Ultrasound Imaging Flashcards
What is the typical frequency of ultrasounds used in practice?
Typically 5-10 MHz
Can be 2-18 MHz
How do sound waves differ from X-rays?
Need a material to travel through
Velocity depends on material
Have properties of a wave including reflection, refraction, interference
How are ultrasounds produced and what is this process called?
Piezoelectric effect
Voltage applied across crystal - crystal deforms due to its piezoelectric properties - emission of high-frequency sound wave
What is acoustic impedance?
Acoustic impedance = density of tissues x speed of sound in tissue
What affects the proportion of reflection of ultrasound waves in a patient?
Depends on different in acoustic impedance
Relatively little reflection at soft tissue boundaries / larger % at soft tissue/bone interface
What is the difference between specular and non-specular reflection?
Specular = beam hits large smooth surface Non-specular = beam hits small structures e.g. inside of liver, re-radiated in all directions (weak echoes), gives texture to organs
How are ultrasound signals received and used to display an image?
Sound waves reflected from various acoustic interfaces within the body - the echoes deform the crystal, resulting in production of electrical signals - electrical signals displayed as image on screen
What are two display modes of ultrasound images?
B mode (brightness) M mode (motion)
Describe the B mode of image display.
Images a slice through the patient, image built up from lots of lines
Brightness depends on amplitude of signal, position depends on time for signal to return
See movement in ‘real time’
Need to scan organs in >1 plane
Describe the M mode of image display.
Used in cardiac work
B mode image used to position a single line, movement of points along line followed
Image displayed as position vs. time
Continually updated, giving trace of movement
How is ultrasound imaging carried out?
Choose area of body overlying region of interest, avoiding bone / gas where possible
Clip hair
Clean skin (surgical spirit may damage transducer)
Apply liberal quantities of acoustic gel
Place transducer over region of interest
What 3 things should we consider when choosing a transducer?
Type
‘Footprint’
Frequency
What are the 3 types of transducers?
Phased array
Linear array
Microconvex/convex
Describe a phased array transducer.
Beam is steered electronically Cone-shaped image Easy to manipulate Small contact area Wide field at depth
Describe a linear array transducer.
Multiple elements Triggered in groups Rectangular image Large contact area Large field of view near skin - good for superficial structures
Describe microconvex/convex transducers.
Elements arranged in a curve Wider cone-shaped image Easy to manipulate Small contact area Wide field at depth
Why is a smaller wavelength preferred for ultrasound?
Velocity = frequency x wavelength
Velocity is constant within soft tissues
As frequency increases, wavelength decreases
Better resolution if wavelength and therefore pulse length is smaller
Why can we not just use the smallest wavelength possible for ultrasounds?
High frequency gives good image resolution
But sound attenuation is proportional to frequency
So sound does not penetrate so far into body
What are the dis/advantages of high frequency ultrasounds?
7.5-18+ MHz
Good resolution
Cannot image deeper structures in larger animals
What are the dis/advantages of low frequency ultrasounds?
2.5-5 MHz
Can image deeper structures and larger animals
Poorer resolution
