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Vascular and Interventional Radiology |
1 From the Department of Radiology, Brigham and Women's Hospital, 75 Francis St, Boston, MA 02115 (S.G.S., K.T., D.F.A., R.D.N., K.H.Z., P.F.J.) and the Department of Health Care Policy, Harvard Medical School, Boston, Mass (K.H.Z.). From the 1998 RSNA scientific assembly. Received November 9, 1998; revision requested January 4, 1999; revision received February 12; accepted March 25. Address reprint requests to S.G.S. (e-mail: silver@ulna.bwh.harvard.edu).
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Abstract |
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 TOP Abstract IntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: A 6-month period of use of CT fluoroscopy to guide abdominal biopsy procedures and catheter drainage was analyzed. Efficacy measures and needle placement and procedure room times were compared with those of the preceding 6 months during which conventional CT was used. CT fluoroscopic times and estimated radiation exposures were compared for two CT fluoroscopic methods.
RESULTS: The sensitivity and negative predictive values for biopsy procedures and the success rate for needle aspiration or catheter drainages for CT fluoroscopy—98%, 86%, and 100%, respectively—were not significantly different from those for conventional CT—95%, 80%, and 97%, respectively. Room time was not reduced significantly, but mean needle placement time for CT fluoroscopy (29 minutes; n = 95) was significantly lower than that for conventional CT (36 minutes; n = 93; P < .005). The mean patient dose index was 74 cGy. Limiting CT fluoroscopy to scanning the needle tip rather than scanning the entire needle pass significantly reduced the dose to the patient and the operator.
CONCLUSION: Although CT fluoroscopy is a useful targeting technique, significant radiation exposures may result. Therefore, radiologists need to be aware of different methods of CT fluoroscopic guidance and the factors that contribute to radiation exposure.
Index terms: Abdomen, interventional procedures, 70.126 • Abscess, drainage, 70.1263 • Biopsies, technology, 70.12119, 70.125 • Computed tomography (CT), guidance, 70.12119, 70.125 • Fluoroscopy, technology, 70.1261 • Radiations, exposure to patients and personnel, 70.12119
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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CT fluoroscopy was developed and reported on by Katada et al (2–7) and Kato et al (8,9). A slip-ring spiral (helical) CT scanner was modified by adding a high-speed array processor to increase the speed of image reconstruction. The system resulted in real-time reconstruction and display of CT images; it was possible to scan continuously and view a biopsy needle as it is advanced in real time on a display screen in the CT suite. Since its inception, CT fluoroscopy has been used to guide intracranial, chest, abdominal, and pelvic biopsy procedures and fluid collection drainage (2–17). While feasibility has been established, the benefits of CT fluoroscopy, particularly as compared with those of conventional CT, to our knowledge, have not been demonstrated. More important, the radiation exposure to patients and personnel incurred while conducting CT fluoroscopy during interventional procedures remains a concern.
CT fluoroscopy has the potential to improve the efficacy of CT-guided interventions, to reduce the time to place a needle into a lesion for performance of biopsy or drainage of fluid collections, and to reduce procedure times. We used a new CT fluoroscopic scanner to guide abdominal biopsy procedures and fluid collection drainage and evaluated whether using CT fluoroscopy would improve procedural efficacy and reduce procedural times compared with those of conventional CT. We assessed patient and personnel radiation exposures incurred with CT fluoroscopy and evaluated two CT fluoroscopic guidance methods.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Patients and Procedures
Of 143 cases referred for CT-guided interventions in the abdomen and pelvis during a 6-month period (August 15, 1997 to February 14, 1998), 107 procedures were performed with use of CT fluoroscopy. Conventional spiral CT was used in 36 procedures early in the study period mainly owing to unfamiliarity with the CT fluoroscopic system. Of the 107 procedures performed with use of CT fluoroscopy, 95 abdominal biopsy and needle or catheter drainage procedures (61 biopsies, 13 fluid aspirations, 21 catheter drainages) in 87 patients (43 women, 44 men; age range, 29–88 years; mean age, 55 years) were selected for study. Of the 107 procedures, 12 were excluded, seven owing to insufficient data and five because they were special procedures (celiac ganglion block, catheter manipulation, percutaneous gastrostomy), and our purpose was to study a homogeneous population of procedures.
