Semiotica 158 (2006), 297-308.
This is a phenomenological study of the mental processes involved in the reading of Roentgen images. Its conceptual basis is in Roentgen semiotics, an adaptation of Peircean semiotics that incorporates the linguistic concept of markedness. It is shown that the notions of 'abnormality' and 'change' in Roentgen diagnosis are represented by signs of 'difference'. Differential signs for the detection, localization or identification of 'abnormality' or 'change' are constructed by means of Roentgen semiotic grammar.
The most frequently asked questions in diagnostic radiology are: 'Is there an abnormality?' and 'Is there a change?' The diagnostic radiologist attempts to answer these questions by interpreting signs of 'difference'. Both 'abnormality' and 'change' involve events that are unexpected under predetermined conditions. For 'abnormality', the conditions are established by convention. For 'change', the conditions are established by precedent. In this way, both 'abnormality' and 'change' refer to a difference between what is observed and what is expected. This opposition between the observed and the expected is characteristic of diagnostic signs of 'difference.' In the sections that follow, we shall briefly review the semiotic grammar that generates differential signs. In this context, we shall examine the ways in which Roentgen signs are combined to produce differential detectors, localizers and identifiers. We shall conclude with a discussion of integral signs, which are signs of 'sameness'. These signs are used to represent 'normality' or 'absence of change'.
In Roentgen semiotics, the elementary forms of difference are characterized by the application of markedness to binary relations (Cantor 2000, 2004). For our purposes, a binary relation is defined as the coexistence of two distinct thoughts (termed correlates) in the same moment of awareness in the mind of an interpreter. A binary opposition is a binary relation in which one correlate is marked and the other unmarked. There are two logically distinct types of markedness. In markedness of the general type, there is an attribute of the marked correlate that is not an attribute of the unmarked correlate. In markedness of the specific type, the correlates of a binary relation share an attribute where the attribute of the marked correlate is in some sense more or less than the attribute of the unmarked correlate. A binary opposition may be marked in different ways, for different attributes.
In this section, we shall briefly review the morphology of diagnostic Roentgen signs (cf. Cantor 2003b). A Roentgen sign is a triadic relation between a Roentgen image (the representamen), the anatomic event that determines the image (the object) and an interpretation of the object-event (the interpretant). In current practice, the interpretant is a product of the mind of an image reader (the interpreter). On the basis of the three Peircean categories of thought, the representamen is the first category concept of 'attribution', the object is the second category concept of 'opposition' and the interpretant is the third category concept of 'mediation'.
A semiotic ground is a context in which a correlate of a Roentgen sign has a specific meaning (Cantor 2004). In this sense, a ground in semiotics is analogous to a universe of discourse in logic. The interpretation of a Roentgen sign involves a choice by the interpreter of a semiotic ground for each of its correlates. The representamen of a Roentgen sign may be thought of as a percept in an image or a perception in the mind of the interpreter. Hence, the representamen ground of a Roentgen sign may be a physical image or the mind of the interpreter. The object of a Roentgen sign is an anatomic event in the body of a patient. Such an event may be thought of as occurring in a specific anatomic structure at a specific time or as a memory retrieved by the interpreter. Hence, the object ground of a Roentgen sign may be an anatomic structure or the mind of the interpreter. The interpretant ground of a Roentgen sign may be the mind of the interpreter or an artificial neural net. If there is a common ground for the correlates of a Roentgen sign, it will be termed the ground of the sign. For example, Roentgen signs for which the representamen is regarded as a perception and the object a retrieved memory will be called mental signs (Cantor 2005).
