RADIATION EXPOSURE IN THE CATH LAB – SAFETY AND PRECAUTIONS

Dr S M S Raza, MB BS, MD, MRCP,Dip.Card.(UK)

Specialist Registrar (Cardiology)Leeds Teaching Hospitals NHS Trust UK.

Ionising radiation is a workplace hazard that cannot be detected by the human senses. The cardiovascular laboratory or cath lab is one such place where ionising radiation is much in use. The cath lab is a closed atmosphere where the working staff (i.e. cardiologists, cardiac technicians, radiographers, nurses and trainees) is at a potential risk to radiation exposure almost on a daily basis. Compared to other departments (radiology, urology, operating rooms, etc.) that also use x-ray equipment, the cardiac cath lab is generally considered an area where exposure to radiation is particularly high. Factors such as the configuration of the of the x-ray equipment, the number of cases per day, and the often long period of screening required for a study, contribute to this relatively high level of exposure and monitoring results for staff members in the cath lab who wear single badges at the collar outside their lead aprons are generally amongst the highest in the hospital.
Exposure rates exceeding 7.14 Gy/hr( i.e. 5 sievert/hr) in the cath lab have been reported (3) and interventional procedures such as percutaneous coronary intervention (PCI) and electrophysiological studies (EPS)/ pacing result in the highest radiation exposure to patients and staff (3).

Radiation in the cath lab is generated using two different modes: fluoroscopy or cine angiography (cine). Fluoroscopy is used for catheter placement and involves 95% of the total x-ray operation time but only causes 40% of the total radiation exposure to staff and patients. This is due to pulsed screening that reduces exposure dose. Cine is used to acquire diagnostic images and to generate a permanent record of the procedure and, although representing only 5% of the total x-ray tube operation time, 60% of the total radiation exposure to staff and patients occur during cine. This is primarily due to use of relatively high dose rapid sequence screening required to record onto film. Significant reductions in exposure can be realised by being aware of when cine is/will be used and applying radiation safety measures accordingly.

It is important to effectively measure radiation doses acquired by cath lab personnel but exact dosage quantities are difficult to derive due to the non-uniformity of irradiation and differences in X-ray intensity as well as the relatively low energies generated by modern equipment. Therefore the International Commission on Radiological Protection (ICRP) recommend the use of effective dose (E) to evaluate the effects of partial exposure and relate this to the risk of equivalent whole body exposure. It is expressed in Sievert units (Sv) ( 1Gray unit = 0.7 sievert unit). Modern cardiac interventional procedures (coronary angiography and PCI) produce effective doses of 4 to 21 mSv and 9 to 29 mSv respectively and are therefore relatively high (1 mSv is the equivalent of approximately 10 chest x-rays) (4). The intensity of the biological effect of X-rays is dependent on the absorbed dose (total radiation energy per unit mass) of sensitive tissue and is expressed in gray units (Gy). The average dose per procedure for the cardiologist is estimated as 0.05 mGy (6). To allow better comparison of patient and staff doses this value can be expressed as the dose area product (DAP). The DAP is calculated as the product of dose in air in a given plane and the area of the irradiating beam and is independent of the distance from the x-ray source. Coronary angiography and PCI produce mean-patient DAPs in the range 20 to 106 Gy.cm2 and 44 to 143 Gy.cm2 respectively (3).

POTENTIAL HAZARDS OF RADIATION EXPOSURE:

These include:

Injury to skin i.e. placing your hands within the primary beam at all the time. Typically, one minute of screening leads to 20 mGy skin dose. Threshold for transient skin erythema is about 2 Gy.
Radiation may also interact with and alter cellular DNA. The majority of these interactions are inconsequential since the damage can be repaired.
Damage to eye: Relatively high doses of radiation can damage the conjunctiva ,iris, sclera, and blood vessels of the retina. The lens however is the critical site, for it may sustain irreversible damage from a relatively low dose of radiation and subsequently formation of cataract. Radiation induced cataracts are distinct from naturally occurring cataracts in that they form in the posterior pole of the lens.

