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Advanced medical & Biometric Imaging Part 1: By Dr David Maddison One of the greatest advances of modern times has been the ability to non-destructively look inside people or animals to aid in diagnosing diseases or other conditions. This article describes the history of that technology plus the latest innovations in medical imaging. Image source: www.pexels.com/photo/person-holding-silver-round-coins-4226264/ M any imaging technologies have been developed to date; too many to cover in one article. So this article aims to cover the most important, popular and interesting ones. Next month, we will have a follow up article describing similar imaging systems that look inside machines, vehicles and other objects. X-rays One of the first and most significant medical imaging techniques to be used, still in widespread use today, involved X-rays. Wilhelm Conrad Röntgen is credited with the discovery of X-rays in 1895. However, others had previously noted mysterious rays emanating from various gas discharge tubes such as Crookes tubes, which were used to produce cathode rays (see Figs.1 & 2). It is believed that X-rays were first inadvertently and unknowingly produced by a gas discharge apparatus in 1785 by William Morgan (born 1750). In 1888, Philipp Lenard discovered that something came out of a Crookes tube, causing photographic plates to become exposed. In 1889, Ivan Puluj (Іва́н Пулю́й) published his observation that emanations from a gas discharge tube would darken photographic plates. Then Fernando Sanford described “electric photography” in a letter sent in 1893. Then in 1894, Nikola Tesla observed that his photographic film was damaged by unknown radiation seemingly associated with his Crookes tube experiments (including when he photographed Mark Twain). X-rays were adopted for imaging purposes soon after the first demonstrations by Röntgen (Fig.3). The hazard of over-exposure to X-rays was almost immediately recognised. How X-rays are generated X-rays can be generated by a variety of methods. One common approach is Fig.2: a Crookes tube, shown energised at the bottom. The cathode is at the left and the anode underneath. Source: Wikimedia user D-Kuru. Fig.1: early medical experiments using a Crookes tube to generate X-rays in 1896. The man at the back is examining his hand using a fluoroscope screen while the other is taking a radiograph with a photographic plate under his hand. The tube is powered by an induction coil in the background; its drive pulses are generated by a motor-operated interrupter with a rheostat to vary the coil current and thus the voltage. 12  Silicon Chip Australia’s electronics magazine siliconchip.com.au ► ► Fig.3: one of the first published X-ray images, by Wilhelm Röntgen, of Albert von Kölliker’s hand. It was taken at a public lecture on the 23rd of January 1896. The very first picture was of Röntgen’s wife’s hand, but is of inferior quality. Fig.4: a typical X-ray emission spectrum with a tungsten target. Original source: ARPANSA. releasing high-speed electrons from a hot cathode and colliding them with a target, which is also the anode; in modern X-ray tubes, it is typically made of tungsten. The anode and cathode are housed in an evacuated tube. The energy of the X-rays produced is determined by the voltage by which the electrons are accelerated. X-rays are produced when electrons hit the target by one of two processes: 1) When electrons of a high enough energy knock electrons from the inner orbitals of atoms, and electrons fill such vacancies from higher energy levels, X-rays of a particular frequency are emitted. 2) By the process of Bremsstrahlung (“braking radiation”), where electrons are deflected in the vicinity of charged atomic nuclei of the target, which results in X-ray emission with a continuous range of frequencies. The result of these processes is an X-ray emission spectrum with a continuous range from (2) plus some peaks from (1) – see Fig.4. Crookes tubes were initially used to investigate cathode rays, leading to the development of the cathode ray tube. The production of X-rays was an unintended byproduct of this, leading to their discovery. X-rays are produced when electrons bypass the shadow mask and impinge upon the glass, causing the glass to fluoresce and emit X-rays. X-rays are also produced when the high-speed electrons hit the anode at the bottom. After the discovery of X-rays, siliconchip.com.au specialised Crookes tubes were developed which were optimised to produce X-rays. They had a heavy metal anode made from a metal such as platinum, angled to produce a beam of X-rays from the side of the tube. This is more or less the arrangement for a modern X-ray tube – see Fig.5. Incidentally, the CRTs used in older TVs and oscilloscopes could produce X-rays, although generally