,.

.,,

:

t_

:;

PACS Mini Refresher Archival

Image Meryll

M. Frost,

A typical day

Course

Jr, BSEE

radiology

and

much

data

decreases

.Janice

C. Honeyman,

department

as much

which

Technologies1

as

can

1 terabyte

are substantial

create

of data

storage

#{149} Edward

many

per

problems.

data

PhD

gigabytes

year.

One

and

is data

and

MD

of image

Archiving

solution

requirements

V. Staab,

increases

data

per

accessing

this

compression, the

rate

of data

transfer; however, standards are not yet available. Other solutions involve improvements in archival media. Jukebox subsystems allow automated access

to multiple

units.

Digital

magnetic

large amounts of information more practical technologies videotape allows storage of transfer. Optical disks, now a large storage capacity and tape is also being investigated technology to choose depends tution and the cost, stability, tern. U INTRODUCTION One of the ultimate

outcomes

tape,

the

of picture

archiving

and

(PACS) will be the elimination of film as the preferred ogy. Although with present technologic and economic unlikely

in the

near

future,

standard

medium,

can

store

and enables easy updates or replacements; have been introduced in recent years. Digital digital video data and features a high rate of data the preferred permanent archival medium, have provide excellent long-term stability. Optical as a solution to the archiving dilemma. Which on many factors, including needs of the institransfer time, and storage capacity of the sys-

many

pertinent

clinical

communication

systems

medium in diagnostic constraints this goal problems

in image

nadiolis highly

storage

can

be solved available

with existing technologies

PACS technology. In this article, we describe the currently for storage or archival of digital images in a typical PACS and

evaluate

their

capacities

U BASIC Different

storage

CONCEPTS IN diagnostic imaging

quirements. sional

A digital

array

of picture

Abbreviations: read

MO

term:

Picture

RadloGraphics From

1992;

the

Department

Gainesville, Address C

data

transfer

STORAGE modalities that

magneto-optical,

CAPACITY produce widely

would

elements

be stoned

called

PACS

=

characteristics.

pixels.

picture

The

archiving

varying

on a PACS

and

value

digital

consists

stored

storage

at each

communication

ne-

of a two-dimen-

system,

pixel

WORM

=

is a nu-

write

once

many

Index

I

image

and

RSNA,

FL 32610. reprint

requests

archiving

and

communication

system

(PACS)

12:339-343 of Radiology, From

the

1991

University RSNA

of Florida scientific

College

assembly.

of Medicine, Received

November

BoxJ-374 18,

JHMHC. 1991;

1600

accepted

SW Archer

Rd.

November

19.

to M.M.F.

1992

339

approach

merical representation of the magnitude of image data (ie, level of intensity) at the particular x and y coordinates of the original image.

How

accurately

the digital

image

latter

bits,

is referred

10 bits,

quoted

to in numbers

12 bits,

and

ofbits:

referred

plained

in terms

a yardstick.

Ifwe

inch,

are

there

8

age such

reside for some magnetic disks,

storage.

as magnetic

tape,

optical

ex-

something to the

possible

with

nearest

values.

digital

systems

normally

tape.

‘/

Ifwe

and

of the individual

eral also

so

digi-

possible forms been proposed

U DATA In lossless nal digital

of data (3,4).

COMPRESSION data compression, the image can be reproduced

ated that, when decompressed, a close approximation of the

storage, 2 bytes

is i0

Ifwe

consists

bytes,

assume

of 50

5 12 pixels

with

2 bytes

is used

would be A megabyte

512

and

that

images,

lossy

tomographic X

since

the CT image on 0.5 Mbytes.

a gigabyte

bytes.

5 12

but

the

is

average

total

1012

CT study

storage

re-

quired for that study would be 25 Mbytes. If 50 studies are performed per day, 1.25 Gbytes of data will need to be stored.

