Medical ultrasound includes diagnostic techniques (mainly imaging) using ultrasound, as well as therapeutic applications of ultrasound. In diagnosis, it is used to create an image of internal body structures such as tendons, muscles, joints, blood vessels, and internal organs, to measure some characteristics (e.g., distances and velocities) or to generate an informative audible sound. The usage of ultrasound to produce visual images for medicine is called medical ultrasonography or simply sonography. Sonography using ultrasound reflection is called echography. There are also transmission methods, such as ultrasound transmission tomography. The practice of examining pregnant women using ultrasound is called obstetric ultrasonography, and was an early development of clinical ultrasonography. The machine used is called an ultrasound machine, a sonograph or an echograph. The visual image formed using this technique is called an ultrasonogram, a sonogram or an echogram.
Ultrasound is composed of sound waves with frequencies greater than 20,000 Hz, which is the approximate upper threshold of human hearing. Ultrasonic images, also known as sonograms, are created by sending pulses of ultrasound into tissue using a probe. The ultrasound pulses echo off tissues with different reflection properties and are returned to the probe which records and displays them as an image.
A general-purpose ultrasonic transducer may be used for most imaging purposes but some situations may require the use of a specialized transducer. Most ultrasound examination is done using a transducer on the surface of the body, but improved visualization is often possible if a transducer can be placed inside the body. For this purpose, special-use transducers, including transvaginal, endorectal, and transesophageal transducers are commonly employed. At the extreme, very small transducers can be mounted on small diameter catheters and placed within blood vessels to image the walls and disease of those vessels.
Types
Mode The imaging mode refers to probe and machine settings that result in specific dimensions of the ultrasound image. Several modes of ultrasound are used in medical imaging:The Gale Encyclopedia of Medicine, 2nd Edition, Vol. 1 A-B. p. 4 A-mode: Amplitude mode refers to the mode in which the amplitude of the transducer voltage is recorded as a function of two-way travel time of an ultrasound pulse. A single pulse is transmitted through the body and scatters back to the same transducer element. The voltage amplitudes recorded correlate linearly to acoustic pressure amplitudes. A-mode is one-dimensional. B-mode: In brightness mode, an array of transducer elements scans a plane through the body resulting in a two-dimensional image. Each pixel value of the image correlates to voltage amplitude registered from the backscattered signal. The dimensions of B-mode images are voltage as a function of angle and two-way time. M-mode: In motion mode, A-mode pulses are emitted in succession. The backscattered signal is converted to lines of bright pixels, whose brightness linearly correlates to backscattered voltage amplitudes. Each next line is plotted adjacent to the previous, resulting in an image that looks like a B-mode image. The M-mode image dimensions are however voltage as a function of two-way time and recording time. This mode is an ultrasound analogy to streak video recording in high-speed photography. As moving tissue transitions produce backscattering, this can be used to determine the displacement of specific organ structures, most commonly the heart.
Most machines convert two-way time to imaging depth using as assumed speed of sound of 1540 m/s. As the actual speed of sound varies greatly in different tissue types, an ultrasound image is therefore not a true tomographic representation of the body.
3D imaging Three-dimensional imaging is done by combining B-mode images, using dedicated rotating or stationary probes. This has been referred to as C-mode.
A hybrid rotating ultrasound/photoacoustic system produces 3D images, including blood and tissue without the use of magnets or ionizing radiation. It uses an arc of detectors to create a volumetric image. A laser pulse causes hemoglobin molecules to vibrate and generate acoustic signal also absorbed by the detectors.
Technique An imaging technique is the method of signal generation and processing that supports in a specific application. Most techniques use B-mode. Doppler sonography: This imaging technique makes use of the Doppler effect in detection and measuring moving targets, typically blood. Harmonic imaging: backscattered signal from tissue is filtered to comprise only frequency content of at least twice the centre frequency of the transmitted ultrasound. Harmonic imaging used for perfusion detection when using ultrasound contrast agents and for the detection of tissue harmonics. Common pulse schemes for the creation of harmonic response without the need of real-time Fourier analysis are pulse inversion and power modulation.
B-flow is an imaging technique that digitally highlights moving reflectors (mainly red blood cells) while suppressing the signals from the surrounding stationary tissue. It aims to visualize flowing blood and surrounding stationary tissues simultaneously. It is thus an alternative or complement to Doppler ultrasonography in visualizing blood flow.
Therapeutic ultrasound aimed at a specific tumor or calculus is not an imaging mode. However, for positioning a treatment probe to focus on a specific region of interest, A-mode and B-mode are typically used, often during treatment.
