Semiotica 141 (2002), 29-41.
It has been shown by Cantor (2000) that conventional radiographic signs of disease are the interpretants of Peircean signs. Such triadic relations between representamen, object and interpretant were called Roentgen signs. In this paper, we develop a typology of Roentgen signs based on the categories of Charles S. Peirce and the pragmatics of Charles Morris. Using this typology, it will be shown that the linguistic concepts of syntagm and paradigm may be applied to Roentgen semiotics (cf. Sebeok [1994]: 27-28). Furthermore, it will be shown that the three temporal types of Roentgen signs entail the three Peircean types of argument.
In this section, the terminology of Peircean semiotic grammar (Liszka 1996: chapter 2) will be applied to the study of Roentgen signs.
A Roentgen object is an anatomic event in the human body that is referred to by a Roentgen representamen (image). Hence, the object ground of a Roentgen sign is the human body. Historically, Roentgen images have been two-dimensional distributions of brightness intensity produced by transillumination of radiographs (films). However, the same visual effect may now be produced by electronic video monitors (cathode ray tubes) in digital radiography (Raff 1997). Therefore, the image ground of a Roentgen sign may be a film or a video monitor. It has been shown that a Roentgen interpretant is a conventional radiographic sign. Until recently, Roentgen interpretation has been solely a function of the human mind. However, with the advent of computer aided diagnosis, the interpretant ground of a Roentgen sign may be either the human mind or an artificial neural net (Swett et al 1997).
We note that a radiographic image is a representamen of a Roentgen sign and that any part of such an image may also be a representamen of some Roentgen sign. A Roentgen sign may be true or false depending on the truth-value of its interpretant. In this paper, we will deal only with true Roentgen signs. False signs (Keats 1996) present special problems and will be discussed separately. Although Roentgen signs may be interpreted as being normal or abnormal, only abnormal signs will be included in the typology.
Imaging systems are designed, for reasons of efficacy, to produce Roentgen images that are persistent in their image ground. On the other hand, abnormal Roentgen objects may be either transient or persistent in their object ground, the human body. Transience of such events may be due to their natural history or the healing process. Persistence of abnormal events may be due to a prolonged natural history (e.g. chronic disease) or an irreversible change in the object ground (e.g. replacement by neoplasia or healing with deformity). Recall that a representamen of a Roentgen sign of disease refers to an abnormal event in the human body at a specific moment in time. Also note that, under the controlled conditions of the radiographic examination, abnormal events amenable to imaging exhibit the property of temporal continuity (Russell 1914:143). It follows that the instant of image formation may be contained within a sufficiently small time interval during which there is no significant change in the object event. This implicit interpretant of any Roentgen sign will be called the Roentgen present of the sign. The duration of this time interval depends on whether the event is persistent or transient and, if transient, on its rate of presentation or progression. Clearly, if a Roentgen sign has an implicit present, it will also have an implicit past and future. In the next section, we will see how these three temporal interpretants form the basis for a pragmatic typology of Roentgen signs of disease.
A gnostic Roentgen sign, by definition, provides the interpreter with knowledge of abnormal events in the human body. This may be knowledge of past, present or future events. Signs that provide knowledge of their Roentgen present will be called diagnostic signs. Signs that provide knowledge of their Roentgen future will be called prognostic signs. So far, this terminology is consistent with current medical usage. However, in this paper we will refer to signs that provide knowledge of their Roentgen past as anagnostic signs. The term 'anagnostic' (literally 'backward knowing') is to be understood in analogy with the term 'prognostic' (literally 'forward knowing'). Current medical terminology does not distinguish between diagnostic and anagnostic signs. The same radiographic image may be the representamen of Roentgen signs of different temporal types i.e. of diagnostic, prognostic or anagnostic signs.
After defining the concept of a sign in behaviorist terms, Charles Morris (1946) suggested that any comprehensive classification of signs should take into account the use of signs to the interpreter. On the basis of this organizing principle, we developed a pragmatic typology for gnostic Roentgen signs i.e. for Roentgen signs that are used to obtain knowledge of disease in humans. For this typology, we will use a pragmatic nomenclature that is in the style of Charles Morris i.e. terms like detector, localizer, descriptor, etc. These particular terms belong to the 'folklore' of image interpretation and are used by radiologists without reference to a source.