However, there is a small probability that the DNA damage will remain altered and can potentially cause cancer induction (carcinogenesis) and genetic defects. The incidence of these effects increases with the increasing radiation exposure. The organ tissues that have the greatest risk for cancer formation are brain skin, and thyroid. The gonads however are at lower risk of developing cancer.

The incremental fatal cancer risk is estimated at 4% per Gy. unit. (15). Thus a 7.14 Gy unit exposure per year for 30 yrs will have an incremental risk of 0.6 % in addition to 20-22% incidence of cancer in the general population. Genetic effects caused by radiation have also been demonstrated in animal models . However genetic effects are yet to be observed in humans, even when exposed to relatively high levels of radiation.

METHODS OF REDUCING RADIATION EXPOSURE:

TIME

An average the procedure time for a diagnostic coronary angiogram is approximately 30 minutes and an interventional procedure PCI or EPS/pacing would take between 90 to 120 minutes. However the fluoroscopic and the cine screening time are highly variable depending on the nature of the procedure and the experience of the operator. The lower the amount of time spent in a radiation area, the lower the exposure will be. Significant reductions can be achieved when an activity is delayed until after cine imaging is completed. Every effort should be made by the operating cardiologist in the cath lab to minimise fluoroscopy and cine screening time.

DISTANCE

Increasing the distance from the radiation beam decreases the risk of exposure. doubling the distance between the primary beam and operator, reduces the exposure by a factor of four. In addition, the radiation exposure varies according to the angle at which the camera is projected Oblique views (left and right anterior oblique) and steep angulations increase radiation exposure but are often employed to improve visualisation. 60-degree angulations give up to three times the operator dose than 30-degree angulations (11). The second operator or assistant is generally less exposed to radiation compared to the first operator but certainly more at risk than the other staff in the room.

SHIELDING

Lead shields and shielding will significantly reduce the risk of exposure but only if appropriately used and in proper working order. Protective equipment includes lead aprons, thyroid collars and leaded glasses. With the newly designed frames and ultra light lenses, protective leaded eyewear is now used by more of the cardiologists and staff in cardiac cath lab. Some cath labs also use overhanging lead screens to prevent radiation exposure to brain. The staff should wear a protective apron of at least 0.25 mm lead equivalent. Protective gloves should be of at least 0.35 mm lead equivalent. All such protective clothing should bear an identifying mark and should be examined at yearly intervals. Defective items should be withdrawn from use.

ADHERING TO GUIDELINE AND PROTOCOLS

Every unit or work place that deals with ionising radiation should have their own local guidelines and rules for radiation safety. These must be read, understood and strictly adhered to in daily practice. Staff must comply with these local rules in order to insure that the Trust and all their employees do not contravene statutory requirements of the ionising radiation regulations and other relevant legislation.

MINIMISING RISK OF EXPOSURE TO STAFF AND PATIENTS:

The occupational limit of radiation exposure in the UK currently is estimated at 20 mSv per year averaged over five consecutive years (5). Every operator who undertakes a cardiovascular procedure in the cath lab is responsible for the amount of radiation exposure to the patient, his or her co-staff and to themselves. In the event of an incident where the patient might have been exposed to inadvertent excess radiation either due to clinical circumstances, malfunctioning of the equipment or operation errors, the radiation protection adviser should be informed of the incident. It is their duty to estimate the radiation dose received by the patient and also advise whether the incident is to be reported.

Only essential staff shall be in the cath lab during radiation exposure. All persons not required in the room should leave the room during serial radiographic exposure. The operator shall stand behind a barrier if possible. People who must move around the room during the procedure should wear a wraparound protective garment. When possible, the cardiologist and all other personnel required in the room should step back from the table and behind portable shields during cine and serial radiography procedures. This action can decrease the exposure of the cardiologist and the other nearby personnel by a factor of three or more (10).

