Computed tomography turns 50

Computed tomography (CT) uses thousands of x-ray transmission measurements taken at different angles around a patient to produce three-dimensional cross-sectional images the human body. That technology allows physicians visualize their patients’ internal anatomy, as shown in opening image, and reveals presence acute chronic diseases consequences injury remarkable detail. Prior integration CT into clinical radiology, relied on exploratory surgery diagnose many serious symptoms. Thankfully, that’s now relic past.Image courtesy Hiroshi Moriya, MD, PhD, Ohara General Hospital, Japan.PPT|High resolutionCT had curious birth. The Central Research Laboratories British music company EMI were well known for development stereophonic sound, broadcast television devices, radar. Yet late 1960s, with financial support from UK’s Department Health Social Security, radar-operator-turned-electrical-engineer Godfrey Hounsfield developed first scanner (see reference 11. G. N. Hounsfield, Br. J. Radiol. 46, 1016 (1973). https://doi.org/10.1259/0007-1285-46-552-1016 box 1). On 1 October 1971, radiologist James Ambrose performed scan22. Ambrose, 1023 https://doi.org/10.1259/0007-1285-46-552-1023 Atkinson Morley Hospital UK.Box 1. Nobel PrizeCT inventors Allan Cormack awarded Prize Physiology or Medicine 1979. That’s an interesting distinction, given that neither researcher was physiologist physician. (Both physical scientists—an engineer physicist, respectively; lectures, see 1212. “Computed medical imaging,” lecture presented Karolinksa Institute, Stockholm, 8 December 1979;A. M. Cormack, “Early two-dimensional reconstruction recent topics stemming it,” 1979..) A South African physicist who worked US time prize awarded, investigated mathematical principles1313. A. Appl. Phys. 34, 2722 (1963). https://doi.org/10.1063/1.1729798,1414. 35, 2908 (1964). https://doi.org/10.1063/1.1713127 tomographic imaging using cobalt-60 radiation source early 1960s.Working independently UK, hardware tube between 1967 1971. His used make patients. An affable man, he became celebrity radiology world, recognized humble changed worldwide practice medicine forever. community named grayscale values “Hounsfield units” his honor.Box honor.Since use, has grown ever more sophisticated versatile. years advancements coincided other technological developments. Those include improvements precision machining, advanced bearing systems, solid-state detectors, lasers, semiconductor integration, composite materials—all which enabled faster precise performance. Computers also continued get faster, computer algorithms led improved image quality, dramatically expanded commercial market. fields been applied are remarkably diverse. Box 2 describes few beyond field medicine.Box 2. Big small systemsComputed systems extend far applications, small-specimen huge devices designed nondestructive testing. At end spectrum, can tiny specimens, gemstones fruit flies, pixel dimensions below 20 nm. extreme, 2009 government engineers constructed able evaluate structure nuclear warheads, aerospace built two-story-tall system scan components such rocket assemblies turbine blades imperfections.In familiar scanners airports scanning interiors checked bags. Most specimen industrial rotate specimen, rather than detector array. geometry greatly simplifies mechanical design system. For scanning, however, array, not patient, rotated.Box rotated.More 90 million examinations each year US. pervasiveness reflects value information provides both diagnosis treatment planning. large number is driven by front lines medicine, order exams, radiologists, interpret scans prescribed referring physician colleagues.CT requires some 106–109 positions along long (z) axis body reconstructed image. During data acquisition, generates relatively uniform beam x rays incident photons S0. collimated z-axis only cross section exposed. exponentially attenuated patient’s smaller S detected element Thus, geometry, S=S0e−μt, where t thickness tissues µ sum linear attenuation coefficients trajectory. then transformed projection according P=µt=ln{S0S0e−μt}.(1) equation assumes single energy. But actually polyenergetic; hence considered effective coefficient. P reconstruct images, logarithm linearizes respect µ. means physically dependent It implies affected entrance exposure levels unlike film-based radiography, never over- underexposed.Medical energies about 30 150 keV. those photon energies, Compton scattering dominates interactions tissue. depends electron density ρe ρe≈ρN0ZA, (2) ρ mass density, N0 Avogadro’s number, Z atomic number. turns out Z/A same (½) prevalent elements—carbon, oxygen, nitrogen—that compose soft Hence, primarily shows map dependence due photoelectric interaction, pronounced higher elements, including bone (calcium), metal implants (titanium, tantalum, steel), injected contrast agents (iodine).Panel figure illustrates