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State-of-the-Art Digital Radiography .
1
.
A
Kyo Rak Edward Arch W. SamuelJ. Mark D. Louis H.
,
,
:
,
,
Lee, MD L. Siegel, MD Templeton, MD Dwyer III, PhD Murphey, MD Wetzel, MD
Technologic advances in digital radiography have improved the ways in which radiographic images are acquired, displayed, transmitted, recorded, and archived. With computed radiography, performed with storage phosphor plates and interactive high-resolution workstations, radiation dose is reduced and repeat exposures necessitated due to technical errors are eliminated. Digital fluorography allows reductions in dose, procedure time, and film costs. These digital imaging modalities have been well accepted clinically and are equal in diagnostic accuracy to conventional methods. Teleradiology has advanced with the development of laser film digitization, fiberoptic networks, and dial-up circuit switching technology. Laser film printers yield improved hard copies of transmitted images, but further work is needed to faithfully reproduce the images displayed on high-resolution workstations. Although the capacity for archiving digital image data has increased (260,000 examinations or 23,500 Gbytes can be stored in a six-unit optical disc library), higher capacity storage media are needed. Further technologic advances in the speed of image transmission and storage capacity are anticipated. U
INTRODUCTION
Digital
radiography
involves
image
acquisition,
electronics,
and
computers.
In the
past 3 years, there has been signfficant technologic progress in the development and implementation of digital radiography. Advances have been made in the areas of image acquisition, display and image processing, image transmission, hard copy recording, and archiving. New equipment, such as high-resolution interactive display
.
Abbreviation: Index Video
terms: systems
,
RadloGraphics
,
City, KS 66103. receivedjune
I
From
the
CRT
cathode
Picture
archiving
1991;
11:1013-1025
Department
ray tube and
of Diagnostic
From the 1990 3; acceptedJune
communication
Radiology,
system
University
(PACS)
of Kansas
#{149} Radiography,
Medical
RSNA scientific assembly. Received February 5. Address reprint requests to K.R.L.
digital
Center, 22,
1991;
39th revision
#{149} Radiography,
and
technology’
Rainbow
Blvd,
requested
April
Kansas 23 and
#{176}RSNA,1991 See
the
commentary
by Ritenour
foilowing
this
article.
1013
X-ray
THE SUBSYSTEMS OF COMPUTED RADIOGRAPHY
C 0 C
Dose
1. Figures 1-3. (1) Flow diagram ofcomputed radiography system that uses storage phosphor plates (Digiscan; Siemens Medical Systems). A = phosphor plate, B = image reader, C = image processor, CR = computed radiographic, D = laser film printer and film processor, E = interactive gray-scale workstation. (2) Storage phosphor screens have a dynamic range of 10’ and screen-film systems have a dynamic range ofover 102. The storage phosphor screens have a linear response over the entire range of exposure. (3) The latent x-ray image stored in the phosphor plate is acquired by a Iaser beam scanner. The resultant luminescence is collected by a photomultiplier detector and is transmitted to the digitizer.
monitors, discs,
laser
image
as well
recorders,
as improved
and
meth-
are now available. At our own institution in the past 3 years, computed radiography has routinely been used for pediatric, emergency, and bedside radiography (i). In addition, a teleradiology system that links our institution with two others has been implemented. Laser film digitization,
high-resolution
cording,
and
in this
system.
In this digital
displays,
fiberoptic
laser
networks
article,
we describe
radiography,
including
image
are
re-
all used
state-of-the-art computed
radi-
performed with storage phosphor plate technology, digital fluorography, digital scanning radiography, film digitization, image ography
display,
own), images.
teleradiology
hard
copy
systems
recording,
(including
and
archival
our
of
U IMAGE ACQUISITION Image acquisition technology includes computed radiography, digital fluorography, digital scanning radiography, and film digitization.
U
RadioGraphics
U
Lee
et at
3.
.
optical
teleradiology
ods,
1014
(uGy)
2.
Computed
The
basic
sists
of four
Radiography computed
radiography
components:
the
system latent
con-
image
sensor (phosphor plate), a laser image reader, an image processor capability, and a laser image recorder (2 3) A workstation can be added to this system. At our institution, we use a Digiscan system with an interactive grayscale workstation (both by Siemens Medical Systems, Iselin, NJ) (Fig 1). The image sensor is a thin plate coated with minute crystals of photostimulable phosphor (europium-activated barium fluorobromide). The phosphor crystals have the unique capa,
bility
of storing
.
the
energies
of absorbed
photons over a wide dynamic to i#{248}pGy (Fig 2). The plate intensifying
screen
and
film
in a cassette.
cassette is used for overhead spot imaging in any existing unit. Latent
images
captured
x-ray
range from i0’ replaces the The
or fluoroscopic radiographic on
plates
are
read
3). The image data, emitted in the form of luminescent light, are converted to electronic signals, which are subsequently digitized and transferred to the image processor. by a helium-neon
laser
beam
Volume
(Fig
11
Number
6
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b. Figure 4. (a) Computed radiograph ofthe abdomen obtained with an underexposed plate due to a technical error. (b) Image data from a were subjected to image processing workstation. Resultant computed radiograph has excellent quality. a.
The processed image data are transferred to the image recorder. A pain of images, with and without edge enhancement, are recorded automatically on 15 x 36-cm laser-sensitive film. The exposed plate is flooded with ultraviolet to erase the stored plate can be reused for The laser-scanned matrix 1,760 x 2,140 x 10 bits to 2,000 x 2,510 x 10 plate). Accordingly, the light
ies
in the
range
meter (lp/mm). In our system, high-resolution
of 2.5-5.0
the
1991
line
workstation
(1,023
and is directly interfaced cessor (Fig 1). The station
November
image so that the another examination. size ranges from (i4 X 17-inch plate) bits (8 X 12-inch spatial resolution van-
lines/60
with has
pairs
has Hz)
per
milli-
two
tions,
(eg,
including
interactive
windowing,
cation),
edge
image
pression
departmental images
image
be
magnifi-
reversible
2: i),
and
network.
can
processing
enhancement,
storage,
(3: 1 or
storage phosphor on the interactive
data
interfacing
Desired
transferred
processed
back
to the
recorder for hard copy production. image data are also stored on optical The major advantages of computed raphy
include
of repeat plate has tude
greater
ceeds
the
dose
exposures a linear, than response
reduction
comto our
and
image
The raw discs. radiogelimination
(Fig 4). The phosphor wide range of exposure iO,000
to one,
which
of a conventional
latiex-
screen-
monitors
the image promultiple func-
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a. Figure
b. 5.
