- Email: [email protected]
Optimisation of post mortem cardiac computed tomography compared to optical coherence tomography and histopathology – Technical note
Journal of Forensic Radiology and Imaging 2 (2014) 85–90
Contents lists available at ScienceDirect
Journal of Forensic Radiology and Imaging journal homepage: www.elsevier.com/locate/jofri
Technical note
Optimisation of post mortem cardiac computed tomography compared to optical coherence tomography and histopathology – Technical note Helle Precht a,b,n, Peter Mygind Leth c, Jesper Thygesen d, Michael Hardt-Madsen e, Bjarne Nielsen f, Erling Falk g, Kenneth Egstrup h, Oke Gerke i,j, Alexander Broersen k, Pieter H. Kitslaar k, Jouke Dijkstra k, Jess Lambrechtsen h a
Odense University Hospital Svendborg, Medical Research Department, Valdemarsgade 53, 5700 Svendborg, Denmark Conrad Research Center, University College Lillebelt, Blangstedgaardsvej 4, 5220 Odense SØ, Denmark c University of Southern Denmark, Institute of Forensic Medicine, J.B. Winsløws Vej 17B, 5000 Odense C, Denmark d Aarhus University Hospital Skejby, Department of Medico Engineering, Brendstrupgaardsvej 100, 8200 Aarhus N, Denmark e Odense University Hospital Svendborg, Department of Clinical Pathology, Valdemarsgade 53, 5700 Svendborg, Denmark f Hospital of Southern Jutland, Department of Pathology, Sydvang 1, 6400 Sønderborg, Denmark g Aarhus University Hospital Skejby, Department of Cardiology, Brendstrupgaardsvej 100, 8200 Aarhus N, Denmark h Odense University Hospital Svendborg, Medical Research Department, Valdemarsgade 53, 5700 Svendborg, Denmark i Odense University Hospital, Department of Nuclear Medicine, Sdr. Boulevard 29, 5000 Odense C, Denmark j University of Southern Denmark, Center of Health Economics Research, J.B. Winsløws Vej 9, 1. Sal, 5000 Odense C, Denmark k Leiden University Medical Center, Department of Radiology, Division of Image Processing, Albinusdreef 2, 2300 RC Leiden, the Netherlands b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Received in revised form 5 December 2013 Accepted 21 December 2013 Available online 2 January 2014
Introduction: Coronary atherosclerosis is a leading cause of mortality. New technological developments in computed tomography (CT), including dual energy, iterative reconstructions and high definition scanning, could significantly improve the non-invasive identification of atherosclerosis plaques. Here, a new method for optimising cardiac coronary CT with optical coherence tomography (OCT) and histopathology is presented. Materials and methods: Twenty human hearts obtained from autopsies were used. A contrast agent that solidifies after cooling was injected into the coronary arteries. CT scanning was performed on the heart alone as well as with the heart in a chest phantom. We used eight different CT protocols and the newest CT technique to image every heart. The OCT and CT images were compared with their corresponding histological sections. A procedure for ensuring the correct alignment of the images was also developed. Results: We have succeeded in developing a new method for post-mortem coronary CT angiography in which an autopsy heart is placed in a chest phantom to simulate clinical CT. Conclusion: The new method permits comparison of CT with OCT and histopathology. This method can also be used for evaluating coronary artery disease, including characterising plaques, and will eventually allow for the detection of rupture-prone plaques, which we will assess in a future study. Clinical testing is our ultimate goal. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Atherosclerosis Angiography Computerised tomography CT-scanning Coronary artery disease
1. Introduction Ischaemic heart disease (IHD) is one of the leading causes of morbidity and mortality in Western countries [1] and is nearly always a result of coronary atherosclerosis leading to coronary thrombosis by plaque rupture [1,2]. Procedures that could improve the diagnosis of the arteriosclerotic component and rupture-prone plaques are therefore essential. New technological developments in computed tomography (CT), including dual energy, iterative reconstructions and high definition scans, could improve non-
n Corresponding author at: Conrad Research Center, University College Lillebelt, Blangstedgaardsvej 4, 5220 Odense SØ, Denmark. Tel.: þ 45 23 27 19 31; fax: þ45 63 18 32 26. E-mail addresses: [email protected], [email protected] (H. Precht).
2212-4780/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jofri.2013.12.006
invasive plaque characterisation [3–7]. Optical coherence tomography (OCT) is a high-resolution, minimally invasive imaging modality that can identify fibrous caps, necrotic cores, lipid plaques and calcified regions and thus seems to be a promising method for plaque characterisation [8,9]. We present a technical note on the methodology developed for optimising cardiac coronary CT and compare these new developments with OCT and histopathology in preparation for clinical testing. 2. Description of the method The idea of developing this method was inspired by former study recommendations [1,10–14] and was preceded by a pilot study with 10 pig hearts. The ethical committee of the Region of Southern Denmark approved the study.
