Radiologic Issues and Radiation Safety During ERCP

Fluoroscopic Imaging Systems

Real-time image guidance for ERCP is most commonly provided by fluoroscopy. The basic components of a fluoroscopic system include an x-ray tube, generator, image receptor, and video system for image display and recording ( Fig. 3.3 ). Modern fluoroscopy systems incorporate multiple operational modes and configurations. Therefore it is important that physicians using fluoroscopy have adequate knowledge of its appropriate use.

An x-ray tube produces the primary beam with adjustment of x-ray energy (tube potential, or kVp) and beam intensity (tube current, or mA) provided by generator. An important operational feature of fluoroscopic systems is automatic exposure control (AEC). As patient attenuation changes when the primary beam is panned across the body, x-ray beam energy and intensity must be adjusted to maintain consistent quality in the displayed fluoroscopic image. With AEC, these adjustments are made continuously during imaging without operator intervention. X-ray tube heat can build up quickly during procedures involving long fluoroscopy exposure and acquisition of multiple images. When heat loading limits are reached, fluoroscopic equipment will typically terminate operation or exclude selection of high-dose imaging modes to allow for cooling to occur.

On the exit port of the x-ray tube, a collimator is used to define the shape of the x-ray beam. The collimator automatically limits the x-ray beam to the image receptor field of view (FOV) as changes are made in the magnification mode selection or source-to-image distance. Additionally, the operator can further limit the exposed area by manually moving collimator blades closer to the area of clinical interest.

Fluoroscopy can be performed with continuous x-ray generation or x-ray pulses with frame rates ranging from 30 frames per second (fps) to 1 fps or below. One advantage of pulsing is an improvement in temporal resolution. Motion blur occurring within each image is reduced because of the shorter acquisition time, making pulsed fluoroscopy useful for examining moving structures. In addition, pulsing with low pulse rates can reduce radiation dose. When fluoroscopic imaging is stopped, last-image-hold allows for display of the last image on the monitor for continued study and review. Fluoroscopy loop recording is available on some models to store and review short video clips. Higher-dose image acquisition can consist of a single exposure or a series of exposures at frame rates ranging from 30 to 1 fps or below. Methods for image recording include capture of fluoroscopic video, which can be stored and reviewed with captured endoscopic video, and output of acquired images for review and long-term archive.

Two different types of image receptors are currently available on fluoroscopy systems: image intensifiers and flat panel detectors. Image intensifiers produce a circular image by converting x-rays into light with electronic intensification. A video camera is used to capture the output image and display it on a monitor. Image intensifiers are available with input surface diameters ranging from 10 to 40 cm, with selection of one or more magnification modes. To maintain consistent output image quality as the FOV is decreased, the x-ray exposure rate is increased, resulting in higher patient dose rates when smaller FOVs are used.

Flat panel detectors are solid-state detectors that produce a digital electronic signal. Fluoroscopy flat panels are available in square or rectangular formats with sizes ranging from 17 × 17 cm to 40 × 40 cm. Selection of magnification modes is also possible. In contrast to image intensifiers, which produce an image in which the peripheral area has decreased magnification (pincushion distortion) and reduced brightness level (vignetting), flat panel detectors are distortion-free with uniform image brightness.

Before display of the image, modern fluoroscopic systems apply digital image processing techniques to enhance image appearance. Processing options include gray-scale processing, edge enhancement, and temporal frame averaging. Gray-scale processing adjusts displayed image contrast and brightness to accentuate contrast in a desired density range. This may also mitigate the appearance of flare, which is a bright area in an image where the signal has become saturated, reducing or completely eliminating contrast. Flare is particularly apparent in image intensifier systems, which have reduced dynamic range compared with flat panel detectors. Edge enhancement increases the sharpness of small objects and boundaries between areas of differing density. For temporal frame averaging, the current video frame is averaged with one or more previous video frames to decrease the appearance of image noise. Objects moving in the image may appear blurred when this technique is applied, or multiple ghost images of a high-contrast moving object may be seen if the object is moving rapidly. Adjustment of image processing parameter settings to optimize image quality for the clinical application and user preference is a critical step in equipment configuration.

