Radiation Safety Office

Henry Ford Health System

3. Fluoroscopy System Description and Operation

1. Radiation Physics
2. Radiation Biology
3. Fluoroscopy System Description and Operation
4. Case Studies of Radiation Injury
5. Reducing Radiation Exposure
Fluoro Training Exam (12/01)
Fluoro Training References

Chapter 3: Fluoroscopy System Description and Operation

Modern fluoroscopy imaging systems (Figure 3-1) consist of: the X-ray tube which produces X-rays; an Image Intensifier (I-I) that captures or stops the X-rays and converts the X-ray energy into light; a closed-circuit video system which ultimately producing a "live" image on a monitor. On some systems the light output can also be distributed to a spot film or cinematography recording systems, though the X-ray output must be greater for these imaging modalities.

Figure 1-2: Fluoroscopy System Components

Courtesy of Scott Sorenson, 2000

Generator Control

The X-ray generator controls the quantity (number) and quality (spectrum of energy) of the X-rays produced.  There are three basic controls to the generator:

kVp - voltage applied accross the X-ray tube.

mA -Current across the X-ray tube.

Time - starting and stopping the exposure.

From a safety perspective, the beam- on- time is the most important control.

Beam-on-Time (Foot Pedal/Switch)

Radiation exposure during fluoroscopy is directly proportional to the length of time the unit is activated by the foot pedal or switch. Unlike regular X-ray units, fluoroscopic units do not have an automatic timer to terminate the exposure after it is activated. Instead, depression of the switch determines the length of the exposure which ceases only after the switch is released. Thus, cognizance of beam-on-time is extremely important to fluoroscopy safety.  

X-ray tube Current (mA)

The X-ray tube current (mA setting) essentially controls the quantity of X-rays produced per unit time.  If you double the mA, you double the patient and operator exposure.  The higher current mode (High Level Control or "boost" mode) dramatically increases the number of X-rays produced and thereby improves image quality.  The boost mode is activated in some systems when the foot switch is firmly depressed (a light push activates the normal, low dose, mode).  An audible alarm will indicate when the high level mode is being used.  HFH policy limits the rate for the boost mode to 20 R/minute (compared with maximum of 10 R/minute for the normal, Automatic Brigtness Control (ABC) mode and 5 R/minute for the manual mode). Both the patient and fluoroscopy operator dose are proportional to the total amount of current used by the machine.  Thus, a light touch on the pedal (educated use of the boost mode) will minimize both patient and worker doses.

During normal mode fluoroscopy, the average patient Entrance Skin Exposure (ESE) is approximately 2 R/min. The level of radiation exposure falls off exponentially with increasing tissue depth due to attenuation and inverse-square effects. Only approximately 1% of the original radiation beam reaches the I-I for image generation.

The ESE exposure rate can be as high as 20 R/min under certain conditions using a high dose rate or "boost" mode if the patient’s skin is close to the X-ray tube. During cineangiography, ESE may exceed 90 R/min.

Fluoroscopy Timer

Fluoroscopy machines are equipped with a timer and an alarm which sounds at the end of 5 minutes of fluoroscopy use. This system is designed alert the user when the usage is becoming significant and provide additional warnings 5 minutes after each reset. Fluoroscopy systems also display the total fluoroscopy time for a procedure.  There are internal requirements for recording fluoroscopy times discussed 

Fluoroscopy Monitoring

The Radiation Safety Committee requires that the total elapsed fluoroscopy time be recorded for every procedure that exceeds 10 minutes of fluoroscopy.  Certain areas, primarily in the Henry Ford Hospital, are also required to record all fluoroscopy times.  Sheets used to record fluoroscopy times are available from the Radiation Safety Office.

Imaging Modes

Automatic Brightness Control (ABC)

Modern fluoroscopy machines produce images from an I-I that captures the radiation exiting the patient. The machine can be operated in either a manual mode or in an Automatic Brightness Control (ABC) mode (Figure 3-2). When the ABC mode is selected, the ABC circuitry controls the X-ray intensity measured at the I-I so that  a proper image can be displayed on the monitor. 

Figure 3-2: Automatic Brightness Control System

Courtesy of Phil Rauch, 2000

ABC mode was developed to provide a consistent image quality during dynamic imaging, When using ABC, the I-I output is constantly monitored and machine factors are then adjusted automatically to bring the brightness to a constant, proper level for adequate I-I function. Both patient and operator factors influence the number of X-rays reaching the I-I. The ABC compensates brightness loss caused by decreased I-I radiation reception by generating more X-rays (increasing mA) and/or producing more penetrating X-rays (increasing kVp). Conversely, when the image is too bright, the ABC compensates by reducing mA and decreasing kVp. 