All procedures were performed by both a staff abdominal radiologist and a fellow or resident. Radiologists and nurses wore protective lead aprons. All procedures began in the usual fashion with a spiral CT scan of the region of interest after a radiopaque surface grid was placed on the skin. This scan was used to plan the percutaneous approach to the lesion, and the skin was then marked. The operators then chose CT fluoroscopic parameters (ie, tube voltage, tube current, collimation) available on the scanner. The parameters available were the following: for 80 kVp, 75, 105, or 135 mA; for 120 kVp, 50, 70, or 90 mA; and for 140 kVp, 43, 60, or 77 mA. Available collimations were 1, 2, 3, 5, 8, and 10 mm. For all procedures, 6 frames per second was used. Images were acquired with a gantry rotation speed of 0.75 seconds per 360°. After sterile preparation, draping of the skin, and application of local anesthesia, the first needle, used as the guiding reference needle for subsequent needle or catheter placements, was placed by using one of two CT fluoroscopic guidance methods selected at the time of the procedure by the operator.
Real-time method.—The biopsy or aspiration needle was held by using a plastic clamp to keep the operator's hand approximately 10 cm outside of the primary beam (Fig 1). Needles were advanced during CT fluoroscopy. The radiologist stepped on the scanning foot pedal while adjusting the table position (and therefore the scanning plane) by using his or her free hand on the joystick to follow the needle tip. Once the needle tip was followed into the lesion, the foot pedal was released, and the last image was saved onto the hard disk and printed with other spiral CT images (Fig 2).
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Diagnostic test characteristics (sensitivity and negative predictive value of biopsy results, success rate of fluid collection drainage), mean needle placement times, mean CT procedure room times, and mean target sizes (ie, mass or fluid collection) were determined for both CT fluoroscopy– and conventional CT–guided groups. Needle placement time, defined as the time elapsed between the start of the initial localizing spiral CT scan and the start of the scan showing the needle tip in the target, was determined retrospectively by using images obtained during the procedure. Room time, representing the total time the patient was in the CT procedure room, was logged by CT technologists for all cases in each group. In addition to the procedure time, this time included monitor hookup and removal and in some cases, intravenous catheter placement. When a biopsy was performed, the time required for the cytopathology team to examine initial specimens for adequacy was included. Some fluid aspiration and drainage procedures included the time needed to obtain a Gram stain result. Catheter drainage procedures also included time spent securing the catheter to the skin site.
All patients provided written informed consent prior to entering the room. Statistical comparisons were performed by using two-sample Student t tests of the times described. Logarithmic transformation was used to offset skewed distributions in the data for procedure times and lesion sizes. Sex, mean patient age, and mean target size were compared.
For the 95 procedures in which CT fluoroscopy was used, total CT fluoroscopy times were obtained retrospectively. Per departmental policy, they were recorded by technologists for each case. The real-time method (ie, scanning that depicted the entire needle pass) was used in 75 procedures, and the quick-check method (ie, short scanning of the needle tip) was used in 20. Total CT fluoroscopy times represented the total time used during CT fluoroscopy for each procedure, and in some instances it consisted of several individual CT fluoroscopic scans. The two methods were compared by using two-sample Student t tests for their mean CT fluoroscopy times, and mean target sizes were compared by using a logarithmic transformation of the available data. The two methods also were compared with the data stratified according to target size, again by using Student t tests of logarithmically transformed data.
The CT parameters (ie, tube voltage, tube current, collimation) chosen by the operators during the 95 CT fluoroscopy–guided procedures were recorded. Assessments of patient doses were calculated by multiplying the CT fluoroscopy time for each procedure by the CT dose index rate in a 20-cm Plexiglas phantom for each technique used (20). This product was called the "patient dose index." If one wishes to imagine the patient dose index as a patient skin dose, it is the upper bound of the maximum skin dose to a patient of size comparable to the Plexiglas phantom. The upper bound means that the maximum skin dose to such a patient will not exceed the patient skin dose. The CT dose index was estimated at a 2-cm depth from the surface in the Plexiglas phantom by using a 10-cm pencil ionization chamber for a 10-second CT fluoroscopy time (20). The CT dose index rates were 3.1–13.8 mGy/sec.
The patient dose index values provide an overestimation of the patient skin dose because the same patient tissue is not always within the section thickness. The mean patient dose index for all procedures was calculated by averaging the patient dose index for each procedure. The mean patient dose indexes in the real-time and quick-check method groups also were compared by using a two-sample Student t test. The real-time and quick-check methods were compared for the frequency of use of different exposure techniques by using a Fisher exact test. A P value less than .05 was considered to indicate a statistically significant difference.
