ULTRASOUND


DEFINITION

Ultrasound imaging (sonography) uses a transducer or probe to generate sound waves and produce pictures of the body's internal structures.

It provides a clear picture of soft tissues that don't show up well on x-ray images.

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 often used to help diagnose unexplained pain, swelling and infection. It may also be used to provide imaging guidance to needle biopsies or to see and evaluate conditions related to blood flow.

HOW IT WORKS

Ultrasound imaging is based on the same principles involved in the sonar used by bats, ships and fishermen. When a sound wave strikes an object, it bounces back, or echoes. By measuring these echo waves, it is possible to determine how far away the object is as well as the object's size, shape and consistency (whether the object is solid or filled with fluid).

In medicine, ultrasound is used to detect changes in appearance, size or contour of organs, tissues, and vessels or to detect abnormal masses, such as tumors.

In an ultrasound examination, a transducer both sends the sound waves into the body and receives the echoing waves. When the transducer is pressed against the skin, it directs small pulses of inaudible, high-frequency sound waves into the body. As the sound waves bounce off internal organs, fluids and tissues, the sensitive receiver in the transducer records tiny changes in the sound's pitch and direction. These signature waves are instantly measured and displayed by a computer, which in turn creates a real-time picture on the monitor. One or more frames of the moving pictures are typically captured as still images. Short video loops of the images may also be saved.

WHEN IT IS USED

  • B-mode image (the acoustic impedance of a two-dimensional cross-section of tissue)

  • Blood flow

  • Motion of tissue over time

  • The location of blood

  • The presence of specific molecules

  • The stiffness of tissue

  • The anatomy of a three-dimensional region

  • to guide injecting needles when placing local anaesthetic solutions near nerves

  • gaining vascular access such as central venous cannulation and difficult arterial cannulation

RISKS AND BENEFITS

Compared to other prominent methods of medical imaging, ultrasound has several advantages. It provides images in real-time, it is portable and can be brought to the bedside, it is substantially lower in cost, and it does not use harmful ionizing radiation. Drawbacks of ultrasonography include various limits on its field of view including patient cooperation and physique, difficulty imaging structures behind bone and air, and its dependence on a skilled operator.

TYPES AND USAGE:

DOPPLER

Doppler ultrasonography is 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 color scale. Color Doppler images are generally combined with grayscale (B-mode) images to display duplex ultrasonography images.

ANGIOLOGY

Duplex ultrasound (B Mode vessels imaging combined with Doppler flow measurement) is daily used to diagnose arterial and venous disease all over the body. This is particularly important in neurology, where ultrasound is used for assessing blood flow and stenoses in the carotid arteries (Carotid ultrasonography) and the big intracerebral arteries (Transcranial Doppler).

Intravascular ultrasound (IVUS) is a methodology using a specially designed catheter with a miniaturized ultrasound probe attached to the distal end of the catheter. The proximal end of the catheter is attached to computerized ultrasoundequipment. It allows the application of ultrasound technology, such as piezoelectric transducer or CMUT, to see from inside blood vessels out through the surrounding blood column, visualizing the endothelium (inner wall) of blood vessels in living individuals.

Intravascular ultrasound image of a coronary artery (left), with color-coding on the right, delineating the lumen (yellow), external elastic membrane (blue) and the atherosclerotic plaque burden (green).

IMAGE: Intravascular ultrasound image of a coronary artery (left), with color-coding on the right, delineating the lumen (yellow), external elastic membrane (blue) and the atherosclerotic plaque burden (green).

CARDIOLOGY

Echocardiography is an essential tool in cardiology, to diagnose e.g. dilatation of parts of the heart and function of heart ventricles and valves.

  • Carotid Intima-Media Thickness Test (to determine the extent of plaque buildup in the walls of the arteries supplying blood to the head. If a CIMT shows increased thickness in the inner layers of the carotid artery, you may be at risk for cardiovascular disease. CIMT uses ultrasound imaging to measure the thickness of the two inner layers of the carotid artery—called the intima and media.)

IMAGE: An abnormal echocardiogram showing a midmuscular ventricular septal defect. The trace in the lower left shows the cardiac cycle and the red mark the time in the cardiac cycle when the image was captured. Colors are used to represent the velocity and direction of blood flow.

GASTROINTESTINAL

In abdominal sonography, the solid organs of the abdomen such as the pancreas, aorta, inferior vena cava, liver, gall bladder, bile ducts, kidneys, and spleen are imaged. Sound waves are blocked by gas in the bowel and attenuated in different degree by fat, therefore there are limited diagnostic capabilities in this area. The appendix can sometimes be seen when inflamed (as in e.g.: appendicitis). 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. Ultrasonography of liver tumors allows for both detection and characterization.

(a) Third-degree sphincter tear with good repair as demonstrated on tomographic slicing (white arrow). (b) Third-degree tear with residual defect between 10 and 1 o'clock, as demonstrated on tomographic slicing (white arrow).

