Exam 5 Digital Imaging

absorption material is the part that decides if the IR is direct or indirect technology (made of DELS)

TFT flat panel digital image detector

what are the three components of each DEL

1. detector surface area

2. capacitor

3. TFT

stores electrical charge until read-out

capacitor

what is the job of the detector surface area

capture signal

isolates each DEL

reacts like a switch (sends charges to image processor)

Thin Film Transistor (TFT)

layer of microscopic detector elements each containing their own TFT

Active Matrix Array (AMA)

Process of a direct system

x-rays photos are absorbed by photoconductor

photoconductor converts the photons into an electrical signal, which is then stored in the DELs

information is sequentially discharged and read-out with dedicated electronics

low noise, high-sensitivity: amplifiers perform read-out amplification, and analog-to-digital conversion

Line scanning sequence (lifeguard telling the swimmers at the top to go)

what is the main photoconductor for the direct system

amorphous selenium (a-se)

describe the process of an indirect system

photons strike the scintillator

scintillator converts photons into light

light is then converted into an electrical charge by a photodetector 

electrons stored into the capacitor within the DEL until the time for line sequencing and read out

T/F TFT arrays are the same for both systems

True

Gd2O2S (unstructured and turbid)

Gadolinium Oxysulphide

what does the gadolinium oxysulphide detector do

air pockets allow light (indirect) to escape laterally before it reaches the TFT surface, reduces efficiency, lowers spatial resolution

Csl (structured)

thallium doped cesium iodide 

most popular a-Si detector

Csl

describe the process of using the Csl detector

tubules act as a guide for light (indirect) form the scintillation layer, almost no light speed, increased efficiency, increased spatial resolution (higher DQE)

non-functioning pixels

dead pixels

what are some of the reasons that cause dead pixels

dust, scratches, static damage, chemical corrosion, age

what are the software programs that identify and isolate dead pixels

interpolation (fills in dead space using surrounding pixels as a guide)

gain calibration

“flat-fielding” corrects flaws in the detector

gain calibration

what makes gain calibration important

removes potential of having artifacts that interfere with diagnoses

removes densities on an image by taking out the unwanted densities and leaving only diagnostic information

faint image from previous exposure still visible, previous image’s signal not getting completely erased (double exposure artifact)

image lag

what are the causes of image lag

rapid succession of images

overexposure in an area with little beam attenuation (lead marker)

ways to prevent image lag

increase time between exposures

proper collimation

appropriate technical factors

how to correct image lag

detector software

dark noise

offset correction

how much noise can be tolerated in the image

Signal-to-noise ratio

relationship between noise and patient exposure

inversely proportional

a decrease in noise leads to what in patient exposure

higher exposure (high mAs, less noise)

spatial resolution and DQE are what type of measurements

numerical measurements for performance

how efficiently a system converts the x-ray input signal into a useful output image

Detective Quantum efficiency (DQE)

Higher DQE = ___ quality images at lower doses

higher

Increased DQE (higher DQE) does what to patient dose

lowers it

anything that has a DEL, assessment of quality of a flat-panel detector

fill factor

a fill factor is a ratio of ___ areas to the __ __ area of the DEL

sensitive, non-sensitive

High fill factor does what to DQE

increases it

what are the things that a high fill factor effects

high contrast resolution (increased SNR)

less patient exposure and dose

measures from the center of one DEL to the center of an adjacent DEL

DEL pitch

DEL includes both ___ and ___ areas, unlike Fill Factor

sensitive, non-sensitive

spatial resolution =

½ times the DEL pitch

equation for DEL pitch

SR=1/2P

what are the alternative options to TFT indirect technology (replace flat panel array)

CCD and CMOS

describe the process of the CCD device

1. x-ray photons interact with scintillation material

2. signal transmitted by the lenses or fiber optics to the CCD

3. lenses reduce image size

4. image is transferred to small capacitors where light is converted into an electrical charge

5. charge is released sequentially line by line and sent to ADC

which of the two is more susceptible to noise

CMOS

what is the basic construction and function of CMOS

• light photons stored in individual capacitors within each pixel

• each pixel has its own amplifier that converts light photons into electrical signal

• created voltage from amplifiers is converted by the ADC

between CCD and CMOS which is more sensitive to light

CCD 

between CCD and CMOS which uses more power

CCD

between CCD and CMOS which is more expensive

CCD

between CCD and CMOS which has a greater pixel fill factor

CCD

difference between cassette-based vs cassette-less PSP

cassette processing with a reader vs internal processing of plate without a cassette (they are both still PSP because they utilize photosimulation of the imaging plate)

Film or PSP the cassette doesn’t need to be light-tight and is read with a CR reader

PSP

Film or PSP may be single or double emulsion

film

Film or PSP barium fluorohalide is the active element

PSP

Film or PSP cassette needs to be completely light tight and must be processed in a dark room

