Safety Code 35: Safety Procedures for the Installation, Use and Control of X-ray Equipment in Large Medical Radiological Facilities

Section A: Responsibilities and Protection

3.0 Procedures for Minimizing Radiation Exposure to Patients

The largest single contributor of man-made radiation exposure to the population is dental and medical radiography. In total, such use of X-rays accounts for more than 90 % of the total man-made radiation dose to the general population.

The risk to the individual patient from a single radiographic examination is very low. However, the risk to a population is increased by increasing the frequency of radiographic examinations and by increasing the number of persons undergoing such examinations. For this reason, it is important to reduce the number of radiographs taken, the number of persons examined radiographically, and the doses associated with the examinations.

To accomplish this reduction, it is essential that patients must only be subjected to necessary radiological examinations and, when a radiological examination is required, patients must be protected from excessive irradiation during the examination.

The required and recommended procedures for the protection of the patient, outlined in this section, are directed toward the physician/practitioner, radiologist, and technologist. They are intended to provide guidelines for elimination of unnecessary radiological examinations and for minimizing doses to patients when radiological examinations are necessary.

3.1 Guidelines for the Prescription of X-ray Examinations

Unnecessary radiation exposures of patients can be significantly reduced by ensuring that all examinations are clinically justified. This can be done by adhering, as much as possible, to certain basic recommendations. These recommendations are presented below.

1. The prescription of an X-ray examination of a patient should be based on clinical evaluation of the patient and should be for the purpose of obtaining diagnostic information or patient treatment.
2. X-ray examinations should not be performed if there has been no prior clinical examination of the patient.
3. Radiological screening must not be performed unless, it has been proven that the benefit to the individual examined or the population as a whole is sufficient enough to warrant its use.
4. It should be determined whether there have been any previous X-ray examinations which would make further examination unnecessary, or allow for the ordering of an abbreviated examination. Relevant previous images or reports should be examined along with a clinical evaluation of the patient.
5. When a patient is transferred from one physician or hospital to another any relevant images, or reports should accompany the patient and should be reviewed by the consulting physician.
6. When prescribing a radiological examination, the physician should specify precisely the clinical indications and information required.
7. The number of radiographic views required in an examination must be kept to the minimum practicable, consistent with the clinical objectives of the examination.
8. Before performing X-ray examinations on females of child bearing age (11-55 years), the patient must be asked whether there is any chance that they may be pregnant. Radiological examinations of the pelvic area in women of childbearing age should be undertaken in the ten-day period following the onset of menstruation, since the risk of pregnancy is very small during this period. Care must be taken to protect the foetus from radiation when the X-ray examination is not avoidable. The guidelines in section A3.2 for radiological examinations of pregnant women should be followed.
9. If a radiograph contains the required information, repeat procedures must not be prescribed simply because the radiograph is not of the "best" diagnostic quality.
10. Specialized studies should be undertaken only by, or in close collaboration with a qualified radiologist.
11. A patient's clinical records should include details of X-ray examinations carried out.

More specific guidance for the prescription of imaging examinations is available from the Canadian Association of Radiologists (CAR) in their Diagnostic Imaging Referral Guidelines (CAR 2005). These guidelines provide recommendations on the appropriateness of imaging investigations for the purpose of clinical diagnosis and management of specific clinical/diagnostic problems. The objective of these guidelines is to aid the referring physician/practitioner to select the appropriate imaging investigation and thereby reduce unnecessary imaging by eliminating imaging that is not likely to be of diagnostic assistance to a particular patient and by suggesting alternative procedures that do not use ionizing radiation but offering comparable diagnostic testing accuracy.

3.2 Guidelines for Radiological Examinations of Pregnant Women

Radiological examinations of the pelvic area of a woman known to be pregnant simultaneously irradiate the patient's gonads and the whole body of the foetus. Irradiation of the unborn foetus increases the infant's risk of somatic effects and also carries the risk of genetic effects in subsequent offspring. Therefore, every effort should be made to avoid unnecessary irradiation of any woman known to be, or who might be pregnant. Clearly, however, in spite of the possibility of radiation damage, if a radiological examination is required for the diagnosis or management of an urgent medical problem, it must be done, irrespective of whether the patient may or may not be pregnant.

