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Use of Hounsfield units in cone-beam computed tomography
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Letters to the editor* Use of Hounsfield units in cone-beam computed tomography he article “Hounsfield unit change in root and alveolar bone during canine retraction”1 focused on HU changes in the alveolar bone and root surface as the canines were moved by 2 treatment strategies— translation and controlled tipping—that were evaluated with cone-beam computed tomography (CBCT). The authors stated that although the use of HUs from CBCT images has not been considered a reliable way to quantify absolute bone mineral density, it is still the best method available to monitor changes of bone mineral density in terms of HUs through longitudinal studies as long as the same CBCT machine and the same scan settings are used. Although the absolute values were not described in this study, the alterations found for root and bone mineral densities do not correspond to the variations of HUs and, therefore, should not be referred to as such. The gray values obtained with CBCT show a linear relationship with the attenuation coefficients of the materials, and the HUs obtained with medical computed tomography and density values from dual energy x-ray absorptiometry. Errors are expected when CBCT images are used to define the quality of the scanned structures because these images show inconsistencies and arbitrariness in the gray values, particularly when related to abrupt changes in the density of the object, x-ray beam hardening effects, scattered radiation, projection data discontinuity-related effect, differences between CBCT devices, changes in the volume of the field of view (FOV), and changes in the relationships of size and position between the FOV and the object evaluated. According to the studies available to date, CBCT should not be considered the examination of choice for the determination of mineral density of osseous and soft tissues, especially when the values obtained are compared with predetermined standard values.2 In the methodology used in the study, the standardization of the equipment, the image-acquisition setting, the patient's head posture, and the FOV during image acquisition allowed the standardization of errors for image acquisition, minimizing the problems associated with the technique. However, the values obtained from
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*The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.
204
the images were not from HUs, but of gray values. Our intention is not to criticize the methodology in the article, but to call attention to the appropriate nomenclature to be used. Robert Willer Farinazzo Vitral Marcelo Reis Fraga Marcio Jose da Silva Campos Juiz de Fora, Minas Gerais, Brazil Am J Orthod Dentofacial Orthop 2015;148:204 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists.
http://dx.doi.org/10.1016/j.ajodo.2015.05.005
REFERENCES 1. Jiang F, Liu SS, Xia Z, Li S, Chen J, Kula KS, Eckert G. Hounsfield unit change in root and alveolar bone during canine retraction. Am J Orthod Dentofacial Orthop 2015;147:445-53. 2. Campos MJ, de Souza TS, Mota J unior SL, Fraga MR, Vitral RW. Bone mineral density in cone beam computed tomography: only a few shades of gray. World J Radiol 2014;6:607-12.
Authors' response
W
e want to thank the authors of this letter to the editor for their comments regarding our article.1 We are fully aware of the limitation of using Hounsfield units (HU) to predict bone mineral density (BMD) from cone-beam computed tomography (CBCT) images; this was the reason that we designed this longitudinal study and only reported the relative HUs or HU changes. We chose HUs because they are commonly associated with BMD. In general, HUs are linked to gray values through linear transformation. In CBCT images, the absolute values of both gray values and HUs are not very reliable because they are affected by many parameters, as indicated in the authors' referenced article.2 The parameters include object difference, beam hardening, scattered radiation, discontinuity-related effect, differences between the CBCT device and field of view (FOV) changes, and size and position changes. Our study design ensured that these parameters remained the same in this longitudinal study, meaning that the changes of HUs or gray values are reliable, although their absolute values may not be reliable. Some of the uncontrollable errors were handled statistically. Thus, the results of the study were reliable, and we acknowledged this.
Letters to the editor* Use of Hounsfield units in cone-beam computed tomography he article “Hounsfield unit change in root and alveolar bone during canine retraction”1 focused on HU changes in the alveolar bone and root surface as the canines were moved by 2 treatment strategies— translation and controlled tipping—that were evaluated with cone-beam computed tomography (CBCT). The authors stated that although the use of HUs from CBCT images has not been considered a reliable way to quantify absolute bone mineral density, it is still the best method available to monitor changes of bone mineral density in terms of HUs through longitudinal studies as long as the same CBCT machine and the same scan settings are used. Although the absolute values were not described in this study, the alterations found for root and bone mineral densities do not correspond to the variations of HUs and, therefore, should not be referred to as such. The gray values obtained with CBCT show a linear relationship with the attenuation coefficients of the materials, and the HUs obtained with medical computed tomography and density values from dual energy x-ray absorptiometry. Errors are expected when CBCT images are used to define the quality of the scanned structures because these images show inconsistencies and arbitrariness in the gray values, particularly when related to abrupt changes in the density of the object, x-ray beam hardening effects, scattered radiation, projection data discontinuity-related effect, differences between CBCT devices, changes in the volume of the field of view (FOV), and changes in the relationships of size and position between the FOV and the object evaluated. According to the studies available to date, CBCT should not be considered the examination of choice for the determination of mineral density of osseous and soft tissues, especially when the values obtained are compared with predetermined standard values.2 In the methodology used in the study, the standardization of the equipment, the image-acquisition setting, the patient's head posture, and the FOV during image acquisition allowed the standardization of errors for image acquisition, minimizing the problems associated with the technique. However, the values obtained from
T
*The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.
204
the images were not from HUs, but of gray values. Our intention is not to criticize the methodology in the article, but to call attention to the appropriate nomenclature to be used. Robert Willer Farinazzo Vitral Marcelo Reis Fraga Marcio Jose da Silva Campos Juiz de Fora, Minas Gerais, Brazil Am J Orthod Dentofacial Orthop 2015;148:204 0889-5406/$36.00 Copyright Ó 2015 by the American Association of Orthodontists.
http://dx.doi.org/10.1016/j.ajodo.2015.05.005
REFERENCES 1. Jiang F, Liu SS, Xia Z, Li S, Chen J, Kula KS, Eckert G. Hounsfield unit change in root and alveolar bone during canine retraction. Am J Orthod Dentofacial Orthop 2015;147:445-53. 2. Campos MJ, de Souza TS, Mota J unior SL, Fraga MR, Vitral RW. Bone mineral density in cone beam computed tomography: only a few shades of gray. World J Radiol 2014;6:607-12.
Authors' response
W
e want to thank the authors of this letter to the editor for their comments regarding our article.1 We are fully aware of the limitation of using Hounsfield units (HU) to predict bone mineral density (BMD) from cone-beam computed tomography (CBCT) images; this was the reason that we designed this longitudinal study and only reported the relative HUs or HU changes. We chose HUs because they are commonly associated with BMD. In general, HUs are linked to gray values through linear transformation. In CBCT images, the absolute values of both gray values and HUs are not very reliable because they are affected by many parameters, as indicated in the authors' referenced article.2 The parameters include object difference, beam hardening, scattered radiation, discontinuity-related effect, differences between the CBCT device and field of view (FOV) changes, and size and position changes. Our study design ensured that these parameters remained the same in this longitudinal study, meaning that the changes of HUs or gray values are reliable, although their absolute values may not be reliable. Some of the uncontrollable errors were handled statistically. Thus, the results of the study were reliable, and we acknowledged this.