Radiation Safety in the Cardiac Catheterization Laboratory

C-Arm

The x-ray tube and image detector are affixed to opposite ends of the c-arm. The tube is mounted in a fixed orientation with respect to the detector on the c-arm typically positioned below the procedure table. The detector is mounted on a movable suspension above the table. This suspension permits the operator to raise or lower the detector in relation to the patient. The entire c-arm support system may be mounted directly on the floor, ceiling, or a robotically controlled device.

Most c-arms are capable of rotation speeds of up to 35 degrees/second and up to 100 degrees/second for CT-angiography and rotational angiography. Movement of the c-arm is commonly limited by proximity sensors that slow or stop rotation at a certain distance from the patient or the x-ray table.

X-Ray Tube

The x-ray tube consists of an evacuated glass or metal-enclosed assembly that contains a circular anode (positive electrode) and a cathode with one or more filaments (negative electrode). When an electric current is passed through the filament, its temperature increases and electrons are released through thermionic emission. These electrons are accelerated through a potential difference, focused on a small area of the rapidly rotating anode known as the focal spot track. The x-rays produced have a heterogeneous energy distribution dependent on the anode material and tube voltage. Typically, less than 1% of the energy applied to the x-ray tube is converted into x-rays; the majority is lost to heat. Management of this heat production is a major consideration in the design of x-ray tubes.

Characteristics of the x-ray tube include the following:

• 1.

  MA (milliamperes): The tube current or the number of electrons traveling across the anode-cathode gap per second. The x-ray output is linearly proportional to the tube current.

• 2.

  Pulse width: The duration of time that x-rays are used to create a single fluoroscopic image. A shorter pulse width can better capture an image of a moving object. The pulse width for cardiac angiographic procedures varies from approximately 6 msec to 10 msec. X-ray output is linearly proportional to pulse width.

• 3.

  MAs (milliamperes*sec): The measure of the total charge, the product of the mA, and the pulse width (in seconds) for a given fluoroscopic image. X-ray output is linearly proportional to mAs.

• 4.

  KVp (peak kilovoltage): The measure of voltage applied across the anode-cathode gap that characterizes the distribution of photon energies within the x-ray beam. Increasing the kVp increases the mean photon energy of the x-ray spectrum, resulting in a more penetrating beam. The tube voltage has a complex relationship to x-ray output, which can be approximated as a power relationship. For example, doubling the peak kilovoltage approximately quadruples the x-ray output.

• 5.

  Focal spot: A well-defined region on the anode where the accelerated electrons are focused and x-rays are produced. Most x-ray tubes have two or more focal spots, each paired with a dedicated cathode filament. The focal spot size can vary from approximately 0.3 mm to 1.0 mm for cardiac angiographic systems. The limited heat capacity of the anode dictates that when the total heat deposited exceeds a certain threshold, the focal spot must change to a larger size to distribute the electrons over a larger area to prevent anode damage.

• 6.

  X-ray filtration: The x-ray beam passes through numerous materials prior to reaching the patient. These include substances inherent to the tube (glass/metal assembly, oil, exit port) and those added for spectral shaping (aluminum and/or copper sheets) contained within the collimator. Modern systems allow for variable filtration that can be changed within a procedure or between protocols. This added filtration disproportionately reduces the number of lower energy photons, thereby increasing the average photon energy. This process is referred to as “beam hardening,” which can reduce the skin entrance dose for a given detector dose.

Image Detector

Digital flat-panel detectors have replaced the older image intensifier technology in virtually all modern CCL equipment. The vast majority of these detectors uses an amorphous silicon detector coupled with a two-dimensional thin-film transistor (TFT) array. The physical size of detector elements (pixels) ranges from 80 to 200 microns, or one twelfth to one fifth of a millimeter.

• 1.

  Image detector function: Digital flat panel detectors convert x-ray energy into a digital signal via a process that converts the x-rays into light, which is then converted into an electric signal via photodiodes. Once the x-ray pulse has terminated, the stored information is sampled, amplified, digitized, and transmitted to the nearby workstation. The resulting data then undergo image processing prior to display.

• 2.

  Automatic dose rate control (or automatic brightness control): In all fluoroscopy systems, the detector acts as the critical component of a feedback loop that regulates the output of x-rays (mA, kVp, pulse width, filtration, and focal spot size). Automatic dose rate control (ADRC) ensures that the dose to the detector is sufficient to provide adequate image quality, accounting for changes in patient size, thickness, and presence of highly attenuating structures. For instance, changing from narrow to steep angulations increases the effective patient thickness. The ADRC increases output parameters to ensure that the image quality is similar to previously obtained images.