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Hearts from 20 deceased patients with suspected coronary atherosclerosis (median age 747 9 years; 16 men, 4 women) who underwent autopsy were included. 3. Specimen preparation for OCT After removal from the body, the hearts were rinsed in water. The coronary arteries were flushed with water and exposed to a gentle vacuum to remove loose blood clots. 4. OCT The OCT examination was performed with the Ilumien™ PCI Optimisation system (model 900-700-00, Software version D.0.2, LightLab Imaging, St. Jude Medical, Minnesota, USA). An intravascular OCT catheter (C7 Dragonfly™ Imaging Catheter, model 100100-00, LightLab Imaging, St. Jude Medical, Minnesota, USA) was inserted sequentially through the diaphragm of the sheath, guided by a guidewire (Pressurewire Certus, model C12008, St. Jude Medical, Minnesota, USA) at 0.014 In in the right and left coronary orifices. Serial OCT images were obtained using an automatic pullback device at a speed of 20 mm/s displaying the proximal 5 cm of the two major coronary arteries (right coronary artery [RCA] and left anterior descending coronary artery [LAD] with the orifice at the end. Saline (0.9%, 37 1C) was infused just before OCT imaging was performed, and the pull-back function was activated through the side arm of the sheath. The pressure inside the coronary artery was maintained at a physiologic level (60–80 mm Hg) by a sphygmomanometer connected to the infusion port. 5. OCT data analysis
of Qlvus 2.2 software of Medis medical imaging systems by Leiden, the Netherlands). The software uses a combination of transversal and longitudinal models and knowledge-guided contour detection techniques. Luminal contour points are transformed to individual cross-sections. The software provides an initial segmentation of the lumen and allows for manual correction afterwards [15]. 6. Preparation for CT To simulate an arterial enhancement similar to in vivo CT coronary angiography, a mixture of CT contrast media (9%) (Visipaque Iodixanol 320 mg I/ml, GE Healthcare, USA), 0.9% saline solution (85%), blue food colouring (3%) and powder gelatine (3%) was prepared with an attenuation of approximately 350 Hounsfield Units (HU) [1] after injection. The contrast agent mixture was heated to 60 1C until the gelatine was dissolved. The contrast agent (15 ml) was injected into the right and left coronary arteries with a pressure of 60–80 mm Hg through a 6 Fr. artery sheath catheter (Terumo Radifocus Introducer II, Terumo Europe, Leuven, Belgium), fixed with a suture around the artery and connected to a manometer. After injection, the heart was cooled in a plastic bag on crushed ice to allow the contrast agent mixture to consolidate. Aluminium circles marked the beginning of each artery. To prevent the hearts from collapsing, two fluid filled condoms were placed in the right and the left atrium/ventricle, which were filled, respectively, with saline medium and saline/contrast mixture with a HU value of 270. These settings are comparable to in vivo cardiac CT. After preparation, the heart was transported to the Medical Imaging Department in a cooler box. The entire process is shown in Fig. 1. The hearts were warmed to 25 1C before CT examination to simulate the x-ray attenuation of the heart present for in vivo examinations [16]. 7. CT
The OCT images were processed and analysed using proprietary software version D.0.2 from LightLab Imaging, St. Jude Medical, USA and OCT analysis software (QCU-CMS 4.68, Research version
A 64 multi-slice CT scanner was used (750HD GE CT scanner, GE Healthcare, Milwaukee, USA). For the first scan, the heart was
Fig. 1. The process for preparing the hearts for the CT scans.
Fig. 2. The position of the hearts in the CT scanner. (A): In a foam cranium holder on the scanner bed; (B): in the chest phantom; and (C): chest phantom in the CT scanner.
H. Precht et al. / Journal of Forensic Radiology and Imaging 2 (2014) 85–90
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Table 1 Technical CT parameters used for each post-mortem heart. (A): Hearts outside the phantom at a full dose and with a high SNR; (B): hearts outside the phantom at a reduced radiation dose and with a similar SNR as for in vivo cardiac CT; and (C): hearts in the chest phantom and with a similar SNR as for in vivo Cardiac CT.
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Fig. 3. Artery sample fixed on polystyrene and fastened on the self-developed alignment component.
Fig. 4. CT images of the post-mortem heart as an example for the image quality achieved. (A): heart directly on the scanner bed. (B): heart in phantom. (C) and (D): heart with contrast filling.
