Diagnostic imaging

Chapter 6


Diagnostic imaging




Chapter Contents


Radiography





Practical positioning



1. Spine



2. Head and neck



3. Thorax



4. Abdomen



5. Appendicular skeleton


A The forelimb



B The hindlimb



Use of contrast media



Diagnostic Ultrasound



Endoscopy



There is no doubt that the use of ‘pictures’ makes it much easier to confirm a diagnosis and nowadays there are several methods of taking this ‘picture’. The majority of practices have facilities for radiography and most have ultrasound. This chapter will describe the basic use of both techniques, but it is not the brief of the book to describe the more complicated ways of creating an image such as magnetic resonance imaging (MRI), computed tomography (CT) or nuclear scintigraphy. These three imaging techniques are advanced and require expensive, and in some cases cumbersome, equipment making them currently unavailable to all but the most sophisticated referral practices.



Radiography


Radiography may be defined as all the procedures involved in the production of a radiograph of diagnostic quality and is very often carried out by a veterinary nurse. It involves cleaning and maintenance of the equipment, whatever its type, and the vital correct positioning of the patient. In many practices, positioning is the responsibility of the nurse and the veterinary surgeon acts as a supervisor checking that everything is as he / she wants it before an exposure is made; in other practices it is the vet who positions the patient. Whichever applies in your practice, it is important that you understand the recommended methods of positioning to achieve the optimal image.


Radiology is the interpretation of the radiograph and it is recommended that you refer to specific textbooks for guidance on this subject.


Whatever your role in the production of an image, it is important to remember that radiation is potentially hazardous and the use of x-rays should never be taken lightly. The legislation concerning the use of radiation is embodied in the Ionizing Radiation Regulations (IRR) 1999 and all practices are required to have a copy. Practices are also recommended to have a copy of the guidance notes, which provide a more digestible form of the regulations. Remember that it is scatter that is the unseen and unpredictable hazard; wearing a dosemeter at all times, wearing protective clothing when appropriate and avoiding the use of manual restraint are three rules that should prevent you developing problems in later life. Screen films and grids should be used as often as possible.


When positioning the patient, manual restraint should be avoided if possible as this increases the risk of exposure to scatter even if you are wearing protective clothing. Chemical restraint necessitates the use of sedation or general anaesthesia and when reading the instructions for the following procedures you should assume that the patient is anaesthetized. When you are examining the chest, the condition of the patient may be such that general anaesthesia is considered to be dangerous and each patient must be assessed in the light of the individual clinical signs.






Procedure: Preparing the patient for a radiographic examination:



1. Action: Make sure that there is a valid clinical reason for the examination.


    Rationale: All radiographic examinations must be clinically justified and exposures must be kept to a reasonable minimum.


2. Action: Use some form of chemical restraint (i.e. sedation or general anaesthesia as appropriate to the patient).


    Rationale: Manual restraint should be used only in extreme circumstances to reduce the risk of radiation to personnel.


3. Action: Remove any potential artefacts from the patient (e.g. collar, clips, matted hair, etc.).


    Rationale: An artefact may overlie the area of interest, may distract from the main point and may lead to an incorrect diagnosis.


4. Action: If required for the procedure, make sure that the patient is properly prepared (e.g. use of an enema, emptying the bladder, starvation).


    Rationale: In some views the presence of food in the stomach, faeces in the colon, or urine in the bladder may restrict the view of some diagnostic points.


5. Action: Position the patient correctly for the radiograph.


    Rationale: Following the correct procedure will ensure that the appropriate area is visible on the radiograph and is not obscured by an overlying part of the anatomy. This reduces the numbers of repeat exposures.



Practical positioning (Tables 6.1 and 6.2)


Correct positioning is vital if you are to produce a radiograph of diagnostic quality at the first attempt. If you have to repeat an exposure you have doubled the amount of scatter produced, which is bad for all personnel involved, and you have increased the time for which the patient is anaesthetized. It goes without saying that you have also added to the cost of the procedure.



