Chapter 30: FlashCards

 Medical Imaging:

Q. What happens when mechanical stress is applied to piezoelectric crystals?

The crystals are forced to change shape. As a result, an emf is generated across them.

Q. What happens when a potential difference is passed across piezoelectric crystals?

An internal stress is felt within the crystals, causing them to change shape.

Q. How can the piezoelectric effect be used to produce ultrasound waves?

If an alternating potential difference is applied across a piezoelectric material, the material will vibrate at the same frequency as the applied voltage. If applied at the natural frequency of the material, resonance will occur. The amplitude of a resonating piezoelectric material is sufficient to produce ultrasound waves.

Q. How is it possible that a piezoelectric transducer can receive as well as emit ultrasound waves?

The alternating potential difference is only applied across piezoelectric materials for a few short pulses. After this, the transducer is open to receiving reflected ultrasound waves. When receiving reflected waves, the pressure changed inflicted by the ultrasound causes the piezoelectric crystals to change shape, generating a potential difference in the transducer.

Q. Explain how X-rays can be produced through electron bombardment?

Passing a current through a tungsten filament causes it to heat up and emit electrons: an effect known as thermionic emission. The electrons are attracted to a positively charged tungsten anode and collide with it. This interaction gives off energy in the form of heat and X-ray photons. X-rays are reflected in a particular direction such that an electron beam is created.

Q. Why are piezoelectric transducers useful in the context of prenatal screenings?

Ultrasound is partially reflected at tissue boundaries as a result of acoustic impedance variations between tissues. Consequently, the ultrasound will reflect off several tissue boundaries and subsequently be picked up by the transducer. These reflected waves can be used to establish a picture of the unborn child, checking that there are no prenatal defects in the foetus. If there are defects, these can then be treated.