Atlas of Pain Medicine Procedures 1st Edition

SECTION I

BASIC APPLICATIONS

CHAPTER 1

Fluoroscopy in Interventional Pain Medicine

David M. Schultz

Since their discovery in 1895, x-rays have revolutionized the practice of medicine. By allowing doctors to view the inside of the living body, x-rays have greatly increased our ability to diagnose and treat disease and to precisely deliver targeted therapies. The fluoroscope was the first x-ray machine and has evolved from its humble beginnings into a powerful and sophisticated device that has become the basis for the new field of interventional pain management. Modern fluoroscopes enable the interventional pain practitioner to use continuous, real-time x-ray imaging to guide interventional procedures that target the physical generators of pain with a high degree of precision and safety. Since fluoroscopy is essential for most invasive pain procedures, it is imperative that interventional pain physicians have a firm understanding of the fluoroscope in order to use it safely and effectively in daily practice.

HIGHLIGHTS IN THE HISTORY OF FLUOROSCOPY

• 1895: German physicist Wilhelm Roentgen discovers x-rays and takes the first fluoroscopic image, purportedly of his wife’s hand, winning the 1903 Nobel Prize in Physics for his efforts.
• 1896: Thomas Edison invents the first fluoroscope, which is quickly adopted for medical uses.
• 1897: Madame Curie discovers radium, which is then used to illuminate games of chance in New York City.
• 1898: The inappropriate, nonmedical use of fluoroscopy becomes increasingly common.
• 1899: Compensation is awarded in the first medical malpractice suit involving x-ray injury.
• 1900: Radiation injuries become increasingly common, and practitioners become increasingly aware of the dangers of x-ray exposure.
• 1910: After several years of practice, radiology pioneer Dr. Mihran Kassabian suffers severe radiation burns to his hands and ultimately dies of radiation-induced cancer at age 34.
• 1929: The National Council on Radiation Protection and Measurement (NCRP) is created to protect patients, healthcare workers, and the public from the harmful effects of radiation.
• 1994: Patient injuries resulting from excessive use of fluoroscopy during medical procedures prompt the FDA to issue a Public Health Advisory.¹

THE PHYSICS OF X-RAYS

X-rays are a form of ionizing radiation that can be created in the fluoroscope and harnessed for medical imaging. X-rays are produced in the x-ray tube of the fluoroscope, which contains a cathode and metal anode.

• When high-velocity electrons leave the cathode and collide with the metal anode, the kinetic energy contained within the electrons is converted to electromagnetic energy and released in the form of x-rays.
• X-rays are close to light photons on the electromagnetic spectrum but have shorter wavelength and higher energy (Figure 1-1).

Figure 1-1. X-rays are close to light photons on the electromagnetic spectrum but have shorter wavelength and higher energy.

The x-rays produced by the x-ray tube are directed through body tissues. When these x-rays contact matter, they interact in one of three ways:

1. They are absorbed.
2. They are deflected and scattered.
3. They pass through matter unheeded.

Since x-rays are a form of ionizing radiation, they interact with certain media in ways that allow the human eye to view their presence. In fluoroscopy, the x-rays cause the phosphorous in a fluorescent screen to emit visible light. Modern fluoroscopic systems use zinc-cadmium sulfide as an effective phosphor.

The fluoroscopic image is essentially composed of shadows created as body tissues of various densities preferentially absorb x-rays. As the density of matter increases, x-rays are absorbed or scattered to a greater extent, giving rise to the 5 radiologic densities commonly used to describe radiographs:

1. Air
2. Fat
3. Water (soft tissue)
4. Bone
5. Metal

Air allows most emitted x-rays to penetrate through to the underlying imaging medium. Bone and metal are denser and absorb or deflect x-rays, allowing fewer x-rays to penetrate through to the imaging medium. Consequently, higher-density tissues cast shadows that appear darker on the displayed image because the x-rays contacting the phosphor create light (Figure 1-2). This is in contrast with traditional x-ray imaging, which uses a photographic plate to capture the effects of photons (Figure 1-3). The plate starts out as a white background that is exposed by the x-rays reaching it. The fluoroscopic image is analogous to the photographic negative whereas the developed x-ray film is analogous to the photograph.

Figure 1-2. Fluoroscopic image with higher density structures appearing darker; note the bubbles of air contained within the injected x-ray contrast medium.

Figure 1-3. Plain chest radiograph with denser tissues appearing lighter. (Reprinted with permission from Fuster V, Walsh RA, Harrington RA: Hurst’s The Heart, 13th Edition: www.accessmedicine.com © The McGraw-Hill Companies, Inc. All rights reserved.)

THE C-ARM FLUOROSCOPE

The primary function of the fluoroscope is to generate a controllable beam of x-rays that can be directed through tissue and then captured on a viewing medium to form a visible image. Interventional pain physicians commonly use the C-arm fluoroscope because of its maneuverability and compact design (Figure 1-4).

Figure 1-4. Modern fluoroscope with a shielded x-ray tube contained within a movable C-arm.