An investigation will be taken when any employee records a cumulative whole total dose during the year which exceeds:

2.0 mSv – Interventional Cardiologist and Radiologist
1.5 mSv – Conventional Radiology
1.0 mSv - Radiographers
0.5 mSv – Non radiographic /Radiological staff (e.g.nurses, students)

A record of the investigation should be kept for at least 2 years. The investigation level for eye and extremity doses will be 15 mSv and 50 mSv respectively(9)

There should be adequate guidelines provided to the users of angiography and cardiology cath lab fluoroscopic x-ray equipment that satisfies the regulatory standards under clinical use conditions. The radiation safety department should review exposures on a regular basis and should be in accordance with the ALARA (As Low As Reasonably Achievable) policy and procedure.

X-ray Equipment Performance and Calibration:

The following requirements are necessary:

Adequate total filtration is present.
The fluoroscopy timer terminates the exposure or produces an audible signal at the end of a five-minute accumulative time interval.
During fluoroscopy, x-ray field collimation and alignment with the image intensifier is appropriate.
Fluoroscopic exposure rates do not exceed the regulatory standards.
Patient exposure information has been obtained for the simulated clinical conditions and is posted where it is readily available to the physician during the fluoroscopic procedure.

PREGNANCY AND RADIATION

As a general rule, the sensitivity of a tissue to radiation is directly proportional to its rate of proliferation. Therefore, one could infer that the human foetus, because of its rapid progression from a single cell to a formed organism in nine months, is more sensitive to radiation than the adult .This inference is supported by the results of experiments in animal models, and the experience with human populations that have been exposed to very high doses of radiation (atomic bombing victims). In humans, the major deleterious effects on the foetus include foetal wastage (miscarriage), teratogenicity (birth defects), mental retardation, intrauterine growth retardation and the induction of cancers such as leukaemia that appear in childhood. Birth defects and mental retardation are the adverse effects that are of the most immediate concern for the expectant mothers. Fortunately, not all exposures to ionising radiation result in these outcomes. The risk to the foetus is a function of both gestational age at exposure and the radiation exposure.
The threshold for childhood cancer induction is not clear cut. Despite these uncertainties in the dose –effect relationship, some broad generalizations based on fetal dose ranges may be made.
Fetal Dose less than 0.01 Gy unit – There is no evidence supporting the increased incidence of any deleterious developmental effects on the fetus.
Fetal Dose between 0.01 Gy unit and 0.1 Gy unit – The additional risk of gross congenital malformations, mental retardation, intrauterine growth retardation and childhood cancer is believed to be low compared to the baseline risk.
Fetal Dose exceeding 0.1 Gy unit – The lower limits for threshold doses for effects such as mental retardation and diminished IQ and school performance fall within this range.(16)
Most countries have limits on the annual amount of occupational radiation exposure a pregnant woman can receive and the amount the foetus can receive. In the UK radiation dose limits are 200 mGy and 500 mGy between 1-5 weeks and 5-7 weeks respectively. Women are strongly encouraged to declare their pregnancy with the Radiation Safety officer as soon as possible. A dosimeter can then be issued to such employees to monitor foetal dose. This should be worn at the waist, under the lead apron. Thus the Radiation Safety Officer and the mother can keep track of the foetal dose throughout pregnancy.

The pregnant staff should generally minimise their time spent in the cath lab and should ideally try to stand in the room where the possibility of radiation exposure is minimum.

Pregnant staff often has two major complaints:

The lead aprons are heavy. That is why many cath staff members now use lead vests along with lead skirts – by distributing the weight better, the vest /skirt apron combination causes less back strain.
As the foetus grows so does the women’s body. The available sizes of vests and skirts are often limited. So the pregnant women often use the single piece, non-wrap around style used regularly in x-ray areas.

Again, although doses to the cath lab personnel are generally high, they may be minimised by practicing ALARA. By minimising the dose to the mother, the dose and the risk to the foetus will also be minimised.
Counselling the Pregnant Patient Exposed to Ionising Radiation:
It is essential that pregnant staff or patient who has been accidentally exposed to excess radiation is adequately counselled and the following parameters should be considered in the evaluation:

Gestational age at the time of exposure.
.Calculation of fetal exposure using dose reconstruction techniques.
Maternal age
Other potentially harmful environmental factors ( malnutrition, smoking)
Attitude of mother toward pregnancy.

