781

Digital SubtractionAngiography: Overview of

Technical Principles

Donald P. Harringto&Lawrence M. Boxt

Philip D. Murray

Received June 8, 1 982; accepted after revisionJuly 12, 1982.

1 All authors: Department of Radiology, Harvard

Medical School, Brigham and Women’s Hospital,75 Francis St., Boston, MA 021 i 5. Address re-print requests to D. P. Harrington.

AJR 139:781 -786, October 1982o36i-8o3x/ 82/1394-0781 $00.00© American Roentgen Ray Society

The rapid development of equipment for digital subtraction angiography (DSA) hascreated a new diagnostic imaging method, the limits of which have not been scientifi-cally determined. Yet through aggressive marketing, the technique is already beginningto permeate radiologic practice. The radiologist requires technical understanding ofthe instrumentation for informed judgment on clinical applications. DSA depends onthe mating of high-resolution image-intensifier and television technology with comput-erized information manipulation and storage. In this overview, the individual compo-nents of the system are analyzed, from the generator to the image intensifier to thetelevision system to the associated computer. By examining the role of each compo-nent, the current limitations and the areas of possible future development of DSA canbe understood. This provides a basis for dealing with current technology and forevaluating the rapid technological changes that will occur over the next few years.

Digital subtraction angiognaphy (DSA) is an emerging technology that has

many characteristics in common with the development of the CT scanner during

the i 970s. Like CT scanning, DSA is a computer-assisted technique, integrating

digital data collection and computer processing to produce a medical image. A

further similarity is simultaneous development and marketing of equipment. In 5

years, CT evolved through four generations of scanners with profound changes

between the first and last generation, each of which was vigorously marketed. It

is reasonable to assume that DSA will also evolve significantly oven the next

several years, making selection of current equipment more difficult because of

concerns for premature technological obsolescence.

The theoretical basis of DSA also presents difficulties. Computer technology

and its associated jargon are one source. While the computer underlies CT

scanning, its operation does not need the active direction of the user. This is not

the case with DSA where image processing and a variety of viewing modes are

under user control.

A second cause for difficulty in evaluating DSA equipment is the need for a

component-by-component evaluation of the whole imaging chain. To achieve

optimum low-contrast visualization, the principal advantage of DSA, all links in

the imaging chain must be individually evaluated and optimized. In evaluating

individual components, four factors, contrast sensitivity, x-ray exposure, temporal

resolution, and spatial resolution, must each be considered for an understandingof current and future DSA equipment.

This report defines the concepts that underlie the development of DSA and

evaluates each component in the imaging chain. Figure 1 provides a schematic

representation of the components of a composite DSA system. An attempt is

made to define some of the jargon associated with the computer technology and

to offer a background for evaluation of specific equipment and the possibilities

of further improvement.

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782 HARRINGTON ET AL. AJR:139, October 1982

Basic Principles of Digital Subtraction Angiography

DSA is part of the larger phenomenon known as digital

radiology. For DSA, the televised fluoroscopic image is

converted point-by-point to digital data. These data are then

manipulated in two operations [1 -5]. The first is the process

of image subtraction, also known as temporal or time sub-

traction. For this process, an image (the mask) obtained

before arrival of contrast material at the area of interest, is

placed into one of two digital memories. Then one or more

subsequent images are obtained after arrival of a contrast

bolus and placed into a second digital memory. The mask

image is digitally subtracted from the succeeding contrast

image, with the result that contrast-filled structures are

rendered visible free of background detail.

The second operation occurs after subtraction and is

expansion of the dynamic range of the subtracted image

which results in enhancement of the final image. This is

necessary because the range of contrast within the initial

subtracted image is very small. Subtraction and enhance-

ment are performed in real time, which means that the

processing of data is sufficiently rapid that the results are

available in time to influence the clinical examination. The

speed and apparent simplicity of computerized subtraction

are two major advantages of DSA over standard film sub-

traction angiography.

Several further steps are a routine part of the process.

Amplification of the output of the image intensifier is one.

