Preparing effective medical illustrations for publication (Part 1): pixel-based image acquisition
Department of Diagnostic Radiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore
The explosion in medical literature over the last 20 years
has seen a huge increase in the number of articles and books written, as well
as an increase in the number of journals being published, often in increasingly
narrow areas of research and educational focus. The net result has been an
increase in the amount of literature that doctors have to go through in order
to have some semblance of keeping up with the rapid pace of change and
expansion in medical knowledge.
Even though the widespread availability of online tools
such as PubMed, the Cochrane Database and even tools such as Google Scholar has
made searching for publications and their key information easier than ever
before, for radiologists at least, the publication of appropriately high
quality medical images and illustrations remains of crucial importance for our
highly visually oriented discipline.
For many busy radiologists reading hardcopy literature
(despite online access and PDF soft copy versions, printed matter remains more
portable, convenient and easier to read for many people than electronic
versions of the same documents), there is a commonly adopted strategy for
dealing with the very large and increasing body of published imaging
- First skim the titles
- Then look at the pictures and illustrations
- Maybe read the abstract if these look interesting
- Occasionally, read the paper or chapter
Even the most erudite articles with the greatest
scientific impact lack some force on the page without highly effective,
well-designed and clear illustrations and images. In short, images help to
�sell� the message of a radiology article or chapter, help to distill key
points into graphical form, and help to ensure that the piece is actually read.
In the last decade, digital acquisition, storage and
transmission of medical images has become widespread and pervasive. The
time-honoured tradition of photographing the hardcopy film with a film camera
to produce glossy prints for publication has almost disappeared as publishers
have adopted totally digital workflows, and as radiologists have obtained
increasing access to digital acquisition methods from DICOM images to film scanners
and digital cameras. However, two things remain unchanged in this digital image
era: a) the original source image has to be of good quality, and b) the image
must still be optimised for the intended publication. Today publication can
range from a low resolution web-page image to a huge mural-sized poster, and
the images should be modified prior to export to a suitable standard image file
format to ensure that the final reproduction is of suitable quality.
This article is the first of a two-part series which
highlights and describes important approaches to producing high quality medical
illustrations for publication, which has differing requirements to electronic,
web-based or computer presentations. This part deals with the image sources
available to radiologists, and how we should use them to capture images at an
optimal quality suitable for publication, whatever the eventual medium. The
second part will deal with software manipulation of such images and the use of
dedicated illustration packages to create medical diagrams, annotations and
Preparing for Publication
In general, hardcopy publication has significant
constraints in length and the number and type of illustrations that can be
included. This forces author(s) to be highly selective and careful in their
choice of images that convey the key points in the most economical and
Preparing for publication then depends on the type of
piece that is to be authored. In general, case reports are the shortest
publications and are limited to only a few illustrations pertinent to the
specific case being reported. Subject or technique reviews, book chapters and
pictorial essays need many more illustrations, and the emphasis is on breadth,
range and quality of images, with often fairly lengthy captions. Original
articles demand mainly well-organised tabulated data and charts, with usually
fewer specific images and illustrations that highlight key points.
In general terms, all hard copy publications have usually
very explicit, clearly defined written guidelines for authors. These are
crucial and should be carefully read before embarking on creation of the
graphics, since the ultimate output must be designed for the likely printed
size, resolution and even potentially paper of the final product. So find the
instructions, read them and then follow the guidelines when creating the
images. Standardising on specific fixed image sizes and resolutions for
publication greatly simplifies the eventual publication.
A medical image or set of images often need alphanumeric
identification and appropriate arrows or other symbols highlighting specific
features, with an appropriate legend. A caption is usually required in order to
ensure the reader can quickly grasp what the images are intended to depict.
Today, all illustrations are digital in nature, or become
so in the course of publication preparation, simply because publishing
technology is now universally digital, with all journals now laid out using
computers and dedicated professional software tools.
