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  Digital Radiography Receptors Beth Schueler, Ph.D.

  Mayo Clinic Rochester, Minnesota

  

Overview: Projection Radiography

• Computed Radiography (CR)
• – Basic technology
• – Characteristics – Recent developments
• Digital Radiography (DR)
• – Types – Basic technology
• – Characteristics

  

Computed Radiography (CR)

• Based on use of a photostimulable storage phosphor
• Can be configured as:
• – Cassette-based
• – Cassette-less
• Manufacturers include:
• – Fuji – Agfa – Carestream (formerly Eastman Kodak)
• – Konica Minolta – Lumisys

  

CR: Cassette-based

• Photostimulable storage phosphor
• – Absorbs x-rays and stores a latent image
• Cassette
• – Used like a film-screen cassette
• CR Reader
• – Extracts the latent image

CR: Cassette-less

• No cassette handling
• Use is similar to digital radiography detectors

Image Plate

  Protective Laminate Photostimulable Phosphor (PSP) 0.

  Layer

  5 mm Support Light Shielding Layer Backing Layer

  

CR Image Formation

2+

• PSP material: BaFX:Eu
• – Commonly contain barium

  X-rays and fluorine with bromine (Br), iodine (I) and/or strontium (Sr) in a crystal structure

• – Europium added trace

  Patient amount as an activator

• With x-ray exposure
• – About half of signal is prompt light emission
• – Remaining is stored as a trapped electron

  Readout Process

• Exposed image plate is scanned by a laser

  Laser

• – red-emitting diode:

  Beam

  670-690 nm

• Stored energy in image plate is released as a photon
• – blue-violet: 400 nm
Readout Process

• Emitted light is

  PMT collected by a light guide

  Light

• – Light guide: curved

  Guide

  acrylic plate

• Light is detected by a photodetector (PMT) and converted to a voltage signal

Lightguide

Readout Process Laser scan Output Voltage Signal

  Readout Process

• Residual signal is removed by scanning with a high intensity erasure light
• – Some low level residual signal remains
• Image plate is ready to expose again

  

New Developments in CR

• Line scan readout
• – single laser beam is replaced with a laser line source
• – single PMT is replaced by a linear CCD array
• – readout time is 10X faster than point scan system

  Laser Line Source Linear CCD Array Image

  Dual-sided Readout

• Image plate is

  Light Guide mounted on a clear support/backing layer

• Emitted light is detected from both sides with 2 light collection systems
• Decreases image

  Light noise

  Guide

Structured Phosphors

• Image plate has needle-like rods of phosphor material
• Rods channel emitted light to improve spatial resolution
• Allows for use of a thicker phosphor layer to improve x-ray absorption

  

Digital Radiography (DR)

Different types based on choice of x-ray absorber:

  1. Indirect

• use a phosphor to produce light, then convert light into photoelectrons
• Gd O S – intensifying screen
₂ 2<
• CsI – image intensifiers

  2. Direct

• Use a photoconductor to produce electron-hole pairs for direct signal capture
• Amorphous selenium (a-Se)

  

Digital Radiography (DR)

Different types based on choice of signal collection

method:

  1. CCD

• Detectors are indirect with CCD collecting light

  2. Thin-film-transistor (TFT) array

• Large area signal collection array
• Used with both indirect and direct x-ray absorbers

  

CCD-based DR

• Phosphors used
• – Gd
₂ O S 2<
• – CsI
• Configurations available
• – Large area phosphor
• – Linear array with slot scanning
• Coupling of phosphor to CCD through
• – Lens – Fiberoptic

  

CCD-based DR

Manufacturer Model Size (cm) Pixel spacing (m) X-ray Absorber

  Swissray ddR 35x43 167 CsI Imaging Dynamics

  Xplorer 1700

  43x43 108 Gd ₂ O ₂ S

  IMIX

  IMIX2000 40x40 200 Gd ₂ O ₂ S

  

CCD-based DR: Issues

• For large area detectors, light collection efficiency from the phosphor is very low
• Coupling system requires space – thick detector

  35 cm cm

  5x5 cm

  4

  

TFT Arrays

• Made from amorphous silicon (a-Si:H)
• – Layers deposited onto glass
• – Etched to create pixel elements and connection lines for readout of signal
• Same technology that is used for flat panel display systems
• – Large market for TFT displays has allowed for improved fabrication methods and decreasing cost

  

TFT: Pixel Elements

Switching element

  Active area

• Indirect detector
• – Active area is a photodiode to collect light
• Direct detector
• – Active area is a storage capacitor to collect charge

