Radiation exposure of radiologists during different types of CT-guided interventions: an evaluation using dosimeters placed above and under lead protection

Abstract

Background

Computed tomography (CT) is widely used not only for diagnostic purposes but also for image guidance during different types of interventions. Therefore, radiation exposure of both patients and interventional radiologists remains a much-discussed topic.

Purpose

To quantify radiation exposure of interventional radiologists during multiple CT-guided interventions using dosimeters placed under and outside standard protective lead clothing.

Material and Methods

A total of 113 consecutive interventions covering three different types of procedures (grouped as periradicular infiltration therapy, biopsies, and drain placement) and performed using routine clinical protocols were prospectively analyzed. The interventions were performed by two radiologists of different experience levels with identically placed dosimeters outside and underneath their protective clothing. Personal doses (right hand, eye lens, thyroid gland, thorax, gonads) were cumulatively measured for each type of intervention and separately for the two radiologists.

Results

Personal dose was below the detection limit of the dosimeters during periradicular infiltration therapy. In the biopsy and drain placement groups, the highest dose was found for the right hand (maximum cumulative dose = 1.84 ± 1.30 mSv in 19 consecutive drain placements). Under the protective gear, exposure was only observed for drain placements performed by the less experienced radiologist (maximum = 0.05 ± 0.04 mSv for the eye lens).

Conclusion

Personal doses measured here were far below annual thresholds published by the International Commission on Radiological Protection. Therefore, performing multiple CT-guided interventions appears to be safe for interventional radiologists in terms of radiation exposure.

Keywords

Interventional computed tomography dosimetry radiation safety

Introduction

The recent past has seen an increasing use of computed tomography (CT) for diagnostic purposes (1,2). Moreover, a wide range of interventional procedures are performed with CT guidance, among them punctures, markings, drain placements, and pain therapy (3,4). Interventional procedures can help avoid more invasive open surgery in many instances, thus benefiting patients and at the same time reducing healthcare costs (5,6). However, the advantage of image guidance with detailed anatomical information on soft and hard tissues comes at the cost of radiation exposure for patients and interventional radiologists alike (7). The latter are mostly exposed to scattered radiation. This remains a much-discussed topic because possible stochastic and in case of the eye lens also deterministic damage caused by ionizing radiation has raised concerns (8 –11).

Basically, exposure can be lowered in one of two ways: either by reducing the radiation output of the CT tube or by protective methods for the interventionalist. A lower tube output can be accomplished by manipulating the parameter set-up including scan length, tube current-time product, and tube voltage. Protective methods for the radiologist include wearing lead gear and, if possible, increasing the distance to the radiation source (12). Routinely, the amount of radiation is only monitored for the patient, while structured data on the exposure of interventional radiologists are not available.

This study was conducted to systematically measure the radiation exposure of interventional radiologists by calculating the cumulative dose received during a range of different interventions using a set of dosimeters placed on different body parts both above and underneath the protective gear. All interventions were performed according to protocols established in clinical routine at our institution.

Material and Methods

Patients and performing interventional radiologists

This prospective study was approved by the Institutional Review Board (IRB) and included different CT-guided interventions performed in 113 patients over a period of three months. No patients were excluded. Interventions were assigned to one of three categories of intervention types (periradicular infiltration therapy [PRT], drain placement, and biopsy). For each of the three categories, two subgroups were formed according to the level of experience of the radiologist performing the intervention (radiologist 1 = 14 years of experience; radiologist 2 = resident with three years of experience). Details of the interventions are summarized in Table 1.

Table 1.

Patients included in the six subgroups (defined by type of intervention and level of experience of interventional radiologist).

Group	n	Body site of intervention
PRT 1	21	11 lumbar	10 cervical
PRT 2	22	11 lumbar	11 cervical
Drain 1	17	13 abdominal	4 thoracic
Drain 2	19	17 abdominal	2 thoracic
Biopsy 1	17	8 abdominal	6 thoracic	3 bone
Biopsy 2	17	6 abdominal	8 thoracic	2 bone

PRT: periradicular infiltration therapy; 1: experienced radiologist; 2: resident.

