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DOI:10.2214/AJR.06.1146
AJR 2007; 188:1648-1650
© American Roentgen Ray Society

Technical Innovation

MOSFET Dosimetry for Radiation Dose Assessment of Bismuth Shielding of the Eye in Children

Srinivasan Mukundan, Jr.1, Page Inman Wang, Donald P. Frush, Terry Yoshizumi, Jeffrey Marcus, Emily Kloeblen and Meredith Moore

1 All authors: Department of Radiology, Duke University Medical Center, DUMC Box 3808, Durham, NC 27705.

Received August 27, 2006; accepted after revision November 21, 2006.

 
S. Mukundan is a 2005–2006 American Roentgen Ray Society Scholar.

Address correspondence to S. Mukundan, Jr.


Abstract
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 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to measure radiation dose to the orbit during pediatric cranial CT with and without bismuth shielding using a novel dosimetry system. Cranial CT was performed on a pediatric anthropomorphic phantom, with and without bismuth eye shields. A solid-state metal oxide semiconductor field effect transistor (MOSFET) dosimeter was used to obtain real-time dose measurements.

CONCLUSION. Bismuth shielding reduced radiation dose to the eye by up to 42%; shield artifact fell outside the diagnostic area of interest.

Keywords: bismuth shielding • CT • dosimetry • eye • head and neck imaging • pediatric imaging • radiation dose • safety


Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
The advent of MDCT has resulted in improved spatial resolution and faster scan acquisitions. Consequently, MDCT has become a more widely used diagnostic procedure, responsible for a greater proportion of medical radiation exposure to patients. Although CT represents 11% of all radiographic examinations, it accounts for 67% of medically induced radiation exposure [1]. Ten percent of all CT examinations are performed in children (age range, birth–15 years) to assess for disorders including trauma, congenital anomalies, metabolic diseases, inflammatory lesions, and tumors [1].

The eye is one of the most radiosensitive tissues, and the threshold for inducing cataracts in adults has been documented as low as 0.5–2 Gy (50–200 rad) [2]. In children this threshold is even lower, with the development of cataracts having been documented at less than half this dose of radiation [2]. Therefore, it is of paramount importance to shield the pediatric orbit from any unnecessary radiation during CT. Typically, the dose to the eye is about 50 mGy (5 rad), depending on the instrument and protocol [1]. Moreover, the as-low-as-reasonably-achievable (ALARA) principle would dictate that the radiation dose be limited.

Our objective for this study was to assess a contemporary method for measuring radiation dose to the pediatric lens and orbit and to apply this method to determine whether the novel use of a bismuth-impregnated latex eye shield could offer protection from radiation without a loss in di agnostic quality.


Materials and Methods
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 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Metal oxide semiconductor field effect transistors (MOSFETs) were used to measure radiation dose rather than traditional thermoluminescence dosimeters (TLDs) because MOSFETs are sensitive to a few milligrays and have a linear response at these doses [3]. Because MOSFETs have immediate readout and reuse, they are convenient for MDCT use [3]. The MOSFET high-sensitivity dosimeters (TN-1002RD, Best Medical Canada, Ltd.) were individually calibrated at each CT energy using a National Institute of Standards and Technology (NIST) traceable calibrated ionization chamber. Before the study, a linearity test was performed to verify the actual dose response to prescribed instrument mA settings for the range 80–300 mA.

The 5-year-old pediatric anthropomorphic phantoms (705-D, CIRS) used in this study were constructed of human-tissue-equivalent materials to represent the configuration and size of a typical 5-year-old child. Before exposure in the CT scanner, 20 MOSFET dosimeters were deployed throughout the phantom eye lens well, retinal well (eye globe), brain, chest, and thyroid (Fig. 1). The eye shield was fabricated from a double layer of bismuth-impregnated latex (1.7 g Bi/cm2, equivalent to 0.45 mg/cm3 of lead) that was placed on top of a 1.0-cm-thick radiolucent foam step-off pad.


Figure 1
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  		Fig. 1 —Photograph of pediatric anthropomorphic phantom with bismuth eye shield and metal oxide semiconductor field effect transistor (MOSFET) dosimeters deployed.

 
The anthropomorphic phantoms were scanned using a commercially available 16-MDCT scanner (LightSpeed, GE Healthcare). Both axial brain (5.0-mm-thick slices, 140 mAs, 140 kVp) and helical craniofacial (2.5-mm-thick slices; 170 mAs; 140, 120, and 100 kVp) pediatric protocols were performed. Each mAs–kVp combination was imaged three times and averaged with and without the eye shield in position.


Figure 2
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  		Fig. 2 —Bar graph shows radiation dose (cGy) to eye globe and lens versus energy (kVp). Vertical lines represent SDs.

 


Figure 3
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  		Fig. 3A —CT images of pediatric anthropomorphic phantom. Axial bone window setting image of bismuth shield (high density, ventral to orbits) and of step-off pad (intermediate density, ventral to orbits).

 


Figure 4
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  		Fig. 3B —CT images of pediatric anthropomorphic phantom. Because significant artifact was not present using conventional bone window setting (A), windows were artificially widened to show artifact (arrows). Bismuth shield with step-off pad successfully deflected artifact away from regions of anatomic interest.

 

Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
To determine the dose reduction due to bismuth shielding, the following formula was used: (NS – S) / NS x 100, in which NS represented the unshielded dose, and S represented the shielded dose, both measured in cGy. Using the craniofacial protocol, the average dose reduction to the eye as a result of bismuth shielding was 39%, 42%, and 41% at 140, 120, and 100 kVp, respectively (Fig. 2). The lens of the eye experienced a smaller dose reduction at 36%, 25%, and 45% at 140, 120, and 100 kVp, respectively, again using the craniofacial protocol (Fig. 2).

