1

Fundamentals of

CR, DR and PACS

J. Anthony Seibert, Ph.D.

Professor of Radiology

Sacramento, California

Learning Objectives

• Explain technology of available and future digital radiography technology

• Understand underlying system operation and characteristics

• Discuss Exposure Indices and the new international standard for CR and DR

• Illustrate integration with PACS architecture and DICOM

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DIGITAL IMAGING IN RADIOLOGY

• Digital imaging is an essential component of Electronic Imaging, Telemedicine and Remote Diagnosis

• Steps for digital imaging – Acquisition

– Display

– Diagnosis

– Distribution

– Archive

The “Big” Picture

• “Digital” Radiology

MRI

CT Nuclear Medicine Ultrasound

Interventional Angio

PACS Digital Radiography

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Imaging Exams

• Projection imaging 70%

• Fluoroscopy 3%

• Computed tomography 8%

• MRI 6%

• Ultrasound 10%

• Nuclear Medicine 3%

Medical Imaging Modalities

• Common thread

– Digital data (and lots of it!!)

• Problems

– Proprietary structures

– Unknown data format

• Solutions

– DICOM and PACS

– HL-7 and RIS

– Networking and Informatics

– IHE integration “profiles”

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But First…… CR & DR

• CR: Computed Radiography using photo-

stimulable phosphors and passive detection

• DR: Direct / Digital Radiography using a

variety technologies and active detection

• An intrinsic part of the PACS

• Historically, the last electronically integrated

• Many advances in the past decade

Digital x-ray detector

Transmitted x-rays

through patient

Charge collection

device

X-ray converter

x-rays electrons

Analog to Digital

Conversion

1. Acquisition

2. Display

3. Archiving

Digital

processing

Digital to Analog

Conversion Digital Pixel

Matrix

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Exposure Latitude

Log relative exposure

Sig

na

l o

utp

ut

10000:1

Digital

Analog versus Digital

Film

100:1

Spatial Resolution

Digital sampling

MTF of pixel aperture (DEL)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10 11

Frequency (lp/mm)

Mo

du

lati

on

500 mm

200 mm

100 mm

Detector

Element,

“DEL”

Sampling

Pitch

Fourier transform of Rect (Dx) = sinc(Dx)

Cutoff frequency fC = (Dx)-1

With sampling pitch = sampling aperture, Dx

Nyquist Frequency fN = (2 Dx)-1

6

Storage Phosphors

Slot-scanner

Optical Lens

CsI

Gadox

CsI

CCD

Flat-panel

a-Si

Indirect

Direct Selenium

Gadox

Cesium Iodide

Fuji, Agfa, Kodak, Konica….

Hologic, Toshiba, Shimadzu…

Canon, Carestream

GE, Trixell, (Philips, Siemens..)

Varian, others….

Imaging Dynamics, Imix,

Swissray, Wuestec

Delft Dx Imaging, LoDox

CMOS

x-Si Indirect CsI Bioptics

Photon counters

Gas detector

Solid State Si

Direct

Sectra

X-Counter

Digital Radiography 2012: detectors and manufacturers

BaFBr

CsBr

Digital Radiography common themes

• Separation of Acquisition, Display, and Archive

• Wide dynamic range

– ~0.01 to 100 mR (~0.1 to 1000 mGy) incident exposure

• Variable detector exposure operation

– 20 to 2000 “speed class”

• Appropriate SNR & image processing are crucial

for image optimization

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Available digital radiography technology,

2012

• CR: Photostimulable Storage Phosphor (PSP) – Cassette-based detectors/readers

– Flying spot mechanical changers

– Line scan integrated detectors

• CCD: Charge-Coupled Device – 2-D lens coupled systems

– 1-D slot-scan systems

• Thin-Film-Transistor (TFT) flat panel – Indirect detection (scintillator)

– Direct detection (semi-conductor)

– Portable wireless implementations

Future “in-progress” technologies

• Hybrid direct / indirect flat panel TFT with variable gain

• Complementary Metal Oxide Semiconductor (CMOS)

• X-ray “light valve” Liquid Crystal and reflective scanner detector system

• X-ray photon counters based on gas or silicon strip detectors

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PSP Radiography (CR)

• Currently the major technology available for large field-of-view digital imaging