Uses
Sonography (ultrasonography) is widely used in medicine. It is possible to perform both diagnosis and therapeutic procedures, using ultrasound to guide interventional procedures such as biopsies or to drain collections of fluid, which can be both diagnostic and therapeutic. Sonographers are medical professionals who perform scans which are traditionally interpreted by radiologists, physicians who specialize in the application and interpretation of medical imaging modalities, or by cardiologists in the case of cardiac ultrasonography (echocardiography). Sonography is effective for imaging soft tissues of the body. Superficial structures such as muscle, tendon, testis, breast, thyroid and parathyroid glands, and the neonatal brain are imaged at higher frequencies (7–18 MHz), which provide better linear (axial) and horizontal (lateral) resolution. Deeper structures such as liver and kidney are imaged at lower frequencies (1–6 MHz) with lower axial and lateral resolution as a price of deeper tissue penetration.
Anesthesiology In anesthesiology, ultrasound is commonly used to guide the placement of needles when injecting local anesthetic solutions in the proximity of nerves identified within the ultrasound image (nerve block). It is also used for vascular access such as cannulation of large central veins and for difficult arterial cannulation. Transcranial Doppler is frequently used by neuro-anesthesiologists for obtaining information about flow-velocity in the basal cerebral vessels.
Angiology (vascular)
In angiology or vascular medicine, duplex ultrasound (B Mode imaging combined with Doppler flow measurement) is used to diagnose arterial and venous disease. This is particularly important in potential neurologic problems, where carotid ultrasound is commonly used for assessing blood flow and potential or suspected stenosis in the carotid arteries, while transcranial Doppler is used for imaging flow in the intracerebral arteries. Intravascular ultrasound (IVUS) uses a specially designed catheter with a miniaturized ultrasound probe attached to its distal end, which is then threaded inside a blood vessel. The proximal end of the catheter is attached to computerized ultrasound equipment and allows the application of ultrasound technology, such as a piezoelectric transducer or capacitive micromachined ultrasonic transducer, to visualize the endothelium of blood vessels in living individuals.
In the case of the common and potentially, serious problem of blood clots in the deep veins of the leg, ultrasound plays a key diagnostic role, while ultrasonography of chronic venous insufficiency of the legs focuses on more superficial veins to assist with planning of suitable interventions to relieve symptoms or improve cosmetics.
Cardiology (heart)
Echocardiography is an essential tool in cardiology, assisting in evaluation of heart valve function, such as stenosis or insufficiency, strength of cardiac muscle contraction, and hypertrophy or dilatation of the main chambers. (ventricle and atrium)
Emergency medicine
Point of care ultrasound has many applications in emergency medicine. These include differentiating cardiac from pulmonary causes of acute breathlessness, and the Focused Assessment with Sonography for Trauma (FAST) exam, extended to include assessment for significant hemoperitoneum or pericardial tamponade after trauma (EFAST). Other uses include assisting with differentiating causes of abdominal pain such as gallstones and kidney stones. Emergency Medicine Residency Programs teach ultrasound to residents .[citation needed]
Gastroenterology/Colorectal surgery
Both abdominal and endoanal ultrasound are frequently used in gastroenterology and colorectal surgery. In abdominal sonography, the major organs of the abdomen such as the pancreas, aorta, inferior vena cava, liver, gall bladder, bile ducts, kidneys, and spleen may be imaged. However, sound waves may be blocked by gas in the bowel and attenuated to differing degrees by fat, sometimes limiting diagnostic capabilities. The appendix can sometimes be seen when inflamed (e.g.: appendicitis) and ultrasound is the initial imaging choice, avoiding radiation if possible, although it frequently needs to be followed by other imaging methods such as CT. Endoanal ultrasound is used particularly in the investigation of anorectal symptoms such as fecal incontinence or obstructed defecation. It images the immediate perianal anatomy and is able to detect occult defects such as tearing of the anal sphincter.
Hepatology
Ultrasonography of liver tumors allows for both detection and characterization. Ultrasound imaging studies are often obtained during the evaluation process of Fatty liver disease. Ultrasonography reveals a "bright" liver with increased echogenicity. Pocket-sized ultrasound devices might be used as point-of-care screening tools to diagnose liver steatosis.
Gynecology and obstetrics
Gynecologic ultrasonography examines female pelvic organs (specifically the uterus, ovaries, and fallopian tubes) as well as the bladder, adnexa, and pouch of Douglas. It uses transducers designed for approaches through the lower abdominal wall, curvilinear and sector, and specialty transducers such as transvaginal ultrasound.
Obstetrical sonography was originally developed in the late 1950s and 1960s by Ian Donald and is commonly used during pregnancy to check the development and presentation of the fetus. It can be used to identify many conditions that could be potentially harmful to the mother and/or baby possibly remaining undiagnosed or with delayed diagnosis in the absence of sonography. It is currently believed that the risk of delayed diagnosis is greater than the small risk, if any, associated with undergoing an ultrasound scan. However, its use for non-medical purposes such as fetal "keepsake" videos and photos is discouraged.
Obstetric ultrasound is primarily used to: Date the pregnancy (gestational age) Confirm fetal viability Determine location of fetus, intrauterine vs ectopic Check the location of the placenta in relation to the cervix Check for the number of fetuses (multiple pregnancy) Check for major physical abnormalities. Assess fetal growth (for evidence of intrauterine growth restriction (IUGR)) Check for fetal movement and heartbeat.