Christopher R. Lamb, MA, VetMB, DACVR, DECVDI, ILTM, MRCVS
Department of Veterinary Clinical Science, The Royal Veterinary College, North Mymms, Herts, UK
The radiologic (roentgen) signs are abnormal:
Memorize this list and apply it routinely when considering any potential abnormality that you have identified in any diagnostic image. Although this sounds very basic, these abnormalities are the basis for all radiologic diagnoses and this list of criteria applies equally well to radiography, ultrasonography, CT and MRI.
Ability to count structures radiographically depends on their correct identification. This is not always possible because, for example, many normal anatomical structures are not visible radiographically (e.g., adrenal glands). Conversely, certain structures that are normally visible may be obliterated as a result of physiological variants (e.g., lack of body fat) or disease.
Potential causes of abnormal number in the abdomen:
Congenital absence (rare)
Small organ, e.g., kidney, bladder
Organ not recognised, e.g., because it is deformed
Enlargement of normally invisible structures
Lesion mimicking normal organ
Paraprostatic cyst mimicking bladder
Adrenal mass mimicking kidney
Potential causes of abnormal position in the abdomen:
Volvulus, e.g., stomach
Torsion, e.g., spleen
Problems with Assessment of Organ Position in the Abdomen
Radiographic descriptions of organ position and borders are often limited because they are deduced from relatively few projections, say lateral and ventrodorsal.
Abnormal position, i.e., as a result of displacement of an organ, may be the principal radiographic sign that a mass is present, and is a particularly important sign if the mass itself is not visible. Consider a dog with a hepatic mass: caudal displacement of the stomach may be the only radiographic abnormality. The caudal surface of the liver that contacts the stomach is not normally visible radiographically; therefore the presence of a mass cannot change its appearance. On the other hand, the position of the lumen of the stomach is influenced by the position of the caudal border of the liver and is normally visible because of the gas it contains. Therefore, a change in the position or shape of the gastric lumen is likely to be the only sign of a hepatic mass bulging from the caudal surface of the liver.
It is often possible to make an assessment of the origin of a mass by the direction of displacement of adjacent organs. Mobile abdominal viscera, principally the small intestine, are displaced away from the point of origin of the mass. For example, a mass originating by the bladder will be expected to displace small intestine cranially; a mass originating in the mesenteric root will displace the intestine peripherally. The larger the mass, the more marked any displacement is likely to be. Even relatively fixed organs, such as the kidneys may be displaced by a gradually enlarging mass.
Potential causes of abnormal size in the abdomen:
Compensatory hypertrophy of contralateral kidney in unilateral renal disease
Infiltration (usually diffuse)
Neoplasia, e.g., lymphoma
Fat, e.g., affecting the liver in diabetes mellitus
Glycogen, e.g., affecting the liver in hyperadrenocorticism
Hypovolaemia, e.g., small liver in hypoadrenocorticism
Problems Associated with Recognition of Abnormal Organ Size in the Abdomen
We can recognise abnormal organ size only when we know how fairly precisely big the normal organ is in an animal of the same body weight as the patient in question; however, there are marked variations in conformation between different breeds of dog and between individuals of the same breed that tend to limit the usefulness of organ measurements. As a result there are very few measurements that are useful aids to diagnosis--don't bother trying to measure the liver or spleen, for example.
Rather than a direct measurement, radiographic measurements of organ size frequently use a bone landmark as a reference point to take account of the effects of magnification and variations in body size, e.g., the canine kidney is about 3xL2.
Note that the silhouette of an irregular object varies greatly depending on its orientation whereas the silhouette of a rounded, regular object does not. Perhaps radiologists are fortunate that many important pathological structures, including primary neoplasms and metastases, are roughly spherical so may be recognised whatever the radiographic projection.
Only a very uniform, diffuse disease (e.g., congestion/oedema, acute inflammation) is likely to change the size of an organ without affecting its shape. Whereas a change in shape is usually the result of a focal or multifocal lesion(s).
Potential causes of abnormal shape in the abdomen:
Physiological enlargement, e.g., urinary bladder
Inflammatory lesion, e.g., abscess
Obstruction causing dilatation
Cyst (or pseudocyst)
Torsion, e.g., spleen, retained testicle
Impingement by adjacent structure
Summary of potential opacity changes in disease.
Change in opacity
Intraluminal gastric mass
Bone in GI tract
Hollow foreign body
Bone cyst, lytic lesion
A difference in opacity must be present between adjacent structures for their edges to be visible radiographically.
Imagine a typical radiograph of an animal. The bones are visible because they are more opaque than the adjacent tissues (cartilage, muscle, ligament) as a result of their higher physical density and calcium content. The edge of the bone (= periosteal surface) is a bone: soft tissue interface. Changes in the appearance of bone margins, such as irregular or indistinct margins, which may represent a periosteal reaction, are important signs of diseases affecting the bone.
Similarly, the edges or margins of various abdominal organs are normally visible because they are more opaque that the surrounding fat, which appears as a darker grey around the liver, spleen, kidneys, etc. If no fat is present, for example if the patient is very thin due to chronic illness, the difference in opacity that produced the organ edges initially is no longer present and therefore the organs are invisible. If the fat is infiltrated by inflammatory cells or displaced by peritoneal fluid the margins of the abdominal organs may become indistinct or blurred. Conversely the margins of abdominal organs may be particularly sharp in an animal with pneumoperitoneum. Hence in the abdomen, margination of the various organs tends to reflect changes in their surroundings rather than organ disease per se.
Potential causes of abnormal margination in the abdomen:
Decreased clarity of organ margins
Adhesions (chronic peritonitis)
Adherent clots (blood, proteinaceous)
Lack of surrounding fat
Chronic disease causing weight loss