• Based upon the principles of photostimulated luminescence; 20+ years of experience

• Operation emulates the screen-film paradigm in use and handling.. (flexible but labor intensive)

• Manufacturing trends: – Smaller, faster, less expensive

Computed Radiography “reader”

Information panel

Plate

stacker

Fuji

Agfa

Konica

Carestream

Various capabilities, sizes, throughput

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Phosphor Plate Cycle

PSP

Base support

reuse

plate erasure:

remove residual signal

light erasure

plate exposure:

create latent image

x-ray exposure

plate readout:

extract latent image

laser beam scan

“Direct” Radiography (DR)

....refers to the acquisition and capture of the x-ray image without user intervention (automatic electronic processing and display)

– “Indirect” detector: a conversion of x-rays into light by a scintillator, and light into electrons for signal capture

– “Direct” detector: a conversion of x-rays to electron-hole pairs with direct signal capture

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Flat panel portability

• Initial products introduced by Canon

– Tethered, thick profile

• Wireless products now on the market

– Trixell, Carestream, Canon, Source one…

CR and DR mobile radiography

Tethered cassette CR reader / processor

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Wireless DR cassette

• Integrated

• Battery powered

• On-board computer

and processing

Point of service direct imaging

• 14x17inch

cassette…

12

• Preview

image in

2-3 s

Point of service direct imaging

• For Processing image…. 15 seconds

• QC

• Annotation

• Send

Point of service direct imaging

13

DR replacement trends

• Passive for active detector technology

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.5 1.5 2.5 3.5 4.5 1.0 2.0 3.0 4.0 5.0

Frequency (lp/mm)

Modula

tion

Spatial Resolution – MTF

a-Selenium: 0.13 mm

Screen-film CsI-TFT: 0.20 mm

CR: 0.10 mm

CR: 0.05 mm

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Detective Quantum Efficiency (DQE)

• A measure of the information transfer efficiency of a

detector system

• Dependent on:

– Absorption & conversion efficiency

– Spatial resolution (MTF)

– Conversion noise & electronic noise

– Detector non-uniformities / pattern noise

– Not necessarily indicative of clinical performance

2 2

out

2

in N

SNR MTF( )DQE( ) =

SNR NPS ( )

ff

f q

DQ

E(

f )

Spatial Frequency (cycles/mm)

0.0 0.5 1.0 1.5 2.0 2.5 0.0

0.2

0.4

0.6

0.8

Detective Quantum Efficiency Radiography

CsI - TFT

Screen-film

CR Conventional

a-Se - TFT

CR “dual-side”

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High DQE does not “guarantee” good

image quality

• Appropriate radiographic technique is required

• Optimization of acquisition technique – kV, mAs, SID, filtration, anti-scatter grid

• For similar acquisition techniques and grid use, the SNR requires radiation dose proportional to DQE-1

• “Effective DQE” concept takes into account clinical situations (magnification, grid)

Attribute CR DR CCD

Positioning flexibility **** ** **

Replacement for screen/film **** ** **

DQE / dose efficiency ** *** **

Patient throughput * *** **

X-ray system integration ** **** ****

Access to advanced

technology applications ** **** ***

Cost for comparable image

throughput *** ** ***

Radiographer ease of use

(manufacturer dependent) * *** **

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31

• CR & DR systems have variable speed, wide dynamic range, and internal signal scaling

• Consistent (and often inconsistent) image appearance eliminates exposure feedback loop

• There is no direct link between image appearance and detector “speed class”

• Overexposures can easily be unnoticed, resulting in needless overexposure to the patient

• Underexposures have increased image noise that can reduce diagnostic accuracy

Standardized Exposure Index for Digital Radiography – Technical Issues

32

Screen-Film system indicators

Traditional screen-film systems use overall film density as an exposure indicator

Direct feedback to the technologist regarding exposure

J. Shepard and M. Flynn

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33

CR & DR system indicators

CR & DR systems use image processing to align the grayscale with the signals

Direct visual cues (dark/light) are lost regarding exposure

J. Shepard and M. Flynn

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Noise

The image processing adjusts the grayscale, however;

• Images with low signals are noisy and

• Images with high signal are associated with high dose

Exposure Indicators describe

image quality in

terms of the signal to

noise ratio

(SNR)

Underexposed, low SNR Overexposed?, high SNR

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Exposure Indicators

CR and DR systems assess the recorded signal to indicate whether the radiographic technique used is appropriate

• Tests with defined beam conditions are used to verify that correct indicators are being reported

• Recommended exposure indicator ranges are used by technologists to check each radiographic exposure

36

Region to assess signal indicator

Systems vary in the region used to assess the signal for an image.

