Screening and other tools Flashcards
Safety of USS
There have been no proven adverse biological effects associated with obstetric USS
ALARA principle = ‘as low as reasonably achievable’
Lowest acoustic power output, for the shortest duration, with the least exposure to sensitive target tissues, while achieving the optimum diagnostic information
Doppler can cause significant rise in temperature
Routine examination in the first trimester is not advisable
Other risk factors
- prolonged scanning
- increased depth of scanning
- Scanning at times of maternal pyrexia
Types of USS used in O&G
B-mode = 2D ultrasound
Mode of scanning most often associated with clinical ultrasound imaging
M-mode = motion mode
Doppler ultrasound
When to use what measurements to date a pregnancy?
CROWN RUMP LENGTH
- Measure length excluding limbs
- Use maximal length up to 10/40
- Common practice is to adjust the EDD if disparity between menstrual and ultrasound dates is greater than 4 days
At 11-13/40 use CRL and BPD to give EDD
At 14+ weeks, use BPD, HC, FL
Common practice is to adjust the EDD if disparity between menstrual and ultrasound dates is greater than 10 days from 18-22/40
BPD and HC measurements
Midline horizontal Obtain axial section which includes: - Falx cerebrum - Cavum septum pellucidum - Thalami
BPD - place callipers on outer skull margin of the near to the inner skull margin of the far parietal bone
HC - Measure outer perimeter of bone with ellipse
- Exclude subcutaneous tissue
How to measure the FL
Obtain horizontal image, parallel to the USS probe
Measure longest length along the diaphyseal shaft excluding cartilaginous epiphyses in the measurement
How to measure the AC
Transverse section of the abdomen that includes stomach, umbilical vein and spine
- Umbilical vein section is that adjacent to its junction with the left portal vein
- Correct transverse plane is where the right and left portal veins are continuous with one another
whole single rib
Measure with spine at 3 or 9 o’clock
Measure using ellipse function on skin edge
Macrosomia associated with higher margin of error
General principles for 1st tri USS
Pulsed Doppler (spectral, power and colour flow imaging) USS should not be used routinely Displayed thermal index should be <1.0 and exposure time as short as possible (max 5-10 mins)
At what gestation should you see what features on USS?
Gestational sac seen = 4+3/40
- double decidual sign highly suggestive of IUP
Yolk sac = 5/40
Always present in 10mm MSD
Fetal pole - Always present in a 25mm MSD
- Should be seen by 6-7 weeks
Heart motion
- Always present in a CRL of 7mm
- Mostly as soon as CRL is visible
Fetal pole and GS size increases at approx 1mm/day at 5-7 weeks
Miscarriage USS diagnosis
Absent fetal heart motion with CRL >/=7mm
- Observed for period of at least 30-60s
No live fetus visible in GS with an MSD >/=25mm
IF:
MSD >/= 12mm - repeat 7 days
MSD <12mm - repeat in 14
If yolk sac, repeat in 11+ days
no guidelines exist for TA assessment of pregnancy
- If woman declines TVS, low threshold for f/u imaging, ideally with a 14 day interval
Incomplete misc: >15mm AP diameter
Complete misc: =15mm AP diameter
Early scan features of twin pregnancy
Twin peak / lambda sign = dichorionicity
- Tongue of trophoblast extending into the intertwin membrane
- Appears ~9-10/40
T sign - thin intertwin membrane inserted into the single placental mass at a perpendicular plane
Number of placental masses
- ~3% of MC placentas have two placental masses (bilobed)
Should be performed in the first trimester - sensitivity and specificity 98-100%
If uncertain, refer to specialist urgently and make photographic record for review
- Manage as MC until proven otherwise
USS features of corpus luteum
Size: up to 5cm
Critical role until placenta takes over at 7-9 weeks
Most characteristic feature if a peripheral ring of vascularity on either colour or power Doppler imaging
Colour flow is more prominent around a CL than ectopic
CL wall is less echogenic (<2mm) than an ectopic
Ectopic has a thick echogenic rim >6mm
How is Doppler principle used in USS?