Semiotic syntax is a set of rules by which signs are used to produce other signs. Such rules are called semiotic operators (Cantor 2003b). A unary semiotic operator is a rule by which one sign is used to produce another sign. For example, the process of reinterpretation as applied to the interpretant of a sign produces a new interpretant and therefore a new sign (cf. Cantor 2000). A binary semiotic operator is a rule by which two signs are combined to produce a third sign (Cantor 2003b). Semiotic union and semiotic product are two binary operators. Semiotic union operates on pairs of signs that present in the same Roentgen image. The resultant of a semiotic union is a sign in which the representamen is the perception of the copresence of the component representamina in the same Roentgen image, the object is a knowledge of the copresence of the component object-events in the same patient and the interpretant is the logical conjunction of the component interpretants. A semiotic product operates on pairs of signs that present in different Roentgen images. The resultant of a semiotic product is a sign in which the representamen is the perception of the coexistence of the component representamina in different Roentgen images, the object is knowledge that the coexistent representamina refer to the same anatomic event in the patient and the interpretant is the logical conjunction of the component interpretants together with collateral knowledge of the projectional and temporal relationships of the component images. In general, the resultant of a binary semiotic operator will be called a compound sign. A monadic compound sign is derived from a single Roentgen image. A dyadic compound sign is derived from a pair of Roentgen images. In the sections that follow, we shall examine the pragmatic types of diagnostic signs from the perspective of differential semiotics ie. differential detectors, differential localizers and differential identifiers (cf. Cantor 2002).
In Roentgen diagnosis, an object-event is said to be abnormal if the interpreter is aware of a difference between a perception of the event and an expectation of the event. By expectation we mean a habit of thought acquired by the interpreter from repeated past experience. An object-event is said to be changed if the interpreter is aware of a difference between perceptions of the same event at different times in its history. Hence, the detection of abnormality or change entails an awareness of a difference. This state of awareness is accompanied by a feeling of 'surprise' that provokes a direction and intensification of attention. Detectors of 'difference' may be found in single or paired images. Such signs will be called monadic or dyadic detectors, respectively. In the sections that follow, we deal with local and regional Roentgen signs. The representamen of a local sign presents in an arbitrarily small area of an image. The representamen of a regional sign presents in a sufficiently large area of an image.
In this study, a diagnostic sign is defined to be a marked Peircean sign. Hence, a diagnostic sign is a triadic relation comprised of a binary opposition of a marked representamen and an unmarked object that is mediated by an interpretant. We have seen that binary oppositions, ie. marked binary relations, are used in the detection of abnormality. Two general types of markedness determine two types of detector. In one type, markedness consists of the presence in the representamen of an intrinsic attribute that is not expected in a faithful representation of the object. In another type, markedness consists of the absence from the representamen of an intrinsic attribute that is expected to be in a faithful representation of the object. For both types of detector, the representamen is a perception (the marked correlate), the object is an expectation (the unmarked correlate) and the interpretant contains the predicate 'abnormal'. Since perception, expectation and interpretation are all grounded in the mind of the interpreter, these two elementary types of detector are mental signs. The most frequently used monadic detectors are local signs. These include the separation signs, gradient signs and density signs previously described by Cantor (2000).
Another type of monadic detector is the resultant of a semiotic union of two regional signs that present in the same Roentgen image. Such detectors may be of two types. Most commonly, the components of the semiotic union present in regions of the body where there is an expectation of midline or bilateral symmetry. In this type of detector, asymmetry is perceived where symmetry is expected. These are mental Peircean signs where the representamen is the perception of asymmetry (the marked correlate), the object is the expectation of symmetry (the unmarked correlate) and the interpretant contains statements such as 'symmetry has been lost' or 'asymmetry has been gained'. In the less common type of detector, the components of the semiotic union present in regions of the body where there is an expectation of midline or bilateral asymmetry. In this case, symmetry is perceived where asymmetry is expected. These are mental Peircean signs where the representamen is the perception of symmetry (the marked correlate), the object is the expectation of asymmetry (the unmarked correlate) and the interpretant contains statements such as 'asymmetry has been lost' or 'symmetry has been gained'. Hence, two general types of markedness determine two types of monadic symmetry sign e.g. the 'loss of symmetry' and 'loss of asymmetry' signs (cf. Cantor 2002, for examples).
