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Introduction To Diagnostic Ultrasound

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sihirlifil's version from 2017-10-01 17:41

Section

Question Answer
What happens to sound when it hits a solid structure?Reflected back at the same angle as it hit
Sound waves consist of?Series of pulses made up of compressions and rarefactions (spread out)
Ultrasound waves are high or low frequency?High (cycles/sec)
Ultrasound waves unit?Hertz (1 cycle/second)
Range of human hearingUp to 20KHz
Human medicine uses frequencies in the order of ___ MHz2-15 (or 20 for ocular)
How does ultrasound work?Cyrstals in transducer send out pulse when stimulated, then waits for return (echo) of the signal (receiver)
The transducer crystals are ___ 1% of the time, and ___ 99% percent of the timeEmitter 1%
Transducer 99%
Piezoelectric effect: What happens when the current is switched on?Crystal vibrates, emits sound, sound collides with structure, goes back and vibrates the crystal again, generates another pulse that machine reads
Distance that sound travels =Time it takes to hit structure x speed of that sound (machine calculates this)
Strength of returning echo converted to?Brightness pixel
B-mode: how does brightness compare to strength of echo?Brightness proportional to strength of echo
Icon at top of picture tells youCranial or lateral, depending on orientation of transducer
How is ultrasound image displayed?Real-time. Contantly analyzing data & converting to moving image
How do some systems give us the width?Crysals lined up in row, series of sound waves emitting to give fan shape (Beam sweep)
Sound has different ___ in different tissuesVelocities
Bone 4080m/s
Soft tissue 1540m/s (average)
Air 331m/s
How does sound travel through bone?Fast! majority of sound absorbed
How does sound travel through air?Poorly
Acoustic impedence =Inablility of sound to transmit (resistance)
Different tissues have different capacities
Why can we differentiate liver & intestine on US scan?Acoustic impedence. Liver is soft tissue, intestines contain air (have different transmission entities)
Speed of sound and frequency: related how?Not! Speed of sound does not depend on frequency
When sound interacts with matter: reflection?Specular- reflection deviated in wrong direction (not at 90*), losing information because it isn’t being returned to the transducer
Scatter- hit substance at 90* but may be moving, so some of sound deviated in all directions
When sound interacts with matter: refraction?Some of the sound hitting the transducer is reflected, but some penetrates through so machine miscalculates, shows with deviated angle (spoon in glass of water)
When sound interacts with matter: absorption?(Depends on ultrasound frequency) Sound emitted is very strong in near field (close to transducer), but less sound comes back in father field because of tissue absorption
How do we want the transducer arranged?90 degrees to area of interest
High frequency transducers have ___ wavelengthsSmall (e.g. horse jugular, superficial. Waste of time for horse heart deep in thorax)
High frequency transducers = ___ resolutionHigher
High frequency transducers = ___ penetration into tissue depthsPoor
Low frequency transducers = ___ resolutionLower
Low frequency transducers = ___ penetration into tissue depthsBetter
What’s going on here?
8MHz transducer: Near field really well defined (can see wall of intestine). Good resolution, poor depth
5MHz: Poor definition in near field (can’t see intestinal wall), but reasonable at far field. Poor resolution, good depth
Types of transducersMechanical (spin left to right, generate wedge-shaped image)
Linear (esp. reproductive & tendon work, series of crystals lined up in row that fire sequentially)
Arrays (most expensive, lined in row but configured to create wedge shape)
Types of transducers based on frequency:Single
Range
Multiple frequencies simultaneously
Types of probes:Linear
Curvilinear
Sector
Mechanical
Phased-array
Returning echo is strongest when…Greater difference in acoustic impedance
2.5MHz transducer: high or low frequency? cm travel? quality?Low frequency, 24-30cm, poor
7.5MHz transducer: high or low frequency? cm travel? quality?High frequency, only 5-8cm, excellent
Selection of transducer depends onArea of examination
Transmission of sound through air: how much reflection?99% of sound waves (so…. avoid air)