Operator Consoles

The angiographer employs the bedside console to control movements of the c-arm and table, field of view, magnification mode, and clinical protocol/techniques. A foot pedal is used to control the duration of x-ray exposure and the type of image that is desired (fluoroscopy/acquisition). The operating console in the nearby control room provides the interface between the operator and the imaging and EKG/hemodynamic systems. It communicates with the image display system, the PACS (Picture Archiving and Communication System), and the EMR (Electronic Medical Record).

Image Display Monitor

Visualization of images on the overhead monitor is an integral part of all angiographic procedures. Monitors vary in size from 17 to 60 inches and should be routinely evaluated to ensure appropriate luminance, grayscale performance, contrast, resolution, spatial linearity, and absence of artifacts. Any issue that impairs the ability of the angiographer to evaluate fluoroscopic images may prolong the procedure and unnecessarily increase the radiation dose. It is not only the monitor that is important but also the display environment, where important variables include the following:

• Distance: The optimal viewing distance between the angiographer to the monitor is a factor of 1.5 to 2.0 of the diagonal. For instance, with a 19-inch display, this distance is about to 3 feet or slightly more than an arm's length.

• Viewing conditions: Bright ambient lights within the angiographic suite increase glare on the image monitors, which decreases the ability of the angiographer to visualize differences in shades of gray. Also, glare and reflections from spot lighting near the operator have the potential to interfere with image evaluation.

Terminology of Fluoroscopic Modes

Historically, the terms “fluoro” and “cine” have been used to denote two different modes of radiographic image observation and/or recording. Fluoro has been used to describe real-time observation of lower dose radiographic temporal imaging without recording. Cine has been used to describe the recording of higher dose and image quality radiographic temporal imaging. However, the term cine implies the use of motion picture film in the recording of the radiographically produced images. Modern systems no longer employ film; recording is exclusively digital and available for all operational modes, including fluoroscopy.

These two modes of operation shall be defined for this chapter as follows:

• 1.

  Fluoro ( or fluoroscopic observation) describes the real-time temporal imaging performed at or below radiation output limits established by regulatory agencies. Fluoro typically defaults to nonrecorded imaging; however, an operator can choose to save either a single fluoro image or image sequence at the operator controls.

• 2.

  Acquisition describes the mode of operation that requires recording of the real-time imaging, employing increased radiation output that is needed for high-quality images. This mode of imaging is not governed by regulatory limits, and it is limited only by hardware capability or by design parameters established by the vendor not typically accessible to the end user without service support.

Within the fluoroscopic mode of operation there are typically three radiation output/image quality levels selectable by the operator. These settings are customizable and can vary greatly between fluoroscopic units. Generally there is a “low dose” fluoro level that is nominally set at 50% of the “standard dose” and a “high dose” level set at 200% of the standard dose level. In the United States, federal regulations pertaining to manufacturers limit the radiation output for the fluoroscopic imaging modes under specific conditions. For the standard and low dose fluoroscopic imaging modes, the air kerma limit is 88 mGy/min (10 R/min in traditional units). The high dose fluoro mode may, given certain additional requirements, extend the air kerma limit to 176 mGy/min (20 R/min). For c-arm fluoroscopes these limitations are defined at 30 cm from the face of the image receptor, regardless of the source to image distance (SID). Again, the acquisition imaging mode of operation does not include regulatory radiation output limitations. Acquisition rates can range from approximately 10 mGy/min to 3000 mGy/min and under most circumstances fall between 100 mGy/min to 300 mGy/min.

Terminology of Radiation Doses

It is important for angiographers to become familiar with the nomenclature of radiation doses to understand basic concepts of radiation dose reduction. This nomenclature is complicated due to different aspects of a “dose” and the proliferation of different systems for naming dose quantities. Dosimetric units within this chapter follow the S.I. standard (Système International) ( Table 6-2 ).

• 1.

  Absorbed Dose/Peak Skin Dose (D _skin,max )

• Absorbed dose is defined as the amount of energy absorbed per unit mass in units of gray (Gy). This quantity does not reflect the total amount of energy deposited because it is normalized to mass. Since dose in fluoroscopic cases is distributed, it is the peak skin dose (D _skin,max ), which is the best indicator of radiation-induced cutaneous injury. D _skin,max is defined as the maximum radiation dose to any one area of skin at the entrance of the x-ray beam into the body during a fluoroscopic exam. This dose estimate is not displayed on current fluoroscopic equipment because there is no convenient method for its measurement. One method that can provide D _skin,max in delayed fashion is radiochromic film dosimetry. A thin sheet of this film can be placed between the source and the patient's skin in the direct path of the primary x-ray beam. At the end of the procedure, the film will reveal the two-dimensional radiation dose distribution, including backscatter. With appropriate calibration curves applied, the scanned film can be used to determine the D _skin,max . A similar result can be obtained by using multiple small calibrated thermo-luminescent dosimeters applied at the anticipated location(s) of maximal dose.

