X-ray imaging

Welcome to X-ray Imaging!

Hello! In this chapter, we are going to explore one of the most famous tools in medical history: the X-ray. Have you ever wondered how doctors can see a broken bone without doing surgery? It’s all about the physics of high-energy light and how different parts of your body cast "shadows." Whether you find Physics a bit daunting or you're a math whiz, these notes will help you master the AQA Physics 7408 requirements for Medical Physics.

1. How X-rays are Produced

X-rays are high-energy electromagnetic waves. To make them, we need to take electrons, give them a massive amount of energy, and then crash them into a metal target.

The Rotating-Anode X-ray Tube

In a hospital, X-rays are made in a special glass vacuum tube. Here is the step-by-step process:
1. A wire filament (the cathode) is heated until it releases electrons.
2. A huge voltage (up to 100,000 Volts!) pulls these electrons toward a metal target (the anode).
3. The electrons hit the anode and stop suddenly. This "sudden stop" converts their kinetic energy into X-ray photons.
4. Did you know? About 99% of that energy turns into heat! To stop the metal from melting, we use a rotating anode. By spinning the target, the heat is spread over a larger area.

The Energy Spectrum

When you look at the X-rays produced, they aren't all the same energy. The "spectrum" (graph of intensity vs. energy) has two parts:
• Continuous Spectrum: This is created by electrons slowing down as they pass near the nuclei of the target atoms. This is called Bremsstrahlung or "braking radiation."
• Characteristic Spectrum: These are sharp "peaks" on the graph. They happen when high-speed electrons knock an inner electron out of a target atom. When another electron drops down to fill that gap, it releases a very specific amount of energy as an X-ray.

Maximum Photon Energy

An X-ray photon cannot have more energy than the electron that created it. We use this equation:
\(hf_{max} = eV\)
Where \(h\) is the Planck constant, \(f\) is frequency, \(e\) is the charge of an electron, and \(V\) is the accelerating voltage.
Quick Tip: Increasing the voltage (\(V\)) makes the X-rays more "penetrating" (higher energy).

Key Takeaway: Use the Voltage to control the energy (quality) of the X-rays and the Current to control the number of X-rays (intensity).

2. Controlling the Image

Just like taking a photo with a camera, we need to adjust settings to get a clear X-ray image.

Sharpness and Contrast

• Sharpness: This is how clear the edges of the image are. To get a sharp image, the X-ray source (the "focal spot" on the anode) should be as small as possible. This is like using a tiny flashlight instead of a big lantern to cast a shadow.
• Contrast: This is the difference between the black and white areas. High contrast means bones look very white and soft tissue looks very dark.

Image Enhancement

Sometimes, organs like the stomach don't show up well on X-rays because they aren't dense enough. To fix this, we use:
• Contrast Media (Barium Meals): The patient drinks a liquid containing Barium. Barium is very dense and absorbs X-rays well, making the stomach or intestines show up clearly as "shadows."
• Intensifying Screens: This is a layer of scintillator material next to the film. It turns one X-ray into many visible light photons, which helps create an image using a lower dose of radiation for the patient.
• Fluoroscopic Image Intensification: This is like a "live video" X-ray. It uses a special screen to brighten the image so it can be viewed in real-time on a monitor.

Flat Panel (FTP) Detectors

Modern hospitals use FTP detectors instead of old-fashioned film.
1. X-rays hit a scintillator layer, which flashes with light.
2. This light hits photodiode pixels, which turn the light into electrical signals.
3. These signals are scanned electronically to make a digital image.
Why are they better? They are faster, the images can be stored on computers, and they usually require a lower dose of radiation than film.

Key Takeaway: Sharpness depends on the size of the source; Contrast depends on the density of the tissue (or Barium meals).

3. X-ray Absorption (The Math)

Don't worry if the math looks scary at first! It follows a simple rule: the thicker the material, the more X-rays are absorbed. This is called Exponential Attenuation.

The Attenuation Equation

The intensity \(I\) of the X-rays after passing through a thickness \(x\) is:
\(I = I_0 e^{-\mu x}\)
• \(I_0\) is the initial intensity.
• \(\mu\) is the linear attenuation coefficient (how well the material stops X-rays).
• \(x\) is the thickness.

Mass Attenuation Coefficient

Sometimes we care more about the density of the material than just its thickness. We use:
\(\mu_m = \frac{\mu}{\rho}\)
Where \(\rho\) is the density. This helps us compare how different tissues (like bone vs. muscle) absorb radiation.

Half-Value Thickness (HVT)

The Half-Value Thickness is the thickness of a material required to reduce the X-ray intensity to exactly half of its original value.
Analogy: If a piece of lead has an HVT of 2cm, then 2cm of lead stops 50% of the X-rays. Another 2cm stops 50% of what's left (leaving only 25% total).

Quick Review: Bone has a high \(\mu\) (absorbs more), which is why it looks white on an X-ray. Soft tissue has a lower \(\mu\) (absorbs less), so more X-rays reach the detector, making it look darker.

4. The CT Scanner

A standard X-ray is a 2D "shadow" of a 3D body. This can be a problem if organs are overlapping. A CT (Computerised Tomography) scanner solves this.

How it works

• The X-ray tube and an array of detectors rotate around the patient.
• It uses a narrow, monochromatic (single energy) X-ray beam.
• The tube takes many "slices" as it moves along the patient.
• A computer processes all these 1D signals to build a 3D image of the body.

Pros and Cons

• Advantages: You get a clear 3D image; you can see soft tissues much better than a normal X-ray; doctors can "rotate" the image on a screen to see every angle.
• Disadvantages: It is much more expensive; the radiation dose is significantly higher than a single X-ray; the patient must stay very still inside a tunnel.

Key Takeaway: CT = many 2D slices combined by a computer to make a 3D map.

Common Mistakes to Avoid

• Mixing up Intensity and Energy: Remember, Current = how many photons (brightness); Voltage = how fast/strong each photon is (penetration).
• Forgetting the Vacuum: X-ray tubes must be a vacuum so electrons don't hit air molecules on their way to the anode!
• Barium Meal Logic: Barium doesn't "glow" or emit radiation; it is opaque, meaning it stops X-rays from passing through, just like a brick wall stops light.

Final Summary Table

Component — Purpose
Rotating Anode — Dissipates heat so the target doesn't melt.
Barium Meal — Increases contrast for soft tissues like the gut.
FTP Detector — Digital, fast, and high-resolution detection.
CT Scanner — Produces 3D images by rotating the tube and detectors.
Filter (Lead/Alum) — Removes low-energy X-rays that would only harm the patient without helping the image.