TABLE OF CONTENTS

Abstract Basic Radiographic Standards Radiation sources Films New European Standardization Projects for digital Radiology in NDT Radioscopy Computed Radiography References

Abstract

New digital radiological techniques have been developed which may replace classical techniques in medical and industrial radiology in the coming years. However, a slow standardization process could hamper the application of these techniques. The rapid development of proper application guidelines and minimum requirements promotes their safe and successful application in the industrial sector (See also [1]). Significant differences between industrial NDT and medical needs and usage have to be considered. Careless application of techniques which were optimized for medicine may lead to considerably reduced probability of industrial flaw detection compared to the classical film radiography.

Good workmanship criteria for industrial radiography were developed during the last decades in all countries. The corresponding rules are defined in a system of radiographic standards. But these rules are not harmonized in the different countries, because of differences in the traditions and training contents. This leads to deviations in the understanding of requirements for quality and expense of testing. European countries were forced to spend extraordinary efforts for the harmonization of rules due to the process of European centralization. The traditional European schools of NDT had to agree on common standards which allow a harmonized procedure and mutual agreement of NDT quality and certificates. These standards are generally submitted to ISO on the basis of the Vienna agreement between ISO and CEN. This provokes a new process of international harmonization. Nevertheless, we still observe differences in the standards between USA, Japan, Europe and others, which complicates the international mutual recognition.

During the last decade, several joint committees have developed European standards on industrial radiography from the previous national standards. They include guidelines on the measurement of instrumental parameters, as well as minimum requirements for instruments, practice and evaluation.

Basic Radiographic Standards

	Radiography	Radioscopy
Vocabulary	EN 1330-3/ISO 5576	EN 1330-3/ISO 5576
Equipment	prEN 12543,12544,12579 EN 584, EN 462,EN 25580	prEN 13068-1,2
General Rules	EN 444 ISO 5579	prEN 13068-3
Sectors(Inspection Rules) Welding Casting Steel Tubes
	EN 1435
	prEN12681
	prEN 10246-10
Sectors(Evaluation Rules) Welding Casting	[EN 25817 (Quality Levels)]
	EN 12062(General Rules) EN 12517(Acceptance levels)
Table 1: Overview about basic standards for industrial radiology

Fig 1: Maximum energy for X-ray inspections up to 500keV

European standards are conceptually very strict in guidelines for radiographic procedures. Two testing classes with different expenditure and testing quality are described in the general rules and the important application standards. Different minimum requirements are defined to produce a certain spatial resolution and/or contrast. To ensure sufficient spatial resolution, the minimum film-to-object distance is computed from the focal spot size or source size and the object thickness. Different wire IQI-values must be achieved, depending on the penetration geometry and testng class, and a minimum optical film density is required. Additionally, maximum X-ray voltages (figure 1) and minimum film system class qualities are defined depending on the wall thickness and testing class. The usage of gamma sources is permitted for certain wall thickness ranges, depending on the testing class (figure 2).

Fig 2: Wall Thickness Ranges Corresponding to EN444, EN1435 and ISO 5579

The basic parts of European and international standards (see tab. 1) are the general rules (EN444 corresponding to ISO5579) for radiographic examinations and the standard for radiographic weld inspection (EN1435) and foundry inspection (prEN12681). These are accompanied by the vocabulary (EN1330-3=ISO5576) and a set of standards for equipment properties (See figure 4).

Considerable differences appear in the understanding about the required sharpness of radiographs. These differences can be found e.g. between the European and US-American system. Figure 3 shows the differences of the minimum required sharpness (maximum geometric unsharpness) for some selected international, European and US-standards.

Fig 3: Comparison of limits for geometric unsharpness of different international standards. ISO 5579, EN444, EN 1435, prEN 12681 class A and B require the highest sharpness. Radioscopy (prEN 13068-3) and ASME V/2 allow relatively unsharp testing. The 1% and 2% lines shall support the interpretation of this figure. Due to the definitions, some ASTM-penetrameters (e.g.E1742-95) are suitable to check unsharpness in the frame of EPS down to 500 or 250 µm only.

Fig 4: Scheme of standards for measurement of equipment properties, classification and minimum requirements (pr: proposal).

Radiation sources
The image quality of the radiographs depends on the equipment properties. Two source parameters are of importance: the size of the radiator and the tube voltage. The corresponding standards are under final vote now. The most significant change is the new definition of the focal spot size or gamma source size, which now corresponds to the largest measured dimension (length or width). These values are normally significantly greater than those of IEC 336. This will influence the daily test practice. The standards for the measurement of the voltage of x-ray tubes are less critical concerning the influence on the daily NDT-work. They are important for in-service quality control and for a better comparison of manufacturer's parameters.

