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Medical Physics |
1 From the Dotter Interventional Institute, Oregon Health Sciences University, 3181 SW Sam Jackson Park Rd, MC L-605, Portland 97201. Received February 7, 2000; revision requested April 4; revision received April 24; accepted May 1. Address correspondence to R.T.A. (e-mail: andrewro@ohsu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Clinical and procedural factors were evaluated for 42 consecutive procedures performed in 39 patients by one operator. Seven patients were excluded because of early termination (n = 1) or unusual conditions that necessitated extended procedures (n = 6). Fluoroscopic time, number of images acquired, height, and weight were available in the 35 remaining patients, and dose-area product (DAP) was available in 20. Equipment factors were evaluated by using a Lucite phantom in four angiography units from three manufacturers.
RESULTS: The mean fluoroscopic time per case decreased from 30.6 to 14.2 minutes between the 1st and 5th quintiles. Mean DAP decreased from 211.4 to 30.6 Gy · cm2 with dose reduction techniques; this primarily reflected a decreased number of acquired images. Phantom studies demonstrated many significant dose variations with magnification and equipment position. Low-dose and pulsed fluoroscopic modes reduced exposure rates in units so equipped, but roadmapping caused a silent switch to continuous fluoroscopy in two such units, which doubled the exposure rate.
CONCLUSION: With operator experience and careful technique, uterine arterial embolization can be performed at radiation exposures comparable to those used in routine diagnostic studies. However, operators must be familiar with the technical parameters of their angiographic equipment.
Index terms: Arteries, therapeutic embolization, 969.1264 • Dosimetry • Leiomyoma, 854.318 • Radiations, exposure to patients and personnel • Uterine neoplasms, therapy, 854.1264, 854.318
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Given the huge potential volume of uterine arterial embolization cases and the dual goals of organ conservation and retained fertility, the issue of radiation exposure to patients during this procedure has, in our opinion, been critically underevaluated. The uterus and ovaries are in the direct path of the radiation beam throughout uterine arterial embolization and cannot be shielded. The gonads are among the most radiation-sensitive organs in any individual, regardless of age, sex, or fertility, and the potential for malignant degeneration increases directly with cumulative radiation dose (6). Furthermore, it must be remembered that, at birth, the ovaries contain all the oocytes that a woman will ever produce. Thus, the potential for genetic injury to her offspring also varies directly with dose (7). Despite these crucial facts, to our knowledge, only a single study of uterine arterial embolization dosimetry (8) had been published, and only two related abstracts (9,10) had been presented at the time this article was written.
The purpose of this study was to investigate patient radiation exposures during uterine arterial embolization and to determine the degree to which various clinical and procedural factors affect the dose delivered to the patient.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Seven procedures (17%) were considered atypical and were not included in the data analysis. Three of the seven constituted a "second look" in patients with persistent symptoms and were primarily a search for collateral blood supply. Although the first procedure in each of these patients was included in our data, the second was not. One patient was excluded because an incidentally discovered ovarian arterial collateral vessel necessitated subselective microcatheter embolization via the ovarian artery (11). This atypical aspect of the procedure represented the majority of our fluoroscopic time and image acquisitions. Another patient had essentially normal uterine arteries bilaterally, with no visualization of a hypervascular uterine mass, and we elected not to proceed with embolization. Two additional patients were excluded because they had unusually tortuous pelvic arterial anatomy that necessitated complex catheter manipulation. The total fluoroscopic time, number of images acquired, and patient height and weight were tabulated in the remaining 35 cases.
Embolization of both uterine arteries was performed through a 4- or 5-F Berenstein catheter (Cook, Bloomington, Ind) and with a right common femoral arterial approach. Coaxial microcatheters (Tracker; Boston Scientific/Meditech, Watertown, Mass) were used on a case-by-case basis when a uterine artery was found to be small or prone to spasm. The primary embolic agent in all cases was 355–500-µm polyvinyl alcohol from different vendors (Cook, Boston Scientific/Meditech, and the Cordis Division of Johnson & Johnson, Miami, Fla).