Estimates of personnel exposure at the level of the operator's hand and neck or body were obtained by multiplying the CT fluoroscopy times by the scatter exposure rates obtained from phantom measurements at 10 cm from the x-ray beam, as an approximation of the position of the operator's hand, and at 100 cm, as an approximation of the position of the operator's neck or body, by using 120 kVp, 50 mA, and 10-mm collimation (20). Estimates of exposure to the operator's hand were obtained by using only the cases (n = 39) in which the real-time method and 120 kVp, 50 mA, and 10-mm collimation were used, which were the parameters for which scatter rate phantom experiment data were available. This estimation of exposure to the hands was based on the assumption that the hands were at 10 cm, the approximate distance when holding the towel clamp attached to the needle, for the entire CT fluoroscopy time. Because quick-check scans were used in some of the real-time cases, this estimate is a maximum.
Hand exposures were not assessed for the cases in which only the quick-check method was used because the hands do not manipulate the needle during quick-check scanning. Estimates of exposure to the operator's neck or body were obtained for all cases in which 120 kVp, 50 mA, and a 10-mm section thickness were used (n = 49). Mean personnel exposure was calculated by averaging the estimates of personnel exposure per procedure.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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All aspirations and drainages were successful, except in one case in the conventional CT group in which the collection was considered inaccessible. There were three self-limited small hematomas in each group and one mild vasovagal reaction in the conventional CT group. There were no significant complications in either group. The mean needle placement time for CT fluoroscopy–guided procedures (29 minutes; standard error = 1.1; range, 6–67 minutes; n = 95) was significantly shorter than the mean needle placement time for conventional CT (36 minutes; standard error = 1.6; range, 17–100 minutes; n = 93; P < .005) and was independent of target size. The one case of an unsuccessful attempt at fluid aspiration by using conventional CT could not be analyzed for mean needle placement time or room time.
There was no significant difference in mean target size between CT fluoroscopy–guided (5.2 cm; standard error = 0.38; range, 1.2–15.0 cm; n = 95) and conventional CT–guided (6.0 cm; standard error = 0.28; range, 0.8–25.0 cm; n = 93) groups. The mean room time for CT fluoroscopy–guided procedures (87 minutes; standard error = 3.4; range, 31–260 minutes; n = 95) was not significantly different from the mean room time for procedures guided with conventional CT (90 minutes; standard error = 3.0; range, 48–190 minutes; n = 93). There was no significant difference in patient demographics (age, sex) between the two groups.
The overall mean CT fluoroscopy time was 79 seconds (range, 8–546 seconds). A CT fluoroscopy time of 546 seconds was used during a difficult procedure involving two catheter drainages and two needle aspirations of four separate fluid collections in the same patient. The mean CT fluoroscopy time was significantly shorter with use of the quick-check method (41 seconds; standard error = 7.4; n = 20) than with use of the real-time method (90 seconds; standard error = 12.2; n = 75; P < .005). The mean target size was significantly smaller in the latter group (4.9 cm; standard error = 0.3; range, 1.2–15.0 cm; n = 75 vs 6.1 cm; standard error = 0.5; range, 3.0–10.0 cm; n = 20; P = .02).
Stratified analysis indicated that radiologists chose real-time more often than quick-check when performing biopsies in masses smaller than 5 cm. Moreover, when biopsies were performed in masses smaller than 5 cm or collections smaller than 5 cm were drained, the mean size of the target when the real-time method was chosen (2.9 cm; standard error = 0.1; n = 40) was smaller than the mean target size when the quick-check method was chosen (3.8 cm; standard error = 0.2; n = 6; P = .02). In contrast, when targets 5 cm or larger were encountered, we did not find a significant difference in target size between the group in which the real-time method was used (7.3 cm; standard error = 0.4; n = 35) and the group in which the quick-check method was used (7.1 cm; standard error = 0.4; n = 14).
The most commonly chosen CT parameters, or tube voltage and tube current, were 120 kVp and 50 mA (n = 50), 120 kVp and 90 mA (n = 26), and 80 kVp and 135 mA (n = 10). In a minority of cases, 80 kVp and 75 mA (n = 4), 120 kVp and 70 mA (n = 3), 80 kVp and 105 mA (n = 1), and 140 kVp and 43 mA (n = 1) were used. Collimation was 10 mm for all but seven cases in which a 5-mm collimation was used. The mean patient dose index (n = 95) was 74 cGy (range, 3–494 cGy) and decreased during the 6-month study period (Fig 4). In the case with a resultant mean patient dose index of 494 cGy, 546 seconds of CT fluoroscopy time was used. There was an initial increase and then a modest monthly decrease in mean CT fluoroscopy times (Fig 5). There was a progressive decrease in the use of relatively higher CT parameters (eg, 120 kVp and 90 mA) and more frequent use of lower parameters (eg, 120 kVp and 50 mA) during the study period (Fig 6).
