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{{Image frame|width=400|content=[[Image:Spiral 4DCT Lung (Post).gif|200px]][[Image:Spiral 4DCT Lung (Lateral).gif|200px]]
|caption=Lung motion in 4DCT|align=right}}

'''Four-dimensional computed tomography''' ('''4DCT''') is a type of [[CT scan]]ning which records multiple images over time. It allows playback of the scan as a video, so that physiological processes can be observed and internal movement can be tracked. The name is derived from the addition of time (as the [[Four-dimensional space|fourth dimension]]) to traditional [[Three-dimensional space|3D]] computed [[Physics:Tomography|tomography]]. Alternatively, the phase of a particular process, such as [[Biology:Respiration (physiology)|respiration]], may be considered the fourth dimension.<ref>{{cite book|last1=Cox|first1=James D.|last2=Chang|first2=Joe Y.|last3=Komaki|first3=Ritsuko|title=Image-Guided Radiotherapy of Lung Cancer|date=2007|publisher=CRC Press|isbn=9780849387821|page=85|url=https://books.google.com/books?id=T68ZrSRgoGkC&pg=PA85|language=en}}</ref>

Fluoroscopy is a similar technique to 4DCT, however it refers to the introduction of a time element to [[Two-dimensional space|2D]] planar radiography, rather than to 3D CT.<ref>{{cite book|last1=Menze|first1=Bjoern|last2=Langs|first2=Georg|last3=Tu|first3=Zhuowen|last4=Criminisi|first4=Antonio|title=Medical Computer Vision: Recognition Techniques and Applications in Medical Imaging|date=2011|publisher=Springer|isbn=9783642184215|page=63|url=https://books.google.com/books?id=nDHABAAAQBAJ&pg=PA63|language=en}}</ref><ref>{{cite book|last1=Stack|first1=Brendan C. Jr.|last2=Bodenner|first2=Donald L.|title=Medical and Surgical Treatment of Parathyroid Diseases: An Evidence-Based Approach|date=2016|publisher=Springer|isbn=9783319267944|page=152|url=https://books.google.com/books?id=p1C7DQAAQBAJ&pg=PA152|language=en}}</ref><ref>{{cite book|last1=Chin|first1=Lawrence S.|last2=Regine|first2=William F.|title=Principles and Practice of Stereotactic Radiosurgery|date=2015|publisher=Springer|isbn=9781461483632|page=191|url=https://books.google.com/books?id=2vUSBgAAQBAJ&pg=PA191|language=en|quote=The ITV can be assessed with a 4D CT scan or fluoroscopy...}}</ref><ref>{{cite book|last1=Jeremic|first1=Branislav|title=Advances in Radiation Oncology in Lung Cancer|date=2011|publisher=Springer Science & Business Media|isbn=9783642199257|page=160|url=https://books.google.com/books?id=tWYTxrb0Z30C&pg=PA160|language=en}}</ref>

==Applications==
===Radiotherapy===
4DCT is used in [[Physics:Radiation therapy|radiation therapy]] planning to reduce doses to healthy organs such as the heart or lungs. Most radiation therapy is [[Medicine:Radiation treatment planning|planned]] using the results of a 3D CT scan. A 3D scan largely presents a snapshot of the body at a particular point in time, however due to the time of the acquisition, in which the patient is likely to have moved in some way (even if only breathing), there will be an element of blurring or averaging in the 3D scan.<ref name="Korreman2012">{{cite journal|last1=Korreman|first1=Stine S|title=Motion in radiotherapy: photon therapy|journal=Physics in Medicine and Biology|date=7 December 2012|volume=57|issue=23|pages=R161–R191|doi=10.1088/0031-9155/57/23/R161|pmid=23165229|bibcode=2012PMB....57R.161K|doi-access=free}}</ref> When it comes to treatment planning, this motion can mean there is less accuracy in the positioning of treatment beams, and reduce the likelihood of a repeatable set-up on the linear accelerator when it comes to treatment.<ref>{{cite journal|last1=De Ruysscher|first1=Dirk|last2=Faivre-Finn|first2=Corinne|last3=Nestle|first3=Ursula|last4=Hurkmans|first4=Coen W.|last5=Le Péchoux|first5=Cécile|last6=Price|first6=Allan|last7=Senan|first7=Suresh|title=European Organisation for Research and Treatment of Cancer Recommendations for Planning and Delivery of High-Dose, High-Precision Radiotherapy for Lung Cancer|journal=Journal of Clinical Oncology|date=20 December 2010|volume=28|issue=36|pages=5301–5310|doi=10.1200/JCO.2010.30.3271|pmid=21079134|doi-access=free}}</ref>

To minimise physical movements of the patient, some sort of immobilisation is typically used. To overcome physiological motion, such as breathing, 4DCT acquires images at a range of times and positions, allowing the extent of motion to be visualised (e.g. from maximum inspiration to maximum exhalation). The treatment plan can then be designed with a knowledge of the full range of possible positions of important organs, and the tumour (target) itself.<ref>{{cite book|last1=Schlegel|first1=Wolfgang C.|last2=Bortfeld|first2=Thomas|last3=Grosu|first3=Anca Ligia|title=New Technologies in Radiation Oncology|date=2006|publisher=Springer Science & Business Media|isbn=9783540299998|page=83|url=https://books.google.com/books?id=tdpDAAAAQBAJ&pg=PA83|language=en}}</ref>

