Acceptability requirements for x-ray equipment used in health care

Transcription

1 DECISION 17 August /310/06 Translation. Original text in Finnish. ACCEPTABILITY REQUIREMENTS FOR X-RAY EQUIPMENT USED IN HEALTH CARE Radiography and fluoroscopy equipment and computed tomography appliances Under section 30 of the Decree of the Ministry of Social Affairs and Health on the Medical Use of Radiation (no. 423 of 2000) measures causing exposure to radiation must be performed using equipment that is suited for the said purpose. The Decree also prescribes that the requirements and criteria of acceptability for specific equipment functions to be considered from the point of view of radiation safety are to be confirmed by the Radiation and Nuclear Safety Authority (STUK). This decision confirms the attached acceptability criteria, i.e. the acceptability requirements for radiography and fluoroscopy equipment and computed tomography appliances. These acceptability requirements do not apply to mammography equipment, dental x-ray imaging equipment or digital imaging sensors. This decision shall be in force as of 1 October Director General Jukka Laaksonen Director Eero Kettunen APPENDIX Acceptability requirements for x-ray equipment used in health care STUK RADIATION AND NUCLEAR SAFETY AUTHORITY Visiting address Laippatie 4, FI Helsinki, Finland Postal address PO Box 14, FI Helsinki, Finland Tel Fax

2 Appendix 1 (9) 17 August 2006 Translation. Original text in Finnish. ACCEPTABILITY REQUIREMENTS FOR X-RAY EQUIPMENT USED IN HEALTH CARE Radiography and fluoroscopy equipment and computed tomography appliances 1 Background X-ray tubes and generators Filtration of primary radiation X-ray tube voltage X-ray tube current Exposure Time Electrical charge X-ray tube radiation output Radiation beam indicators and focusing X-ray equipment operation Dose display Imaging sensors based on a film-intensifier screen combination Image quality Film cassette Film processing Automatic exposure control unit function Fluoroscopy equipment Image quality Dose rate automation in fluoroscopy Relation between radiation field and image field sizes Computed tomography equipment Image quality CT numbers Slice thickness Table movement and patient positioning... 8 Bibliography... 9 STUK RADIATION AND NUCLEAR SAFETY AUTHORITY Visiting address Laippatie 4, FI Helsinki, Finland Postal address PO Box 14, FI Helsinki, Finland Tel Fax

3 2 1. Background Health care X-ray equipment and the accessories and instruments that are associated with its use must satisfy the acceptability requirements that are specified in this decision (see also section 30 of Decree no. 423 of 2000 of the Ministry of Social Affairs and Health [9] and Guide ST 3.3 [8]). These acceptability requirements denote the minimum requirements or acceptability limits that are imposed on the performance capacity of the equipment when the equipment is used [1]. The equipment must either be repaired and its performance capacity restored to the acceptable standard or it must be decommissioned where necessary, no later than at the time when acceptability limit requirement is not met. Acceptability requirements typically concern the precision of equipment settings and the operating condition of the equipment. They are not limiting values for optimal equipment performance. Responsible parties (the party running a radiation practice) procuring new equipment, performing acceptance tests and inspecting the quality of equipment at the time of use should apply stricter requirements, which may be based, for example, on the equipment specifications or performance tolerances shown in equipment standards. The performance measurement results will depend on the conditions of measurement and may also depend on the method of measurement. The methods of measurement and test samples shown in the bibliography [references 2 7] are used in measurements of compliance with acceptability requirements unless otherwise shown in this decision. 2. X-ray tubes and generators 2.1 Filtration of primary radiation Total filtration of primary radiation must correspond to a minimum of 2.5 mm of aluminium. The total radiation filtration must be marked on the protective housing of the x-ray tube. If the additional filtration is adjustable, then the fixed filtration must be marked on the protective housing of the x-ray tube and it must be possible to verify the selected additional filtration. 2.2 X-ray tube voltage The deviation in voltage from the preset value in an X-ray tube may not exceed 10 per cent. When adjusting the voltage from one value to another the change in actual voltage must also be not less than 0.5 times and not more than 1.5 times the difference between the preset voltages. 2.3 X-ray tube current The deviation in electric current from the preset value in an X-ray tube may not exceed 20 per cent. NB. Direct measurement of X-ray tube current is not always necessary in practice, and compliance with the requirement may instead be monitored on the basis of constancy and linearity of radiation output. It may also be necessary to verify the absolute accuracy of the X-ray tube current if the result of measuring radiation output differs from the radiation output reference value determined in the equipment acceptance test by more than the action threshold imposed in the quality control programme (see Guide ST 3.3 [8]), or if the dose *) as a function of electrical charge fails to satisfy the linearity requirement stipulated in section 2.6 of this decision. *) The dose referred to in this decision may be measured as air kerma and the dose rate may be measured as air kerma rate.