Charles Sanders Peirce described his three irreducible categories of thought or experience for the first time in 1867 (Peirce 1867). In subsequent work he used the abstract terms 'Firstness, Secondness and Thirdness' consistently (Buchler 1955: 74-97) but never arrived at a definitive set of descriptive terms for the categories (Ransdell 1998). As a result, later writers on the Peircean categories have been free to assemble their own nomenclatures from the multitude of terms originally proposed by Peirce. For this paper, the author has chosen to use a set of terms that have intuitive value for Roentgen semiotics. Therefore, we will refer to Firstness as attribution, Secondness as opposition and Thirdness as mediation.
Roentgen signs may be of two major topological types which will be termed local and nonlocal. Local signs present in an arbitrarily small neighborhood in an image. Nonlocal signs present in a sufficiently large neighborhood in an image. The separation signs that present at radiographic boundaries are local detectors that have been previously described (Cantor 2000). These signs are based upon the presence or absence of an expected visual separation effect at an image boundary. There are three types of separation signs, each with two subtypes. These are the gain of separation (by line or edge), the loss of separation (by line or edge) and the exchange of separation (line for edge or edge for line) signs. These separation signs are interrelated by dualities.
The asymmetry signs are nonlocal detectors. These signs are based on the presence or absence of asymmetry relative to an anatomic midline in an image. There are three types of asymmetry signs: the gain of asymmetry, the loss of asymmetry and the exchange of asymmetry signs. In a gain of asymmetry sign, symmetry is expected but asymmetry is observed. In a loss of asymmetry sign, asymmetry is expected but symmetry is observed. In an exchange of asymmetry sign, a unilateral finding is expected on one side but is observed on the other. Clearly, there is a duality between the gain of asymmetry and loss of asymmetry types. There is also a duality between the two exchange of asymmetry signs.
Gnostic Roentgen signs will first be classified on the basis of their temporal reference. These types of signs will be called diagnostic, prognostic and anagnostic when they refer to events in the Roentgen present, future and past, respectively. The diagnostic signs will then be classified on the basis of their use to the interpreter. This division into pragmatic types will yield detectors, localizers and identifiers. These basic pragmatic types will in turn be divided into subtypes. Each pragmatic type or subtype will then be placed in one of the three Peircean categories. These categories will be referred to as attribution (Firstness), opposition (Secondness) and mediation (Thirdness). Similarly, the prognostic and anagnostic sign types will be divided into categorical subtypes. Concrete examples of each sign type will be given. Since Roentgen signs are numerous to the point of being uncountable, only a limited number of examples will be given. The author has chosen to include only three examples of each sign type. To emphasize the inclusive nature of the typology, one example of each type was drawn from each of the three basic anatomic domains of general radiology i.e. chest, abdomen and bone. Conventional names for radiographic signs will be used when possible. However, some signs that do not have conventional names will be referred to by description. A convenient collection of radiographic signs by Eisenberg (1984) contains many of the examples cited. The other signs may be found in the books by Felson (1973), Gore et. al. (1994: The Abdominal Plain Film, chapters 12-14) and Helms (1995).
By definition, diagnostic signs provide their interpreter with knowledge of events in their Roentgen present. This is the most numerous type of gnostic sign and has the most highly developed typology. The diagnostic signs are divided into subtypes called detectors, localizers and identifiers.
Detectors
Detection involves the perception of a presence or an absence, both of which are irreducible attributes of an image. Therefore, detection is a first category concept. Detectors may be of three types, depending on the presence or absence perceived. These are the attribute of place (topic), the attribute of measure (metric) and the attribute of shape (morphic).
