CALCIFICATION, OSSIFICATIONS, MATRICES, RADIODENSE FOREIGN BODIES, AND THEIR MIMICS
- Calcification, ossification, and the presence of a matrix are frequently crucial to the diagnostic process.
- Calcification, ossification, and matrices are more easily and accurately appreciated on computed tomography.
- Radiodense foreign bodies, blood, impacted secretions, and reactive changes may mimic calcification and ossification. This may impede or enlighten the diagnostic process.
- Protocols for imaging approaches to clinical problem solving must take into account the relative strength of computed tomography over magnetic resonance imaging for identifying calcification, ossification, matrices, and foreign bodies that are radiodense on computed tomography.
The presence of calcification, ossification, or their mimics is often not important if such findings are physiologic or just part of the natural aging process (Fig. 12.1). Sometimes, calcification, when present, is not a critical factor in evaluating a related disease process (Fig. 12.2). However, sometimes calcification can prove crucial for differential diagnosis and medical decision making (Fig. 12.3). The cause of some calcifications often remains unknown. In all cases, calcifications and all calcified matrices are better seen and appreciated for what they are on computed tomography (CT) and plain films than magnetic resonance (MR), and this becomes a critical issue for planning the proper imaging approach to clinical problems. The latter point is the main issue of interest in this introductory material. Calcifications are marginally appreciable on ultrasound.
THINGS THAT MAY LOOK CALCIFIC BUT ARE NOT
The range of CT density that might appear on images as calcific is wide. Sometimes, dense keratin debris in lymph nodes, densely desiccated secretions, or metals other than calcium manifest a similar density, signal intensity, or pattern of echogenicity— things that may look like calcifications but are not that at all (Figs. 12.4A,B). Small metallic foreign bodies and even wood can mimic calcific density (Fig. 12.5) on CT.1 Focal areas of contrast enhancement and even normal vessels may mimic or mask calcification from accurate observation. Acute or subacute blood products can be mistaken for calcification (Fig. 12.5B). Sometimes, the foreign bodies are actually calcified even though they originate extrinsically; this appearance may evolve over time (Fig. 12.6). It is particularly difficult to recognize foreign bodies on magnetic resonance imaging (MRI) compared to CT; however, MRI may provide clues to their nature. The variations that such findings can create on MRI can be even more confusing (Fig. 12.7).
CALCIFICATION AS SEEN ON COMPUTED TOMOGRAPHY AND MAGNETIC RESONANCE IMAGING
Calcification and ossification are often very difficult to recognize on MR studies as such, but calcification can be anticipated in certain situations. A focal or diffuse area of markedly diminished signal intensity or signal void on all pulse sequences suggests calcification, ossification, fibrosis, or very chronic blood products (Fig. 12.8). Other possible explanations for such decreased signal include flow void, air, surgical clip, foreign body (Figs. 12.6 and 12.9), or other very proton poor substance or at least something lacking enough mobile protons to create a signal (Fig. 12.10). Small foci of calcification or ossification (several millimeters) are very likely to go unnoticed on standard spin echo (SE) sequences depending on the nature of the surrounding tissues. Gradient echo techniques can improve the rate of detection, or at least suspicion, of calcifications since these sequences are highly sensitive to the local field distortions produced by calcium-related susceptibility effects (Fig. 12.11 and Chapter 2). Gradient echo and susceptibility-weighted sequences are useful in the brain for this purpose, but their value is sometimes limited in the extracranial head and neck because these sequences are more prone to magnetic susceptibility artifacts that variably degrades image quality outside the brain depending on the adjacent anatomy and its vulnerability to susceptibility artifacts such as a mandible full of metal due to dental appliances.
If the diagnosis is already known, then the MR depiction of the process may be adequate or yield more information than the CT. However, whenever such suggestive areas of consistently diminished signal are present and the nature of the low-intensity area is important for treatment or diagnostic purposes, a CT should be done to clarify the situation (Fig. 12.12).
Calcium may also be the cause of areas of increased signal intensity on T1-weighted images. This has been observed in patients with neurofibromatosis as areas of increased signal intensity in the basal ganglia (Fig. 12.13).2 One group2has demonstrated that the effect is probably related to the physical state of the calcium in these regional areas of disordered brain maturation and thus altered MR signal. A similar situation may exist in calcium in solution in a fluid collection.
In general, the calcifications and areas of ossification on CT and plain films are very easily recognized as areas of discreet increased density with a morphology conforming to the source of their origin (Figs. 12.14 and 12.15). Occasionally, they may be confused with retained noncalcific foreign bodies, acute blood products, desiccated impacted secretions, or focal contrast enhancement (Fig. 12.16 compared to 12.5). If important to diagnosis, pre- and postcontrast CT images may be necessary. Currently, this is not a routine in any CT protocol except the initial evaluation of a potential retinoblastoma.