Table 6.1


Positioning aids*



























Type Use Effect on the radiograph
Troughs – range of sizes To restrain the animal on its back and prevent rotation of the trunk Radiolucent – can be placed over the cassette if necessary
Foam wedges – range of sizes. Often covered in wipeable fabric To provide support of trunk and limbs; can be used to prevent rotation of the spine and to maintain it in a horizontal plane Radiolucent
Sandbags – loose filling allows bending and twisting. Covered in wipeable fabric Can be wrapped around limbs to hold them in place or placed over the neck Radio-opaque – do not place in the primary beam
Tapes or ties – range of lengths Looped around limbs to pull them into position and then tied to the cleats on the table Radiolucent
Wooden blocks For raising the cassette closer to the x-ray tube head Radio-opaque – do not place over the cassette

*After Aspinall 2003a, p 216, with permission of Elsevier Butterworth-Heinemann.



Table 6.2


General principles of positioning*



























Action Rationale
Centre the primary beam over the main area of interest To prevent distortion of the area by an oblique view
Place the area of interest as close as possible to the film If there is an excessive object–film distance the part in question may be magnified and blurred
Ensure that the centre of the primary beam is at right angles to the film To avoid distortion of the image; this is important when examining joints or intervertebral spaces
Collimate the beam to as small an area as is realistically possible To reduce the amount of scattered radiation and thus improve the sharpness of the image
Take two views at right angles to each other To assist in location of a lesion and to visualize the area completely
Try to contain the whole area of interest on a single film To reduce the number of exposures; however, if this means that important parts are viewed obliquely (e.g the whole spine) then it is better to take views of several smaller areas
When imaging the spine, the body must be supported in areas which may drop down or rotate (e.g. neck and lumbar spine) so that the entire vertebral column is in the same horizontal plane To prevent distortion and magnification of individual vertebrae and of the intervertebral discs

*After Aspinall 2003a, p 216, with permission of Elsevier Butterworth-Heinemann.



1 Spine


Imaging of the spine may be used to investigate prolapsed discs, spinal tumours, traumatic injuries and in conjunction with a myelogram.




Procedure: Lateral spine (Fig. 6.1):




1. Action: Place the patient in right lateral recumbency.


    Rationale: It is traditional to have the head to the left on the radiograph.


2. Action: Place supporting pads under the natural curves of the spine (i.e. the neck and lumbar spine).


    Rationale: This support prevents these areas dropping down towards the table and keeps the spine in a level horizontal line.


3. Action: Place pads under the sternum and between the limbs.


    Rationale: These prevent rotation, which will pull the spine out of its horizontal position.


4. Action: If the lower cervical spine is to be examined, pull the limbs caudally (Fig. 6.1).


    Rationale: This ensures that the musculature of the shoulder does not overlie C6 and C7.


5. Action: If the cervical spine as a whole is to be examined, place a pad under the nose.


    Rationale: This prevents rotation of the head. This should also be done if the whole spine is to be examined.


6. Action: Centre the primary beam (as indicated by the cross wires in the light-beam diaphragm) over the point of interest. Include muscle mass, but not fat and skin.


    Rationale: Centring must be accurate and care must be taken to avoid covering too large an area in one view because divergence of the beam at the edges of the field will cause artificial narrowing of the intervertebral spaces. Aim to cover 3–4 intervertebral spaces in each view.


7. Action: If the entire spine is to be examined, ensure that each image overlaps with the ones on either side.


    Rationale: By ensuring overlap a complete study of each vertebra and its associated intervertebral spaces can be achieved with a minimum of distortion.


8. Action: If the first film fails to identify an abnormality, take repeat films on either side of the initial film.


    Rationale: To ensure that you have thoroughly examined the area of the spine.


NB Centring points for the whole spine are as follows:



Fewer films may be sufficient for cats and small dogs.