Ultrasonography of the liver with some standard measurements.

GYNECOLOGY AND OBSTETRICS

Gynecologic ultrasonography examines female pelvic organs (specifically the uterus, the ovaries, and the Fallopian tubes) as well as the bladder, the adnexa, and the Pouch of Douglas. It commonly uses vaginal ultrasonography. Obstetrical sonography is commonly used during pregnancy to check on the development of the fetus.

IMAGE: Left hydrosalpinx on gynaecological ultrasonography

OTOLARYNGOLOGY

Most structures of the neck, including the thyroid and parathryoid glands, lymph nodes, and salivary glands, are well-visualized by high-frequency ultrasound with exceptional anatomic detail. Ultrasound is the preferred imaging modality for thyroid tumors and lesions, and ultrasonography is critical in the evaluation, preoperative planning, and postoperative surveillance of patients with thyroid cancer. Many other benign and malignant conditions in the head and neck can be evaluated and managed with the help of diagnostic ultrasound and ultrasound-guided procedures.

IMAGE: Orthogonal planes of a 3 dimensional sonographic volume with transverse and coronal measurements for estimating fetal cranial volume.

NEONATOLOGY

In neonatology, transcranial Doppler can be used for basic assessment of intracerebral structural abnormalities, bleeds, ventriculomegaly or hydrocephalus and anoxic insults (Periventricular leukomalacia). The ultrasound can be performed through the soft spots in the skull of a newborn infant (Fontanelle) until these completely close at about 1 year of age and form a virtually impenetrable acoustic barrier for the ultrasound. The most common site for cranial ultrasound is the anterior fontanelle. The smaller the fontanelle, the poorer the quality of the picture.

IMAGE: Normal mid coronal view at the level of the brain stem.

OPTHAMOLOGY

In ophthalmology and optometry, there are two major forms of eye exam using ultrasound:

  • A-scan ultrasound biometry, commonly referred to as an A-scan (short for Amplitude scan). It is an A-mode that provides data on the length of the eye, which is a major determinant in common sight disorders.

  • B-scan ultrasonography, or B-scan, which is a B-mode scan that produces a cross-sectional view of the eye and the orbit. It is commonly used to see inside the eye when media is hazy due to cataract or any corneal opacity.

PULMONOLOGY

In pulmonology, endobronchial Ultrasound (EBUS) probes are applied to standard flexible endoscopic probes and used by pulmonologists to allow for direct visualization of endobronchial lesions and lymph nodes prior to transbronchial needle aspiration. Among its many uses, EBUS aids in lung cancer staging by allowing for lymph node sampling without the need for major surgery.

UROLOGY

Ultrasound is routinely used in urology to determine, for example, the amount of fluid retained in a patient's bladder. In a pelvic sonogram, organs of the pelvic region are imaged. This includes the uterus and ovaries or urinary bladder. Males are sometimes given a pelvic sonogram to check on the health of their bladder, the prostate, or their testicles (for example to distinguish epididymitis from testicular torsion). In young males, it is used to distinguish more benign testicular masses (varicocele or hydrocele) from testicular cancer, which is highly curable but which must be treated to preserve health and fertility. There are two methods of performing a pelvic sonography – externally or internally. The internal pelvic sonogram is performed either transvaginally (in a woman) or transrectally (in a man). Sonographic imaging of the pelvic floor can produce important diagnostic information regarding the precise relationship of abnormal structures with other pelvic organs and it represents a useful hint to treat patients with symptoms related to pelvic prolapse, double incontinence and obstructed defecation. It is used to diagnose and, at higher frequencies, to treat (break up) kidney stones or kidney crystals (nephrolithiasis).

MUSCOSCELETAL

Musculoskeletal ultrasound in used to examine tendons, muscles, nerves, ligaments, soft tissue masses, and bone surfaces. Ultrasound is an alternative to x-ray imaging in detecting fractures of the wrist, elbow and shoulder for patients up to 12 years (Fracture sonography).

Quantitative ultrasound is an adjunct musculoskeletal test for myopathic disease in children; estimates of lean body mass in adults;[10] proxy measures of muscle quality (i.e., tissue composition)[11] in older adults with sarcopenia.

NEPHROLOGY

In nephrology, ultrasonography of the kidneys is essential in the diagnosis and management of kidney-related diseases. The kidneys are easily examined, and most pathological changes in the kidneys are distinguishable with ultrasound. US is an accessible, versatile inexpensive and fast aid for decision-making in patients with renal symptoms and for guidance in renal intervention. Renal ultrasound (US) is a common examination, which has been performed for decades. Using B-mode imaging, assessment of renal anatomy is easily performed, and US is often used as image guidance for renal interventions. Furthermore, novel applications in renal US have been introduced with contrast-enhanced ultrasound (CEUS), elastography and fusion imaging. However, renal US has certain limitations, and other modalities, such as CT and MRI, should always be considered as supplementary imaging modalities in the assessment of renal disease.

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