Medical Imaging: Seeing Inside the Body with Radiation

Hey everyone! Ever broken a bone and had an X-ray? Or heard of a CT scan on a medical TV show? This chapter is all about the amazing physics that lets doctors see inside our bodies without surgery. We'll explore how we can use special types of radiation, called ionizing radiation, for diagnosis.

We'll look at three main techniques:

  • X-ray Imaging: The classic method for checking bones.
  • CT Scans: A powerful 3D version of an X-ray.
  • Radionuclides: Using tiny radioactive 'tracers' to see how organs are working.

Don't worry if this sounds complicated! We'll break it down step-by-step with simple explanations and real-world examples. Let's get started!


1. X-ray Radiographic Imaging: The Original 'Inside' Look

An X-ray image, or radiograph, is like a shadow picture. But instead of blocking visible light, our body parts block X-rays to different extents.

What is Attenuation?

When X-rays pass through matter, some of them are absorbed or scattered. This reduction in the intensity of the X-ray beam is called attenuation.

Analogy: Think of X-rays as light and your body parts as different pairs of sunglasses. Air is like clear glass (almost no attenuation). Soft tissue (muscle, fat) is like light sunglasses (some attenuation). Bone is like very dark sunglasses (high attenuation).

This difference in attenuation is the key to creating an image! Bones absorb a lot of X-rays, while soft tissues let more pass through.

The Attenuation Formula

We can calculate how much the X-ray intensity decreases using this formula:

$$ I = I_0 e^{-\mu x} $$

Let's break that down:

  • I is the final intensity of the X-ray beam after passing through the material.
  • I₀ is the initial intensity of the X-ray beam before it enters.
  • μ (the Greek letter 'mu') is the linear attenuation coefficient. This is just a number that tells us how good a material is at blocking X-rays. A bigger μ means more attenuation (e.g., bone has a high μ).
  • x is the thickness of the material the X-rays pass through.
  • e is a mathematical constant (approx. 2.718).
Quick Review: Key Concept

Half-Value Thickness (HVT or x₁/₂): This is a very important idea. The HVT is the thickness of a material required to reduce the intensity of the X-ray beam to half its original value (i.e., I = 0.5 I₀).

A material with a high attenuation coefficient (μ) will have a low HVT, because you don't need much of it to block half the X-rays. They are inversely related! The formula connecting them is $$ HVT = \frac{\ln(2)}{\mu} $$.

How an X-ray Image is Formed

An X-ray machine shoots a beam of X-rays through your body onto a detector (like photographic film or a digital sensor).

  • Areas where X-rays pass through easily (like lungs, full of air) hit the detector with high intensity, making that part of the image look dark.
  • Areas where X-rays are heavily attenuated (like bones) don't reach the detector, leaving those parts of the image looking bright or white.

So, a radiograph is really a map of the attenuation of the X-ray beam as it passes through the body.

Making Things Clearer: Contrast Media

It's hard to see soft tissues like the stomach or intestines on an X-ray because they have very similar attenuation coefficients to their surroundings. To fix this, doctors use contrast media.

Example: A barium meal. A patient drinks a liquid containing barium sulphate. Barium has a very high attenuation coefficient. It coats the inside of the stomach and intestines, making them show up brightly on an X-ray.

Advantages and Disadvantages of X-rays

Advantages:

  • Quick and painless.
  • Relatively inexpensive.
  • Excellent for imaging dense structures like bones.

Disadvantages:

  • It produces a 2D image, so overlapping structures can be confusing (this is called superimposition).
  • Poor at showing details in soft tissues without contrast media.
  • Uses ionizing radiation, which carries a small health risk.
Key Takeaways for X-rays
  • X-ray images are formed based on the different attenuation of X-rays by body tissues.
  • Bone has high attenuation (appears bright); soft tissue has low attenuation (appears dark).
  • The formula $$ I = I_0 e^{-\mu x} $$ describes attenuation.
  • Contrast media (like barium) are used to make soft tissues visible.

2. Computed Tomography (CT) Scans: 3D X-rays

A CT scan takes X-ray imaging to the next level. Instead of one flat image, it creates detailed 3D images of the body.

How a CT Scanner Works

Analogy: Imagine a loaf of sliced bread. A regular X-ray is like looking at the loaf from the side – you can't see the details inside. A CT scan is like taking out each slice and looking at it individually, then stacking them back together to see the whole loaf in 3D.