4DCT will usually involve a [[Gating signal|gating]] technique, such as breathing tracking, so that image acquisition is automatically triggered at set points.<ref>{{cite journal|last1=Maciejczyk|first1=Adam|last2=Skrzypczyńska|first2=Iga|last3=Janiszewska|first3=Marzena|title=Lung cancer. Radiotherapy in lung cancer: Actual methods and future trends|journal=Reports of Practical Oncology & Radiotherapy|date=November 2014|volume=19|issue=6|pages=353–360|doi=10.1016/j.rpor.2014.04.012|pmc=4201776|pmid=25337407}}</ref> This gating can also be applied at treatment, where the radiotherapy beam is only switched on at certain points in the breathing cycle (as in the [[Biology:Deep inspiration breath-hold|deep inspiration breath-hold]] technique).<ref>{{cite journal|last1=Glide-Hurst|first1=Carri K.|last2=Chetty|first2=Indrin J.|title=Improving radiotherapy planning, delivery accuracy, and normal tissue sparing using cutting edge technologies|journal=Journal of Thoracic Disease|date=2014|volume=6|issue=4|pages=303–318|doi=10.3978/j.issn.2072-1439.2013.11.10|issn=2072-1439|pmc=3968554|pmid=24688775}}</ref>

===Diagnostic radiology===
4DCT has started to be used for diagnostic radiology procedures, for example looking at [[Biology:Joint|joint]] problems, the [[Medicine:Cardiac cycle|cardiac cycle]] and parathyroid washout of [[Chemistry:Radiocontrast agent|contrast]]. Downsides of 4DCT for diagnostic purposes include large and complex datasets, and increased [[Physics:Effective dose (radiation)|radiation dose]] to the patient.<ref>{{cite journal|last1=Kwong|first1=Yune|last2=Mel|first2=Alexandra Olimpia|last3=Wheeler|first3=Greg|last4=Troupis|first4=John M|title=Four-dimensional computed tomography (4DCT): A review of the current status and applications|journal=Journal of Medical Imaging and Radiation Oncology|date=October 2015|volume=59|issue=5|pages=545–554|doi=10.1111/1754-9485.12326|pmid=26041442|s2cid=25440312}}</ref>

==Reconstruction methods==

4DCT aims to visualise the temporal dynamics of a 3D sample with a sufficiently high [[Engineering:Temporal resolution|temporal]] and [[Spatial resolution|spatial resolution]]. Successive time frames are typically obtained by sequential scanning, followed by independent reconstruction of each 3D dataset. Such an approach requires a large number of projections for each scan to obtain images with sufficient quality (in terms of artefacts and [[Physics:Signal-to-noise ratio (imaging)|SNR]]). Hence, there is a clear trade-off
between the rotation speed of the gantry (i.e. time resolution) and the quality of the reconstructed images.
Motion vector based [[Iterative reconstruction|Iterative]] Techniques are available which reconstruct a particular time frame by including the projections of neighbouring time frames as well. Such a strategy allows to improve the trade-off between the rotation speed and the SNR.
<ref>{{cite journal|last1=Van Nieuwenhove|first1=Vincent|last2=De Beenhouwer|first2=Jan|last3=Vlassenbroeck|first3=Jelle|last4=Brennon|first4=Mark|last5=Sijbers|first5=Jan|title=MoVIT: A tomographic reconstruction framework for 4DCT|journal=Optics Express|year=2017|volume=25|issue=16|pages=19236–19250|doi=10.1364/OE.25.019236|pmid=29041117|bibcode=2017OExpr..2519236V|doi-access=free}}</ref>

For [[Physics:Fluid dynamics|fluid dynamics]], specialized reconstruction algorithms have been developed that model the attenuation course throughout time.<ref>{{cite journal|last1=Van Eyndhoven|first1=Geert|last2=Batenburg|first2=Joost|last3=Kazantsev|first3=Daniel|last4=Van Nieuwenhove|first4=Vincent|last5=Lee|first5=Peter|last6=Dobson|first6=Kathy|last7=Sijbers|first7=Jan|title=An iterative CT reconstruction algorithm for fast fluid flow imaging|journal=IEEE Transactions on Image Processing|year=2015|volume=24|issue=11|pages=4446–4458|doi=10.1109/TIP.2015.2466113|pmid=26259219|bibcode=2015ITIP...24.4446V|hdl=10044/1/44267|s2cid=643273|url=https://discovery.ucl.ac.uk/id/eprint/10096686/|hdl-access=free}}</ref> An example of such fluid dynamics is [[Biology:Perfusion CT|perfusion CT]] in which the propagation of contrast agent is modelled and simultaneously estimated with the CT images.
<ref>{{cite journal|last1=Van Nieuwenhove|first1=Vincent|last2=Van Eyndhoven|first2=Geert|last3=Batenburg|first3=Joost|last4=Vandemeulebroucke|first4=Jef|last5=De Beenhouwer|first5=Jan|last6=Sijbers|first6=Jan|title=Local Attenuation Curve Optimization (LACO) framework for high quality perfusion maps in low-dose cerebral perfusion CT|journal=Medical Physics|year=2016|volume=43|issue=12|pages=6429–6438|doi=10.1118/1.4967263|pmid=27908148|doi-access=free|hdl=10067/1389790151162165141|hdl-access=free}}</ref>

==References==
{{reflist}}

[[Category:Medical imaging]]
[[Category:Radiation therapy]]
[[Category:Medical physics]]
[[Category:Radiology]]
[[Category:Radiography]]

{{Sourceattribution|4DCT}}

Physics:4DCT - Revision history

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