Pixel-based graphics are the mainstay of radiology;
virtually all medical images are pixel-based. These also include photographs
and scans of hardcopy images. The key aspect of these images is that they are
resolution-dependent, and cannot be infinitely enlarged or shrunk without a
sometimes major loss of quality. The key information all pixel-based images
possess are listed in Table 1.
It should be clear from Table 1 that the linear size, very
relevant to final printed output, is dependent on both the pixel resolution and
the pixel dimensions of the image.
Vector images (drawings and charts) will be dealt with in
the second part of this article.
For print and presentation applications, images are
limited in gamut, or range of colours and shades of grey reproducible.
Typically, only a small fraction of the 24-bit colour (16 million colours) and
8-bit grey-scale (256 shades of grey) range can be reproduced on paper, because
of limitations in paper and ink technology. Furthermore, all grey-scale and
colour printing involves a process called line-screening, where any
given pixel�s shade of grey is reproduced by a pattern of small dots of varying
size and ink density (see Figure 1). This is because for routine applications,
grey-scale printing is performed only with black ink, and colour printing with
only 4 inks, the CMYK (Cyan, Magenta, Yellow and
blacK) system. Shades of grey and colours have to be simulated by a
variable dot density/size/colour pattern. There are sophisticated print methods
using stochastic screening and other technologies to eliminate screening
altogether, and some very high end printing presses use 6 or more colours in
the process, but these are limited to niche publications and are not relevant
The line-screen resolution is invariably lower than the
pixel resolution. Line-screen resolution is most commonly expressed in lines
per inch (lpi). As a rule of thumb, an image�s pixel resolution should be
150-200% that of the line-screen resolution. Although higher resolutions can be
used, this leads to much larger file sizes, longer image transmission times and
printing times without any improvement in final quality.
Common line screen resolutions used for print applications
and their corresponding pixel resolutions required are listed in Table 2.
Native vs. Interpolated Resolution
We are so used to seeing medical and other images
displayed on computer screens at varying resolutions that we do not appreciate
that for the most part these are not displayed at the original resolution of
the image, but rather are interpolated. This means that the display
software artificially increases or decreases the number of pixels displayed to
match the resolution and size of the screen.
Medical images range in native pixel size from 256 x 256
(65,536 or 64K pixels) for some CT, MRI and Ultrasound, up to 5 megapixels (MP)
for computed radiography (CR) and 10 MP for digital mammography. If the native
medical images were used directly for printing, a high end print publication
could only reproduce a 2562 image at about 1 inch square and a 5122
image at 2 inches square, which is too small for normal use.
Correspondingly, a digital mammogram or CR file is far too large for routine
use and if unadjusted would lead to excessively large document and printing
files. Furthermore, most DICOM medical images are typically 10-bit or 12-bits
per channel for greyscale (i.e., there are thousands or millions of shades of
grey), and computer displays and printed publications cannot reproduce more
than 8-bit greyscale (256 shades of grey) routinely. All DICOM images must
therefore be manipulated and adjusted prior to export into formats suitable for
online or print publication.
There are a number of ways to obtain images for publication
from medical imaging systems. Until the advent of the digital era, each medical
image intended for publication underwent a lengthy and time-consuming
- The image was printed to hardcopy film, ideally without the patient name
and distracting annotation. This was sometimes done as a separate print from
the normal hardcopy for interpretation and storage.
- The image was photographed with a 35mm camera with appropriate cropping
and scaling to fit the frame, and exposure adjusted to highlight specific
features (especially for x-ray images)
- The negative was processed and printed to glossy paper prints at the
size stipulated by the publisher. In order to capture the specific greyscale
information of the key finding, this step was often repeated to alter the
- The prints were labelled and any arrows, text or other overlay
annotation added using sticky transfer lettering (e.g., Letraset).
- Once accepted, subsequently each image was rephotographed by the
publisher and converted to a line screen image for printing.