Intervention protocols

All interventions included in this analysis were performed with fluoroscopy protocols already implemented in clinical routine. Periradicular infiltration therapy was accomplished on an 80-slice scanner (Aquilion Prime, Canon Medical Systems, Ottawara, Japan) with a tube voltage of 100 kV (lumbar) or 80 kV (cervical) and a tube current-time product of 5 mAs while iterative reconstruction was activated (AIDR 3D, standard level). All drain placements and biopsies were performed on a 64-slice scanner (first generation Somatom Definition, Siemens Healthineers, Erlangen, Germany) using fluoroscopic parameters according to body region. While a constant tube voltage of 100 kV was used, the tube current-time product was in the range of 30–60 mAs. The radiologist performing the intervention chose the tube current-time product based on the anatomical detail resolution required to safely complete the procedure. Therefore, in all thoracic drain placements and biopsies, 30 mAs was sufficient, while interventions close to abdominal structures at risk of injury (e.g. bowel loops or vessels) required up to 60 mAs. A summary of protocol specifications is presented in Table 2. If necessary, the radiologist fixed the coaxial needle, true-cut needle, or drain with a needle holder to achieve alignment with the ray path. The hand itself was kept out of the ray path. Complications during the interventions (pneumothorax, bleeding) were recorded.

Table 2.

Summary of CT fluoroscopy protocol parameters used for the three groups of interventions analyzed.

Type	Body site	Scanner	kV	mAs	Reconstruction
PRT	Lumbar	Canon	100	5	AIDR 3D
PRT	Cervical	Canon	80	5	AIDR 3D
Drain	Thoracic	Siemens	100	30	FBP
Drain	Abdominal	Siemens	100	30–60	FBP
Biopsy	Thoracic	Siemens	100	30	FBP
Biopsy	Abdominal/bone	Siemens	100	30–60	FBP

PRT: periradicular infiltration therapy; AIDR 3D: Adaptive iterative dose reduction 3D; FBP: filtered back projection.

Radiation protection and dose measurement

Radiologists wore protective lead clothing, consisting of a skirt, vest, and thyroid collar with 0.35-mm lead equivalent (MAVIG, Munich, Germany). Additionally, a protective visor with 0.10-mm lead equivalent covered the face from the forehead to the cheek bones (MAVIG, Munich, Germany). Nine dosimeters were assigned to each of the six subgroups and cumulatively exposed during all interventions and placed as follows: on the forehead (under the visor) and on the right outer side of the visor (to estimate eye lens dose); the right annular finger (both radiologists were right-handed); and at the levels of the thyroid gland, the chest, and the gonads, one each outside and underneath the lead protection (distribution shown in Fig. 1).

Measurements were made over a period of three months, which was taken into account when performing correction for background radiation. Two different types of dosimeters were used to measure personal doses as a basis for sound estimation of effective or organ dose. Exposure of the head and hand was measured using thermoluminescent dosimeters (TLD 100, lithium fluoride [LiF]: Mg, Ti), which were calibrated for skin dose H_p (0.07) with a lower detection limit of 30 µSv. Measurement uncertainty at the lower detection limit is ±100% and ±30% for doses >1 mSv. Dosimetry at the levels of the thyroid, the breast, and the gonads was performed using optically stimulated luminescence dosimeters (OSL, beryllium oxide, BeO), which were calibrated for depth dose H_p (10) with a lower detection limit of 20 µSv. Measurement uncertainty at the lower detection limit is ±100% and ±30% for doses >0.5 mSv. The dose report generated by the CT scanner software was used for documentation of dose outputs during interventions.

Data analysis

Statistical analysis was performed using RStudio (Version 1.1.383, RStudio, Boston, MA, USA). Plots were generated using the “lattice” package. The Shapiro–Wilk test showed that data were not distributed normally (P < 0.01), so differences in mean dose-length product (DLP) were tested using the Wilcoxon rank-sum test with continuity correction.

