Overview of Digital Detector Technology

Overview of Digital Detector Technology J. Anthony Seibert, Ph.D. Department of Radiology University of California, Davis

Disclosure • Member (uncompensated) – BarcoBarco-Voxar Medical Advisory Board – ALARA (CR manufacturer) Advisory Board

Learning Objectives • Describe digital versus screenscreen-film acquisition • Introduce digital detector technologies • Compare cassette and cassettecassette-less operation in terms of resolution, efficiency, noise • Describe new acquisition & processing techniques • Discuss PACS/RIS interfaces and features

1

Conventional screen/film detector 1. Acquisition, Display, Archiving Transmitted xx-rays through patient

Film processor

Exposed film

Developer

Fixer

Wash

Dry

Gray Scale encoded on film

Intensifying Screens

Film

x-rays → light

Digital xx-ray detector

2. Display Digital to Analog Conversion

Digital Pixel Matrix

1. Acquisition Transmitted xx-rays through patient

Digital processing Analog to Digital Conversion

Charge collection device

X-ray converter x-rays → electrons

3. Archiving

Analog versus Digital Spatial Resolution MTF of pixel aperture (DEL) 1 100 µm

Modulation

0.8

200 µm

0.6

1000 µm

0.4 0.2 0 0

1

2

3

4

5

6

7

8

9

10 11

Frequency (lp/mm)

Sampling Pitch

Detector Element, “DEL” DEL”

2

Characteristic Curve: Response of screen/film vs. digital detectors 5 Useless

10,000

FilmFilm-screen (400 speed)

Digital 1,000

3 2

Overexposed

Useless

100 Correctly exposed

1

10

Relative intensity

Film Optical Density

4

Underexposed

0 0.01 20000

0.1

1

10

100

1 Exposure, mR

2000

200

20

2

Sensitivity (S)

Analog versus digital detectors • Analog – – – –

Coupled acquisition and display Higher resolution Limited dynamic range, fixed detector contrast Immediate exposure feedback

• Digital – – – –

Separated acquisition and display Lower resolution Higher dynamic range and noisenoise-limited contrast Proper exposure potentially hidden

Image Processing • Crucial for optimal image presentation • Flexibility adds potential advantage – DiseaseDisease-specific image processing – Computer aided detection

3

Digital System Technologies Projection Radiography

• Computed Radiography (CR) • CCD “Direct” Radiography (DR)

• CMOS • Flat Panel (TFT) arrays

Consideration: “Cassette” Cassette” vs. “Cassetteless” Cassetteless” operation

Computed Radiography (CR) ...is the generic term applied to an imaging system comprised of:

Photostimulable Storage Phosphor to acquire the xx-ray projection image

CR Reader to extract the electronic latent image

Digital electronics to convert the signals to digital form

Image Acquisition

CR QC Workstation

Patient information Latent image produced

CR Reader Latent image extracted

DICOM / PACS

Laser film printer

Display / Archive

4

CR: Photostimulated Luminescence

τ

phonon

Conduction band tunneling

τ recombination

Energy Band BaFBr

4f 6 5d

Laser stimulation

F/F+ e-

PSL 3.0 eV

8.3 eV

2.0 eV

τ Eu 4f 7

/

Eu 3+ Eu 2+

e

Incident x-rays

Valence band PSLC complexes (F centers) are created in numbers proportional to incident xx-ray intensity

CR: Latent Image Readout Reference detector

f -θ lens

Cylindrical mirror Light channeling guide

Laser Source

Output Signal

Polygonal Mirror

ADC

PMT

Laser beam: Scan direction

x= 1279 image y=To1333 z=processor 500

Plate translation: SubSub-scan direction

Phosphor Plate Cycle PSP x-ray exposure

Base support plate exposure: create latent image

reuse laser beam scan

plate readout: extract latent image light erasure plate erasure: remove residual signal

5

CR Innovations • HighHigh-speed line scan systems (