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Radiographic Testing - A comparison of Standards for Classical and Digital Industrial Radiology Uwe Ewert, Uwe Zscherpel, BAM-Berlin, Germany Contact

Introduction

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. Significant differences between industrial NDT and medical needs and usages 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.

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.

New digital radiological techniques are compared with the potential of classical film radio-graphy. The major parameters are spatial resolution, contrast sensitivity and optical density range. Derived from the properties of X-ray NDT film systems and application ranges minimum requirements are defined. An European standard committee works out a standard on radiographic film digitization systems. Due to the required international harmonization the standard reference film is taken over from ASME/ASTM. Additionally, minimum requirements for digitizer systems are derived. Three system classes are currently proposed with different requirements for density range, density contrast sensitivity and spatial resolution.

Good workmanship criteria for industrial radiography were developed during the last decades in all countries[1]. 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.

A new proposal on "general principles for examination of metallic materials by Computed Radiography" was worked out in a committee of the German Society of NDT and has been accepted as European standard project at CEN TC138/WG1. The concept of the standard is the classification of systems and the definition of minimum requirements to ensure a certain spatial resolution and contrast sensitivity, which shall be similar to those as defined by ISO5579 and EN444 (film radiography). It is required to carry out all measurements with two IQI types, the wire IQI for contrast and the duplex wire IQI for the spatial resolution. This concept was taken over from the European standard proposal prEN 13068 for industrial radioscopy. Tables define the minimum wire and duplex wire read out values in dependence on the testing class, the wall thickness range and radiation energy.

Basic Differences between European and North American Radiography

Of cause there are no differences in Physics on both sides of the world, but traditions and rules for radiography are quite different. This is due to the historical development and national specialties.

The most important standards for radiography can be found in tab. 1. In Europe each radiographer must know the following basic standards:

• EN 444 General rules(r) ISO 5579
• EN 1435 Welding
• prEN 12681 Foundry (Casting)

The idea behind this standards is: Standards shall guarantee a minimum image quality by 'Minimum Requirements' for the testing procedure. This is basically:

• Contrast- opt. Density, IQI-values, max. X-ray energies,
• wall thickness limits for gamma sources
• Film system classes-definition of film system classes in dependence on energy and wall thickness
• Unsharpness- minimum focus-object-distances
• Handling- Radiation geometries,
  Filter, Shielding, Masking, Marking

The radiographic techniques are subdivided in Europe into two classes:

Class A: basic technique;
Class B: improved technique.

Class B technique will be used when class A may be insufficiently sensitive. All test tasks shall be preferably performed by class A or class B testing.

ASTM standards (E 94, E 1742, E 1032, E1030) do not require testing classes. The quality of testing is basically defined through the Image Quality Indicator (IQI) perceptibility, defined by the responsible person of the company (or cognisant engineering organisation). This IQI's are mostly step hole penetrameters corr. to E 1025. 1997 ASTM issued a standard for wire type IQI's, E747, which corresponds to the old British standard but does not correspond to the valid European E 462.

The ASTM standards do mainly describe the method and workmanship. They do not consequently work with minimum requirements. The background is: People widely believe that testing problems are very different and specific. Therefore, a simple subdivision into two testing classes was not accepted. For each problem a written procedure shall define the needed IQI perceptibility and geometric unsharpness. The standard is the broad basic for all testing tasks. Well educated personal shall carry out the radiography with a quality corresponding to the requirements.

New Standards on Digital Industrial Radiology (DIR)

Nevertheless, new developments in the field of industrial digital radiology (DIR) open the opportunity to bring up the discussion about harmonisation of standards on upcoming technologies. A new tendency of harmonisation can be observed in the field of qualification standards for old and new methods.

An example for harmonisation in the classical field of radiography is the new set of standards for classification of industrial X-ray films. Tab.2 shows the comparison of national and international standards for film classification.

