X-ray imaging:Fundamentals and planar imaging

Mikael Jensen1 and Jens E. Wilhjelm2

1Hevesy Laboratory, DTU Nutech, 2Biomedical Engineering, DTU Elektro

Technical University of Denmark(Ver. 4 28/8/14) 2004 - 2014 by M. Jensen and J. E. Wilhjelm

1 IntroductionX-ray imaging is the most widespread and well-known medical imaging technique. It dates back to

the discovery by Wilhelm Conrad Rntgen in 1895 of a new kind of penetrating radiation coming from an evacuated glass bulb with positive and negative electrodes. Today, this radiation is known as short wavelength electromagnetic waves being called X-rays in the English speaking countries, but Roengten rays in many other countries. The X-rays are generated in a special vacuum tube: the X-ray tube, which will be the subject of the first subsection. The emanating X-rays can be used to cast shadows on photographic films or radiation sensitive plates for direct evaluation (the technique of pla-nar X-ray imaging) or the rays can be used to form a series of electronically collected projections, which are later reconstructed to yield a 2D map (thus, a tomographic image). This is the so-called CAT or CT technique (see the chapter on CT imaging).

1.1 Characterization of X-rayX-rays are electromagnetic radiation (photons) with wavelengths, 10 pm < < 10 nm. They travel

with the speed of light, c0 300 000 km/s and has a frequency of = c0/ [Hz]. The energy of the individual photon is E = h [J], where h = 6.6210-34 Js is Plancks constant. The energy of an X-ray is typically measured in electron volts (eV). 1 eV is the energy increase that an electron experiences, when accelerated over a potential difference of 1 V. Thus, 1 eV = qeV = 1.60210-19 J, where the charge of an electron is qe (both qe and V is negative in this context).

I

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Figure 1 X-ray tube showing cathode and anode with electrons accelerated from cathode towards anode. The tube generates X-rays in all directions, but due to the encapsulation most are lost and only a fraction is used for imaging.

Problem 1 Calculate the frequency and energy for monochromatic x-rays with = 1 nm.Answer: = c0/ 31017 Hz = 300 000 THz. E = h 1.9910-16 J = 1.24 keV.

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1.2 X-ray generation: The X-ray tube

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Characteristic X-ray emission from Wolfram

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Figure 2 X-ray emission from Wolfram anaode X-ray tube. Observe that for a given tube voltage, the higher the energy of the photons, the less there are. And if the number of photons are to increase, then the tube voltage should increase. Data from [1].

A typical X-ray tube is depicted in Figure 1. It consists of an evacuated glass bulb with a heated fil-aments (Danish: gldetrd) as the negative electrode and a heavy metal positive anode. Thermic elec-trons emitted by the heated filament are accelerated across the gab to the anode. If the voltage between cathode and anode is U volts and the current in the tube being I amperes, each electron will be hitting the anode with a kinetic energy of U eV. The power deposited in the anode will be I times U, and the total energy transferred to the anode in an exposure lasting t seconds will be IUt.

The electrons will be slowed down in the anode material, mainly releasing their energy as heat, but to a small degree (few percent) the energy is transformed to either Bremsstrahlung or characteristic X-rays. The Bremsstrahlung originates from the sudden deacceleration and direction changes of the primary electrons in the field of the anode atoms, the characteristics X-rays originates from the knock-out and subsequent level filling of inner electrons in the atoms of the anode material. The highest pos-sible quantum energy of emanating X-rays (measured in eV) will be equal to U. Typical energy spectra as a function of voltages are shown in Figure 2. Please note that the spectra are all taken at the same current, only the voltage has been varied. This demonstrates that the total number of X-ray pho-tons are heavily dependent on tube voltage. In addition to the information in Figure 2, a general rule of thumb says that 15 keV increase in voltage corresponds to a doubling of the photon output. For practical medical applications, the low energy part of the photons are normally not used but removed by filtering either inside or just outside the X-ray tube. Normal filter materials are either aluminium or copper. The thicker the filter and the higher the atomic number of the filter, the greater the cut-off of low energy photons.

