Radiation safety in interventional nephrology

Abstract

Fluoroscopic imaging is widely utilised for diagnostic and therapeutic procedures and is fundamental to the establishment and maintenance of dialysis vascular access. To optimise outcomes and avoid injury to patients and healthcare providers, radiation technology must be applied effectively and safely in clinical practice. Radiation safety may be overlooked by nephrology training curricula. This narrative review discusses the theoretical and practical principles of radiation management in fluoroscopy-guided procedures and is intended as a primer for trainees and nephrologists working in interventional settings.

Key Words: Interventional nephrology; Radiation; Fluoroscopy; X-ray; Image-guided; Procedural

Core Tip: Image-guided procedures are a core of interventional nephrology practice. Safe and appropriate utilisation of ionising radiation is essential. Radiation management is not widely incorporated into training curricula. This review summarises the principles and practice of procedural fluoroscopic-guidance for the nephrologist.

INTRODUCTION

Over 8 million interventional radiologic procedures are performed in the United States every year[1]. Radiologic guidance improves a range of procedural outcomes; however, such techniques expose patients and providers to substantial amounts of radiation. Although cumulative health effects of occupational ionising radiation exposure can be serious, appropriate implementation reduces the risk of radiation-induced injury significantly.

Training pathways for specialties that involve fluoroscopic interventions, such as radiology, cardiology, and surgery, typically mandate competence in radiation management. Conversely, formal training in image-guided procedures is lacking from most nephrology programmes. Nephrologists are responsible for dialysis access in many centres. Nephrologists may utilise X-ray guidance for haemodialysis (HD) catheter insertion, percutaneous peritoneal dialysis catheter insertion, and endovascular arteriovenous (AV) fistula interventions. To optimise clinical outcomes as well as the safety of patients and clinicians, interventional nephrologists must possess a sound knowledge of radiation management principles and be able to effectively operate X-ray equipment.

The following discussion is aimed at providing a practical overview for physicians learning and performing interventional nephrology procedures. The article forms an educational resource and is not intended to supplant dedicated instruction. It reviews the physics of X-ray radiation, the adverse health effects of excess exposure, the equipment that is commonly found in the fluoroscopy suite, and the relevance of dialysis access interventions.

RADIATION PHYSICS

Radiation is defined as the movement of electromagnetic energy in the form of waves or high-speed particles[2]. The electromagnetic spectrum comprises different types of radiation which vary in frequency, wavelength and energy. Ionising radiation carries enough energy to detach electrons from atoms to form ions, giving it the potential to disrupt molecular bonds and alter structures. Non-ionising radiation lacks the necessary energy to ionise atoms. Ionising electromagnetic radiation, such as X-ray and computed tomography (CT), can injure biological tissues, whereas non-ionising sources, such as ultrasound, are considered less hazardous.

X-rays are commonly utilised for medical and scientific applications. X-ray radiation is characterised by high energy. X-rays can be used to generate images because of variegated penetration through the human body, in which the beam is attenuated more by some tissues than others. Approximately 5% of incident X-ray photons pass through the patient and are received by the detector, leaving most to be absorbed by the patient or scattered. The interaction between an X-ray beam and tissue surfaces is an example of a phenomenon known as Compton scattering. In a medical X-ray machine, electrons are emitted from the negatively charged cathode of an X-ray tube and received by the positively charged anode of the detector after passing through the patient.

Radiation dose is defined as the local concentration of energy from the radiation field that interacts with matter. Dose is quantified in a range of units. The absorbed radiation per unit mass is measured in grays (Gy) while the impact of this absorption on biologic matter is measured in Sieverts (Sv). Radiation dose can be difficult to estimate precisely in practice because exposure depends on patient, technical and operator factors. Dose is generally estimated based on calibration testing[3]. Though there are small theoretical differences, 1 Gy is approximately equivalent to 1 Sv in clinical terms.

