| Ultrasound - Obstetric
What is Obstetric Ultrasound Imaging? Back to Top
Ultrasound
imaging, also called ultrasound scanning or sonography, is a method of
obtaining images of internal organs by sending high-frequency sound
waves into the body. The sound wave echoes are recorded and displayed
as a real-time visual image. No ionizing radiation (x-ray) is involved
in ultrasound imaging. Obstetric ultrasound refers to the specialized
use of sound waves to visualize and thus determine the condition of a
pregnant woman and her embryo or fetus.
What are some common uses of the procedure? Back to Top
Obstetric ultrasound should be performed only when clinically indicated. Some indications may be:
- To establish the presence of a living embryo/fetus.
- To estimate the age of the pregnancy.
- To diagnose congenital abnormalities.
- To evaluate the position of the fetus.
- To evaluate the position of the placenta.
- To determine if there are multiple pregnancies.
- To determine the amount of amniotic fluid around the fetus.
- To check for opening or shortening of the cervix or mouth of the womb.
How should I prepare for the procedure? Back to Top
You
should wear a loose-fitting two-piece outfit for the examination. Only
the lower abdominal area needs to be exposed during this procedure;
consequently, a two-piece outfit will prevent you from having to
readjust or remove all of your clothing.
If an ultrasound is ordered by your clinician early in your
pregnancy, you may be instructed to have a full bladder for the
procedure. Air interferes with sound waves, so if your bladder is
distended, the air-filled bowel is pushed out of the way and an image
of the uterus and embryo or fetus is obtained. About an hour before the
procedure you should empty your bladder. You may be instructed to drink
up to six glasses of water and avoid urinating until the procedure is
completed. A full bladder is not necessary in most cases, especially
after the first two to three months of pregnancy. The radiologist or
sonographer may elect to examine an early pregnancy by means of
transvaginal ultrasound. This requires an empty urinary bladder. You
should ask for instructions when you make your appointment.
What does the equipment look like? Back to Top
The equipment
consists of a transducer connected to a monitoring system by a long
cord. The transducer is a small hand-held device that resembles a
microphone. The radiologist or sonographer spreads a lubricating gel on
the area being examined and then presses this device firmly against the
skin.
The ultrasound image is immediately visible on a nearby screen that
looks much like a computer or television monitor. The radiologist or
sonographer watches this screen during an examination and captures
representative images for storage. Often the patient is able to see it
as well.
How does the procedure work? Back to Top
Ultrasound imaging
is based on the same principles involved in the sonar used by bats,
ships at sea and anglers with fish detectors. As a controlled sound
bounces against objects, its echoing waves can be used to identify how
far away the object is, how large it is, its shape and its internal
consistency (fluid, solid or mixed).
The ultrasound transducer functions as both a loudspeaker (to create
the sounds) and a microphone (to record them). For obstetric
ultrasound, when the transducer is pressed against the skin, it directs
a stream of inaudible, high-frequency sound waves into the lower
abdomen and pelvis. As the sound waves echo from the embryo or fetus
and surrounding structures in the uterus, the sensitive microphone in
the transducer records tiny changes in the sound's pitch and direction.
These signature waves are instantly measured and displayed by a
computer, which in turn creates a real-time picture on the monitor. The
live images of the examination can be recorded on videotape. In
addition, still frames of the moving picture are usually "frozen" to
capture a series of images. Conventional ultrasound displays the images
as thin sections (like looking at single slices of bread in a loaf).
3-D ultrasound is the result of modern computer technology that can
reformat data into three-dimensional images (like looking at the entire
loaf of bread from various projections). 4-D ultrasound is 3-D
ultrasound in motion.
Doppler ultrasonography is the application of diagnostic ultrasound
to detect moving blood cells and measure their direction and speed of
movement. The Doppler effect is used to evaluate blood flow by
measuring changes in the frequency of the echoes reflected from blood
cells.
The movement of the embryo or fetus and the heart beat can be seen
as an ongoing ultrasound "movie." Most ultrasound devices also have an
audio component that processes the echoes produced by blood flowing
through the fetal heart, blood vessels and umbilical cord. This sound
can be made audible to human ears and has been described by patients as
a "whooshing" noise.
How is the procedure performed? Back to Top
You will be
asked to lie on your back or side. You will also be asked to expose
your lower abdominal area. The sonographer or radiologist then spreads
a warm water-soluble gel over your lower abdomen. This gel allows
better transmission of the sound waves by making it easier to move the
transducer over your abdomen and by sending the sound beam directly
into the body without the interference from even a tiny amount of air
on the skin. The transducer emits high-frequency sound waves as the
sonographer or radiologist moves it over your abdomen. The transducer
also detects the echoes that bounce off anatomic structures as
reflections. Sometimes the radiologist determines that a transvaginal
scan will need to be performed. Instead of a transducer being moved
over your abdomen, the high-frequency waves will be emitted by a
transducer placed in the vagina. This technique often provides
improved, more detailed images of the uterus and ovaries. It is
especially useful in early pregnancy. With this approach the urinary
bladder needs to be empty. Shown is an example of a transvaginal
transducer. Only two to three inches of the transducer are inserted
into the vagina. The rest of the transducer is a handle for use by the
operator.
The obstetric ultrasound examination takes about 20 minutes.
What will I experience during the procedure? Back to Top
This
is a painless procedure. There may be varying degrees of discomfort
from pressure as the sonographer or radiologist guides the transducer
over your abdomen, especially if you are required to have a full
bladder. At times the sonographer may have to press more firmly to get
closer to the embryo or fetus to better visualize the structure. This
discomfort is temporary. Also, you may dislike the feeling of the
water-soluble gel applied to your abdomen. With transvaginal scanning,
there may be minimal discomfort as the transducer is moved in the
vagina, especially when the bladder begins to refill.
Who interprets the results and how do I get them? Back to Top
A
radiologist, who is a physician experienced in obstetric ultrasound and
other radiology examinations, will analyze the images and send a signed
report with his or her interpretation to the patient's personal
physician. The patient receives ultrasound results from the referring
physician who ordered the test. Digital technology also allows for
distribution of diagnostic reports and images over the Internet at many
facilities.
What are the benefits vs. risks? Back to Top
Benefits
- Ultrasound does not use x-rays to produce an image—neither the mother nor her unborn child is exposed to ionizing radiation.
- Ultrasound
has been used to evaluate pregnancy for nearly four decades and there
has been no evidence of harm to the patient, embryo or fetus.
Nevertheless, ultrasound should be performed only when clinically
indicated.
- Ultrasound allows the doctor to see inside the uterus and provides much information about the pregnancy.
Risks
- For standard diagnostic ultrasound there are no known harmful effects to humans.
What are the limitations of Obstetric Ultrasound Imaging? Back to Top
Obstetric
ultrasound cannot identify all fetal abnormalities. Consequently, when
there are clinical or laboratory suspicions for a possible abnormality,
a pregnant woman may have to undergo nonradiologic, invasive testing
such as amniocentesis (the evaluation of fluid taken from the sac
surrounding the fetus) or chorionic villus sampling (evaluation of
placental tissue) to determine the health of the fetus, or she may be
referred by her primary care provider to a perinatologist (an
obstetrician specializing in high-risk pregnancies). Back to Top |