Transmission of sound through bone: how much reflection?20% (70-80% absorbed)
Acoustic shadowing =Marked differences in acoustic impedance results in black shadow beyond that point
How would an acoustic shadow in the bladder be useful?Means it’s definitely a calculus causing acoustic shadow, and definitely in bladder
Why can we tell the difference between certain types of tissue by attenuation?Transmission of sound through tissue results in weakening (attenuation) of the beam, and not all tissue attenuates equally
How do fluid-filled structures attenuate vs. soft tissue?Fluid-filled do not weaken the beam as much
Acoustic enhancement =Brighter shadow beyond fluid-filled structures
Sound beam is relatively stronger when it exits…? What does this mean?Fluid structure surrounded by soft tissues. So we can identify fluid-filled structures in a tissue (e.g. gall bladder in liver) by acoustic enhancement
Acoustic enhancement useful to e.g.:Differentiate tumors from fluid-filled cysts
Between 2 white arrows (blue are ultrasound beam)=
Acoustic enhancement
Edge shadowing =Sound bends when it collides with the edge of a circular structure, so sound wave doesn’t get back to the transducer. Machine sees void (black stripe)
Arrows pointing to
Edge shadowing
Lack of echoes beyond bone =
Acoustic shadow (Can see edge of calcaneous well, would be able to see remodeling/new bone formation)
Echogenic =Echoes visible
Echogenicity =Ability of a tissue to generate echoes
Echolucent/anechoic =Tissue generates no echoes
Hyperechoic =Bright or white echoes
Hypoechoic =Shades of gray echoes
Black arrow bottom right pointing to?
"Dirty" acoustic shadow- air in colon
Resolution =Ability of soundwaves to locate and recognize 2 separate points in tissue
Lateral resolution =Ability to discern 2 objects close together in lateral plane
Example of when good lateral resolution is necessary?Need to be able to see twins in mare (usually don’t come to term)
For reproductive work, you want ___ frequency transducersHigh (better lateral resolution)
When the beam is really wide (aka lower frequency), how is visibility of lateral structures?Less
The higher the frequency the ___ the lateral resolutionGreater
Axial resolution =Ability discern 2 objects close together in vertical plane
When the beam is really thick (aka lower frequency), how is visibility of adjacent (vertical) points?Less ability to resolve, don’t see as separate
The higher the frequency the ___ the axial resolutionGreater
The higher the frequency the ___ the depth penetrationLower
Important factors to consider choosing a transducerFrequency (what needs to be scanned)
Size of footprint (3.5 linear array for horses won’t work in cats)
Cost
Need standoff? (Tendons)
What is a standoff used for?E.g. superficial flexor (only 1 cm in), transducers need to be backed off skin. Sits as sleeve or wraparound
Footprint =
What modes do we use to look at images?B-mode: brightness/2-D (general work)
M-mode: motion (heart)
(A-mode: amplitude for eyes)
Doppler: specialized cardiac
Describe B-mode2D, Real-time (image constantly updated, seems to be moving)
Frame rate controlled by machine, allows to select for abdomen (low framerate) or cardiac (high framerate)
Machine controls: (6)Depth
Frequency
Power (strength of outgoing frequency)
Gain (strength of returning echoes)
Time-gain compensation (TGC)
Focus
How should you set the power?Soundwaves not too strong (returning echoes overwhelming), enough echoes back for signal
Higher power =Stronger beam, more artifacts
Lower power =Returning echoes not strong enough to create adequate image
Gain =Amplification to enhance returning echoes (controls brightness)
Gain too high:One of the most common operator mistakes
Meaningless echoes displayed in image (too bright)
Gain too low:Image too dark
Time gain compensation =Set of sliding switches/buttons. Each slider controls signal amplification within corresponding band (near vs. far) in image, need to adjust to achieve even brightness
Focus =Narrowest point of the beam. Possible to move up and down in most machines (e.g. need to move further down for liver compared to kidneys)
Can be single or multiple focus points
Fixed in mechanical probes
Set to depth of structures of interest
What does the focus look like in the image? (and in this particular image?)
Not adjusted correctly (should be at the level of the area of interest)
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