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Medical ultrasonography
Medical ultrasonography (sonography) is an ultrasound-based diagnostic imagingtechnique used to visualize muscles and internal organs, their size, structure and any pathological lesions, making them useful for scanning the organs. Obstetric sonographyis commonly used during pregnancy. The choice of frequency is a trade-off between the image spacial resolution and the penetration depth into the patient. Typical diagnostic sonography scanners operate in the frequency range of 2 to 13 megahertz. Whilst in physics the term "ultrasound" applies to all acoustic energy with a frequency above human hearing (20,000 hertz), its common usage as a term of medical imaging applies to just a band of frequencies hundreds of times higher.
UsesImage:Sonograph.jpg Ultrasonography (sonography) is widely utilized in medicine. It is possible to perform diagnosisor therapeuticprocedures with the guidance of ultrasonography (for instance biopsiesor drainage of fluid collections). Typically uses a hand-held probe (often called a scan head or transducer) that is placed directly on and moved over the patient: a water-based gel ensures good coupling between the patient and scan head. Medical ultrasonography is used in, for example:
Pelvic ultrasoundA pelvic ultrasound is a major diagnostic tool used to detect Polycystic Ovarian Syndromeand to image the uterusand ovariesor urinary bladder. Ultrasoundsare used during pregnancyto check on the development of the fetus. Men are sometimes given a pelvic ultrasound to check on the health of their bladder and prostate. There are two methods of performing a pelvic ultrasound - externally or internally. The internal pelvic ultrasound is perfomed either transvaginally(in a woman) or transrectally (in a man). See:-
From sound to imageThe creation of an image from sound is done in three steps - producing a sound wave, receiving echos, and interpreting those echos. Producing a sound waveIn medical ultrasonography, a sound wave is produced by creating short, strong pulses of sound from a phased arrayof piezoelectrictransducers(usually a type of ceramic). The electrical wiring and transducers are encased in a probe. The electrical pulses vibrate the ceramic to create a series of sound pulses from each. The frequenciespresent in this sound wave can be anywhere between 2 and 10 MHz; well above the capabilities of the human ear. Any frequency above the capabilities of the human ear is referred to as 'ultrasound'. The goal is to produce a single focused arc-shaped sound wave from the sum of all the individual pulses emitted by the transducer. To make sure the sound is transmitted efficiently into the body (a form of impedance matching), the transducer is coated with rubberand a special gel. The sound wave, which is able to penetrate bodily fluids, but not solids, bounces off the solid object and returns to the transducer, this return is an echo. Receiving the echosThe return of the sound wave to the transducer results in the same process that it took to send the sound wave, just in reverse. The return sound wave vibrates the transducer and turns that vibration into an electrical pulse that is sent through the probe and into sonographer's computer where it can be interpreted and transformed into a digital image. Interpreting the echoThe computer must determine three things from each electrical impulse received: 1.) Which wire did the impulse come from (There are multiple receiving wires on a transducer). 2.) How strong was the impulse. 3.) How long did it take the impulse to be received from when it was sent. Once the computer determines these three things, it can locate which portion of the monitor to light up and what color. Transforming the electrical signal into a digital image can be best explained by using a blank Microsoft Excel Worksheet as an analogy. The wire receiving the impulse determines the 'Column' in our Excel Worksheet (A,B,C,etc.). The time that it took to receive the impulse determines the 'Row' (1,2,3,etc.), and the strength of the impulse determines the color that the cell should change too (white for a strong pulse, black for a weak pulse, and varying shades of grey for everything in between.) InstrumentationUltrasonography (sonography) uses a probe containing one or more acoustic transducersto send pulses of sound into a material. Whenever a sound wave encounters a material with a different acoustical impedance, part of the sound wave is reflected, which the probe detects as an echo. The time it takes for the echoto 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 impedences, the larger the echo is. The difference between gases and solids is so great that most of the acoustic energy is reflected, and so imaging of objects beyond that region is not possible. The speed of sound is different in different materials, and is dependent on the acoustical impedanceof the material. Part of the acoustic energy is lost every time an echo is formed. Sound in the range of hearing and ultrasound can be focused. The echoes received by a stationary probe will result in a single dimensional signal showing peaks for every major material change. To generate a 2D-image, the probe is swivelled, either mechanically or electronically through a phased arrayof acoustic transducers. The data is analyzed