• Full Image

• Regular regions

• Anatomic regions

J. Shepard and M. Flynn presentation

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37

Region to assess signal indicator IEC 62494-1

• Gray histogram for the entire image

• Black histogram for the anatomic region (relevant region)

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Computation of an exposure indicator

…. computed from the probability distribution of signal values in the relevant image region, based on the median

Manufacturers have adopted proprietary methods

• Algorithms, values, and calibration methods are widely different, leading to confusion amongst users

• Inappropriate image segmentation or histogram ‘values of interest’ range can produce inaccuracies

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39

Summary of manufacturer Exposure Indices Manu-

facturer Indicator

Name Symbol Units

Exposure Dependence

Calibration Conditions

Fujifilm S Value S Unitless 200/S X (mR) 80 kVp, 3 mm Al “total

filtration” S=200 @ 1 mR

Kodak Exposure Index EI mbels EI + 300 = 2X 80 kVp + 1.0 mm Al + 0.5

mm Cu EI = 2000 @ 1 mR

Agfa Log of Median of histogram

lgM bels lgM + 0.3 = 2X for 400 Speed Class, 75

kVp + 1.5 mm Cu lgM=1.96 at 2.5 µGy

Konica Sensitivity Number

S Unitless for QR = k,

200/S X (mR) for QR=200, 80 kVP S=200

@ 1 mR

Canon Reached

Exposure Value REX Unitless

Brightness = c1, Contrast = c2,

REX X 1

for Brightness = 16, Contrast = 10,

REX ≈ 106 @ 1 mR1

Canon EXP EXP Unitless EXP X

80 kVp, 26 mm Al HVL = 8.2 mm Al DFEI = 1.5

EXP = 2000 @ 1 mR 1 From empirical data

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Summary of manufacturer Exposure Indices Manu-

facturer Indicator

Name Symbol Units

Exposure Dependence

Calibration Conditions

GE Uncompensated

Detector Exposure

UDExp mGy Air KERMA UDExp X (μGy) 80 kVp, standard filtration,

no grid

GE Compensated

Detector Exposure

CDExp mGy Air KERMA CDExp X (μGy)

GE Detector Exposure Index DEI Unitless DEI ≈ 2.4X (mR) 1 Not available

Swissray Dose Indicator DI Unitless Not available Not available

Imaging Dynamics Company

Accutech f# Unitless 2f#=X(mR)/Xtgt(mR) 80 kVp + 1 mm Cu

Philips Exposure Index EI Unitless 100/S X (mR) RQA5, 70 kV, +21 mm Al, HVL=7.1 mm Al

Siemens Medical Systems

Exposure Index EXI mGy Air KERMA X(mGy)=EI/100 RQA5, 70 kV +0.6 mm Cu,

HVL=6.8 mm Al

Alara CR Exposure Indicator Value EIV mbels EIV + 300 = 2X 1 mR at RQA5, 70 kV, +21 mm Al,

HVL=7.1 mm Al => EIV=2000

iCRco Exposure Index none Unitless Exposure Index log [X (mR)] 1 mR at 80 kVp + 1.5 mm Cu => =0

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Manufacturer Symbol 50 mGy 100 mGy 200 mGy

Canon (Brightness =16, contrast = 10

REX 50 100 200

IDC (ST = 200) F# -1 0 1

Philips EI 200 100 50

Fuji, Konica S 400 200 100

Kodak (CR, STD) EI 1700 2000 2300

Siemens EI 500 1000 2000

Approximate EI Values vs. Receptor Exposure

….. The need for a standard clearly evident

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Standardization

• American Association of Physicists in Medicine Task Group 116 and International Electrotechnical Commission (IEC)

• Collaborative effort • Physicists • Manufacturers/Vendors representatives • MITA (Medical Imaging and Technology Alliance)