A dyadic detector is the resultant of a semiotic product of two regional signs that present in two different Roentgen images. As in the case of monadic symmetry signs, the components of the semiotic product present in opposite regions of the body where there is an expectation of bilateral symmetry or asymmetry. In the same way, two general types of markedness determine two types of dyadic symmetry sign. When such signs are derived from pairs of Roentgen images formed at the same time, they may be termed synchronic detectors.
In contrast, diachronic detectors are derived from pairs of Roentgen images formed at different times with the same field of view. Hence, a diachronic detector is the resultant of a semiotic product of two Roentgen signs that represent the same anatomic event as it appears at different times in its history. Diachronic detectors are used for the detection of 'change'. In the mind of the interpreter, the detection of change entails an awareness of a perceptual difference between Roentgen images formed at different times. For diachronic detectors, a difference is perceived where sameness is expected. These are mental signs where the representamen is the perception of a difference (the marked correlate), the object is the expectation of sameness (the unmarked correlate) and the interpretant includes the statement 'there has been a change'. The sense of the change, e.g. improvement or worsening of disease, is determined by collateral knowledge that includes the temporal order in which the images were formed.
A clinical Roentgen image is a two-dimensional representation of a three-dimensional region of the body. This loss of dimension is problematic for localization. Radiographic images are transmission images produced by an x-ray beam, transformed by differential absorption in its passage through the body. In contrast, photographic images are reflection images produced by visible light, transformed by differential absorption at the outer surfaces of the body. Depth localization in reflection images is based on depth cues (i.e. signs) such as occlusion and shading (cf Gibson 1950; Hoffman 1998). Clearly such signs are not present in transmission images. In general, localization involves the specification of a spatial relation between the object to be localized and another object, the location of which is known. In transmission images, a depth localizer is a sign that indicates the depth of an object or structure along a particular line of sight, relative to the depths of known structures. In Roentgen semiotics, depth localization is based on differential brightness, sharpness, magnification, projection or displacement.
A monadic depth localizer is derived from a single Roentgen image. The most frequently used monadic localizer is the so-called 'silhouette sign' in chest radiology (cf. Cantor 2000). This classic Roentgen sign may also be used as a detector. The use of this sign as a depth localizer is based on the principle that a silhouette sign at an interface in an image is produced by physical contact at the corresponding interface in the object.
On frontal projections of the chest, some bilateral structures are asymmetrically located (e.g. the right hemidiaphragm is higher than the left) and others are symmetrically located (e.g. the ribs). Such structures with known locations may be used to distinguish right from left on lateral projections where direct signs of laterality are absent. If a pulmonary opacity produces a silhouette sign along the edge of one hemidiaphragm, the opacity is localized in the right or left lung by the relative height of the hemidiaphragm. This type of localizer is a semiotic union of a silhouette sign (a local sign) and a sign of relative height (a regional sign). Its representamen is the perception of focal unsharpness or obliteration of the edge of one of the two hemidiaphragms (the marked correlate), its object is the expectation of sharp diaphragmatic edges and a specific diaphragmatic height difference (the unmarked correlate) and its interpretant includes statements such as 'obliteration indicates contiguity' and 'higher indicates the right side'.
Another type of monadic localizer is based on differential magnification. In a radiographic imaging system, the object is placed between the radiation source and the imaging surface. A divergent beam produces images by central projection with image magnification that varies directly with the distance between object and imaging surface. Positioning the patient so that bilaterally symmetric structures are at different distances from the imaging plane will result in differential magnification of symmetric structures. This differential magnification may be used to distinguish right from left on lateral projections. A magnification sign is the semiotic union of two regional signs that present in the same Roentgen image. Its representamen is the perception of differential magnification of corresponding structures (the marked correlate), its object is the expectation of bilateral symmetry (the unmarked correlate) and its interpretant includes the statement 'larger indicates farther from the imaging surface'. With collateral knowledge of standard radiographic positioning, this may be reinterpreted in terms of laterality. For example, an abnormality within a rib may be localized to the right or left on a lateral projection of the chest.
