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(Radiology. 2000;216:180-184.)
© RSNA, 2000

Medical Physics

Patient and Personnel Exposure during CT Fluoroscopy-guided Interventional Procedures1

Richard D. Nawfel, MS, Philip F. Judy, PhD, Stuart G. Silverman, MD, Stuart Hooton, RT, Kemal Tuncali, MD and Douglass F. Adams, MD

1 From the Department of Radiology, Brigham and Women's Hospital, 75 Francis St, Boston, MA 02115. From the 1998 RSNA scientific assembly. Received July 22; revision requested September 2; revision received October 21; accepted November 22. Address correspondence to R.D.N. (e-mail: nawfel@bwh.harvard.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To estimate patient dose and personnel exposure from phantom measurements during computed tomographic (CT) fluoroscopy, to use the estimates to provide users with dose information, and to recommend methods to reduce exposure.

MATERIALS AND METHODS: Surface dose was estimated on a CT dosimetric phantom by using thermoluminescent dosimetric (TLD) and CT pencil chamber measurements. Scatter exposure was estimated from scattered radiation measured at distances of 10 cm to 1 m from the phantom. Scatter exposures measured with and without placement of a lead drape on the phantom surface adjacent to the scanning plane were compared.

RESULTS: Phantom surface dose rates ranged from 2.3 to 10.4 mGy/sec. Scattered exposure rates for a commonly used CT fluoroscopic technique (120 kVp, 50 mA, 10-mm section thickness) were 27 and 1.2 µGy/sec at 10 cm and 1 m, respectively, from the phantom. Lead drapes reduced the scattered exposure by approximately 71% and 14% at distances of 10 and 60 cm from the scanning plane, respectively.

CONCLUSION: High exposures to patients and personnel may occur during CT fluoroscopy–guided interventions. Radiation exposure to patients and personnel may be reduced by modifying CT scanning techniques and by limiting fluoroscopic time. In addition, scatter exposure to personnel may be substantially reduced by placing a lead drape adjacent to the scanning plane.

Index terms: Computed tomography (CT), guidance, 70.12119 • Computed tomography (CT), radiation exposure, 70.12119 • Fluoroscopy • Phantoms • Radiations, exposure to patients and personnel, 70.12119 • Radiations, measurement, 70.12119 • Radiations, protective and therapeutic agents and devices, 70.12119


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Computed tomography (CT) has been used to guide interventional procedures for many years (1,2). A limitation in the use of conventional CT is the lack of real-time imaging information for the radiologist. CT fluoroscopy, which involves the reconstruction and display of CT images in real time, has been developed (3,4). Real-time CT scanning provides the physician with immediate feedback of images during the intervention; it may also improve targeting of lesions and reduce procedure time. However, unlike conventional CT, CT fluoroscopy requires personnel to be in the procedure room while the x-ray exposure occurs.

During CT fluoroscopy, the patient is exposed to radiation at or near the needle puncture site. The duration of exposure may range from seconds to a few minutes (3–6). Since the fluoroscopic exposure to the patient at the needle entry site is cumulative, deterministic effects can be substantial, as they are during conventional fluoroscopy (7,8). Hence, there is a potential radiation hazard to the patient during CT fluoroscopy–guided interventional procedures. Furthermore, the level of scatter exposure to personnel could be high at locations close to the scanning plane with use of CT fluoroscopic techniques during these procedures. Personnel exposure, especially exposure to the hands of the physician, is of concern, since the hands are closest to the scanning plane during the intervention.

Since there is a possibility for high exposures to occur during CT fluoroscopy, the purpose of our study was to assess the potential for high radiation exposure by using data from phantom measurements. The exposure rate during CT fluoroscopy was measured at the surface of an acrylic CT dosimetric phantom and at locations adjacent to the phantom. In addition, we investigated the effect of lead drape shielding for the reduction of radiation exposure to personnel.

Results were used to develop guidelines for exposure time during CT fluoroscopy with the goal of minimizing patient dose and personnel exposure. These guidelines are specific for the particular CT scanner and scanning conditions used during CT fluoroscopic interventional procedures (6).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The CT scanner used was a third-generation Somatom Plus 4 model with the Combined Applications to Reduce Exposure, or CARE, Vision fluoroscopic CT option (Siemens Medical Systems, Forchheim, Germany). Fluoroscopic CT scanning with this unit was performed at 750 msec per 360° rotation. Images were reconstructed and displayed at 6 frames per second. The x-ray tube filtration was specified as 10-mm aluminum equivalent, and the focal spot to the center of rotation was 57 cm.

