Diagnostic Imaging - Ultrasound Flashcards
what type of waves does diagnostic ultrasound use?
high frequency sound waves
what are the frequencies typically used in diagnostic ultrasound?
2-18 MHz
what level of Hz is audible sound?
20-20,000 Hz
how do sound waves and x rays differ?
sound waves need a material to travel through
why are soundwaves unable to move through a vacuum?
rely on compression and relaxation of the medium that they are traveling through
what does the velocity of a soundwave depend on?
the material it is traveling through - higher density will lead to higher velocity
what effect is relied upon to produce ultrasound?
piezoelectric effect
how does the piezoelectric effect work?
electrical voltage is applied to a disc within a transducer
the disc expands or contracts due to electrical current
movement is proportional to voltage and gives rise to sound wave
what is the piezoelectric effect?
conversion of kinetic energy to sound energy
what are the 2 types of disc used in production of ultrasound?
PZT
PVDF
what does PZT stand for?
lead ziconate titanate
what does PVDF stand for?
polyvinylidine difluoride
describe how ultrasound is produced
voltage applied across crystal
crystal deforms due to its piezoelectric properties
leads to emission of high frequency sound waves
is production of ultrasound in the transducer continuous?
no - transducer sends out a pulse of sound
how long is the pulse of sound emitted by the the transmitter usually?
typically 3 wavelengths - 1.5 mm
3 compressions and relaxations
what happens between pulses of ultrasound?
scanner waits for echos from tissues
what percentage of the time it is being used it the transducer producing ultrasound?
1% of the time
what percentage of the time it is being used it the transducer receiving ultrasound?
99%
how is the ultrasound signal received and an image produced?
sound wave returns from tissues to transducer
pressure of sound waves distorts disc and so piezoelectric crystal
generates a voltage proportional to pressure
voltage is then processed by the machine and displayed
describe the piezoelectric effect
voltage deforms piezoelectric disc in transducer pulse of sound into tissue hits tissue interface reflection from tissue interface returning echo of sound deforms piezoelectric disc in transducer voltage
define acoustic impedance
density of tissue x speed of sound in tissue
what has the most effect on acoustic impedance?
density of tissues
how do tissues vary?
in acoustic impedance
what happens when a sound crosses a boundary between tissues of different acoustic impedance?
some is reflected back to transducer
what does the proportion of reflected sound depend on?
difference in acoustic impedance of tissues
how much reflection of ultrasound is seen at soft tissue boundaries?
relatively little (e.g. fat / kidney interface)
how much reflection of ultrasound is seen at interface between soft tissue and bone?
much larger percentage - bone surface will appear very bright
when does specular (mirror) reflection occur?
when ultrasound beam hits a large smooth surface (e.g. small intestine wall)
when does non-specular reflection occur?
beam hits small structures which have density variations
the beam is re-radiated in all directions leading to weak echoes
what is enabled by non-specular reflection?
texture is given to organs which allows assessment
describe how echoes are detected
sound waves reflected from various acoustic interfaces within the body
the echoes deform the crystal, resulting in the production of electrical signals
electrical signals are displayed as image on screen
what are the 3 display modes of ultrasound images?
A mode
B mode
M mode
what is B display mode for ultrasound?
brightness
what is M display mode for ultrasound?
motion
what is A display mode for ultrasound?
amplitude - oldest method, only used in opthalmology
how is the image produced in b mode?
images a slice thorough a patient with a line of ultrasound
beam of ultrasound scans back and forth
image is produced from lots of lines
what does the brightness of an ultrasound image in B mode depend on?
amplitude of signal - stronger echo will be brighter
what does the position of a structure on the screen depend on?
time for the signal to return (longer time = further depth)
why must organs be scanned in more than one plane in B mode?
as it only images a slice so full impression cannot be gained from one view only
when is M mode most commonly used?
cardiac work
how is B mode utilised to aid M mode?
B mode image is used to position a single line - movements along this line are followed
how is the image in M mode displayed?
position vs. time
how is a trace of movement developed in M mode?
continuous updating
what is being shown by the yellow line in this B mode ultrasound?
the area being imaged in M mode
in what state of consciousness can an ultrasound exam be completed?
tolerated well fully conscious or with light sedation
deeper sedation may be required for abdominal exam
what are the main benefits of ultrasound exam?
relatively quick (e.g. diagnosis of pyometra)
non-invasive
safe
what considerations should be made when choosing the area of the body to place the transducer?
area of the body which overlies the region of interest, avoiding interveneing bone or (where possible) gas
how should the area be prepared for ultrasound?
clip
clean the skin
apply ultrasound gel
when cleaning the skin why may surgical spirit not be the best option?
may damage transducer
what are the 2 types of arrays found in transducers?