• Develop common “Exposure Indices” and “Deviation Indices” across detectors and manufacturers/vendors

• Provide means for placing data in DICOM metadata

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43

AAPM TG 116 The AAPM TG 116 report on exposure indicators was published in July of 2009

IEC Standard IEC published a standard for Exposure Index definitions in August of 2008

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Description of Exposure Indices Parameters

AAPM

TG116 Med Physics 2009

IEC

62494-1 IEC:2008

Exposure Index

Air-kerma at the receptor

KIND = KCAL (μGy)

EI = KCAL × 100 μGy-1

(unitless)

Calibration Energy

RQA-5

66 - 74 kVp

RQA-5

66 - 74 kVp

Calibration Filtration

RQA-5 Equivalent

0.5 mm Cu (+ 0-3 mm Al) or 21 mm Al

6.8 ± 0.2 mm Al HVL

RQA-5 Equivalent

0.5 mm Cu + 2 mm Al or 21 mm Al

6.8 ± 0.3 mm Al HVL

Deviation Index

Deviation Index

DI = 10*log10(KIND/KTGT)

Deviation Index

DI = 10*log10(EI/ET)

DI format Signed decimal string with

1 decimal point Unspecified

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Deviation Index (DI)

• EIT is a target index value that is to be determined for each

body part b, view v, procedure type, and clinical site

• When EI equals EIT, DI = 0 • DI = +3.0 for 2x target exposures • DI = -3.0 for ½ target exposure • ± 1 is one step on a standard generator mAs control or

AEC compensation (ISO R5 scale)

Exposure Indices

1010( . )T

EIDI Log

EI b v

46

Num

be

r o

f p

ixe

ls

Pixel Value

Values of Interest

EI = EIT

DI = 0.0 EI and DI calculated from

this pixel value

What about VOI modification by the technologist?

Need to have robust methods of determining DI

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47

Num

be

r o

f p

ixe

ls

Pixel Value

VOI recognition algorithm fails

• Gonadal shields, prosthetics, etc.

• False DI reported

Correct

Values of Interest

EI = EIT

DI = 0.0

EI and DI calculated from

this pixel value

Incorrect Values of Interest

EI and DI incorrectly calculated

from this pixel value

EI EIT

DI = -1.3

Need robust methods of determining EI & DI

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Need robust methods of determining EI & DI

Num

be

r o

f p

ixe

ls

Pixel Value

User adjusts VOI for proper grayscale rendition manually,

and DI returns to zero

Incorrect Values of Interest

EI and DI incorrectly calculated

from this pixel value EI EITGT

DI = -1.3

Correct

Values of Interest

EI and DI calculated from

this pixel value

EI = EITGT

DI = 0.0

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Need to determine recommendations for repeats

• DI target is -2.0 to +2.0

• Check for noise. Consult with radiologist on need for repeat if EI is 63% of target (DI -2)

• Investigate cause (do not repeat) if EI is between 160% and 200% of target (+2.0 DI +3.0)

• Consult with radiologist (check for saturation) on need for repeat and counsel of technologist if EI is 200% of target (DI +3.0)

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IEC 62494-1: Target Exposure Index EIT

• EIT may depend on detector type, examination type, diagnostic question and other parameters

• Establishing target exposure index values needs medical knowledge – may be done by professional societies

• EIT values should be provided as a data base in the digital imaging system

Ulrich Neitzel Project Leader,

Convenor IEC SC62B WG 43

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Caveats

• The EI does not describe patient dose

• EI is derived from detector signal (dose at the detector)

• The EI is not a dose measurement tool

• Dose calibration only valid at one radiation quality

• Images with same EI obtained on different digital systems might not have similar image quality

• Influence of detector DQE, scattered radiation, beam quality differences

Ulrich Neitzel Project Leader,

Convenor IEC SC62B WG 43

52

Exposure Index & Deviation Index monitoring

• Collect EI and DI for every image and analyze • By technologist • Technique factors • X-ray system • Plate scanning unit (CR) • Processing unit (CR - DR) • Anatomical view

• Longitudinal studies • Track performance over time • Mean and Standard Deviation of EI and DI • Watch for trends upward (Dose Creep)

