Module 2

Describing a radiograph and interpreting what you see

Dentists have a unique position among the health professions in that, with the exception of MRI and CAT technology which must be executed and interpreted in the hospital, they produce, develop and interpret their own radiographs. This necessitates early introduction and training in radiographic interpretation for dental students. In oral and maxillofacial pathology, many of the lesions you will learn about are discovered as manifestations on radiographs.

Radiographs, as you know, consist of black and white and intervening gray-scale, two dimensional images of bone and overlying soft tissues. Most radiographs depend on the principle of the "focal plane" to deliver a clear image of the object. Interpretation of changes visible on radiographs depends on the dentist's ability to recognize normal and abnormal variations of gray between black and white and understand what these changes mean in terms of pathophysiologic activity and function.

Describing and Interpreting

The first step in learning to interpret radiographs is to learn to describe what you see. In the sections below, both description and interpretation are dealt with at the same time. Distinction is made between description (simply reporting what something looks like in terms of shape, measurement etc) and interpretation (lending a meaning or an alleged meaning to a specific appearance we see on a radiograph). We describe radiographic lesions similarly to the way we describe soft tissue lesions:

Shape

We begin with shape. Is the lesion round, oval, square or whatever shape? Some specific lesions are associated with a specific shape. Is the shape regular or irregular? Regular shape is often associated with even growth. Irregular shape is associated with uneven growth such as you might see with a lesion that is growing from several different centers, or a malignant lesion which has no coordinated growth pattern.

Size

Quite simply we always measure the size of a lesion in millimeters or centimeters. If the lesion is single and regular or irregular in shape, then measure it at its greatest diameter in two dimensions roughly at 90 degrees to each other. If the lesion is several parts close together, we measure the greatest two dimensions of the several parts taken as a single unit. If the lesion is multiple and clearly separated into individual lesions, then a two dimensional measurement of each lesion is made. If you are not good at estimating dimensions, and many are not, carry a millimeter ruler with you, or use a perio probe as a guide.

Anatomic location

Again, simply locate the lesion in as exact a location as possible. For example, even in an edentulous mandible, #32 area is more desirable than right mandible. If the lesion is clearly associated with a specific anatomic structure, describe the location in relation to that. For example, distal to partially impacted #31,or periapical to #6.

Degree of lucency or opacity

In this case, we describe the degree to which the lesion is absorbing the photon beam. A fully radiolucent lesion is black on radiograph and is allowing most of the radiation to pass through it. Portions of the maxillary sinus are fully radiolucent because they contain air. A fully radiopaque lesion is "white" on a radiograph. That is, it absorbs most of the radiation. The best example of this is gold or silver restorations. There are many shades of gray in between these two extremes, and at some point in between radiolucency stops and radiopacity takes over. Much of the interpretation of this point rests with experience, and learning by mistakes. However, we do use rough rules of thumb, such as the dentin and pulp in teeth. If it is whiter than the pulp, it is considered radiopaque, and if it is darker than dentin it is radiolucent. In bone, if a lesion shows up as darker than the bony trabeculae, it is considered radiolucent. If it shows as whiter than the trabeculae it is considered radiopaque. Some lesions contain both radiopaque and radiolucent structures. These are termed mixed radiolucent/radiopaque lesions. Not so strange, is it?

Interpretation of radiolucency and radiopacity is general and imprecise. Radiolucency means that bone is being destroyed (resorbed). This makes the hole in the bone less dense than the surrounding bone, thus radiolucent. The destroyed bone may or may not be in the process of being replaced by a less dense tissue such as granulation tissue or neoplastic tissue. Radiopaque lesions indicate that a substance more dense than the surrounding bone is being or has been laid down. Sometimes this denser material is harder, more calcified bone, but it also may be very dense, fibrous connective tissue with or without calcification, or perhaps a denser but inferior bone. In the case of mixed radiolucent/radiopaque lesions, bone may be in the process of being destroyed and being replaced by inferior bone or connective tissue almost simultaneously. This may be confusing and imprecise, but you can practice by looking a numerous radiographs.

Recognizable structures

Can you recognize specific structures within a lesion? The best example is the odontogenic tumor called Odontoma. This tumor contains fragments of dentin, pulp, enamel and frequently miniature teeth. These structures are clearly identifiable in an adequate radiograph.

