First published online November 26, 2007
British Journal of Radiology (2008) 81, 35-45
© 2008 British Institute of Radiology
doi: 10.1259/bjr/26551424
An investigation into patient and staff doses from X-ray angiography during coronary interventional procedures
O W E Morrish, BSc, MSc and
K E Goldstone, BSc, MSc, CSci, FIPEM
East Anglian Regional Radiation Protection Service, Box 191, Cambridge University Hospitals NHS Foundation Trust, Hills Road, Cambridge CB2 2QQ, UK
Correspondence: Mr Oliver W E Morrish, Radiation Protection Physicist, East Anglian Regional Radiation Protection Service, Cambridge University Hospitals NHS Foundation Trust, Box 191, Hills Road, Cambridge, Cambs CB2 2QQ, UK. E-mail: oliver.morrish{at}addenbrookes.nhs.uk
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Abstract
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Radiation doses to patients from interventional coronary X-ray procedures are relatively high when compared with conventional radiographic procedures. These high patient doses can translate into high staff doses owing to scattered radiation. This study investigates patient doses by means of dose–area product (DAP) meters installed in six rooms in two hospitals. DAP measurements in each room ranged from 28.0–39.3 Gy cm2 for coronary angiography and from 61.3–92.8 Gy cm2 for percutaneous transluminal coronary angioplasty, with the mean effective doses calculated to range between 5.1–6.6 mSv and 11.2–17.0 mSv, respectively. These values are comparable with those found in recent literature. DAP measurements were found to correlate strongly (correlation coefficient of 79%) with patient weight. The non-uniform scatter radiation fields surrounding the irradiated area during coronary angiography were also investigated using a tissue equivalent phantom and an ionization chamber. Exposure rates of scattered radiation from digital acquisition were found to be around 16 times higher than those generated from fluoroscopy, and oblique-angled imaging led to greater amounts of scatter owing to the increase in related exposure factors. The distribution of scatter from oblique projections confirms that X-ray photons in the diagnostic energy range are preferentially scattered backwards, toward the X-ray tube. These concepts are a major consideration when training individuals working in the angiography suite in order to keep doses "as low as reasonably practicable".
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Introduction
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X-ray angiography is a technique used to image the arterial system of the body. Coronary angiography plays an important role in the diagnosis and treatment of vascular disease and conditions that can lead to heart attack and stroke. The patient is injected with contrast media through a catheter and the blood vessels in the anatomical region of interest are then highlighted on a sequence of radiographical images. Coronary angiography can be used to detect the narrowing of coronary arteries. As such, it may be performed if the patient is suffering from symptoms of unstable angina, chest pain, aortic stenosis or unexplained heart failure. Following its detection, an obstruction can be opened by percutaneous transluminal coronary angioplasty (PTCA), whereby a balloon catheter is used to open the narrow vessels. A stent can then be inserted to keep the vessel open.
In modern practice, some of these procedures can last up to an hour or more, especially where a combination of interventions are carried out on the same patient during a single session. This can result in high radiation doses to the patient and to those staff working close by. Coronary interventional procedures can generate highly localized doses to the skin of patients, which may be above the threshold for deterministic injuries as well as carrying an increased risk of cancer induction. Without due care and understanding, multiple procedures could lead to serious injury. This highlights the need to optimize the imaging equipment used during angiography and to properly use any dose saving techniques. The training of staff working in the vicinity of X-ray equipment is also of paramount importance and provides the motivation for this study.
Staff doses are linked to patient doses because they result from secondary scattered radiation arising mainly from the patient. Staff may also be exposed to primary leakage radiation that is generated at the X-ray target and which has penetrated the leaded X-ray tube housing.
Efstathopoulos et al [1] measured mean staff doses during coronary angiography and intervention with the use of thermoluminescent dosemeters (TLDs) during 40 consecutive procedures. They estimated effective doses from coronary angiography and coronary angioplasties to be 0.04–0.05 mSv year–1 for the primary operator, and 0.03–0.04 mSv year–1 for assisting operators during their annual workload (240 procedures). Estimates of the effective dose to other staff in the room were below detectable levels. The same study gave a mean effective dose for patients undergoing a single procedure of 5±2 mSv for coronary angiography and 14±6 mSv for coronary angioplasties. Staff doses to the primary and assisting operators from a single procedure are approximately four or five orders of magnitude lower than patient doses.