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The estimated maximal mean personnel hand exposure was 78.6 µC/kg (305 mR) (n = 39) with use of the real-time method at 120 kVp, 50 mA, and 10-mm section thickness. Estimated maximal operator neck or body exposure for all cases with use of 120 kVp, 50 mA, and 10-mm collimation was 2.6 µC/kg (10 mR) (n = 49).
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Compared with results obtained with conventional CT, our study results show the successful use of CT fluoroscopy to guide abdominal biopsy procedures and drainage, but we could not demonstrate an increase in efficacy or a reduction in room time. Test characteristics of CT-guided biopsy and catheter drainage were so good that they were difficult to improve. Needle placement time was reduced, however. This is probably because CT fluoroscopy helped target more effectively. CT fluoroscopy likely helped avoid errors in targeting owing to patient movement or breathing.
The inability to reduce room time was in part because room time at our institution included many aspects of the procedure unrelated to needle placement, which was the only aspect of the procedure affected by this new technology. Although we were able to demonstrate a significantly shorter needle placement time with use of CT fluoroscopy, the reduction was only 7 minutes, and its effect on room time was lessened by the fact that our room times averaged approximately 90 minutes.
There are two major factors that contributed to the length of our room times. First, intraprocedural time was used to teach trainees. Second, cytologists rendered a preliminary interpretation of all biopsy results. This process added approximately 10–20 minutes.
Because the study was performed within weeks of the equipment's installation, there was probably a start-up or learning factor that prolonged needle placement time, as well as the entire procedure. It is possible that with more experience, needle placement time could be shortened further. Also, our needle placement time included acquisition of the initial localizing spiral CT scan. With CT fluoroscopy, targeting could have begun by placing the needle over the patient and scanning the region to determine the puncture site. This method may have resulted in shorter needle placement time but probably would have required more CT fluoroscopy and therefore more radiation exposure to personnel.
CT fluoroscopy has been reported to reduce procedure times (17). Although the absolute reduction in procedure time reported by others was similar to ours, it was considered significant because their definition of procedure time did not include the entire time the patient was in the procedure room (17). It is difficult to compare procedure times between studies because the definition of procedure time (eg, start time, end time, included components of the procedure) differs. Furthermore, procedure times are a result of numerous factors, including procedure type, level of difficulty of the procedure, operator experience, and whether and how much time is used to train residents and fellows.
Our study results demonstrated the potential for high dose. The mean patient dose index was 74 cGy. As a comparison, the CT dose index during two needle-tip localizing spiral abdominal CT scans (120 kVp, 240 mA, 10-mm collimation, pitch of 1, 6 seconds) would be approximately 6 cGy delivered to 6 cm of tissue. Our mean CT fluoroscopy time with use of the real-time method (90 seconds) is comparable to the scanning times reported by others (9,10,12,14,16). The patient dose during CT fluoroscopy could have been reduced with shorter CT fluoroscopy times and lower CT dose parameters (ie, lower tube voltage, lower tube current, and thinner collimation).
During our initial 6-month experience, the patient dose decreased progressively as the use of lower CT parameters was adopted (Figs 4, 6). We now believe that the lower CT parameters (80 kVp and 135 mA) would be qualitatively satisfactory in most upper abdominal cases in which the arms can be raised cephalad to the imaging plane (Fig 7). The higher CT parameters of 120 kVp and 50 mA would be satisfactory in the remainder of cases (Fig 8). Currently, when a tube voltage of 120 kVp is used, we no longer use tube currents of 70 and 90 mA; we begin by using settings of 80 kVp and 135 mA, and sometimes lower, and a tube voltage of 120 kVp and a tube current of 50 mA only when necessary. Thinner collimation may be used if the increased noise that results from thinner sections does not prevent the ability to target the lesion.
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There are ways to reduce radiation exposure to personnel by using needle holders that remove the hands from the beam. One such needle holder removes the hands 4 cm from the beam (9). In our method, we used a towel clamp that removed the hands approximately 10 cm from the beam. We found this method effective and inexpensive for holding and guiding the needle. However, there were times when the needle became dislodged from the clamp, and at other times it was difficult to exert sufficient inward force. Further research in needle holder development is warranted, with the goal of removing the hands as much as possible while maximizing tactile feedback and the ability to maneuver the needle (9,15).
