At a glance......
- 1 Types of Ultrasound
- 2 According to the imaging types of Ultrasound
- 3 According to the Body Position of Ultrasound
- 4 What are some common uses of the procedure?
- 5 Modes of Ultrasound
- 6 Contrast Ultrasonography (Ultrasound Contrast Imaging)
- 7 Molecular Ultrasonography (Ultrasound Molecular Imaging)
- 8 Elastography (Ultrasound Elasticity Imaging)
- 9 Interventional Ultrasonography
- 10 Medical Uses of Ultrasound
- 11 Risks and side-effects
- 12 Studies on the safety of ultrasound
- 13 References
Ultrasound (also known as diagnostic sonography or ultrasonography) is a diagnostic imaging technique based on the application of ultrasound. It is used to see internal body structures such as tendons, muscles, joints, blood vessels, and internal organs. Its aim is often to find a source of a disease or to exclude any pathology. The practice of examining pregnant women using ultrasound is called obstetric ultrasound and is widely used. Ultrasound is sound waves with frequencies which are higher than those audible to humans (>20,000 Hz). Ultrasonic images, also known as sonograms, are made by sending pulses of ultrasound into tissue using a probe. The sound echoes off the tissue; with different tissues reflecting varying degrees of sound. These echoes are recorded and displayed as an image to the operator.
Types of Ultrasound
Most ultrasounds are done using a transducer on the surface of the skin. Sometimes, however, doctors and technicians can get a better diagnostic image by inserting a special transducer into one of the body’s natural openings:
- In a transvaginal ultrasound, a transducer wand is placed in a woman’s vagina to get better images of her uterus and ovaries.
- A transrectal ultrasound is sometimes used in the diagnosis of prostate conditions.
- A transesophageal echocardiogram uses the transducer probe in the esophagus so that the sonographer can obtain clearer images of the heart.
Additionally, ultrasound technology has advanced to allow for different types of imaging:
- Doppler is a special type of ultrasound that creates images of blood flow through vessels.
- Bone sonography helps doctors diagnose osteoporosis.
- Echocardiograms are used to view the heart.
- 3D imaging adds another dimension to the ultrasound image, creating three-dimensional interpretations rather than the flat two-dimensional images that are made with traditional ultrasound.
- 4D ultrasounds show 3D images in motion.
The twelve major types of ultrasound procedures described in other articles, make use of four kinds of the available ultrasound image. The choice of which type of image to use depends on the goals for a particular test, the phenomena being investigated and what equipment is available.
The most common and type of ultrasound picture is a series of flat, two-dimensional cross-section images of the scanned tissue. Referred to simply as 2d ultrasound, this mode of scanning is still standard for many diagnostic and obstetric situations after a half-century of use.
In recent years, 2d ultrasound images have also been projected into three-dimensional representations. This is achieved by scanning tissue cross sections at many different angles and reconstructing the data received into a three-dimensional image. A common use for 3d ultrasound pictures is to provide a more complete and realistic image of a developing fetus.
4D Ultrasound Imaging
By updating 3d ultrasound images in rapid succession, sonographers can also create 4d ultrasound pictures. In the 4d ultrasound, the fourth dimension, time, adds movement and creates the most realistic representation of all.
Evaluating blood flow as it moves through blood vessels is a common component of many of the types of ultrasound. While traditional 2d ultrasound and its three-dimensional offshoot show internal tissues and structures, a different kind of ultrasound is required to evaluate blood flow and pressure within a blood vessel.
Doppler ultrasonography employs the Doppler effect to assess whether structures (usually blood) are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example, flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualized.
Color Doppler is the measurement of velocity by a color scale. Color Doppler images are generally combined with grayscale (B-mode) images to display duplex ultrasonography images. Uses include
- Doppler echocardiography – the use of Doppler ultrasonography to examine the heart. An echocardiogram can, within certain limits, produce an accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. Velocity measurements allow assessment of cardiac valve areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves (valvular regurgitation), calculation of the cardiac output and calculation of E/A ratio (a measure of diastolic dysfunction). Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.
