The medical specialty of Radiology is intimately connected to the themes of the International Year of Light and Light-based Technologies 2015 (IYL 2015). More than any other profession, radiologists and radiologic technologists put theoretical quantum physics to practical use to improve the health and lives of their patients. Although quantum light theory can explain everything from the tiniest subatomic particles to immense galaxy-devouring black holes, radiologists apply this technology at the human level to diagnose and treat disease and thus alleviate human suffering.
More than 100 years ago in 1895, Wilhelm Conrad Roentgen discovered a form of radiation which had strange new properties. These new rays were so unique and mysterious that he named them “X-rays”, for the unknown. Although often described as a fortuitous discovery, chance favors the prepared mind, and Roentgen’s astute observations back then are still accurate today.
- transmit in complete darkness
- are invisible to the human eye
- originate from a cathode ray tube
- expose covered photographic plates
- have diminished intensity that follows the inverse square law of light emission
- make human flesh appear transparent, but metal and bone appear opaque. The transparency of intervening objects depends on their molecular density and thickness
- are not reflected by mirrors nor deflected by glass prisms
- travel at a constant speed – the speed of light
- share some properties with visible light, yet also have uniquely different properties
For the very first time, doctors (without using a scalpel) could see beyond the skin surface of their patients and peer deep inside the human body. It was later found that X-rays were a form of electromagnetic radiation with wavelengths shorter and with energies greater than visible light. Subsequent research into particle theory by Albert Einstein and others led to the physics principles that not only laid the groundwork for state-of-the-art medical imaging but also changed the understanding of our entire universe, from the mechanics of the atom to the largest objects in the universe. In 1901, Roentgen received the very first Nobel Prize awarded in physics, an indication that his discovery of a form of invisible light was the beginning of a remarkable scientific journey.
Albert Einstein’s theories of relativity soon followed and would explain the space time continuum and the equivalence of mass and energy. Throughout his brilliant career, Einstein was fascinated and preoccupied with the strange properties of light. Einstein once said, “For the rest of my life I will reflect on what light is.” His concept of special relativity came to him when he was riding his bicycle towards a lamp post. He realized that the speed of light was the only constant for all observers and that the classic Newtonian measurements of mass, distance, and time were all subject to change at velocities approaching the speed of light. Einstein’s relativity means that the science fiction adventures of galaxy-hopping space travel in Star Trek and Star Wars are mere fantasy. The vast distances of space and the universal speed limit of light make intergalactic travel too impractical. If a hypothetical space craft approaches the speed of light, time slows, length compresses, the mass of the space craft increases, and impossibly high amounts of energy are required. At a certain point, the space craft stops accelerating, despite greater and greater energy input.
A result of Einstein’s special theory of relativity has been called the most famous equation in all of science. Energy (E) equals mass (m) multiplied by the speed of light squared (c2), that is E=mc2. This simple equation, which states that energy and mass are interchangeable quantities, is often misinterpreted as the formula of the atomic bomb. The principle of the atomic bomb is bombardment of a uranium atom with a neutron that splits the uranium atom into two smaller atoms and more neutrons that trigger a fission chain reaction. Although tremendous energy is released, it is the energy of internuclear binding forces, and there is no appreciable change in mass. A much better demonstration of E=mc2 is the physics of positron emission tomography (PET scanning), in which an electron and positron (the antiparticle of an electron) annihilate each other and convert their masses into pure light energy, consisting of photons traveling in opposite directions. This light is detected and calculated as a three dimensional image of the patient. Einstein was another founder of radiology because his theory of the Photoelectric Effect (published in 1905 and awarded the Nobel Prize in 1921) explained how X-rays interact with matter. This theory also showed that light was absorbed and emitted in discreet packets of energy, leading to the Quantum Theory revolution in physics.
Following Einstein’s ideas that light was transmitted in packets of energy, Niels Bohr and Werner Heisenberg developed a model of the atom that diverged from classic Newtonian physics. The Rutherford atomic model consisting of electrons orbiting the central nucleus was inadequate because charged particles changing direction in an orbit would lose energy and fall into the nucleus. Bohr’s model had to explain the Photoelectric Effect, chemical reactions, and the inherent stability of atoms. A carbon atom can undergo countless chemical reactions yet remains a carbon atom. As Bohr further investigated the atom, the simplistic idea of light just being a wave and electrons just being particles was no longer valid. With the Photoelectric Effect, Einstein showed that light could be a photon particle. Louis de Broglie then showed that particles could be waves. Both photons and electrons have particle-wave duality. The electron therefore could exist as a standing wave around the nucleus, absorb and emit quanta of light energy, and yet remain stable. The paradoxes that resulted from Bohr’s quantum theory shook the foundations of science. Werner Heisenberg found that the method of investigation alters the result of an experiment. He explained this idea mathematically in his Uncertainty Principle, which remains a major tenet of quantum mechanics. The light used to measure particles imparts energy, altering the momentum or location of the particles, thus changing the results by the mere act of observation. An experiment can be designed to measure either momentum or location precisely, but not both (the experimenter must choose).
