tag:theconversation.com,2011:/africa/topics/the-science-of-medical-imaging-6650/articlesThe Science of Medical Imaging – The Conversation2013-09-26T04:54:48Ztag:theconversation.com,2011:article/150302013-09-26T04:54:48Z2013-09-26T04:54:48ZThe science of medical imaging: magnetic resonance imaging (MRI)<figure><img src="https://images.theconversation.com/files/29647/original/gxp3tcjs-1377055247.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Magnetic resonance imaging uses the body's most abundant molecule – water.</span> <span class="attribution"><span class="source">Berkeley Lab</span></span></figcaption></figure><p><em>Our short series, the Science of Medical Imaging, examines the technology behind non-invasive methods of creating images of the human body. In this third and final instalment, we look at the basics of magnetic resonance imaging (MRI).</em></p>
<p>In the first two articles in this series we saw that <a href="https://theconversation.com/the-science-of-medical-imaging-spect-and-pet-14086">emission imaging</a> provides functional information and <a href="https://theconversation.com/the-science-of-medical-imaging-x-rays-and-ct-scans-15029">transmission imaging</a> provides structural context in the body. </p>
<p>The previously-discussed techniques are hugely beneficial; however, their repeated use on a given patient must be limited as they utilise <a href="http://en.wikipedia.org/wiki/Ionizing_radiation">ionising radiation</a>.</p>
<p>In the context of this discussion, it is important to remember that we are exposed to ionising radiation every day and our bodies do an excellent job in ameliorating any effects. </p>
<p>It remains true, though, that exposure to ionising radiation even as a part of effective clinical care can have a negative effect. One key technique that circumvents some of these issues is magnetic resonance imaging (MRI).</p>
<h2>Mapping water</h2>
<p>MRI was first used for imaging in the 1970s and since then, has seen many improvements. One of these, functional MRI (<a href="http://en.wikipedia.org/wiki/Functional_magnetic_resonance_imaging">fMRI</a>), allows changes in neuronal function to be observed while the patient performs a task.</p>
<p>MRI most commonly maps the distribution of water in the body. Water is composed of two hydrogen atoms and a single oxygen atom. </p>
<p>We can imagine these two hydrogens at the end of oxygen’s two “arms”. It is this specific molecular shape that allows MRI to pick out the hydrogens within water molecules. </p>
<p>Your brain and other soft tissue are 60-70% water compared to your bones which are only 30% water, so MRI is much better at mapping soft tissue. As you can see in the MRI image below, the aqueous humour of the eye shows up white, while skull bones are comparatively dark. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=241&fit=crop&dpr=1 600w, https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=241&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=241&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=303&fit=crop&dpr=1 754w, https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=303&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/29661/original/43kh6wjw-1377065251.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=303&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="attribution"><a class="source" href="https://www.flickr.com/photos/foltzwerk/281516702/sizes/z/in/photostream/">Foltzwerkp</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Generating magnetic resonance</h2>
<p>A complete description of MRI requires an understanding of <a href="http://en.wikipedia.org/wiki/Quantum_mechanics">quantum mechanics</a> and <a href="http://en.wikipedia.org/wiki/Spin_%28physics%29">spin</a>, but in the interests of article brevity, let’s ignore them.</p>
<p>All we really need to note is that we rely on protons acting as <a href="http://en.wikipedia.org/wiki/Magnetic_dipole">magnetic dipoles</a>. Simply put, a dipole is an object with two opposing poles. Thus, a magnetic dipole - such as a bar magnet - has north and south magnetic poles. </p>
<p>The orientation of the bar magnet in space can be described by the magnetic <a href="http://en.wikipedia.org/wiki/Dipole">dipole moment</a> which is a <a href="http://en.wikipedia.org/wiki/Euclidean_vector">vector</a> (as it has a direction, being from the south to the north pole).</p>
<p>The nucleus of the hydrogen atom in water has just one proton and it is the magnetic property of the proton that generates the signal in MRI.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=155&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=155&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=155&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=195&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=195&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27874/original/2by5ymjv-1374549972.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=195&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<p>To understand magnetic resonance, first start by making the assumption that the dipole moment of a single proton in a uniform magnetic field behaves like a compass needle. </p>
