Thoracic and abdominal radiology pearls (Proceedings)

We often have patients present to us for coughing. Our job is to determine if it is heart disease or pulmonary disease. So the purpose of this lecture is to review thoracic radiology and some radiographic pitfalls you should avoid. To show the importance of differentiating between heart and pulmonary disease, the Journal of Prehospital Emergency Care evaluated the appropriateness of diurectic administration in humans. They found that diuretics were inappropriately administered in 42% of cases. Administration was potentially harmful in 17% of patients. In patients with pneumonia, dehydration, and sepsis diuretic administration is contraindicated. One dose of diuretic decreases mucociliary apparatus function in pneumonia.

In humans one dose of diuretic increased mortality rate of patients without cardiac disease in 7 of 9 patients or 78%. Not proven in animals....but could it be the same for our patients?

So our coughing patient again. We take thoracic radiographs and need to evaluate the cardiac silhouette first. We ask , "Is it big or not???" Remember normal cardiac dimensions on the lateral view the height of the heart is ⅔ of thoracic cavity. The height of the heart evaluates the left heart – specifically the left ventricle. Also on the lateral view, a normal heart will be 2.5-3.5 intercostal spaces in width. The width of the heart evaluates the right heart. On the DV/DV view, the distance between the heart and chest wall should be relatively equal on each side. The heart is less than 50% of thoracic cavity on the DV/VD view. The trachea deviates away from the vertebral bodies on the lateral view. The left and right main stem bronchi are superimposed just caudal to the carina. The caudal cardiac waist curves cranially to the carina on the lateral view. A lot of people talk about "sternal contact" to evaluate the heart. This is quite a variable finding and Increased sternal contact may be breed or age associated. Obliquity can make evaluation of the heart difficult. The trachea can artifactually be parallel to the vertebral bodies with obliquity. Mild obliquity can cause the left main bronchi to be dorsally displaced and also make the caudal cardiac silhouette straight. You can see that a lateral view is obliqued by looking at the rib heads, they should be superimposed. Obliquity of the ventrodorsal view can cause the right (or left) heart appear larger. Make sure you look at the dorsal spinous processes to assess whether the VD view is straight. They should appear as triangles centered over the vertebral bodies.

Next we'll look at the thoracic vasculature. It is important to be able to look for congestion. Cardiac patients will have large vessels or a large vein, whereas patients with pneumonia or sepsis will have small vessels. Normal pulmonary vasculature on the lateral view is easiest to evaluate in the cranial ventral thorax. The artery is dorsal and the vein is ventral and between these two is a bronchus. The artery and vein should be similar in size. These should be compared to the width of the proximal 4th rib. You should compare the vessels to the smaller rib (which is the down side rib – there is less magnification). In patients with pulmonary venous congestion, the vein is larger than the artery and larger than the proximal 4th rib. Another vessel that you can look at is the caudal vena cava. Normally it extends from caudal to cranial and angles ventrally. When the heart is enlarged, the caudal vena cava is either horizontal or angles dorsally between the diaphragm and caudal cardiac waist. And lastly when there is pulmonary venous congestion, you are evaluating the left heart. If right heart disease is present, look for hepatic venous congestion. IN order to do this, let's review the location of a normal liver silhouette. On the lateral view the gastric axis should be parallel with the intercostal spaces. IN deep chested dogs, the gastric axis can be perpendicular to the vertebral bodies and still be normal. On the VD view, the gastric axis is normally perpendicular to spine at the level of the 10th - 12th thoracic vertebral bodies. By comparison to the normal liver, when hepatic venous congestion is present, the gastric axis is caudally deviated and the caudal hepatic margins are rounded. Ascites may be present as well.

