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| Comments may be sent to: | Center for Devices and Radiological Health Mail Code HFZ-140 5600 Fishers Lane Rockville, Maryland 20857 |
Concepts for proposed amendments to the performance standard for diagnostic x-ray systems.
CDRH is proposing to develop amendments to the Performance Standard for Diagnostic X-ray Systems and their Major Components (21 CFR 1020.30 - 1020.33) to address changes in the technology and clinical application of radiographic and fluoroscopic x-ray systems. Concepts for the amendments are described below with a brief rationale for each amendment and with a very preliminary first draft of potential wording for some of the amendments. The draft wording is provided only for the purposes of discussion and should not be interpreted at this stage as indicative of the final approach which may be taken with any particular amendment. These concepts for amendments were discussed with the Technical Electronic Product Radiation Safety Standards Committee, an advisory committee the FDA at the committee's April 8, 1997 meeting. Interested parties are invited to comment on the need for and content of possible amendments to the performance standard to address these areas in response to an Advanced Notice of Proposed Rulemaking which will be published in the Federal Register in the near future.
Topics for which amendments are proposed and the affected sections of 21 CFR 1020.30 - 1020.33
1. Change to SI Quantities and Units
This proposal would replace in all sections of the performance standard the radiation quantity "exposure" with "air kerma" and would change the units used accordingly.
Rationale
The International System of Units (SI) was named and adopted at the 11th General Conference on Weights and Measures (CGWM) in 1960 as an extension of the earlier MKS and MKSA (metric) systems. SI, also referred to as the metric system, is the approved system of units for the United States. The U.S. Department of Commerce published an "Interpretation and Modification of the International System of Units for the United States" in the FEDERAL REGISTER on December 10, 1976, which set forth the interpretation of the SI system for the U.S. The Omnibus Trade and Competitiveness Act of 1988 amended the Metric Conversion Act of 1975 and required each Federal agency to use the metric (SI) system in its activities. In a memorandum dated March 19, 1990, from the Associate Commissioners of Regulatory Affairs and Public Affairs, the FDA policy for use of metric measurement was described. This guidance calls for use of the metric units followed by a parenthetic "inch-pound" declaration unless there is a cogent reason not to utilize dual metric and "inch-pound" measurement. It was noted that there should be a few such exceptions.
The International Commission on Radiation Units and Measurements (ICRU) has as one of its principal objectives the development of internationally acceptable recommendations regarding quantities and units of radiation and radioactivity. These recommendations often form the basis of CGWM actions. The ICRU published ICRU Report 33, Radiation Quantities and Units, in 1980 which used the SI units, including the special names for some radiation units. The ICRU suggested dropping usage of certain special units, not part of the SI system, by 1985. The special unit of exposure, the roentgen, was in this category.
The current diagnostic x-ray equipment performance standard uses "roentgen," a non-SI unit of exposure. In amendments published as a final rule on May 3, 1993 (59 FR 26386), the Agency made a partial transition to SI units by changing the unit for exposure from "roentgen" to "coulomb/kilogram." This change to SI units required using an awkward conversion factor of 2.58 x 10-4 coulomb/kilogram per roentgen.
Recent reports of the National Council on Radiation Protection and Measurements (NCRP) have adopted the use of the SI quantity air kerma to describe the radiation output of an x-ray system. This change in the NCRP recommendations was made without significant concern that previous limits in the voluntary recommendations were slightly raised by this change. Current and draft IEC standards for diagnostic x-ray systems use the SI quantity kerma to describe the radiation output of systems. Use of SI quantities and units is now required by most scientific journals in this field. In spite of these recommendations, several comments were received on a previous proposed rule which opposed the use of the SI units for exposure as unwarranted or as leading to further confusion in the field.
The current limits on fluoroscopic exposure rates were established in agreement with recommendations of the NCRP. The change to SI units maintains agreement between the limits in the standard and in recently published NCRP Report 102, Medical X-ray, Electron Beam and Gamma-Ray Protection for Energies up to 50 MeV. The FDA acknowledges the slight increase in the radiation output permitted for what is essentially worst-case operation of fluoroscopic systems with this proposed change in quantities and adoption of the limits of the current NCRP recommendation. The limits set in the original and recently revised NCRP recommendations are somewhat arbitrary, reflecting equipment capabilities, required image quality and application of the ALARA principle, and are not based on criteria which suggest a specific value for the limits as a safety threshold.
In view of current trends, scientific practice, U.S. policy and FDA directives, FDA proposes that the standard be amended to use the quantity air kerma in place of the quantity exposure and that a complete conversion be made to SI quantities and units. It is further recommended that in making this change that integer values be used for the limits concerning radiation output in order to maintain agreement with current NCRP recommendations and current or draft IEC standards and for convenience of use.