<p>Figure A shows that a compass needle placed in a magnetic field aligns with the field. The direction of the field is indicated by the vector <strong>B</strong> in the diagram. </p>
<p>We can then “kick” (excite) the system by introducing a second magnetic field (figure B) and then subsequently removing it (figure C). This will cause the needle to rock back and forth like a pendulum until it returns to its rest position. </p>
<p>If we plotted the position of the red part of the needle relative to its start position, we would get something like the shape shown in figure D. </p>
<p>This shape of graph is called a <a href="http://en.wikipedia.org/wiki/Damped_sine_wave">damped sinusoidal wave</a>. It is given this name as oscillates like a cosine (or sine) wave but its magnitude decreases (is damped) with time.</p>
<p>If, instead of removing the magnet entirely, we slide the second magnet back and forth (along the x-axis) at the correct frequency, we give the maximum “kick” to the needle and achieve a <a href="https://en.wikipedia.org/wiki/Resonance">resonance</a>. </p>
<p>This is analogous to choosing the correct times at which you push a child on a swing to gain maximum height.</p>
<p>Now, back to a practical discussion of MRI. Imagine a patient is laying in the scanner along the z-axis (see figure E) - the same direction as the strong magnetic field. We have already demonstrated what happens if the protons in the water in your body behaved like compass needles (see figure A) - they align with the magnetic field. </p>
<p>However, because the protons have <a href="http://en.wikipedia.org/wiki/Spin_magnetic_moment">spin</a> (angular momentum), they can align along or at 180 degrees to the field. These are two different energy states. There is not an exact 50:50 split and slightly more (an excess) of the protons align in the same direction as the magnetic field. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=240&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=240&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=240&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=302&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=302&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27876/original/sn29brph-1374550848.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=302&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<p>Figure F shows a cartoon of the excess of aligned protons (red arrows) in the patient. They appear as only a single red arrow because the dipole moment for each proton is sat on top of all the others. </p>
<p>Now, we need to “kick” the system. The way we do this is not with a bar magnet (as in figures B and C), but with a suitable radio frequency signal, approximately 60 MHz. </p>
<p>A radio frequency wave has both electric and magnetic fields. By generating a radio frequency wave and passing it through the body we disturb the protons, giving them energy. The protons have spin and so they <a href="http://en.wikipedia.org/wiki/Precession">precess</a> around the z-axis (as opposed to just rocking back and forth like the compass needle). </p>
<p>An example of the protons precessing after the radio frequency wave has “kicked” the system is shown in figure G. The frequency of this precession is proportional to the strength of the magnetic field and is called the <a href="http://en.wikipedia.org/wiki/Larmor_precession">Larmor frequency</a>. Again, in figure G, all of the protons are precessing at the same frequency and so the arrows are sat on top of one and other.</p>
<p>Up until now, all we have done is prepare the system by putting in energy in terms of electromagnetic fields - we haven’t actually measured anything. After the radio frequency pulse is applied, the precessing protons return to their equilibrium positions by emitting radio frequency waves (see figure H). </p>
<p>It is the magnetic field of this radio frequency wave - the magnetic resonance - that provides us with a signal that we can measure, and we do so with an <a href="http://en.wikipedia.org/wiki/Induction_coil">induction coil</a>. </p>
<p>Like we showed for the compass, the signal measured in magnetic resonance is also a damped sinusoidal wave that is caused by the dipole moments returning to their original orientations. </p>
<p>The signal shape is determined by the precession frequency and another effect called <a href="http://en.wikipedia.org/wiki/Dephasing">dephasing</a> (see the second definition on this Wikipedia page). The dephasing causes the dipole moments of the protons to separate out rather than sitting on top of each other as is shown in figures F to H.</p>
<h2>Locating the signal</h2>
<p>There is still a problem as we have not mentioned anything about determining where in the patient the signal comes from. After all, if we want to produce a map of the distribution of proton density in the patient (the image), we need to know the position of the emission of the radio frequency wave.</p>
<p>When we look at the signals, they have two main properties of interest:</p>
<ol>
<li>Amplitude, which is related to the number of protons in the region.<br></li>
