Rad-103: Ch 5 Digital Image Characteristics, Receptors, and Image Aquisition

how is raw data stored

as electronic data set

what happens to electronic data set

processed by computer for viewing

what does computer processing alter in an electronic data set

brightness and contrast

what is advantage of automatic processing through computer

any anatomic structure can be well visualized

how is a digital image recorded

as a matrix or combination of rows and columns of pixels

what is a pixel

small, usually square, ‘picture element’

how is the size of a pixel measured

in microns

100 microns = .1mm

how is each pixel recorded

as a single numerical value represented as a single brightness and shade of gray

what does the location of a pixel in an image matrix correspond to

an area within patient or volume of tissue

what is a spatial domain of an image matrix

when the location of a pixel within a matrix corresponds to ana area within patient or volume of tissue

what happens when a data point is reconstructed

placed in precise spatial position, representing anatomy

what is the field of view (FOV)

anatomic area of interest

what is field of view made of

matrix

what effect does matrix size have on digital image

larger matrix = greater number of pixels = improved quality

what is negative effect of larger matrix

increased computer processing time, network transmission time, digital storage space

what is the formula for relationship between pixel size, FOV, and matrix

pixel size = FOV / matrix size

what happens to pixel size, matrix, or FOV is either is increased

increased FOV for fixed matrix = increased pixel size (directly related)

increased matrix size for fixed FOV = decreased pixel size (inverse relationship)

how is the numerical value assigned to each pixel determined

by relative attenuation of x-rays passing through corresponding volume of tissue

how does assigned numerical change for pixels representing higher attenuation

different value assigned for higher brightness

what does pixel bit depth determine

amount of precision in digitalizing analog signal and number of gray that can be display in image

what is bit depth determined by

analog-to-digital converter (ADC)

how is bit depth expressed

2 to the power of n or number of bits (2ⁿ)

what does larger bit depth allow

greater number of shades of gray to be displayed

what kind of system has better contrast resolution

one that can digitize and display greater number of shades of gray

what does increased contrast resolution do to an image

increase visibility of anatomic details and ability to distinguish among small anatomic area of interest

relationship between pixel amount and size

inverse relation

what is pixel density

number of pixels per unit area

what does large pixel density improve

spatial resolution

what factors determine pixel pitch

pixel size, spacing, or distance from center of a pixel to adjacent pixel

what do smaller-sized pixels have

decreased pitch

increased spatial resolution

what is spatial frequency

unit of line pairs per millimeter (lp/mm)

what is a resolution test pattern

device used to record and measure line pairs

how does spatial frequency change with object size

smaller objects have higher

larger objects have lower

how does changing lp/mm affect spatial resoslution

increasing lp/mm increases spatial frequency

what is spatial resolution range for DR

3.5 - 5.5 lp/mm

what is directly impacted by pixel size in digital imaging systems

ability to resolve or demonstrate specific spatial frequency

what does dynamic range on an imaging system refer to

ability of an IR to accurately capture range of photon intensities that exit the patient

how does wise IR range affect under or over exposure

allows for image to still be diagnostic

what is exposure latitude

range of exposure that should be used to produce a diagnostic image

how many intensities can digital IR capture

16,000 - 65,000 intensities of exit radiation depending on bit depth

what does air kerma do

specifies intensity of x-rays at a given point in air at a known distance from focal spot/x-ray source

what is dose area product (DAP)

measure of exposure in air used by DR systems

what happens after DAP is measured

computation occurs to measure patient dose

where is DAP located

embedded in collimator

what does DAP value depend on

exposure factors and field size

what does DAP reflect

patient exposure

total volume of tissue being irradiated

what is kerma area product (KAP)

product of total air kerma and area of x-ray beam at entrance of patient

how are KAP and DAP unites expressed

in micro, milli, and centigray per area squared

what do KAP and DAP provide indication of

patient risk

what is modulation transfer function (MTF)

measure of imaging system’s ability to display contrast and spatial resolution of anatomic objects varying in size

what to MTF values mean

0 = no difference in brightness levels/

1 = maximum difference between brightness levels / exact representation of feature in terms of contrast and spatial resolution

