Principles of Radiation Physics Part 2: The X-Ray Beam (ARRT Registry Prep)

Understanding the nature of the x-ray beam is essential for any radiologic technologist aiming to master both the science and the art of imaging. In this second installment on radiation physics, we go beyond the tube and into the beam itself—unpacking how it behaves, what defines its characteristics, and why its properties matter in clinical practice.

Let’s start with two fundamental descriptors of electromagnetic energy: frequency and wavelength.

Frequency and Wavelength: The Invisible Architects of Energy

X-rays are a form of electromagnetic radiation, traveling as oscillating waves of electric and magnetic energy. These waves vary in two key ways: frequency (how often the wave cycles per second) and wavelength (the distance between successive peaks or troughs).

Frequency is measured in hertz (Hz), where one hertz equals one cycle per second. Wavelength is commonly expressed in angstroms (Å) or nanometers (nm). The diagnostic x-ray spectrum typically operates within a wavelength range of 0.1 to 0.5 angstroms—extremely short wavelengths, indicating extremely high frequencies.

Why does this matter? Because frequency and wavelength determine the energy of an x-ray photon. They are inversely related: as wavelength decreases, frequency increases, and with it, photon energy rises. A photon with a short wavelength and high frequency carries more energy and penetrates tissues more effectively.

This relationship sets the stage for understanding the quality and quantity of the x-ray beam.

Beam Characteristics: Quality and Quantity

In radiologic science, the x-ray beam isn’t just a flood of random energy—it’s a carefully engineered stream with specific traits that influence how well it can create an image and how much radiation dose it delivers to the patient. These traits fall into two categories: quality and quantity.

Beam Quality: Penetrating Power

The quality of an x-ray beam refers to its penetrating power, determined primarily by the kilovolt peak (kVp) setting on the control panel. A higher kVp accelerates electrons across the tube with greater force, which results in higher-energy x-ray photons with shorter wavelengths and higher frequencies.

Beam quality affects:

• Image contrast: Higher kVp lowers image contrast, while lower kVp increases it.

• Tissue penetration: Higher-energy photons can penetrate denser tissues.

• Patient dose: Higher kVp settings often allow for reduced mAs, decreasing overall exposure time.

Quality is critical when imaging various anatomical regions. A chest x-ray, for example, may require a higher kVp to penetrate the lungs and mediastinum effectively, while a hand x-ray benefits from lower kVp to preserve contrast and detail.

Beam Quantity: Photon Count

Quantity refers to the number of photons in the x-ray beam. This is determined by the milliampere (mA) setting and the exposure time, collectively known as mAs (milliampere-seconds). The mA controls how many electrons are boiled off at the cathode, and therefore, how many x-ray photons are produced when those electrons strike the anode.

Beam quantity affects:

• Image brightness (in traditional film or exposure in digital systems)

• Radiation dose to the patient

• Signal-to-noise ratio in digital imaging

The relationship is direct: double the mAs, and you double the number of photons. But unlike kVp, increasing mAs doesn’t affect photon energy—it only increases the volume of x-rays, not their strength.

Primary vs. Remnant Beam: Two Halves of the Imaging Story

To truly understand how an x-ray image is formed, it’s essential to distinguish between the primary beam and the remnant beam—each plays a critical role in the journey from energy to image.

The Primary Beam

The primary beam refers to the stream of x-ray photons emitted directly from the x-ray tube, prior to interacting with the patient. It originates at the focal spot on the anode target, travels through the tube window and collimator, and enters the patient’s body. This beam is polyenergetic and divergent, meaning it contains photons of varying energies and spreads outward in all directions from a point source.

All rays in the primary beam diverge except the central ray, which travels perpendicular to the image receptor. This central ray is often aligned with the center of the anatomy being imaged and the center of the IR, ensuring accurate representation and minimizing distortion.

Understanding the divergence of the primary beam is crucial because the intensity of the beam diminishes as it spreads—leading us directly to the inverse square law, which we’ll explore shortly.

The Remnant (Exit) Beam

The remnant beam—also known as the exit beam or signal—is what remains of the primary beam after it has passed through the patient. This is the portion that carries the anatomical information and ultimately forms the image on the detector.

The remnant beam is made up of:

• Transmitted photons: those that pass through the patient without interaction, providing information about less dense structures.

• Scattered photons: those deflected by interactions (mostly Compton scatter) within the body, which can degrade image quality.

• Secondary radiation: radiation produced inside the patient when photons interact with tissue atoms.

Critically, the remnant beam represents only a small fraction of the primary beam—typically less than 1%. But this fraction is what delivers the visual contrast between different tissues, forming the diagnostic image.

The Inverse Square Law: Intensity and Distance

One of the most important physical principles governing the behavior of the x-ray beam is the inverse square law. This law describes how the intensity of radiation decreases with increased distance from the source.

Mathematically, the law is written as:

$$I_1 / I_2 = (D_2 / D_1)^2$$

Where:

• $I_1$ and $I_2$ are the original and new intensities

• $D_1$ and $D_2$ are the original and new distances

What This Means Clinically

If you double the distance from the x-ray source to the image receptor, the intensity of the beam decreases by a factor of four (since $2^2 = 4$). Conversely, if you halve the distance, the beam becomes four times more intense.

