TECHNOLOGY REFRESHER COURSE

I

The
multiformat

camera

Whilst very few multitormat
cameras, based on cathode ray tube
(CRI) technology; are purchased these
days, it is important to review that
technology in order to assess both the
impact and potential oflaser imagers.

Early imaging cameras bolted
directly onto the diagnostic monitor
(Figure 1) had special lenses to

Figure 1: The imaging camera is bolted
directly on to the diagnostic monitor.
compensate for spherical aberration
because of the curved screen. The
applications of these cameras were in the
fields of ultrasound and first generation
CTscanners.

Next came the dedicated
multifotmat camera, so called because
multiple exposures could be made on
the same film. These cameras are video
based, that is they require a video signal
in order to work Such a camera featured
a tlatscreen CRftube, in-line opties and
electronic control. The various
challenges faced with this type of
camera are as follows:

Stability
The image on the CRT is built up

from the electronic beam crossing the
tube at least 625 times from top to
bottom. Each sweep constitutes a line
(a raster line) with a set of 625lines
constituting a frame.A frame is built up
25 times a second. With conventional
illm an exposure of about 10 frames will
be necessary to form an image. It follows,
therefore, that each line on each frame
must be in exactly the same position on
the screen in order to produce
maximum sharpness in the image.Any
instability will result in the raster lines
being blurred with a resultant loss in
image quality.

Exposure
As stated above, 25 frames a second

are produced on the monitor screen.
The minimum exposure time is thus
1I25th ofa second. On latermultiformat
cameras a method of determining
exposure by number of frames (or
fields) was developed.

An additional issue in determining
exposure is the monitor brightness. A
change in exposure time causes a
sensitometric change in the film's
characteristic curve similar to the effect
caused by increasing monitor
brightness. Additionally, an increase in
monitor brightness can cause a
deterioration in image sharpness due to
phosphor flare. It will be appreciated,

32 SAJOURNAL OF RADIOLOGY- March 1996

r

therefore, that the monitor is set for a
maximum sharpness / brightness
compromise with exposure time
determining blackness of the image.

Optical system
There are various problems

associated with an optical system:
• Vignetting - a loss of brightness at
the edges of the image. This is reduced
by using a CRf screen that islarger than
the image field

The laser
Imager

1987 saw the advent of the laser
imager with 3M introdudng an infra red
system to the American market.Today
there are seven major manufacturers of
laser imagers,namely Agfa, DuPont, Fuji,
Kodak, Konica,3M and Polaroid.

The output, that is the illm, of the
laser imager may look very much like
that of a CRT multiformat camera, but
that iswhere the similarity ends.

Input
Although provision can be made

on most systems for a video input.laser
imagers almost exclusively use a digital
interface to the imaging modality. This
has far reaching implications with regard
to increased departmental productivity
where different modalities can be
connected to the same imager. Much
research and development is underway
developing Local Area Networks
(LAN), Image Gateways, Picture
Achiving and Communications Systems
(PACS) to fully utilise the potential of
laser imagers.

• Spherical aberration - this manifests
itself as geometric distortion. This is
overcome by the use ofin-line optics
and a flat screen monitor
• Density uniformity - a falling off of
brightness at the edges of the screen due
to variations in electron density on the
phosphor. This is compensated for
electronically by alterïngthe brightness
proffie across the screen

Basic principles
The following is a description of the

operation of a Helium Neon (HeNe)
laser: (Figure 2)

The data from the imaging
modality is inputted via the interface
electronics and an image buffer which
acts as a storage device.The data are then
split into x-axis; y-axis and intensity
information.

tDPSIS 88



The development of the imaging
camera CRT to Laser

,pompaus 82

The intensity of the laser beam is
varied by means of an Acoustic Optical
Modulator. This is a very stable system
which delivers 4096 grey levels
(compared to 200 on the CRT). The
beam is then swept to and fro across the
film by a system oflenses and a rotating
or galvanometric mirror (x-axis).Atthe
same time the film is transported in the
longitudinal direction by a precise
mechanical drive system (y-axis). The
image is thus "painted" on the film from
top to bottom in sequential lines by a
very fine laser beam which achieves a

pixel size of 80llm. By this means
problems of geometric and density
distortion common to CRT imagers are
avoided.

Exposure
The one main disadvantage of the

laser imager against the CRT imager is
exposure time. On the CRT it is
approximately 40 ms. The laser imager
takesanythingJrom 17to45 seconds per
film. Because of its digital architecture,
however, the data for an image can be
stored in a queue to maximise

throughput. In addition multiple copies
can be made independently of the
imaging system (CT Scanner, MRl, ete.).

Resolution
The resolution of a laser imager is

in the region of63oo X 7650 pixels.This
applies to the whol Hlm. With a CRT
camera the resolution of 1250 lines
applies to each image With 12expoo.tres
on a 35 X 43 cm film the resolution of
each system is comparable. With less
than 12 exposures on a film, the laser
imager is superior.

This is thefimin the series 'Technolog;
Refresher Course". We would welcome
feedbackastowhetherlhesubjectmaterial
iscomct, whethertheinfonnationshould
be more in-depth, and suggestions about
what subjects should be covered in future
isI'ues.

The author wishes to thankAgfa,
DuPont and Konica for the infannatian
they provided. towards this article.

Roger Short
Publisher

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Journal of Radiology c/o Cannon
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ore-mailedtoGll1non@so!o.pipex.co.za

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33 SA JOURNAL OF RADIOLOGY- March 1996

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to P'fIII 86