by computer and used to construct the image. In a similar way, 3Dimages can be generated by computer using a specialised probe. Some sonographic machines can produce colour images, of sorts. From the amount of energy in each echo, the difference in acoustic impedance can be calculated and a colour is then assigned accordingly. The frequencies used for medical imaging are generally in the range of 1 to 10 MHz. Higher frequencies have a correspondingly lower wavelength, and so images can have a greater resolution. However, the attenuation of the sound wave is increased at higher frequencies, so in order to better penetration of deeper tissues, a lower frequency (3-5 MHz) may be used. MicrobubblesThe use of microbubble contrast media in medical sonography to improve ultrasound signal backscatter is known as contrast enhanced ultrasound. This technique is currently used in echocardiography, and may have future applications in molecular imaging and drug delivery. Doppler sonographyUltrasonography can be enhanced with Doppler measurements, which employ the Doppler effectto 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 a jet of blood flow over a heart valve, its speed and direction can be determined and visualised. This is particularly useful in cardiovascular studies (ultrasonography of the vasculature and heart) and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension. The Doppler information is displayed graphically using spectral Doppler, or as an image using colour Doppler or power Doppler. It is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, sound. Strengths of sonography
Weaknesses of ultrasound imaging
Dangers of ultrasound imagingThere have been disputes whether ultrasound is safe. Since ultrasound is energy, there are questions such as "What are the energy waves doing to my tissue?". There are some reports of low birth weight babies being born to mothers who had more than the recommended ultrasound examination. There may be these side-effects:-
However, there are no substantiated side-effects documented in studies. HistoryUnited StatesUltrasonic energy was first applied to the human body for medical purposes by Dr. George D. Ludwig at the Naval Medical Research Institute, Bethesda, Maryland in the late 1940s. For more on the history of medical ultasonography in the United States see: [{{fullurl:Template:FULLPAGENAME}}#endnote_www.aium.org.599] also [1] The first demonstration of color Doppler by Geoff Stevenson, MD, who was also involved in the early developments and medical use of Doppler shifted ultrasonic energy. See: [2] SwedenMedical ultrasonography was used 1953at Lund Universityby cardiologistInge Edlerand Carl Hellmuth Hertz, the son of Gustav Ludwig Hertz, who was a graduate student at the department of nuclear physics. Edler had asked Hertz if it was possible to use radarto look into the body, but Hertz said this was impossible. However, he said, it might be possible to use ultrasonography. Hertz was familiar with using ultrasonic reflectoscopes for nondestructive materials testing, and together they developed the idea of using this method in medicine. The first successful measurement of heart activity was made on October 29, 1953using a device borrowed from the ship construction company Kockumsin Malmö. On December 16the same year, the method was used to generate an echo-encephalogram (ultrasonic probe of the brain). Edler and Hertz published their findings in 1954. ScotlandParallel developments in Glasgow, Scotland(coincidentally also a major shipbuilding centre) by Professor Ian Donaldand colleagues at the Glasgow Royal Maternity Hospital(GRMH) led to the first diagnostic applications of the technique. Donald was an obstetricianwith a self-confessed "childish interest in machines, electronic and otherwise", who, having treated the wife of one of the company's directors, was invited to visit the Research Department of marine boilermakers Babcock & Wilcox at Renfrew, where he used their industrial ultrasound equipment to conduct experiments on various morbid anatomical specimens and assess their ultrasonic characteristics. Together with the medical physicist Tom Brown and fellow obstetrican Dr John MacVicar, Donald refined the equipment to enable differentiation of pathology in live volunteer patients. These findings were reported in The Lancet on 7th June 1958 as "Investigation of Abdominal Masses by Pulsed Ultrasound" - possibly one of the most important papers ever published in the field of diagnostic medical imaging. At GRMH, Professor Donald and Dr James Willocks then refined their techniques to obstetric applications including fetal head measurement to assess the size and growth of the foetus. With the opening of the new Queen Mother's Hospitalon Yorkhillin 1964, it became possible to improve these methods even further. Dr Stuart Campbell's pioneering work on fetal cephalometry led to it acquiring long-term status as the definitive method of study of fetal growth. As the technical quality of the scans was further developed, it soon became possible to study pregnancy from start to finish and diagnose its many complications such as multiple pregnancy, fetal abnormality and placenta praevia. Diagnostic ultrasound has since been imported into practically every other area of medicine. References
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