Fluoroscopy, Ultrasonography, Computed Tomography, and Radiation Safety

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

Stanley Golovac

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:

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.

Box 3-1 Ultrasound Gel

Ingestion: nonsignificant

Eye contact: flush with water for 15 minutes

Overexposure: reddening or blistering of the eyes

Carcinogenicity: none

OSHA regulated: no

Boiling point: >212° F

Specific gravity: 1.1 ²

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.

Fig. 3-2 Ultra-fast computed tomography scanner Helicoil GE.

CT may be the best method to accurately place a needle at small individual sites laden with blood vessels, nerves, and organs that should not be violated. Many studies have compared results of guidance with ultrasound (US) to CT, which is commonly accepted as the gold standard. However the use of CT is rising dramatically, and there are more significant risks.

Safety Concerns

The individual risk from radiation associated with a CT scan is quite small compared with the benefits that accurate diagnosis and treatment can provide. Still, unnecessary radiation exposure during medical procedures should be avoided. This is particularly important when the patient is a child because children exposed to radiation are at a relatively greater risk than adults. The American College of Radiology has noted, “Because they have more rapidly dividing cells than adults and longer life expectancy, the odds that children will develop cancers from x-ray radiation may be significantly higher than adults”³(Fig. 3-3). Unnecessary radiation may be delivered when CT scanner parameters are not appropriately adjusted for patient size. When a CT scan is performed on a child or small adult with the same technique factors used for a typically sized adult, the small patient receives a significantly larger effective dose than the full-sized patient.

Fig. 3-3 A, Estimated number of computed tomography scans performed annually in the United States. B, Estimated dependence of lifetime radiation-induced risk of cancer on age at exposure for two of the most common radiogenic cancers.

(Modified from Brenner DJ, Hall EJ: Computed tomography—an increasing source of radiation exposure, N Engl J Med 357:2277-2284, 2007.)

The absorbed dose is the energy absorbed per unit of mass and is measured in grays (Gy). One gray equals 1 joule of radiation energy absorbed per kilogram. The organ dose (or the distribution of dose in the organ) largely determines the level of risk to that organ from the radiation. The effective dose, expressed in sieverts (Sv), is used for dose distributions that are not homogeneous (which is always the case with CT); it is designed to be proportional to a generic estimate of the overall harm to the patient caused by the radiation exposure. The effective dose allows for a rough comparison between different CT scenarios but provides only an approximate estimate of the true risk. For risk estimation, the organ dose is the preferred quantity.

A recent study by Academic Emergency Medicine⁴ confirms what many doctors already believed; people may be receiving doses of radiation, sometimes unnecessarily, that puts them at a heightened risk for cancer. Researchers found that a typical patient who visited the emergency department received a cumulative radiation dose of 40 mSv over a 5-year period. Ten percent of patients ended up with a staggering 100 or more mSv. Both levels are well above the safety threshold for lifetime radiation exposure. Exposure above the threshold leaves patients vulnerable to increased long-term risk of cancer. As a point of comparison, one chest CT is around 10 mSv of radiation, and a traditional chest radiograph is only 0.02 mSv (Table 3-1).

Table 3-1 Typical Organ Radiation Doses from Various Radiologic Studies

Relevant Organ Dose*	Study Type	Organ (mGy or mSv)
Dental radiography	Brain	0.005
Posterior-anterior chest radiography	Lung	0.01
Lateral chest radiography	Lung	0.15
Screening mammography	Breast	3
Adult abdominal CT	Stomach	10
Barium enema	Colon	15
Neonatal abdominal CT	Stomach	20

* The radiation dose, a measure of ionizing energy absorbed per unit of mass, is expressed in grays (Gy) or milligrays (mGy); 1 Gy = 1 joule per kilogram. The radiation dose is often expressed as an equivalent dose in sieverts (Sv) or millisieverts (mSv). For x-ray radiation, which is the type used in computed tomography (CT) scanners, 1 mSv = 1 mGy.

