doppler. otolaryngology (head and neck) neck ultrasound most structures of the neck, including the thyroid and parathyroid 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 its use is important 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 differentiated, evaluated, and managed with the help of diagnostic ultrasound and ultrasound-guided procedures. neonatology in neonatology, transcranial doppler can be used for basic assessment of intracerebral structural abnormalities, suspected hemorrhage, ventriculomegaly or hydrocephalus and anoxic insults (periventricular leukomalacia). it 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 by which time they have formed a virtually impenetrable acoustic barrier to ultrasound. the most common site for cranial ultrasound is the anterior fontanelle. the smaller the fontanelle, the more the image is compromised. ophthalmology (eyes) in ophthalmology and optometry, there are two major forms of eye exam using ultrasound: a-scan ultrasound biometry, is commonly referred to as an a-scan (amplitude scan). a-mode provides data on the length of the eye, which is a major determinant in common sight disorders, especially for determining the power of an intraocular lens after cataract extraction. b-scan ultrasonography, or b-scan, is a b-mode scan that produces a cross-sectional view of the eye and the orbit. its use in the emergency department for the timely diagnosis of conditions such as retinal or vitreous detachment, retinal and vitreous hemorrhages, and intra-ocular foreign bodies is common and important. pulmonology (lungs) ultrasound is used to assess the lungs in a variety of settings including critical care, emergency medicine, trauma surgery, as well as general medicine. this imaging modality is used at the bedside or examination table to evaluate a number of different lung abnormalities as well as to guide procedures such as thoracentesis, (drainage of pleural fluid (effusion)), needle aspiration biopsy, and catheter placement. although air present in the lungs does not allow good penetration of ultrasound waves, interpretation of specific artifacts created on the lung surface can be used to detect abnormalities. lung ultrasound basics the normal lung surface: the lung surface is composed of visceral and parietal pleura. these two surfaces are typically pushed together and make up the pleural line, which is the basis of lung (or pleural) ultrasound. this line is visible less than a centimeter below the rib line in most adults. on ultrasound, it is visualized as a hyperechoic (bright white) horizontal line if the ultrasound probe is applied perpendicularly to the skin. artifacts: lung ultrasound relies on artifacts, which would otherwise be considered a hindrance in imaging. air blocks the ultrasound beam and thus visualizing healthy lung tissue itself with this mode of imaging is not practical. consequently, physicians and sonographers have learned to recognize patterns that ultrasound beams create when imaging healthy versus diseased lung tissue. three commonly seen and utilized artifacts in lung ultrasound include lung sliding, a-lines, and b-lines. § lung sliding: the presence of lung sliding, which indicates the shimmering of the pleural line that occurs with movement of the visceral and parietal pleura against one another with respiration (sometimes described as 'ants marching'), is the most important finding in normal aerated lung. lung sliding indicates both that the lung is present at the chest wall and that the lung is functioning. § a-lines: when the ultrasound beam makes contact with the pleural line, it is reflected back creating a bright white horizontal line. the subsequent reverberation artifacts that appear as equally spaced horizontal lines deep to the pleura are a-lines. ultimately, a-lines are a reflection of the ultrasound beam from the pleura with the space between a-lines corresponding to the distance between the parietal pleura and the skin surface. a-lines indicate the presence of air, which means that these artifacts can be present in normal healthy lung (and also in patients with pneumothorax). § b-lines: b-lines are also reverberation artifacts. they are visualized as hyperechoic vertical lines extending from the pleura to the edge of the ultrasound screen. these lines are sharply defined and laser-like and typically do not fade as they progress down the screen. a few b-lines that move along with the sliding pleura can be seen in normal lung due to acoustic impedance differences between water and air. however, excessive b-lines (three or more) are abnormal and are typically indicative of underlying lung pathology. lung pathology assessed with ultrasound pulmonary edema: lung ultrasound has been shown to be very sensitive for the detection of pulmonary edema. it allows for improvement in diagnosis and management of critically ill patients, particularly when used in combination with echocardiography. the sonographic feature that is present in pulmonary edema is multiple b-lines. b-lines can occur in a healthy lung; however, the presence of 3 or more in the anterior or lateral lung regions is always abnormal. in pulmonary edema, b-lines indicate an increase in the amount of water contained in the lungs outside of the pulmonary vasculature. b-lines can also be present in a number of other conditions including pneumonia, pulmonary contusion, and lung infarction. additionally, it is important to note that there are multiple types of interactions between the pleural surface and the ultrasound wave that can generate artifacts with some similarity to b-lines but which do not have pathologic significance. pneumothorax: in clinical settings when pneumothorax is suspected, lung ultrasound can aid in diagnosis. in pneumothorax, air is present between the two layers of the pleura and lung sliding on ultrasound is therefore absent. the negative predictive value for