X - rays & Ultrasound

(a) The Nature and Properties of X-rays

X-rays are a form of electromagnetic radiation with wavelengths ranging from 0.01 to 10 nm.

They have high energy and can penetrate most materials, making them useful for medical imaging.

X-rays are produced when high-energy electrons decelerate upon collision with a metal target.

They exhibit properties such as ionization, fluorescence, and diffraction.

X-rays travel in straight lines and are absorbed to varying degrees by different tissues, leading to image contrast.

(b) The Production of X-ray Spectra

X-ray spectra consist of two components:

1. Continuous spectrum (Bremsstrahlung radiation): Produced when high-speed electrons are decelerated by the target nucleus.

2. Characteristic spectrum: Produced when electrons from the target atom are ejected, and other electrons drop to fill vacancies, emitting X-rays with specific energies.

Beam intensity and photon energy are controlled by:

• Tube current (mA): Controls the number of X-rays produced.

• Tube voltage (kV): Affects the maximum photon energy and penetration ability.

• Filtration: Removes low-energy X-rays to improve image quality.

• Collimation: Limits the X-ray beam to the area of interest, reducing exposure.

(c) High-Energy and Low-Energy X-rays in Medicine

High-energy X-rays (Therapy):

• Used in radiotherapy to destroy cancerous tissues by damaging DNA.

• High-energy X-rays penetrate deep into tissues, targeting tumors while sparing surrounding healthy tissue.

Low-energy X-rays (Diagnosis):

• Used in radiography for imaging bones and soft tissues.

• Lower energy allows for better contrast in diagnostic images.

(d) Attenuation of X-rays

Described by the equation:

I = I₀ exp( μ−x )

Where:

• I = Intensity after passing through material.

• I₀ = Initial intensity.

• μ = Linear attenuation coefficient.

• x = Thickness of material.

Higher attenuation occurs in denser materials like bone, leading to image contrast.

(e) X-rays in Soft Tissue Imaging and Fluoroscopy

Soft tissues absorb X-rays weakly, requiring contrast agents (e.g., barium, iodine) for better visualization.

Fluoroscopy provides real-time X-ray imaging using image intensifiers to enhance visibility.

Used in procedures like angiography and gastrointestinal studies.

(f) Radiography Techniques with Digital Image Receptors

Modern X-ray systems use digital detectors instead of film.

Digital receptors include:

• Flat-panel detectors (direct or indirect conversion).

• Computed radiography (CR) plates.

• Digital radiography (DR) systems.

Digital imaging improves efficiency, reduces radiation dose, and allows image enhancement.

(g) Rotating Beam X-ray CT Scanner

• Uses a rotating X-ray beam and multiple detectors to create cross-sectional images.

• Provides 3D reconstructions of internal structures.

• High resolution, useful for diagnosing complex conditions.

(h) Ultrasound Generation and Detection

Uses piezoelectric transducers made of materials like quartz or PZT (lead zirconate titanate).

Alternating voltage causes the crystal to vibrate, emitting ultrasound waves.

Echoes are detected and converted back into electrical signals for imaging.

(i) Ultrasound Scanning: A-scans and B-scans

A-scan (Amplitude scan):

• One-dimensional representation of echoes.

• Used in ophthalmology to measure eye structure dimensions.

B-scan (Brightness scan):

• Two-dimensional grayscale images.

• Used for abdominal, cardiac, and fetal imaging.

(j) Acoustic Impedance and Coupling Medium

Acoustic impedance:

Z = cρ

Where:

• Z = Acoustic impedance.

• c = Speed of sound in tissue.

• ρ = Tissue density.

A mismatch at tissue boundaries causes reflection.

A coupling medium (e.g., gel) minimizes reflection losses.

(k) Doppler Ultrasound and Blood Flow Measurement

Doppler shift equation:

Δf / f₀ = 2v / c

Where:

• Δf = Frequency shift.

• f₀ = Transmitted frequency.

• v = Blood velocity.

• c = Speed of ultrasound.

Used in vascular studies and fetal monitoring.