H. Precht et al. / Journal of Forensic Radiology and Imaging 2 (2014) 85–90
placed in a foam cranium holder on the scanning bed in a supine position with the long axis of the heart along the z-axis of the CT-scanner (Fig. 2A) to simulate the correct anatomical position. For the second scan, the heart was placed in a chest phantom (Kyoto Kagaku, Multipurpose chest phantom N1 – Lungman, Kyoto, Japan) with padding to simulate an extra layer of fat (Fig. 2B). The technical scanning parameters and the CT-scans are included in Table 1. All post-mortem scans and series were conducted prospectively with simulated ECG gating and a heart rate of 55, including a stable rhythm, to emulate clinical cardiac CT [17].
8. CT data analysis All datasets were transferred to an offline workstation for analysis using semi-automated plaque analysis software (QAngioCT Research Edition version 1.3.8, Medis Medical Imaging Systems, Leiden, the Netherlands) [18]. First, a central line originating from the ostium was automatically extracted and then straightened multiplanar reformatted images were generated. The software detected the lumen and vessel borders longitudinally on 4 different vessel views. On the basis of these longitudinal contours, cross-sectional images at 0.5-mm intervals were calculated to create transversal lumen and vessel wall contours, which were examined and, if necessary, adjusted by an experienced observer. Gradient magnitude images, which are derived from the MSCT images and display the degree of CT density change, were used to facilitate detection of the lumen and vessel wall borders.
9. Alignment After the CT acquisition, the heart was fixed in 10% buffered formalin and after 24 h, the proximal 5 cm of the RCA and LAD were cut out, placed on a 4 mm thick polystyrene plate and fixed with thin needles. A CT scan of each artery placed on an alignment component (shown in Fig. 3) was performed. An experienced, board-certified cardiologist registered all plaques visible on the CT images, and these were then marked on both the polystyrene plate and directly on the tissue sample with a special blue pencil to ensure the correct position in case of shrinking. Each plaque and its specific location from the ostium were registered in a computer database to manually ensure alignment between the CT scan of the entire heart, the second CT and the histopathology. The RCA and LAD were fixed for an additional 24 h in 10% buffered formalin and then decalcified in formic acid for 48 h. Additionally, correct alignment between OCT, CT and histopathology was secured digitally as part of the image analysis by aligning the three examination images side by side, which was measured by the distance in mm from the ostium [19].
10. Histopathology Standard paraffin embedding was performed for 3-5 mm blocks with the CT-marked plaque in the middle. For every 200 mm of the RCA and LAD, 4 series of 3 mm-thick sections were cut. Two series were stained with haematoxylin–eosin and modified elastic Verhoef van Gieson, and two series were stored in a freezer for later use. All histopathology slices were mounted in the correct spatial orientation on the glass slide for comparison with and validation of the CT and OCT modalities [20]. The modified American Heart Association classification of atherosclerotic lesions was used to classify plaques [21]. For fibroatheromas, the maximal size of the necrotic core and the minimal thickness of the fibrous cap covering the core were determined.
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11. Discussion Previous studies comparing post-mortem cardiac CT to histopathology have either only included the coronary arteries [10,11] or the entire heart after removal from the body [1,12,13]. A realistic attenuation of the X-ray beam cannot be obtained with these methods because scattered radiation from the surrounding muscles and organs is missing and no weakening of the x-ray beams by the passage through the body occurs. The result is a completely different image with an unrealistic, clear visualisation of the plaque [22,23]. We have attempted to avoid these problems by scanning the heart while it is in a chest phantom. To ensure that the CT protocols match those used in clinical practice, we simulated an ECG on the monitor with a pulse rate of 55. Jackowski et al. [24] developed the most optimal procedure for post-mortem cardiac studies; they investigated three human corpses using CT angiography with iodine contrast injected in the right femoral artery. With a full CT angiography of a human corpse, the penetration of the radiation is similar to in vivo CT. Their study has some limitations such as the low number of corpses studied, low body temperature, presence of intravascular putrefaction gas resulting in artefacts and an arterial pressure lower than that for in vivo CT. Proper alignment of the CT images, OCT images and the histology sections is critical for accurate comparison of the images. Muscle and soft tissues shrink during the fixation and decalcification processes, and the tissue samples are obtained after the artery has been straightened. Earlier studies have either not described the alignment [11,24] or selected the tissue samples according to the distance from the ostium [1,12]. To ensure correct alignment between the CT images and the histopathology slices, we supplemented the primary CT-scans with a CT-scan of the arteries after they had been separated from the heart. It was then possible to compare these CT images with the images from the first CT scan and thus ensure that the plaques were correctly identified in the histopathology slices. The objective of this technical note was to achieve a method that closely matches in vivo CT imaging and clinical practice by fine-tuning the physical factors and CT techniques. We used a phantom, prepared the heart with a contrast mixture, returned the heart to room temperature (correct HU values), simulated ECG and ensured the alignment between CT, OCT and histopathology. The limitations of this method are primarily that our post-mortem hearts did not move and that we did not have contrast in the aorta. Evaluating the CT image quality, there was close correlation to clinical in vivo scans with a small visible difference in the demarcation of the vessel border (see Fig. 4 for examples).
