diagnostic ultrasound for postgraduates in obstetrics

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

diagnostic ultrasound for postgraduates in obstetrics

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