Rather than directly converting the current limits to equivalent limits of the quantity air kerma, resulting in unwieldy and inconvenient numbers, the SI quantities would be expressed in whole numbers where practical to facilitate ease of use. This would result in a slight relaxation in the performance standard for previously used tolerances expressed as exposure in roentgens. For example, the previous exposure limit of 100 mR needs to be converted to 115 mR to be equivalent to the whole number value of 1 mGy when the limit is expressed as air kerma in the unit mGy. The previous limits for fluoroscopic entrance exposure rates become 50 mGy/min or 200 mGy/min, for example, which are equivalent to 5.8 R/min or 23.0 R/min, respectively, compared to the old values of 5 R/min or 20 R/min. This change is not considered to have a significant public health impact: the previous limits, expressed in roentgen, were chosen largely on the recommendations of the NCRP to have an ample margin of safety, and limits were expressed as whole numbers for convenience of use. Use of whole number limits expressed in units of gray will allow agreement with recently revised NCRP recommendations and international standards.
This suggested change in quantities and units will increase the limits for a number of requirements in the standard by about 15 percent. Examples of the changes are as follows:
There is a potential for objection to this proposal by those who might view this as a relaxation of current requirements and a compromise of public health. One former member of the Technical Electronic Product Radiation Safety Standards Committee (TEPRSSC) voiced such concern during an earlier discussion of this change at a previous TEPRSSC meeting.
"Exposure" has also a second, common meaning in this
standard which does not refer to a quantity of radiation as defined
here. The second meaning of "exposure" refers to the
process or condition during which the x-ray tube is activated
by a flow of current to the anode and radiation is produced. The
second meaning of exposure will continue to be used where appropriate.
For the purposes of the performance standard, the term "kerma"
will normally refer to kerma in air.
2. Clarification of Applicability to deal with Technological Developments.
The current organization and structure of the standard which utilizes
the presence of an x-ray image intensifier as the basis for many
of the requirements may be inappropriate under the situation of
digital fluoroscopic systems. These systems use digital means
to perform both radioscopy and radiography (using the IEC definitions
of these terms) and may not have an image intensifier tube in
the future, as they may use other devices as the image receptor.
The structure and organization of the standard may need revision
with the aim of dealing with these developments and clarification
and harmonization with the IEC terminology. It is not anticipated
that this reorganization would result in any change in performance
requirements.
3. Amendment to incorporate draft Compliance Policy Guide on Information to be Provided to Users.
This proposal would require that users be provided specific information on the patient irradiation rate capabilities for certain modes of operation. This amendment would incorporate into the standard a draft Compliance Policy Guide which has been developed, but not yet issued, and is intended to interpret section 1020.30(h) for certain "unique" modes of fluoroscopic system operation.
Rationale
[Note that in the following discussions, the quantities "air kerma" and "air kerma rate" with units of Gy and Gy/min are used in place of the quantity exposure (current SI unit coulomb/kilogram or the traditional unit, the roentgen) or exposure rate. This is in anticipation of the change to SI units described above. In this discussion and in the proposed amendments, the quantity air kerma is the quantity which will be used to describe the x-ray radiation impinging on the patient. For x-ray beams in the diagnostic energy range, air kerma is approximately equivalent to the absorbed dose to air free-in-air.]
The Performance Standard for Diagnostic XRay Systems and Their Major Components (Performance Standard), as amended on May 19, 1994, limits the entrance exposure rate of fluoroscopic xray systems during normal fluoroscopy to 10 roentgens per minute (R/min) unless an optional high-level control (HLC) is activated. If HLC is activated, the entrance exposure rate must be limited to 20 R/min. Provision for the HLC was made for those situations in which radiation output at the normal level was insufficient to provide acceptable fluoroscopic images. The entrance exposure rate limits do not apply during the recording of images from the image intensifier when in a pulsed exposure mode of operation. Recording includes, but is not limited to, digital image acquisition and storage, video recording using analog or digital modes, and cinefluorography. At the time the Performance Standard was developed, cinefluorography was the major reason for the absence of an exposure-rate limit during recording.
Recent developments in the technology of fluoroscopic systems have resulted in equipment being increasingly provided with a variety of special modes of operation and methods of recording fluoroscopic images. Some of these modes of operation may significantly increase the entrance exposure rate to the patient. There is concern that the operating instructions provided with the equipment lack sufficient information concerning the characteristics of these special modes of operation to permit the operator to adequately evaluate the increased radiation exposure risk to the patient and operator from these modes of operation.
