LETTERS TO THE EDITOR
Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights.
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Letters to the Editor
Spatial Resolution for Finite Element Analysis From Francesco Carbonetti, MD Department of Radiology, Sapienza University of Rome, Sant’Andrea Hospital, Via Di Grottarossa 1035Cap 00189, Rome, Italy e-mail: francescocarbonetti799@ hotmail.com
Editor: We read with interest the article by Dr Chang and colleagues (1), which appeared in the August 2014 issue of Radiology. This is an important subject, and the article was a pleasure to read. I do, however, have questions about the spatial resolution used in this study. Others have emphasized the importance of spatial resolution and isotropic voxel size for finite element analysis (FEA). In this context, it is important to remember that the only reason that structural information can be obtained with relatively low spatial resolution is related to the large spacing (approximately 800–1300 mm) between the trabeculae (2). For this reason, section thicknesses of 500 mm and less have been commonly used. A PubMed search reveals several publications where much higher spatial resolution was achieved in vivo. For example, a recent article (2) shows in vivo hip images with 700-mm-thick sections (that is less than half the voxel size used by Dr Chang and colleagues). There is also another article (3) in which in vivo hip images were obtained with 500-mm-thick sections in a reasonable clinical imaging time. The section thickness used in the study by Dr Chang and colleagues was 1.5 mm, which is considerably higher than that in previous investigations. Do the authors believe that the voxel size used is sufficient?
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It seems that the field has progressed much further in that regard, and their opinion of what spatial resolution is required for FEA and how the spatial resolution used in their study may have influenced the outcomes would be of great interest. Disclosures of Conflicts of Interest: disclosed no relevant relationships.
References 1. Chang G, Honig S, Brown R, et al. Finite element analysis applied to 3-T MR imaging of proximal femur microarchitecture: lower bone strength in patients with fragility fractures compared with control subjects. Radiology 2014;272(2):464–474. 2. Rajapakse CS, Magland J, Zhang XH, et al. Implications of noise and resolution on mechanical properties of trabecular bone estimated by image-based finite-element analysis. J Orthop Res 2009;27(10):1263–1271. 3. Han M, Chiba K, Banerjee S, Carballido-Gamio J, Krug R. Variable flip angle three-dimensional fast spin-echo sequence combined with outer volume suppression for imaging trabecular bone structure of the proximal femur. J Magn Reson Imaging doi: 10.1002/ jmri.246732014. Published online June 23, 2014. 4. Krug R, Burghardt AJ, Majumdar S, Link TM. High-resolution imaging techniques for the assessment of osteoporosis. Radiol Clin North Am 2010;48(3):601–621.
Response From Gregory Chang, MD,* Ryan Brown, PhD,* Ravinder R. Regatte, PhD,* and Chamith S. Rajapakse, PhD† Department of Radiology, NYU Langone Medical Center, 660 First Ave, 4th Floor, New York, NY 10016* e-mail: [email protected] Department of Radiology, University of Pennsylvania School of Medicine, Philadelphia, Pa† We appreciate the insightful comments by Dr Carbonetti. For FEA,
radiology.rsna.org n Radiology: Volume 274: Number 2—February 2015
LETTERS TO THE EDITOR
images with high spatial resolution, high signal-to-noise ratio (SNR), and isotropic voxels would be ideal. Although the section thickness in our study was 1.5 mm, we did use the same resolution in patients and control subjects. The microstructural changes are visually evident on the images comparing both groups. In vivo, achieving the same resolution and/or SNR in the hip as in the distal radius or tibia (0.137 mm × 0.137 mm × 0.400 mm) (1) will be challenging, given that the proximal femur is deep anatomy and SNR decreases dramatically as distance from the receive coil increases (2). The study by Han et al (3) is important because it demonstrates the feasibility of achieving thinner sections in the proximal femur by using a three-dimensional fast spin-echo pulse sequence. In the future, the combination of hardware and such pulse sequence advances will likely make it possible to achieve 0.4mm isotropic voxel sizes. It will also be important to compare magnetic resonance imaging–derived bone strength predictions with computed tomography (CT)–derived bone strength predictions by using the gold standard of mechanical testing. Continuum FEA of lower-resolution CT images of the proximal femur (0.674–1.08-mm inplane voxel size, 3-mm-thick sections) has been shown in one study to help predict bone strength in vitro with r = 0.95–0.96 (4). Disclosures of Conflicts of Interest: G.C. disclosed no relevant relationships. R.B. disclosed no relevant relationships. R.R.R. disclosed no relevant relationships. C.S.R. disclosed no relevant relationships.
