NDT Wiki X-ray - the Digital X-ray Encyclopedia

Wiki X-ray offers a wide variety of definitions of terms and concepts from the digital X-ray field. Click on the terms and learn more about technical concepts, analytic tools, professional analytic terms and more. Examine the images included to enhance your understanding and experience.

 

 

Undercut

It is when the weld reduces the cross-sectional thickness of the base metal. In other words, it is an erosion of the base metal next to the weld. The undercut causes a reduction of the strength of the weld and work piece.

 
There are 2 types of undercuts which can be found in welds:
1. Root/internal undercut - when it funds next to the root of the weld.
2. Crown/external undercut - when it funds next to the crown of the weld.

 

Figure 1: Internal Undercut

 

Figure 2: External Undercut

Slag

A residue of the flux coating in MMA (Manual Metal Arc) welds. It is principally a de-oxidation product from the reaction between the flux, air and surface oxide.

 
During welding, the slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs.

 

Slag is normally seen in X-ray as elongated lines either continuous or discontinuous along the length of the weld. This is easily identified in a radiograph , as seen in the image below.
 

Weld slag inclusion

Fatigue Crack

Every material in service (e.g. – Pipes in the Oil & Gas Industry, Aircraft wings etc) is susceptible to stress, which can eventually cause cracks in the material. These are called “Fatigue Cracks” and are one of the primary reasons for the failure of structural components.

 
Fatigue cracks are typically a thin (straight or jagged), linearly disposed discontinuities that occur when a material is subjected to cyclic loading. Fatigue cracks will eventually transpire even if the maximum stress values applied to the structure are substantially lower than the yield stress limit of the material. This is mainly due to the cyclic loading, or, when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators (welding area, curvatures, bolt holes etc) and the latter can develop into a failure (this is more likely to happen when the stress applied on the material is perpendicular to the propagation direction of the crack itself).

 

Detecting fine cracks such as fatigue cracks is not easy and requires the proper technique and high quality system with very low noise.
 

Fatigue crack in an aircraft landing gear (taken with Vidisco’s RayzorX Pro system)

Gas Voids (Porosity)

Gas voids are actually one form of porosity typically found in casting. When gas is trapped in a casting, it naturally produces a void. The gas can erupt spontaneously from molten metal, and it can develop from water vapor or green sand in a mold. In addition, during the pouring of a mold, gas voids can form as a result of simple turbulence.

 

A gas void appears in radiography as a smooth dark spot. It can be rounded, oval, or elongated. The size of these spots can vary considerably.

 

Here is an example of how gas voids look like when using a DR system:

Casting X-ray with FlashX Pro system - showing porosity

Erosion

Similar to corrosion, erosion is also wearing away of the material. But, unlike corrosion which is an electro-chemical process (caused by a chemical reaction with moisture) erosion is a process that carries away particles of the material simply by physical process.

 
The physical process is actually a constant friction caused by running fluid (liquid or gas) inside a pipe, i.e. high pressure steam, running oil etc.

 

Just like corrosion, the damages caused to the industrial sector by erosion, are estimated at large amount of capital each year. These days there are special computer software tools that allow estimating the amount of damage caused by the erosion/corrosion. This is done by measuring the wall loss (remaining wall thickness) of a pipe in a quick and automated way while maintaining high accuracy.

 

A portable DR system such as Vidisco’s products is an efficient tool that allows quick and reliable detection in almost every scenario. The example below shows the X-ray excellence and accuracy of measurement of pipe erosion, achieved with the RayzorX Pro system.

 

Erosion inside a pipe

Corrosion

Corrosion is an electro chemical process that gradually causes a destruction of material, usually metals, by chemical reaction with its environment. It can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term “Degradation” is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

 

Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface.

 

The damages caused by corrosion to the industry, are costly. This is why it is crucial to detect and treat corrosion on time. A good tool allowing quick, fast and reliable detection in almost every scenario is portable DR systems, such as Vidisco’s products.