Most fluoroscopy systems include a display of patient dose (required for equipment manufactured in the United States since 2006). The dose parameter displayed is the entrance skin air kerma with units of milligray (mGy). During fluoroscopy, the air kerma rate is displayed, and after fluoroscopy, the patient's cumulative air kerma is shown. Typical air kerma rates for an average-sized adult abdomen range from 20 to 60 mGy/min. Regulatory requirements limit the maximum air kerma rate to 88 mGy/min in the normal mode. Fluoroscopy equipment may include high-level control mode, which can be activated to allow for higher exposure levels, up to 176 mGy/min. For ERCP procedures, Buls et al. reported a median cumulative entrance air kerma of 271 mGy (maximum 1180 mGy).

These basic fluoroscopic equipment components are available in several different configurations to meet the requirements of specific diagnostic and interventional applications. One common stationary fluoroscopy configuration used for ERCP has an incorporated patient table with an undertable x-ray tube and image receptor above the patient. Another stationary configuration employed for ERCP fluoroscopy has a reverse design with an overtable x-ray tube and the image receptor located under the table. Mobile fluoroscopy units are also frequently used. In a mobile fluoroscopy system, the x-ray tube and image receptor are mounted on a C-arm positioner that allows for angulation of the image chain around the patient. It should be noted that a radiolucent procedure table is needed for this fluoroscopy imaging configuration. A mobile C-arm can also be moved between procedure rooms for a more flexible clinical workflow. More recently, stationary multiple-purpose fluoroscopy systems have been introduced that incorporate a tilting C-arm positioner with a right side–mounted table for easy ERCP access. These units offer the advantage of C-arm angulation with table-side control. Consideration of the x-ray source is important when shielding the patient is necessary, such as during ERCP in pregnant patients (see Chapter 30 ).

Radiation Dose Management in Fluoroscopic Procedures

The endoscopist has control over multiple parameters that can be adjusted to alter patient radiation dose during a fluoroscopic procedure. Though the risk of radiation injury is low, deterministic effects (including skin burns and cataract formation) and stochastic effects (increased risk of cancer) are possible. Deterministic injuries occur only after the radiation dose to the tissue exceeds a given threshold dose. For fluoroscopic procedures, patient skin injury may occur. A threshold dose of 2000 mGy results in transient erythema, with more severe effects (including epilation and desquamation) resulting from higher dose levels. Although a single ERCP procedure is not likely to reach the threshold dose level for skin injury, if patients have had fluoroscopic exposure in the same anatomic area in the past 60 days, the total skin dose should be evaluated and actions taken to minimize dose to that entry area should be considered if needed. Additional information on radiation injury can be found in several recommended reviews. Because of the potential of radiation injury, care must be taken to minimize radiation exposure when performing procedures where fluoroscopy is used. Dose optimization requires attention to several basic principles summarized below.

Limiting fluoroscopy time is the most direct dose reduction technique. Fluoroscopy should never be activated unless the operator is looking at the image display. Short taps of fluoroscopy are generally sufficient for observation instead of continuous operation. Last-image-hold or fluoroscopy loop recording is useful for consultation and review without the need for additional fluoroscopic exposure. Last-image-hold may also be stored for image archive as an alternative to an additional acquired image. Note that reducing exposure time will also limit x-ray tube heat buildup, which will minimize procedure delays required for tube cooling.

Low dose rate fluoroscopy modes should be used whenever possible. A pulsed fluoroscopy mode with a low frame rate is generally the best selection for dose reduction. Fluoroscopy systems should be configured to default to a low dose setting, allowing the operator to increase the dose rate if needed to achieve adequate image quality for the task. The use of high-level control fluoroscopy should be limited.