The complex and attentive nature of coronary interventional procedures requires a range of professional groups to be involved, each having a role to perform. The Ionising Radiations Regulations 1999 [2] require that measures are taken to minimize the radiation dose received by those working in a radiation environment. This is normally achieved by ensuring that those persons working within "Controlled Areas" are adequately trained in matters relating to radiation protection. For some of these groups (e.g. cardiologists and radiographers), training in such matters forms a significant part of their basic training. Others, e.g. cardiac technicians or nurses for whom the main component of their professional qualifications has not been radiation related, might have to rely on mandatory training to receive such knowledge. Additionally, the Ionising Radiation (Medical Exposure) Regulations 2000 [3] require that the doses received by patients are kept "as low as reasonably practicable" and that special attention is paid to "high dose" procedures. For this reason, doses to both patients and staff from coronary interventional procedures should be monitored.
As part of the UK National Service Framework for Coronary Heart Disease (CHD), investment has been made to try to reduce the incidence of CHD. From June 2002 to the beginning of 2006, there have been 72 new catheterization laboratories installed and 17 more have been replaced [4]. Additionally, there was an increase of 47% in the number of cardiologists between September 1999 and March 2004.
The British Heart Foundation's 2006 report on coronary heart disease statistics [5] indicates that death rates from CHD have been falling since the 1970s, and in the 10 years to 2003 there was a 44% reduction in deaths in England among those under 65 years old. In the same period, however, there has been an increase in its prevalence from 6.0% to 7.4% in men, and from 4.1% to 4.5% in women. This has led to an increase in X-ray angiography carried out in the diagnosis and treatment of CHD. For example, the number of percutaneous coronary interventions carried out in the UK in the 10 years leading up to 2004 has quadrupled, and has increased by 21% between 2003 and 2004 alone.
Angiography and interventional procedures account for 16% of the collective effective dose attributable to X-ray examinations of the UK population each year. These same procedures account for only 2% of the number of X-ray examinations [6]. These figures demonstrate that the individual dose received from one of these procedures is much higher than doses received during conventional radiographical examinations. Numerous studies list patient and staff doses from these procedures, reflecting the concern that they generate. 
Tables 1 and 2 illustrate the results of dose audits that have been published in recent years; as can be seen, the range of these doses is wide.
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Table 2. Mean radiation exposure to patients undergoing percutaneous transluminal coronary angioplasty published between 1995 and 2004
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This study aimed to investigate the nature and magnitude of radiation doses from coronary interventional procedures to staff and patients, so aiding the provision of advice on dose reduction techniques.
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Methods and materials
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This work is presented in two parts: the first part discusses the scattered dose distribution surrounding a patient during a simulated angiographic procedure; the second forms an analysis of a dose audit of coronary angiograms and PTCAs in two cardiology departments.
Scatter measurements
The chest section (slices 11–23) of an Alderson Rando-Phantom (Alderson Research Laboratories Inc., Stamford, CT) was used to simulate a patient undergoing coronary angiography (Figure 1). The phantom consists of a human skeleton contained within a resin that is radiologically equivalent to soft tissue. Although the phantom was originally designed for use at energies commonly found in radiotherapy, the phantom's linear attenuation coefficient is within 10% of tissue equivalence at diagnostic energies [19]. The procedure was carried out on a Philips Integris HM 3000 (Philips Medical Systems, Best, The Netherlands), installed with leaded protection in the form of ceiling-suspended eye shields on both sides of the couch and curtains also hanging off both sides. The ceiling-suspended eye shields are designed to protect the head area — specifically the lens of the eye — of the operator and are shaped to fit around the patient (see Figure 1 ) to ensure that their placement is as close to the source of scatter as possible. The lead curtains protect the operator's legs from scatter and are a feature of most radiology equipment with an under-couch X-ray tube and when the operator is likely to be standing next to the couch.
Before measuring the exposure from radiation scattered in the phantom, the automatic exposure control of the unit was assessed. All results were within 10% of those measured at the previous quality assurance visit, and it was accepted that the unit was operating within normal acceptable parameters as defined by the Institute of Physics and Engineering in Medicine's (IPEM) Report 77 [20] (since superseded by Report 91).
