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What terms are used to describe performance and specifications
transmit power, dynamic range, signal, noise, noise floor, signal to noise ratio (SNR), Compression, Pre-processing, Post processing
What is transmit power also called
acoustic power, output power, transmit gain, power gain, acoustic gain, output intensity, transmit voltage, output voltage
What does transmit power control
the amplitude of the excitation voltage that drives the crystal(s)
Higher voltage= ___ amplitude mechanical oscillation of the crystal = ____ amplitude sound wave
higher, higher
increasing transmit power and resulting intensity increase can cause
bioeffects and tissue damage
What is the dynamic range
the ratio of maximum to minimums of any quantity
name the different types of dynamic range
input, output, display, and gain
what is input dynamic range
ratio of the maximum input signal to the minimum possible input signal
what is output dynamic range
ratio of the maximum to the minimum output signal
what is the default dynamic range
input dynamic range
What is the input dynamic range formal definition
range of the signal amplitudes a system can receive and process without causing harmonic distortion
what is signal
any phenomenon desired to be measured
what is noise
unwanted signals
what is noise floor
amplitude level below which no signals are visible because of the presence of noise
what is signal to noise ratio (SNR)
amplitude of the signal divided by the amplitude of the noise
Signal to noise ratio specifies...
the quality of the signal and how much faith we should put in the data
What does a Higher SNR imply
better imaging situation excluding artifact and more trustworthy data
A strong signal by itself does/does not guarantee a good SNR
does not
a weak signal by itself does/ does not guarantee bad SNR
does not
Changing the receiver gain does/does not improve the signal to noise ratio
does not, it changes both unless some component of the signal is saturated
So even if the true SNR does not change the
apparent SNR might
A certain amount of what is necessary to make the signal bright enough so that it can be visualized on the display within the dynamic range of the monitor and the human eye
gain
What happens if the gain doesn't map the signal into the proper dynamic range
the signal will appear weak at best because of the eyes or the displays lack of sensitivity
When you increase the gain the signal and noise...
are amplified by the same amount, giving the appearance of improved SNR "apparent SNR"
What is electronic noise
randoms signals caused by electric amplification of small returning echoes
What creates electronic noise?
random excitations of electrons within the electronics
What does electronic noise looks like in doppler spectrum, color doppler, or image
random white speckle with high receiver gain, or random color pixels where there is no flow
When does clutter happen?
when larger returning echoes obliterate weaker signals
How is clutter classified?
as noise but the signal from clutter and other noise sources are distinctly different
where does haze come from
artifact
give an example of when haze is produced
sidelobe returning echoes, poor skin contact, beam distortion (aberration) from tissue characteristic.
What is electrical interference
when the transducer receives energy from other electrical devices or electromagnetic waves such as radio transmission
How does electrical interference occur
it can be carried through the air or from the power supplying the system
what does electrical interference look like
a bright flashlight down the middle of an image or a barber pole flashing
what does electrical interference appear as on spectral doppler?
bright white horizontal or zigzag lines in the spectrum called doppler tones
what is preprocessing
signal conditioning that occurs in real time and cannot be removed from an image once acquired
what is postprocessing
any processing which can be changed after the data is acquired
where is postprocessing usually done?
in the scan converter
postprocessing can be performed on...
frozen data as well as real time
what are examples of preprocessing?
receiver gain, receive focusing, received compression
what are examples of postprocessing?
data compression colorization, and rejection
what is the pulser also known as
the transmit beamformer
what does the pulser do
it creates electrical signals that excite the transducer crystal thus forming sound beams
What must the pulser be able to do
produce infinite numbers of electrical waveforms to drive the variety of transducers used in ultrasound, and produce continuous and pulsed waves
what does the pulser also do
regulates the amplitude of the signals produced, therefore changing the acoustic power output
what is the beamformer
a part of the transmitter that functions with array transducers during transmission and reception
what are two important functions of the beam former
creates the appropriate phase delays and pulse sequencing to create the transmit beam and creates the appropriate phase delays and pulse sequencing to create the receive beams
what does the beamformer do to the signals that return from each element of the transducer?