- Transcranial Doppler (TCD) and transcranial color Doppler (TCCD) – which measure the velocity of blood flow through the brain’s blood vessels transcranially (through the cranium). They are used as tests to help diagnose emboli, stenosis, vasospasm from a subarachnoid hemorrhage (bleeding from a ruptured aneurysm), and other problems.
- Doppler fetal monitors – although usually not technically -graph but rather sound-generating, use the Doppler effect to detect the fetal heartbeat for prenatal care. These are hand-held, and some models also display the heart rate in beats per minute (BPM). Use of this monitor is sometimes known as Doppler auscultation. The Doppler fetal monitor is commonly referred to simply as a Doppler or fetal Doppler. Doppler fetal monitors provide information about the fetus similar to that provided by a fetal stethoscope.
According to the Body Position of Ultrasound
Abdominal Ultrasound Imaging
An abdominal ultrasound is a useful way of examining internal organs, including the liver, gallbladder, spleen, pancreas, kidneys, and bladder. Because US images are captured in real time, they can show the movement of internal tissues and organs and enable physicians to see blood flow. This can help to diagnose a variety of conditions and to assess the damage caused by illness.
Carotid and Abdominal Aorta Ultrasound Imaging
Ultrasound of the carotid arterial system provides a fast, noninvasive means of identifying blockages of blood flow in the neck arteries to the brain that might produce a stroke or mini-stroke. Ultrasound of the abdominal aorta is primarily used to evaluate for an aneurysm which is an abnormal enlargement of the aorta usually from atherosclerotic disease.
Obstetric 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.
Obstetric ultrasound should be performed only when clinically indicated.
The prostate gland is located directly in front of the rectum, so the ultrasound exam is performed transrectally. A protective cover is placed over the transducer, lubricated, and then placed into the rectum so the sound need only travel a short distance. The images are obtained from different orientations to get the best view of the prostate gland. Ultrasound of the prostate is most often performed with the patient lying with his left side down on the table and with his knees bent up slightly toward the chest.
For the transabdominal approach, the patient has a full urinary bladder and is positioned on an examination table. A clear gel is applied to the lower abdomen to help the transducer make secure contact with the skin. The sound waves produced by the transducer cannot penetrate air, so the gel helps to eliminate air pockets between the transducer and the skin. With transabdominal ultrasound, you will lie on your back on an examining table. The sonographer will spread some gel on your skin and then presses the transducer firmly against the skin and sweeps it back and forth to image the pelvic organs. Doppler sonography can be performed through the same transducer. There may be varying degrees of discomfort from pressure as the transducer is moved over your abdomen, especially if you are required to have a full bladder.
Transvaginal ultrasound involves the insertion of the transducer into the vagina after the patient empties her bladder and is performed very much like a gynecologic exam. The tip of the transducer is smaller than the standard speculum used when performing a Pap test. A protective cover is placed over the transducer, lubricated with a small amount of gel, and then inserted into the vagina. Only two to three inches of the transducer end is inserted into the vagina.
Pelvic Ultrasound Imaging
Pelvic ultrasound is most often used to examine the uterus and ovaries and, during pregnancy, to monitor the health and development of the embryo or fetus. In men, a pelvic ultrasound usually focuses on the bladder and the prostate gland.
Millions of expectant parents have seen the first “picture” of their unborn child thanks to pelvic ultrasound examinations of the uterus and fetus (see the Ultrasound-Obstetric page).
What are some common uses of the procedure?
Ultrasound examinations can help to diagnose a variety of conditions and to assess organ damage following an illness.
