Chapter 2 Prep Questions

The answer to this question is a bit tricky and requires you to interrelate density with stiffness. Let's start with question b. There's an inverse relationship between density and speed. This means that without any other variables, increases in density cause decreases in speed. However there is another variable present and that present is stiffness. Stiffness, unlike density, is directly proportional to speed. But density and stiffness don't exactly offset each other. Stiffness is the more dominating factor. That's important when we evaluate question a. Another thing that's important is the fact that density and stiffness are generally directly proportional to each other (this is that interrelationship I mentioned at the beginning of this answer). Say for example you fill an empty plastic bottle with water. Then you empty it and fill it with mud instead. The mud is more dense with the water, and it's also true that this second bottle would be stiffer. In other words, you wouldn't be able to bend the bottle as easy as if it had water in it. So with all that... the answer to a. is that denser tissues usually cause faster sound propagation velocities. When you encounter quiz and test questions on this topic, read that wording carefully. The relationship between the 2 variables is not always the same thing as the outcome/result.

Answer both of these questions, then explain why the difference between these 2 questions is important: a.) "Do denser tissues usually cause faster sound propagation velocities or slower?", and b.) "is there a direct or an inverse relationship between density of tissues and propagation velocity?".

1/rate = time and 1/time = rate. Be sure to correctly change units (e.g. Hz to seconds, ms to kHz, etc...)

How do you quickly convert a time into a rate and vice versa?

The transmitter creates electrical energy which is sent to the transducer through wires in the cable. The transducer converts that electricity into sound and pulses it into the body. The body causes echoes to form from that pulse which go back to the transducer. There the transducer re-converts the sound energy back into electricity. From there the energy goes on to the receiver, and then processing, and then display on the monitor and finally archived for storage. If only it was just that simple The rest of this course will fill in the gaps.

Briefly explain how a sonographic image is created.

An acoustic variable is when a property of the medium's energy changes. This change is what defines a 'sound wave'. The four changes in the medium, AKA 4 acoustic variables, are pressure, density, temperature, and particle vibration.

Define 'acoustic variable' and list the 4 acoustic variables.

Cyclical means repetition. In waves, there are half cycles of compressions where variables are at their highest change away from the undisturbed state, and there are half cycles of rarefactions where variables are at their lowest change away from the undisturbed state. A complete cycle includes 1 compression and 1 rarefaction.

Describe the word 'cyclical' as it relates to waves.

Propagation means there is a change in the location of energy concentration over time. Oscillation refers to the wave's energy vibrating back and forth.

Explain the difference between wave oscillation and wave propagation

A small proportion as shown on figure 31. Furthermore, there is no end to the upper end of frequency and so it goes on infinitely. This makes the diagnostic range even much smaller than shown on this figure, since the figure stops once the frequency reaches 20MHz.

Is the diagnostic range of ultrasound a large proportion or a small proportion out of the entire spectrum of ultrasound? Explain.

In reality, few humans can hear all the way up to 20kHz. Typically we can only hear up to 17kHz but it depends on the person, and age becomes inversely proportional to frequency range. Each of you will have an opportunity to test your range in lab 2!

Is the physics definition for human audible sound the same as the reality of human audible sound? Explain.

It's an approximation because tissues vary from patient to patient. Furthermore, the tissue orientation relative to the beam may have some influence (muscle propagates sound differently depending on whether your beam is parallel vs. perpendicular to the muscle fibers for example).

Is the tissue propagation velocity table within your book demonstrating exact velocities or an approximation? Explain why.

Detail resolution which includes subtypes of axial (vertical), lateral (horizontal), slice thickness/elevation (hidden front to back dimension), and spatial quality (pixel density). Detail resolution is how well you can see separation between 2 close structures. Temporal resolution is the rate of real time motion, expressed in hertz. It's the frames per second (fps), and is more important when scanning moving structures like blood flow or the heart. Motion artifact/blurriness can occur when it's suboptimal, or real-time lag can be seen as well. Contrast resolution relates to the number of possible grayshades that the image can produce. Sometimes it's important to have more grays in our pics, and sometimes less. It depends on the purpose of the picture needed to make the best diagnostic quality. Signal to Noise Ratio (SNR) is mathematically the number of parts signal (real anatomy) divided by the number of parts noise (random static/junk). Higher SNRs are pretty important for good image quality.

List and describe the 4 types of image quality mentioned in that lesson.

A mode is amplitude modulation and plots a graph over time (x) and amplitude (y). B mode is brightness modulation which converts the above mentioned graph into a brightness scaled picture. Both A mode and B mode are considered outdated (with few exceptions in A mode). Real-time is B mode in motion, typically around 20 - 30 frames per second (Hz) M-mode is motion mode and can detect a moving structure along its line of path. It's commonly used for calculating fetal heart rates. Doppler modes relate to blood flow and will be discussed later.