The description of the exposure characteristics of a given X-ray tube will comprise the voltage (in units of kV), the current (in units of mA), the time of exposure (in units of s) and the degree of filtering (for example a plate of Al, one mm thick next to a plate of Cu, 0.5 mm thick). As the total number of photons produced for a given high voltage setting only depends on the product of current and time this is often stated as a product in units of mAs.

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1.3 Anode material, power dissipationHeavy elements are normally preferred for anode materials as the high Z-number gives efficient pro-

duction of the part of the X-ray that originates from Bremsstrahlung. The characteristic X-ray lines, which add to the total energy spectrum, normally appear in the range 50-70 keV, which ofte is in the middle of the medical useful energy range.

The thermal load on the anode material both during the short exposures and averaged over time when performing rapid, multiple exposures heats the anode dramatically. For this reason, normally a high melting point material is used. Anodes made out of Tungsten (Danish: Wolfram), abbreviated W, are very common. The area of thermal dissipation can be enlarged by rotating the anode during exposure. An example of such a device is shown in Figure 3.

2 Typical X-ray system

Figure 3 Rotating anode X-ray tube. "RTM" anode designates a Molybdenum anode mixed with 5% Rhenium to improve the thermal stability. The metal anode is supported by graphite to improve the total thermal capacity. Source: Siemens.

Figure 4 shows a typical X-ray system. The X-ray is generated by the X-ray tube (Danish: Rnt-genrr). Low energy photons are removed by the Al filter, since as they cannot penetrate the object and contribution to the information on the film, they would only add needless to the dose received by the object. X-ray radiation outside the image region on the film is removed by the collimator (Danish: primrblnde). Attenuation (what is measured on the film) and Compton scattering take place at the object. Only photons moving directly from the source to the film are allowed through the grid at the bottom (Danish: sekundrblnde).

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Figure 5 provides an interactive illustration of a simple X-ray system allowing for translation of a simple homogeneous phantom. As the distance is changed, different edge effects can be observed. By pressing the Lines button, the geometry of the edge effect is visualized.

3 Geometrical considerations

Al-filter (removes low energy radiation)

Secondary radiation(Compton scattering)

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Figure 4 Schematic illustration of a typical X-ray system.

Referring to Figure 5 we can define the distance from the X-ray origin (the focus1) to the objects as FOD. The distance from the focus to the film2 or any other medium of radiation detection can be de-fined as FFD. Any object will to some degree attenuate the X-ray, and variation in X-ray absorption across the objects will create a corresponding variation in the radiation impinging on the film. An un-avoidable and sometimes desirable geometrical magnification of the image relative to the object can be deduced from the triangle in Figure 5. The enlargement factor F, can be defined as:3

F = size of film image / size of object = FFD / FOD (1)

If near to normal picture size and little variation in enlargement is sought for organs having different depths in the body, the geometrical magnification should be minimized by using a large focus to film distance (FFD) and a small object to film distance. A small object to film distance also improves im-age contrast, as blurring by scattering increases with increasing distance between object and film. This is because the origin of the scatter is mainly inside the object: The longer the scattered radiation is

1. The "focus" is in this context the source of the X-ray photons, as it is the name for the electron spot on the anode of the X-ray tube.

2. The term "film" is still common language, even though the conventional x-ray film to a large degree has been replaced by various other imaging plates.

3. Here size means any distance e.g. the length of a given object.

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allowed to travel between the object and the film, the more this radiation diverges from the true un-scattered photons.

Figure 5 Simple illustration of the geometry of the planar X-ray system corresponding to Figure 4. The X-ray image of the green homogeneous box is shown to the right. The focal point is the origin of the X-rays. The film is identical to the detector. FOD = focus to object distance. FFD = focus to film distance.

Similar geometrical considerations (i.e. similar triangles) can demonstrate that extended size of the focus will generate blurring on the film.

4 Origins of contrast in the X-ray imageX-rays are attenuated according to the normal linear attenuation law:

I(x) = I0 exp(-x) = I0 exp(-/ x) (2)

where x is the distance transversed in the material and is the so-called linear attenuation coefficient in units of m-1. I0 is the intensity at the entrance to the material (x = 0) and I(x) is the intensity at dis-tance x. In the latter part of (2), is the density of the material. By giving the attenuation coefficient in units of / and the thickness in length times density (area weigth, e.g. g/cm2) the attenuation co-efficient (now called mass attenuation coefficient) becomes independent on the physical state of the material. Mass attenuation coefficients for some common tissues are given in Table 1.

