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Monday, October 10, 2011

ULTRASOUND BIRDS VIEW PART 2

ECHO RANGING

In diagnostic ultrasound reflection of sound beam from interface along the ultrasonic path are of primary importance ,An ultrasound wave is transmitted from the body strikes an interface and is partially reflected back to the transducer as determined by the percentage of reflection formula .The reflected ultrasound waves ,arising from the various acoustic impedance mismatch results in ultrasonic detection of interfaces within the body

D=ct

T=time of travel

STATIC IMAGING PRINCIPLES AND INSTRUMENTATION

SONAR plays a major role in development of medical diagnostic ultrasound instrumentation particularly A mode scanning, Transmission mode scanning, the only tech that does not rely on ECHO RANGING principle

A MODE SCANNING PRINCIPLE AND INSTRUMENTATION

This is based on ECHO ranging principle. The term A refers to amplitude which is nothing but the magnitude of the signal as a spike (strength) in one dimension, against the depth of the signal ,depth is directly proportional to time .An increase in the amplitude or strength of the signal give rise to an increase in height of the spike .The variation in strength is the result of percentage of reflectance from the different interfaces and the attenuation of the beam as the ultrasound wave travels to and from the respective interfaces. A mode is used in echoencephalography for detection of midline shift and for localization of foreign bodies in the eye

B MODE IMAGING PRINCIPLE AND INSTRUMENTATION

In B mode imaging, the amplitude of the signal (echo strength) is represented by the brightness of a DOT. The A mode spike on CRT is converted into a DOT .The amplitude of the signal is represented by a DOT, the position of the DOT represents the depth of the interface from the transducer, but our interest is a 2 Dimensional image rather than acquiring signal from a single scan line which could be accomplished by Compound B scanning where in multiple set of dots are combined to delineate the echo pattern from internal structures within the body .There are two problems in B mode scanning which are not present in A mode is determination of transducer position and storage of the scan line information ,to overcome this problem the transducer is mounted on a scanning arm. If the detected echo signal is strong enough ,a positive charge region is created on the strong mesh which is behind the phosphorus screen of the CRT ,this would produce a bistable image where most of the internal organ details are lost so special system called scan converter are used for B mode imaging ,.Gray scale images enables different echo amplitude to be displayed in varying shades of gray so that fine internal structures of organs could be visualized ,the analog scan converter is similar to CRT except that the phosphor face is completely replaced by a wafer of silicon called the dielectric matrix. Another important characteristic of Scan converter is OVERWRITE –PROTECTOR CIRCUIT, each interface is scanned from many different orientation or direction during a B mode scan ,variety of signals are detected for each pixel depending on transducer orientation ,each time the signal is received ,the appropriate location in the scan converter is identified via sensor in the registration arm and the position generator .the system would add the new signal amplitude to the memory only if the new signal strength is higher than the previous

GATED MODE IMAGING PRINCIPLES AND INSTRUMENTATION

The difference in B mode and the C mode that is constant depth mode is a gating circuit ,In B mode all the echoes reflec ted back are taken where as in gated mode echoes only within specified interval of time is taken this is due to the gating circuit ,thus scan data obtained from a specific depth is displayed ,thus creating a plane of interest or a slice

ELECTRO CARDIOGRAPHY MODE

ECG –gated scanning is designed to reduce motion artifact obtained from static B mode scanning of the heart ,the position of the heart creates a blurred image because it is not at a constant position while acquiring image in B mode for this reason ECG acts as a gated circuit By selecting Q part of the wave as gating circuit an image of the heart at the beginning of the ventricular contraction is formed

In Real time gray scale scanner, the displayed image is continuously and rapidly updated with new scan data as the beam is swept repeatedly throughout the field view. .The increased use of Real time scanning is the result of advances in transducer technology.

In A mode the transducer is placed on the area of interest where ultrasound pulse last for 1micro sec and silent period is for about 999 microsec, the returning echoes are amplified and displayed .In B mode the transducer is placed at the starting position and information is recorded along the single line of sight, for a 2 dimensional image, transducer is manually moved by sonographer to a different position

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