Welcome to Non-Ionising Imaging!
In this chapter, we are going to explore how doctors look inside the human body without using "ionising" radiation like X-rays or Gamma rays. This is really important because non-ionising methods are generally much safer, especially for sensitive patients like unborn babies. We will look at Ultrasound, Endoscopy, and MRI. Don't worry if some of these sound complicated—we'll break them down step-by-step!
1. Ultrasound Imaging
Ultrasound uses high-frequency sound waves (higher than humans can hear, usually above 20,000 Hz) to create images. Think of it like a bat using sonar to see in the dark!
Acoustic Impedance (\(Z\))
When an ultrasound wave hits a boundary between two different types of tissue (like between fat and muscle), some of the wave reflects back, and some of it passes through. How much reflects depends on a property called Acoustic Impedance.
The formula for acoustic impedance is:
\(Z = \rho c\)
Where:
• \(\rho\) is the density of the material (in \(kg \: m^{-3}\))
• \(c\) is the speed of sound in that material (in \(m \: s^{-1}\))
Reflection at Boundaries
The amount of sound reflected at a boundary depends on the difference in impedance between the two materials. We use the Intensity Reflection Coefficient (\(\frac{I_r}{I_i}\)):
\(\frac{I_r}{I_i} = \left( \frac{Z_2 - Z_1}{Z_2 + Z_1} \right)^2\)
Analogy: Imagine running from a field of grass into a swimming pool. The "impedance" of the water is much higher than the air, so you slow down suddenly. In physics, if the two materials have very different impedances, almost all the sound reflects back. If they are similar, most of the sound passes through.
Quick Review: Why use Coupling Gel?
Air and skin have very different impedances. Without gel, almost 100% of the ultrasound would reflect off the skin and never enter the body. The gel has an impedance similar to skin, acting as a "bridge" for the sound waves.
Piezoelectric Devices
How do we make and hear these sounds? We use a Piezoelectric Crystal (like Quartz).
1. Generation: When we apply an alternating voltage to the crystal, it vibrates at the same frequency, producing ultrasound waves.
2. Detection: When a reflected ultrasound wave (an echo) hits the crystal, it squeezes it. This squeezing creates a voltage that a computer can record.
A-scans and B-scans
• A-scan (Amplitude scan): This is the simplest type. It produces a graph showing the strength of reflections over time. It’s used to measure distances, like the diameter of an eye.
• B-scan (Brightness scan): This creates a 2D image. The "brightness" of dots on the screen represents the strength of the reflection. By moving the probe, the computer builds up a full picture, like an ultrasound of a baby.
Key Takeaway: Ultrasound is safe (non-ionising) and provides real-time images, but it has lower resolution than X-rays and cannot see through bone or air easily.
2. Fibre Optics and Endoscopy
An endoscope is a long, flexible tube that lets doctors see inside the body (like the stomach) without surgery. It works using the principle of Total Internal Reflection (TIR).
How it Works
Light travels down glass fibres. Because the glass core has a higher refractive index than the cladding around it, light reflects off the inside surface of the fibre repeatedly, staying trapped inside even if the fibre is bent.
Two Types of Fibre Bundles
Doctors use two different types of bundles in one endoscope:
1. Non-coherent Bundles: These fibres are all mixed up. They are cheap and easy to make, and their only job is to carry light down to illuminate the inside of the patient. Think of this as a flexible torch.
2. Coherent Bundles: These fibres are kept in the exact same relative positions at both ends. This is vital because it allows an image to be carried back. If the fibre at the "top-left" of the stomach end is also at the "top-left" of the eyepiece end, the doctor sees a clear picture.
Did you know? Endoscopes can also have "working channels" to slide in tiny tools for taking biopsies (tissue samples) or performing laser surgery!
Key Takeaway: Endoscopes use TIR to carry light and images. Coherent bundles are the "camera" (ordered), while non-coherent bundles are the "light" (unordered).
3. Magnetic Resonance (MR) Scanner
MRI is one of the most advanced imaging tools. It uses powerful magnetic fields and radio waves. It is great for looking at soft tissues like the brain.
The Process (Step-by-Step)
Don't worry if this seems tricky at first! Just remember these steps:
1. Alignment: The patient is placed in a very strong superconducting magnet. This causes the protons (hydrogen nuclei) in the body to align their spins parallel to the magnetic field.
2. Precession: The protons don't just sit still; they "wobble" around the magnetic field lines. This wobble is called precession.
3. Excitation: The scanner sends a short pulse of Radio Frequency (RF) waves. If the frequency matches the precession frequency, the protons absorb energy and flip their spin state.
4. De-excitation and Signal: When the RF pulse stops, the protons flip back to their original state. As they do this, they emit their own RF signal.
5. The Image: These signals are detected by coils and processed by a computer. Because different tissues (like fat vs. water) have different concentrations of hydrogen, they emit different signals, allowing the computer to build a 3D image.
What are Gradient Coils?
To know where the signal is coming from, the scanner uses gradient coils. These coils slightly change the strength of the magnetic field across the patient's body. Because the precession frequency depends on the magnetic field strength, each "slice" of the body responds to a slightly different radio frequency. This lets the computer map the signals to exact locations.
Common Mistake to Avoid: Students often think MRI uses ionising radiation because it’s a big machine. It doesn't! It uses radio waves, which have very low energy and are non-ionising.
Key Takeaway: MRI uses magnets to align protons and radio pulses to make them emit signals. It provides amazing detail for soft tissue without any radiation risk.
Summary Review
Ultrasound: Best for babies and moving organs. Uses sound echoes and piezoelectric crystals.
Endoscopy: Best for looking inside hollow organs (stomach/intestines). Uses Total Internal Reflection in fibre bundles.
MRI: Best for brain and soft tissue. Uses magnets, spinning protons, and radio waves.