Organ doses from CT scanning are considerably larger than those from corresponding conventional radiography (Table 3-2). For example, a conventional anterior-posterior abdominal radiographic examination results in a dose to the stomach of approximately 0.25 mGy, which is at least 50 times smaller than the corresponding stomach dose from an abdominal CT scan.

Table 3-2 Total Radiation for Body Parts

Body Part	Dose
Whole body, critical organs	5 rems in any year
Gonads, lens of eye	(Prospective annual limit) 10-15 rems in a year
Bone marrow	10-15 rems in any 1 year (Retrospective annual limit) (N-18) × 5 rems (long-term accumulation)
Skin	15 rems in any 1 year
Hands	75 rems in any 1 year (25/qtr)
Forearms	30 rems in any 1 year (10/qtr)
Other organs, tissues, and organ systems	15 rems in any 1 year

qtr, Quarter; rem, roentgen-equivalent-man.

Radiation doses vary by operator, radiology technician, and even the radiologist who may request additional images to verify areas of concern. Because there is no universal policy as to how many images should be used per examination, concerns among radiologists are on the rise. Precautions for radiation safety are presented in Box 3-2.

Box 3-2 Basic Radiation Safety

By using the ALARA (as low as reasonably achieved) technique, any radiation dose, no matter how small, can have some adverse effects. Therefore, radiation exposure can be minimized in the following ways.

Time: The shorter the exposure, the less radiation received.

Distance: Doubling the distance from the exposure means reducing the dose by one-fourth; tripling the distance means only one-ninth the dose.

Shielding: Lead barriers, aprons, sliding walls, and eyewear, along with hand and arm protectors, all add improved safety and reduce the total dose of radiation exposure.

A skirt can be applied to a carbon fiber imaging table that will help prevent scatter from irradiating the user that is particularly close to the patient being treated.

Thyroid shielding also is imperative to help reduce excess exposure of the user’s thyroid.

When females are to be imaged, a small, lightweight apron can be placed over female reproductive organs to add extra protection.

The use of a CT-guided image to perform most interventional pain procedures does expose a patient and staff to small doses of radiation. Very few procedures actually require a CT scanner to be able to correctly place needles in clinically difficult areas.⁵ Waldman⁵ notes that celiac plexus blockade and sacroiliac joint blocks have been performed with CT guidance, but each of these procedures can be performed with fluoroscopy guidance.

Fluoroscopy

The beginning of fluoroscopy can be traced back to November 8, 1895, when Wilhelm Röntgen noticed a barium platinocyanide screen fluorescing as a result of exposure to what he would later call x-rays.⁶ Within months of this discovery, the first fluoroscopes were created. Early fluoroscopes were simply cardboard funnels, open at the narrow end for the eyes of the observer. The wide end was closed with a thin cardboard piece that had been coated on the inside with a layer of fluorescent metal salt. The fluoroscopic image obtained in this way was rather faint. Thomas Edison quickly discovered that calcium tungstate screens produced brighter images, and he is credited with designing and producing the first commercially available fluoroscope.

Over the past several years, the use of fluoroscopy has allowed interventional pain physicians to accurately perform injections with precision guidance. What was once considered the gold standard (performing injections blind) is now taboo. A typical injection is performed with either a local anesthetic to confirm the cause of a pain source or in combination with a corticosteroid to help reduce the inflammatory effect caused by an injury or a chronic condition. Confirmation of a painful etiology is necessary to aid in the diagnosis of suspected painful areas. (Fig. 3-4).

Fig. 3-4 Portable fluoroscope (OEC 9900 GE) at the Space Coast Pain Institute.

