Behind the Cover: September 2021

Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.

The cover: The longest bone in the human body, the femur connects to the pelvis in a classic ball-and-socket joint, which defines the hip. The high-resolution computed tomography (CT) scan of the bone on this month’s cover, which shows a thin coronal (or frontal) cross section created from a three-dimensional data set, reveals the bone’s microstructure. The overlaid color scale indicates the fraction (by volume) of bone mineral, from low (blue) to high (red). The mineral and its distribution in the bone can be used to define a risk metric for bone fracture in a patient. (Image courtesy of Lance Frazer/Southwest Research Institute.)

The science: The CT scan is an oft-used diagnostic procedure that can inform physicians about bone strength, fracture, soft tissue injury or disease, and many abnormalities. CT images are produced from millions of individual x-ray transmission measurements, and the color scale reveals both mass density and effective atomic number.

This year marks the 50th anniversary of clinical CT. The article by medical physicists John Boone and Cynthia McCollough provides an account of the technological evolution of the important diagnostic tool, from its birth at the Central Research Laboratories of the British music company EMI to its current use in 90 million medical examinations each year in the US. The collaborators and friends discuss how concurrent advances in technology—including composite materials, x-ray tube design, precision bearing manufacture, faster computers, and new computer algorithms—all contributed to the technical sophistication and clinical success of CT.

The design: Once the art and editorial teams selected the femur image for the cover, the art team picked out the most vibrant color from the CT scan for the Physics Today logotype to amplify its contrast against the dark background.