Therapeutic angiographic procedures: differences in dose area product between analog image intensifier and digital flat panel detector

Abstract

Background

Radiation exposure remains an unceasing concern in angiographic procedures. Modern angiography machines such as analog image intensifiers (AII) or the new flat panel detectors (FPD) aim at a further dose reduction.

Purpose

To present dose area products (DAP) in a broad spectrum of therapeutic angiographic procedures, comparing an AII to an FPD angiography system.

Material and Methods

A total of 999 peripheral therapeutic angiography procedures performed with an FPD (n = 562) and an AII system (n = 437) were evaluated. DAP, fluoroscopy time, and patients' body mass index (BMI) were recorded. Interventions were classified into five main groups: percutaneous transluminal angioplasty (PTA); PTA and stent placement; intra-arterial thrombolysis; embolization procedures; and specialized interventions.

Results

DAP values in therapeutic angiographic procedures were significantly higher when performed with the FPD compared to the AII system. The increase of the FPD versus AII system was 100.1% for PTA, 39.9% for PTA and stent placement, 187% for intra-arterial thrombolysis, 31.3% for embolization procedures, and 361% for specialized interventions. These differences persisted after standardizing DAP values to the geometric mean fluoroscopy duration of each procedure. Fluoroscopy times were shorter in all interventions performed at the FPD as compared to the AII system. DAPs increased with higher BMI, but the DAP increase of both systems with elevated BMI was variable, depending on the individual intervention.

Conclusion

In therapeutic angiographic procedures, the FPD system required higher DAPs despite shorter fluoroscopy times as compared to an AII system. Better ergonomics and speediness of the FPD system may be advantageous in the emergency setting.

Keywords

Therapeutic angiography intervention radiation exposure technical aspects

Introduction

Due to new minimally invasive therapy and improvements in skills and techniques, the number of peripheral therapeutic angiographies continuously increases: Percutaneous transluminal angioplasty (PTA) with or without stent placement and intra-arterial thrombolysis have evolved to safe and effective treatments of ischemia and vascular stenoses (1,2). Transarterial chemoembolization in liver tumors (3) and embolization procedures in acute arterial bleeding or arteriovenous malformations are routinely performed. Transjugular hepatic portosystemic shunts decrease portal venous pressure in patients with portal hypertension (4) and vena cava filters are placed to reduce the risk of fatal pulmonary thromboembolism in venous thrombosis (5).

However, radiation exposure remains an unceasing concern in all diagnostic and therapeutic angiographic procedures (6). To minimize radiation doses, we place our hopes on the development of new angiography machines. Analog image intensifiers (AII) are used since the 1980s and enable high image quality and dose reductions (7). Pulsed fluoroscopy can further decrease radiation exposure but needs to be balanced against the decrease in visibility (8,9). There is conflicting evidence about a possible dose reduction with the advent of modern flat panel detectors (FPD). Tsapaki et al. report as far as 2.5 times better resolution and up to five times lower radiation doses with the use of FPD systems (10), whereas the results of Davies et al. speak against an improvement in image quality or dose efficiency of FPD systems over AII systems (11). In a previous in vivo study on diagnostic angiographic procedures, using the same angiography systems we found higher DAP values despite shorter fluoroscopy times and greater DAP gain with higher body mass index using an FPD system compared to an AII system (12).

In the current literature only few studies exist on the radiation exposure in selected therapeutic non-cardiac angiographic procedures (13 –16) and we did not find a comparative analysis of the radiation exposure in FPD versus AII systems in the therapeutic non-cardiac setting. Here, we set out to retrospectively determine whether the findings of diagnostic angiography (12) also apply to a therapeutic angiographic setting. Additionally, we present up-to-date DAP values for a broad spectrum of peripheral therapeutic angiographic interventions in a large patient population (n = 999).

