.

Image Calibration of Laser Digitizers, Printers, and GrayScale Displays1 Edward Arch W. Larry T DonaldA. Linda A. SamuelJ.

Laser

L. Siegel, MD Templeton, MD Cook, PhD Eckard, MD Harrison, MD DwyerHl, PhD

film

digitizers, interactive gray-scale monitors, and are necessary to transmit digital image information. must be standardized so that hardand soft-copy

printers

devices similar

as possible.

Standardization

of appropriate

laser

film These images are

calibration

as

proce-

dures digital tions

is necessary to attain this goal. Radiographs are converted into data representations by a laser film digitizer. These representa(and those obtained with other modalities) are transferred to a

laser

film

To obtain should be calibrated functions calibration

printer

or

to an

interactive

monitor

with

gray-scale

display.

the best gray-level fidelity, printer output optical densities identical to those of the input film. Laser printers should be regularly to ensure uniform results. A gray-scale controller as an adjunct to the host computer and can automate the process. Gray-scale controller functions may someday be

incorporated

into

an

accelerator

or

array-processor

board.

U INTRODUCTION Digital radiology requires high-quality hard-copy recordings and the best possible gray-scale renditions of digital data. Laser film digitizers convert the optical densities of a film into digital data. Laser printers map digital data into the proper optical densities

of the

radiology) limited

the

Abbreviations: terms:

chiving

and

RSNA,

direct

1992;

the

Department

Rainbow

BIvd, 1 and

of a laser

=

of any

cathode

tube.

Images,

D

display

analysis

#{149}

data

Images,

#{149}

gray

density,

display

to a laser

significant

and

into

maximum

=

digitizer

of the

gray-scale

of digital

ray

system

film

reproduction

mapping

Computers

requestedjuly C

range

communication

RadioGraphics From

connection consistent

CRT

Index

I

The

dynamic

inhibit

St and

film. requires

its reduced

levels.

0,,.

Images,

=

printer

The

brightness

of interactive

minimum

(tele-

densities.

screen

Use

processing

#{149}

film

optical

window

density Images.

#{149}

transmission

Picture

ar-

#{149}

(PACS)

12:329-335 of Diagnostic Kansas received

City,

Radiology. KS 66103.

September

2 1 55 Bell From

4; accepted

the

Memorial

1990 November

RSNA

Hospital, scientific 19. Address

University

of Kansas

Medical

assembly.

Received

March

reprint

requests

to E.L.S.

Center.

5, 1991

;

39th

revision

1992

329

Image

Digital

(CT.MR

Figure ways.

1.

Three

digital

Path A is from to the gray-scale

data

path-

the laser display.

digiPath

PATH

Data

...

PATH B

A

tizer B is from the gray-scale display to the laser film printer. Path C is from the film digitizer to the film printer.

Printed Film Output

Electron Gun

Fluorescent Screen

Interactive Device Figure 2. The on a CRT. Image

array

function of a gray-scale display is to transform a digital data are mapped by a table look-up into 8 bits (0-255).

(DAC) and subsequent amplifiers convert the 8-bit output of the table accelerate electrons at the fluorescent phosphor screen. The phosphor 0-205 cd/m2, as determined by its response.

and

level

ing

with

range This

functions the

provides

limited

of gray-scale article

contrast

a means and

for deal-

dynamic

and

discusses

the

unique characteristics of the three digital devices and their connecting pathways. The iterative calibration method developed to properly digitize, print, and display digital data from images is presented. A personal cornputer-based gray-scale controller is descnibed, which continuously samples and selects the best-fitting hard copy and display image parameters. U CHARACTERISTICS OF DEVICES AND DIGITAL DATA PATHWAYS Local or wide-area transmission of digital information from images requires digital data pathways that interconnect devices such as

laser

330

U

RadioGrapbics

film digitizers,

U

Siegel

interactive

et al

gray-scale

into

a visible

digital-to-analog

look-up screen

picture

converter voltages

into appropriate emits light with

that

an intensity

of

stations,

and laser film printers (Fig 1). films are converted into digital data representations with a laser film digitizer. The digitized radiographic film data may Radiographic

displays.

identifies

play

of numbers

The

dis-

be transferred (Fig

i, path

tion

with

Digitally

directly C) or

a gray-scale

data

(CT),

If a digital

(Fig

aging these and output digital contrast cal. All devices

1, path

computed

resonance

A). to-

imaging,

ultrasonography, radioand chest radiography can on the interactive workstahard

copy

data is desired, it must the display workstation printer

worksta-

(Fig

from

magnetic

digital fluorography, nuclide imaging, also be displayed

film printer

interactive

display

formatted

mography

tion.

to a laser

to an

1, path

image optical

manipulated

transferred

to the laser

B). The

challenge

from

film in man-

data is to ensure that input densities of the film and

ranges must

of the

be

are

perceptually

provide

Volume

identi-

similar

images.