Recent

research

indicates

tnix of at least 2,048 x 2,048 for digital chest radiography we assume use of 12-16-bit of storage (or the equivalent diskettes)

will

be

If a department and lateral chest Gbytes ofstorage These numbers

required

that

a pixel

ma-

may be required (1,2). Again, if pixels, 8 Mbytes of six 3.5-inch for

that

one

image.

obtains 75 postenoantenion radiographs per day, 1.2 will be required. indicate that the typical

radiology

department

gigabytes

of image

can

data

create

many

pen day,

and

upward

of 1 Thyte of data per year. The problem is, How do we maintain this much digital data in an archive and still have reasonable access to many years worth of digitized images? The digital

image

archive

typically

has

a staged

the

a reduced

is currently

amount

tolerated diagnostic

facIn

set

is cre-

to determine

compression Images

as 10: 1 may

onigifrom its

produces only original image. that

still maintaining

image.

much

exact

data

underway

oflossy

while

have

Lossless compression are readily achievable.

compression,

Research x

is 106 bytes,

a terabyte

the

12

for

x 512

(eg,

compression

example,

computed

media

oped, such as the optical or tape jukebox. To maximize the available storage capacity, sev-

mea-

store

each

compressed form. tors of two to four

contains

on

optical disk, magnetic tape) does not have the total storage volume necessary, methods of handling multiple volumes have been devel-

Most

pixel,

disk,

be

values,

scan

Eventually

can

possible

per

of

in bits

1,024

(CT)

period usually

from the working permanent stor-

tal data in multiples of 8 bits (1 byte). Therefore, 8-bit data are stored in 1 byte, but 10-, 12-, and 16-bit data are stoned in 2 bytes. For

bits

digital

acquisition

optical

measure

a typical

The the

typically

10 bits

forth.

from

are

sure to the nearest ‘/8 inch, there are 288 possible values. The number of bits needed to represent a pixel value in a digital image also depends on the precision of the measurement. Eight bits represents 256 possible values,

retrieval.

16 bits

of measuring 144

and archive

images are migrated to a form of long-term

Because

as measured

the

to as working

these store

by manufacturers.

Precision

enter

system and usually time on high-speed

represents

the original image depends on both the sampling frequency and the precision of the numerical value used to represent each pixel.

The

to storage

images

can

compressed

still

be

an acceptable by as

be clinically

acceptable

(5).

Data

compression

fits. First is the age requirements.

duced

data

can have

direct

several

bene-

reduction in data storSecond, since only a re-

set is sent

to the archive,

an

apparent increase in the rate of effective data transfer is obtained. The negative aspects of compression are the lack of standards and intolerance of data errors. Eventually, data compression will be added to the standards of the committee formed by the American College of Radiology and the National Electrical Manufacturers

until

that

vendor’s general addition,

Association

time

the

compression use of image because

(ACR-NEMA),

proprietary

algorithm compression

of the

ture of a compressed can be allowed from

nature

highly

but

of each

will make difficult. encoded

the In na-

image data set, no errors the archive system. With

an uncompressed image, a single-bit error in an image of several million bits will have no impact on the diagnostic quality of the image. In the case of highly compressed image data,

even

a single-bit

error

can have

devastating

results.

340

U

RadioGraphics

U

Frost

et al

Volume

12

Number

2

U

TYPES

.

Magnetic

OF

developed have been the primary method of software distribution, system software backup, and data interchange. Only in recent years have practical replacements such as CDROM (compact disk, read-only memory) and floppy digital disks been introduced. Al-

MEDIA

ARCHIVAL

Disk

The front end of any will contain a number

imaging

digital

archive

of conventional

mag-

netic disks. Normally, fault-tolerant designs must be incorporated to ensure image data integrity until migration to the permanent archive media has been confirmed. Fault tolerance is usually achieved by one of two

though

methods. The first is disk mirroring, in which the data are duplicated in two identical places. The second method, usually used with multiple-disk-array systems, involves the addition of one or more parity drives. The parity drive allows efficient restoration of data after the replacement of any failed drive.

