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FAQ for DoctorsArrow


» What is Ultrasound and how these images are produced?
Diagnostic Ultrasound uses sound energy with wave frequencies that exceed 20000Hz,the audible range in humans. When a sound beam crosses an interface between objects of different densities, some of the energies are reflected and some are transmitted ,the amount of each proportional to the difference in density. Reflected waves are detected by a transducer that creates an image of the object.
» What generates Ultrasound ?
The ultrasound waves are generated by a piezo-electric transducer which is capable of changing electrical signals into mechanical (ultrasound) waves. The same transducer can also receive the reflected ultrasound and change it back into electrical signals. Transducers are both transmitters and receivers of ultrasound.
» What are the different modes of Ultrasound ?
The various modes show the returning echoes in different ways.
-- A-mode: (Amplitude mode)- the echoes are shown as peaks and the distances between the various structures and can be measured.
-- B-mode :-the echoes are shown as two dimensional images in which the echo amplitude is depicted as dots of different brightness.
-- Real-time: This mode displays motion by showing images of the part of the body under the transducer as it is being scanned. The images change with each movement of the transducer or if any part of the body is moving(for example ,a moving fetus or pulsating artery ).The movement is shown on the monitor in real time ,as it occurs.
-- M-mode: another way of displaying motion . The result is a wavy line .This shows movement as a function of time. Used in cardiac scanning
» What is Doppler effect? Explain?
" The Doppler effect is the phenomenon by which the frequency of a wave received after reflection by a moving target is shifted from that of the source."
When ultrasound is transmitted towards a stationary reflector, the reflected waves will be of the same frequency as those originally transmitted. If the reflector is moving towards the transmitter, the reflected frequency will be higher than the transmitted frequency. Conversely, if the reflector is moving away from the transmitter, the reflected frequency will be lower than the transmitted frequency.
The difference between the transmitted and received frequencies is proportional to the speed with which the reflector is moving away from or approaching the transmitter. This phenomenon is called the Doppler effect and the difference between the frequencies is called the Doppler shift.
» What are the basic types of Doppler Ultrasound units?
The Doppler effect makes it possible to detect and measure the rate of movement of any fluid such as blood. In blood, the moving reflectors are the red blood cells. To measure this movement there are two basic types of Doppler Ultrasound unit, the continuous wave(CW) and the pulse wave(PW).
1. In a continuous wave Doppler unit, the ultrasound is continuous and the unit measures high velocities accurately, but there is no depth resolution so that all the movement among the ultrasound beam is shown together.
2. In a pulse wave Doppler unit, The Ultrasound is transmitted in pulses of ultrasound into the body, with good depth resolution. It can be aimed directly to measure the speed of blood vessels in a particular vessel.
» What is Duplex Doppler?
In a duplex Doppler system, a blood vessel is located by B-mode ultrasound imaging and then the blood flow is measured by Doppler ultrasound. This combination of a B-mode and Doppler system allows the Doppler beam to be directed more accurately at any particular blood vessel.
» What is Color Doppler?
In a color Doppler unit, the distribution and direction of the flowing blood are shown as a two dimensional image in which the velocities are distinguished by different colors.
» What is Power Doppler?
An alternative to the display of frequency information with color flow Doppler imaging is it use a color map that displays the integrated power of the Doppler signal instead of its mean frequency shift. The image does not provide any information related to flow direction or velocity, and power mode Doppler imaging is much less angle dependent than frequency based color flow Doppler display. In contrast to the color flow Doppler, where noise may appear in the image as any color, power mode Doppler permits noise to be assigned to a homogenous background color that does not greatly interfere with the image This result in a significant increase in the usable dynamic range of the scanner, permitting higher effective gain settings for flow detection and increased sensitivity for flow detection.
» How many scans are required in a pregnancy?
The number of scans required in a pregnancy depends on the clinical condition of the patient. The indications for scan can be to confirm intrauterine pregnancy, bleeding per vaginum, to rule out congenital anomalies, to asses fetal weight and growth, to asses fetal well being, to know placental location, suspected multiple gestation, suspected fetal death etc It is necessary to get at least one scan in a pregnancy, even if no complications are seen. The ideal time of US scan is 16-18wk size to rule out congenital anomalies.