Film

Film or PSP usually just a single emulsion

PSP

Film or PSP silver halide is the active element

film

its durable, lightweight and plastic material

the antistatic material

• protects against static electricity

• dust collection

• mechanical damage to plastic

backed by a thin sheet of aluminum

• absorbs back scatter

structure and function of the PSP cassette

the protective layer (1st)

thin, tough clear plastic (protects phosphor layer)

phosphor layer (2nd most important layer)

active layer, barium fluorohalides work here (traps electrons during the exposure)

light reflective layer (3rd)

some detail is lost in this step

• sends light forward once its released from the reader (black, reduces light spread)

conductive layer (4th)

absorbs and reduces static electricity

support layer (5th)

semi-rigid material for plate strength

light shielding layer (6th)

prevents light from erasing data and leaking through the back

backing layer (7th)

soft polymer, protects back of cassette

barcode must be scanned and connected to patient position from the exam menu

(barcode may be linked with patient info)

cassette based

match the exam to the patient through the worklist

no barcode on the plate

cassette-less

what are orientation markers

cassette-based systems- colored marker or sticker (indicates appropriate orientation of cassette in relation to patient)

• requires less post-processing manipulation when oriented correctly

smallest imaging plate possible = ___sampling rate

highest

how ist eh entire cassette read

by the CR reader

the size of the matrix determines the ___ of the pixels

size

fitting the matrix within our cassette size is going to influence __ ___

spatial resolution

if the matrix is 512 × 512

cassette is 14×17

creates ___ pixels

less spatial resolution

bigger

if the matrix is 512 × 512

cassette is 10×12

creates ___ pixels

more spatial resolution

smaller

how is film different from CR when the image is being taken

1. light from intensifying screen +photons interact with silver halide

2. electron ejected from halide

3. attracted to sensitivity speck

4. silver ions attracted to now negative charge

how does photostimulation work

some light is given off when XR strikes PSP

• fluorohalides luminesce from XR interaction

• not all of the energy is released with the light

• trapped energy forms latent image

what are the four steps of reading and erasing the imaging plate

using laser to read the plate

digitizing the signal

erasing the image

preprocessing, processing, and forwarding

single laser scan that radiates imaging plate

point scan

scan that has a photomultiplier tube

point scan

scan that has several linear laser units and optical light collection lenses

line scan

scan that has a ccd linear array photodetector collecting equipment

line scan

movement of the laser across the imaging plate “scan”

fast scan direction

movement of the imaging plate through the reader “translation” or sub-scan direction

slow scan direction

Light Amplification of Stimulated Emission of Radiation

LASER

what is the main purpose of a laser

release stored electrons

what does beam shaping optics do

ensures the same beam output no matter the location (consistent spatial resolution)

what are the three steps to beam shaping

1. shaped by special optics that keep the beam size, shape, and speed largely independent of beam position

2. beam deflector- moves laser beam rapidly back and forth across the IR to stimulate the phosphors

3. light collection optics- direct released phosphor energy to an optical filter and then to photodetector

imaging plate is scanned with__ light in a raster pattern, the light that is emitted is __ when its relaxed into lower energy levels

red, blue

converting light signal to an electrical signal

digitization

returns electrons to lower energy state (removes image from plate)

erasure

how often should the erasure process occur

once a week to remove buildup of background signal and scatter

(surface of imaging plate is flooded with light)

exposure

photostimulation of IR

enter IR into reader

Laser excited electrons to release latent image

photodetector captures emitted light

signal is digitized

plate erased

preprocessing, processing, and forwarding the image

the whole process of taking an image

the ___ the phosphor layer

the ___the pixel size

the higher the resolution

thinner, smaller

sampling frequency must be greater than 2x the frequency of the input signal (changing the sampling frequency affects the spatial resolution of the image)

nyquist theorem

more data in our bins (more sampling frequency) = ___ accurate representation of signal coming off the plate

more

• sampling occurs less than twice a cycle

• results in lost information and a fluctuating signal

• when the signal is reproduced, frequencies above the nyquist frequency

• (grid errors)

aliasing

five causes of PSP artifacts

1. imaging plate

2. image processing

3. plate reader

4. printer

5. operator (picnic)

image processing errors

incorrect LUT selected

inappropriate technique

inappropriate positioning

plate reader errors

white lines are parallel to direction of plate travel

double imaging plates loaded into single cassette

insufficient erasure after exposure

incorrect alignment of grid lines to laser scan direction

printer errors

fine white lines- debris on mirror in the laser printer (on hard copy images)

operator error (PICNIC)

Problem In Chair, Not in Computer

• insufficient collimation

• exposure on the wrong side of the cassette

• underexposure= quantum mottle

image display in DR is ____ of image acquistation

independent