Original Research
Vascular and Interventional Radiology
October 2011

Radiation Dose Exposure for Lumbar Spine Epidural Steroid Injections: A Comparison of Conventional Fluoroscopy Data and CT Fluoroscopy Techniques

Abstract

OBJECTIVE. The purpose of this article is to compare the radiation dose of conventional fluoroscopy–guided lumbar epidural steroid injections (ESIs) and CT fluoroscopy (CTF)–guided lumbar ESI using both clinical data and anthropomorphic phantoms.
MATERIALS AND METHODS. We performed a retrospective review of dose parameters for 14 conventional fluoroscopy ESI procedures performed by one proceduralist and 42 CTF-guided ESIs performed by three proceduralists (14 each). By use of imaging techniques similar to those for our clinical cohorts, a commercially available anthropomorphic male phantom with metal oxide semiconductor field effect transistor detectors was scanned to obtain absorbed organ doses for conventional fluoroscopy–guided and CTF-guided ESIs. Effective dose (ED) was calculated from measured organ doses.
RESULTS. The mean conventional fluoroscopy time for ESI was 37 seconds, and the mean procedural CTF time was 4.7 seconds. Calculated ED for conventional fluoroscopy was 0.85 mSv compared with 0.45 mSv for CTF. The greatest contribution to the radiation dose from CTF-guided ESI came from the planning lumbar spine CT scan, which had an ED of 2.90 mSv when z-axis ranged from L2 to S1. This resulted in a total ED for CTF-guided ESI (lumbar spine CT scan plus CTF) of 3.35 mSv.
CONCLUSION. The ED for the CTF-guided ESI was almost half that of conventional fluoroscopy because of the shorter fluoroscopy time. However, the overall radiation dose for CTF-guided ESIs can be up to four times higher when a full diagnostic lumbar CT scan is performed as part of the procedure. Radiation dose reduction for CTF-guided ESI is best achieved by minimizing the dose from the preliminary planning lumbar spine CT scan.
Imaging-guided lumbar epidural steroid injections (ESIs) can be performed with conventional fluoroscopy or CT fluoroscopy (CTF). Although conventional fluoroscopy reveals only bony anatomy, CTF combines the excellent soft-tissue contrast resolution of conventional CT with the real-time capabilities of conventional fluoroscopy, thus providing numerous potential benefits over conventional fluoroscopy during interventional procedures [1]. Compared with conventional fluoroscopy, CTF involves imaging the epidural space in cross-section, which allows more accurate placement of the needle tip and less contrast material required to confirm the epidural location [2].
A concern regarding the use of CTF may be radiation exposure to the patients from CT, because the dose per time is substantially higher for CTF than for conventional fluoroscopy [3]. However, this factor may be offset by the potentially shorter fluoroscopy time with CTF [2]. Few prior studies have addressed the radiation dose difference between these two procedures for spine pain management intervention [2, 4]. The purpose of this study is to compare the radiation dose for conventional fluoroscopy–guided lumbar ESI and CTF-guided lumbar ESI using both clinical data and anthropomorphic phantoms.

Materials and Methods

There were two parts to this study. First, patient data for ESI technique, including fluoroscopy time and scan parameters, were retrospectively collected for both CTF- and conventional fluoroscopy–guided lumbar spine ESI procedures. Subsequently, the information from the patient data was used to obtain organ dose measurements in an anthropomorphic phantom for conventional fluoroscopy– and CTF-guided lumbar spine ESI procedures. Institutional review board approval was obtained for retrospective review of previously obtained clinical and imaging data. The study was HIPAA compliant.
Fig. 1 Frontal view of anthropomorphic phantom (model 701-D, CIRS) with leads attached to metal oxide semiconductor field effect transistor dosimeters.