(a) Conventional
screen-film
radiograph
(3.0 mAs, 55 kVp) of a newborn with bronchopulmonary dyspla.sia. (b) Computed radiograph of the same newborn obtained 1 day later with a storage phosphor plate (1.5 mAs, 55 kVp). (c) Computed radiograph (0.5 mAs, 55 kVp) ofthe same newborn obtained 2 days after the conventional screen-film examination. Image quality is diagnostic, although quantum noise is visible due to the lower exposure.
The left percutaneous
venous
catheter
is better
seen
(arrows).
..,,‘,
C.
film receptor by a factor of over 1 ,000 (Fig 2). The system has been well accepted for all aspects of general radiography, including imaging
of the
chest,
gastrointestinal
and
geni-
tracts, and bone (Figs 4-7) (3-6). An average of 25-30 chest examinations can be performed per hour. The smaller hand copy image formats have caused no problems once the radiologists and clinicians become acquainted with them. tourinary
1016
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6
a.
Figure 6. (a) Spot image conventional double-contrast of the upper gastrointestinal (b)
Storage
phosphor
plate
from a double-contrast upper gastrointestinal gastric
mucosa
onstrated
from a study tract. image
study of the tract. The
is more
sharply
dem-
on b.
b.
Figure 7. Computed demonstrates excellent tion. A right pneumothorax
November
1991
radiograph of the chest spatial and contrast resoluis clearly
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Figure 8. Diagram of a digital fluorography system that faced to the University of Kansas Medical Center radiology
is interdepart-
ment network. The high-resolution mens Medical Systems) is interfaced by means of a DR-i i device (Digital
crocomputer 2,048 radiology
.
system
high-resolution
is connected gray-scale
video digitizer (DS-i000; Sieto a microcomputer system Equipment, Boston). The mito a laser film printer, a 2,048 x display, and a gateway to the tele-
system.
Digital
Fluorography spot imaging and storage of digital images can be used in an independent system or as part of a digital departmental network (7,8). The system is based on conventional image-intensifier technology and incorporates a microprocessor-controlled, general-purpose fluonoscopy unit, video chain, and video digitizer (Fig 8). The installation is designed for digital fluonoscopy and radiography of the gastrointestinal tract or for venography, myelography, arthrography, and genitourinary tract procedures (Figs 9, iO) (9, 10). A typical system is capable of acquiring 1 ,024 x 1,024 x 8- or i2-bit images at up to six frames pen Digital
second
and
system
can
of storing be
operated
up to 100 in two
images. modes,
The fluoro-
Figure
9.
Digital
fluorography
spot
image obtained during a double-contrast barium enema examination.
1018
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Figure 10. Digital knee (b), and thigh
Table 1 Comparison
venograms of the lower extremity. The images were obtained at the (c) at the rate ofone image per i.5 seconds with a digital fluorography
of Digital
phic
Fluorogra
and
Conventional
size
33-cm
Actual field size Spatial resolution Contrast resolution (adequate visualization of blood clots Exposure 86 kVp 76 kVp
scopic
and
Digital
digital
during sultation
the
senior
staff)
November
66.8 97.5
images
examination. This with other radiologists and
1991
referring
mR mR
(0.017 (0.025
field
35.5-cm
ofview
mg
mC/kg) mC/kg)
mode.
costs
ages are printed. needed because
facilitates (resident
clinicians.
Film
conand
settes
ble
are
reduced
is eliminated.
1) and
(i4-inch)
(a),
film
31 cm (i2.2 inch) (rectangular) 3.0 lp/mm 7.5% contrast material (36.5 of iodine/mL) 109 294
are
acquired
leg
Conventional
27 cm (10.6 inch) (circular) 1.5 lp/mm 7.5% contrast material (36.5 of iodine/mL)
[9])
radiographic
fluonographic
(i3-inch)
of the
unit.
Systems
Digital
Characteristic
Image
5creen-Film
level
procedure
mR mR
(0.028 (0.076
because
mC/kg) mC/kg)
only
pertinent
No extra personnel manual changing Total
time
Lee
et at
im-
are of film
cas-
dose
(Ta-
radiation
are
mg
reduced.
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C
11. Figures
11, 12.
(ii)
Diagram
of a
digital scanning equalization radiography system illustrates the fore slit, the modulator unit, the detector and electronic feedback loop, and
the x-ray film cassette. tronic feedback intensity of the
The elec-
loop modulates x rays by means
the of
the modulator unit. (12) Diagram of a video film digitizer system shows the video camera, the analog-todigital
converter,
the
microcom-
puter system and disc, and an interface. The camera is focused on the radiograph, which is positioned on a view box. The camera can zoom on any portion of the radiograph.
in
12.
I Digital Scanning Radiography In chest examinations, the intensity of the x nays transmitted through the lungs, mediastinum, and bones varies widely. The dynamic range ofx-ray film, however, is not adequate to record this variation ofx-nay transmission (1 i). The problem of the required dynamic range has been addressed by such techniques as unsharp masking (12,13), digital beam attenuators (14), and scanning equalization radiography (15-20). A prototype digital scanning chest unit was developed and evaluated in the early i980s. This
system
uses
a small
x-ray
beam
that
scans
across the patient in a raster fashion. A detecton behind the patient measures the exposure and, by means of a feedback loop connected to an x-ray modulator, maintains a constant
1020
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RadioGraphics
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film exposure by adjusting the put (Fig 1 1). Scatter is reduced slits
and
a grid.
The
x-ray
x-ray tube outby fore and aft
modulator
consists
of an absorber controlled by a piezoelectric actuator. In a study of the quality of chest images produced with scanning equalization and conventional screen-film systems, the images were comparable (21). .
Film
Digitization
The quality of digitized images the film digitization technology ity of the
original
radiograph
depends and the to be
on qual-
digitized
(22). The digitization of an image can be achieved with a video camera and an ordinary light view box (Fig 12). Video camera systems have been the most common method offilm digitization. However, these systems have limited spatial resolution (0.8-mm pixel size in chest nadiographs of adults) and have been used primarily for
Volume
11
Number
6
Figures
13, 14.
