Computerized Tomography – Introduction and History

Computerized Tomography – Introduction and History

Images created from present day xray scanners are the result of computer processing. The methods used in radiology have evolved since its discovery. It is safe to assume that computers will continue to be used to advance this field of understanding.

Here you will be introduced to this technology. Together, we will cover the history of radiographic imaging and how it has arrived to were it is today. I will also cover one of the techniques used in computerized tomography using GIMP. We will step manually through this process. From that, we will gain understanding why automated computer processing is central to the speed and accuracy of modern xray imaging.


Computerized Tomography, also know as CT scanning, is a technique used in present day radiographic imaging. Radiographic imaging is the creation of pictures using xrays. It is similar to conventional photography, but instead of visible light, xrays are used as an exposure source. Visible light photography is different because light is mostly absorbed or reflected by a subject and that level of reflection is what is captured in the picture. Radiographic images contain the opacity of a subject in respect to the xray energy passing through it.

One of the limitations of traditional radiographic imaging is accurately imaging a subject that has varying opacity. This was especially true when x raying the human skull, with the intent of imaging the brain. The bone of the skull has less opacity than the brain. Worse yet, the brain is surrounded by a layer of fluid that also masks the final image. As a result, the contrast is too narrow to accurately distinguish between the different materials. It is like listening to a whisper in a room full of people shouting.

Tomography was introduced to solve this problem by imaging an object from various angles. Using a series of images, they could be layered to create a level of detail that was previously unattainable.

History of Radiographic Imaging

Professor Wilhelm Conrad Roentgen made the discovery of xrays late in the afternoon on the 8th of November, 1895 when conducting experiments with vacuum tubes and fluorescent screens positioned nearby. The discovery was by chance observation of glowing fluorescent screens in a dark room when the vacuum tube had high voltage applied to it.

The vacuum tube he used was a Hittorf-Crookes tube that was covered with black cardboard. It was powered using an induction coil that created high voltage sparks from a low voltage DC power source. When power passes through the vacuum tube, the electrons collide with the anode and decelerate quickly. This sudden deceleration releases energy, one form of this energy are xrays. The fluorescent screen that glowed, does this because the xray energy reacts with the platinocyanide material painted on the screen.


When Professor Roentgen had his wife place her hand in front of the fluorescent screen, they could see the translucent skeletal image of her hand on the screen.

The technique was adopted by medicine as a way to photograph the inside of the body. Patients were place between a xray tube and a photographic plate that was light proof. Inside the light proof case contained a photographic sensitive material and a fluorescent screen. The glow of the fluorescent screen exposed the photographic material. This same technique is still used today.

Computerized Tomography

As I mentioned, traditional radiographic techniques fail to capture images of the living brain. This is due to the highly translucent material of the brain behind the highly opaque skill.

The technique of computerized tomography was introduced by Godfrey Hounsfield in the 1960’s. This method created several radiographic images from different angles through a subject. It was based on the projection reconstruction transform developed by mathematician Johann Radon in 1917. What Mr. Hounsfield was attempting to accomplish was determining the depth of a feature.

The method isn’t as simple as layering radiographic images from different sources. What Mr. Hounsfield’s method did was take a slice of the radiographic image, stretch it to the length of the original image, then use this final image to layer with successive angular images. The principle if fairly abstract and is represented well in this visual example made by North Star Imaging (

The unique method moved away from traditional photographic plates and used digital sensors that recorded electrical values. These values were then processed by a computer to generate the final images. These digital sensors can be nothing more than a fluorescent materials and a photo-sensor.

Since the medium was electronic, numeric values could be assigned to the electrical levels read. The values can be digitized and then processed by a computer. It took Mr. Housfield’s computer several days to compute the final results, he was correct in his assumptions. The experiments that followed involved tomographic images of a brain, which led to his lecture at the Neuroradiology Postgraduate Course on May 15th, 1972. One of the attendees was Dr. Forrest Clore. With the mainstream medical community taking notice, the CT scanning method soon became adopted.

GIMP and image layering

Using GIMP is a good way to see how the computer processing works. In this example, we will take a series of radiographic image that were imaged from various angles. In each image, we will copy a set horizontal scan line and create a new image. This new image will be our source for the tomographic method. In this next example, I’ll be use a radiographic image with a resolution of 640 x 640.

The process requires that the image be grayscale. Use the menu Color > Hue-Saturation and set the Saturation value to -100.

Next, make a selection of a scan line. We will be using a set horizontal scan line value of 240. Choose the Selection Tool and in the Tool Options windows set the selection values. Choose a Position of 0 x 240 for the left side of the image and 240 pixels from the top edge of the image. Next set the Size of 640 x 1 for the width of the image and the hight of 1 pixel. Now we can copy the selection and paste it as a new image.

With this new image we will want to resize the canvas to be the same hight as our original radiographic image, 640 pixels. From the menu bar select Image > Scale Image. Unlink the Image Size width and height proportions. Now set the Height value to 640 and choose scale. We should have our first angular tomographic image source.

Repeat these steps for each angular image. If we were to take an image at every 5th degree, we should end up with 35 tomographic image sources. We only need a 180 degree view since the subject is transparent.

Now we will need to rotate each image based on its angular position. From the Toolbox Tool Options window, choose the rotate tool. We have 35 images, so we only need to modify 34 of them. Our first image was shot from 0 degrees, no need to change this. Our next shot was done from 5 degrees. Set the value in the Tool Options to 5 degrees, then select the image. The Rotate window should appear, set the angle to the correct value and press rotate. The image should now appear tilted to its correct angular point of reference. Repeat this for the remaining images.

At this point, you should be gaining an appreciation for computers and the nature of how mundane manual work is.

Next, we’ll adjust the Brightness-Contrast so that the brightness is set at full.

Now, we will set a transparency value, mostly known as an alpha channel. From the menu choose Layer > Transparency > Color to Alpha. I typically select white as the alpha value, since my canvas is typically white. Again, repeat this for all the tomographic image sources.

Once that has been completed, now we should be able to layer our tomographic image sources together and view the final rendering.


As you can see, the manual process is a painstaking. GIMP supports the use of scripting to help automate this process. I will not cover that in the post, but do keep that in mind when considering GIMP as a tool for a job like this.

I hope you have enjoyed this simple introduction into the world of radiology. It is an exciting and rewarding field of work. The techniques are constantly being refined and as with anything, the best is yet to come.

Credit and References

Much thanks to following for making this post possible.
Dr. Scott Klioze, MD for his video Modern Medicine – History of Computerized Tomography (CT Scanner)

North Star Imaging (

Ben Krasnow’s DIY radiographic methods
and his mention of – 3D slicer.


If you’re intrested in purchasing digital xray sensors, be aware that the differences between the sale price and parts costs are significant.  Here are some selling on ebay.  In contrast, the basic materials to make your own sensor can also be purchased on ebay.

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