Radiology staff were consulted to obtain the appropriate projections and methods for carrying out a coronary angiogram, including placement of the available protective equipment. Each room has a procedure manual detailing the appropriate field sizes and modes that the radiographers are to use. In this case, the manual indicated the use of a 17 cm field size for digital acquisition, and a 23 cm field size for fluoroscopy. The collimators were positioned so that they were only just visible on the 17 cm field and were not moved for the 23 cm field. The collimators did not move between changes in field size, and therefore similar areas were irradiated by both field sizes at the 106 cm focus-to-intensifier distance used. The fluoroscopy was carried out on the automatic "low" dose mode at eight frames per second with added filtration which, according to specification, is 1 mm of aluminium and 0.4 mm of copper in addition to the permanent 2.5 mm of aluminium. The digital acquisition was carried out on the automatic "12.5 frames per second coronary" mode, which has no additional filtration.
A semi-transparent wedge filter, which is designed to equalize the image brightness in the field of view, is available for use at the discretion of the operator to improve visible image quality. In practice, the use of this filter tends to be ad hoc and was not used for the simulated procedure. The use of this filter would reduce patient radiation doses (including measured doses) and, to a certain extent, the magnitude of the scattered radiation. However, no attempt has been made in this work to quantify this reduction. It is not thought that its use would significantly affect the scatter distribution, as it intercepts only part of the field and, being wedge shaped, has the greatest effect at the periphery (less so towards the centre). It should be noted that the patient doses presented later were recorded at the exit of the X-ray tube housing, and therefore the effect of this filter would be taken into consideration if used.
Table 3 lists five projections commonly used in coronary angiography of the left coronary artery, the right coronary artery and the left ventricle (see also Figure 2). The phantom was set up on the couch without the mattress, with the centre 87 cm from the floor. The image intensifier was positioned 20 cm from its anterior surface and rotated using a fixed focus-to-intensifier distance of 106 cm. This meant that there was a slight variation in the intensifier-to-skin distance with the shape of the phantom in various positions.
An 1800 cm3 ionization chamber (Radcal Corporation, Monrovia, CA), suitable for measuring low dose rates with a calibration traceable to a national standard, was used to measure radiation exposure around the phantom during irradiation. Readings were corrected for temperature and pressure, and are accurate to within 4% between 33 kV and 1.33 MV according to the ion chamber's manufacturer specifications. Measurements were taken in three planes parallel with the floor, representing different parts of an operator's body: 60 cm from the floor for the knees, 120 cm for the waist, and 170 cm for the head. The ionization chamber was placed 1 m laterally from the unit's isocentre to measure exposure surrounding the phantom in each plane. Measurements were taken every 30°, with 0° being the head position, 90° being the left side of the patient and 270° being the right side (Figure 3). Chamber positions were marked out on the floor prior to measurements being taken. It was not possible to take readings at 90° or 180° because of the positions of the c-arm and the couch, respectively.
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Figure 3. Diagram showing the positions of the cardiologist and the nurse. The shown angles orientate the positions for the scattered exposure measurements.
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In the room in which scatter measurements were taken, the cardiologist stands at the right of the patient in the 210° position, and the assisting nurse stands to the left in the 150° position. The lead protection integral to the unit is designed to protect these two positions. Measurements were taken with and without these features in place. The left-hand-side under-couch lead curtain could not be removed and thus no scatter measurement was made in the unprotected 150° position at 60 cm from the floor.
Patient doses
Dose audits of coronary angiograms and PTCAs were carried out in five rooms across two hospitals. These audits were carried out by using dose–area product (DAP) meters with a correction factor applied, which was derived from inter-comparison in situ at 80 kVp against a meter with a calibration traceable to a national standard (as per the protocol detailed by the IPEM) [21]. The use of DAP measurements is a convenient way of collecting dose data, as most modern X-ray units have a DAP meter integrated into their design. DAP measurements are also useful when measuring doses from examinations such as angiography, in which several different projections of the patient are imaged, for giving an indication of the total energy absorbed by the patient. Conversely, TLDs are useful for determining radiation doses to the skin. When using TLDs in this type of exposure situation, the user has to be careful when placing them, and multiple TLDs need to be used to ensure coverage of the whole patient. The situation may be further complicated by TLDs measuring entrance doses in one projection and exit doses in another. TLDs have not been used in this study.
