Book of Medical Disorders in Pregnancy - Tintash

depends on the lines of sight and frame
rate.
The higher the line density per frame the
better the resolution. The time machines
a very well balanced and changeable line
density and frame rate is electronically
achieved therefore these give a very
good resolution. The type of contact 'B'
scanning described above has several
disadvantages, it is slow and the
instrument is cumbersome and expensive
and needs a skilled operator. It is
particularly difficult when applied to
moving organs. Its only advantage is a
very good resolution of the anatomical
detail in the hands of an expert operator.
To over come the difficulties
experienced with static 'B' machines real
time machines were produced, where the
transducer consists of a group of piazza
electric crystals as opposed to single
crystal used in static 'B' scanners.
Initially due to technical difficulties the
resolution of these machines was not as
good as the static 'B' scanners, but in the
newer machines this problem is no
longer there, therefore the real time
machines are gradually replacing static
'B' machines especially in obstetrics.
Time gain control - Time gain control
is the electronic compensation of tissue
attenuation, when an echo that comes
from twice the distance is given four
times the amplification and similar echo
amplitude emanates from similar reflectors
independent of the distance from the
transducer and the effects of attenuation,
it is called time gain control.
Preparation of the patient - A full
urinary bladder is essential to visualize
the pelvic organs in a non pregnant state
or in early pregnancy. This does not only
provide a sonic window but also a
212
landmark. The visualization is made
easier as the bowel is pushed away and
the uterus changes its position from a
relatively caudal to a more favorable
cranial.
Measurements - Since ultrasonographic
measurements are used for important clinical
decisions in management of the patient
< 11 measurements should be made
as accurately as possible. The student
must be aware of the Mutations imposed
by the equipment and the fundamental
physical principles involved in these
modern techniques. All measurements
made with presently available instruments
depend upon the assumption that
the velocity of sound in tissue is constant.
This is because the measuring instrument.
The cathode ray tube has a
sweep speed which is related to the
velocity of sound in tissue. The amount
of time required for a signal to return to
the transducer is thus interpreted by the
instrument as a distance. Instruments
which are used in
North America takes as their standard, a
velocity of 1540 meters per sec. This
velocity is not correct for certain tissues,
in particular the fetal skull. It also does
not help to make an arbitrary change in
the velocity calibration since the exact
change one should make is not precisely
known. The thickness and degree of
calcification of the fetal skull does not
remain constant throughout pregnancy
therefore very little can be done in
improving the accuracy. The accuracy of
measurement also depends upon the
depth resolution and lateral resolution of
the instrument.
In all cases lateral resolution is superior
to depth resolution. The wave from
which it is produced by the transducer