 

Figure 1: Corroded steel pipe

 

Figure 2: Corroded vs. clean steel pipe (X-ray and 3D/Emboss effect)

Lack of Fusion / Penetration

Incomplete/lack of fusion or penetration occurs when the weld metal does not form a cohesive bond with the base metal or when the weld metal does not extend into the base metal to the required depth, resulting in insufficient throat thickness.

 

Figure 1: Incomplete fusion, incomplete joint penetration

 

Figure 2: Lack of fusion - taken with the RayzorXPro system

 

Figure 3: Lack of penetration - shot with the RayzorXPro system

Porosity

Porosity is a collective name describing cavities or pores caused by gas and non-metallic material entrapment in molten metal during solidification (welds and casting).

 
The effects of porosity on performance depend on quantity, size, alignment, and orientation to stresses. E.g. when clustered at a weld’s center, porosity is not considered a dangerous fatigue promoter, or detrimental to fatigue resistance, although it may reduce the static stress carrying capacity of the weld.

 

This is how porosity looks like in welded plate using Vidisco DR System
 

Porosity seen in weld X-ray, taken with RayzorX Pro system

Duplex Wire IQI

The duplex wire IQI measures the Basic Spatial Resolution (BSR) of a radiographic image produced by Radioscopy, CR, DR or film. In other words, it measures the total unsharpness of the radiographic image taking into consideration all the different parameters that can influence the resolution.

 
In Digital Radiography (DR), the image resolution is determined by the DDA’s resolution (pixel size), the scintillator type and thickness, the overall noise level (SNR) in the image and the examined object itself.

 
In Computerized Radiography (CR) the image resolution is determined by the phosphor plate thickness and type, the overall noise level (SNR) in the image, the readout time and the diameter of the laser beam use to scan the plate.

 

The duplex wire type IQI, consists of a series of 13 pairs of wires of high density material (tungsten and platinum) where each pair of wires of diameter (d) is spaced at a distance (d) apart. The diameter (d) of a pair is a measure of the total effective unsharpness of the radiographic image.

 

Figure 1: Duplex wire IQI

 

IQI values

D=Duplex

 

Unsharpness (mm)

– EN 462

Wire

(mm)

13D

0.10

0.050

12D

0.125

0.063

11D

0.16

0.080

10D

0.20

0.100

9D

0.26

0.130

8D

0.32

0.160

7D

0.40

0.200

6D

0.50

0.250

5D

0.64

0.320

4D

0.80

0.400

3D

1.00

0.500

2D

1.26

0.630

1D

1.60

0.800

 

The measurement is conducted by placing the Duplex wire IQI on the source side of the object examined in an angle between 2-5 degrees (vertical or horizontal).

 
To determine the Basic Spatial Resolution (BSR), one need to see at least 20% dip, in the gray levels values, between the two lines in a pair. The tool which allows you to measure this is called a line profile tool. Some of the radiographic standards e.g. ISO and ASTM standards define the Basic Spatial Resolution (BSR) as the first pair that does not meet the 20% gray levels dip while other regulations define it as the last pair that does show the 20% gray values dip.

 

Figure 2: 20% dip

 

In the example above, a 5mm steel plate was examined using the RayzorX Pro panel. When taking into consideration all the unsharpness of the radiogram, the 8th pair meets the requirement of 20% dip in the gray levels values while pair number 9th is the first pair that does not meet the needed dip. This means that according to the ISO and ASTM standards, the Basic Spatial Resolution (BSR) of this image is 0.130 mm while based on other standards the BSR will be considered 0.160mm.

 

Figure 3: 50 micro wire seen with BoltXPro

Scatter Radiation

Scatter radiation, often referred to as secondary radiation, is radiation that is due to atomic interactions within the specimen itself or material within the primary cone of radiation, i.e. wall table, floor, fixuring, etc.  and strikes the DDA. It is not the primary radiation, which is the radiation created by the X-ray/radiation source.

 
Secondary or scatter radiation must often be taken into consideration when positioning/setting up a DR system. The scattered photons create a loss of contrast and definition, resulting in reduced image integrity/ clarity (a blurry image).