Ultrasound is used to help physicians evaluate symptoms such as
Ultrasound is a useful way of examining many of the body’s internal organs, including but not limited to the
- Heart and blood vessels, including the abdominal aorta and its major branches
- Uterus, ovaries, and unborn child (fetus) in pregnant patients
- Thyroid and parathyroid glands
- Scrotum (testicles)
- Brain in infants
- Hips in infants
- Spine in infants
Ultrasound is also used to
- Guide procedures such as needle biopsies, in which needles are used to sample cells from an abnormal area for laboratory testing.
- Image the breasts and guide biopsy of breast cancer (see the Ultrasound-Guided Breast Biopsy page.
- Diagnose a variety of heart conditions, including valve problems and congestive heart failure, and to assess damage after a heart attack. Ultrasound of the heart is commonly called an “echocardiogram” or “echo” for short.
Doppler ultrasound images can help the physician to see and evaluate
- Blockages to blood flow (such as clots
- Narrowing of vessels
- Tumors and congenital vascular malformations
- Reduced or absent blood flow to various organs
- Greater than normal blood flow to different areas, which is sometimes seen in infections
With knowledge about the speed and volume of blood flow gained from a Doppler ultrasound image, the physician can often determine whether a patient is a good candidate for a procedure like angioplasty..
An ultrasound exam may be performed throughout pregnancy for the following medically-necessary reasons
- Confirm viable pregnancy
- Confirm heartbeat
- Measure the crown-rump length or gestational age
- Confirm molar or ectopic pregnancies
- Assess abnormal gestation
- Diagnose fetal malformation
- Weeks 13-14 for characteristics of potential Down syndrome
- Weeks 18-20 for congenital malformations
- Structural abnormalities
- Confirm multiples pregnancy
- Verify dates and growth
- Confirm intrauterine death
- Identify hydramnios or oligohydramnios – excessive or reduced levels of amniotic fluid
- Evaluation of fetal well-being
- Identify placental location
- Confirm intrauterine death
- Observe fetal presentation
- Observe fetal movements
- Identify uterine and pelvic abnormalities of the mother
Modes of Ultrasound
Several modes of ultrasound are used in medical imaging These are
- A-mode – A-mode (amplitude mode) is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for the pinpoint accurate focus of the destructive wave energy.
- B-mode or 2D mode – In B-mode (brightness mode) ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on the screen. More commonly known as 2D mode now.
- B-flow – is a mode that digitally highlights weak flow reflectors (mainly red blood cells) while suppressing the signals from the surrounding stationary tissue. It can visualize flowing blood and surrounding stationary tissues simultaneously.
- C-mode – A C-mode image is formed in a plane normal to a B-mode image. A gate that selects data from a specific depth from an A-mode line is used; then the transducer is moved in the 2D plane to sample the entire region at this fixed depth. When the transducer traverses the area in a spiral, an area of 100 cm2 can be scanned in around 10 seconds.
M-mode – In M-mode (motion mode) ultrasound, pulses are emitted in quick succession – each time, either an A-mode or B-mode image is taken. Over time, this is analogous to recording a video in ultrasound. As the organ boundaries that produce reflections move relative to the probe, this can be used to determine the velocity of specific organ structures.
Doppler mode – This mode makes use of the Doppler effect in measuring and visualizing blood flow
- Color Doppler – Velocity information is presented as a color-coded overlay on top of a B-mode image
- Continuous wave (CW) Doppler – Doppler information is sampled along a line through the body, and all velocities detected at each time point are presented (on a timeline)
- Pulsed wave (PW) Doppler – Doppler information is sampled from only a small sample volume (defined in the 2D image), and presented on a timeline
- Duplex – a common name for the simultaneous presentation of 2D and (usually) PW Doppler information. (Using modern ultrasound machines, color Doppler is almost always also used; hence the alternative name
- Pulse inversion mode – In this mode, two successive pulses with opposite sign are emitted and then subtracted from each other. This implies that any linearly responding constituent will disappear while gases with non-linear compressibility stand out. Pulse inversion may also be used in a similar manner as in
- Harmonic mode – In this mode, a deep penetrating fundamental frequency is emitted into the body and a harmonic overtone is detected. This way noise and artifacts due to reverberation and aberration are greatly reduced. Some also believe that penetration depth can be gained with improved lateral resolution; however, this is not well documented.