The following recommendations apply to X-ray examinations involving pregnant or potentially pregnant women:

1. Only essential investigations should be taken in the case of pregnant or suspected pregnant women.
2. When radiological examinations of the pelvic area or abdomen are required, the exposure must be kept to the absolute minimum necessary and full use must be made of gonadal shielding and other protective shielding if the clinical objectives of the examination will not be compromised.
3. If a radiological examination of the foetus is required, the prone position should be used. This has the effect of shielding the foetus from the softer X-rays and hence reducing the foetal dose.
4. Radiography of the chest, extremities, etc., of a pregnant woman, for valid clinical reasons, should only be carried out using a well-collimated X-ray beam and with proper regard for shielding of the abdominal area.

3.3 Guidelines for Carrying Out X-ray Examinations

Next to elimination of unnecessary X-ray examinations, the most significant factor in reducing patient exposure is ensuring that only necessary examinations are performed with good technique. It is possible, for example, to obtain a series of diagnostically-acceptable radiographs and have the patient exposures vary widely because of choice of technique and loading factors used. It is the responsibility of the operator and radiologist to be aware of this and to know how to carry out a prescribed examination with the lowest possible exposure to the patient.

The requirements and recommendations that follow are intended to provide guidance to the operator and radiologist in exercising their responsibility toward reduction of patient exposure.

3.3.1 General Requirements and Recommendations

1. The operator must not perform any examination which has not been prescribed.
2. The exposure of the patient must be kept to the lowest practicable value, consistent with clinical objectives and without loss of essential diagnostic information. To achieve this, techniques appropriate to the equipment available should be used.
3. Particular care, consistent with the recommendations of section A3.2, must be taken when radiological examinations of pregnant or potentially pregnant women are carried out.
4. The X-ray beam must be well-collimated to restrict it as much as is practicable to the area of diagnostic interest.
5. The X-ray beam size must be limited to the size of the image receptor or smaller.
6. For systems with multiple AEC sensors, the AEC sensor(s) covering the area of diagnostic interest should be selected.
7. The X-ray beam should not be directed toward the gonads unless it is absolutely essential, in which case gonads shields must be used whenever the value of the examination is not impaired by such use. Guidelines on the use of gonad shielding are given in section A3.4 of this Code.
8. Shielding must be used where appropriate and practicable to limit the exposure of body tissues. It is particularly important that special effort be made to protect the blood forming organs, gonads and thyroid of children.
9. The focal spot-to-skin distance should be as large as possible, consistent with good radiographic technique.
10. Radiological examinations of infants and children should only be performed using techniques and loading factors which have been modified for size and age.
11. For very young children, special devices should be employed to restrict movement.
12. Full details of the radiological procedures carried out should be noted on the patient's clinical records.
13. All images captured , whether on film or on digital imaging systems, must remain with the patient study unless they are rejected by the operator for valid predefined quality issues. All rejected images must be collected for use during routine rejection analysis. The facility must have a program established to prevent the total loss of any images without review for reject analysis.

3.3.2 Requirements and Recommendations for Radiographic Procedures

1. The edges of the X-ray beam should be seen on all X-ray images to ensure that no more than the desired area has been irradiated. The image receptor size used should be as small as possible, consistent with the objectives of the examination.
2. For film-based imaging, the most sensitive screen-film combination, consistent with diagnostically-acceptable results, should be used.
3. To ensure that patient exposure is kept to a minimum, consistent with image quality, full advantage should be taken of a combination of techniques, such as:
   1. use of an anti-scatter grid or air gap between the patient and the image receptor;
   2. use of the optimum focal spot-to-image receptor distance appropriate to the examination;
   3. use of the highest X-ray tube voltage which produces images of good quality;
   4. use of automatic exposure control devices designed to keep all irradiations and repeat irradiations to a minimum.
4. The operator of the X-ray equipment should evaluate the resulting images to verify that the techniques being used are producing diagnostic quality images and that the X-ray equipment is functioning correctly.
5. To avoid the necessity of retakes, it is particularly important before taking a long series of images that a single preliminary image of the series should be taken to verify correctness of settings.