Although we did not use it during the study, we currently are using a lead drape positioned over the patient immediately caudad to the cutaneous access site because it has been shown to reduce scatter exposure to personnel significantly (20). The patient radiation dose and potential exposure to the CT fluoroscopist that we report are considerably lower than those previously reported with use of the same model scanner (17). However, our phantom-derived dose rates are consistent with the manufacturer's specifications.
Our report describes two basic guidance methods of using CT fluoroscopy and their implications for radiation exposure. Others have used methods similar to our real-time method (2–17). Because of the possibility of high radiation exposures when using CT fluoroscopy to either continuously or intermittently view an advancing needle, we developed and evaluated the quick-check method to reduce radiation exposure. The quick-check method, which uses less CT fluoroscopy time, used less radiation than the real-time method but was used in slightly larger targets. Another factor contributing to the lower mean dose in the quick-check group may be that the relatively lower dose exposure parameters of 80 kVp and 75 mA or 80 kVp and 135 mA were more likely to be used with this method, and the higher dose parameters of 120 kVp and 90 mA were more likely to be used with the real-time method. Radiologists may have preferred the parameters of 120 kVp and 90 mA when using the real-time method because the lesions were smaller in this group.
The same targeting methods are used with the quick-check method as with conventional CT, but needle-tip location can be more easily and quickly checked with a short CT fluoroscopic scan and the in-room monitor (Fig 3). Large targets also afford the use of lower CT dose parameters because, given a fixed target-to-background contrast-to-noise ratio, they are easier to visualize. It is possible in some cases to use the quick-check method to view the position of a drainage catheter with the stiffening cannula in place when entering anteriorly, or posteriorly in a patient in the prone position (Fig 9). This is not possible with many CT systems, as there is not enough space to fit the catheter in the gantry, particularly with side entries. The quick-check method can be used to image the catheter after the stiffening cannula has been removed and during catheter placement with use of the Seldinger technique (Fig 10).
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Our patient dose index and personnel exposure values are reference doses, that is, they represent maximal values and provide a basis for comparisons between devices or protocols. The actual patient doses were lower because the CT fluoroscopic table is moved about the needle, and therefore CT fluoroscopy did not occur always at the same location. The case in which the highest mean patient dose index was estimated is an overestimation because four different body regions were scanned. We have no information regarding patient location during CT fluoroscopy and could not estimate the magnitude of dose reduction. More accurate estimates of patient dose could be provided by obtaining a record of patient locations during CT fluoroscopy or by estimating skin exposures by using thermoluminescent dosimetry, or TLD, measurements. To further evaluate personnel exposure, we now wear ring radiation badges from which we can measure radiation exposure.
To optimize the use of CT fluoroscopy for this initial evaluation, we chose conservative estimates of patient dose and personnel exposure. Also, as noted earlier, our study was conducted soon after the arrival of the equipment. These factors would tend to increase CT fluoroscopy times, and hence radiation exposures, and room times. The learning curve associated with new technology also likely affected the choice of CT dose parameters during the procedures, with higher doses used more often initially.
The comparison of the quick-check and real-time methods may have been affected by selection bias. Radiologists chose the real-time method when targeting small masses, as shown by results of statistical comparison of lesion sizes in both groups, and they probably chose the real-time method when performing technically difficult procedures. These factors tended to increase CT fluoroscopy times in the real-time group.
In conclusion, use of CT fluoroscopy reduced the time required to target abdominal masses and fluid collections. While it did not reduce the CT procedure room time or improve the already high efficacy of conventional CT–guided biopsy, it has been embraced enthusiastically by our interventional team because of its ease of use and because it facilitates the performance of difficult abdominal interventions. Although CT fluoroscopy is a useful targeting technique, because significant radiation exposures may result, radiologists need to be aware of different methods of CT fluoroscopic guidance and the factors that contribute to radiation exposure.
Because radiation exposures to both patient and personnel were high in our study, we now recommend use of CT fluoroscopy in selected cases and use of the quick-check method whenever possible. However, the real-time method is a useful way to target small or deep targets. In this case, the lowest possible CT parameters necessary to view the target should be used. A comprehensive educational program of the potentially high patient and personnel exposures should be implemented to teach radiologists, nurses, and technologists. Use of lead drapes on the patient (20) or needle holders (9,15) and development of ways to reduce radiation exposure at the time of image acquisition (23) are warranted.
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Acknowledgments |
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Footnotes |
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