Penetrating the Invisible: Uncovering Hidden Structures with X-Rays
For over a century, radiography has been a cornerstone of medical diagnostics, offering unprecedented insight into the human body. The advent of X-rays, discovered at the dawn of modern medicine, brought a revolution, as it became possible to see the body's inner structures without invasive procedures.
However, X-rays themselves are invisible, intangible, their presence often revealed only through artifacts on some individuals' eyes or the smell of ozone when passing through the nasal cavity. To be diagnostically useful, X-rays emitting varying intensities as they traverse living tissue must be translated into images. Let's delve into how this transformation occurs.
Snapshots in the Dark
Until the late 19th century, photographic film was the primary means to capture medical X-rays. X-rays had already caused fogging in photographic plates during experiments prior to Wilhelm Conrad Roentgen's systematic study of them in 1895. It was Roentgen's wife, who held her hand between one of his tubes and a photographic plate, that resulted in the first intentional medical X-ray and sparked the field of radiography.
The chemical reaction that makes photographic film sensitive to X-rays is a mirror of the process behind light photography. X-ray film is composed of a thin layer of photographic emulsion on a transparent substrate—originally celluloid, laterpolyester. The emulsion is a mixture of gelatin and silver halide crystals, which react to incident X-rays, creating a latent image on the film that is developed through chemical means.
Direct X-ray imaging onto photographic emulsions was rare, as it required high energy and proper contrast, especially for soft tissues. This led to the development of screen-film radiography, where X-rays passing through the patient were converted to light by intensifying screens, made from plastic sheets coated with phosphorescent materials. These screens were attached to the front and back covers of light-proof cassettes, with double-emulsion film sandwiched between them, exposing the film as the screens glowed.
By amplifying one incident X-ray photon into thousands or millions of visible light photons, intensifying screens dramatically reduce the radiation dose necessary to create diagnostically useful images. However, this comes at a cost as the phosphors spread each X-ray photon across a larger area, leading to a loss in image resolution. When higher resolution is required, single-screen cassettes with one-sided emulsion films can be used, at the expense of increased X-ray dose.
Visualizing Motion
Fluoroscopy, the continuous imaging of body structures in motion, was an early development in radiography. Initially, direct viewing of images created by X-rays passing through the patient onto a phosphor-covered glass screen was utilized. This method necessitated an X-ray tube engineered for a higher duty cycle and placed the radiologist directly in the path of X-rays, posing significant hazards, such as cataracts.
As technology advanced, image intensifiers replaced direct screens in fluoroscopy suites, amplifying the X-ray signal and reducing the radiation dose. The intensifiers employed vacuum tubes with a large input window coated with a fluorescent material like zinc-cadmium sulfide or sodium-cesium iodide. The resulting visible light was immediately converted to photoelectrons by a photocathode, focusing and accelerating the electrons to an anode, and projecting the image onto a smaller, phosphor-covered output screen.
Radiologists initially viewed the output screen through a microscope; later, mirrors, lenses, and cameras were incorporated to project the images onto screens, moving the doctor's head out of the direct line of X-ray exposure. While these advancements improved safety, they could not eliminate the inherent dangers associated with long-term exposure to ionizing radiation.
Points of Interest
Planar detectors, while useful, are not always sufficient, especially when examining a defined volume of tissue. Point detectors, such as Geiger tubes and ionization chambers, measure current created when X-rays ionize low-pressure gas inside an electric field. While these devices are often used in radiological safety and nuclear medicine applications, ionization chambers have been utilized as autoexposure controls in conventional radiography.
Another type of point detector is the scintillation counter, which uses a crystal like cesium iodide or sodium iodide doped with thallium. This crystal releases a few visible light photons when it absorbs ionizing radiation, which is greatly amplified by photomultiplier tubes to produce a pulse of current proportional to the amount of radiation absorbed.
The Digital Age
As solid-state image sensors emerged in the 1980s, the era of traditional film-based radiography began to wane. Digital radiography came with considerable advantages over film development, including reduced infrastructure, expense, and storage requirements.
Solid-state sensors can be categorized as either indirect or direct. Indirect sensor systems consist of a large matrix of photodiodes on amorphous silicon that measure the light produced by a scintillation layer directly above it. Direct sensors, on the other hand, do not rely on converting X-rays to light; they utilize a large flat selenium photoconductor, and X-rays absorbed by the selenium produce electron-hole pairs that migrate to a matrix of fine electrodes on the underside of the sensor. The current across each pixel is proportional to the amount of radiation, allowing for the construction of detailed digital images.
With this restructured understanding of how X-rays are detected and transformed into medical miracles, we now have a deeper appreciation for the science that drives one of medicine's most valuable diagnostic tools.
- In the development of technology, the use of sensors has played a pivotal role in enhancing diagnostics, such as in the case of Geiger tubes and ionization chambers, which function as point detectors, measuring current created when X-rays ionize low-pressure gas.
- In medical diagnostics, advances in technology have gone beyond planar detectors, with visualization of motion achieved through the use of radioactive signals amplified by image intensifiers and continuously projected onto smaller screens in fluoroscopy suites.
- The field of medical diagnostics has expanded beyond mere imaging, thanks to the advent of digital radiography; utilizing sensors like solid-state detectors, we can now construct detailed digital images that offer a more efficient, cost-effective, and storage-friendly alternative to traditional film-based radiography.