Section 1020.30(h)(1)(i) of the Performance Standard states that the information to users shall contain "Adequate instructions concerning any radiological safety procedures and precautions which may be necessary because of unique features of the equipment... ." The FDA considers any mode of operation which yields an entrance air kerma rate above 10 cGy/min to be a unique feature of the specific fluoroscopic equipment. The FDA is also of the opinion that, for modes of operation where the entrance air kerma rate exceeds 10 cGy/min, the manufacturer should provide information to permit the user to assess the exposure to the patient relative to that delivered in the normal mode of operation. This information will give operators important radiation safety data with which to make better judgments on the hazards involved with a particular procedure.
On April 9, 1993, by multiple-address letter (REF:MA:OCS:DSE:XPB:394), FDA informed manufacturers of fluoroscopic x-ray controls and systems that additional information was required in the product reports (Initial Reports) for components and systems. This information was required for any new model fluoroscopic x-ray control or system introduced after August of 1993 or when existing models had high-level controls or recording modes added after August of 1993. This proposed change in the regulations is an additional step to ensure that information needed for proper and safe use of the equipment be provided in the user information documents required by Section 1020.30(h)(1)(i). FDA will review the user information documents for completeness.
Issues to be addressed
Because industry has already been requested to provide much of this information in their product Initial Reports, the requirement to provide it as part of their user information should not be a great burden upon the manufacturer. However, because the manufacturer subsequently becomes responsible for what they provide the user in their manuals, the manufacturer may wish to make sure their statements are precise, and this will cause them some additional testing and quality control expenses. The manufacturer should be able to supply information to the operators of their equipment concerning the relative increase in radiation exposure to the patient during modes of operation delivering higher air kerma rates as compared to the normal fluoroscopic mode of operation.
Proposed draft amendment for discussion
Amend 21 CFR 1020.30(h) Information to be provided to User by adding the following:
1020.30(h)(5) Fluoroscopic imaging systems. For fluoroscopic equipment manufactured after (enter date one year after date of final rule) and which provides modes of operation resulting in entrance air kerma rates greater than 10 cGy/min, there shall be provided for each such mode:
| (i) For systems with modes of operation which produce images at frame rates having less than 0.25 seconds between successive frames:
(A) Detailed instructions on how the mode is engaged and disengaged. Included with this information shall be the identification of how the operator is alerted that such mode is engaged. (B) Description of the control of technique factors provided with the mode of operation, including any system settings that affect entrance air kerma rate, e.g., kVp, mA, irradiation time, grid, aperture size, and/or electronic magnification. (C) The maximum entrance air kerma rate, and the system control settings or technique factors that produce this entrance air kerma rate, for each mode of operation. (D) For the automatic exposure rate control (AERC) modes of operation, (1) the entrance air kerma rate for a specific phantom or specific amount of attenuating material providing xray attenuation representative of the attenuation of a typical patient during normal fluoroscopy, and the system control settings and technique factors that produce this entrance air kerma rate; and (2) the entrance air kerma rate for the same phantom or attenuating material when the mode of operation is activated, and the system control settings and technique factors that produce this entrance air kerma rate. A table, graph, or written explanation may be provided to meet this requirement. The specifications of the phantom or attenuating material shall also be provided, including the patient size it is intended to simulate. (E) A warning that use of this mode will subject the patient to an increased entrance air kerma rate and should be used only as needed. This warning should also include a statement of any specific clinical procedure(s) for which the mode is recommended or designed, and instructions on how it should be used. (ii) For systems that produce images (frames) separated by intervals equal to or greater than 0.25 sec: (A) Detailed instructions on how the mode of operation is engaged and disengaged. Included with this information shall be the identification of how the operator is alerted that such mode is engaged. (B) Description of the control of technique factors provided with the mode of operation, including any system settings that affect entrance air kerma per frame, e.g., kVp, mA, irradiation time, grid, aperture size, frame rate, total number of frames per run, and/or geometrical magnification. (C) The entrance air kerma per frame to a typical patient at a specified xray tube-to-patient distance and the typical total number of frames, including frames from the examination and during automatic technique selection, if such is provided, and the maximum number of frames possible for these conditions. (D) If the technique factors can be adjusted to increase the entrance air kerma per frame to values higher than provided in subparagraph (ii)(C) of this paragraph, such as a reduction in the optical aperture, the following shall be provided: (i) The maximum selectable entrance air kerma per frame at the specified distance under these conditions; (ii) The maximum selectable number of frames possible for the conditions in (i). A table, graph, or written explanation may be provided to meet this requirement. The specifications of the phantom or attenuating material selected to represent the typical patient shall also be provided. (E) A warning that use of this mode will subject the patient to increased entrance air kerma and should be used only as needed. This warning shall also include any specific clinical procedure(s) for which the special mode of operation is recommended or designed, and instructions on how it should be used. (iii) When referring to entrance air kerma and entrance air kerma rate, the measurement geometry of 1020.32(e)(3) shall be used with the specified phantom in the beam. The air kerma and air kerma rate values provided shall include a statement of the maximum deviation from the values given. |
4. Amendment to add additional requirements for minimum half-value
layer for systems designed for interventional radiology.