Contrast Ultrasonography (Ultrasound Contrast Imaging)
A contrast medium for medical ultrasonography is a formulation of encapsulated gaseous microbubbles to increase echogenicity of blood, discovered by Dr. Raymond Gramiak in 1968 and named contrast-enhanced ultrasound. This contrast medical imaging modality is clinically used throughout the world, in particular for echocardiography in the United States.
Microbubbles-based contrast media is administrated intravenously inpatient bloodstream during the medical ultrasonography examination. The microbubbles being too large in diameter, they stay confined in blood vessels and cannot extravasate towards the interstitial fluid. An ultrasound contrast media is therefore purely intravascular, making it an ideal agent to image organ microvascularization for diagnostic purposes. A typical clinical use of contrast ultrasonography is the detection of a hypervascular metastatic tumor, which exhibits a contrast uptake (kinetics of microbubbles concentration in blood circulation) faster than healthy biological tissue surrounding the tumor. Other clinical applications using contrast exist, such as in echocardiography to improve delineation of left ventricle for visually checking contractibility of heart after a myocardial infarction. Finally, applications in quantitative perfusion .(relative measurement of blood flow ) emerge for identifying early patient response to an anti-cancerous drug treatment (methodology and clinical study by Dr Nathalie Lassau in 2011), enabling to determine the best oncological therapeutic options.
In the oncological practice of medical contrast ultrasonography, clinicians use the method of parametric imaging of vascular signatures invented by Dr Nicolas Rognin in 2010. This method is conceived as cancer aided diagnostic tool, facilitating characterization of a suspicious tumor (malignant versus benign) in an organ. This method is based on medical computational science to analyze a time sequence of ultrasound contrast images, a digital video recorded in real-time during a patient examination. Two consecutive signal processing steps are applied to each pixel of the tumor
- calculation of a vascular signature (contrast uptake difference with respect to healthy tissue surrounding the tumor);
Automatic classification of the vascular signature into a unique parameter, this last coded in one of the four following colors:
- green for continuous hyper-enhancement (contrast uptake higher than healthy tissue one),
- blue for continuous hypo-enhancement (contrast uptake lower than healthy tissue one),
- red for fast hyper-enhancement (contrast uptake before healthy tissue one) or
- yellow for fast hypo-enhancement (contrast uptake after healthy tissue one).
Once signal processing in each pixel completed, a color spatial map of the parameter is displayed on a computer monitor, summarizing all vascular information of the tumor in a single image called parametric image (see the last figure of press article as clinical examples). This parametric image is interpreted by clinicians based on predominant colorization of the tumor: red indicates a suspicion of malignancy (risk of cancer), green or yellow – a high probability of benignity. In the first case (suspicion of a malignant tumor), the clinician typically prescribes a biopsy to confirm the diagnostic or a CT scan examination as a second opinion. In the second case (quasi-certain of benign tumor), only a follow-up is needed with a contrast ultrasonography examination a few months later. The main clinical benefits are to avoid a systematic biopsy (a risky invasive procedure) of benign tumors or a CT scan examination exposing the patient to X-ray radiation. The parametric imaging of vascular signatures method proved to be effective in humans for characterization of tumors in the liver. In a cancer screening context, this method might be potentially applicable to other organs such as breast or prostate.