applies the appropriate processing summing together all the signals
what is apodization
it is the limiting or restricting of which elements are active
what are active elements collectively known as
the aperture
each active element is connected to an ___ and ____ _____
amplifier and processing chain
what are the amplifier and processing chain collectively known as
the receive channel
how many active receive channels are found in high end systems
256-512
the benefit of more receive channels is
greater processing flexibility
the benefits of greater processing flexibility can only really be recognized when transducers exist that have...
as many elements ad the system has channels (512 channels and 500 elements vs 512 channels and 6 elements) Ferrari in a school zone
what operations does the receiver perform?
amplification, compensation, compression, demodulation, rejection, and alalog to digital conversion.
what is amplification also known as
receiver gain
why is amplification necessary
the returning signals from the body are too small to be processed within the electronics or visualized on a monitor
what is amplification controlled by?
the user and partly by the machine
If the gain reads 0, does this mean there is no gain applied to the image?
no, the machine always applies gain, it is always necessary
what does the gain nob do
amplifies or deamplifies the gain already on the image based on the unpredictability of the size of the returning signals from the patient.
what is preamplification
a process to improve the quality of the signal before it gets amplified
where is preamplification done?
as close to the transducer as possible (in the chain)
what are preamplifiers designed to do?
prevent electronic noise from contaminating the tiny signals created by the crystal transducers
what is receiver gain system control also known as
receiver gain, gain, amplification
what does receiver gain do
it affects the amount of amplification of the received signal and does not affect eh intensity on the patient
what is TGC or DGC
time or depth gain compensation
what do TGCs do
its the application of extra amplification to compensate for increasing attenuation with depth
what are the TGC pods normally ranged at
0-60dB to avoid over sensitivity of the pods
are TGCs modifiable from a tech stand point?
no, there are internally applied ones that cannot be adjusted
We want to use the ___ as our coarse adjustment and the ___ as our fine adjustment of the image
gain, TGCs
what is dynamic range?
maximum to minimum range in anything
___ dynamic range is generally much greater than the ___ dynamic range and usually the ___ dynamic range will exceed our ___ dynamic range
signal, display, display, visual
what ranges are usually outside our visible dynamic range
range of returning signals and monitor or display ranges
what is compression
a general term used for any technique that maps a larger dynamic range into a smaller dynamic range
what does compression have to do with the range of returning signals
it maps the range of signals into a smaller dynamic range that our eyes can distinguish
about how many shades of gray can the eye distinguish
20
what does compression allow
for us to visualize different tissues within the 20 visible shades
is compression performed with or without altering the rank between signals
without
can the user control compression?
yes, but there is internal compression that is not modifiable
is the receiver function of compression user controlled?
no its set by manufacturer
receiver function of compression is
a preprocessing function of compression
compression process that happens later in the signal processing can be though of as
video compression, it compresses the displayed gray scale appearance
what does compression warn us of
the limitation of ultrasound
compressing information gives potential to
compress important signals out of visibility relative to the surrounding tissue
what is bistable
an image that looks black and white
what is demodulation
a two part process that changes the electrical signals within the receiver into a form more suitable for display
as a wave propagates through a medium the interactions cause changes or ___ in the wave
modulations
demodulation is a process by which
modulations are removed or detected
what is demodulation also known as
signal detection
what are the two parts of demodulation
smoothing and rectification
what is smoothing also known as
envelope detection
what does rectification do
it converts the negative components of a signal to positive
what does smoothing do
it traces the signal peaks and valleys, applying some averaging or smoothing
when is envelope detection done?
after rectification
what is demodulation like?
the early modality of a mode
what is rejection
it sets threshold below which signals will not be visible on the display
what does rejection do
it suppresses low level noise signals caused by signal through the body, transducer, cable, or system electronics.