This proposal would increase the minimum half-value layer for fluoroscopic systems designed for interventional radiology. Systems which permit the x-ray beam axis to be moved relative to the normal to the table top would be assumed to be designed for interventional radiology. This proposed requirement would not apply to systems in which the x-ray beam direction is fixed with respect to the plane of the tabletop, such as conventional radiographic/fluoroscopic systems.
Rationale
The use of filtration to increase the spectral homogeneity of an x-ray beam through selective absorption of the low energy photons has been a recommended practice for many decades. The values of beam quality in terms of half-value layer (HVL) for specified voltage ranges in the Performance Standard are a performance-oriented filtration requirement. The values of HVL in the standard would result if one used the NCRP1 suggested values of filtration of 0.5, 1.5, and 2.5 mm Al equivalent in the x-ray beam. These prescribed values of HVL represent a consensus for x-ray fluoroscopy systems circa 1974.
Over the past 20 years, there has been a substantial increase in the use of x-ray fluoroscopy as a visualization tool for a wide range of diagnostic and therapeutic procedures. Because of the long catheter manipulation times and the need, in some cases, for a stationary x-ray field, these procedures have the potential, sometimes realized, for high radiation dose to patients as well as clinical personnel.2 In fact, the Agency has been actively involved in promoting recommendations for the avoidance of serious x-ray-induced skin injuries to patients during fluoroscopically-guided procedures. As a result, there continues to be an interest in dose reduction techniques for x-ray fluoroscopy. For these reasons, it is proposed to require higher minimum values of HVL for x-ray systems suitable for fluoroscopically-guided interventional procedures.
In general, the addition of either beam-hardening or K-edge x-ray filters can provide a significant reduction in the exposure, particularly skin exposure, to the patient but with an attendant increase in tube load.3 It should be noted that one of the recommendations at the ACR/FDA Workshop on Fluoroscopy4 was to increase the minimum HVL. The recommendation was for a minimum HVL of 3.0 mm of aluminum at an x-ray tube voltage of 80 kVp. A similar requirement is included in the draft of an IEC standard for interventional radiology equipment. In the IEC draft standard, the recommendation was a minimum HVL of 2.5 mm of aluminum at an x-ray tube voltage of 70 kVp. Using these two minimum HVL values, a table of values at a range of x-ray tube voltages can be generated and is represented in the table of the proposed amendment.
References for HVL discussion
Proposed draft amendment for discussion
Amend 21 CFR 1020.30(m) Beam quality-(1) Half-value layer to change the wording of the first sentence in the requirement and the table of half-value layers as shown below. (The rest of the subparagraph (m) requirement after the "..." shown below would be unchanged.)
1020.30(m) Beam Quality - (1) Half-value layer.
The half-value layer (HVL) of the useful beam for a given x-ray
tube potential shall not be less than the appropriate value shown
in Table 1 under "Specified interventional fluoroscopy x-ray
systems" for stationary systems designed for interventional
radiology manufactured after (insert date one year after date
of final rule); under "Specified dental systems" for
any dental ...
(Kilovolt Peak) | (Millimeters of Aluminum) | |||
|---|---|---|---|---|
Operating Range | Operating Potential | Dental Systems | Fluoroscopy X-ray Systems | |
51 to 70............. | 40 50 51 |
1.5 1.5 1.5 | 0.4 0.5 1.2 | 1.8 |
70 71 80 90 100 110 120 130 140 150 | 1.5 2.1 2.3 2.5 2.7 3.0 3.2 3.5 3.8 4.1 | 1.5 2.1 2.3 2.5 2.7 3.0 3.2 3.5 3.8 4.1 | 2.5 2.9 3.2 3.6 3.9 4.3 - - - | |
(In addition to the above change in requirements, the following definition will be needed to explicitly define what is meant by an "interventional" fluoroscopy x-ray system.)
Amend 21 CFR 1020.30(b) Definitions to add the following definition:
Interventional fluoroscopy x-ray system means an x-ray
system in which the beam axis of the x-ray beam is not constrained
to be perpendicular to the plane of the x-ray table.
5. Amendments to require improved x-ray field limitation.
This proposal would require improved limitation of the x-ray beam to match the area of the image receptor being used for image capture and so reduce the amount of nonuseful beam striking the patient.
Rationale
A reduction in unnecessary patient irradiation is the basis for all of the x-ray field limitation requirements in the Performance Standard. For example, any x rays falling outside the visible area of the image receptor provide no useful diagnostic or visualization information and therefore represent unnecessary patient irradiation. Once it is recognized that restricting the size of the x-ray field provides an effective control of unnecessary irradiation, the question shifts to what is the value of the tolerance technically achievable by the manufacturers for the matching of the x-ray field and the visible area of the image receptor.