12

Number

2

3.5

lan raster,

N

2.8

N N

II.

N

U)

2 ‘U 0

an

1

est

I8.

A table translates

0

0-255

4 U

0.2 0

GRAY 3.

Figure

Laser

LEVEL

digitizer

4095

has a linear

relationship

film optical densities and digital data gray levels (dashed line). Laser printer table look-up used for Figures 4-6 has an optical density range of between

0.2-2.8

(solid

line).

is visualized

on

(8 bits).

Devices digitizer

converts

radiographic film into ning laser beam moves graphic film in a raster

beam

passes

by the

through

optical

in the

have

films

Conven-

optical

densi-

[Dmj,,]

as base

density

levels). The spot size of the determines spatial resolution 200 p.m (2,048 x 2,048-array) x

4,096-array)

for

The resultant digital memory of the digitizer,

units. beam gray

x 17-inch

each

point

reads

back

digitizer

described

squares -

the

is

painted

linear

1,218

OD

the input

in this article,

fit gave + 4,159,

optical

relation:

g

OD

represents

interactive

tube

March

(CRT)

1992

(Fig

the

a least-

this

density

verts digital data played in a raster

With =

and g is the come=

The

Pearson

.998,

with

con-

an asso-

data. workstation

which

in-

light

output

and its physical characteristics screen

con-

display.

corresponding

of the

The

raster

point

of the

fluores-

properties. of a gray-scale

brightness

is subject

of 0-205

to electronic

or geometric

distortion.

put of a typical view film is 1,370 cd/rn2. to match

copy obtained nonlinearity film.

where

sponding gray-level output. relation coefficient was r ciated P < .0005 for these A gray-scale

values.

its

signal. The brightness of

on the raster

of the

difficult

image is stored in the which uses a logarith-

numeric

that of

with

to an electric modulates the

include

noise

mic amplifier to attain a linear output. The linearity of a specific digitizer is tested by digitizing a standard film strip with 2 1 steps in density. The digitized gray levels are then sampled by using standard display software that

look-up,

converter,

number signal

display

laser beam and may be or 80 pm a 14

device values

candela/rn2 and resolution of 1,100 (1,024) or 2,100 (2,048) raster lines. The relatively low light output of the CRT may be further diminished by strong lights nearby. The quality of the picture seen on the monitor is quite dependent on the type of phosphor used. The

it is modulated film.

fog) to 3.5 (maximum density [D]) The intensity of the transmitted laser sensed and digitized to 12 bits (4,096

(4,096

by

monitor.

table

the digital-to-analog verts the resultant

display

array. A scanthe radioAs the laser

the film,

densities

tional radiographic ties of 0.02 (minimum

a conventional

a digital across fashion.

the

The

cent phosphor The principal

film

is modulated

teractive window and level functions, is used to map up to 12 bits of raw digital image data from the frame buffer into the 8 bits requested. This pixel contrast value is sent to

is a function I

beam

look-up is a computer gray levels into pixel

brightness

A laser

electron

the output of the digital-to-analog converter and associated amplifiers so that the energy is proportional to the gray level. The digital numbers presented to the digital-to-analog converter were modified with a table look-up, through which the original digital data can be adjusted so that the gray-level range of inter-

by its

gamma,

is the

light

the

is 80

out-

through makes it

image

to the

a

hard

lim.

is approximately

2.3.

If L

V is the voltage input, and then L = kV. information is printed onto

film

laser

on 12 bits and is modulated

measured disparity

CRT

which output,

laser-sensitive

spot

phosphor light

from the laser film printer. The of screen brightness is measured

k is a constant, Digital image scanning

box This

and The

by

beam.

outputs The by

the

exposing

the

Printers

of 12 bits. intensity digital

film

have

of 8

The laser

of the data

to a

inputs laser

after

they

beam pass

through a table look-up. Compared with conventional film with an optical D of 3.5, the laser output film has a range ofO.20 (Dmj to about 3.0 (Dm) (Fig 3). The printer table look-up must compensate for the specific gamma characteristics of the laser film being used.

con-

into an image that is disfashion on a cathode ray 2).