In one of the hospitals, the audit comprised retrospective data collected from the electronic patient record system in which details of procedure, room and DAP were recorded by radiology staff. These data covered a 6-month period from January 2004 to June 2004. In the second hospital, the data were collected by radiographers in one room over a 4-week period in June 2004 and July 2004.
The retrospective dose audit contains data from 3123 procedures. These were then sorted into four individual angiography rooms: Angios 1, 2, 3 and 4. Angio 2 underwent a refurbishment midway through the sampling period. The data for that room was further split into Angio 2 (old) and Angio 2 (new). The equipment installed in the rooms is listed in Table 4.
The other hospital had a single catheterization laboratory containing a Philips Integris H5000F (Philips Medical Systems). In this department, the prospective dose audit was carried out using a simple form that contained information on the date of procedure, procedure, height and weight of patient, average kVp and mA, number of digital acquisition runs, total "beam-on" time, DAP and any additional comments. Only data for coronary angiography were collected, as PTCA was not routinely carried out at this hospital. Patients undergoing typical procedures, i.e. complete cases without complications, were selected for inclusion by the radiographer. Data for 15 procedures were collected in this way.
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Results and discussion
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Scatter distribution
The effectiveness of the lead protection is demonstrated in Figures 4 and 5 where the results with and without the lead protection in the operator positions are shown. Results are presented for fluoroscopy, and dose rates measured under digital acquisition were found to be, on average,16 times higher that those shown here.
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Figure 4. Measurements during fluoroscopy at the 210° position (cardiologist's position) with and without lead protection. RAO, right anterior oblique; LAO, left anterior oblique; CA, caudal; CR, cranial.
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Figure 5. Measurements during fluoroscopy at the 150° position (nurse's position) with and without lead protection. RAO, right anterior oblique; LAO, left anterior oblique; CA, caudal; CR, cranial.
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The head and knee areas (170 cm and 60 cm from floor) at the cardiologist's position are the most protected, with a mean of 93% and 96% of the incident radiation, respectively, being attenuated by the relevant protective device. The efficacy of the protection reduces at waist height (120 cm from the floor) with a mean attenuation of 89%. This is likely to result from the fact that the protection comprises the ceiling-suspended eye shields and the couch-side lead curtain. Any gap between the two will result in higher doses to the operator. Such a gap arises owing to the need to fit the shield, as close as possible, around patients of various girth. The size of the gap will increase with patent size. The fact that there is any attenuation of the scatter here is probably a result of a partial shielding effect by the edge of the eye-shield, which reflects the operator's situation. In practice, the operators will be wearing lead aprons, which will further absorb any scatter that has not already been attenuated.
The operator will also be subjected to a small amount of penetrating primary leakage radiation from the X-ray tube. Such leakage is subject to regulatory limits, and quality assurance measurements quantify this leakage as being no greater than 0.15 µGy min–1 at 1 m from the focal spot in any direction; therefore, this is very small when compared with the scatter from the patient.
The projection is also important, as illustrated by the difference in the effectiveness of the protection in the cardiologist's position for the 30° right anterior oblique (RAO) 25° caudal projection shown in Figure 6. In this position, the intensifier is close to the cardiologist's head and the X-ray tube is opposite. This means that any scatter that has been generated in the direction of the cardiologist's head will have been scattered through a small angle; the X-ray photons will therefore have lost little of their initial energy and will be more penetrating than those scattered through a larger angle. However, the proportion of this scatter in the forward direction compared with that in the backward direction is small at diagnostic X-ray energies, as illustrated by the distributions shown in Figures 7–11.
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Figure 6. Mean effectiveness of lead protection at the 210° position (cardiologist's position). RAO, right anterior oblique; LAO, left anterior oblique; CA, caudal; CR, cranial.
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Figure 7. Distribution of scattered radiation from digital acquisition on the 45° left anterior oblique (LAO) projection at 72 kVp. The radial axis shows the dose in µGy min–1, whereas the ionization chamber position is indicated on the circumference. The figure shows the patient position from above, and the arrow shows the direction of the primary beam. Data for points not measured at 90°, 180° and 150° at 60 cm have been interpolated.
