medical imaging

medical imaging

1. CT scanner

   • uses X-rays to produce images of one slice of the body at a time
   • very good for examining soft tissue.
     • advantage:
       • More readily available than MRI in the UK
         • More detailed information (particularly soft tissue)
     • disadvantage:
       • Significantly higher radiation doses
         • Very expensive
           • Requires cooperative or sedated patients
   • The X-rays from the beams are detected after they have passed through the body and their strength is measured. Beams that have passed through less dense tissue such as the lungs will be stronger, whereas beams that have passed through denser tissue such as bone will be weaker. A computer can use this information to work out the relative density of the tissues examined. Each set of measurements made by the scanner is, in effect, a cross-section through the body.

2. MRI scanner

   • Magnetic resonance imaging (MRI) uses nuclear magnetic resonance to make detailed images of slices through the body. The images produced are in many ways similar to those of the CAT scanner, but without the radiation hazard
   • Detecting brain and spinal cord tumours and other neurological diseases. Analysis of joints, such as the ankle, knee or shoulder. Shows presence of abnormal body water (e.g. swelling, infection, bleeding, cysts). Fluid flow (e.g. blocked blood vessels, heart studies). Looking at cancerous tissues
     • advantages:
       • Does not involve ionising radiation
         • No known side effects
           • Non-invasive
             • Better soft tissue contrast than CT
               • 3D data
     • disadvantages:
       • very high cost
         • Cannot scan patients with metallic implants
           • Unsuitable for claustrophobic or obese patients

3. ultrasound

   • High frequency sound waves Can be used in many different fields – including medicine.
     • Ultrasound scans use sound waves to produce images of the internal organs, vessels and tissues. The images are produced when the sound waves are directed into the body then reflected back to a scanner that measures them.
       • advantages:
         • Does not involve ionising radiation
           • No known sound effects
             • Good soft tissue resolution
               • Non-invasive
                 • Images over time/ equipment relatively cheap
       • disadvantages:
         • All ultrasound reflected at the air/tissue interface
           • Nothing can be seen beyond bone

4. x-rays

   • A conventional X-ray machine generates electrons when its cathode, a tungsten filament, is heated. The cathode is positioned in the head of a tube which is kept under vacuum conditions. The electrons emitted are attracted to the an anode by a large potential difference. The anode gives off X-ray photons. Most of the radiation is absorbed by the tube but a window allows some to escape.
   • The X-ray beam is focussed onto the body or structure under investigation. The X-rays pass through the body to impact with a detector. The detector is photographic film sandwiched between two fluorescent screens. X-rays cause light to be emitted of the screens. The light causes blackening of emulsion to occur when the film is developed. As a result an X-ray image is produced on the film.
     • advantages:
       • Relatively cheap and easy
       • Good bone resolution
       • Can be interpreted by non-radiologists
     • disadvantages:
       • Poor soft tissue resolution
       • Contrast media can be unpleasant and hazardous
       • Ionising radiation harmful/ increased cancer risk/ dose accumulative
       • High-voltage supply hazardous