Student questions and answers

Have a question? It's very likely that another student has had the same question. This page contains a compilation of recent student questions that I thought might be of benefit to all.

You can scroll through or use your browser's "find" feature (usually CTRL-F) to search this page for specific terms. Alternatively, you can use the "Search Site" tool in the upper right corner of any page within the site.

Question: At a specular interface, does the angle of reflection always equal the angle of incidence?

Yes! At a specular interface the angle of reflection is always equal to the angle of incidence, regardless of the incident angle.

For example, if the angle of incidence is 0 degrees (normal incidence), the angle of reflection is 0 degrees. If the angle of incidence is 23 degrees, the angle of reflection is 23 degrees. And so forth.

The principle is similar to the way billiard balls bounce off the cushions at the edge of a billiard table. Regardless of the angle at which the ball strikes the cushion, it will always bounce off the cushion at exactly the same angle at which it struck the cushion. The angle of reflection always equals the angle of incidence.

This principle of reflection at a specular interface applies to all waves including sound, light, and water.

Here's a link to a video clip demonstrating the principle with light.

In diagnostic ultrasound, we want to image specular reflectors at angles close to normal (0 degrees). This will ensure that the angle of reflection is also close to 0 degrees enabling the echo to return to the transducer. When imaging specular reflectors a good rule of thumb is " Get perpendicular!"

Question: How can I differentiate average / below average / above average attentuation? Is it by the clarity of the tissue border?

Structures with lower than average attenuation will typically display two features: 1. a bright backwall (due to the large impedance difference at that interface 2. acoustic enhancement (brighter echoes deep to the structure due to the decreased attenuation)

A good example of a low attenuating structure is a cyst.

Structures with higher than average attenuation typically will display some degree of acoustic shadowing (a vertical band of darker echoes deep to the structure). A good example of a structure with higher than average attenuation is a stone or a calcification.

Question: Is the output power of the transducer independent of the power output control?

No, the output power control determines the output power from the transducer. If the output power control is increased, the acoustic power within each pulse is increased. There is a direct relationship between the two.

Question: The notes have mentioned that the ultrasound gel which we use should have intermediate acoustic impedance. What does that mean?

All ultrasound gel is designed to provide the best transmission and reception of sound from the transducer to the patient. Consequently, the acoustic impedance of ultrasound gel is manufactured to have an intermediate impedance that falls between the high impedance of the transducer and the much lower impedance of skin. By having an intermediate impedance, the gel reduces the large acoustic impedance mismatch between the transducer and the skin. This results in improved transmission and reception of sound. You will never need to determine the actual acoustic impedance value of transducer gel. You just need to know that the acoustic impedance value for ultrasound gel is lower than the impedance of the transducer and higher than the impedance of the skin. For example, if the acoustic impedance of the transducer is 80 Rayls and the acoustic impedance of the skin is 30 Rayls, the best choice for the acoustic impedance of the gel would be a value that falls between the two, such as 50 Rayls.

Question: What is the "usable beam length"? What does it mean?

Useable beam length is a term that describes the length of the beam over which the beam diameter is less than or equal to the diameter of the beam's source. It is, as the term suggests, the length of the beam that is diagnostically useable. Once the distance from the transducer exceeds the useable beam length, the beam width will be increasingly wider than the source, and as such, becomes diagnostically less and less useful. Wide beams have very poor lateral resolution, so the beam width at all points farther from the transducer than the "useable beam length" is just too wide to produce the kind of lateral resolution required for diagnostic imaging.

For unfocused beams, the "useable beam length" can be easily calculated. It is equal to the NZL x 2.

Question: Why is the best axial resolution equal to exactly one half of the length of the pulse?

Axial resolution is the resolution along the beam. It is determined strictly by the SPL (spatial pulse length). In general, the shorter the SPL, the better the axial resolution. The formula for the theoretical calculation of axial resolution is SPL/2. You should know this formula for the registry exams. The reason for the formula is a little more complicated and has to do with the fact that the reflected pulse will turn back on itself when incident on an interface. In effect, at the point where the pulse turns back on itself, its actual axial dimension is SPL/2. Thus the calculation for the theoretical axial resolution is SPL/2. There is more detail than this available, however it will not be required for the registry and is certainly not of any concern when scanning in practical situations.

Question: How can we change the Doppler transmit frequency?

The Doppler transmit frequency is determined by the system and the transducer. When the sonographer selects a Doppler mode, the system will typically operate with a lower transmit frequency than it would use for imaging. Know this point for the registry.

For example, when using a 5 MHz curved array transducer to examine the kidney, the transmit frequency in grayscale imaging mode will be 5 MHz. However, if the sonographer decides to do some Colour Doppler with the same transducer, the transmit frequency in Colour Doppler mode will be typically 3 MHz.

On most modern systems, the sonographer has the ability to adjust the transmit frequency in both grayscale mode and Doppler mode using a system control (usually from the touch screen or using a button). In either mode, the transmit frequency can be slightly adjusted up or down as needed.

For example, if the transducer is transmitting in 3 MHz when doing Colour Doppler, the sonographer will likely have a control that will allow the transmit frequency to increase to 4 MHz or decrease to 2 MHz.

Old ultrasound systems could not do that. However, all modern broad bandwidth transducers will be able to provide the sonographer with the ability to slightly vary the transmit frequency using a system control. Look for that control on your system. It can be very useful when trying to optimize your Doppler display.

Question: How is the Near Field Length equation for soft tissue derived?

The key factor you should know about the Near Field Length is that it is directly proportional to Frequency and also proportional to crystal diameter squared (D²). This is shown in the Near field Length equation for soft tissue: NFL = D² x F/6. Using the formula to do calculations is not that important. Knowing the relationships is the key skill. For those inquiring minds who would like to know how the Near field Length equation for soft tissue is derived from the basic Near Field Length equation (NFL = D² /4 lambda), I have posted an attachment at the bottom of this page that you can download. However, please remember that the derivation of the formula is not the critical skill. The most important skill is to know the factors that affect the Near Field Length and the relationship.