This amplification may occur before or after digitization of

the data and may be fixed or selectable, depending on the

individual system. Choices for amplification include linear

and logarithmic modes. In the former, the unsubtracted

signal is amplified linearly, independent of its numerical

value. This is appropriate if there is uniform tissue density

in the field, such as in abdominal imaging. Logarithmic

amplification, in which amplification of the input signal is in

proportion to the signal’s strength, provides images of opa-

cified blood vessels unaffected by overlying high- and low-

density structures. This is the technique used in carotid-

vertebral imaging and represents the most common form of

image amplification. Square-root amplification is an expeni-

mental form being investigated at the University of Arizona.

After amplification, the image must be converted from its

analogue form to a digital form. The analogue image, which

is a representation (or analogue) of a fixed quantity by

means of a physical variable, such as shades of gray in the

radiographic film or the brightness of the image intensifier,

must be converted to the digital form in which a discrete

value, rather than a continuous variable, represents the x-

rays exiting an object. The digital signal is more accurately

and easily transmitted than an analogue signal, and the well

defined digital data make computer manipulation a relatively

simple task. It is much easier to perform image processing

on digital data than on analogue data.

This process of digitization is performed by an analogue-

to-digital converter (A/D converter), the efficiency of which

can be defined by the rate of digitization or conversion time

and intensity resolution or depth of digitization. This device

assigns the output signals of the television camera, specific

IMAGE PROCESSOR � ADVANCED � IMAGESTORAGE �

Fig. 1 -Flow of information in generalized digital subtraction system.Display console contains TV monitors for display of unprocessed imagesfrom TV chain on left or processed images from image storage. Controlfunctions include x-ray generator settings and choice of images for manipu-lation. Image manipulation can be simple subtraction/enhancement per-

formed by the image processor/computer or more complex procedures,such as signal filtration or edge enhancement performed by “advanced”image processor/computer. Different systems allocate these functions todifferent hardware in different configurations. Current systems transfer pro-cessed images to either analogue or digital short-term storage module.Images stored in analogue system can be redigitized and further processedor archived on disk or tape. Images stored in digital form can be directlyreprocessed or archived.

discrete digital values. The rate of digitization is in micro-

seconds for all systems. The depth of digitization (or exact-

ness of the assigned value) is important because it is related

to the number of shades of gray that can be displayed; the

greater the depth of digitization, the larger number of shades

of gray become available for the final image. Commercially

available A/D converters provide 256 gray (28 or 8 bits) to

1 ,024 gray (210 or 1 0 bits) levels. The latest commercial

entry on the market claims selective 4,096 gray levels (212

or 1 2 bits) for each discrete point in the image. The A/D

conversion process influences the ultimate resolution of the

system in that noise may be introduced by this process,

affecting image quality. At the depth of digitization de-

scnibed, there is no significant limitation to image quality in

the present DSA systems, but noise may be significant when

quantification of the image data is attempted. The concept

of noise in the DSA system will be discussed subsequently.

It is at this point after digitization that the image is entered

into one of the two memories and the subtraction and

enhancement are performed.

DSA may be performed in several different modes. Serial

imaging, the most common mode, is used where image

acquisition rates can be relatively low (from one to eight

images per second). This mode allows for the greatest

degree of flexibility in image acquisition and may be

achieved with a short-pulsed x-ray exposure or by a longer

exposure with variable addition and averaging of individual

television frames. The difference between these methods

will also be discussed later. A second mode, the continuous

or dynamic mode, uses an acquisition rate of 30 fnames/

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AJR:139, October 1982 DIGITAL SUBTRACTION ANGIOGRAPHY 783

The modern cesium-iodide image intensifier is one of thestrongest links in the DSA image chain, but the character-

sec with resultant shorter x-ray exposure times, decreased

photon flux per image, and decreased resolution in com-

panison to the serial mode. This mode is used for rapidly

changing dynamic processes, such as occur in cardiac

imaging. In both modes, a precontrast image is used as the

mask, followed by contrast injection. With a third imaging

mode, termed the time-interval-difference mode, a rate of

30 frames/sec is used, but the subtraction is performed

between each successive image rather than from a single

first image (i.e. , frame 2 minus frame 1 , frame 3 minus frame

2, and so on). This mode which displays the differences

between the two successive images has not been fully

explored and it is not available from all manufacturers.

Variations in the way data are handled between different

manufacturers become evident after the subtraction and

enhancement process. The images formed must either be

further processed for viewing and other manipulation or be

stored. This choice introduces several major considerations.