At acceptance testing, dose measurements of the computed tomography dose index (CTDI) (9–17) obtained by using the head (16-cm-diameter) and body (32-cm-diameter) dosimetric phantoms (Nuclear Associates; Carle Place, NY) were 13.0 mGy per 100 mAs and 8.8 mGy per 100 mAs, respectively, at 120 kVp with a 10-mm section thickness.

Surface Dose and Scatter Exposure Rate Measurements
Surface dose and scatter exposure were assessed during CT fluoroscopy by using a 20-cm-diameter acrylic CT dosimetric phantom. This phantom was used because it better characterizes the attenuation caused by a human abdomen, which has an intermediate size between those of the 16-cm–diameter head and 32-cm–diameter body phantoms.

Dose rates were estimated at the surface and at a 2-cm depth in the phantom from both thermoluminescent dosimetric (TLD) and pencil ionization chamber (model PC 4P; Capintec, Ramsey, NJ) measurements. These data were collected with the table at a fixed position during CT fluoroscopic scanning so that the entire exposure occurred at the same location (section) in the phantom. The TLD chips (TLD100; Harshaw Bircon, Solon, Ohio) were 3 x 3 x 1-mm and were arranged in a linear pattern with no spacing between them. The TLDs were sent to be read out (Landauer; Glenwood, Ill).

Calibration was performed by exposing the TLDs to a diagnostic x-ray beam that was calibrated with an ionization chamber traceable to the National Institute of Standards and Technology (NIST). The tube potential of the calibration beam was 120 kVp, with a half-value layer of 8 mm of aluminum.

Scatter exposures were estimated from measurements of phantom scatter radiation. A digital dosimeter (model 192X; Capintec) and diagnostic chamber (model PM-30 30-mL diagnostic chamber with NIST traceable calibration; Capintec) were used to measure scatter radiation. Scatter exposures were specified as air kerma. Tube potentials were 80 and 120 kVp, and tube currents were 50–135 mA. Section thicknesses were 2–10 mm. Dose rates (both surface and scatter) were calculated by dividing the total cumulative exposure by the exposure time (10 seconds). The reproducibility determined from six measurements of the CTDI was 3.3%.

Scatter Radiation Exposure
Secondary exposure was measured at points relative to the scanning plane and phantom. Figure 1 illustrates the geometry and experimental setup for the measurement of the scatter exposure rate, with measurement points chosen at distances from the scanning plane toward the foot end of the table.



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  		Figure 1. Drawing depicts the measurement of scatter exposure at distances of 10-100 cm from the scanning plane along the z axis. Position of the measurement point in the x-y plane is constant.

 
The x and y coordinates of the measurement point were fixed for a given set of measurements, and the distance along the z axis was varied. Measurement points were 10–15 cm from the central axis in the positive y direction. The tabletop was 100 cm from the floor. With a method analogous to that used for the surface dose rate, the scattered exposure rate was calculated from measurements of the cumulative exposure and was converted to air kerma by using the following equation: Er = (E/t)f, where E is the total exposure for acquisition of a scan with exposure time t, and f is the conversion factor (8.76 µGy/mR). The exposure E depends on tube potential, tube current, and section thickness. The exposure measurements were performed with the scanning parameters (tube potential, tube current, section thickness) used in patient examinations (6).

Scatter Exposure Reduction
The potential for high exposures to personnel during CT fluoroscopy persuaded us to investigate a practical method for controlling scatter exposure. Several of the previous scatter measurements were repeated with a lead drape placed on top of the phantom adjacent to the scanning plane (Fig 2). Reduction in scatter was estimated at various distances from the scanning plane and with different section thicknesses. It was assumed that the measured exposure rate at distances along the z axis was due to scattered radiation from the phantom.



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  		Figure 2. Photograph depicts the phantom with the lead drape placed adjacent to the scanning plane for the measurement of scatter exposure.