In the current Performance Standard, 21 CFR 1020.32(b)(2)(i), "neither the length nor the width of the x-ray field in the plane of the image receptor shall exceed that of the visible area of the image receptor by more than 3 percent of the SID." In addition, "the sum of the excess length and the excess width shall be no greater than 4 percent of the SID." Simply stated, this requirement results in worst-case values of geometrical efficiency (as defined by the area of a circular visible area divided by the area of a square, relatively uniform x-ray field) enumerated in Table 1 for what are typical geometrical and operating conditions of fluoroscopy systems. It should be noted that the requirements in the current IEC standard with respect to x-ray field limitation are more stringent than those in the Performance Standard.1 The IEC standard requires a limitation of the area of the x-ray field to less than the size of the visible area of the image intensifier. Thus, if the x-ray field is centered on the visible area of the image intensifier, the x-ray field would exceed the visible area of the image intensifier only in the corners of a rectangular x-ray field, unlike the US Performance Standard.
Table 1. - Worst-case geometrical efficiency in percent
for a fluoroscopy system with an SID of 100 cm, an x-ray
field size at the tolerances in the requirements of 21 CFR 1020.32(b)(2),
and image intensifiers with 12 cm, 15 cm, 23 cm and 30 cm diameter
visible areas.
(cm) | (cm2) | (cm2) | (%) |
As can be seen from the table, the current Performance Standard allows the possibility of relatively low geometrical efficiency, particularly, in modes of operation corresponding to small visible areas on the image intensifier. It should be noted that many fluoroscopically guided interventional procedures involve the use of small visible areas on the image intensifier.2 These low values of geometrical efficiency are a direct result of using a square collimator for the x-ray field with a circular visible area of the image receptor. Obviously, the use of a continuously adjustable, circular collimator could reduce the low values of geometrical efficiency.
A continuously adjustable, circular collimator is provided as a basic and/or optional capability on many currently marketed x-ray systems suitable for fluoroscopically guided interventional procedures.3 Thus, a continuously adjustable, circular collimator is technically feasible, albeit at additional cost to the user community. Fluoroscopy systems with this feature provide for a substantial increase in geometrical efficiency which is particularly important for interventional procedures resulting in high skin exposure. For these reasons, it is proposed to require geometrical efficiencies of 80 percent or more for x-ray systems suitable for fluoroscopically guided interventional procedures when the visible area of the image receptor is circular. When the visible area of the image receptor is greater than 34 cm in diameter, a geometrical efficiency of 80 percent may not be stringent enough. For this reason, the requirement is changed to a sizing tolerance at that point, i.e., the x-ray field measured along a diameter in the direction of greatest misalignment with the visible area of the image receptor shall not extend beyond the visible area of the image receptor by more than 2 cm. This over-sizing tolerance will ensure geometrical efficiencies of better than 80 percent. A similar set of requirements is included in the draft of an IEC standard for interventional radiology equipment.
Our intent is to promote the incorporation of continuously adjustable, circular collimators into these types of systems but we understand that the new requirements could be met through the use of less expensive rectangular collimation and under-framing. For example, the amount of under-framing (as defined by the difference in the width of the x-ray field versus the diameter of the visible area) of a rectangular x-ray field needed to meet the new requirements are enumerated in Table 2 for the same geometrical and operating conditions on fluoroscopy systems described in Table 1. We invite comment and input on the ramifications of this amount of under-framing which is closely aligned to the current IEC requirements.
Table 2. - Amount of under-framing of a rectangular x-ray field needed to meet the new requirements for a fluoroscopy system with an SID of 100 cm and image intensifiers with 12 cm, 15 cm, 23 cm and 30 cm diameter visible areas.
Diameter (cm) | Width (cm) | (cm) |
References for field limitation discussion
Proposed draft amendment for discussion
Amend 21 CFR 1020.32(b)(2) to include a new section (b)(2)(v) as follows:
1020.32(b)(2) Image-intensified fluoroscopy... (v) For x-ray systems manufactured after (insert date one year after date of final rule) suitable for fluoroscopically guided interventional procedures and when the visible area of the image receptor is circular, the requirements of (i) of this subparagraph are superseded by the following requirement. At the plane of the image receptor, the maximum area of the x-ray field shall conform with one of the following constraints:
Note: See 21 CFR 1020.30(b) for the definition of the x-ray field
and the visible area of the image receptor.
6. Amendment to clarify the requirements on the minimum source-skin distance for small, mobile or portable "C-arm" fluoroscopic systems.
The purpose of this amendment is to address numerous requested and granted variances for systems which have limited source-to-image-receptor distances to deviate from the current requirement for the minimum source-skin distance. The amendment is intended to specify the conditions under which a reduced source-skin distance is permitted and to obviate the need for continued variances from the standard.
