UT Equipment and System Performance
What you must know
A UT system is transducer, instrument, cables, and couplant-each contributes error. Understand piezoelectric element types (quartz for stability, ceramics like PZT for sensitivity), damping to shorten pulse and improve axial resolution, and matching layers to improve transmission. Know the difference between resolution (axial and lateral) and penetration, and how frequency, bandwidth, and damping trade off.
Instrument controls matter: gain (dB, logarithmic), time base/velocity calibration, reject (avoid using it to hide signals), and the DAC/TCG concepts. Know how a distance-amplitude correction curve is built from reference reflectors and why TCG electronically compensates depth amplitude loss.
Traps in questions
- Confusing vertical linearity (amplitude fidelity) with horizontal linearity (time-base accuracy).
- Assuming higher gain improves resolution-it only raises displayed amplitude and noise.
- Mixing up DAC (amplitude vs distance reference curve) with a straight-beam reference block calibration.
- Forgetting cable length/impedance can shift pulse shape and apparent gain.
- Treating screen height percentage as absolute-it is relative to calibrated gain and reference.
Worked example
A reference hole gives 80% FSH at 25 mm. At 50 mm the same reflector reads 40% FSH, a 6 dB drop (halving amplitude ≈ 6 dB). To compare equal-amplitude reflectors at different depths, apply the DAC offset rather than raw screen height. Always record gain, reference, and transfer corrections so another examiner can reproduce the reading.