To be able to clearly visualize critical structures such as nerves and blood vessels or unwanted intrathecal spread of an agent that may lead to a subarachnoid block is a major reason to perform fluoroscopically-guided procedures. The use of a fluoroscope permits the precise targeting of injections. The performance of a selective nerve root injection, for example, requires the placement of a predetermined anesthetic to carefully anesthetize a spinal nerve deemed to be the pain generator. By visualizing the anatomical spread of the contrast material being injected, the enhancement of the suspected painful area can be evaluated for pain relief. Fluoroscopy also is a very good predictor of how difficult the placement of complex devices such as spinal cord stimulator leads may be. With the use of the fluoroscope, the evaluation of spinal segments for optimal needle entry points can be studied prior to the performance of the procedure. Examination under fluoroscopy (visualizing the spine prior to performing the procedure coupled with physical examination) may also be possible. Fluoroscopy also allows one to see the pitfalls that might have been encountered if only surface landmarks had been used.

Disadvantages of Fluoroscopy

Fluoroscopy has several disadvantages. Acquisition and maintenance costs are a huge factor to physicians in private practice. The cost of the device may take several years to recoup. Availability of space in an office is another disadvantage, as the unit requires a large footprint.

Safety Concerns

Radiation safety is a major concern, and in some cases imaging rooms may need to have special construction, including lead shielding. The daily use of fluoroscopy requires a skilled technician to aid in proper device function and positioning of patients in order not to overexpose either the patient or other personnel in the room. Regular and routine maintenance is required for the machine’s warranty procedures and to adequately ensure safe delivery of radiation.

As fluoroscopy became increasingly useful as an interventional imaging tool, concerns about increasing exposure times increased scrutiny regarding radiation safety for patients and radiology professionals.^7,⁸ In 1994, the U.S. Food and Drug Administration entered the picture, issuing public health advisories dealing with serious radiation-related skin injuries resulting from some fluoroscopic procedures.⁹ Today’s newer techniques and equipment have contributed to lower dose rates, but fluoroscopy procedures still produce the greatest radiation exposures in diagnostic radiology. Investigators continue to study methods to further reduce exposure rates.^7,¹⁰

A key point with the use of the fluoroscope is distance, protection, and exposure. Distance is the main factor at reducing the amount of radiation exposure. The use of a continuous mode of radiation vs. pulsed emission is also a way to reduce exposure. The use of key protective gear is mandatory to ensure that eyes, thyroid, reproductive organs, and extremities are properly protected. The use of a standard dosimeter is also a key safety factor to quantify the amount of radiation one receives. Some physicians will also wear a “ring” dosimeter to measure radiation exposure of hands. There are various shields and safety garments that can be used for almost full body protection.

Pulsed Fluoroscopy Imaging

Some modern fluoroscopes have the capability of pulsed fluoroscopy, whereby the x-ray beam is emitted as a series of short pulses rather than continuously. At reduced frame rates, pulsed fluoroscopy can provide substantial dose savings. Images may be acquired at 15 frames per second rather than the usual 30 frames per second. Each image is displayed multiple times in sequence to provide a 30-frames-per-second display. Pulsed fluoroscopy can also be performed at even lower frame rates (e.g., 7.5 or 3 frames per second), but the display appears choppy when imaging rapidly moving regions such as the heart. This is because simply reducing the number of pulses results in an increase in image noise. Manufacturers may increase the milliamperage setting to achieve a similar visual appearance.

Image Storage

A typical portable fluoroscope used today is versatile and mobile and occupies less space in confined quarters than fixed units (Fig. 3-5). These units also allow one to store and archive images for scanning, reprinting, or illustrating details as to where needle placements are located. PACS (picture archiving and communication system) is a combination of hardware and software dedicated to the short- and long-term storage, retrieval, management, distribution, and presentation of images. Electronic images and reports are transmitted digitally via PACS; this eliminates the need to manually file, retrieve, or transport film jackets. The universal format for PACS image storage and transfer is DICOM (digital imaging and communication in medicine). Non-image data, such as scanned documents, may be incorporated using consumer industry standard formats such as PDF (portable document format) after being encapsulated in DICOM.