View source for Gamma camera

{{short description|Camera to record gamma radiation}}
{{Refimprove|date=September 2016}}

[[Image:Lung scintigraphy keosys.JPG|thumb|An example of lung scintigraphy examination]]
A '''gamma camera''' ('''γ-camera'''), also called a '''scintillation camera''' or '''Anger camera''', is a device used to image gamma radiation emitting radioisotopes, a technique known as [[scintigraphy]]. The applications of scintigraphy include early [[drug development]] and [[nuclear medicine|nuclear medical imaging]] to view and analyse images of the human body or the distribution of medically injected, inhaled, or ingested [[radionuclides]] emitting [[gamma ray]]s.

==Imaging techniques==
[[File:Coded aperture mask (for gamma camera).jpg|thumb|[[Coded aperture]] mask for gamma camera (for [[SPECT]])]]
[[Scintigraphy]] ("scint") is the use of gamma cameras to capture emitted radiation from internal radioisotopes to create two-dimensional<ref>[http://medical-dictionary.thefreedictionary.com/scintigraphy thefreedictionary.com > scintigraphy] Citing: Dorland's Medical Dictionary for Health Consumers, 2007 by Saunders; Saunders Comprehensive Veterinary Dictionary, 3 ed. 2007; McGraw-Hill Concise Dictionary of Modern Medicine, 2002 by The McGraw-Hill Companies</ref> images.

[[SPECT]] (single photon emission computed tomography) imaging, as used in nuclear [[cardiac stress test]]ing, is performed using gamma cameras. Usually one, two or three detectors or heads, are slowly rotated around the patient.



==Construction==
[[Image:GammaCamera.jpg|thumb|right|Gamma camera]]
[[Image:Gamma camera cross section.PNG|thumb|right|Diagrammatic cross section of a gamma camera detector]]
[[Image:Gamma Camera Cross Section detail.png|thumb|right|Details of the cross section of a gamma camera]]
A gamma camera consists of one or more flat crystal planes (or detectors) optically coupled to an array of [[Photomultiplier|photomultiplier tubes]] in an assembly known as a "head", mounted on a gantry. The gantry is connected to a computer system that both controls the operation of the camera and acquires and stores images.<ref name="SahaGC">{{cite book|last1=Saha|first1=Gopal B.|title=Physics and radiobiology of nuclear medicine|date=2006|publisher=Springer|location=New York|isbn=978-0-387-30754-1|edition=3rd|doi=10.1007/978-0-387-36281-6}}</ref>{{rp|82}} The construction of a gamma camera is sometimes known as a compartmental radiation construction.

The system accumulates events, or counts, of [[Gamma ray|gamma]] [[photon]]s that are absorbed by the crystal in the camera. Usually a large flat crystal of [[Sodium iodide#Thallium-doped NaI.28Tl.29 scintillators|sodium iodide]] with thallium doping NaI(Tl) in a light-sealed housing is used. The highly efficient capture method of this combination for detecting gamma rays was discovered in 1944 by [[Samuel Curran|Sir Samuel Curran]]<ref>[https://openlibrary.org/b/OL17868379M/Counting_tubes "Counting tubes, theory and applications", Curran, Samuel C., Academic Press (New York), 1949]</ref><ref>{{cite ODNB|last1=Fletcher|first1=W W|title=Oxford Dictionary of National Biography|date=2004|location=Oxford|chapter-url=http://www.oxforddnb.com/view/article/69524|chapter=Curran, Sir Samuel Crowe (1912–1998)|doi=10.1093/ref:odnb/69524 }}</ref> whilst he was working on the [[Manhattan Project]] at the [[University of California at Berkeley]]. Nobel prize-winning physicist [[Robert Hofstadter]] also worked on the technique in 1948.<ref>{{cite web|title=Robert Hofstadter – Biographical|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1961/hofstadter-bio.html|website=Nobel Prize|access-date=29 September 2016}}</ref>

The crystal [[scintillation (physics)|scintillate]]s in response to incident gamma radiation. When a gamma photon leaves the patient (who has been injected with a [[radiopharmaceutical|radioactive pharmaceutical]]), it knocks an electron loose from an iodine atom in the crystal, and a faint flash of light is produced when the dislocated electron again finds a minimal energy state. The initial phenomenon of the excited electron is similar to the [[photoelectric effect]] and (particularly with gamma rays) the [[Compton effect]]. After the flash of light is produced, it is detected. [[Photomultiplier]] tubes (PMTs) behind the crystal detect the fluorescent flashes (events) and a computer sums the counts. The computer reconstructs and displays a two dimensional image of the relative spatial count density on a monitor. This reconstructed image reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged.<ref name="Khalil10">{{cite book|last1=Khalil|first1=Magdy M.|title=Basic sciences of nuclear medicine|date=2010|publisher=Springer|location=Heidelberg|isbn=978-3-540-85961-1|chapter=Elements of Gamma Camera and SPECT Systems}}</ref>{{rp|162}}