Molecular Ultrasonography (Ultrasound Molecular Imaging)
The future of contrast ultrasonography is in molecular imaging with potential clinical applications expected in cancer screening to detect malignant tumors at their earliest stage of appearance. Molecular ultrasonography (or ultrasound molecular imaging) uses targeted microbubbles originally designed by Dr. Alexander Klibanov in 1997; such targeted microbubbles specifically bind or adhere to tumoral microvessels by targeting biomolecular cancer expression (overexpression of certain biomolecules occurs during neo-angiogenesis or inflammation processes in malignant tumors). As a result, a few minutes after their injection in blood circulation, the targeted microbubbles accumulate in the malignant tumor; facilitating its localization in a unique ultrasound contrast image. In 2013, the very first exploratory clinical trial in humans for prostate cancer was completed at Amsterdam in the Netherlands by Dr Hessel Wijkstra.
In molecular ultrasonography, the technique of acoustic radiation force (also used for shear wave elastography) is applied in order to literally push the targeted microbubbles towards microvessels wall; firstly demonstrated by Dr Paul Dayton in 1999. This allows maximization of binding to the malignant tumor; the targeted microbubbles being in more direct contact with cancerous biomolecules expressed at the inner surface of tumoral microvessels. At the stage of scientific preclinical research, the technique of acoustic radiation force was implemented as a prototype in clinical ultrasound systems and validated in vivo in 2D and 3D imaging modes.
Elastography (Ultrasound Elasticity Imaging)
Ultrasound is also used for elastography, which is a relatively new imaging modality that maps the elastic properties of soft tissue. This modality emerged in the last two decades. Elastography is useful in medical diagnoses as it can discern healthy from unhealthy tissue for specific organs/growths. For example, cancerous tumors will often be harder than the surrounding tissue, and diseased livers are stiffer than healthy ones. There are many ultrasound elastography techniques.
Interventional ultrasonography involves a biopsy, emptying fluids, intrauterine Blood transfusion (Hemolytic disease of the newborn).
- Thyroid cysts – The high-frequency thyroid ultrasound (HFUS) can be used to treat several gland conditions. The recurrent thyroid cyst that was usually treated in the past with surgery, can be treated effectively by a new procedure called percutaneous ethanol injection, or PEI. With the ultrasound-guided placement of a 25 gauge needle within the cyst, and after the evacuation of the cyst fluid, about 50% of the cyst volume is injected back into the cavity, under strict operator visualization of the needle tip. The procedure is 80% successful in reducing the cyst to minute size.
- Metastatic thyroid cancer neck lymph nodes – The other thyroid therapy use for HFUS is to treat metastatic thyroid cancer neck lymph nodes that occur in patients who either refuse surgery or are no longer a candidate for surgery. Small amounts of ethanol are injected under ultrasound guided needle placement. A blood flow study is done prior to the injection, by power Doppler. The blood flow can be destroyed and the node becomes inactive, although it may still be there. Power Doppler visualized blood flow can be eradicated, and there may be a drop in the cancer blood marker test, thyroglobulin, TG, as the node become non-functional. Another interventional use for HFUS is to mark a cancer node one hour prior to surgery to help locate the node cluster at the surgery. A minute amount of methylene dye is injected, under careful ultrasound-guided placement of the needle on the anterior surface, but not in the node. The dye will be evident to the thyroid surgeon when he opens the neck. A similar localization procedure with methylene blue can be done to locate parathyroid adenomas at surgery.
Medical Uses of Ultrasound
Relatively high power ultrasound can break up stony deposits or tissue, accelerate the effect of drugs in a targeted area, assist in the measurement of the elastic properties of tissue, and can be used to sort cells or small particles for research.
- Focused high-energy ultrasound pulses can be used to break calculi such as kidney stones and gallstones into fragments small enough to be passed from the body without undue difficulty, a process known as lithotripsy.
- Cleaning teeth in dental hygiene.
- Focused ultrasound sources may be used for cataract treatment by phacoemulsification.
- Ultrasound can ablate tumors or other tissue non-invasively. This is accomplished using a technique known as High-Intensity Focused Ultrasound (HIFU), also called focused ultrasound surgery (FUS surgery). This procedure uses generally lower frequencies than medical diagnostic ultrasound (250–2000 kHz), but significantly higher time-averaged intensities. The treatment is often guided by Magnetic Resonance Imaging (MRI); the combination is then referred to as Magnetic resonance-guided focused ultrasound (MRgFUS).