<li>Frequency, which is related to the strength of the magnetic field (<strong>B</strong>) at that location.<br></li>
</ol>
<p>This is demonstrated in the diagrams below. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=153&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=153&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=153&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=192&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=192&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27885/original/k7byfwk5-1374553572.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=192&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<p>Figure I shows yellow, red and green glasses of water which represent the head, chest and abdomen of a patient in the scanner. We can see in figure I that the magnetic field - shown as <strong>B</strong> - is the same strength all along the z-axis. Figure J shows that after we “kick” the system, all three glasses of water respond by emitting radio waves of the same frequency. The signals have different amplitudes due to the different water levels.</p>
<p>Figures K and L show what happens if we make the field stronger at one end (<strong>B+</strong>) and weaker at the other (<strong>B-</strong>). We see in figure L that this gradient changes the Lamor frequency of the protons along the z-axis and the signals now have different frequencies. </p>
<p>So, by applying a magnetic field gradient and tailoring the frequency of the wave that “kicks” (excites) the protons we can select to only excite a thin slice of the patient. </p>
<p>Anyone who has had an MRI scan, will know that they are in the scanner for a significant length of time. This is because the patient has to be scanned slice-by-slice by applying the gradient and incrementing the frequency of the excitation. This is repeated many times to sequentially collect signals and build up an image from the slices of the patient. </p>
<p>While this discussion has brushed over a lot of the technical details, it serves to give a basic introduction to MRI and complements the articles on emission and transmission imaging.</p>
<p>Ideally (from ionising radiation considerations), all CT scans would be replaced with MRI. But, due to material differentiation, time and cost arguments these techniques must be used in unison. </p>
<p>Each of the techniques discussed in this series of articles - along with others than have not been covered, such as ultrasound - give clinicians a diagnostic arsenal to deploy when fighting our health problems.
<br></p>
<p><strong><em>Further reading:</em></strong> <br>
<strong><a href="https://theconversation.com/the-science-of-medical-imaging-spect-and-pet-14086">SPECT and PET</a></strong><br>
<strong><a href="https://theconversation.com/the-science-of-medical-imaging-x-rays-and-ct-scans-15029">X-rays and CT scans</a></strong></p><img src="https://counter.theconversation.com/content/15030/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matthew Dimmock does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Our short series, the Science of Medical Imaging, examines the technology behind non-invasive methods of creating images of the human body. In this third and final instalment, we look at the basics of…Matthew Dimmock, Researcher in Medical Imaging and X-ray Science, Australian SynchrotronLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/150292013-09-16T02:43:00Z2013-09-16T02:43:00ZThe science of medical imaging: X-rays and CT scans<figure><img src="https://images.theconversation.com/files/29631/original/szcjshkq-1377050500.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Computed tomography uses computer-analysed X-rays to produce 'slices' of the body.</span> <span class="attribution"><span class="source">U.S. Pacific Fleet</span></span></figcaption></figure><p><em>Our short series, the Science of Medical Imaging, examines the technology behind non-invasive methods of creating images of the human body. In this article, we discuss the technique of transmission imaging, in which the radiation source originates from outside the patient.</em></p>
<p>Since the first incarnation of the computed tomography (<a href="https://en.wikipedia.org/wiki/X-ray_computed_tomography">CT</a>) scanner in 1975, devised by British engineer <a href="http://www.nobelprize.org/nobel_prizes/medicine/laureates/1979/hounsfield-bio.html">Godfrey Hounsfield</a> (for which he was awarded the Nobel prize in 1979), the use of the technique has exploded. </p>
<p>In the US alone, approximately <a href="http://www.usatoday.com/story/news/nation/2013/03/21/ct-scan-concerns/2005729/">70 million CT scans</a> are performed annually. </p>
<p>So why are they so popular?</p>
<p>Compared to <a href="https://theconversation.com/the-science-of-medical-imaging-spect-and-pet-14086">emission imaging</a> - a useful technique for detecting whether a tumour is present - transmission imaging, such as CT, can show precisely where the tumour is located with respect to surrounding organs. </p>