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Figure 8. Distribution of scattered radiation from digital acquisition on the 10° right anterior oblique (RAO) projection at 68 kVp. The radial axis shows the dose in µGy min–1, whereas the ionization chamber position is indicated on the circumference. The figure shows the patient position from above, and the arrow shows the direction of the primary beam. Data for points not measured at 90°, 180° and 150° at 60 cm have been interpolated.
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Figure 9. Distribution of scattered radiation from digital acquisition on the 30° right anterior oblique (RAO) projection at 68 kVp. The radial axis shows the dose in µGy min–1, whereas the ionization chamber position is indicated on the circumference. The figure shows the patient position from above, and the arrow shows the direction of the primary beam. Data for points not measured at 90°, 180° and 150° at 60 cm have been interpolated.
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Figure 10. Distribution of scattered radiation from digital acquisition on the 30° right anterior oblique (RAO) 25° caudal projection at 69 kVp. The radial axis shows the dose in µGy min–1, whereas the ionization chamber position is indicated on the circumference. The figure shows the patient position from above, and the arrow shows the direction of the primary beam. Data for points not measured at 90°, 180° and 150° at 60 cm have been interpolated.
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Figure 11. Distribution of scattered radiation from digital acquisition on the 45° left anterior oblique (LAO) 25° cranial projection at 75 kVp. The radial axis shows the dose in µGy min–1, whereas the ionization chamber position is indicated on the circumference. The figure shows the patient position from above, and the arrow shows the direction of the primary beam. Data for points not measured at 90°, 180° and 150° at 60 cm have been interpolated.
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Under automatic control of exposure factors, the X-ray equipment selected a higher kVp for fluoroscopy than for digital acquisitions, with means across all projections of 78.6 kVp and 70.2 kVp, respectively. This results from the selection of different exposure protocols programmed into the system for each of the two modes of operation. This difference in kVp increased with the greater exposure factors required for the more oblique projections. For the 45° left anterior oblique (LAO) 25° cranial projection, the unit delivered 86 kVp for fluoroscopy and 75 kVp for acquisitions whereas, for the 10° and 30° RAO projections, the unit delivered 75 kVp and 68 kVp, respectively. Our results indicate that this difference in kVp had little effect on the overall directional distribution of scatter, and so diagrams for digital acquisition only are presented. The magnitude of the scatter was increased by use of the digital acquisition mode: on average 16 times that generated by fluoroscopy.
Points to note from the scatter distribution diagrams are:
- Points on the graphs where the scattered radiation field drops to very low levels coincide with the positions of the X-ray tube or image intensifier. Both tube and intensifier are housed in lead shielding and are effective at attenuating scattered radiation. The reason that this is not the case for the 10°RAO (Figure 11
) view is because, in this view, these items do not obscure the source of scatter from the measuring points.
- The greater the oblique angle of the projection, the more intense the radiation field becomes. This is a result of the larger exposure factors selected for oblique angles. The scatter produced by an object exposed to radiation increases with rising mA and kVp values.
- The radiation field is less intense at 170 cm from the floor than at 120 cm and 60 cm. This is because of the backward direction of the scatter from the phantom. If the unit had an over-couch X-ray tube, these distributions would be inverted.
These results clearly have practical implications to staff carrying out their duties in the angiography suite. The distribution of scattered radiation can be predicted with knowledge of the basic physics of the exposed situation. The importance of imparting this knowledge to staff cannot be underestimated and should be incorporated into any radiation protection training. Specific points to impart are:
- Digital acquisitions lead to much higher doses that fluoroscopy.
- When imaging oblique angles, the scatter on the X-ray tube side is greater than that on the intensifier side.
- Lead protection must be carefully placed to ensure continuity of protection.
- Distance from the patient is an effective method of dose reduction.
Retrospective dose audit
The results of the retrospective dose audit are shown in Tables 4 and 5. All doses have been corrected for individual DAP correction factors (determined at 80 kVp). There are no results for PTCA doses in the new Angio 2 because few of these procedures were carried out in this room during the period of the audit. Data for PTCA doses in Angio 3 was also excluded because there were only 13 patients in the sample.
