ULTRASOUND BIRDS VIEW PART 3

DIGITAL SIGNAL AND IMAGE PROCESSING

Some accuracy is sacrificed when digital scan converter is used, the loss of accuracy occurs in the translation of information from detection system (signal amplitude and location of the reflecting structure) to a form that is understood by the computer, this process could be called as analog to digital conversion and limited by the number of bits available in the digitization process. In ultrasound the analog signal is a voltage wave form generated by the returning echo striking the Transducer , increased intensity relates to increased voltage

Now let us study using a example rather than theory ,for this let us take two ADC for two different types of scanner just for better understanding and these scanner are configured to digitize the signal over a range of 0v to 5v,Scanner A has a 2bit (4 discrete steps)ADC and Scanner B has a 3(8 discrete steps) bit ADC.The corresponding step size is calculated by dividing the range (5V) by the total number of discrete steps available (either 4 for 2bit OR 8 for 3 bit) which would give a value of( 5/4=1.25V) and (5/8=0.625V).Now if the signal peak corresponds to 1.25V then when digitilasied gives a value of (1.25/1.25=1) for a 2 bit scanner would be (01) in case for a 3 bit scanner if the voltage level is 1.875V volts then for a 3 bit scanner it would be (1.875/0.625=3) that would be (011). The accuracy of the scanner can be increased by increasing the bit depth

SPATIAL REPRESENTATION

The information obtained from the scanned area is divided into small, square picture elements called “pixels” which are combined as building blocks to form the image .During data collection the digitalized signal is stored in a particular location of memory specified by the address .Each pixel has one location reserved in memory for holding the amplitude of the signal associated with that pixel. The address is determined from x and y coordinates, generated by transducer positioning –sensing electronics and time between the transmitted pulse and the received echo

This is analogues to a Puzzle where in the piece of puzzle is restricted to a square, each of which has a uniform shade, but only the value is stored in memory for each pixel .The same value is assumed to exist throughout the pixel ,here some accuracy is sacrificed by digital representation of spatial resolution