fixed-head

by

Although

the

storage

capacities

of individual

archival media (eg, digital tape cassette, optical disk) are rapidly expanding, ready access to multiple units will be a necessary part of any imaging archive. To solve this problem,

jukebox

subsystems

cess have been system consists

back

units,

that

allow

designed. of one

storage

automated

The or

typical

more

shelves

Jukebox

systems

magnetic

digital

designed videotape,

and

then

time

usually

.

to the record-playback

access

required

8-20

Magnetic

the data to fetch

data and expanding

programs) industry.

produced In addition,

dardized

method

dates and The digital

March

1992

the

of providing

the

oxide

on the

tape

as the

to the

a signal

originally

this

are

sensed

ideally

recorded

the

The

is

informa-

is grossly

describe

mechanism.

Durhead,

that

explanation

it does

of

by the

patterns

creating

created

material.

passes

recorded

head,

identical tion.

tape

field alignment

over-

basic

storage

recordcapacity

of

that

and

is usually

unit,

The is

of both

by this rapidly an easy, stan-

software

causes

previously the

a magnetic

head

bits

medium

form

the

designs.

and

has been an archive stabeginning of the combeginning, computer dethe need to provide a ofstoring the enormous (in

on

for

Tape

of information

reading,

playback

seconds.

Digital magnetic tape pie almost since the puter age. From the velopers recognized safe, secure method amount

desired

of

a magnetic tape is determined by multiplying the usable length of the tape by the track density and the number of tracks on the tape. The track density refers to the number of digital

on that volume.

the

ing

Although

optical disks. Storage capacities in excess of 28 Thytes are possible with such configurations. In the case ofjukebox systems, the process to access any digital image requires that the robotics locate the correct medium, trans-

fer that medium

head

the head,

simplified,

for the individual

have been tape,

magnetic

jukebox

record-play-

to be a multitude

technologies available all implementations two basic categories,

moving

record

the

by

ac-

and

passes the

the

media, and the necessary robotics to allow fast movement of the media to and from the record-playback units.

digital

may seem

In fixed-head technology, the record-playback magnetic heads remain stationary while the tape is moved mechanically past the heads. The number of heads and tracks on the tape vary from type to type, but the basic pninciple is the same. During recording, as the

tape

Jukebox

.

there

different digital tape the market, in general can be classified into

can

example, as the

be

stored

the

digital

computer

mation

pen

expressed

unit

tape

used

industry

exchange

length

in bits

was

pen for

inch

For

many

standard ‘/2

of tape

inch. for

wide,

years infor-

stored

1,600 bits per inch, and contained nine tracks. The rate of data transfer from digital magnetic tape is a function of the track density

multiplied

the

rate

sec.

Although

can

by

the

be as high this

speed

tape

speed. was

Digital

Kbytes/

adequate

die digital audio needs, another necessary to expand digital tape tal video arena.

.

Typically,

as 200-300

to

approach into the

han-

was digi-

Videotape

The moving-head originally designed this system, both

magnetic tape machine was to record analog video. In the tape and the heads

up-

new software was also necessary. magnetic tape standards that were

Frost et al

U

RadioGraphics

U

341

move, larger

allowing bandwidth.

accommodation Typically,

of a much or more

one

heads are mounted on a rotating drum, the tape is positioned such that contact the drum is maintained for most of the diameter.

in addition,

lower

on one

the

side

tape

an optical sensor reads the reflected light, thereby recreating the original data. Sustained data transfer rates of up to 1 Mbyte/sec for both reading and writing have been demonstrated.

and with drum

One

is positioned

of the drum

and

higher

on

the other. With the head mounted on a drum, the path traced by the head on the tape is a helix, thus the term helical scan tape. Normally, the rotation of the drum is synchronized with the video information such that one video field is stored per rotation of the drum. The D 1 and D2 digital video standards were created to allow the utilization of helical scan techniques for storing digital video information.