(13)
Diagram
of a
laser film digitizer system demonstrates the laser beam scanning across the radiographic film. The
collected through
light intensity passing the film is collected and
digitized. Low digital numbers
cal densities. the essential
values of resultant indicate high opti-
(14) Diagram depicts elements of an interac-
tive digital gray-scale workstation. host computer provides the image
A
data and the image processing software. Text generation and interacInteractive
tive interrupts are graphics processor.
Device
managed
by the
14.
initial
diagnosis
and
consultation
in teleradi-
ology.
Image quality can be improved by using a laser film digitizer (Fig 13). Pixel size, number ofgray scales, and scanning time are the important specifications that determine the physical characteristics of a film digitizer system (23-25).
memory board that reads the digital image data at the scanning rate of the cathode ray tube [CRT]), lookup tables (which map the digital data to the desired shades ofgray for display on the CRT), and the gray-scale CRT display monitor (Fig 14). The dynamic range of the phosphor on the face
U IMAGE DISPLAYS Digital radiography requires high-resolution, interactive workstations that address a wide range of user requirements. The basic elements of an interactive gray-scale workstation include a graphics processor (a special-punpose microcomputer used to control the workstation),
November
a frame
1991
buffer
of the
distinguishable
CRT
is limited luminance
and
provides
levels
50-60
as deter-
mined by a selected choice of discernible gray levels. Discernible gray levels are the minimum detectable gray levels displayed on the CRT as a function of the digital numbers be-
(a high-speed
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Figure 15. Diagram of the teleradiology system connecting two military hospitals and the University of Kansas Medical Center.
lnteracth,e
Gray
Scale
Display
ing applied to the CRT. Lookup tables map the pixel values of a digital image into luminance values to be displayed as gray-scale 1evels on the CR1. On the basis of our experience with 2,048 x 2,048 x 8- or 12-bit grayscale displays, we have devised a set of ideal parameters
The number ing:
for
these
monitors
(Table
use of display workstations of issues (26), including
perception
displayed contrast
of disease
images,
the
resolution,
workstation
focuses suspected
about
on
radiographic
the specific features disease (27).
on
CRT-
spatial
and
platforms,
findings
that
present
in a
U TELERADIOLOGY SYSTEMS Teleradiology systems transmit images from one site to another by means ofwide-area networks (28). Teleradiology wide-area networks employ a variety of public digital communication
links,
including
dedicated
point-to-point
circuit switching, packet or message switching, fiberoptic links, and microwave or satellite links. Dedicated point-to-point digital data links are installed for transmission between two sites. Circuit switching is a dial-up system, in which a data path is established by switching multiple circuits. For packet or message switching, the data to be transmitted are divided into smaller sizes. These packets or messages are labeled with a destination address and are transmitted from node to node until they finally arrive at the destination. links,
1022
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for Gray-Scale
Characteristic Pixel
Ideal 0.2
size
Display Specification
mm
(2,048
required software, display protocols, and the use of image-processing algorithms. It is possible to develop a workstation that can present knowledge
2
Ideal CharaCteristiCs Monitors
2).
raises a the follow-
as seen
required
Table
Spatial
resolution
Contrast
range
Contrast
ratio
2.5
x 2,048)
cycles/mm
12 bits (4,096 levels)
gray
0:1
Brightness Screen refresh rate Data transfer rate
60 foot-lamberts 72 Hz 1 second, full screen
Fiberoptic links use light to transmit image data. Data to be transmitted are encoded into a high-speed (4 Gbits/sec) serial bit stream. Microwave links are high-frequency radio wave systems that transmit encoded data in a line-of-sight manner. Satellite links are used to transmit data over large distances. Teleradiology
systems
are
used
to transmit
images either for consultation or diagnosis. Consultation systems are low-cost, low-nesolution systems that are used when subsequent review of images is needed. Diagnostic systems are high-resolution systems that transmit images between medical facilities for the punpose of diagnosis. The radiographic films are digitized, transmitted, and displayed on a high-resolution
gray-scale
display
or printed by a laser film printer. The diagnostic teleradiology
workstation
system
in our
institution is a digital wide-area network that is connected to two military hospitals by a U.S. Sprint (Kansas City, Kan) switched fiberoptic network and a copper Ti carrier service
Volume
11
Number
6
Table
3 of CommuniCation Multiplexer
Comparison
56-kbits/seC
Sites Teleradiology
50-bed
No. of Examinations Transmitted per Day*
on the System
military
data
e Achieved
wi th a Ti
Megabits Transmitted per Montht
Access Charges per Months
C arrier
or N x
TiCarrier Costs per Month (point to point)1
NX56 kbits/sec Costs per Month (dial up)
28
$3,832
$1,701
$3,169
$2,619
$1,700
32,413
hospital
from
24** computed
Megabits per month Access charges/month
=
288
tomography
and recorded on a laser film printer. screen-film radiographs are digitized transmitted and recorded on a laser t
Servic
No. of Images Transmitted per Day
7#
100-bed military (i5Omiles) Image
f or DS-1
hospital
(48miles)
*
Costs
23 days/month
vary depending
(CT),
nuclear
89,355 medicine,
$354 and ultrasound
(US) are transmitted
Special procedures images are not included. All conventional to 2,048 pixels per row x 2,048 rows x 12 bits per pixel and are film printer. x bits/image X images/day. on digital exchange equipment
provided
by the
local
carrier
service. § Ti carrier charges/month = $i,8i0/month + $6.70/mile + access charges/month. I N X 56 kbits/sec charges/month = access charges/month + (tariff/minute x 1 minute/60 seconds second/56 kbits x bits/month x 1.2), where the tariffis $0.06/minute and 1.2 is protocol overhead. # All conventional radiographic examinations. ** Includes 17 conventional radiographic, two CT, two nuclear medicine, and three US examinations.
(Fig 1 5). The data rate service is a Digital Senvice (DS)-1 (i.544 Mbits/sec). Equipment at each military hospital includes a laser film digitizer, an interactive gray-scale display, a computen system, and a network access controllen. The network access controller is a multiplexer that dials up multiple 56-kbits/ sec digital channels for transmitting data in parallel. The user selects the value of n (n = 1, . . ., 24), and the multiplexer dials up n available circuit-switched 56-kbits/sec channels. The data to be sent are encoded and transmitted to the receiving site. The data are decoded, and hard copy is recorded by a laser film printer. The cost of communications in teleradiology systems is about i5%-20% ofthe total cost. One method for reducing this cost is to use an n X 56-kbit/sec multiplexer. The advantages of the n X 56-kbits/sec multiplexer are the dial-up capability and a usage cost of $0.06 per minute (Table 3). U HARD COPY RECORDING If a digital radiography system has a matrix size ofless than i,024 x 1,024 x 8 bits, multiformat video film recorders may be used (28).