• 2.

  Kerma/Air Kerma at the Reference Plane (K _a,r )

• Kerma (K) is defined as the kinetic energy released in matter, which is expressed in units of Gy. The displayed quantity K _a,r , refers to the kerma in the air at the interventional reference plane (IRP) expressed in units of Gy. The IRP was defined by the International Electrotechnical Commission (IEC) as the plane located 15 cm below the isocenter of the c-arm in the direction of the focal spot. This plane is irrespective of the table height, source to image distance, or c-arm positioning. This location puts the patient's skin surface at or near the IRP for a 30-cm thick patient with the axial center of the body at isocenter of the c-arm. Since there is no convenient way to directly measure D _skin,max , K _a,r is commonly used as a surrogate.

• There are several reasons why K _a,r does not always accurately reflect the D _skin,max :

  • a.

    The IRP and the plane of the skin entrance commonly differ, depending on the rotation and angulation of the c-arm, the distance from the x-ray tube to the patient, the thickness of the patient, and the height of the table used for the procedure. Two examples illustrate the problems with using K _a,r as a direct surrogate for D _skin,max :

    • 1.

      In a scenario where the patient is thin (20 cm thick), the table is maximally elevated, the x-ray tube is located as far as possible from the patient, the image detector is close to the patient's chest, and two views are acquired at two very shallow, nonoverlapping fields: the displayed K _a,r may overestimate the D _skin,max by a factor of 3.

    • 2.

      In a scenario where the patient is thick (50 cm in lateral dimension), the x-ray tube is close to the patient, and one lateral view is obtained, the K _a,r may underestimate the D _skin,max , giving a displayed estimate of skin dose that is less than half of the actual dose ( Figure 6-1 ).

      

  • b.

    Angiographers commonly employ multiple angulations during fluoro and acquisitions, which results in spreading the skin dose over a wide area, often with no consistent overlap of one area of skin. However, the K _a,r includes the contributions of all fluoro and acquisitions as if they had overlapped. The result is that K _a,r will overestimate the D _skin,max when there is no consistent overlap.

  • c.

    The quantity air kerma is defined only in air, whereas skin dose refers to an absorbed dose that is affected by the absorption characteristics of skin.

  • d.

    D _skin,max includes backscattered radiation from within the body, but K _a,r does not. Backscatter may increase the D _skin,max by 10% to 40%, depending on beam area and energy.

  • e.

    The current requirement for accuracy of the displayed air kerma is ±35% (IEC standard). This means that a procedure in one CCL vs. the same procedure on the same patient in another laboratory may potentially result in a difference of 70% in the displayed air kerma. The accuracy of these quantities should be evaluated annually by a qualified medical physicist and included in any formal radiation dose evaluation.

  • f.

    The procedure table and cushion are usually in the path of the primary x-ray beam before the patient's skin entrance. These objects may attenuate the beam by approximately 20% to 40%, depending on photon energy and other secondary factors. This attenuation is not accounted for in the displayed K _a,r .

As a result of the above reasons, K _a,r may overestimate or underestimate the D _skin,max by as much as a factor of 4.

TABLE 6-2

Terminology of Radiation Doses

TYPE of DOSE	S.I. UNIT	DEFINITION/PURPOSE
Kerma (K)	Gray (Gy)	Sum of initial kinetic energies released in material per unit mass
Air kerma at IRP (K a,r )	Gray (Gy)	Kerma in air at the IRP
Air kerma rate at IRP ( a,r )	Gray/sec (Gy/s)	Instantaneous rate of air kerma at IRP
Air kerma area product (KAP or DAP)	Gy* cm 2	Air kerma × field size; indicative of stochastic risk
Absorbed dose (D)	Gray (Gy)	Mean energy/unit mass by ionizing radiation to a material
Peak skin dose (D skin,max )	Gray (Gy)	Maximum absorbed dose at one skin area; indicates skin injury risk
Effective dose (ED)	Sievert (Sv)	Estimate of stochastic risk due to x-ray exposure