The principle of ultrasound Flashcards
- In what planes can we investigate using ultrasound?
– Any plane
While in other modalities, we usually make pictures in given planes and projections, in ultrasound, we can take sections in various planes and in various body positions. The ultrasound picture changes not only with the place we put the probe on, but also by the inclination of the probe towards the body surface. This is why the best way to learn about ultrasound is by physically doing the examination and seeing the images for yourself.
- Ultrasound –
is a dynamic examination
- Ultrasound has -
a higher frequency than audible sound
What are the physical principles behind ultrasound imaging? Well, the thing we hear is sound. Ultrasound is physically the same thing, but it has a different – much higher – frequency usually between 2 and 15 MHz.
- Choose the correct statement about piezoelectric effect
- the voltage causes volume changes in the crytal resulting in the emission of ultrasonic waves
- What is the name of the physical phenomenon that generates an ultrasonic signal in the probe?
Piezoelectric effect
The main physical principle is called the piezoelectric effect. For this, we need a piezoelectric crystal – this crystal can increase in volume when exposed to an electric current – this change in volume is accompanied by emitting of a sound. This crystal can also do the same thing but in the opposite direction – that is – if the crystal receives an ultrasound wave, it changes volume and this can generate an electrical current. The probes work like this – first, the crystal emits the ultrasound wave and then it waits for the waves to get back from the tissues – this creates the image.
- What is the physical phenomenon that records the ultrasonic signal in the probe?
piezoelectric effect
- In which of these tissues does ultrasound spread the fastest? -
Bones
The ultrasound waves travels through tissues with different speeds depending on the density of the tissue (E.g. – in bone it travels faster than muscle tissue)
- The reflection of ultrasound happens
- in places with different acoustic impedance
The waves can reflect from the tissue, scatter, bend or be absorbed. Reflection is the most important feature – and if you think about it, reflection of sound is usually called an echo.
An important factor that determines the effect in tissues is called acoustic impedance – this shows us the resistance of the tissue towards the sound waves. You can determine this by the product of tissue density and sound velocity in given tissue. The greatest acoustic impedance is therefore in the bones. The most important effect in the tissues is reflection – since we need the sound waves to come back to the probe, and this reflection happens most often in places where the acoustic impedance changes rapidly.
- The acoustic impedance (Z) is -
the product of the density of an environment multiplied by the velocity of the ultrasound propagation in a given environment
- In ultrasound we use the so-called B-mode. The letter B means -
Brightness
The image we form is black and white, which we refer to as B mode (B stands for brightness). Reflection and scatter are the most improtant effects for making this image. As we mentioned on the previous slide, the reflection or echo happens in the places where the acoustic impedance changes rapidly. The greatest changes in the body are between soft tissues and bone or soft tissue and gas.
- The greatest reflection of the ultrasound signal arises between
– soft tissue and bone; soft tissues and air
- If the US signal is fully reflected, a hyperechogenic line is formed and behind it a phenomenon that we call -
Acoustic shadow
When the ultrasound hits an area with great acoustic impedance difference nearly all of the waves rebound from the surface or are decelerated greatly as in the case of air, thus there is an acoustic shadow under this area.
- Acoustic shadow occurs -
if all of the ultrasound signal is reflected (most of the time behind air, bone or calcification)
- White ultrasound structures are called -
hyperechogenic
In ultrasound we use term „echogenicity“. Structures that can reflect a lot of US waves are whiter than the surrounding tissues and therefore are called hyperechogenic. The more homogenous a tissue is the less US waves get reflected, so these structures became darker and are called hypoechogenic. The term isoechogenic is used to describe structures that have similar echogenicity to their surroundings.
- Darker (not entirely black) ultrasound structures are called -
hypoechogenic
In ultrasound we use term „echogenicity“. Structures that can reflect a lot of US waves are whiter than the surrounding tissues and therefore are called hyperechogenic. The more homogenous a tissue is the less US waves get reflected, so these structures became darker and are called hypoechogenic. The term isoechogenic is used to describe structures that have similar echogenicity to their surroundings.
- Fully black ultrasound structures are called -
anechogenic
Structures that do not reflect ultrasound waves appear anechogenic, and they apear as black. The signal from the tissues under such a structure is whiter than all the same tissues in the same depth – this is called acoustic enhancement and it is specific for fluid deposits within a structure.
- An isoechogenic structure
- has almost the same echogenicity as its surroundings
- Which of these structures is the most hyperechogenic? –
bone
- Which of these structures is the least echogenic? -
fluid
- Which structure is physiologically anechoic? –
Gall bladder and urinary bladder
- Foreign bodies on ultrasound are most often -
hyperechogenic
- Using doppler ultrasound examination -
we can measure blood flow in blood vessels
We use this principle in ultrasound to visualize blood flow in vessels. If the blood flows toward the probe, the frequency of ultrasound increases. The difference between emitted and accepted frequency is called the Doppler frequency shift. This shift is described as 2x emitted frequency multiplied by cosine of the angle of the probe, divided by ultrasound velocity in given tissue (2f x cos(angle of probe) / velocity of US). This difference in frequencies can be recognized by the USG machine and visualized as colors.
- What is the Doppler principle? -
If a source of constant pitch (i.e. constant frequency) is moving towards an observer, he perceives this sound as a higher frequency (increasing pitch)
- When a constant frequency sound source moves towards a listener, the pitch height -
rises
- When a constant frequency sound source moves from an observer, the pitch -
decreases
- The frequency shift of the Doppler effect on ultrasound is calculated by –
(2 x frequency transmitted x cosin of insonation angle) ÷ the propagation velocity of ultrasound in the given environment
- In Doppler ultrasound imaging when we produce the doppler curve, it is called -
Spectral doppler
- Spectral Doppler can show us dependence of blood flow velocity in time, we can see a Doppler curve.
- Color Doppler shows us a map of colors in a certain area, which gives us a rough idea about blood flow.
- Triplex mode is an image of all three combined – that is B-mode, Doppler curve and color Doppler.
- Colours in the colour doppler US imaging –
can be set as required
- The ultrasonic probe is -
both the transmitter and the ultrasonic signal receiver
- The most expensive part of the ultrasound machine is –
the probes
- Convex vs. Linear probes –
Both use the same physical principle.
There are several types of probes according to their shape and frequency. *Convex probes (the left one in the picture) are ideal for abdominal imaging. *Linear probes (the middle one) for vessels and soft *tissues. Sector probe (left one) for echocardiography
There are lot of special probes for special investigations – bioptic probes (seen below), endosonographic probes etc.
- Which statement is true about ultrasonic probes?
The lower the probe’s frequency, the better the tissue penetration, but the lower the resolution; the higher the frequency, the lower the penetration, but the better resolution.
The frequency of the probe determines how deep we can see with the probe, or where the best resolution of the picture is. The lower the fruequency is, the better the penetration is, but the resolution is worse. Higher frequencies have good resolution, but the penetration is bad, so we can use them for surface imaging. In pratice – linear probes have higher frequencies and they are great for surface imaging, but we cant see anything in deeper areas. For deeper areas convex probes are better but the resolution decreases with the distance from the probe
- A linear probe has higher frequencies, indicating that
- it has excellent resolution but only a few centimeters from the probe
- A convex probe has a lower frequency, indicating that
- it has good penetration and displays deeper structures
- With the distance from the probe -
the image resolution decreases
- Ultrasonic gel –
is used to eliminate air between the probe and the skin of the patient
- Preparation for ultrasound examination of the abdomen includes –
Fasting prior to examination
- Which of the following features in a patient limits ultrasound imaging? –
Obesity and bowel pneumatology