Rationale
In the mid-1980s and possibly earlier, the FDA granted variances from the requirement for minimum source-skin distance (MSSD) for devices that were designed as "mini C-arms." These were devices that were hand held and could be used at sporting events for a quick confirmation of orthopedic injuries. In fact some of the early ones used a radioisotope instead of an x-ray tube and were, therefore, outside the purview of the FDA under the Radiation Control for Health and Safety Act (but still a medical device). With time, designers of these devices began enlarging the gap or clear opening between the source and the image receptor to allow for larger size extremities. The argument was that professional football players had larger extremities and more opening was needed to use the units on them. The systems were marketed under a variance for the MSSD and labeled for extremity use only. As the size of the clear opening has been increasing from about 20 cm to 35 cm, and manufacturers have been asking about neo-natal or pediatric use for the equipment, the Center has become concerned about the loss of the skin dose sparing properties of the MSSD requirement. In addition, because the variance is time limited, renewal of the variance and the addressing of new conditions for use have resource implications for FDA and the manufacturers.
The justification used by many of the mini C-arm manufacturers is geometrical scaling. They have stated in their applications that the MSSD remains in proportion to the source to image receptor distance. Although extremities scale geometrically in a similar manner, other body parts do not scale as the system does. In Figure 1, it can be seen for small body parts that there is only a small increase in EER [Ref 1]. As the body part thickness increasesabove 15 or 16 cm the EER ratio starts increasing quickly, reaching a factor of two increase for a patient thickness of about 26 cm. In their original configuration these devices had a very small clear opening and could not accommodate anything other than a limb. The latest configurations can easily accommodate a neo-natal whole body or pediatric patient's whole body.
It is clear at some point that these systems are no longer "mini
C-arms" and are simply slightly smaller versions of conventional
C-arms. Exactly where this point is has not been determined. There
are presently three manufacturers of mini C-arms on the market.
These types of devices have been on the market for a dozen years
or more. It is time to amend the standard to allow for systems
designed for extremity use only to meet a smaller minimum SSD
regulation.
Reference for minimum source-skin distance
1. R. Gagne, FDA/CDRH, private communication.
Proposed draft amendment for discussion
Amend 21 CFR 1020.32(g) by adding a new subparagraph (2) as follows:
1020.32(g) Source-skin distance. (1) Means shall...
| (2) For fluoroscopic systems manufactured after (insert date one year after date of final rule) and designed and labeled for extremity use only and having a separation distance between the spacer or means limiting the source-skin distance and the image receptor support of 35 centimeters or less, means shall be provided to limit the source-skin distance to not less than 19 centimeters. In addition, for those systems intended for specific surgical application that would be prohibited at the source-skin distances specified in this paragraph, provisions may be made for operation at shorter source-skin distances but in no case less than 10 centimeters. When provided, the manufacturer must set forth precautions with respect to the optional means of spacing, in addition to other information as required in Sec. 1020.30(h). |
7. Amendment to require indication of cumulative irradiation time of a patient.
The purpose of this amendment is to establish a requirement for an indication of the cumulative irradiation time of a patient during a fluoroscopic procedure.
Rationale (See discussion under item 9 below.)
Proposed draft amendment for discussion
Amend 21 CFR 1020.32(h) to read as follows:
1020.32(h) Fluoroscopic time and timer. For fluoroscopic systems manufactured before (insert date one year after date of final rule) means shall be provided to preset the cumulative...until the timing device is reset. Fluoroscopic equipment manufactured on or after (insert date one year after date of final rule) shall include:
| (1) the means to display the cumulative ontime of the fluoroscopic tube from the beginning of a patient examination or procedure. The display shall be able to be reset to zero prior to the commencement of a new examination or procedure. | |
| (2) a means to alert the fluoroscopist via an audible alarm to the completion of any preset cumulative ontime of the fluoroscopic tube. The utilization and duration of a preset time shall be selectable at the discretion of the fluoroscopist. If any interval of cumulative fluoroscopic ontime is preset, a signal audible to the fluoroscopist shall indicate the completion of this interval. Such signal shall continue to sound while xrays are produced until the timer is reset. |
| (i) The timer shall not be interlocked to the production of xrays. Xrays may be produced whether an audible signal is sounding or not. | |
| (ii) The timer shall not be interlocked to the display of the cumulative ontime of the fluoroscopic tube. At the completion of any preset time interval, the cumulative ontime displayed according to 1020.32(h)(1) shall not be automatically reset to zero. |
8. Amendment to require provision of "last-image hold"
feature on fluoroscopic systems.
This amendment will require that all fluoroscopic x-ray systems be provided with a means to continuously display the last image acquired following termination of exposure.
