Digital Image Display and Assessment

Pixel

picture elements, smaller size make better resolution, x-y dimensions

Matrix

number of rows and columns that make up image on CRT, larger size create better resolution

Voxel

depth of pixel(gray scale), determined by gray scale bit depth that determine dynamic range of system, 3 dimensional

Field of View(FOV)

overall dimensions of image, fixed for digital radiography(same size of IR), selectable for CT/MRI

Gray Scale Bit Depth

available number of bits used to display image, 2¹⁴ digital radiography

Dynamic Range

range of exposures over which a detector can acquire an image, relates to sensitivity of IR

Exposure Latitude

amount of exposure error that will still lead to a useable image, in digital 50% underexposure to 200% overexposure ok

Noise

detected background information that does not contribute useful information to image, quantum mottle, measured by SNR which determines if object can be detected, high SNR best

SNR

signal(number of x-ray photons that strike receptor(mAs), noise(negative factors of final image),

High SNR

signal(mAs) greater than noise allowing low contrast soft tissue structures to be demonstrated

Image Contrast and Noise Relationship

inverse, higher noise decrease image contrast

Commercial S/N

200

High Resolution Digital Fluoroscopy S/N

1,000

Detective Quantum Efficiency(DQE)

ability of radiography detector to convert x-ray to useable signal based on measurement of combined effects of contrast and noise on imaging system, try to use lowest dose

DQE Ratio

output signal²/input signal² of IR or SNR²out/SNR²in

DQE Ranges

between 0 and 1, 1 perfect with no lost information

Excessive DQE

image noise becomes more evident at lower exposure levels

Best DQE

provide lowest dose and yields diagnostic image at given spatial resolution(lp/mm), gains ability to view small low contrast objects(mammography)

Noise Sources

quantum statistics, electronic, fixed pattern, sampling, anatomical, quantum

Quantum Statistics

random pattern of x-ray photons

Electronic Noise

noise from electrical systems

Fixed Pattern Noise

variation of intensities created by pixel function

Sampling Noise

aliasing, improper sampling of electronic signal

Anatomical Noise

created by different absorptions of body

Quantum Noise

lack of mAs or kVp

High DQE

high SNR/CNR ratio and decreased/low exposure to patient

Digital X-Ray DQE

impact both patient dose and image noise, PSP’s(BaFl) lowest, indirect digital better, direct capture digital best, higher allow higher speed class operation

MTF

best in direct DR then indirect Si then CR

Superior Image Quality and Object Detectability

require both low noise and high contrast

S Number

exposure indicator number in cassettes

Fluoroscopy Resolution Factors

progressive scanning, slow scanning, 1024 vs 525 line systems

Bandwidth

amount of information a computer can handle per unit of time, higher allow more resolution capabilities

Progressive Scanning

scans all 1023 lines of image monitor in order instead of using raster pattern

Slow Scanning

improves resolution but cannot be used with real time fluoro

High Resolution Video(1000 Line System)

scans 1050 lines not just 525 and uses higher bandwidth(20 MHZ), improves resolution 5-7lp/mm

CR Spatial Resolution Factors

size(width) of laser beam and fast read sweep(sampling frequency), pixel and matrix size, cassette size

Spatial Resolution Formula

FOV/matrix

Pixel Sampling Pitch

measurement from center to center of adjacent ultimately determines spatial resolution, defined by Nyquist Frequency in CR and its resulting MFT

Monitor Spatial Resolution

½ of pixel density because it takes 2 pixels to see image detail

Indirect Si Light Spread

from scintillator resulting in doubling of aperture size effectively negating small pixel size

Direct Digital Light Spread

use no scintillator and spatial resolution equal to detector size

Digital Contrast Resolution

smallest exposure change captured by IR, impacted by sensitivity of detector, controlled by dynamic range

Contrast-Noise Ratio(CNR)

amount of contrast in image divided by noise, high desirable with its ability to detect small low contrast anatomy

CNR Improvement Methods

decrease scatter radiation(reduce kVp, collimation), use contrast agents, reduction of quantum noise by increasing exposure to IR, post-processing of image(low pass filter)

CR Image Quality Factors

kVp, resolution, dynamic range

CR kVp

contributing factor for contrast, excessive cause fog and loss of contrast, below 80kVp needed for non-grid, inadequate penetration result in lack of contrast(controlled by LUT)

CR Resolution

2.5-5lp/mm, speed class choice impacts noise in image

CR Dynamic Range

number of shades of gray that can be displayed by imaging system, response linear in CR and DR, all digital have WIDE range

Closely Spaced Objects

need both high spatial and contrast resolution to be appreciated

Modulation Transfer Function(MTF)

accuracy of image compared to original object, perfect is 1, recorded contrast of image compared to subject contrast, best measure of image contrast in digital radiography

Size of Object Impact on MTF

as object size or distance between objects increase the MTF increases

Spatial Resolution

ability to delineate one thing from another or differentiate closely spaced objects

