--- title: "Piezoelectric pulse transmitter, A-mode vs B-mode" subject: "Biomedical Instrumentation" chapter: 5 key: "semester_7/biomedical_instrumentation:ultrasound-pulse-a-b-mode" source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/ultrasound-pulse-a-b-mode site: BCT Engineering --- # Piezoelectric pulse transmitter, A-mode vs B-mode Biomedical Instrumentation · Chapter 5: Non- Invasive Diagnostic Instruments - Asked in: 2075 Chaitra, 8 marks - Video: [The Principles of Ultrasound Imaging](https://www.youtube.com/watch?v=X8ab6NAIV5I) — Medical Aid Films - Films For Life - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/ultrasound-pulse-a-b-mode - Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-5 ## Question How a transmitter circuit discharges a capacitor through the crystal of a piezo-electric transducer to generate large amplitude of pulses? How these pulses are utilized to view internal structures of the human body? Compare A-mode and b-mode display format. ## Short answer A high-voltage supply charges a capacitor; a fast switch discharges it through the crystal, producing a brief ultrasound pulse. Echoes from tissue boundaries are received; depth is computed by d = c·t/2. A-mode shows signal amplitude vs depth as spikes, while B-mode displays brightness-modulated dots to form a cross-sectional image. ## Answer #### Transmitter circuit The piezoelectric crystal needs a short, high-amplitude electrical pulse to produce a strong ultrasound pulse. ```text HV supply --[R]--+--[C]--+-------------+--> T/R switch --> receiver amp | | | [SCR / switch] (crystal) [R damp] | | | gnd gnd gnd ^ trigger pulse (clock) ``` **Working:** the capacitor C charges slowly through the resistor R from a high-voltage supply (hundreds of volts). A trigger from the clock turns on the fast switch (SCR, thyratron or MOSFET), so C discharges almost instantly through the crystal (and a damping resistor). The sharp high-voltage spike shocks the crystal into oscillation at its resonant frequency (1 to 10 MHz), producing a short damped burst of ultrasound (about 1 µs). The cycle repeats at the pulse repetition frequency (1 to 5 kHz). Between pulses, the same crystal acts as the receiver for the echoes. #### Use of pulses to view internal structures (pulse-echo) 1. The transducer, placed on the skin with coupling gel, sends a pulse into the body. 2. At each boundary between tissues of different acoustic impedance, part of the pulse is reflected as an echo. 3. The echo returns to the crystal, which converts it to an electrical signal; this is amplified (with time-gain compensation) and detected. 4. The depth of the reflector is **d = c·t/2** (c = 1540 m/s, t = time between pulse and echo); the echo strength shows the reflectivity. 5. The signals are displayed in A, B or M mode to show the organs (liver, kidney, foetus, heart). #### A-mode versus B-mode | Point | A-mode (amplitude) | B-mode (brightness) | |---|---|---| | Display | Spikes: amplitude vs depth | Dots of brightness: grey-scale image | | Dimension | One-dimensional | Two-dimensional cross-section | | Transducer | Fixed, single | Moved or scanned (or array) | | Echo strength shown by | Height of the peak | Brightness of the dot | | Image | No picture | Anatomical picture | | Uses | Distance measurement, eye (ophthalmology), brain midline | Obstetrics, abdomen, heart, tissue imaging | | Complexity | Simple, cheap | More complex (scan converter and memory) | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does the transmitter generate a high-amplitude pulse for the crystal? A. C charges slowly through R from a HV supply and is discharged almost instantly through the crystal by a fast switch triggered by the clock B. The crystal is driven by a continuous sine wave C. C is charged quickly and discharged slowly D. The HV supply is connected directly to the amplifier Answer: A. Slow charging through R and rapid discharge via an SCR or MOSFET gives a sharp high-voltage spike that excites the crystal. (Key point: Transmitter circuit) 2. How is the depth of a reflector found in pulse-echo imaging? A. d = c·t B. d = c·t/2 C. d = c/(2t) D. d = 2c/t Answer: B. The pulse travels to the reflector and back, so depth = c·t/2 with c about 1540 m/s. (Key point: Depth calculation) 3. Why do echoes occur inside the body? A. Tissue absorbs all the ultrasound B. Part of the pulse is reflected at boundaries between tissues of different acoustic impedance C. Because of electromagnetic induction D. Because the gel reflects the beam Answer: B. Reflection happens at interfaces with an acoustic impedance mismatch; echo strength shows reflectivity. (Key point: Pulse-echo principle) 4. Which statement correctly compares A-mode and B-mode? A. A-mode shows echo strength as brightness in a 2D image B. B-mode is one-dimensional with peak height C. A-mode shows spikes of amplitude versus depth with a fixed transducer; B-mode shows a 2D grey-scale image by scanning D. Both need a scanned transducer Answer: C. A-mode is 1D with peak height; B-mode shows brightness dots forming a 2D cross-section using a scanned or array transducer. (Key point: A-mode vs B-mode) ## Related in Biomedical Instrumentation - [Electrode theory and ultrasonic bloodflow meter](https://www.bctengineer.com/question.md?key=semester_7%2Fbiomedical_instrumentation%3Aelectrode-theory-ultrasonic-flowmeter) — Biomedical Instrumentation, chapter 2 ## Related in other subjects - [Piezoelectric transducer sensitivity and output voltage](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Apiezoelectric-transducer-output-voltage) — Instrumentation I, chapter 3 - [FS of unit-amplitude rectangular pulses](https://www.bctengineer.com/question.md?key=semester_4%2Fsignals_and_systems%3Arectangular-pulse-train-fs) — Signals and Systems, chapter 2 - [BCC, FCC and HCP crystal structures](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Abcc-fcc-hcp-structures) — Material Science, chapter 2 - [Piezoelectric load cell and charge amplifier](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Apiezoelectric-load-cell-charge-numerical) — Instrumentation and Measurement, chapter 7