Rationale
The advent of various methods for recording and display of fluoroscopic images makes possible the provision of a "last-image hold" or "freeze" frame capability on fluoroscopic xray systems. By this capability is meant the ability of the fluoroscopic xray system to continuously present a static image of the last fluoroscopic scene following termination of the fluoroscopic exposure. Such a feature provides the physician using fluoroscopic imaging the ability to conveniently study a fluoroscopic image "offline" without the necessity to continuously irradiate the patient.
This feature is especially useful in procedures such as fluoroscopicallyguided needle placement for biopsy or drainage, catheter or tube placement, and other diagnostic or therapeutic interventional procedures. Fluoroscopic systems provided with this feature permit reductions in fluoroscopic exposure times while permitting convenient examination and study of a fluoroscopicallyguided procedure and planning the next stage of the procedure.
This feature is provided as a basic or optional capability on many currently marketed fluoroscopic systems. Many have expressed the opinion that because of the radiation dose reduction afforded by such a feature in many clinical circumstances it should be provided on all new fluoroscopic system. Such a recommendation was strongly endorsed at the American College of Radiology/Food and Drug Administration sponsored Workshop on Fluoroscopy in 1992. In addition, a requirement for this capability is included in the draft of an IEC standard for the safety of xray equipment for interventional radiology currently under development. Establishing this requirement would assure that all new fluoroscopic systems have this patient radiation dose reduction feature and that it is available when its use is appropriate. Without such a requirement some systems may continue to be purchased without this feature, due to economic reasons, denying the availability of this means of dose reduction to patients.
Issues to be addressed
1. Should this requirement apply to all fluoroscopic systems (regardless of design or intended use)?
2. Should it apply to mobile and portable systems?
3. Should it apply to mobile Carm systems intended for use in surgical suites?
4. Should the requirement apply only to systems with digital imaging capabilities?
5. Should the requirement apply only to systems with digital or certain analog recording capabilities?
6. Should it apply only to systems intended for interventional procedures?
7. How can meaningful data on impact and cost of the proposed amendment be obtained?
Proposed draft amendment for discussion
Amend 21 CFR 1020.32 by adding the following:
1020.32 (j) Display of last fluoroscopic image. Fluoroscopic
equipment manufactured on or after (insert date one year from
date of final rule) shall be equipped with means to display the
last fluoroscopic image obtained prior to termination of fluoroscopic
exposure. This image may be a sum or other combination of a series
of images obtained immediately prior to termination of exposure,
provided the number and method of combination is selectable prior
to initiation of fluoroscopic exposure and that one option is
for display of the last image obtained prior to exposure termination.
Means shall be provided to indicate to the user whether any displayed
image is the result of the last-image hold feature or is an image
resulting from concurrent radiation exposure. The options for
the last image displayed and the impact of the selected option
on image characteristics shall be described in the information
required by 1020.30(h).
9. Amendment to require display of air kerma rate and cumulative air kerma.
This amendment will require a real-time display of the air kerma rate and the cumulative air kerma.
Rationale
Fluoroscopic air kerma rate, fluoroscopic irradiation time, and cumulative air kerma are radiological variables of fundamental importance for radiation safety. Evaluated at reference locations representative of xray entry points on the patient skin surface, these quantities can serve as indices of risk to skin susceptible to acute injury from potentially prolonged irradiation in interventional procedures [13]. Moreover, for diagnostic fluoroscopic examinations in general, knowledge of skinentry irradiation levels is the starting place for assessment of internaltissue absorbed dose [4,5]. Such doses are stochastically linked to cancer morbidity, mortality, and to genetically transmissible defects [6,7]. The availability of estimates of cumulative absorbed doses to tissues would facilitate risk communication between medical staff and patients. Realtime display air kerma rate and air kerma has been shown to be an efficacious tool in the quality assurance of radiological examinations and procedures. Display feeds back information to clinical staff, enabling them to adjust their techniques and protocols to minimize radiation burden to patients and themselves as they maintain requisite levels of imaging for visualization tasks [8].
The need for display of irradiation information has been recognized nationally by the Workshop on Fluoroscopy, October 1992, organized and sponsored by the American College of Radiology, Commission on Physics and Radiation Safety, with partial support from the Food and Drug Administration [9], and internationally by the Workshop on Efficacy and Radiation Safety in Interventional Radiology, October 1995, sponsored jointly by the World Health Organization and the Institute of Radiation Hygiene, Radiation Protection Ministry, Federal Republic of Germany [10]. Most recently a requirement for display of air kerma rate and cumulative air kerma has been incorporated into a draft IEC standard [11]. With the advent of commercially available and relatively inexpensive means to measure and display realtime air kerma rate and cumulative air kerma produced by fluoroscopic systems, it is feasible as well as desirable to require that this information be provided to fluoroscopists at their operating positions. The proposed amendments express this requirement.