Procedure: Ventro-dorsal spine (Fig. 6.2):




1. Action: Place the dog in dorsal recumbency supported in a trough or by foam pads or sandbags.


    Rationale: The spine must be positioned so that the sternum and spine are in the same vertical plane. It may be difficult to prevent rotation, particularly in deep-chested dogs. Remember that sandbags are radio-opaque.


2. Action: Extend the hind- and forelimbs and secure with ties.


    Rationale: This prevents rotation and flexion of the lumbar spine.


3. Action: Extend the neck and hold in place with tape or a sandbag overlay. It may help to place a pad under the neck.


    Rationale: This ensures that the spine is fully extended.


4. Action: Centre the primary beam (as indicated by the cross wires in the light-beam diaphragm) over the point of interest.


    Rationale: Try to select the areas radiographed in the lateral view so that you have two planes per area of the spine, which makes location of the lesion much easier.


5. Action: If the entire spine is to be examined, ensure that each image overlaps with the ones on either side.


    Rationale: By ensuring overlap a complete study of each vertebra and its associated intervertebral spaces can be achieved with a minimum of distortion.


6. Action: If the first film fails to identify an abnormality, take repeat films on either side of the initial film.


    Rationale: To ensure that you have thoroughly examined the area of the spine.


NB Centring points for the whole spine are as follows:



Fewer films may be sufficient for cats and small dogs.



2 Head and neck


Imaging of the skull may be required for the investigation of trauma or of swellings related to infection or neoplasia, and of neurological signs associated with the CNS and the cranial nerves. The anatomy of the skull is complex and the interpretation of radiographs relies on accurate positioning.




Procedure: Rostro-caudal view – open mouth: This is used to demonstrate the tympanic bullae, the foramen magnum, C1 and C2 and the atlanto-occipital joint.



1. Action: Place the animal in dorsal recumbency with the hard palate vertical to the cassette. Tip the nose backwards slightly past the vertical.


    Rationale: This position ensures that the tympanic bullae are as close as possible to the cassette. Tilting the head ensures that the bones of the skull do not obscure the view of the bullae.


2. Action: Hold the mouth open to form a V shape (Fig 6.3) using tapes, or place an old needle case (with one end cut off to form a hole) between the teeth of the upper and lower jaws.



    Rationale: This removes the mandible and maxilla from the area of interest.


3. Action: Orientate the primary beam towards the hard palate and centre it (indicated by the cross wires in the light-beam diaphragm) on the base of the tongue.


    Rationale: In this position the tympanic bullae are located directly behind the base of the tongue.


4. Action: If the animal is intubated, remove the endotracheal tube just before exposure.


    Rationale: The tube may be superimposed on the tympanic bullae.




Procedure: Ventro-dorsal view: The ventro-dorsal (VD) view is used for the examination of the tympanic bullae and the external auditory meatus on either side.



1. Action: Place the animal in dorsal recumbency.


    Rationale: This ensures that the skull is as close as possible to the film.


2. Action: Extend the neck (Fig 6.4).



    Rationale: Extension makes sure that the head is horizontal. This position may be quite difficult to hold because the sagittal crest tends to tilt the head to one side or the other.


3. Action: Place a pad under the neck.


    Rationale: This forces the head back so that the hard palate is parallel to the cassette.


4. Action: Secure a tape around the upper canines and tie it to the table top (Fig. 6.4).


    Rationale: This ensures that the hard palate remains in a horizontal plane.


5. Action: Place a right or left marker on the cassette beside the head as appropriate.


    Rationale: It is important to be able to identify the location of a lesion or mass.


6. Action: Centre the beam (as indicated by the cross wires in the light-beam diaphragm) in the midline at a point halfway along the interpupillary line.


    Rationale: This point may vary with the area to be examined.


7. Action: Collimate the beam to include the entire skull.


    Rationale: If necessary, collimate more tightly over the area of interest.