Dyadic localizers are derived from a pair of Roentgen images with the same field of view. The most common type of dyadic localizer is based on differential projection.
This ubiquitous localizer has no common name. In practice, depth localization is usually successful when based on paired frontal and lateral projections. Frontal projections exhibit mediolateral spatial relations without direct anteroposterior depth cues. Lateral projections exhibit anteroposterior spatial relations without direct mediolateral depth cues. Hence, in orthogonal projections, depth cues that are missing in one image are present in the other.
An orthogonal localizer is the semiotic product of two regional signs that present in orthogonal projections. Its representamen is the perception of mediolateral and anteroposterior spatial relations combined in short term memory (the marked correlate), its object consists of expected spatial relations (the unmarked correlate) and its interpretant is a predicate such as 'within...', 'on the surface of...', 'anterior (posterior) to...' or 'medial (lateral) to...'. Another type of dyadic localizer is based on differential displacement in the image. In standard radiographic technique, the central ray of a divergent radiation beam is perpendicular to the imaging surface. Along any ray, an object point and its image are colinear. Hence, the images of all structures interacting with the beam are superimposed. This superposition may be eliminated by rotating the patient, which results in differential displacement or separation of the superimposed images. Specifically, if the longitudinal axis of the body is parallel to the imaging plane, rotation along this axis will produce opposite displacements of the images of structures on opposite sides of the axis. Also, the amount of displacement will vary directly with the distance of the structures from the axis of rotation. In this way, differential displacement produced by rotation is an indicator of relative depth. A pair of Roentgen images is required to represent this differential displacement, one obtained before and the other after rotation. A parallax sign is the semiotic product of two regional signs derived from a rotational pair of Roentgen images with component signs representing the position of the structure to be localized relative to a known reference structure, prior to and following rotation. Its representamen is the perception of a relative displacement of structures, based on a comparison of their relative positions in short term memory (the marked correlate), its object is the expected displacement of known structures under the rotation (the unmarked correlate) and its interpretant includes a statement such as 'differential displacement indicates differential depth'. With collateral knowledge of the sense of rotation, this localizer may be reinterpreted with greater specificity such as 'the structure is anterior (posterior) to...'.
In Roentgen semiotics, the identification of an object-event involves the mental processes of description, exclusion or specification. In categorical terms, description entails the recognition of an attribute, exclusion entails the recognition of an opposition and specification entails the recognition of a convention (cf. Cantor 2002).
Differential descriptors are based on the recognition of change in an attribute of an image. We define change to mean a difference between the perception and the expectation of an attribute at a given time or a difference between perceptions of an attribute at different times in the history of the object-event. A synchronic descriptor may be the resultant of a semiotic union derived from a single image or the resultant of a semiotic product derived from two images formed at the same time. Its representamen is the perception of an attribute (the marked correlate), its object is an expectation based on prior experience (the unmarked correlate) and its interpretant includes the predicate 'significant change'. This refers to a clinically significant difference between observed and expected appearances, where significance is determined by the interpreter on the basis past experience and training. Changes in boundary density gradient, interior density, size and shape may be interpreted as abnormal and represent signs of disease. With collateral knowledge, some synchronic descriptors may be reinterpreted as 'recent onset' or 'chronic' disease.
A diachronic descriptor is the resultant of a semiotic product derived from two images formed at different times. Its representamen is the perception of an attribute in the later image not present in the earlier image (the marked correlate), its object is an expectation produced by the earlier image (the unmarked correlate) and its interpretant includes the predicate 'active disease'. With collateral knowledge, diachronic descriptors may be reinterpreted as a process such as 'early onset', 'progression' or ' resolution' of disease.