12. Conclusion This method for coronary CT angiography is the first to simulate clinical CT by placing the heart in a phantom and by using a new dual energy CT technique. The method presented permits comparison with OCT and histopathology. This method could be used to characterise plaques in the coronary arteries and eventually to detect rupture-prone plaques. We plan to follow up with a clinical study if we find that our method can detect atherosclerotic plaque more precise or even detect rupture-prone plaques.
Acknowledgements We would like to thank St. Jude Medical, Denmark for lending their OCT equipment, sponsoring the OCT catheter and providing technical support. The investigators acknowledge the excellent
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assistance of all medical personnel in the pathologic laboratory and the CT radiographers for supporting this project at the Odense University Hospital Svendborg, Denmark. The study was funded by the Centre for Regional Health Research, the Odense University Hospital Svendborg and the Region of Southern Denmark. References [1] T. Henzler, S. Porubsky, H. Kayed, N. Harder, U.R. Krissak, M. Meyer, T. Sueslbeck, A. Marx, H. Michaely, U.J. Schoepf, S.O. Schoenberg, C. Fink, Attenuation-based characterization of coronary atherosclerotic plaque: comparison of dual source and dual energy CT with single-source CT and histopathology, Eur. J. Radiol. 80 (1) (2011) 54–59. [2] T. Thim, M.K. Hagensen, D. Wallace-Bradley, J.F. Granda, G.L. Kaluza, L. Drouet, W.P. Paaske, H.E. Bøtker, E. Falk, Unriliable assessment of necrotic core by virtual histology intravascular ultrasound in porcine coronary artery disease, Circ. Cardiovasc. Imaging 3 (2010) 384–391. [3] E. Falk, M. Nakano, J.F. Bentzon, A. Finn, R. Virmani, Update on acute coronary syndromes: the pathologists0 view, Eur. Heart J. 34 (10) (2013) 719–728. [4] M.A. De Graaf, A. Broersen, P.H. Kitslaar, C.J. Roos, J. Dijkstra, B.P. Lelieveldt, J.W. Jukerha, M.J. Schalij, V. Delgado, J.J. Bax, J.H. Reiber, A.J. Scholte, Automatic quantification and characterization of coronary atherosclerosis with computed tomography coronary angiography: cross-correlation with intravascular ultrasound virtual histology, Int. J. Cardiovasc. Imaging 29 (5) (2013) 1177–1190. [5] F. Schwarz, B. Ruzsics, U.J. Schoepf, G. Bastarrika, S.A. Salvatore, A. Chiaramida, et al., Dual-energy CT of the heart–principles and protocols, Eur. J. Radiol. 68 (2008) 423–433. [6] J.T. Bushberg, J.A. Seibert, E.N. Leidholdt, J.M. Boone, The Essential Physics of Medical Imaging, 3rd international edition, Lippincott Williams and Wilkins, USA, 2012. [7] M.J. Willemink, P.A.J. Jong, T. Leiner, L.M. Heer, R.A.J. Nievelstein, R.P.J. Budde, A. M.R. Schiham, Iterative reconstruction techniques for computed tomography part 1: technical principles, Eur. Radiol. 23 (2013) 1623–1631. [8] S.I. Negi, O. Rosales, The role of intravascular optical coherence tomography in peripheral percutaneous interventions, J. Invasive Cardiol. 25 (3) (2013) E51–E53. [9] H.G. Bezerra, G.F. Attizzani, V. Sirbu, G. Musumeci, N. Lortkipanidze, Y. Fujino, W. Wang, S. Nakamura, A. Erglis, G. Guagliumi, M.A. Costa, Optical coherence tomography versus intravascular ultrasound to evaluate coronary artery disease and percutaneous coronary intervention, JACC Cardiovasc. Interv. 6 (3) (2013) 228–236. [10] H. Zachrisson, E. Engström, J. Engvall, et al., Soft tissue discrimination ex vivo by dual energy computed tomography, Eur. J. Radiol. (2010).