The location of the patient relative to the x-ray tube and image receptor also affects radiation dose levels. X-ray beam intensity is inversely proportional to the inverse square of the distance from the x-ray tube. Therefore, when C-arm positioners are used, the patient should be positioned as far as possible from the x-ray tube. Because the exposure rate can be very high at the exit port of the x-ray tube assembly, a spacer cone should be installed on C-arm positioners to keep the x-ray tube a safe distance from patient anatomy. Moreover, reducing the source–to–image receptor distance by positioning the image receptor as close as possible to the patient exit surface will also reduce patient dose. For example, when using a stationary undertable x-ray tube fluoroscopy system, the operator should lower the image receptor to near the patient's body when possible.

Collimator blades should be manually adjusted to include only the area of interest in the exposure field. This action reduces patient dose by reducing the exposed volume of tissue. Tight collimation will improve image quality by reducing flare, especially when imaging near the lung fields or near the edge of the body. Another detriment to image quality is caused by scattered x-rays. When primary beam x-rays interact in patient tissue, some of the x-rays are scattered and are emitted from the body in all directions. When these scattered x-rays strike the image receptor, they increase signal intensity throughout the image, masking the shadow of patient attenuation formed by transmitted x-rays. As a result, image contrast is reduced. Because a larger volume of exposed tissue produces more scattered radiation, collimation results in increased image contrast.

Magnification modes are useful to improve visualization of image detail during fluoroscopy by increasing both spatial resolution and image contrast. However, as the magnification is increased, the dose rate must be increased to maintain image quality. Therefore magnification modes should be used sparingly, when subtle pathology dictates.

Be aware that patient dose rates are higher for larger patients. As patient thickness increases, the patient entrance dose rate is approximately doubled with every additional 3 cm of tissue up to the maximum dose rate. The added tissue also results in increased scatter radiation. This scatter radiation, along with the increased x-ray beam energy needed for adequate penetration, results in reduced image contrast, making fluoroscopy of obese patients problematic.

The displayed patient cumulative radiation dose should be monitored throughout the procedure. Postprocedure, the final dose value should be recorded in the patient's medical record. This information is necessary for monitoring dose administered to patients receiving multiple procedures over time. Monitoring and recording of radiation dose also helps endoscopists maintain awareness of potential radiation harms. Similarly, monitoring of personnel radiation exposure is essential to ensure worker safety. Specific protocols for monitoring will vary by facility as determined by the facility radiation safety officer. Radiation monitoring may use a single dosimeter worn at the collar level outside a protective apron or two dosimeters, one worn at the collar and the other worn under a protective apron. For accurate estimation and reliable tracking of occupational dose, dosimeters should be worn and exchanged consistently. The recommended annual dose limit for personnel is 50 mSv whole body and 150 mSv to the lens of the eye.

The primary x-ray beam is the major source of radiation exposure for the patient. Scattered x-rays emanating from the exposed patient tissue are the major source of radiation exposure for personnel in the room during fluoroscopy. Scatter dose rates are typically 1 to 10 mGy/h adjacent to the irradiated patient volume and decrease in intensity in proportion to the inverse square of the distance from that volume. As the patient entrance dose rate increases, the scatter dose rate increases proportionally. Therefore implementation of the patient dose reduction techniques above will also result in decreased scatter levels.

Fig. 3.4 shows a representative scatter isodose plot for a C-arm fluoroscopy configuration with the x-ray tube positioned under the table. Note that radiation intensity is concentrated in the area below the procedure table near the x-ray tube. Although x-ray tube leakage results in a small amount of radiation released from the sides of the x-ray tube, scattered radiation levels are substantially higher. This distribution is caused by higher levels of scattered x-rays produced at the primary x-ray beam patient input port. Forward scattered x-rays from the first few centimeters of tissue depth are heavily attenuated by the rest of the patient tissue, resulting in lower radiation levels in the region back near the image receptor. The intensity of scatter radiation for an overtable x-ray tube fluoroscopy system is shown in Fig. 3.5 and a lateral projection is shown in Fig. 3.6 .