Physics | Medical imaging techniques

Your Friendly Guide to Medical Imaging!

Hey everyone! Ever wondered how doctors can see inside your body without a single cut? It's not magic, it's Physics! This chapter, "Medical Imaging Techniques," is all about the amazing physics principles that let us peek inside the human body to diagnose illnesses and check on our health. We'll explore everything from using sound waves to see a baby before it's born, to using X-rays to check for broken bones. It might sound complicated, but we'll break it down step-by-step. Let's get started on our journey into the invisible world within us!

Part 1: Imaging with Non-Ionizing Radiation

First up, we're looking at techniques that are super safe because they don't use high-energy radiation that can harm cells. The key players here are ultrasound and endoscopy.

Ultrasound Scans: Seeing with Sound

You've probably seen this in movies, used for checking on pregnant women. But how does it work? It's all about echoes!

The Physics Behind Ultrasound

• What is Ultrasound? It's just sound with a very high frequency (typically > 2 MHz for medical use), so high that humans can't hear it.
• How it's Made (and Heard): It uses a special device called a piezoelectric transducer.
  Analogy: Think of it like a special crystal. When you apply a voltage, it vibrates and sends out an ultrasound pulse. When an echo (a returning ultrasound pulse) hits it, the crystal gets squeezed and produces a voltage. It's a two-way street!
• Acoustic Impedance (Z): This is a crucial concept! It basically measures how much a material resists sound waves passing through it. Every tissue in your body (skin, fat, muscle, bone) has a different acoustic impedance.
  The formula is: $$Z = \rho c$$ Where:
  Z = Acoustic Impedance
  ρ (rho) = density of the tissue
  c = speed of sound in the tissue
• Reflection and Transmission: When ultrasound travels from one tissue to another (e.g., from fat to muscle), some of it gets reflected back, and some passes through. The amount reflected depends on the difference in the acoustic impedance (Z) of the two tissues.
  A big difference in Z means a strong reflection (a strong echo).
  A small difference in Z means a weak reflection.

Quick Review Box: Acoustic Impedance

High Z difference (e.g., tissue to bone, or tissue to air) = Strong Echo. This is why ultrasound can't see through bone or lungs very well.
Low Z difference (e.g., liver to kidney) = Weak Echo. This allows us to see the boundaries between different soft tissues.

Calculating the Echo Strength

We can calculate how much of the ultrasound intensity is reflected using the intensity reflection coefficient (α).

$$ \alpha = \frac{I_r}{I_o} = \left( \frac{Z_2 - Z_1}{Z_2 + Z_1} \right)^2 $$

Where:
I_r is the reflected intensity
I_o is the original intensity
Z₁ and Z₂ are the acoustic impedances of the two tissues.

Don't worry if this formula seems tricky! The key idea is that the bigger the difference between Z₁ and Z₂, the closer the value of α gets to 1, meaning more reflection.

Attenuation: Why the Signal Fades

As ultrasound travels deeper into the body, it gets weaker. This is called attenuation. It's caused by the energy being absorbed or scattered by tissues.
Important: Attenuation is worse for higher frequency ultrasound. This leads to a trade-off:

• High Frequency Ultrasound: Gives a clearer, more detailed image (better resolution) but can't penetrate very deep. (Used for things near the surface, like eyes).
• Low Frequency Ultrasound: Can penetrate much deeper into the body but gives a less detailed image (lower resolution). (Used for organs deep inside, like the liver or a developing fetus).

Types of Ultrasound Scans

1. A-scan (Amplitude Scan)

• How it works: Sends a single ultrasound pulse and measures the time it takes for echoes to return. The strength of the echo is shown as the height (amplitude) of a peak on a graph.
• What it's for: It's basically a range-finder. It measures the depth of different boundaries. For example, it's used to measure the length of the eyeball before cataract surgery.
• Interpreting an A-scan display: You'll see a series of spikes. The position of the spike on the horizontal axis tells you the depth of the tissue boundary, and the height of the spike tells you how strongly the ultrasound was reflected.

2. B-scan (Brightness Scan)

• How it works: A B-scan is essentially a string of many A-scans put together. The transducer is swept across the skin. Instead of showing spikes, the strength of the echo is shown as the brightness of a dot. Strong echoes are bright dots, weak echoes are dim dots.
• What it's for: This creates a 2D image of a "slice" of the body. This is the one you see in movies for pregnancy scans!
• Estimating size: Doctors can use a scale on the B-scan image to measure the size of organs or a fetus.

Advantages and Limitations of Ultrasound

Advantages:

• Non-ionizing: Very safe, no harmful radiation.
• Real-time: You can see movement as it happens (like a baby kicking!).
• Portable and relatively cheap.

Limitations:

• Cannot pass through bone or air: The acoustic impedance mismatch is too high, so it's not good for imaging the brain (blocked by skull) or lungs (full of air).
• Resolution can be limited: The image quality is not as high as some other methods.

