I argue historical information on the relationship between Copernicus’s work and Islamicate astronomy, which came to light when Kuhn was writing The Copernican Revolution, complicates the depiction of Copernicus’s work as revolutionary, or discontinuous with previous astronomy. I consider Saliba’s claim that Tusi’s work was “a Scientific Revolution before the Renaissance.” I conclude that, although Tusi’s work is important, it is better understood as extending normal science. Similar arguments undermine the claim that Copernicus’s work was revolutionary. Earlier histories of the Copernican revolution have given too little credit to the innovations of Tycho Brahe, and his imitators and opponents, and the solid scientific reasons for preferring Tycho’s system over Copernicus’s as late as 1650. However, the situation in European astronomy and cosmology from the career of Copernicus to the death of Newton does look like a Kuhnian crisis state. It is also possible to locate incommensurabilities between heliocentric and geocentric cosmologies, especially beginning with Kepler. The concept of incommensurability remains an important resource for understanding the history of science.

Introduction

The flow of blood to an organ is a fundamental physiological factor affecting tissue health, growth, and repair. Blood flow and volume are perturbed in many disease conditions, most notably in vascular disease and in tumors. The ability to determine non-invasively blood flow and blood volume using imaging methods therefore has important diagnostic and therapeutic implications. Since the early days of radiological imaging, scientists and physicians have been searching for methods that can accurately and non-invasively depict the major blood vessels of the body, and measure blood flow in tissue. For instance, X-ray projection imaging of blood vessels (angiography) was first demonstrated in 1927 by Moniz [1], using iodinated contrast agents injected intravascularly, while early measurements of tissue blood flow were based on the inhalation of freely diffusible tracers (e.g., nitrous oxide [N2O] [2], or radioactive xenon or krypton [3]). Subsequently, stable (i.e., non-radioactive) xenon was used in conjunction with X-ray computed tomography (CT) to image cerebral blood flow (CBF) [4], while other methods such as single-photon emission CT (SPECT) [5, 6] and positron emission tomography (PET) [7, 8] imaging using a variety of radiotracers also became available. More recently, dynamic CT perfusion imaging using bolus injection of iodinated contrast agents has been growing in popularity [9], particularly as fast multi-slice CT scanners have become widely available.

Blood flow is one of the most fundamental physiological parameters. Maintenance of adequate blood flow is vital for the health of biological tissue. The growth and function of many organ systems are linked tightly to their blood supply. In addition, many disease processes are associated with either increases or decreases in flow compared with normal values. The development and validation of non-invasive tools for the measurement of flow have been longstanding goals, both in biomedical research and in clinical practice.

Traditionally, the imaging of flow, or perfusion, has been accomplished using either nuclear medicine-based techniques involving radioactive isotopes, or X-ray computed tomography (CT) methods using radio-opaque contrast agents. However, soon after the introduction of magnetic resonance imaging (MRI) for anatomical imaging, research began on techniques for depicting flow. Since then, progress has been rapid, not least because MR methods have the advantage of not involving radiation, and in the case of arterial spin labeling-based techniques, are completely non-invasive. This makes them particularly appealing for use in a wide range of populations, including children and normal subjects. In addition, MR perfusion can be combined with the armamentarium of other structural, vascular, physiological, metabolic, and functional techniques available with MR to provide a comprehensive, “one-stop” examination for the patient.