4 3 2.4 Exposure Time The deviation in exposure time from the preset value may not exceed 15 per cent + 1 ms. 2.5 Electrical charge The deviation in electrical charge, i.e. the product of the X-ray tube current and the exposure time, from the preset value may not exceed 20 per cent mas. NB. Direct measurement of X-ray tube electrical charge is not always necessary in practice, and compliance with the requirement may instead be monitored on the basis of constancy and linearity of radiation output. It may also be necessary to verify the absolute accuracy of the X-ray tube electrical charge if the result of measuring radiation output differs from the radiation output reference value determined in the equipment acceptance test by more than the action threshold imposed in the quality control programme (see Guide ST 3.3 [8]), or if the dose*) as a function of electrical charge fails to satisfy the linearity requirement stipulated in section 2.6 of this decision. 2.6 X-ray tube radiation output When manual adjustment values, which correspond to the clinical use of equipment, are used, the standard deviation of dosage *) measured in the radiation beam may not exceed 10 per cent. The standard deviation will be calculated from at least five repeated measurements. If a film-screen combination and suitable phantom are used as a detector when measuring, then the deviation in x-ray image densities from the average must not exceed When using manual adjustment values the dose *) measured in the radiation beam of an X-ray appliance must be proportional to the electrical charge applied, such that K Q K Q K Q K + Q where K 1 is the dose corresponding to the electrical charge Q 1, K 2 is the dose corresponding to the electrical charge Q 2 and Q 2 < 2 Q Radiation beam indicators and focusing The radiation beam signal lights must be clearly visible under normal working illumination conditions. The deviation between the signal lights or other indicators of the radiation beam and the edges of the radiation beam at the image receptor must not exceed 1 per cent of the distance between the focus of the X-ray tube and the image receptor on any edge of the radiation field. The deviation of the central axis of the radiation beam from the centre point of the radiation beam at the image receptor as shown by the signal lights describing the radiation beam or by some other radiation beam indicator, and from the centre point of the image receptor, may not exceed 1 per cent of the distance between the focus and the image receptor. When using automatic adjustment of the radiation field or image partition automation the radiation beam must be directed onto the image receptor in a manner that is expedient and intended by the appliance manufacturer. *) The dose referred to in this decision may be measured as air kerma and the dose rate may be measured as air kerma rate.

5 4 2.8 X-ray equipment operation 3. Dose display The X-ray equipment, its accessories and its premises of use must be conducive to safe operation of the equipment. The appliance warning and indicator lights and other safety device and accessories must be undamaged and operating in the intended manner. The error of any meter or display indicating the radiation exposure of a patient from X-ray equipment may not exceed 25 per cent (50 per cent in fluoroscopy appliances). 4. Imaging sensors based on a film-screen combination 4.1 Image quality 4.2 Film cassette Image quality must satisfy the clinical requirements for X-ray examinations made with the appliance. This may be assessed in the manner specified in Guide ST 3.3 [8]. NB. No detailed technical acceptability requirements for image quality are specified in this decision, as compliance with technical requirements constitutes no direct demonstration of the clinical acceptability of an image, and because the required clinical image quality essentially depends on the medical indications that justify the X-ray examination. Control of the operating condition and performance characteristics of equipment employs a phantom suited to technical assessment of image quality and appropriately selected performance levels (e.g. resolution and contrast sensitivity) (see Guide ST 3.3 [8]). The locking mechanism of the film cassette must function reliably and the cassette must be lightproof. The compression material must be resilient and compression must be effective and even throughout the film area. Contact between the intensifying screen and film must be good. Cassettes must be classified by film speed and the speed must be clearly marked on them. The difference in film density with intensifying screens in the same speed class and images made in the same way may not exceed Film processing The darkroom and storage box for X-ray film must be lightproof. The safelight must be suitable for the type of film used, and indicator and other lights must not expose the film. Ordinary film processing in the darkroom must cause no visible change (about 0.05) in the density of pre-exposed film. The optical density of unexposed developed X-ray film must not exceed 0.3. Deviations in speed index and contrast index (figure 1) from the reference value (the value obtained in the acceptance test) in film development monitoring must not exceed 0.2. The film processor must not cause damage to X-ray films. Film processors used for developing similar films should yield similar film processing results. If this does not occur, then the film processors are to be adjusted so that at least the film speed index is the same.