 
Three  names are used to describe the scattered radiation. These names are derived from the way this scattered radiation is formed.


1. Forward scatter is generated either within the specimen itself or by such things as fixturing devices used to position the specimen within the primary beam. The primary method for reducing forward scatter is by the use of beam filtration to harden the beam. By removing some of the longer wavelength or soft radiation scatter can be reduced yet never eliminated.

   

2. Side scatter radiation originates from walls, or objects on the source side of the DDA. Control of side scatter can be achieved by moving objects in the room away from the DDA, moving the X-ray source to the center of the room, or placing a collimator at the exit port, thus reducing the diverging radiation surrounding the central beam. An additional way is to place a filter between the DDA and the object. This way both the side scatter and the forward scattering from the object are being minimized.

 

3. Backscatter radiation is radiation that originates from objects behind the DDA. Industry codes and standards often require that a lead letter "B" be placed on the back of the DDA to verify the control of backscatter. If the letter "B" shows as a "ghost" image on the radiogram, a significant amount of backscatter radiation is reaching the DDA. The control of backscatter radiation is achieved by placing a sheet of material with high absorption coefficient and, if possible, increase the distance between the back of the DDA and objects i.e. wall, floor.
 

Panel with filter and protective cover (reduces scatter radiation)

Window Leveling Tool

Window Leveling is a software tool which allows the operator to view various segments of the image grey levels spectrum rather than looking at the entire spectrum on the computer screen. It does not cause any changes to raw data; rather it allows viewing important scales of gray which the relevant data is mainly found while ignoring the areas outside the ROI.

 

Why not display the entire gray levels scale on the computer screen? Because there are two limitations: technical and biological.

 
Technically, it is not possible to display the entire scale (16,384-65,536 gray levels) on a standard computer display screen, because it can only display a maximum of 256 gray levels at a time. The second limitation, which is even more significant, is the human eye which can only distinguish between 64 to 100 gray levels. This means that even if there wasn’t a technological limitation, we still would have not been able to see beyond our biological limitations. The window Leveling tool ensures that we can see any detail we need, because we can view the relevant spectrum.
 

 

Manual Window Leveling tool

Dynamic Range

Latitude (in film), or dynamic range (in DR) is the range of panel exposures over which an image and contrast will be formed.  Due to film characteristics, and mainly because the way images are formed with the silver halide crystals, the latitude range is narrow and there is a tradeoff between it and the radiogram contrast; this means that you can either receive image with a high contrast and low latitude or vise verse but not combined.

 
In Digital Radiography (DR), digital sensors do not have this limitation. This means that the sensor responds to X-ray exposure and produce digital data over a wide range of X-ray exposure values (16,386 – 65,536 gray scale values) while maintaining high contrast images.

 

A typical digital radiography panel has a linear relationship between exposure and the resulting pixel value (the amount of gray values each pixel can display).

  

Dynamic range is represented in bits

Sharpening (unsharp Mask)

Sharpening or Unsharp Mask is an image processing tool often used in digital imaging.

 
An "unsharp mask”, in contrary to what its name implies, is an algorithm used to sharpen images. Sharpening can help emphasize details (such as discontinuities) in an image. Unsharp masks are probably the most common type of sharpening and can come in a variety of forms. An unsharp mask cannot create additional detail, but it can greatly enhance the appearance of details in an image. In practice, the algorithm amplifies high-frequency elements in the image (e.g. cracks) which are surrounded by a low frequency element (e.g. an even surface) and thereby helping the human eye to distinguish more details.

 
Vidisco combines different methods of Sharpening into one easy-to-use tool.

 

Figure 1: LP-MM test pattern X-ray image - with non-digitizing zooming and sharpening tools, 20LP/MM clearly seen

 

Figure 2: Sharpening Tool helps with the detection of fatigue cracks|

 

Figure 3: Sharpen Tool in the Vidisco XbitPro proprietary software for NDT operators, utilized in pipe NDT

 

Histogram Equalization

An image Histogram is the graphical representation of the color (in Radiography – the Grey levels) distribution in a digital image.

