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Back to CT basics: Why 100 kVp?

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Philips CT Clinical Science Philips Healthcare

CT basics

In producing a CT image both mA and kVp must be carefully chosen to ensure sufficient delivery of X-rays to achieve acceptable image quality at a radiation dose as low as reasonably achievable.

 

When X-rays leave the tube and pass through an object, the beam intensity is reduced. This is known as attenuation and results in some of the X-ray energy being absorbed by the scanned object (Figure 1). Given that image quality is dependant upon recording sufficient X-ray events by the detector, beam attenuation (photon starvation) becomes a primary consideration.

Figure 1
Figure 1

 

CT images are a display of the amount of attenuation that has occurred when the X-ray beam penetrates the body - this is known as the linear attenuation coefficient. The density measurement of this attenuated beam is assigned a CT number (Hounsfield Units) which are related to this linear attenuation coefficient. The selection of kVp has a direct effect on these linear coefficients values.

 

For example, water at 60 and 122 kVp have values of 0.206 and 0.166 respectively. Lower kVp values have increased beam attenuations, i.e. less of the beam penetrates the body (or tissue). This simply means that kVp selection has a direct affect on CT numbers.

 

kVp selection

The selection of kVp sets the energy of the X-rays reaching the patient and should take the scanned object into consideration. Each object will vary in density and atomic number, both of which will impact the X-ray beam's attenuation. Larger, denser objects will require higher or more intense kVp selections to ensure a sufficient number of photons exit the object and are subsequently captured by the detector.

 

Each substance/tissue has its own attenuation and absorption coefficient for a specific kVp setting. For example, at lower kVp's there is greater opacification of calcifications and iodinated contrast media. Lower kVp settings inherently have more photoelectric affect and lower Compton scatter. Various clinical scenarios require specific X-ray spectra and hence lower kVp settings for optimal image quality. However, these lower settings will decrease the overall signal and will result in an increase in noise. To maintain signal to noise ratios while lowering the kVp there must be an increase in the mAs.

 

Reducing radiation dose

Reduction in radiation dose while maintaining image quality is a primary focus of diagnostic imaging. This can be achieved through system and operator controls. System controls include automatic selection of beam filtration (Intellibeam filtration), beam shaping (Smart Shape Wedges), and beam limitation (Eclipse Collimation). For the operator, careful consideration of scan length, selections in mAs, and kVp will impact the radiation dose for each scan.

Table 1 Cardiac scan using constant 950 mA and 0.27 rotation time with kV varied.
Table 1
Cardiac scan using constant 950 mA and 0.27 rotation time with kV varied.

 

Some facts to consider when choosing a kVp:

  • Lower kVp settings at a maintained mAs will reduce radiation dose to the patient (See Table 1 above).
  • Some publications are claiming a reduction from 120 kV to 100 kVp can reduce radiation dose by as much as 60%.1
  • Various clinical scenarios require a specific kVp setting for optimal image quality.
    • Improved delineation of smaller structures especially those filled with iodinated contrast.
  • When IV contrast has been administered.
    • Reducing the kVp should not alter the contrast to noise.

1 AJR: 194 March 2009

 

 

100 kVp benefits

  • Minimizing radiation dose while maintaining image quality (Figure 2 - abdomen image).
    • Improved contrast and small vessel conspicuity (Pulmonary Embolism, Figure 3).
  • Iodine enhancement (CTA - coronary arteries, Figure 4).
  • Imaging of smaller adults and children (Figure 5).
Figure 2 Figure 3
Figure 2
Figure 3
Figure 4 Figure 5
Figure 4
Figure 5

 The articles listed in the table above are suggested reading.
The articles listed in the table above are suggested reading.


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abdomen, abdomen CTA, Body, Cardiac, chest, chest CTA, contrast, coronary angiography, coronary arteries, dose, dose reduction, Eclipse Collimator, Head, image quality, kVp, Neck, Neuro, PE, Pediatric, peripheral runoff CTA, peripheral runoffs, Thorax, Vascular
 

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