phased
linear
is on type of transducer going to work for all imaging?
no - need a range of frequencies and types for optimal examination
what 3 factors can be considered when choosing transducers?
type
footprint
frequency
what are the 3 types of transducer?
phased array
linear array
microconvex / convex
how is the beam steered in phased array?
electronically
what size footprint does a phased array have?
small
why does phased array produce a triangular shaped image?
sound waves diverge the further into tissue they move
how does a linear array transducer work?
multiple elements are triggered in groups
image is the width of the transducer as waves don’t diverge
how are the elements on microconvex / convex transducers arranged?
in a curve
what is a benefit of microconvex / convex transducers?
contour better with the animal
does the wave diverge in a microconvex / convex transducer?
yes
what is the key difference between microconvex and convex transducers?
size of footprint - larger in convex
what are phased array transducers useful for?
cardiac work - sit between ribs but larger image can be produced at depth due to divergence of beam
what is a linear array transducer useful for?
limbs
describe the similarities between phased array and microconvex transducers
easy to manipulate
small contact area
wide field at depth
describe the linear array transducers
large contact area
large field of view near skin
why are linear array transducers useful for superficial structures?
large field of view near the skin
what happens to wavelength as frequency increases?
wavelength decreases
what is a benefit of shorter wavelengths?
better resolution as pulse length is smaller
how does shorter pulse length lead to better resolution?
better separation between structures as there is no overlap of reflections that can be seen with longer wavelengths
what frequencies of ultrasound give good image resolution?
7.5-18 mHz
what is the cost of using high frequency ultrasound?
sound attenuation is proportional to frequency so sound will not penetrate as far into the body
what can high frequency ultrasound be used to image?
superficial structures in larger animals (e.g. eyes, tendons)
what can low frequency ultrasound be used to image?
deeper structures (e.g. the liver in large dogs) large animals (e.g. horses)
what is the resolution like of lower frequency ultrasound transducers?
poor
what cna be imaged with lower frequency ultrasound transducers?
deeper structures
what range are lower frequency transducers?
2.5 - 5 MHz
when should patients be starved overnight for ultrasound?
abdominal ultrasound
why should patients be starved for abdominal ultrasound?
empty stomach is prefurrable
improves ability to examine organs
can safely sedate or GA if needed
what is essential when performing ultrasound?
good contact with skin
still patient
how can good contact with the skin during ultrasound be achieved?
clipping
surgical spirit
coupling gel
what are the issues with using surgical spirit alongside coupling gel?
can damage transducer
describe clipping required for an abdominal ultrasound
xiphisternum to pubis
line of costal arch up to lumbar muscles
include last 2-3 intercostal spaces
describe the clipping site for heart ultrasound
right side
(and left for full echo)
4th to 6th intercostal space
costochondral junction to sternum
where can the 4th to 6th intercostal space be located?
just behind elbow
describe clipping sites for left kidney
behind last rib
below lumbar muscles
describe clipping site for right kidney
include last 2-3 intercostal spaces
below lumbar muscle
what is involved in ultrasound machine care?
regular cleaning of transducers and key board
removal of gel and hair after each use
safe storage of transducers and leads
regular servicing of machine
what should be kept on record regarding ultrasound?
patients and areas imaged
how may a permanent record of ultrasound findings be kept?
digital archive
thermal printer
video
what does the appearance of tissues on ultrasound relate to?
echogenicity
how does fluid appear on ultrasound?
black - anechnoic
what does anechoic mean?
no echoes produced
how does fat appear on ultrasound?
white - echgenic
how to soft tissues appear on ultrasound?
variable - hyper/hypoechoic relative to surrounding structures
how does a soft tissue / gas interface appear on ultrasound?
total reflection of sound
cannot see beyond gas
is it possible to ultrasound through bone?
no - sound is reflected or absorbed
what part of bone can be examined through ultrasound?
surface
what are the advantages of ultrasound?
widely available safe for operator and patient relatively quick non-invasive (except biopsy) rarely need GA sedation is optional but can be helpful
what clinical information can be gained from ultrasound?
good soft tissue detail
functional information / movement
can guide aspirates / biopsies
what is routine ultrasound used for?
pregnancy diagnosis and monitoring
what is difficult to predict with pregnancy monitoring ultrasound?
number of foetuses
when can ultrasound reliably detect pregnancy in dogs?
28 days
when can ultrasound reliably detect pregnancy in cats?
20 days
what are the disadvantages of ultrasound?
equipment relatively expensive and easily damaged
need to clip hair (issue in show animals)
need experience ot interpret images
what are the issues with clinical information gained from ultrasound?
gas/fat/bone hinders exam
many findings are non-specific so biopsy is needed for diagnosis
what may hinder ultrasound of the heart?
panting so lots of air in lungs