Procedure: Dorso-ventral view: The dorso-ventral (DV) view is used for examination of the ear and the temporo-mandibular joint.



1. Action: Place the animal in sternal recumbency with the neck extended (Fig. 6.5).



    Rationale: The head is now in a stable position. This is preferred to the VD position even though there is some magnification because of the distance of the skull from the cassette.


2. Action: Place a sandbag over the neck.


    Rationale: This keeps the hard palate parallel to the cassette especially in deep-chested dogs.


3. Action: Place a right or left marker on the cassette beside the head as appropriate.


    Rationale: It is important to be able to identify the location of a lesion or a mass.


4. Action: Centre the beam (indicated by the cross wires in the light-beam diaphragm) on a line midway between the eyes.


    Rationale: This point may vary according to the point of interest.




Procedure: Lateral oblique view: This is used to show masses or lesions identified in other views and to view the tympanic bullae or temporo-mandibular joints. It is also used to view the dental arcades (e.g. for the diagnosis of a malar abscess).



1. Action: Place the animal in lateral recumbency.


    Rationale: This is the most stable position for this view.


2. Action: Tilt the head along its long axis and support with foam pads.


    Rationale: This will separate each side of the head.


3. Action: Alternatively tilt the animal’s nose up by about 15° and support with pads.


    Rationale: The aim of this view is to raise the lesion or mass so that it is on the ‘skyline’; the position will vary according to the location of the lesion.


4. Action: If viewing the teeth, place foam wedges under the appropriate arcade.


    Rationale: Remember to keep the mandible or maxilla in a horizontal plane parallel to the cassette; foam wedges will help to do this. The side under investigation should be closest to the cassette.


5. Action: If necessary, open the mouth and hold it open with a dental gag or needle case between the teeth.


    Rationale: This will prevent the mandible from obscuring the view of the upper arcades.


6. Action: Centre the beam (indicated by the cross wires in the light-beam diaphragm) on the point of interest (Fig. 6.6).



    Rationale: This will vary according to the case. If viewing a dental arcade, centre the beam half way along it.


NB This view can be varied according to what you want to investigate in more detail having identified a lesion in other positions.



Procedure: Dorso-ventral intra-oral view: This is used for the investigation of the premaxilla, upper incisors, rostral portions of the maxilla and premolar teeth, and for the nasal cavity.



1. Action: The animal must be fully anaesthetized.


    Rationale: The cassette is placed in the animal’s mouth – anaesthesia will prevent it chewing the film.


2. Action: Place the animal in sternal recumbency and extend the neck. Support with foam pads (Fig. 6.7).



    Rationale: The position of the head is straighter if the neck is extended.


3. Action: Place a sandbag over the neck.


    Rationale: To prevent rotation of the head.


4. Action: The endotracheal tube should be tied to the lower jaw.


    Rationale: To keep it in place and to allow correct placement of the cassette.


5. Action: Place a non-screen film above the tongue and the tube as far as possible into the mouth.


    Rationale: Non-screen film will provide better definition, particularly of the maxillary arcades, but screen film in flexible oral cassettes may be used for the incisors.


6. Action: Place a left or right marker beside the head as appropriate.


    Rationale: It is important to be able to identify the location of any lesions.


7. Centre the beam as follows:




Procedure: Ventro-dorsal intra-oral view: This is used for investigation of the body of the mandible, the rostral parts of the horizontal rami of the mandibles and the mandibular incisor teeth.



1. Action: The animal must be fully anaesthetized.


    Rationale: To enable accurate positioning and to prevent the animal chewing on the cassette placed in its mouth.


2. Action: Place the animal in dorsal recumbency and support it.


    Rationale: Use a plastic trough and / or foam wedges to keep the body from tilting.


3. Action: Push the tongue firmly to one side or place it symmetrically in the centre of the oral cavity.


    Rationale: To avoid any confusion when evaluating the radiograph.