“The violent reaction on the recent development of modern physics can only be understood when one realizes that here the foundations of physics have started moving; and that this motion has caused the feeling that the ground would be cut from science.” – Werner Heisenberg
This finding was unsettling for physicists who strove for precise measurements, because precision was not possible at the atomic and subatomic levels. Heisenberg showed that every experiment (and radiologic examination) is subject to limitation. Einstein objected to this inherent fuzziness, stating that “God does not play dice with the Universe.”
The Doppler Effect
Christian Doppler was a professor who studied mathematics, physics, and astronomy. He published a paper on spinning binary star systems, noting that starlight shifts to the violet spectrum when a star is moving toward an observer on Earth, and that starlight shifts to the red when a star is moving away. The explanation was that the wavelength of the light wave was compressed or elongated depending on the motion of the source relative to the observer. When the Doppler Effect is applied to sound, it explains the tone of an approaching or departing train whistle; when applied to radar it predicts violent weather; when applied to ultrasound (another radiology modality) it determines the direction and velocity of blood flow; and when applied to distant starlight it explains our expanding (red shifted) universe. Using Doppler ultrasound, a technologist can screen for: the risk of stroke from carotid artery stenosis, renal arterial causes of hypertension, abdominal aortic aneurysms, peripheral vascular disease, deep vein thrombosis, portal vein thrombosis and varices, and post-catheterization pseudo-aneurysms. Countless lives have been saved or improved because of a phenomenon originally observed in starlight. Doppler’s idea extends well beyond the sonography suite and even tells us about the origins of our universe. Edwin Hubble demonstrated that all objects observed in deep space have a Doppler red-shifted velocity that is proportional to the object’s distance from the Earth and all other interstellar bodies. This tells us that our universe is expanding and supports the theory that the universe was created by the Big Bang, which occurred about 13.7 billion years ago.
Old Master Painters
Artists such as Rembrandt and Vermeer (17th century) were adept at depicting light to create the illusion of realistic three dimensional subjects on two dimensional canvases. These artists studied the interaction of light with their models and understood visual perception of subtle shading and light to make their artwork dramatic and convincing. Rembrandt van Rijn’s famous portraits and self-portraits displayed skill with light source positioning and intensity, later duplicated by movie director Cecil B DeMille who coined the term “Rembrandt lighting,” a technique that is still used today by portrait photographers. Johannes Vermeer was skilled at depicting subjects in naturally lit interiors with a subtle photorealistic style that is considered uncanny even today. Some believe Vermeer used special optics and mirrors because his depiction of light was too subtle for the naked eye to detect. For example, scientific analysis showed that his backgrounds demonstrated the inverse square law, with exponential diffusion of light, which is difficult to capture when using only an artistic eye. Experienced radiologic technologists use artistic vision when they create radiographs. By positioning and framing their subjects and by adjusting contrast and exposure, each image can be a work of art, not only pleasing to the eye but also containing a wealth of information.
Light as the Medium for Medical Imaging
Light, as visual information, is portrayed in art. Light also is the medium for medical imaging, whether in the form of a backlit film, cathode ray tube monitor, liquid crystal display screen, or plasma monitor. The eye is our most complex and highly evolved sense organ, capable of detecting subtle changes in light and color, and transferring this information (via the optic nerves and optic tracts) to the visual cortex of our occipital lobes. However, what distinguishes artists and seasoned radiology professionals from other people is post-processing (i.e., the thinking that occurs after perceiving visual data). Much of science and medicine is about logic, language, analysis, and categorization (left brain functions). However, visual processing (the artistic eye) is about conceptualization, spatial orientation, and pattern recognition (right brain functions). These right brain skills are harder to teach and measure but are just as important in radiology. With the rapid increases in digital image resolution and in the number of multi-planar images involved with each case, developing the right brain is crucial to make sense of this visual information overload. Knowingly or unknowingly, seasoned radiologists develop the right side of their brains through the experience of viewing thousands of medical images. This “artistic eye” can be further enhanced in radiologists and radiologic technologists who appreciate the techniques used by great artists. Or better yet, they can train their right brains by creating original art themselves.
Radiologists and radiologic technologists use light technology and artistic vision in their daily work. They sense subtle shades, recognize patterns, and use symmetry and balance to detect abnormalities. When this artistic skill is applied in combination with an appreciation for the underlying physics that created the images, a thorough knowledge of human anatomy, and an understanding of the pathophysiology of disease, they serve their patients by providing timely diagnosis and excellent medical care.
To learn more about Dr. Hom’s writings, concepts, and artwork, please refer to his recent articles and new book:
The Art and Science of Light: An Illustrated Retrospective, Mark Hom, Radiologic Technology, July/Aug 2015 86 (6), 702-708.
The Artistic Eye and the Radiologist, Mark Hom, American Roentgen Ray Society, Senior Radiologists Section Notes, Fall 2014.
The Science of Fitness: Power, Performance, and Endurance, Greg LeMond and Mark Hom Publisher: Elsevier, December 2014.
Mark Hom, M.D. is a Johns Hopkins University trained biologist, an award winning artist and medical illustrator, a practicing interventional radiologist, a book author published by Elsevier/Academic Press, and an assistant professor of radiology affiliated with the Virginia Commonwealth University School of Medicine.