designs system, overcoming physics inertia major challenge. mounted opposite rigid but movable gantry—the framework head placed. first-generation, translate–rotate mechanism fundamentally limited lead-shielded gantry, accelerate decelerate measurement. Eighty acquired during translation one side other, after gantry rotated 1°. translated across again another repeated until total 180°; process 14 400 4.5 minutes.Figure evolved considerably since (a) In another, placed them. together width direction, frame they secured 1°; back direction. processes device 180° patient. (b) rotate–rotate array detectors sits emits fan-shaped beam. Both synchronously acquire data. Producing significantly shorter times, all today. (c) rotate–stationary stationary, ring-shaped within expensive less dose-efficient ability block scattered abandonment this geometry. (Courtesy John Boone/University California, Davis.)PPT|High resolutionThe 80 × representing scanned via reconstruction. determined width—either 13 mm—of To greater length body, technologist entire process, time. complete brain consisted six acquisition took minutes, needed hold perfectly still. Such times initially procedure head, because it could be immobilized.Despite created tremendous excitement among radiologists physicians, feedback encouraged corporations invest heavily technology’s development. Next-generation incorporated somewhat wider process. Third-generation scanners, panel b 1, curved angular coverage 60°. fixed position arrangement reduced times.After rotation’s worth collection, either mechanism, gantry’s rotation braked prevent severing cables connected stationary frame. 1990s, slip-ring introduced conduct power signal rotating eliminating need stop rewind cables. slip ring did allow continuously; allowed helical, spiral, scanning. sequential time—during table exposure—in helical moved through while taken. result trajectory completely eliminated inertial constraints scan.As decreased, tubes introduction thus achieve production rates. fundamental heat conduction tube’s vacuum environment remained limitation. innovations helped address limitations power. Instead just covered 10 mm researchers multiple arrays z-axis. 1999, sixteen 1.25 available, although four channels available first. Current exist up 320 channels. With expansion came increase width—from 160 mm. 16-fold solid angle produced 16 efficiently.Having thinner spatial resolution resolution, 5–10 outdated single-detector 1970s 0.5 less. improvement, routinely coronal sagittal allowing 3D 2D planes z-axis, no longer referred CAT scan, axial plane (the “A” stood axial). Since addition arrays, speeds have reached 240 rotations per minute. Acquisition gone minutes 1971 5 seconds chest, abdomen, pelvis, comprise over 500 50 years, decreased factor 25 000.Figure Sagittal perspectives torso. Images thorax (top) abdomen pelvis (bottom) demonstrate clarity computed depict anatomy. Excellent air, tissue, makes lung essential diagnosing pulmonary disorders. vascular agent containing high-atomic-number iodine enhance blood vessels cardiac chambers visible. Canon Medical Systems, USA.)PPT|High resolutionFrom imagesSection:ChooseTop pageABSTRACTFrom i... <<Quantitative imagingRadiation levelsClinical implicationsReferencesCITING ARTICLESFigure 3 how P(x, θ) called filtered projection. recorded terms distance θ, mathematically projected backward onto digital matrix I(x, y) simple trigonometry. adds path acquired. As data, reinforce location attenuating objects build picture anatomy.33. T. Bushberg et al., Essential Physics Imaging, 4th ed., Wolters Kluwer (2020). Although description conceptually accurate, modern use diverging beam, whose spread fan. rebinned Cartesian coordinate prior filtering projection.Figure 3. Filtered back-projection These represent object detector. direction uniformly distributes measured line sets typically thousand projections, comprising individual points After projected, representation emerges. overcome blurring inherent resolutionA may 1000 projections θ measurement n (with ranging 320). collect position, z), form set, y, z). 4, display specific z location, z) y I(y, location. displayed volume-rendering techniques.Figure 4. Blood brain. Once (computed tomography) plane, submillimeter pixels enable high-resolution volumetric viewed (top left), right), (bottom techniques right). Cynthia McCollough/Mayo Clinic.)PPT|High resolutionBecause increasing computers iterative (IR) methods available. They go statistical considerations model lower noise. And reducing noise doses. even powerful computers, IR still require minutes-to-hours unacceptable busy department patients immediate attention. Artificial intelligence convolutional neural networks (CNNs) recently become cases replacing approaches. perform better levels.Quantitative imagingSection:ChooseTop i...Quantitative <<Radiation ARTICLESUnlike modalities department, quantitatively