[11] Y. Tanami, E. Ikeda, M. Jinzaki, et al., Computed tomographic attenuation value of coronary atherosclerotic plaques with different tube voltage: an ex vivo study, J. Comput. Assist. Tomogr. 34 (2010) 58–63. [12] S. Leschka, S. Seitun, et al., Ex vivo evaluation of coronary atherosclerotic plaques: Characterization with dual-source CT in comparison with histopathology, J. Cardiovasc. Comput. Tomogr. 4 (2010) 301–308. [13] T. Kume, T. Akasaka, T. Kawamoto, N. Watanabe, et al., Assessment of coronary arterial plaque by optical coherence tomography, Am. J. Cardiol. 97 (2006) 1172–1175. [14] C. Pøhlsgaard, P.M. Leth, Post-mortem CT-coronary angiography, Scand. J. Forensic Sci. 13 (2007) 8–9. [15] G.J. Tearney, E. Regar, T. Akasaka, T. Adriaenssens, P. Barlis, H.G. Bezerra, B. Bouma, et al., Consensus standards for acquisition, measurement, and reporting of intravascular optical coherence tomography studies, J. Am. Coll. Cardiol. 59 (12) (2012) 1058–1072. [16] G.D. Pandeya, Klaessens J.H.G.M., M.J.W. Greuter, B. Schmidt, T. Flohr, R. Hillegersberg, M. Oudkerk, Feasibility of computed tomography based thermometry during interstitial laser heating in bovine liver, Eur. Radiol. 21 (2011) 1733–1738. [17] A. Hassan, S.A. Nazir, H. Alkadhi, Technical challenges of coronary CT antiography: today and tomorrow, Eur. J. Radiol. 79 (2) (2011) 161–171. [18] Papadopoulou, et al., Reproducibility of computed tomography angiography data analysis using semiautomated plaque quantification software: implications for the design of longitudinal studies, Int. J. Cardiovasc. Imaging suppl 5 (2013) s28–s37. [19] P.L. Maurovich-Horvat, C.L. Schlett, H. Alkadhi, M. Nakano, F. Otsuka, P. Stolzmann, H. Scheffel, M. Ferencik, M. Kriegel, H. Seifarth, R. Virmani, U. Hoffmann, The napkin-ring sign indicates advanced atherosclerotic lesions in coronary CT angiography, JACC 5 (12) (2013) 1243–1252. [20] T. Thim, E. Falk, Spatial orientation of cross-sectional images of coronary arteries: point of view in intracoronary imaging, Cardiovasc. Ultrasound 10 (2012) 12. [21] R. Virmani, F.D. Kolodgie, A.P. Burke, A. Farb, S.M. Schwartz, Lessons from sudden coronary death, a comprehensive morphological classification scheme for atherosclerotic lesions, Arterioscler. Thromb. Vasc. Biol. 20 (2000) 1262–1275. [22] J.A. Seibert, J.M. Boone, X-ray imaging physics for nuclear medicine technologists. Part 2: X-ray interactions and image formation, J. Nucl. Med. Technol. 33 (2005) 3–18. [23] N.S. Paul, H. Kashani, D. Odedra, A. Ursani, C. Ray, P. Rogalla, The influence of chest wall tissue composition in determining image noise during cardiac CT, Cardiopulm. Imaging 197 (2011) 1328–1334. [24] C. Jackowski, M. Sonnenschein, M.J. Thali, E. Aghayev, et al., Virtopsy: postmortem minimally invasive angiography using cross section techniques – implementation and preliminary resultsn, J. Forensic Sci. 50 (5) (2005) 1–12.