The first is whether these images should now be converted

back into an analogue format or remain digital. Ideally an

all-digital system is preferable, but storage is limited to a few

images unless improved and highly expensive technology is

used. But if the images are converted back to the analogue

format for storage, the number of images is no longer a

problem. Another issue is what memory and computing

capacities are necessary for further processing and/or

quantification, and finally, how should the data be ultimately

stored? This sketch of basic principles can be usefully

amplified by evaluating each of the individual components

of the imaging chain [6, 7].

DSA System Components

X-ray Tube and Generator

Two components can be taken together, the x-ray tube

and generator. While a high photon flux is a necessity for

DSA, standard angiographic x-ray tubes and generators are

generally adequate to the task. One notable exception is the

0.3-mm-focal-spot magnification tube, which does not allow

sufficient photon flux for DSA. A compromise x-ray tube

might have 0.6 and 1 .2 mm dual focal spots. DSA is routinely

accomplished with the larger focal spot, since focal-spot

size is not a major factor in this relatively low-resolution

system [8].

As to the generator, a highly desirable feature is computer

control. Such a system allows rapid determination of expo-

sure factors, without multiple trial and error exposures which

waste time and radiation. In a computer-controlled system,

a first approximation of the exposure factors is based on

the size of the patient, the area of interest, and such factors

as the number of video frames to be integrated if frame

averaging is used. A particular amperage is then picked,

and a test exposure (or preferable fluonoscopic exposure)

is made with automatic computation of the final exposure

factors to provide for the proper light output from the image

intensifier. This is a critical factor, as the light range within

which the TV camera functions is quite narrow compared tothe output range of the intensifier.

Another advanced design is control of the photon flux at

the intensifier face in relation to the opacity of injected

contrast material. For example, in an elderly patient with

poor renal function, a high x-ray dose could be chosen in

order to reduce the contrast dose necessary for the study.

In other circumstances, larger contrast injections allow

lesser x-ray exposure.

X-ray exposure is a misunderstood factor in DSA. The

procedure is lauded as a relatively noninvasive angiographic

substitute, which is also said to be possible with lessen x-

ray exposure compared to standard angiography. In a pre-

viously published clinical series, entrance skin dose rates

varied from 1 30-1 80 mR [2] to 46-i 75 mR [9] per image.

Our own experience suggests that dose rates are higher,

200-700 mR pen image. Radiation requirements to the

intensifier face are i -2 mR pen image depending on the

manufacturer. If 2 mR is delivered to the intensifier, then at

a minimum the entrance skin dose would be in the range of

200 mrad (2 mGy) per image. Dose reduction is not gener-

ally a high priority item but can be realized with a better

understanding of x-ray dose vs. contrast sensitivity of the

system. This subject should be examined when one consid-

ens individual DSA units. It is also worth pointing out that the

expected tube life in the DSA system may be shorter than

for angiographic tubes because of the large demands for

high photon flux in all systems.

A future prospect is that DSA may be performed with

energy subtraction rather than temporal subtraction. Energy

subtraction, an alternative being tested in several centers,

is based on subtraction of images of different kilovoltage,

rather than in different time [i 0]. The advantage is that the

different kilovoltages can be programmed within millisec-

onds of each other, so that motion no longer introduces an

artifact. Current fluonoscopically based DSA units are incap-

able of energy subtraction although one manufacturer is

planning to provide such a system in the future. The circuitry

of the generator must be extensively altered for this tech-

nique.

An important purchasing consideration is whether DSA

units can be added to existing equipment. Some companies,

mainly full-range x-ray equipment manufacturers, sell theirDSA units only as a package with their x-ray equipment,while other companies provide an add-on unit to alreadystanding equipment. One reason given for not mixing equip-ment is the need for optimization of all links in the imagingchain. For example, a 5-year-old image intensifier may notmeet the specifications of a new cesium-iodide image inten-sifier. A further argument is that synchronization of the DSAequipment and the x-ray generators is complicated, andincompatibilities may exist between different manufacturers’generators. However, the continued success of add-on in-stallations tends to deny that argument, but the controversycontinues along well defined parochial lines.

Image Intensifiers

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784 HARRINGTON ET AL. AJR:139, October 1982

istics of these intensifiers are somewhat different from stan-dand fluorographic intensifiers [1 1 , 1 2]. For DSA applica-tions, the image intensifier must operate at a 1 -2 mR perimage exposure rate without loss of contrast or resolution.This is higher than for conventional fluoroscopy or evencinefluoroscopy. This operation level alone can lead toabnormalities such as pulse-charge defocusing or saturationof the output phosphor in standard image intensifiers [6].

