Medical Ultrasound Imaging
Thursday, 21 November 2024
 Info Sheets  Out- side     | 'MegaHertZ' Searchterm 'MegaHertZ' found in 6 articles 6 definitions [ • ] Result Pages : •
(Hz) The standard SI unit of frequency. Definition: The number of repetitions of a periodic process per unit time. It is equal to the old unit cycles or oscillations each second of a simple harmonic motion. The unit is named for the German physicist Heinrich Rudolf Hertz. Larger units are kilohertz (kHz) = 1 000 Hz megahertz (MHz) = 1 000 kHz gigahertz (GHz) = 1 000 MHz •
Sonography [aka: ultrasonography] is a term that encompasses the entire process of performing ultrasound examinations and interpreting the obtained images. Sonography involves the skilled application of ultrasound technology by trained professionals known as sonographers or ultrasound technologists. These specialists operate the ultrasound equipment, manipulate the transducer, and acquire the necessary pictures for diagnostic imaging purposes. Sonography requires in-depth knowledge of anatomy, physiology, and pathology to accurately interpret the ultrasound images and provide valuable information to the treating physician. Sonography uses equipment that generates high frequency sound waves to produce images from muscles, soft tissues, fluid collections, and vascular structures of the human body. Obstetric sonography is commonly used during pregnancy. Sonography visualizes anatomy, function, and pathology of for example gallbladder, kidneys, pancreas, spleen, liver, uterus, ovaries, urinary bladder, eye, thyroid, breast, aorta, veins and arteries in the extremities, carotid arteries in the neck, as well as the heart. A typical medical ultrasound machine, usually a real-time scanner, operates in the frequency range of 2 to 13 megahertz. See also Musculoskeletal and Joint Ultrasound, Pediatric Ultrasound, Cerebrovascular Ultrasonography and Contrast Enhanced Ultrasound.  Further Reading: Basics:
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Submicron ultrasound contrast agents are gas-filled, double-walled microspheres with a diameter smaller than 1 μm that rupture when exposed
to ultrasound energy at megahertz frequencies. These agents differ from traditional ultrasound contrast microbubbles in that the submicron bubbles may serve as extravascular agents. They are small enough to travel through the lymphatic system and to be extravasated from tumor neovasculature. The detection of these agents is limited by their hard shell, which requires high-pressure ultrasound insonation
for shell rupture and excitation of the gas bubble. After shell rupture, the gas diffuses rapidly from submicron sized agents. The optimal processing of each echo is important.
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A transducer is a device, usually electrical or electronic, that converts one type of energy to another. Most transducers are either sensors or actuators. A transducer (also called probe) is a main part of the ultrasound machine. The transducer sends ultrasound waves into the body and receives the echoes produced by the waves when it is placed on or over the body part being imaged. Ultrasound transducers are made from crystals with piezoelectric properties. This material vibrates at a resonant frequency, when an alternating electric current is applied. The vibration is transmitted into the tissue in short bursts. The speed of transmission within most soft tissues is 1540 m/s, producing a transit time of 6.5 ms/cm. Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the transducer can be used to determine the depth of the object causing the reflection. The waves will be reflected when they encounter a boundary between two tissues of different density (e.g. soft tissue and bone) and return to the transducer. Conversely, the crystals emit electrical currents when sound or pressure waves hit them (piezoelectric effect). The same crystals can be used to send and receive sound waves; the probe then acts as a receiver, converting mechanical energy back into an electric signal which is used to display an image. A sound absorbing substance eliminates back reflections from the probe itself, and an acoustic lens focuses the emitted sound waves. Then, the received signal gets processed by software to an image which is displayed at a monitor. Transducer heads may contain one or more crystal elements. In multi-element probes, each crystal has its own circuit. The advantage is that the ultrasound beam can be controlled by changing the timing in which each element gets pulsed. Especially for cardiac ultrasound it is important to steer the beam. Usually, several different transducer types are available to select the appropriate one for optimal imaging. Probes are formed in many shapes and sizes. The shape of the probe determines its field of view. Transducers are described in megahertz (MHz) indicating their sound wave frequency. The frequency of emitted sound waves determines how deep the sound beam penetrates and the resolution of the image. Most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency, but multi-frequency probes are also available. See also Blanking Distance, Damping, Maximum Response Axis, Omnidirectional, and Huygens Principle. Further Reading: News & More:
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(US) Ultrasound is very high frequency sound above about 20,000 Hertz. Any frequency above the capabilities of the human ear is referred to as ultrasound. Diagnostic ultrasound imaging uses much higher frequencies, in the order of megahertz. The frequencies present in usual sonograms can be anywhere between 2 and 13 MHz. The sound beam produce a single focused arc-shaped sound wave from the sum of all the individual pulses emitted by the transducer. See also Medical Imaging. Further Reading: Basics:
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[last update: 2023-11-06 01:42:00]