Films
The standard EN584 defines 6 film system classes for Europe instead of the four classes in ISO 11699. ASTM (E1815-96) and JIS (K 7627-97) followed the same philosophy, but with a different definition of the system classes (table 2). The given parameter ranges for the film system classes are now defined for the complete system consisting of film type and development process in all standards. Peripherals: The IQIs and film illuminators (light boxes) are defined in the EN462 and EN25580 (ISO5580) respectively.

System Class				Minimum gradient at		Minimum gradient-noise ratio at	Maximum granularity at
World ISO 11699-1	Europe CEN 584-1	USA ASTM E1815-96	Japan K7627-97	D=2 above Do	D=4 above Do	D=2 above Do	D=2 above Do
				G2	G4	G2/sD	sD
T1	C1	Special	T1	4,5	7,5	300	0,018
	C2	I		4,3	7,4	270	0,018
T2	C3		T2	4,1	6,8	180	0,023
	C4			4,1	6,8	150	0,028
T3	C5	II	T3	3,8	6,4	120	0,032
T4	C6	III	T4	3,5	5,0	100	0,039
		W-A	W-A	3,8	5,7	135	0,027
		W-B	W-B	3,5	5,0	110	0,032
		W-C	W-C	<3,5	<5,0	80	0,039
Table 2: Definition of film system classes

New European Standardization Projects for digital Radiology in NDT

Film Digitization and Computed Radiography standards are under development in the CEN subworking group 3 of TC138 WG1, headed by Dr. Zscherpel [2] and of the German Society for NDT, subworking group 'Computed Radiography' headed by Dr. Ewert [1]. The intention is to harmonize all activities with current ASTM/ASME activities.

The CEN standard project of subworking group 3 is entitled: "Fundamental parameters of radiography digitizers". Two drafts are proposed. Part 1 is: "Definitions, quantitative measurements of image quality parameters, standard reference film and qualitative control". Here, the standard reference film is taken over from ASTM with the required modification in the CEN-texts for the same film. Part 2 defines the minimum requirements to digitizer systems.

Film digitization is used for different reasons. The applications of image processing, quantitative analysis, and archiving are the most important tasks. While PC-based image scanners are available for paper images, transparencies and slides in a wide variety and at low prices, they are generally not suitable for X-ray film scanning. Industrial X-ray films are used with optical densities up to 4 and sometimes up to 5. Only a few available scanners are able to cover that density range. Furthermore, the NDT X-ray films are characterized by a high signal-to-noise ratio (SNR) which is defined in EN 584, ISO 11699, ASTM E1815-96and JIS 7627-97 by maximum granularity values related to an optical density of 2 (above fog). A suitable digitizer must not only be able to reach the density of 4 or 5, it also must not increase the image noise by its own detector noise.

Beside the requirements for maximum density and SNR, X-ray films require a very high spatial resolution. The limiting structure for very low X-ray energies is the grain size of the photoactive silver based crystals, which is below 1 µm. This is particularly important for micro radiography. General NDT applications require X-ray energies between 50 and 12000 keV. In medicine, the application range is normally below 150 keV only. Due to this large energy range for NDT radiography, it was decided to reduce the requirements for spatial resolution to the unsharpness which is caused by interaction of high energy X-rays with the screen film system. Measured functions provide unsharpness values between 30 and 800 µm (Klasens criterion), depending on the energy and the screen film system. Based on these measurements, the following tables define the minimum requirements. Table 3 defines the minimum working range of the radiographic film digitization system. In this working range, the digitizer shall provide an optical density contrast sensitivity DD_cs which is DD_cs £ 0.02 O.D. The minimum digital resolution is given for all devices converting the digital value proportional to the optical density. If the digital value is converted proportional to the light intensity, the digital resolution must be increased by at least 2 additional bits.

	Class DS	Class DB	Class DA
density range* DR [OD]	0.5 - 4.5	0.5 - 4.0	0.5 - 3.5
digital resolution [bit]	³12	³ 10	³ 10
density contrast sensitivity DDCS [OD] within DR	£0.02	£0.02	£0.02
Table 3: Minimum density range of the radiographic digitisation system with a minimum density contrast sensitivity

Table 4 specifies the minimum spatial resolution as a function of the X-ray energy.