3.3.3 Requirements and Recommendations for Radioscopic Procedures

1. In view of the relatively high exposure that results from radioscopy, such procedures should only be carried out when an equivalent result cannot be obtained from radiography. Radioscopy must not be used as a substitute for radiography.
2. Equipment operators must be trained in radioscopic procedures before carrying out radioscopy on patients. Training must meet with pertinent provincial/regional regulations. Continuing professional development must meet with the requirements of the Maintenance of Certification Program of the Royal College of Physicians and Surgeons of Canada. (CAR 2005).
3. Where included in the scope of practice of technologists, a technologist who has been properly trained in radioscopic procedures can perform radioscopy on patients. It is recommended to consult provincial or territorial statutes and regulations governing the scope of practice of medical technologists.
4. All radioscopic procedures should be carried out as rapidly as possible with the smallest practical X-ray field sizes.
5. When operating equipment with automatic brightness control, the operator must monitor the X-ray tube current and voltage since both can rise to high values without the knowledge of the operator, particularly if the gain of the intensifier is decreased.
6. When performing radioscopy, the operator must at all times, have a clear line of sight to the output display.
7. Mobile radioscopic equipment should only be used for examinations where it is impractical to transfer patients to a permanent radioscopic installation.
8. Cinefluorography produces the highest patient doses in diagnostic radiography because the X-ray tube voltage and current used are generally higher than those used in radioscopy. Therefore, this technique should not be used unless significant medical benefit is expected.

3.3.4 Requirements and Recommendations for Angiography

1. Exposure to the patient's eyes and thyroid can result during neurological examinations, such as cerebral angiography and cardiac catheterization and angiography. The technique of the procedure should take into consideration the risk to the eyes and thyroid. Where it does not interfere with the diagnostic information sought, appropriate shielding should be used.
2. To reduce doses to patient, keep irradiation time to a minimum.
3. If possible, use an increased tube filtration to reduce low energy X-rays, and use a lower time frequency in pulse radioscopy.
4. Keep the X-ray tube as far as possible and the image intensifier as close as possible from the patient.
5. In children and for small adults, the removal of the grid will reduce doses to patients.
6. Be aware that magnification mode may increase the dose to the patient.
7. Use cine-run only as long as necessary, and if possible, use automated injection systems.
8. If the procedure is long, reposition the tube so that the same area of skin is not subjected to X-ray beam.
9. Facilities should have documented, for each type of interventional procedure, a statement on the radiographic images (projections, number and loading factors), radioscopy time, air kerma rates and resulting cumulative skin doses and skin sites associated with the various part of the interventional procedure.

3.3.5 Requirements and Recommendations for Computed Tomography Procedures

1. The number of slices produced and the overlap between adjacent scans should be kept to the minimum practicable, consistent with clinical objectives of the examination.

3.4 Guidelines for Reduction of Dose to Sensitive Tissues

Ionizing radiation has the ability to produce gene mutations and chromosome aberrations in cells. These effects are especially important in two circumstances, exposures to reproductive cells, and rapidly dividing cells. When such effects occur in a reproductive cell(gametes and the stem cells they arise from), undesirable mutations may be transmitted to subsequent generations. If damage is caused in rapidly dividing cells mutations will rapidly be passed on to the cell progeny, amplifying the deleterious radiation effects.

Medical X-ray exposures are, at present, the major contributor of gonadal radiation exposure to the population. By reducing the gonadal dose to individual patients one can, in fact, make a significant contribution toward the reduction of the genetically significant dose to the population. It is generally presumed that there is no threshold dose below which genetic effects cannot occur. Therefore, it is important that even small radiation exposures to the sensitive tissues of patients be avoided, unless such exposures can be shown to be medically necessary.

