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Patient and staff dose during CT guided biopsy, drainage and coagulation

Departments of ¹Radiology and ²Clinical Oncology, Leiden University Medical Center, Albinusdreef 2, 2333 ZA Leiden, The Netherlands

Correspondence: W Teeuwisse, Department of Radiology, Leiden University Medical Center, PO Box 9600, 2300 RC Leiden, The Netherlands

	Abstract

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Patient and staff dose during CT guided coagulation of osteoid osteoma, tissue biopsy and abcess drainage were evaluated retrospectively on a conventional CT scanner and prospectively on a scanner equipped with fluoroscopic CT. The computed tomography dose index (CTDI) and the individual dose equivalent, i.e. the penetrating dose for workers at a depth of 10 mm tissue, were measured. Evaluation of CTDI enabled effective dose and maximum skin entrance doses for the patient to be determined. Doses were assessed for 96 CT guided interventions, including 16 drainages with average effective doses of 13.5 mSv and 9.3 mSv for the conventional CT scanner and the scanner with spiral CT fluoroscopy, respectively, 49 biopsies (effective doses of 8 mSv and 6.1 mSv, respectively), and 31 coagulations of osteoid osteoma (effective doses of 2.1 mSv and 0.8 mSv, respectively). Effective doses to patients were in the same range as those observed for regular diagnostic CT examinations. Entrance skin doses were well below the 2 Gy threshold for deterministic skin effects on the CT scanner equipped with fluoroscopic function (0.03–0.33 Gy), whilst skin doses on the conventional scanner were considerably higher (0.09–1.61 Gy). This is mainly owing to the fact that on the conventional scanner mAs was rarely reduced for scans evaluating needle position whereas low mAs per rotation was selected on the scanner with the fluoroscopy option. The maximum dose to a worker measured outside the lead apron was 28 µSv for one single procedure. The mean dose per procedure was below 10 µSv for radiologists and below 1 µSv for radiographers. Correcting for attenuation of the lead apron, the doses to workers are very low.

	Introduction

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During conventional CT guided interventions, operation of the CT scanner is performed at the operating console outside the CT room. In this case, personnel always remain outside the CT room during patient exposure. The procedure for CT guided interventions changed with the introduction of CT fluoroscopy [1]. Connection of a footswitch and control panel directly to the gantry of the CT scanner now enables image acquisition and real-time image viewing in the CT room. Application of CT fluoroscopy inevitably implies radiation exposure of personnel. The transition from conventional CT guided procedures to CT fluoroscopic guidance was also expected to have an impact on patient dose.

The main radiation risk associated with exposure to X-rays is the stochastic risk, i.e. tumour induction and hereditary effects. However, during CT guided interventions repeated CT scans are made at the same anatomical location of interest. Such repeated exposures might lead to relatively high local skin doses. As such, there is concern about the occurrence of skin damage that may occur when threshold dose levels for deterministic skin effects, such as erythema or epilation, are exceeded [2].

This study evaluates the radiation exposure of workers and patients during three types of CT guided interventions; CT guided coagulation of osteoid osteoma, biopsy and drainage. Special care was taken to select the most appropriate dosimetric quantities and methods. A comparison of the radiation exposure of the patient is made for the situation before and after the introduction of fluoroscopic CT in our department.

	Instruments and methods

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Procedure
Before fluoroscopic CT was installed at our hospital, CT guided interventions were performed on a conventional Philips LX CT scanner (Philips Medical Systems, Best, The Netherlands). Recently, optional fluoroscopic CT was installed on the Philips AV-El spiral CT scanner and all interventions are now performed on this scanner. During fluoroscopy, the CT image is refreshed six times per s, with a minimum fluoroscopy duration of 1 s.