Material and Methods

Angiography systems

This retrospective study used the Axiom Artis flat panel system (FPD) and the Fluorospot TOP analog image intensifier-based system (AII) (both Siemens Healthcare, Erlangen, Germany). The Axiom Artis is a monoplane system using conversion. Cesium-iodide crystals form a scintillator layer and convert Röntgen-photons to light photons which are then detected by photodiodes. Fluoroscopy pulsing can be performed with a frame rate of up to 30 pulses/min, spotfilm, and three magnification modes. The FPD system uses a dedicated automatic dose control and automatic spectral beam selection in fluoroscopy. Start values of the tube voltage were 66 kV without filter and 81 kV using a filter placed at the radiation detector side (3.6 µGy nominal dose/fluoroscopy image, 440.6 mA per 96.8 ms pulse time, 42.7 mAs tube current-time-product, 0.2 mm copper filter, 1 picture/s image frequency).

The Fluorospot TOP (AII) is also a monoplane system with three magnification views. It uses a charge-coupled device (CCD) camera with fluoroscopy pulsing of up to 30 pulses/s and spotfilm. The X-ray tube is equipped with automatic dose control and spectral beam filters resulting in a 1024 × 1024 pixel matrix of the CCD camera. Similar to the FPD system, start values of the tube voltage were 64.5 kV without filter and 81 kV using a filter placed at the radiation detector side (5.4 µGy nominal dose/fluoroscopy image, 494.3 mA per 73.1 ms pulse time, 36.0 mAs tube current-time-product, 0.3 mm copper filter, 1 picture/s image frequency).

Interventions and dosimetry

We retrospectively evaluated 999 therapeutic angiographic interventions performed with the FPD system (installed 2005, updated 2006) and AII system (installed 2004). The interventions were performed by four interventional radiologists using both machines evenly. Interventions were classified into five main groups: percutaneous transluminal angioplasty (PTA); PTA and stent placement; intra-arterial thrombolysis; embolization procedures; and specialized interventions. A subsequent subdivision of each group according to body region or explicit intervention enabled an additional dedicated within-group comparison. Images were obtained in supine position with a preferentially small distance between patient and detector/image amplifier. Body parts not showing the treated vessels were shielded.

Dose area products are an established proxy for the effective radiation dose (17), and were measured intrinsically by both systems. The fluoroscopy time and number of series were also recorded. The accuracy of the systems' intrinsic DAP measurements was tested by the manufacturer and was continuously controlled every 3 months by our institute's radiation physicists, as follows: both radiation detectors were positioned over the table and the radiation sources under the table (focus-detector distance 112 cm for both machines). Then, the same plastic phantom was placed on both tables with a roentgenologic density comparable to a standard patient. A squared plate subdivided into multiple 1 cm² squares was placed under the phantom on the table and an ionization chamber (Diados PTW at 70 kV, 2.5 mm Al filter) back-fixed under the table. The distance between ionization chamber and focus was 76 cm for both systems. Then pulsed fluoroscopy was applied at 30 pulses/min (AII system: 70 kV, 0.2 mm Cu filter; FPD system: 66 kV, 0.3 mm Cu filter). The radiation field (cm²) was calculated as:

Radiationfield = a \times b \times (\frac{dist . focustoradiationchamber}{dist . focustosquaredplate})^{2}

(a = length [cm] and b = height [cm] of radiation field, dist. = distance).

The radiation field was multiplied by the DAP measured by the Diados PTW ionization chamber. This value serves as reference for the intrinsically measured DAP of each machine, respectively. The DAP values can be compared and related to each other under the presumption of similar radiation qualities, kV values, and filters. When comparing the intrinsic DAP measurements to the reference of the ionization chamber, an overestimation of the radiation dose was noted as 10.7% with the FPD system and 15.4% with the AII system. Therefore, intrinsic AII-system DAP measurements were calculated to be only 4.7% ( ± 4%) higher than intrinsic FPD system DAP measurements, and no further adjustments of intrinsic DAP measurements were performed.