Results

Fig 2 and Table 3 provide data on dose-related parameters as displayed in the dose reports generated by the CT scanner software. In general, the less-experienced radiologist 2 needed more acquisitions than radiologist 1 to complete an intervention, resulting in higher DLPs. The differences in DLP were significant in the drain placement and biopsy groups but not for PRT.

Table 3.

Acquisition and dose parameters by subgroup.

Group	kV	mAs	Acquisitions	DLP	P value
Group	Mean	Mean	Total/mean	Total/mean	P value
PRT 1	90.47	5.00	178/8.48	27.90/1.33	0.90
PRT 2	90.00	5.00	192/8.73	32.20/1.46	0.90
Drain 1	100.00	37.06	397/23.36	498.00/29.29	<0.01
Drain 2	100.00	46.32	563/29.63	837.00/44.05	<0.01
Biopsy 1	100.00	38.24	461/27.11	559.00/32.88	0.02
Biopsy 2	100.00	37.06	638/37.53	743.00/43.71	0.02

P value indicates significance of differences (Wilcoxon rank-sum test) in DLPs (unit: mGy*cm) between the subgroups (defined by type of intervention and level of experience of interventional radiologist).

PRT: periradicular infiltration therapy; DLP: doselength product; 1: experienced radiologist; 2: resident.

Fig. 3 and Table 4 present the cumulative doses measured by each dosimeter. The dose was below the detection limit for all dosimeters in PRT groups 1 and 2. In the drain placement and biopsy groups, the dose was highest for the right hand. Among the dosimeters positioned under the lead protection, only those in drainage group 2 identified exposure (maximum of 0.05 ± 0.04 mSv for the lens).

Table 4.

Cumulative radiation exposure measured by each of the nine dosimeters worn by the experienced and the less experienced radiologist (unit: mSv).

Group	H	L*	L	T*	T	B*	B	G*	G
PRT 1	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl
PRT 2	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl
Drainage 1	0.63 ± 0.45	<dl	<dl	<dl	<dl	<dl	0.03 ± 0.02	<dl	<dl
Drainage 2	1.84 ± 1.30	0.05 ± 0.04	0.22 ± 0.16	0.02 ± 0.02	0.10 ± 0.05	<dl	0.05 ± 0.03	<dl	0.03 ± 0.02
Biopsy 1	0.86 ± 0.61	<dl	0.04 ± 0.03	<dl	<dl	<dl	<dl	<dl	<dl
Biopsy 2	1.33 ± 0.94	<dl	0.04 ± 0.03	<dl	<dl	<dl	<dl	<dl	<dl

*The dosimeters are placed under lead protection.

PRT: periradicular infiltration therapy; H: hand; L: lens; T: thyroid; B: breast; G: gonads; <dl: below detection limit.

One patient each in biopsy groups 1 and 2 developed pneumothorax during pulmonary biopsies. In biopsy group 2, pneumothorax had to be treated with an additional coaxial needle; the other patient was observed with acquisitions over a time span of 5 min. These events required additional acquisitions: 12 and 24, respectively.

In one patient from drainage group 2, suspected bleeding from a vessel close to the targeted abscess in the major psoas muscle was monitored with 15 additional acquisitions until bleeding was ruled out.

Discussion

Our study shows that there is only minimal radiation exposure of interventional radiologists during different types of CT-guided interventions. This might be attributable to the protocols used here, which are well established in our institution and have been optimized to minimize radiation dose. While there is no generally accepted definition of “standard-dose” or “low-dose” protocols, we may reasonably assume that our protocols are low-dose when compared with recently published reports on dose reduction of intermittent fluoroscopy in interventional procedures. For instance, Sarti et al. used a set-up with 120 kV and 50 mA for pulmonary biopsy, resulting in a DLP of 54 mGy*cm (12). The 14 pulmonary biopsies included in our analysis were performed with a set-up of 100 kV and 30 mAs, resulting in a median DLP of 34.5 mGy*cm. Lamba proposes dose-optimized protocols for CT-guided interventional procedures in the abdomen and pelvis, taking into account patient diameter (13). Their parameter set-up starts with a tube voltage of 120 kV and a tube current-time product of 30 mAs, similar to our study protocols. Lucey et al. also used a tube current-time product of 30 mAs to perform thoracic and abdominal interventions while they varied tube voltage in the range of 120–140 kV. With this protocol, they achieved the same technical success rates as in earlier interventions performed using standard-dose protocols (14). Paik suggested low-dose protocols for cervical and lumbar epidural steroid injection (ESI) procedures, which are very similar to periradicular infiltration therapy. During intermittent fluoroscopy, he used a set-up of 120 kV and 40 mAs for the cervical segments, resulting in a median DLP of 6.60 mGy*cm (15), and 120 kV and 30 mAs for the lumbar segments, resulting in a median DLP of 3.71 mGy*cm (16). Compared to Paik, our set-up with 100 kV (lumbar) or 80 kV (cervical) and 5 mAs for both leads to DLPs of 0.7 mGy*cm (cervical) and 2.2 mGy*cm (lumbar).

