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7/27/2019 Diagnostic Ultrasound for Postgraduates in Obstetrics
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Diagnostic Ultrasoundfor Postgraduates in
Obstetrics andGynaecology
Max Brinsmead PhD FRANZCOGSeptember 2008
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Potential uses for ultrasound in the 1st
trimester of pregnancy: Locate the pregnancy exclude ectopic
Assessment of viability
Diagnosis of molar pregnancy
Determining gestational age
Diagnosis of multiple pregnancy
Assessment of other pelvic masses
Screening for fetal abnormalities
Assisting CVS and amniocentesis
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Other uses for ultrasound in obstetrics:
Screening for placenta previa
Assessment of APH
Cervical length monitoring
Assessment of fetal growth
Evaluation of polyhydramnios and hydrops
Diagnosis and management of malpresentation
Assessment of fetal welfare
Assessment of the postpartum uterus
Directing intrauterine interventions
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Potential uses for ultrasound ingynaecology:
Assessment of adnexal pelvic masses IUCD and Implanon location
Treatment of ovarian cysts (aspiration) and ectopicpregnancy (methotrexate)
Investigation of postmenopausal bleeding
Evaluation of pelvic pain
Investigation of menorrhagia
Diagnosis of polycystic ovaries
Tubal patency studies in infertility
Evaluation of primary amenorrhoea Screening for ovarian cancer
Monitoring of follicle number and growth for IVF
Egg recovery for IVF and ICSI
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But before you cando all this
You must know how to drive an
ultrasound machine
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What is Medical Ultrasound?
Sound waves whose frequency is beyond the
human ear
That is >20 kHz
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Advantages of Ultrasound:
Can be directed in a beam
Obeys the laws of reflection and refraction Reflected by objects of quite small size
Can be converted to analogue or digital signals for
image production
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An ultrasound image is produced by:
Producing a beam of sound waves
Transmitting this through the object of interest Receiving echoes
Converting the echoes into electric signals
Interpreting and displaying those signals
Can be snapshot or in real time
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The ultrasound beam AND the receipt ofechoes is achieved by piezoelectric
crystals:
Mounted in an array on a probe
The probe can be fixed or oscillating The wave of sound can be focused to a point of
interest
The image is displayed on an oscilloscope (or TV
screen)
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The image is formed by:
The direction of the echo
The strength of the echo The time taken for the echo to return
These 3 characteristics determine which pixels on
the screen will light up
And with what intensity
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So the ultrasound image will be:
White = Area of high acoustic impedance e.g. bone
Black = Areas of low acoustic impedance e.g fluid All shades of grey in between
Shadowed by area of non penetration or areas
behind those of high acoustic impedance e.g.
behind bone
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Disadvantages of Ultrasound:
Travels poorly through gas
The amount reflected depends on the degree of
acoustic mismatch
The piezoelectric crystals are quite delicate
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Diagnostic ultrasound:
Typically involves frequencies of 2 15 mHz
Lower frequencies will give greater penetration And thereby you can see further
Higher frequencies allow you to see more detail
But the penetration is less
And very high frequencies have the potential for
adverse biological effects
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Types of Probes:
A linear array of crystals Produces parallel sound waves
And a rectangular image
Good for surface structures
A sector scanning probe Produces a fan-like image
Can fit ito narrow spaces
Has poor near-field resolution
A curved array of crystals Will fit curved surfaces of the body
The density of scan decreases proportionally to the distance
from the transducer
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Probe Types
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Machine Controls
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Maxs Maxim Number 17
Using an ultrasound machine without using
a few of its knobs is like driving a car only
in the first gear
Its a safe to go
But you dont get very far
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Some tips: Dont be intimidated by all the knobs
Just like driving a car, You only need to know a fewbasic controls
Practice and play!
The first challenge is to find the switch to turn it on There may be more than one
Next find the machine pre set for the exam you are
about to do And do all this before you get to the patient
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Machine Controls:
Gain Controls brightness or contrast
Also in a array of sliding levers
Use maximum gain and minimum power
Depth Reach to the area of interest then
Zoom To enlarge your view then
Freeze For measurements (or stored image)
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Machine Controls
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Machine Controls 2:
Tracker Ball This is the mouse for your computer, usually with right
and left click buttons to execute functions
Used to superimpose things on the screen May have several functions
Calipers To measure distance between 2 points
Ellipse To measure area
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Machine Controls
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Some traps:
Doing patients in succession when data from one iscarried forward onto the next
When you find a fetal heart make sure that it inside auterus
Pseudo sac within the uterus with an ectopic Measuring the yolk sac as a part of the CRL
Image duplication resulting in the false diagnosis oftwin sacs
A small amount of free fluid in the pelvis can benormal
Know the many variations of a corpus luteum
Using a too-narrow field of view
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Unproven uses for ultrasound in pregnancy:
Screening for Aneuploidy Cost effectiveness of universal screening debated
Ethical issues and patient choice involved
Screening for structural malformations Sensitivity is 13 50% depending on expertise & equipment And only half of these before 20 w gestation
False positives occur
Screening for IUGR in the 3rd trimester Sensitivity is 80-90%
But the positive predictive value of neonatal morbidity is only 25-
50%
The rest have constitutional smallness
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Harmful Effects of ultrasound in pregnancy:
It is not ionising radiation
However, thermal effects and cavitation can occur
in tissues exposed to high power ultrasound
One RCT of repeated routine ultrasound withDopplers in the 3rd trimester found a small but
significant decrease in birth weight in the exposed
cohort
A meta analysis showed males exposed toultrasound in uterus are more likely to be left-
handed
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Caring for your ultrasound machine:
Treat your probes as if they were made of glass
Wash, clean and dry probes
Sterilisation options
Dont use oil or alcohol Transport probes safely stowed
If you changed the machine defaults set them backto the original
rasoun n e rs r mes er o
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rasoun n e rs r mes er opregnancy:
Start with the abdominal probe
Counsel the patient about your expected findingsand expertise
First find the cervix and/or uterine body Its not as far in as you think
Look for embryo at the edges of a sac 2 cm Measure CRL up to 12w, thereafter BPD, HC, AC
and FL
Remember ectopic and multiple pregnancy
If you are not sure say so Exclude ectopic and recheck in 7 14 days
Check the POD and ovaries before you finish
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Assume ectopic & proceed accordingly
>1000 iu/L
Diagnostic laparoscopy if clinically
suspicious
500 - 1000 iu/L
Observe
Repeat HCG in 24 - 48 hrsRescan when >1000 iu/L
or follow to
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Ultrasound in the third trimester ofpregnancy:
Start with abdominal palpation
Tell patient purpose of examination
Quick scan for presentation and lie
Measure BPD, HC, AC and FL Remember that this does not predict dates
Liquor volume
Find placenta and examine lower edge in
relationship to the presenting part Suspected placenta previa best evaluated by PV or
TV scan
Ovaries virtually never seen
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Ultrasound for the non pregnant woman:
Start with abdominal probe Preferably with a full bladder
I measure uterine dimensions in two planes
Then send patient to empty bladder
And switch to vaginal probe First find the cervix
Acutely anteverted/flexed uterus is tricky
Find and measure endometrium
Then evaluate myometrium Ovaries can be anywhere
And cannot be found 25 30% of the time
I measure ovaries in two dimensions