Individuals performing X-ray examinations of patients must pay special attention to the following factors that are important for reducing doses to sensitive organs:

1. Correct collimation of the X-ray beam. It is not sufficient merely to limit the beam to the size of the image receptor. Care should be taken to further restrict the beam to the region of the patient's body that is of diagnostic interest. Irradiation of any part of the body outside that region contributes nothing to the objective of the examination and only increases the dose to the body. Irradiation of gonads, female breast tissue, and the thyroid should be avoided, if possible.
2. Examinations of children and adolescents. X-ray examinations of young children and adolescents whose body tissues are developing should not be performed unless a condition exists such that the benefit of the diagnostic information outweighs the radiation risk.
3. Gonad Shields. Appropriate use of specific area gonad shielding is strongly advised when:
   1. the gonads lie within, or are in close proximity to, the X-ray beam;
   2. the patient is of reproductive age; and
   3. clinical objectives will not be compromised.
4. Appropriate selection of loading factors and technique. An appropriate selection of tube voltage, current and filtration is particularly important for diagnostic procedures in which the gonads or breast tissues lie within or near the X-ray beam. For example, in radioscopy, use of higher tube voltage and filtration and lower tube current will almost always reduce the gonad dose.
5. Sensitivity of imaging system. Doses are related to the sensitivity of the imaging system. Thus, an increase in the sensitivity of the imaging system reduces the dose; conversely, decreasing the sensitivity increases the dose. It is therefore, very important to maintain the sensitivity of the imaging system at its optimum value and to be alert for any significant deterioration.

3.5 Diagnostic Reference Levels (DRLs)

3.5.1 Introduction

Doses for medical diagnostic procedures can vary widely between equipment and facilities. Numerous surveys have demonstrated that, for typical procedures, the difference in radiation doses can be as wide as a factor of 50 to 100. For interventional procedures, this difference can be even wider. In diagnostic radiology, the use of surface air kerma limits is not sufficient since these dose limits are usually set at a level high enough so that any doses greater than the limit is clearly unacceptable, but this limit does not help in optimising patient doses. For this reason, the concept of Diagnostic Reference Levels (DRLs) is introduced, instead of using maximum dose limits.

The purpose of DRLs is to promote a better control of patient exposures to X-rays. This control must be related to the clinical purpose of the examination. DRLs must not be seen as limits but instead as guidance to optimise doses during procedures. DRLs are based on typical examinations of standardized patient or phantom sizes, and for a broad type of equipment. While it is expected that facilities should be able to attain these levels when performing procedures using good methodologies, it is not expected that all patients should receive these dose levels but that the average of the patient population should. DRLs are useful where a large reduction in patient doses may be achieved, such as for computed tomography (CT) procedures, where a large reduction in collective doses may be achieved, such as for chest X-rays, or where a dose reduction will result in a large reduction in risk, such as for paediatric procedures. However, interventional procedures are not going to be addressed at this time since it is difficult to establish DRLs for them due to the variability in techniques, the frequency of procedures, the difficulty in dose measurement, and the lack of published data.

3.5.2 Application

The tables shown in section A3.5.3, list representative ranges of DRLs for radiographic procedures, performed on adults and children, radioscopic procedures and CT procedures. It is obvious that not all facilities will perform all of the listed procedures. Therefore, each facility should establish DRLs for those procedures relevant to them and where the number of patients undergoing the procedures is sufficiently high. A facility may set DRLs for other procedures not presented in the tables but which are being performed. At least one procedure should be evaluated for each X-ray equipment.

DRL measurements can be performed in two different ways; with a phantom specifically designed for the procedure, or using patients. In general, it is preferable to use phantoms since the measurements can be more easily replicated and offer more flexibility in the type of procedures which can be performed. Appropriate phantom, such as phantoms for chest, lumbar spine and abdomen representing a patient thickness, in the PA projection, of 23 cm are acceptable for DRL measurements, as long as they are consistently used. DRLs for CT are based on the weighted CT Dose Index, or CTDI_w which can be determined by using CT Dosimetry Phantoms, described in Table 22, section C3.6.3.

