Implementation of a Low Frame-Rate Protocol and Noise-Reduction Technology to Minimize Radiation Dose in Transcatheter Aortic Valve Replacement

Abstract: Objectives. Limiting radiation exposure is necessary in radiological procedures. This study evaluates the impact of a radiological low frame-rate protocol in a standard angiographic system and the implementation of a noise-reduction technology (NRT) on patient radiation exposure during transcatheter aortic valve replacement (TAVR). Methods. Transfemoral TAVR procedures performed between February 2016 and February 2017 were analyzed according to two angiographic systems, Standard and NRT, and further divided in four subgroups: (1) Standard 15 frames per second (fps) with 15 fps for both fluoroscopy and cine acquisitions; (2) Standard 7.5 fps with 7.5 fps for both fluoroscopy and cine acquisitions; (3) NRT 15 fps with 15 fps for both fluoroscopy and cine acquisitions; and (4) NRT 7.5 fps with 15 fps for fluoroscopy and 7.5 fps for cine acquisitions. Study endpoints were kerma area product (KAP) and cumulative air kerma at interventional reference point (AK at IRP). Results. Significant differences were found in KAP (153 Gy•cm² [IQR, 95-234 Gy•cm²] vs 78.3 Gy•cm² [IQR, 54.4-103.5 Gy•cm²]; P<.001) and AK at IRP (1.454 Gy [IQR, 0.893-2.201 Gy] vs 0.620 Gy [IQR, 0.437-0.854 Gy]; P<.001) between Standard system and NRT. Within the procedures conducted with Standard protocol, a reduction of KAP and AK at IRP was found between Standard 15 fps and Standard 7.5 fps groups (184 Gy•cm² [IQR, 128-262 Gy•cm²] vs 106.8 Gy•cm² [IQR, 76.87-181 Gy•cm²] [P<.01] and 0.973 Gy [IQR, 0.642-1.786 Gy] vs 0.64 Gy [IQR, 0.489-0.933 Gy] [P<.01], respectively). Conclusions. The present study suggests that the low frame-rate protocol in Standard system and NRT implementation allows a marked reduction of patient radiation exposure in TAVR procedures. 

J INVASIVE CARDIOL 2018;30(5):169-175.

Key words: radiation dose, transcatheter aortic valve implantation, transfemoral

Transcatheter aortic valve replacement (TAVR) has now become a suitable alternative treatment for patients with severe AV stenosis who are considered at high risk for conventional surgical AV replacement. Several registries and randomized clinical trials have demonstrated the feasibility and efficacy of the TAVR procedure.^1-3 The latter involves the use of radiation, with subsequent potential tissue reactions and stochastic adverse effects for both patients and operators. Recently, the opportunity to make this procedure available for intermediate-risk^4,5 (and therefore younger) patients stresses even further the need for a greater application of radioprotection.⁶ Therefore, according to the As Low As Reasonably Achievable (ALARA) principle, it is essential to limit radiation exposure to patients and operators as much as possible, and this is currently facilitated by the appropriate use of new tools implemented in modern angiographic systems. Our study aim was to evaluate the impact of a radiological low frame-rate protocol in a standard angiographic system and the implementation of a noise-reduction technology (NRT) on patient radiation exposure during TAVR.

Methods

The study was conducted in accordance with the provisions of the Declaration of Helsinki and was approved by the local ethics committee. Data presented in this single-center study were retrospectively collected at San Raffaele Hospital Interventional Cardiology Unit in Milan, Italy.

Two different time periods were considered: the 1st period, ranging from February 2016 until July 2016, when the angiography system was equipped with Philips Allura Xper FD 10 single-plane C-arm with flat-panel detector (FPD) technology (Philips Healthcare); and the 2nd period, ranging from August 2016 until February 2017, following the implementation on the same system of ClarityIQ imaging technology (Philips), which provides an optimization of x-ray image acquisition with image NRT and allows radiation exposure reduction.⁷ Data on radiation exposure were collected by a dedicated radiological technician from an integrated dosimeter on the x-ray tube collimator.

