Direct and Indirect Measurement of Patient Radiation Exposure during Endovascular Aortic Aneurysm Repair

Article Outline

• Abstract
• Introduction
• Methods
  • Skin Dose Measurement
  • Statistical Analysis
• Results
• Discussion
• References
• Copyright

With the increasing complexity of endovascular procedures, concern has grown regarding patient radiation exposure. Abdominal aortic aneurysm (AAA) repair represents the most common complex endovascular procedure currently performed by vascular specialists. Our study evaluates the patient radiation dose received during endovascular AAA repair. Over a 3-month period we prospectively monitored the radiation dose in a series of consecutive patients undergoing endovascular AAA repair. All patients underwent standard endovascular AAA repair with one of two commercially available grafts using the GE OEC 9800 unit. Direct measurement of maximum radiation dose at skin level (peak skin dose, PSD) was recorded using GAFCHROMIC radiographic dosimetry film. Indirect measurements of radiation dose (fluoroscopy time and dose-area-product [DAP]) were recorded with the C-arm dosimeter. A total of 12 consecutive patients undergoing standard endovascular AAA repair were evaluated. Mean PSD was 0.75 Gy (range 0.27-1.25). Mean total fluoroscopy time was 20.6 min (range 12.6-34.2) with an average of 92% spent in standard fluoroscopy and 8% spent in cinefluoroscopy. Regarding total fluoroscopy time, 49% was spent in normal field of view and 51% in magnified view. Mean DAP was 15,166 cGy · cm² (range 5,207-24,536). PSD correlated with DAP (r = 0.9, p < 0.05) but not total fluoroscopy time (r = 0.18, p > 0.05). PSD also correlated with body mass index (BMI; r = 0.82, p < 0.05). Obese patients had a mean PSD of 1.1 Gy compared to 0.5 Gy in nonobese patients. PSD of all patients was well below the accepted 2.0 Gy threshold for skin injury. PSD correlated with DAP but not total fluoroscopy time. PSD also correlated with BMI, and the mean PSD was significantly increased in obese compared to nonobese patients. Despite the complexity and duration of endovascular AAA repair, the procedure can be performed safely without excessive radiation exposure.

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Introduction 

With the increasing complexity and length of endovascular procedures, concern has grown regarding patient radiation exposure.¹, ², ³ Abdominal aortic aneurysm (AAA) repair is one of the most complex endovascular procedures currently performed by vascular specialists. The primary concern during endovascular procedures is skin injury,⁴ reflected by the peak skin dose (PSD), the maximum radiation dose to any point on the patient's skin.⁵ Radiation skin dose is most commonly assessed utilizing indirect methods through measurement of the dose-area-product (DAP). Direct methods have previously been cumbersome or expensive, and as a result, most studies reporting on patient radiation exposure during endovascular operations utilize indirect methodology for their measurements.⁶, ⁷ Direct methods are superior to indirect ones because they are much more precise and exact in quantifying PSD.⁸ In the present study we sought to use both direct and indirect measurements to assess the patient radiation skin dose during standard endovascular AAA repair.

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Methods 

The study was undertaken as a quality-improvement process at the Nebraska Medical Center in conjunction with the Nebraska/Western Iowa Veterans Administration Hospital. Over a 3-month period we prospectively monitored the skin radiation exposure in a series of 12 consecutive patients undergoing standard endovascular AAA repair. All patients underwent modular bifurcated endograft placement via open femoral artery access with one of two commercially available endografts (Zenith AAA Endograft from Cook, Bloomington, IN; Gore Excluder AAA Endograft from W. L. Gore, Flagstaff, AZ) for infrarenal aortic aneurysm repair. Standard operative techniques including open bilateral common femoral artery exposure were performed by three board-certified vascular surgeons in conjunction with a general surgery chief resident. Procedures were performed in the operating room with an OEC 9800 portable C-arm with 12-inch image intensifier (GE Medical Systems, Waukesha, WI) and a carbon fiber table with a four-way motorized tabletop (Allegro 6800 Mobile Imaging Table; Orthopedic Systems, Union City, CA). Imaging was obtained in standard fluoroscopic mode without the use of pulsed or low-dose fluoroscopy. The maximum cinefluoroscopy frame rate was 15 frames per second. All data were prospectively collected.