November
1991
However,
for digital
systems
with
x 1
spatial
neso-
lution of i,024 x i,024 x iO bits or greaten, laser film printers are required to generate hard copy recordings (29). The laser film printer uses a laser beam to scan across a sheet of laser-sensitive film. The laser scanning beam is modulated by the digital data be recorded. A laser film printer prints a 4,096
x 5, i20
x
i2-bit
array.
The
images
to to
be printed are interpolated onto the 4,096 x 5, 120 x i2-bit array. It is still difficult to faithfully reproduce on the laser printed image what the user saw on the display workstation. U ARCHWING One of the naphy
major
is that
chived
by
of digital
advantages
the
digital
electronic
image
states
require
tamed
until
data
systems.
archiving that
the
of digital
image
film
The
are
patient
is 21
legal
serious.
image
radiog-
can
data
an-
issues
Some be
years
be
main-
of age.
Other states require that consultation reports be maintained. Liability issues require maintaming image data for an adequate period of time.
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The on-line, high-speed, 30-day archiving storage requirements for a department with workload of 130,000 examinations per year range from 30 Gbytes to 1,410 Gbytes. The storage requirements depend on the fraction of the workload that is digital. In our institution,
about
digitally
30%
of such
formatted
a workload
imaging
involves
modalities
(i
Gbyte/day). Ifthis were increased to 90% of the workload due to the replacement of conventional screen-film radiography with computed radiography, 47 Gbytes/day (or i,4i0 Gbytes per 30-day period) of digital data storage would be needed. The current technology used to manage on-line, high-speed archiving of these data consists of magnetic discs (600 Mbytes per disc drive). Erasable optical discs (14-inch-diameter optical discs, 6.8 Gbytes per disc, and i-Mbyte/sec transfer rates) will be used in the future. For intermediate 2-year archiving storage, an electronic system that archives from 500 Gbytes (1 Gbyte/day x 250 days/year x 2 years) to 23,500 Gbytes (47 Gbytes/day X 250 days X 2 years) is required. The technology available for intermediate 2-year archiving is an optical disc library (or “jukebox”). Such a system might contain 1 50 discs (each disc storing 6.8 Gbytes) for a total storage of 1,020 Gbytes. Thus, six optical disc library units with a 4: 1 compression can archive the 2 years of image data generated at 47 Gbytes/ day. The long-term, 5-year archiving storage requirements range from 1,250 Gbytes (i.25 Thytes or i.25 x iO’2 bytes) to 58,750 Gbytes (58.75 Thytes). It is thought that optical tape will be able to archive 1 Thyte per 2,400-foot reel. Optical tape drives are currently in research development.
a
to-noise ratio. Ultra-high-resolution (2,048 x 2,048) gray-scale workstations are being implemented. Manufacturers are increasing the screen brightness with new monitor
designs.
gray-scale significant
FUTURE
EFFORTS
IN
U 1.
2.
3.
reductions,
and
the
screen
examinations. AppI Radiol 1990; Sonoda M, Takano M, MiyaharaJ,
19:27-34. Kato H.
148:833-838. Merritt CRB,
Tutton
RH,
Bell
KA, et al.
Clini-
cal application of digital radiography: computed radiographic imaging. RadioGraphics 4.
i985; 5:397-4i4. Schaefer CM, Greene
Improved
5.
6.
DIGITAL
control
RE, OestmannJw,
of image
optical
et al.
density
with low-dose digital and conventional radiography in bedside imaging. Radiology 1989; 173:713-716. Schaefer CM, Greene R, Hall DA, et al. Mediastinal abnormalities: detection with storage phosphor digital radiography. Radiology 1991; 178:169-173.
Prokop
M, Galanski
Storage
phosphor
phy:
are
radiographic
being imaging
accomplished systems.
in dig-
7.
A general-
8.
9.
effect
M, Oestmann versus
ofvarying
unsharp ofcortical
mask bone
JW, et al.
screen-film
exposure filtering defects.
radiogra-
parameters on the detectabilRadiology 1990;
109-i 13. DM, Edmonds EW, RowlandsJA, Krameta KR, Pack WK. Videofluorography and pulsed fluoroscopy using a 5 12 x 5 12 pixel digital image system. Radiology 1985; 155: Hynes
S 19-523. Cox GG, Templeton
AW, McMillanJH,
LH, Lee KR.
Digital
fluoroscopy
raphy system: cal examples.
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technical
498. Lee KR, Templeton McClure CB. lower extremity.
RadioGrapbics
i,024)
with
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ity
purpose, single-plate, storage phosphor plate system is being implemented with 3,072 x 3,072 spatial resolution. Improvements are being made in digitized fluorography to achieve fasten frame rates (at a resolution of i,024 x 1,024 x 8 bits) and improved signal-
U
x
improved
i77:
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1024
being
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cost
(1,024
are
brightness on these displays is excellent. There is a need to develop laser printers that reproduce faithfully the images displayed on gray-scale monitors. Laser film printers that generate 4,096 x 5, 120 x i2-bit images are being developed. Laser film digitizers and lasen film printers are required for digital teleradiology. Communication costs are being reduced by the use of dial-up, multiple digital public switched 56-kbits/sec channels.
and
U
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and radiog-
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Templeton AW, Dwyer SJ III, Cox GG, et al. A digital radiology imaging system: descrip-
.
tion and clinical evaluation. AJR 1987; 149: 847-851. Barnes GT, Sones RA, Tesic MM. Digital chest radiography: performance evaluation of a prototype unit. Radiology 1985; 154:80 1806. Plewes DB, Wandtke JC. A scanning equalization system for improved chest radiography. Radiology 1982; 142:765-768. SorensenjA, Nikiason LT, NelsonjA. Photographic unsharp masking in chest radiography. Invest Radiol 1981; 16:281-288. Hasegawa BH, Naimudlin 5, DobbmnsJT, et al. Digital beam attenuator technique for com-
pensated 15.
chest
radiography.