This concept is more than just physics—it has direct clinical implications:

• Changing the source-to-image distance (SID) affects the brightness of the image and the exposure to the patient.

• If SID increases, you need to increase mAs to maintain consistent exposure.

• If SID decreases, you must reduce mAs to avoid overexposure.

Understanding the inverse square law allows radiographers to anticipate how adjustments in positioning or equipment setup will impact image quality and patient dose. It's also key in mobile imaging scenarios, where distance between the source and receptor is often less consistent than in stationary radiography.

Fundamental Properties of the X-Ray Beam

To round out our exploration of the x-ray beam, we’ll focus on the intrinsic properties that define how x-rays behave in the physical world. These properties are not settings that technologists can adjust—they are constants rooted in the nature of electromagnetic radiation itself. Understanding these traits allows you to predict how x-rays interact with matter and informs every aspect of clinical radiography.

1. X-rays Travel in Straight Lines

One of the simplest yet most important properties: x-rays travel in straight lines. This principle underpins the entire practice of radiographic imaging. Because x-rays don’t curve or bend around objects, precise positioning is essential. Misalignment between the beam, patient, and image receptor can lead to distortion, magnification, or missed anatomy.

This property also explains why collimation is so effective. By shaping the x-ray field, collimators ensure only desired anatomy is irradiated—nothing outside the beam path will be exposed.

2. X-rays Can Penetrate Matter

Unlike visible light, x-rays have the power to penetrate materials, especially soft tissues. The extent of penetration depends on the energy of the x-ray photons and the density and thickness of the object in their path.

Higher-energy photons (shorter wavelengths, higher frequencies) penetrate more deeply. This property is why x-rays can pass through the human body and create images of internal structures. It also explains the importance of selecting appropriate kVp levels—too low, and the beam can’t reach the image receptor; too high, and image contrast suffers.

3. X-rays Can Be Absorbed or Scattered

When x-rays interact with matter, they may be:

• Absorbed (as in the photoelectric effect)

• Scattered (as in Compton interactions)

• Transmitted (pass through unchanged)

Absorption is what creates image contrast—bone absorbs more radiation than soft tissue, leading to varying degrees of brightness on the image. Scatter, however, is undesirable. It adds noise, reduces contrast, and increases radiation exposure to both patients and staff.

That's why radiation protection strategies like collimation, grids, and proper positioning are crucial to managing scatter and preserving image quality.

4. X-rays Can Ionize Matter

Ionization is the process of removing or adding electrons to atoms, creating charged particles. X-rays are capable of ionizing atoms, particularly in human tissue. This is both a powerful tool and a potential hazard.

In diagnostic imaging, ionization allows x-rays to interact with film, digital detectors, and biological tissues, producing the contrast needed to form an image. However, ionization also carries biological risks, including cellular damage, DNA mutations, and—in excessive doses—cancer.

This is why radiation safety protocols and adherence to ALARA (As Low As Reasonably Achievable) principles are essential in every imaging environment.

5. X-rays Have No Mass and No Charge

X-ray photons are pure energy. They have no mass and no electrical charge, which allows them to pass through tissues without being deflected by magnetic or electric fields. This property also means they do not leave a trail behind—they only interact at the moment they encounter matter.

6. X-rays Travel at the Speed of Light

In a vacuum, x-rays travel at the speed of light—approximately 186,000 miles per second (or 3 × 10⁸ meters per second). This rapid travel time ensures that exposure and image formation occur almost instantaneously, making real-time imaging techniques like fluoroscopy possible.

7. X-rays Are Polyenergetic and Heterogeneous

The beam produced in diagnostic radiography is not uniform. It contains photons of varying energies due to how electrons interact with the anode. This polyenergetic nature contributes to image contrast but also requires careful control of beam quality to avoid excessive patient dose or poor image detail.

8. X-rays Cannot Be Focused by a Lens

Unlike visible light, x-rays cannot be refracted or focused using glass lenses. Their extremely short wavelength and high energy mean they pass through most materials. Instead, beam direction and shape must be controlled mechanically—through collimators, shutters, and angulation.

9. X-rays Can Cause Fluorescence and Biological Changes

X-rays can stimulate certain materials (like phosphors) to emit light—a property used in intensifying screens and image receptor technology. More critically, x-rays can also induce biological changes in living tissue, including both therapeutic effects (as in radiation therapy) and harmful side effects.

Final Thoughts

The x-ray beam is more than a stream of invisible energy—it is the product of carefully balanced physics, precision engineering, and clinical decision-making. By understanding the principles of frequency and wavelength, recognizing the implications of beam quality and quantity, mastering the inverse square law, and respecting the fundamental properties of x-rays, technologists gain control over their imaging practice.

Whether you are positioning a patient, selecting exposure factors, or optimizing image quality, your understanding of the beam is what turns routine imaging into diagnostic excellence.