 
Histogram Equalization is one of a few methods in image processing which artificially adjusts the contrast in order to allow the human eye to distinguish more details in an image.

 
In a Black & White (X-ray) image the algorithm stretches the Grey Levels to the extreme. This means that the shades of grey which have a light shade (closer to white) will be stretched to near-white or converted into white. The darker shades (closer to black) will be stretched to near-black or converted into black.  Additionally, the grey levels in-between the extreme ends will be more dispersed; these actions increase the total contrast in the image, a crucial factor in visualizing defects.

 
The histogram equalization is effective only on homogeneous areas in a digital X-ray image. This means that when the ROI (Region of Interest) contains both ends of the gray level spectrum this tool will not create a significant change. In other words, on an entire image this feature will be less effective than on a selected area of interest in the image, as there will almost always be large variations in grey levels in an entire image.

 
Vidisco developed a smart, one click, histogram equalization tool called “Adaptive Histogram” , which overcomes this challenge and enhances the entire image at once. This tool can be helpful in viewing different materials in one image.

 

Figure 1: Pipe under insulation X-ray; taken with Ir-192 and Vidisco RayzorX Pro System - Adaptive Histogram

 

Figure 2: Adaptive Histogram Tool in the Vidisco XbitPro proprietary software for NDT operators, utilized in Art NDT

Pixels

In digital imaging, a pixel is the smallest screen element in a display device. It is the smallest unit of picture that can be represented, measured or controlled. Pixels are normally arranged in a two-dimensional grid and are often represented using dots or squares. Each pixel is a digital sample of the original image. More samples typically provide more accurate representations of the original (high resolution). The smaller the pixel size, the higher the theoretical resolution, yet at the same time, the relative noise level would be higher and the quantity of light (“penetration”) per given dose, will be lower.

 
It should be noted that pixel size is one of a few factors which will determine whether we see a small discontinuity or not; Signal to Noise ratio (see definition) is even more crucial.

 
The real (effective) resolution of a DDA is determined by the size of the pixel, the type and thickness of the fluoroscopic screen (Scintillators) as well as the level of noise.

 
Below is an example of a 143µ pixel size which shows that in 1mm² there are 49 (!) pixels.

Figure 1: 143 µ pixel size, 49 pixels in 1 sqmm

 

Figure 8: Pixel Structure

 

Fluoroscopic Screens (Scintillators)

Fluoroscopic Screens (Scintillators) are found in DDAs and act as scintillating materials which convert ionizing or penetrating radiation into visible light.

 
There are two commonly used Fluoroscopic Screens:

 
• The crystalline material called sodium-activated Cesium Iodide - or CsI (Na) – this material is widely used in DDAs for medical X-ray. CSI’s main advantage is producing a small un-sharpness in the image (due to good light spreading characteristics) but at the same time, it has one big disadvantage which is the memory effect (“Ghost Image”) known to last even for weeks (especially when working at high energies, with hard-to-penetrate objects) and consequently interfering with interpretation of newly acquired images.

 

• Gadolinium Oxysulfide (Gd202S – also known as Gaddox) - this material is widely used in DDAs mainly for industrial applications. Gaddox has a very low memory effect, thus enabling extremely fast X-ray imaging. It was also proven to be suitable for a wide energy range (starting from low energies and even up to the extreme energies such as those produced by Cobalt 60).
Based on over a decade of experience Vidisco realized that for the overwhelming majority of NDT applications, the most suitable Scintillator is Gaddox.
 

 
Figure 1: Two main scintillator types

 


Figure 2: Scintillator types and corresponding light spreading

 

 

Digital Detector Array (DDA)

Digital detector array (DDA) sometimes referred to as Flat Panel, Detector or Imager, is a sensor or device that converts penetrated radiation into digital information.

 
In practice, the DDA converts ionizing radiation into analog signals, which are then digitized and transferred to a computer for display as a digital image, corresponding to the energy pattern imparted upon it. The conversion of the ionizing or penetrating radiation into an electronic signal may emerge by first converting the ionizing or penetrating radiation into visible light through the use of a scintillating material (see Fluoroscopic Screens). The light photons will then strike a matrix of elements that are sensitive to light and a readout on the X and Y axis will be made in order to acquire the digital signal.