The

D2

standard

specifies

data

Optical

Disk

The large storage capacity of optical disks along with their excellent long-term stability have made them the preferred permanent archive medium. Laser optical disks come in several varieties, although only the writeonce-read-many (WORM) and magneto-optical (MO) or erasable optical varieties are typically

used

The

in digital

capacity

imaging

archives.

ofWORM

disks

has

reached

more than 10 Gbytes, with 2 1 Gbytes expected in the near future. The WORM medium has the advantage of being very stable after being written. It is not influenced by external magnetic fields and will retain the data for at least 10 years. WORM technology uses a high-energy ing

laser

disk.

If the

burned

into

beam

laser

By modulating

digital

data,

digital

data

beam

beam

the surface

dium.

laser

focused

an

is on,

a pit

laser

accurate

actually

the

rotatis

of the optical the

is stored

on

me-

beam

with

representation on

the

is focused

the of the

optical

disk.

on the

The

disk

sun-

face by a mirror-lens system mounted on a movable arm. By moving the arm, random access

to any

individual

track

on

the

disk

can

a digital

to the disk, mirror-lens

data

pattern

a lower-power system irradiates

has been laser with the the pattern

written same and

drawbacks

MO

disks

and erased. The MO earth material coated This

material

ofWORM

tech-

can

can

be

written

on,

read,

disk medium is a rareonto the disk substrate.

maintain

localized

magnetic

The polarity can be changed by apa magnetic field while the material is

polarity.

heated.

A focused

laser

beam

is used

to pro-

duce the local heat necessary to create the polarity change. With this disk material, the light polarization is different for each of the two material states. Therefore, once a digital pattern

has

heating

laser

been

written

with

the

by modulating

incoming

the

digital

data,

tion pattern. The MO tive

than

reading the

process

WORM

is much

technology,

less

sensi-

resulting

in

significantly slower rates of data transfer. Data transfer rates are on the order of 100 Kbytes/ sec, although recent advances indicate that this limit will significantly improve in the futune. . Optical Tape Recent developments in flexible optical make it a viable technology for long-term age of digital images. Thirty-five-millimeter

flexible

optical

as high

as

media

1 Thyte

per

with

storage

reel

have

media

stor-

capacities been

demon-

strated. Data transfer rates of up to 250 Kbytes/sec have been routinely achieved. To date, no jukebox has been designed to use this technology, but the evolution of this product bears close scrutiny. U

SUMMARY

are undergoing and performance.

rates

for digital radical For

of MO drives

have

image

changes example,

archives in both price the transfer

increased

so dramati-

that they may become the preferred cal medium for image archiving. The real lemma is that there is no clear-cut answer cally

342

U

RadioGraphics

U

Frost

Ct

al

a

lower-power laser can be used to read back the data by filtering the respective polariza-

The technologies

be achieved.

Once

technology.

plying

transfer

rates as high as 14.5 Mbytes/sec, greater than all but the fastest magnetic disks. In addition, four full digital audio channels are simultaneously recorded with the digital video. I

of the major

nology is the lack of standardization. There is no guarantee that the disks produced by one manufacturer’s equipment can be used by another. In fact, even the size of the media has not yet been agreed on. The MO disk is another rapidly evolving

Volume

12

optidito

Number

2

the question ofwhich technology should be chosen for digital image archiving. The choice must be based on careful evaluation of the particular needs of a specific institution. The long-term stability of both WORM optical disk and optical tape makes them attractive as permanent archive media. Unfortunately, the fact that the information cannot be

changed tient

will lead

records,

to fragmentation

leading

this

increase

in operating

If the

problem

to increased

major

with

only

access

effective

function by other

of the factors

archival such

is the

rates

time

sets, digital rates of 14 benefit. Howare

media as network

not but

just are

speed,

1992

for

fault-tolerant

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ne-

work activity, working storage speed, and display speed. The most feasible approach is to incorporate any digital imaging archive into a cohesive network resource system (6). This approach will allow one to add newer technologies to an existing archive scheme rather

March

1.

cal

consideration

transfer

U

time

a moderate

to provide

it

operation.

costs.

quired to transfer large image video technology with transfer Mbytes/sec can be ofsignificant ever,

tant,

of the pa-

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RadioGraphics

U

343