159:537-543. Plewes DB, Vogelstein
Radiology
E.
i983;
1986;
A scanning
for chest radiography with regional control: practical implementation.
17.
i8.
Korhola
20.
DB.
Improved
0, Riihim#{228}kiE, Savikurki scatter device.
scanning
multiple-beam
for chest
radiography. Kool
Advanced
chest
S.
1991
28.
Reduc-
Kool
U.
AMBER:
equalization
Radiology
U, Busscher
multiple-beam
DLT,
29.
a
performance
in se-
digital
skeletal
imaging.
Radiology
of
1990;
174:865-870. Arenson RL, Chakraborty DP, Seshadri SB, Kundel HL. The digital imaging workstation. Radiology 1990; 176:303-3 15. Swett HA, Fisher PR, Cohn Al, Miller PL, Mutalik PG. Expert system-controlled image display. Radiology 1989; 172:487-493. Batnitzky 5, Rosenthal SJ, Siegel EL, et al. Teleradiology: an assessment. Radiology 1990; 177:11-17. Lee KR, Anderson WFJr, Wetzel LH. Hardcopy recordings of analog and digital images. Invest Radiol 1988; 23:933-939.
system
1988;
169:
Vlasbloem
H,
equalization
radiography in chest radiology: a simulated nodule detection study. Radiology 1988; 35-39.
November
27.
of observer
lected cases. Radiology 1986; 18:11-19. MacMahon H, Vyborny CJ, Metz CE, Doi K, Sabeti V, Solomon SL. Digital radiography subtle pulmonary abnormalities: an ROC study of the effect of pixel size on observer performance. Radiology 1986; 18:21-26. Murphey MD, BrambleJM, Cook LT, Martin NL, Dwyer SJ III. Nondisplaced fractures: spatial resolution requirements for detection
with
with a multiple penRadiology 1987;
165 :257-259. Vlasbloem H, Schultze
et al.
24.
26.
Plewes
WandtkeJC, Plewes DB. Comparison of scanning equalization and conventional chest radiography. Radiology 1989; 172:641-645. Kundel HL, MezrichJL, Brickman I, et al. Digital chest imaging: comparison of two film image digitizers with a classification task. Radiology 1987; 165:747-752. Lams PM, Cocklin ML. Spatial resolution requirements for digital chest radiographs: an
ROC study
25.
10:655-663.
disease detection with scanning equalization radiography. AJR 1985; 145:979-983. Plewes DB, Wandtke JC. A scanning equalization system for improved chest radiography. Radiology 1982; 142:765-768.
29-34. Schultze
23.
system
WandtkeJC,
tion of radiation cil-beam imaging
22.
exposure Med Phys
16.
19.
2i.
169:
Lee
et at
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RadioGrapbics
U
1025
.
r
y &..
State-of-the-Art Digital Radiography .
1
.
A
Kyo Rak Edward Arch W. SamuelJ. Mark D. Louis H.
,
,
:
,
,
Lee, MD L. Siegel, MD Templeton, MD Dwyer III, PhD Murphey, MD Wetzel, MD
Technologic advances in digital radiography have improved the ways in which radiographic images are acquired, displayed, transmitted, recorded, and archived. With computed radiography, performed with storage phosphor plates and interactive high-resolution workstations, radiation dose is reduced and repeat exposures necessitated due to technical errors are eliminated. Digital fluorography allows reductions in dose, procedure time, and film costs. These digital imaging modalities have been well accepted clinically and are equal in diagnostic accuracy to conventional methods. Teleradiology has advanced with the development of laser film digitization, fiberoptic networks, and dial-up circuit switching technology. Laser film printers yield improved hard copies of transmitted images, but further work is needed to faithfully reproduce the images displayed on high-resolution workstations. Although the capacity for archiving digital image data has increased (260,000 examinations or 23,500 Gbytes can be stored in a six-unit optical disc library), higher capacity storage media are needed. Further technologic advances in the speed of image transmission and storage capacity are anticipated. U
INTRODUCTION
Digital
radiography
involves
image
acquisition,
electronics,
and
computers.
In the
past 3 years, there has been signfficant technologic progress in the development and implementation of digital radiography. Advances have been made in the areas of image acquisition, display and image processing, image transmission, hard copy recording, and archiving. New equipment, such as high-resolution interactive display
.
Abbreviation: Index Video
terms: systems
,
RadloGraphics
,
City, KS 66103. receivedjune
I
From
the
CRT
cathode
Picture
archiving
1991;
11:1013-1025
Department
ray tube and
of Diagnostic
From the 1990 3; acceptedJune
communication
Radiology,
system
University
(PACS)
of Kansas
#{149} Radiography,
Medical
RSNA scientific assembly. Received February 5. Address reprint requests to K.R.L.
digital
Center, 22,
1991;
39th revision
#{149} Radiography,
and
technology’
Rainbow
Blvd,
requested
April
Kansas 23 and
#{176}RSNA,1991 See
the
commentary
by Ritenour
foilowing
this
article.
1013
X-ray
THE SUBSYSTEMS OF COMPUTED RADIOGRAPHY
C 0 C
Dose
1. Figures 1-3. (1) Flow diagram ofcomputed radiography system that uses storage phosphor plates (Digiscan; Siemens Medical Systems). A = phosphor plate, B = image reader, C = image processor, CR = computed radiographic, D = laser film printer and film processor, E = interactive gray-scale workstation. (2) Storage phosphor screens have a dynamic range of 10’ and screen-film systems have a dynamic range ofover 102. The storage phosphor screens have a linear response over the entire range of exposure. (3) The latent x-ray image stored in the phosphor plate is acquired by a Iaser beam scanner. The resultant luminescence is collected by a photomultiplier detector and is transmitted to the digitizer.
monitors, discs,
laser
image
as well
recorders,
as improved
and
meth-
are now available. At our own institution in the past 3 years, computed radiography has routinely been used for pediatric, emergency, and bedside radiography (i). In addition, a teleradiology system that links our institution with two others has been implemented. Laser film digitization,
high-resolution
cording,
and
in this
system.
In this digital
displays,
fiberoptic
laser
networks
article,
we describe
radiography,
including
image
are
re-
all used
state-of-the-art computed
radi-
performed with storage phosphor plate technology, digital fluorography, digital scanning radiography, film digitization, image ography
display,
own), images.
teleradiology
hard
copy
systems
recording,
(including
and
archival
our
of
U IMAGE ACQUISITION Image acquisition technology includes computed radiography, digital fluorography, digital scanning radiography, and film digitization.