To plan the procedure, all CT guided interventions are preceded by a scan projection radiograph (scanogram) and a routine CT scan of the region of interest. An array of radio-opaque line markers are then attached to the patient's skin with the markers running parallel to the z-axis. The array covers the anticipated entrance point of the needle. Subsequently, a single slice is scanned at the puncture level, showing the array of markers as a series of dots on the image. This image is used to select the exact entrance point, in-slice angulation and puncture depth of the needle. Puncture depth is marked on the needle and a laser device placed at the end of the patient support provides a lightbeam that marks the desired in-slice angulation.

In fluoroscopic CT, a useful distinction has been introduced, a "quick-check" method and a"real-time" method [3]. The "quick-check" method involves very short fluoroscopic CT scans, for checking only the position of the needle. In this case there is no manipulation of the needle during the scans. The "real-time" method involves simultaneous needle manipulation and CT scanning. During our study, all CT fluoroscopy was performed as a "quick-check" method. For this reason, no significant radiation exposure to the hand of the radiologist was expected, therefore this parameter was not measured.

Dosimetry
The most pragmatic dosimetric quantity for assessing the risk of deterministic effects to the patient is the maximum skin dose. In most cases, maintaining entrance skin dose (ESD) well below the threshold for deterministic effects, e.g. 2 Gy for temporary erythema [4], can be considered adequate for avoidance of deterministic effects to most organs and tissues. The quantity that best expresses the stochastic risk to the patient is the effective dose E [5]. However, E cannot be measured directly since it is calculated as the weighted sum of averaged organ and tissue doses. In this study, ESD and E were derived from the computed tomography dose index (CTDI) (Gy) [6], the preferred operational dosimetric quantity, defined as:

where T is the nominal slice thickness and D(z) is the dose profile along a line parallel to the z-axis of rotation. CTDI is also used by manufacturers of CT scanners for characterization of absorbed dose. Unfortunately, there are several approaches for defining and measuring CTDI. For our purpose, the most appropriate definition of CTDI comes from the recent International Electrotechnical Commission standard [7]. This involves CTDI measurements in cylindrical polymethylmethacrylate (PMMA) dose phantoms. The integration length of the dose profile along the axis of rotation (z-axis) is 100 mm and dose is specified as absorbed dose to air. In general, two different phantoms are used, one representing the head (cylinder diameter 16 cm) and another the body (cylinder diameter 32 cm). In this study, for CT scans of the head, neck or extremities, the CTDI values for the head phantom were applied. For scans of the chest, abdomen or pelvis the values for the body phantom were used. The weighted CTDI (CTDI_w) is the approximation of average dose over a single slice, defined as:

where CTDI_c and CTDI_p are measured at the centre and periphery of the CT dose phantoms, respectively [7, 8]. For this study, CTDI values were actually measured at the CT scanners. However, for a first estimate it may be sufficient to use the CTDI values specified in the data sheets of the manufacturer. On more recent models these CTDI data are displayed at the operating console of the CT scanner. If these data are used for initial dose assessment, it should be carefully checked which definition of CTDI is used by the manufacturer and, specifically, whether CTDI is corrected for helical pitch. Scanner-specific CTDI values can sometimes also be extracted from published dosimetric databases.

CTDI cannot be used directly as a measure of radiation risk for stochastic effects as it relates only to the dose during the acquisition of a single slice or during one rotation of the X-ray tube. A dosimetric quantity that characterizes exposure for a complete examination is the dose–length product (DLP) (Gym) defined as:

where i represents each scan sequence forming part of an examination and N is the number of slices (conventional CT) or number of rotations (spiral CT).