[[Image:Collimated and penetration.ogv|thumb|Animated schematic of gamma-camera physics and main constituents]]

==Signal processing==
[[Hal Anger]] developed the first gamma camera in 1957.<ref>{{cite journal|last1=Tapscott|first1=Eleanore|title=Nuclear Medicine Pioneer, Hal O. Anger, 1920–2005|journal=Journal of Nuclear Medicine Technology|date=2005|volume=33|issue=4|pages=250–253|url=http://tech.snmjournals.org/content/33/4/250.full|pmid=16397975}}</ref><ref>{{cite journal|last1=Anger|first1=Hal O.|title=Scintillation Camera|journal=Review of Scientific Instruments|date=1958|volume=29|issue=1|pages=27–33|doi=10.1063/1.1715998|bibcode=1958RScI...29...27A}}</ref> His original design, frequently called the Anger camera, is still widely used today. The Anger camera uses sets of [[vacuum tube]] [[photomultiplier]]s (PMT). Generally each tube has an exposed face of about {{nowrap|7.6 cm}} in diameter and the tubes are arranged in hexagon configurations, behind the absorbing crystal. The electronic circuit connecting the photodetectors is wired so as to reflect the relative coincidence of light fluorescence as sensed by the members of the hexagon detector array. All the PMTs simultaneously detect the (presumed) same flash of light to varying degrees, depending on their position from the actual individual event. Thus the spatial location of each single flash of fluorescence is reflected as a pattern of voltages within the interconnecting circuit array.

The location of the interaction between the gamma ray and the crystal can be determined by processing the voltage signals from the photomultipliers; in simple terms, the location can be found by weighting the position of each photomultiplier tube by the strength of its signal, and then calculating a mean position from the weighted positions.<ref name="SahaGC" />{{rp|112}} The total sum of the voltages from each photomultiplier, measured by a [[pulse height analyzer]] is proportional to the energy of the gamma ray interaction, thus allowing discrimination between different isotopes or between scattered and direct photons.<ref name="Khalil10" />{{rp|166}}

==Spatial resolution==
In order to obtain spatial information about the gamma-ray emissions from an imaging subject (e.g. a person's heart muscle cells which have absorbed an intravenous injected radioactive, usually thallium-201 or [[technetium-99m]], medicinal imaging agent) a method of correlating the detected photons with their point of origin is required.

The conventional method is to place a [[collimator]] over the detection crystal/PMT array.  The collimator consists of a thick sheet of [[lead]], typically {{convert|1|to|2.2|in|mm|order=flip|round=5}} thick, with thousands of adjacent holes through it. There are three types of collimators: low energy, medium energy, and high energy collimators. As the collimators transitioned from low energy to high energy, the hole sizes, thickness, and septations between the holes also increased. <ref>{{Cite journal|last1=Razavi|first1=Seyed Hossein|last2=Kalantari|first2=Faraz|last3=Bagheri|first3=Mahmoud|last4=Namiranian|first4=Nasim|last5=Nafisi-Moghadam|first5=Reza|last6=Mardanshahi|first6=Alireza|last7=Emami-Ardekani|first7=Alireza|last8=Sobhan Ardekani|first8=Mohammad|last9=Razavi-Ratki|first9=Seid Kazem|date=2017-07-01|title=Characterization of low, medium and high energy collimators for common isotopes in nuclear medicine: A Monte Carlo study|url=http://irjnm.tums.ac.ir/article_26411.html|journal=Iranian Journal of Nuclear Medicine|volume=25|issue=2|pages=100–104|issn=1681-2824}}</ref> Given a fixed septal thickness, the collimator resolution decreases with increased efficiency and also increasing distance of the source from the collimator.<ref name="Sorensen 1987"/> Pulse-height analyser determines the [[Full width at half maximum]] that selects certain photons to contribute to the final image, thus determining the collimator resolution.<ref>{{cite web |title=Joint CI-JAI advanced accelerator lecture series Imaging and detectors for medical physics Lecture 5: Gamma cameras |url=https://www.cockcroft.ac.uk/wp-content/uploads/2016/04/Lecture-5-gamma-cameras.pdf |publisher=The Cockroft Institute |access-date=14 January 2022 |archive-url=https://web.archive.org/web/20210131201044/https://www.cockcroft.ac.uk/wp-content/uploads/2016/04/Lecture-5-gamma-cameras.pdf |archive-date=31 January 2021}}</ref><ref name="Sorensen 1987">{{cite book |vauthors=Sorensen JA, Phelps ME |title=Physics in Nuclear Medicine |date=1 January 1987 |publisher=Elsevier - Health Sciences Division |isbn=978-0808918042 |page=336 |url=http://websites.umich.edu/~ners580/ners-bioe_481/lectures/pdfs/Sorenson_chpt-16.pdf |access-date=14 January 2022 |archive-url=https://web.archive.org/web/20220114020335/http://websites.umich.edu/~ners580/ners-bioe_481/lectures/pdfs/Sorenson_chpt-16.pdf |archive-date=14 January 2022}}</ref>

