3. Attenuation

As ultrasound waves travel through tissue, their intensity decreases. This loss of signal strength is called attenuation, and it happens for two main reasons:

• Absorption: The energy of the wave is transferred to the tissue, slightly heating it.
• Scattering: Boundaries and small structures in the tissue scatter the wave in different directions, taking energy away from the main beam.

Higher frequency ultrasound provides better resolution (clearer images) but it attenuates much faster than lower frequency ultrasound. A compromise must be made: high frequencies are used for imaging shallow structures like the eye, while lower frequencies are used for deeper structures like the heart.

4. A-scans and B-scans

Depending on how the transducer is used and how the data is processed, ultrasound produces different types of scans.

A-scan (Amplitude scan)

An A-scan is a simple, one-dimensional measurement. A single pulse of ultrasound is sent into the body. As it hits boundaries, echoes bounce back.

• The data is displayed on an oscilloscope or a graph of voltage (amplitude) against time.
• The height of the peak represents the strength of the reflection.
• The position of the peak on the time axis tells you how long the echo took to return.

If you know the speed of sound $c$ in the tissue, you can use the time delay $t$ to calculate the distance $d$ to the boundary.

A common application of A-scans is ophthalmology (measuring the dimensions of the eye).

B-scan (Brightness scan)

A B-scan creates the classic two-dimensional image we associate with ultrasound (like a baby scan).

• An array of many small piezoelectric transducers is used.
• Instead of displaying a graph, each returning echo is displayed as a dot on a screen.
• The brightness of the dot represents the amplitude of the echo (a strong reflection makes a bright white dot).
• The position of the dot is calculated from the time delay.
• By firing pulses from the array in sequence, a 2D slice of the body is quickly built up in real-time.

5. Pros and cons of ultrasound imaging

When choosing an imaging technique, doctors must weigh the advantages and disadvantages of ultrasound compared to alternatives like X-rays, CT, and MRI.

Advantages:

• It uses non-ionising radiation, meaning there is no risk of cell mutation or cancer. It is safe for pregnant women.
• Excellent for imaging soft tissues (unlike simple X-rays which pass straight through soft tissue).
• Provides live, real-time images, allowing doctors to watch organs moving (e.g. a beating heart) or guide a needle during a biopsy.
• Cheaper and far more portable than MRI or CT machines.

Disadvantages:

• Ultrasound cannot easily penetrate bone (as nearly all of it reflects at the tissue-bone boundary due to the massive difference in acoustic impedance). Consequently, it is virtually useless for scanning the brain, which is encased in the skull.
• Ultrasound cannot pass through air spaces (like the lungs or the bowel) for the same reflection reasons.
• The image resolution is generally lower than that of MRI or CT.