SOURCES OF RADIATION

The human population is perennially exposed to radiation from natural and unnatural sources. Natural sources account for approximately 80% of overall exposure[4], predominantly through the natural decay of organic and mineral earth elements. This natural radiation dose is between 1-3 mSv annually per person. The healthcare industry is a major contributor to man-made radiation exposure. Patients undergoing repeated diagnostic imaging tests or radiologic interventions are subjected to a significantly higher overall radiation dose than the general population[5]. In a study of maintenance HD patients at a single centre in Ireland, the median healthcare-related ionising radiation dose was 7 mSv per year[6]. Most of the burden originated from CT scanning and a smaller amount resulted from interventional procedures.

Occupational radiation exposure has not significantly decreased over the recent decades despite growing experience and proficiency with the use of medical radiation[7]. This is because technological advances have been matched by rising case complexity and frequency. Steps to minimise radiation dose in the interventional suite are therefore increasingly important. The radiation doses associated with common procedures are listed in Table 1. To provide context, a passenger’s effective radiation dose during a 20-hour international aeroplane flight is approximately 0.06 mSv[8,9].

Table 1 Representative radiation doses received by patients and providers (with standard shielding garments) during common radiologic procedures.

Intervention	Patient radiation dose (mSv)	Typical screening time (minute)	Primary operator radiation dose (mSv)
Diagnostic
Chest X-ray	0.02
Barium swallow	2
CT pulmonary angiogram	8
CT coronary angiogram	1
Interventional
Coronary angiogram	6	2	0.003
Percutaneous coronary intervention	15	10	0.006
Nephrostom	3	3	0.006
Percutaneous nephrolithotomy	4	2	0.013
Neurologic embolisation	40	30	0.02
Peripheral revascularisation	20	15	0.02
Dialysis access
Tunnelled haemodialysis catheter	0.1	1	0.0005
Percutaneous tenckhoff catheter	0.1	1	0.0005
Arteriovenous fistuloplasty	2	4	0.005

CT: Computed tomography.

Interventionalists are exposed to small levels of ionising radiation with every procedure carried out under X-ray guidance. Occupational doses occur in addition to the clinician’s background exposure to natural radiation. Interventional cardiologists, for example, are subjected to occupational radiation doses of 1-4 mSv each year[2,10]. The estimated annual dose exposure for a vascular surgeon performing around 150 endovascular cases per year is 1 mSv[11], compared to 0.4 mSv per year for an interventional nephrologist undertaking 200 fluoroscopic cases annually[12]. By comparison, a small study of intensive care medicine registrars demonstrated an average annual work-related radiation dose of less than 0.4 mSv[13].

The primary operator receives the highest radiation dose in the interventional laboratory after the patient. Procedures are still performed safely because the effective dose is greatly reduced by adequate personal protections. For instance, during a typical diagnostic coronary angiogram the patient and cardiologist doses are around 6mSv and 0.003 mSv, respectively[14,15]. Similarly, in an endovascular aortic aneurysm repair the expected patient and surgeon doses approximate 12 mSv and 0.02 mSv, respectively[16,17]. A primary operator must therefore perform many hundreds of endovascular interventions per year to exceed legislated maximum dose thesholds[11].

RADIATION INJURY

Ionising radiation can lead to cell mutation or death through free radical formation and disruption of chemical bonds. Tissue damage is referred to as radiation injury. Radiation injury may be an acute process from a single incident or may result after years of subclinical exposure. Understanding of the relationship between radiation dose and clinical adverse effects is predominantly extrapolated from its use in the therapeutic radiotherapy setting, and from laboratory data and observations of survivors of detonated atomic weapons[18].

The most important tenet of radiation management is to minimise harm. Radiation injury is rare in contemporary medical settings with judicious use. While the biologic consequences of a single X-ray dose are generally insignificant, cumulative harm from repeated peri-procedural exposure is more concerning. Radiation injury may be broadly classified as either a stochastic or deterministic effect. Deterministic effects are dose-related and of predictable severity, such as skin damage. Stochastic effects have an incidence that is proportionate to exposure but a severity that is unpredictable, such as cancer.

