1. This unit explores scanning techniques such as MRI and CT
The Professional Development Service for Teachers is funded by the Department of Education and Science under the National Development Plan

2. This unit explores scanning techniques such as MRI and CT

3. Objectives By the end of this unit you should be able to
Understand how an MRI scanner works
Understand how a CT scanner works
Understand when doctors would decide to prescribe these scans for their patients
Have the basic knowledge needed for Expert Group Tasks

4. What is an MRI scanner and how does it work?
MRI stands for Magnetic Resonance Imaging.  This type of scan developed in the early 80’s uses magnetism and radio waves to build up a picture of the inside of the body instead of X-rays. 
This picture shows an MRI
scan of the head.

5. How does an MRI scanner work?
The patient lies inside a large, cylinder-shaped magnet. Radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are then sent through the body. This affects the body's atoms, forcing the nuclei into a different position. As they move back into place they send out radio waves of their own. The scanner picks up these signals and a computer turns them into a picture. These pictures are based on the location and strength of the incoming signals.

6. How are the scans created?
Our body consists mainly of water, and water contains hydrogen atoms. For this reason, the nucleus of the hydrogen atom is often used to create an MRI scan in the manner described on the previous slide.

7. What does an MRI scan show?
Using an MRI scanner, it is possible to make pictures of almost all the tissue in the body. The tissue that has the least hydrogen atoms (such as bones) turns out dark, while the tissue that has many hydrogen atoms (such as fatty tissue) looks much brighter. By changing the timing of the radio wave pulses it is possible to gain information about the different types of tissues that are present.

8. When would an MRI scan be useful?
An MRI scan is able to provide clear pictures of parts of the body that are surrounded by bone tissue, so the technique is useful when examining the brain and spinal cord.
Because the MRI scan gives very detailed pictures it is the best technique when it comes to finding tumours (benign or malignant abnormal growths) in the brain. If a tumour is present the scan can also be used to find out if it has spread into nearby brain tissue.
The technique also allows us to focus on other details in the brain. For example, it makes it possible to see the strands of abnormal tissue that occur if someone has multiple sclerosis and it is possible to see changes occurring when there is bleeding in the brain, or find out if the brain tissue has suffered lack of oxygen after a stroke.

9. Is an MRI scan dangerous?
There are no known dangers or side effects connected to an MRI scan. The test is not painful; you cannot feel it. Since radiation is not used, the procedure can be repeated without problems. There is a small theoretical risk to the foetus in the first 12 weeks of pregnancy, and therefore scans are not performed on pregnant women during this time.

10. What is a CT or CAT scan and how does it work?
CT scan or CAT scan as it is sometimes called stands for Computerised (Axial) Tomography scan. This just means a scan that takes pictures from all around your body and uses a computer to put them together.
Below is a CT scan of the pelvis marked for radiotherapy treatment.

11. How a CT scanner works
A CT scanner uses X-rays. Like an X-ray, it is painless.  The CT machine takes a lot of pictures of your body from different angles.  These pictures are fed into a computer. 
The computer puts them together to give a series of cross sections or 'slices' through the part of the body being scanned.  A very detailed picture of the inside of the body can be built up in this way.

12. A CT Scanning Machine
A CT scanning machine is a large machine that is shaped rather like a doughnut.  There is a couch that you lie on.  The couch can slide backwards and forwards through the hole of the doughnut.  The pictures are taken as you move through the machine.

13. How does an MRI scan differ from a CT scan?
With an MRI scan it is possible to take pictures from almost every angle, whereas a CT scan only shows pictures horizontally.
There is no ionizing radiation (X-rays) involved in producing an MRI scan.
MRI scans are generally more detailed, too. The difference between normal and abnormal tissue is often clearer on the MRI scan than on the CT scan.

14. Here Are Some Examples Of What MRI scans can show
The Arrow points to an ACOUSTIC NEUROMA of the inner ear.
An acoustic is a benign tumour that usually results in total hearing
loss in the affected ear as the tumour invades the nerves of the inner ear.
The image is performed with a contrast injection,. Contrast or dye, if needed, is given intravenously and usually has very few complications.
This example is a large tumour and will need surgery to reduce any more symptoms for the patient. Hearing in the left ear will be sacrificed as a result unfortunately.
If surgery is not performed, the tumour may grow and invade other vital structures in the base of skull.

15. Knee Injury
MRI is a great way to diagnose Ligament ruptures cartilage damage, bone bruising, joint fluid and meniscal tears.
Prior to MRI the patient would have had to undergo surgery to diagnose the problem.
This patient has ruptured his anterior cruciate ligament and will need total knee reconstruction.

16. Blood Flow in the Brain
Flowing blood is well visualised with MRI. In this case the arteries in the brain are seen very well. Blockages and aneurysms can be clearly seen. The fact that the blood is moving in the arteries whilst the brain tissue is still gives a different signal.
Using a special MRI technique the arteries can be made bright whilst the stationary brain tissue is dark on the scan.
Prior to this procedure the patient would have undergone a distressing angiogram.