When patients are be used to establish DRLs, measurements should be done only on patients whose individual weight is 70 ± 20 kg, and the average weight measurement of the patients should be 70 ± 5 kg. It is recommended that the minimum sample size for a specific procedures or equipment be 10 patients. Patients should not be used for paediatric procedures.

Entrance surface doses for establishing DRLs can be measured using thermoluminescent dosimeters (TLDs) placed on the tube side of the patient, by using dose area product (DAP) meters, or through information retrieved from the Radiology Information System (RIS) , or other means. The use of DAP is more practical since the whole procedure is recorded and their use is less complicated than TLDs, while with the use of RIS, the patient weight may not be available.

The values presented in section A3.5.3 are provided to facilities for guidance. The values presented are dependent of patient size and, as such, a facility will need to evaluate whether their patient population falls within the range of patient size for the procedure. While this safety code recommends representative DRLs, a hospital or clinic can set their own local DRLs if enough data is available. The facility should create a list of reference doses for their patient population and use these values within their quality assurance program. DRL values should be reviewed from time to time to assess their appropriateness. It is recommended that this review be done annually.

Radiological facilities which fall under provincial or territorial jurisdiction should contact the responsible agency in their respective region for information on any provincial or territorial statutory or regulatory requirements concerning dose limits. A listing of these responsible agencies is provided in Appendix V.

DRL values must not be used for comparison with individual patients. The values should be compared only with the average of a collection of patients of a specific weight. The evaluation of conformity with DRLs should be done at the X-ray room level or X-ray equipment type, i.e., mobiles, CT. For each examination under consideration, the mean patient doses for each room should be compared to the DRL for the examination. If the mean dose is found to significantly and consistently exceed the suggested DRL, an investigation of the performance of the equipment, the radiological technique used, and the methodology of dose measurement should be done in order to reduce patient doses. It is recommended that this action level be set at a defined proportion (i.e., 25% of the mean) and at least twice the standard error of the mean of the measurements.

3.5.3 Recommended DRL Values

Table 1 presents representative DRL values for radiographic procedures performed on adults. Table 2 presents DRL values for a 5 year old child along with the mean body thicknesses for each examination. It should be noted that the range of values provided for the entrance surface dose is reflective of the variation of values found in published data. Representative DRLs for radioscopic and CT examinations are shown in Table 3 and Table 4 respectively.

Table 1: Representative DRLs for radiographic procedures performed on adults (IPEM 2004), (IAEA 1996)

Examination	Entrance Surface Dose (mGy)
Chest (PA)	0.2 - 0.3
Chest (LAT)	0.7 - 1.5
Thoracic Spine (AP)	5 - 8
Thoracic Spine (LAT)	7 - 10
Lumbar Spine (AP)	7 - 10
Lumbar Spine (LAT)	15 - 30
Abdomen (AP)	7 - 15
Pelvis (AP)	5 - 10
Skull (AP)	4 - 5
Skull (LAT)	2 - 3

Table 2: Representative DRLs for radiographic procedures performed on a five-year-old child (IPEM 2004)

Examination	Mean Body Thickness (cm) (Hart et al.)	Entrance Surface Dose (mGy)
Chest (AP/PA)	13.4	0.05 - 0.15
Chest (LAT)	18.8	0.15 - 0.25
Abdomen (AP/PA)	13.6	0.5 - 1.0
Pelvis (AP/PA)	13.7	0.6 - 1.0
Skull (AP)	17.8	1.0 - 2.0
Skull (LAT)	13.8	0.8 - 1.5

Table 3: Representative DRLs for radiographic procedures (IPEM 2004)

Examination	Dose Area Product (Gy·cm2)
Abdominal Radioscopy	20 - 70
Barium Enema	30 - 60
Coronary Angiography	35 - 75

Table 4: Representative DRLs for CT procedures (IPEM 2004), (Aldridge 2006), (Shrimpton 2004)

Examination	CTDIw (mGy)	Dose Length Product (mGy·cm)
Head	60	930 - 1300
Face and Sinuses	35	360
Chest	30	580 - 650
Abdomen - Pelvis	35	560 - 1100
Liver and Spleen	35	470 - 920