IMPROVING YOUR Thoracic Radiography Indications Radiography ...

unkindnesskindUrban and Civil

Nov 15, 2013 (4 years and 1 month ago)

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IMPROVING YOUR THORA
CIC RADIOGRAPHY


INDICATIONS


Radiography of the canine and feline thorax is an invaluable aid to diagnosis of many thoracic
diseases and lesions, but a poor radiograph can be worse then none at all, because it may
provide false in
formation.


The indications include the following clinical presentations:


1.

Coughing

2.

Dyspnoea

3.

Cardiovascular disease

4.

Thoracic trauma

5.

Assessment of primary and secondary neoplasia

6.

Lesions of the chest wall

7.

Regurgitation of food

8.

Invest
igation of other abnormalities detected by palpation, auscultation or percussion

9.

Miscellaneous


RESTRAINT FOR THORAC
IC RADIOGRAPHY


The Ionising Radiations Regulations (1999) require that animals may only be manually
restrained for radiography in except
ional circumstances when clinical consideration precludes
the use of sedation or anaesthesia.


With patience and skill, most patients can be restrained using sedation with positioning aids
such as troughs, sandbags, foam wedges and ties. General anaesthes
ia may be safer then
frantic struggling and may prevent unnecessary repeat radiography.


RADIOGRAPHIC TECHNIQUE


1.

Film/screen combination


Rare
-
earth screens reduce the exposure required and thus allow the shortest
exposure times to be used. In addition
, rare
-
earth screens are less sensitive to scattered
radiation than are calcium tungstate screens. This may allow a grid to be dispensed with.


2.

Cassettes


Large cassettes (up to 43 x 35 cm) should be available so that the whole chest can
be included on

a single film. It is very difficult to interpret a mosaic of smaller films.


3.

Grids


A grid should be used to reduce the effect of scattered radiation if the chest is more
than 15
-
20 cm thick. However this will require an increase in exposure time of
approximately
2
-
4 fold.


4.

Exposure Factors


A high KV low mAs technique is usually employed. It has two advantages : the
higher KV reduces the inherently very high contrast in the chest and produces a greater
number of grey shades, and the low mAs reduc
es movement blur.


Exposure factors will need to be increased if pleural fluid or diffuse lung pathology are
present and reduced if pneumothorax is present.


5.

Collimation


Close collimation of the beam will reduce scattered radiation and improve detail,
but
the margins of the lung field should not be coned off as they may contain useful diagnostic
information.


6.

Radiation Safety


If manual restraint is required then all measures to protect personnel must be taken.


7.

Inspiratory Films


The exposure sho
uld be made at full inspiration so that the lung fields are fully
expanded and aerated. This may be difficult in animals that are panting or breathing
shallowly. Sedation may reduce the respiratory rate. Temporary occlusion of the nostrils or
blowing on

the nostrils may cause the animal to inhale deeply. In the anaesthetised animal,
the lungs may be held inflated by the use of the rebreathing bag taking care to avoid
overdistension.


8.

Darkroom Technique


Films should be developed with care to avoid ar
tefacts. Underdevelopment is the
most common film fault and results in loss of contrast.


RADIOGRAPHIC PROJECTIONS


1.

Lateral View


The right lateral recumbent view should be taken as standard since i) the heart lies in
a more consistent position ii) the
re is more air
-
filled lung between the heart and lower chest
wall giving better cardiac detail and iii) the diaphragm obscures less of the caudal lung field.
However, as the dependent lung field is less well aerated, small soft tissue densities may be
mas
ked. For this reason, it is often useful to take both right and left lateral recumbent views
since lesions in the uppermost lobes will be better visualised.


The patient should be padded into the true lateral position and the forelimbs pulled
well forward
s. Centre to the caudal border of the scapula, midway between the scapula and
the sternum.


2.

Dorsoventral/ventrodorsal View


The DV projection is essential if the heart is under investigation. If the animal is in
respiratory distress, a VD view must no
t be attempted. For both views it is important to
ensure symmetrical positioning with the spine and sternum superimposed.


Centring is level with caudal scapular border (DV) or midway along the sternum (VD).


Additional Views


3.

Horizontal beam standing
lateral view


Occasionally a very dyspnoeic animal may be impossible to position for the standard
views. Extreme care must be taken when using a horizontal beam. The forelimbs will
obscure the cranial thorax and only the caudodorsal area is visible.


4.

Decubitus lateral view


This view is rarely required but can be of value in assessing small pleural effusions or
pneumothorax. Particular attention must be paid to radiation safety. Free air will be seen
below the uppermost chest wall whilst free fluid w
ill drain towards the lower side.


5.

Erect VD view


Rarely indicated. May be of value in assessing cranial mediastinal lesions.


6.

Oblique views


Lesion
-
orientated oblique views can be used to skyline lesions of the chest wall such
as rib tumours. Expo
sure factors usually need to be significantly reduced.