  Indirect Type X-rays

  X-rays

  Light Light

  Photodiode

  Photodiode

  Cesium Cesium

  Iodide Iodide

  

Indirect DR Detectors

Manufacturer Model Size (cm) Pixel spacing (m) X-ray Absorber

  Trixell (Siemens, Philips)

  Digital Diagnost, Aristos

  43x43 143 CsI General Electric

  Revolution, Definium

  41x41 200 CsI Varian PaxScan 30x40 194 CsI,

  Gd ₂ O ₂ S Canon CXDI-40 43x43 160 CsI,

  Gd ₂ O ₂ S

  

Direct Type

• Converts x-rays directly into electron-hole pairs
• – High voltage draws + charge to electrode pixels

  X-rays

  X-rays

• H ig h V olt ag e -
• H ig h V olt ag e -

  Selenium Selenium

  

Direct DR Detectors

Manufacturer Model Size (cm) Pixel spacing (m) X-ray Absorber

  Hologic (DRC

  Kodak Directview, Fischer VersaRad

  35x43 139 a-Se Anrad Toshiba 35x35 150 a-Se Shimadzu RadSpeed 43x43 150 a-Se

• – Direct Radiography Corp)

  

TFT: Pixel Elements

• Fill factor
• – Dead space surrounds the active area
• – Fraction of pixel area that is sensitive = fill factor
• – Required separation of components reduces fill factor as pixel spacing is reduced

  TFT: Array Operation

  Data lines Switch control lines

• 5 V -5 V
• – Control voltage at -5 V for all
• 5 V

  control lines

• – All switching elements are off

  TFT: Array Operation is made

• – Signal is stored in each active area
• 5 V -5 V -5 V
TFT: Array Operation

• 5 V

  set to +10 V for one row

• – Signal for this
• 5 V

  row of pixels is transferred out data lines

• 10 V

TFT: Array Operation

• 10 V for next row

  5. And so on until each row is emptied

  6. Entire array readout takes 300-500 ms

• 10 V
• 5 V -5 V

  

TFT: Reinitialization

• Process of preparing the detector for the next exposure
• If not done properly, there is residual signal left over resulting in a ghost image
• Approaches vary depending on detector manufacturer, but commonly include:
• – Use of an applied bias voltage
• – Use of a light field
• – Injection of signal offset charge

  

CR/DR Characteristics

• Image pre-processing
• – initial image corrections
• Characteristic response
• – dynamic range
• Detector x-ray absorption vs energy
• – scatter sensitivity
• Exposure indicators
• – under and over-exposure cue

  

CR/DR Image Pre-Processing

• CR
• – Laser beam and light guide variations
• DR
• – Variations in phosphor or photoconductor thickness
• – Pixel-to-pixel variation in TFT array components
• – Pixel and line malfunctions
• Image correction routines
• – CR: Shading correction
• – DR: Pixel defect correction
• – DR: Offset correction
• – DR: Gain correction

CR Shading Correction

  Uncorrected Image Corrected Image

DR Pixel Defect Correction

• Pixels that do not produce signal are identified
• Signal for that location is replaced with average of surrounding pixels

• • Manufacturers have specifications for number of

  dead pixels, lines and pixel clusters that are allowed before detector replacement is required

  

DR Offset Correction

• Requires acquisition of a dark image
• – Image is readout without x-ray exposure
• Offset signal depends on
• – previous exposures – residual signal left
• – detector temperature

  

DR Offset Correction

• Requires acquisition of a dark image
• – Image is readout without x-ray exposure
• Offset signal depends on
• – previous exposures – residual signal left
• – detector temperature

  

DR Gain Correction

• Requires uniform field images
• – Multiple image are acquired and averaged to reduce quantum noise
• Gain depends on
• – x-ray beam energy
• – SID
• – exposure level
• Also removes heel effect

  Uncorrected Image CR-DR Image Quality

  0.8

  0.7

  0.6 Indirect - CsI

  0.5 Direct - a-Se

  f) ( E

  0.4 CR - Dual-side Q D Screen-film

  0.3 CR - General

  0.2

  0.1

  0.5

  1

  1.5

  

2

  2.5

  3 Frequency (per mm)

X-Ray Absorption for CR-DR

  100

  90 n

  80 io ct

  70 ra CsI

  BaFBr o

  50 ti Gd2O2S rp

  40 a-Se so b

  30 r

   A tte

  20 % ca

  Primary S

  10

  20

  40

  60 80 100 120 X-ray Energy (keV)

  

Scatter Rejection for CR-DR

• Increased absorption in the scattered x-ray

  energy range for BaFBr and CsI make scatter

  rejection especially important
• Grids use is important for CR and indirect-type detectors
• – Need to be used in more situations (such as portable radiographs)
• – Need to have Q (quality factor)

