In x-ray imaging (radiography and CT) the contrast between the tissues in the image is generated by the difference between the x-ray attenuation (influenced by density and atomic number). In this post we demonstrate the material dependence for x-ray attenuation.
For all radiographers, radiologic technologists and students understanding the basic principles of x-ray interactions, which lead to the image contrast in x-ray and CT images is the building block for understanding how x-ray imaging works. These interactions also provide the physical reason behind many of the protocol optimizations on x-ray (RF, interventional, mammography) or CT equipment.
Basic Principle of Image Signal (Contrast) in X-Ray Images
During an x-ray exposure, for diagnostic x-rays or a CT scan, x-rays travel through the human body and measurements are made on an image receptor of the x-rays which pass through the body.
The x-rays which do not interact and are transmitted travel in straight lines. The image signal is generated by the x-rays that interact and are detected by either film, or a detector.
Alternatively, some of the x-rays are stopped in the body and they deposit they energy locally. This is via photoelectric effect, which is described in detail below.
The way that contrast is generated in the images is actually the difference between areas where more x-rays are stopped and areas where fewer x-rays are stopped.
The other way that the x-rays can interact with matter is via a Compton scattering event. In that case, the x-ray comes in a straight line, it scatters in the body, and then goes off in an angle.
This in general will cause artifacts in our x-ray images because we don’t know where this x-ray came from.
We implicitly assume that it came from a different path because we assume that it was a transmitted photon travelling in a straight line.
Rad Take-home Point: The signal in x-ray images is generated by the difference in x-ray attenuation of different materials, and contrast is generated by materials with different x-ray attenuation.
Attenuation Coefficients of Different Materials based on atomic number (Z)
Attenuation of x-rays is a function of energy and the materials that the x-ray beam is passing through. (see Figure)
This plot is a useful schematic if you know the average energy of the x-ray beam you can estimate the contrast between different materials.
The coefficient for fat is lower than that of water (oil is less dense and floats in water), and water’s coefficient is lower than that in muscle. All three are relatively close while bones have a much higher attenuation coefficient, and pure calcium even a higher attenuation coefficient.
Lower energy x-rays form image with higher contrast because of high attenuation coefficient. See the figure which shows the contrast of bones in an x-ray at two different energies, and the bones are much brighter in the lower energy image.
In the next section we will discuss why contrast agents like Iodine and Barium are used in x-ray and CT imaging as they have much higher x-ray attenuation compared with that of soft tissues.
Rad Take-home Point: The largest differences in x-ray attenuation are for high Z materials and at lower x-ray energies.
Contrast Agents in X-Ray and CT
The intrinsic contrast of blood compared with background tissue in CT is not very high since the soft tissue and blood have similar x-ray attenuation.
In comparison when an iodine based contrast agent is injected into the blood stream the contrast is improved significantly due to the high Z material. The figure demonstrates the large difference in x-ray attenuation between soft tissue and the contrast agents frequently used in x-ray imaging (Barium and Iodine).
In the CTA (CT Angiography) we can clearly see the delineation of the blood vessels given the contrast agent injection.
Also obvious in the plot of x-ray attenuation for these materials is that lower kVp, will give higher contrast between the contrast agent and the background.
The practical tradeoff which is needed is that a higher kVp may be required in order to penetrate through larger objects.
So, depending on your given task, if the object is relatively small then lower kVp can be beneficial and especially if you’re looking at something like bone or like an iodinated contrast where there’s really a significant boost in the contrast level.
Higher kVp can be beneficial for greater penetration and for artifact mitigation, for instance, in the case of beam hardening in CT.
Rad Take-home Point: The contrast agents in x-ray and CT take advantage of the large x-ray attenuation for high Z materials.
X-Ray Attenuation Coefficients (Thickness dependence)
X-ray images have one significant limitation, in that all of the anatomy along the direction of the x-rays is overlapping. In a chest radiograph for instance: bones, lung tissue, and vessels all overlap with each other in the image. The signal in a x-ray radiograph is dependent on the thickness of the material and the types of material that the x-rays pass through.
Such an image is sum of all x-rays that is shown as anatomic clutter. This leads to a lower contrast in x-ray images as higher intensity structures can block the impact of subtle low contrast changes.
In CT, reconstruction of the image takes place so that a fully 3D map of the attenuation coefficients can be reconstructed.
This has the advantage of significantly improving the ability to differentiate low contrast structures in comparison with x-ray imaging.
In the chest image for instance in this figure we can very well see the delineation of the soft tissue with the lung and the bone. We are no longer seeing all of the overlap of different materials as in the x-ray image.
Rad Take-home Point: The signal in x-ray is the sum of all the attenuation along the ray paths and CT provides superior low contrast detectability by removing the anatomic clutter in x-ray images.