what is MTF a numerical expression of

recording systems ability recreate an image with optimum contrast and spatial resolution

what is detective quantum efficiency (DQE)

measurement of efficiency of IR in converting x-ray exposure into a usable electronic signal that can be converted to a quality radiographic image

what does high DQE indicate about radiation exposure

less required to produce quality image

what is DQE value impacted by

type of material used in IR to capture exit radiation (greater for DR)

energy of x-ray beam (higher for amorphous selenium)

spatial frequency

MTF (directly proportional)

what is signal-to-noise (SNR) ratio

method of describing the strength of the radiation exposure in comparison to the amount of noise apparent in a digital image

what does increasing SNR cause

improves digital image quality (high signal strength compared to noise)

how does quantum noise occur

too few x-ray photons captured by IR to create raw image data

what are sources of image noise

electronics that capture, process, and display image

how is electronic noise minimized

with special design of circuit components

what is contrast-to-noise ratio (CNR)

method of describing the contrast resolution compared with the amount of noise apparent in a digital imahe

what does noise affect

SNR

anatomic visibility

contrast of digital image

what factors determine spatial resolution

sampling frequency (CR only)

pixel size

pixel density

pixel spacing

detector element (DEL) size (DR only)

how do DR receive and convert exit radiation into electronic signals

self-scanning readout mechanism with an array of x-ray detectors

how are DR images available quicker

combine image capture and readout

what are flat-panel detectors (FPD)

solid state IR that use a large-area active matrix array of electronic components

FPD size range

43 × 35 cm to 43 × 43 cm

what is the purpose of FPD layers

receive and convert x-ray photons to electrical charges for storage and readout

what features are integrated in a FPD

signal storage

signal readout

digitizing electronics integrated into the FPD

what are the FPD layers composed of

top layer = x-ray converter

second layer = houses thin-film transistor (TFT) array

third layer = glass substrate

what is the TFT array divided into

square detector elements

what is each TFT array detector element have

capacitator to store electrical charges

switching transistor for readout

where is the electrical signal send for digitzation

ADC

what does a smaller DEL size do

increase spatial resolution

what is DEL size range

200 - 60 micron

how can an accurate signal be created in DEL technology

digitizing electronic signal within DEL

what can affect original digital signal

magnetic disturbances from surrounding

radiofrequency (R/F) disturbances from surrounding

what does the DEL x-ray sensitive (sensing) area represent

each pixel in image matrix

why can a pixel only capture a percentage of x-rays

it is smaller than DEL

what is fill factor

percentage of x-ray capture (80% - 95%)

what does a scintillator is indirect conversion detector do

convert remnant radiation into visible

scintillator exampnes

CsI

gadolinium oxysulfide (Gd₂O₂S)

what does the phosphorous-type material in scintillators do

produces light following absorption of x-rays

what happens to visible light by photodetectors

converted into electrical charges

how are the electrical charges temporarily stored

by capacitors in TFT array

what happens after electrical charges are stored

digitized by ADC and processed in computer

what is a scintillator

material that absorbs x-ray energy and emits light in response

what are the two ways a design of scintillator can be

structured

unstructured

what do structured scintillator phosphors reduce

spread of visible light

what type of scintillator design produces images with higher spatial resolution

structured

what type of scintillator design uses less radiation to produce quality image

structured

examples of other DR indirect conversion devices

charge-coupled devices (CCD)

complementary metal oxide semiconductor (CMOS)

how does CCD react to light

light sensitive

can respond to low light intensities and wide range

what is scintillator for CCD

CsI phosphor plate

what is CsI

highly efficient hygroscopic material (absorbs moisture) that must be hermitically sealed to avoid water absorption and prevent rapid degeneration

what can CsI be paired with and how

CCD

using fiberoptic bundle or optical lens system

how does x-ray work in CCD devices

light energy transmitted to device → converted to electrical signal → sent to computer workstatsion for procession and display

why could an x-ray receptor have array of closely spaced CCD

due to technical limits of the size of CCD devices

what is tiling

process where seveal CCD detectors adjoin to create a larger detector

what can tiling cause

seams with unequal response