6.1 Principle of ultrasound imaging

Diagnostic ultrasound forms images by sending short pulses of high-frequency sound (typically 2 to 15 MHz) into the body and listening for the echoes that return from boundaries between tissues. A transducer acts as both loudspeaker and microphone: it converts electrical pulses to sound, then converts returning echoes back to electrical signals. The machine measures how long each echo takes to return, which gives the depth of the reflecting boundary, and how strong it is, which sets the brightness of its dot on the screen. Because it uses no ionising radiation, ultrasound is the imaging method of choice in pregnancy and for repeated examinations. Example: the maternity unit at Chogoria Hospital in Tharaka Nithi uses ultrasound for every antenatal dating scan, with no radiation risk to mother or baby.

The pulse-echo principle

1. Transmission: The transmitter applies a short voltage pulse to the piezoelectric elements of the transducer, which vibrate and send a pulse of a few cycles of sound into the tissue.
2. Propagation: The pulse travels through tissue at about 1540 m/s (the average speed assumed for soft tissue).
3. Reflection: At each boundary between tissues of different acoustic impedance, part of the sound is reflected back towards the transducer and the rest continues deeper.
4. Reception: The transducer stops transmitting and listens. Returning echoes make the elements vibrate and produce small voltages.
5. Processing: The signals are amplified, corrected for depth (time gain compensation), detected and converted to digital values.
6. Display: Each echo is shown as a dot at a position set by its depth and the beam direction, with brightness set by its strength.

The range equation

The machine calculates the depth of a reflector from the round-trip time of its echo:

Depth = (speed of sound × time) ÷ 2

The division by 2 allows for the pulse travelling to the reflector and back. Example: an echo returns after 65 µs.

• Depth = (1540 m/s × 0.000065 s) ÷ 2
• Depth = 0.1001 ÷ 2
• Depth = 0.050 m = 5.0 cm

A useful rule: each 13 µs of round-trip time corresponds to 1 cm of depth in soft tissue.

Display modes

• A-mode (amplitude): Echoes shown as spikes along a line, height proportional to strength; now used mainly in ophthalmology to measure eye length.
• B-mode (brightness): Echoes shown as dots of varying brightness; many scan lines side by side form a two-dimensional grey-scale image. This is the standard imaging mode.
• M-mode (motion): One scan line displayed against time, showing the movement of structures such as heart valves.
• Doppler modes: Show blood flow using the frequency shift of echoes from moving blood (topic 6.4).
• 3D and 4D: Volume data acquired and displayed in three dimensions, and in real time (4D).

Real-time imaging and frame rate

An array transducer forms one scan line after another by firing groups of elements electronically, building a full frame many times per second. The frame rate is limited by the speed of sound: the machine must wait for the deepest echo of each line before firing the next.

Frame rate = speed of sound ÷ (2 × depth × number of lines per frame)

Example: depth 15 cm, 128 lines: frame rate = 1540 ÷ (2 × 0.15 × 128) = 1540 ÷ 38.4 = about 40 frames per second. Increasing the depth or the number of lines lowers the frame rate.

Main parts of an ultrasound system

Time gain compensation (TGC)

Sound is attenuated as it travels deeper, so echoes from deep structures are weaker than identical echoes from shallow ones. TGC increases the receiver gain progressively with time (depth), so that similar tissues appear equally bright at all depths. The operator adjusts it with slide controls, each acting on a band of depth.

Safety of diagnostic ultrasound

Ultrasound is not ionising, but it deposits energy as heat and can cause mechanical (cavitation) effects at high intensities. Modern machines display two indices: the thermal index (TI) and the mechanical index (MI). Users follow the ALARA principle (as low as reasonably achievable), keeping the indices low and scan times short, especially in early pregnancy and when scanning the eye. The technician checks that the output display works and that transducer surfaces are intact, since a cracked lens can expose the patient to electrical leakage.