<p>The best-known form of transmission imaging is the medical <a href="https://en.wikipedia.org/wiki/Radiography">radiograph</a>, commonly referred to as an X-ray, as it uses an <a href="https://en.wikipedia.org/wiki/X-ray_generator">X-ray generator</a> as the source of photons to collect data on the medium that they pass through. </p>
<h2>The science behind radiographs</h2>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=649&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=649&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=649&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=815&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=815&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27148/original/x8fxp956-1373349194.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=815&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<p>Figure A shows a simplified cartoon of a radiograph of a broken bone in 2D. </p>
<p>In the cartoon, the X-ray generator (source) is shown in red. The source produces 100 X-rays (yellow) at all six locations. The number of X-rays that reach the film (shown in the figure, left) is determined by the thickness of bone each X-ray beam (yellow arrow) intersects (assuming the bone is of uniform density). </p>
<p>The thicker the section of bone, the more electrons an X-ray encounters and the more likely it is the X-ray gets scattered or absorbed, and won’t reach the film. </p>
<p>The collective term for scattering and absorption is <a href="http://en.wikipedia.org/wiki/Attenuation">attenuation</a>. Therefore, a thicker section of bone attenuates more of the incident X-rays giving a <a href="http://en.wikipedia.org/wiki/Negative_%28photography%29">negative</a> image of the bone on the film.</p>
<p>Each material has an <a href="http://en.wikipedia.org/wiki/Attenuation_coefficient">attenuation coefficient</a>. This is a number showing how many X-rays of a given energy would pass through a material of a given density and thickness. </p>
<p>In reality your bones are not uniform in density and are surrounded by muscle and tissue, all with varying densities. So the radiograph maps the average attenuation coefficient along the path of the X-ray beam. </p>
<p>As we all know, a traditional radiograph only gives you a single 2D projection. This works well for a broken arm, but in parts of the body where there are different organs in the way, the 2D image gives clinicians no sense of the depth perspective as all of the body parts in the scan appear to sit on top of each other. </p>
<h2>Two to three dimensions</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=572&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=572&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=572&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=719&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=719&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27147/original/w5kx92zm-1373349082.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=719&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<p>In order to obtain a more useful 3D image, clinicians have to take a series of 2D images (projections) at different angles and then perform a reconstruction step in software - known as CT.</p>
<p>Figure B (above right) shows a cartoon of a CT scanner at its starting position with no rotation. </p>
<p>The detector (green) is divided into small pixels (p1 to p20) to record the number of X-ray interactions (intensity) at each pixel location. </p>
<p>Placed between the source and the detector is a cranial phantom called the <a href="http://en.wikipedia.org/wiki/Shepp-Logan_Phantom">Shepp-Logan Phantom</a>. The word “phantom” refers to an industry standard representation that is agreed on by researchers so that they know they are comparing apples with apples.</p>
<p>The cranial phantom is in greyscale. The different colours of the components of the phantom are materials of different density, where white is the most dense and represents the skull.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=405&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=405&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=405&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=509&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=509&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27137/original/xcvvnsg7-1373345166.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=509&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p>In Figure C, the source of X-rays has been turned on. The varying numbers of counts in each detector pixel are due to differing densities and thicknesses of material that each X-ray beam intersects. </p>
<p>Instead of writing the actual number of X-rays that each pixel will detect, the intensity is shown here as a horizontal strip of colour on the right side of the image. In this greyscale colour strip, black represents a high intensity (lots of X-rays passing through this particular section of the head) and white is low intensity. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=443&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=443&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=443&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=556&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=556&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27130/original/b4m3wdd5-1373343629.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=556&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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</figcaption>