A second aspect of the intensifier operation is that at high

radiation levels, there is a correspondingly high gain or lightlevel which adversely affects the TV camera, and appropni-ate variable filtration or aperture control is necessary forproper light-level control [6]. This control is also necessaryto allow variability in the photon flux to the image intensifier

face as mentioned above.The cesium-iodide image intensifier is the standard for

production DSA units, but state-of-the-art image intensifiers,such as the Thompson CSF 96 intensifier and the Philips1 4-inch (36 cm) intensifier, are proposed for future DSAunits. Further modifications will include thicker image phos-phors in order to better control and use the light output [6].

The size of the image intensifier is also an importantfactor, especially for demonstrating large vascular areas, asin the extremities. Large intensifiers, such as the Philips 14-inch (36 cm), offer superior contrast resolution capabilities,but have the disadvantages of decrease in spatial resolutiondue to the fixed matrix size of the image processor and theconsiderable increase in cost.

Television System

The TV system serves to convert the optical image of theimage intensifier to an electronic signal that will be digitized.Many believe this limits the overall resolution of the system,and most manufacturers agree that the television compo-nent of the DSA system must be state-of-the-art. Newlydeveloped video tubes, such as the Amperex 45-XQ (‘ ‘frog’shead’ ‘ plumbicon), as incorporated into a Sierra Scientificcamera, and the lead oxide Videcon tube are examples [1,2]. Concepts of noise and signal-to-noise ratio in TV systemsneed special consideration.

Noise is anything that obscures a signal that is beingmeasured. In the context of the DSA system, it can becaused by another electrical signal (interference) whichoriginates primarily from the TV camera, by some physicalprocess such as the quantum noise which originates from a

limitation in the number of x-rays per image, and from thedigitization noise which reflects the uncertainty associatedwith quantizing the video signal into a finite number of digitallevels [8]. Signal-to-noise ratio (SNR) is the ratio of thesignal voltage to the noise voltage (100:1 , 1000:1 , etc.).The SNR of a standard radiographic system is roughly1 00:1 , while the standard fluoroscopic system raises this to200:1 . These systems have poor low-contrast detectability(low-contrast sensitivity). CT has an SNR of 2000:1 and,like DSA, has high contrast sensitivity. In any ideal radio-graphic system, the SNR should be limited by the quantumstatistics of the photon flux and not the components of thesystem. Since the TV system is an important source of noise

for DSA, one method for improving the entire system is toachieve the highest possible signal-to-noise ratio for the TVlink, which is obtained with state-of-the-art television sys-tems.

Work by Mistretta et al. [4, 5] in Wisconsin indicates that

a standard plumbicon television system will give a 500:1SNR and the newer ‘ ‘frog’s head’ ‘ plumbicon, 800:1 , andthey have suggested an alternative method for improvingthe SNR of the imaging chain, which is known as signalaveraging. In this technique, a series of serially acquiredtelevision frames are ‘ ‘averaged’ ‘ ; that is, one collects asum of repetitive analogue signals and an ‘ ‘ average’ ‘ signalis derived. This technique can provide an SNR of 1200:1[4]. The disadvantages of this technique are that motionartifacts may be introduced as the number of averagedframes is increased, and that the need for long exposuretimes increases the radiation exposure to the patient. Suchan averaging technique also demands a high-quality televi-sion system with low lag and good image stability.

A second alternative is termed the ‘ ‘ snapshot’ ‘ mode [1],which incorporates a slow-scan video technique where theimage is stored on the target of the TV pick-up tube andthen read out and digitized. This method is limited by theneed for a progressive TV readout where the entire videoframe, consisting of 525 lines per field is read out progres-sively rather than the standard interlaced system where twofields of 262.5 lines are alternatively placed on the videoframe. A further advantage of this latter system is economyof radiation exposure in both dose and time, which de-creases the chance for motion artifact. The disadvantage ofthe ‘ ‘snapshot’ ‘ method is that it will not serve for rapidacquisition of images.