Our study adds important data compared with earlier reports. It was designed to measure cumulative doses during multiple interventions (on the order of 17 to 21 per type and radiologist). The primary reason for this was to obtain robust data on true intervention-related exposure rather than background dose. This is a major difference to a similar study published by Rathmann et al., where dosimeters were read out after single interventions (17). Moreover, Rathmann et al. did not investigate doses under protective lead gear worn by interventional radiologists.

Another focus of our study was on obtaining dose exposure data for radiologists with different levels of experience as the overall dose of fluoroscopy is mainly determined by the number of acquisitions required to complete the procedure. In our study, the highest doses were found for the right hand. This is not surprising and can be explained by the occasional need to align the instruments with the ray path projected by the laser. It is also not surprising that the less-experienced radiologist’s hand was more exposed than the hand of the experienced radiologist. However, with the doses observed here, even the less-experienced radiologist could perform > 5000 drain placements before exceeding the annual hand dose limit of 500 mSv recommended by the most recent publication of the International Commission on Radiological Protection (ICRP) from 2007 (18). Assuming a workload of 780 drain placements per year (three per day and five days per week), the same less-experienced radiologist would be registered with an annual hand dose of approximately 76 mSv. Regarding doses measured under the lead protection, the less-experienced radiologist could perform >7000 drain placements before exceeding the annual eye lens dose limit of 20 mSv averaged over five years with no single year exceeding 50 mSv (19). A total of 780 drain placements per year would lead to an annual eye lens dose of >2 mSv. It is also of note that this new dose limit, which was introduced in 2011 following research proving the lens to be highly radiosensitive tissue, drastically lowered the threshold compared with the former dose limit of 150 mSv (18,20,21). Additionally, recent research indicates that awareness of this issue among young physicians is still unsatisfactory (22). Our study underlines the effectiveness of lead protection in general and specifically shows that the visors worn by radiologists during interventions provide adequate lens protection.

Our study is limited by some general facts regarding dosimetry that need to be considered. The above-mentioned background radiation is a standard variable in the readout process and it is more difficult to separate from intervention-related exposure when the dose levels detected are very low. Hence, very low dose levels as detected in our study should always be interpreted with caution. Exposure of the eye lens was measured by dosimeters calibrated for skin dose H_p (0.07). Dosimeters calibrated for H_p (3) were not provided in our study but could yield more precise data to estimate dose at the real depth of the eye lens instead of the cornea. The data presented in Table 1 indicate that interventions were fairly evenly distributed to the six subgroups. Applying a scoring system to objectify difficulty levels of each intervention could have added information on the comparability of the subgroups. On the other hand, it was not the main focus of our study to compare doses needed to complete interventions between radiologists of different experience levels but to quantify cumulative radiation exposure when using low-dose protocols and adequate radiation protection.

In conclusion, during different types of CT-guided interventions, the radiation exposure of interventional radiologists is far below current annual limits set by the ICRP. With the dose-optimized protocols used in this study, even complex interventions such as drain placements could have been performed >5000 times before exceeding the annual hand dose limit and even more often before exceeding different annual dose limits for organs covered by protective lead gear. We therefore conclude that even frequent performance of CT-guided interventions is safe for the interventional radiologist in terms of radiation exposure.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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