The main components of a typical mobile C-arm fluoroscope include the x-ray generator, x-ray tube, collimator, image intensifier, optical coupling chain and viewing monitor (Figure 1-5).

Figure 1-5. Modern fluoroscope with components labeled.

The x-ray generator converts alternating current to high voltage direct current, which is delivered to the x-ray tube.

• The current determines thenumber of x-rays produced by the x-ray tube and therefore controls the density and intensity of the x-ray beam.
• The voltage determines theenergy of the x-rays produced and the penetrating ability of the x-ray beam.
• The current and voltage can be automatically or manually adjusted from the base unit of the fluoroscope (Figure 1-6).

Figure 1-6. The amount of current supplied to the x-ray tube is measured in milliamps (mA) and determines the density and intensity of the x-ray beam.

The x-ray tube is housed in one end of the C-arm and is balanced by the image intensifier at the other end.

• The beam that is released from the x-ray tube diverges as it moves toward the image intensifier.
• The x-ray beam is most concentrated as it exits the x-ray tube at the center point aperture.
• Severe patient injuries occur when body tissue remains in close proximity to the origin of the x-ray beam for prolonged periods.²

Fluoroscopic images are dim and difficult to view without some mechanism to brighten the viewable image. The image intensifier was introduced in 1934 and functions to convert x-rays into light photons, which amplify brightness by 5000 to 20,000-fold.

Collimation allows the fluoroscope operator to reduce the size and shape of the x-ray beam to better conform to the field of view (Figures 1-7 and 1-8). As the fluoroscope moves across areas of varying body tissue density, the collimator automatically adjusts the x-ray beam to conform to the viewing field. The operator can also adjust the collimation window manually to conform to a region of clinical interest (Figure 1-9).

• By reducing the x-ray beam to include only the tissue targeted for viewing, less tissue is irradiated, and patient exposure to radiation is reduced. In addition, there is less scattered radiation exposure to personnel in the room.
• Unattenuated x-rays also cause glare on the image screen resulting in poor image quality. Collimation reduces glare and improves the clarity of the image.

Figure 1-7. Without collimation, the x-ray beam is not optimized to fit the field of view, and there is a large amount of scattered radiation from the patient and table as well as a monitor image that is relatively degraded.

Figure 1-8. By using collimation, the x-ray beam is shaped to better fit the field of view resulting in fewer x-rays leaving the x-ray tube, less scatter radiation, and a clearer image on the monitor.

Figure 1-9. Controls for both radial (iris) and rectangular collimation on a typical C-arm fluoroscope.

To process the images that have been brightened by the image intensifier for optimal viewing, modern fluoroscopic systems incorporate optical coupling chains that route the image signal to a video camera.

Optical coupling enables the x-ray image to be viewed via closed-circuit television, displaying real-time video imaging of continuous fluoroscopy.

A typical mobile C-arm fluoroscope can also display simultaneous static images on a second television monitor for viewing of the last image in a video sequence. This “last image hold” capability allows the interventionalist to minimize radiation exposure by performing procedures that utilize a series of static images to follow needle placement.

Using digital image conversion technology, analogue video signals are digitized and stored in computer memory. Using less radiation, subsequent digital enhancement of the fluoroscopic image can achieve image clarity approaching that of x-ray film. Digital images can also be quickly and conveniently distributed via computer networks and stored on computer workstations or archived into various digital storage media for later retrieval.

Since materials of differing density cause x-rays to be absorbed or deflected to varying degrees, fluoroscopy table and pad materials with high or inconsistent density can result in poor image quality.

• Denser materials in table and pad attenuate x-rays, which will result in increased patient radiation exposure and loss of image contrast.
• Materials that are inconsistent in density tend to cast artifactual shadows on the imaging screen.

It is therefore imperative to use only x-ray tables and pads designed to optimize fluoroscopic imaging. Newer composite materials such as carbon fiber in fluoroscopy tables provide adequate strength to support large patients while minimizing x-ray attenuation and distortion. Likewise, thin foam pads overlying the table have minimal effect on x-rays, but large gel supports or irregularly folded pillows may create significant x-ray attenuation, distortion, and artifact. A diving-board table configuration allows for easier imaging of upper body structures during interventional pain procedures.

RADIATION SAFETY

Exposure to ionizing radiation—including x-rays—can result in the formation of free radicals that cause damage to cell structures.

• Chemical chain reactions may trigger changes in cell membrane permeability, resulting in cellular dysfunction.
• Damage to DNA may cause somatic mutations, causing harm to subsequent generations.
• Radiation protection standards are designed to prevent unintended radiation and to maintain exposure for therapeutic purposes “As Low As is Reasonably Achievable” (ALARA).

Dosimetry badges are solid-state radiation detection devices used to measure cumulative radiation dose. For optimal monitoring, two badges are worn, one inside and one outside of a protective lead apron, to determine both overall exposure and the efficacy of the lead apron protection.