Single or multiple

If the lesion is one of a kind, it is single, simple enough. Where the judgement comes in is with multiple lesions. If several lesions exist close enough to each other to form a unit, they are considered a single lesion with multiple parts. However, if the lesions are clearly related by their radiographic features, but are far enough apart that they should be considered separately, they are multiple related lesions. How far apart? you ask. There is no fixed point, and much has to do with how you interpret the behavior of the lesion and the similarity of their radiographic characteristics. For example, a periapical radiolucency on three different teeth whether the teeth are adjacent or in three different sextants, would be considered multiple periapical pathoses. A periapical radiolucency on #4 and a periapical radiopacity on #5 would be considered 2 single pathologic entities even if they are only millmeters apart. An odontoma. with a dozen clearly defined small tooth structures would still be considered a single mixed radiolucent/radiopaque lesion. As you can see, it takes some practice and getting used to, but you'll eventually catch on. Mistakes are not fatal and may only cost you time.

Unilocular or multilocular

These terms apply to radiolucent lesionsUnilocular is a radiolucent lesion with a single compartment, and multilocular is a radiolucent lesion with several of multiple compartments.

Quality of the border

Here we use the terms well defined and poorly defined. Well defined means the border is clearly visible around most of its perimeter, given a radiograph of acceptable contrast and density. A well defined border can be irregular, or regular, but it needs to be reasonably visible and demarcated, given the condition of the film. A poorly defined border is one that is not well demarcated. It can be partially well defined, but in areas it seems to fade into the surrounding structure (usually bone). It can be totally poorly defined and fade into the surrounding structure all the way around. Poorly defined does not necessarily need to be irregular. although many poorly defined lesions are also irregular. Well defined borders often mean the lesion is expanding (as opposed to invading) slowly and evenly, giving the surrounding bone time to build up cortex in an attempt to resist the expansion. Benign lesions and benign growth patterns tend to be well defined on radiograph. Poor definition may, but not always, imply aggressive. faster growth pattern in the form of invasion of surrounding bone. Invasion is often a characteristic of malignant growth, however it may also represent spreading infection. As you can see, the lines are not always clear when it comes to interpreting or describing some aspects of lesions as they appear on radiograph.

Cortical involvement

What is the relationship between the lesion your are describing and the cortex of the bone that contains it? Does it involve cortex at all? Yes or No. If it does involve cortex is the involvement invasion or expansion? Is the lesion in proximity to the bony cortex and apparently causing a cortical reaction without direct invasion (such as a thickening or thinning)? Again, benign growth tends to remain localized within medullary bone and at most expand cortex or cause some cortical thickening, though this is not a hard and fast rule. Malignant growth and aggressive infections tend to invade, expand and perforate cortex. Interpretation is not an exact science, therefore remain flexible and 'report and interpret what you see.

Lamina dura

We speak of an intact lamina dura, or that we see loss of lamina dura. This change is related to the cortical involvement. Lamina dura, as you know, is cortical bone that lines the tooth socket. Thus if a lesion invades cortex, it might, if nearby, invade and thus decrease the densi1y of lamina dura making it less visible or invisible. This is what we call loss of lamina dura. Benign lesions displace teeth possibly compressing lamina dura, and benign lesions of the cementum displace lamina dura out away from the tooth, but the lamina dura will be preserved. Malignant lesions frequently destroy lamina dura of adjacent teeth which results in loss of lamina dura on a radiograph. Loss of lamina dura is also seen in non-malignant lesions that change the nature of the bone

Periodontal ligament (PDL)

If the lamina dura has been compromised by a malignant tumor or an infection, and the PDL space invaded, the radiographic appearance is of a wider, more prominent PDL space (which you know is always radiolucent) and loss of lamina dura (which is always radiopaque). Infection, pressing out from the opening of the pulp canal at the root tip can also expand the PDL space and give rise to a radiolucent, widened PDL on radiograph.

Root resorption

Inflammatory, malignant, and some aggressive benign lesions can cause root resorption in adjacent teeth. Root resorption can also be caused by caries, periodontal disease, and orthodontic tooth movement. Sometimes it is idiopathic. Generally, there is no consistency in conditions causing root resorption. For example a malignancy will resorb roots in one patient and the same disease will not resorb roots in, another patient. Thus, root resorption is significant when present, but if it is not present, it does not necessarily mean the lesion is less aggressive or benign.

Take the quiz

This is the end of the unit on Describing Radiographs and Interpreting the Changes You See. Now take the quiz on describing and interpreting radiographic changes.