Radiography

Radiography is a sophisticated imaging technique that employs various forms of radiation, ranging from X-rays and gamma rays to other ionizing and non-ionizing forms, to visualize the internal structures of an object. Its applications are diverse, spanning both the medical field—where it is categorized into "diagnostic" and "therapeutic" radiography—and industrial sectors, such as in industrial radiography for quality control. Even in everyday scenarios like airport security, similar principles are applied, often using backscatter X-ray technology in what are commonly known as "body scanners."

The fundamental principle behind creating a radiographic image in conventional radiography involves generating a beam of X-rays from an X-ray generator and directing it towards the object of interest. As this radiation passes through the object, a portion of it is absorbed, with the degree of absorption being dependent on the object's density and its constituent materials. The radiation that successfully traverses the object then strikes a detector, which can be traditional photographic film or a more modern digital sensor. This process, when used to generate flat, two-dimensional images, is specifically termed projectional radiography.

A more advanced application is computed tomography (CT scanning). In this method, the X-ray source and its detectors perform a circular motion around the subject, which is simultaneously moved through the emitted conical X-ray beam. This allows for multiple beams to intersect any given point within the subject from various angles over time. The collected data, reflecting the attenuation of these beams, is then processed computationally to construct detailed images. These images can be presented in three primary planes—axial, coronal, and sagittal—and can be further manipulated to generate a comprehensive three-dimensional representation of the internal anatomy.

Medical Specialty

In the context of medicine, radiography is a cornerstone of diagnosis and treatment. It falls under the broad umbrella of the medical specialty known as Radiology, which encompasses all imaging modalities. Within this specialty, radiography plays a vital role in identifying a wide array of conditions. The Musculoskeletal system is frequently examined using radiography, making it significant in diagnosing conditions like bone fractures. Beyond skeletal issues, radiography is crucial for detecting and monitoring Cancer, and it is integral to various screening tests.

The field involves numerous diagnostic tests, including standard X-ray examinations, CT scans, MRI (which, while not using radiation, is often housed within radiology departments), PET scans, bone scans, ultrasonography, mammography for breast imaging, and fluoroscopy for dynamic imaging. The professionals who operate the equipment and acquire these images are known as Radiographers, while the physicians who interpret the images and provide diagnoses are Radiologists. The specialty is further subdivided into areas such as Interventional, Nuclear, Therapeutic, and Paediatric radiography, each focusing on specific applications and patient populations.

History

The genesis of radiography is intrinsically linked to the discovery of the X-ray itself. On November 8, 1895, Wilhelm Conrad Röntgen, a German physics professor, stumbled upon this new form of radiation while experimenting with cathode rays and a Crookes tube. He observed that these invisible rays could pass through opaque materials, including his wife's hand, revealing the bones beneath when projected onto a screen coated with barium platinocyanide. Röntgen, acknowledging the unknown nature of this discovery, christened it "X" radiation, a designation that has endured. His groundbreaking work earned him the inaugural Nobel Prize in Physics in 1901.

The precise narrative of Röntgen's discovery remains somewhat shrouded, as he reportedly destroyed his laboratory notes. However, a widely accepted reconstruction by his biographers suggests that while investigating cathode rays, he had covered his Crookes tube in black cardboard to block out its visible glow. It was then that he noticed a faint fluorescence emanating from a screen located about a meter away. This led him to deduce that the tube was emitting invisible rays capable of penetrating the cardboard and causing the screen to luminesce—effectively passing through an opaque barrier to affect a material beyond it.

The medical implications of this discovery were realized almost immediately. The first radiograph, a hauntingly intimate image of his wife's hand, prompted her to remark, "I have seen my death." This stark observation underscored the profound ability of X-rays to reveal the unseen within the human body.

The practical application of X-rays in a clinical setting commenced shortly thereafter. On January 11, 1896, John Hall-Edwards in Birmingham, England, performed the first recorded medical radiograph, capturing an image of a needle lodged in a patient's hand. Hall-Edwards further cemented his place in medical history on February 14, 1896, by being the first to utilize X-rays during a surgical operation.