References 1. Wehrli FW. Structural and functional assessment of trabecular and cortical bone by micro magnetic resonance imaging. J Magn Reson Imaging 2007;25(2):390–409. 2. Wright SM, Wald LL. Theory and application of array coils in MR spectroscopy. NMR Biomed 1997;10(8):394–410. 3. Han M, Chiba K, Banerjee S, Carballido Gamio J, Krug R. Variable flip angle threedimensional fast spin-echo sequence combined with outer volume suppression for
imaging trabecular bone structure of the proximal femur. J Magn Reson Imaging doi: 10.1002/jmri.246732014. Published online June 23, 2014. Accessed September 10, 2014. 4. Keyak JH, Kaneko TS, Tehranzadeh J, Skinner HB. Predicting proximal femoral strength using structural engineering models. Clin Orthop Relat Res 2005;(437):219–228.
Should the Axilla Be Included in Screening US? From Romuald Ferré, MD, Shaza AlSharif, MD, Martine Paré, TR, Ellen Kao, MD, and Benoît Mesurolle, MD Department of Radiology, Cedar Breast Clinic, McGill University Health Center, Royal Victoria Hospital, 687 Pine Ave West, Montreal, PQ, H3H 1A1, Canada e-mail: benoit.mesurolle@muhc. mcgill.ca
Editor: We read with great interest the excellent article by Drs Berg and Mendelson entitled “Technologist-performed Handheld Screening Breast US Imaging: How Is It Performed and What Are the Outcomes to Date?” in the July 2014 issue of Radiology (1). We congratulate the authors for their detailed comprehensive article. Regarding the technique of breast ultrasonography (US), we would like to know the authors’ opinion about the usefulness of routinely scanning the axillary regions when performing screening breast US. Axillary examination is not mentioned in the article, and we wonder if the authors would consider including the axilla as part of screening US. Although not reported in the American College of Radiology Imaging Network (ACRIN) 6666 trial (2), other publications as well as the most recent American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) atlas suggest that “the axilla could be scanned as well the breast parenchyma” (3–5). In our institution, we (technologists and radiologists) routinely include the axillary space in screening US because “it is small, su-
Radiology: Volume 274: Number 2—February 2015 n radiology.rsna.org
perficial and accessible to ultrasound” (6). As part of the provided images, in addition to the four quadrants and nipple region, we always add an image of axillary lymph nodes. Two main reasons might support this approach. First, accessory axillary breast tissue is present in 0.6%–6% of the general population (7,8). Some cancers or other abnormalities can consequently develop in the axilla and can be missed if the axilla is not scanned (9). Second, the presence of an abnormal lymph node can be related to an underlying sonographically occult or subtle breast carcinoma. For these reasons, we think it is certainly desirable that, as suggested in the new BI-RADS lexicon, the axilla could be included in breast US screening. Disclosures of Conflicts of Interest: R.F. disclosed no relevant relationships. S.A. disclosed no relevant relationships. M.P. disclosed no relevant relationships. E.K. disclosed no relevant relationships. B.M. disclosed no relevant relationships.
References 1. Berg WA, Mendelson EB. Technologist-per formed handheld screening breast US imaging: how is it performed and what are the outcomes to date? Radiology 2014;272(1): 12–27. 2. American College of Radiology Imaging Network. http://www.acrin.org/TabID/153/Default.aspx. Published November 30, 2007. Accessed July 28, 2014. 3. Hooley RJ, Greenberg KL, Stackhouse RM, Geisel JL, Butler RS, Philpotts LE. Screening US in patients with mammographically dense breasts: initial experience with Connecticut Public Act 09-41. Radiology 2012; 265(1):59–69. 4. Hooley RJ, Scoutt LM, Philpotts LE. Breast ultrasonography: state of the art. Radiology 2013;268(3):642–659. 5. Mendelson EB, Böhm-Vélez M, Berg WA, et al. Breast Imaging Reporting and Data System, BI-RADS—ultrasound. 2nd ed. Reston, Va: American College of Radiology, 2013. 6. Yang WT. Axilla. In: Berg WA, Yang WT, eds. Diagnostic imaging: breast. 2nd ed. Salt Lake City, Utah: Amirsys, 2013. 7. Seifert F, Rudelius M, Ring J, Gutermuth J, Andres C. Bilateral axillary ectopic breast tissue. Lancet 2012;380(9844):835.