Doppler shift (the apparent change in frequency) can be used to calculate the velocity of movement
Perform Doppler USS quickly and accurately
- Most potential to cause heating effect
How and why do you calculate S/D, PI and RI
Absolute values are more prone to error as they can be affected by the angle at which the transducer meets the blood flow
The ratio is largely unaffected by this
Systolic / diastolic (S/D) = max systolic velocity / minimal diastolic velocity
Pulsatility index (PI) = systolic - diastolic / mean velocity PI is preferable because it can be calculated with AEDF and REDF
Resistance index (RI) = systolic - diastolic / systolic velocity
Key points for performing the UAPI
Free loop of cord, away from insertions
No fetal body, limb or breathing movements
Identify UA with colour Doppler
Position sample volume in a portion of the cord coursing parallel to Doppler beam
- Avoid aiming Doppler beam towards fetal eyes
- Position in the middle of the vessel
- Want as close to 0 degrees as possible (definitely <60)
If PI within normal range, only sample one of the umbilical arteries
If abnormal, sample both and use the more normal (lower) value
Indications of UAPI
Provides a measure of placental resistance
SGA / FGR
HTN in pregnancy
Decreased FM
MC twins
Use of umbilical artery Dopplers in high risk pregnancy compared with no Doppler, has found statistically significant reduction in (2017 Cochrane)
- Perinatal death
- CS
- Emergency CS
- Induction
- Gestational age at delivery
placental insufficiency and abnormal UA PI
placental insufficiency is a/w development of high resistance and low flow circulation within the placenta
This increases impedance to blood flow in the umbilical artery
Results in higher peak systolic velocity being needed to maintain forward flow, and reduction in velocity of flow during diastole, when the heart is not forcing blood towards the placenta
MCA PI - how to perform?
Perform assessment during fetal quiescence
Start with BPD view
Move caudally to visualise the butterfly shape of suprasellar cisterns and the sphenoid
Activate colour Doppler to visualise the MCA
Assess the MCA which is closer to the transducer
Aim for angle of 0 degrees
Indications for MCA PI
FGR / SGA
- With abnormal UA PI at any gestational age
- With normal UA PI >34/40
MCDA twin gestation with TTTS
Why do you get changes in the MCA PI?
Indicates fetal compensation for redistribution of blood flow to the brain, and shows progressive low resistance flow with increasing diastolic flow due to vasodilation
Indications for Middle Cerebral Artery Peak Systolic Velocity (MCA PSV)
Fetal anaemia, e.g. RBC alloimmunisation
Unexplained hydrops
MCDA twins from 20/40, earlier if signs of TTTS
Why do you get changes in Middle Cerebral Artery Peak Systolic Velocity (MCA PSV)
The most probable explanation for the observed increase in MCA PSV is that fetal anemia is associated with decreased blood viscosity leading to increased venous return and preload with consequent increase in cardiac output, the faster the velocity, the higher risk of fetal anemia
Why do you get changes in the ductus venosus?
Ductus venosus shunts a proportion of oxygenated blood from the umbilical vein to the IVC at its inlet to the fetal heart
- 30% of umbilical blood at mid-gestation, by 30/40 falls to 20%, stable until term
Hypoxaemia –> increased shunting through the ductus venosus
- Part of the preferential redistribution mechanisms of the growth-restricted fetus
Waveform of DV reflects relationship with the right side of the fetal heart
- First peak indicates systole (ventricular contraction)
- Second peak = early diastole
- Nadir of the waveform (a wave) occurs during right atrial contraction
Abnormal flow pattern indicated by a reversed flow during atrial contraction
- Pressure in right heart increases with loss of myocardial contractility and cardiac dysfunction –> increased DV resistance –> absent and finally reversed ‘a’ wave
- Blood flows in retrograde through the IVC to the DV during atrial contraction
Associated with fetal academia and imminent fetal death
Indications for DV
Markedly raised UA PI (»95th) and reduced MCA PI in preterm SGA
MCDA twin gestation with TTTS or selective IUGR
Fetal abnormality, e.g. hydrops, cardiac abnormality
Indications for uterine artery doppler
Screen patients at high risk of early PET or early SGA at 20 or 24/40
If abnormal at 20/40, repeat at 24/40
Early onset IUGR
Current hypertensive disorder in pregnancy
Full assessment of suspected SGA
- only do once when first diagnosed
How to perform uterine artery doppler?