  

Grid Selection for CR-DR

• Stationary grids can result in aliasing patterns caused by insufficient sampling by the image receptor
• – not a problem for moving or Bucky grids
• Stationary grids are used for:
• – portable radiographic exams with CR or portable DR
• – free cassette (cross-table) views with CR or portable

  DR

• – table and upright image receptors for some DR manufacturers (Siemens, General Electric)

  Grid Alias Example

  

Grid Aliasing Patterns

• When the sampling rate exceeds 2 X grid frequency, the grid image is preserved

  

Grid Aliasing Patterns

• When the sampling rate is below 2 x grid frequency, an interference pattern is produced

  

Grid Aliasing Patterns

• • 2 X grid frequency is Nyquist sampling frequency

  

Stationary Grid Specification for CR-DR

• For CR, position grid lines perpendicular to reader scan line direction

• • For both CR (when grid lines must be parallel to

  scan lines) and DR, use high frequency grids
• – at least 60 or 70 line/cm is typical recommendation
CR/DR Characteristic Response

  4 10000 R ty Film- si ela screen en

  3 1000 tiv D

  DR l e I n ca

  2 te 100 CR ti n p sit O y

  1 10 m il F

  1

  0.01

  0.1

  1 10 100 1000

Exposure

  

Exposure Indicators

• Underexposure/overexposure
• – In film-screen, film density indicates under- and overexposure conditions
• – In CR and DR, underexposure appears as a noisy image, overexposure is generally impossible to detect
>Exposure indicators provide a way to verify proper radiographic exposure was used

• • There is no standard indicator in use at this time,

  each manufacturer’s method is unique

  Example Exposure Indicators Exposure Indicator Value Exposure

  Indicator Under- Over-

Manufacturer Name exposed Target exposed

  AGFA lgM

  1.9

  2.2

  2.5 Kodak Exposure 1700 2000 2300 index, EI Fuji Sensitivity 400 200 100 number, S General Detector

  1.3

  1.7

  2.7 Electric Exposure Index, DEI

  

CR-DR Implementation

• CR Configurations
• – cassette
• – cassette-less – operates much like DR
• DR Configurations
• – table and wall stand
• – portable – with wire connections and wireless (future)
• – U-arm
• – flexible wall stand

  Portable DR Detector Other DR Configurations

  

CR-DR Implementation

• Configuration should fit application
• – Tables and wall stands:
• Cassette-less CR and DR are more efficient
• – Portables:
• CR – flexible and inexpensive
• portable DR (wired or wireless) - expensive
• – Free cassette views:
>CR – flexible, inexpensive, lightweight
• portable DR (wired or wireless) – heavier than CR, more

  sensitive to impact, wire difficult to handle in a sterile environment

• U-arm or flexible wall stand DR – some movement limitations, large spacing between detector housing edge and imaging area can be problematic for certain views

Digital Mammography Vendors Vendor Model Detector Detector Size (cm) GE Senographe DS, Indirect 19 x 23 2000D GE Essential Indirect 24 x 31 Hologic Lorad Selenia, Direct 24 x 29

  Siemens Mammomat Novation Fischer SenoScan CCD - line scan 21 x 29 Sectra MicroDose Photon counting 24 x 26

• – line scan

  Fuji ClearView CSm CR – Dual- 18 x 24 or 24 x 30

References

  1. Rowlands JA. The physics of computed radiography. Phys Med Biol 2002; 47:R123–R166.

  

2. Schaetzing R. Computed Radiography Technology, in Advances in Digital

Radiography Categorical Course in Diagnostic Radiology Physics, eds.

  Samei E, Flynn MJ, RSNA 2003, 7-22.

  3. Seibert JA, et al., ‘‘Acceptance testing and quality control of photostimulable phosphor imaging systems,’’ Report of the American Association of Physicists in Medicine #93, 2006.

  4. Yorkston J. Digital Radiography Technology, in Advances in Digital

Radiography Categorical Course in Diagnostic Radiology Physics, eds.

  Samei E, Flynn MJ, RSNA 2003, 23-36.

  5. Seibert JA, Computed Radiography/Digital Radiography: Adult in From Invisible to Visible—The Science and Practice of X-ray Imaging and

Radiation Dose Optimization Digital Radiography Categorical Course in

Diagnostic Radiology Physics, eds. Frush DP, Huda W, RSNA 2006, 57-71.

  6. Mahesh M, Digital Mammography: An Overview, Radiographics 2004: 24; 1747-1760.