Clinical Cohort Review for Dose Settings

We retrospectively reviewed lumbar spine ESI procedures performed by four proceduralists. Data collected consisted of fluoroscopy time (seconds), tube current (milliamperes), and peak beam energy (peak kilovoltage). Fourteen consecutive conventional fluoroscopic lumbar spine ESIs were performed in December 2006 by a single anesthesiologist with 12 years of injection experience. Radiation dose assessment of these data has been previously reported [5]. To match these data, we selected 14 consecutive ESIs performed in January 2010 under CTF guidance by each of three neuroradiologists (with 10, 7, and 2 years of experience) for a total of 42 CTF-guided ESIs. The three neuroradiologists were selected to reflect a wide range of experiences, which may affect the procedure time. Despite the differences in imaging guidance and dates performed, the ESI procedures were otherwise performed with the translaminar approach using identical needles, contrast material, drugs (bupivacaine and triamcinolone acetonide), and technique. The conventional fluoroscopy and CTF equipment did not change during this period. The conventional fluoroscopy–guided ESIs were performed at the pain management clinic, whereas the CTF-guided ESIs were performed in the radiology department.

Dose Measurements in an Anthropomorphic Phantom

A commercially available anthropomorphic male phantom (model 701-D, CIRS; Fig. 1) was used for measurement of absorbed organ doses. The phantom is 173 cm in height and weighs 73 kg. Twenty metal oxide semiconductor field effect transistor (MOSFET) dosimeters (model 1002RD, Best Medical) with active detector areas of 200 × 200 μm (total dimensions, 2.5 mm width × 1.3 mm thickness × 8 mm length) were placed in defined anatomic locations in the chest, abdomen, and pelvis as designated by the phantom’s manufacturer (Table 1). Each detector was calibrated for the appropriate beam energy, and individual calibration factors for all 20 detectors were stored in a database. Detailed calibration methods and validation of MOSFET methods have been described by Yoshizumi et al. [6]. The lower limit of detection of absorbed dose for the MOSFETs with AutoSense system (Best Medical) is 1.50 mGy. The detectors used for both imaging guidance modalities were the same diagnostic MOSFET model, but they were different batches from the manufacturer. This does not affect the results, because two groups of detectors were calibrated independently using different x-ray beams.
TABLE 1: Location of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) Detectors

ESI Technique and Imaging Parameters for the Clinical Cohort and the Phantom

The technique for performing translaminar imaging-guided ESI procedures has been described in the literature [2, 7, 8]. Both conventional fluoroscopy and CTF methods involve placing the patient in prone position. Briefly, the steps are as follows: the skin is sterilized and anesthetized; a spinal or epidural needle is advanced under imaging guidance through the skin toward the spinal canal; a syringe containing iodinated contrast agent is attached to the needle; the epidural location of the needle tip is confirmed by loss of resistance technique, negative aspirate, and visualization of iodinated contrast agent in the epidural space; and, finally, the drugs are injected. The mean fluoroscopy time and imaging parameters from the patient cohort were used for the conventional fluoroscopy- and CTF-guided ESI examinations of the anthropomorphic phantom.
For the conventional fluoroscopy–guided ESI procedure, the site of injection was identified using fluoroscopy at the time of injection. Conventional fluoroscopy was performed using a mobile x-ray fluoroscopy system (OEC 9800 Super C-Arm, GE Healthcare). The imaging system consisted of a rotating anode x-ray source (Varian RAD-99, Varian Medical Systems) with a trimode image intensifier (9/6/4-inch) attached to a C-arm, which was controlled manually or with a motorized controller switch. The fluoroscopy technique was set for automatic brightness control to optimize tube current and peak beam energy dose settings for the body habitus. The frame rate was 15 frames per second. Pulsed fluoroscopy was not used. Anteroposterior, lateral, and oblique projections were used while performing fluoroscopy. The geometric details are as follows: image intensifier to table, 53 cm; image intensifier to surface of phantom, 30 cm; and table to x-ray source, 20 cm.
For the CTF-guided ESI procedure, the total radiation dose consisted of the procedural CTF dose as well as the dose from a preliminary planning lumbar spine CT scan. Lumbar spine CT scan was performed before the procedure for evaluation of spinal canal stenosis and for planning the needle placement. All CT procedures were performed on a 16-MDCT scanner (LightSpeed, GE Healthcare). The CT parameters for the planning lumbar spine CT scan were the same as those for our standard clinical protocol for lumbar spine CT scan: 120 kVp, automatic tube current modulation with Smart mA (minimum, 100 mA; maximum, 440 mA; noise index, 10), contiguous slice thickness of 2.5 mm, and 25-mm FOV. The area scanned (z-axis) varied for the clinical cohort from one lumbar level to four lumbar levels (L2 to S1), depending on the following factors: patient’s symptoms, available prior imaging, previous lumbar spine injections, and proceduralist’s preference. After the preliminary lumbar spine CT scan, the radiologist selected the tube current for the CTF procedure on the basis of the patient’s body habitus and the radiologist’s personal preference. Three CT images were obtained each time the radiologist stepped on the pedal, and each image was equivalent to 0.33 second of exposure time.
To obtain radiation dose for conventional fluoroscopy and the procedural CTF, the phantom was scanned with fixed tube current and peak beam energy parameters according to the clinical data. Fluoroscopy time for scanning the phantom was set higher than the actual mean clinical times (set at 10 minutes for conventional fluoroscopy and 6 seconds for CTF) to ensure accurate detection of the dose by the MOSFET detectors. The doses recorded by the MOSFET detectors were then scaled linearly to generate the true dose per second for each procedure. The phantom was imaged three separate times at L4–L5 with both conventional fluoroscopy and CTF to obtain an average dose per second. In addition, the phantom was scanned for lumbar spine CT with the maximum coverage used in the clinical cohorts (L2 to S1).