Energy	Class DC		Class DB		Class DA
[keV]	Pixel size [µm]	MTF 20 % [lp/mm]	Pixel size [µm]	MTF 20 % [lp/mm]	Pixel size [µm]	MTF 20 % [lp/mm]
£ 100	15	16.7	50	5	70	3.6
£ 200	30	8.3	70	3,6	85	3
£ 450	60	4.2	85	3	100	2.5
Se-75, Ir-192	100	2.5	125	2	150	1.7
Co-60	200	1.25	250	1	250	1
Table 4: Proposed minimum spatial resolution of film digitisation systems

On the basis of the image quality of film radiography and the state of the art of digitizing systems, the committee has defined three quality classes; DA, DB and DC. The user may select the testing class based on the needs of the problem:

DS -	the enhanced technique, which performs the digitisation with an insignificant reduction of signal-to-noise-ratio and spatial resolution, Application field : digital archiving of films (digital storage)
DB -	the enhanced technique, which permits some reduction of image quality, Application field : digital analysis of films, films have to be archived,
DA -	the basic technique, which permits some reduction of image quality and further reduced spatial resolution, meeting the needs of radiographs according to ISO 5579 and EN 444 class A above 5 mm wall thickness.

Due to the required international harmonization, the standard reference film is taken over from ASTM for test and evaluation as well as for long term stability tests of digitizers.

Each radiographic film digitization system for NDT applications shall be identified with all working ranges of optical densities. It shall be classified corresponding to table 3 and the maximum MTF 20 % value, which can be performed by this system. So for instance, a digitization system of class DS 5 can be applied for archiving of radiographs taken with X-rays above 200 keV or gammarays and can be applied for all class DB and DA digitization tasks.

Radioscopy
New standards (Draft: prEN13068/1-3) were proposed for radioscopic testing. The first two parts on equipment properties are in final voting now. The third one, "general principles", passed successfully the 6 month inquiry. It requires to carry out all measurements with two IQI's, the wire IQI for contrast and the duplex wire IQI for the spatial resolution (figure 5). Tables 5 defines the minimum wire and duplex wire values, depending on the testing class (SA or SB) and the wall thickness for metallic materials. Another table is given for lightalloy materials. Advantages of real time testing are considered here.

Fig 5: Image quality indicators. Left: Wire IQI for contrast measurement. Right: Duplex wire IQI for spatial resolution measurement. Both IQI's are required for radioscopy and Computed Radiography.

The equipment is subdivided into three system classes SC1 - SC3, which depend on the test problem. Lower requirements for spatial resolution than in film radiography (EN 444, ISO 5579) are compensated by increased requirements for contrast.

Testing Class SA			Testing Class SB
System Class	SC2		System Class	SC3
	Wire no.	Duplex no.	Wire no.	Duplex no.
Wall thickness					Wall thickness
1.2 - 2.0 mm	W17	11D	W19	13D	-1.5 mm
2.0 - 3.5 mm	W16	10D	W18	12D	1.5 - 2.5 mm
3.5 - 5.0 mm	W15	9D	W17	11D	2.5 - 4.0 mm
5.0 - 7.0 mm	W14	8D	W16	10D	4.0 - 6.0 mm
7.0 - 10 mm	W13	7D	W15	9D	6.0 - 8.0 mm
10 - 15 mm	W12	7D	W14	9D	8.0 - 12 mm
15 - 25 mm	W11	7D	W13	9D	12 - 20 mm
25 - 32 mm	W10	7D	W12	9D	20 - 30 mm
32 - 40 mm	W9	7D	W11	9D	30 - 35 mm
40 - 55 mm	W8	7D	W10	9D	35 - 45 mm
55 - 85 mm	W7	6D	W9	9D	45 - 65 mm
Table 5: System performance for metallic materials testing class SA and SB

Computed Radiography
A new proposal on "General Principles for Examination of Metallic Materials by Computed Radiography" has been developed in a committee of the German Society for NDT. Again, the concept of the standard is the definition of minimum requirements to ensure a certain spatial resolution and contrast which should be similar to the requirements of the EN444 (or ISO5579). In analogy to radioscopy, measurements are needed with two IQI's, the wire IQI for contrast and the duplex wire IQI for the spatial resolution (figure 5). Table 6 defines the minimum wire and duplex wire values depending on the testing class (IPA or IPB), the wall thickness and radiation energy. The values were derived from the measured film unsharpness as function of energy and the geometric unsharpness corresponding to the wall thickness, on the basis of EN 444 and ISO 5579. The requirements for spatial resolutions are higher than those for radioscopy and are similar to film digitization.

