What is an ultrasound?
While the patient's history and physical examination are the initial steps of making a medical diagnosis, the ability to peer inside the body can be a powerful tool. Ultrasound is an imaging technique that provides that ability to medical practitioners.
Ultrasound produces sound waves that are beamed into the body causing return echoes that are recorded to "visualize" structures beneath the skin. The ability to measure different echoes reflected from a variety of tissues allows a shadow picture to be constructed. The technology is especially accurate at seeing the interface between solid and fluid filled spaces. These are actually the same principles that allow SONAR on boats to see the bottom of the ocean.
What is ultrasonography?
Ultrasonography is a form of body imaging using sound waves to facilitate makeing a medical diagnosis. A skilled ultrasound technician is able to see inside the body using ultrasonography to answer questions that may be asked by the medical practitioner caring for the patient. Usually, a radiologist will oversee the ultrasound test and report on the results, but other types of physicians may also use ultrasound as a diagnostic tool. For example, obstetricians use ultrasound to assess the fetus during pregnancy. Surgeons and emergency physicians use ultrasound at the bedside to assess abdominal pain or other concerns.
A transducer, or probe, is used to project and receive the sound waves and their echoes. A gel is wiped onto the patient's skin so that the sound waves are not distorted as they cross through the skin. Using their understanding of human anatomy and the machine, the technician can evaluate specific structures and try to answer the question asked by the patient's physician. This may take a fair amount of time and require the probe to be repositioned and pointed in different directions. As well, the technician may need to vary the amount of pressure used to push the probe into the skin. The goal will be to "paint" a shadow picture of the inner organ that the health care practitioner has asked to be visualized.
The physics of sound can place limits on the test. The quality of the picture depends on many factors.
- Sound waves cannot penetrate deeply, and an obese patient may be imaged poorly.
- Ultrasound does poorly when gas is present between the probe and the target organ. Should the intestine be distended with bowel gas, organs behind it may not be easily seen. Similarly, ultrasound works poorly in the chest, where the lungs are filled with air.
- Ultrasound does not penetrate bone easily.
- The accuracy of the test is very much operator dependent. This means that the key to a good test is the ultrasound technician.
Ultrasound can be enhanced by using Doppler technology which can measure whether an object is moving towards or away from the probe. This can allow the technician to measure blood flow in organs such as the heart or liver, or within specific blood vessels.
For what purposes are ultrasounds used?
Ultrasound is not limited to diagnosis, but can also be used in screening for disease and to aid in treatment of diseases or conditions.
Diagnostic uses for ultrasound
Obstetrics
Ultrasound is routinely used for assessing the progression of a pregnancy. Pelvic ultrasounds can be obtained trans-abdominally where the probe is placed on the abdominal wall, or trans-vaginally, where the probe is placed in the vagina. For example ultrasound in obstetrics/gynecology is used to diagnose growths or tumors of the ovary, uterus, or, Fallopian tubes.
Cardiology
Echocardiography
Echocardiography (echo=sound + cardio=heart + graphy=study) evaluates the heart, the heart valve's motion, and blood flow through them. It also evaluates the heart wall motion and the amount of blood the heart pumps with each stroke.
Echocardiography can be performed in two ways:
- trans-thoracic: the probe is place on the chest wall to obtain images, and
- trans-esophageal: where the probe is placed through the mouth into the esophagus.
Anatomically, the esophagus sits near the heart and allows clearer images. However, this approach is a little more invasive.
Different groups of illnesses can be assessed by echocardiography:
- Valves in the heart keep blood flowing in one direction when the heart pumps. For example, when the heart beats, blood is pumped from the left ventricle through the aortic valve into the aorta and the rest of the body. The aortic valve prevents blood from back-flowing into the heart as it fills for the next beat. Echocardiography can determine if the valve is narrow or leaking (regurgitating, insufficient). By following how the patient fares clinically, repeated echocardiograms can help determine whether valve replacement or repair is warranted. The same principles apply to the mitral valve which keeps blood flowing from the left atrium to the left ventricle.
- The heart muscle pumps blood to the body. If the heart weakens, the amount of blood it pumps with each beat can decrease, leading to congestive heart failure. The echocardiogram can measure the efficiency of the heart beat and how much blood it pumps; which assists in determining whether medications are needed. It also is used to monitor how well medications are working.
- Echocardiography can visualize the heart chambers to detect blood clots in conditions such as atrial fibrillation (an irregular heart rhythm). In other situations, the test can help diagnose endocarditis (an infection of the heart valves) by visualizing "vegetations" (an infected mass) on the valves themselves.
