Background

X-rays are not directly detectable by the five human senses. However, comparing x-rays with light gives perspective.

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.

Review available imaging before starting your procedure. Imaging (x-ray, MRI, CT, etc) review uses no radiation. 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 Figure 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, before acquiring the image.

Use the fluoroscope’s laser beam, if available, to line the needle up parallel to the beam. Some C-arms are equipped with a laser pointer that indicates alignment with the center of the image intensifier and that is meant to estimate beam trajectory. One practical use for this laser feature is to assist with identifying the skin-entry location and maintaining parallel trajectory during 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 (Figures 4-1 and 4-2).

Figure 4–1 A, For this right-sided L5 transforaminal epidural steroid injection, the needle has been inserted and is not parallel with the beam. Notice the needle position relative to the image intensifier. B, The unnecessary fluoroscopic image only confirms what is obvious in part A of this figure and should have been corrected before the fluoroscopic image was taken.

Figure 4–2 A, The needle has now been adjusted and appears to be inserted parallel to the beam. B, The fluoroscopic image confirms an exposure parallel to the needle, with the hub and needle in line (i.e., a “hubogram”; see Chapter 3).

When performing multilevel procedures (e.g., two-level transforaminal epidural steroid injection, three-level discogram), place needles for all levels with the use of the trajectory view before checking multiplanar imaging. By obtaining anteroposterior and lateral images for all needles simultaneously and by repositioning all needles before subsequent images are obtained, radiation exposure can be limited (as compared with checking each level individually).

Proper C-Arm Operation: Limiting Exposure Time Use Pulsed Mode Fluoroscopy

The default mode of fluoroscopic image acquisition is commonly referred to as “continuous” or “conventional” fluoroscopy. Continuous fluoroscopy requires continuous x-ray tube operation. In continuous mode, image acquisition occurs at 30 images/second (equivalent to 30 frames/second that is typically used for cinema). Modern fluoroscopes can be set to obtain images in various “pulsed modes.” Pulsed fluoroscopy obtains images or “pulses” at intervals rather than continuously. This is typically accomplished in fractions of 30 (e.g., 1/2 mode is 15 images per second, 1/4 would be 8 images per second, and so forth). In this mode the x-ray tube is operating intermittently rather than continuously, and the resulting x-ray exposure is reduced. (Rates as low as 1 image per second are typically available on most modern fluoroscopes.) Higher frame/image rates provide better image quality, at a cost of much higher radiation exposure. Typically, a lower pulse rate (e.g., 4–8 per second) provides adequate temporal resolution. The interventionalist is encouraged to utilize different pulse rates until an optimal setting is determined.

Use Low Dose Mode whenever possible. Many conventional fluoroscopes are capable of operation in various dose modes, commonly referred to as “Low Dose”, “Medium Dose” and “High Dose”. As the name implies, each mode allows a different amount of x-ray exposure per unit time, with Low Dose generating the least amount of x-rays per unit time. One study found approximately a 50% reduction in fluoro times when switching from automatic exposure settings to pulse and low dose mode.² There is a trade-off though, in image quality. 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).

Use Last Image Hold, which automatically saves each image and displays it on a second monitor for comparison to the current image. Another technique, preferred by the authors, is to frequently save images for reference. By saving the last image prior to adjusting the C-arm for multiplanar imaging (e.g., transition from AP to lateral), the interventionalist will always have a reference image available. (See Chapter 3.)

When mid-thoracic, consider using a small gauge needle (e.g., 25 g) to “mark” a level to avoid needing to count again. See Chapter 22 (Thoracic Interlaminar ESI) for examples of marker use.

Proper C-Arm Operation: Maximizing Distance from Source

Maintaining optimal 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 (Figures 4-3 through 4-10).

Figure 4–3 The inverse square law. As the distance from the source increases, radiation intensity decreases exponentially.

Figure 4–4 This experienced interventionalist has stepped away from his patient before the fluoroscopic image is taken. The red portion of this and subsequent figures simulates the direct fluoroscopic beam and the yellow portion simulates the scatter. Our simulation does not properly depict the true extent of the x-ray absorption by the patient or the true amount and direction of the scatter.

Figure 4–5 During skin marking and target localization with live fluoroscopy and live contrast injection, the interventionalist can still minimize exposure by using longer metallic markers (A) and extension tubing (B and C

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