The radiation exposure rate is independent of the patient size, body part imaged and tissue type when the manual mode is used. However, the image quality and brightness are greatly affected (often adversely) by these factors when the operator "pans" across tissues with different thickness and composition. For this reason, most fluoroscopic examinations are performed using ABC.

Obese patients drive the X-ray machine output up considerably.  Thus, obese patients have the greatest risk of skin injury.

Magnification Modes

Many fluoroscopy systems have one or several magnification modes. Magnification is achieved by electronically manipulating a smaller radiation I-I input area over the same I-I output area (Figure 3-3). A reduction in radiation input subsequently results, lowering image brightness. The ABC system, in turn, compensates for the lower output brightness by increasing radiation production and subsequent exposure to patient and staff.  Patient entrance skin exposures can become quite high when small field of views are used.

Figure 3-3: Field of View

Courtesy of Scott Sorenson, 2000

Normal Magnification Mode

Under Normal mode, there is little magnification with the whole beam used to generate a bright image. The "Normal" mode is used in the majority of fluoroscopy procedures. The radiation output is sufficient to provide images for guiding most procedures or observing dynamic functions. The typical exposure rate at the X-ray beam entrance into the patient Entrance Skin Exposure (ESE) is 2 R/min.

Under Mag 1 mode, a smaller beam area is projected to the same I-I output. The resulting object size is larger, but the image is dimmer due to the less beam input. 

The Food and Drug Administration (FDA) regulates the construction of all fluoroscopy systems. For routine fluoroscopy applications, the FDA limits the maximum ESE to 10 R/min.  The use of higher radiation rates ("High Level Control" or "boost" modes) are useful in situations requiring high video image resolution. ESE of up to 20 R/min is permitted for short duration. Special operator reminders, such as audible alarms, are activated during "boost" modes.

Figure 3-4 illustrates the effect of changing Field-Of-View, or magnification modes, on skin entrance exposure (ESE) for a typical fluoroscopy system:

Figure 3-4: Image Intensifier Input Exposure

Courtesy of Phil Rauch, 2000

Cineangiography 

Cineangiography (cine) originally involved exposing cinematic film to the I-I output, providing a permanent record of the imaged sequence. Technological advances in film-less imaging have eliminated the film for most systems.  The cine mode extracts several separate diagnostic quality images per minute.  The amount of information collected for each of these images is essentially equivalent to a normal flat plane X-ray image.   The X-ray machine output required to produce a cine movie is much higher than the level needed for normal fluoroscopy. Consequently, dose rates during cine image collection are usually 10 to 20 times higher than normal fluoroscopy. For this reason, careful use of cineangiography is required.

Field Size and Collimators

The maximum useful area of the X-ray beam, or field size, is machine specific. Most fluoroscopy systems allow the operator to reduce the field size through the use of lead shutters or collimators. Figure 3-5 shows a diagram of an X-ray tube and collimator system.

Figure 3-5: X-ray tube and Collimator System

Courtesy of Phil Rauch, 2000

Irradiating larger field sizes increases the probability of scatter radiation production (Figure 3-6). A portion of the increased scatter will enter the I-I, degrading the resulting image. 

Figure 3-6: Benefits of Collimation

Courtesy of Scott Sorenson, 2000

Prudent use of collimators can also improve image quality by blocking-out "bright areas," such as lung or other low density regions, allowing better resolution of other tissues.

Benefits from using collimation

Limiting beam size by using the collimators provides many benefits:

1. The patient receives less total radiation exposure since less tissue is in the radiation beam (Figure 3-7).
2. The workers in the area receive less radiation exposure since there is less radiation available to scatter toward staff.
3. The image is improved since scatter contributes noise to the image. 

Figure 3-7: Collimation Technique

Courtesy of Scott Sorenson, 2000

Last Image Hold

Newer fluoroscopy units are often equipped with a last-view freeze-frame or video recording. Use of these modes allows the operator to view an image at leisure, avoiding unnecessary patient and staff radiation exposure caused by constant fluoroscopy use.