Skin Dose Measurement 

Indirect measurements of radiation dose included total fluoroscopy time and DAP, which were recorded by the C-arm throughout each procedure utilizing the standard software package. Direct measurement of PSD or maximal radiation dose administered to the patient's skin was recorded using GAFCHROMIC^® XR-RV2 radiographic dosimetry film (International Specialty Products, Wayne, NJ). GAFCHROMIC film is a self-developing radiation-sensitive film utilized for quality control and dosimetric measurement during fluoroscopic procedures. The film, measuring 17 × 14 inches, is placed immediately underneath the patient at the level of the abdomen and pelvis, thus covering the entire field of view and eliminating orientation effects. GAFCHROMIC is opaque yellow prior to radiation exposure and develops a blue color, the intensity of which is proportionate to radiation exposure (Fig. 1). The radiation dose is then quantified with an optical densitometer that has been calibrated using film exposed to a known radiation dose. GAFCHROMIC film is capable of measuring radiation doses of 1 cGy-50 Gy and has a sheet-to-sheet variation of <5%. The cost of GAFCHROMIC film is $21 per sheet.

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• Fig. 1 

  GAFCHROMIC film following endovascular AAA repair. GAFCHROMIC is a radiation-sensitive, self-developing film used for measuring and mapping patient skin exposure. The film's color density increases with radiation dose. The center area corresponds to the main aortic body deployment, with the lower iliac limb deployment regions noted bilaterally. PSD was noted where overlap between fields of view occurred.

Statistical Analysis 

Mean values and standard deviations (SDs) were calculated with Excel (Microsoft, Redmond, WA). Scatter plots were created with Excel (Microsoft), and corresponding Pearson correlation coefficients (r value) were calculated using Sigmastat (Jandel, San Rafael, CA). Student t-tests were used to compare differences between the obese and nonobese groups of patients. p ≤ 0.05 was considered significant.

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Results 

A total of 12 patients underwent standard endovascular AAA repair from May 14, 2007, to August 22, 2007. Demographic information of the patient population is shown in Table I. The mean patient age was 69 years (SD = 9.9, range 54-86), with a mean body mass index (BMI) of 28.4 (SD = 4.4, range 19.9-34.5). One patient in the study was female, and the remaining were male. The mean aneurysm size was 5.65 cm (SD = 1.05, range 4.4-7.8), and mean proximal neck length was 2.8 cm (SD = 1.06, range 1.5-5). All aneurysms had angulations less than 60 degrees. Nine patients received a Zenith endograft and three received a Gore endograft.

Table I. Patient demographics

The mean total fluoroscopy time was 20.6 min (SD = 7.5, range 12.6-34.2). Standard fluoroscopy was used during 92% of the total fluoroscopic imaging time, and cinefluoroscopy was used the remaining 8%. Normal field of view was used for 49% of the fluoroscopy time and magnified view for 51%.

The radiation skin dose measurements are shown in Table II. The mean PSD for all patients was 0.75 Gy (SD = 0.36, range 0.27-1.25), and the maximum PSD among all patients was 1.25 Gy. The mean DAP was 15,166 cGy · cm² (SD = 7,835, range 5,207-24,536).

Table II. Fluoroscopy Time and Radiation Dose

Direct PSD correlated with the indirect measurement of DAP (r = 0.9, p < 0.001) but not total fluoroscopy time (r = 0.18, p = 0.58) (Fig. 2, Fig. 3). PSD also correlated with BMI (r = 0.82, p < 0.01) (Table III, Fig. 4, Fig. 5). The mean PSD of obese patients (BMI > 30) was significantly increased compared to that of nonobese patients (1.1 vs. 0.5 Gy, p < 0.001). PSD did not correlate with aneurysm size (r = -0.35, p = 0.24) but had a negative correlation with aneurysm neck length (r = -0.67, p < 0.015) (Fig. 6, Fig. 7).

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• Fig. 2 

  Scatterplot with trend line demonstrating correlation between PSD and DAP (r = 0.90, p < 0.001).

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• Fig. 3 

  Scatterplot with trend line showing no correlation between PSD and total fluoroscopy time (r = 0.18, p = 0.58).

Table III. BMI and Peak Skin Dose

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• Fig. 4 

  Scatterplot with trend line demonstrating correlation between PSD and BMI (r = 0.82, p < 0.01).