</figure>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=504&fit=crop&dpr=1 754w, https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=504&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/27131/original/3rgx75hh-1373343657.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=504&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
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</figure>
<p>Figure D shows what happens when the source and detector is rotated around the patient by 10 degrees. The intensity detected in pixels p1 to p20 is now stacked below that from the 0 degree projection on the right hand side of the image.</p>
<p>Figure E shows what happens when the process of rotating by 10 degrees repeats, and the intensity measurements are recorded all the way around to 180 degrees. </p>
<p>The clinician ends up with an intensity image with pixel position along one axis and rotation angle along the other axis. This image is called a <a href="http://en.wikipedia.org/wiki/Sinogram">sinogram</a>. Image reconstruction software analyses the sinogram to form an accurate anatomical representation of the patient’s skull.</p>
<h2>Reconstruction</h2>
<p>Getting from a sinogram to the image of what’s inside the patient is not trivial. The mathematics that unlocked this technique were first developed by the mathematician <a href="http://en.wikipedia.org/wiki/Johann_Radon">Johann Radon</a> in 1917. </p>
<p>Modern implementations of image reconstruction that enable fast and accurate reconstruction are based on work published by <a href="http://en.wikipedia.org/wiki/Larry_Shepp">Larry Shepp</a> and <a href="http://en.wikipedia.org/wiki/Benjamin_F._Logan">Benjamin Logan</a> (who have the phantom named after them) in 1974. </p>
<p>While CT scans have many benefits, the doctor must weigh up the pros and cons before prescribing it. On one hand, a CT scan provides essential detail that can aid diagnosis; on the other, radiation doses from CT are typically more than 100 times those from a conventional radiograph. </p>
<p>As such, the risk of inducing DNA damage that can <a href="https://theconversation.com/ct-scans-can-triple-risk-of-brain-cancer-leukemia-7532">lead to cancer</a> must be properly assessed, especially in children.
<br></p>
<p><strong>Further reading:</strong><br>
<a href="https://theconversation.com/the-science-of-medical-imaging-spect-and-pet-14086">SPECT and PET</a><br>
<a href="https://theconversation.com/the-science-of-medical-imaging-magnetic-resonance-imaging-mri-15030">MRI</a></p><img src="https://counter.theconversation.com/content/15029/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matthew Dimmock does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Our short series, the Science of Medical Imaging, examines the technology behind non-invasive methods of creating images of the human body. In this article, we discuss the technique of transmission imaging…Matthew Dimmock, Researcher in Medical Imaging and X-ray Science, Australian SynchrotronLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/140862013-09-09T04:45:48Z2013-09-09T04:45:48ZThe science of medical imaging: SPECT and PET<figure><img src="https://images.theconversation.com/files/29626/original/pzjtx2v2-1377048277.jpg?ixlib=rb-1.1.0&rect=18%2C141%2C2081%2C1389&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A patient undergoes a PET scan – but how does it work?</span> <span class="attribution"><span class="source">Wikimedia Commons</span></span></figcaption></figure><p><em>Our new series, the Science of Medical Imaging, examines the technology behind non-invasive methods of creating images of the human body. In this first instalment, we look at two types of emission imaging: positron emission tomography (PET) and single-photon emission computerised tomography (SPECT).</em></p>
<p>As its name suggests, emission imaging works by detecting radiation emitted from within the patient, enabling clinicians to determine the presence and size of cancerous tumours, and conduct other diagnostic procedures such as <a href="http://en.wikipedia.org/wiki/Coronary_perfusion_pressure">coronary perfusion</a>. </p>
<p>Hospitals use two main forms of emission imaging: </p>
<ul>
<li>positron emission tomography (<a href="http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/PET_scan">PET</a>)</li>
<li>single-photon emission computerised tomography (<a href="http://www.mayoclinic.com/health/spect-scan/MY00233">SPECT</a>)</li>
</ul>