Draft amendments supporting display of air kerma rate and cumulative air kerma are distributed according to precedent categories of the Performance Standard: A new paragraph (6) would be added to 21 CFR 1020.30(h) specifying information to be provided to users. A new class of fluoroscopic equipment is established, namely, that "designed for fluoroscopically-guided interventional procedures," and with it is the attendant requirement, 1020.32(e)(3)(v), for measuring compliance with entrance air kerma rate limits at a newly defined reference point in space. This reference point conforms to the one that is defined and justified in the draft IEC standard [11]. An issue under discussion is whether there ought to be this new reference point added to the four others already cited in 1020.32(e)(3) whether there ought be a new display reference point different from that for measuring compliance with air kerma rate limits, or whether there ought be one common reference point for all fluoroscopic systems.
The requirement for display of fluoroscopic time is stipulated in a revised 1020.32(h); it is not required that this time bedisplayed to the fluoroscopist at the fluoroscopist's working position. This paragraph requires that a timer be made available for use at the discretion of the fluoroscopist, and it eliminates the 5minute maximum interval after which there is an audible signal until reset.
Finally, a new paragraph (k) is added to 1020.32 to require display of air kerma rate and cumulative air kerma. The requirements closely conform to those expressed in the IEC draft standard [11]. Two issues are under discussion: First, the requirement for display applies to all equipment manufactured after an effective date, not solely to equipment suitable for fluoroscopically guided interventional procedures. Is the broad scope of the requirement justified? Second, the manner in which the air kerma rate and cumulative air kerma are displayed is strictly stipulated in draft paragraph 1020.32(k)(2). Would it be more advantageous to allow manufacturers to develop their own ways of displaying this information?
References for display of entrance air kerma rate and cumulative air kerma amendment
Proposed draft amendments for discussion
Amend 21 CFR 1020.30(h) by adding new paragraph (6) as follows:
21 CFR 1020.30(h) Information to be provided to users ...(6) Cumulative air kerma and air kerma rate. For xray systems manufactured on or after (insert date one year from date of final rule) there shall be provided
| (i) A statement of the accuracy of the cumulative air kerma and air kerma rate displayed. | |
| (ii) Instructions for calibrating and maintaining any instrumentation associated with measurement or evaluation of the cumulative air kerma and air kerma rate. | |
| (iii) A statement specifying the reference location used in evaluation of the cumulative air kerma and air kerma rate. | |
| (iv) A rationale for specification of a reference location other than 55 cm from the xray source in systems suitable for fluoroscopically guided interventional procedures. |
Amend 21 CFR 1020.32(e)(3) by adding new subparagraph (v) as follows:
21 CFR 1020.32(e)(3) Entrance air kerma rate limits, Measuring compliance...
| (v) For xray systems designed for fluoroscopically guided interventional procedures and manufactured on or after (insert date one year from date of final rule) the entrance air kerma rate shall be measured either at a point 55 cm from the focal spot along the main xray beam axis or else at point(s) deemed by the manufacturer, and described in the information provided per Section 1020.30(h), to be representative of the intersection of the xray beam axis with the entrance surface of a patient. |
Amend 21 CFR 1020.32 by adding a new paragraph (k) to read as follows:
1020.32(k) Display of indices of fluoroscopic irradiation. Fluoroscopic equipment manufactured on or after (insert date one year after date of final rule) shall include the means to display to the fluoroscopist at the fluoroscopist's working position quantitative values of the cumulative air kerma (measured in Gy) and the air kerma rate (measured in Gy/min) during a diagnostic examination or interventional procedure. The following requirements apply:
| (1) The cumulative air kerma shall represent a realtime total of all contributions of air kerma accruing from all fluoroscopy and radiography used from the beginning of an examination or procedure. The air kerma rate shall represent a realtime value of the air kerma per unit time as delivered when a fluoroscopic mode is activated. | |
| (2) When a fluoroscopic mode is activated, the air kerma rate shall be displayed. When a fluoroscopic mode is not activated, the cumulative air kerma shall be displayed. The display of the cumulative air kerma shall occur within 5 seconds of termination of an activated fluoroscopic mode. | |
| (3) The cumulative air kerma and air kerma rate shall be evaluated for freeinair irradiation conditions at one of the following reference locations: |
| (i) For xray systems designed for fluoroscopically-guided interventional procedures, the reference location shall correspond to the point(s) in space specified in 21 CFR 1020.32(e)(3)(v). | |
| (ii) For xray systems not designed for fluoroscopically-guided interventional procedures, the reference location shall correspond to one of the points in space specified in 21 CFR 1020.32(e)(3)(i)-(iv) according to the type of fluoroscope. |
| (4) The display of the cumulative air kerma and the air kerma rate shall be able to be reset to zero prior to the commencement of a new examination or procedure. |
End.
(Updated October 30, 1997)
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