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Does Digital Acquisition Reduce Patients' Skin Dose in Cardiac Interventional Procedures? An Experimental Study

¹ Department of Radiological Technology, College of Medical Sciences, Tohoku University, 2-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan.
² Department of Radiology, Tohoku University School of Medicine, 1-1 Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan.
³ Department of Internal Medicine and Rehabilitation Science, Tohoku University Graduate School of Medicine, 1-1 Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan.

Received October 16, 2003; accepted after revision March 8, 2004.

 
Address correspondence to K. Chida (chida{at}rad.cms.tohoku.ac.jp).

Abstract

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OBJECTIVE. It is necessary to reduce the exposure doses from both fluoroscopy and angiocardiography. Pulsed fluoroscopy clearly reduces patients' exposure. By contrast, whether digital acquisition reduces patients' exposure is not clear. This study simulated the skin radiation doses of patients in cardiac catheterization laboratories with various radiography systems used in percutaneous transluminal coronary angioplasty to determine whether digital acquisition reduces patient exposure as compared with cine film recording.

MATERIALS AND METHODS. The entrance surface doses with cineangiography and fluoroscopy of acrylic phantoms were compared for 11 radiography systems at seven facilities; each performs more than 100 cardiac intervention procedures per year. The entrance surface dose for an acrylic plate (20 cm thick) was measured using a skin-dose monitor.

RESULTS. The maximum dose exceeded the minimum dose by 6.44 times for cineangiography and by 3.42 times for fluoroscopy. The entrance surface dose with acrylic plate was lower with digital-only acquisition (mean ± SD, 3.07 ± 0.84 mGy/sec) than with film recording (6.00 ± 3.04 mGy/sec). By contrast, the entrance surface frame dose, after correction for the cine frame rate, tended to be higher with digital acquisition than with film recording (0.210 ± 0.053 vs 0.179 ± 0.058 mGy/frame, respectively). CONCLUSION. The entrance surface dose was approximately 50% less with digital-only acquisition than with film recording. However, after correcting the dose for cine frame rate, filmless acquisition did not in itself reduce the exposure. For the surface dose to be reduced for cardiac interventional radiography, even with digital filmless radiography systems, a low recording speed is necessary for angiocardiography.

Introduction

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Cardiac interventional radiography procedures such as percutaneous transluminal coronary angioplasty and cardiac radiofrequency catheter ablation have lower risks than surgical procedures, and their wide acceptance has led to an increasing number being performed [1]. This increase is despite the fact that patient radiation doses from one cardiac interventional radiography procedure (e.g., radiofrequency catheter ablation) are the highest of any commonly performed diagnostic radiography study [2]. The high dose induces potentially harmful radiation skin injuries, such as ulceration [3–5]. The U.S. Food and Drug Administration has suggested that physicians should record the radiation skin dose received by patients undergoing catheterization [6].

Recently, cardiac interventional radiography equipment has tended toward digital acquisition (filmless) archiving instead of cine film recording and pulsed fluoroscopy instead of conventional fluoroscopy. Pulsed fluoroscopy clearly reduces patient exposure [7–10]. By contrast, whether digital acquisition does reduce patient exposure in comparison with cine film recording is not clear, although the American College of Cardiology, Limacher et al. [11], and Aldridge et al. [12] recommend using digital-only cine acquisition to reduce radiation exposure in angiocardiography. For the exposure to patients during cardiac interventional radiography to be reduced, reducing the skin dose from both fluoroscopy and film recording is necessary. Therefore, this study simulated the skin radiation doses of patients in cardiac catheterization laboratories with various radiography systems used in cardiac interventional radiography to determine whether digital acquisition reduces patients' exposure as compared with cine film recording.

Materials and Methods

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This study was conducted from October 2001 to March 2002. We examined 11 radiography systems in seven cardiac catheterization laboratories in and around Sendai City, Japan (Table 1). The seven facilities studied each performed more than 100 cardiac interventional radiography cases per year. Of the 11 radiography systems measured, five used digital-only acquisition and six used cine film recording; eight used pulsed fluoroscopy and three used conventional fluoroscopy. The entrance surface doses with cineangiography and fluoroscopy were compared for the 11 radiography systems using acrylic plates (20 cm thick) and a skin-dose monitor (SDM model 104-101, McMahon Medical) [13]. The X-ray conditions used in the measurements (including the image intensifier field magnification mode and the filming or recording speed [frame rate] for cineangiography and fluoroscopy) were those normally used in the facilities performing percutaneous transluminal coronary angioplasty. The entrance exposure area was that associated with the actual diameter setting of the image intensifier. The X-ray exposure factors for all systems were as follows: the distance of the source to the image intensifier was 100 cm, and the distance of the source to the acrylic entrance surface was approximately 75 cm. A skin-dose-monitor sensor was placed on the center of the entrance surface of the acrylic plate. Therefore, the skin-dose monitor measurements included backscatter from the acrylic plate. Of the 11 radiography systems measured, three (units B, J, and K) used added copper beam filtration with 0.1-mm-thick copper filtration for both cine acquisition and fluoroscopy measurements (Table 1).

