Ultrasound Scans: How Do They Work?

Ultrasound Scans: How Do They Work?


MNT Knowledge Center

An ultrasound scan, also referred to as a sonogram, diagnostic sonography, and ultrasonography, is a device that uses high frequency sound waves to create an image of some part of the inside of the body, such as the stomach, liver, heart, tendons, muscles, joints and blood vessels.

Experts say that as sound waves, rather than radiation are used, ultrasound scans are safe. Obstetric sonography is frequently used to check the baby in the womb.

This article looks specifically at ultrasound scans. We also have articles in our knowledge center about CT scans, MRI scans and PET scans.

Ultrasound scans are used to detect problems in the liver, heart, kidney or the abdomen. They may also be useful in helping the surgeon when carrying out some types of biopsies.

The word “ultrasound”, in physics, refers to all sound with a frequency humans cannot hear. In diagnostic sonography, the ultrasound is usually between 2 and 18 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 can penetrate deeper, but the image quality is inferior.

According to MediLexicon’s medical dictionary:

Diagnostic ultrasound is: “the use of ultrasound to obtain images for medical diagnostic purposes, employing frequencies ranging from 1.6 to about 10 MHz.”

Therapeutic ultrasound is: “high intensity ultrasound causing coagulation necrosis of tissue, used in treatment of some benign tumors, such as uterine leiomyomata.”

Ultrasound scans and sonograms

The difference between an ultrasound scan and a sonogram is:

  • An ultrasound scan is the procedure, the event
  • A sonogram is the image produced when an ultrasound scan is performed

How do ultrasound scans work?

Although ultrasound travels through soft tissue and fluids, it bounces back off denser surfaces. Ultrasound will travel through blood, for example in the heart chamber, but much of it will echo (bounce back) when hitting a heart valve.

If there are no solid gallstones in the gallbladder, ultrasound will travel straight through, but when there are stones, ultrasound will bounce back from them.

The denser the object the ultrasound hits, the more of it bounces back.

The bouncing back, or echo, is what gives the ultrasound image its features – varying shades of gray reflect different densities.

What can ultrasound scans be used for?

Ultrasound probe transducer being used on pregnant woman

Ultrasound is commonly used in medicine today. Health care professionals can use sonography for either diagnosis or treatment (therapeutic procedures), as well as for guidance during procedures that require intervention, such as biopsies.

A medical professional who performs ultrasound scans is called a Sonographer. Scans, or images are then interpreted by radiologists, cardiologists, or other medical specialists. The sonographer usually holds a transducer; a hand-held device which is placed on the skin of the patient.

Below are examples of medical sonography:

Ultrasound in anesthesiology

Ultrasound is often used by anesthetists for guidance (accuracy) when injecting needles with anesthetic solutions near nerves.


Used in echocardiography, also known as cardiac ultrasound. Two-dimensional slices of the heart are imaged. Modern devices can produce 3-dimensional images.

As well as creating images of the cardiovascular system, echocardiograms can accurately assess the speed of blood flow and cardiac tissue at specific points using pulses or continuous wave Doppler ultrasound. The health care professional can assess the function and state of cardiac valve areas, abnormalities between the left and right side of the heart, valvular regurgitation (blood leaking from valves), as well as working out how well the heart pumps out blood.

Arterial sonography can also be used to assess the patency and possible blockage of arteries, as well as diagnosing deep vein thrombosis (DVT). Venosonography may help determine how severe a case of venous insufficiency might be.

Ultrasound in emergency medicine

The use of ultrasound in emergency medicine has grown considerably over the last two decades. In fact, for emergency medicine ultrasound training has become increasingly popular.

Today, in emergency medicine, ultrasound is used in the FAST (Focused Assessment with Sonography for Trauma) for assessing trauma, pericardial tamponade (fluid builds up in the sac in which the heart is enclosed; the pericardium) or hemoperitoneum (blood in the peritoneal cavity).

Sonography is also used to speed up care for patients with suspected gallstones or inflammation of the gallbladder (cholecystitis). These patients usually come in with abdominal pain in the right upper quadrant.

