Ontology as a branch of philosophy is the science of what is, of the kinds and structures of objects, properties, events, processes and relations in every area of reality. The earliest use of the term ‘ontology’ (or ‘ontologia’) seems to have been in 1606 in the book Ogdoas Scholastica by the German Protestant scholastic Jacob Lorhard. For Lorhard, as for many subsequent philosophers, ‘ontology’ is a synonym of ‘metaphysics’ (a label meaning literally: ‘what comes after the Physics’), a term used by early students of Aristotle to refer to what Aristotle himself called ‘first philosophy’. Some philosophers use ‘ontology’ and ‘metaphysics’ to refer to two distinct, though interrelated, disciplines, the former to refer to the study of what might exist; the latter to the study of which of the various alternative possible ontologies is in fact true of reality.
The upper level of Lorhard's own ontology is illustrated in Figure 7.1.
The term – and the philosophical discipline of ontology – has enjoyed a chequered history since 1606, with a significant expansion, and consolidation, in recent decades. We shall not discuss here the successive rises and falls in philosophical acceptance of the term, but rather focus on certain phases in the history of recent philosophy which are most relevant to the consideration of its recent advance, and increased acceptance, also outside the discipline of philosophy.
A clinician making a diagnosis based on medical images looks for a number of different types of indication. These could be changes in shape, for example enlargement or shrinkage of a particular structure, changes in image intensity within that structure compared to normal tissue and/or the appearance of features such as lesions which are normally not seen. A full diagnosis may be based upon information from several different imaging modalities, which can be correlative or additive in terms of their information content.
Every year there are significant engineering advances which lead to improvements in the instrumentation in each of the medical imaging modalities covered in this book. One must be able to assess in a quantitative manner the improvements that are made by such designs. These quantitative measures should also be directly related to the parameters which are important to a clinician for diagnosis. The three most important of these criteria are the spatial resolution, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). For example, Figure 1.1(a) shows a magnetic resonance image with two very small white-matter lesions indicated by the arrows. The spatial resolution in this image is high enough to be able to detect and resolve the two lesions. If the spatial resolution were to have been four times worse, as shown in Figure 1.1(b), then only the larger of the two lesions is now visible. If the image SNR were four times lower, illustrated in Figure 1.1(c), then only the brighter of the two lesions is, barely, visible.
In nuclear medicine scans a very small amount, typically nanogrammes, of radioactive material called a radiotracer is injected intravenously into the patient. The agent then accumulates in specific organs in the body. How much, how rapidly and where this uptake occurs are factors which can determine whether tissue is healthy or diseased and the presence of, for example, tumours. There are three different modalities under the general umbrella of nuclear medicine. The most basic, planar scintigraphy, images the distribution of radioactive material in a single two- dimensional image, analogous to a planar X-ray scan. These types of scan are mostly used for whole-body screening for tumours, particularly bone and metastatic tumours. The most common radiotracers are chemical complexes of technetium (99mTc), an element which emits mono-energetic γ-rays at 140 keV. Various chemical complexes of 99mTc have been designed in order to target different organs in the body. The second type of scan, single photon emission computed tomography (SPECT), produces a series of contiguous two-dimensional images of the distribution of the radiotracer using the same agents as planar scintigraphy. There is, therefore, a direct analogy between planar X-ray/CT and planar scintigraphy/SPECT. A SPECT scan is most commonly used for myocardial perfusion, the so-called ‘nuclear cardiac stress test’. The final method is positron emission tomography (PET). This involves injection of a different type of radiotracer, one which emits positrons (positively charged electrons).
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