Fluoroscopy and cine angiography were used for TAVR procedures. Two different radiological protocols, with the two different angiographic systems, were used for image acquisition at the operator’s discretion. When using the Standard angiographic system (without imaging and dose improvements), the operator could choose between two different radiological protocols for images acquisition: (1) Standard 15 frames per second (fps), which included three different settings (low, normal, and high) for fluoroscopy and 15 fps for cine acquisition; and (2) Standard 7.5 fps, which included 7.5 fps for fluoroscopy, with three different settings (low, normal, and high) and a reduced frame rate of 7.5 fps for cine acquisition (Table 1). When using the NRT system (with imaging and dose improvements provided by ClarityIQ imaging technology), the operator could choose between two different radiological protocols for image acquisition: (1) NRT 15 fps, which included 15 fps and three different settings for fluoroscopy (low, normal, and high) and 15 fps for cine acquisition; and (2) NRT 7.5 fps, which included 15 fps for fluoroscopy and three different settings for fluoroscopy (low, normal, and high) and a reduced frame rate of 7.5 fps for cine acquisition (Table 1). In both systems, the last fluoroscopy hold (LFH) tool, or “fluoro-store” function, was utilized in order to enable dynamic storage of the last fluoroscopy sequences and record fluoroscopic sequences for documentation (similar to cine sequences) and deemed “recorded fluoro runs.”

All interventional fluoroscopes were equipped with integrated dosimetric instrumentation, the performance of which was verified semiannually as part of the Hospital’s Quality Assurance program by the Internal Medical Physics department. Radiation dose during each procedure was recorded using the air kinetic energy released per unit mass (kerma) area product (KAP), also known as the dose area product (DAP). The cumulative KAP (in Gy•cm²) is the product of absorbed dose to air and beam cross-sectional area and is independent of the distance to the x-ray tube focal spot. It is therefore considered a surrogate measure of patient risk of stochastic radiation effects.⁸ Total air kerma at the interventional reference point (AK at IRP), also known as K_a,r, AK, cumulative air kerma, or air kerma at the patient entrance reference point, represents the x-ray energy delivered to air at 15 cm from the isocenter toward the x-ray tube. AK at IRP is used to monitor patient radiation dose and is correlated with tissue reactions.⁹ 

Recorded data included patient clinical characteristics, fluoroscopy time, procedure time, contrast volume, number of overall runs acquired divided by recorded fluoro runs, cine runs, and overall runs (recorded fluoro runs plus cine runs). In accordance with the International Commission on Radiological Protection (ICRP) 120 report¹⁰ and the National Council on Radiation Protection and Measurements (NCRP) 168 report,¹¹ when the procedure exceeded one of the trigger level points for a potential skin injury (peak skin dose of 3 Gy and/or KAP of 500 Gy•cm² and/or air kerma at the patient entrance reference point of 5 Gy), a clinical follow-up at 4 weeks was recommended for early detection and management of potential radiation injuries.

In addition to the aforementioned data, in-hospital clinical outcomes and complications were recorded in accordance with the Valve Academic Research Consortium (VARC)-2 definitions.¹²

Patient and procedural planning including selection of access route was carried out at a dedicated multidisciplinary Heart Team meeting. Transfemoral TAVR procedures were under local anesthesia and superficial sedation. Different valves types were utilized: Sapien and Sapien 3 (Edwards Lifesciences), CoreValve and CoreValve Evolut R (Medtronic), Transcatheter Aortic Valve System (Direct Flow Medical), Lotus valve (Boston Scientific), Portico valve (St. Jude Medical), and Acurate neo valve (Symetis).

Statistical analysis. Continuous data are presented as median and interquartile range (IQR; 25th-75th percentiles) or mean ± standard deviation, according to the non-normal or normal data distribution, as assessed with the Kolmogorov-Smirnov or Shapiro-Wilk tests. Comparisons were performed using the Mann-Whitney U-test or Student t-tests, as appropriate. Categorical data are presented as counts with percentages, and compared with the Pearson’s Chi-square test. Two-sided P-values <.05 were considered statistically significant. Multivariable linear regression analysis was performed with block entry of all baseline and procedural variables in order to ascertain whether the radiological protocol was an independent predictor of KAP and AK at IRP. All data were analyzed with the IBM SPSS Statistics for Windows, version 24.0 (IBM Corporation).