Height and weight were recorded in all patients. The body mass index (BMI) was calculated as:

BMI = \frac{weight [kg]}{height {[m]}^{2}}

Statistical analysis

Statistical analysis was performed using the JMP 7.0.2 software (SAS Institute, Cary, NC, USA). As the DAP data were markedly positively skewed they were mathematically transformed by taking the logarithms prior to further analysis. The logarithmic transformation resulted in a good fit to the normal distribution, enabling a subsequent parametric data analysis. Accordingly, the DAP values are presented as geometric means (18). To objectively compare both systems and exclude differences in fluoroscopy duration as bias we performed a second analysis and additionally standardized DAP values for the mean fluoroscopy time of each procedure by multiplying each intervention's DAP with the quotient of “mean fluoroscopy time / intervention's real fluoroscopy time”. The comparative analyses were performed using the student's t-test. For all tests, P values less than 0.05 were considered as statistically significant.

This retrospective study was approved by our institutional review board, which waived informed consent.

Results

DAPs in therapeutic angiography – FPD versus AII system

We found consistently higher DAP values in therapeutic angiographic procedures performed with the FPD system compared to those performed with the AII system. The geometrically averaged DAPs for PTA were 100.1% higher with the FPD compared to the AII system (Table 1, P < 0.05). For PTA and stent placement, a 39.9% increase was observed with the FPD compared to the AII system (Table 2, P < 0.05). In intra-arterial thrombolysis, geometrically averaged DAPs were 187% higher with the FPD compared to the AII system (Table 3, P < 0.05). Embolization procedures resulted in a 31.3% increase with the FPD compared to the AII system (Table 4, P < 0.05). In specialized interventions geometrically averaged DAPs were 361% higher with the FPD compared to the AII system (Table 5, P < 0.05).

Table 1.

DAPs for percutaneous transluminal angioplasties (PTA) according to body regions.

	n	FT (cGy × cm²)	Time (s) FT	AA (cGy × cm²)	Time (s) AA
PTA (arm)	8	110.5 (16.2–756.3)	711.5	499.6 (50.2–4968.8)	383.8
PTA (pelvis)	14	1500.3 (237.4–9480.0)	1804.6	7098.5 (2943.1–17120.8)	1236.6
PTA (thigh)	145	330.2 (241.6–451.3)	1132.3	573.8 (447.9–735.2)	821.6
PTA (thigh + lower leg)	14	286.0 (90.9–900.3)	1189.7	508.8 (246.6–1049.8)	1040.4
PTA (lower leg)	52	145.9 (109.3–194.9)	910.8	407.3 (312.6–530.8)	852.6
PTA (all regions)	233	297.0 (n = 96)	1123.5	594.2 (n = 137)	850.3

For both, the Fluorospot TOP image intensifier system (FT) and the Axiom Artis FPD system (AA) geometrically averaged DAPs (cGy × cm²) are listed and 95% confidence intervals are given in brackets. Fluoroscopy time (Time) is recorded in seconds.

Table 2.

DAPs for percutaneous transluminal angioplasties with stent placement (PTA + Stent) according to body regions.

	n	FT (cGy × cm²)	Time (s) FT	AA (cGy × cm²)	Time (s) AA
PTA + Stent (pelvis)	62	5880.0 (3756.0–9205.3)	1059.3	6691.0 (5023.8–8911.5)	535.3
PTA + Stent (thigh)	92	484.6 (354.2–663.1)	1051.2	852.0 (697.3–1040.9)	755.6
PTA + Stent (thigh + lower leg)	38	237.9 (167.1–338.8)	1233.8	353.7 (283.5–441.2)	740.2
PTA + Stent (lower leg)	27	237.5 (160.4–351.7)	894.7	654.1 (407.3–1050.3)	833.4
PTA + Stent (all regions)	219	862.8 (n = 94)	1066.8	1206.8 (n = 125)	706.0

Table 3.

DAPs for intra-arterial thrombolysis according to body regions.

	n	FT (cGy × cm²)	Time (s) FT	AA (cGy × cm²)	Time (s) AA
Lysis (pelvis)	25	1260.3 (609.9–2604.3)	693.3	2595.7 (1245.1–5411.2)	444.0
Lysis (thigh)	137	309.7 (201.7–475.5)	869.2	794.0 (614.3–1026.2)	588.3
Lysis (thigh + lower leg)	49	122.8 (75.6–199.5)	608.3	444.0 (290.1–679.6)	477.1
Lysis (lower leg)	21	226.1 (58.5–873.5)	112.6	304.6 (114.2–812.5)	395.7
Lysis (arm)	22	27.2 (15.9–46.4)	162.3	69.9 (47.4–103.2)	96.1
Lysis control	75	216.5 (137.2–341.5)	274.9	481.7 (345.2–672.3)	153.6
Lysis (all regions)	329	211.6 (n = 139)	542.9	606.4 (n = 190)	438.2

Table 4.