Abdominal Sonography (gastroenterology)

Using ultrasound, the healthcare professional is able to see images of the spleen, kidneys, bile ducts, gall bladder, liver, aorta, inferior vena cava, pancreas and other solid organs located in the abdomen. If the appendix is swollen, as may be the case with appendicitis, it can sometimes show up in the imaging. Sonographers say that certain quantities of fat and gas in the bowel can sometimes block the ultrasound waves, making diagnosis more difficult.

Newborn infants (neonatology)

The Sonographer can perform an ultrasound scan on an infant by placing the probe in the newborn’s fontanelle (soft spot in the skull) to check for abnormalities in the brain, hydrocephalus and preiventricular leukomalacia (a form of white-matter brain injury). As the Fontanelle gets smaller in time, the quality of the images becomes poorer.

Ultrasound and neurology

Ultrasound may be used to measure blood flow in the carotid arteries. Known as carotid Ultrasonography, the scan looks out for blood clots and atherosclerotic plaque build-up. A carotid duplex is a form of carotid ultrasonopgraphy using Duplex ultrasonography, which may include a Doppler ultrasound – a test which can reveal how blood cells move through the carotid arteries.

Obstetric Ultrasonography

Ultrasound is used to create images of the fetus or embryo in the uterus. Today, often referred to simply as an ultrasound scan, it is part of standard prenatal care. Obstetric Ultrasonography can reveal various aspects of the fetus’ health, as well as the mother’s. It can also help the health care professional assess the progress of the pregnancy.

Ultrasound of a fetus at 22 weeks (left) and of the feet of a fetus at 18 weeks (right)Photo left: ultrasound of a fetus at 22 weeks.
Photo right: little big feet in the uterus at 18 Weeks.

The probe, or transducer is typically placed on the mother’s abdomen, but may also be placed in her vagina – transvaginal Sonography. A transvaginal scan gives a clearer picture during early pregnancy, and may be a better option for obese mothers. A Doppler Sonography shows the fetus’ heartbeat, and can help the doctor detect signs of abnormalities in the heart and blood vessels.

Ultrasound and urology

Ultrasound is used in urology for many purposes, such as checking how much urine remains in the patient’s bladder after going to the toilet. Organs in the pelvic region can be checked, including the uterus, testicles (to tell testicular torsion from epididymitis). In young adult male patients, ultrasound is sometimes used to distinguish hydrocele or varicocele from testicular cancer. Testicular cancer, even though highly curable, must be treated in order to preserve the man’s fertility and overall health.

Pelvic sonographies can be carried out internally or externally. In a male the internal sonogram may be inserted transrectally, while in a female transvaginally. Ultrasound scans of the pelvic floor can help the doctor determine the extent of, for example, a pelvic prolapse, incontinence or obstructed defecation. At higher frequencies, ultrasound can be used to break up kidney stones or crystals (nephrolithiasis).

Musculoskeletal sonography

Ultrasound can be used to examine ligaments, bone surfaces, soft tissue masses, nerves, muscles and tendons.

Doppler ultrasound

Doppler ultrasound testA doppler ultrasound test on a woman’s carotid artery.

Doppler ultrasound is a type of ultrasound that depends on the Doppler Effect, a change in the wave’s frequency, which can occur from the motion of a reflector, such a red blood cell.

We experience the Doppler Effect when, for example, an ambulance approaches us, passes and then drives away. The received frequency is higher than the emitted frequency during the ambulance’s approach, is identical when it is passing directly by us, and lower when it has passed and is moving away.

The Doppler Effect, or Doppler Shift was named after Christian Doppler (1803 – 1853), an Austrian mathematician and physicist.

Below are some examples of Doppler Ultrasound:

Color doppler ultrasound testA color doppler echocardiogram (cardiac ultrasound)

  • Color Doppler – the blood flow’s average velocity is estimated via a system of color coding the data. Blood flow direction is assigned the color blue or red, one color indicates whether blood is moving towards and the other away from the ultrasound transducer.
  • Pulsed Doppler – this method allows the health care professional to see a full range of blood velocities within a sampling volume (gate). In this method, shades of gray determine how many red blood cells there are.
  • Power Doppler – the power of Doppler signals is depicted, instead of the frequency shift. Small vessels can be better visualized, at the expense of blood velocity and directional data.