In general, E is derived from CTDI normalized organ doses that are calculated using Monte Carlo techniques [9]. Normalized organ doses only apply to a specific type of scanner and a specific technique, e.g. tube potential (kVp) and additional filtration. In this study we applied CTDI normalized organ doses from the Danish National Board of Health [10]. Using their software, DLP normalized E could be derived for both the Philips AV-El and the Philips LX scanner. The results are summarized in Table 1. The normalization applies to the DLP for a 16 cm diameter CT dose phantom (head, cervical spine, extremities) or a 32 cm diameter phantom (chest, abdomen). CTDI_p was used as a measure of local ESD for one single rotation. An estimate of the maximum ESD was derived by calculating CTDI_p for cumulative exposures at each point along the length axis of the patient (or z-axis of the scanner) using increments of 0.5 mm. When different technique settings (e.g. tube potential (kVp), slice thickness) were applied during one examination, an estimate was made for each technique separately. The total ESD for each point along the length axis was then determined by summation of local CTDI_p values of all techniques applied. This approach yields a safe upper limit of the ESD for a procedure like CT fluoroscopy thatinvolves multiple scans at one specific location [11].

Personnel dosimetry
Personnel dosimetry was performed with electronic personal dosemeters (EPD1; Siemens Plessey Controls Limited, Dorset, UK). These dosemeters contain three diodes that are sensitive to X-rays in the 20 keV to 6 MeV range. The accuracy of the dosemeter is specified as ±30%, this being the combined uncertainty due to angular response, i.e. left–right 60°, craniocaudal 60°, and the energy response for photons in the range 20 keV to 1.5 MeV. This dosemeter measures penetrating dose (Hp(10), µSv), i.e. the individual dose at a depth of 10 mm tissue. The cumulative dose value is displayed on LCD display of the EPD1. Dosemeters were worn on the collar outside the lead apron. The radiation exposure of personnel during a CT guided intervention was derived from the difference of the dose reading before and after the procedure, i.e. no correction was made for the contribution of background radiation.

Registration of technical and clinical parameters
The parameters registered immediately after the procedure were only the most relevant parameters such as the CT fluoroscopy time, worker penetrating dose (Hp(10)) and patient identification number. Information required for the estimation of patient ESD and E, e.g. selected tube voltage, tube current, scan time, slice thickness and number of slices, was derived retrospectively from the digital archives. The exact anatomical location of all separate CT slices was derived using the indication of the table position (i.e. the z-axis of the scanner), which was traceable from the archives for each individual CT slice. The match between table position and anatomical location was sometimes disrupted owing to a change of position of the patient, e.g. owing to patient movement. When images revealed such a mismatch, care was taken to correct table position data in such a way that a specific table position corresponded with the same anatomical position for all subsequent scans.

	Results

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On the Philips LX scanner, the localizing scan preceding the puncture was most often a contiguous scan at a tube voltage of 120 kVp and a slice thickness of 10 mm (drainage, biopsy) or 3 mm (coagulation of osteoid osteoma). Tube load per slice varied from 285 mAs to 415 mAs and was in general higher for small slice thicknesses. The single CT slices scanned for checking the position of the needle during the conventional CT guided interventions were in general made with the same scan parameters. The localizing scan on the Philips AV-El scanner for drainages and biopsies on chest or abdomen wasperformed at 120–140 kVp tube voltage, 225–250 mAs per rotation tube load, 5–7 mm slice thickness and 1.4 pitch. The localizing scan preceding the coagulations was performed at 120–140 kVp tube voltage, 125–250 mAs per rotation tube load, 1–3 mm slice thickness and between 1 and 2 pitch. CT fluoroscopy slices during drainages and biopsies were performed at 140 kV (abdomen) or 120 kV (chest) tube voltage, 25 mAs per rotation tube load and 3–7 mm slice thickness. During the coagulation of osteoid osteoma the position of the needle was checked using the same parameters, except that a smaller slice thickness (1–3 mm) was used.

Patient dose, i.e. E and maximum ESD, was assessed for 96 CT guided interventions, including 16 drainages, 49 biopsies and 31 coagulations of osteoid osteoma. These CT guided interventions included 33 procedures analysed retrospectively on the conventional Philips LX CT scanner and 63 procedures performed on the Philips AV-El CT fluoroscopy scanner. A total of 152 measurements of personnel dose were obtained during the procedures on the Philips AV-El scanner. In general, more than one worker was present in the CT room. Consequently, in comparison with patient dose, a relatively large number of personnel dose values were obtained.

