1. ALARA Principle ☢️

As Low As Reasonably Achievable - The golden rule of radiation safety!

TIME

Less time in radiation field = Less dose

DISTANCE

Inverse Square Law: Double distance = 1/4 dose

SHIELDING

Lead aprons, thyroid shields, gonadal shields

2. X-ray Production ⚛️

High-speed electrons hit the anode target, producing X-rays through two mechanisms:

Bremsstrahlung Radiation (Braking Radiation)

Electrons decelerate near nucleus → X-rays with continuous spectrum (most X-rays produced this way)

Characteristic Radiation

Electrons knock out inner shell electrons → Discrete energy peaks (specific to target material)

3. X-ray Tube Components 🔌

CATHODE (Negative)

Filament (tungsten coil) - electron source
Focusing cup - directs electrons
Thermionic emission

ANODE (Positive)

Target (tungsten or rhenium-tungsten)
Focal spot - where electrons hit
Rotating anode for heat dissipation

4. Exposure Factors (mAs and kVp) ⚡

mAs (Milliampere-seconds)

Controls QUANTITY of X-rays (number of photons)

Directly proportional to patient dose

mAs = mA × time

kVp (Kilovoltage Peak)

Controls QUALITY of X-rays (penetrating power)

Higher kVp = More penetrating, lower contrast

15% Rule: ↑15% kVp = Double exposure

5. Image Quality Factors 📸

Density (Optical Density)

Overall darkness of image. Controlled by mAs.

Contrast

Difference between light and dark areas. Controlled by kVp.

Spatial Resolution

Sharpness/detail. Affected by focal spot size, motion, geometry.

💡 Pro Tips for the Board Exam

✓ Remember: mAs controls QUANTITY, kVp controls QUALITY
✓ Inverse Square Law: I1/I2 = (D2)²/(D1)²
✓ Line Focus Principle: Actual focal spot > Effective focal spot

Radiologic Physics