Contents lists available at ScienceDirect
Journal of Forensic Radiology and Imaging journal homepage: www.elsevier.com/locate/jofri
Technical note
Optimisation of post mortem cardiac computed tomography compared to optical coherence tomography and histopathology – Technical note Helle Precht a,b,n, Peter Mygind Leth c, Jesper Thygesen d, Michael Hardt-Madsen e, Bjarne Nielsen f, Erling Falk g, Kenneth Egstrup h, Oke Gerke i,j, Alexander Broersen k, Pieter H. Kitslaar k, Jouke Dijkstra k, Jess Lambrechtsen h a
Odense University Hospital Svendborg, Medical Research Department, Valdemarsgade 53, 5700 Svendborg, Denmark Conrad Research Center, University College Lillebelt, Blangstedgaardsvej 4, 5220 Odense SØ, Denmark c University of Southern Denmark, Institute of Forensic Medicine, J.B. Winsløws Vej 17B, 5000 Odense C, Denmark d Aarhus University Hospital Skejby, Department of Medico Engineering, Brendstrupgaardsvej 100, 8200 Aarhus N, Denmark e Odense University Hospital Svendborg, Department of Clinical Pathology, Valdemarsgade 53, 5700 Svendborg, Denmark f Hospital of Southern Jutland, Department of Pathology, Sydvang 1, 6400 Sønderborg, Denmark g Aarhus University Hospital Skejby, Department of Cardiology, Brendstrupgaardsvej 100, 8200 Aarhus N, Denmark h Odense University Hospital Svendborg, Medical Research Department, Valdemarsgade 53, 5700 Svendborg, Denmark i Odense University Hospital, Department of Nuclear Medicine, Sdr. Boulevard 29, 5000 Odense C, Denmark j University of Southern Denmark, Center of Health Economics Research, J.B. Winsløws Vej 9, 1. Sal, 5000 Odense C, Denmark k Leiden University Medical Center, Department of Radiology, Division of Image Processing, Albinusdreef 2, 2300 RC Leiden, the Netherlands b
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Received in revised form 5 December 2013 Accepted 21 December 2013 Available online 2 January 2014
Introduction: Coronary atherosclerosis is a leading cause of mortality. New technological developments in computed tomography (CT), including dual energy, iterative reconstructions and high definition scanning, could significantly improve the non-invasive identification of atherosclerosis plaques. Here, a new method for optimising cardiac coronary CT with optical coherence tomography (OCT) and histopathology is presented. Materials and methods: Twenty human hearts obtained from autopsies were used. A contrast agent that solidifies after cooling was injected into the coronary arteries. CT scanning was performed on the heart alone as well as with the heart in a chest phantom. We used eight different CT protocols and the newest CT technique to image every heart. The OCT and CT images were compared with their corresponding histological sections. A procedure for ensuring the correct alignment of the images was also developed. Results: We have succeeded in developing a new method for post-mortem coronary CT angiography in which an autopsy heart is placed in a chest phantom to simulate clinical CT. Conclusion: The new method permits comparison of CT with OCT and histopathology. This method can also be used for evaluating coronary artery disease, including characterising plaques, and will eventually allow for the detection of rupture-prone plaques, which we will assess in a future study. Clinical testing is our ultimate goal. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Atherosclerosis Angiography Computerised tomography CT-scanning Coronary artery disease
1. Introduction Ischaemic heart disease (IHD) is one of the leading causes of morbidity and mortality in Western countries [1] and is nearly always a result of coronary atherosclerosis leading to coronary thrombosis by plaque rupture [1,2]. Procedures that could improve the diagnosis of the arteriosclerotic component and rupture-prone plaques are therefore essential. New technological developments in computed tomography (CT), including dual energy, iterative reconstructions and high definition scans, could improve non-
n Corresponding author at: Conrad Research Center, University College Lillebelt, Blangstedgaardsvej 4, 5220 Odense SØ, Denmark. Tel.: þ 45 23 27 19 31; fax: þ45 63 18 32 26. E-mail addresses: [email protected], [email protected] (H. Precht).
2212-4780/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jofri.2013.12.006
invasive plaque characterisation [3–7]. Optical coherence tomography (OCT) is a high-resolution, minimally invasive imaging modality that can identify fibrous caps, necrotic cores, lipid plaques and calcified regions and thus seems to be a promising method for plaque characterisation [8,9]. We present a technical note on the methodology developed for optimising cardiac coronary CT and compare these new developments with OCT and histopathology in preparation for clinical testing. 2. Description of the method The idea of developing this method was inspired by former study recommendations [1,10–14] and was preceded by a pilot study with 10 pig hearts. The ethical committee of the Region of Southern Denmark approved the study.