Locate the maternal ASIS and angle medially
Alternatively visualise the external iliac artery
Uterine artery is typically seen crossing the EIA anteriorly
ABNORMAL:
>95th percentile
Bilateral notching after 24/40 is abnormal
Describe normal and abnormal umbilical vein
Normal umbilical vein
- non-pulsatile flow pattern
Abnormal - pulsatility is present
- Pulsatility can also occur with breathing and hiccups
What is normal fluid?
SDP
Normal range 2-10cm
Oligohydramnios is no SDVP measuring 2cm deep and 1cm wide
AFI
Measure deepest pool in each quadrant excluding cord and fetal parts
Normal range is gestational age dependent
- Often 5-25cm used as normal
Comparing AFI and SDVP
No evidence that either in the prevention of adverse perinatal outcomes (Cochrane review 2008)
Compared to SDVP of <2cm, an AFI <5cm is associated with increased:
- Diagnosis of oligohydramnios
- Induction of labour
- CS delivery for fetal distress
But with no difference in Apgar score, umbilical artery pH <7.1 or improvement in perinatal outcome
Ultrasound features of fetal demise
Fetal heart activity is not present
Oedema or a translucent rim around the head like halo
Spalding’s sign - overlapping of the skull bones
Echogenic areas and fluid in the abdomen
How does MRI work?
No exposure to x-rays or other forms of ionising radiation
Powerful magnetic field affects the body’s atoms, forcing the nuclei into a different position
Termination of the radiofrequency excitation –> protons free to return to their orientation –> give up energy they have absorbed –> detected by receiver coil –> computer turns signals into pictures
Hydrogen (proton) most commonly used nucleus in MRI
- Abundance in the human body (consists mainly of water)
Pros and cons of MRI
PROS:
Enables pictures to be taken from almost any angle
No ionising radiation
Generally more detailed
Often diagnostic in absence of IV contrast
Particularly useful for injuries or diseases of the CNS
CONS:
Must lie still
Issues if claustrophobia
Some tissue heating but at surface, negligible at the core
Recommend avoid 1st trimester if possible
How does CT work
Uses x-rays to produce precise cross-sectional images of anatomical structures
Iodine based contrast media does not appear to be teratogenic but neonatal hypothyroidism has been associated with some iodinated agents taken during pregnancy, therefore avoid
VQ scan pros and cons
Radioisotope study using injected and inhaled markers to detect mismatches between lung perfusion and ventilation
Preferred test in pregnancy women with normal CXR
Excellent negative predictive value
Radiation maternal dose lower than CTPA
Fetal radiation dose, well below a level that would cause fetal abnormality
- Very slightly increased risk of childhood malignancy
Breastfeeding needs to be interrupted for 13h after VQ (discard milk)
CT PA - pros and cons
Contrast angiography of the pulmonary arterial vasculature to identify filling defects
Useful if CXR abnormal, diagnosis is in doubt by VQ scan, alternative diagnosis is in question
Detects clots in smaller vessels
Widespread availability
Radiation maternal dose higher than VQ
Lower radiation dose to the fetus, particularly earlier in pregnancy
No impact on breastfeeding
Increased blood volume and CO in pregnancy make quantifying and timing the IV contrast more difficult
Normal CT pros and cons
PROS
Rapid, accessible
Not user dependent (vs. USS)
CONS
Ionising radiation
Less tissue contrast
USS pros and cons
PROS
No ionising radiation
Portable
Readily available (usually)
CONS
Operator dependent
Gravid uterus limits visualisation
USS physics principles
Sound is produced when something vibrates –> medium around it to vibrate too –> produces a travelling longitudinal wave
Wavelength and the velocity determines the frequency of sound
Velocity = frequency x wavelength
Velocity of sound depends on the material through which the sound is travelling
When sound wave hits a surface including an interface between two types of tissue it is reflected - these echoes are used by the USS probe to locate a tissue interface and time taken for the echo to return tells us how far away the tissue interface is
