radi 462 quiz 4

function of digital detectors

capture the remnant (exit) beam/info from the patient; photoelectric absorption, transmitted photons, scattered photons

xrays → light → electrons/electrical signals

indirect

xrays -→ electrons/electrical signals

direct

indirect detector; has a protective layer, phosphor layer, and support layer

PSP (photostimulable phosphor plate)

what is the phosphor layer of a PSP plate made of?

barium fluorohalide crystals with europium (added to trap electrons so they don’t recombine with the barium fluoride, don’t want to lose the data)

barium fluorohalide _ xrays

absorbs; ionizations result in electrons which get trapped by europium

the plate gets removed travels along rollers into a _ for data extraction

scanning assembly

direction plate moves along roller system

slow scan

direction helium neon (red) laser scans the plate (perpendicular to direction of plate travel)

fast scan

_ determines the pixels/mm (pixel size)

sampling frequency of the laser

starts at _ and goes to the _ when the fast laser scans the entire plate

top, bottom

electrons trapped in _ are the image data

europium

laser scans plate, electrons rise up out of traps and lose energy in the form of light; this light is our image data and gets emitted and captured by a light guide (fiber optic tubes) and travels to PMT (photomultiplier tube); the PMT then converts light into electrons/electrical activity

indirect conversion process

there are fluorescent light bulbs in a reader device that do plate erasure in order to release remaining trapped electrons from the plate that may cause _ and _; data extracted from PMT plate, travels back and turns on erasure lamps

low contrast images, ghost images (superimposing of old images with new image)

if a plate gets grossly overexposed, this creates extra data the reader can’t handle, have to keep running plates through erasure cycle

why we erase plates before scanning

if a plate is not used for _ hours, it needs to be erased before scanning/have to run through erasure cycle, turn on erasure lamps to remove radiation without sending to scanning mechanism

48

FPD with TFT array direct FPD

xrays → electrons

FPD with TFT array indirect FPD

xrays → light → electrons

TFT array consists of _ layered on a glass substrate (base)

individual detector elements

what is each little square on a TFT array called?

DEL (detector element)

each DEL consists of a _ and _

active area/layer of semiconductor material for image acquisition/capture; electrical components in the corner of every DEL for image data storage and release

direct FPD has an active layer made out of…

amorphous selenium (a-Se) which absorbs xrays and energy is directly converted into electrons/image data

electrical components of a direct FPD

capacitors (store energy, don’t want to lose the information) and thin film transistors that do signal readout

with a direct FPD, remnant beam photons enter and x-rays are…

absorbed and converted directly into electrons in the a-Se/amorphous selenium

with an indirect FPD, you now have a _ layer made out of _

scintillator layer (creates light); cesium iodide (CsI) or gadolinium oxysulfide, depending on the manufacturer; this layer absorbs xrays and energy is converted to light

the active layer of an indirect FPD is made out of…

amorphous silicon (a-Si), which absorbs light and energy is converted to electrons

indirect FPD electrical components

capacitors that store electrons and thin film transistors that do signal readout

remnant beam photons enter, xray photons are absorbed and converted to light in the CsI, light is absorbed and converted to electrons in the a-Si, capacitors in each DEL store electrical charges (electrons)

indirect FPD with cesium iodide (lab grown phosphur) - columnar/structured

remnant beam photons enter, xray photons are absorbed and converted to light in the gadolinium oxysulfide, light is absorbed and converted to electrons in the a-Si, capacitors in each DEL store electrical charges (electrons)

indirect FPD with gadolinium oxysulfide (also phosphur crystals) - turbid/unstructured

each DEL has its own electrical configuration/readout connection that allows for signal release

image data (electrical signals) read out by TFTs

data is extracted from each DEL in a _ manner

row and column manner, orderly release

capacitors store charges/electrons/our image data; when electrons need to be released from DEL, current will come through image detector and cause electrons to be released from readout connection in each individual DEL

image extraction

charge coupled device (CCD) is a _ detector and converts _ into electrons so a _ is needed to convert _

indirect; light; scintillator; xrays into light

CCD is used in conjunction with a scintillator, which can be lens-coupled or fiberoptic-coupled

light travels through lens or fiber-optic tubes to chip so it doesn’t get lost

lens-coupled is multiple or tiled with _ CCDs

192

CCD chip consists of individual pixels that have electrical component (semiconductor material) for image data release and an active area/layer also made of semiconductor material that converts light into electrons; the active area takes up the…

entire pixel for image acquisition/capture and storing electrons; the electrical component triggers active area to release its information

the active layer of a CCD (charge coupled device) is made of _ and the function is _

silicon; absorbs light and converts it to electrons, then stores these electrons in the silicon

the electrical component of a CCD is made of _ and the function is _

polysilicon; charge readout, tells the active layer when its time to release those charges

image extraction for CCDs - image data is read out by electrical components and pixel charges are emptied by…

rows, in an orderly fashion

with CCDs, the _ empties into a readout register and charges move _ to exit the chip; succeeding rows follow simultaneously, with charges moving _

top row; horizontally; vertically, then horizontally

CCD data is extracted by _

bucket brigade

CMOS (complementary metal-oxide semiconductor) is a _ detector so it uses…

indirect; a scintillator to convert xrays into light first; uses less power than CCDs to operate and tend to be less expensive to manufacture

