CR (PSP) & DR (Flat Panel)

Imaging Systems ; A Primer

Presenter ;

Mr. S. Patefield, MPhil, TDCR(R), Cert. (Ed),

Senior Lecturer,
Medical Imaging Sciences,
University of Cumbria,
Bowerham Campus,
LANCS., ENGLAND.

June 2012
Note; no reproduction of this presentation is allowed without the authors / institution’s permission
CR (PSP) & DR (Flat Panel)
Imaging Systems ; A Primer

Section 1 ; Imaging Technologies CR/DR

Section 2 ; Exposure Indices

CR (PSP) Imaging Systems ;
@ Lancaster & Carlisle
CR Technology

The CR phosphor;

A CR plate is based on a fluorescent screen and housed

in a conventional ‘cassette’

Most CR systems use a Barium Fluoro-bromide phosphor,

This is then “doped” with Europium

The Europium changes the chemical structure of the

phosphor to “trap” electrons on x-ray exposure
CR (PSP) Imaging Systems ;
CR phosphor
In a fluorescent screen ; K shell electrons are normally
‘bound’ within a low energy band within the chemical
structure of the screen phosphor

High energy
limit

Low energy
band
electrons
CR (PSP) Imaging Systems ;
CR phosphor
When donated energy by an x-ray photon, electrons can
‘jump up’ to a higher energy level

x-ray photon

High energy
limit

Low energy
band +
electrons
CR (PSP) Imaging Systems ;
CR phosphor
However, they leave a positive ‘hole’ behind them and
will gradually fall back giving off the energy they gained
as light
light photon

High energy
limit

Low energy
band
electrons
CR (PSP) Imaging Systems ;
CR phosphor
In a film imaging system we captured the light using
photographic film, the light forming ‘silver centers’ in the
emulsion, ‘the latent’ image

Film detector

Low energy
band
electrons
CR (PSP) Imaging Systems ;
CR phosphor
In a CR phosphor, an impurity is added which creates
‘positive’ holes in the higher energy band

High energy
limit
‘electron
traps’

Low energy
band
electrons
CR (PSP) Imaging Systems ;
CR phosphor

In approx. 50% of interactions, the electron is trapped

x-ray photon
High energy
limit
‘electron
traps’

Low energy
band
electrons
CR (PSP) Imaging Systems ;
CR phosphor
During and after exposure, about 50% of the electrons
remain trapped, the rest fall back almost immediately

Fluorescence during exposure !

‘Digital Latent
Image’

Low energy
band
electrons
CR (PSP) Imaging Systems ;
CR phosphor

Over time, due to thermal energy, trapped electrons fall

back spontaneously over many hours

Afterglow
‘Digital Latent
Image’

Low energy
band
electrons
CR (PSP) Imaging Systems ;
Laser scanning
• As the laser scans, electrons are released from each pixel location,
which produces photons of blue light. These are fed into the
photomultiplier tube via a fibre-optic light guide.

Rotating polygon
mirror
laser
CR (PSP) Imaging Systems ;
Laser scanning
A red laser disturbs electrons in traps
Blue Light
emitted as
electron drops
back
‘Digital
Latent
Image’

Low energy
band
electrons
CR (PSP) Imaging Systems ;
Photomultiplier Tube
As the laser scans, electrons are released from each pixel location,
which produces photons of blue light. These are fed into the
photomultiplier tube via a fibre-optic light guide.

Rotating polygon
mirror
laser

Fibre-optic light Photomultiplier tube

guide
CR (PSP) Imaging Systems ;
Photomultiplier tube

500 - 2000 V

http//micro.magnet.fsu.edu
CR (PSP) Imaging Systems ;
PMT Spectral response

The coating applied to

the photocathode is
crucial to the spectral
sensitivity of the
device, for instance
this coating would
respond to both red
and blue light.

http//micro.magnet.fsu.edu
CR (PSP) Imaging Systems ;
PMT Spectral response

The second coating

would be better as the
response to red light is
very poor.
Across the whole
spectrum, however,
QDE is generally low,
and never more than
35%

http//micro.magnet.fsu.edu
CR (PSP) Imaging Systems ;
Image Matrices
As the laser scans, electrons are released from each pixel location,
which produces photons of blue light.