Manifestations are diverse, including cutaneous and ocular reactions, myelosuppression, infertility and foetal malformation, pulmonary and gastrointestinal toxicity, and cancer. Acute deterministic reactions such as skin lesions and marrow suppression may occur following single a radiation dose of 500 mSv. This level of exposure is rare and requires at least 60 minutes of fluoroscopy time[19,20]. A whole-body dose of 10 Sv, as encountered by people in the epicentre of the Hiroshima nuclear bombing, is rapidly fatal through multi-organ failure and brain injury.

An important deterministic radiation side effect is cataract formation. The lens is a radiosensitive structure. Epidemiologic studies demonstrate a higher likelihood of cataracts with exposures roughly exceeding 15 mSv annually for 5 years[21,22]. The incidence of cataracts has fallen with the uptake of proper eye protection[23,24]. Cataract rates among interventionalists appear to be 2-5 times greater than matched controls[25,26].

Radiation exposure is associated with an increased risk of cancer. Studies suggest a relatively linear trend between dose and cancer incidence; however, the exact relationship has been difficult to quantify. The lifetime probability of fatal cancer is approximately 5% and 1% for every 1000 mSv and 100 mSv of radiation exposure, respectively[27]. A coronary angiogram is thought to raise a patient’s cancer risk by 0.05%[28]. The cancer risk incurred by the primary operator during an isolated interventional procedure is negligible. Studies of interventional cardiologists and radiologists observed a 0.4% increased likelihood of cancer death[29,30]. However, there does not appear to be an increased rate of all-cause mortality[31].

Attentive radiation management is particularly important in paediatric patients, who are more vulnerable to injury than adults. A child’s small body size lowers the safe X-ray dose limits. Radio-sensitivity is also increased in tissues that are vital to the maturing body, such as the thyroid, gonads and marrow. Finally, a child’s greater anticipated lifespan after X-ray exposure allows are longer interval for the development of side effects such as cancer.

The management of radiation injury depends on the chronicity and magnitude of exposure. The treatment of acute deterministic tissue toxicities, such as those which may result from massive X-ray overuse or sudden exposure to a machine malfunction, is largely supportive. Longer-term specific sequelae such as cancer are managed per usual therapeutic approaches.

PRACTICAL APPLICATION OF FLUOROSCOPY EQUIPMENT

Fluoroscopy is the use of a continuous X-ray beam to generate real-time images and is a cornerstone of interventional nephrology practice. Fluoroscopy is indicated if it is determined that the benefits of X-ray guidance outweigh the potential risks and if a suitable non-ionising imaging alternative is lacking. Fluoroscopy should be utilised with the precept that radiation dose is to be kept ‘as low as reasonably achievable’ (ALARA). X-ray techniques require a balance between augmenting procedural success and avoiding ionising radiation-induced harm. The major practical determinants of radiation dose are time, distance and shielding. Each variable can be modified by the primary operator to achieve exposure that is ALARA. Increased operator experience is closely correlated with more efficient fluoroscopy utilisation and lower radiation doses[32].

Room setup

The fluoroscopy suite is a specialised room that accommodates procedures performed using an X-ray system and is equipped with protective features (Figure 1). Room specifications are regulated by healthcare facility construction guidelines. The suite must be large enough to accommodate necessary equipment and to ensure an ergonomic workspace for the procedural team around the patient.

Figure 1 The fluoroscopy suite accommodates procedures performed using an X-ray system and is equipped with protective features. A: Positioning of the proceduralist relative to X-ray equipment; B: Floorplan of a typical fluoroscopy suite. 1: C-Arm; 2: Surgical table; 3: Primary proceduralist; 4: Setup trolley; 5: Portable shield; 6: Ultrasound machine; 7: Display monitor; 8: Wash basin; 9: Control room; 10: Doorways; 11: Equipment cupboard.

The primary operator is responsible for managing intra-procedural radiation in the fluoroscopy suite (Table 2). The proceduralist may choose to handle the apparatus independently or may prefer to instruct and be assisted by a radiographer.

Table 2 Key components of a fluoroscopy suite.

