Fluoroscopy, Ultrasonography, Computed Tomography, and Radiation Safety

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Chapter 3 Fluoroscopy, Ultrasonography, Computed Tomography, and Radiation Safety

Chapter Overview

Chapter Synopsis: A detailed, accurate picture of the body’s internal environment is key for a clinician guiding a needle to a targeted location within the spine, extremities, or viscera. This chapter considers imaging technologies that aid in this guidance for the diagnosis, confirmation, and/or treatment of pain. Ultrasound (US) technology sends very high-pitched sound waves into the body, which are reflected differently depending on the tissue’s makeup, thereby providing a picture of the internal environment and good resolution images of soft tissue structural relationships. US is limited, however, by lack of clarity of many deeper spinal targets because of bone shadowing. Computed tomography scans can provide high-resolution images of the internal environment, including the spine and deeper targets, but carries additional risk from radiation exposure, particularly in children. In addition, most CT techniques are delayed, thus real-time guidance of the needle is not always possible. Fluoroscopy, which utilizes x-rays and is usually portable, is perhaps the most versatile tool. Fluoroscopy does not provide resolution for soft tissues, but instead relies on bone images and the use of real-time contrast dye administration for procedural guidance in interventional pain. Although available in some centers, the use of magnetic resonance imaging (MRI) guidance is not discussed due to the complexity and cost of this modality. The relative merits of US, CT, and fluoroscopic image guidance are emphasized in this chapter, along with known safety concerns.

Important Points:

Clinical Pearls:

Clinical Pitfalls:

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Although this limit varies from person to person, it is approximately 20 kHz (20,000 Hz) in healthy, young adults, and thus 20 kHz serves as a useful lower limit in describing ultrasound (Fig. 3-1).

The production of ultrasound is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium, a property also used by animals such as bats for hunting. The most well-known application of ultrasound is its use in sonography to produce pictures of fetuses in the human womb. There are a vast number of other applications as well.

In diagnostic ultrasonography, also known as sonography, the physician or technician places a transducer, or ultrasound probe, in or on the patient’s body. Pulsed ultrasound waves emitted by the transducer pass into the body and reflect off the boundaries between different types of body tissue. The transducer receives these reflections, or echoes. A computer then assembles the information from the reflected ultrasound waves into a picture on a video monitor. The frequency, density, focus, and aperture of the ultrasound beam can vary. Higher frequencies produce more clarity but cannot penetrate as deeply into the body. Lower frequencies penetrate more deeply but produce lower resolution, or clarity. For uses in the spine or deeper tissues such as the hip joint, a curvilinear (low frequency) probe is generally used. Bone structures such as the posterior elements and lamina reflect sound waves back, causing darker “hypoacoustic” areas, effectively shadowing many soft tissue targets such as spinal nerves in the foramina that may be deeper than these bones, thus the bones obscure the reflected echoes from the nerves. In many cases, the use of color Doppler will add additional clarity by rendering blood flow in either red or blue color to delineate vascular structures from other anatomical tissues in the visual field.

Safety Concerns

Most infants now born in the United States are exposed to ultrasonography before birth, and in Germany, Norway, Iceland, and Austria, all pregnant women are screened with ultrasonography. To date, researchers have not identified any adverse biological effects clearly caused by ultrasonography, even though 3 million babies born each year have had ultrasound scans in utero. This is an enviable safety record. However, the National Council on Radiation Protection and Measurements advocates continued study of ultrasound safety, improvements in the safety features of ultrasound systems, and more safety education for ultrasound system operators.1 Because of the sheer number of people exposed to ultrasonography, any possibility of a harmful effect must be investigated thoroughly.

Ultrasound gel is intended only for external use. If a needle becomes contaminated with gel, every effort should be made to remove the needle and replace it with a sterile new one. Even though the gel initially is sterile, the substance itself may irritate structures either in the epidural space or even intrathecally. Either way, one should err toward needle replacement. Remember, ultrasound gel contains propylene glycol, glycerine, phenoxyethanol, and FD&C Blue #1. For properties and side effects of ultrasound gel, see Box 3-1.

Computed Tomography

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