Effective Dose Estimations

The effective dose (ED), an equivalent uniform dose to the entire body, was calculated from the measured organ doses (Di) by applying tissue weighting factors (WT) published elsewhere [9] and by assuming a radiation-weighting factor (WR) of 1.0 for x-rays. The ED is computed by the following equation:
ED=iWTi×Hi=WR×iWTi×Di
(1)
where Hi equals the equivalent dose for organ i. For the fluoroscopic procedures (conventional fluoroscopy and CTF), we first measured individual organ dose rates and derived the ED rate as follows:
ED=WR×iWTi×D.i
(2)
From the ED rate, the ED was computed by multiplying by the fluoroscopy time (t) as follows:
ED=WR×iWTi×D.i×t
(3)
In determining the absorbed dose to the bone marrow, a bone marrow distribution factor (17.41% for adult) for lumbar spine was used to limit overestimation of the dose to this organ [10].

Statistical Analysis

Data were entered into a spreadsheet (Excel, Microsoft). Statistical analyses were performed using SAS (version 9.1, SAS Institute). The mean organ doses and EDs for conventional fluoroscopy and procedural CTF were compared with the unpaired Student t test. A two-tailed p value of less than 0.05 was considered to be statistically significant.

Results

Clinical Cohort Data

The fluoroscopy time and parameters for conventional fluoroscopy and procedural CTF are shown in Table 2. The 14 patients who underwent lumbar ESI by conventional fluoroscopy (mean [± SD] age, 66 ± 16 years; three men) had a mean exposure time of 37 seconds, which was eightfold greater than the mean exposure time for patients who underwent lumbar ESI (4.7 seconds) by CTF guidance (mean age, 65 ± 14 years; nine men). The exposure time of 4.7 seconds for CTF corresponds to the proceduralist’s obtaining images during the time of needle placement and verification of epidural location with contrast agent. The mean procedural CTF tube current (54 mA) was 13.5 times higher than that of the conventional fluoroscopy tube current (4 mA).
Fifty percent (21/42) of patients undergoing lumbar CTF-guided ESI had preliminary lumbar spine CT examinations scanning from L2 to S1. The remaining patients had preliminary CT studies with limited z-axis coverage targeted at selected single levels of injection, ranging from L2–L3 to L5–S1.

Anthropomorphic Phantom Absorbed Organ Doses and EDs

Table 3 shows the highest individual absorbed organ doses for conventional fluoroscopy, procedural CTF, and lumbar spine CT scan based on the anthropomorphic phantom scans. For both fluoroscopic techniques, the skin recorded the highest absorbed organ dose with statistically significant differences between the two procedural techniques. Other organs with statistically significant differences in recorded organ doses were small bowel, large bowel, and bone marrow. The absorbed dose to the small bowel was particularly high for conventional fluoroscopy compared with other internal organs and was four times greater for conventional fluoroscopy compared with CTF.
The ED for conventional fluoroscopy was 0.85 mSv for 37 seconds of exposure time (0.023 mSv/s), which is nearly two-fold higher than the ED for procedural CTF, 0.45 mSv for 4.7 seconds of exposure time (0.09 mSv/s) (p = 0.0001).
The ED of the preliminary lumbar spine CT scan (obtained from L2 to S1) was 2.90 mSv, which was 6.4 times higher than the ED for the procedural CTF. The combined dose of lumbar spine CT scan and the procedural CTF was 3.35 mSv, which is four times higher than the dose for the conventional fluoroscopy–guided ESI procedure.