not enough to be of concern. Most CRTs had X-ray absorbing glass to minimise the problem. With a Crookes tube, X-rays are generated with the application of 5kV or more; the higher the voltage, the higher the energy of the X-rays produced, leading to greater penetration through targets. Fig.5: X-rays are produced in a tube when high-speed electrons strike the metal target. This is a more efficient method than Crookes tubes. Table 1: X-ray sources for various applications Application Dental Acceleration voltage Source X-ray energy 60kV Tube 30keV General medical 50-140kV Tube 40keV CT scan 80-140kV Tube 60keV Airline bag screening 80-160kV Tube 80keV Shipping container 450kV-20MV Tube or linear accelerator 150keV-9MeV Structural analysis 150kV-450kV Tube 100keV X-ray therapy 10MV-25MV Linear accelerator 3MeV-10MeV Australia’s electronics magazine August 2021  13 linac and are ultimately transferred to the primary storage ring. As the electrons go around the storage ring, they are deflected by magnets, causing them to emit radiation at a range of possible frequencies due to Magnetobremsstrahlung (“synchrotron radiation”; a variation of braking radiation). There are several “beamlines” where different experiments are conducted. One of the beamlines of the Australian Synchotron is the Imaging and Medical Beamline (IMBL). It delivers the world’s widest synchrotron X-ray beam at extremely high resolution, greater even than MRI. How are X-rays recorded? Fig.6: a medical linac (linear accelerator) for producing X-rays for radiotherapy. Original image by The Scientific Sentence. Fig.7: the operation of a linear accelerator. An electron is injected at the left, accelerated to the first “drift tube” and when it gets to the end of that, the polarity changes to the alternating RF current, it is accelerated across the gap to the next one, and so on. The electrons impinge upon a metal target to generate the X-rays. Original image by The Scientific Sentence. Generating X-rays by accelerating electrons onto a target is relatively inefficient, with only about 1% of the electrical energy being converted to X-rays, and the rest into heat. Another way to generate X-rays for imaging purposes is using a linear particle accelerator or ‘linac’ (see Figs.6 & 7 and Table 1). A linac can also be used to produced X-rays for radiotherapy in a medical setting. Linacs generate X-rays by accelerating electrons in a tuned cavity waveguide energised by a radio frequency (RF) electric field. An electron is accelerated through a series of cylindrical electrodes whose polarity is constantly changing due to the RF field; as it gets to the end of one electrode, it is accelerated across the gap into the next one. Another method to generate X-rays is with a synchrotron (Fig.8). The Australian Synchrotron was first discussed in Silicon Chip May 2012 (siliconchip. com.au/Article/671). A synchrotron is another type of particle accelerator, circular rather than linear. Electrons start their journey in a Traditional, two-dimensional planar X-ray images were recorded on film, and many still are. Alternatively, flat-panel sensors can be used. These use ‘scintillating’ materials such as gadolinium oxysulfide (Gd2O2S) or caesium iodide (CsI) to convert X-ray photons into light, which is then detected by an imaging array. Photoconductive materials like amorphous selenium may also be used; these convert X-ray photons into electric charges, which are then read by an electrode array. Fluoroscopy (Fig.9) can be used to produce a two-dimensional “motion X-ray” where the X-rays illuminate a fluorescent screen, or in modern implementations, an X-ray image intensifier and camera or a flat panel sensor, as described above. Fluoroscopy is used for various applications, such as: • Inserting catheters or various electrical leads, such as pacemakers • Investigating the gastrointestinal tract after a “barium meal” has been swallowed (barium blocks X-rays) • Biopsies which require guidance ► Fig.8: the layout of a generic synchrotron showing (1) Electron gun, (2) linac, (3) booster ring, (4) storage ring, (5) beamline (one of many) and (6) end station, where experiments are performed. To give an idea of the size, the main storage ring of the Australian Synchrotron is 216m in circumference. Fig.9: the insertion of pacemaker leads into the heart is a procedure typically done under fluoroscopic guidance, as real-time imagery of the lead is needed. Source: Gregory Marcus, MD, MAS, FACC. 14  Silicon Chip Australia’s electronics magazine siliconchip.com.au • Orthopaedic surgery • Studies of blood vessels such as in the heart, brain and leg • Urology Medical computed tomography (CT) scanning CT scanning, originally known as CAT scanning (computed axial tomography), is a method based on X-rays that can produce cross-sectional slices or three-dimensional