1. What is a CAT/CT Scan?

Simply put, a CAT Scan is a machine that is used to create a 3D image from a series of many 2D images. This machine is used to look inside things that are not easily accessible. Most commonly, a CAT scan is used to examine the internal structures of patients such as their head, spine, and lungs. These scans then allow doctors to identify tumors, masses, bleeds, and other medically related conditions.

The term CAT/CT scan is an acronym for: X-ray computed tomography

2. How does it work?

A CAT scanner is a large doughnut-shaped machine. Inside the machine, there is a large rotating disk on which an x-ray emitting device is mounted. A curved sensor that picks up the x-rays is located opposite the emitter. The patient lies between the x-ray emitter and the sensors. The entire disk rotates around the patient to capture a slice, which is a full 360^o image that is approximately 1-10mm thick. These slices are made of around 1000 "profiles", which are basically individual pictures taken at various angles. These profiles are all then sent to a computer that creates a user-friendly image that doctors are then able to look at.

Here is the outside and inside of a CT Scan.

This image shows the process of capturing, sending and then displaying an image from the scanner.

The x-rays used in the CT Scanner are actually a form of electromagnetic radiation that has both electric and magnetic fields. The wavelength of x-rays can vary between 0.01 to 10 nanometers (x10^-9 m). Due to the nature of the wavelengths, CT Scans are considered slightly harmful, especially for children. The patients are exposed to radiation through the procedure, but in most cases the benefit of getting a CT scan done outweighs the effects.

In a basic x-ray, x-rays pass through the body and project a shadow of the contents of the body onto the detectors. The detectors then record the projection in different shades of grey. An x-ray image of a broken arm would show white bones, dark gray tissues and black surroundings. This is because bone stops a large amount of x-rays, while soft tissue only blocks some. The air surrounding a patient cannot easily stop the x-rays, and therefore appears black.

3. The Physics Behind CT Scans

The x-rays in a CT Scan can be made in one of two ways. Both ways begin the same way where an electron is created by a cathode and accelerated through a large potential difference into a metal anode. Most of the energy is wasted as heat as they electron travels, but a fraction of it will be turned into x-rays. The two ways that x-rays can be made are:

1. The first way that x-rays can be generated is when an electron slows down near the atom. As the electron passes by the nucleus of the metal atom (within the anode) it reduces its momentum and a x-ray is formed. This is called “bremsstrahlung”, from the German word that means "braking radiation".

This type of x-ray can have many varying wavelengths within the x-ray spectrum.

2. Another way that x-rays can be created is when the accelerated electron knocks out an electron from the atom's inner shell. When this electron is knocked out, an outer shell electron falls down to take its place. This exchange then creates a x-ray and it is called a “K-shell” emission.

This method of x-ray creation can create x-rays at one wavelength only, due to the fact that the electrons can only jump from preset shells within the atom.

Bremsstrahlung X-Ray

K-shell Emission

One of the most fundamental formulas used within a CT Scan is:

I _out= I _in* e ^−aΔd

X-rays, made of photons, diminish in intensity exponentially when they pass through a material with a thickness of Δd according to this relationship.

The values for this formula are:

I = Intensity

e = Euler’s constant

a = Attenuation constant (See this link for more information)

Δd = Distance traveled / thickness of a material

4. Interesting Facts / Extra

CT Scans are also very useful not just for current patients, but also historians. CT Scans have been preformed on mummies to determine the cause of death, or to find out any interesting information. The oldest case of arterial disease had been discovered, a mummified princess who died 3500 years ago, with the help of a CT Scanner.

An article about the mummified princess can be found here.

A short video that shows a person's chest through a CT scanner can be found here. In the video, you can clearly see the ribs, spine, and other thicker tissue within the human body. Its images like this that allow doctors to be able to effectively diagnose patients.

When comparing our modern-day CT scanners with some of the first prototypes, we can summarize the results within the following table: (taken from this website)

Specifications	First CT Scanner (circa 1970)	State of the Art CT Scanner (2001)
Time to acquire one CT image	5 minutes	0.5 seconds
Pixel size	3 mm x 3 mm	0.5 mm x 0.5 mm
Number of pixels in an image	6,400	256,000

Using this data, we can see that we have come a long way medically within the past half a century. The CT scanner has been a great contribution to our hospitals, and it allows doctors to more effectively diagnose patients without having to physically look inside.

Here are some videos that explain the basics of a CT Scan:

http://www.youtube.com/watch?v=rN4E8Y5loAs

http://www.youtube.com/watch?v=wcAiLE17sQw

http://www.youtube.com/watch?v=ZdeB8XeAnaM

http://www.youtube.com/watch?v=U4d3nta_jTU (a musical!)

5. Practice Question

The practice question can be downloaded here:
CT Scanner Practice Question.docx

6. Resources & Bibliography

http://en.wikipedia.org/wiki/X-ray_computed_tomography

http://www.medicinenet.com/cat_scan/article.htm

http://health.howstuffworks.com/medicine/tests-treatment/cat-scan.htm

http://www.imaginis.com/ct-scan/how-does-ct-work

http://en.wikipedia.org/wiki/X-ray

http://en.wikipedia.org/wiki/Electromagnetic_radiation

http://www.chemistryexplained.com/Co-Di/CT-Scans.html

http://web2.uwindsor.ca/courses/physics/high_schools/2006/Medical_Imaging/ctphysics.html

http://www.impactscan.org/slides/eanm2002/index.htm

http://www.physicscentral.com/explore/action/scans-1.cfm