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Hearts from 20 deceased patients with suspected coronary atherosclerosis (median age 747 9 years; 16 men, 4 women) who underwent autopsy were included. 3. Specimen preparation for OCT After removal from the body, the hearts were rinsed in water. The coronary arteries were flushed with water and exposed to a gentle vacuum to remove loose blood clots. 4. OCT The OCT examination was performed with the Ilumien™ PCI Optimisation system (model 900-700-00, Software version D.0.2, LightLab Imaging, St. Jude Medical, Minnesota, USA). An intravascular OCT catheter (C7 Dragonfly™ Imaging Catheter, model 100100-00, LightLab Imaging, St. Jude Medical, Minnesota, USA) was inserted sequentially through the diaphragm of the sheath, guided by a guidewire (Pressurewire Certus, model C12008, St. Jude Medical, Minnesota, USA) at 0.014 In in the right and left coronary orifices. Serial OCT images were obtained using an automatic pullback device at a speed of 20 mm/s displaying the proximal 5 cm of the two major coronary arteries (right coronary artery [RCA] and left anterior descending coronary artery [LAD] with the orifice at the end. Saline (0.9%, 37 1C) was infused just before OCT imaging was performed, and the pull-back function was activated through the side arm of the sheath. The pressure inside the coronary artery was maintained at a physiologic level (60–80 mm Hg) by a sphygmomanometer connected to the infusion port. 5. OCT data analysis
of Qlvus 2.2 software of Medis medical imaging systems by Leiden, the Netherlands). The software uses a combination of transversal and longitudinal models and knowledge-guided contour detection techniques. Luminal contour points are transformed to individual cross-sections. The software provides an initial segmentation of the lumen and allows for manual correction afterwards [15]. 6. Preparation for CT To simulate an arterial enhancement similar to in vivo CT coronary angiography, a mixture of CT contrast media (9%) (Visipaque Iodixanol 320 mg I/ml, GE Healthcare, USA), 0.9% saline solution (85%), blue food colouring (3%) and powder gelatine (3%) was prepared with an attenuation of approximately 350 Hounsfield Units (HU) [1] after injection. The contrast agent mixture was heated to 60 1C until the gelatine was dissolved. The contrast agent (15 ml) was injected into the right and left coronary arteries with a pressure of 60–80 mm Hg through a 6 Fr. artery sheath catheter (Terumo Radifocus Introducer II, Terumo Europe, Leuven, Belgium), fixed with a suture around the artery and connected to a manometer. After injection, the heart was cooled in a plastic bag on crushed ice to allow the contrast agent mixture to consolidate. Aluminium circles marked the beginning of each artery. To prevent the hearts from collapsing, two fluid filled condoms were placed in the right and the left atrium/ventricle, which were filled, respectively, with saline medium and saline/contrast mixture with a HU value of 270. These settings are comparable to in vivo cardiac CT. After preparation, the heart was transported to the Medical Imaging Department in a cooler box. The entire process is shown in Fig. 1. The hearts were warmed to 25 1C before CT examination to simulate the x-ray attenuation of the heart present for in vivo examinations [16]. 7. CT
The OCT images were processed and analysed using proprietary software version D.0.2 from LightLab Imaging, St. Jude Medical, USA and OCT analysis software (QCU-CMS 4.68, Research version
A 64 multi-slice CT scanner was used (750HD GE CT scanner, GE Healthcare, Milwaukee, USA). For the first scan, the heart was
Fig. 1. The process for preparing the hearts for the CT scans.
Fig. 2. The position of the hearts in the CT scanner. (A): In a foam cranium holder on the scanner bed; (B): in the chest phantom; and (C): chest phantom in the CT scanner.
H. Precht et al. / Journal of Forensic Radiology and Imaging 2 (2014) 85–90
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Table 1 Technical CT parameters used for each post-mortem heart. (A): Hearts outside the phantom at a full dose and with a high SNR; (B): hearts outside the phantom at a reduced radiation dose and with a similar SNR as for in vivo cardiac CT; and (C): hearts in the chest phantom and with a similar SNR as for in vivo Cardiac CT.
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Fig. 3. Artery sample fixed on polystyrene and fastened on the self-developed alignment component.
Fig. 4. CT images of the post-mortem heart as an example for the image quality achieved. (A): heart directly on the scanner bed. (B): heart in phantom. (C) and (D): heart with contrast filling.
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placed in a foam cranium holder on the scanning bed in a supine position with the long axis of the heart along the z-axis of the CT-scanner (Fig. 2A) to simulate the correct anatomical position. For the second scan, the heart was placed in a chest phantom (Kyoto Kagaku, Multipurpose chest phantom N1 – Lungman, Kyoto, Japan) with padding to simulate an extra layer of fat (Fig. 2B). The technical scanning parameters and the CT-scans are included in Table 1. All post-mortem scans and series were conducted prospectively with simulated ECG gating and a heart rate of 55, including a stable rhythm, to emulate clinical cardiac CT [17].
8. CT data analysis All datasets were transferred to an offline workstation for analysis using semi-automated plaque analysis software (QAngioCT Research Edition version 1.3.8, Medis Medical Imaging Systems, Leiden, the Netherlands) [18]. First, a central line originating from the ostium was automatically extracted and then straightened multiplanar reformatted images were generated. The software detected the lumen and vessel borders longitudinally on 4 different vessel views. On the basis of these longitudinal contours, cross-sectional images at 0.5-mm intervals were calculated to create transversal lumen and vessel wall contours, which were examined and, if necessary, adjusted by an experienced observer. Gradient magnitude images, which are derived from the MSCT images and display the degree of CT density change, were used to facilitate detection of the lumen and vessel wall borders.