It can be appreciated that these steps effectively
reinterpolated the image size. A small image could be effectively scaled up
optically in this manner, and a large one scaled down. This process meant the
final image was 3 to 4 generations removed from the original, leading to a
major loss of quality unless care was taken at each step.
Today this process is universally performed using software
tools and all publishers have a completely digital workflow, ideally without
modification to the submitted image. The commonest methods used are listed in
For most radiologists the best options are either a)
photographing an already printed hardcopy image, or b) exporting an image
correctly scaled and windowed to his/her liking from a DICOM image viewing
software platform, either standalone or web-based through an image web server.
Both of these methods produce typical computer file formats such as JPEG, TIFF
etc, and are much faster and more convenient than scanning the film or
attempting formal DICOM image export and subsequent conversion. In the case of
photography, today this means using a digital camera, and for DICOM conversion
and export, specialised DICOM viewing software (much of it freely available on
the internet) that allows for such image conversion.
The digital camera has become ubiquitous and relatively
inexpensive, and is rapidly replacing the film camera for virtually all
applications. It has several advantages:
- Fast, flexible, lightweight, portable
- Almost free and unlimited image capture
- Instant feedback on exposure and overall image appearance
- Easy input, storage, transmission and management
- Photograph any printed image
- Routinely high resolution pixel matrix (5 megapixels or higher)
However, digital cameras are not created equal, and the
vast majority of available cameras are designed to photograph pictures for
computer screen and snapshot printing output, with only limited control over
image exposure and minimal control over in-camera processing. As such they are
usually limited to 8 bits per channel, have limited or no means of capturing
directly in grey-scale, and have problems with variable levels of image noise
and lens-induced image distortion.
Today, digital cameras range in pixel count from at least
3 MP to 10 MP for common consumer applications. Paradoxically, as the above
discussion shows, a typical printed image 10 cm (4 inches) square for a 150 lpi
screen needs 300 x 300 x 4 (360,000) pixels, or far less than 1 megapixel.
Thus, ANY modern digital camera available far exceeds the pixel resolution
needed for good quality radiology image reproduction in print.
However, the demands of capturing optimal radiology
images, especially from the extremely wide density range of x-rays and
mammograms, require sophisticated controls within the camera. Although
professional equipment is ideal, it is often very expensive. Today, the digital
camera class known as �semiprofessional� or �advanced amateur� usually has the
appropriate features needed for photographing radiology images with suitable
image quality. These include:
- Very high quality lens, with macro photography option and minimal
- Must be able to focus very close to the object (<6 cm minimum
- Complete manual control of exposure
- Low noise sensor and/or large size digital sensor
- Ability to capture in RAW mode
In most cases this list means a digital single lens reflex
(D-SLR) camera. However, a few high end �all-in-one� cameras do have such
capabilities built in and can take very acceptable images, although in general
these have more image noise than those from D-SLR�s. Finally, it should be
noted that no available digital camera is able to capture the full greyscale
range of a properly exposed x-ray or mammogram; optimal exposure requires the
user knows exactly what he or she wishes to show on the final image.
Using a Digital Camera
For photography of images printed to film, the minimum
setup for consistent quality is a good evenly illuminated lightbox in a
darkened room, some shutters or dark sheets of card or film to cut out
excessively bright borders and a tripod. Ideal but much more cumbersome is a
dedicated copy stand, with a flat horizontal lightbox and the camera mounted on
a vertical arm.
Framing the image is important to save time and
post-processing. Although it is tempting to attempt the cleanup work in Adobe
Photoshop or similar pixel editing software, this is time-consuming and cannot
correct for many problems. As much as possible, the best possible image should
be obtained �in camera�. The image should be centred and square to the camera;
i.e., the plane of the film should be parallel to the plane of the camera
detector; otherwise the image will suffer from geometric distortion and will
require software correction. Also, the camera should be positioned so that the
desired image fills as much of the frame as possible with minimal extension
into adjacent images or dead space. The exception to this is if distortion
cannot be easily avoided by better positioning; software distortion correction
always requires some �dead space� around the image.