- Echocardiography also can detect abnormal fluid collections (pericardial effusions) in the pericardium.
- Echocardiograms are used to diagnose and monitor pulmonary artery hypertension.
Blood vessels
Ultrasound can detect blood clots in veins (superficial or deep venous thrombosis) or artery blockage (stenosis) and dilatation (aneurysms). Some examples of ultrasound testing include:
- Carotid ultrasound is performed in patients with transient ischemic attacks (TIAs) or strokes to determine whether the major arteries in the neck are blocked causing the decreased blood supply to the brain.
- The aorta is the large blood vessel leaving the heart that supplies blood to the rest of the body. The walls of the aorta are under significant pressure from the force of the heartbeat and over time, may weaken and widen. This is called an aneurysm, and it can be detected in the abdomen by ultrasound (abdominal aortic aneurysm). For those patients with small aneurysm, observation may be recommended and the aneurysms size followed over time by repeated tests.
- Veins can also be evaluated by ultrasound and it is a common test to assess whether swelling in a leg is due to a blood clot, deep vein thrombosis (DVT) or another cause.
Abdominal structures
Aside from its use in obstetrics, ultrasound can evaluate most of the solid structures in the abdominal cavity. This includes the liver, gallbladder, pancreas, kidneys, bladder, prostate, testicles, uterus, and ovaries.
- Ultrasound is the preferred technique to test for gallstones or an infected gallbladder. The ultrasound can reveal the stones as well as signs of infection, including thickening of the gallbladder wall and fluid surrounding the gallbladder. The ultrasound may find blockage in the bile ducts.
- For those patients where the radiation of a CT scan (computerized tomography) is a potential risk (pregnant patients or children), ultrasound may be used to look for diseases like appendicitis or kidney stones.
- Ultrasound is the test of choice to diagnose testicular torsion.
- Pelvic ultrasound is used in gynecology to help assess non-pregnancy related issues like lower abdominal pain, ovarian cysts, uterine fibroids, uterine growths, and endometriosis.
The neck
The thyroid gland can be imaged using ultrasound looking for nodules, growths, or tumors.
Knee joint
Ultrasound can be used to detect bulging of fluid from a swollen knee joint into the back of the knee, called a Baker's cyst.
Screening uses for ultrasound
Ultrasound may be used to screen for blood vessel diseases. By measuring blood flow and blockage in the carotid arteries, the test can predict potential risk for future stroke. Similarly, by measuring the diameter of the aorta in the abdomen, ultrasound can screen for aneurysm (abnormal dilatation) and the risk of rupture. These tests may be indicated for an individual patient or they may be offered as a community wide health screening assessment.
Therapeutic uses for ultrasound
Ultrasound may be used to help physicians guide needles into the body.
In situations where an intravenous line is required but it is difficult to find a vein, ultrasound guidance may be used to identify larger veins in the neck, chest wall, or groin.
Ultrasound may be used to guide a needle into a cavity that needs to be drained (for example, an abscess) or a mass that needs to be biopsied, where a small bit of tissue is removed for analysis.
What are the risks of ultrasound?
There are no known risks to ultrasound, and as technology has improved, the machines have become smaller, portable and available for use at the patient’s bedside.
How do patients prepare for an ultrasound?
Preparation for ultrasound is minimal. Generally, if internal organs such as the gallbladder are to be examined, patients are requested to avoid eating and drinking with the exception of water for six to eight hours prior to the examination. This is because food causes gallbladder contraction, minimizing the size, which would be visible during the ultrasound.
In preparation for examination of the baby and womb during pregnancy, sometimes it is recommended that mothers drink at least four to six glasses of water approximately one to two hours prior to the examination for the purpose of filling the bladder. This helps improve the images captured during the exam.
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How are the results of ultrasound interpreted and communicated to the physician?
The ultrasound is generally performed by a technician. The technician will notice preliminary structures and may point out several of these structures during the examination. The official reading of the ultrasound is done by a radiologist, a physician who is an expert at interpreting ultrasound images. The radiologist records the interpretation and transmits it to the practitioner requesting the test. Occasionally, during the ultrasound test the radiologist will ask questions of the patient and/or perform an examination in order to further define the purpose for which the test is ordered, or to clarify preliminary findings.
Plain x-rays might be ordered to further evaluate early findings.
A summary of results of all of the above is reported to the health care practitioner who requested the ultrasound. They are then discussed with the patient in the context of the patient's overall health status.