 
Attenuation properties of the drape were evaluated with a series of transmission measurements performed by using a radiographic x-ray tube (GE Medical Systems, Milwaukee, Wis; three-phase, 12-pulse generator; half-value layer, 3.1 mm of aluminum at 80 kVp and 7.0 mm of aluminum at 120 kVp) at 80 kVp and 120 kVp, applying narrow-beam and good geometric conditions. Transmission data indicated that the lead drape provided 99% and 97% attenuation at 80 and 120 kVp, respectively. This attenuation was equivalent to that provided by a 0.5-mm lead-equivalent apron. Reduction of scattered radiation was determined by comparing the scattered exposure rates determined with and without the drape in place.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Surface Dose Rate
At the surface of the phantom, maximum dose rates determined from TLD profiles were 5.1 mGy/sec for a CT scanning technique of 120 kVp, 50 mA, and 10 mm and 4.0 mGy/sec for a CT scanning technique of 80 kVp, 135 mA, and 10 mm. The use of TLDs in the determination of the surface dose for CT fluoroscopic procedures with various scanning parameters would have been time-consuming and costly. Therefore, we developed a pragmatic approach by using the CTDI to predict the surface dose. The maximum surface dose rates and integral doses from TLD data were compared with the CTDI estimated from pencil chamber measurements. Using the maximum dose from the TLD profiles with the pencil chamber measurements, we predicted the surface dose rate with a correction factor for the CTDI. The estimated CTDI was multiplied by a correction factor of 0.75, which was based on TLD results. This method proved to be useful in the prediction of patient surface doses with the use of relatively few measurements. The adjusted dose rates are given in Table 1.


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  		TABLE 1. Dose Rates and Surface Doses for Selected CT Fluoroscopic Protocols
 
With the scanning plane fixed at the same location on the phantom surface during CT fluoroscopic scanning, the predicted surface dose was an estimate of the maximum surface dose. The maximum surface dose was the product of the surface dose rate and the total fluoroscopic scanning time, and the surface dose rate was a function of tube potential, tube current, and section thickness.

Scatter Exposure Rate
Scatter exposure rates for all techniques ranged from 51.7 to 1.22 µGy/sec (5.9–0.13 mR/sec) at distances of 10–100 cm from the scanning plane. The scatter exposure rate at 10 cm from the scanning plane was substantially lower with 80 kVp and 75 mA, as opposed to 120 kVp and 50 mA (Fig 3). With the same CT fluoroscopic scanning time, use of the lower-exposure technique would correspond to a 60% reduction in personnel hand exposure.



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  		Figure 3. Graph depicts the scatter exposure rate as a function of the distance from the scanning plane, with a substantial decrease in scatter exposure with the lower-exposure technique (80 kVp, 75 mA, 10-mm section thickness) and with increased distance from the scanning plane. Measurements were made at a height of 20 cm from the tabletop.

 
Figure 3 illustrates the substantial decrease in scatter exposure with distance from the scanning plane for two CT fluoroscopic techniques. As depicted in Figure 4, a curved fitted to the data measured at distances of 30–100 cm was used to predict the ratio of scatter exposure rate to surface exposure rate. This curve illustrates that, at distances greater than 30 cm from the scanning plane, the scatter exposure rate is approximated by the inverse square law; the ratio of scatter exposure rate to surface exposure rate is independent of tube potential. Thus, this relationship can be used to predict the scatter exposure with different tube potentials at different distances on the basis of a single measurement of the surface exposure rate.



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  		Figure 4. Graph depicts the ratio of scatter to surface exposure rate as a function of distance from the scanning plane. Measured scatter exposures at distances of 30, 40, 60, and 100 cm were used to predict the curve by applying the inverse square law. A 10-mm section thickness was used.

 
Control of Scatter Exposure
An attenuating lead drape placed 2.5 cm caudal to the scanning plane reduced the scattered exposure rate by 71% (from 23.6 to 6.8 µGy/sec [from 2.7 to 0.78 mR/sec]) at a distance of 10 cm from the scanning plane (Fig 5) with a CT fluoroscopic technique of 120 kVp, 50 mA, and 10-mm section thickness. This substantial reduction will be important for physicians who are concerned about exposure to their hands.



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  		Figure 5. Bar graph depicts the reduction in scatter exposure (120 kVp, 50 mA, 10-mm section thickness) with the lead drape positioned 2.5 cm caudal to the scanning plane on the phantom. Measurements were obtained at four distances from the scanning plane and at 24 cm from the tabletop. Percentages indicate the reduction in scatter exposure with the lead drape.

 
Figure 6 demonstrates that the scatter exposure was proportional to section thickness. However, the percent reduction in scattered exposure near the scanning plane (13 cm) was independent of section thickness. The reduction with the lead drape in place was 77%, 74%, and 73% for 2-, 5-, and 10-mm section thicknesses, respectively. Thus, for CT fluoroscopy–guided interventional procedures that require scanning parameters of 120 kVp and 50 mA, scattered exposure could be reduced by 73%–77% with a lead drape in place. This reduction was comparable to the reduction achieved when section thicknesses were decreased from 10 to 2 mm without a lead drape in place.