Results

A total of 212 consecutive patients undergoing transfemoral TAVR from February 2016 to February 2017 were evaluated. Fifty-five patients (25.9%) were treated with the Standard 15 fps protocol, 48 patients (22.6%) with the Standard 7.5 fps protocol, 47 patients (22.3%) with the NRT 15 fps protocol, and 62 patients (29.2%) with the NRT 7.5 fps protocol. Baseline clinical, procedural, radiological, and dosimetric characteristics stratified according to the different groups are presented (Tables 2-4; Figure 1). No significant difference was found among the groups and subgroups in terms of clinical and procedural characteristics (Tables 2-4). Compared to the Standard group, the NRT group had a significant reduction of both KAP (78.3 Gy•cm² [IQR, 54.4-103.5 Gy•cm²] vs 153 Gy•cm² [IQR, 95-234 Gy•cm²]; P<.001) and AK at IRP (0.620 Gy [IQR, 0.437-0.854 Gy] vs 1.454 Gy [IQR, 0.893-2.201 Gy]; P<.001) (Figures 1B, 1C). No differences were found with regard to KAP trigger level dose of 500 Gy•cm² and AK at IRP trigger level of 5 Gy.^10,11 Moreover, the number of cine runs was significantly lower in the NRT group vs the Standard group (9 runs [IQR, 7-12 runs] vs 11 runs [IQR, 9-15 runs]; P<.01) (Table 2).

Additionally, a significant reduction of study endpoints was observed in the Standard 7.5 fps group vs the Standard 15 fps group: KAP (106.8 Gy•cm² [IQR, 76.87-181 Gy•cm²] vs 184 Gy•cm² [IQR, 128-262 Gy•cm²] P<.01); and AK at IRP (0.64 Gy [IQR, 0.489-0.933 Gy] vs 0.973 Gy [IQR, 0.642-1.786 Gy]; P<.01) (Figures 1B, 1C). There was no difference of KAP trigger level dose of 500 Gy•cm² and AK at IRP trigger level of 5 Gy.^10,11

Conversely, no difference in KAP and AK at IRP was found between the NRT 15 fps and NRT 7.5 fps protocols, despite imbalances in the number of recorded fluoro runs and all runs (Figures 1B, 1C; Table 4). There was a higher prevalence of KAP trigger level dose >500 Gy•cm² in the NRT 15 fps group vs the NRT 7.5 fps group (3 patients [6.4%] vs 0 patients [0.0%]; P=.04).^10,11

Importantly, no difference in fluoroscopy time was observed across all group comparisons (Figure 1A). No case of skin injury was reported at 4-week follow-up and no difference in clinical outcomes was found, except for vascular complications in the NRT 15 fps vs NRT 7.5 fps groups (7 patients [14.9%] vs 2 patients [3.2%]; P=.03) (Tables 5-7).

On multivariable linear regression analysis, radiological protocol was independently associated with KAP and AK at IRP. In particular, Standard 7.5 fps was an independent predictor of lower KAP (beta, -59.24; 95% confidence interval [CI], -95.02 to -23.47; P<.01) and AK at IRP (beta, -0.751; 95% CI, -1.081 to -0.420; P<.001) compared with Standard 15 fps. Similarly, the combined NRT group exhibited a lower KAP (beta, -61.57; 95% CI, -89.71 to -33.42; P<.001) and AK at IRP (beta, -0.741; 95% CI, -1.011 to -0.471; P<.001) compared with the combined Standard protocol. However, no difference was observed between the two NRT protocols (Figures 1B, 1C).

Discussion

Our study findings are as follows: (1) the use of NRT significantly reduces TAVR radiation exposure compared with a Standard protocol; and (2) within the Standard angiographic system, use of the Standard 7.5 fps protocol significantly reduces TAVR radiation exposure compared with Standard 15 fps.

The fundamental principles of radiological protection, ie, justification, optimization, and the application of dose limits, are described in the ICRP 103 publication.¹³ Recent studies indicate that digital image tools have the potential to reduce radiation doses for TAVR procedures.^14,15 In particular, data regarding radiation exposure in our cath lab during TAVR demonstrate that improvements in materials, procedural techniques, and angiographic system technology have contributed to a decrease in the amount of radiation delivered over the years.¹⁶

Our study describes the reduction of radiation exposure resulting from the further improvement of angiographic system technology, and therefore indicates that the appropriate use of existing technology can lead to dose reductions for both patients and operators. With the introduction of NRT technology, we obtained a 49% reduction of KAP and 57% of AK at IRP; this is an especially important result, as the opportunity to make this procedure available to patients with an intermediate risk will also expose a younger population to a non-trivial long-term radiological risk in addition to repeated x-ray imaging exposure before and after the procedure.