At each

point

in a panticu-

Siegel

et al

U

RadioGrapbics

U

331

Calibration

. Pathways The laser film digitizer to laser film printer pathway (Fig ic) represents teleradiology. the best possible gray-level fidelity, output printer optical densities should be identical film input optical densities. Unfortunately, this

is not

printed

always

film

possible,

may

have

since

the

a 2.6-2.8

For

ing

to

laser-

D,

and

the

original film may have had an optical density of 3.5 on greaten. Similarly, at the lower end of the scale, the density of the laser film used in this

study

was

objects

about

in the

0.20.

original

Some

low-density

could

be

film

less

than

In the intermediate optical density range, it should be possible to exactly match the original film with the laser-printed film. The use of the laser digitizer or printer data pathway requires that the laser printer consistently reproduces the significant optical denfound

in the

If the input bits

(each

pixel

the

takes

12-bit

range

fonmly

mapped

by

Dimensional

16.

original

radiographic

to the laser

film on

of the

into

one

digitizer

an 8-bit fidelity,

that

from geometric distortion, mechanically stable laser digitizer. Some conventional tions

may

from

a scatter-reduction

aliasing

contain

This

laser-printed

causes

an

routines

equalization ent gamma U

such

as histogram

may be used to modify the apparor slope of the screen response.

We routinely

calibrate

guarantee

laser

uniformity that

pendent

on the internal

a printer

creates

as the and

A new

the

of film,

on

hardware

nal conditions such processing chemicals,

of images

printer. the

printer

If any must

The

film

are

and

exter-

introduction

produced

of these

to de-

film characteristics, the film processor.

chemicals, or changing mechanical trol characteristics of the processor quality

weekly

of performance.

densities

batch

printers

by the

factors

be recalibrated.

are

of new

and conaffect the laser

changed,

density

RadioGraphics

U

Siegel

et al

is printed.

The

with

a densi-

a fixed

number

of contrast

steps

ranging

from

Dmin

the pre-

tO

densitometer is used to assign a density number to each step. These densities are entered into the laser printer through its console keyboard. The calibration procedure is then complete. A

density

through

can

the

step wedge slope yields

to the laser table

translation functional translates

determined

read-back

gray-scale

the

new

be

of the

display.

function

of

Printing

the

with the table look-up of unit a plot that shows how the printer optical densities to gray levels. A table is created, which becomes a part of the table look-up. This table

is assigning

into

usually

use

the interactive

gray

gray

optical

densities

output

printer

process

uses

wedge,

there

levels

levels

from

so that

are

mapped

optical the

digitizer digitizer

into

the

densities.

2 1-step

are

the input

optical

2 1 points

this

density

at which

can be determined

same

Because

step

the

precisely.

table

The

re-

maining entries in the table look-up are obtamed with linear interpolation. The table look-up is linear in a stepwise fashion (Fig 3). With a first-approximation table look-up, actual cases are used to refine it. Findings

from

several

cases

ing technologies radiologist can

printed the

versions

table

look-up

that

and

in an overall process is satisfied,

then

to bring

iterative

feedback

involve

are digitized compare the

together

ogist

U

bars is measured

selected

as the

332

bar

into

look-up

PROCEDURES

CALIBRATION

partitioned

input

image.

Digital data from the digitizer is 12 bits, but all gray-scale displays are limited to 8 bits (Fig 1, path A). Consequently, interactive window and level functions are used. If the window and level parameters are selected a priori, the image can be displayed with no interaction. These preset window and level settings are useful for the consistent reproduction of CRT gray levels as optical densities by laser print-

ers. Automated

density

of each

look-up into the printer. The first step in identifying the best possible table look-up is to digitize and print a 2 1-step density film with the table look-up ofunit slope. The digital number, or gray level, associated with each

a

and laser film exarninainformation

grid.

in the

with

is initiated

The laser digitizer is matched printer by loading an appropriate

is, freedom

high-frequency

artifact

be uni-

by dividing

is ensured

printer

8

levels),

may

range

printer

torneten. The density selected is the maximum that appears on any film printed after the calibration procedure is completed. The least dense bar that is still acceptable should be selected, since the range from Dm to Dm,, is

32

requires

of 256

16 uniform

density

viously

film.

printer

laser

levels. It is advantageous to have the largest number of contrast levels available in critical density ranges to produce the best output picture. The second step of the calibration procedune involves printing a picture that contains

0.20.

sities

of the

when the operator selects the optical Dm to be written by the laser printer. A film contain-

suggest the

visual

in which

tone.

process

Volume

imag-

When

A

changes

versions

in

closer

This

is an

the

radiol-

a radiologist

mechanism.

the

different and printed. original and

is used

is terminated.

12

Number

2

4a.

4b.

L.

4, 5.

(4a) Original radiograph ofa patient with an apical pneumothorax on the right side. (4b) Rathat has been laser digitized to 2,048 x 2,048 x 12 bits, transmitted, and laser printed with our table look-up (as in Fig Sb). (5a) Original radiograph of a patient with multiple myeloma of the hu(5b) Digitized and laser-printed film accomplished by using our universal table look-up.

Figures diograph universal menus.