lung sliding on ultrasound is reported as 99.2–100% – briefly, if lung sliding is present, a pneumothorax is effectively ruled out. the absence of lung sliding, however, is not necessarily specific for pneumothorax as there are other conditions that also cause this finding including acute respiratory distress syndrome, lung consolidations, pleural adhesions, and pulmonary fibrosis. pleural effusion: lung ultrasound is a cost-effective, safe, and non-invasive imaging method that can aid in the prompt visualization and diagnosis of pleural effusions. effusions can be diagnosed by a combination of physical exam, percussion, and auscultation of the chest. however, these exam techniques can be complicated by a variety of factors including the presence of mechanical ventilation, obesity, or patient positioning, all of which reduce the sensitivity of the physical exam. consequently, lung ultrasound can be an additional tool to augment plain chest xray and chest ct. pleural effusions on ultrasound appear as structural images within the thorax rather than an artifact. they will typically have four distinct borders including the pleural line, two rib shadows, and a deep border. in critically ill patients with pleural effusion, ultrasound may guide procedures including needle insertion, thoracentesis, and chest-tube insertion. lung cancer staging: 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. covid-19: lung ultrasound has proved useful in the diagnosis of covid-19 especially in cases where other investigations are not available. urinary tract main article: urinary tract ultrasound urinary bladder (black butterfly-like shape) and hyperplastic prostate (bph) visualized by medical sonographic technique ultrasound is routinely used in urology to determine the amount of fluid retained in a patient's bladder. in a pelvic sonogram, images include the uterus and ovaries or urinary bladder in females. in males, a sonogram will provide information about the bladder, prostate, or testicles (for example to urgently 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 curable but must be treated to preserve health and fertility. there are two methods of performing 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 also used to diagnose and, at higher frequencies, to treat (break up) kidney stones or kidney crystals (nephrolithiasis). penis and scrotum main articles: scrotal ultrasound and penile ultrasonography scrotal ultrasonography is used in the evaluation of testicular pain, and can help identify solid masses. ultrasound is an excellent method for the study of the penis, such as indicated in trauma, priapism, erectile dysfunction or suspected peyronie's disease. musculoskeletal musculoskeletal ultrasound is used to examine tendons, muscles, nerves, ligaments, soft tissue masses, and bone surfaces. it is helpful in diagnosing ligament sprains, muscles strains and joint pathology. it is an alternative or supplement 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; proxy measures of muscle quality (i.e., tissue composition) in older adults with sarcopenia ultrasound can also be used for needle guidance in muscle or joint injections, as in ultrasound-guided hip joint injection. kidneys main article: renal ultrasonography 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 are distinguishable with ultrasound. it is an accessible, versatile, relatively economic, and fast aid for decision-making in patients with renal symptoms and for guidance in renal intervention. 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 (cect) and mri, should be considered for supplementary imaging in assessing renal disease. venous access intravenous access, for the collection of blood samples to assist in diagnosis or laboratory investigation including blood culture, or for administration of intravenous fluids for fluid maintenance of replacement or blood transfusion in sicker patients, is a common medical procedure. the need for intravenous access occurs in the outpatient laboratory, in the inpatient hospital units, and most critically in the emergency room and intensive care unit. in many situations, intravenous access may be required repeatedly or over a significant time period. in these latter circumstances, a needle with an overlying catheter is introduced into the vein and the catheter is then inserted securely into the vein while the needle is withdrawn. the chosen veins are most frequently selected from the arm, but in challenging situations, a deeper vein from the neck (external jugular vein) or upper arm (subclavian vein) may need to be used. there are many reasons why the selection of a suitable vein may be problematic. these include, but are not limited to, obesity, previous injury to veins from inflammatory reaction to previous 'blood draws', previous injury to veins from recreational drug use. in these challenging situations, the insertion of a catheter into a vein has been greatly assisted by the use of ultrasound. the ultrasound unit may be 'cart-based' or 'handheld' using a linear transducer with a frequency of 10 to 15 megahertz. in most circumstances, choice of vein will be limited by the requirement that the vein is within 1.5 cms. from the skin surface. the transducer may be placed longitudinally or transversely over the chosen vein. ultrasound training for intravenous cannulation is offered in most ultrasound training programs. mechanism the creation of an image from sound has three steps – transmitting a sound wave, receiving echoes, and interpreting those echoes. producing a sound wave medical ultrasound scanner a sound wave is typically produced by a piezoelectric transducer encased in a plastic housing. strong, short electrical pulses from the ultrasound machine drive the transducer at the desired frequency. the frequencies can vary between 1 and 18 mhz, though frequencies up to 50–100 megahertz have been used experimentally in a