ULTRASOUND. Flashcards
Define ultrasound scanning
Ultrasound involves exposing part of the body to high frequency sounds waves to produce pictures of the inside of the body
What kind of waves does US create?
SONAR
What is the range of US probe frequencies used in medical diagnostics?
2mHz - 10mHz
How does US work?
US waves are created by passing an electric current through a crystal (contained within a ceramic probe), causing it to vibrate.
The US waves then travel through bodily tissues where they are partially reflected at each tissue interface.
Each returning reflection hits the crystal and causes it to vibrate creating a current.
Echoes have discrete amplitudes which are assigned a specific brightness (using a finite gray scale) and subsequent location on the screen – the image on the screen is a plot of all the reflection returned
How frequently is an US plot refreshed?
25-50 times a second
Probe frequency determines…?
Optimal depth of penetration Higher frequency (>5.0mHz) have less depth of penetration but provide a higher resolution – good for vascular structures Lower frequency (<5.0mHZ) are good for deeper depths – good for visualising the abdomen and heart
What causes different brightness levels in US scanning?
Time taken for the wave to return to the probe and the direction from which it came from
What do the following colours represent on US?
White
Black
Grey
White: soft tissue (echogenic structures) – muscle, fat, vessels, nodes, masses
Black: fluid (anechoic structures) – blood, CSF
Grey: somewhere in between
When would the use of a transvaginal probe be indicated?
Investigating infertility disorders
Diagnosis of ectopic pregnancy
Differentiation of normal and abnormal pregnancies in the 1st trimester
Diagnosis of congenital abnormalities in the 2nd trimester
Name 3 advantages of TVUS
Specially designed high frequency transducers
Higher resolution images
Favorable for obese patients in the early stages of pregnancy
Name 2 disadvantages of TVUS
More invasive procedure
Reduced beam penetration
At what point can the following structures be expected to be seen?
Gestational sac
Yolk sac
Embryo
Gestational sac seen at 4 weeks – fluid filled with an echogenic border
Yolk sac seen at 33 days
Embryonic echoes seen at 38 days
How much daily growth does a gestational sac undergo?
0.6mm
In a normal pregnancy at what point should the gestational sac be visible on US?
If 25mm or larger in diameter
How is gestational age calculated using crown rump lenght?
CRL + 6.5 = gestational age in weeks
What tests are used to screen for T21 (between 11-13+6 weeks)? What is the sensitivity of these tests? What is the false positive rate?
Fetal nuchal translucency
b-hCG
PAPPA
60-85% sensitivity
5% false positive rate
What is considered an abnormal nuchal translucency measurement?
> 3.5mm
What two criteria must be fulfilled when calculated fetal nuchal translucency?
The fetus must be in a neutral position
The measurement must be taken in a sagittal view
What is the change of having a pregnancy affected by T21 at the following ages:
20 years
30 years
40 years
20 years: 1 in 1500
30 years: 1 in 900
40 years: 1 in 100
What is a normal cervix length from 10-36 weeks gestation?
> 2.5cm or more
What should the width of the cervical canal be at the level of the internal os?
<4mm
Define the following views in US scaning:
Transverse
Longitudinal
Transverse: perpendicular to long axis of patient
Longitudinal: parallel to long axis of patient
Which 3 structures need to be included when measuring bi-parietal diameter?
Cavum septum pellucidum
Thalamus
Choroid plexus
What is the most sensitive marker of macrosomnia / IUGR?
Abdominal circumference
