For centuries, medical imaging has been invasive and potentially harmful to the body of the patient (Fig. 1). Frequently, diseases were missed or diagnosed too late because external anatomical changes were observed long after the onset of disease. During the past century, technologies have progressed rapidly and have had an impact on medicine to an extent unseen before. One of the most cited observations that led to the development of an entirely new medical discipline was the discovery of X-rays by Wilhelm Conrad Roentgen in 1895 [1]. The first applications of X-rays were not necessarily medical, let alone clinical, and it took a few years before the full scope of X-ray imaging was explored for the benefit of the patient.
The early 1970s saw the introduction of the first X-ray computed tomography (CT) system, initially for brain imaging and then later for whole-body studies [1]. Following CT, the 1980s witnessed the appearance of clinical magnetic resonance (MR), a technique of particular importance for imaging patients because it does not require the use of ionising radiation. These two techniques, CT and MR, located within radiology departments, came to dominate the imaging of human anatomy. However, in diagnosing and staging disease or monitoring response to therapy, anatomical imaging does not always provide the complete picture. Functional or metabolic changes may occur, even in the absence of a corresponding anatomical correlate.
Nuclear medicine techniques, initiated in the late 1940s, image functional processes by using radioactive tracers and photon detectors. Tomographic imaging with radionuclides actually predates CT, with early attempts dating from 1963 [2]. The first human tomographic images with positron-emitting isotopes were presented in 1972 [3], thus establishing positron emission tomography (PET) on the map of medical imaging technologies, to be joined by single photon emission tomography (SPECT) a year or so later, following on from the pioneering work of the early 1960s [4].
Various methods of imaging have become available over the past century that have made patient observation, disease diagnosis and therapy follow-up feasible, and above all non-invasive. We know that disease originates from physical distress as well as from changes on the molecular and physiological level. In most cases of serious diseases, early diagnosis is key and, therefore, imaging the anatomy of a patient may not suffice in making a correct and timely diagnosis. Therefore, medical doctors typically employ a combination of imaging techniques during the course of diagnosis and subsequent treatment to monitor their patients. In other words, both functional and anatomical information are essential in state-of-the-art patient management. An appreciation for this type of combined information is best illustrated with the introduction of the term “anato-metabolic imaging” [5], in reference to an imaging technique that gathers both anatomical and functional information, ideally within the same examination.
The advantages of integrated, anato-metabolic imaging are manifold [6]. First, a single examination would provide comprehensive information on the state of a disease. Here, functional, and thus, less anatomically accurate information would be gathered and displayed in a widely appreciated anatomical context. Second, patients would be invited for only one, instead of two or multiple examinations. Third, while engineering costs for combined imaging devices may initially be high, customers would benefit from purchasing a single device rather than two independent devices. Fourth, as we will see later on, the combination of complementary imaging techniques can yield synergy effects for the acquisition and processing of image data. Fifth, experts in radiology and nuclear medicine are hopefully forced to discuss and integrate their knowledge into one report.
While each of the above points can be debated, it is generally assumed that combined imaging has revolutionised imaging and medical diagnosis. Nonetheless, while technological innovation always partners enthusiasm and public interest, subsequent devices and imaging techniques must be affordable and assessed for their health benefit to justify their introduction into a public healthcare system [7]. Unfortunately, today, over 10 years after the introduction of the first commercial dual-technique imaging systems, technology assessment and cost-benefit analyses are not applied to the same standards across imaging techniques.
Here, we intend to describe briefly the status of combined imaging and, based on recent developments, hypothesise on the short-term future of dual- and multi-technique imaging. Further, we will highlight requirements for supporting a wider dissemination of hybrid imaging.