<p>Both work on the same basic principles - detecting gamma rays and building a three-dimensional picture of, say, a cancerous tumour.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&rect=0%2C41%2C1024%2C703&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&rect=0%2C41%2C1024%2C703&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=448&fit=crop&dpr=1 600w, https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=448&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=448&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=563&fit=crop&dpr=1 754w, https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=563&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/28912/original/w97z8dzm-1375942408.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=563&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A SPECT scan in action.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span></span>
</figcaption>
</figure>
<h2>The basics of emission imaging</h2>
<p>A radioactive tracer (<a href="http://en.wikipedia.org/wiki/Radioactive_tracer">radiotracer</a>) is injected into the patient. Radiotracers are molecules such as glucose with a <a href="http://www.britannica.com/EBchecked/topic/489027/radioactive-isotope">radioactive isotope</a> attached. </p>
<p>As tumour cells rapidly grow, compared to neighbouring cells, they require large amounts of glucose. Blood carries glucose throughout the patient but it is absorbed mostly at the tumour site, carrying the isotope with it. </p>
<p>The isotope then decays, emitting <a href="http://missionscience.nasa.gov/ems/12_gammarays.html">gamma rays</a> (photons that are much higher in energy than visible light and can pass out of the body). By collecting the gamma rays in detectors placed around the patient, we can build up a picture of where they came from, locating the tumour position and shape.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=567&fit=crop&dpr=1 600w, https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=567&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=567&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=713&fit=crop&dpr=1 754w, https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=713&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/24563/original/j29s5qh3-1369785975.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=713&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p>Many different detector systems are available. The simplest is the pinhole camera, which you may have used to <a href="http://www.exploratorium.edu/eclipse/how.html">observe an eclipse</a>. </p>
<p>To do this, you prick a tiny hole in a sheet of card and place a sheet of paper behind it. A projected image of the eclipse is cast onto the paper. The projection appears back to front and upside down and is viewable without damaging your eyes. </p>
<p>This is shown in Figure A above; but the eclipse has been replaced with The Conversation logo. In the subsequent figures, our theoretical tumour is also replaced by the logo.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=664&fit=crop&dpr=1 600w, https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=664&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=664&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=835&fit=crop&dpr=1 754w, https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=835&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/24556/original/4pygbr8n-1369784700.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=835&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p>And while pinhole cameras are handy for eclipses, the simple design will not suffice for imaging in an oncology department. </p>
<p>In this instance, the sheet of card is replaced with a sheet of dense metal (typically lead, and called a <a href="http://en.wikipedia.org/wiki/Collimator">collimator</a>) and the paper is replaced by a detector divided into pixels that can measure the position and energy of each photon that passes through the pinhole as shown in Figure B. </p>
<p>The detector is divided into pixels by the manufacturing process. The smaller the pixels, the more precisely we will know the location of where the gamma ray interacted. </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=436&fit=crop&dpr=1 600w, https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=436&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=436&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=548&fit=crop&dpr=1 754w, https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=548&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/24558/original/4smb7k6c-1369784924.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=548&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p>The gamma rays emitted from the decay of the radioisotope leave the tumour in all directions. In Figure B, the two diagonal lines show the limits of the directions that they must follow to pass through the pinhole and interact in the detector. </p>
<p>The tumour has a 3D distribution and the detector must be rotated around the patient - see Figure C above - to collect a projection at each angle as is performed in a SPECT scan in a hospital.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/l6V6VLxQlkY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A patient undergoes a SPECT scan.</span></figcaption>
</figure>
<h2>Putting it all together</h2>
<p>After the radiotracer has “washed through” the patient, the aim of the exercise is to build up a picture of the tumour from a series of measured counts in detector pixels. We can’t simply add the counts in the detector at each angle - we have to perform a <a href="http://en.wikipedia.org/wiki/Iterative_reconstruction">reconstruction</a>. </p>