So now we are onto evaluation of the pulmonary parenchyma. When we see something in the lungs we have to determine, "Is the pulmonary opacity real or not?" Pulmonary opacity changes if the radiographs are obtained at the incorrect phase of the respiratory cycle. With inspiratory radiographs, the lumbodiaphragmatic recess extends to the 12th thoracic vertebrae on the lateral view. The accessory lung lobe is large and the cupula of the cranial lung lobe extends beyond the first rib. The retrosternal lucency extends to the 5th sternabrae. On the VD view, the diaphragmatic cupula is caudal to T8 on inspiratory films and the costodiaphragmatic angel to T10. Now a review of technique. For good thoracic soft tissue evaluation many shades of gray are required. High kVp and low mAs are used to obtain these shades of gray. By comparison, with evaluation of the musculoskeletal system we want fewer shades of gray, just mainly black and white. Low kVp and High mAs are used. Overexposed films decrease diagnosis of pulmonary infiltrates and under-exposed films artifactually increase diagnosis of pulmonary disease. Why do we collimate? We will now discuss the "Chabdomen" and why it doesn't work. What is a "chabdomen"? Chest and abdomen in one film. The first problem is that thoracic films are supposed to be obtained during the inspiratory phase of the cardiac cycle, while abdominal radiographs are supposed to be obtained during the expiratory phase of the cardiac cycle. Neither the thorax nor abdomen are evaluated well......Next since you are taking a radiographs of the whole patient, scatter radiation is increased due to inadequate collimation. Scatter radiation is radiation produced by interaction of x-rays with the body tissues. 50-90% of the total number of photons emerging from the patient is scatter radiation. Only 1% of the primary x-rays actually penetrate the patient to create the useful image, increasing scatter radiation detracts from film quality and it contributes no useful information. FIELD SIZE IS THE MOST IMPORTANT FACTOR IN THE PRODUCTION OF SCATTER RADIATION. Radiographs taken of the whole body can miss pulmonary metastasis due to scatter radiation degrading the overall image quality.

A few words on pulmonary metastatic neoplasia. Don't forget that without 3 radiographs when evaluating for pulmonary metastatic neoplasia (both laterals and a DV or VD), up to 15% of pulmonary metastatic disease can be overlooked. If you are unsure if the pulmonary nodule you see is a nipple or not, perform a "nippleogram". Place barium on the nipple in question and repeat the VD film. Pulmonary osteomas are mineralized opacities that are less than 3mm in diameter and they can be diffuse throughout the lungs. Do not confuse these with metastatic pulmonary neoplasia. On careful evaluation, these structures are extremely opaque for being so small. Pulmonary metastatic neoplasia is usually round and somewhat variously sized, not all measuring less than 3mm.

In the abdomen, this is a more random group of things to remind you of. Many of these are easily performed techniques/studies that will aid in your diagnostic ability.

In the abdomen, a VD view is always indicated. DV views are useless and really non-diagnostic. All the organs move with gravity and become centered on midline and you can't really evaluate the organs effectively. If you have a dyspenic or painful patient and you need to look at the abdomen, take a lateral view only to begin with. Taking a radiograph with the dog sitting sternal (yes with the legs underneath them) really only serves to expose your staff to radiation un-necessarily.

When you are obtaining radiographs of the caudal abdomen to evaluate dysuria/ stranguria, don't forget how long the urethra is in a male dog. The femurs overly the urethra on a neutral lateral view. Fabella also summate with the urethra in this view, making evaluation difficult. It takes two lateral views to fully evaluate the urethra. The "legs pulled cranially" view (ie the butt shot) and the "legs pulled caudally" view.

You notice a mass effect in the cranial abdomen on your thoracic films. What should you do? First take abdominal radiographs centered on the abdomen without the thorax included – is the mass still there? If so, take opposite lateral view. If it is a "pyloric mass" or fluid filled pylorus on the right lateral view, it will become gas filled on the left laterally recumbent view. By comparison, a mass in the tail of the spleen will be apparent in the cranioventral abdomen on both lateral views.

Abdominal compression techniques are useful to remember when evaluating for cystic calculi and other structures in the caudal abdomen. If bowel is overlying the bladder use a wooden spoon to push the other structures out of the way so that you can effectively evaluate without superimposition of other structures.

Now some non-pathologic findings/ structures to be remember. Ventral to the sixth lumbar vertebrae is the deep circumflex iliac arteries. They exit the aorta perpendicularly compared to the external iliac arteries which leave at an angle from the aorta. This causes the deep circumflex arteries to be increased in opacity on lateral views. These are located relative "high" up in the abdomen. Do not confuse these normal anatomic structures with ureteral stones. Ureteral stones are usually located slightly "lower" in the retroperitoneal space and are actually bone opacity. Aseptic fat necrosis used to be called a "cholesterol cyst". This is an incidental finding in the mesentery of the abdomen (and rarely in the fat of the thorax). This appears as a rounded, irregularly shaped less than 3cm diameter egg shell mineralized structure that is not associated with any abdominal organ. These are hard to find with ultrasound and really you don't need to go look for them as they are non-pathologic. And finally, adrenal mineralization in feline adrenal glands is common, often bilateral, most often appears as punctuate mineralization cranial to the kidneys, and usually is seen on both radiographic projections.

By comparison, adrenal mineralization in dogs is pathopneumonic for adrenal neoplasia. Prostatic mineralization also is not seen in normal patients. It should be considered adenocarcinoma until proven otherwise.