6 5 3,2 2,8 2,4 Film density 2 1,6 1,2 0,8 0, Density step number Figure 1. Determination of film processing reference values from a density curve derived by exposing a film by a sensitometer. The film fog is defined at step 1 (0.2 in the graph). For a speed index reference value a step is sought where the density is closest to a value of 1.0 above the base line fog (the value shown on the graph is 1.2). The density at this step (the density value (1.1) at step 12 on the graph) expresses the value of the speed index. The contrast index is determined using two density steps, selecting the steps for which the densities are closest to values of 1.0 and 2.0 above the base line fog (steps 12 and 15 (1.1 and 2.3) on the graph). The difference in densities between these steps is the contrast index value of the film (1.2 in the situation shown on the graph). The values of the indices will always be determined in monitoring from the densities at these same density steps (12 and 15 in this example): the density at step 12 is the speed index and the difference between the densities at steps 12 and 15 is the contrast index. 4.4 Automatic exposure control unit function When imaging a phantom that optimally corresponds to real patient imaging using an automatic exposure control unit, at the various X-ray tube voltage values used in such X-ray examinations, the deviation in X-ray film densities from the average may not exceed 0.3 (checking operation of automatic exposure control unit voltage compensation). The thickness of the phantom must correspond to the thickness of body parts to be imaged in these X-ray examinations. 5. Fluoroscopy equipment The following requirements apply to fluoroscopy in addition to the requirements (in chapters 2 and 3) for the X-ray tube and generator and for the dose display. 5.1 Image quality Image quality must satisfy the clinical requirements for fluoroscopy examinations and fluoroscopically guided procedures made with the appliance. This may be assessed in the manner specified in Guide ST 3.3 [8]. NB. No detailed technical acceptability requirements for image quality are specified in this decision, as compliance with technical requirements constitutes no direct demonstration of the clinical acceptability of an image, and because the required clinical image quality essentially depends on the medical indications that justify the examination or procedure. Control of the operating condition and performance characteristics of equipment employs a phantom suited to technical assessment of image quality and appropriately selected performance levels (e.g. resolution and contrast) (see Guide ST 3.3 [8]).

7 6 5.2 Automatic dose rate control in fluoroscopy The level of dose rate, i.e. the air kerma rate regulated by the dose rate control at the surface of the image receptor entrance plate without a grid, may not exceed 0.8 µgy/s when using the most common image receptor field size (e.g. an image diameter of about 25 cm) in fluoroscopic examinations. This limiting value is a recommendation when using other image receptor field sizes. The level of dose rate must be checked, for example using the measurement system shown in figure 2. The primary radiation dose rate (air kerma rate) in normal operation under the measurement system shown in figure 3 may not exceed 100 mgy/min. If the examination or procedure requires a dose rate that is higher than this (in which case the level of the dose rate may also exceed 0.8 µgy/s), then the operation may be accepted subject to the following conditions: The dose rate to the patient s skin in the primary beam of the X-ray tube does not exceed 50 mgy/min. in normal operation. The equipment includes a switch, which the operator must activate continually in order to use the enhanced function in question. The dose rate according to the measuring system shown in figure 3 does not exceed 200 mgy/min. A continuous sound or light signal indicates use of the enhanced function to the person performing the procedure. There is an extra shield for the operator in the vicinity of the equipment corresponding in effectiveness to a layer of no less than 1.0 mm of lead. The equipment includes a DAP display (dose-area product display) or other suitable dose display. Figure 2. Arrangements for measuring the level of dose rate in fluoroscopy equipment. 1 focus of X-ray tube, 2 test phantom (20 mm of aluminium or other test phantom as necessary, e.g. 10 mm of aluminium for paediatric fluoroscopy), 3 table surface (where present), 4 cassette partition automation (where present), 5 radiation meter of an ionization chamber, 6 entrance plate, 7 front surface of image receptor (e.g. image intensifier).

