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 ::

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

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

ULTRASOUND BIRDS VIEW PART 1

Sound is a mechanical energy that is transmitted through a medium .The term MECHANICAL is defined as a physical change induced by a force, sound waves require elastic deformable medium for propagation .Ultrasound is defined as a high frequency mechanical wave that humans cannot hear with frequencies greater than 20K Hz. Waves are divided into two basic types :longitudinal and transverse, Ultrasound waves are longitudinal waves but only bone can cause production of transverse waves ,sometimes referred as shear waves or stress waves. Sound has a greater velocity in air (low density) than in bone (high density),Velocity is inversely proportional to the square root of the compressibility of the medium √B ,compressibility indicates the fractional decrease in volume when pressure is applied to the material, easier the medium to compress ,higher the compressibility, as bone is less compressible velocity of sound is more in bone .Reciprocal of compressibility is given by Bulk modulus, which means bulk modulus is directly proportional to velocity .As bulk modulus increases compressibility decreases in turn increases velocity of sound in the medium referring to this property as stiffness of medium The major interaction of interest for diagnostic ultrasound is that of reflection .if a sound beam is directed at right angles (normal incidence) to an interface (boundary between tissue type) larger than width of the sound beam, the beam is partially reflected back towards the sound source .the interface is called specular reflectors, and they are responsible for major organ outline seen in diagnostic ultrasound examination

The angle of reflection of the sound bean is equal to the angle of incidence of the sound beam. To obtain maximum reflected signal the transducer must be oriented so that the generated sound beam strikes the interface perpendicularly (normal incidence).to understand the concept of reflection of energy let us go back to basic physics of conservation of energy and conservation of momentum .conservation of energy would permit to transfer all its energy from one object to another, where as conservation of momentum prevents this due to differences in masses .Energy cannot be transmitted readily from large objects to small objects or from small objects to loarge subjects .If massive transfer of kinetic energy are required collision between objects of equal masses must occur .for sound waves which are a vibrating molecules ,they to behave in same manner ,as long as the molecule is transmitting energy to an identical sized molecule ,maximum transfer occurs .If there is a difference in the mass of the molecule ,less energy is transferred and the energy that is not transferred is reflected .In ultrasound ,the analogy to momentum is acoustic impedance .here ,one does not look at individual molecules ,but rather at their concerted action ,there by applying the concept of density.

Momentum (mechanics)=Mass × velocity Acoustic impedance( Ultrasound)= Density × Velocity

To simplify the term acoustic impedance I would say it is equal to resistance

High density materials give rise to high sound velocities and therefore high acoustic impedance, similarly low density materials such as gasses have low acoustic impedence .If acoustic impedence is the same in one medium as in another ,then the sound is readily transmitted from one medium to the other .a difference in acoustic impedance causes some portion of the sound to be reflected at the interface .this ist the main reason for ultrasound images .

The reflection coefficient for intensity is expressed as

α =(z2-z1/z2+z1)^2

α =reflection coefficient

z2 =impedance of medium 2

z1=impedance of medium1

The difference between two impedance subtracted any way and squared gives the same number, if acoustic impedance difference is small magnitude of the reflected wave is small. Bone is avoided during an ultrasound examination because acoustic impedance .As the difference between bone and tissue is very large most of the wave is reflected, little of the beam is transferred behind the bone and most of the incident beam returns to the detector, that is the reason why ultrasound is restricted to bony region .Thickness of the medium is never considered in calculation of reflection ,only impedance miss match is a point of concern

Another interaction that occurs between ultrasound and tissue is that of refraction .If the ultrasound beam strikes an interface between two media at an angle of 90 degree with respect to the surface ,a percentage of beam is reflected back to the first medium ,and the rest is transmitted into second medium without a change in direction ,if velocity of sound in both the medium will be equal then no refraction will occur although acoustic impedance may be different .For ultrasound image it is assumed that ultrasound beam always travel in straight lines through tissue .another important aspect is scattering and absorption ,all other modes of interaction except absorption decreases the ultrasonic beam intensity by redirecting the energy of the beam . Absorption is a process where in ultrasonic energy is transformed into another energy form, primarily heat which is used for physiotherapy .The absorption of the medium is related to frequency of beam ,viscosity of medium and relaxation time of the medium .If substances have a short relaxation time ,the molecules return to their original position before the next compression of wave arrives .however if the medium has a long relaxation time ,molecules may be moving back toward their original position as the wave crest strikes them ,more energy is required to stop and then reverse the direction of the molecule there by producing more heat