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• Fig. 5 

  Scatterplot with trend line demonstrating correlation between DAP and BMI (r = 0.76, p < 0.005).

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• Fig. 6 

  Scatterplot with trend line showing no correlation between PSD and aneurysm size (r = −0.35, p = 0.24).

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• Fig. 7 

  Scatterplot with trend line demonstrating correlation between PSD and proximal neck length (r = −0.67, p < 0.015).

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Discussion 

With the increasing complexity of endovascular interventions and need for repeated exposure to ionizing radiation, concern has grown regarding radiation exposure to both the patient and practitioner. Previous studies have measured patient radiation exposure during selected endovascular procedures, but accurate measurements of patient radiation exposure during endovascular AAA repair have not been documented. Our study was undertaken as part of a quality-improvement project to ensure acceptable levels of patient radiation exposure during prolonged fluoroscopic procedures.

Various methods can be used to estimate the peak radiation dose delivered to the skin. Indirect dosimetric methods include fluoroscopy time and DAP.⁴ Fluoroscopy time does not provide information regarding the dose rate and is therefore usually considered insufficient to accurately determine the patient dose.⁴ DAP is defined as the radiation absorbed in air multiplied by the cross-sectional area of the X-ray beam.² DAP is measured by placing an ionization chamber (large enough to completely intercept the X-ray beam) just beyond the X-ray collimators and is an overall measurement of the total radiation energy delivered to the patient.⁴, ⁹ DAP provides no information regarding the spatial distribution of the radiation dose and is therefore often a poor measurement of PSD and risk of skin injury.⁸ In addition, DAP ignores the effect of backscatter from the patient, which can increase the skin dose by 10-40%.⁴ Indirect measurements can be used to estimate the patient skin dose but cannot precisely determine the PSD.⁶ In contrast to indirect measurements, direct measurements can provide precise measurements of skin dose.⁸ Direct measurement of PSD or maximal radiation dose at any given point on the patient's skin has previously been measured by varying techniques which, although accurate, were cumbersome and difficult to perform.⁶ Recently, a commercially available film for direct measurement of PSD was utilized in our institution. GAFCHROMIC film is a self-developing radiation-sensitive film utilized for quality control and dosimetric measurement during fluoroscopic procedures. The film measures 17 x 14 inches covering the entire field of view and eliminates orientation effects. GAFCHROMIC is yellow prior to radiation exposure, and develops a blue color whose intensity is proportionate to the PSD (Fig. 1). The radiation dose is then quantified with an optical densitometer that has been calibrated using film exposed to a known radiation dose. By utilizing the simple and direct measurement of PSD with GAFCHROMIC film, we were able to readily confirm the safety of endovascular AAA repair with regard to PSD.

Our study utilizing direct and accurate measurement shows that endovascular AAA repair results in PSD levels well below the 2 Gy threshold for skin injury.¹⁰, ¹¹ The maximum PSD in the 12 patients studied was 1.25 Gy. Low PSD despite higher fluoroscopy times can be attributed to dispersion of the field of view throughout the abdominal cavity compared to the other documented interventional procedures, where a focused and magnified view is required. Although our current study demonstrated a low patient PSD, it is clear from previous studies that complex endovascular cases can easily result in PSD above the threshold dose of skin injury. Miller et al.⁶ performed an observational study of patient radiation dose delivered during 21 common interventional radiology procedures. PSD was calculated indirectly with skin dose mapping software. All of the 21 procedures evaluated except nephrostomy, pulmonary angiography, and inferior vena cava filter placement had instances of PSD >2 Gy. In 14% of iliac angioplasties with stenting and 33% of visceral angioplasty with stenting the PSD was >2 Gy. Due to this propensity of complex endovascular procedures to result in high patient skin doses, vascular surgeons must be diligent with their use of fluoroscopy despite the findings in our study.

