keywordsALARA, fluoroscopy, Radiation safety
Note: Please see pages ii , iii for a list of anatomic terms/abbreviations used throughout this book.
This chapter provides interventionalists practical ways to minimize our patients’, our coworkers’, and our own radiation exposure. Even one or two seemingly small adjustments in procedure habits can significantly decrease the accumulated radiation exposure received over one’s career. This chapter focuses on minimizing radiation exposure by limiting exposure time, maximizing distance from the source, optimizing use of shielding, and performing other useful exposure limiting techniques. We also discuss dosimeter badge result interpretation.
All C-arm positions (antero-posterior, postero-anterior, and lateral) are made with reference to a prone or supine patient. Please see Chapter 3 , Table 3.1 , for definitions of the different C-arm movements and other conventions used throughout this atlas.
X-rays are not directly detectable by the senses. Comparing X-rays with light gives perspective:
X-radiation (X-ray) is a form of ionizing electromagnetic radiation that has more energy than visible light and can penetrate solid objects.
Unlike visible light, X-ray is undetectable by our senses, which can create a false sense of safety.
There is no lower limit of safe dose of radiation. Any dose of radiation can produce some degree of detrimental effect.
The effects of X-ray exposure (even high levels of exposure) may not be noticeable until long after exposure.
Radiation exposure can result in cataract formation, radiation burns, and other permanent changes at the cellular level (e.g., cancer).
While performing fluoroscopic procedures, the operator should continuously minimize radiation dosage by using all reasonable methods. This principle is referred to as ALARA (As Low As Reasonably Achievable) and is a regulatory requirement for all radiation safety programs.
This chapter will present a logical and rational approach to fluoroscopic usage, emphasizing time, distance, and shielding as three major techniques employed to maintain ALARA dosages.
Proper C-Arm Operation: Limiting Exposure Time
Limiting fluoroscopic time is the most effective way to decrease radiation exposure (i.e., dose). Techniques for doing this will be discussed in this chapter and elsewhere throughout this book.
Operate the C-arm in pulsed mode whenever possible. The total dose delivered to the patient is a result of the X-ray output per unit time and the duration of exposure (dose = dose rate × time). Reduction of radiation exposure dosage can be accomplished by adjusting the mode of operation of the C-arm fluoroscope to reduce the tube X-ray output rate or tube duty cycle (time taken by the tube to operate).
The default mode of fluoroscopic image acquisition is commonly referred to as continuous or conventional fluoroscopy. In continuous mode, the X-ray tube output is continuously operating for the duration of image acquisition to allow image acquisition to occur at 30 images/second (equivalent to 30 frames/second, which is typically used in cinema). Modern fluoroscopes can be set to obtain images in pulsed mode . Pulsed mode operates the X-ray tube intermittently rather than continuously, resulting in a reduced radiation dose. Pulsed mode is typically accomplished in fractions of 30 (e.g., 1/2 mode is 15 images per second, 1/4 mode would be eight images per second). Rates as low as one image per second are typically available on most modern fluoroscopes.
One study found an approximately 50% reduction in fluoro times when switching from automatic exposure settings to pulsed and low-dose mode. 1
1 1 mSv = 100 mrem.However, there is a trade-off in image quality. Higher frame/image rates provide better temporal image quality, at the cost of higher radiation exposure. Typically, a lower pulse rate (4–8 per second) provides adequate temporal resolution.
With experience, the interventionalist will discern when to utilize the various available modes on a given fluoroscope (e.g., continuous mode fluoroscopy provides increased temporal resolution and therefore may be more appropriate than pulsed mode for contrast visualization under “live” or “real-time” injection). The interventionalist is encouraged to explore different pulse rates to determine which setting is appropriate for the procedure being performed.
Review available imaging before starting your procedure. Imaging (X-ray, magnetic resonance imaging, computed tomography [CT], etc.) review uses no radiation. Prior knowledge of the level of a vertebral deformity or spondylolisthesis can serve as a reference point to determine the level at which a procedure will be performed. In comparison, real-time spinal segment counting and other live scans to evaluate levels uses a finite amount of avoidable radiation exposure.
Anticipating the appropriate C-arm position before obtaining an image will limit unnecessary radiation exposure. For example, when setting up a particular trajectory or other view, one can expect that to obtain a “true” anteroposterior view of a segment, the C-arm will need to be tilted appropriately to match the lordosis or kyphosis of the segment of interest (see Fig. 3.1 ). Likewise, the C-arm will need to be obliqued to the anticipated angle before any exposure occurs. Although in many chapters the need to both tilt and then oblique is emphasized, these motions can of course be combined to further minimize radiation time and exposure. When setting up or changing from one view to another (e.g., anteroposterior to oblique), properly center the fluoroscope over the targeted structure, under visual inspection and/or using the laser, before acquiring the image. When the region of anatomic interest is centered in anteroposterior view and the C-arm is then moved to obtain an oblique view, the visualized structure shifts to the side opposite to which the C-arm is obliqued. Anticipating the need to translate (piston) the C-arm toward the side to which the C-arm is being obliqued can save fluoroscopic exposure time. Also, if an atypical angle is needed to get a “true anteroposterior” (AP), anticipate that the associated lateral image will be at a perpendicular angle.
Use the fluoroscope’s laser beam, if available, to line the needle up parallel to the beam ( Fig. 6.1 ). Some C-arms are equipped with a laser pointer that indicates alignment with the center of the image intensifier, which is meant to estimate beam trajectory. One practical use of this laser feature is to assist with the identification of skin entry location and maintenance of parallel trajectory during the needle insertion. After the trajectory view has been established and the target is centered on the fluoroscopic field, the laser can confirm a trajectory parallel to the beam.
While advancing in the trajectory view, the needle should be parallel to the beam (and perpendicular to the flat surface of the image intensifier). If it is not, adjust the needle before obtaining an image ( Figs. 6.2 and 6.3 ).
When performing multilevel procedures (e.g., two-level transforaminal epidural steroid injection [TF-ESI] and three-level discogram), place needles at all levels with the use of trajectory view before checking multiplanar imaging. By simultaneously obtaining anteroposterior and lateral images for all needles and repositioning all needles before subsequent images are obtained, radiation exposure can be limited (as compared with checking each level individually).
Most fluoroscopes provide an audible 5-minute timer warning to maintain awareness of total fluoro time.
The Last Image Hold feature digitally “freezes” the last image on the monitor after X-ray exposure is terminated. This allows the operator to study the last image and plan the next move without additional radiation exposure. The authors refer to this as “thinking with your foot off the pedal.” A technique preferred by the authors is to save the last image and transfer it onto the second monitor prior to adjusting the C-arm to another plane (e.g., transition from AP to lateral). This enables the operator to reference the previous imaging plane to better appreciate the needle location in three-dimensional space (see Chapter 3 ).
For mid-thoracic interventions, consider using a small gauge “marker” needle (e.g., 25 gauge) to provide a visual reference at the level to be injected. This will prevent the need for repeat localation of the level after repositioning the C-arm. See Chapter 20 (Thoracic Interlaminar ESI) for an example of marker needle use.
Proper C-Arm Operation: Maximizing Distance
Maintaining maximal operator distance from the X-ray source throughout the procedure is an important way in which accumulated radiation exposure can be minimized. This is based on the inverse square law ( Figs. 6.4 to 6.10 ). The major source of occupational radiation exposure to the operator is scatter radiation from the patient. The greatest scatter radiation received by the operator is during the lateral view. The operator should be educated to not only maximize his or her distance from the X-ray source but also maintain distance from the patient during image acquisition.