4. Action: Use only radiolucent gags in the mouth.


    Rationale: To prevent the image appearing on the radiograph.


5. Action: Position the endotracheal tube away from the point of interest.


    Rationale: To prevent it interfering with the image.


6. Action: Place the cassette as far as possible into the mouth between the tongue and hard palate.


    Rationale: Flexible oral cassettes (screen film) are used to enable the film to be pushed as far back in the mouth as possible. Non-screen film will provide better definition.


7. Centre the beam as follows:




Procedure: Ventro-dorsal open mouth oblique rostro-caudal view: This is used to investigate the nasal passages and allows better visualization of the more caudal regions than the dorso-ventral intra-oral view.



1. Action: The animal must be fully anaesthetized.


    Rationale: To allow adequate relaxation and thus accurate positioning.


2. Action: Place the animal in dorsal recumbency with the head as straight as possible. Use a trough or wedges to hold the body in position without tilting to one side.


    Rationale: If the body tilts it will tilt the head. This will lead to inequalities in the nasal cavities and may lead to a misdiagnosis.


3. Action: Open the mouth as widely as possible and prop open with a dental gag or a needle case between the caudal molars or the carnassials.


    Rationale: This prevents the mandibles obscuring the view of the maxillae.


4. Action: Pull the tongue and the endotracheal tube to one side.


    Rationale: To prevent them obscuring the view of the maxillae.


5. Action: Place the cassette under the head.


    Rationale: So that the nasal cavities are as close as possible to the cassette thus preventing excessive magnification.


6. Action: Centre the primary beam (indicated by the cross wires in the light-beam diaphragm) on a point halfway along the length of the hard palate and collimate to include the whole hard palate.


    Rationale: This will provide good visualization of the whole of both nasal cavities.



3 Thorax


Radiography of the thorax provides a guide to the diagnosis of many conditions because it allows examination of the lower respiratory tract, the lung tissue and the pleurae, the heart, the mediastinum, the diaphragm and the thoracic wall. Problems may occur because the chest wall and diaphragm are always moving and steps must be taken (e.g. reducing the exposure time) to minimize movement blur.




Procedure: Lateral thorax:



1. Action: Place the animal in right lateral recumbency.


    Rationale: This is the conventional position for a thoracic radiograph because the heart lies in a more consistent position, there is more air-filled lung between the heart and the chest wall, which gives better cardiac detail, and the diaphragm obscures less of the caudal lung field.


2. Action: Extend the forelegs and secure with ties tied to cleats on the table or with sandbags.


    Rationale: Extending the forelegs prevents the soft tissue mass of the shoulder girdle impeding the view of the thoracic contents.


3. Action: Place a pad under the sternum.


    Rationale: To prevent rotation of the chest and ensure that it remains in the same horizontal plane as the spine. This prevents distortion of the thoracic structures.


4. Action: Place sandbags over the neck and the hindlegs.


    Rationale: The hindlegs should be secure, but should not be tied as this leads to rotation of the chest.


5. Action: Centre the beam (as indicated by the cross wires on the light-beam diaphragm) midway between the sternum and spine and level with the caudal border of the scapula (approximately the 5th rib).


    Rationale: This ensures that the primary beam coincides with the base of the heart.


6. Action: Collimate the beam to include the cranial thoracic inlet, the edge of the sternum, the thoracic spine and the full extent of the diaphragm.


    Rationale: All parts of the lung will be included. In large breeds it may be necessary to cover the area with two overlapping films.


7. Action: Expose on inspiration.


    Rationale: This ensures that the diaphragm is flattened, creating maximum space in the cavity for the organs. The lungs are fully inflated, which provides better contrast between the air in the lungs and the soft tissues.

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Related posts:

  1. First aid and other emergencies
  2. Medical diagnostic and treatment techniques
  3. Laboratory techniques
  4. Restraint, handling and administration of medication

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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on Diagnostic imaging

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