The microscopic description of the attenuation comprises photo electric effects and Compton scatter-ing, which are both described in the chapter on nuclear medicine. For the understanding of the X-ray technique, it suffices to say that the linear attenuation coefficient for human tissues varies approxi-mately as the electron density. Thus, it varies roughly proportional with the physical density (kg/m3). Air has the lowest density, lung tissue has lower density than fat, fat has lower density than muscle,

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again having much lower density than the bone mineral of the skeleton. The X-ray attenuation varies accordingly. X-rays transversing parts of the body having high absorbing material will be much more attenuated, and the film or radiation capture device will in this region not receive as much radiation. It should be remembered that the X-ray image is a negative (bright areas correspond to high attenua-tion) and that the image is a 2D projection of the 3D distribution of attenuation.

0,1310,1860,424Bone =1,92 g/cm3

0,1370,1710,227Water =1,00 g/cm3

0,1360,1690,226Muscle =1,05 g/cm3

0,9995,5498,041Lead =11,35 g/cm3

0,1360,1690,212Adipose tissue =0,95 g/cm3

0,1220,1540,208Air =0,0013 g/cm3200 keV100 keV50 keV given in cm2/g

Table 1: Mass attenuation coefficients for typical tissues in /From [2].

Figure 6 Normal chest X-ray image. This image is recorded with a tube voltage of 150 kV to minimize the contribution from bone. Press ? to try to identify tissues.

An example of a normal X-ray image of the chest (one of the most common medical imaging proce-dures) is seen in Figure 6. Notes that the most attenuating areas (ribs, vertebral column, heart) appear white while the lungs with little attenuation appear black on a conventional planar X-ray image.

Problem 2 Does the planar X-ray image have arbitrary units (or, put in other words, are the pixel val-ues relative or absolute)?

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The image information in the planar X-ray is mainly anatomical, actual densitometric measurements on the film are only performed for quality assurance programs and yields little medical information. Today, all planar X-ray images are evaluated by a human observer.

5 Film, intensifyer foils and screensOriginally, the radiation was captured by a normal photographic film. In the film, the energetic X-

ray photons are absorbed in the silver halide (NaB-NaI) crystals, generating very small amounts of free silver. During film processing, any grain with small amounts of free silver are completely con-verted to metallic, nontransparent silver, while the remaining unreduced silver halide is removed by the fixative. X-ray films are of course made to the size necessary for the anatomical situation in ques-tion and can be very large. To increase sensitivity and thus lower radiation dose, the photo sensitive film emulsions are often thicker and occasionally coated on both sides of the film, in contrast to nor-mal photographic film. The silver contents of the films makes X-ray films rather expensive. Any film has a specific range of optimal sensitivity (exposure range from complete transparency to completely blackened). Although modern equipment are normally assisted by electronic exposure meters, the correct choice of film, exposure time, exposure current and high voltage is still left to the judgement of the X-ray technician.

To improve the sensitivity and thus lower radiation exposure to the patient, the film is often brought in contact with a sheet of intensifying screen. The screen contains special chemical compounds of the rare earth elements, that emits visible blue-green light when hit by X-rays or other ionizing radiation. This permits the use photographic film with thinner emulsions and more normal sensitivity to visible light. While increasing the sensitivity, the use of intensifying screen on the other hand blurs the im-ages as the registration of X-ray radiation is no longer a direct, but an indicted process.

The patients or the object is not only the source of X-ray absorption but also of X-ray scattering, mainly due to Compton effect (please see the chapter on Nuclear Medicine). Any part of the patient exposed to the primary X-ray beam will be a source of secondary, scattered, X-rays. These X-rays will have lower energy than the original ray, but as no energy discrimination is used in the registration, also the secondary scattered radiation adds to the blackening of the film. The scattered radiation car-ries no direct geometrical information about the object and thus only reduces the contrast by increas-ing the background gray level of the film. Scattered radiation can to some degree be avoided by the use of special collimators called raster. The raster can be a series of thin, closely lying bars of lead, only allowing radiation coming from the direction of the focus point to hit the film while other direc-tions are excluded. A typical example of a complete radiography cassette content is shown in Figure 7.