rejection affects all
low level signals on the image regardless of location, birhgt or strong reflectors are unchanged.
the reject threshold is set/not set in the receiver
not actually actively set in the receiver but rather is just the limit of the sensitivity of the system
is there a preset reject level?
no, users adjust it
a level is reached below which the signals are not detected, this level is referred to as the
noise floor
what creates greater system sensitivity
the lower the noise floor the smaller the signals that can be detected
what is the balance for reject
pushing the noise floor as low as possible while preserving the required signal input dynamic range and amplification
why is a-mode called thus
the demodulated signal detection was shown along a horizontal line, displaying the amplitude of signals
in a-mode the variation of amplitudes corresponded with
impedance difference between the propagating medium
is a-mode still used
rarely, although b-mode uses that same principles
how is a mode translated to b mode
a mode line is converted into a brightness mapped line, the horizontal line is now represented as depth instead of time, amplitude is reflected as brightness of reflector
where is a to b mode conversion done
the scan converter
what are the two main functions of the scan converter
conversion of a mode lines to b mode lines and organization of the successive lines of data into a formatted image
is the conversion of a to b mode easier on a linear or a sector
linear
what is the scan converter responsible for
keeping track of which line of data should be presented at what location on the screen
once the frame is complete the scan converter is sent a
flag, that tells it the next line received is the first line of the next frame
pre and post processing functions vary
vendor to vendor
what is one of the pluses of more processing power and memory
the machine can store more raw RF data, which can make it easier to pre and post process at any time
post processing system controls which are user controlled:
compression, dynamic range, grayscale, post processing curves or maps, contrast (display)
adding colorization to an image
extends the dynamic range of the eye (theoretically)
colorization is intended to improve visualization of
low level signals preserving a greater dynamic range
the overall effect of colorization depends on the ___ ___ uses and the colorization __ chosen.
compression maps, hue
does colorization really cause enhancement of the image?
no; compression, gain, etc are much more important
where does the analysis package reside?
the back end software
what allows for scrolling back in time, freezing data, and placing calipers?
data storage in digital format in the memory in the back end of the system
with area measurements you want to be
perfectly orthogonal to the structure with the transducer
what plane are lateral and AP measurements best?
transverse
name three different kinds of video displays and monitors
CRT, LCD, and DLP
CRT is
cathode ray tube which up until 5 yrs ago was the most common monitor used in ultrasound systems
LCD stands for
liquid crystal display
DLP stands for
Digital light processing chips
which display types are used in flat panel displays?
LCD and DLP
What is the great thing about flat panel displays?
the matte finish reduces ambient glare
CRT vs LCD
LCD is much lighter, more flexible as far as swiveling, and the monitor is larger
what is NTSC
the National Television Standards Committee, they set US standards
What is the NTSC standard for black and white
525 horizontal lines and 30fps (frames per second), interlaced monitor
Color was added to the NTSC standard which took
longer for frame creating dropping the frame rate to 29.97 Hz
If the machine has a higher FR then the monitor, we.... so...
lose data, so FR of monitor must be more than machines
Interlaced monitors can display approx
30fps
Non-interlaced monitors were developed to
compensate for HD broadcasts, frame rates are double of interlaced to 60fps
What is a pixel
the smallest division of the monitors display
each pixel can be representes as ___ ___ which can stay light or dark
multiple layers
each layer cna be lit or not chanign the level of
brightness of the pixel, regulating the shade of gray
ranme each layer of a pixel with the
bit
if there is only one layer of a pixel it is either _ or _
lit up (1) or dark (0)
one bit displays are
either black or white (bistable)
two layers or a two bit would give us _ shades of gray
4
What is the equation for gray levels
gray levels = 2 raised to the power of bits
most current monitors are
8 and 10 bit monitors (which are well beyond our visual discrimination of shades of gray)
Whare are most current monitors outside our visual discrimination?
this has to do with room ambient light