If using a D-SLR with a dedicated macro lens, there will
be no significant lens distortion � in short, straight lines will be straight
throughout the entire image. However, most integrated digital cameras have zoom
lenses that invariably show some barrel distortion (see Figure 2) at the
widest angle settings where the focussing distance is closest; this is manifest
by outward curvature of straight lines near the edge of the image. Although
this distortion can be corrected with appropriate software, this is too
specialised and tedious for the general user, and is best avoided or minimised
at the capture stage.
Most lenses have minimal barrel distortion at mid-zoom or
telephoto settings, but the closest focussing distance is much greater and the
maximum magnification is actually reduced. For most cameras, a mid-zoom range
setting produces acceptable results, with still-useful close focussing and
A tripod is generally mandatory because the exposure times
for most cameras using a low ISO rating (50-100) to minimise image noise is in
the range of 0.5 to 1/15 second; handshake is reliably eliminated only at
exposures of 1/125 second or higher, so for most people the tripod is essential
for consistent image quality. D-SLR cameras typically have very low image noise
with high ISO ratings, which would allow for handheld exposures. It is possible
to reduce, but not entirely eliminate such image noise using software tools
However, one of the major reasons for using a mechanical
support is that it is much easier to consistently frame an exposure, especially
when using a copy stand.
Obtaining the correct exposure can be difficult,
especially for x-rays. The use of center-weighted or an averaged automatic
exposure setting on the camera usually results in the correct exposure for most
images. However, such averaged exposures can be problematic for mostly dark
images (e.g., diffusion-weighted MRI with a single small bright lesion) or
mostly bright images (e.g., narrow lung windows on HRCT of the chest). This is
because the majority of the image is not grey; all automatic exposure systems
are calibrated to ensure the midrange image intensity is forced towards the
centre of the illumination scale (i.e., grey). So for very dark images the user
should force the camera to deliberately under-expose by as much as �2 or �3 EV,
and conversely over-expose bright images by as much as +2 or +3 EV. These steps
will ensure that blacks appear black and white appears white on the final
For x-rays and mammograms, the density range is usually
greater than the digital camera can capture. This means that exposure has to be
optimised to show the feature of specific interest at its best, which may mean
the overall image may be over- or under-exposed, and that spot metering or even
manual exposure will be needed to ensure the appropriate finding is clearly
visible. Sometimes, image exposure bracketing (3 or more exposures varying by �
1 EV) is required to ensure accurate capture of the desired greyscale.
More recently, many cameras have been equipped with RAW
image capture capability. This is standard on all available single-lens reflex
digital cameras, but is only available on a limited range of high end
�semi-professional� all-in-one cameras. The RAW file represents the original
unmodified digital data captured by the camera sensor, and offers a major
advantage over the typical JPEG or TIFF images produced by most cameras, in
that no in-camera postprocessing is performed and often less image noise and
more image detail, especially at the extremes of shadow and bright areas, can
be retrieved by dedicated computer-based RAW image processing software.
Applications such as Adobe Photoshop are now supplied with such processing
capability as standard, and camera vendors routinely supply dedicated RAW
processing software packages with their RAW-capable cameras.
Unfortunately, there is no industry standard method of
creating a RAW image file format, and every vendor takes a different approach,
sometimes significantly changing the format for different cameras in their own
product lineup. It typically takes other software vendors some weeks, even
months, to update their software to handle the latest file formats as new
cameras are released. The end-user thus may be restricted in their choice of
post-processing software. Adobe has created the closest thing to a common
format, the DNG file, an intermediate format that almost all RAW image formats
can be converted to using Adobe�s free DNG conversion software. However, this
remains a somewhat unsatisfactory solution, requiring an extra step in image
conversion in the workflow from camera to final image.