Tables 2 and 3 summarize radiation exposure of patients and workers, respectively. When comparing patient effective dose E in Table 2 with penetrating dose to the worker in Table 3, it should be noted that patient dose is expressed in mSv and Hp(10) in µSv and that Hp(10) is not corrected for attenuation of the lead apron. Radiation exposure of the worker did not, of course, occur on the conventional Philips LX scanner, therefore Table 3 only provides information for the Philips AV-El scanner. The maximum measured dose to one worker during one procedure was 28 µSv. Average doses per procedure to the radiologists are well below 10 µSv and average doses to the radiographers are below 1 µSv per procedure.

Figure 1 shows examples of the skin dose along the length axis of the patient for two CT guided drainages and for two coagulations of osteoid osteoma, for both the Philips AV-El scanner and the conventional Philips LX scanner. The contribution to the patient exposure by the CT scan preceding the actual CT guided intervention can easily be recognized as the broad base of the graphs. The dose distribution concentrates around the anatomical region of interest. Figure 1a clearly demonstrates the separate entrance points of the two drains that were placed during the procedure. Figures 1a–d show the variation in the position of the slices scanned during CT guided procedures owing to adjustment of the table position by the clinician to achieve accurate needle placement. For assessment of the maximum ESD it is unrealistic to calculate the cumulative skin dose as the sum of the dose from all separate slices [3].

View larger version (20K): [in this window] [in a new window]   Figure 1. Graphs showing examples of skin dose (mGy) to the patient for spiral and conventional CT guided interventions. (a) Abdominal drainage (2 drains) using the Philips AV-El scanner; (b) wrist osteoid osteoma coagulation using the Philips AV-El scanner; (c) abdominal drainage using the Philips LX scanner; and (d) ankle osteoid osteoma coagulation using the Philips LX scanner.

	Discussion

Top Abstract Introduction Instruments and methods Results Discussion References

One radiologist (WRO) performed about 70% of the procedures evaluated during this study. It is estimated that during 1 year he would receive anHp(10) of 0.4 mSv owing to the radiation exposure from the CT guided interventions. For a realistic estimation of the radiation exposure, this value has to be corrected for attenuation of the lead apron. A conservation correction would be dividing Hp(10) by a factor of five [13, 14]. Thus, for this radiologist, the cumulative annual dose due to CT fluoroscopy would be less than 0.1 mSv.

CT guided interventions on conventional CT scanners should be performed at low mAs. Unnecessarily high ESDs may otherwise be received. With regard to the CT guided interventions that we currently perform, we do not expect additional merits from the "real-time" fluoroscopic CT method. The additional value of multislice fluoroscopic CT for the CT guided procedures such as those described in this paper will probably be moderate. Application of "real-time" CT fluoroscopy and multislice CT fluoroscopy should therefore be considered carefully, taking into account the probable higher exposures to patient as well as to workers. The method for estimation of ESD applied in this study can be easily extended to these applications. The assessment of E is relatively simple nowadays owing to the availability of specific conversion factors. Dose assessment will become even more straightforward on the newest CT scanners since they provide the operator with values of the actual CTDI_w. Further evaluations of E as well as bench-marking between hospitals should be stimulated and could contribute to further optimization of CT. This is, of course, not only relevant to CT guided interventions but also for regular diagnostic CT scans.

	Acknowledgments

 
This work was supported by the IRS—J.A. Cohen Institute, Leiden, The Netherlands. We thank Fokje Spoelstra, PhD, for her useful comments on the draft manuscript.

Received for publication July 31, 2000. Accepted for publication January 31, 2001.

	References

Top Abstract Introduction Instruments and methods Results Discussion References

 

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