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Monday, October 10, 2011

ULTRASOUND BIRDS VIEW PART 4

IMAGE ACQUISITION

After transducer sends the short duration pulse and the echo is reflected back ,the transducer converts the echo into an electronic pulse that can be converted into digital format using the ADC ,The X and Y co-ordinates are specified by the position generator that uses the time of travel for the ultrasonic pulse and position sensor for beam direction .The signal amplitude and position co-ordinates for multiple line of sight are placed temporarily in the buffer before final storage in RAM .Changing the shade of Gray level could improve the quality of image and it can be done well processing techniques that could be a TGC ,Edge enhancement ,logarithmic compression ,fill in interpolation ,selective enhancement ,Image updating and write zoom

· Time Gain Compensation:: Ultrasound attenuates as it propagates through the body. When used to image the biological tissues, the echo signals need to be progressively amplified to allow the visualization of the shallow regions simultaneously with the deeper regions. Time Gain Compensation, or TGC, is that amplification step. The amplification coefficient, or gain, needs to increase with depth, as the signal gets smaller and smaller. But in ultrasound imaging, the speed of sound is usually assumed to be constant, creating a direct equivalence between distance and time (TGC) Time gain compensation is the signal processing control that allows the sonographer to amplify the returning signal from deeper structures in the body. The TGC control compensates for the attenuation of sound waves.

· Edge Enhancement :: ultrasound imaging require to detect the interface or boundaries between one structure and another . Edge enhancement is a filtering technique

Pixel no

1

2

3

4

5

6

7

8

Original value

200

200

200

200

100

100

100

100

Kernel

-1

3

-1

Edge enhanced value

200

200

200

300

0

100

100

100

Suppose eight pixels along one line of sight have initial values as in the table, the filtering process uses a Kernel (collection of weighing factors or convolution coefficient) to calculate the edge enhanced value for pixel number 4

New value =200 × -1 + 200 × 3 + 100 × -1 ==300

Similarly move the window for other pixel values

· Logarithmic Compression :: Compression is necessary because the display device is unable to process an extremely wide range of signal level

· Fill in Interpolation:: The scanned area are represented as pixel values in the image matrix and some matrix area are not filled which could have a value “0” and it could like a chess box so the area with “0” matrix is replaced by averaged signals of the nearby matrix

· Selective Enhancement :: In addition to TGC there are keys provided for selective enhancement ,this is because amplification is in a non uniform manner ,by selective enhancement the amplification is done as a function of depth ,each key corresponds to certain depth

· Write Zoom :: also called as regional expansion ,this is required to zoom an area of a region

POST PROCESSING TECHNIQUES

· Black And white inversion :: Pixel value of 240 which is normally depicted as white is depicted as near black on the inverted image

· Freeze Frame :: when freeze frame is used the interested area is used for prolonged viewing and updating of output buffer is discontinued

· Frame averaging or Persistence :: this helps in increasing signal noise ratio ,frame averaging allows successive frames to be added together to increase SNR

· Contrast enhancement :: Contrast enhancement is an powerful technique ,suppose Gray value from 0-56 are of no use as they are weak signals and not of interest they can be eliminated by redistributing these gray scales

· Smoothing :: is an image processing technique used to reduce random noise ,Random noise is a parameter that describes the variation of signal level associated with a particular interface

· Distance calculation ::

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