QUALITY AND INTENSITY OF X-RAYS QUALITY

QUALITY AND INTENSITY OF X-RAYS QUALITY 

The term quality describes the penetrating power of the radiation. If a radiation consist photons of single energy (mono-energetic), then the quality can be described either by photon energy or wavelength. But, X-ray beam consist many photon energies (heterogeneous), and hence, its quality cannot be described by the photon energy. Therefore, the X-ray beam quality is usually specified by the following: half-value layer, applied voltage (kV), filtration and effective photon energy.

HALF-VALUE LAYER (HVL)

The half-value layer or half-value thickness of a radiation beam is the required thickness of a material, which reduces the beam intensity to one half. The half-value layer is always stated together with the value of the applied voltage and the filtration. Aluminum and copper are the materials commonly used to specify HVL.

INTENSITY

The intensity is a measure of quantity of radiation. The intensity of a radiation beam is the energy flowing in unit time through a unit area. It is equal to the number of photons in the beam multiplied by the energy of each photon. The intensity is commonly measured in roentgens per minute (R/min). The term exposure is often used in radiology, which is proportional to the energy fluence of the X-ray beam. It refers both quality and quantity of the beam. The term quantity refers the number of X-ray photons in the beam.

 FACTORS AFFECTING QUALITY AND INTENSITY

The X-ray production efficiency, quality, quantity and intensity are affected by seven factors, namely, applied voltage, tube current, filtration, target material, exposure time, generator waveform and distance.

The Applied Voltage (kVp)

 The applied voltage affects both the quality and intensity of the X-rays. The energy of the photon emitted from the X-ray tube depends on the energy of the electrons that bombard the target. The energy of the electron is, in turn, determined by the peak kilo-voltage used. As the applied voltage increases, the effective photon energy in the bremsstrahlung also increases. The maximum photon energy is proportional to the peak value of the applied voltage. In addition, the X-ray production efficiency is related with applied voltage. The intensity increases with increase of applied voltage . The amount of radiation produced increases as the square of the kilo voltage,

i. e. Radiation exposure µ (kVp)2

Thus, increase in kVp increases the quality, quantity and efficiency of X-ray production.

Tube Current (mA)

The number of X-rays produced depends on the number of electrons that strike the target of the X-ray tube. The number of electrons depends directly on the tube current (mA) used. Greater the mA, higher the electrons that are produced, and hence, more X-rays will be obtained. The tube current affects only the intensity but not the quality of the X-rays. As the tube current increases the intensity also increases.

i. e. the intensity is µ mA.

Increase of applied voltage is compensated by the reduction of tube current, which is required to maintain same exposure. The ratio of mAs varies with 5th power of kVp ratio

(kVp1/kVp2)5 = mAs2/mAs1

Filtration

 Filters are thin sheet of material (Al,Cu,Mo), which offer high attenuation for low energy photons. The purpose of using filter is to reduce patient exposure at the skin level. Filters alter both the quality and quantity of X-rays by selectively removing the low energy photons in the spectrum.This reduces the photon number (quantity) and shifts the average energy to higher values by increasing the quality. A filtered beam consists of higher photon energies and is said to be hardened.

Target Material (Z)

The atomic number of the target material affects the intensity of the X-rays. The intensity increases with increase of atomic number. The X-ray production is more efficient if higher the atomic number of the target material. For example, tungsten (Z = 74) would produce much more bremsstrahlung than tin (Z = 50), if both were used as target material under identical kVp and mA.

 The atomic number of the target material also determines the energy (quality) of characteristic X-rays. Thus, the atomic number of the target material determines the intensity of bremsstrahlung and quality of characteristic X-rays produced.

Exposure Time

 Exposure time determines the length of X-ray production. The total quantity of X-rays is directly proportional to the product of the tube current and exposure time (mAs).

The Generator Waveform 

The generator wave form (single-phase, 3-phase or constant potential) directly affects the quality of the emitted X-ray spectrum because of the average potential difference across the tube. For example, a single phase generator provides a lower average applied voltage potential difference, than a 3-phase generator.

Distance

The X-ray beam intensity decreases with distance from the target because of the divergence of the X-ray beam. The decrease in intensity is proportional to the square of the distance from the target. The nonlinear fall-off in intensity with distance is called the inverse square law. In general, if the distance from the X-ray source is changed from x to y, then the X-ray beam intensity changes by (x/y)2. If x = 1 m and y = 2 m, then the intensity decreases by a factor 4. Thus, doubling the distance from the X-ray source, decreases the X-ray beam intensity by a factor of 4.

 In summary, the intensity (I) of X-ray radiation is given by the relation

kVp 2 × mAs × Z I 

                                                 ---------------------

          d2

Where d is the distance between the target and the point of measurement.