Reversal Trend of Hounsfield Unit Values of Substances with High and Low Effective Atomic Numbers

Document Type : Original Paper


1 Medical Imaging Research Center, Shiraz University of Medical Sciences, Shiraz, Iran

2 Ongil, 79 D3, Sivaya Nagar, Reddiyur Alagapuram, Salem 636004, India

3 Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India

4 Nuclear Medicine and Molecular Imaging Research Center, Shiraz University of Medical Sciences, Shiraz, Iran


Introduction: In dual-energy computed tomography (DECT), the Hounsfield values of a substance measured at two different energies are the basic data for finding the chemical properties of a substance. The trends of Hounsfield unit (HU) alterations following the changes in energy are different between the materials with high and low Zeff. The present study aimed to analyze the basic principles related to the attenuation coefficient of x-ray photons and a quantitative explanation is given for the mentioned behavior or trend.
Material and Methods: A mathematical expression was derived for the HU difference between two different scanner voltages. Attenuation coefficients of diverse substances, such as methanol, glycerol, acetic acid, the aqueous solution of potassium hydroxide, and water were calculated for x-ray scanners operating differently at distinct applied voltages and with diverse inherent or added filters.
Results: Findings of the current study demonstrated that the negative or positive outcome of HU(V1) - HU(V2) equation is not determined by the electron density of a substance. However, it is affected by the effective atomic number (Zeff) of the material and machine parameters specified by the source spectrum.
Conclusion: According to our results, the sign of HU difference [HU(V1) – HU(V2)] for the variable cases of V2 and V1 gives an indication of the effective atomic number of the material under study. The obtained results might be of diagnostic value in the DECT technique.


Main Subjects


    1. Rutherford RA, Pullan BR, Isherwood I. Measurement of effective atomic number and electron density using an EMI scanner. Neuroradiology. 1976;11(1): 15-21.
    2. Heismann BJ, Leppert J, Stierstorfer K. Density and atomic number measurements with spectral x-ray attenuation method. J. Appl. Phys. 2003; 94(3): 2073-9.
    3. Bazalova M, Carrier JF, Beaulieu L Verhaegen F. Dual-energy CT-based material extraction for tissue segmentation in Monte Carlo dose calculations. Phys. Med. Biol. 2008; 53(9): 2439-56.
    4. Alvarez RE, Macovski A. Energy selective reconstruction in x-ray computerized tomography. Phys. Med. Biol. 1976; 21(5): 733-44.
    5. Heismann BJ, Balda B. Quantitative image-based spectral reconstruction for computed tomography. Med. Phys. 2009; 36(10): 4471-85.
    6. Haghighi RR, Chatterjee S, Kumar P, Vani VC. Numerical analysis of the relationship between the photoelectric effect and Energy of the x-ray photons in CT. Front. Bio. Tech. 2014 1(4): 240-51.
    7. Seeram E. Computed Tomography, Physical Principles, Clinical Applications, Quality Control. W.B. Saunders Co. Philadelphia, 2001.
    8. Zata LM, Alvarez RE. An inaccuracy in computed tomography: The energy dependence of CT values. Radiology. 1977; 124(1): 91-7.
    9. Ay MR, Shahriyari M, Sarkar S, Adib M, Zaidi H. Monte Carlo simulation of x-ray spectra in diagnostic radiology and mammography using MCNP4C. Phys. Med. Biol. 2004; 49(21), 4897-917.
    10. Okayama S, Soeda T, Takami Y, Kawakami R, Somekawa S, Uemura S, et al. The influence of effective energy on computed tomography number depends on tissue characteristics in monoenergetic cardiac imaging. Radiology research and practice. 2012;2012.
    11. Raptopoulos V, Karellas A, Bernstein J, Reale FR, Constantinou C, Zawacki JK. Value of dual-energy CT in differentiating focal fatty infiltration of the liver from low-density masses. AJR. 1991; 157(4): 721-25.
    12. Haghighi RR, Chatterjee S, Vyas A, Kumar P, Thulkar S. X-ray Attenuation Coefficient of Mixtures: Inputs for Dual-Energy CT. Med. Phys. 2011; 38(10): 5270-9.
    13. Hubbell JH, Seltzer SM. Table of x-ray mass attenuation coefficients from 1 keV to 20 MeV for elements Z ¼ 1–92 and 48 additional substances of dosimetric interest. Available URL:
    14. Sunga R. Solutions for Technicians. Available From:
    15. Boone JM, Seibert JA. An accurate method for computer-generating tungsten anode   x- ray spectra from 30 to 140 kV. Med. Phys. 1997; 24(11): 1661-70.
    16. Haghighi RR. Evaluation of coronary artery plaque. Thesis submitted to the faculty of All India Institute of Medical Sciences, New Delhi for the degree of Doctor of Philosophy, Medical Physics Unit, Dr. B. R. A. Institute Rotary Cancer Hospital, All India Institute of Medical Sciences, New Delhi-110029. 2013.
    17. Johnson TRC, Krauß B, Sedlmair M, Grasruck M, Bruder H, Morhard D, et al. Material differentiation by dual energy CT: initial experience. Eur Radiol. 2007; 17: 1510–7.
    18. Mahmoudi R, Jabbari N, Aghdasi M, Khalkhali HR. Energy dependence of measured CT numbers on substituted materials used for CT number calibration of radiotherapy treatment planning systems. PLoS ONE. 2016; 11(7): e0158828.
    19. Haghighi RR, Chatterjee S, Tabin M, Sharma S, Jagia P, Ray R, et al. DECT evaluation of non-calcified coronary artery plaque. Med. Phys. 2015; 42(10): 5945-54.
    20. Sidky EY, Yu L, Pan X, Zou Y, Vannier M. A robust method of x-ray source spectrum estimation from transmission measurements: Demonstrated on computer simulated, scatter free transmission data. J. Appl. Phys. 2005; 97(12): 124701-11.
    21. Halvorsen RA, Korobkin M, Ram PC, Thompson WM. CT appearance of focal fatty infiltration of the liver. American Journal of Roentgenology. 1982;139(2):277-81.



Volume 17, Issue 5
September and October 2020
Pages 340-349
  • Receive Date: 29 August 2019
  • Revise Date: 29 November 2019
  • Accept Date: 12 December 2019