Key Takeaways for Ultrasound

- Uses high-frequency sound echoes.
- Piezoelectric transducer sends and receives pulses.
- Image is formed by reflections at boundaries between tissues with different acoustic impedances (Z).
- A-scan measures depth; B-scan creates a 2D image.
- It's safe and real-time, but can't see through bone or air.

Fibre Optic Endoscopy: The Light Tunnel

An endoscope is like a tiny, flexible camera that doctors can guide into the body through natural openings (like the mouth) to look at internal organs like the stomach or intestines.

The Physics Behind Endoscopy

• Optical Fibres: The key technology is the optical fibre, a very thin, flexible strand of glass or plastic.
• Total Internal Reflection (TIR): This is the magic principle! Light is shone down a bundle of optical fibres. As the light travels down the fibre, it keeps hitting the inside wall at a large angle (greater than the critical angle) and is completely reflected back inside. It's like a perfect mirror tunnel, so the light can travel around corners without escaping.
• The Endoscope Structure: An endoscope has two main bundles of optical fibres:
  1. One non-coherent bundle to carry light *into* the body to illuminate the organ. (The fibres in this bundle are arranged randomly).
  2. One coherent bundle to carry the image (the reflected light) *out* of the body to a camera/eyepiece. In a coherent bundle, the fibres are arranged in exactly the same position at both ends, so the image isn't scrambled.
• Extra Channels: Endoscopes also have extra channels to pump in air or water, or to insert tiny tools for taking a sample (biopsy) or performing minor surgery.

Advantages and Limitations of Endoscopy

Advantages:

• Direct viewing: Doctors can see the actual surface of the organ in full colour.
• Minimally invasive: No major surgery required.
• Can take samples (biopsy): Very useful for diagnosing conditions like cancer.

Limitations:

• Invasive: Although it's not major surgery, it can be uncomfortable for the patient.
• Limited reach: Can only be used to examine areas accessible from the outside (e.g., digestive tract).

Key Takeaways for Endoscopy

- Uses a flexible tube with optical fibres to see inside the body.
- Works on the principle of Total Internal Reflection (TIR).
- A coherent bundle of fibres preserves the image on its way out.
- Allows for direct, colourful viewing and taking tissue samples.

Part 2: Imaging with Ionizing Radiation

Now we move to techniques that use high-energy radiation, like X-rays and gamma rays. This radiation is called ionizing because it has enough energy to knock electrons out of atoms, which can damage living cells. This is why safety is super important with these methods!

X-ray Radiographic Imaging: The Classic Skeleton Picture

This is the oldest and most common type of medical imaging. Perfect for spotting broken bones!

The Physics Behind X-rays

• How X-rays are made: Fast-moving electrons are slammed into a heavy metal target. The rapid deceleration of the electrons produces X-rays. (This is from the Radioactivity chapter syllabus).
• Differential Attenuation: This is the key to forming an image. As a beam of X-rays passes through the body, different tissues absorb (attenuate) it by different amounts.
  • Dense materials like bone absorb a lot of X-rays.
  • Less dense materials like soft tissue and muscle let most X-rays pass through.
• The Image: The X-rays that pass through the body hit a detector (like photographic film or a digital sensor).
  • Areas where many X-rays passed through (soft tissue) appear dark/black on the image.
  • Areas where few X-rays passed through (bone) appear white.
  Therefore, an X-ray image is essentially a shadow map of X-ray attenuation.

Calculating Attenuation

The intensity of the X-ray beam decreases exponentially as it passes through a material. The formula is:

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

Where:
I = final intensity after passing through the material
I_o = initial intensity
μ (mu) = linear attenuation coefficient (a value that depends on the material and the X-ray energy)
x = thickness of the material

Half-Value Thickness (HVT or x_1/2): This is the thickness of a material required to reduce the X-ray intensity to half its original value. It's related to μ by:

$$ x_{1/2} = \frac{\ln(2)}{\mu} \approx \frac{0.693}{\mu} $$

Improving the Image: Contrast Media

Sometimes, doctors need to see soft tissues that have very similar attenuation coefficients (e.g., the stomach and intestines). To make them visible, they use a contrast medium. This is a substance that strongly absorbs X-rays.
Example: A patient might drink a barium meal. Barium is very dense and will fill the stomach and intestines, making them show up clearly white on an X-ray.

Advantages and Disadvantages of X-rays

Advantages:

• Excellent for imaging dense structures like bone.
• Quick, painless, and widely available.

Disadvantages:

• Uses ionizing radiation: There is a small health risk, especially with repeated exposure.
• Poor at imaging soft tissues: Different soft tissues look very similar on a standard X-ray.
• 2D image: A 3D structure is flattened into a 2D image, so overlapping organs can be confusing.

Computed Tomography (CT) Scan: The 3D X-ray

Imagine taking X-ray pictures from many different angles all around the body and then using a computer to put them together to form a detailed 3D image. That's a CT scan!