In the United States, the initial medical X-ray was achieved using a discharge tube designed by Ivan Pulyui. Upon learning of Röntgen's discovery in January 1896, Frank Austin of Dartmouth College identified the unique capabilities of the Pulyui tube, which incorporated an oblique "target" of mica. This feature allowed for the observation of fluorescence from samples placed upon it. Subsequently, on February 3, 1896, Gilman Frost, a professor of medicine at Dartmouth, along with his brother Edwin, a physics professor, used the Pulyui tube to radiograph the wrist of a patient named Eddie McCarthy, who had sustained a fracture. The resulting image, capturing the broken bone, was recorded on gelatin photographic plates provided by local photographer Howard Langill, who was also captivated by Röntgen's work.

The year 1897 saw the creation of a sciagraph (an X-ray photograph) of a frog, Pelophylax lessonae (then known as Rana Esculenta), by James Green and James H. Gardiner, featured in their publication "Sciagraphs of British Batrachians and Reptiles."

Diagnostic applications of X-rays were adopted with remarkable speed. For instance, Alan Archibald Campbell-Swinton established a radiographic laboratory in the United Kingdom in 1896, even before the full extent of the dangers associated with ionizing radiation was understood. During World War I, Marie Curie actively advocated for the use of radiography to aid wounded soldiers on the battlefield. Initially, the operation of radiographic equipment in hospitals was carried out by a diverse range of personnel, including physicists, photographers, physicians, nurses, and engineers. Over time, the specialized field of radiology gradually evolved around this nascent technology. As new diagnostic tests emerged, it became a natural progression for radiographers to acquire training in and adopt these advancements. Today, radiographers are proficient in operating not only X-ray machines but also fluoroscopy, computed tomography, mammography, ultrasound, nuclear medicine, and magnetic resonance imaging. While a basic dictionary definition of radiography might limit it to "taking X-ray images," this represents only a fraction of the responsibilities held by those working in "X-ray departments," radiographers, and radiologists. Early radiographic images were referred to as roentgenograms, a term derived from Röntgen's name. The term "skiagrapher," originating from Ancient Greek words for "shadow" and "writer," was in use until approximately 1918 to denote a radiographer. Interestingly, the Japanese term for a radiograph, rentogen (レントゲン), shares its etymological roots with the original English term.

Medical Uses

The application of radiography in medicine is extensive, serving as a critical tool for both diagnosis and, in some forms, treatment.

Medical Diagnostic Method

The fundamental principle of medical radiography relies on the differential interaction of radiation with the various tissues and substances within the human body. Since the body is composed of materials with varying densities, ionizing and non-ionizing radiation can be employed to delineate internal structures. This is achieved by highlighting differences in attenuation – the reduction in intensity of a radiation beam as it passes through matter. In the case of ionizing radiation like X-rays, denser substances, such as calcium-rich bones, absorb more photons, appearing lighter on the resulting image. The study of anatomy through these radiographic images is termed radiographic anatomy. Typically, the acquisition of medical radiographic images is performed by radiographers, while the interpretation and diagnostic analysis are the responsibility of radiologists. However, some radiographers also specialize in image interpretation. Medical radiography encompasses a spectrum of modalities, each producing distinct image types tailored for specific clinical applications.

Projectional Radiography

The process of creating images by exposing an object to X-rays or other high-energy forms of electromagnetic radiation and then capturing the remaining radiation, often referred to as the "shadow," is known as "projection radiography." This captured remnant beam forms a latent image that can then be rendered visible. The "shadow" can be converted into visible light using a fluorescent screen, which is then recorded on photographic film. Alternatively, the image can be captured by a phosphor screen that is later "read" by a laser (in computed radiography, CR), or it can directly activate a matrix of solid-state detectors (in direct radiography, DR), functioning similarly to a large version of a CCD found in digital cameras. Projectional radiography is particularly well-suited for visualizing bone and certain organs, such as the lungs, due to their inherent differences in density and composition. This method is generally cost-effective and offers a high diagnostic yield, making it a staple in medical imaging. The contrast observed between soft tissues and denser structures like bone is largely attributable to the significantly lower X-ray cross-section of carbon compared to calcium.