The individual holes limit photons which can be detected by the crystal to a cone shape; the point of the cone is at the midline center of any given hole and extends from the collimator surface outward.  However, the collimator is also one of the sources of blurring within the image; lead does not totally attenuate incident gamma photons, there can be some [[crosstalk]] between holes. 

Unlike a lens, as used in visible light cameras, the collimator attenuates most (>99%) of incident photons and thus greatly limits the sensitivity of the camera system. Large amounts of radiation must be present so as to provide enough exposure for the camera system to detect sufficient scintillation dots to form a picture.<ref name="SahaGC" />{{rp|128}}

Other methods of image localization ([[pinhole camera|pinhole]], rotating slat collimator with [[Cadmium Zinc Telluride|CZT]]) have been proposed and tested;<ref>{{cite journal|last1=Zeng|first1=Gengsheng L.|last2=Gagnon|first2=Daniel|last3=Matthews|first3=Christopher G.|last4=Kolthammer|first4=Jeffery A.|last5=Radachy|first5=Jason D.|last6=Hawkins|first6=William G.|s2cid=13092740|title=Image reconstruction algorithm for a rotating slat collimator|journal=Medical Physics|date=20 June 2002|volume=29|issue=7|pages=1406–1412|doi=10.1118/1.1485057|pmid=12148720|bibcode=2002MedPh..29.1406Z}}</ref> however, none have entered widespread routine clinical use.

The best current camera system designs can differentiate two separate point sources of gamma photons located at 6 to 12&nbsp;mm depending on distance from the collimator, the type of collimator and radio-nucleide. Spatial resolution decreases rapidly at increasing distances from the camera face. This limits the spatial accuracy of the computer image: it is a fuzzy image made up of many dots of detected but not precisely located scintillation. This is a major limitation for heart muscle imaging systems; the thickest normal heart muscle in the left ventricle is about 1.2&nbsp;cm and most of the left ventricle muscle is about 0.8&nbsp;cm, always moving and much of it beyond 5&nbsp;cm from the collimator face. To help compensate, better imaging systems limit scintillation counting to a portion of the heart contraction cycle, called gating, however this further limits system sensitivity.

==See also==
*[[Nuclear medicine]]
*[[Scintigraphy]]

==References==
{{reflist}}

==Further reading==
{{refbegin|2}}
* H. Anger. A new instrument for mapping gamma-ray emitters. Biology and Medicine Quarterly Report UCRL, 1957, 3653: 38. (University of California Radiation Laboratory, Berkeley)
* {{cite journal|last1=Anger|first1=HO|title=Scintillation camera with multichannel collimators|journal=Journal of Nuclear Medicine|date=July 1964|volume=5|pages=515–31|pmid=14216630}}
* {{cite book|last1=Sharp|first1=Peter F.|last2=Gemmell|first2=Howard G.|last3=Murray|first3=Alison D.|title=Practical nuclear medicine|date=2005|publisher=Springer|location=London|isbn=978-1-85233-875-6}}
* {{cite patent |country=US|number=6359279|status= |title=Detector for nuclear imaging |pubdate= |gdate=19 March 2002 |fdate= |pridate= |invent1=Gagnon, Daniel|invent2=Matthews, Christopher G. |assign1= |assign2= |class= |url=}}
* {{cite patent |country=US|number=6552349|status= |title=Detector with non-circular field of view |pubdate= |gdate=2 April 2003|fdate= |pridate= |invent1=Gagnon, Daniel|invent2=Matthews, Christopher G. |assign1= |assign2= |class= |url=}} 
* {{cite book|last1=Cherry|first1=Simon R.|last2=Sorenson|first2=James A.|last3=Phelps|first3=Michael E.|title=Physics in nuclear medicine|date=2012|publisher=Elsevier/Saunders|location=Philadelphia|isbn=978-1-4160-5198-5|edition=4th}}
{{refend}}

==External links==
*{{commonscat-inline|Gamma cameras}}
{{Nuclear technology}}

[[Category:Nuclear medicine]]
[[Category:Image sensors]]
[[Category:Medical physics]]
[[Category:American inventions]]
[[Category:Gamma rays]]
[[Category:Articles containing video clips]]