In addition, specialised processing software has been
developed to create High Dynamic Range (HDR) images from 3, 5 or even more
different wide-ranging exposures of a single subject. While this has been
mostly confined to creating spectacular high resolution colour images of
geography, scenery and paintings, it is ideally suited to the creation of
exceptional quality capture of static medical images with a very wide dynamic
range, such as x-rays and mammograms. The production of these images is however
still tedious and somewhat experimental, and is neither within the scope of the
average user or necessary for the vast majority of medical images.
While pressing the shutter button directly is simple
enough, once a tripod has been set up and the image framed correctly, use of
the self-timer system is ideal; most have the option for a 2-second delay,
which is enough time for any vibration to settle before exposure.
The image resolution captured, as noted above, is
generally far greater than needed for publication. Nevertheless, the maximum
resolution, best quality images possible should be captured at all times.
In-camera reduction of resolution to say, 2 megapixels, will result in
considerable image data being discarded with potentially serious loss of
quality. It is much safer and more controllable to capture at the maximum
resolution possible and then adjust the image size using computer software
Ultimately, wherever the highest quality is needed, the
RAW format should be used if the camera supports it and the user is willing and
able to use this less convenient approach.
Regardless of the image format at acquisition, whether
from a DICOM image, film scanner or digital camera, page layout applications
used by professional publishers require one of two standard formats in most
cases: lossless Tagged Image File Format (TIFF) or high-quality, minimal lossy
compression Joint Photographic Expert Group (JPEG) images. All pixel-based
editing or capture applications will allow export or import of either of these
The old computer adage, �garbage in, garbage out� applies
very well to medical image capture for publication. We must always start with
the best quality image possible; for many radiologists today this is often the
original DICOM image itself, exported directly from the scanner or PACS system,
and exported to JPEG or TIFF with the appropriate cropping, zoom and
window-level settings to optimise the key feature of the image. The next best
solution is a high quality print to film, which is then captured digitally
either by a dedicated film scanner or carefully performed digital photography
as described above. Older processes, such as photography of a paper print or
the scanner display itself, are no longer considered acceptable.
There are many more steps required to ensure that a
captured image becomes optimised for printing or publication; today these are universally
post-acquisition manipulations in software applications on a computer, and are
covered in Part 2 of this article.
Figure 1 Typical examples of printed matter at standard viewing resolution (left images) and when examined with a high magnification (right images) using line screening. Top images are printed in greyscale using a single black ink line screen; the enlargement of the model�s left eye clearly shows the different sized dots used to create the appearance of continuous shading. Bottom images are printed in colour using the CMYK system; the enlargement of the model�s right eye clearly shows the rosette pattern of different coloured ink dots used to create the appearance of continuous shades of colour.
Figure 2 An image with severe barrel distortion showing bowed straight lines (left) and the corrected image (right). The effect has been exaggerated, but when photographing usually rectangular medical images, this can be easily seen when present.
Figure 3 Strong image noise in the form of colour botches are seen in the left hand image, and is very commonly seen in most �all-in-one� digital cameras. After de-noising with custom software, the right hand image appears cleaner and smoother. However, the noise is not completely eliminated and the image appears softer as a result of the post-processing.
Table 1 Pixel-Based Images and Pixel.
Table 2 List of print/output applications, the image resolution required and its dependence on the publication line screen resolution.
Table 3 Image capture/export methods and their strengths and weaknesses.
|Received 18 December 2007; accepted 10 January 2008
Correspondence: Department of Diagnostic Radiology, Yong Loo Lin School of Medicine, National University of Singapore, 5 Lower Kent Ridge Road, Singapore 119074. Tel.: (65) 6772-4211; Fax: (65) 6773-0190; E-mail: email@example.com (Shih-Chang Wang).
Please cite as: Wang SC,
Preparing effective medical illustrations for publication (Part 1): pixel-based image acquisition, Biomed Imaging Interv J 2008; 4(1):e11