Topic detectors. Topic detectors refer to a presence or absence in a place. Since place is itself an irreducible attribute, it is a first category concept. Signs that indicate the presence of a finding that is unexpected in a place will be called paratopic detectors. The term paratopic means literally 'present at a place'. Signs that indicate the absence of a finding that is expected in a place will be called apotopic detectors. The term apotopic means literally 'absent from a place'. Signs that indicate both the presence of an unexpected finding and the absence of an expected finding in a place will be called syntopic detectors where syntopic means literally 'together in a place'. The previously mentioned separation signs that present at boundary lines and edges belong to a family of local topic detectors consisting of paratopic, apotopic and syntopic signs. Examples of these detectors have been given previously (Cantor 2000) and will not be repeated here. However, examples of the asymmetry signs which are nonlocal topic detectors will be given. Mediastinal shift indicating loss of lung volume, an absent kidney and asymmetry of the sacral foraminal lines indicating fracture are paratopic nonlocal detectors (gain of asymmetry signs). The 'symmetric' cardiovascular silhouette in total anomalous venous return and the symmetric liver in asplenia are apotopic nonlocal detectors (loss of asymmetry signs). The right aortic arch and situs inversus of the intestines are syntopic nonlocal detectors (exchange of asymmetry signs). In the skeletal system, loss of asymmetry and exchange of asymmetry signs do not occur since bilateral symmetry is the norm.
Metric detectors. Metric detectors refer to a presence or absence in expected measure. Since measure involves an opposition between something and a scale, it is a second category concept. We distinguish three types of measure in Roentgen images: measures of optical density, optical density gradients and size. The density signs and the boundary gradient signs are local metric detectors, examples of which have been given previously (Cantor 2000). However, examples of nonlocal metric detectors will be given here. Note that detectors of size must be nonlocal signs. The unilateral radiolucent lung, the opacity of an abdominal mass and osteosclerosis are nonlocal density detectors. The 'vertebral fade-off' sign of pulmonary disease, the 'ground-glass' abdomen in ascites and the periarticular osteoporosis of rheumatoid arthritis are nonlocal gradient detectors. The enlarged pulmonary hilum, the enlarged spleen and the reduced intervertebral disc height are nonlocal size detectors.
Morphic detectors. Morphic detectors refer to a presence or absence in expected shape. Since shape is a mediation between part and whole, it is a third category concept. Note that signs of shape must be nonlocal.
There are two topological subtypes of nonlocal Roentgen signs that will be termed global and regional. A global sign involves an entire anatomic compartment. A regional sign presents in a part of an anatomic compartment. The 'water-bottle' heart of pericardial effusion, the 'horseshoe' kidney and signs of congenital skeletal dysplasia are global morphic detectors. A focal mediastinal bulge, a focal bulge in a renal contour and the cortical 'expansion' produced by some bone tumors are regional morphic detectors since they do not involve an entire anatomic compartment. This completes the typology for Roentgen detectors.
Localizers
Localization involves an opposition between an object and a reference object. Therefore, localization is a second category concept. Localizers may be of three types, depending on the opposition observed. These are the opposition of proximity (proximate), the opposition of depth and an opposition inferred at a distance (remote).
Proximate localizers. Proximate localization is based on the opposition of proximity. Proximity is an irreducible attribute and therefore is a first category concept. We may distinguish two types of proximity: contiguity and containment. The 'silhouette' sign, which is based on contiguity, is used for localization of air space disease, 'mass effect' or displacement is used in all anatomic fields as a sign of contiguity and 'pressure erosion' is a sign of a soft tissue mass in contact with bone. Superposition on orthogonal projections is used as a sign of containment in all anatomic fields of radiology.
Depth localizers. Depth localization is based on the opposition of relative depth. Since depth itself implies an opposition between different levels, depth localization is a second category concept. Depth localization is accomplished in all anatomic fields by differential magnification (a property of central projection) and differential displacement (the parallax effect) produced by rotation of the patient or angulation of the x-ray beam.
Remote localizers. Remote localization is based on an opposition between distant objects that is mediated by inference. Therefore, remote localization is a third category concept. Examples of remote localizers include: the thoracoabdominal sign for localizing subdiaphragmatic masses on a chest film, uniform opacity of the dome of the liver on an abdominal film indicating pneumonia in the right lung base and a depressed fracture of the radial head on an elbow film indicating dislocation of the distal radioulnar joint in the wrist. This completes the typology for Roentgen localizers.
