Seeing hidden structures

1. Doctors often need to examine structures inside the body that are hidden from direct view, such as bones, organs or an unborn baby.
2. Different types of wave behave differently as they travel through solids and liquids such as bone, muscle, fat and blood.
3. Some waves are absorbed strongly by one tissue but only weakly by another.
4. Some waves are partly reflected at the boundary between two different tissues.
5. Waves also travel at different velocities in different tissues, which affects how long they take to cross them.
6. Measuring these differences from outside the body lets a machine build up an image of what is inside.
7. The main methods use ultrasound, X-rays, gamma rays and infrared.

Ultrasound scans

1. A probe placed on the skin sends short pulses of ultrasound into the body, with a layer of gel to stop air gaps reflecting the waves at the skin.
2. At each boundary between two tissues, part of the pulse is reflected back as an echo and the rest carries on into the next tissue.
3. The more different the two tissues are, the more of the pulse is reflected, so each boundary returns an echo of a different strength.
4. The probe detects each echo and measures the time between sending the pulse and receiving the echo.
5. Ultrasound travels at about $1500m/s$ in soft tissue, so a computer uses $distance=speed\times time$ to work out how deep each boundary is.
6. The time measured is for the journey there and back, so the depth of the boundary is half of the total distance travelled.
7. Some ultrasound is absorbed as it travels, so echoes from deep structures are weaker and the machine has to amplify them.
8. Ultrasound is not ionising, so it is used to scan unborn babies and soft organs such as the heart, liver and kidneys.
9. Almost all of a pulse is reflected at a boundary with bone or air, so ultrasound cannot show structures behind bone or inside the lungs.

X-ray images

1. X-rays are absorbed much more strongly by bone than by soft tissue such as muscle and fat.
2. To take an X-ray image, X-rays are passed through the patient onto a detector on the other side.
3. Where the X-rays pass through soft tissue, many reach the detector and the image is dark.
4. Where bone absorbs the X-rays, few reach the detector, so the bones appear white and form a shadow picture.
5. A CT scanner takes many X-ray images from different angles, and a computer combines them into detailed images of slices through the body.
6. X-rays are ionising, so the dose is kept as low as possible and they are avoided during pregnancy unless they are essential.

Gamma rays and infrared

1. For a gamma-ray scan, a substance that emits gamma rays is injected into or swallowed by the patient, and it collects in the organ being studied.
2. Gamma rays are weakly absorbed by body tissue, so they pass out of the body and are detected by a gamma camera.
3. The image shows where the substance has collected, which reveals how well an organ such as a kidney is working, not just its shape.
4. Gamma rays are ionising, so the patient receives a dose, and a substance is chosen that stops emitting after a short time.
5. Infrared cameras detect the infrared emitted by the skin, so they show temperature differences across the body.
6. A warmer patch can reveal inflammation or a problem with blood flow just below the surface.
7. Infrared cannot show structures deep inside the body, because it is absorbed by the skin, but infrared imaging is not ionising.