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Note: This copy is for your personal non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, contact us at www.rsna.org/rsnarights.
COMMUNICATIONS n
Letters to the Editor
Spatial Resolution for Finite Element Analysis From Francesco Carbonetti, MD Department of Radiology, Sapienza University of Rome, Sant’Andrea Hospital, Via Di Grottarossa 1035Cap 00189, Rome, Italy e-mail: francescocarbonetti799@ hotmail.com
Editor: We read with interest the article by Dr Chang and colleagues (1), which appeared in the August 2014 issue of Radiology. This is an important subject, and the article was a pleasure to read. I do, however, have questions about the spatial resolution used in this study. Others have emphasized the importance of spatial resolution and isotropic voxel size for finite element analysis (FEA). In this context, it is important to remember that the only reason that structural information can be obtained with relatively low spatial resolution is related to the large spacing (approximately 800–1300 mm) between the trabeculae (2). For this reason, section thicknesses of 500 mm and less have been commonly used. A PubMed search reveals several publications where much higher spatial resolution was achieved in vivo. For example, a recent article (2) shows in vivo hip images with 700-mm-thick sections (that is less than half the voxel size used by Dr Chang and colleagues). There is also another article (3) in which in vivo hip images were obtained with 500-mm-thick sections in a reasonable clinical imaging time. The section thickness used in the study by Dr Chang and colleagues was 1.5 mm, which is considerably higher than that in previous investigations. Do the authors believe that the voxel size used is sufficient?
622
It seems that the field has progressed much further in that regard, and their opinion of what spatial resolution is required for FEA and how the spatial resolution used in their study may have influenced the outcomes would be of great interest. Disclosures of Conflicts of Interest: disclosed no relevant relationships.
References 1. Chang G, Honig S, Brown R, et al. Finite element analysis applied to 3-T MR imaging of proximal femur microarchitecture: lower bone strength in patients with fragility fractures compared with control subjects. Radiology 2014;272(2):464–474. 2. Rajapakse CS, Magland J, Zhang XH, et al. Implications of noise and resolution on mechanical properties of trabecular bone estimated by image-based finite-element analysis. J Orthop Res 2009;27(10):1263–1271. 3. Han M, Chiba K, Banerjee S, Carballido-Gamio J, Krug R. Variable flip angle three-dimensional fast spin-echo sequence combined with outer volume suppression for imaging trabecular bone structure of the proximal femur. J Magn Reson Imaging doi: 10.1002/ jmri.246732014. Published online June 23, 2014. 4. Krug R, Burghardt AJ, Majumdar S, Link TM. High-resolution imaging techniques for the assessment of osteoporosis. Radiol Clin North Am 2010;48(3):601–621.
Response From Gregory Chang, MD,* Ryan Brown, PhD,* Ravinder R. Regatte, PhD,* and Chamith S. Rajapakse, PhD† Department of Radiology, NYU Langone Medical Center, 660 First Ave, 4th Floor, New York, NY 10016* e-mail: [email protected] Department of Radiology, University of Pennsylvania School of Medicine, Philadelphia, Pa† We appreciate the insightful comments by Dr Carbonetti. For FEA,
radiology.rsna.org n Radiology: Volume 274: Number 2—February 2015
LETTERS TO THE EDITOR
images with high spatial resolution, high signal-to-noise ratio (SNR), and isotropic voxels would be ideal. Although the section thickness in our study was 1.5 mm, we did use the same resolution in patients and control subjects. The microstructural changes are visually evident on the images comparing both groups. In vivo, achieving the same resolution and/or SNR in the hip as in the distal radius or tibia (0.137 mm × 0.137 mm × 0.400 mm) (1) will be challenging, given that the proximal femur is deep anatomy and SNR decreases dramatically as distance from the receive coil increases (2). The study by Han et al (3) is important because it demonstrates the feasibility of achieving thinner sections in the proximal femur by using a three-dimensional fast spin-echo pulse sequence. In the future, the combination of hardware and such pulse sequence advances will likely make it possible to achieve 0.4mm isotropic voxel sizes. It will also be important to compare magnetic resonance imaging–derived bone strength predictions with computed tomography (CT)–derived bone strength predictions by using the gold standard of mechanical testing. Continuum FEA of lower-resolution CT images of the proximal femur (0.674–1.08-mm inplane voxel size, 3-mm-thick sections) has been shown in one study to help predict bone strength in vitro with r = 0.95–0.96 (4). Disclosures of Conflicts of Interest: G.C. disclosed no relevant relationships. R.B. disclosed no relevant relationships. R.R.R. disclosed no relevant relationships. C.S.R. disclosed no relevant relationships.
