Introduction

Biomedical Imaging: Past, Present, and Future

Clinical imaging is an essential part of the diagnostic process for an enormous number of diseases. Many tens of millions of investigations are carried out per year, ranging from a relatively simple ultrasound sonogram all the way up to a whole body positron emission tomography (PET) scan. The number of scans continues to increase by several percent year-on-year with ultrasound and computed tomography (CT) being the most common, closely followed by magnetic resonance imaging (MRI), and nuclear medicine at about half these numbers. This chapter gives a brief historical overview of the different clinical imaging modalities in terms of instrumental and technique developments, and then discusses where medical imaging is heading in the future, including a section on the increasingly important role that artificial intelligence (AI) is starting to play.

Listing important historical discoveries, and linking them with their corresponding protagonists, is a dangerous task since there are often multiple claims to simultaneous academic publications, commercial patents, or public demonstrations. This is particularly true given the tendency towards a western-world centered viewpoint, which has often ignored research performed in continents where the dissemination language is not English. To minimize the risks of controversial assignations, in this short chapter the major developments have not been associated with specific scientists unless they are unambiguous. The further reading section provides a wealth of articles where much more detail can be found concerning the development of each of the imaging modalities.

With the exception of ultrasound, which is a mechanical wave, each of the major imaging modalities transmits electromagnetic (EM) energy into the body. Figure 1 shows a schematic of the relevant ranges of wavelength, frequency, and energy. Radiation can be classified as either ionizing or nonionizing. In general, EM energy with a frequency above the near-UV region is defined as ionizing, whereas below this it is termed nonionizing.

Figure 1 Electromagnetic spectrum highlighting the range of wavelengths/frequencies/energies relevant to medical imaging modalities.

Historical Developments

On 8 November, 1895, Wilhelm Röntgen saw the bones of his hand on a photographic plate placed on one side of a Crookes cathode ray tube. Röntgen also imaged his wife's hand, which showed her wedding ring very clearly, and is probably the most reproduced medical image in history. Röntgen was awarded the Nobel Prize for Physics in 1901. Since the technology was relatively simple, it was very quickly taken up by physicians across the world, finding widespread application in military medicine during the first World War.

X-ray and Computed Tomography

X-ray technology continued to advance throughout the twentieth century, and was essentially the only imaging modality for many decades. The introduction of film technology, replacing glass plates, was a major step forward, and made the storage of a patient's data possible. Improvements in all parts of the instrumentation slowly increased the diagnostic quality of the images, with barium- and iodine-based contrast agents added to the imaging capabilities. A major breakthrough was the development of CT in the late 1960s by Godfrey Hounsfield and Alan Cormack, with the first patient scanned in 1971. CT enabled multiple thin slices, rather than a single projection image, to be acquired. Cormack and Hounsfield jointly received the Nobel Prize for Physiology or Medicine in 1979. Scans at that time took tens of minutes, with resolutions in the several millimeters range. Nowadays an entire CT of the body takes only a few seconds, with spatial resolutions of fractions of a millimeter. Key developments included the instrumentation required to perform multislice spiral CT (in which the patient bed slides through the X-ray beam), highly efficient semiconductor-based flat panel detectors, and iterative reconstruction techniques.

1890s X-rays first discovered by Roentgen 1900 First X-ray fluoroscopy demonstration 1913 Development of the anti-scatter grid 1913 Development of the thermionic emission X-ray tube 1918 X-ray film developed 1920s Barium used as a contrast agent for abdominal X-rays 1930 First clinical mammography trial 1950s Concept of subtraction angiography described 1970s First CT system designed by Hounsfield and Cormack 1980s Multislice CT developed 1980s Development of slip ring technology and solid state detectors 1990s Spiral multi-detector CT and reconstruction techniques developed 2000s Flat panel detector technology introduced 2010s Dual energy CT introduced commercially Iterative reconstruction techniques integrated into commercial systems 2020 First commercial photon counting devices available

Nuclear Medicine

Radioactivity was first "discovered" by Antoine Henri Becquerel in 1896, with Marie and Pierre Curie making many of the early breakthroughs in elucidating the nature of the phenomenon. In the 1920s and 1930s, the production of artificial radioactivity through nuclear bombardment was studied in many countries, resulting in the development of the cyclotron. The first radiotracer experiments were performed in rabbits using bismuth-210 labeled antisyphilitic drugs. Human nuclear medicine has its origins in both therapy and imaging. In 1946, radioactive iodine-131 was used in the treatment of thyroid tumors, and it was noted that if the ?-rays could be detected then an image of the radioactivity could be produced. The instrumentation required to perform such imaging was developed in the 1950s, with the result being termed an "Anger camera" after its inventor Hal Anger. This camera formed the basis for planar imaging of many different radioactive isotopes. Clinical utilization was significantly enhanced by the development and commercialization of the technetium generator, which could be delivered on a weekly basis to a nuclear medicine department. Parallel developments were occurring in instrumentation development for PET, again with a key development being the synthesis of still the most widely-used imaging agent, 18F-fluorodeoxyglucose. Clinically, two-camera single-photon emission computed tomography (SPECT) systems were introduced in the 1980s for both brain and cardiac applications. PET scanners were also introduced commercially into the clinic, with improved spatial resolution using time of flight (TOF) becoming available in the 1990s. Improvements in detector technology for both SPECT and PET in the last two decades has resulted in higher spatial resolution and faster imaging times. The other major development has been the integration of a CT scanner into essentially all SPECT and PET systems, in order to provide efficient attenuation correction and also a high resolution anatomical image on which to overlay the functional radioactivity scans.

1940s Production of radionuclides for medical purposes (Oak Ridge National Laboratory) 1950s Development of the first "rectilinear scanner" by Cassen Development of the gamma camera by Anger 1960s Development of the 99mTc generator 1961 First single slice PET scanners 1970s 18F-fluorodeoxyglucose first produced 1980s Development of SPECT technology First TOF PET scanners demonstrated 1990s First SPECT/CT prototype first PET/CT prototype 2000s Introduction of iterative reconstruction schemes for PET 2010s First commercial PET scanners with LSO scintillators 2020 First commercial total body PET system

Ultrasound

Similar to the case of nuclear medicine, ultrasound was first developed for therapeutic applications, specifically noninvasive procedures in which a focused beam was used to thermally destroy various pathologies. The history of ultrasound imaging began in the 1940s, with the first gynecological images of the unborn fetus, uterus, and pelvis published in 1958 by Ian Donald. Advances in transducer technology enabled real-time images to be acquired, and many researchers,...