U
RadioGraphics
U
Lee
et at
3.
.
optical
teleradiology
ods,
1014
(uGy)
2.
Computed
The
basic
sists
of four
Radiography computed
radiography
components:
the
system latent
con-
image
sensor (phosphor plate), a laser image reader, an image processor capability, and a laser image recorder (2 3) A workstation can be added to this system. At our institution, we use a Digiscan system with an interactive grayscale workstation (both by Siemens Medical Systems, Iselin, NJ) (Fig 1). The image sensor is a thin plate coated with minute crystals of photostimulable phosphor (europium-activated barium fluorobromide). The phosphor crystals have the unique capa,
bility
of storing
.
the
energies
of absorbed
photons over a wide dynamic to i#{248}pGy (Fig 2). The plate intensifying
screen
and
film
in a cassette.
cassette is used for overhead spot imaging in any existing unit. Latent
images
captured
x-ray
range from i0’ replaces the The
or fluoroscopic radiographic on
plates
are
read
3). The image data, emitted in the form of luminescent light, are converted to electronic signals, which are subsequently digitized and transferred to the image processor. by a helium-neon
laser
beam
Volume
(Fig
11
Number
6
p
....
‘1’
.
,..
:
:
-
.
.:_ .
: .
.
.
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.
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-S
b. Figure 4. (a) Computed radiograph ofthe abdomen obtained with an underexposed plate due to a technical error. (b) Image data from a were subjected to image processing workstation. Resultant computed radiograph has excellent quality. a.
The processed image data are transferred to the image recorder. A pain of images, with and without edge enhancement, are recorded automatically on 15 x 36-cm laser-sensitive film. The exposed plate is flooded with ultraviolet to erase the stored plate can be reused for The laser-scanned matrix 1,760 x 2,140 x 10 bits to 2,000 x 2,510 x 10 plate). Accordingly, the light
ies
in the
range
meter (lp/mm). In our system, high-resolution
of 2.5-5.0
the
1991
line
workstation
(1,023
and is directly interfaced cessor (Fig 1). The station
November
image so that the another examination. size ranges from (i4 X 17-inch plate) bits (8 X 12-inch spatial resolution van-
lines/60
with has
pairs
has Hz)
per
milli-
two
tions,
(eg,
including
interactive
windowing,
cation),
edge
image
pression
departmental images
image
be
magnifi-
reversible
2: i),
and
network.
can
processing
enhancement,
storage,
(3: 1 or
storage phosphor on the interactive
data
interfacing
Desired
transferred
processed
back
to the
recorder for hard copy production. image data are also stored on optical The major advantages of computed raphy
include
of repeat plate has tude
greater
ceeds
the
dose
exposures a linear, than response
reduction
comto our
and
image
The raw discs. radiogelimination
(Fig 4). The phosphor wide range of exposure iO,000
to one,
which
of a conventional
latiex-
screen-
monitors
the image promultiple func-
Lee
et at
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RadioGraphics
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1015
a. Figure
b. 5.
(a) Conventional
screen-film
radiograph
(3.0 mAs, 55 kVp) of a newborn with bronchopulmonary dyspla.sia. (b) Computed radiograph of the same newborn obtained 1 day later with a storage phosphor plate (1.5 mAs, 55 kVp). (c) Computed radiograph (0.5 mAs, 55 kVp) ofthe same newborn obtained 2 days after the conventional screen-film examination. Image quality is diagnostic, although quantum noise is visible due to the lower exposure.
The left percutaneous
venous
catheter
is better
seen
(arrows).
..,,‘,
C.
film receptor by a factor of over 1 ,000 (Fig 2). The system has been well accepted for all aspects of general radiography, including imaging
of the
chest,
gastrointestinal
and
geni-
tracts, and bone (Figs 4-7) (3-6). An average of 25-30 chest examinations can be performed per hour. The smaller hand copy image formats have caused no problems once the radiologists and clinicians become acquainted with them. tourinary
1016
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RadioGrapbics
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Lee
Ct at
Volume
11
Number
6
a.
Figure 6. (a) Spot image conventional double-contrast of the upper gastrointestinal (b)
Storage
phosphor
plate
from a double-contrast upper gastrointestinal gastric
mucosa
onstrated
from a study tract. image
study of the tract. The
is more
sharply
dem-
on b.
b.
Figure 7. Computed demonstrates excellent tion. A right pneumothorax
November
1991
radiograph of the chest spatial and contrast resoluis clearly
Lee
et at
seen.
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1017
Figure 8. Diagram of a digital fluorography system that faced to the University of Kansas Medical Center radiology
is interdepart-
ment network. The high-resolution mens Medical Systems) is interfaced by means of a DR-i i device (Digital
crocomputer 2,048 radiology
.
system
high-resolution
is connected gray-scale
video digitizer (DS-i000; Sieto a microcomputer system Equipment, Boston). The mito a laser film printer, a 2,048 x display, and a gateway to the tele-
system.
Digital
Fluorography spot imaging and storage of digital images can be used in an independent system or as part of a digital departmental network (7,8). The system is based on conventional image-intensifier technology and incorporates a microprocessor-controlled, general-purpose fluonoscopy unit, video chain, and video digitizer (Fig 8). The installation is designed for digital fluonoscopy and radiography of the gastrointestinal tract or for venography, myelography, arthrography, and genitourinary tract procedures (Figs 9, iO) (9, 10). A typical system is capable of acquiring 1 ,024 x 1,024 x 8- or i2-bit images at up to six frames pen Digital
second
and
system
can
of storing be
operated
up to 100 in two
images. modes,
The fluoro-
Figure
9.
Digital
fluorography
spot
image obtained during a double-contrast barium enema examination.
1018
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Volume
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Number
6
Figure 10. Digital knee (b), and thigh
Table 1 Comparison
venograms of the lower extremity. The images were obtained at the (c) at the rate ofone image per i.5 seconds with a digital fluorography
of Digital
phic
Fluorogra
and
Conventional
size
33-cm
Actual field size Spatial resolution Contrast resolution (adequate visualization of blood clots Exposure 86 kVp 76 kVp
scopic
and
Digital
digital
during sultation
the
senior
staff)
November
66.8 97.5
images
examination. This with other radiologists and
1991
referring
mR mR
(0.017 (0.025
field
35.5-cm
ofview
mg
mC/kg) mC/kg)
mode.
costs
ages are printed. needed because
facilitates (resident
clinicians.