One limitation of most current television systems is thatthey are formatted to 525 television raster lines. This is nota limiting factor to resolution, however, if the matrix sizeremains at 51 2 x 51 2. Manufacturers are introducing newhigh-resolution television systems which will incorporate1 ,000-raster-line (on more) television cameras and monitors.This will improve the display of the image but will be mostuseful only when larger matrices, such as 1 024 x 1024,are incorporated into future image processing systems.

Image Processor

The image processor is the heart of DSA because it is thepart of the system where subtraction and image enhance-ment takes place. Two principal types have been developed.

The design concept put forth by Mistretta [1 3] at the Uni-vensity of Wisconsin was that a subtraction system shouldbe fast, easy to operate, and relatively inexpensive as com-pared to standard film subtraction. The image processordescribed by Mistretta is a hard-wined system, which meansthat the subtraction circuits are fixed and not alterable orprogrammable.

The design concept developed by Nudelman and col-leagues at the University of Arizona (Ovitt et al. [1 1) calls fora more complex system for subtraction and image enhance-ment requiring considerable operator interaction withchoices to be made of types of amplifications and methods

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AJR:139, October 1982 DIGITAL SUBTRACTION ANGIOGRAPHY 785

of enhancement. In this more complicated system, program-mable computer hardware (as opposed to Mistretta’s non-programmable subtraction circuits) control the system op-eration. These systems are capable of handling patientinformation and an array of alternative and more compli-cated image manipulations (limited by the ability of program-mers to write programs for them). A large number of addi-tional algorithms are available as a part of some DSA units.One such algorithm is for image-edge enhancement. This isaccomplished by increasing the contrast density in thepicture elements so that edges of areas where there is ashift in contrast density are enhanced, thus making theobject more prominent. The application of such algorithmsare under active investigation, but the diagnostic valuebeyond basic subtraction is still unknown.

Despite the increasing complexity of systems, subtractionof the background still remains the most important functionof any DSA system. There are further options available forimage manipulation. An array processor is a device capableof performing a series of image manipulations very rapidly.One such manipulation is image reregistration, which shiftsthe contrast image up on down in relation to the mask imageto compensate for motion between the mask and the con-trast image. It is not unreasonable to suggest that thegreatest percentage of the image in DSA is due to thesubtraction process, while a further improvement is pro-duced from contrast enhancement. Other manipulations ofthe images, as previously described, have not yet had asignificant impact on the imaging process.

The matrix of the image processor (mentioned above)needs further description. For our purpose, a matrix is arectangular array of picture elements (pixels). The size ofthe matrix is defined by the number of pixels on a side. Themost common sizes in DSA are 256 x 256 and 51 2 x 512.As the pixel is the smallest element in the picture, theresolution of the system is defined by the pixel size. Thematrix size should also be considered in the context of theimage intensifier. If the field size is 6 inches (1 5 cm) indiameter and the matrix size is 51 2 x 51 2, there are 3.3pixels/mm (1 .6 line pairs/mm) or for a 9-inch-diam (23 cm)field, 2.2 pixels/mm (1 .1 line pairs/mm). With a fixed matrix,improved spatial resolution is achieved by decreasing thefield size. The loss in spatial resolution with increasing fieldsize is important in a system that is already deficient inspatial resolution, as has been discussed with reference tolarge-field intensifiers. Some users of large-field image in-tensifiers have found that the loss of spatial resolution isoutweighed by the increased field size and contrast sensi-tivity afforded by the advanced design of such intensifiers.This points out that theoretical advantages and disadvan-tages of any system or component should be carefullyevaluated in a clinical situation before conclusions aboututility or limitations are drawn. The same controversy willarise when enlarged matrix sizes (e.g. , 1 024 x 1024)

become available. It is difficult to say whether the cost ofdevelopment and extra instrumentation needed (such asextra memory and high-resolution TV system) will be offsetby the increased spatial resolution they may provide.

Data Transfer and Storage

The matrix size also defines the problem of data storageand transfer. For example, a matrix size of 51 2 x 512contains 262,1 44 bits of data on pieces of information forstorage in digital form. Data are conveniently quantified interms of bits and bytes (8 bits = 1 byte; 1 06 bytes = 1megabyte). To double the matrix size from 51 2 x 51 2 to1 024 x 1 024, one needs to quadruple the memory neces-sary to store each image.