How a CT Scanner Works

1. An X-ray source and a ring of detectors are mounted on a rotating gantry.
2. The gantry spins around the patient, taking hundreds of X-ray "slice" images from all angles.
3. A powerful computer then takes all this data and performs image reconstruction. It calculates the attenuation coefficient for thousands of tiny cubes (called voxels) within the body.
4. The final CT image is a cross-sectional map of these attenuation coefficients, displayed as different shades of grey.

Comparing CT Images with X-ray Radiographs

• X-ray: A 2D shadowgram. Overlapping structures are a problem. Poor soft tissue detail.
• CT Scan: A detailed 2D slice or a full 3D model. No overlapping structures. Excellent for distinguishing between different types of soft tissue because it's much more sensitive to small differences in attenuation.

Radionuclides in Medicine: Imaging from the Inside Out

Also known as nuclear medicine, this technique is unique. Instead of sending radiation *through* the body, we put a tiny, safe amount of radioactive material *inside* the body and detect the radiation it emits!

The Physics of Tracers

• Radioisotopes as Tracers: A radioactive isotope (radionuclide) is attached to a biological molecule that the body naturally uses (like glucose). This combination is called a radiotracer. It is injected into the patient.
• Targeting Organs: The tracer travels through the body and accumulates in the specific organ or tissue we want to study. For example, a tracer using iodine will accumulate in the thyroid gland.
• Gamma Emission: The radionuclide decays and emits gamma rays, which can easily pass out of the body.
• Gamma Camera: A special detector called a gamma camera is used to detect these gamma rays and build an image showing where the tracer has accumulated.
• The Image: The final image is a map of the radioisotope's distribution. "Hot spots" (bright areas) show high accumulation, while "cold spots" (dark areas) show low accumulation. This tells us about the *function* of an organ, not just its structure. For example, a tumour might be metabolically very active and show up as a hot spot.

Characteristics of a Good Medical Radionuclide

The syllabus highlights Technetium-99m (Tc-99m) as a perfect example. Here's why:

• Short physical half-life (6 hours): Long enough to do the scan, but decays quickly so the patient isn't radioactive for long.
• Emits only gamma rays: No alpha or beta particles, which would be absorbed by the body and cause unnecessary cell damage.
• Ideal gamma energy (~140 keV): Strong enough to exit the body, but weak enough to be detected efficiently and safely.

Biological Half-Life

The body is constantly removing substances through biological processes (like urination). The biological half-life is the time taken for the body to remove half of a substance. The overall clearance of a radiotracer depends on both its physical decay and its biological removal.

Comparing Radionuclide Images with X-rays

• X-ray: Shows anatomical structure. Based on attenuation from an external source.
• Radionuclide Image: Shows physiological function. Based on radiation emitted from an internal source. The image resolution is generally much lower than an X-ray.

Key Takeaways for Ionizing Radiation Imaging

- X-ray Radiography: A 2D shadow map based on differential attenuation. Great for bones.
- CT Scan: A 3D image constructed from many X-rays. Excellent soft tissue detail.
- Radionuclide Imaging: Uses an internal radiotracer (like Tc-99m) and a gamma camera to create a map of organ function.

Part 3: Safety and Comparisons

Health Risks and Safety Precautions

Ionizing radiation is dangerous, so we must always minimise the dose! The risk-benefit balance must be considered for every patient.

• Health Risks: High doses of radiation can cause cell death (radiation sickness). Lower doses increase the long-term risk of cancer due to DNA mutations.
• Effective Dose: This is a measure used to compare the risk of different procedures. It's measured in sieverts (Sv). For example, a CT scan of the chest delivers a much higher effective dose than a simple chest X-ray.
• Safety Precautions (ALARA principle - As Low As Reasonably Achievable):
  • Justification: Only perform a scan if the medical benefit outweighs the risk.
  • Minimise Exposure Time: Use the shortest possible scan time.
  • Shielding: Doctors and radiographers use lead aprons and shields and stand behind protective screens.
  • Use of tracers with short half-lives: For radionuclide imaging, this ensures the patient's exposure is limited.

Did You Know?

The effective dose from one chest X-ray is roughly the same as the amount of natural background radiation you receive in about 10 days. A CT scan of the chest is more like 2 years' worth of background radiation!

Final Summary: Which Tool for Which Job?

No single imaging technique is the "best" – they are all tools with specific uses.

• To check for a broken bone? -> X-ray (quick, cheap, perfect for bone).
• To check on a developing fetus? -> Ultrasound (safe, non-ionizing, real-time).
• To look for a stomach ulcer? -> Endoscopy (direct view, can take a biopsy).
• To investigate a serious head injury? -> CT Scan (fast, detailed 3D view of bone and soft tissue like brain bleeds).
• To check if your thyroid gland is working properly? -> Radionuclide Scan (shows function, not just structure).