Identifiers
Descriptors. Descriptive identifiers (descriptors) are used for the recognition of attributes. Hence, description is a first category concept. Descriptors are the most numerous type of Roentgen sign. There are correspondingly numerous descriptor subtypes. Only a few descriptors will be mentioned here. One common type is distribution (focal or diffuse). Another is 'mass' with interior descriptors (cystic or solid), boundary descriptors (sharp or ill-defined) and morphological descriptors (contour or shape). Still another type describes the growth pattern of tumors (frontal expansion or infiltration). These same descriptors are used in all anatomic fields of radiology.
Excluders. Exclusive identifiers (excluders) are used for the recognition of incompatibility which is an opposition between what is possible and what is observed. In this sense, exclusion is a second category concept. Examples of excluders are: the 'air-bronchogram' sign within a pulmonary opacity that excludes solid tumor, a normal intestinal gas pattern that excludes bowel obstruction and 'segmental' loss of articular cartilage in a synovial joint that excludes rheumatoid arthritis.
Specifiers. Specific identifiers (specifiers) are used for the recognition of an interpretation. Since interpretation is a form of mediation, specification is a third category concept. We distinguish three types of specifiers, depending on the nature of the interpretation involved. Interpretation may be material, anatomical or pathological. The corresponding signs will be called material, anatomical, and pathological specifiers.
Material specifiers. Material specifiers are used for the recognition of substances with characteristic attributes. Therefore, material specification is a first category concept. The substances encountered in diagnostic radiology are characterized by their physical state (gas, liquid, solid) and their composition (air, fat, water, bone). Material specifiers of physical states are based on gravitational effects. Gas rises, liquid flows and solids fall under the influence of gravity. These specifiers of state occur in all anatomical domains of diagnostic radiology. Material specifiers of composition are based on the radiographic gray scale (or brightness scale). In this scale, image brightness increases along with physical density from air (black) to bone (white). These specifiers of composition occur in all anatomical domains of diagnostic radiology.
Anatomical specifiers. Anatomical specifiers are used for the recognition of the anatomical substrate of an abnormality. The abnormality is interpreted as being in opposition to its substrate as figure is to ground. In this way, anatomical specification involves an opposition and is a second category concept. The anatomical substrates of Roentgen objects are tissues, organs and organ systems. Examples of anatomical specifiers include: lung, kidney and bone tumors.
Pathological specifiers. Pathological specifiers are identifiers used for the recognition of the meaning of an abnormality. Since meaning involves the mediation of thought, pathological specification is a third category concept. We distinguish two types of pathological specifiers. These are termed categorical and etiological specifiers. Examples of categorical specifiers include: the pulmonary edema pattern, the pattern of small bowel obstruction and the pattern of ischemic necrosis of the femoral head. Examples of etiological identifiers include: calcified basilar pleural plaques in asbestosis, large bowel obstruction due to sigmoid volvulus and the vertebral 'picture frame' sign of Paget disease. Due to the nonspecificity of most Roentgen signs of disease, there are relatively few etiological specifiers. Identification usually involves the use of descriptors and anatomical or categorical specifiers. This completes the typology for diagnostic Roentgen signs.
A prognostic sign provides the interpreter with knowledge of a probable course of events in its Roentgen future. As such, prognostic signs are used in the evaluation of treatment options. They are much less numerous than diagnostic signs and their typology is less well developed. Prognostic signs may be divided into three pragmatic subtypes called graders, stagers and predictors.
Graders
Graders indicate the rate of onset or progression of abnormal events. Since grading refers to an attribute of biological behavior, it is a first category concept. Abnormalities may be spoken of as high-grade or low-grade. Signs of high-grade abnormalities include: invasion of the chest wall by a lung tumor, severe dilatation of the intestines in bowel obstruction and the 'sun-burst' pattern of periosteal reaction due to osteogenic sarcoma.
Stagers
Stagers involve a comparison between an event and an expected sequence of events. Therefore, staging involves an opposition and is a second category concept. Examples of stagers include: the presence of Kerley-B lines indicating interstitial pulmonary edema, appendicolith as a sign of chronic appendicitis and the 'crescent sign' indicating late stage avascular necrosis of the femoral head.