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  		Figure 6. Bar graph depicts the reduction in scatter exposure with the lead drape positioned 2.5 cm caudal to the scanning plane on the phantom. Measurements were obtained at 13 cm from the scanning plane and at 24 cm from the tabletop by using different section thicknesses and a CT technique of 120 kVp and 50 mA. Scatter exposure is proportional to section thickness. Percentages indicate reduction in scatter exposure with the lead drape.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The dose rates given in Table 1 can be used to predict patient skin doses during abdominal CT fluoroscopy–guided interventions and to provide guidelines to minimize doses during CT fluoroscopy. These guidelines permit conservative planning in the management of radiation safety on the basis of proposed upper limits of CT fluoroscopic scanning time. The dose estimates are specific for our scanner and scanning conditions.

During 80 seconds of CT fluoroscopic scanning, the dose is substantially reduced with the use of lower-exposure CT fluoroscopic techniques. For example, surface doses at 120 kVp and 50 mA and at 80 kVp and 135 mA are, respectively, 52% and 44% of the surface dose at 120 kVp and 90 mA. Eighty seconds was used, since this is the maximum duration of CT fluoroscopic exposure before the timer on the system resets. It was also approximately the mean scanning time of the CT fluoroscopy–guided abdominal biopsies that were performed in patients over a 6-month period (6). The estimated skin dose was a maximum, since we assumed that continuous CT fluoroscopic scanning was used in the same section location. In clinical procedures, this may not always occur.

The radiation dose from CT scanning is high when compared with the dose from other radiographic exposures (10,17). The typical CTDI estimated for an abdominal CT examination at our institution is 31.4 mGy. This is comparable to the range of doses reported in the literature (10, 17–19,20). Actual patient doses may vary considerably because of variations in patient size, scanner geometry (source collimation, scanning field diameter), and scanning parameters (tube potential, tube current, scanning time).

With the highest-exposure technique considered in our study (120 kVp, 90 mA, 10-mm section), the dose would be approximately 830 mGy. This skin dose is comparable to doses at approximately 20–30 minutes of fluoroscopy during cardiac catheterization or angiographic procedures at our institution and to skin doses at fluoroscopy, as others report (21,22).

Radiation-induced deterministic effects in the patient's skin, such as erythema, may occur at certain dose thresholds during CT fluoroscopy. We hope that dose rate estimates provide information to physicians for the modification of the scanning technique to reduce exposure. In Table 2, the CT fluoroscopic scanning times required to reach dose thresholds of 1,000 and 2,000 mGy with various techniques are given. Transient erythema may occur at a threshold skin dose of approximately 2,000 mGy, but it has been observed at 1,000 mGy (7,8). Thus, these thresholds were used in the development of a dose management plan to reduce patient and personnel exposure during CT fluoroscopic procedures.


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  		TABLE 2. CT Fluoroscopic Scanning Time Required to Reach Dose Thresholds
 
In our procedures, levels 1 and 2 have been designated as 180 and 360 seconds of CT fluoroscopic scanning time, respectively. These times represent the CT fluoroscopic time necessary to reach doses of 1,000 and 2,000 mGy, respectively, by using the recommended CT scanning technique of 120 kVp and 50 mA with a 10-mm section. This was the technique used most often during CT fluoroscopy–guided abdominal biopsies performed in patients during a 6-month period (6); the technique was also used during CT fluoroscopy–guided interventions reported by others (5).

Our dose management plan requires technologists to notify the physician performing the procedure and other staff when the cumulative CT fluoroscopic scanning time reaches level 1. Subsequently, technologists are required to inform the physician when the scanning time is approaching level 2. If level 2 is exceeded, the rationale for the long exposure time is investigated, and the medical physicist is consulted to provide an estimate of patient skin dose. In addition, we recommend that the patient be examined for radiation-induced skin changes after completion of any procedure when the scanning time exceeds level 2. The ideal intervals for follow-up are at 24 hours and at 1–2 weeks after the procedure. The intent of this plan is to provide the physician with additional information that may assist him or her in the reduction of unnecessary exposure.

In addition to high patient exposures, the exposure to personnel can be substantial and comparable to personnel exposures at fluoroscopy during cardiac catheterization and interventional radiology procedures (23–26). Although the physician's hands will most likely receive the highest levels of exposure, the thyroid and lens of the eye are also at risk.

