Chapters 13-17 Flashcards
What is harmonics?
multiples of the transducer frequency
Where are harmonics created?
In the tissues
What creates harmonics?
non-linear behavior
Where does sound move slightly faster?
in regions of compression (higher pressure)
where does sound travel slightly slower?
in regions of rarefraction (lower pressure)
what is the major disadvantage of pulse invertion imaging?
the frame rate is half that of fundamental imaging
What does pulse inversion do?
degrades temporal resolution while improving spatial resolution
What is brightness related to?
the brilliance of an image
What does contrast determine?
the range of brilliancies that are displayed (Bistable images are high contrast)
Analog (real world)
a variable attains a continuum of values
what is a pixel?
the smallest element of a digital picture
what does pixel density improve?
spatial or detail resolution
what is spatial resolution related to?
the number of lines per frame (more lines, better spatial resolution)
what do more bits per pixel give you?
more shades of gray and more contrast resolution
what is a byte?
a group of 8 bits
preprocessing
TGC, log compression, write magnification, fill-in interpolation, persistance, spatial compounding
how can write magnification improve temporal resolution?
if imaging is shallower
fill in interpolation improves what?
spatial or detail resolution
how is artifact created?
by the interference effects of scattered sounds, both constructive and destructive
how are scan lines steered?
by the transducer in different directions or views
dynamic apeture
optimizes lateral resolution @ all depths
edge enhancement
increases the contrast @ a boundary; ideally suited to distinguish interfaces
where does coded excitation take place
in the pulser
rendering
creates an element of realism to a 3D or 4D image
what is dynamic range?
the ratio of the largest to the smallest signal strength that each component processes, the number of choices
PACS
Picture Archiving and Communication Systems
DICOM
Digital Imaging and Communications in Medicine
NAS
Network storage devices
what are two types of laminar flow?
plug and parabolic, found in normal physiologic states
turbulence
may be identified as spectral broadening
units of viscosity
poise
what causes increased blood viscosity?
increased hematocrit or hemoglobin
what does anemia reduce?
viscosity because blood is thinner
where does velocity change @?
@ a stenosis
where in a stenosis is pressure lower?
@ the point of maximum narrowing
what are the units of electrical resistance?
Ohms
what are the resistance vessels in the circulation?
arterioles
what does valsalva accomplish?
increases pressure in the thorax and reduces all venous flow
what is a positive doppler shift?
when source and receiver are approaching each other, reflected frequency is higher than transmitted frequency
Units of Doppler shift?
Hertz, cycles per second
what are the typical values of Doppler shift?
20 Hz- 20KHz
Doppler shift =
2 x reflector speed x incident frequency x cos (angle) /propagation speed
what must be known in order to accurately determine velocity?
the angle between the directions of flow and the sound beam
what does doppler measure?
velocity, not speed
what does the magnitude of shift depend upon?
the cosine of the angle between the sound beam & the direction of motion