The entrance surface dose corrected for the cine frame rate (i.e., the frame dose) was determined as follows: frame dose (mGy / frame) = [original entrance surface dose with cineangiography (mGy / sec) / cine recording speed (frames / sec)].

The Student's t test was used to compare data for the two groups (digital-only acquisition vs film recording, pulsed fluoroscopy vs conventional fluoroscopy, and without vs with copper filtration). Statistical significance was defined as a p value of less than 0.05.

Results

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Cineangiography Dose of the 11 Radiography Systems
For cineangiography with the 11 radiography systems, the average entrance surface dose of the 20-cm-thick acrylic plate was 4.40 mGy/sec (range, 1.71–11.01 mGy/sec) (Table 1). The average frame dose, after correction for the cine frame rate, of the 20-cm-thick acrylic plate was 0.196 mGy/frame (range, 0.114–0.262 mGy/frame) (Table 1). The maximum surface and maximum frame doses exceeded the minimum doses by 6.44 and 2.30 times, respectively, for cineangiography with a 20-cm-thick acrylic plate.

Cineangiography Dose
Digital versus film recording.—The entrance surface dose with the 20-cm-thick acrylic plate was significantly (p < 0.05) lower with digital-only acquisition (mean ± SD, 3.07 ± 0.84 mGy/sec) than with film recording (6.00 ± 3.04 mGy/sec). By contrast, the entrance surface frame dose, after correction for the cine frame rate, tended to be higher with digital acquisition (0.210 ± 0.053 vs 0.179 ± 0.058 mGy/frame, respectively), although the difference was not significant (p = 0.375).

Without versus with added copper filtration.—The entrance surface frame dose of the 20-cm-thick acrylic plate was not significantly lower in the systems with added copper filtration than in the systems without added beam filtration (0.172 ± 0.050 vs 0.205 ± 0.057 mGy/frame, respectively; p = 0.412).

Fluoroscopy Dose of the 11 Radiography Systems
For fluoroscopy, the average entrance surface dose of the 20-cm-thick acrylic plate was 19.3 mGy/min (range, 8.9–30.4 mGy/min) (Table 1). The maximum dose exceeded the minimum dose by 3.42 times for the 20-cm-thick plate. The surface doses were identical for continuous fluoroscopy (mean ± SD, 23.93 ± 2.77 mGy/min) and pulsed fluoroscopy at 30 pulses/sec (25.20 ± 7.35 mGy/min).

Discussion

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It is necessary to keep the exposure doses from both fluoroscopy and angiocardiography as low as is reasonably achievable to avoid radiation skin injuries in patients undergoing cardiac interventional radiography. Recently, there has been a trend toward digital acquisition and pulsed fluoroscopy in the radiography equipment that is used for cardiac interventional radiography. In 1987, Cascade et al. [14] reported that fluoroscopy contributed 74% of the total radiation exposure dose during percutaneous transluminal coronary angioplasty. Nowadays, the contribution percentage of the fluoroscopy dose to the total radiation exposure is speculated to be lower than or equivalent to the cine dose because the radiation dose of fluoroscopy has been reduced with the development and spread of the pulsed fluoroscopy technique for cardiac interventional procedures. Therefore, a reduction of the exposure dose from angiocardiography should be emphasized.

Filmless laboratories have reduced the radiation exposure in cardiac interventional radiography. Some reports have indicated that filmless laboratories provide important further reductions in radiation exposure [11, 12]. Nevertheless, another report showed that the radiation dose was higher with a digital system than that with a conventional system in 13 of the 15 groups [15]. In our study, the entrance surface dose was approximately 50% less with digital-only acquisition than with film recording. Our results support the reports that skin radiation dose reduction is achieved in filmless laboratories.

One of the most important factors in reducing exposure is the use of a slower frame rate, even with digital-only acquisition. The use of digital acquisition in angiocardiography does not in itself reduce the surface dose, which varies markedly with the radiography system used. The maximum surface dose exceeded the minimum dose by 6.44 times for cineangiography and 3.42 times for fluoroscopy in the 11 radiography systems that we measured. By contrast, the maximum frame dose, after correction for the cine frame rate, exceeded the minimum frame dose by 2.30 times for cineangiography. The frame doses in digital-only acquisition were identical to those in cine film recording. In our study, the recording frame rate used in digital-only cine acquisition was 15 or 12.5 frames/sec and that of cine film recording was 15, 30, or 60 frames/sec. Therefore, the entrance surface dose of the acrylic plate tended to be lower for filmless acquisition than for cine film recording. For the surface dose to be reduced, the frame rate must be reduced in angiocardiography—both in cine film recording and digital acquisition. Baim [16] stated that some laboratories record coronary images at speeds as low as 15 frames/sec. Therefore, to reduce the skin dose that patients receive from cineangiography during cardiac interventional radiography, we recommend using digital-only cine acquisition with a low recording speed.