<p>For SPECT and PET this requires tracing lines back from the detector pixel to where the photons came from (inside the tumour). We do this for each pixel that records energy from a gamma ray. </p>
<p>We know we need two points to form a line. In SPECT, the two points forming each line are the pixel location and the pinhole location.</p>
<p>In order to increase the sensitivity of the device, we can punch more holes in the collimator, which will allow more gamma rays through. This means we can give the patient a lower dose of radiotracer, reducing the <a href="https://theconversation.com/ct-scans-have-small-increased-risk-of-cancer-14555">risk of causing secondary tumours</a> by the very act of administering the radiotracer in the first place. </p>
<h2>Differences between SPECT and PET</h2>
<p>To demonstrate why SPECT and PET have different types of detectors, first we must understand the difference in radioisotope that is administered to the patient. </p>
<p>Figure D (below) shows two cartoon representations of a patient with a tumour that has absorbed the SPECT radiotracer (on the left) and the PET radiotracer (on the right). </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=145&fit=crop&dpr=1 600w, https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=145&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=145&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=182&fit=crop&dpr=1 754w, https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=182&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/25192/original/547gptzq-1370568171.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=182&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Left: three gamma rays emitted in the case of a SPECT scan. Right: the presence of an intermediate positronium in PET means twice as many gamma rays emitted from the tumour.</span>
</figcaption>
</figure>
<p>For both cartoons we have shown three of the millions of decays of the radiotracer that happen while it is inside the patient. For the SPECT case, the nuclear decay is straightforward and we get three gamma rays. </p>
<p>For the PET case, an intermediate <a href="http://en.wikipedia.org/wiki/Positronium">positronium</a> is formed, resulting in two gamma rays at each of the three decay sites, giving a total of six gamma rays. </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=587&fit=crop&dpr=1 600w, https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=587&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=587&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=738&fit=crop&dpr=1 754w, https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=738&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/25191/original/8q8g6bmv-1370568008.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=738&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<p>The great thing about PET radioisotopes is that each pair of gamma rays are primarily emitted at 180 degrees to each other. Figure E shows why this is so useful. </p>
<p>As mentioned already, the two points that form the line in the reconstruction in SPECT are the pinhole in the collimator and the pixel of the detector. </p>
<p>For PET radioisotopes, all we have to do is detect the two gamma rays for each decay and this enables us to trace the line that finds where the emission came from. </p>
<p>We can do this by including a second detector and throwing away the collimator, which means we get an enormous gain in sensitivity (up to 1,000 times). </p>
<p>Generally, SPECT radiotracers last longer in the patient and are primarily used in cardiology where myocardial stress imaging takes about three to four hours, whereas PET radiotracers emit gamma rays with shorter lives and higher energies and are more useful in brain imaging where scans last about 30 minutes.</p>
<p>As with everything in science, no system is perfect. PET does suffer from several <a href="http://www.sciencedirect.com/science/article/pii/S0168900210026276">resolution limitations</a> and there are issues with the range of radioisotopes that decay via positron emission. </p>
<p>But they are two useful tools in the clinician’s arsenal for diagnosing disease in patients.</p>
<p><strong>Further reading:</strong><br>
<a href="https://theconversation.com/the-science-of-medical-imaging-x-rays-and-ct-scans-15029">X-rays and CT scans</a><br>
<a href="https://theconversation.com/the-science-of-medical-imaging-magnetic-resonance-imaging-mri-15030">MRI</a></p><img src="https://counter.theconversation.com/content/14086/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Matthew Dimmock does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Our new series, the Science of Medical Imaging, examines the technology behind non-invasive methods of creating images of the human body. In this first instalment, we look at two types of emission imaging…Matthew Dimmock, Researcher in Medical Imaging and X-ray Science, Australian SynchrotronLicensed as Creative Commons – attribution, no derivatives.