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