In the current study, PSD dose strongly correlated with DAP (r = 0.90) but not fluoroscopy time (r = 0.18). This is in agreement with the findings from the Radiation Dose in Interventional Radiology Procedures Study, which also reported a strong correlation between PSD and DAP (r = 0.85), despite a wide variation between cases.⁶ Fletcher et al.¹ also found significant correlation between PSD and DAP in various interventional radiological procedures, with the closest correlation found for port placement (r = 0.94) and the lowest (although still significant) for aortogram with runoff (r = 0.6). Based on the current study and previous reports, although not ideal, DAP is likely an adequate real-time estimate of PSD for the practicing vascular surgeon. What is clear, however, is that total fluoroscopy time is a poor predictor of PSD, with our data demonstrating no correlation between the two parameters. This is likely due to fluoroscopy time not accounting for dose rate variability due to patient size or technique. In addition, fluoroscopy time provides no information regarding the X-ray field size, area of the body irradiated, position, magnification, cinefluoroscopy, or distance from the skin, all of which are important determinants of the radiation dose delivered.¹, ⁴, ⁶ Therefore, when assessing radiation to the patient, one must rely on measurement other than total fluoroscopy time.

In our study PSD had a strong correlation with patient BMI. The correlation is not surprising considering that obesity is common among patients with radiation-induced skin injury.³, ¹² Fluoroscopy utilizes low-energy X-ray radiation, which is rapidly attenuated as it passes through tissue.³ In obese patients the X-ray beam must penetrate more tissue to reach the image detector.², ¹³ Automatic exposure control as found in mobile C-arms increases the dose rate to increase the penetration of the X-ray beam. A photodiode detects the low light output by the image intensifier and produces a feedback signal to increase the radiation dose until there is sufficient penetration to produce an adequately bright image.¹⁴ In our series, obese patients received two to three times higher DAP and PSD than patients with normal BMI. With the prevalence of obesity being >30% in the United States and rapidly growing in a number of countries worldwide, high radiation exposure should always be considered when planning endovascular procedures on obese patients.¹⁵ Attempts at reducing radiation exposure with the use of collimators, pulsed fluoroscopy, and minimized fluoroscopy time should always be made.¹⁶

In contrast to patient radiation exposure, the surgeon's exposure during endovascular AAA repair has been addressed by both Lipsitz et al.⁷ and Ho et al.¹⁷ These studies found the accumulated radiation dose of the surgeon to be well below the maximum yearly occupational limit recommended by the International Commission of Radiation Protection (ICRP). Ho et al.¹⁷ measured the yearly effective body, eye, and hand radiation doses for vascular surgeons performing standard aortic repairs, aortograms, and percutaneous transluminal angioplasty and stenting. The median yearly effective dose of the surgeons was approximately 1% of the maximum yearly occupational radiation exposure recommended by the ICRP. Endovascular aortic repairs had the highest radiation exposure, but a vascular surgeon would still need to perform 2,597 endovascular aortic repairs to reach the yearly dose limit.¹⁷ Although these reports suggest that radiation injury to the surgeon should be minimal, the stochastic effects of radiation are still present; therefore, surgeons must adhere to the “as low as reasonably achievable” practice. The above studies also addressed patient PSD during endovascular AAA operations but in a relatively limited fashion. Ho et al.¹⁷ measured the patient skin dose by attaching a single mini-thermoluminesecent dosimeter (3 mm² × 1 mm) to a predetermined point on the patient where they predicted the maximum patient exposure was located. The mean patient surface dose was 0.0127 Gy, far less than found in our study. Unfortunately, this methodology is not accurate at predicting PSD.¹⁸ Lipsitz et al.⁷ did not measure the PSD but calculated the patient entrance dose using the fluoroscopic energy and positions recorded during the procedure (mean 0.36 Gy, range 0.12-0.86). Similar to the study by Ho et al.,¹⁷ PSD levels were low, in contrast to the current study, again likely secondary to the measurement technique.