Computed Tomography

Computed tomography, commonly known as CT or CAT (Computed Axial Tomography) scanning, utilizes ionizing X-ray radiation in conjunction with sophisticated computer processing to generate detailed images of both soft and hard tissues. The resulting images provide cross-sectional views, akin to "slices" of the body, hence the term "tomography" (from the Greek tomo, meaning "slice"). While CT scans employ a higher dose of ionizing radiation than standard diagnostic X-rays, advancements in technology have led to significant reductions in both radiation levels and scan durations. CT examinations are typically brief, often completed within the time it takes for a patient to hold their breath. The use of contrast agents is frequently employed to enhance the visibility of specific tissues or structures. Radiographers conduct these examinations, often in collaboration with radiologists, particularly in cases of CT-guided biopsy procedures where the radiologist guides the needle using real-time CT imaging.

Dual Energy X-ray Absorptiometry

DEXA, also referred to as bone densitometry, is primarily utilized for assessing osteoporosis by measuring bone mineral density. Unlike projectional radiography, DEXA employs two narrow X-ray beams emitted at different energy levels that are scanned across the patient at a 90-degree angle to each other. The examination typically focuses on the hip (specifically the head of the femur), the lower back (lumbar spine), or the heel (calcaneum). The amount of calcium present in the bone is quantified, yielding a T-score that indicates the level of bone density. It is important to note that DEXA is not suitable for diagnosing fractures or inflammation due to its limited image quality for such purposes. While it can also be used to estimate total body fat, this application is less common. The radiation dose administered during DEXA scans is notably low, significantly less than that associated with projectional radiography.

Fluoroscopy

The term Fluoroscopy was coined by Thomas Edison during his early investigations into X-rays, inspired by the fluorescence he observed when a plate was bombarded with these rays. This technique generates dynamic, moving projection radiographs, allowing for the visualization of motion within the body. Fluoroscopy is predominantly used to observe the movement of tissues, the flow of a contrast agent, or to guide medical interventions. Examples include procedures like angioplasty, pacemaker insertion, or joint repair and replacement surgeries. In surgical settings, a portable fluoroscopic unit known as a C-arm can be maneuvered around the operating table to provide real-time digital images to the surgeon. Biplanar Fluoroscopy enhances this capability by displaying images in two planes simultaneously, which is particularly beneficial in orthopedic and spinal surgeries, potentially reducing operative times by eliminating the need for patient repositioning.

Angiography is a specific application of fluoroscopy focused on visualizing the cardiovascular system. A contrast agent, typically iodine-based due to its high density, is injected into the bloodstream and tracked as it circulates. This allows for the detection of abnormalities such as aneurysms, leaks, blockages (thromboses), the growth of new vessels, and the precise placement of catheters and stents. Balloon angioplasty is frequently performed in conjunction with angiography. The image displays a transverse projection of the vertebro basilar and posterior cerebral circulatory systems.

Contrast Radiography

Contrast radiography involves the administration of a radiocontrast agent, a type of contrast medium, to enhance the visibility of specific structures against their background. These agents are essential for conventional angiography and can be utilized in both projectional radiography and computed tomography, where the examination is then referred to as contrast CT.

Other Medical Imaging

While not strictly radiographic techniques as they do not utilize X-rays, modalities such as PET and MRI are often grouped under the umbrella of radiography within hospital radiology departments because these departments manage all forms of medical imaging. It is also important to distinguish radiography from radiotherapy, which is the treatment of disease using radiation.

