Skip to main content
PMC home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2014 Mar 1.
Published in final edited form as: Eur Radiol. 2012 Sep 13;23(3):640–641. doi: 10.1007/s00330-012-2645-5

Letter to the Editor re: Spectral Hounsfield units – a new radiological concept

David P Cormode 1,*, Zahi A Fayad 2,*
PMCID: PMC3529963  NIHMSID: NIHMS407613  PMID: 22971961

Modified units for computed tomography

We read with interest the recent paper in your journal by Hurrell et al.[1]. In this paper, they observed that in spectral CT measurements of materials such as iodine, iron, calcium and oil in a phantom using their Medipix-based system, the attenuation of the samples was highly dependent on the X-ray energy window. This corresponds with our observations of gold and iodine contrast media using both a conventional CT system run at different voltages and a spectral CT system[2; 3]. These results are due to the characteristic X-ray attenuation profiles of different materials,[4] with K-edge absorbances having a large effect, in particular.

In the conclusions to their paper, Hurrell et al. proposed a modified scale of Hounsfield Units, termed spectral HU, to account for the differing attenuation at different energies, where a subscript denoting the mean energy used is appended, e.g. 400 HU90, when the mean energy used is 90 keV.[1] We support this new scale of Units in principle. However, it seems to us that the use of solely the mean energy to denote the attenuation will be insufficient. For example, HU90 could equally refer to a voltage window of 80–100 keV or 20–160 keV. It is likely that a material will have differing attenuation in these two windows. Therefore we suggest the use of the energy window in the subscript, e.g. 400 HU80–100.

Lately, we and others have developed a variety of novel CT contrast agents based on nanoparticles[3; 5–9]. Furthermore, in a recent paper we proposed the use of a unit to denote the performance of CT contrast agents, i.e. attenuation rate (attenuation per unit concentration, given in HU/mM).[2] As mentioned, these attenuation rates were found to be highly dependent on the X-ray tube voltage used. In light of the above discussion, it seems that attenuation rates should be given with subscripts denoting the X-ray voltages in question, i.e spectral attenuation rates should be used. For example, HU80–100/mM should be used to describe contrast agent performance in a 80–100 keV window. Adoption of spectral HU and spectral attenuation rates should provide a framework for evaluating contrast with spectral CT or dual energy CT, the easy comparison of novel contrast agents and swifter advancement of the field.

Key points

  • —

    Dual energy and spectral CT are novel techniques becoming more widely available.

  • —

    These technological advancesrequire a new attenuation scale: spectral Hounsfield Units.

  • —

    We suggest a modification to this new attenuation scale proposed by Hurrell et al.

  • —

    We also propose a new scale for assessing novel CT contrast media.

Acknowledgements

The authors wish to thank the National Heart, Lung, and Blood Institute (NHLBI) and the US National Institutes of Health (NIH), as a Program of Excellence in Nanotechnology (PEN) Award (Contract no. HHSN268201000045C), as well as by the NIH grants R00 EB012165 (D.P.C.) and R01 EB009638 (Z.A.F.) for support.

References

  • 1.Hurrell AH, Butler APH, Cook NJ, Butler PH, Ronaldson JP, Zainon R. Spectral Hounsfield units: a new radiological concept. Eur. Radiol. 2012;22:1008–1013. doi: 10.1007/s00330-011-2348-3. [DOI] [PubMed] [Google Scholar]
  • 2.Galper MW, Saung MT, Fuster V, et al. Effect of Computed Tomography Scanning Parameters on Gold Nanoparticle and Iodine Contrast. Invest. Radiol. 2012;47(8):475–481. doi: 10.1097/RLI.0b013e3182562ab9. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Cormode DP, Roessl E, Thran A, et al. Atherosclerotic plaque composition: analysis with multicolor CT and targeted gold nanoparticles. Radiology. 2010;256(3):774–782. doi: 10.1148/radiol.10092473. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 4.Lumbroso P, Dick CE. X-ray attenuation properties of radiographic contrast media. Med. Phys. 1987;14:752–758. doi: 10.1118/1.595999. [DOI] [PubMed] [Google Scholar]
  • 5.van Schooneveld MM, Cormode DP, Koole R, et al. A fluorescent, paramagnetic and PEGylated gold/silica nanoparticle for MRI, CT and fluorescence imaging. Contrast Media Mol. Imaging. 2010;5(4):231–236. doi: 10.1002/cmmi.376. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Mieszawska AJG, Anita, Cormode David P., Zhao Yiming, Meijerink Andries, Langer Robert, Farokhzad Omid C., Fayad Zahi A., Mulder Willem J. M. Engineering of lipid-coated PLGA nanoparticles with a tunable payload of diagnostically active nanocrystals for medical imaging. Chem. Commun. 2012;48:5835–5837. doi: 10.1039/c2cc32149a. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.Pan D, Roessl E, Schlomka J-P, et al. Computed tomography in color: nanoK-enhanced spectral CT molecular imaging. Angew. Chem. Int. Ed. 2010;49:9635–9639. doi: 10.1002/anie.201005657. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.de Vries A, Custers E, Lub J, van den Bosch S, Nicolay K, Grull H. Block-copolymer-stabilized iodinated emulsions for use as CT contrast agents. Biomaterials. 2010;31(25):6537–6544. doi: 10.1016/j.biomaterials.2010.04.056. [DOI] [PubMed] [Google Scholar]
  • 9.Guo R, Wang H, Peng C, et al. X-ray Attenuation Property of Dendrimer-Entrapped Gold Nanoparticles. J. Phys. Chem. 2010;C 114(1):50–56. [Google Scholar]

RESOURCES