1.1.4 Ultrasound (US): Sound Waves & Echoes

“Real-time imaging turned diagnosis into something you can watch.” — Why ultrasound is everywhere in clinical practice

After CT, MRI, and X-ray (the “anatomical imaging trio”), ultrasound stands out as the real-time, radiation-free, portable, and cost-effective modality that clinicians reach for first in many scenarios.

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🔊 From SONAR to medical imaging

Early roots

Ultrasound (frequency > 20 kHz) was first widely used in SONAR for submarine detection in World War I. Later, it became a workhorse in industrial nondestructive testing.

Bringing it into medicine was harder: biological tissues are complex, and clinicians need fast and repeatable imaging.

Clinical pioneers

• 1942: Karl Dussik attempted ultrasound transmission imaging for brain tumors (limited by the skull).
• 1958: Ian Donald demonstrated obstetric ultrasound and helped establish medical ultrasound as a clinical tool.

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📡 Imaging principle: echoes + piezoelectric transducers

Frequency vs penetration

Medical ultrasound typically operates at 1–20 MHz:

• Low frequency (1–5 MHz): deeper penetration (abdomen)
• High frequency (7–20 MHz): higher resolution (thyroid, vessels)

Piezoelectric effect

The transducer uses piezoelectric crystals (e.g., PZT):

• Apply voltage → crystal vibrates → emits ultrasound
• Echo returns → crystal vibrates → generates voltage (signal)

A linear-array ultrasound probe with multiple piezoelectric elements

Interaction with tissue

• Reflection: the main information source; depends on acoustic impedance mismatch
• Scattering: creates speckle/texture
• Attenuation: increases with depth and frequency
• Refraction: may cause artifacts

Common modes

Mode	What it shows	Typical use
B-mode	2D grayscale	general imaging
M-mode	motion over time	cardiac valves
Color Doppler	flow overlay	vascular / cardiac

Doppler equation

Blood flow velocity can be estimated by:

$$v=\frac{c\cdot \mathrm{\Delta }f}{2f_0\cos \theta }$$

💡 Why gel is required

Air has a very different acoustic impedance than tissue, causing near-total reflection at the air–skin interface. The coupling gel removes air gaps so ultrasound energy can enter the body.

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🚀 Technology evolution (high-level)

Era	Key breakthroughs	Typical impact
1970s–1980s	Real-time B-mode	true dynamic imaging
1980s–1990s	Doppler	flow/hemodynamics
1990s–2000s	3D/4D, harmonic imaging	better visualization & contrast
2000s–2010s	Contrast US, elastography	functional assessment
2010s–today	AI + handheld devices	accessibility & workflow

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Where to go next

• PET/SPECT: 1.1.5 PET/SPECT: Metabolism & Function (/en/guide/ch01/01-modalities/05-pet)
• Practical preprocessing topics (attenuation correction / denoising): Chapter 2.3 (/en/guide/ch02/03-pet-us-preprocessing)