Practical Demonstration Topographical Features of the Thoracic Cavity
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What do you think the different options might be for restraining animals for thoracic radiography?
- Unconscious - GA
- Conscious - sedation / unseated animal is quiet or sick
- Physical restraint - positioning aids eg sandbags, ropes, etc
- Manual restraint - held by person in protective lead lined clothing
In the UK which option would only be indicated in the case of an animal with severe dyspnoea/respiratory distress and why is this the case?
Manual restraint - risk to human from ionising radiation should be minimised and permitted dose when performing veterinary radiology is zero so need very good reason to do this in the UK
How do you think the presence of dyspnoea/respiratory distress would affect the quality of the resulting radiograph?
Poor quality image due to movement blur. Sedating/anaesthetising animals with respiratory distress may be contraindicated but the converse argument would be that at least you would be more likely to get a diagnostic radiograph
Why do you think the dorsoventral view is more commonly used in practice than ventrodorsal view?
Conscious dogs tolerate this position much better and it is easier to position. Also dogs breath better when in sternal recumbency rather than dorsal recumbency which is an important consideration when imaging dogs with respiratory problems.
Why is it important to pull the forelegs as far cranially as possible when radiographing the thorax?
To remove the triceps muscle mass from overlying the cranial region of the thorax on the radiograph
The ventral margin of the lung field looks different in the right and left lateral view - why?
The size and shape of the cardiac notch differs between the right and left lungs and this difference is visible in radiographs
Why are the blood vessels in the lung visible?
They are a combination of fluid (blood) and soft tissue (vessel walls) and so are radio-opaque (grey) while the surrounding lung is airfield and therefore radiolucent (dark)
When a dog is positioned in lateral recumbency for any length of time what do you think happens to the two different sides of the lung?
The lower lung becomes compressed and congested therefore is smaller and more opaque. The uppermost lung increases in size and hyper inflates to compensate so becomes darker.
If a dog had a mass in its left lung field do you think it would be easier to see in a right or left lateral view?
Right lateral view. The left side of the lung is uppermost and so hyper inflated so there is good contrast between the air filled lung and soft tissue mass. The right side of the lung becomes compressed and contains less air so soft tissue masses would not be as well delineated.
The heart appears to be a different shape int her goth and left lateral view - why?
The heart is fixed in position at the base by the great vessels but the apex is mobile and so in lateral recumbency the apex of the heart moves with gravity. The apex naturally sits to the left so when the animal is in right lateral recumbency the apex falls towards midline while in left lateral recumbency it falls towards the dependent thoracic wall
Why do you think right lateral views are more commonly performed in practice than left lateral views?
The apex of the heart falls towards midline so there is less distortion of the cardiac shadow allowing better assessment of the heart
Why do you think blood within the heart is not visible on radiographs?
Fluid produces the same opacity as soft tissue on radiographs so the fluid and myocardium are superimposed onto each other
When assessing a dog with cardiac abnormalities what information does radiography provide about the heart?
Size, shape, position, specific chamber enlargement, evidence of congestive failure e.g. pulmonary oedema
What information does radiography not provide about the heart?
Internal structures of the heart, myocardial function, valve incompetence
Why do you need to clip the dog and apply ultrasound gel before starting the scan?
Remove air from between the transducer and skin surface to allow penetration of the beam and therefore produce good quality images
The heart is located within a bony ribcage. How do you think we can overcome the fact that the ultrasound beam cannot penetrate through bone in order to scan the heart?
Use a transducer with a small face or area of contact with the skin surface and direct the beam through the intercostal spaces between ribs.
What anatomical feature of the lungs can we use to allow the ultrasound beam to reach the heart without air filled lung getting in the way?
The cardiac notch
Echocardiographic image quality is best when the dog is in lateral recumbency and the transducer is placed against the dependent thoracic wall. Why do you think this might be the case?
The cardiac apex is mobile and falls under gravity towards the dependent thoracic wall. It falls into the cardiac notch and pushes the lung out of the way. The lower lung also becomes congested and reduces in size so the cardiac notch becomes bigger and there is less interference from the lung.
Do you think we would be able to image the heart from the uppermost thoracic wall?
It is possible but image quality is poor becomes the heart falls way from the transducer. The uppermost lung also hyper inflates therefore the cardiac notch becomes smaller and there is more interference from the lung.
How do you know where to place the transducer to start the scan?
Feel for the apical beat and place the transducer over it
The orientation of the heart is different in images taken from the left and right sides of the thorax - why?
- In right lateral recumbency the apex of the heart falls towards middling and this allows the beam to image the heart from the side.
- In left lateral recumbency the apex of the heart falls further to the left so the heart becomes more vertically orientated and this allows the beam to image the heart along its length from apex to base
Which side of the heart is in the near field (top) of the ultrasound image in the right thoracic views?
Right side - the nearer the transducer is to a structure the further up the image it is displayed and when imaging the right thoracic wall the right side of the heart is nearest to the transducer
When assessing a dog with cardiac abnormalities what information does echocardiography provide about the heart?
Chamber size, myocardial function, blood flow and leakage through valves, disease of internal cardiac structures
What information does it not provide in cases with left sided cardiac failure?
Evidence of congestive changes in the lungs e.g. pulmonary oedema
In cases with right sided heart failure which abdominal organ might you find changes in on ultrasound examination?
In the left thoracic views which direction is blood flowing relative to the transducer? Do you have any idea why these views are therefore ideal for measuring blood flow through the heart?
- Flow through the AV valve is directly towards the transducer and flow through the aortic valve is directly away from the transducer
- Doppler ultrasound most accurately records the speed of flow when it is directly towards or away from the transducer
Which major arteries arise from the brachiocephalic trunk in the dog?
Right and left common carotid aa and right subclavian a. (The left subclavian a arises independently from the aorta just distal to the origin of the brachiocephalic trunk)
Which four major arteries arise from the right subclavian artery in the dog before it continues as the axillary artery?
Vertebral artery, costocervical artery, superficial cervical artery and internal thoracic artery
From a left lateral view where is the oesophagus positioned relative to the aorta just before they penetrate the diaphragm?
Ventral and to the right
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