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

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:

At the present time, multiple forms of radiological imaging devices are used to evaluate and confirm the diagnosis of clinical conditions.

Safety is still a very real concern, and awareness of risk that is taken with each procedure is important.

Clinical Pearls:

Interventional pain procedures are now required to be performed with the use of radiological guidance in most situations. Without imaging, patients may be placed at greater risk unnecessarily.

Minimizing radiation exposure is still most important. MRI or US allows the performance of evaluations without radiation exposure, and may be considered in specific situations.

Clinical Pitfalls:

Careless use of and overexposure to radiation can compromise the health and well-being of both the operator and the patient being examined. Careful attention to lead apron quality, sizing, and regular safety inspections of all equipment are important.

The use of pulsed or reduced dose techniques of fluoroscopy are important safety measures to consider, particularly in higher dose procedures such as spinal cord stimulation electrode placement.

The effects of US gel are not well known, and efforts to limit the introduction of gel internally are important.

Introduction

In the last three decades, image guidance for interventional pain management procedures has become standard, replacing the less accurate and less reproducible use of surface landmark–based techniques. There are many reasons for this evolution in procedural therapy, including the advent of new procedures (e.g., transforaminal epidural injections largely replacing interlaminar epidural injections); increases in both the number of procedures performed and an associated rise in the number of complications; and the need for image storage to ensure the appropriateness of the diagnostic and therapeutic procedures that were performed. First used in 1895, fluoroscopy has become the most popular technique for image-guided procedures, particularly for those procedures in the cervical, thoracic, and lumbar spine. It is one of many different forms of imaging, including ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI), all of which can be used to help guide needle placement to diagnose, confirm, and treat areas believed to be the pain generators.

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).

Fig. 3-1 Ultrasound kilohertz. NDE, Nondestructive evaluation.

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.

Disadvantages of Ultrasound

Unfortunately, the use of ultrasonography does not confirm contrast spread to enhance structures that need to be identified. This is important in techniques such as transforaminal epidural injections where the desire is to ensure that medications and particulate steroids not be injected intravascularly. Ultrasonography allows visualization of tissue density and depth but not unlike fluoroscopy one cannot see radiographic contrast enhancement. Deep spinal injections utilizing ultrasound are particularly difficult and require a large learning curve to become proficient, compared to the relatively simple use of fluoroscopy techniques.

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

CT was discovered independently by a British engineer named Sir Godfrey Hounsfield and Dr. Alan Cormack. Cormack was the first to analyze the possibility of such an examination of a biological system, in 1963 and 1964, and to develop the equations needed for computer-assisted x-ray reconstruction of pictures of the human brain and body. It has become a mainstay for diagnosing medical diseases. For their work, Hounsfield and Cormack were jointly awarded the Nobel Prize in 1979.

CT scanners first began to be installed in hospitals around 1974. Currently, 6000 scanners are in use in the United States. Advances in computer technology have vastly improved patient comfort because CT scanners are now much faster. These improvements have also led to higher resolution images, which improve the diagnostic capabilities of the test. For example, the CT scan can show doctors small nodules or tumors, which they cannot see on radiography.

The CT scanner is an expensive yet sophisticated way to guide needle placement (Fig. 3-2). It is somewhat expensive for the routine use of image-guided procedures, especially in an office-based practice or even an ambulatory surgery center. One could justify the use of such a device if looking at a study or working in a hospital with access to a scanner. Most scanners are used daily for diagnostic workups but not for pain management procedures. They allow for excellent needle placement and biopsies that are performed.