• Badges are analyzed on a monthly basis and allow for monitoring of cumulative radiation exposure.
• Current occupational exposure recommendations set the upper effective dose equivalent of 50 mSv/y (5 rem/y) and a cumulative dose not to exceed 10 mSv (1 rem) times the age of the worker. Thus, lifetimeexposure for a 50-year-old radiation worker would be 500 mSv (50 rem).

Modern x-rays are completed within milliseconds, and typical radiation exposure is a small fraction of what it was 100 years ago; however, prolonged patient exposures to x-rays may occur as increasingly complex procedures are performed using continuous fluoroscopy.

More than 50 reports of patient injury from prolonged exposure to x-rays during fluoroscopic procedures occurred in 1994 alone, resulting in the health advisory issued by the Food and Drug Administration.¹

• Several reports of serious patient injuries from radiation during fluoroscopy have been recently published.^3,4
• Recognition of patient injury from fluoroscopy is often delayed, since the effects of excessive radiation exposure are usually not immediately apparent.
• Fluoroscopy injuries documented in the past 20 years have included skin burns serious enough to require skin grafting.
• Since reporting of fluoroscopy injury is not mandatory, the actual extent of this problem is unknown.

Occupational radiation exposure can be reduced to as low as is reasonably achievable through adherence to 3 basic principles:

• Reduce exposure time.
• Increase the distance from the radiation source.
• Shield yourself and your patient from direct and scattered radiation.

The longer one is exposed to a radiation field, the greater the total radiation dose. Thus, limiting time of exposure is a simple, common-sense method for reducing risk.

• With modern fluoroscopic systems, a short burst of radiation used with “last image hold” capability allows the operator to identify needle position with minimal x-ray exposure time.
• This technique allows the interventionalist to take a fluoroscopic “snapshot” of the field, hold the static image on the monitor, move the needle a small distance toward the target, and then obtain another brief fluoroscopic image to determine the next needle position.
• A recent study found the average time required for needle placement during various interventional pain procedures was 7.7 seconds, which is quite low compared to other dosimetry-measured radiation exposure times in other medical specialties using fluoroscopy.⁵

Based on the inverse square law, the amount of radiation exposure is proportional to the inverse square of the distance from the source.

• Therefore, the amount of radiation exposure declines exponentially as the operator moves away from the source.
• With fluoroscopy, distances of 6 ft or more from the x-ray tube, and from scatter radiation coming off the patient and table, result in minimal radiation exposure.

Lead aprons are mandatory, and thyroid shields are recommended, as standard garb within the procedure room during fluoroscopy (Figure 1-10).

• Lead shielding can be tailored to body contours and made reasonably comfortable while providing an effective barrier to radiation exposure especially to the thyroid and pelvis.
• A wide variety of lead glass screens can be placed between the operator and the x-ray source and/or patient in order to reduce direct and scatter radiation exposure, respectively.
• The interventional procedure room can be configured with leaded glass screens that are attached to ceiling mounts or used with rolling frames on the floor to fit a particular space.
• Leaded glass lenses offer an effective barrier to eye exposure and can be configured with optical correction; however, they may be heavy and uncomfortable. Regular glass lenses also afford some protection and other alternative materials for eye protection are becoming available (Figure 1-10).

Figure 1-10. Radiation protection with lead apron, glasses, and screen.

Some experts have advocated the use of leaded gloves in order to reduce hand exposure to x-rays during fluoroscopic procedures.

• With lead gloves, there is less chance of hand exposure to scattered x-rays.
• However, the increased density of lead gloves placed within the x-ray field will cause the fluoroscope to increase output as it tries to penetrate the high-density lead.
• Therefore, leaded gloves within the field of view of the monitor will cause an automatic increase in direct and scattered radiation.
• Lead gloves are also expensive and may decrease the tactile sensation, hindering safe and accurate needle placement.
• Keeping hands completely out of the beam is advisable with or without lead gloves.

SUMMARY

The fluoroscope has revolutionized the treatment of chronic pain. For optimal safe and effective use, the pain specialist physician should have an in-depth understanding of fluoroscopy and the fluoroscope to accurately diagnose and treat the physical generators of pain. Skill with the fluoroscope combined with needle placement skills and an understanding of the anatomy and pathophysiology of chronic pain will allow the interventional pain specialist to help patients with chronic pain more effectively than ever before possible.

References

FDA Public Health Advisory. Avoidance of serious x-ray induced skin injuries to patients during fluoroscopically guided procedures. Rockville, MD: Food and Drug Administration, September 9, 1994.

Wong L, Rehm J. Radiation injury from a fluoroscopic procedure. N Engl J Med. 2004;350:e23.

Sovik E, Klow NE, Hellesnes J, Lykke J. Radiation-induced skin injury after percutaneous transluminal coronary angioplasty: case report. Acta Radiol. 1996;37:305-306.

Knautz MA, Abele DC, Reynolds TL. Radiodermatitis after transjugular intrahepatic portosystemic shunt. South Med J. 1997;90:352-356.

Manchikanti L, Cash KA, Moss TL, Pampati V. Radiation exposure to the physician in interventional pain management. Pain Physician. 2002;5:385-393.