Fittings	Description
Operating table	The patient is placed on the fluoroscopy table. The fluoroscopy table is radiolucent and lightweight to enable X-ray penetration
C-arm	A movable C-shaped semi-circular frame, usually mounted on a gantry, which girdles the patient. The X-ray tube is located on the lower arm and the flat panel detector and image intensifier are located on the top arm
Display monitor	Television or computer screens located within the suite which display fluoroscopic images for the proceduralist. Film was historically used but has been replaced by digital picture archiving and communication systems
Control console	Buttons and dials control various parameters, such as distance or acquisition. Many settings are computer-automated, which is often more efficient than manual controls
Shielding	Radiation shields skirt the underside of the operating table and are embedded within the external room walls. Transparent mobile protective shields can be mounted on wheels or fixed to the ceiling
Consumables	It is preferable that instruments, such as wires, catheters, and a portable ultrasound machine are easily accessible in or near the room. This prevents radiation breaches and maintains sterility

Time

Radiation dose is directly proportional to time and energy. Prolonged screening is accompanied by higher chances of injury. Judicious direct screening and replacing screening with periodic single shots or ultrasound can help in ensuring that ionising radiation dosing is ALARA. The frame rate, which is the number of X-ray images captured per second, determines temporal resolution. Temporal resolution describes the sensitivity of feedback in time. The energy emitted per pulse determines spatial resolution, which describes image clarity. Pulse energy is usually measured in nano-grays.

The dosing protocol of standard fluoroscopy is gentler than cine acquisition mode or digital subtraction angiography. The lower temporal and spatial resolution of standard fluoroscopy should thus be preferred where feasible. Halving the frame rate from 30 fps to 15 fps, for instance, will also essentially halve the screening time and radiation dose. Slower frame rates are adequate for simpler tasks such as catheter insertion. A frame rate such as 5 fps is typically suitable for catheter insertion whereas a rate of at least 15 fps is usually indicated for more complex endovascular procedures. Devices such as catheters are frequently impregnated with trace amounts of radio-opaque materials like barium or tungsten to improve visualisation.

Distance

Radiation dose is inversely proportional to distance. According to the inverse square law, doubling the distance from the radiation source results in a quartering of the effective dose. Separation between the X-ray tube and the image intensifier should be minimised. Ideally, the radiation source should be further from the patient than the detector to reduce divergence. The primary operator’s effective dose is reduced by up to 50% by standing back from the X-ray source by 50 cm[7]. Other staff in the fluoroscopy suite can reduce their radiation exposure by 80% by maintaining a distance of at least 2 m from the operating table[33,34].

Dose is also proportional to the imaged area, in a relationship known as the dose-area product. Collimation refers to adjustment of the line of sight of the X-ray beam. Because the product of dose multiplied by area is constant, collimating to a small target area results in greater image resolution but also an increased delivered dose. Fluoroscopic images should be zoomed out as far as practicable. Scattering is diminished by adequate collimation and by utilising a vertical orientation as opposed to oblique. Avoidance of scattering enables greater picture quality at lower energy.

Shielding

Shielding and personal protective garments are simple and effective ways for staff in the interventional suite to reduce their radiation exposure. Some degree of shielding is mandatory in all facilities with radiation equipment. Materials that are commonly used to prevent X-ray penetration include lead, leaded plastic, bismuth, and tin. The walls of the fluoroscopy suite are proofed to contain radioactivity. Barriers and screens are usually installed to protect staff within the room. Protective clothing should be worn by all staff within the fluoroscopy suite. The biggest source of X-ray exposure to theatre staff is scatter rebounding directly off the patient. Personal shields are particularly advisable for the primary operator. As described earlier, effective shielding can reduce primary operator exposure by up to 1000 times relative to the patient dose[27,35].

Leaded gowns, aprons and thyroid collars each block more than 90% of local dose penetration. Protective gloves and head caps are controversial and are uncommonly worn. Although the operator’s hands are exposed to relatively high levels of radiation, there is little evidence that this occurrence leads to harm[36]. Movable transparent shields inhibit penetration by a further 85%; these shields may be mounted from the ceiling or may be portable on wheels.