DAPs for hepatic embolization procedures according to explicit intervention.

	n	FT (cGy × cm²)	Time (s) FT	AA (cGy × cm²)	Time (s) AA
Embolization (portal vein)	44	10,442.6 (7427.6–14,681.6)	1466.7	16,083.7 (11,067.9–23,372.4)	1632.4
Embolization (hepatic artery)	12	6823.2 (174.3–267,129.8)*	4095.7	10,816.3 (3196.5–36,599.8)	1852.9
Chemoembolization (liver)	85	10,867.0 (8941.2–13,207.4)	1711.6	12,634.6 (8315.2–19,197.6)	1641.6
All hepatic embolizations	141	10,602.3 (n = 76)	1767.0	13,925.9 (n = 65)	1666.3

Embolization of the portal vein, the hepatic artery, and transarterial chemoembolization of the liver are distinguished. For both, the Fluorospot TOP image intensifier system (FT) and the Axiom Artis FPD system (AA) geometrically averaged DAPs (cGy × cm²) are listed and 95% confidence intervals are given in brackets. Fluoroscopy time (Time) is recorded in seconds.

*The very high upper end of the confidence interval in the field “Embolization (hepatic artery)” via “FT” is due to a small sub-sample size (n = 3) and the large variance of the values (DAPs were 2958.7 cGy × cm², 2861.4 cGy × cm², and 37,522.5 cGy × cm²).

Table 5.

DAPs for specialized interventions.

	n	FT (cGy × cm²)	Time (s) FT	AA (cGy × cm²)	Time (s) AA
TIPSS	22	4545.8 (3046.5–6782.8)	1121.9	8577.8 (4556.2–16,148.9)	1462.9
TJLB	15	151.7 (60.3–381.9)	622.7	499.9 (126.4–1977.0)	690.2
Aortic prosthesis	20	2696.0 (–)	464.0	17,576.0 (11,001.2–28,080.1)	1002.5
Vena cava filter	20	477.0 (165.4–1375.5)	1127.1	515.1 (173.0–1533.9)	2141.4
All specialized interventions	77	982.1 (n = 32)	1005.2	4523.1 (n = 45)	936.9

Transjugular portosystemic shunt (TIPSS), transjugular liver biopsy (TJLB), and the placement of aortic prostheses and vena cava filters are distinguished. For both, the Fluorospot TOP image intensifier system (FT) and the Axiom Artis FPD system (AA) geometrically averaged DAPs (cGy × cm²) are listed and 95% confidence intervals are given in brackets. Fluoroscopy time (Time) is recorded in seconds.

DAPs in therapeutic angiographies after standardization to fluoroscopy time – FPD versus AII system

Differences in DAPs of FPD versus AII systems could be driven by variations in fluoroscopy time. We therefore standardized this parameter to the geometric mean fluoroscopy duration of each procedure. A standardized fluoroscopy time of 731 s for PTA yields a 42.9% higher radiation dose of the FPD versus the AII system (FPD 901.9 cGy × cm², AII 631.0 cGy × cm²) (P < 0.05). Analogously, the increase in radiation dose with the FPD against the AII system was 50.2% in PTA and stent-placement (standardized fluoroscopy time 703.8 s, FPD 887.7 cGy × cm², AII 591.1 cGy × cm²), 235% in intra-arterial thrombolysis (standardized fluoroscopy time 210.2 s, FPD 647.8 cGy × cm², AII 193.3 cGy × cm²), 45.3% in embolization procedures (standardized fluoroscopy time 1245.1 s, FPD 14,702.6 cGy × cm², AII 10,121.8 cGy × cm²), and 341% in specialized interventions (standardized fluoroscopy time 659.1 s, FPD 4437.4 cGy × cm², AII 1006.4 cGy × cm²) (P < 0.05, respectively).

