this presentation deals with various modes of image acquisition,its storage,processing & the artifacts which are obtained in this imaging modality

1. ULTRASOUND
INSTRUMENTATION

2. Presentation modes
• A mode
• B mode
• M mode

3. A Mode
• A-mode systems have no memory, and a permanent record is obtained by
photographing the CRT monitor
• The CRT represents a graph of echo voltage on y-axis to time on x-axis
• A-mode may be used in ophthalmology or when accurate distance
measurements are required

4. B Mode
• B-mode is the electronic conversion of the A-
mode and A-line information into brightness-
modulated dots on the display screen
• The brightness of the dots is proportional to
the echo signal amplitude
• The B-mode display is used for M-mode and
2D gray scale imaging

5. M Mode
• M-mode or T-M mode displays time on the
horizontal axis and depth on the vertical axis
• The spikes of A-mode are converted into dots, and
brightness replaces amplitude

6. Image
acquisition

7. Pulser : The pulser produces electric pulses that drives the
transducer (T) through the beam former. It also includes a clock
that determines the pulse repetition frequency (PRF) and
synchronizes the various components of the instrument.
Beam former : The beam former performs all the tasks necessary
for beam steering,transmit focusing, dynamic aperture and any
other additional timing requirements for phase arrays.

8. Receivers
• The receiver performs the following functions:
(1) amplification
(2) compensation
(3) compression
(4) demodulation
(5) rejection
• Amplification
Amplification is the conversion of the small voltages received from
the transducer to larger ones suitable for processing and storage.

9. Compensation (Gain Compensation or Depth Gain
Compensation)
• Compensation equalizes differences in received echo amplifications
because of reflector depth.
• The attenuation depends on depth, reflectors with equal reflection
coefficients will not result in equal amplitude echoes arriving at the
transducer if their travel distances are different.

10. Compression
• Compression is the process of decreasing the differences between
the smallest and largest amplitudes.
• This is accomplished by logarithmic amplifiers that amplify weak
inputs more than strong ones. In other words, compression lowers
the systems’ dynamic range.

11. Demodulation
• Demodulation is the process of converting the voltage delivered to the
receiver from one form (radio frequency, RF) to another (video).
• This is done by rectification and smoothing.

12. Rejection
• Rejection (also called suppression or threshold) eliminates the smaller
amplitude voltage pulses produced by weaker echoes (multiple scattering
from within tissue) or electronic noise.

13. Overall receiver process

14. Signal processing

15. Digitization(preprocessing)
To store echo information in digital memory the demodulated voltage
amplitudes representing echoes must pass through an analog-to-
digital converter (ADC).
Digital pre-processing is performed to assign numbers to echo
intensities.

16. Contrast resolution
• For linear pre-processing assignments, the echo dynamic range (40 dB
below) is equally divided throughout the gray levels of the system.
• The more gray levels (bits/pixel) that are used, the better the contrast
resolution between adjacent pixels

17. Image Memory
• Image memory used in ultrasound instrumentation are of the digital
type. These memories are some times called digital scan converters
because they provide a means for displaying, using a television scan
format information acquired in a linear or sector scan line format.
• The image plane is divided into a 512 x 512 pixel matrix, with each
pixel being 8 bit (256 gray values) deep.

18. Image Storage
• Image storage (without compression) is typically 0.25 MB/image
• For real-time imaging (10 to 30 frames/s) this can amount to hundreds of
megabytes
• Color images used for Doppler studies increase the storage requirements
further because of larger numbers of bits needed for color resolution (full
fidelity color requires 24 bits/pixel, 1 byte each for the red, green and blue
primary color)

19. Image Display (Post-processing)
• Digital post-processing is performed to assign specific display brightness
to numbers derived from specific pixel locations in memory.
• Since monitors are analog output devices a digital-to-analog (DAC) must
be used to convert the digital pixel value to a brightness value on the
monitor.
• Look-up tables (LUT) can be used to alter the way the image can look on
the monitor.

20. Dynamic range
• The ratio of the largest to the smallest echoes processed by
components of an ultrasound device is known as the dynamic
range of the device.
• In general, the dynamic range decreases as signals pass
through the imaging system because operations such as TGC
and rejection eliminate small and large signals.
• Echoes returning from tissues can have dynamic ranges of 100
to 150 dB.
• The dynamic range in decibels may be easily converted to a
ratio of amplitudes or intensities

21. Ultrasound
image artifacts

22. Speckle
The unscattered reflections are referred to as speckle
The speckle pattern changes from frame to frame, even for stationary
objects, and therefore is not a simple indicator of even the number of
such objects.
However, various attempts have been made to determine the presence or
absence of disease in organs from statistical analysis of speckle patterns

23. Shadowing and Enhancement
• If some object within the patient has a larger attenuation coefficient than
the material that lies beyond it, then the settings of the TGC circuit that
would provide appropriate compensation for normal tissue will under
compensate and cause the region beyond the object to appear less
echogenic.
• This phenomenon is referred to as acoustic shadowing.
• Similarly, if the object in the path of the ultrasound beam has a lower
attenuation coefficient than its surroundings, acoustic enhancement may
result.

24. Multiple pathway
• Various types of multiple-pathway artifacts occur in ultrasound images
When an echo returns to a transducer, the imaging device assumes that
the sound traveled in a straight line following a single reflection from some
interface in the patient.
• The scan converter then places the brightness value at an appropriate
location in the image.
• If the actual path of the echo involved multiple reflections, the echo would
take longer to return, and the scan converter would place the interface at a
greater depth in the image

26. Refraction
• Refraction sometimes causes displacement of the sound beam as it crosses
tissue boundaries.
• Because the scan converter assumes that the ultrasound travels in straight
lines, refraction causes displacement errors in positioning reflective
interfaces in the image.
Ultrasound image of aorta duplication artifact.
A refraction artifact which results from the
difference in the velocity of sound between
muscle and fat tissues