9. Alignment After the CT acquisition, the heart was fixed in 10% buffered formalin and after 24 h, the proximal 5 cm of the RCA and LAD were cut out, placed on a 4 mm thick polystyrene plate and fixed with thin needles. A CT scan of each artery placed on an alignment component (shown in Fig. 3) was performed. An experienced, board-certified cardiologist registered all plaques visible on the CT images, and these were then marked on both the polystyrene plate and directly on the tissue sample with a special blue pencil to ensure the correct position in case of shrinking. Each plaque and its specific location from the ostium were registered in a computer database to manually ensure alignment between the CT scan of the entire heart, the second CT and the histopathology. The RCA and LAD were fixed for an additional 24 h in 10% buffered formalin and then decalcified in formic acid for 48 h. Additionally, correct alignment between OCT, CT and histopathology was secured digitally as part of the image analysis by aligning the three examination images side by side, which was measured by the distance in mm from the ostium [19].
10. Histopathology Standard paraffin embedding was performed for 3-5 mm blocks with the CT-marked plaque in the middle. For every 200 mm of the RCA and LAD, 4 series of 3 mm-thick sections were cut. Two series were stained with haematoxylin–eosin and modified elastic Verhoef van Gieson, and two series were stored in a freezer for later use. All histopathology slices were mounted in the correct spatial orientation on the glass slide for comparison with and validation of the CT and OCT modalities [20]. The modified American Heart Association classification of atherosclerotic lesions was used to classify plaques [21]. For fibroatheromas, the maximal size of the necrotic core and the minimal thickness of the fibrous cap covering the core were determined.
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11. Discussion Previous studies comparing post-mortem cardiac CT to histopathology have either only included the coronary arteries [10,11] or the entire heart after removal from the body [1,12,13]. A realistic attenuation of the X-ray beam cannot be obtained with these methods because scattered radiation from the surrounding muscles and organs is missing and no weakening of the x-ray beams by the passage through the body occurs. The result is a completely different image with an unrealistic, clear visualisation of the plaque [22,23]. We have attempted to avoid these problems by scanning the heart while it is in a chest phantom. To ensure that the CT protocols match those used in clinical practice, we simulated an ECG on the monitor with a pulse rate of 55. Jackowski et al. [24] developed the most optimal procedure for post-mortem cardiac studies; they investigated three human corpses using CT angiography with iodine contrast injected in the right femoral artery. With a full CT angiography of a human corpse, the penetration of the radiation is similar to in vivo CT. Their study has some limitations such as the low number of corpses studied, low body temperature, presence of intravascular putrefaction gas resulting in artefacts and an arterial pressure lower than that for in vivo CT. Proper alignment of the CT images, OCT images and the histology sections is critical for accurate comparison of the images. Muscle and soft tissues shrink during the fixation and decalcification processes, and the tissue samples are obtained after the artery has been straightened. Earlier studies have either not described the alignment [11,24] or selected the tissue samples according to the distance from the ostium [1,12]. To ensure correct alignment between the CT images and the histopathology slices, we supplemented the primary CT-scans with a CT-scan of the arteries after they had been separated from the heart. It was then possible to compare these CT images with the images from the first CT scan and thus ensure that the plaques were correctly identified in the histopathology slices. The objective of this technical note was to achieve a method that closely matches in vivo CT imaging and clinical practice by fine-tuning the physical factors and CT techniques. We used a phantom, prepared the heart with a contrast mixture, returned the heart to room temperature (correct HU values), simulated ECG and ensured the alignment between CT, OCT and histopathology. The limitations of this method are primarily that our post-mortem hearts did not move and that we did not have contrast in the aorta. Evaluating the CT image quality, there was close correlation to clinical in vivo scans with a small visible difference in the demarcation of the vessel border (see Fig. 4 for examples).
12. Conclusion This method for coronary CT angiography is the first to simulate clinical CT by placing the heart in a phantom and by using a new dual energy CT technique. The method presented permits comparison with OCT and histopathology. This method could be used to characterise plaques in the coronary arteries and eventually to detect rupture-prone plaques. We plan to follow up with a clinical study if we find that our method can detect atherosclerotic plaque more precise or even detect rupture-prone plaques.
Acknowledgements We would like to thank St. Jude Medical, Denmark for lending their OCT equipment, sponsoring the OCT catheter and providing technical support. The investigators acknowledge the excellent
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