Here’s the step-by-step process:

  1. An X-ray source and a ring of detectors are housed in a large donut-shaped machine called a gantry.
  2. The gantry rotates around the patient, shooting thin beams of X-rays through the body from hundreds of different angles.
  3. The detectors measure the intensity of the X-rays that pass through.
  4. A powerful computer takes all this data and performs a complex process called image reconstruction. It calculates the attenuation coefficient for every tiny point within that "slice" of the body.
  5. The result is a detailed cross-sectional image. The patient's bed then moves slightly, and the process is repeated to create another slice. This builds up a full 3D view.
Comparing CT Images with X-ray Images

A CT image is much more detailed than a standard X-ray. Here's a quick comparison:

  • Image Type:
    X-ray: 2D image with superimposition.
    CT Scan: Cross-sectional "slices" that can be combined into a 3D image. No superimposition.
  • Detail:
    X-ray: Good for bone, poor for soft tissue.
    CT Scan: Excellent detail for both soft tissue and bone. It can distinguish between tissues with very similar densities.
  • What it shows:
    X-ray: A map of total attenuation through the body.
    CT Scan: A detailed map of the attenuation coefficients of the tissues within a slice.
  • Radiation Dose:
    X-ray: Lower dose.
    CT Scan: Significantly higher dose because it's essentially taking hundreds of X-rays.
Key Takeaways for CT Scans
  • A CT scanner uses a rotating X-ray source and detector to create cross-sectional images ("slices").
  • A computer reconstructs these slices into a detailed 3D image, avoiding the problem of superimposition.
  • CT images are maps of attenuation coefficients and provide much more detail of soft tissues than X-rays.
  • CT scans involve a higher radiation dose than standard X-rays.

3. Radionuclides in Medicine: Functional Imaging

While X-rays and CT scans are great at showing structure (what things look like), radionuclide imaging is used to show function (how things are working).

The basic idea is to introduce a radioactive substance (a radionuclide) into the body and track where it goes. This substance acts as a tracer.

Analogy: It’s like asking someone to drink a glow-in-the-dark liquid and then using special goggles to see which parts of their body light up. If the kidneys are working well, they should light up as they filter the liquid.

What Makes a Good Medical Tracer?

For a radionuclide to be safe and useful for diagnosis, it must have specific characteristics:

  1. Type of Radiation: It must emit only gamma (γ) rays. Alpha and beta particles are too ionizing and are stopped by the body, causing damage without being detected. Gamma rays can easily pass out of the body to be detected by a camera.
  2. Short Half-Life: The physical half-life should be short (usually a few hours). This ensures it provides a strong enough signal for imaging but decays away quickly so the patient isn't radioactive for long.
  3. Correct Energy: The gamma ray energy must be high enough to escape the body but low enough to be accurately detected by the equipment.
  4. Chemically Suitable: It must be possible to attach the radionuclide to a molecule that the body will send to the specific organ of interest (e.g., attach it to a compound that is absorbed by the thyroid gland).

The most common radionuclide used in medicine is Technetium-99m (Tc-99m). It’s almost perfect: it has a 6-hour half-life and emits gamma rays of a suitable energy.

Half-Life: Physical, Biological, and Effective

This is a common point of confusion, so let's clear it up!

  • Physical Half-Life (Tₚ): The time it takes for half of the radioactive nuclei in a sample to decay. This is the half-life we learned about in the core radioactivity topic.
  • Biological Half-Life (Tₑ): The time it takes for the body to remove half of a substance through natural biological processes (e.g., urination). This is not related to radioactivity.
  • Effective Half-Life (Tₑ): This combines both physical decay and biological removal. It represents the actual time that half of the original radionuclide is removed from the body. This is the most important value for calculating patient dose. $$ \frac{1}{T_e} = \frac{1}{T_p} + \frac{1}{T_b} $$

Common Mistake: Don't just add the half-lives! You must add their reciprocals. The effective half-life will ALWAYS be shorter than both the physical and biological half-lives.

How the Image is Made: The Gamma Camera

After the tracer is introduced into the patient, a special machine called a gamma camera is used. It doesn't emit any radiation; it just detects the gamma rays coming from the patient. The camera builds up an image, called a scintigram, which is a map of the radioisotope's distribution in the body.

Bright spots ('hot spots') on the image show areas of high tracer concentration, which might indicate high metabolic activity (like a tumour). Dark spots ('cold spots') might indicate areas of low function.

When comparing a radionuclide image with an X-ray, the most important difference is that the radionuclide image shows physiological function, whereas the X-ray shows anatomical structure.

Did you know?

The unit for measuring the biological risk of radiation is the sievert (Sv). It accounts for both the amount of energy absorbed and the type of radiation. A chest X-ray gives a dose of about 0.1 millisieverts (mSv), while a full-body CT scan can be over 10 mSv. We all receive about 2-3 mSv per year just from natural background radiation!

Health Risks and Safety Precautions

All ionizing radiation carries a health risk, primarily an increased risk of cancer due to potential DNA damage. For medical imaging, the principle is that the benefit of the diagnosis must outweigh the small associated risk.

To ensure safety, medical staff and patients follow key precautions:

  • Minimise Exposure Time: Spend as little time as possible near radiation sources.
  • Maximise Distance: The intensity of radiation decreases rapidly with distance.
  • Use Shielding: Use materials like lead (in lead aprons or shielded walls) to block radiation.
  • Handle with Care: Radioactive sources must be stored and handled according to strict safety protocols.
Key Takeaways for Radionuclides
  • Radionuclides are used as tracers to show organ function.
  • A good tracer (like Tc-99m) emits only gamma rays and has a short half-life.
  • Effective half-life combines physical decay and biological removal.
  • A gamma camera detects the radiation to create a map of the tracer's distribution.
  • Safety is managed by minimizing time, maximizing distance, and using shielding.