8 7 a) Over coutch tube Focus Measurement chamber Minimum distance 30 cm Table surface b) Under coutch tube Measurement chamber Table surface Minimum distance Focus c) C or U arm Front surface of image receptor entrance plate 30 cm Measurement chamber Focus Figure 3. Primary radiation dose rate measurement geometry of fluoroscopy equipment.

9 8 5.3 Relation between radiation field and image field sizes The ratio of sizes of the radiation field and the image field (surface area of the image receptor front panel) must not exceed It is considered good practice for the edges of the radiation beam limiter to show on the monitor image. 6. Computed tomography equipment The following requirements apply to computed tomography equipment in addition to the requirements (in chapters 2 and 3) for the X-ray tube and generator and for the dose display. 6.1 Image quality 6.2 CT numbers Image quality must satisfy the clinical requirements for computed tomography examinations made with the appliance. This may be assessed in the manner specified in Guide ST 3.3 [8]. NB. No detailed technical acceptability requirements for image quality are specified in this decision, as compliance with technical requirements constitutes no direct demonstration of the clinical acceptability of an image, and because the required clinical image quality essentially depends on the medical indications that justify the examination or procedure. Control of the operating condition and performance characteristics of equipment employs a phantom suited to technical assessment of image quality and appropriately selected performance levels (e.g. resolution and contrast) (see Guide ST 3.3 [8]). The CT numbers at the reference point of the image field must not deviate from the manufacturer s notified values by more than 10 HU when measured in the manner specified by the manufacturer. 6.3 Slice thickness The true slice thickness (detector sensitivity profile half-width value) in axial imaging must not deviate from the preset value by more than 1 mm when the preset slice thickness is greater than 2 mm 50 per cent, when the preset slice thickness is 1-2 mm 0.5 mm when the preset slice thickness is less than 1 mm. 6.4 Table movement and patient positioning When the table moves a distance of 20 cm the true movement of the table must not deviate by more than 2 mm from the value shown on the table movement display. The indicated starting point of CT imaging may not deviate from the true starting point by more than 3 mm.

10 9 Bibliography [1] Criteria for acceptability of radiological (including radiotherapy) and nuclear medicine installations. Radiation Protection 91, European Commission [2] EN (IEC) Medical electrical equipment Part 1: General requirements for safety. 3. Collateral standard: General requirements for radiation protection in diagnostic X-ray equipment. [3] EN (IEC) Medical electrical equipment Part 2: Particular requirements for the safety of highvoltage generators of diagnostic X-ray generator. [4] IEC Medical electrical equipment Part 2: Particular requirements for the safety of X-ray source assemblies and X-ray tube assemblies for medical diagnosis. [5] IEC Medical electrical equipment Part 2: Particular requirements for the safety of X-ray equipment for interventional procedures. [6] IEC Medical electrical equipment Part 2: Particular requirements for the safety of X-ray equipment for computed tomography. [7] IEC Medical electrical equipment Part 2: Particular requirements for the safety of X-ray equipment for computed tomography, Amendment 1. [8] Röntgentutkimukset terveydenhuollossa, Ohje ST 3.3, STUK (X-ray examinations in health care. Guide ST 3.3. Radiation and Nuclear Safety Authority, 20 March 2006.) [9] Sosiaali- ja terveysministeriön asetus säteilyn lääketieteellisestä käytöstä, 423/2000. (Decree of the Ministry of Social Affairs and Health on the medical use of radiation, no. 423 of 2000.)