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The Science of X-ray and CT

An overview of the fundamental principles of radiography and the technology involved in the imaging process.

Kevin Stevens, ARRT RT,R,MR

Let’s take a brief look at the utility and advancements of x-ray and CT in medical radiography. In order to do this we first need to go back to the beginning to see how this all came about.

 

History:

 

In the beginning was radiation, and a man named Wilhelm Conrad Roentgen discovered it. Really it was November 8, 1895 and he didn’t know what he had discovered so he called it X-radiation due to its “unknown” origin. Once he was able to reproduce and harness this otherworldly energy he quickly understood its ability to pass through an object relative to the density of that object. The basic relationship between radiation and matter is that radiation passes freely through air and is absorbed more easily by matter the more dense it becomes. Inversely, when radiation exists at a higher energy state it can more easily pass through matter. The next thing to note is that when the amount of radiation increases the amount that will pass through an object also increases. These relationships are the basis for every way that we utilize x-rays in medical imaging. But how do we create and control radiation to make an image?

 

The X-Ray Tube:

X-ray photons are a high energy component of the electromagnetic spectrum and it takes a lot of energy to create them. I will try to keep this explanation as simple as possible. First, we increase the energy of electrons in a coil of wire (cathode) until they release from their atoms at half the speed of light. Those projectile electrons travel across a vacuum tube and collide with a target (anode). The interaction of these electrons with the target atoms produces a large amount of heat, infrared radiation, and x-ray photons. These photons exit the tube through a small window and are focused by lead plates (collimators) which allow us to control the size of the area that will be irradiated. X-ray production is controlled by three predetermined settings, kV, mA, and mAs. The energy, and therefore penetrating power of the x-rays produced is controlled by the preset kilo voltage (kV). Milliamperage or mA controls how many electrons are available for x-ray production. Lastly, the milliampere seconds or mAs is a factor of exposure time (how long the tube is producing radiation) and this primarily controls the amount of radiation that exits the tube. Over the years these x-ray tubes have evolved in many ways, bringing new advancements to our imaging modalities. 

 

Detectors:

 

As the x-rays pass through the patient they interact with the atoms in the tissues in different ways. These interactions occur as absorption of the photons, attenuation or changing the direction and velocity of the photons, production of scatter radiation, or by passing through the subject completely. As we discussed before, the higher the tissue density the more radiation will be absorbed or attenuated. One of the many ways radiation interacts with atoms inside the body is by ejecting or adding electrons to the items creating free radicals and ions. For this reason x-rays are known as a form of “ionizing” radiation which can be dangerous or even deadly if not used in a safe manner. The remaining x-rays that do make it all the way through the patient must be received by some form of detector in order to capture that information. Originally this was accomplished by a screen containing a fluorescent material that would glow when exposed to x-rays. The more radiation, or more intense the radiation, the brighter the screen will glow. This fluorescent intensifier screen is placed above a piece of photosensitive film and the exposure produces a latent image on the film. Then, just like in photography, the film is developed and “voila!”, you have an x-ray image or radiograph. The resulting image shows greater exposure in the areas with the most radiation passing through the subject and appearing black on the image. While areas with no exposure due to complete absorption of the x-rays will appear white on the image. The “blackness” of the image is known as the image density, and the scale of differences between adjacent densities is known as image contrast. This means that the density is primarily controlled by the mAs and amount of radiation, while the kV primarily controls contrast. The ability of the detector to accurately resolve the latent image is referred to as image quality and resolution. When your detector resolution increases you are able to accurately image smaller objects with better definition of those objects. As detectors have advanced over the years we have moved away from film/screen systems to electronic systems that could convert the photons into light information . Modern systems have evolved into completely digital systems that receive the radiation and convert it directly to a digital signal. Digital detectors have made it possible for immediate image processing and a vast increase in the flexibility of x-ray systems. Today, radiography (X-ray) is used primarily for evaluation of bones, joints, and general overviews of the chest, abdomen, and pelvis. 

 

Radiation Safety:

 

Before we move on we need to cycle back to something and that is the danger of radiation. As we are all well aware, radiation is bad for you. Too much radiation exposure can cause severe illness and even death as we’ve seen throughout history. Roentgen’s own wife was the first documented case of death from radiation exposure. His wife was his “test subject” and his exploration of x-rays caused him to expose his wife over and over again while he performed his tests. This ultimately made her very sick and eventually claimed her life. Though we may not see the damage it causes, it doesn’t mean that the radiation exposure isn’t causing some form of cellular damage inside of our bodies. This is true not just for the patient being imaged but anyone in the area that can suffer a secondary exposure. Though the patient is the only one that should be exposed to the direct beam of radiation some of the x-rays are scattered by their interactions with the patient or the environment around. This scatter radiation has much less energy than the primary beam but still contains enough energy to cause cellular damage. Radiation safety is guided by the ALARA principle which states that radiation exposure should always be As Low As Reasonably Achievable. Time, distance, and shielding are the primary concepts that we use to protect ourselves and our patients from unnecessary radiation exposure. Obviously the length of exposure time directly contributes to radiation exposure by producing more or less radiation in the first place. Distance refers to the idea that the further you are away from the radiation source the less exposure you will receive. A guiding principle for this concept is the Inverse Square Law which states that the exposure to an object from a radiation source is inversely proportional to the square of the distance between them. Shielding relates to using lead shields to protect areas of the body, as well as anyone outside of the room, from radiation. Aside from good radiation safety practices it is important to monitor the exposure level of radiation workers in order to ensure that they don’t suffer any permanent damage from repeated exposure to radiation. In order to monitor exposure all radiation workers must wear a radiation dosimeter any time they are in the x-ray environment. These dosimeter badges are routinely evaluated to determine the amount of radiation they have been exposed to. The results are then reported back to the personnel, their employer, and any necessary regulatory agencies.