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• RIS-PACS Integration – Data Synchronization, Validation

– Interpretation & Results Reporting

• Modality Worklist (MWL) – RIS-driven data transfer of exam information

– Aid the technologist for protocol and room setup

• Image Distribution (The Internet) – Clinical Review, OR, Patients, Conferences

– Enterprise Integration - EMR

– Teleradiology

Image Integration and Distribution

• TCP/IP – Standard Communications Protocol

– The Internet

• HL7 – Health Level 7

– RIS / HIS

• DICOM 3.0 – Digital Imaging COmmunications in Medicine v3.0

– PACS

• HTTP – Hyper-Text Transport Protocol

– The World Wide Web

Communication Protocols

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A typical imaging solution

Typical PACS workstation configuration

•1536x2048

•10 bit

•Grayscale or

color

•400 Cd/m2

Navigation

Monitor(s)

1920x1080

color

Digital voice

dictation

support

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Basic Workstation Software Reqt’s

• Filters: sort studies by modality, location, time, etc.

• Worklist functionality: automate workflow

• Hanging protocols: arrange images and display

• Retrieving priors: pre-fetch or all spinning disk

• Graphic user interface: tool palette parameters

• Mechanical interface: keyboard, mouse, other

• User preferences: individual preferences for above

• American College of Radiology

• National Electrical Manufacturers Association

(NEMA)

• Established 1983, first published 1985

• Followup standards in 1988 (ACR-NEMA 2.0)

• 1993, DICOM 3.0 published and continuously

updated

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What does DICOM do?

• Addresses 5 areas of functionality

– Transmission and persistence of complete objects

(images, waveforms, documents)

– Query & Retrieval of such objects

– Performance of specific actions (e.g. film printing)

– Workflow management (support of worklists)

– Quality and consistency of image appearance

(both display and print)

• Network configurations

– Application Entity (AE) title, IP address, TCP/IP port

number

What does DICOM do?

• DICOM storage

– e.g., CT image storage SOP class, CR image storage SOP class

• DICOM print

– Basic grayscale print management SOP class (SCU only)

• Query / Retrieve

– Poll a DICOM device for a list of studies or patients, then retrieve

one or more

• Worklist Management

– Download a list of “scheduled procedures” to the modality from

the RIS through a worklist management provider (PACS Broker)

• Modality Performed Procedure Step (MPPS)

– Modality tells RIS that the procedure has been performed

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DICOM Metadata……

PACS capability & needs

• Hanging protocols—radiologist specific

• API – application program interface

• 3rd party add-ons

• Presentation State

• VOI LUT (Value Of Interest Look-up-table)

• De-identification / anonymization (HIPAA)

• Part 14 DICOM export

• ……… and so on…..

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IMAGE DISTRIBUTION:

Web and the Internet and Teleradiology

Image

Archive

(TB)

Control

Software

Review

Station

Review

Station

Web

Servers

Hospital

PCs

Radiology Departments

US CR

Clinic

PCs

Office

PCs

Home

PCs

Radiologists

Remote

Radiologists

Imaging

Centers CT MRI

Courtesy of Dr. Keith Dreyer

PACS installation planning

• Location of server and major hardware

• PC requirements for workstations,

environment and power considerations

• “Mini-PACS” for ultrasound, nuclear medicine

– Color monitors

– Application specific workstation requirements

• Reading room

– Lighting

– Furniture

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Challenges

• Integration of different systems

• IHE: Integrating the Healthcare Enterprise

– Addresses INTEROPERABILITY of systems

– Provides INTEGRATION PROFILES and a

framework for performing needed functionality and

workflow

• http://www.IHE.org

PACS quality control

• Interfaces

• Redundancy and emergency backup

• Software verification (distance accuracy

measurements, quantitative measurements)

• MONITORS

• Verification of correct data

• Display and viewing conditions

• Image compression & archiving

• Disaster Recovery and backup plans

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SUMMARY Enterprise distribution of images is crucial

for implementation and application of technology

• Cassetteless, active detector radiography devices (flat panel) are becoming the detectors of choice

• Proliferation of web-based PACS and unified patient database (instead of Radiology centric orientation) is here

• New opportunities

– Image acquisition and image processing tools

– Imaging technology innovation for diagnosis and intervention