Medical Physics P1 Flashcards
What is ultrasound
Sound wave with frequency above 20kHz.
What is used to produce ultrasound?
Piezo-electric transducer.
What is the main material used in a piezo-electric transducer?
Quartz crystal (SiO₂).
How are ultrasonic waves generated in a transducer?
Alternating voltage causes crystal to compress and expand, vibrating at same frequency.
How does a piezo-electric transducer receive ultrasound?
Ultrasound causes pressure changes → crystal atoms shift → voltage induced → signal processed.
What happens when ultrasound hits a boundary?
It is partly reflected and partly refracted.
What is the law of conservation of energy for ultrasound?
I = IR + IT (incident = reflected + transmitted intensity).
What is Specific Acoustic Impedance (Z)?
Z = ρ × c (density × speed of sound in medium).
What does difference in acoustic impedance cause?
Determines how much ultrasound is reflected at boundary.
What is the Intensity Reflection Coefficient (α)?
α = ((Z₂ - Z₁)/(Z₂ + Z₁))².
What happens at air-tissue boundary?
Very large fraction of wave is reflected.
What happens at tissue-bone boundary?
Large fraction of wave is reflected.
What happens at soft tissue boundaries?
Very little reflection.
Why is gel used before ultrasound scan?
Gel matches skin impedance and avoids reflection at air-skin boundary.
What are the two components of X-ray spectrum?
Braking radiation and characteristic radiation.
What is braking radiation?
X-rays from decelerating electrons near anode nuclei.
What is characteristic radiation?
X-rays from electron transitions inside anode atoms.
How are X-rays produced?
Fast electrons hit metal target → X-rays emitted.
What causes electrons to accelerate in X-ray tube?
High voltage between cathode and anode.
What is thermionic emission?
Heated filament releases electrons.
Why is the metal target cooled?
To prevent overheating from kinetic energy of electrons.
What is tube current?
Rate of electrons hitting target per unit time.
What does X-ray intensity depend on?
Number of electrons (heater current).
What does X-ray hardness depend on?
Voltage between anode and cathode.
