Ultrasound imaging is a medical technology that uses sound waves to create pictures of structures inside the body. It is widely used because it is fast, portable, and does not use ionizing radiation. Doctors can view organs, blood flow, muscles, tendons, and a developing fetus in real time.
The central tool is the handheld transducer, which sends sound into the body and listens for echoes that return from tissue boundaries.
Inside the transducer, piezoelectric crystals convert electrical pulses into high-frequency sound waves, usually above 1 MHz. When these waves reach a boundary between tissues with different acoustic properties, some sound reflects back as an echo. The machine measures echo return time to calculate depth, then uses echo strength to set brightness on a grayscale image.
By rapidly repeating this process along many directions, ultrasound builds a live image of a slice through the body.
Key Facts
- Ultrasound uses sound frequencies above human hearing, typically f > 20,000 Hz, with medical imaging often using 1 MHz to 15 MHz.
- Depth is found from echo time using d = vt/2, where v is sound speed in tissue and t is round-trip travel time.
- The average speed of sound in soft tissue is about v = 1540 m/s.
- Higher frequency gives better image detail but lower penetration depth.
- Echo brightness depends on how much sound reflects at a boundary between tissues with different acoustic impedances.
- Acoustic impedance is Z = ρv, where ρ is density and v is sound speed in the material.
Vocabulary
- Transducer
- A handheld device that sends ultrasound pulses into the body and detects returning echoes.
- Piezoelectric effect
- The property of certain crystals that lets them convert electrical signals into vibrations and vibrations back into electrical signals.
- Echo
- A reflected sound wave that returns to the transducer after meeting a tissue boundary.
- Acoustic impedance
- A measure of how strongly a material resists sound wave motion, equal to density times sound speed.
- B-mode image
- A brightness-mode ultrasound image in which stronger echoes appear as brighter pixels.
Common Mistakes to Avoid
- Forgetting the divide by 2 in d = vt/2 is wrong because the measured time includes the trip from the transducer to the tissue and back again.
- Thinking ultrasound uses radiation like X-rays is wrong because ultrasound forms images with mechanical sound waves, not ionizing electromagnetic radiation.
- Assuming higher frequency is always better is wrong because higher-frequency waves give sharper detail but are absorbed more quickly and cannot reach as deeply.
- Confusing echo strength with echo timing is wrong because echo timing gives depth, while echo strength mainly affects pixel brightness.
Practice Questions
- 1 An ultrasound echo returns 52 microseconds after a pulse is sent into soft tissue. Using v = 1540 m/s, calculate the depth of the reflecting boundary.
- 2 A transducer sends ultrasound at 5.0 MHz through soft tissue where v = 1540 m/s. Calculate the wavelength using λ = v/f.
- 3 A doctor switches from a 3 MHz probe to a 10 MHz probe for a shallow tendon scan. Explain why the image detail may improve and why the probe would be less useful for imaging a deep organ.