Discussion

Although CT provides better visualization of the epidural space, conventional fluoroscopy may be favored over CTF for lumbar ESI procedures because of concern about higher radiation exposure resulting from CT techniques. On the basis of our clinical data and anthropomorphic phantom dose measurements, we found that the ED for the CTF-guided ESI procedure was half that of a conventional fluoroscopy procedure because of a shorter fluoroscopy time. However, both conventional fluoroscopic and CTF procedural doses are relatively small compared with the ED from the preliminary lumbar spine CT scan used for planning CTF-guided ESI.
TABLE 2: Fluoroscopy Times and Settings for Lumbar Spine Epidural Steroid Injections Using Conventional and CT Fluoroscopy for Clinical Cohort
Schmid et al. [4] previously reported the radiation dose of CT-guided (not CTF-guided) and conventional fluoroscopy–guided lumbar spine injections using anthropomorphic phantoms and found the ED to be similar for both modalities. This finding is in contrast to that of our study, in which the procedural CTF dose was significantly lower than the conventional fluoroscopy radiation dose. One of the main differences was that, in our study, the conventional fluoroscopic dose was higher because of higher tube current settings (4.0 vs 1.9 mA). We propose that this difference could reflect a larger body habitus in our patient group. Although we did not evaluate individual patient size, our scan parameters and examination times were based on clinical cohorts, whereas the study by Schmid et al. did not state the source of their assigned dose settings and fluoroscopy times.
The literature shows that fluoroscopy times can vary widely for spine procedures depending on the experience of the operator and practice setting [11]. Our fluoroscopy times for conventional fluoroscopy–guided ESI are lower than the average in the literature (47–60 seconds), possibly because of the experience of our operator [4, 11]. There is no literature, to our knowledge, reporting fluoroscopy times for CTF-guided lumbar ESI, but we compared our times to those of Wagner [12], who found that the average fluoroscopy time for CTF-guided lumbar nerve root injections was 2 seconds. The additional CT fluoroscopy time to perform ESI compared with nerve root injection is likely the result of the additional step of requiring contrast agent injection to confirm the needle location.
TABLE 3: Absorbed Doses for Conventional Fluoroscopy, CT Fluoroscopy (CTF), and Preliminary Lumbar Spine CT From 16-MDCT Scanner Using an Anthropomorphic Male Phantom
The radiation dose associated with a CTF-guided ESI procedure is small compared with the doses associated with other procedures requiring CT fluoroscopy. Joemai et al. [13] studied patient doses for CTF-guided radiofrequency ablations, vertebroplasty, and percutaneous ethanol injections of tumors. The median patient ED from 107 procedures was 10 mSv (range, 0.1–235 mSv). In the present study, the ED resulting from CTF-guided ESI was 0.45 mSv. This occurred because of the lower complexity of the ESI procedure, which resulted in a shorter fluoroscopy time and a lower tube current selection.
The greatest contributor to the radiation dose for the CT-guided technique was the preliminary lumbar spine CT scan used to plan the procedure. The overall ED from a CTF-guided ESI procedure (lumbar spine CT scan plus CTF) can be up to four times higher than that of conventional fluoroscopy if a full lumbar spine planning CT scan is performed as the preliminary lumbar spine CT scan. Thus, a reduction in radiation dose for the CT-guided technique will be best achieved by minimizing the dose from the preliminary lumbar spine CT scan. The most practical methods to achieve this will be to limit the area and levels scanned (smaller z-axis), use a lower tube current (milliam-peres) or peak beam energy (peak kilovoltage), and increase the noise settings for tube current modulation [14, 15]. For the fluoroscopic portion of the CTF-guided ESI, the radiation dose can also be minimized by reducing the fluoroscopy time and selected scan parameters, such as tube current. Figure 2 shows the change in ED with variable exposure time and tube current for procedural CTF. Because exposure time and tube current are directly proportional to radiation dose, the ED resulting from CTF at 30 mA for 18 seconds is equivalent to that resulting from CTF at 90 mA for 6 seconds. After selecting an optimal tube current, operators should be mindful of their fluoroscopy time. In the present study, the mean fluoroscopy time for CTF was 4.7 seconds. Doubling this time to 10 seconds (while keeping tube current unchanged at 50 mA) would result in an equivalent radiation dose for CTF and conventional fluoroscopy. Finally, with new software and equipment, there are several methods by which radiation dose could be minimized with conventional fluoroscopy and CTF techniques. These techniques include pulsed fluoroscopy for conventional fluoroscopy and angular beam modulation for CTF [16, 17].
