Ultrasound Flashcards
1
Q
Ultrasound
A
- Ultrasound (US) waves are spread over wide range of frequencies: 20 kHz – 20 MHz
- US waves carry higher energy compared to same amplitude sound (due to their high frequency).
- US waves can interact with the biological tissues (due to the similarity in size of different bio-structures and US wavelength),
- determines its significance in medicine
- US vibrations are well absorbed by tissues
- Being reflected or scattered by small objects, US serve as information carrier in diagnostic imaging.
2
Q
Production of US
A
- US is produced by means of Piezoelectric effect
- PEE occurs in some crystals due to their specific crystal lattice: normally electric charges are placed symmetrically but if being deformed the crystal the symmetry of the lattice is impaired and polarization occurs – this phenomenon is called direct piezoelectric effect
- Conversely – if the electric charge is loaded on the opposite walls of the crystal, it experiences electric forces deforming the lattice (reverse piezoelectric effect). If this deformation appears at more than 20000 times per sec the crystal will vibrate with US frequency and it becomes US source
- The crystals of this type can simultaneously generate and receive US waves
- The device build on piezo-crystal is called transducer and serves as both - agenerator and a receiver of US.
3
Q
Physical properties of US
A
- The wavelength of US is shorter than that of the audible sound
(example: if two sound waves with frequencies of 2 KHZ and 2 MHz are spreading in air their wavelengths will be 75 cm and 0.75 mm respectively) - Short wavelength enables US to interact with small structures (dimensions of mm and less) in human tissues
- As a consequence US can produce resonant vibration within tissues and being reemitted or reflected by them it carries useful information about their location, intactness, constitution.
4
Q
US Imaging
A
- US sonography is based on analysis of US reflected by tissues.
- Reflections occur when US encounters boundary between different types of tissues (due to difference in acoustic impedances).
- The distribution of reflected/penetrated US beam is determined by: Reflection coefficient :
αr2 = (Z2 – Z1)2/ (Z2 + Z1)2
- When US falls from the air to the patient’s skin αr equals approximately 1 (Z2 skin >> Z1 air)
- therefore almost 100% of the wave reflects back to the air
- Thus the US wave does not penetrate into the body and no diagnostic information is received
- This problem is abolished by use of contact gel
- The gel has the same acoustic impedance as the skin (tissues). The transducer is immersed in gel and US penetrates into body without losses when it crosses the border gel-skin
5
Q
US sonography types: Amplitude scan
A
- The image represents a straight line with spikes
- Each spike corresponds to a border between tissues
- Main diagnostic parameter is the amplitude of spikes – the larger the difference between acoustic impedances of tissues, the greater the amplitude
- This method is applicable to simple structures
6
Q
US sonography types: Brightness scan
A
- images represents each small detail of the tissue observed as a bright/dark spot, depending on the value of acoustic impedance
- This method is suitable for compound structures examination
- This is the default mode that is produced by any ultrasound / echo machine
- It is a 2 dimensional cross sectional view of the underlying structures - This is the most intuitive of all modes to understand
- The field of view is the portion of the organs or tissues that are intersected by the scanning plane
- Depending on the probe used, the shape of this field could be a sector - commonly seen with Echo and abdominal ultrasound probes or rectangular or trapezoid - seen with superficial or vascular probes
- Multiple images of the field or frames are generated every second on the screen, giving an illusion of movement. A frame rate of at least 20 frames per second is needed to give a realistic illusion of motion.
7
Q
Describe a brightness scan image
A
- On a grey scale, high reflectivity (bone) is white; low reflectivity (muscle) is grey and no reflection (water) is black
- Deeper structures are displayed on the lower part of the screen and superficial structures on the upper part.