The use of pulsed fluoroscopy can also reduce the skin exposure dose [7–10]. Our results showed that the entrance surface dose was significantly (p < 0.01) higher with continuous fluoroscopy and pulsed fluoroscopy at 30 pulses/sec (mean ± SD, 23.93 ± 2.77 mGy/sec) than with pulsed fluoroscopy at 15 or 7.5 pulses/sec (15.07 ± 4.13 mGy/sec). Therefore, pulsed fluoroscopy should be used at a low pulse rate, such as 15 pulses/sec, to reduce the skin dose [8].

In summary, digital acquisition and pulsed fluoroscopy do not necessarily reduce the radiation dose in cardiac interventional radiography; it is necessary to lower the recording speed with digital-only cine acquisition and to use pulsed fluoroscopy at a pulse rate lower than 30 pulses/sec.

The patient skin dose is usually reduced when added copper beam filtration is used [10, 11]. In our study, however, the entrance surface frame doses of the systems with added copper filtration were not significantly lower than those of the systems without added beam filtration, although we did not compare the same system with and without copper filtration, because we compared different systems in a clinical setting. Consequently, the effect of added copper filtration might be masked.

Physicians should also pay attention to the image intensifier field size used, degeneration of the image intensifier gain, patient body weight, and acquisition angle. A small image intensifier field (e.g., 9 inch [23 cm]) is frequently used instead of a large one (e.g., 11 inch [28 cm]). The radiation exposure with a small-field image intensifier is higher than that with a large-field image intensifier, which makes it even more important to use a lower recording speed with the filmless and pulsed fluoroscopic techniques. The image intensifier has variable magnification modes, so magnification should be used only when necessary. A much greater entrance skin dose is required in large patients and with steeply angled beam orientations.

Degeneration of the image intensifier gain increases the radiation dose. Our dose measurements for two identical pieces of equipment (B and K) differed. This disparity might have been caused by the difference in the number of years the image intensifier had been used (B, 32 months; K, 1 month) because the performance of an image intensifier deteriorates rapidly.

The maximum frame dose, after correction for the cine frame rate, exceeded the minimum frame dose by 2.30 times for cineangiography. Similarly, the maximum dose exceeded the minimum dose by 3.42 times for fluoroscopy. These differences might be caused mainly by the different tube potential (peak kilovoltage), degeneration of the image intensifier gain, insufficient quality control of the radiography unit, or a combination of these factors. Therefore, radiography units should be checked on an annual basis for optimal imaging performance, appropriate radiation dose, and so forth.

Physicians should be aware of the entrance surface dose for cineangiography and fluoroscopy with the radiography system that they use for cardiac interventional radiography. In a clinical setting, the radiation dose varies markedly with the procedure and the physician. In cardiac interventional radiography, the most prevalent factor in radiation injury is prolonged exposure of one area of skin; physicians should therefore change the X-ray and viewing angles. The use of digital-only cine acquisition with a lower recording speed and of pulsed fluoroscopy at a pulse rate lower than 30 pulses/sec should reduce the skin dose. Nevertheless, the entrance surface doses in the radiography systems tested were too large. If the surface dose of the radiography system that they use is high, physicians should determine why and make efforts to reduce patients' exposure.

This study has a limitation in that there were insufficient controls for the different radiography units examined. However, it was difficult to establish a control because the measurements were made using the clinical techniques practiced at each site under clinical conditions.

In conclusion, the entrance surface doses with cineangiography and fluoroscopy of acrylic phantoms were compared for 11 radiography systems at seven facilities; each performs more than 100 cardiac interventional radiography cases per year. The maximum surface dose exceeded the minimum dose by 6.44 times for cineangiography and by 3.42 times for fluoroscopy. The entrance surface dose of the acrylic plate tended to be lower with digital acquisition (filmless) than with cine film recording. However, after correcting the dose for cine frame rate, filmless acquisition did not in itself reduce the exposure. For a reduction in the surface dose for cardiac interventional radiography, even with digital filmless radiography systems, a low recording speed is necessary in angiocardiography and a low pulse rate in pulsed fluoroscopy. To reduce the risk of skin injury to patients, physicians should be aware of the entrance surface dose of the radiography system that they use for cardiac interventional radiography.

Acknowledgments
 
We thank Masatoshi Sasaki of the Tohoku University Hospital, Takashi Kanetomo of the Sendai National Hospital, Souichiro Kamio of Sendai City Hospital, Yoshikatu Kawamura of the Tohoku Kosai Hospital, Takashi Terui of the Sendai Kousei Hospital, Hiroo Chiba of the Tohoku Kouseinenkin Hospital, and Masami Sato of the Sendai Open Hospital for their invaluable assistance.

References

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