References 1. Wehrli FW. Structural and functional assessment of trabecular and cortical bone by micro magnetic resonance imaging. J Magn Reson Imaging 2007;25(2):390–409. 2. Wright SM, Wald LL. Theory and application of array coils in MR spectroscopy. NMR Biomed 1997;10(8):394–410. 3. Han M, Chiba K, Banerjee S, Carballido Gamio J, Krug R. Variable flip angle threedimensional fast spin-echo sequence combined with outer volume suppression for
imaging trabecular bone structure of the proximal femur. J Magn Reson Imaging doi: 10.1002/jmri.246732014. Published online June 23, 2014. Accessed September 10, 2014. 4. Keyak JH, Kaneko TS, Tehranzadeh J, Skinner HB. Predicting proximal femoral strength using structural engineering models. Clin Orthop Relat Res 2005;(437):219–228.
Should the Axilla Be Included in Screening US? From Romuald Ferré, MD, Shaza AlSharif, MD, Martine Paré, TR, Ellen Kao, MD, and Benoît Mesurolle, MD Department of Radiology, Cedar Breast Clinic, McGill University Health Center, Royal Victoria Hospital, 687 Pine Ave West, Montreal, PQ, H3H 1A1, Canada e-mail: benoit.mesurolle@muhc. mcgill.ca
Editor: We read with great interest the excellent article by Drs Berg and Mendelson entitled “Technologist-performed Handheld Screening Breast US Imaging: How Is It Performed and What Are the Outcomes to Date?” in the July 2014 issue of Radiology (1). We congratulate the authors for their detailed comprehensive article. Regarding the technique of breast ultrasonography (US), we would like to know the authors’ opinion about the usefulness of routinely scanning the axillary regions when performing screening breast US. Axillary examination is not mentioned in the article, and we wonder if the authors would consider including the axilla as part of screening US. Although not reported in the American College of Radiology Imaging Network (ACRIN) 6666 trial (2), other publications as well as the most recent American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) atlas suggest that “the axilla could be scanned as well the breast parenchyma” (3–5). In our institution, we (technologists and radiologists) routinely include the axillary space in screening US because “it is small, su-
Radiology: Volume 274: Number 2—February 2015 n radiology.rsna.org
perficial and accessible to ultrasound” (6). As part of the provided images, in addition to the four quadrants and nipple region, we always add an image of axillary lymph nodes. Two main reasons might support this approach. First, accessory axillary breast tissue is present in 0.6%–6% of the general population (7,8). Some cancers or other abnormalities can consequently develop in the axilla and can be missed if the axilla is not scanned (9). Second, the presence of an abnormal lymph node can be related to an underlying sonographically occult or subtle breast carcinoma. For these reasons, we think it is certainly desirable that, as suggested in the new BI-RADS lexicon, the axilla could be included in breast US screening. Disclosures of Conflicts of Interest: R.F. disclosed no relevant relationships. S.A. disclosed no relevant relationships. M.P. disclosed no relevant relationships. E.K. disclosed no relevant relationships. B.M. disclosed no relevant relationships.
References 1. Berg WA, Mendelson EB. Technologist-per formed handheld screening breast US imaging: how is it performed and what are the outcomes to date? Radiology 2014;272(1): 12–27. 2. American College of Radiology Imaging Network. http://www.acrin.org/TabID/153/Default.aspx. Published November 30, 2007. Accessed July 28, 2014. 3. Hooley RJ, Greenberg KL, Stackhouse RM, Geisel JL, Butler RS, Philpotts LE. Screening US in patients with mammographically dense breasts: initial experience with Connecticut Public Act 09-41. Radiology 2012; 265(1):59–69. 4. Hooley RJ, Scoutt LM, Philpotts LE. Breast ultrasonography: state of the art. Radiology 2013;268(3):642–659. 5. Mendelson EB, Böhm-Vélez M, Berg WA, et al. Breast Imaging Reporting and Data System, BI-RADS—ultrasound. 2nd ed. Reston, Va: American College of Radiology, 2013. 6. Yang WT. Axilla. In: Berg WA, Yang WT, eds. Diagnostic imaging: breast. 2nd ed. Salt Lake City, Utah: Amirsys, 2013. 7. Seifert F, Rudelius M, Ring J, Gutermuth J, Andres C. Bilateral axillary ectopic breast tissue. Lancet 2012;380(9844):835.
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