Film
conand
settes
ble
are
reduced
is eliminated.
1) and
(i4-inch)
(a),
film
31 cm (i2.2 inch) (rectangular) 3.0 lp/mm 7.5% contrast material (36.5 of iodine/mL) 109 294
are
acquired
leg
Conventional
27 cm (10.6 inch) (circular) 1.5 lp/mm 7.5% contrast material (36.5 of iodine/mL)
[9])
radiographic
fluonographic
(i3-inch)
of the
unit.
Systems
Digital
Characteristic
Image
5creen-Film
level
procedure
mR mR
(0.028 (0.076
because
mC/kg) mC/kg)
only
pertinent
No extra personnel manual changing Total
time
Lee
et at
im-
are of film
cas-
dose
(Ta-
radiation
are
mg
reduced.
U
RadioGrapbics
U
1019
C
11. Figures
11, 12.
(ii)
Diagram
of a
digital scanning equalization radiography system illustrates the fore slit, the modulator unit, the detector and electronic feedback loop, and
the x-ray film cassette. tronic feedback intensity of the
The elec-
loop modulates x rays by means
the of
the modulator unit. (12) Diagram of a video film digitizer system shows the video camera, the analog-todigital
converter,
the
microcom-
puter system and disc, and an interface. The camera is focused on the radiograph, which is positioned on a view box. The camera can zoom on any portion of the radiograph.
in
12.
I Digital Scanning Radiography In chest examinations, the intensity of the x nays transmitted through the lungs, mediastinum, and bones varies widely. The dynamic range ofx-ray film, however, is not adequate to record this variation ofx-nay transmission (1 i). The problem of the required dynamic range has been addressed by such techniques as unsharp masking (12,13), digital beam attenuators (14), and scanning equalization radiography (15-20). A prototype digital scanning chest unit was developed and evaluated in the early i980s. This
system
uses
a small
x-ray
beam
that
scans
across the patient in a raster fashion. A detecton behind the patient measures the exposure and, by means of a feedback loop connected to an x-ray modulator, maintains a constant
1020
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RadioGraphics
U
Leeetat
film exposure by adjusting the put (Fig 1 1). Scatter is reduced slits
and
a grid.
The
x-ray
x-ray tube outby fore and aft
modulator
consists
of an absorber controlled by a piezoelectric actuator. In a study of the quality of chest images produced with scanning equalization and conventional screen-film systems, the images were comparable (21). .
Film
Digitization
The quality of digitized images the film digitization technology ity of the
original
radiograph
depends and the to be
on qual-
digitized
(22). The digitization of an image can be achieved with a video camera and an ordinary light view box (Fig 12). Video camera systems have been the most common method offilm digitization. However, these systems have limited spatial resolution (0.8-mm pixel size in chest nadiographs of adults) and have been used primarily for
Volume
11
Number
6
Figures
13, 14.
(13)
Diagram
of a
laser film digitizer system demonstrates the laser beam scanning across the radiographic film. The
collected through
light intensity passing the film is collected and
digitized. Low digital numbers
cal densities. the essential
values of resultant indicate high opti-
(14) Diagram depicts elements of an interac-
tive digital gray-scale workstation. host computer provides the image
A
data and the image processing software. Text generation and interacInteractive
tive interrupts are graphics processor.
Device
managed
by the
14.
initial
diagnosis
and
consultation
in teleradi-
ology.
Image quality can be improved by using a laser film digitizer (Fig 13). Pixel size, number ofgray scales, and scanning time are the important specifications that determine the physical characteristics of a film digitizer system (23-25).
memory board that reads the digital image data at the scanning rate of the cathode ray tube [CRT]), lookup tables (which map the digital data to the desired shades ofgray for display on the CRT), and the gray-scale CRT display monitor (Fig 14). The dynamic range of the phosphor on the face
U IMAGE DISPLAYS Digital radiography requires high-resolution, interactive workstations that address a wide range of user requirements. The basic elements of an interactive gray-scale workstation include a graphics processor (a special-punpose microcomputer used to control the workstation),
November
a frame
1991
buffer
of the
distinguishable
CRT
is limited luminance
and
provides
levels
50-60
as deter-
mined by a selected choice of discernible gray levels. Discernible gray levels are the minimum detectable gray levels displayed on the CRT as a function of the digital numbers be-
(a high-speed
Lee
et at
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RadioGraphics
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1021
Figure 15. Diagram of the teleradiology system connecting two military hospitals and the University of Kansas Medical Center.
lnteracth,e
Gray
Scale
Display
ing applied to the CRT. Lookup tables map the pixel values of a digital image into luminance values to be displayed as gray-scale 1evels on the CR1. On the basis of our experience with 2,048 x 2,048 x 8- or 12-bit grayscale displays, we have devised a set of ideal parameters
The number ing:
for
these
monitors
(Table
use of display workstations of issues (26), including
perception
displayed contrast
of disease
images,
the
resolution,
workstation
focuses suspected
about
on
radiographic
the specific features disease (27).
on
CRT-
spatial
and
platforms,
findings
that
present
in a
U TELERADIOLOGY SYSTEMS Teleradiology systems transmit images from one site to another by means ofwide-area networks (28). Teleradiology wide-area networks employ a variety of public digital communication
links,
including
dedicated
point-to-point
circuit switching, packet or message switching, fiberoptic links, and microwave or satellite links. Dedicated point-to-point digital data links are installed for transmission between two sites. Circuit switching is a dial-up system, in which a data path is established by switching multiple circuits. For packet or message switching, the data to be transmitted are divided into smaller sizes. These packets or messages are labeled with a destination address and are transmitted from node to node until they finally arrive at the destination. links,
1022
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et at
for Gray-Scale
Characteristic Pixel
Ideal 0.2
size
Display Specification
mm
(2,048
required software, display protocols, and the use of image-processing algorithms. It is possible to develop a workstation that can present knowledge
2
Ideal CharaCteristiCs Monitors
2).