Beam Quality (kVp)

The tube voltage (kV) controls the maximum energy of electrons produced by the X-ray tube and the maximum energy of the resultant X-ray spectrum (kilovolt-peak energy or kVp). Use of high kVp techniques can reduce patient dose but contrast between differing tissues is also reduced. Experienced operators can optimize the choice between image quality and patient dose through careful adjustment.  The ABC on most X-ray systems can change the kVp and mA used to optimize imaging.

Image Display Monitor

The image quality available to the operator is dependent on proper adjustment of the image monitor.   These image monitor settings are adjusted during service and annual testing.  Operator adjustment of the brightness and contrast controls can degrade image sharpness, contrast and distortion and should be done if problems occur during use.  If this happens, service should be contacted to correct the problems. In addition, the eye's ability to discern detail on the image monitor is improved under low light conditions. The need for good lighting for surgical needs must be balanced with imaging considerations.  Therefore, the monitor settings should be calibrated for the area in which it is being used.

System Quality Control Checks

Quality Control (QC) checks are extremely important to ensure proper performance of the equipment. Daily testing is required by JCAHO standards and should be utilized to track system performance and optimize display monitor settings.  A simple test tool (Fig 3-8) can be used to monitor system performance.  This tool should be used to evaluate the X-ray system each day prior to use and any discrepancies corrected immediately.

Figure 3-8: Quality Control Checks

Courtesy of Phil Rauch, 2000

Operator Exposure Profile

No portion of the operator's body should be in the primary beam during imaging.  Thus, the majority of the radiation dose received by the operator is due to scattered radiation from the patient. After interacting with the patient, radiation is scattered more or less uniformly in all directions (Figure 3-9). 

Figure 3-9: Collimation Technique

Courtesy of Scott Sorenson, 2000

It is important to note that the patient does not uniformly emit this scattered radiation since some of the scattered radiation is absorbed or reduced in intensity by passing through the patient. The intensity of scatter decreases with increasing distance, due to inverse square law effects (see Chapter 1). Consequently, scatter radiation is highest near its source (i.e. the X-ray beam entry point on the patient). Because radiation scattered in the forward direction (into the patient) is subject the most tissue attenuation, radiation levels are significantly lower on the I-I side than the X-ray tube side. 

Highest scatter radiation levels are often where the operator stands. Radiation levels increase with decreasing distance from the point of X-ray entry (Figure 3-10). In general, an operator positioned 3 feet from the X-ray beam entrance area will receive 0.1% of the patient’s ESE. Staff members positioned further away receive much less exposure due to inverse square law effects. In almost all cases, the tableside operator will receive the highest occupational radiation exposure during the fluoroscopic procedure.  Tableside fluoroscopy receive among the highest occupational radiation exposures within the health system.

Figure 3-10: Radiation Level vs. Entry Point

Courtesy of Scott Sorenson, 2000

Radiation levels are highest beneath the table (when the X-ray tube is below the patient) because the patient provides an effective beam stop (Figure 3-11). Highest levels are directed at the operator's waist (See bar chart on figure).

Figure 3-11: Patient Shielding

Courtesy of Scott Sorenson, 2000


The scatter radiation profile tilts with the X-ray tube. Higher exposure to the operator’s head and eyes (which have low dose limits) results during oblique angle projections where the X-ray tube is tilted towards the operator (I-I is tilted away from the operator). Conversely, radiation exposure is decreased when the X-ray tube is tilted away from the operator (I-I tilted towards the operator) (Figure 3-12). When possible, the operator should work on the I-I side of the table when oblique angles are being imaged.  While it is contrary to your instincts, generally you should work closer to the image intensifier than the X-ray tube. 

Figure 3-12: Tilt Exposure Profile

Courtesy of Scott Sorenson, 2000 

Courtesy of Scott Sorenson, 2000

Effect of rotating X-ray system. Images taken with the I-I away (Figure 3-12) result in higher radiation exposure to the operator's eyes compared to images with the I-I towards the operator (Figure 3-13).

Figure 3-13: Tilt Exposure Profile

Courtesy of Scott Sorenson, 2000

Courtesy of Scott Sorenson, 2000
Image Quality Versus Patient Dose

A basic principle in Radiology is that you collect dose along with image information.  The clearest, least jittery, images produce the highest doses.  Thus, it is very important for clinicians to judiciously use appropriate judgment when increasing the X-ray beam output and to learn to work with most amount of (necessary) imaging imperfections as possible which still allow the needed clinical outcome.  

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Copyright © 2001 Radiation Safety Office at Henry Ford Health System
Last modified: 08/15/05