- Delivering chemotherapy to brain cancer cells and various drugs to other tissues is called acoustic targeted drug delivery (ATDD). These procedures generally use high-frequency ultrasound (1–10 MHz) and a range of intensities (0–20 W/cm2). The acoustic energy is focused on the tissue of interest to agitate its matrix and make it more permeable for therapeutic drugs.
- Ultrasound has been used to trigger the release of anti-cancer drugs from delivery vectors including liposomes, polymeric microspheres and self-assembled polymeric.
- Ultrasound is essential to the procedures of ultrasound-guided sclerotherapy and endovenous laser treatment for the non-surgical treatment of varicose veins.
- Ultrasound-assisted lipectomy is Liposuction assisted by ultrasound.
There are three potential effects of ultrasound. The first is the increase in blood flow in the treated area. The second is the decrease in pain from the reduction of swelling and edema. The third is the gentle massage of muscle tendons and/ or ligaments in the treated area because no strain is added and any scar tissue is softened. These three benefits are achieved by two main effects of therapeutic ultrasound. The two types of effects are – thermal and nonthermal effects. Thermal effects are due to the absorption of the sound waves. Nonthermal effects are from cavitation, microstreaming and acoustic streaming.
Cavitational effects result from the vibration of the tissue causing microscopic bubbles to form, which transmit the vibrations in a way that directly stimulates cell membranes. This physical stimulation appears to enhance the cell-repair effects of the inflammatory response.
The effectiveness of therapeutic ultrasound for pain, musculoskeletal injuries, and soft tissue lesions remains questionable. A 2017 meta-analysis of randomized controlled trials concluded that there is no benefit of low-intensity pulsed ultrasound on bone healing. An associated guideline published in the British Medical Journal recommends against the use of ultrasound for bone healing.
Risks and side-effects
Ultrasonography is generally considered safe imaging, with the World Health Organizations saying”Diagnostic ultrasound is recognized as a safe, effective, and highly flexible imaging modality capable of providing clinically relevant information about most parts of the body in a rapid and cost-effective fashion”.
Diagnostic ultrasound studies of the fetus are generally considered to be safe during pregnancy. This diagnostic procedure should be performed only when there is a valid medical indication, and the lowest possible ultrasonic exposure setting should be used to gain the necessary diagnostic information under the “as low as reasonably practicable” or ALARP principle.
However, medical ultrasonography should not be performed without a medical indication to perform it. To do otherwise would be to perform unnecessary health care to patients, which bring unwarranted costs and may lead to another testing. Overuse of ultrasonography is sometimes as routine as screening for deep vein thrombosis after orthopedic surgeries in patients who are not at heightened risk for having that condition.
Similarly, although there is no evidence ultrasound could be harmful to the fetus, medical authorities typically strongly discourage the promotion, selling, or leasing of ultrasound equipment for making “keepsake fetal videos”.
Studies on the safety of ultrasound
- A meta-analysis of several ultrasonography studies published in 2000 found no statistically significant harmful effects from ultrasonography but mentioned that there was a lack of data on long-term substantive outcomes such as neurodevelopment.
- A study at the Yale School of Medicine published in 2006 found a small but significant correlation between prolonged and frequent use of ultrasound and abnormal neuronal migration in mice.
- A study performed in Sweden in 2001 has shown that subtle effects of neurological damage linked to ultrasound were implicated by an increased incidence in left-handedness in boys (a marker for brain problems when not hereditary) and speech delays.
- The above findings, however, were not confirmed in a later follow-up study.
- A later study, however, performed on a larger sample of 8865 children, has established a statistically significant, albeit weak association of ultrasonography exposure and being non-right handed later in life.