List and describe the sonographic modes.

frequency, period, propagation speed, wavelength, amplitude, power, and intensity.

List the basic acoustic parameters

1. Stronger sound wave = higher (improved SNR). 2. Stronger sound wave = increased risk of patient bioeffects. Sonographers need to balance this using the ALARA principle (as low as reasonably achieveable... use enough energy to make an excellent diagnostic picture but don't overdo it).

List the two consequences of setting a higher voltage power, and explain why this is a practical concept for a sonographer to use while scanning.

1. . Wavelength is related to both frequency and propagation speed. It increases when propagation speed is increased (directly proportional) and decreases when frequency is increased (inversely proportional). Frequency is only determined by the sound source. Propagation speed is only determined by the medium in which the sound is travelling through, specifically its stiffness (direct proportionality) and density (inverse proportionality).

Page 18 demonstrates a very important interrelationship between frequency, wavelength, and propagation speed. In 1 short paragraph, describe the important take-aways that page 18 is stating.

Electromagnetic waves can travel through both a medium and a vacuum. Mechanical waves on the other hand always require a medium.

State the defining characteristic of an electromagnetic wave that's different than that of a mechanical wave.

They transport energy from one place to another.

What do all waves have in common?

The diagrams on pages 21 and 22 show us that wave amplitude can change when 2 waves collide with each other, and depending how they collide. If the compressions and rarefactions were to line up with each other they'd be 'in phase', and the amplitudes would add to each other essentially forming into a single wave that's double the amplitude. If these waves were out of phase though, the compression of one would line up with the rarefaction of the other when they collided, essentially turning the amplitude into 0. Most commonly waves collide in a partial interference pattern (called partial constructive or partial destructive... they mean the same thing). In this case the sum of the waves would be somewhat bigger than the individual waves, but not double like they'd be in pure constructive interference.

What does it mean for a wave to be in-phase or out of phase? How does this affect interference?

A pulse is a burst of sound energy sent into the body. An echo is a piece of the pulse's energy that breaks off and bounces back to the transducer. The brightness of a dot corresponds with the strength of a returning echo. The location of a dot corresponds with the round trip travel time.

What is a pulse? What is an echo?

Directly related to mass and inversely related to volume. Depending on how you interpreted the question, you could also add that density is inversely related to propagation speed.

What is density directly related and inversely related to?

Directly related to force and inversely related to area.

What is pressure directly related and inversely related to?

Depth is the # of cm deep into the body. Distance travelled includes the round trip from the pulse and its echo. What that means is that distance travelled is twice that of depth. Let's say you're going to walk to your friend's house who lives a mile away, visit for a while, then walk home. Your friend lives a mile away from your starting point. That's synonymous with the depth. You walk 2 miles to make the round trip. That's synonymous with distance travelled.

What is the difference between 'distance travelled' and 'depth'? Why is this an important concept?

Sonographers only work with diagnostic ultrasound. HIFU is used for tissue destruction, and there is promise in this area of medicine for destroying cancerous tumors among other thereapeutic applications. Although frequency ranges might be similar to that of diagnostic values, HIFU uses much higher energy output levels, up to 1500W/cm2 which is over 100x diagnostic values. This can heat tissues up to 56 degrees celcius, and can only be maintained for a few seconds. HIFU should not be confused with therapeutic ultrasound, which also uses higher energy levels than diagnostic values (although much less than HIFU) and is used by physical therapists as a treatment tool for MSK injuries and disorders.

What is the difference between HIFU and Diagnostic Ultrasound? Do you think sonographers work with both?

The assumption is that sound moves in a straight line of path. Therefore direction of travel isn't relevant and it's basically the same thing to say speed and velocity (technically velocity means both speed and direction).

What is the vector assumption that allows for sonographers to use the terms 'propagation speed' and 'propagation velocity' interchangeably?

Continuous wave sound is always on. Pulsed wave sound has intervals of a burst of sound followed by listening time.

What's the difference between continuous and pulsed sound?

Scanned modalities scan over a field of view. Real-time imaging and color Doppler are 2 examples. A picture of the anatomy is created. Non-scanned modalities collect sonographic data over a single scan line. Spectral Doppler, M-mode, and A mode are examples.

What's the difference between scanned and non-scanned modalities?

Transverse waves are easier to draw and doesn't require motion to be inferred by the person evaluating the diagram.

Why are longitudinal sound waves drawn as transverse waves on diagrams?