Industrial Radiography

Industrial radiography is a critical method of non-destructive testing used to examine manufactured components and verify their internal structure and integrity without causing damage. This technique can be implemented using either X-rays or gamma rays, both of which are forms of electromagnetic radiation. The distinction between these forms of radiation lies in their wavelength; X-rays and gamma rays possess the shortest wavelengths, enabling them to penetrate various materials, including carbon steel and other metals. Industrial computed tomography is a specific application within this field. Radiography has also found use in paleontology, exemplified by the radiographs taken of the Darwinius fossil, affectionately known as Ida.

Image Quality

The quality of a radiographic image is determined by two primary factors: resolution and density. Resolution refers to the ability of the imaging system to distinguish between closely spaced structures within the object as separate entities in the final image. Density, in this context, relates to the image's "blackening power," or how dark it appears. The sharpness of a radiographic image is significantly influenced by the size of the X-ray source, which is dictated by the area of the electron beam impinging upon the anode. A larger photon source leads to increased blurring in the resultant image, a phenomenon exacerbated by a greater distance between the object and the image receptor. This blurring can be quantified as a component of the modulation transfer function of the imaging system.

Radiation Dose

The amount of radiation administered during radiographic procedures varies considerably depending on the specific examination. For instance, a standard chest X-ray typically involves an effective dose of approximately 0.1 millisieverts (mSv), whereas an abdominal CT scan can range up to 10 mSv. Organizations such as the American Association of Physicists in Medicine (AAPM) have posited that the risks associated with medical imaging at patient doses below 50 mSv for single procedures, or 100 mSv for multiple procedures within a short timeframe, are either too low to be detectable or potentially non-existent. Similar conclusions have been reached by other scientific bodies, including the International Organization of Medical Physicists, the UN Scientific Committee on the Effects of Atomic Radiation, and the International Commission on Radiological Protection. Nevertheless, leading radiological organizations, such as the Radiological Society of North America (RSNA) and the American College of Radiology (ACR), along with various governmental agencies, maintain stringent safety standards to ensure that radiation doses are kept as low as reasonably achievable.

Shielding

Lead is the material of choice for shielding against X-rays due to its high density (11,340 kg/m³), its remarkable stopping power, and its relative affordability and ease of installation. The attenuation of a photon beam through matter follows an exponential decay. This means that doubling the thickness of the shielding material will square its shielding effect. The table below outlines the recommended minimum thickness of lead shielding required for X-ray rooms, dependent on the peak voltage generated by the X-ray machine.

X-rays generated by peak voltages below	Minimum thickness of lead (depending on machine)
75 kV	1.0 mm
100 kV	1.5 mm
125 kV	2.0 mm
150 kV	2.5 mm
175 kV	3.0 mm
200 kV	4.0 mm
225 kV	5.0 mm
300 kV	9.0 mm
400 kV	15.0 mm
500 kV	22.0 mm
600 kV	34.0 mm
900 kV	51.0 mm

Historically, beginning in the 1950s, personal lead shielding was routinely placed directly on patients during abdominal X-rays to protect the gonads or a developing fetus in pregnant individuals. Similarly, dental radiography often included lead shielding for the thyroid. However, a consensus has emerged between 2019 and 2021, supported by multiple studies and recommendations, that routine gonadal shielding during diagnostic X-rays is generally unnecessary and may even be counterproductive in certain situations. Nevertheless, personal shielding for medical professionals and others present in the examination room remains a standard recommendation.

Campaigns

In response to heightened public concern regarding radiation exposure and the continuous evolution of best practices, The Alliance for Radiation Safety in Pediatric Imaging was established under the auspices of the Society for Pediatric Radiology. In collaboration with the American Society of Radiologic Technologists, the American College of Radiology, and the American Association of Physicists in Medicine, this society launched the Image Gently campaign. This initiative aims to maintain the highest quality of diagnostic imaging while employing the lowest radiation doses and adhering to the most effective radiation safety practices for pediatric patients. The campaign has garnered widespread endorsement and adoption by numerous international medical organizations and has received support from leading equipment manufacturers in the radiology sector.