meaningful related µ, coefficient imaged material. numbers defined units (HU), honor inventor scanner, HUv=1000{µv−µwµw}. (3) HUv HU voxel v, µv µw water. When contains water, equals 0 HU, when −1000 HU. quantitative nature assists making correct diagnosis. example, lesions tend benign calcified, effect increases relative cancerous lesions. nodules very high benign. Other examples mineral broadly predictor fracture risk, perfusion measurements, reveal reductions flow organs heart.The accurate. well-defined required CT, made straight elements. Because radiation-based cancer treatments accurately planned performed; biopsy needles positioned; linear, areal, made. size stent artery diameter volume tumor assessed outlining its boundaries, response drug therapy deduced measuring changes time.Radiation levelsSection:ChooseTop <<Clinical ARTICLESImaging amazing look provide lifesaving diagnostic information, involves exposing ionizing radiation, doses alterations DNA potentially lead cancer. low dose accurately, resulting risks difficult quantify. weak carcinogen. epidemiological studies individuals exposed perform. vary conclusions numerous uncertainties assumptions; health benefit, show effect, exhibit detriment.What’s more, upon factors, distribution organs; timing exposures; age, sex, race, genetics, status, influence radiobiological impact exposure.44. Boone Radiology 265, 544 (2012). https://doi.org/10.1148/radiol.12112201 children’s cells actively reproducing adult, most child would risk negative effects adult amount radiation. Fortunately, obtain diagnostic-quality children, size.Despite difficulty predicting exact person’s getting compared baseline developing people, will develop sometime lifetime. contrast, receive them might scan. critical care ill injured. almost situations recommends exam, potential benefit outweighs any risk.Still, scary patients, details scientific statistics hard explain. One helpful approach describe dose.33. naturally occurring sources, radon gas cosmic rays, Denver, Colorado, 5.2 mSv/year, whereas San Francisco, 3.1 mSv/year. Moving Denver Francisco therefore background mSv/year—the equivalent year. No likely worry increased mSv/year considering move yet hesitate advised type comparison help families put perspective. Even negative, normal, important turn next.Clinical implicationsSection:ChooseTop implications <<ReferencesCITING ARTICLESThe caring continues proven hands experienced radiologists. demand scanner’s exquisitely detailed wide availability exams: seconds. Recently, two manufacturers resolve 125–150 microns.55. Hernandez Med. 48, 2760 (2021); https://doi.org/10.1002/mp.14789S. Leng Radiographics 39, 729 (2019). https://doi.org/10.1148/rg.2019180115 achievement promises further bone, lung, tissues.Because so fast, artifacts motion few, pediatric sedation necessary cases. special electron-beam past,66. Lipton 152, 579 (1984). https://doi.org/10.1148/radiology.152.3.6540463 heart cardiac-gating techniques.33. Some multi-energy imaging, processed isolate materials,77. R. C. Johnson, AJR, Am. Roentgenol. 199, S3 organic crystals cause gout; identify classes kidney stones; differentiate types arterial plaque. Relatively low-cost cone-beam emerged highly specialized breast,88. 221, 657 (2001). https://doi.org/10.1148/radiol.2213010334 orthopedic,99. Q. Cao 45, 114 (2018). https://doi.org/10.1002/mp.12654 dental imaging.1010. W. Scarfe, Farman, P. Sukovic, Can. Dent. Assoc. 72, 75 (2006).The disciplines exceeded expectations pioneers. Now, tool medicine—a stethoscope steroids. Indeed, survey leading magnetic resonance ranked innovation 20th century.1111. V. Fuchs, H. Sox Jr, Aff. 20, https://doi.org/10.1377/hlthaff.20.5.30 celebrate CT’s birthday, millions lives treatment-planning capabilities raise glass unseen generations scientists contributed enjoy well-deserved pat back.Conflict interestJohn received research funding CT-component manufacturers, Varian Siemens TeleSecurity Sciences, Imatrex. He stockholder cofounder Izotropic focuses breast CT. patent royalties university-owned patents licensed Samsung Electronics Imaging. McCollough principal investigator grant Mayo Clinic manufacturer Healthcare. She Mayo-owned Healthcare.ReferencesSection:ChooseTop implicationsReferences <<CITING ARTICLES1. https://doi.org/10.1259/0007-1285-46-552-1016, Google ScholarCrossref, ISI2. https://doi.org/10.1259/0007-1285-46-552-1023, ISI3. Scholar4. https://doi.org/10.1148/radiol.12112201, ISI5. https://doi.org/10.1002/mp.14789, ISIS. https://doi.org/10.1148/rg.2019180115, , ISI6. https://doi.org/10.1148/radiology.152.3.6540463, ISI7. ISI8. https://doi.org/10.1148/radiol.2213010334, ISI9. https://doi.org/10.1002/mp.12654, ISI10. (2006). Scholar11. https://doi.org/10.1377/hlthaff.20.5.30, ISI12. 1979; ScholarA. Scholar13. https://doi.org/10.1063/1.1729798, ISI14. https://doi.org/10.1063/1.1713127, ISI© 2021 American Institute Physics.