NATURALLY OCCURRING PARAMAGNETIC SUBSTANCES INCLUDING BLOOD PRODUCTS.
- How to understand the effects of naturally occurring paramagnetic substances on computed tomography and magnetic resonance images.
- The location, origin, and evolution of abnormal blood products in the head and neck region have a significant impact on how they appear on computed tomography and magnetic resonance images.
- Paramagnetic effects on magnetic resonance images can create both very specific diagnoses as well as ambiguous and sometimes perplexing diagnostic situations.
- A thorough knowledge of paramagnetic effects and the physical principles that produce them is necessary for the accurate interpretation of images.
Paramagnetic substances shorten T1 and T2. Their visible effects vary with their concentration, location, field strength, and pulse sequences used. The most commonly encountered of these substances are the blood breakdown products also discussed elsewhere (Chapter 3). Heavy metals produced by fungi, and other metallic elements and possibly naturally occurring free radicals may produce significant effects on magnetic resonance (MR) images. These substances also have more marginal effects on computed tomography (CT) images in some normal and pathologic states.
Bleeding occurs as part of the natural history of some head and neck lesions and will affect the imaging findings. Bleeding may be a prime clinical concern but more often is a secondary phenomenon that may aid or hinder differential diagnosis. Blood products may cause confusion during MR interpretation.
On CT examination of the extracranial head and neck, a fresh blood clot will generally appear more dense than skeletal muscle. However, fresh hemorrhage usually only is recognized following facial or neck trauma (Fig. 11.1) or an active vascular leak, where it is obviously more dense than the usual secretions and mucosal thickening seen in inflamed sinuses or the surrounding soft tissues, respectively (Fig. 11.2). It might also be recognized by the pattern of the bleed, as in the eye with a choroidal hemorrhage (Fig. 11.3). Foreign bodies can also be mistaken for blood, but the morphology of the presumed hematoma may a clue to avoid such error (Fig. 11.4).1 Increased CT density of blood is due to increased protein concentration within the clot (Fig. 11.5).2
Outside of the brain, this increased density is short-lived because clot lysis and resorption progress at a more rapid rate than within the brain and related intracranial spaces; thrombolytic substances have greater access to the clot in the absence of a blood–brain barrier and in a space with lesser contact with a vascular healing response (Fig. 11.2). An extracranial subacute hematoma often shows increased density relative to cerebrospinal fluid but really looks no different, based on its density, from any somewhat proteinaceous fluid collection.
On MR, the relative protein and water content and paramagnetic properties of degraded hemoglobin combine to produce often characteristic findings at the site of bleeding, especially in the brain.3 The appearance is linked to the rate of clot lysis with breakdown of the red cells and degradation of the hemoglobin molecule. These factors are further influenced by location and whether there have been repeated episodes of hemorrhage. Since the area of bleeding is usually sampled at one point in its evolution, a rigid approach that involves assigning specific time frames to certain MR findings may lead to confusion. It is essential to recognize that the findings are related to hemorrhage in order to construct an accurate diagnosis. The basic timetable that follows in the next paragraph is a useful outline for evaluating the approximate age of hemorrhage. The physical principles that explain the effect of blood on proton relaxation are discussed in conjunction with the basic physical principles of MR image formation (Chapters 1 and 3).
Acute or hyperacute hemorrhage, as seen within several hours to 24 hours, is basically a fluid collection. It will have low signal intensity on T1-weighted and high signal intensity on T2-weighted sequences (Fig. 11.6). However, hemorrhage is usually encountered in a subacute (24 to 72 hours) or early chronic phase (3 to 7 days to several weeks). The formation of clot results in a compacting of the cellular elements of blood that concentrates protein and accelerates relaxation on T1-weighted images. After 24 to 48 hours, blood will begin to brighten relative to brain or muscle on T1-weighted sequences and be isointense to brain and slightly hyperintense to muscle on T2-weighted images (Fig. 11.7). The brightness may be even more obvious on fat-suppressed images. Over several days to weeks, the T2-weighted images will show the clot becoming brighter on both T1- and T2-weighted images as red cells lyse and fluid content increases. At the same time, a zone of darkening may appear within the hematoma or at its periphery (Figs. 11.8–11.11). The changes at the periphery are related to hemosiderin deposition—a common pattern in the evolution of parenchymal brain hematomas (Fig. 11.8).4,5 The relative conspicuity of these findings is, to some extent, field strength dependent.