The basic difference in the testing philosophy cannot easily be overridden in the new field of DIR too. But in principle there exist no obstacles for harmonisation of the qualification and classification standards for the new methods and new digital equipment. The new digital detectors (e.g. flat panel detector, like a -Si or a -Se detector, imaging plates and others ) are world wide conquering NDT-applications. Due to the industrial globalisation, there exist also a serious need for harmonised standards.

Unfortunately several countries started already own standardisation activities. Also in USA and Europe different standard proposals were worked out. Since four years common discussions led to better harmonisation. Beside the classical film radiography three fields are of importance:

1. Radioscopy
2. Computed Radiography with phosphor imaging plates
3. Film digitisation

All digital methods have the problem, that their spatial resolution is considerably lower than provided by film radiography. Therefore, it was necessary to have a better measure for spatial resolution and to measure it separately of contrast resolution. For this purpose the British duplex wire IQI (CERL B) was included in the European standard EN 462-5 and in the American standard ASTM E 2002-99.

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

Here, it should be mentioned that European standards do not accept EPS (Equivalent Penetrameter Sensitivity). Contrast resolution is measured by wire or step hole penetrameters. The step hole penetrameter have in Europe only 1T-holes. No 2T and 4T holes are allowed. Fig. 1 shows the comparison of the wire IQI's.

Fig a: Marking wire IQI/EN 462-1	Fig b: Duplex wire IQI/EN 462-5
Fig 1: 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 basic new idea is the application of always 2 IQI's for radiological testing with new digital detectors:

• contrast by step hole or wire penetrameters and
• spatial resolution by a duplex wire IQI

Radioscopy (RS)

Radioscopy standards have been developed widely independently in ASTM and CEN. Therefore, these standards show quite cosiderable differences. Especially the new part 3 of the European EN 13068 is based on the testing class system of the EN 444 basis.

EN 13068 is confirmed now in Europe. Part 3 requires to carry out all measurements with two IQI's, the wire IQI for contrast and the duplex wire IQI for the spatial resolution (fig. 1). Table 3 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 light-alloy materials. Advantages of real time testing are considered here.

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 3: System performance for metallic materials testing class SA and SB

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.

Computed Radiography (CR)

ASTM has already issued two standard part on CR. The part "Qualification of CR" is under development [3]. 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 1). Table 4 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. Pixel[1] Size [µm]	Double wire IQI-number[2]	Max. Pixel[1] size [µm]	Double wire IQI-number[2]
X-ray Up £ 50 kV	w < 4	40	> 13[3]	30	>> 13[4]
	4 £ w	60	13	40	> 13[3]
X-ray 50 < Up £150 kV	w < 4	60	13	30	>> 13[4]
	4 £ w < 12	70	12	40	> 13[3]
	w ³ 12	85	11	60	13
X-ray 150 < Up £ 250 kV	w < 4	60	13	30	>> 13[4]
	4 £ w < 12	70	12	40	> 13[3]
	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:	If magnification technique is used, duplex wire IQI-readout is required only The given IQI-numbers indicate the readout value of the first unresolved wire pair corr. to EN 462-5. The symbol ">13" requires the 13th wire pair to be resolved. The ">>13" is not readable with the double wire IQI corr. To EN 462-5. Up - Tube voltage.
Table 4: 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 6). 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 5), 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 5: 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 6: 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 "Generic NDE data format model" published under CEN-CR13935. 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 based on the CEN data model.

Computed Tomography (CT)

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 fife 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.

Film Digitisation (FD)

FD 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 E1815-96 and 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 7 defines the minimum working range of the radiographic film digitization system[2]. 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	0.5 - 4.5	0.5 - 4.0	0.5 - 3.5
digital resolution [bit]	³ 12	³ 10	³ 10
density contrast sensitivity DDCS within DR	£0.02	£0.02	£0.02
Table 7: Minimum density range of the radiographic digitisation system with a minimum density contrast sensitivity

Table 8 specifies the minimum spatial resolution as a function of the X-ray energy.

Energy	Class DS		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 8: 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 7 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 gamma-rays and can be applied for all class DB and DA digitization tasks.

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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