| Comment Record |
|
Commentor |
Mr. Joseph Arnaudo |
Date/Time |
2003-04-09 12:07:39 |
|
Organization |
Philips Medical Systems SSR NA |
|
Category |
Company |
| Comments for FDA General |
| Questions |
|
1. General Comments
|
Comments on 21 CRF 1020 proposed regulations(Docket No. 01N-0275:
Increase in minimum half-value layer:
The proposed new values are based on a scientific research of todays state-of-the-art equipment. But these values, valid as reasonable average technical state-of-the-art values, must not be used as minimum legal values. Assume that we produce an equipment that technically is built as anticipated (12º anode angle, 2.5 mm Al, < 10% HV ripple). Then this equipment, within certain torerances, will comply with the proposed requirements. But we would have severe difficulties to demonstrate definitive compliance for each single piece. For any acceptance test we would have to set a certain margin for measurement inaccuracies, and as a result of production and measurement tolerances, a certain percentage of systems would not comply. The only possible manufacturer’s measure then would be to increase the minimum filtration significantly above the todays 2.5 mm Al value. This would cause significant cost on the manufacturer’s side, but might also influence X-ray tube lifetime or uptime, because, for the same amout of radiation, more heat is produced and must be dissipated, which in turn will increase owner’s cost.
Proposal: Reduce HVL values (Table 1, right Column) by 0.1 mm for kVp 71, 80, 90, 100, by 0.2 mm for kVp 110, 120, by 0.3 mm for kVp 130, 140, and by 0.5 mm for kVp 150. Such legal imits would result in the proposed actual values.
Display of fluoroscopic irradiation time at operator’s position:
Since it will be required to display air kerma rate and cumulative air kerma at the operator’s position during fluoroscopy, there seems to be little added value in also displaying the fluoroscopic irradiation time at that position. To the contrary, it might be annoying for the operator to be distracted by a lot of figures and “unwanted” information, while his primary concern is “the image”. It must be noted that in the majority of cases all this information might be good to know and important to keep, but has little to no influence on the conduct of the examination.
Proposal: Display of fluoroscopic irradiation time at any position (like kV, mA, etc.)
Display of air kerma rate in units of mGy/min
According to IEC 60601-2-43, which was written with participation of FDA representatives, X-ray equipment for interventional procedures must not only display air kerma rate during fluoroscopy, but also during exposure series longer than 3 s in duration. During exposure series, the air kerma rate is roughly a factor of 100 higher than during fluoroscopy, and an indication in units of mGy/min would lead to long figures, difficult to display, and difficult to not misread.
Example: a 4-digit display could go up to 9999 mGy/min, which might not be enough for all cases. A 5-digit display showing something like 12340 might easily be mistaken for nearly the same as 1234 (!); many people have difficulties with intuitively judging such large numbers.
Proposal: Display of air kerma rate in units of mGy/s should be required. Or leave it to the judgement of the manufacturer, in which units to indicate air kerma rate, taking into account the user interface capabilities. This would also leave some room for technical progress in that area.
Accuracy of air kerma rate and cumulative air kerma display
An accuracy better than ± 25% is required, but since no conditions are given, it must be assumed that this is meant for all possible operating conditions. According to IEC 60580, ionisation chamber based dose area product meters are required to achieve an overall accuracy of ± 25%, under certain circumstances (certain radiation qualities, kerma rate ranges, air pressure range, air humidity range, etc.). For the calculation of air kerma (rate), the resulting values would have to be multiplied by the the irradiated area, which would add some more percent in error. Accuracies in that standard are expressed as “limits of variation” which are not linearly added. These accuracies are to be understood as “there is only a small probability that limits are exceeded” compared to a “legal limit”, which is meaning “any possible value outside that range is illegal”. There are other sources of error for calculations of air kerma based on technique factors, but the similar considerations apply. For good reason, IEC 60601-2-43 only requires an accuracy of ± 50% for these indications, and gives operating ranges for which such accuracy is required.
Proposal: Reduce required accuracy to ± 30% or more, and indicate the range for which such accuracy is required.
Joseph S. Arnaudo
|
|