Ch 8 Signal Processor + Image Processor Flashcards
What is the signal processor?
-Receives digital signals from beam former
-After processing, it sends digital signals to the image processor
List 3 functions of the signal processor that occur in order after the beam former?
-Bandpass filtering
-Amplitude detection
-Compression (dynamic range reduction)
What is a bandpass filter?
-A filter that rejects frequencies above + below the accepted bandwidth
-The filter eliminates frequencies outside the echo bandwidth + only retains the accepted most useful frequencies
What is amplitude detection?
-Aka demodulation
-Is the conversion of echo voltages from radio frequency form to video form, where it retains amplitudes of echo voltages
What is compression + dynamic range?
Compression:
-Process of decreasing the smallest + largest echo amplitudes to a usable range
Dynamic range (dB):
-Ratio of the largest to smallest echo amplitudes from the pt
-Display DR is much higher than visual DR (which is range of signals visual to human eye)
Why is compression needed?
B/c display dynamic range is much higher than the visual dynamic range of the human eye
Is compression changeable by the sonographer?
-Some is done by the receiver (not our control)
-Video compression is controlled by us, as a post processing technique
Is compression + dynamic range directly or inversely related?
Inversely related
High compression = less DR of signals
(meaning now high contrast as signals are black/white)
Low compression = high DR of signals
(meaning low contrast as signals are more shades of grey)
Which setting gives us the highest level of contrast resolution?
High compression, low dynamic range
(images will make anechoic structures stand out + grey structures stand out - big contrast)
Why is compression important?
-When imaging for thrombi + masses
-If the compression settings are incorrect it can lead to misinterpretation of pathology
-Be mindful of the compression when imaging tissue parenchyma as well (ex. liver, kidneys)
What type of resolution is compression important for?
Contrast resolution
What is the image processor?
-Converts digitized, filter, detected + compressed scan line data that is stored in image memory
-Locates each series of echoes to each scan line for each pulse, filling up the memory with echo info
-Is accomplished in a fraction of a second
List functions of the image processor?
-Preprocessing + postprocessing
-Persistence
-3D acquisitions
-Storing image frames
-Cine loops
-Gray scale
-Color scale
-3D presentation
-Digital to analog conversion
What is scan conversion?
-Signals from the beam former + image processor are still in A-mode
-Scan conversion is the process of converting A-mode signals into B-mode signals
-It assists grayscale values to pixel locations on the screen, resulting in an image
(in B-mode each amplitude is mapped to a grayscale level/brightness)
List the 2 roles of scan conversion?
-To convert A-mode lines into B-mode lines
-To organize the lines of data into a formatted image
Explain image memory?
-After the echo signal converts from A-mode into B-mode, it gets stored in the image memory
-Numbers correspond to the level of brightness from the location of the echo along the matrix
Differentiate frame vs FR?
Frame: single image produced by 1 complete scan of the sound beam
FR: # of frames stored per second
What gets stored in image memory?
Frames (either a freeze frame or cineloop)
Define pixels + bits?
Pixels:
-2D image is divided into squares called pixels in a matrix
-each pixel is represented by multiple layers of brightness
-these layers can be renamed bits (which describes the total # of grayscale levels possible)
Bits:
-digital representation of the shade of grey
-binary digits consist of only 0 or 1
The greater the matrix size, the more or less pixels in the matrix?
More (this improves detail resolution)
The smaller the matrix size, the more or less pixels in the matrix?
Less (this creates poor resolution)
What does 1 + 2 mean in regards to bits?
1: bright display
2: dark display
The human eye can see how many shades of grey?
<64 shades
(u/s shows 256 shades)
How can we calculate bit depth?
2^n
(ex. 2^8 = 256 shades)