It is normal practice in a patient dose audit to select data so that the mean weight of the sample population lies within 70±5 kg for general radiographic procedures [21]. In the retrospective dose audit, the method of data collection did not allow for inclusion of the patient's weight. For a large data set, such as that presented here, it can be assumed that the mean weight of the sample reflects the mean weight of the population undergoing these examinations. It is quite likely that patients undergoing coronary procedures are, on average, heavier than the general population, making the standard weight range stated above unsuitable for this dataset. All patients in the retrospective dose audit have been accepted into the dose audit without reference to individual characteristics.
In 1994, the National Radiological Protection Board (now the Radiation Protection Division of the Health Protection Agency) produced a report documenting a method for calculating effective dose from DAP measurements [22]. This method used Monte Carlo techniques and information gathered from dose audits to simulate specific radiographical examinations on a geometric phantom. These simulations determined the equivalent dose to individual organs; the International Commission on Radiological Protection's tissue weighting factors were then used to convert DAP in Gy cm2 to effective dose in mSv. There are two ways of using this conversion factor to calculate effective dose for a complete procedure. The first determines a conversion factor for each of the projections used in an examination, which is then multiplied by the DAP contribution to the total procedure from that view. The dose from all of the projections is then summed to obtain a total effective dose for that procedure. The second method assumes that the projections used in coronary procedures are standard, as are their relative contributions to the total dose. This allows a conversion factor to be calculated for a representative procedure, which can then be used for all procedures of that type. Betsou et al [12] used the latter method to calculate a DAP to effective dose conversion factor of 0.183 mSv Gy–1 cm–2 for cardiac catheterization procedures, and we have applied this factor to both procedures investigated.
Using the first method mentioned above, the National Radiological Protection Board has published effective patient doses of 3–10 mSv for coronary angiography and 7–15 mSv for PTCAs [6]. To put this into the context of other common radiographical procedures, the same report gives the dose from a typical abdominal CT to be 10 mSv and a typical chest X-ray to be 0.02 mSv.
The mean DAP values given here are comparable with, and fall within the range, of those presented in recent literature (Tables 1 and 2), all of which used DAP meters to audit patient doses. The mean values for the total audit are 33.4±0.01 Gy cm2 for coronary angiography and 80.5±0.07 Gy cm2 for PTCA. The errors quoted are the standard error of the mean. The DAP to effective dose conversion factor of 0.183 mSv Gy–1 cm–2 reported by Betsou et al [12] was used to calculate the mean effective doses, which are shown in Tables 4 and 5. It should be noted that the conversion factor is for a standard patient undergoing a representative procedure, and as such should not be used to calculate individual doses to patients.
A dose audit similar in size to the one presented here was carried out by the East Anglian Regional Radiation Protection Service in 2001. Mean DAP values for Angio 1 and Angio 2 (old) were 36.8±0.5 Gy cm2 and 35.0±0.7 Gy cm2, respectively, for coronary angiography, and 51.4±2.3 Gy cm2 and 70.7±3.2 Gy cm2 for PTCA. The results are similar to those presented in this study with the exception of coronary angiography carried out in Angio 1, which increased in dose by 19% between 2001 and 2004.
The two currently installed rooms that gave the highest mean doses are Angio 2 (both before and after replacement) and Angio 4 — the rooms installed with digital detectors. It is claimed that the use of digital detectors can provide adequate image quality for a lower dose than can conventional systems. This claim is reinforced by Tsapaki et al [23], who optimized the use of a flat-panel fluoroscopy system and reduced doses for coronary angiography by 30% when compared with an older conventional fluoroscopic system. There is, therefore, possible scope for further optimization of these units.
It is difficult to make definitive comparisons of radiation dose between rooms without considering the type of workload and staff concerned. It is not known if the complexity of cases or staff expertise is spread evenly across all the rooms in a department. If a particular room is used for cases that could require more attention owing to a complex clinical situation, or for training, then radiation doses may be increased if more radiographical imaging is required. The low median coronary angiography dose in Angio 4 suggests that a few high-dose procedures may be increasing the mean value. Clearly, these factors need to be considered before taking any action based on these results.
