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| Prior exposure of detector leads to differences in sensitivity of x-rays at different parts of the detector |
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| Two or more highly reflective structures cause "ping-pong" of the beam between the structures causing multiple bands of noise to show up |
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Series of parallel bands posterior to gas collection
When US beam hits air bubble, the energy causes vibration of the bubble; the vibration of the bubbles creates a continuous sound wave that gets transmitted back to transducer |
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Def:
photons hit detector from something other than x-ray beam |
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superimposed image
cassette used twice without erasing receptor |
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| insufficient number of x-rays pass through to produce a useful image |
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| Through Transmission (US) |
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Increased signal distal to a lesion
Lesion (e.g. cyst) attenuates less sound than adjacent structures so the area beyond the lesion appears brighter than its surroundings |
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Reduced amplitude of multiple echoes caused by structures of high attenuation (e.g. stone)
Sound passing through the structure is weaker and echoes from tissue behind the stone are attenuated
The structure is so dense that it attenuates all the US beam and echoes from the beam cannot penetrate beyond it |
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Low level echoes in the shadow deep to gas
2/2 high degree of reflection at gas/tissue interface
transmission through gas is essentially the same as the original transmitted pulse |
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Multiple closely spaced small bands of echoes
Form of reverberation
Two reflective interfaces (and thus sequential echoes) are closely spaced, maybe so closely that individual signals are not perceived
later echoes have decreased amplitude as a result of attenuation which leads to a triangular/tapered shape |
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Magnetic susceptibility describes the behavior of material when placed in external mag field
Diamagnetic, Para, Ferro are classes of materials by susceptibility
Ferromagnetic substances develop strong internal magnetic field parallel to direction of external field which dramatically increases it
Susceptibility artifacts observed where magnetic susceptibility varies rapidly (such as nasal cavity) and result from the presence of metal objects
Commonly in T2-weighted, GE, and Echo planar (DWI) sequences
Appears as image distortion w/ warped anatomy
Appears as signal voids, surrounded by bright rim |
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| ALIASING (wrap-around) (MR) |
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Tissue excited outside FOV is recorded artificially w/in FOV
occurs when FOV is smaller than the surrounding tissue |
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Duplicated copies of anatomic structures with a reduced intensity
may appears as a blurring or motion
Caused by physiologic and/or patient motion or pulsatile flow (aorta) |
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Multiple, alternating bright/dark lines (ringing)
Fourier-based imaging technique leads to image errors near sharp boundaries
Typically involved w/ bright signal boundaries such as spinal cord b/t CSF/tissue, other T2 fluid images, or T1 fat |
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shifting of fat in one direction 2/2 misregistration of fat and water
TYPE 1 - fat/water interfaces separate on images - bright band is where fat and water are seen to overlap while dark band is where fat and water are separate
TYPE 2 ("India ink") - boundaries of tissues with large differences in fat/water content are outlined in black - THIS IS OUT OF PHASE IMAGING |
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caused by detection of extraneous RF signals; usually malfunctioning or incompatible medical equipment (syringe pumps, open door to magnet)
thin stippled/dashed band through the image |
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