Fluoroscopy is the method that provides real-time X ray imaging that is especially useful for guiding a variety of diagnostic and interventional procedures. The ability of fluoroscopy to display motion is provided by a continuous series of images produced at a maximum rate of 25-30 complete images per second. This is similar to the way conventional television or video transmits images.
While the X ray exposure needed to produce one fluoroscopic image is low (compared to radiography), high exposures to patients can result from the large series of images that are encountered in fluoroscopic procedures. Therefore, the total fluoroscopic time is one of the major factors that determines the exposure to the patient from fluoroscopy.
Because the X ray beam is usually moved over different areas of the body during a procedure, there are two very different aspects that must be considered. One is the area most exposed by the beam, which results in the highest absorbed dose to that specific part of the skin and to specific organs. The other is the total radiation energy imparted to the patient’s body, which is related to the Kerma Area Product (KAP or PAK), a quantity that is easily measurable.
The absorbed dose to a specific part of the skin and other tissues is of concern in fluoroscopy for two reasons: one is the need for minimizing the dose to sensitive organs, such as the gonads and breast, by careful positioning of the X ray beam and using shielding when appropriate. The second is the possible incidence of the radiation beam to an area of the skin for a long time that can result in radiation injuries in cases of very high exposure.
On the other hand, the total radiation energy imparted to the patient’s body during a procedure is closely related to the effective dose ) and to the risk of radiation induced cancer.
In fluoroscopy, as in all types of X ray imaging, the minimum exposure required to form an image depends on the specific image information requirements.An important characteristic of a fluoroscopic system is its sensitivity, i.e. the amount of exposure required to produce images. The use of intensifier tubes and more modern digital flat panel receptors make it possible to optimize the balance of patient exposure with image quality so as not to expose the patient to unnecessary radiation. Non-intensified fluoroscopy with just a fluorescent screen for a receptor should not be used because of the excessive exposure to the patient.
Yes, in general, increasing the kV reduces the exposure of the patient and especially to the skin exposed by the beam. This is because the higher kV produces radiation beams with increased penetration through the patient’s body and less radiation is required at the entrance surface (*) to produce the necessary exposure to the image receptor. The other factor that must be considered in selecting the appropriate kV value is the effect on image contrast. In general, lower kV values produce increased image contrast. This can be especially significant in fluoroscopy when using iodine contrast media.
(*) The number of photons and the total energy carried by these photons per unit area. For a more rigorous definition and detailed discussion (energy fluence) see ICRU Publication 74 and IAEA Tecdoc 457
While there are several factors under your control that have an effect on exposure, one of the most significant is the time that the X ray beam is on. Good practice is to use the shortest fluoroscopic times that are consistent with the clinical requirements of the procedure. This can be aided by being aware of and making a record of the exposure time for each procedure. The use of pulsed fluoroscopy with the pulse rate set as low as practical for the specific type of clinical procedure provides a significant reduction in the absorbed dose, although not all equipment may follow this and it is desirable to understand your equipment. If the equipment has more than one mode of operation, the high-dose rate mode should be used with caution, only for the time in which a low-noise image is required. Read more »
Not in all cases. While the ABC is useful for adjusting the exposure to produce a good image for different patient thicknesses and densities, the actual exposure depends on the exposure level that has been set by the manufacturer or the engineer who maintains the equipment. The optimum setting of the ABC exposure level is one that delivers only the exposure to the image receptor that is required to produce the necessary image quality in terms of the visual noise.
Yes with regard to the absorbed dose (*) and no with regards to the energy imparted. Changing from a large field of view to an increased magnification increases the exposure required by the image intensifier tube. Therefore, the absorbed dose to tissues within the beam is also increased. Decreasing the field of view by a factor of two increases the dose rate by a factor of four. Attention has to be paid to magnification Example:
Field of view, diameter 25 cm Dose rate= 0.3 mGy/s
Field of view, diameter 17 cm Dose rate = 0.6 mGy/s
Field or view, diameter 12 cm Dose rate = 1.23 mGy/s.
However, since the X ray beam is covering a smaller area, the total energy imparted is about the same as with the larger field of view that produces a lower dose rate.
Yes. Since the absorbed dose to a specific tissue is influenced by the number of photons impinging on the same area of the skin, moving the beam spreads the radiation over more of the patient’s body and reduces the absorbed dose to any one area of the skin.
The highest absorbed dose to a specific tissue occurs when the X ray beam is not moved but remains at the same location on the patient’s body during a procedure.
Special attention should be paid to avoiding overlapping image areas combined with projections through the body at relatively low angles of the X ray beam (obliquity, e.g. cranio caudal or caudio cranial projections).
In summary, moving the beam can help avoid radiation injuries to the skin. The PKA, and the total energy imparted, is not changed by moving the beam during a procedure. The total energy imparted to a given region of the body is related to the probability of radiation induced cancer.
Yes. There are several parameters that have an effect on the exposure rate (mGy/min). The basic exposure rate is set by the ABC, as discussed in a previous question. Some fluoroscopic equipment is designed for pulsed-mode operation. With the pulsed mode, it can be set to produce less than the conventional 25 or 30 images per second. This is likely to reduce the exposure rate in most equipment. But patient dose management is a complex topic and practitioners should know the operational capabilities of their X ray systems. The reduction of unnecessary cine series or number of frames per series, the proper use of collimation, the influence of C-arm angulations and position of the table and image detector (geometry), to avoid the use of “high quality” of cine modes, etc ... could have a substantial influence on patient (and staff) doses.
Collimation of the X ray beam to the smallest practical size and keeping the distance between the patient and image receptor as short as possible contribute to good exposure management.
It can be complex, especially if the X ray beam is moved during the procedure. However, let’s consider a simple case in which the X ray beam was in the same location for five minutes. The absorbed dose to the skin in the beam can be obtained by multiplying the dose rate (mGy/min) by exposure time. For this example we will use a dose rate of 30 mGy/min. This is within the range of normal fluoroscopic dose rates but is subject to considerable variation with factors such as patient size, kV, and the magnification mode used.
Our estimate for this case will be an absorbed dose to the skin of 30x5=150 mGy.
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There are two general sources of unnecessary exposure. The first one is equipment deficiency and the second is operational deficiency.
Physicists working in the context of radiation safety and quality assurance procedures can detect deficiencies in design and equipment performance.
Physicians using fluoroscopic equipment can also have an effect on the radiation exposure of the patient, as there are many variables associated with the procedure that are controlled by the physician. These include selection of kV values, field of view, fluoroscopic time, use of beam limitation, and the use of specific imaging modes. Some modes such as pulsed fluoroscopy can reduce exposure while the high-dose-rate mode contributes to increased exposure.
A major factor is the use of appropriate fluoroscopic equipment. The use of non-intensified (dark room) fluoroscopy is not recommended both because of low image quality and excessive radiation exposure to patients and staff.
Typical values in terms of effective dose and dose area product (DAP) values are presented in Table 1 below:
Table 1: Mean effective doses and DAP values from contrast procedures involving fluoroscopy
|Radiography / Fluoroscopy procedures||Mean Effective Dose (mSv)||Mean DAP (Gy.cm2)||Equivalent number of PA chest radiographs (each 0.02 mSv)|
|Orthopaedic pinning (hip)||0.7||35|
|Micturating Cystourethrogram (MCU)||1.2||6.4||60|
|Barium meal ||2.6||130|
|Barium meal follow through ||3||150|
|Barium enema ||7.2||360|
|Small bowel enema ||7.8||30||390|