images. This is unlike conventional X-ray images, which simply project an X-ray image onto a film or digital sensor. An X-ray beam is passed through an object to be examined, and the intensity of the beam is measured as it exits. Different structures will absorb the beam by different amounts; hard tissue such as bone will absorb more and soft tissue such as brain will absorb less. This gives information about the totality of what the beam has encountered on the way through but no information as to the individual structures encountered. Additional information is gathered by rotating the beam and corresponding sensor to a different angle and repeating the measurement. This is done thousands of times to build up a comprehensive amount of information about many beamlines passing through the object – see Fig.10. This is then transformed into a two dimensional ‘tomographic’ slice by an appropriate mathematical transformation, and by further interpretation of these slices, 3D images can be generated. As with any X-ray procedure, CT scanning exposes the patient to X-rays, although the dose is kept to the minimum possible. Another disadvantage is that certain tissues are not highly visible. To get around this problem, sometimes so-called radiocontrast agents are used, which strongly block X-rays. These are injected to enhance images of specific soft tissues which would otherwise not be sufficiently visible. Substances containing iodine can be used for blood vessels, and substances containing barium for the gastrointestinal tract. Specialised medical uses of CT scanners There are several specialised uses and imaging modes of CT scanning. Two of note are CT coronary angiograms (Fig.11), and the use of CT scans siliconchip.com.au Fig.10: in a CT scanner, the X-ray beam and detectors are rotated about the patient. Three different positions are shown here. The patient also moves through the imaging plane of the beam orthogonal to the page. Original source: Elizabeth Swanson. Fig.11: an image produced by a CT coronary angiogram. Source: Macquarie Medical Imaging (MMI), siliconchip.com.au/link/ab90 in combination with 3D printing to make bone replacement parts to repair bone defects (Fig.12). In a CT coronary angiogram, a highspeed CT scanner is used to image the heart’s arteries. They are made more visible by the injection of a contrast agent. Disease or the location and functional status of stents can be detected. The blood vessels are revealed more clearly this way compared to MRI or ultrasound. Detection of CT X-rays X-ray detectors in CT scanners are generally based on scintillator materials that generate visible light when struck with a charged particle or high energy photon (such as an X-ray photon). Some common materials used are caesium iodide, gadolinium oxysulfide and sodium metatungstate (H2Na6O40W12). This is similar to fluorescence but based on a different physical principle (see Fig.13). The light is coupled to a photodiode matrix or photomultiplier tube to convert it into electrical signals (see Australia’s electronics magazine Fig.12: a titanium skull and facial implant that was created based on a patient CT scan, then 3D printed for implantation. Source: Open Biomedical Initiative (www.openbiomedical.org). llator Scinti ray de Ar dio Photo ut IC Reado trate Subs ector Conn ock ng Bl Cooli Fig.13: a typical X-ray detector array in a modern CT machine. Each element of the photodiode array corresponds to a pixel (picture element). Source: ams (https://ams.com). August 2021  15 The first clinical CT (a brain scan) was performed in 1971 by a scanner invented by Godfrey Hounsfield at EMI Central Research Laboratories in England (see Figs.16-18). It was publicly announced in 1972. Pictures from the original machine had a resolution of only 80x80 pixels. See the YouTube video titled “Radiographer Films Inside of a CT scanner spinning at full speed” at https://youtu.be/pLajmU4TQuI MRI ► Fig.14: a normal CT scan of an abdomen. Source: Dr Ian Bickle, radiopaedia.org Fig.15: an illustration from Oldendorf’s patent for the CT scanner. Fig.14). Gamma-ray detectors as used in scintigraphy; SPECT and PET, discussed later under gamma-ray imaging, work similarly. History of CT The mathematics that was to be later used for computed tomography was introduced in 1917 by Johann Radon and is known as the Radon Transform. It has many uses apart from CT, such as in barcode scanners. Stefan Kaczmarz did additional theoretical work in 1937, followed by Allan McLeod Cormack in 1963-64. This paved the way for the image reconstruction method used by Godfrey Hounsfield (see below). Fig.16: the world’s first commercial CT head scanner, made by EMI in 1971. Image processing was done on a Data General Nova 1200 minicomputer. Source: Wikimedia user Philipcosson. 