Radiation source	Wall thickness w [mm]	Class IPA		Class IPB
		Max. Pixel1) Size [µm]	Double wire IQI-number2)	Max. Pixel1) size [µm]	Double wire IQI-number2)
X-ray Up £ 50 kV	w < 4	40	> 133)	30	>> 134)
	4 £ w	60	13	40	> 133)
X-ray 50 < Up £150 kV	w < 4	60	13	30	>> 134)
	4 £ w < 12	70	12	40	> 133)
	w ³ 12	85	11	60	13
X-ray 150 < Up £ 250 kV	w < 4	60	13	30	>> 134)
	4 £ w < 12	70	12	40	> 133)
	w ³ 12	85	11	60	13
X-ray 250 < Up £ 350 kV	12 £ w < 50	110	10	70	12
	w ³ 50	125	9	110	10
X-ray 350 < Up < 450 kV	w < 50	125	9	85	11
	w ³ 50	160	8	110	10
Yb 169,Tm 170		85	11	60	13
Se 75, Ir 192	w < 40	160	8	110	10
	w ³ 40	200	7	125	9
Co 60		250	6	200	7
X-ray Up > 1MeV		250	6	200	7
NOTE:	1) If magnification technique is used, duplex wire IQI-readout is required only 2) The given IQI-numbers indicate the readout value of the first unresolved wire pair corr. to EN 462-5. 3) The symbol ">13" requires the 13th wire pair to be resolved. 4) The ">>13" is not readable with the double wire IQI corr. To EN 462-5. Up - Tube voltage.
Table 6: Required spatial system resolution in dependence on energy and wall thickness

Phosphor imaging plate system classes are derived from EN584 (or ISO 11699, ASTM E1815-96, JIS K 7627-97) on the basis of the Signal-to-Noise Ratio (SNR) (see Table 8). Detailed guidelines are given on how to determine the exposure time to provide the specified SNR. Even with the same CR-phosphor imaging plate (IP) and scanner, different IP-classes can be obtained by using different exposure times, if the homogeneity of the phosphor layer is sufficient. Furthermore, a minimum lead filter thickness is specified to reduce the influence of scattered radiation (table 7), which is generally more intensive than for X-ray film.

	100KeV	200KeV	300KeV	400KeV	Ir-192	Co-60
Screen for film	27 mm Pb	27 mm Pb	100 mm Pb	100 mm Pb	100 mm Pb	500 mm Fe
Screen for IP's	100 mm Pb	100 mm Pb	200 mm Pb	300 mm Pb	350 mm Pb	1 mm Pb 0.5 mm Fe
Specific contrast for IP's to film	100%	100%	100%	100%	100%	80%
Table 7: Equivalent screen thickness for IP's(BAS III)

IP System classes
System class	Minimum Signal-noise ratio
IP 2	110
IP 3	90
IP 4	70
IP 5	60
IP 6	50
Table 8: IP-scanner system classes, depending on the minimum SNR

Further activities for DIR-standardization are the "standard data format" and "Computed Tomography". The standard data format was the attempt to unify and simplify the manifold of available data formats for NDT technology. This is a very difficult task, because different data formats are already installed in a variety of equipment and used for different applications. The original intend to develop a standard was changed to the publication of a technical CEN report, consisting of two parts: part 1 "General format for NDE data", part 2 "...application programming interfaces" . The ASTM has opened a new project with a similar goal. The data format DICOM, which is widely used in medical applications, shall be modified to NDT-applications under the name DICONDE.

Computed Tomography (CT) is a method which is routinely used in medicine. Now it can be observed that CT becomes a more and more established method for NDT-applications. Therefore, corresponding standards were developed during the last fifty years by ASTM.

The following ASTM - Standards are valid:

CT Examination	E 1570-95a
CT Examination of Castings	E 1814-96
Measurement of CT System Performance	E 1695-95
CT Imaging	E 1441-97
CT System Selection	E 1672-95
Calibrating and Measuring CT Density	E 1935-97
X-Ray Compton Scatter Tomography	E 1931-97

The most of these standards were submitted to ISO.

References

1. U. Ewert, H. Heidt, Current Status of European Radiological Standards for NDT, ASNT spring conference and IIW micro symposium, Orlando, 03/22-03/27, 1999, proceedings p. 171-173.
2. U. Zscherpel, Internet page: Standardization of fundamental parameters of radiography digitizers at http://trappist.kb.bam.de/CEN-NFD/cenahg3.html.
3. U. Ewert, Internet page: Standardization of Computed Radiography - IP in Radiology at http://trappist.kb.bam.de/UA-CR/welcome.html.

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