All procedures were performed by using one of two angiography units (V3000 or I2000; Philips Medical Systems North America, Shelton, Conn). The former was equipped with an integrated dosimeter that automatically measured the product of radiation absorbed dose in grays and the area exposed in square centimeters. The resultant dose-area product (DAP) was reported as fluoroscopic (live) and acquisition (imaging "run" and spot radiography) contributions to the total (in grays multiplied by square centimeters). No such capability was present in the latter unit. Both angiography units automatically recorded the total fluoroscopic time. Procedures were assigned to either unit on the basis of room availability and daily caseload. Early in the study, access to the V3000 unit was frequently limited by construction activity in the adjacent area. Acquisition and fluoroscopic DAP were tabulated for the 20 patients treated in the DAP-capable unit.
Clinical Group 1
This group included the first 16 patients who were treated. In the first four patients, we did not perform initial nonselective pelvic arteriography but rather proceeded directly to uterine arterial catheterization by performing roadmapping or digital subtraction arteriography (DSA) in the internal iliac artery as necessary. After the serendipitous identification of a large ovarian arterial collateral vessel in an excluded patient, initial DSA in the pelvis by using a nonselective (pigtail) catheter (Cook) at the level of the renal arteries was incorporated for all subsequent patients. In the first six patients, our end point was distal arterial stasis, with preservation of slow flow in the proximal uterine artery. After treating the second patient who had persistent symptoms and required an additional procedure, we used a slurry of Gelfoam (Pharmacia & Upjohn; Kalamazoo, Mich) to provide complete stasis in the uterine artery after polyvinyl alcohol embolization in all subsequent patients.
Clinical Group 2
This group included the remaining 19 of the 35 patients reported on. After performing the 20th uterine arterial embolization procedure (16 reported as clinical group 1 and four excluded), we reviewed patient dosimetry for quality control purposes. By that point, four patients had been treated with the DAP-capable angiography unit. The results of our review led us to begin a dose-phantom evaluation of imaging procedures, which was followed by an aggressive clinical effort to minimize dose. We discontinued all DSA examinations, with the exception of the initial nonselective pelvic study. Instead, angiographic findings were documented by recording the last image obtained with contrast material injection under fluoroscopic guidance. The single DSA study was tightly collimated to show only the true pelvis (a square field of approximately 15 cm per side) and recorded at one frame every other second. Tight collimation was also used during fluoroscopy (a square field of approximately 10 cm per side), and roadmapping was performed liberally to facilitate catheterization and, as discussed later, to increase our sensitivity to reflux. Finally, we discontinued magnification and raised the procedure table as far from the radiographic tube as possible without inconveniencing the operator. Neither pulsed nor low-dose fluoroscopy was available in either angiography unit.
Phantom Studies
Dosimetric measurements with use of the block phantom were obtained by using four angiography units: a model V3000 (Philips Medical Systems North America), which was used for many of our clinical procedures, a model V5000 (Philips Medical Systems North America), a model OEC 9600 (GE Medical Systems, Milwaukee, Wis), and a Multistar (Siemens Medical Systems, Iselin, NJ). These units were selected solely on the basis of their presence at either our facility or the adjacent Veterans Administration Medical Center. The specific capabilities and configurations tested for each unit are shown in Table 1.
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In this study, "fluoroscopic mode" referred to the nature of the radiation beam generated during live imaging. Continuous fluoroscopy involves a steady uninterrupted beam. Pulsed fluoroscopy involves an intermittent beam that is produced at a predetermined rate of 2–30 pulses per second. Low-dose fluoroscopy can be continuous or pulsed and typically results from the interposition of an absorbent filter between the source tube and the patient, in the imaging tube housing. With one exception (the model V3000), each angiography unit could generate a pulsed or low-dose fluoroscopic beam. Finally, "roadmap imaging" referred to a fluoroscopic mode in which a selected background image was subtracted from the live image.
