Patient Radiation Dose in Percutaneous Transluminal Coronary Angioplasty Using Various Types of Flat-panel Digital X-ray Systems

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DOI
http://dx.doi.org/10.15420/ecr.2006.0.2.1

Digital medical imaging has progressed immensely in recent years, providing the opportunity to store images in a picture archiving and communications system (PACS) and eliminating the need for film storage. Images can be viewed simultaneously in many monitors during or after the radiological technique, enabling more accurate and efficient treatment of the patient. The result is acceleration of patient throughput in the medical department, the elimination of running costs due to film processing and the reduction of retakes.1 Recently, dynamic flat-panel (FD) digital detectors were introduced, which replaced conventional image intensifier detectors. The superior image quality, distortion free images and increased sensitivity claimed by the manufacturers make the use of FP systems very promising.2-3

Dynamic FP detectors have recently been applied in interventional cardiology (IC), a medical speciality widely known to generate high radiation doses for patients as well as for the medical staff involved.4-6 These procedures occasionally result in deterministic effects such as erythema.7-9 There is a growing concern regarding doses from interventional procedures10 and instructions are given by formal bodies so as to avoid skin injuries due to X-ray radiation.11

A few studies can be found among recent literature regarding the use of FP detectors in IC.12-14 Three dynamic cardiac FP machines from different manufacturers have been installed the last three years in Athens, two of which are situated in the largest public hospitals in Greece and the third in a dedicated cardiac centre. Measurements of patient doses were performed during percutaneous transluminal coronary angioplasty (PTCA). The scope of the study was to investigate the factors that influence radiation dose imparted to the patient and compare the results with recent literature.

Materials and Methods

Two hundred and fifty-four patients participated in the study, 46 of whom were treated in Hospital A, 40 in Hospital B and 168 in Hospital C. The main technical characteristics of the dynamic FP X-ray machines are found in Table 1.

The dose performance of the X-ray systems was evaluated by measuring patient dose rate in fluoroscopy and patient dose per frame in cine mode. The patient was simulated by blocks of polymethyl methacrylate (PMMA), 20cm x 20cm in size and 20cm in total width, which attenuate the X-ray beam in a similar way to a normal-sized patient. The blocks were placed on the patient table. The dose measurements were made using a dosemeter (Solidose 400, RTI Electronics, M├Âlndal, Sweden) with a solid state detector (R100) calibrated with a calibration traceable to a standard laboratory. The detector was placed on the patient table at 65 cm from the X-ray tube and the PMMA blocks were placed on top of the detector. For comparison purposes, 20 cm field of view (FOV), normal mode in fluoroscopy and 12.5 frames/sec were applied in all the machines.

Patient dose was measured in terms of dose area product (DAP) (as well as DAP in fluoroscopy (DAPf) and in cine imaging (DAPc) in Hospitals B and C). All X-ray systems comprised a DAP meter for patient dose measurements. The DAP meters were calibrated following the UK National Protocol for Patient Dose Measurements in Diagnostic Radiology.15 The uncertainty in the reading of the instruments, as quoted by the manufacturers, was ┬▒4% for tube potentials ranging from 50 kVp to 100 kVp. Apart from dose data, patient weight (W), height (H), fluoroscopy time (T) and total number of frames (F) (images) were also recorded.

Results

Table 2 provides the results on dose measurements using the PMMA phantom. The results reveal a large range of doses that reach up to a factor of two for fluoroscopy and even more for cine (Hospitals B and C) for the same test procedure. This shows that the internal characteristics of the machine play a very important role in dose performance.

Median values of total patient dose in terms of DAP as well as fluoroscopy time (T) and total number of images (F) are presented for the three hospitals in Figure 1. Preliminary reference levels for image intensifier machines are also presented in the same figure.16 RL for FP systems are not yet determined by official bodies. The results reveal a large range of patient doses with max/min value equal to 4.6 (Hospital A presenting the lowest DAPt value and Hospital B the highest).

The interesting point is that despite the fact that Hospital B presented the lowest dose performance both in fluoroscopy and cine modes with the PMMA phantom in comparison for example with Hospital C, which presented the highest (ratio B/C approximately: 0.5), patient doses found in Hospital B were the highest (DAPt ratio B/C: 1.45). This reveals that the PTCA technique applied by the operator or the clinical condition of the patient can practically invert the 'good dose performanceÔÇÖ of an X-ray system.

Figure 2 and Figure 3 show the correlation of DAPt with T and F. It appears that practically no linear correlation exists due to the large number of factors influencing patient dose. The results make it impossible to correlate the dose imparted to patient with the fluoroscopy time of the total images. This practically implies that increasing T or F does not necessarily mean an increase in patient dose.

The variability in dose found in PTCA can be explained by the fact that it is a therapeutic procedure that depends on the pathology of the patient. Bernardi17 found an increase of T and F in complex PTCA procedures. Padovani et al.18 found an increase of about 50% in radiation dose for medium complex procedures and an increase of 100% for complex procedures.

Tsapaki19 showed that increasing number of stents or their position, as well as the grade of tortuosity and the stage of occlusion, can increase patient dose.

Conclusion

PTCA is routinely performed in a number of hospitals and is considered a safe therapeutic IC procedure in the hands of experienced operators. However, as the technology evolves in terms of X-ray equipment, catheters and stents the procedure is becoming more and more complex. This has great impact on the patient dose.

The standardisation of PTCA procedure and new RL for digital FP systems will definitely limit the range of patient doses encountered. Ôûá

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