Predictors
Predictors involve the anticipation of future events. Anticipation is mediation by inference that is based upon personal or collective past experience. Therefore, prediction is a third category concept. Examples of predictors include the left ventricular enlargement that precedes congestive heart failure, dilation of the cecum with intramural gas which indicates impending perforation and intra-articular fracture with deformity that predicts post-traumatic arthrosis. This completes the typology for prognostic Roentgen signs.
By definition, anagnostic signs provide the interpreter with knowledge of their Roentgen past i.e. knowledge of events that preceded the abnormal events in their Roentgen present. Hence, it is natural to base a typology of anagnostic signs on prior limiting events (limiters), prior ordering of events (orderers) and likely initiating events (retrodictors).
Limiters
Limiting events for abnormalities include healing with sequellae, chronicity and death. The last mentioned is the province of forensic radiology. Since healing and chronicity are both attributes of biological behavior, limitation is a first category concept. Signs of chronicity include: the 'honeycomb' pattern of pulmonary fibrosis, pancreatic calcifications in pancreatitis and a sequestrum within an abscess cavity in bone.
Orderers
The order of occurrence of past events may be due to causation, reactivation or malignant transformation. Temporal ordering involves an opposition of the type preceding/following. Therefore, ordering is a second category concept. Examples of ordering signs include: coexistent pneumomediastinum and pneumothorax indicating that the pneumothorax followed the pneumomediastinum, 'gallstone ileus' indicating that the small bowel obstruction followed a complicated cholelithiasis and coexistent arthrosis with joint deformity from healed fracture indicating that the arthrosis followed the deformity.
Retrodictors
An initiating event will be thought of as an inferred cause in the Roentgen past of an abnormal event in the Roentgen present of a sign. In this sense, knowledge of an initiating event involves the mediation of inference. Therefore, retrodiction is a third category concept. Examples of retrodictors include: calcified granulomata and lymph nodes indicating previous pulmonary histoplasmosis, pneumoperitoneum and dilated bowel indicating previous perforation and the inference of fracture mechanism from fracture pattern. This completes the typology for anagnostic Roentgen signs.
A schematic representation of the pragmatic typology of gnostic Roentgen signs is given in Figure 1.
The phenomenological inclusion rule (Liszka 1996:46) that orders the Peircean categories induces relations of entailment between the pragmatic types of gnostic signs. Hence, identification entails localization and localization entails detection. This ordering will be called pragmatic entailment. In pursuing the analogy between image and text, both of which are 'read', we may identify syntagmatic and paradigmatic relations (Ducrot and Todorov [1972]:109) in the typology of gnostic Roentgen signs. On a temporal level, there are only three gnostic sign types and temporal ordering induces syntagmatic relations between them. On a pragmatic level, each temporal type is divided into three pragmatic subtypes with paradigmatic relations between them, based on the Peircean categories. Furthermore, in the highly developed class of diagnostic signs, this process may be continued. Pragmatic entailment induces syntagmatic relations between the three types of diagnostic signs. In turn, each type of diagnostic sign may be divided into three subtypes with corresponding paradigmatic relations (see the full typology of diagnostic signs).
There is a correspondence between the three types of gnostic signs (diagnostic, prognostic and anagnostic) and the three Peircean types of argument (deduction, induction and abduction [see Liszka 1996, chapter 3]). By this correspondence, interpretation of a diagnostic sign employs deductive arguments which are based on implicit relations between events in the Roentgen present of the sign. Interpretation of a prognostic sign employs inductive arguments which are based on a knowledge of the probable temporal sequence of events in the Roentgen future of the sign. Finally, interpretation of an anagnostic sign employs abductive arguments which are based on hypotheses concerning events in the Roentgen past of the sign.
In this paper, we have developed a typology for an important class of Roentgen signs. These are signs that provide the interpreter with knowledge of anatomical abnormalities in patients. The existence of such a typology suggests that fundamental patterns of thought are involved in the creation and use of signs by radiologists. This typology may serve as the basis for future studies on the logic of Roentgen diagnosis. It is possible that some of the concepts of Roentgen semiotics may be applied to visual semiotics in other contexts.