6 Digital radiography and direct captureDuring the last years the conventional film based radiography has gradually been replaced by newer

digital techniques. The end points is of course the acquisition and storage of the X-ray image infor-mation as computer files. It should be noted that X-ray images are normally of very high resolution (more than four thousand by four thousand pixels) with large dynamic range (12 to 16 bits). The cor-rect handling and display of such image information without loss or compression is still the subjects of specialized workstations. The digital X-ray images are normally stored and displayed in so-called PACS systems (Picture Archiving and Communications System). The image information is either temporarily captured on phosphor plates for subsequent transfer to digital storage by so-called phos-phor plate readers, in function much related to the old X-ray film processors) or by direct, position

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sensitive electronic X-ray detection devices, the so-called direct capture systems. At present (2003) the geometrical resolution of the various digital techniques is still somewhat inferior to the best pos-sible film technique. However, the benefits of rapid viewing, interactive image availability, postpro-cessing and digital transmission often outweighs the reduction in image quality. For special applications, like breast cancer detection by mammography, the photographic film is still the system of choice.

X-ray not passing through the raster

X-ray passing through the raster

Raster

ISFilm

Figure 7 X-ray cassette, containing double coated film, intensifying screen (IS) and lead raster.

7 Analysis of image of phantomConsider the conventional planar X-ray image of a typical phantom in the course shown in Figure 8.

This X-ray system has a high dynamic range, thus the image has many levels. To include as many details as possible, this X-ray is shown - very untraditional - in color. Normal X-ray images are shown in shades of gray.

It is not trivial to analyze such an image, unless one is very careful. First one should remember that what one see (the contrast in the image) is total attenuation which again is dependent on what kind of material is present and how much of this material is present in the trajectory between focus and a given detector. And since the colorbar is representing relative values, both of these observations should be considered relative to other places in the image.

The analysis goes as follows:

Since the image shows more than the box of the phantom itself, the connectors can be identified. Those appear dark yellow and since they obviously must attenuate more than the nearby air (which appears nearly white) it can be concluded, that a high pixel value corresponds to low attenuation (in contrast to normal X-ray). Be aware of a potential pitfall: if the film is larger than the area that is ex-posed (as controlled by the collimator at the x-ray tube), then the image has an additional outer frame, which, obviously, should not be used in the analysis.

Next consider the fiducial markers. Since the lid was in place during recording of the image, we have two series of materials that are penetrated by the X-rays: one through the markers and another between the markers:

agar, acrylic base plate

acrylic lid, agar, acrylic base plate.

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Problem 3 Please make a cross-sectional drawing of this, and be sure that the total distance penetrat-ed by the X-ray is the same (for this particular problem, it is assumed that the X-rays hits perpendic-ular to the surface).

Since air-free agar is mainly water and thus has very much the same density as water and since acryl-ic has a higher density, the fiducial markers must attenuate less, and thus appear brighter than the sur-roundings. This is also the case in the image in Figure 8.

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Figure 8 Left: Top photo of phantom 4 from 2009. Right: Planar X-ray image of same phantom. The image is shown in color to enhance contrast. The two images are sought aligned as well as possible.

Next consider the tube. The tube contains air inside and thus the wall is the most attenuating. But even if the tube contained water inside, the wall would still be the most attenuating. Thus, the tube periphery should be darker than the center when looking at the projection image. The shape of the actual attenuation profile through the tube is considered in Problem 13 in the exam of year 2008.

8 References[1] Johannes Jensen og Jens Munk: Lrebog i Rntgenfysik, Odense 1973.2.udg. s.121.

[2] Data from http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html

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X-ray imaging: Fundamentals and planar imaging1 Introduction1.1 Characterization of X-ray1.2 X-ray generation: The X-ray tube1.3 Anode material, power dissipation

2 Typical X-ray system3 Geometrical considerations4 Origins of contrast in the X-ray image5 Film, intensifyer foils and screens6 Digital radiography and direct capture7 Analysis of image of phantom8 References