raises a the follow-
as seen
required
Table
Spatial
resolution
Contrast
range
Contrast
ratio
2.5
x 2,048)
cycles/mm
12 bits (4,096 levels)
gray
0:1
Brightness Screen refresh rate Data transfer rate
60 foot-lamberts 72 Hz 1 second, full screen
Fiberoptic links use light to transmit image data. Data to be transmitted are encoded into a high-speed (4 Gbits/sec) serial bit stream. Microwave links are high-frequency radio wave systems that transmit encoded data in a line-of-sight manner. Satellite links are used to transmit data over large distances. Teleradiology
systems
are
used
to transmit
images either for consultation or diagnosis. Consultation systems are low-cost, low-nesolution systems that are used when subsequent review of images is needed. Diagnostic systems are high-resolution systems that transmit images between medical facilities for the punpose of diagnosis. The radiographic films are digitized, transmitted, and displayed on a high-resolution
gray-scale
display
or printed by a laser film printer. The diagnostic teleradiology
workstation
system
in our
institution is a digital wide-area network that is connected to two military hospitals by a U.S. Sprint (Kansas City, Kan) switched fiberoptic network and a copper Ti carrier service
Volume
11
Number
6
Table
3 of CommuniCation Multiplexer
Comparison
56-kbits/seC
Sites Teleradiology
50-bed
No. of Examinations Transmitted per Day*
on the System
military
data
e Achieved
wi th a Ti
Megabits Transmitted per Montht
Access Charges per Months
C arrier
or N x
TiCarrier Costs per Month (point to point)1
NX56 kbits/sec Costs per Month (dial up)
28
$3,832
$1,701
$3,169
$2,619
$1,700
32,413
hospital
from
24** computed
Megabits per month Access charges/month
=
288
tomography
and recorded on a laser film printer. screen-film radiographs are digitized transmitted and recorded on a laser t
Servic
No. of Images Transmitted per Day
7#
100-bed military (i5Omiles) Image
f or DS-1
hospital
(48miles)
*
Costs
23 days/month
vary depending
(CT),
nuclear
89,355 medicine,
$354 and ultrasound
(US) are transmitted
Special procedures images are not included. All conventional to 2,048 pixels per row x 2,048 rows x 12 bits per pixel and are film printer. x bits/image X images/day. on digital exchange equipment
provided
by the
local
carrier
service. § Ti carrier charges/month = $i,8i0/month + $6.70/mile + access charges/month. I N X 56 kbits/sec charges/month = access charges/month + (tariff/minute x 1 minute/60 seconds second/56 kbits x bits/month x 1.2), where the tariffis $0.06/minute and 1.2 is protocol overhead. # All conventional radiographic examinations. ** Includes 17 conventional radiographic, two CT, two nuclear medicine, and three US examinations.
(Fig 1 5). The data rate service is a Digital Senvice (DS)-1 (i.544 Mbits/sec). Equipment at each military hospital includes a laser film digitizer, an interactive gray-scale display, a computen system, and a network access controllen. The network access controller is a multiplexer that dials up multiple 56-kbits/ sec digital channels for transmitting data in parallel. The user selects the value of n (n = 1, . . ., 24), and the multiplexer dials up n available circuit-switched 56-kbits/sec channels. The data to be sent are encoded and transmitted to the receiving site. The data are decoded, and hard copy is recorded by a laser film printer. The cost of communications in teleradiology systems is about i5%-20% ofthe total cost. One method for reducing this cost is to use an n X 56-kbit/sec multiplexer. The advantages of the n X 56-kbits/sec multiplexer are the dial-up capability and a usage cost of $0.06 per minute (Table 3). U HARD COPY RECORDING If a digital radiography system has a matrix size ofless than i,024 x 1,024 x 8 bits, multiformat video film recorders may be used (28).
November
1991
However,
for digital
systems
with
x 1
spatial
neso-
lution of i,024 x i,024 x iO bits or greaten, laser film printers are required to generate hard copy recordings (29). The laser film printer uses a laser beam to scan across a sheet of laser-sensitive film. The laser scanning beam is modulated by the digital data be recorded. A laser film printer prints a 4,096
x 5, i20
x
i2-bit
array.
The
images
to to
be printed are interpolated onto the 4,096 x 5, 120 x i2-bit array. It is still difficult to faithfully reproduce on the laser printed image what the user saw on the display workstation. U ARCHWING One of the naphy
major
is that
chived
by
of digital
advantages
the
digital
electronic
image
states
require
tamed
until
data
systems.
archiving that
the
of digital
image
film
The
are
patient
is 21
legal
serious.
image
radiog-
can
data
an-
issues
Some be
years
be
main-
of age.
Other states require that consultation reports be maintained. Liability issues require maintaming image data for an adequate period of time.
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1023
The on-line, high-speed, 30-day archiving storage requirements for a department with workload of 130,000 examinations per year range from 30 Gbytes to 1,410 Gbytes. The storage requirements depend on the fraction of the workload that is digital. In our institution,
about
digitally
30%
of such
formatted
a workload
imaging
involves
modalities
(i
Gbyte/day). Ifthis were increased to 90% of the workload due to the replacement of conventional screen-film radiography with computed radiography, 47 Gbytes/day (or i,4i0 Gbytes per 30-day period) of digital data storage would be needed. The current technology used to manage on-line, high-speed archiving of these data consists of magnetic discs (600 Mbytes per disc drive). Erasable optical discs (14-inch-diameter optical discs, 6.8 Gbytes per disc, and i-Mbyte/sec transfer rates) will be used in the future. For intermediate 2-year archiving storage, an electronic system that archives from 500 Gbytes (1 Gbyte/day x 250 days/year x 2 years) to 23,500 Gbytes (47 Gbytes/day X 250 days X 2 years) is required. The technology available for intermediate 2-year archiving is an optical disc library (or “jukebox”). Such a system might contain 1 50 discs (each disc storing 6.8 Gbytes) for a total storage of 1,020 Gbytes. Thus, six optical disc library units with a 4: 1 compression can archive the 2 years of image data generated at 47 Gbytes/ day. The long-term, 5-year archiving storage requirements range from 1,250 Gbytes (i.25 Thytes or i.25 x iO’2 bytes) to 58,750 Gbytes (58.75 Thytes). It is thought that optical tape will be able to archive 1 Thyte per 2,400-foot reel. Optical tape drives are currently in research development.
a
to-noise ratio. Ultra-high-resolution (2,048 x 2,048) gray-scale workstations are being implemented. Manufacturers are increasing the screen brightness with new monitor
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