The basic problems with transfer and storage of data arethe time necessary to transfer large amounts of data andthe cost of storage. Until the introduction of CT scanning, x-ray film storage was an acceptable mode for storage of allradiographic images, but this is not the case for CT imagesor DSA. In CT, the digital data are stored on magnetic tapeor magnetic disks. The average CT study will consist of20-30 images, which requires 1 0 megabytes (about 1/3 mB/image) to store. This is a large storage requirement. DSA issimilar. The DSA image processing is done in the computer’s

central processing unit (CPU) which allows for rapid pen-formance of subtraction. But the CPU cannot be used forstorage of images. The processed images must be trans-ferred to an auxiliary storage device. The rate of transfer ofdata within the CPU is about 1 0 million bytes/sec, whichallows images obtained at 30 frames/sec to be subtractedand enhanced. Transfer of the processed information tostorage is limited to about 500,000 bytes/sec. Thus, a 512x 51 2 image contains so much information, that only twoimages/sec can be transferred in digital form. To go fasterentails more advanced technology and expense. One solu-tion that preserves the digital nature of this information is todecrease the matrix size and therefore the amount of infon-

mation to be stored. Thus, at a 1 28 x 1 28 matrix, 30frames/sec may be processed. The solution proposed byMistretta is to transfer images in an analogue form. In thisform, 512 x 512 matrix images can be acquired and pro-

cessed at 30 frames/sec without difficulty. The drawbackto this solution is that the images must be nedigitized forfurther processing.

There are three further questions that must be addressedconcerning the storage and retrieval of images generatedby DSA systems. In what form will be data be stored? Whichdata will be stored? What storage devices will be used?

Conversion of data from digital to analogue causes im-mediate loss of information content and subsequent degra-dation of image quality. It is not clear, however, to whatextent this affects the diagnostic quality of the study. Fur-thermore, conversion to analogue necessitates reconver-sion to digital for later manipulation. Digital data can behandled faster by DSA systems and is the basis for allmanipulation. Thus, it would be advantageous to keep allstoned data in digital form. The present state of the technol-ogy dictates, however, that analogue data storage ischeaper and the only means for real-time, high-resolution,high-frame-rate image acquisition. As digital technology ad-vances, there will be greaten imperative to store all collecteddata in the digital mode. Manufacturers of DSA systems are

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786 HARRINGTON ET AL. AJR:i39, October 1982

currently working to fulfill this imperative.The question of which data to save is by no means a new

problem. Radiologists have always had this problem andhave yet to solve it. There exists a temptation to save alldata generated in a digital subtraction examination becauseit appears to be less voluminous than a conventional filmexamination. This, however, is misleading. The number ofimages generated and processed by these electronic tech-niques can be as many as hundreds per patient per exami-nation, and this amount of data adds up quickly.

It is clear that unlimited space for electronic data storage

is no more available than for film storage. Thus, somecriteria for image selection must be developed. Should only, ‘diagnostic’ ‘ images be archived, all other image dataerased and lost forever? Is there any advantage to savingonly enhanced, as opposed to ‘ ‘ raw’ ‘ or unprocessed, im-ages? Certainly there are more questions than answers.

Finally, a word about the image storage medium. Ana-logue storage media (video magnetic tape and disk, andfilm) theoretically limit the range of later reprocessing andmanipulating of archival data. For the present, video disk istoo expensive for large-scale image storage. This leavesvideotape or hard-copied film of CRT (cathode-ray-tubevideo screen) images. The former is suboptimal; the lattertakes us back to the original problem of giving up electronicinformation for analogue image. Most archived digital sub-traction images are in the form of hard-copied film transpan-encies of CRT images. This form of image storage is mad-equate because it has been difficult to reliably reproduce

high-resolution CRT images on film. Although there aretheoretical reasons for optimism in its solution, this problemhas yet to be solved. A soon-to-be-available method ofdigital image storage is the laser disk [14]. This techniquewould provide permanent digital images stored in a randomaccess manner. It is analogous to a phonograph record, in

that one can select any spot on the disk to display. Magnetictape is a reasonably inexpensive means of digital datastorage, but has the drawbacks of being cumbersome (nomore than five or six patients’ images per 2,400 foot [732m] reel) and necessitating sequential access to images. Thatis, one image after another has to be passed until the

desired image data is reached. Until digital disks becomecommercially available at reasonable cost, tape is probablythe best means of long-term digital data storage.

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1982:151 -1 57

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