technique known as biomicroscopy in special regions, such as the anterior chamber of the eye. older technology transducers focused their beam with physical lenses. contemporary technology transducers use digital antenna array techniques (piezoelectric elements in the transducer produce echoes at different times) to enable the ultrasound machine to change the direction and depth of focus. near the transducer, the width of the ultrasound beam almost equals to the width of the transducer, after reaching a distance from the transducer (near zone length or fresnel zone), the beam width narrows to half of the transducer width, and after that the width increases (far zone length or fraunhofer's zone), where the lateral resolution decreases. therefore, the wider the transducer width and the higher the frequency of ultrasound, the longer the fresnel zone, and the lateral resolution can be maintained at a greater depth from the transducer. ultrasound waves travel in pulses. therefore, a shorter pulse length requires higher bandwidth (greater number of frequencies) to constitute the ultrasound pulse. as stated, the sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner, in the beamforming or spatial filtering technique. this focusing produces an arc-shaped sound wave from the face of the transducer. the wave travels into the body and comes into focus at a desired depth. materials on the face of the transducer enable the sound to be transmitted efficiently into the body (often a rubbery coating, a form of impedance matching). in addition, a water-based gel is placed between the patient's skin and the probe to facilitate ultrasound transmission into the body. this is because air causes total reflection of ultrasound; impeding the transmission of ultrasound into the body. the sound wave is partially reflected from the layers between different tissues or scattered from smaller structures. specifically, sound is reflected anywhere where there are acoustic impedance changes in the body: e.g. blood cells in blood plasma, small structures in organs, etc. some of the reflections return to the transducer. receiving the echoes the return of the sound wave to the transducer results in the same process as sending the sound wave, in reverse. the returned sound wave vibrates the transducer and the transducer turns the vibrations into electrical pulses that travel to the ultrasonic scanner where they are processed and transformed into a digital image. forming the image to make an image, the ultrasound scanner must determine two characteristics from each received echo: how long it took the echo to be received from when the sound was transmitted. (time and distance are equivalent.) how strong the echo was. once the ultrasonic scanner determines these two, it can locate which pixel in the image to illuminate and with what intensity. transforming the received signal into a digital image may be explained by using a blank spreadsheet as an analogy. first picture a long, flat transducer at the top of the sheet. send pulses down the 'columns' of the spreadsheet (a, b, c, etc.). listen at each column for any return echoes. when an echo is heard, note how long it took for the echo to return. the longer the wait, the deeper the row (1,2,3, etc.). the strength of the echo determines the brightness setting for that cell (white for a strong echo, black for a weak echo, and varying shades of grey for everything in between.) when all the echoes are recorded on the sheet, a greyscale image has been accomplished. displaying the image images from the ultrasound scanner are transferred and displayed using the dicom standard. normally, very little post processing is applied. sound in the body linear array transducer ultrasonography (sonography) uses a probe containing multiple acoustic transducers to send pulses of sound into a material. whenever a sound wave encounters a material with a different density (acoustical impedance), some of the sound wave is scattered but part is reflected back to the probe and is detected as an echo. the time it takes for the echo to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. the greater the difference between acoustic impedances, the larger the echo is. if the pulse hits gases or solids, the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to progress further. the frequencies used for medical imaging are generally in the range of 1 to 18 mhz higher frequencies have a correspondingly smaller wavelength, and can be used to make more detailed sonograms. however, the attenuation of the sound wave is increased at higher frequencies, so penetration of deeper tissues necessitates a lower frequency (3–5 mhz). penetrating deep into the body with sonography is difficult. some acoustic energy is lost each time an echo is formed, but most of it (approximately 0.5 db cm depth ⋅ mhz \cdot ) is lost from acoustic absorption. (see acoustic attenuation for further details on modeling of acoustic attenuation and absorption.) the speed of sound varies as it travels through different materials, and is dependent on the acoustical impedance of the material. however, the sonographic instrument assumes that the acoustic velocity is constant at 1540 m/s. an effect of this assumption is that in a real body with non-uniform tissues, the beam becomes somewhat de-focused and image resolution is reduced. to generate a 2-d image, the ultrasonic beam is swept. a transducer may be swept mechanically by rotating or swinging or a 1-d phased array transducer may be used to sweep the beam electronically. the received data is processed and used to construct the image. the image is then a 2-d representation of the slice into the body. 3-d images can be generated by acquiring a series of adjacent 2-d images. commonly a specialized probe that mechanically scans a conventional 2-d image transducer is used. however, since the mechanical scanning is slow, it is difficult to make 3d images of moving tissues. recently, 2-d phased array transducers that can sweep the beam in 3-d have been developed. these can image faster and can even be used to make live 3-d images of a beating heart. doppler ultrasonography is used to study blood flow and muscle motion. the different detected speeds are represented in color for ease of interpretation, for example leaky heart valves: the leak shows up as a flash of unique color. colors may alternatively be used to represent the amplitudes of the received echoes. expansions an additional expansion of ultrasound is bi-planar ultrasound, in which the probe has two 2d planes perpendicular to each other, providing more efficient localization and detection. furthermore, an omniplane probe can rotate 180° to obtain multiple images. in 3d ultrasound, many 2d planes are digitally added together to create a 3-dimensional image of the object. doppler ultrasonography main article: doppler ultrasonography duplex scan of the common carotid artery doppler ultrasonography employs the doppler effect to assess whether structures (usually blood) are moving towards or away from the probe, and their relative velocity. by calculating the frequency shift of a particular sample volume, flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualized, as an example. color doppler is the measurement of velocity by color scale. color doppler images are generally combined with gray scale (b-mode) images to display duplex ultrasonography images. uses include: doppler echocardiography is the use of doppler ultrasonography to examine the heart. an echocardiogram can, within certain limits, produce 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, abnormal communications between the left and right side of the heart, leaking of blood through the valves (valvular regurgitation), and calculation of the cardiac output and 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 measurements of interest. transcranial doppler (tcd) and transcranial color doppler (tccd), measure the velocity of blood flow through the brain's blood vessels through the cranium. they are useful in the diagnosis of emboli, stenosis, vasospasm from a subarachnoid hemorrhage (bleeding from a ruptured aneurysm), and other problems. doppler fetal monitors use the doppler effect to detect the fetal heartbeat during 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 and provides information similar to that provided by a fetal stethoscope. contrast ultrasonography (ultrasound contrast imaging) main article: contrast-enhanced ultrasound 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 used throughout the world, for echocardiography in particular in the united states and for ultrasound radiology in europe and asia. microbubbles-based contrast media is administrated intravenously into the patient blood stream during the ultrasonography examination. due to their size, the microbubbles remain confined in blood vessels without extravasating towards the interstitial fluid. an ultrasound contrast media is therefore purely intravascular, making it an ideal agent to image organ microvasculature for diagnostic purposes. a typical clinical use of contrast ultrasonography is 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, as in echocardiography to improve delineation of left ventricle for visualizing contractibility of heart muscle after a myocardial infarction. finally, applications in quantitative perfusion (relative measurement of blood flow) have emerged for identifying early patient response to anti-cancerous drug treatment (methodology and clinical study by dr. nathalie lassau in 2011), enabling the best oncological therapeutic options to be determined. parametric imaging of vascular signatures (diagram) in oncological practice of medical contrast ultrasonography, clinicians use 'parametric imaging of vascular signatures' invented by dr. nicolas rognin in 2010. this method is conceived as a 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 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, the latter 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 is 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 a parametric image (see 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 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 systemic biopsy (with inherent risks of invasive procedures) 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 current future of contrast ultrasonography