The exposures generated in any configuration or mode were measured by using a model 9015 dosimeter with a 6-mL ion chamber (Radcal, Monrovia, Calif) and a 20-cm-thick Lucite block phantom (Nuclear Associates, Carl Place, NY). For each measurement, the dosimeter was centered in the direct path of the beam and under the phantom (ie, at the table surface). The measured radiation exposure rate (in millicoulombs per kilogram) thus corresponded to the skin entry dose that would have been experienced by the average patient under identical imaging conditions. The exposure rate and automated imaging parameters responsible for the dose—peak kilovoltage and milliamperage (recorded from the angiography units’ control panel)—were recorded simultaneously during fluoroscopy performed under the conditions described as follows.
The effects of imaging configuration, fluoroscopic mode, and magnification were evaluated by using constant collimation (standardized to a 14-cm square indicated by metallic beads on the block phantom). The maneuvers used are shown in Figure 1. Beginning with minimum source-image, source-object, and object-image distances (configuration A), the exposure variables (millicoulombs per kilogram per minute, kilovolt potential, and milliamperage) were recorded for each available combination of magnification and fluoroscopic mode.
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The effect of roadmap imaging was studied for each unit, with source-image distance, source-object distance, object-image distance, and magnification minimized and with constant 14-cm collimation. Exposure rate, kilovolt potential, and milliamperage were noted in each fluoroscopic mode before and after activation of the roadmap feature. Other than initiation of the roadmap, no other changes were made between measurements.
Because our goal was to evaluate the effects of various imaging parameters on radiation dose rather than compare angiography units, the exposure rates presented for each configuration in a given unit (Ep) were internally standardized. This was accomplished by dividing each measured exposure rate (Em) by the lowest rate measured in any configuration with the same unit (El). Thus, Ep = Em/El, and the lowest exposure rate presented for each angiography unit was 1.00. For the roadmap test, the measured exposures were standardized to those recorded immediately before activation of the roadmap feature.
Statistical Analyses
Clinical and phantom data were analyzed for differences by performing the Student t test. The degree of dependent interaction between data sets was determined by performing the Pearson product-moment correlation. The significance of the correlation coefficient r was then determined by using t = r/ ([1 - r2]/[n - 2])
to obtain the Student t test statistic. The effects of imaging configuration on exposure were evaluated by comparing the true linear measurements of source-image distance, object-image distance, and source-object distance with the internally standardized exposures in standard fluoroscopic mode, without magnification. This comparison pooled the combined data for all recorded imaging configurations from each unit. For all evaluations, the P value considered to indicate a significant difference was less than .05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Fluoroscopic Time
By quintile, the mean fluoroscopic times were 30.6 minutes ± 5.8, 25.6 minutes ± 6.3, 17.3 minutes ± 6.0, 14.1 minutes ± 1.9, and 14.2 minutes ± 3.2 (Fig 2). The mean fluoroscopic times for groups 1 and 2 were 27.0 minutes ± 6.6 and 14.8 minutes ± 3.8, respectively (Table 2, Fig 3). Only three of the first 16 patients had fluoroscopic times less than 20 minutes. By contrast, only two of the subsequent 19 patients had fluoroscopic times greater than 20 minutes. The correlation between study patient number and fluoroscopic time (r = -0.80) was significant (P < .001). Nonetheless, our efforts to reduce dose did not clearly accelerate the preexisting downward trend: By that point in our experience, the fluoroscopic time per patient had already reached a single-patient low of 15.5 minutes. A reproducibly short fluoroscopic time was noted after patient 20; the cumulative SD in patients 1–20 was 7.7 minutes (31% of the mean for that group), whereas the SD in patients 21–35 was 2.5 minutes (18% of the mean). Fluoroscopic time did not correlate with the clinical features (height and weight) of individual patients.
