Diagnostic Radiology Overview

Diagnostic Radiology Branches

• Radiology has three major branches: diagnostic radiology, radiation therapy, and nuclear medicine.

X-Ray Beam Production

• High-speed electrons convert some or all of their energy into an x-ray photon when striking an atom.

Modern X-Ray Unit Components

• A source of electrons (filament or cathode)
• An evacuated space for accelerating electrons
• A high positive potential to accelerate negative electrons
• A target (anode) for electron impact to produce x-rays

X-Ray Tube Operation

• The number of electrons accelerated towards the anode depends on the filament temperature.
• The maximum energy of x-ray photons is determined by the accelerating voltage (kilovolt peak, kVp).
• An 80 kVp x-ray tube produces x-rays with energies up to 80 keV.

Kilo Volt Peak (kVp) Considerations

• The kVp used for an x-ray study depends on the patient's thickness and the type of study.
• Mammography uses 25-50 kVp, while chest x-rays can use up to 350 kVp.
• The intensity of the x-ray beam depends on the anode material. Higher atomic number targets produce x-rays more efficiently.

Tungsten Targets

• Tungsten (Z=74) is commonly used in x-ray tubes due to its high melting point (3400°C).
• The power (P) at the target of an x-ray tube is calculated as P = IV, where I is the current in Amperes and V is the voltage in Volts.

Line-Focus Principle

• A technique used to increase the targeted area without increasing blurring to avoid overheating the target.
• The anode rotates at 3600 rpm, spreading the heat over a large area.
• Special high-speed anodes operate at rates of up to 10,000 rpm.

Bremsstrahlung

• Electrons are deflected from their path by the nucleus of the target atom, emitting an x-ray photon. Also known as braking radiation.

X-Ray Absorption

• Attenuation of an x-ray beam is due to absorption and scattering of photons.
• I = I₀ e^(-μx), where I₀ is the unattenuated beam intensity, I is the intensity after attenuation, μ is the linear attenuation coefficient, and x is the thickness of the attenuator.

Half-Value Layer (HVL)

• The thickness of a material to reduce beam intensity by half.
• HVL = 0.693/μ

Mass Attenuation Coefficient

• Used to compare attenuation in different materials, removing the density effect.
• μm = μ / ρ, where μ is the linear attenuation coefficient and ρ is the density.

X-Ray Energy Loss

• X-rays lose energy through three processes: photoelectric effect (photon energy to photoelectron), Compton effect (some to electron, some to scattered photon), and pair production.

Contrast Media

• Materials with high atomic numbers (Z) are injected to enhance contrast in certain body parts during x-ray imaging.
• Iodine-containing compounds are often injected to visualize arteries, or sprayed into lungs to make airways visible.
• Barium compounds (upper GI and enema) allow visualization of the upper and lower gastrointestinal tracts.
• Air can also be used as contrast medium

Scattered Radiation and Grids

• Scattered radiation affects the image quality.
• Grids consist of alternating lead and plastic strips to reduce scattered radiation by absorbing it. Unscattered rays pass through the plastic strips.

Intensifying Screens

• Intensifying screens increase image quality.
• Screens are coated with crystals that emit light when struck by x-rays. This light exposes film. The screens are held tightly to the film by the cassette structure.

Image Quality and Exposure

• Exposure must be adjusted to obtain an appropriate image darkness. Lead strips reduce unscattered radiation and increase exposure needs.