How do ultrasound scans work?
Ultrasound scans, or sonography, are safe because they use sound waves or echoes to make an image, instead of radiation.
Ultrasound scans are used to evaluate fetal development, and they can detect problems in the liver, heart, kidney, or abdomen. They may also assist in performing certain types of biopsy.
The image produced is called a sonogram.
Here are some key points about ultrasound scans. More detail is in the main article.
- Ultrasound scans are safe and widely used.
- They are often used to check the progress of a pregnancy.
- They are used for diagnosis or treatment.
- No special preparation is normally necessary before an ultrasound scan.
Ultrasound scans are carried out by a sonographer.
The person who performs an ultrasound scan is called a sonographer, but the images are interpreted by radiologists, cardiologists, or other specialists.
The sonographer usually holds a transducer, a hand-held device, like a wand, which is placed on the patient's skin.
Ultrasound is sound that travels through soft tissue and fluids, but it bounces back, or echoes, off denser surfaces. This is how it creates an image.
The term "ultrasound" refers to sound with a frequency that humans cannot hear.
For diagnostic uses, the ultrasound is usually between 2 and 18 megahertz (MHz).
Higher frequencies provide better quality images but are more readily absorbed by the skin and other tissue, so they cannot penetrate as deeply as lower frequencies.
Lower frequencies penetrate deeper, but the image quality is inferior.
How does it capture an image?
Ultrasound will travel through blood in the heart chamber, for example, but if it hits a heart valve, it will echo, or bounce back.
The denser the object the ultrasound hits, the more of the ultrasound bounces back.
This bouncing back, or echo, gives the ultrasound image its features. Varying shades of gray reflect different densities.
The transducer, or wand, is normally placed on the surface of the patient's body, but some kinds are placed internally.
These can provide clearer, more informative images.
- an endovaginal transducer, for use in the vagina
- an endorectal transducer, for use in the rectum
- a transesophageal transducer, passed down the patient's throat for use in the esophagus
Some very small transducers can be placed onto the end of a catheter and inserted into blood vessels to examine the walls of blood vessels.
Ultrasound images are made from reflected sound, and a diagnosis can then be made.
Ultrasound is commonly used for diagnosis, for treatment, and for guidance during procedures such as biopsies.
It can be used to examine internal organs such as the liver and kidneys, the pancreas, the thyroid gland, the testes and the ovaries, and others.
Doppler ultrasound can assess the flow of blood in a vessel or blood pressure. It can determine the speed of the blood flow and any obstructions.
An echocardiogram (ECG) is an example of Doppler ultrasound. It can be used to create images of the cardiovascular system and to measure blood flow and cardiac tissue movement at specific points.
A Doppler ultrasound can assess the function and state of cardiac valve areas, any abnormalities in the heart, valvular regurgitation, or blood leaking from valves, and it can show how well the heart pumps out blood.
It can also be used to:
- examine the walls of blood vessels
- check for DVT or an aneurysm
- check fetal heart and heartbeat
- evaluate for plaque buildup and clots
- assess for blockages or narrowing of arteries
A carotid duplex is a form of carotid ultrasonography that may include a Doppler ultrasound. This would reveal how blood cells move through the carotid arteries.
Ultrasound in anesthesiology
Ultrasound is often used by anesthetists to guide a needle with anesthetic solutions near nerves.
Ultrasound in emergency medicine
Ultrasound is commonly used in emergency medicine to assess various conditions, including:
- traumatic injuries
- pericardial tamponade
- fluid buildup around the heart
- hemoperitoneum, or blood leakage in the abdomen
Gastroenterologists use ultrasound to generate images of the spleen, kidneys, bile ducts, gall bladder, liver, aorta, inferior vena cava, pancreas, and other solid organs located in the abdomen.
It can evaluate patients for suspected gallstones or inflammation of the gallbladder, known as cholecystitis.
It can detect if the appendix is swollen or inflamed, which would suggest appendicitis. Blood work would confirm an infection.
Fat and gas in the bowel can sometimes block the ultrasound waves, making diagnosis more difficult.
The sonographer can perform an ultrasound scan on a newborn by placing the probe on the fontanelle, the soft spot on the top of the skull.
This can check for abnormalities in the brain, hydrocephalus, and periventricular leukomalacia, a form of white-matter brain injury.
As the fontanelle grows smaller in time, the quality of the images becomes poorer.
Ultrasound devices emit a high-frequency sound from their wand and can be used to give an image of the inside of a person's body, for example, during pregnancy.
Ultrasound is part of standard prenatal care. It gathers images of the fetus or embryo in the uterus.
Obstetric ultrasonography can reveal various aspects of both fetal and maternal health. It can also help doctors assess the progress of the pregnancy.
The probe or transducer is typically placed on the mother's abdomen, but sometimes it is placed in the vagina.
A transvaginal scan can provide a clearer picture during early pregnancy, and it may be a better option if the mother has obesity.
Doppler sonography shows the fetal heartbeat. It can help the doctor detect signs of abnormalities in the heart and blood vessels.
Ultrasound and urology
In urology, ultrasound can check:
- how much urine remains in the bladder after urinating
- the health of the organs in the pelvic region, including the uterus and testicles
In young, adult males, ultrasound can distinguish different types of swelling from testicular cancer.
Pelvic sonographies can be internal or external.
In a male, the internal sonogram may be inserted into the rectum. In a female, it might be inserted into the vagina.
This can provide information about the prostate gland, ovaries, or uterus.
Ultrasound scans of the pelvic floor can help the doctor determine the extent of, for example, a pelvic prolapse, incontinence, or obstructed defecation.
Ultrasound can be used to examine ligaments, bone surfaces, soft tissue masses, nerves, muscles, and tendons.
What to expect
An ultrasound can be done at a doctor's office, at an outpatient clinic, or in the hospital.
Most scans take between 20 and 60 minutes. It is not normally painful, and there is no noise.
In most cases, no special preparation is needed, but patients may wish to wear loose-fitting and comfortable clothing.
If the liver or gallbladder is affected, the patient may have to fast, or eat nothing, for several hours before the procedure.
For a scan during pregnancy, and especially early pregnancy, the patient should drink plenty of water and try to avoid urinating for some time before the test.
When the bladder is full, the scan produces a better image of the uterus.
The scan usually takes place in the radiology department of a hospital. A doctor or a specially-trained sonographer will carry out the test.
The sonographer puts a lubricating gel onto the patient's skin and places a transducer over the lubricated skin.
The transducer is moved over the part of the body that needs to be examined. Examples include ultrasound examinations of a patient's heart or a fetus in the uterus.
The patient should not feel discomfort or pain. They will just feel the transducer over the skin.
During pregnancy, there may be slight discomfort because of the full bladder.
If the internal reproductive organs or urinary system need to be evaluated, the transducer may be placed in the rectum for a man or in the vagina for a woman.
To evaluate some part of the digestive system, for example, the esophagus, the chest lymph nodes, or the stomach, an endoscope may be used.
A light and an ultrasound device are attached to the end of the endoscope, which inserted into the patient's body, usually through the mouth.
Before the procedure, patients are given medications to reduce any pain.
Internal ultrasound scans are less comfortable than external ones, and there is a slight risk of internal bleeding.
Most types of ultrasound are noninvasive, and they involve no ionizing radiation exposure. The procedure is believed to be very safe.
However, since the long-term risks are not established, unnecessary "keepsake" scans during pregnancy are not encouraged. Ultrasound during pregnancy is recommended only when medically needed.
Anyone who is allergic to latex should inform their doctor so that they will not use a latex-covered probe.