Excluders are used to distinguish false signs or simulators from true signs (cf. Cantor 2003a, on simulators). The presence or absence of a single attribute of an image may be sufficient to distinguish a simulator from a true sign. An excluder identifies an incompatibility between a perception and an expectation. Its representamen is a perception of the presence/absence of a specific attribute (the marked correlate), its object is an expectation of the absence/presence of the same attribute (the unmarked correlate) and its interpretant is the inference that 'due to the inconsistency between appearance and reality, the sign is false and to be excluded'. In one type of excluder, bilateral symmetry of an unfamiliar attribute is considered as most likely due to anatomic variation and this excludes pathology. Other examples of excluders may be found in chest, abdomen and skeletal radiography. The appearance of an opaque edge in the periphery of an image of a lung is not a sign of pneumothorax and usually represents a skin fold artifact. A discrete calcific density outside of the expected location of calculus forming organs is usually not a calculus (except in extraordinary circumstances) and most likely represents ingested material or vascular calcification. A dark line that extends beyond the edge of a bone cannot be a fracture and must be an artifact that simulates fracture.
In Roentgen semiotics, a diagnostic sign is characterized as a binary opposition between a perception and an expectation that is mediated by an interpretation. Previously, we have dealt with signs that involve expectations of specific events. We now consider signs that involve expectations of possible events. Differential diagnosis is the mental process by which a perception is compared with a gamut of possibilities, producing a gamut of diagnostic signs, from which the most likely diagnosis is selected. In our terminology, a differential specifier is a binary opposition between a perception and a possible event that is mediated by a graded interpretation. Its representamen is the perception of an image (the marked correlate), its object is a possible determining event (the unmarked correlate) and its interpretant includes a judgment of faithfulness of representation. The most likely diagnosis is taken to be the interpretant of the most faithful representation generated by the gamut of possibilities. In other words, a differential diagnosis may be thought of as the interpretant of a semiotic union of differential specifiers in a diagnostic gamut, an interpretant that includes a judgment of the relative faithfulness of the component specifiers. An exhaustive compilation of Roentgen diagnostic gamuts has been provided by Felson and Reeder (1987).
This completes our categorical survey of differential diagnostic signs. It must be emphasized, that every identifier is also a localizer and every localizer a detector, by the inclusion rule for the Peircean categories (cf Liszka 1996:46).
In Roentgen semiotics, the absence of abnormality and the absence of change are represented by signs of 'sameness' which we shall term integral signs. Recall that, in general, a sign may be thought of as a triadic relation consisting of a binary opposition between a marked representamen and an unmarked object that is mediated by an interpretant. For diagnostic signs, the marked correlate is a perception by the interpreter and the unmarked correlate is an expectation in the mind of the interpreter. The interpretant of an integral diagnostic sign entails a perceptual judgment of 'sameness', based on either the absence of a perceived difference between the representamen and the object or the exclusion in the process of interpretation of perceived differences, judged by the interpreter to be of no clinical significance. Such judgments make use of collateral knowledge, including the clinical context of the case in question. A pragmatic typology of integral diagnostic signs consists of integral detectors, localizers and identifiers. An integral detector represents a sameness of intrinsic attributes of the perceived and the expected. An integral localizer represents a sameness of location of the perceived and the expected. An integral identifier represents a sameness of clinical identity of the perceived and the expected. By clinical identity we mean an identity that may serve as a basis for clinical action (intervention or nonintervention). Integral signs are the most common diagnostic signs. Whenever an interpreter concludes a diagnostic report with the words 'normal' or 'essentially normal' or with the words 'unchanged' or 'essentially unchanged', the diagnosis is based on the use of integral signs. While signs of normality may be monadic or dyadic, signs of the absence of change are necessarily dyadic.
Peircean semiotics and pragmaticism provide a natural context for understanding the use of signs in Roentgen diagnosis. In this study, we have seen that events referred to as 'abnormal' or 'changed' are represented by signs of 'difference' and events referred to as 'normal' or 'unchanged' are represented by signs of 'sameness'. We have seen how Roentgen semiotic grammar may be used to characterize signs of 'difference' and 'sameness'. We have verified that this characterization holds for all the pragmatic types of diagnostic Roentgen signs. It is hoped that continued study of the semiotic logic of diagnosis will improve our understanding of this activity of the mind and thereby improve our performance.