DAPs in therapeutic angiography subgroups – FPD versus AII system

As each angiographic procedure comprises different subgroups, a dedicated within-group comparison of DAPs in each intervention is needed to exclude a sampling bias. The geometrically averaged DAPs of each angiographic procedure in every subgroup per se were consistently higher in those performed with the FPD system compared to those performed with the AII system (Tables 1 –5).

DAPs in therapeutic angiography – influence of BMI

With both systems, increasing BMI resulted in higher DAP values (P < 0.05, respectively). For PTA (n = 233), the increase was more pronounced with the FPD system compared with the AII system (P < 0.05). No difference in the slopes of the DAP/BMI curves was seen in PTA and stent placement (n = 219) and embolization procedures (n = 141) (P > 0.05, respectively). For intra-arterial thrombolysis (n = 329), the DAP gain was higher with the AII system compared with the FPD system (P < 0.05), whereas in specialized interventions (n = 77), the FPD curve seemed higher-angled than the AII curve which however did not reach statistical significance due to the limited sample size (P > 0.05).

Discussion

There are several studies on radiation doses in cardiac interventions (19 –21) but rather scarce data in selected subgroups of peripheral therapeutic angiographies (13 –16). Here, we present DAP values for a broad spectrum of peripheral therapeutic angiographic interventions. A comparison of our DAP values with the data existing in the current literature reveals lower radiation doses in our patient population (Table 6, 22 –24). We did not find studies concerning DAP values for intra-arterial thrombolysis, embolization of the portal vein, and transjugular liver biopsies in the literature and therefore suggest our measurements as reference (Tables 3 –5).

Table 6.

Comparison of DAPs – own data versus prior publications.

Therapeutic intervention	Prior publication (Gy × cm²)		Own results (FT / FPD system) (Gy × cm²)
PTA thigh	Tsalafoutas et al. (13)	6.1	3.30 / 5.74
PTA pelvis and abdomen	Miller et al. (22,23)	171.4 (pelvis) 157.5 (abdomen)	15.0 / 70.9 (pelvis and abdomen)
PTA + stent pelvis	Miller et al. (23)	244.9	58.8 / 66.9
Embolization (hepatic artery)	Livingstone et al. (15)	161.97	68.2 / 108.2
Chemoembolization (liver)	Miller et al. (23)	270.1	108.7 / 126.3
TIPSS	Miller et al. (23)	431.1	45.5 / 85.8
Vena cava filter	Miller et al. (23)	53.9	4.8 / 5.2
Aortic prosthesis	Walsh et al. (24)	72.3–150	27.0 / 176

DAP values (Gy × cm²) from prior publications (all obtained with analog image intensifier systems) are compared to own results obtained with the Fluorospot TOP image intensifier system (FT) and the Axiom Artis FPD system (AA).

The FPD system is regarded as a technological advancement enabling a higher grayscale resolution and a broader dynamic range. Additionally, the dose for a single fluoroscopy picture per se is higher for the AII system than for the FPD system (25). However, despite identical matrices, the actual diameter of the image intensifier format (Fluorospot TOP) is smaller than the digital image converter of the FPD system (Axiom Axis). This results in a larger pixel size in the FPD system and entails a diminished contrast resolution with reduced perceptibility of relevant structures, such as stents (25), which may be compensated for in clinical routine through more series. Indeed, we found consistently higher radiation doses in all peripheral therapeutic angiographic interventions performed with the FPD system compared to the AII system (Fluorospot TOP), which is in line with our previous study on diagnostic angiographies (12). Consecutively increased DAP values of the FPD-system were also reported in coronary angioplasties (26,27). Prasan et al. obtained similar results to ours when comparing a Toshiba flat panel fluoroscopic system with a Toshiba image intensifier that was less than 1 year old (26). This speaks against the notion that our findings must be regarded as singular case or are restricted to only one manufacturer but hints to general concerns that need to be raised over the FPD technology. One advantage of FPD systems over image intensifiers is their much slower degradation of image quality over time (28). This could lead to gradually increasing radiation doses with the image intensifier system to maintain image quality and may explain opposite results reported by other groups (10). Furthermore, Prasan et al. discuss the possibility that vendors may adjust the machines' base settings to enhance the image quality of FPD systems at the expense of increased radiation exposure (26). When confronted with our results, neither the manufacturer nor our radiation physicists were able to explain the observed differences in radiation exposure. Nevertheless, our results made us request the manufacturer to re-check the machines which did not reveal any problems or misadjustments.