Put simply, Doppler ultrasound is used to determine how well blood is flowing in a vessel – this includes determining blood velocity and whether there are any obstructions, and how badly they affect blood flow.

Doppler ultrasound can be used to check the fetus’ heart and heart beat.

On the next page we look at ultrasound transducers, ultrasound’s use in treatment, how you prepare for an ultrasound scan and the scan processes themselves.


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Ultrasound transducers

In the majority of cases, a transducer is used that is placed on the surface of the patient’s body. However, there are transducers that are placed internally – these tend to give better and more informative images.

The following ultrasound transducers can be used for internal use:

  • An endovaginal transducer – it goes in through the vagina
  • An endorectal transducer – it goes in through the rectum
  • A transesophageal transducer – it goes through the esophagus (down the patient’s throat)

Some tiny transducers can be placed onto the end of catheters and inserted into blood vessels so that images of vessel walls can be examined.

Ultrasound’s use in treatment

Ultrasound can be used for therapy by bringing agitation or heat to the targeted area in the body. In such cases, higher energy levels are required than are needed for ultrasound scans. Frequency varies considerably, depending on what kind of treatment is needed.

Ultrasound can also be used by dentists to clean teeth.

Ultrasound has been used in cancer treatment, physical therapy and occupational therapy. As far as musculoskeletal therapies are concerned, ultrasound therapy has had mixed results.

Focused Ultrasound Surgery (FUS) or High Intensity Focused Ultrasound (HIFU) can generate localized heat to treat tumors and cysts. Such ultrasound therapies use much lower frequencies than those used for imaging scans, although energy levels are much higher.

Lithotripsy can be used to break up kidney stones.

Cataracts can be treated via phacoemulsification.

Low-intensity ultrasound may help stimulate bone-growth, as well as helping drugs pass the blood-brain barrier.

Preparing for an ultrasound scan

In most cases, no special preparation is needed before going for a routine ultrasound scan. Experts advise patients to wear loose-fitting and comfortable clothing.

If the doctor wants to check your liver or gallbladder, you will be usually asked to eat nothing (fast) for several hours before the procedure.

If you are going for a scan during pregnancy, especially early pregnancy, the doctor or nurse will ask you to drink several glasses of water and not go to the toilet (no pass urine) for several hours before the test. When the bladder is full the nearby loops of the intestine are displaced, and the scan produces a better image of the uterus.

Ultrasound scan process

The majority of scans take between 15 to 45 minutes and will usually occur in the X-ray department in a hospital. Many doctors’ own practices today have ultrasound devices. The test is performed either by a doctor or a sonographer.

In most countries, the sonographer has specialized training in viewing, analyzing and modifying the scan to optimize the data in the image. Sonographers need to understand the physics involved in ultrasound, as well as having good knowledge cross sectional anatomy, pathology, and physiology.

  • External ultrasound – an example is when the doctor wants to examine the patient’s heart, or the fetus in the uterus. The sonographer places lubricating gel onto the patient’s skin and then places a transducer over the lubricated skin. The transducer is moved over the part of the body that needs to be examined.

    The patient should feel no discomfort, and only the transducer over the skin. Pregnant women may feel slight discomfort because they have to come in with a full bladder.

  • Internal ultrasound – internal ultrasounds may be used when the doctor wants to have a better look at the prostate gland, ovaries or uterus. The probe is placed into the vagina (in females) or the rectum (in males).

  • Endoscopic ultrasound – an endoscope is inserted into the patient’s body, usually through his/her mouth. This type of scan is used when the doctor wants to have a better look at the esophagus, chest lymph nodes or the stomach.

    There is a light as well as an ultrasound device fitted to the end of the endoscope. Patients are given painkilling medications, and/or sedatives before the procedure begins.

There is more discomfort with internal ultrasound scans compared to external ones, as well as a slight risk of internal bleeding.

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