Ultimately, vascular surgeons must take into account the deterministic and stochastic effects of radiation for their patients.⁷ The deterministic effects of radiation are those that occur when a threshold radiation dose is exceeded. Radiation-induced skin injury is an example of a deterministic effect, where increased likelihood and severity of injury correlate with increasing radiation dose.¹⁰ Radiation-induced skin injury ranges from erythema to necrosis.¹¹ The first indication of skin injury is transient erythema. The erythema occurs within hours of radiation exposure and resolves in approximately 48 hr.¹⁰, ¹¹ The threshold PSD for erythema is 2 Gy.¹⁰, ¹¹ Patient factors such as diabetes or connective tissue disease should be taken into account as these may result in increased sensitivity to radiation.¹⁰ The incidence of radiation injury is small compared to the number of procedures in which fluoroscopy is used. However, since 1990, hundreds of cases of fluoroscopically induced dermatitis, including numerous cases of dermal necrosis, have been reported.¹⁹ Although the incidence is small, when skin injury does occur it may be severe. In a review of 73 cases of radiation-induced skin injury, there were 38 cases of severe injury such as chronic ulceration and necrosis and 18 cases that required skin grafts.³, ¹⁰ Of these 73 reported cases, 59 were cardiac procedures, seven were transjugular intrahepatic portosystemic shunt (TIPS) placement, three were neuroradiological interventions, and four were other procedures in the abdomen and chest including two renal angioplasties.³, ¹⁰ In contrast to skin injury, radiation-induced cancer is a stochastic effect of radiation where there is no threshold value and severity is independent of dose. The probability of cancer occurrence increases with dose, but the cancer may occur at any dose.⁷, ¹⁰ The distinction between deterministic and stochastic effects is important when placing the current study into perspective. Our data demonstrate that AAA patients receive minimal radiation exposure during their AAA repair procedure with regard to deterministic skin injury. However, patients having endovascular AAA repair are in need of multiple radiographic evaluation, treatment, and follow-up studies; and their increased risk of stochastic effects must be considered. There is assumed to be a linear dose-response relation between radiation dose and cancer, and as the patient dose increases over years of surveillance, so will their cancer risk.²⁰ Following endovascular AAA repair, patients are often followed with a computed tomographic (CT) scan at 3, 6, and 12 months and then yearly. Based on a conservative effective dose estimate of 10 mSv per CT of the abdomen and pelvis,²¹, ²² patients may receive 40 mSv in the first year from CT scans alone. Brenner and Hall²³ recently used the increased cancer incidence in atomic bomb survivors to estimate the cancer risk associated with a CT scan. There was a significant increase in cancer risk in a subgroup of atomic bomb survivors who received a maximum dose of 150 mSv, with a mean of 40 mSv. Brenner and Hall²³ estimated an increased cancer risk of 0.01-0.02% in those who are exposed to radiation from a single CT in their mid-30s or older. The risk of cancer to the individual is low, but with approximately 62 million CT scans performed each year, 1.5-2% of all cancers diagnosed may ultimately be attributable to CT scans. One may argue that our patients are older and the likelihood of cancer developing and leading to death is low. However, it is clear that there is a growing concern about the use of repetitive CT scans and one must take into account the total amount of radiation exposure that an AAA patient will be subjected to over time.

In conclusion, endovascular AAA repair utilizing standard fluoroscopic techniques results in a PSD that is well below the level to induce skin injury. Based upon our current experience, direct skin dose measurements are easily performed with GAFCHROMIC film but, due to cost and need for real time measurements, DAP using fluoroscopic equipment is a readily obtainable and reasonable estimate of patient dose. Care should be utilized to limit radiation exposure to the patient and provider, especially in obese patients. Finally, consideration to overall patient radiation dose is necessary in endograft patients in need of long-term follow-up with CT imaging.