CCD pixels (image data) are read out _

individually (like a FPD)

data formats // data is _ in nature, whereas _ data is _

analog/continuous; digital, discrete

_ format is necessary for computer processing, data manipulation, storage, etc.

digital

image data conversion // extracted analog data is sent to _ for sampling and quantization

an analog-to-digital converter (ADC or A/D)

_ extracts data points from the analog signal (sampling) and measures electrical signals and assigns discrete numerical values to the data (quantization)

ADC

quantization values are based on _

bit-depth

smallest area depicted in an image; two dimensional square; contains discrete gray shade and each one has its own distinct gray shade

pixel (picture element)

two dimensional array of pixels; “X” and “Y”

matrix

in digital imaging, pixels are assigned numerical values (signal values) that are relative to the _

signal intensity received by the detector; dependent on xray interactions in tissue like absorption and scatter, xray transmission, and exposure factor selection

high value of pixel means

decreased brightness/luminance

low value of pixel means

increased brightness/luminance

image data in each pixel (what’s visually represented) consists of pieces of data called _; smallest quantity of information with two possible values of 0 or 1

bit (binary digit)

computers handle data in groupings called _

byte

1 bite = ? bits

8

a group of 8 bits represents a _ numerical value (gray shade) for a particular pixel

single

all 1s show up as _

black ; air, transmitted

all 0s show up as _

white ; complete absorption, no signal received

with digital images, image data consists of more than 8 bits/pixels, for example…

10, 12, 14, 16 bits

bits are still bundled together in groups of 8, so each group of 8 bits = ? and 10, 12, 14, or 16 bits = ?

1 byte; 2 bytes

determines the number of gray shades available per pixel (grayscale)

bit depth

formula for bit depth

2^n where n = how many bits can be in one system; 2 for 2 byte system

more bits = greater bit depth = _ contrast resolution

higher; better representation of image data

the ability to render subtle differences in grayscale, and how well structures with similar attenuation characteristics can be distinguished from one another

contrast resolution

detector exposure is converted into _ values where every pixel is assigned a signal value relative to exposure intensity; contrast results from the _ between adjacent exposure intensities/level

signal; signal differences

high contrast is _ scale and low contrast is _ scale

short; long

subject contrast is impacted by:

photon energy (kvp, tube filtration, filters), part composition (atomic #, mass and electron density), tissue volume (part thickness, beam restriction)

compton typically involves _ photon energy than photoelectric

higher

filament vaporization causes _ in tube filtration

increase

beam hardening causes _ in average energy

increase

low subject contrast tissues due to tissue _

similarities (like atomic number and mass density); example: fat and muscle in the abdomen

high subject contrast tissues due to tissue _

differences (in atomic number and mass density, like in chest or extremities)

as part thickness increases, amount of attenuation _

increases

increasing OID (air gap) _ contrast

increases; increasing the "air gap" (OID) between the patient and the IR decreases the amount of scatter reaching the IR - increasing OID decreases scatter and increases contrast

initial processing

look-up table (LUT)

post-processing

window width (windowing)

ability to delineate between two adjacent structures, in particular reference to the smallest object that can be detected

spatial resolution

measuring spatial resolution

line pairs/millimeter (lp/mm) test tool; parallel lead strips separated by a space equal to the width of the strips; tool grouped as sets of lines and spaces; spatial resolution determined by the number of lines and spaces clearly visualized; blurring has a greater impact on distinguishing smaller structures

object size (mm) = ?

1 / 2*lp/mm

at 5 lp/mm, what is the resolvable object size?

1 / 2×5 = 1/10 = 0.1 mm

factors impacting spatial resolution captured during image acquisition:

detector characteristics, motion, geometric factors, distortion

how well spatial resolution is visualized is impacted by:

scatter, processing, monitor settings, viewing conditions

end-to-end pixel sized

the physical distance from one edge of a single pixel element to the opposite edge

center to center pixel pitch

the distance in millimeters from the center of one pixel to the center of the immediately adjacent pixel

pixel size measurement unit

micron

as pixel density increases, what happens to pixel size?

decreases

what is the relationship between pixel size and spatial resolution?

as pixel size increases, spatial resolution decreases (inverse relationship)

most common cause of image blur

motion

to prevent motion, use _ exposure time

lower