dose–length product values provided by the scanner before one performs the MDCT. If the values are higher than expected, the localizer radiograph may be examined to identify errors in patient positioning that may be corrected and thus avoid the radiation associated with a repeated localizer radiograph.

Most important, though the protocol presented here for MDCT of the elbow minimizes radiation from the examination, the best means of limiting radiation exposure from CT in children is careful selection of patients who will benefit from the information gained. To this end, we are currently investigating the use of elbow MDCT in children for various clinical situations to provide evidence for its appropriate clinical use. In conclusion, for the posttraumatic pediatric elbow, 16-MDCT using z-axis automatic tube-current modulation is optimal at 100 kVp with a noise index of 20 and a minimum amperage of 25 mA.

References

Top Abstract Introduction Subjects and Methods Results Discussion References

 

1. Petit P, Sapin C, Henry G, et al. Rate of abnormal osteoarticular radiographic findings in pediatric patients. AJR2001; 176:987 –990[Abstract/Free Full Text]
2. Rhea J, Rao P, Novelline R. Helical CT and three-dimensional CT of facial and orbital injury. Radiol Clin North Am1999; 37:489 –513[CrossRef][Medline]
3. Sheridan R, Peralta R, Rhea J, Ptak T, Novelline R. Reformatted visceral protocol helical computed tomographic scanning allows conventional radiographs of the thoracic and lumbar spine to be eliminated in the evaluation of blunt trauma patients. J Trauma2003; 55:665 –669[Medline]
4. Guillamondegui O, Pryor J, Gracias V, Gupta R, Reilly P, Schwab C. Pelvic radiography in blunt trauma resuscitation: a diminishing role. J Trauma 2002; 53:1043 –1047[Medline]
5. Pretorius E, Fishman E. Volume-rendered three-dimensional spiral CT: musculoskeletal applications. RadioGraphics1999; 19:1143 –1160[Abstract/Free Full Text]
6. Frush DP, Slack CC, Hollingsworth CL, et al. Computer-simulated radiation dose reduction for abdominal multidetector CT of pediatric patients. AJR 2002; 179:1107 –1113[Abstract/Free Full Text]
7. Kalra MK, Maher MM, Kamath RS, et al. Sixteen-detector row CT of abdomen and pelvis: study for optimization of Z-axis modulation technique in 153 patients. Radiology 2004;233 : 241–249[Abstract/Free Full Text]
8. Kalra MK, Maher MM, Toth TL, et al. Strategies for CT radiation dose optimization. Radiology 2004;230 : 619–628[Abstract/Free Full Text]
9. Franklin PD, Dunlop RW, Whitelaw G, et al. Computed tomography of the normal and traumatized elbow. J Comput Assist Tomogr 1988; 12:817 –823[Medline]
10. Jurik AG, Albrechtsen J. The use of computed tomography with two- and three-dimensional reconstructions in the diagnosis of three- and four-part fractures of the proximal humerus. Clin Radiol1994; 49:800 –804[CrossRef][Medline]
11. Frush DP. Pediatric CT: practical approach to diminish the radiation dose. Pediatr Radiol 2002;32 : 714–717[CrossRef][Medline]

CiteULike

Complore

Connotea

Del.icio.us

Digg

Reddit

Technorati

This article has been cited by other articles:

				S. Singh, M. K. Kalra, M. A. Moore, R. Shailam, B. Liu, T. L. Toth, E. Grant, and S. J. Westra Dose Reduction and Compliance with Pediatric CT Protocols Adapted to Patient Size, Clinical Indication, and Number of Prior Studies Radiology, July 1, 2009; 252(1): 200 - 208. [Abstract] [Full Text] [PDF]

				N. C. Dalrymple, S. R. Prasad, F. M. El-Merhi, and K. N. Chintapalli Price of Isotropy in Multidetector CT RadioGraphics, January 1, 2007; 27(1): 49 - 62. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Chapman, V. M. Articles by Jaramillo, D. Search for Related Content PubMed PubMed Citation Articles by Chapman, V. M. Articles by Jaramillo, D. Social Bookmarking                      What's this? Hotlight (NEW!) What's Hotlight?