Veterinary Cardiorespiratory Centre
Martin Referral Services, Thera House, 43 Waverley Road, Kenilworth, Warwickshire CV8 1JL
Tel: 01926 863445
INFORMATION SHEET
Tips for Thoracic Radiography
Factors Involved In Obtaining Good Quality Radiographs
Sufficiently powered x-ray unit, eg. greater than 150 mA output. Good quality films and screens; rare earth
films and screens will help to reduce exposure time. Automatic processing greatly reduces processing time
and minimises processing faults. A selection of restraining and positioning devices are required such as
sandbags, foam shapes (the wedge is most commonly used), ties, positioning troughs. Good centring and
coning down with the light beam diaphragm to minimise scatter radiation and thus improve contrast.
Although the use of grids will reduce scatter radiation, the increased exposure time required will increase
movement blur and are probably better avoided with low output units.
The most common thoracic radiographic problems in veterinary practice
The guidelines below highlight the common problems that encountered from films submitted for
interpretation.
1. Poor processing – primarily under developing because chemicals are not changed regularly
2. Recumbency atelectasis – an artifact that can mimic lung pathology
3. Inappropriate exposure – under or over penetrated
4. Inadequate positioning – rotated views, forelegs not pulled forward
5. Under aerated lungs – not taken during inspiration
6. Inadequate collimation or centring
1. Film Processing
 Films that are under-developed appear flat and grey, lacking good black and white contrast and thus
reducing detail. In a well developed film, the directly exposed parts of the film (ie. without body part) will
be very black if the film is well processed.
 The main reasons for under-developing are: not replacing the chemicals frequently enough (typically
every 4 weeks) and if processing manually, not waiting for the chemical temperature to reach the correct
level or leaving the films in each tank for long enough.
 It is a popular misconception that leaving the films in the developer for longer will balance out the
problems of old or cool developer.
 Light fogging will reduce film quality, this can be due to: light leakage under the darkroom door, a
darkroom light-bulb with too high a wattage (should be < 25W), having an incorrect dark-room light filter
for the films used (e.g. green-sensitive film with an amber filter), forgetting to put the top on the film box.
TIP: When the chemicals have been freshly changed and the processor is up to temperature - radiograph a
cassette directly using your usual middle-sized dog exposure. Develop the film and note how black the film
is - keep it for future comparison. When there is a suspicion that radiographs are becoming flat due to
under developing, take out the blank film and compare that with the black parts of the current film.
2. Recumbency atelectasis
 This is primarily a problem encountered in dogs, when chest radiographs are taken under sedation or
anaesthesia.
 In lateral recumbency there is atelectasis of the dependant lungs field, due to a combination of the
hypoventilation that occurs because of sedation/anaesthesia and the chest and heart weight on the
dependant lungs.
 The atelectasis is then seen on the DV (or VD) view and can be mistaken for lung pathology. The extent
of lung atelectasis is very variable and can be a mild increase in pulmonary density to almost the
appearance of unilateral lung disease When atelectasis occurs, there is an associated mediastinal shift
towards the collapsed lungs (best appreciated as a shift in the heart).
Tip: Intubate dogs in sternal recumbency and then take the DV (or VD) view before the animal is placed in
lateral recumbency.
3. Radiographic Exposure
 High kV – low mAs technique is preferred for thoracic radiographs. Lung detail is improved and it
allows shorter exposure times (thereby reducing movement blur). For feline chests, generally a kV range of
58 – 66 is adequate. For canine chests, generally a kV range of 70 – 90 is adequate (depending upon the
size of dog).
 A grid should be used if the width of the thorax exceeds 12 cm. However the increased exposure time
required will increase movement blur and may be better avoided with low power x-ray machines.
 The effort should be made to develop a Technique Chart. A technique chart reduces the frequency of
over- and under-exposed films. Although there are a number of fairly complex methods in developing such
a chart, a simple listing of the correct exposures found through trial and error for cats, small dogs, medium
dogs and large dogs will suffice in most instances. Simplifying the film-screen combinations used also
helps.
4. Positioning
The routine views are a DV and a right lateral view for ‘cardiac’ radiographs. For ‘respiratory’ radiographs
a DV and/or a VD view and both right and left laterals are useful. A ready supply of sandbags and foam
wedges all help in positioning the animal correctly. When in lateral recumbency the forelimbs should be
pulled cranially out of the way so as not to obscure the cranial heart border or cranial lung lobes and a foam
wedge should be placed under the sternum to prevent rotation of the body.
The DV or VD view: The sternum and spine should be super-imposed indicating no rotation. On ‘cardiac’
radiographs the aortic line should be evident. ‘Respiratory’ radiographs are usually a slightly softer
exposure to show lung detail better.
The Lateral: The costochondral junctions should be super-imposed indicating no rotation. The caudal
vena cava and cranial lobe vessels should be clearly evident. Thrall’s triangle (the triangle formed by:
ventral border of vena cava, caudal border of heart and the diaphragm) should be large indicating an
inspiratory timed exposure. The distance along the ventral border of the caudal vena cava from the heart to
diaphragm, should be at least 1 intercostal space, but ideally nearer 2.
5. Inspiratory views
These are most important for assessing lung detail. Exposures are best taken during inspiration (if not
manually inflated). In respiratory cases in dogs it is often useful to take x-rays under GA, in order to inflate
the lungs during the exposure (to normal tidal volume), thus maximising air to soft tissue contrast detail
and also minimising motion blur. In cats – manual lung inflation is not necessary and should not be
performed. Expiratory timed exposures can also give the optical illusion of cardiomegaly - use the vertebral
heart score to assess heart size if in doubt.
Tip: In sedated (conscious) animals aim to take the radiographic exposure during inspiration, rather than
peak inspiration. In dogs under GA, inflate the lungs to normal tidal volume by squeezing the rebreathing
bag during the exposure.
6. Collimation & Radiation Safety
With adequate chemical restraint animals should rarely be held for thoracic radiographs (see our
information sheet on sedation for cardiorespiratory cases). All radiographs should be centred over the area
of interest and collimated down to it, this reduces scatter and improves film quality.
References
Mahoney P (2000). "The chest x-ray - are we making the effort." VCS Autumn Meeting
Martin & Corcoran (2006) Notes on: Cardiorespiratory disease of the dog and cat, 2nd ed
Blackwell Science. ISBN 0-632-03298-7