Each

examination

trointestinal,

ferent

optical

ments. universal

image cedure gists

March

type-chest,

skeletal

density

With this method, table look-up

study,

upper or

laser-printer we that

(Figs

1992

dif-

require-

developed maintains

fidelity (Fig 3). The results have been well accepted

gas-

CT-has

a good

The

laser

digitizer

to gray-scale

display

pathway (Fig 1, path A) is limited because the gray-scale monitor amplifiers used have electronic noise equal to one of 256 levels. This limits

any

displayed

image

to a maximum

of 8

of this proby radiolo-

4-6).

Siegel

et al

U

RadioGraphics

U

333

6.

Figure

left

renal

axial CT scan (1,024 x 1,024) ofa patient with an abdominal aortic aneurysm and a CT scan has been digitized, transmitted, and laser printed by using our universal ta-

(a) Original cyst. (b) Axial

ble look-up.

bits.

A second

limitation

is the

nonlinearity

of

to 4,096

screen brightness as a function of the input video signal. This limitation may be compensated for by the display table look-up or the digitizer-to-analog converter. In current applications, selection of the window parameter is

since

made

by the

from

tions,

such

sive

care

user.

However,

as the

emergency

unit,

users

do

in many

room

not

want

applica-

or inten-

the

film

substantially

on

higher

is difficult, a light

brightness

box

the response

tive

box.

judgments

to a high

Therefore,

level

we

of a radiologist

the

gray-scale

The table

modified

look-up

after

of the laser

each

qualita-

as the

find an acceptable match between and the laser printer. The digitized

for the

than

of brightness

use

sented. The calibration

procedure

with

levels

mechanism. comparison

feedback

printer

is

in an

effort

to

the CRT 21-step

subjective feedback from the digitized 2 1-step density wedge mentioned earlier. If an adjustrnent is required, it is accomplished with the

density wedge was used as a test image. To achieve a calibration of this important pathway, a controller is required that would automatically provide information on how to map

display

the luminance

reading

optical

on

is currently

table

accomplished

by using

display

to laser

way (Fig 1, path B) is limited put picture from the gray-scale plays no more than 256 gray substantially

ferences.

the

look-up.

The gray-scale

fewer

The

output

because monitor

levels

perceptible

laser

printer

paththe

and

gray-level

printer

indis-

displays

U

density

GRAY-SCALE

The gray-scale dif-

up

an adjunct

RadioGraphics

U

Siegel

et al

the

from

a CRT into

laser-printed

an

film.

CONTROLLER

controller to the

host

(Fig 7) functions computer,

a personal central

monitor controller

computer processing

Volume

as

controlling

the calibration of the gray-scale the laser printer. The gray-scale designed uses own specialized

U

Comparison

is viewed

with the display. Thus, the table look-up needs to be automatically set so that the image with the best diagnostic detail is pre-

display

334

levels.

those of the gray-scale monitor. The observer’s visual response to a low level of brightness on a gray-scale display screen is different in a light

to interact

gray

12

with unit.

and we its It

Number

2

Gray

Scale

Display

Figure 7. Gray-scale controller. Optical density read from the laserprinter output is used to adjust the table look-up of the printer. Light output from the gray-scale monitor is used

Laser

stores CRT,

all of the local the laser printer,

The output

characteristics and the laser

of the CRT is monitored

light-sensing solid sensors is lowered A preprogrammed

to the CRT. The

sensors

anism controls

the brightness and the CRT is maintained

to modify so that

standard state. densitometer, the

laser

printer

Printer

Similarly, a standard

use a feedback

through picture

is interrogated.

look-up

can

automatically

for the printer.

ups for the may depend Specification

compute

Different

with

or array-processor

mechcontrast in a

the use of a produced by Feedback

a new

table

table

look-

printer and the gray-scale display on the type of examination. of the examination type causes

the gray-scale controller propriate table look-ups

future,

tions

through the gray-scale controller instructs the user to recalibrate the printer so that the controller

In the

of the digitizer.

state devices. An array of over the front of the CRT. set of test patterns is sent

to

adjust

the

table

look-up

of the gray-scale display. D/A = digital to analog, LUT = table look-up.

can

currently

the

are

gray-scale

incorporated but

computer just

being

controller into

board.

available,

appropriate nithms

be

an

These user

func-

accelerator

boards

are

functions

connections developed

and

and and

algo-

imple-

mented. One accelerator board could run our gray-scale controller routines and many other highly specialized related tasks such as digital data compression, contrast enhancement, or spatial-frequency modification. An image sent to the CRT might need edge enhancement to compensate for the characteristic loss of

edges inherent in interactive displays. accelerator board would automatically an edge-enhancement the enhanced image

are now celeraton

clinically

algorithm on the CRT

testing

The apply

and display screen. We

a multifunction

ac-

board.

to download the apto both output de-

vices.

March

1992

Siegel

et al

U

RadioGrapbics

U

335