Leaded glasses reduce eye exposure by approximately 50%[30,37]. Glasses are somewhat ineffectual because penetration still occurs around the sides of the frames. Leaded glasses are recommended for physicians practicing more than 20 minutes of fluoroscopy screening time per week, principally to prevent cataracts. Glasses may be particularly relevant to radiologists and cardiologists rather than surgeons and interventional nephrologists whose case volumes are smaller.

All staff working in the interventional suite should have their radiation exposure monitored. High-volume proceduralists are generally offered personalised dosimeters. These small metered devices are attached to the outside of the lead gown. Recommendations for pregnant clinicians are not significantly different to general shielding advice[38], although particular care is prudent.

FLUOROSCOPIC PROCEDURES IN NEPHROLOGY

Dialysis access practices vary at local and regional levels. The prevalence and patterns of interventional nephrology do not appear to depend on country income status. However, procedural work may be particularly relevant for nephrologists in suburban compared with urban areas[39]. The main procedures performed by nephrologists are non-tunnelled CVCs and kidney biopsies, which do not need fluoroscopic guidance. The number of centres offering a full interventional nephrology service remains restricted but is steadily growing[40,41].

HD catheters and peritoneal dialysis catheters are frequently placed under fluoroscopic control, which is helpful to ascertain guidewire placement, for serial dilation, for contrast studies, and to confirm final catheter positioning. Tunnelled dialysis catheterisation is a relatively short intervention with limited X-ray screening time. In one study of tunnelled central venous catheter placements, the median radiologist exposure was 0.0001 mSv per procedure[42].

Image-guided endovascular interventions for dysfunctional AV access include diagnostic fistulogram, fistula angioplasty, thrombectomy, and stenting. In these procedures, continuous screening is required to facilitate safe navigation of instruments through complex vascular anatomy. Angiography is performed by injecting radio-contrast with real-time fluoroscopic visualisation. Endovascular AV access interventions are intricate and incur a moderate radiation dose, although specific exposure data are lacking. Inferences can be drawn from similar technics in other disciplines. For example, in a systematic review of 14 studies reporting radiation management in orthopaedic surgery, the median operator dose was 0.009 mSv per case[43]. The cases were characterised by a median fluoroscopy time of 4 minutes and primary operators would be expected to stand at similar distances from the patient as in nephrology interventions.

Radiation utilisation should be part of an informed consent process with patients. It has been suggested that alternative image-guided techniques may minimise radiation exposure in fluoroscopic dialysis access interventions. For example, tunnelled HD catheter insertion can also be effectively performed using only ultrasound guidance or with intermittent plain radiographs. However, comparative evidence for the best use of image guidance during interventional nephrology procedures is lacking and fluoroscopic control generally remains the current standard of care.

TRAINING AND CERTIFICATION

Most nephrology training pathways do not provide dedicated education in radiation management. An interventional nephrologist must have proficiency with the fluoroscopy apparatus. Didactic teaching and simulations in radiation management are included as required learning in the process to become a specialist in other specialties such as cardiology and surgery. Medical radiation courses held by local colleges and hospitals are often available to interested nephrologists. Formal certification and licenses are necessary for practitioners to operate fluoroscopy equipment in some centres. Nephrologists should be aware of their institution’s policies.

Radiation safety is tightly regulated. Hospital facilities maintain standards in accordance with health department and government legislation. Regulations vary across jurisdictions. The Australian Radiation Protection and Nuclear Safety Agency mandates an occupational exposure to ionising radiation of less than 20 mSv per year[44]. In Europe, conversely, occupational exposure must generally be lower than 100mSv every 5 years with no single year exceeding 50 mSv[38].

CONCLUSION

Procedures performed with X-ray guidance are a mainstay of interventional nephrology practice. Interventional nephrologists should possess a sound understanding of the principles and operation of fluoroscopic equipment. To balance technical outcomes with patient safety, radiation must be utilised in doses that are ALARA. The incidence of radiation injury is minimised with appropriate protective measures.