Only few reports exist about the performance of newer generations of FPD machines with improved dose reduction programs. Sadek et al. describe a case report where a preoperative magnetic resonance angiography was fused with intraoperative rotational imaging in an Artis Zeego fusion program limiting the amount of contrast and radiation exposure during endovascular repair of an iliac artery aneurysm (29). Honarmand et al. examined 23 DSA image series of nine patients obtained with an Artis Zeego and found no diagnostically significant reduction in image quality when reducing DSA detector dose by a factor of 2.5 through diminution of typical tube voltage, tube current, focal spot size and the addition of a 0.1/0.2 copper filter (30). With respect to further dose reductions, Sapoval et al. describe the successful application of a low-dose/low-frame fluoroscopy (7.5 pulses/s with catheterization, 3 pulses/s with embolization) with an Axiom Artis for uterine artery embolization in leiomyomata (31). In this body region the catheter is not subjected to relevant movements which would cause kinetic blurring at such a low frame rate in other body parts. We did not find conclusive data about the performance of the new generation of FPD machines with improved dose reduction programs in the literature yet, but we are currently evaluating this topic in a follow-up study.

Furthermore, in our patient population DAPs considerably increased with higher BMI, a finding which parallels the results of Vano et al. (32). The DAP increase we observed in both systems with elevated BMI was variable, depending on the individual intervention and no clear-cut disadvantage of the FPD versus the AII system became evident in this respect. Even though some subgroups escaped this paradigm, the conjoint analyses (summary lines Tables 1 –5) show that fluoroscopy times were shorter in interventions performed at the FPD system as compared to the AII system, which may be attributable to the better ergonomics and workflow of the FPD-system (e.g. easier pursuit of the guide wire). Intervention speed is especially important in the emergency setting, such as embolization procedures in acute bleeding, where the FPD system is advantageous compared to the AII system.

This study holds some limitations that need to be discussed. First, DAP values were measured intrinsically by both systems which could have led to a systematic error. However, the accuracy of the systems' intrinsic DAP measurements was continuously controlled every 3 months by our institute's radiation physicists, as previously described. Second, the fields of view and collimations for the respective examinations were not recorded. However, the interventions were performed by four interventional radiologists using both machines evenly, making the interventionalist per se and differences in field of view or collimation an unlikely confounding bias. Third, the performance of both machines may depend on the electronic and software configuration of the systems and the multitude of different adjustment options may explain the heterogeneity of data on FPD systems found in the literature. Therefore, the base settings of FPD systems can differ considerably (even when using the same machines) depending on the adjustments made by the manufacturer. When comparing two machines the question arises whether under the given base settings they produce images of similar quality. In a preparatory work we showed for the identical machines under the exact same base settings that the objective stent detection rates of both machines were similar and that the subjective radiopacity scores were even better for the AII system – excluding drawbacks in image quality as consequence of the DAP differences we observed (25). Fourth, our prior work showed that an increase of neither the magnification factor nor the pulsing rate resulted in a significant DAP increase, excluding both variables as direct causes for the measured DAP differences (25). However, both parameters may have a bearing on perceived image quality, as the magnification factor will affect resolution and the pulsing rate has an influence on the lag. Differences in perceived image quality, in turn, may affect time spent and cannot be excluded with certainty in our analysis.

In conclusion, therapeutic angiographic procedures using the FPD system required higher DAPs despite shorter fluoroscopy times as compared to an AII system, when base settings are adjusted to produce similar image quality. Nevertheless, even the higher DAP values of the FPD system were still similar to or lower than those reported in the literature.

Footnotes

Conflict of interest

None declared.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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