Building on the success of the Image Gently campaign, the American College of Radiology, the Radiological Society of North America, the American Association of Physicists in Medicine, and the American Society of Radiologic Technologists introduced a parallel initiative for the adult population, known as Image Wisely. This campaign addresses similar concerns regarding radiation safety in adult medical imaging. Furthermore, the World Health Organization and the International Atomic Energy Agency (IAEA), both United Nations entities, are actively engaged in efforts to promote best practices and reduce patient radiation doses through various ongoing projects.

Provider Payment

Contrary to the principle of performing radiographs solely when indicated for patient benefit, recent evidence suggests that the frequency of radiographic examinations may increase when dental practitioners are compensated under a fee-for-service model. This indicates a potential influence of financial incentives on diagnostic decision-making.

Equipment

The apparatus used in radiography has evolved significantly since its inception.

Sources

In medical and dental applications, projectional radiography and computed tomography images are predominantly generated using X-rays produced by X-ray generators, which typically house X-ray tubes. The resultant images are formally known as "radiograms" or "roentgenograms" for projection radiography, and "tomograms" for CT scans.

Beyond conventional X-ray tubes, other sources of X-ray photons exist and are utilized in industrial radiography and scientific research. These include betatrons, linear accelerators (linacs), and synchrotrons. For generating gamma rays, radioactive sources such as 192 Ir, 60 Co, or 137 Cs are commonly employed.

Grid

An anti-scatter grid is a device that can be strategically placed between the patient and the detector. Its purpose is to absorb scattered X-rays, which are radiation photons that have been deflected from their original path, thereby reducing the amount of scattered radiation that reaches the detector. While this significantly improves the contrast resolution of the image, it comes at the cost of increased radiation exposure for the patient, as more radiation is required to achieve an adequate signal-to-noise ratio.

Detectors

Detectors in radiography can be broadly categorized into two main types: imaging detectors and dose measurement devices. Imaging detectors are responsible for capturing the radiation and converting it into a visible image. Historically, this included photographic plates and X-ray film (photographic film), though these have largely been superseded by various digitizing technologies. Modern imaging detectors include image plates (used in CR systems) and flat panel detectors (used in DR systems). Dose measurement devices, such as ionization chambers, Geiger counters, and dosimeters, are used to quantify the local radiation exposure, dose, and/or dose rate. These devices are crucial for verifying the effectiveness of radiation protection measures and ensuring adherence to safety protocols on an ongoing basis.

Side Markers

To ensure accurate anatomical identification of radiographic images, radiopaque anatomical side markers are incorporated into the imaging process. For example, if a patient's right hand is being X-rayed, a radiopaque "R" marker is placed within the field of the X-ray beam. This marker serves as a definitive indicator of which side of the body has been imaged. In instances where a physical marker cannot be included, the radiographer may digitally add the correct side marker during post-processing of the image.

Image Intensifiers and Array Detectors

As an alternative to direct X-ray detectors, image intensifiers are analog devices that efficiently convert an acquired X-ray image into a visible image displayed on a video screen. These devices consist of a vacuum tube with a large input surface coated with caesium iodide (CsI). When X-rays strike the CsI phosphor, it emits light, which in turn causes a photocathode adjacent to it to release electrons. These electrons are then accelerated and focused by electron lenses within the intensifier towards an output screen coated with phosphorescent materials. The intensified image on the output screen can be recorded by a camera and displayed.

In contemporary fluoroscopy, digital devices known as array detectors are increasingly prevalent. These detectors are composed of discrete pixelated elements, often thin-film transistors (TFT). They can function either indirectly, utilizing photodetectors that capture light emitted from a scintillator material like CsI, or directly, by capturing electrons generated when X-rays strike the detector. Direct detectors tend to exhibit superior sharpness and reduced blurring compared to indirect detectors, as they are activated directly by X-ray photons, circumventing the spreading effects associated with phosphorescent scintillators or film screens.

Dual-Energy

Dual-energy radiography is a technique where images are acquired using two distinct tube voltages. This method is the standard for bone densitometry (DXA). It is also employed in CT pulmonary angiography to reduce the required dose of iodinated contrast agents.