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DAP information for these 20 patients was further evaluated to identify the source of statistical difference. The mean numbers of DAP runs and acquired images per patient decreased from five runs, 65.3 images in group 1 to one run, 4.8 images in group 2. DAP rates (ie, DAP per minute of fluoroscopy or per acquired image) also had decreasing trends. The fluoroscopic DAP rate (in Gy · cm2 per minute) decreased from a mean of 4.5 ± 1.9 for the first four patients to 1.6 ± 0.3 for the subsequent 16 patients, whereas the mean acquisition DAP rate (in Gy · cm2 per image) decreased from 2.1 ± 1.2 to 1.3 ± 0.8 in the same patient groups. Neither trend was significant, although the P value for the fluoroscopic DAP rate was close to significant at .051. DAP rates did not correlate with height in either patient group or with weight in group 1. However, among the patients treated after the initiation of dose-reducing techniques, there was strong correlation between DAP per acquired image and weight (r = 0.82; P < .01) and body mass index (weight in kilograms/height in meters2) (r = 0.76; P < .01). Height and weight relationships are demonstrated in Figure 4.
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Imaging parameters.—The overall mean kilovolt potential for all measurements and units was 90 ± 12, with a range of 71–110. In general terms, kilovolt potential varied directly with source-image (P < .01) and object-image distance (P < .05). The greatest change with position was seen in the V3000 unit, in which an adjustment from 78 to 85 kVp was seen during the transition from configuration C to configuration E. The smallest change was seen in the Multistar unit, in which 72 kVp was maintained in all imaging configurations. Kilovolt potential also varied directly with magnification, but this trend was significant in only the V3000 and V5000 units (P < .01). The response of kilovolt potential to fluoroscopic mode differed among units; it was unchanged by mode in the Multistar unit, increased by roughly 20 kV with low-dose mode in the V5000 unit, and increased by 5 and 10 kV in pulsed and low-dose modes, respectively, in the OEC 9600 unit.
The mean milliamperage for all measurements and all units was 7.6 ± 4.2, with a range of 0.9–24.5. A correlation existed between milliamperage and source-image distance in the V3000 and Multistar units (P < .001), whereas no correlation existed in the OEC 9600 and V5000 units. The OEC 9600 and Multistar units each demonstrated a correlation between milliamperage and magnification (P < .05), whereas the V3000 unit showed no change with magnification and the V5000 unit had an inverse correlation (P < .05). The Multistar unit demonstrated the greatest changes in milliamperage with table configuration and magnification, with an increase from 4.2 to 6.3 during the transition from configuration A to configuration E and from 6.3 to 24.5 when magnification was then increased from a factor of 1 to a factor of 2.86.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Whereas fluoroscopic time varied inversely with operator experience, the DAP per case was dependent on factors under the operator’s direct control; acquisition and total DAP decreased abruptly and significantly with the initiation of dose-reducing techniques. The mean total DAP among patients in group 2 was 30.6 Gy · cm2 ± 13.6. Multiplying this number by 0.368 mSv/Gy · cm2, the published conversion coefficient for a similar field of view (imaging of the urinary bladder) at 90 kVp with 2.5-mm aluminum filtration indicates an effective dose of 11.3 mSv (12). By comparison, the DAP for barium enema has been reported at 8–29 Gy · cm2, or up to 11 mSv (13,14). Computed tomography of the abdomen and pelvis generates an effective dose of 3.9–20.0 mSv (15–17).
Since the fluoroscopic DAP did not change significantly, the observed DAP reduction must be attributed primarily to the reduction in acquisition DAP. Furthermore, since the DAP per image did not change significantly, the reduction in acquisition DAP must, in turn, be attributed to the reduction in acquired images. Solving for the ratio of mean acquisition DAP rate to mean fluoroscopic DAP rate among the patients in group 2 reveals that each acquired image was equivalent to 49.9 seconds of fluoroscopy. The implication is that the dose can be reduced considerably by limiting the number of images acquired. Because the dose per image paralleled the patient weight, image reduction appears to be of even greater importance in heavier patients. In both of the angiography units used for uterine arterial embolization at our facility, it was possible to record and print on film the last fluoroscopic image from each injection of contrast material. Doing so allowed us to avoid dedicated DSA runs and thus made possible the marked reduction in the number of images acquired among the patients in group 2.