The axial head protocol at 140 kVp revealed a radiation dose reduction from 4.6 (unshielded) to 2.8 cGy (shielded) for the orbit and a radiation dose reduction from 2.9 (unshielded) to 2.1 cGy (shielded) for the lens. Therefore, the dose reduction at 140 kVp was 39% for the orbit and 28% for the lens.


Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Other studies, using traditional approaches, have looked at the utility of orbital shielding and found similar dose reduction of approximately 40% [4–6]. However, one study, by McLaughlin and Mooney [7], used an angled gantry to exclude direct exposure to the eyes and found that bismuth shielding reduced the radiation dose by only 18%. Another study found that when taken to an extreme and optimized gantry angles were used, the dose reduction due to bismuth shielding was less than 2–5% [5]. Therefore, it has been suggested that it should be routine practice to avoid the eye when possible in head CT and align the scan with the supraorbital meatal baseline [8].

Exclusion of the orbits as a means of reducing the dose, however, is impractical. Yeoman et al. [8] reported that only 32% of radiology centers surveyed routinely avoided the orbits in head CT examinations. Even if all centers used a policy to configure pediatric CT along the supraorbital meatal baseline, axial scanning would still be necessary to evaluate a child for craniosynostosis or any orbital, sinus, or mastoid problems. Therefore, excluding the orbit would not be possible, and bismuth shielding would be helpful to reduce the dose from radiation exposure.

Concern about artifact has limited the usage of bismuth shielding; however, in most instances, this artifact is minimal [4–7]. Using a step-off pad, the artifact from the bismuth shields was not evident on the initial scan and only became visible after artificially widening the window settings of the image. The step-off pads below the bismuth shields moved the artifact to a more ventral location outside the area of diagnostic interest (Fig. 3A, 3B).

Because all new CT scanners use automatic tube current modulation as a tool to decrease radiation dose, it is important to evaluate the effect bismuth shielding might have on automatic current modulation. However, to the best of our knowledge, there has been no systematic evaluation of bismuth shields for in-plane shielding using automatic tube current modulation in patients. The method of tube current modulation varies depending on the manufacturer [9]. For example, one form of tube current modulation modulates the tube current based on regional changes in density assessed on the topogram (scout image). In this setting, placement of a shield could arguably offset both the dose reduction through the shield and the benefits of tube current modulation because the increased density through the shielded region seen on the topogram would result in an increase in tube current. Preliminary investigation of this phenomenon has shown that placement of the shield after the topogram has been obtained reduces this effect (Frush DP, unpublished data).

In summary, we found that the use of a bismuth shield with a step-off pad significantly reduces the radiation dose to the eye by 42%. This dose reduction can be reliably detected by a MOSFET dosimeter in an MDCT scanner, which is important because the rapid development of scanner technology has resulted in the lack of development and validation of mathematic models for dose estimation for all scanner types. Given the sensitivity of the pediatric eye to radiation exposure, bismuth shields should be used in pediatric patients when the orbits are included in the CT examination.


Acknowledgments
 
We thank Carolyn Lowry for technical support. In addition, this study was performed as part of a senior-year engineering project by Jesse Riley and Jena Jamal, students in the Pratt School of Engineering at Duke University. Giao Nguyen and Greta Toncheva of the radiation safety office played key roles in the data acquisition.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Task Group on Control of Radiation Dose in Computed Tomography. Managing patient dose in computed tomography: a report of the International Commission on Radiological Protection. Ann ICRP2000; 30:7 –45[Medline]
  2. Merriam GR Jr, Focht EF. A clinical study of radiation cataracts and the relationship to dose. Am J Roentgenol Radium Ther Nucl Med 1957; 77:759 –785[Medline]
  3. Brenner DJ. Is it time to retire the CTDI for CT quality assurance and dose optimization? Med Phys 2005;32 :3225 –3226[CrossRef][Medline]
  4. Colombo P, Pedroli G, Nicoloso M, Re S, Valvassori L, Vanzulli A. Evaluation of the efficacy of a bismuth shield during CT examinations. Radiol Med (Torino) 2004;108 : 560–568[Medline]
  5. Perisinakis K, Raissaki M, Theocharopoulos N, Damilakis J, Gourtsoyiannis N. Reduction of eye lens radiation dose by orbital bismuth shielding in pediatric patients undergoing CT of the head: a Monte Carlo study. Med Phys 2005;32 :1024 –1030[CrossRef][Medline]
  6. Hopper KD, Neuman JD, King SH, Kunselman AR. Radioprotection to the eye during CT scanning. Am J Neuroradiol2001; 22:1194 –1198[Abstract/Free Full Text]
  7. McLaughlin DJ, Mooney RB. Dose reduction to radiosensitive tissues in CT: do commercially available shields meet the users' needs? Clin Radiol 2004;59 : 446–450[CrossRef][Medline]
  8. Yeoman LJ, Howarth L, Britten A, Cotterill A, Adam EJ. Gantry angulation in brain CT: dosage implications, effect on posterior fossa artifacts, and current international practice. Radiology 1992;184 : 113–116[Abstract/Free Full Text]
  9. McCollough CH, Bruesewitz MR, Kofler JM Jr. CT dose reduction and dose management tools: overview of available options. RadioGraphics 2006;26 : 503–512[Abstract/Free Full Text]

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