Basics of X-Ray : Residency Radiology

Introduction

Radiology or Radio diagnosis deals with radiation involving investigations, nuclear medicine, and radiotherapy. Ionizing radiation is the amount of damage that occurs when DNA emits free radicals due to radiation. Ionizing Radiation is also known as Damaging Power or LET (Linear Energy Transfer).

Types of Radiation

Radiation can be of two main types. The first one is Particulate Radiation, which includes Alpha (α) rays, Beta (β) rays, and Neutrons. The second type of Radiation is Electromagnetic Radiation (EMR), which includes X-rays and Gamma (γ) rays. 

The Ionizing power of Alpha (α)particles is the greatest, followed by Beta (β) rays and lastly by X-rays and Gamma (γ) rays. The Penetration power is also the highest for Alpha (α) rays.

Alpha particles have minimum penetration power. They stay in the body for maximum periods and show effects.

Brief comparison of X-rays with Gamma Rays

The energy, frequency, and penetration of X-rays are lesser than gamma rays. However, the wavelength of X-ray is a lot higher than Gamma rays. The origin of X-rays is extranuclear, and the gamma rays originate from the Nucleus.

X-ray Production

X-rays are produced by the sudden stopping of a fast-moving electron beam. The beam converts kinetic energy to 99% heat and 1% x-rays.

W.C. Roentgen accidentally discovered X-rays on November 8, 1895. This day is now known as the International Day of Radiology. Roentgen took the first human X-ray of his wife. Generally, anything unknown in mathematics is described as 'x' so, these unknown rays were named X-rays.

Working Principle of Xray

The Cathode (Filament) is an electron source that produces electron beams. The anode is a Tungsten target that stops electron beams. The electron beam travels from the cathode to the anode. The thermoionic emission cathode is heated to a high temperature and emits electrons. The anode blocks the beam, and 1% of X-rays escape through the window onto a film. The remaining 99% of heat dissipation is by radiation.

It is crucial to remember that the X-ray tube's filament is made of tungsten. The atomic number of tungsten (W) is 74. It has a high melting point. Nowadays, tungsten is added along with thorium. The Anode is made of tungsten and rhenium. Nowadays, rotating anodes are used to avoid focal points as in stationary anodes. This increases the shelf life of an anode. X-ray tubes are made of Pyrex glass, ceramic, or metal.

The majority of anodes are rotatory anodes, which is most suitable. Stationary anodes are used in case of portable X-ray units, Emergency Departments, and Dental clinics

A Focusing cup is used to focus the beam in a straight line and prevent reflection. It is made of nickel.

X-ray Tube Components

The main components of an X-ray tube are:

• Glass envelope which is made up of Lead (Pb). 
• The cathode is made up of Tungsten and Thorium.
• The anode is made up of Tungsten and Rhenium.
• The focusing cup is made up of Nickel.

Bremsstrahlung versus Characteristic radiation

When an electron (negatively charged) beam is traveling toward the anode, the nucleus (positively charged) gets deflected and travels in a different direction. The speed of the beam reduces. For example, 100 kmph reduces to 40 kmph.

Energy can neither be created nor destroyed; it can only be transferred to another form. The remaining energy is converted to another form. This is known as Bremsstrahlung radiation or braking radiation. It is a polyenergetic, continuous spectrum. It depends upon the angle of deflection. Characteristic radiation depends on the atomic number of the element. The incoming beam interacts with the innermost electron (k-shell), which has the lowest energy and ejects the electron. This creates an empty space, which is filled with outer shell electrons. For Ex: An inner shell electron with an energy of 40 is replaced with an outer shell electron with an energy of 100. The remaining energy of 60 is emitted. This emitted radiation is known as characteristic radiation. Tungsten has the same energy for outer and inner shell electrons. This makes the difference between characteristic radiation constants.

Also Read: Musculoskeletal Radiology: Bone Tumors

Fate of X-rays Produced

A. Photoelectric Effect

The Photoelectric effect is similar to characteristic radiation. The beam ejects the innermost electron, which is replaced with the outer electron and then emits radiation. This is responsible for producing contrast in the X-ray film. The energy of the outer electron should be relative to the innermost electron.

B. Crompton Effect

The Crompton effect is when the beam interacts with the outermost shell electron and ejects it. The remaining energy is converted to scatter radiation. The Crompton effect is bad. The High energy X-ray photons interact with the outermost electron.

The X-ray beam is polyenergetic. High-energy beams are useful for high penetration, whereas Low-Energy beams cannot penetrate the body, stay in the body, and are more damaging. A filter is placed between the X-ray tube and the patient. It is used to remove low-energy X-rays. The filter is made of aluminum and copper and is 2.5 mm thick.

It is important to remember that lead is not used as a filter, as it stops all the rays. The filter is made up of particles with a low atomic number, which stops all the low-energy X-rays and permits high-energy X-rays.

Scatter radiation(Crompton effect) is also seen, which is bad.

• The radiation may reach people surrounding it.
• It may also interfere with X-ray film and reduce the film quality (blur or fog).

To overcome the Crompton effect, a lead apron is given to other person(lab technician/radiologist). The lead apron protects from PE and Crompton effects.

According to the Indian guidelines, the thickness of the apron should be 0.25mm, but 0.5mm is the more commonly used one.

According to the International guidelines 0.5mm thickness apron is to be used.

Scattered radiation can be reduced by:

• GRID
  • It is placed between the patient and the cassette.
  • It is made up of lead and aluminum.
  • It blocks the scattered radiation, allowing primary radiation.
• Collimator
  • It is a Beam limiting device.
  • It narrows the beam onto the patient.
  • It decreases the scattered radiation production.

Also Read: Genitourinary Radiology- Horseshoe Kidney

Factors to be Considered while doing X-Ray

Kilovoltage Peak (Kvp)

Kilovoltage peak is the unit for the potential difference between cathode and anode. Kvp is directly proportional to the velocity of the X-ray beam and penetration power. Increased penetration power may reduce the contrast of X-ray film. To increase the contrast of an X-ray film, Kvp should be decreased.

Distance

According to the inverse square law, the intensity reaching a point is inversely proportional to the square of the distance.

• I ∝ 1/D²

For example, Distance is 3, the intensity will be 1/9th. In children, X-ray is taken from long distances to avoid higher intensity.

Frequently Asked Questions/ Previously Asked Questions

Q1. Which one has the highest penetrating power?

1. Alpha
2. Beta
3. Gamma
4. Neutrons

Ans: Neutrons have more penetration power than Gamma rays.

Q2. Radiological Investigations that have ionizing radiation? (FMGE, NEET PG) 

Ans: X-rays and Gamma rays

Q3.Radiological Investigations that do not have ionizing radiation?

Ans: USG, MRI, Thermography

Q4. What is FAST?

Ans: Focused Assessment Sonography for Trauma- it is non-ionizing radiation.

Q5. Do X-rays originate from nuclei of radioactive isotopes?

Ans: No

Q6. Scatter radiation is produced because of what effect?

Ans: Crompton effect.

Q7. What is the most commonly used gonadal shield thickness in India?

Ans: 0.5mm

Q8. What should we do while taking X-ray of  an obese patient?

Ans: Kvp is to be increased as Obese patients have more soft tissues. This may increase the penetration power and reduce contrast.

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