RADIATION SAFETY- European Guidelines and Regulations

The basic requirement for equipment based elements of medical radiation protection is found in the Medical Device Directive (EUROPEAN DIRECTIVE 93/4 1993)
Human exposure to ionising radiation is based on the EURATOM Directive (European directives 96/29 1976 and 97/43 1997).The results allow regional deviation for the purpose of increasing safety. Thus, there are different regulations in different European countries.
There are requirements on equipment, training and special procedures (including interventional fluoroscopy and cine)
All doses due to medical exposure shall be kept as low as reasonably achievable consistent with obtaining the required diagnostic information, taking into account economic and social factors.
‘Member states’ shall promote the establishment and use of diagnostic reference levels for radio diagnostic examinations .Diagnostic reference levels are expected not to exceed for standard procedures when good and normal practice regarding diagnostic and technical performance is applied.
Interventional procedures are viewed as too dependant on individual patient situations to be managed by reference levels

SUMMARY AND DISCUSSION:

The last two decades have seen a continuous increase in the frequency of diagnostic and interventional cardiac catheterization procedures. It is paramount that radiation protection in the cath lab must be a matter of primary concern. In addition to this, interventional cardiologists must realise that their patients are becoming increasingly aware and concerned about radiation hazards acquired during interventional procedures.
Strict measures should be taken to avoid any unnecessary radiation exposure not only to medical staff but also to patients. Local guidelines and precautions to prevent radiation hazard should be adhered to. As we know that the effects of radiation exposure are not apparent immediately but long term consequences can be serious, it is therefore desired that the radiation exposure dose is strictly monitored and the equipments as well as the shielding materials are examined from time to time.
Education on radiation hazard, safety and its prevention is badly needed. Training and awareness in this direction is equally important.

References

1. ’On Health Protection Of Individuals Against the Dangers of Ionising Radiation in Relation to Medical Exposure’ CEC Council Directive 97/43/Euratom,Euratom Amtsblatt,30 June 1997, L 180 pp. 22-27
2.D Hart, D C Jones and B F Wall (1994), ‘’ Estimation of Effective Dose in Diagnostic Radiology from Enterance surface Dose And Dose Area Product Measurements’’ National Radiological Protection Board (NRBP-R 262),HMSO publications centre, London pp 1-57
3.K C Leung and C J Martin, Effective Doses for Coronary Angiography’ British Journal Of Radiology, 69(1996),pp 426-431ath
4.D M Bakalyar , MD Castellani and R D Saffian ,Radiation Exposure to Patients Undergoing Diagnostic and Interventional Cardiac Catheterization Procedures ‘
Catheterization and Cardiovascular Diagnosis ,42 (1997), pp 121-125 (Comments ,pp .126-129)
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pp 634-639
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9. Radiation safety within the department of Radiology –Ionising Radiation Regulations 1999, Version:2/03 March 2003 ,Royal Liverpool &Broadgreen University Hospitals NHS Trust.
10.Radiation exposure to patients and operators during diagnostic cardiac catheterization and coronary angioplasty.
Zoretto M, Bernardi G, Morocutti G, Fontanelli A. Cathet cardiovas Diagn :1997 Apr; 40 (4):348-51
11.Patient Radiation doses during cardiac catheterization procedures. Beston S, Efstathopolous EP, Katritsis D,Faulkner K,Panayiotakis G. Br.J. Radiol. 1998 Jun:71 (846):634-9
12.Radiation protection in cardiac radiology in recent imaging and intervention in cardiovascular disease ;Sharma S, Springer- Verlay; Singapore 1996 ; 117-122
13Radiation exposure in invasive cardiology – A continuing challenge for cardiologists, industry and control organs. Knon E , Kaye A. Business Briefing of European Pharmacotherapy 2003.
14. Radiation safety in Cardiology; Limacher et. al. JACC vol. 31, No. 4; March 15, 1998: 892-913
15.www.saintlukeshealthsystem.org/slhs/com/slh/Radiology/Radiation
16. www.safety.duke.edu/RadSafety/fdose/fdrisk.asp

First Published October 2006

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