After the elimination of extraneous image acquisition, fluoroscopic DAP became the largest factor in patient dose. Among the patients in group 2, fluoroscopic DAP composed, on average, 82% of the total radiation exposure. Thus, although the strong trend toward decreasing fluoroscopic dose rate (DAP per minute) was not significant in our overall findings (P = .051), the rate nonetheless became a critical component of patient exposure among later cases. Our phantom data indicate that the fluoroscopic exposure rate can be significantly reduced with favorable imaging configuration (maximal source-object distance and minimal object-image distance), regular use of low-dose or pulsed fluoroscopic modes, and limited use of magnification. Similar results have recently been reported with the use of a water phantom (18).
In electing to use any or all of these mechanisms for fluoroscopic dose reduction, one must consider their secondary effect on image quality and operator comfort. Increasing the source-image distance generally increased kilovolt potential and would thus be expected to reduce image contrast (19). At the same time, the introduction of an air gap (increased source-object distance) reduces photon scatter and the image penumbra and thereby improves spatial resolution, although this factor was not measured in our study (19). Finding an acceptable balance between these competing effects and finding a table height that does not interfere with comfortable and effective catheter manipulation are subjective matters that are best left to the individual operator. However, our results indicate that the most favorable configuration for dose reduction is with the table as high and the image intensifier as close to the patient as possible. It should be remembered that an air gap increases radiation scatter to the operator, so the use of additional floor-level shielding should be considered when the source-image distance is increased (20).
Pulsed fluoroscopy at very low rates can result in a lurching, noncontinuous image and discontinuous visualization of the catheter. However, pulsed fluoroscopy at higher rates, as compared with continuous fluoroscopy, may actually improve image resolution by eliminating motion-related blur. Again, the appropriate balance between these effects is a subjective issue. We have anecdotally found pulse rates of less than 15 per second to be unacceptable.
We believe that the potential value of roadmap imaging and/or magnification during small vessel catheterization is self evident. However, we have not found magnification to be helpful during the actual embolization procedure. Instead, we rely on "reverse roadmapping" to improve our sensitivity to the reflux of embolic material. To use this technique, one initiates the roadmap feature and then acquires a mask image without the intravenous injection of contrast material. Because contrast and embolization material are subsequently injected during roadmap fluoroscopy, the contrast material is easily identified against an otherwise homogeneously blank background.
Our results indicate that the potential benefit of roadmap imaging—in the standard or reversed method—may come at the cost of increased dose delivery to the patient. Whether such an increase occurred was entirely dependent on the angiographic equipment and baseline fluoroscopic mode used. We found a significant increase in fluoroscopic exposure rate when the roadmap feature was activated from pulsed or low-dose modes in the OEC 9600 and Multistar units but no such change in the V5000 unit. Because the change, when it occurred, was not indicated to the operator, it would be possible for roadmapping in such circumstances to more than double the rate of patient exposure without this fact being recognized.
Our clinical results differ from those reported by Nikolic and colleagues (8) in the only published study of uterine arterial embolization dosimetry known to us. Investigators in that group used endovaginal and external thermoluminescent dosimeters to measure patient exposures during uterine arterial embolization. With a mean fluoroscopic time of 21.89 minutes (range, 8.9–52.5 minutes), the absorbed skin-entry and ovarian doses were estimated to be 162.32 and 22.34 cGy, respectively. The ovarian dose was said to be 30–100 times higher than that in conventional fluoroscopic studies, including barium enema. In the patients in group 2, the estimated effective dose (in millisieverts) at uterine arterial embolization was similar to that at barium enema. In addition, if we assume a mean field size of 100 cm2 (10-cm2 collimation, as was typical in our procedures), the mean total DAP for the patients in group 2 corresponded to a skin-entry dose of 31 cGy. Although differences in measurement technique (thermoluminescent dosimeters vs DAP) make direct comparison of these studies difficult, it is noteworthy that the means of fluoroscopic time and number of images acquired (44 images; range, 21–62) during uterine arterial embolization among Nikolic and colleagues’ patients were substantially greater than among the patients in group 2.