 
Digital Detector Arrays (DDAs) may be of various types: e.g. Amorphous Silicon, CMOS, etc. and each type may consist of various Fluoroscopic Screens options (Scintillators).
X-ray images conducted with DDAs offer marked advantages including reduced exposure (due to very sensitive detectors - up to 100 times more than film), reduced energy required to create an image, high image quality, high Signal to Noise Ratio (SNR), high dynamic range, short overall inspection time, cost savings and immediate imaging which eliminates the need for later repositioning

 

Digital Radiography (DR)

Digital Radiography refers to an X-ray Imaging method that uses digital X-ray sensors (also known as DDAs – Digital Detector Array or Imagers) to produce a digital image rather than using traditional film or phosphor imaging plates (as is done in CR- Computerized Radiography).

 
When a digital detector is exposed to X-rays it forms an image almost instantly and transfers it to a computer screen. Digital Radiography offers several advantages over conventional film and CR such as an overall increase in work efficiency (extremely short exposure times, no developing time), lower costs and environmental friendliness due to the elimination of chemical processing and increased X-ray efficiency due to low doses.  Images are available immediately on the operator’s computer screen, which means having the ability for on-the-spot interpretation before moving on to the next “shot”.
Images can be enhanced with the use of specialized software and can also be easily stored, transferred and shared without ever loosing the original image. Operator safety is substantially increased due to the fact that far less radiation is required to produce a high quality, high contrast image.

 
In the majority of cases, the image quality with Vidisco’s DR will be higher than achieved with conventional radiography mainly due to the wider dynamic range of the DDA.

 

X-ray Source

X-rays are a form of electromagnetic radiation (penetrating energy) which is used in the industrial and medical fields in order to visualize the insides of objects and structures.

 

An X-ray source is a device that generates X-rays and is comprised of the following main components: Vacuum tube; high voltage generator; a filament, and a target.

 

X-rays are generated by shooting a stream of high speed electrons (formed by the filament) towards a target material which has a high atomic number (usually Tungsten). X-rays are formed when the electrons strike the target and are either slowed down or stopped by the interaction with the atomic particles of the target. Increasing the Current (mA) causes more electrons to form, which in turn, creates more X-ray photons (flux). The energy of the photons is determined by the Voltage (kV) of the tube.  For dense and thick materials a higher kV is needed. During the formation of X-rays considerable heat is produced, thus requiring cooling down using various methods.

 

Some X-ray sources (which are not constant potential) require synchronization, hence, control and connection from the Digital Radiography systems to the X-ray source’s controller is required in order to work in an optimum manner. Vidisco’s systems support connections to most X-ray sources on the market today and are compatible with various radiation sources (portable and stationary) such as Pulsed, CP, Pulsed wave/High frequency, Betatron, Linac and Isotopes.

 

How X-rays are generated

Non Destructive Testing (NDT)

Non Destructive Testing refers to a category of analysis techniques used in science, medicine and industry in order to evaluate the properties/structure of a material, components or system without causing any damage to it.  To name a few: Liquid Penetrants, Magnetic particles, Visual testing, Ultrasonic, Radiography, Eddy Current and more.


One of the main NDT techniques is Radiographic testing (also referred to as RT) which traditionally utilized radiography with X-rays or Gamma rays (Isotopes) in combination with film. Major developments in imaging technologies were quickly realized by many Industry sectors and caused a shift to Computerized Radiography (also referred to as CR), utilizing X-rays or Gamma rays (Isotopes) in combination with imaging plates; or Digital Radiography (also referred to as DR), utilizing X-rays or Gamma rays (Isotopes) in combination with digital detectors  (also referred to as Digital Detector Array - DDA) in addition to or instead of Film.


Vidisco’s inspection systems are based on the Radiographic technique which utilizes unique industrial grade DDAs.

 

NDT with Vidisco's Portable Digital Radiography Systems

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