 

Fluoroscopy:

 

Aright, back to the x-ray systems. The next big advancement in radiography was the development of fluoroscopy. With fluoroscopy a tube detector was developed that allowed a radiologist to see the image as it was being produced giving them the ability to view the internal structures of the body in motion. This is what you see when a cartoon character walks in front of a screen and you suddenly see their bones appear on the screen and then watch their bones move as they walk away. This was revolutionary, now you could watch someone drink a radiopaque material and watch them swallow it, or perform studies of the rest of the digestive system. Fluoro also made it possible to perform procedures and surgeries while evaluating the process throughout. Now, this also meant that the personnel was being exposed to much higher levels of radiation. Especially with early fluoroscopic systems. But real time imaging had huge implications that they didn’t even realize at the time. 

 

Tomography:

 

As x-ray systems evolved someone had the idea to figure out a way to create three dimensional images of the internal structures of the body. This began with stereoscopic imaging in which you take two exposures offset 15 degrees from each other. Then using a stereoscope held up to your face the mirrors inside superimpose the two images making the resulting image appear three dimensional. Eventually they realized that they could achieve the same effect if the tube and detector moved together and multiple exposures were made over top of each other at different angles. Now you didn’t need to use a stereoscope to have a 3D image. The next hurdle was whether or not it was possible to get a cross sectional image of the patient without cutting them in half. 

 

Computed Axial Tomography:

 

CT, is a high resolution radiographic imaging modality. The nature of image acquisition makes CT an ideal option for evaluating boney structures, and high contrast areas of the body such as the thorax and sinuses. 

 

Originally this technology was called CAT scan or computed axial tomography scanning, then computer assisted tomography, and now it is just known as computed tomography or CT scanning. Essentially someone found a way to combine all of these technological advancements into one master radiographic modality. If you take the high energy x-ray tubes, the analog to digital image detectors and converters, the movement of tomographic tube and detector relationships, and the real time image acquisition concept of fluoroscopy you’d have something really astounding. With CT scanning the x-ray tube and detector rotate around the patient opposite each other while the patient passes through the x-ray beam on a moving table. The digital detector receives the constant exposure and sends the information to a computer system that processes that information into cross sectional images. The first CAT scanners were primitive by today’s standards and very slow. These systems would take one cross sectional image at a time with every 180 degree rotation of the tube. Soon two big advancements were made in the field. First, the ability to rotate the tube 360 degrees around the patient providing twice as much information per slice. The second advancement was the development of the multi-slice detector which made it possible to acquire more than one slice at a time. These systems just kept increasing from 4, to 8, to 16, 32, 64, 128, 256 and up! The two biggest benefits to this technology were the reduction of radiation exposure and much shorter acquisition times. Now you could perform a CT scan of a patient’s entire abdomen in a few seconds as opposed to an hour!

 

Conventional vs Spiral (Helical) CT:

 

To back up a bit, conventional CT scanning was performed by the table advancing incrementally for each rotation of the tube. With multidetector scanning another advancement was made which allowed for constant image acquisition while the table was moving resulting in a spiral or helical exposure pattern around the patient. The relationship between the speed of the table movement divided by the slice thickness is called Pitch and it makes accurate seamless spiral imaging possible. Spiral CT reduced the inherent artefacts from motion and partial volume averaging and increased spatial resolution with the overlapping of image information. 

 

Reconstructions:

 

The next big advantage of digital cross sectional imaging is the ability to further manipulate the images with the use of a computer. First the computer allows the operator to change the contrast, density of an image by adjusting the window width and window level. When evaluating a CT image each pixel is assigned a specific value known as the CT number which corresponds to the specific tissue density that is contained in that pixel. The scale of these CT numbers is known as the Hounsfield units and different tissues and pathologies are known to have specific Hounsfield units. The Hounsfield scale greatly increased the accuracy of diagnostic image evaluation, interpretation, and diagnosis. Computer software could also be designed to manipulate the sharpness and resolution of that image via software filters and Kernels (convolution algorithms). This also meant that the computer software could reconstruct the images into different image planes or create 3D volume renderings of the area that was scanned. Because the image information was digitally stored one person could be reconstructing the images on one computer while the technologist was already scanning a different patient.  


 

Contrast Administration:

From the advent of x-ray imaging people have been looking for ways to garner additional information by ingesting or injecting different contrast materials. Oral contrast agents containing barium or iodine allow for more accurate evaluation of the digestive system as they coat the mucous membranes of the esophagus, stomach, and intestines. Intravenous contrast agents are typically iodine based and interact with the organs and tissues of the body as well as vascularly connected abnormalities and pathologies. The information gained from the introduction of a contrast agent can significantly narrow down the differential diagnosis of a disease process.


Conclusion:

Radiography and CT are very versatile diagnostic imaging modalities and are heavily relied on to diagnose a myriad of conditions. Understanding the equipment and the science behind these modalities will greatly increase your ability to produce exceptional imaging studies for your patients. It is my hope that even this brief overview has helped increase your knowledge and your ability to utilize these modalities in your daily practices.

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