Spatial Frequency and MTF Relationship

inverse, as spatial frequency of object increases the MTF decreases

Contrast Resolution

ability to see objects or features with similar brightness values

MTF Mathematically

percentage of object contrast to image contrast at particular spatial frequency

Exposure Indicator CR

read from center of plate in CR and resulting number indicates exposure to imaging plate and indirectly indicates patient exposure

Exposure Indicator DR Flat Panel

exposure indicated by dose area product(DAP) using meter mounted near collimator assembly

DAP

dependent on size of collimated field and mAs/kVp levels not SID, does not directly indicate over or under exposure to patient but notes total exposure to IR, registered in cGy/cm² or mGy/cm² or Gy

Sensitivity

how sensitive the IP is to exposure shown by exposure indicator

Exposure Indicator

number of indicating average radiation exposure to IP in order to form CR image, indicate under or over exposure, used by CR reader to compare with LUT

S-Number (Fuji)

number that is inversely proportional to exposure reaching IR, 150-250 properly exposed, over 250 underexposure

Exposure Index (Kodak)

number directly proportional to radiation received by IR, 1800-2200 indicate properly exposed, over 2200 indicates over exposure

Log Medium Exposure(LgM) (Agfa)

compares exposure to baseline exposure developed for each department, .3 increase means exposure doubled

Standardized Digital EI

developed in 2010 by IEC and AAPM in cooperation with vendors, new equipment uses standardized and deviation index

Exposure Index Measure

exposure to IR, NOT the patient

Manufactures EI Responsibilities

calibration of image detector, providing methods to segment anatomical data, generating EI proportional to detector exposure

User EI Responsibilities

define each patient and anatomy-specific information with target index(EIt), subjective based on speed class at which operator and physician decide to operate detector

DI 0

correct exposure

DI +1

25% over target exposure

DI +3

2 times over target

DI -1

20% underexposure

DI -3

2 times underexposure

DI Indication

if technique is appropriate for specific anatomical body part with acceptable S/N

Dose Index Registry(DIR)

extracts dose index from image headers for comparison with other institutions, developed by ACR

Exposure Indicator Factors

scatter(direct), SID/OID(indirect), collimation(diagonal bad), image receptor size(too large bad), positioning(must center to part), VOI(hardware cause incorrect number)

Et Ranges

DI -1(EI 20% below), DI +1(EI 25% above), green(optimal -3 to +2 under or over exposed 50%), yellow(acceptable -5 to +4 under or over exposed 75%), red(out of range(<-5 or >+4)

Quinn B Carroll EIt Recommendation

-.5 to .5

Dose Area Product

measurement of amount of radiation absorbed by patient, determined by Kerma(entrance dose x field area)

Kerma

kinetic energy released in matter used to express radiation concentration delivered to point by radiographic and fluoroscopic equipment, amount of energy released per kilogram of air in Gya

Kerma Area Product(KAP)

used in fluoroscopic units to express DAP, takes Kerma value and multiplies it times exposed area and shown in same units cGy-cm² as DAP

Acquisition Artifacts

scratches and dust artifacts; moire pattern and phantom images

Post-Acquisition Artifacts

algorithm artifacts, histogram analysis error, banding

Display Artifacts

density, brightness, image enhancement artifacts

CR IP Scratches

show white linear lines, depend on polarity of monitor, requires plate be replaced

Dust Artifacts

seen on CR light guide reader or laser mirror in printer, white linear line across ENTIRE image

Algorithm Artifacts

mathematical errors related to image construction

Moire Pattern

appearance of grid lines caused by scan frequency and direction matching that of grid

Phantom Images

appearance of previous images on IP, most often occurs because of incomplete erasure, plates need erased at minimum every 48hrs

Image Lag

image data left over from previous exposure on IR or unattenuated radiation(near leaded marker), corrected using offset voltage or waiting longer between exposures

Flat Fielding

pre-processing calibration used to correct response differences between pixels, offset(many a day) and gain(every few months) images automatically used to calibrate images

Dead Pixels

pixel that is always off, may result in missed data and misdiagnosis, tested for with QC tests, corrections made through pixel interpolation

Signal Interpolation

response of pixels surrounding defective pixel averaged and that value is assigned to defective pixel

Electronic Shutter Failure

image processing algorithm incorrectly identified the collimator blades and incorrectly applied electronic shutters, fixed through reshuttering/reprocessing image

Image Quality Assessment

noise, brightness/contrast, spatial resolution, artifacts, exposure number

PACS

picture archiving and communication system, computer system used to acquire/display/transmit/store images from digital modalities

PACS Functions

integrates medical record on HIS with RIS, displays acquired images through hospital network, allows images to be accessed over internet

PACS Advantages

images manipulated without additional exposure, rapid retrieval, less physical storage space required, images sent across internet and viewed in many locations

PACS Parts

1.image acquisition

2.display stations

3.archive servers with database/image manager server, short and long term storage, work-flow manager computer

Film Digitizers

used to convert analog to digital images after advent of digital