However, the speed at which the laser scans the plate is variable.
For a given ‘quantisation rate’ in the ADC, the pixel matrix obtained
can be related to the size of the plate used.
Plate size mm Pixels in x Pixels in y X / Pixels in x Y/ Pixels in Y

350 x 430 [x,y] 2072 2520 0.17 mm 0.17 mm

240 x 300 2400 3020 0.10 mm 0.10 mm

180 x 240 1792 2392 0.10 mm 0.10 mm

As shown in the table above ; the pixel matrices have been altered
according to psp size, giving smaller pixels using small plates
compared to the largest plates, for similar overall scan times.
CR (PSP) Imaging Systems ;
Digital image processing
As the laser scans, electrons are released from each pixel location,
which produces photons of blue light. These are fed into the
photomultiplier tube via a fibre-optic light guide.

Rotating polygon
mirror
laser
12 bit ADC
001100111010
LUT PIXEL CODE

Fibre-optic light Photomultiplier tube

guide
CR (PSP) Imaging Systems ;
Digital image processing
At a known mAs the histogram should fall
in the middle of the range yielding the
‘optimum’ EI value for that manufacturer
12 bit ADC
001100111010
LUT PIXEL CODE

Signal
amp.

Histogram bin

white black
0 2047 4095
CR (PSP) Imaging Systems ;
Phosphor anealing (erase)
Electron traps cleared ready for next exposure
Strong white
light exposure
approx. 20s

‘Digital
Latent
Image’

Low energy
band
electrons
Image Acquisition - DR

DR Detectors ;

Are multiple coated TFT’s, in a large ‘flat plate’ format

Most modern ‘Radiography’ DR systems use a Cesium Iodide [CsI]

crystal fluorescent layer coated above an electron emissive layer of
amorphous Silicon [a-Si] on a glass substrate

Image acquisition is a two stage process, x-ray conversion to light,

followed by light conversion to electrons

The electrons are ‘trapped’ in the silicon semi-conductor until read-

out as ‘charge packets’
DR Imaging Systems –
@ Lancaster & Carlisle
DR Imaging Systems –
DR Imaging Systems –
DR Imaging Technology –
DR Imaging Technology –

a-Si ‘patches’
DR Imaging Technology –

The ‘active pixel array’ is of a

‘fixed’ size or matrix. The physical
size of this active area can be up
to 45 x 45 cms. With a typical
matrix of around 9 Mega-pixels,
the pixel pitch is around 125
microns or 0.125 mm2

The active array is often ‘tiled’,

into 4 quadrants to ease
manufacturing problems.
DR Imaging Technology –

The ‘pixels’ are individually Light photon/s

isolated within the a-Si ‘patches’
making them a ‘fixed’ size.

The active array switches are e

closed during the x-ray exposure
to allow ‘charge’ to gather in each
pixel patch.

Finally the switches are opened,

column by column, and the
charge packets read out and
‘quantised’ via an ADC [analogue
to digital conveter]
CR & DR Comparisons

DETECTIVE QUANTUM EFFICIENCY

An expression of the efficiency of an imaging system’s transfer, from its

input to its output, as a percentage of signal to noise ratios (SNR).

DQE is the measure most representative of image quality in terms of an

observer’s ability to detect objects of interest in an image.

DQE has superseded reliance upon previous measurement criteria such

as measuring MTF or resolution performance as a function of visible line
pairs.
DQE = SNR2 at detector output / SNR2 at detector input
Measures transfer of both signal and noise.

DQE limited in practice to about 70%

CR & DR Comparisons

CR DR

DQE 0.25 – 3.0 0.5 – 0.70

Spatial resolution 0.1mm – 0.17mm 0.125mm

Bit depth 12 bit 12 bit

Dynamic range ~ ~

Cost effectiveness high ???? life-span

Throughput film-like 60s cycle 2-6s

CR (PSP) & DR (Flat Panel)
Imaging Systems ; A Primer
Seeram E., [2009], “Digital Radiography; An Introduction”, Delmar,
Cengage Learning, NY, ISBN 1-4018-8999-9

This costs just £20 !

My advice is ;
1. Buy it,

2. Read it ?