Prospective dose audit
15 patient doses were recorded in this dose audit. With such a small sample size, the weight of the patient becomes a significant factor and it is important to select patients that represent a typical population of those undergoing the procedure. As previously mentioned, it is recommended that 70±5 kg represents an average patient for general radiographic procedures [21] but it can be argued that the population undergoing coronary angiography is more likely to be overweight. The mean weight of this sample is 82.6±11.4 kg and so it was decided that patients with weights between 71.2 kg and 94.0 kg (i.e. within one standard deviation) would be included in the dose audit. This leaves 10 data points, the minimum recommended by the Health Protection Agency for patient dose measurements.
The mean DAP for this audit is 28.0 Gy cm2 — a similar value to that found in Angio 1 in Department A, which has a similar model of radiographical equipment. The mean effective dose using the same conversion factor used previously is 5.1 mSv. The standard error of the mean for this audit was 0.6 Gy cm2 despite this being a small dataset. This is most likely a result of the selection criteria imposed by the radiographical staff, i.e. routine cases which, assuming a consistent level of expertise and experience of staff, would be expected to receive similar doses.
The average tube potential (or kVp) and total screening time was also collected for all 15 patients in this audit, and can be further examined. There is a statistically weak relationship between screening time and DAP (correlation coefficient of 35%), which is unsurprising given that the DAP meter reading includes the dose contribution from the digital acquisitions, which contributes a significant amount to the total dose but not to the screening time [12]. There is a statistically strong relationship between patient weight and tube potential (correlation coefficient of 76%), which again is expected because it is through varying exposure factors that a constant image brightness in maintained. Similarly, the DAP has a strong relationship with the tube potential (correlation coefficient of 79%), as air kerma is approximately related to kVp2.
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Conclusions
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Lead protection found in a catheterization laboratory was shown to be 93% effective at absorbing incident radiation at head height and 96% effective at knee height. The protection was least effective at waist height, with 89% of the incident radiation being attenucted. This resulted from difficulties in positioning the ceiling-suspended eye shield in such a way that continuity of protection with the couch-side lead apron is provided. This result highlights the importance of the correct placement of such shielding.
The radiographic projection is relevant in determining the scatter distribution around a patient. Oblique angles lead to higher exposure factors and therefore more scatter. At diagnostic energies, the Compton interaction leads predominantly to back-scatter in the direction of the X-ray tube. This means that there are higher levels of exposure on this side of the patient, which is an important result for the radiation protection education of staff. Digital acquisitions were shown to lead to much higher levels of scatter exposure than did fluoroscopy (on average 16 times greater).
Dose audits were also completed in two cardiology departments; one retrospectively using computerized patient data, the other manually with radiography staff recording the data. Six rooms in total were audited. The DAP values were found to range from 28.0–39.3 Gy cm2 for coronary angiography and from 61.3–92.8 Gy cm2 for PTCA. Effective doses for the same procedures were calculated using a DAP to effective dose conversion factor of 0.183 mSv Gy–1 cm–2. They were 5.1–6.6 mSv for coronary angiography and 11.2–17.0 mSv for PTCA. These values are comparable with those found in recent literature. Furthermore, doses from units with digital flat-panel detectors were not any lower than those with conventional image intensifiers, making it clear that if it is possible for flat-panel detectors to reduce radiation doses in X-ray angiography it will only be achieved through optimization of the system.
A strong correlation (76%) was found to exist between the weight of a patient and the tube potential automatically selected. This is expected if a constant image brightness is maintained. A strong correlation (79%) between weight and DAP was also found.
The aim of this study was to detail the nature and magnitude of radiation doses from coronary interventional procedures to staff and patients, but the practical implications of these findings will be in the education of the operators using the angiographic equipment. Information about the physics of scattered radiation, incorporated into an effective training program, should help to reduce the doses received by staff.
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Figure 2. Diagram showing the naming conventions of projections in relation to the patient's position. Left anterior oblique (LAO), right anterior oblique (RAO), cranial (CR) and caudal (CA) refer to the position of the image intensifier, with 0° being above the patient.
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Acknowledgments
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The authors would like to thank the staff in the Radiology and Cardiology departments involved for their assistance in the data collection for this study.
Received for publication June 6, 2006.
Revision received March 21, 2007.
Accepted for publication May 8, 2007.
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References
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Abstract
Introduction