Scatter exposure to personnel during CT fluoroscopic procedures was estimated from the exposure rate given by the equation, Er = (E/t)f, and the scanning time, tf, for a particular CT fluoroscopic scanning technique (6). With a procedure consisting of an 80-second scanning time with 120 kVp, 50 mA, and a 10-mm section thickness, our results predict a maximum dose of 2.2 mGy to the physician's hand (10 cm from the scanning plane) with a potential head and/or neck dose of 0.1 mGy (1 m from the scanning plane). If the physician's workload were 10 procedures per month, the corresponding monthly hand dose to the physician would be 22 mGy. This assumes that the physician's hands are located approximately 10 cm from the scanning plane for the entire scanning time. The hand doses estimated from a single ring badge exposure for one of our radiologists was 17.7 mGy for 1 month and 16.7 mGy for a second consecutive month. The radiologist performed approximately 10 procedures during each month. Each of these ring badge exposures is consistent with the hand dose predicted from our measurements.

Kato et al (4) demonstrated that the use of needle holders would allow physicians to perform substantially more procedures without exceeding the maximum permissible dose, as compared with not using needle holders. Our surface dose rate at 80 kVp (3.1 mGy per 100 mAs) compares reasonably well with that of Kato et al (3.8 mGy per 100 mAs). Thus, we would expect similar patient doses and personnel exposures with use of the same clinical procedures and fluoroscopic scanning times.

A decrease in the patient dose during CT fluoroscopy will decrease scatter radiation exposure, as is true with other diagnostic radiologic procedures. The patient dose can be reduced by decreasing the CT technique factors and/or CT fluoroscopic scanning time. The choice of technique will depend to large degree on patient habitus and type of procedure. The use of a lower tube potential and tube current could yield reductions of 60% or greater in personnel hand exposure, while a reduction in the section thickness from 10 to 5 or 2 mm can result in personnel exposure reductions of 50%–80%. A decrease in the tube potential, tube current, or section thickness, however, may compromise image quality. The physician should choose the technique with the lowest exposure that provides sufficient visualization of the target lesion.

Our results also indicate that positioning a lead drape adjacent to the scanning plane is an effective method for the reduction of scattered radiation during CT fluoroscopy. It would allow sufficient room for needle placement and provide considerable protection to personnel. The greatest effect of the lead drape on the reduction of scatter exposure is close to the patient surface. At greater distances from the patient, the reduction due to inverse square law most likely dominates, and the lead drape becomes less advantageous.

Substantial reduction in personnel hand exposure is possible when a lead drape is placed over the patient. If the drape provides an overall reduction in scattered radiation of 71%, the physician's hand exposure (at a distance of 10 cm from the scanning plane) would be reduced from 2.2 to 0.64 mGy, with the assumption of 80 seconds of scanning time with the use of procedures performed with 120 kVp, 50 mA, and a 10-mm section thickness. The physician could then be allowed to perform substantially more procedures per year without exceeding the maximum permissible dose.

In conclusion, we determined that there is a potential for high exposures to occur during CT fluoroscopy; personnel hand exposure may be reduced by positioning a lead drape caudal to the scanning plane at the needle entry site. In addition, patient dose and personnel exposure can be controlled by using the lowest CT fluoroscopic scanning techniques that are adequate to image the lesion and surrounding tissue structures and by limiting CT fluoroscopy scanning time to only that which is necessary.

The reduction in personnel exposure may be substantial for physicians who perform multiple procedures. Physicians and other users of this technology must develop an awareness of the factors that influence these exposures; they should use standard radiation protection principles regarding time, distance, and shielding, when appropriate. Assessment of radiation dose will provide physicians with useful information for the development of standard imaging protocols as the use of this technology becomes more prevalent.


   ACKNOWLEDGMENTS
 
The authors thank Sally Edwards and Kristin Cervero for their editorial comments.


   FOOTNOTES
 
See also the editorial by Wagner (pp 9–10 ) in this issue.

Abbreviations: CTDI = computed tomography dose index, NIST = National Institute of Standards and Technology, TLD = thermoluminescent dosimeter

Author contributions: Guarantor of integrity of entire study, R.D.N.; study concepts and design, all authors; definition of intellectual content, all authors; literature research, R.D.N., P.F.J., S.G.S., K.T.; experimental studies, R.D.N., S.H.; data acquisition, R.D.N., S.H.; data analysis, R.D.N., P.F.J., S.G.S., K.T.; manuscript preparation, R.D.N., P.F.J., S.G.S., K.T.; manuscript editing, R.D.N., P.F.J., S.G.S.; manuscript review, all authors.


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 DISCUSSION
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