Fig. 2 Effective dose of anthropomorphic phantom data for procedural component of CT fluoroscopy–guided lumbar epidural steroid injection at beam energy of 120 kVp with variable exposure time and tube current selections of 90 (□), 60 (▵), and 30 (○) mA.
There are several limitations to the present study. First, the radiation dose for the preliminary lumbar spine CT scan may overestimate the dose for any individual patient, because many patients have a limited preliminary planning CT scan performed only at the level of injection (smaller z-axis coverage). Ideally, scanning the phantom at each lumbar level individually would provide a better estimate of radiation dose for the preliminary lumbar spine CT scan for which a limited z-axis was scanned. Radiation dose for the two procedural modalities may also be influenced by differences in body habitus in the clinical cohorts [18]. Unfortunately, in this retrospective study, there were no available data from the medical records regarding height and weight. Second, individual organ doses may not represent true absorbed doses for the entire organs because of the point dose measurement method used with the MOSFET detectors (i.e., the inherent limitation resulting from the maximum number of detectors that can be placed in the organs). However, care was taken to place the detectors close to the sections where the primary fluoroscopic x-ray beam passes. In determining the absorbed dose to the bone marrow, a bone marrow distribution factor (17.41% for adult) for lumbar spine was used to limit overestimation of the dose to this organ [10]. Finally, we have examined a small retrospective sample of clinical data and an unequal number of proceduralists performing ESIs under conventional fluoroscopy and CTF guidance. There were only data from one proceduralist performing conventional fluoroscopy–guided ESIs, but for CTF-guided ESIs, we chose to obtain clinical data from three different proceduralists with differing clinical experience, because CTF is less frequently used and there is a paucity of prior studies for comparison.
In conclusion, the ED for procedural CTF in our study was half that of conventional fluoroscopy for lumbar ESI procedures because of the shorter fluoroscopy times. However, this difference is not absolute and will be highly dependent on the fluoroscopy time and dose settings, especially for CTF. The overall radiation dose for a CTF-guided lumbar ESI was higher when a full lumbar spine preliminary planning CT scan was performed as part of the procedure (lumbar spine CT plus CTF). Radiologists performing CTF-guided lumbar spine procedures should carefully consider techniques to reduce radiation dose for the preliminary lumbar spine CT scan by reducing the z-axis length and reducing the tube output.

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 778 - 782
PubMed: 21940563

History

Submitted: November 4, 2010
Accepted: February 23, 2011

Keywords

  1. conventional fluoroscopy
  2. CT fluoroscopy
  3. lumbar epidural injection
  4. radiation dose
  5. spine intervention

Authors

Affiliations

Jenny K. Hoang
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.
Terry T. Yoshizumi
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Greta Toncheva
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Linda Gray
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Andreia R. Gafton
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Billy K. Huh
Department of Anesthesiology, Duke University Medical Center, Durham, NC.
James D. Eastwood
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Christopher D. Lascola, MD
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.
Lynne M. Hurwitz
Department of Radiology, Duke University Medical Center, Box 3808, Erwin Rd, Durham, NC 27710.

Notes

J. K. Hoang is a GE-AUR fellow for 2010–2011.
Address correspondence to J. K. Hoang ([email protected]).

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