Currently, the TAVR population is elderly (median age, 83 years in our study), and the main radiological concern is represented by skin injuries. However, the risk of cancer induction should also be considered if younger populations are to be treated.⁶

In terms of optimization, we demonstrated that the use of a low frame rate radiation protocol in the Standard system (Standard 7.5 fps) allowed a marked reduction of patient radiation dose (42% for KAP and 34% for AK at IRP) while maintaining the same performance quality. This is an important result, as in the absence of sophisticated tools, operators using a standard angiographic system protocol might want to consider several opportunities to reduce radiation exposure to patients and themselves, and one of the most effective strategies is to decrease the acquisition frame rate.¹⁷ 

However, the same reduction was not reproduced in the two NRT groups. This can be explained by the proportion and type of acquired runs. Dose delivered is lower with fluoro run recording than with cine acquisition.¹⁸ This variable represents almost two-thirds of all acquisition runs in our study. Furthermore, we must consider that the two NRT system groups have the same 15 fps fluoroscopy setting and the same entrance dose rate limits. This is different from the two Standard groups, where the frame rate for fluoroscopy differed between Standard 7.5 fps (7.5 fps for both fluoroscopy and cine acquisitions) and Standard 15 fps (15 fps for both fluoroscopy and cine acquisitions). Between the two Standard protocol groups, the difference in the dosimetric parameters was significant. Another reason for the absence of difference in study endpoints between the two NRT groups can be found in the relatively small sample.

Over the years, TAVR has become a highly standardized procedure at our center. The use of preprocedural multidetector computed tomography imaging to determine type, measurements, optimal angiographic projection for valve deployment, and optimal access site has decreased the use of several angiographic cine runs, thereby reducing contrast volume used and radiation dose. In addition, the use of intraprocedural echocardiography in order to immediately evaluate the severity of periprosthetic and internal valve regurgitation has reduced the need for further angiography.

The homogeneity of our TAVR procedural workflow is demonstrated by the fact that fluoroscopy times did not differ across groups. This also indicates that fluoroscopy time can be used as a surrogate for procedural complexity, but should not be considered as a patient radiation exposure parameter.¹⁹ In patients with similar procedural, clinical, and anthropometric characteristics, similar fluoroscopy times did not correspond to similar values of radiation exposure in terms of KAP and/or AK at IRP, which are considered the main descriptors of patient dose.^10,11 

In our study, image quality comparison with the different angiographic systems was not performed since a scientifically sound assessment of this issue would require the use of an anthropomorphic phantom, under the same radiological and geometry conditions. However, image quality was not affected up to a clinically meaningful degree by the implementation of NRT, as none of the operators involved in our study complained of image-quality related issues. It seems reasonable to say that fluoroscopic image quality is less affected by NRT in structural heart disease procedures than in percutaneous coronary interventions, due to difference in size of the anatomic structures involved and detail of the possible pathophysiological alterations observed (eg, dissections).

Study limitations. This is a retrospective, single-center, observational analysis and is thus subject to the limitations of such analyses. The sample size is relatively small and may be a limitation in the interpretation of the data. Furthermore, although all operators were highly experienced in TAVR, differences in operator skill may have influenced radiation dose delivered. Finally, operator radiation doses were not evaluated. 

Conclusion

A significant reduction in radiation exposure was achieved during TAVR with the implementation of image NRT and the appropriate use of fluoroscope radiological protocols. NRT was independently associated with lower KAP and AK at IRP when compared with the Standard protocol. In the absence of NRT, operators using a Standard protocol might want to consider reducing the frame rate from 15 fps to 7.5 fps, since this was also associated with a relative decrease in radiation dose.

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From the ¹Cardio-Thoracic-Vascular Department, San Raffaele Hospital, Milan, Italy; and ²AITRI (Italian Association of Interventional Radiographers), Milan, Italy.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr Latib reports research support and honoraria from Medtronic, Abbott Vascular, and Edwards Lifesciences. Dr Azzalini reports research support from ACIST Medical Systems and honoraria from Guerbet. The remaining authors report no conflicts of interest regarding the content herein.

Manuscript submitted August 11, 2017, provisional acceptance given August 22, 2017, final version accepted August 28, 2017.

Address for correspondence: Davide Maccagni, RT, Interventional Cardiology, San Raffaele Scientific Institute, Via Olgettina 60, 20132 Milan, Italy. Email: maccagni.davide@gmail.com