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References 

1. Fletcher DW, Miller DL, Balter S, et al. Comparison of four techniques to estimate radiation dose to skin during angiographic and interventional radiology procedures. J. Vasc. Interv. Radiol. 2002;13:391–397
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (170 KB)
   • CrossRef
2. Hirshfield JW, Balter S, Brinker JA, et al. ACCF/AHA/HRS/SCAI clinical competence statement on physician knowledge to optimize patient safety and image quality in fluoroscopically guided invasive cardiovascular procedures: a report of the American College of Cardiology Foundation/American Heart Association/American College of Physician Task Force on Clinical Competence Training. Circulation. 2005;111:511–532
   • View In Article
   • CrossRef
3. Koenig TR, Mettler FA, Wagner LK. Skin injury from fluoroscopically guided procedures: part 2. Review of 73 cases and recommendations for minimizing dose delivered to patient. A.J.R. Am. J. Roentgenol. 2001;177:13–20
   • View In Article
   • MEDLINE
4. Miller DL, Balter S, Wagner LK. Quality improvement guidelines for recording patient radiation dose in the medical record. J. Vasc. Interv. Radiol. 2004;15:423–429
   • View In Article
   • Full Text
   • Full-Text PDF (87 KB)
   • CrossRef
5. McParland BJ. Entrance skin dose estimates derived from dose-area-product measurements in interventional radiologic procedures. Br. J. Radiol. 1998;71:1288–1295
   • View In Article
   • MEDLINE
6. Miller DL, Balter S, Cole PE, et al. Radiation dose in interventional radiology procedures: the RAD-IR study. Part II: Skin dose. J. Vasc. Interv. Radiol. 2003;14:977–990
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (440 KB)
   • CrossRef
7. Lipsitz EC, Veith FJ, Ohki T, et al. Does the endovascular repair of aortoiliac aneurysms pose a radiation safety hazard to vascular surgeons?. J. Vasc. Surg. 2000;32:704–710
   • View In Article
   • Abstract
   • Full Text
   • Full-Text PDF (64 KB)
   • CrossRef
8. Balter S. Methods of measuring fluoroscopic skin dose. Pediatr. Radiol. 2006;36:136–140
   • View In Article
   • CrossRef
9. Miller DL, Balter S, Noonan PT, et al. Minimizing radiation-induced skin injury in interventional radiology procedures. Radiology. 2002;225(2):329–336
   • View In Article
   • MEDLINE
   • CrossRef
10. Koenig TR, Wolff D, Mettler FA, et al. Skin injuries from fluoroscopically guided procedures: part 1. Characteristics of radiation injury. A.J.R. Am. J. Roentgenol. 2001;177:3–11
    • View In Article
    • MEDLINE
11. Frazier TH, Richardson JB, Fabré VC, Callen JP. Fluoroscopy-induced chronic radiation skin injury. Arch. Dermatol. 2007;143:637–640
    • View In Article
    • MEDLINE
    • CrossRef
12. Wagner LK, Archer BR, Cohen AM. Management of patient skin dose in fluoroscopic guided interventional procedures. J. Vasc. Interv. Radiol. 2000;11:25–33
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (207 KB)
    • CrossRef
13. Bryk SG, Censullo ML, Wagner LK, et al. Endovascular and interventional procedures in obese patients: a review of procedural technique modifications and radiation management. J. Vasc. Interv. Radiol. 2006;17:27–33
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (86 KB)
    • CrossRef
14. Geise RA. Fluoroscopy: recording of fluoroscopic images and automatic exposure control. Radiographics. 2001;21:227–236
    • View In Article
    • MEDLINE
15. World Health Organization. InfoBase Online WHO Global Comparable Estimates for BMI (2005), http://www.who.int/ncd_surveillance/infobase/web/InfoBasePolicyMaker/CountryProfiles/QCStart.aspx (accessed May 29, 2007).
    • View In Article
16. Ector J, Dragusin O, Adriaenssens B, et al. Obesity is a major determinant of radiation dose in patients undergoing pulmonary vein isolation for atrial fibrillation. J. Am. Coll. Cardiol. 2007;50:234–242
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (1112 KB)
    • CrossRef
17. Ho P, Cheng S, Wu PM, et al. Ionizing radiation absorption of vascular surgeons during endovascular procedures. J. Vasc. Surg. 2007;46:455–459
    • View In Article
    • Abstract
    • Full Text
    • Full-Text PDF (238 KB)
    • CrossRef
18. Nicholson R, Tuffee F, Uthappa MC. Skin sparing in interventional radiology: the effect of copper filtration. Br. J. Radiol. 2000;73:36–42
    • View In Article
    • MEDLINE
19. Wagner LK, Archer BR. Minimizing Risks from Fluoroscopic X Rays. Bioeffects, Instrumentation, and Examination. 4th ed.. The Woodlands, TX: Partners in Radiation Management; 2004;
    • View In Article
20. Brenner DJ, Doll R, Goodhead DT, et al. Cancer risks attributable to low doses of ionizing radiation: assessing what we really know. Proc. Natl. Acad. Sci. USA. 2003;100:13761–13766
    • View In Article
21. Board on Radiation Effects Research, Division on Earth and Life Studies. Executive Summary. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII-phase 2. Washington DC: National Academy Press; 2005;
    • View In Article
22. Semelka RC, Armao DM, Elias J, Huda W. Imaging strategies to reduce the risk of radiation in CT studies, including selective substitution with MRI. J. Magn. Reson. Imaging. 2007;25:900–909
    • View In Article
    • MEDLINE
    • CrossRef
23. Brenner DJ, Hall EJ. Computed tomography—an increasing source of radiation exposure. N. Engl. J. Med. 2007;357:2277–2284
    • View In Article
    • CrossRef