- The main uses for 2-D mode are to measure cardiac chamber dimensions, assess valvular structure & function, estimate global & segmental ventricular systolic function, and improve accuracy of interpretation of Doppler modalities
- While this mode is useful to accurately represent the 2- dimensional structure of the underlying tissues, it does not resolve rapid movements well and may misrepresent 3-dimensional nature of structures
8
Q
M-mode
A
- This mode allows investigation of movable structures in US sonography
- Initially a 2-D image of the object is produced and a single scan line is placed along the area of interest
- The M-mode will then show how the structures intersected by that line move toward or away from the probe over time
- The M-mode has good temporal resolution, so it is useful in detecting and recording rapid movements
- We can also correlate and time events with ECG or respiratory pressure wave forms traced alongside the M- mode tracings
- The M-mode is commonly used for measuring chamber dimensions and calculating fractional shortening and ejection fraction.
9
Q
Doppler’s effect
A
- the change in frequency of a wave as perceived by an observer moving relative to the source of the wave
- occur in any wave-involving process
- a person (receiver) who is moving towards a sound source with velocity, v, will perceive sound with higher frequency than the source frequency (respectively – the wavelength will be shorter than at the source) because moving against the sound propagation he will encounter more than one wave front per sec.
- If the person (receiver) is going away from the source – the effect is reverse: perceived sound has a lower frequency and a longer wavelength, compared with the source
- These phenomena are called blueshift and redshift respectively and are present in other wave phenomena such as light
10
Q
Measurement of Doppler shift
A
- allows calculation of bloodstream velocity
- freq. of reflected US changes with:
Δf = 2ν cosθ f0/c
Δf = f0+ fr
fr– reflected wave frequency
v – velocity of blood stream
c – US velocity in soft tissues (~1540 m/s)
f0 – US frequency
θ – the angle between blood flow and US axis of propagation (Doppler’s angle)
11
Q
US applications in therapy
A
- US can influence tissues producing favorable therapeutic effects upon them
- The character of the effects derived is related to US physical properties and type of tissues
- Different influences can be achieved by selection of US mode of generation – continuous or pulsed
12
Q
Pulsed US
A
(duty cycle of 20%)
- can not produce thermal effects because of dissipation of energy absorbed during the pause of the cycle
- Ultrasound energy also creates mechanical forces independent of thermal effects, thereby causing biologic effects that are not related to temperature rise alone, such as cavitation, torque forces, oscillatory shear, radiation, pressure and microstreaming
- It uses low intensity and pulsed mechanical waves in order to induce regenerative and anti-inflammatory effects on biological tissues, such as bone, cartilage, and tendon
- it is plausible that the treatment relies on non-thermal phenomena, such as microbubbles and microjets induced by cavitation, acoustic streaming, and mechanical stimulation
13
Q
Cavitation
A
- The interaction of ultrasound with gas bubbles or contrast agents causes rapid and potentially large changes in bubble size
- may increase temperature and pressure within the bubble and thereby cause mechanical stress on surrounding tissues, precipitate fluid microjet formation, and generate free radicals
- Gas-containing structures (e.g., lungs, intestines) are most susceptible to the effects of acoustic cavitation
14
Q
US wavelength and bubble formation and growth
A
- short wavelength ultrasound (observed at higher frequencies) does not provide sufficient time for significant bubble growth
- therefore, cavitation is less likely under these circumstances compared with long wavelengths
- The short half-life of cavitation nuclei prevents most cavitation-related biological effects, unless ultrasound contrast agents are also present.
15
Q
Contrast agents effect
A
- Contrast agents markedly reduce the threshold intensity for cavitation
- However, because of the relatively high viscosity of blood and soft tissue, significant cavitation is unlikely
- cavitation has not been shown to occur with the ultrasound exposure commonly used during a diagnostic examination
20
Q
US Sonophoresis
A
- method for drug delivery into localized areas assisted by collimated ultrasound beam directed toward these areas
- Pharmaceuticals are inserted through the skin (per cutis) and can penetrate into the tissues in depths of 4-6 cm
- Medications are impasted on the skin in the form of gel or cream
21
Q
HIFU, High intensity focused US
A
- system to treat cancers pathologies and/or to improve the quality of life of patients
- Compatible with other therapies and may be a viable alternative to traditional surgery
- Despite the great advances made in the field of preventive and therapeutic, cancer remains the second leading cause of death in developed countries and is among the top three leading causes of death in developing countries.
