Book of Medical Disorders in Pregnancy - Tintash

that is, 1 to 10 MHZ. Usually around 2
MHZ frequency is considered safe. Sonography
has made examination of Obstetrical
patients easy, quick and safe. The
interpretation of ultrasound images (sonograms)
still requires considerable knowledge
about clinical obstetrics.
Principle - Ultrasound can be propagated
and directed as a beam. When this
beam passes through tissues at a constant
velocity" it is attenuated according to the
density of the tissue and on passing
through the boundary (interface) bet-
ween tissues of different physical
properties some of the energy is
reflected. If the beam passes through the
interface at right angle then this reflected
energy is passed back to the source and
can be recorded. The returning echoes
can be converted into electrical signals
and passed on to a cathode ray
oscillograph where an Ultra sonogram
can be pro-duced.
Modes and displays - There are several
methods of detecting and displaying the,
reflected ultrasonic information. They
include A mode, B mode, M mode, and
real time display.
A-Mode - (amplitude modulation). In
this method echoes are displayed as
vertical spikes along the baseline of the
cathode ray tube, where the height of the
spike is related to the amplitude of the
detected echo.
This is one dimensional scan and can be
used to measure a diameter whose position
is known. The sound is passed
through the fetal skull but echoes return
only if it strikes the interface (the skull
bones) at right angles. These echoes can
be displayed as a vertical deflection or
blips. Two blips can only be shown
210
simultaneously if the beam traverses the
true biparietal diameter. The distance
between the two blips can be accurately
measured to within 0.5 to 1 millimeter.
This procedure requires skill in palpation
and manipulation of the probe over the
parietal eminence. The measurement
may be erroneous if the head is deeply
engaged in the pelvis where it will be
impossible to manipulate the parietal
eminence.
This mode is quite useful in clinical
ultrasound and is used in conjunction
with the B. scan. The anatomical information
provided by a single beam of
sound is little difficult to interpret. It is
susceptible to changes produced by
slight angulation of the transducer.
Fig16.3: Shows ultrasound scan of
fetus in utero.
When the two methods are used simultaneously
B scan provides anatomical
information while the A mode is used
for the measurement of amplitude and
distance.
B-Mode (brightness modulation): In
this method the amplitude of echo is
represented by a spot of light on the
cathode ray tube. Commonly used variety
of B.scan in obstetrical practice is the
compound contact B scan. It is obtained
by moving the transducer over the
surface of the body and displaying the