By definition, DAP describes the amount of radiation delivered over a specified area rather than the amount actually absorbed. Identical exposure rates produce different DAPs if the collimation is allowed to vary. Thus, a specific organ dose can be estimated from DAP only if the beam geometry is constant and known. Nikolic and colleagues’ measurement technique (8), placement of a dosimeter in the vagina, may be more accurate. However, in our opinion, the technique is invasive and impractical on a routine basis and does not provide continuous feedback during the procedure, as does use of a DAP meter. Furthermore, in a patient with very large fibroids, the ovaries can be far from a dosimeter in the vaginal vault. Accordingly, Nikolic and colleagues excluded three patients from their analysis because their vaginal and skin measurements were discordant, which suggested that the endovaginal dosimeters were inadvertently excluded from the field of view.
The purpose of DAP measurement is to provide continuous patient dosimetry during a procedure and to facilitate interprocedural standardization. Investigators in some studies (21,22) have found DAP to be superior to other dose measurement techniques, including skin-entry dose. DAP meters are readily available, easily standardized, and do not require an invasive procedure for their use. For these reasons, DAP meters are a standard feature on many current-model angiography units and are required in many European countries. The relative DAP reductions observed in the patients in our study are good, if circumstantial, evidence that absorbed ovarian and uterine doses were reduced. Such issues do not apply to our phantom studies, in which the collimation was constant and the measured variable was the exposure delivered to the entry site, not the DAP.
A limitation of this study was that all of the units tested had been tailored to suit operator preferences. Although basic imaging protocols are incorporated into each angiography unit, most units can undergo after-market modification of these parameters to suit individual preferences. These modifications can and do include such critical factors as the presence and type of beam filtration and the frequency of imaging pulses during pulsed fluoroscopy. It would be neither accurate nor appropriate for the reader to assume that our examinations encompassed all possible imaging configurations in any of the units tested. It was for this reason that our investigation focused on operational differences within rather than among angiography units.
In conclusion, although initial clinical experience and patient morphology are beyond the control of the operating physician, uterine arterial embolization can be accomplished by using patient radiation exposures equivalent to those used in routine diagnostic imaging procedures.
Operator experience is critical in the reduction of overall fluoroscopic time, and in our experience, technical proficiency required the performance of 20 uterine arterial embolizations. Procedural technique also plays a deciding role in patient exposure during uterine arterial embolization and is under the control of the operator, regardless of his or her experience. Because image acquisition constitutes an important aspect of patient exposure, extraneous imaging should be avoided. This can be accomplished by recording on film the last fluoroscopic image in lieu of a dedicated DSA acquisition. During fluoroscopy and image acquisition, the table height (source-image distance) should be maximized and the distance between patient and image intensifier (object-image distance) minimized, with due consideration of image quality and radiation scatter to the operator. Magnification should be avoided; when necessary, it should be strictly limited. Roadmap imaging may be associated with silent increases in dose, an issue that must be investigated individually for each angiography unit.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, R.T.A.; study concepts and design, R.T.A.; definition of intellectual content, R.T.A.; literature research, R.T.A., P.H.B.; clinical and experimental studies, R.T.A., P.H.B.; data acquisition and analysis, R.T.A., P.H.B.; statistical analysis, R.T.A.; manuscript preparation, R.T.A.; manuscript editing and review, R.T.A., P.H.B.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) and fluoroscopic DAP (vertical white bars) begins to stabilize after patient 20. Patients in whom fluoroscopic time alone is represented were those treated by using an angiography unit without DAP recording capability.
= OEC 9600, 
= V3000, 
= V5000. 1 R = 0.258 mC/kg.
