Ultrasound Level 1 Tutorial: Ultrasound Physics without Physics
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Tutorial: Ultrasound Physics without Physics
This module will explain how ultrasound works in simple terms.
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Tutorial: Ultrasound Physics without Physics Pick Your Probe
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1 Tissue Properties Tissue Properties
Current lesson 2 Pick Your Probe Pick Your Probe
3 The knobs The knobs
4 Artifacts Artifacts
Pick Your Probe
Flat linear array?   Curved linear array?   Phased array?   How many megahertz did you want with that?

We will cover those questions shortly.

All probes emit sound waves that are generated by a pizoelectric crystal. These cystals are amazing because when you send an electric current into them, they vibrate (and make a sound that you can't hear). The opposite is also true: when incoming sound waves hit them, the vibrations from the sound waves cause the crystal to generate an electrical signal.

It is the same concept as a speaker and a microphone, like what are built into your phone. The difference is that a small electromagnet does the converting of sound waves and electrical signals in the phone.

The electrical signal generated by the crystal goes into the computer and after some complicated processing, a picture is formed on the screen.


How many Megahertz?

Hertz (abbreviated Hz) is a measure of frequency. 1 Hz = 1 cycle per second. Most humans can hear air vibrating from 20 to 20,000 Hz.

Megahertz, abbreviated as MHz = 1,000,000 Hz. Humans cannot hear sounds at this frequency. Neither can your dog nor the neighbourhood bat.

Typical ultrasound probes use sounds in the range of 3-10 MHz. What's that? I can't hear you.


What is the significance of Megahertz and frequency?

Think of your neighbour's extra loud stereo at 2 am when you are trying to sleep. That jerk! You can hear the bass thumping through the walls, but everything else is muffled. What you are experiencing is the fact that low frequency has higher penetration. Low frequency waves travel further and are attenuated less. This means that if you want to take images of structures deep in the body (like the abdomen), a low frequency probe is better.

Consider the next image. It shows 2 structures (blue circles) that are very close to each other. If you want to see both of those strucures in high detail, you will need high resolution. You can see that high frequency has higher resolution because there are more waves going through the structures so the electrical signal will be more detailed. You might want a high frequency probe to look at the thyroid gland or vessels in the neck.


        High Frequency = high resolution                  Low frequency = low resolution
high frequency gives high axial resolutionlow frequency gives low axial resolution


Do you see the tradeoff? You cannot have high penetration (low frequency) and high resolution (high frequency) both at the same time. There is a trade-off due to physics. Sorry about that. We submitted a complaint on your behalf already.


A quick clarification of some different types of resolution:

  • axial resolution is what we just talked about: differentiating 2 objects that are in the same axis (one is closer to the probe and one is further) where frequency is very important

  • lateral resolution is a different resolution where 2 objects are side by side. When we talk about the focus of the ultrasound beam (which we will do later), we will discuss lateral resolution. Lateral resolution is not dependant on frequency.

  • temporal resolution is a third type and refers to moving objects. Temporal = time, so we are referring to the ability of the image to show moving images. This is where frame rate comes into play.


Summary:
  • best for deep structures = low frequency
  • best for high "axial" resolution = high frequency


Setting Frequency on the Ultrasound machine:

Each probe has a range of frequencies that it can function at. You can adjust these frequencies on the ultrasound machine. Many ultrasound machines use shortcuts; instead of telling us the MHz value, the settings are:
  • Res which stand for (high) resolution; this is the higher frequency setting for that probe
  • Gen which stands for general and is medium frequency
  • Pen which stand for (deep) penetration and is the lower frequency


The Different Probes

They are called:
  • Flat linear
  • Curved linear
  • Phased Array

These terms refer to the arrangement of the crystals within the probe and the shape of the probe.

Flat linear means the crystals are in a single row and in a straight line. A flat linear probe will send out the sound signals all parallel to each other and creates a rectangular image on the screen. These probes are common for neck and superficial vessel ultrasound.



A curved linear arranges the crystals along a curve but are still in a single line. A curved linear probe will send out the sound signals in an arc and this creates a triangle shaped or more correctly, an arc shaped image. These probes are common for abdominal ultrasound. They are physically larger than the other 2 probes.
curved array probe


The phased array is a little more complicated. The probes have a flat surface, but the crystals are arranged in a grid and are charged with electricity at different times. In other words, some of the crytals are vibrating in-phase with, and some are vibrating out-of-phase with the other crystals. The waves emitted from these probes are more complex and produce an arc shaped image despite the fact that the probe has a flat surface. These probes are common for cardiac ultrasound.
phased array probe