16  Silicon Chip William H. Oldendorf submitted a patent for a CT scanner in 1960, and it was awarded in 1963. The title is “Radiant energy apparatus for investigating selected areas of interior objects obscured by dense material” and you can view it at siliconchip.com.au/link/ ab91 (see Fig.15). However, his idea was rejected by a manufacturer who said: “Even if it could be made to work as you suggest, we cannot imagine a significant market for such an expensive apparatus which would do nothing but make a radiographic cross-section of a head.” Oldendorf’s work also led to the development of MRI, SPECT and PET imaging. Fig.17: the world’s first clinical CT scan of a human head, at 80x80 pixels resolution, performed in the Atkinson Morley Hospital, England, October 1971. Australia’s electronics magazine MRI stands for Magnetic Resonance Imaging. It uses the principle of Nuclear Magnetic Resonance or NMR. The word “nuclear” was dropped when the technique was introduced because they thought people would be worried that nuclear radiation was involved when that is not the case. In fact, unlike CT scans and X-rays, MRIs do not involve potentially harmful ionising radiation. MRI detects the presence of hydrogen, which is mostly in water (H2O) and fat molecules in the body in abundance. By mapping these molecules and their position within the body, the overall structures within can be imaged (see Fig.19). The position of hydrogen atoms is determined by causing them to emit radio signals and measuring the strength, frequency, phase and timing of those signals, then processing them with a computer. Fig.18: a modern CT image of a stroke victim’s brain. Compare the detail in this image to Fig.17. Source: James Heilman, MD. siliconchip.com.au Fig.19: an MRI image of osteochondroma of the knee. Source: M.R. Carmont, S. Davies, D.G. van Pittius and R. Rees. MRI machines generate a powerful, uniform magnetic field using a superconducting magnet cooled with liquid helium to a temperature of 4K or -269°C. A second magnet is used to impose a gradient over the uniform magnetic field just described. They also contain an RF pulse generator and RF receiver, and a powerful computer to process the data that is produced. The magnetic field strength generated is typically between 1.5T and 3.0T (teslas), compared with the earth’s magnetic field of 0.00006T. As shown in Fig.20, the hydrogen atom of (in this case) a water molecule has a spinning nucleus consisting of one proton, with north and south poles like a magnet. These are randomly oriented under normal circumstances and precess about their axis like a spinning top at a certain frequency. When a powerful and highly uniform magnetic field is applied in the direction indicated in the diagram (the B0 field), all the protons of the hydrogen atoms align along with it, although some are ‘up’ and some are ‘down’. Each of these protons generates a magnetic field, and if the numbers of ‘up’ and ‘down’ protons were even, there would be no net magnetic field as they would cancel each other out. However, it so happens that due to the laws of quantum mechanics, slightly more protons have a preference for the ‘up’ direction, and this means the magnetic fields of the individual protons do not cancel each other, but leave a slight net magnetic field. It is this small net field that is measured in MRI. The magnetic field not only causes siliconchip.com.au Fig.20: hydrogen is found in water and virtually all other molecules in the body. Each nucleus (proton) is randomly aligned with respect to other hydrogen protons. All are aligned by a powerful magnetic field, then are subjected to an RF pulse. Original source: Kathryn Mary Broadhouse. Fig.21: (A) shows the different resonant frequency of protons depending upon the applied magnetic field strength. (B) different structures within organs produce different signal strengths, allowing them to be distinguished. (C) Some of the brain imagery produced. Original source: Kathryn Mary Broadhouse. the protons to align, but the precessional frequency of the protons is also dependent on the strength of the magnetic field. The stronger the magnetic field, the faster the precession. So, once we apply the magnetic field, all the protons align and precess at a specific frequency. A powerful repetitive radio frequency (RF) pulse is applied. That interacts with the small net magnetic field that remains. Suppose that repetitive pulse is Australia’s electronics magazine applied at the same frequency as the precessional frequency of the protons (as determined by the strength of the magnetic field). In that case, they will resonate at that frequency and absorb energy and move their spin axis away from the B0 magnetic field. When the pulse stops, they return to their original position and emit radio waves to release the absorbed energy. These emitted radio waves are recorded (see Fig.21). August 2021  17 Fig.22: cross-sectional and lateral views of an MRI Scanner. Original source: Wikimedia user Fbot. Fig.24: the Siemens MAGNETOM Terra 7T MRI machine, the world’s first 7T machine for clinical applications. Fig.23: the world’s first experimental 10.5T MRI machine with a 110-tonne magnet, designed to image humans. It is at the University of Minnesota. The hole in the middle is where the person goes. Source: www.cmrr.umn.edu Fig.25: an image from the Siemens MAGNETOM Terra, showing small blood vessels in a human brain. Source: Siemens. MRI is used to look at ‘slices’ through the body. If the magnetic field were uniform over the entire body or area of interest, all the resonating protons would emit radio waves at once, and we would not be able to determine their position in the body. As previously mentioned, the resonant frequency of the protons is dependent upon the magnetic field strength. A stronger field means a higher frequency of resonance. This is the reason for the superimposition of the additional magnetic field from the “gradient coils”. The gradient coils are simply loops of wire or metal sheets inside or close to the inner bore of the machine where the patient is located, like those shown in Fig.22. These generate a secondary magnetic field that predictably distorts the uniform electric field, such as shown in Fig.21. There may be other magnetic 10.5T is more than three times stronger than the most powerful commercial machines now in common use, typically 1.5-3.0T. A 3T machine gives a resolution of about 1mm, a 7T machine gives 0.5mm (see Figs.24 & 25) and a 10.5T machine is of course better than that (in this case resolution refers to the smallest feature that is visible). See the YouTube video of a 7T machine titled “Siemens MAGNETOM Terra - 7 Tesla MRI Scanner” at https://youtu.be/PYNGCxQaXrw Hazards involved in high magnetic fields such as 7T or beyond include temporary patient discomfort or overheating. In higher magnetic fields, hydrogen nuclei resonate at a higher frequency, and thus more powerful RF pulses are needed. These are more easily absorbed by the body, which can cause heating if not managed correctly. Small MRI machines are also possible – see Fig.26. 18  Silicon Chip field patterns depending on the specific application. MRI machines usually have three sets of gradient coils corresponding to the X, Y and Z directions. This allows virtually any ‘slice’ of the patient to be imaged by energising some combination of these coils with different intensities. Magnetic field strength With MRI, the more powerful the magnet, the greater the maximum possible image resolution and the faster the image acquisition for a given resolution (due to an improved signalto-noise ratio). Currently, the most powerful fullsize MRI capable of imaging a person is rated at 10.5T with a magnet weighing 110 tonnes and 600 tonnes of iron shielding. It is located at the University of Minnesota’s Center for Magnetic Resonance Research (see Fig.23). Australia’s electronics magazine siliconchip.com.au History of MRI The first clinically useful wholebody MRI scan was obtained in 1980 by a machine developed throughout the 1970s by John Mallard at the University of Aberdeen (see Fig.27). Functional MRI Functional MRI or fMRI machines measure brain activity by detecting blood flow in the brain. Activity is measured based upon the differences in the magnetic response of oxygen-rich arterial blood and oxygen-poor venous blood. Diffusion MRI With this method, MRI parameters are tuned to highlight the movement of certain molecules by looking at the response as a function of time. For example, water molecules that can tumble freely give a different signal to those that are relatively constrained (see Fig.28). Medical gamma-ray imaging Gamma-ray imaging is a medical imaging technique whereby a patient consumes small amounts of radioactively ‘tagged’ chemical agents or ‘radiopharmaceuticals’. These emit gamma rays, and a gamma-ray detector is used to create an image. In a sense, it is like an X-ray but with the radiation source on the inside of the body instead of the outside. The metabolic activity of cells is measured due to the uptake of the radiopharmaceutical by targeted cells. To make the agent, a radioisotope replaces a non-radioactive element in a biologically active chemical compound. Common agents include: • Calcium-47 chloride for investigating bone metabolism • Sodium iodide-123 for thyroid imaging • Krypton-81m for lung ventilation imaging (the m stands for “metastable” since it has a very short half-life of 13s in its isomeric transition form) • The positron emitter fluorine-18 as fluorodeoxyglucose (18F) or 18F FDG for imaging tumours and studies of glucose metabolism in the heart, brain and elsewhere • Rubidium-82 for cardiac imaging Several similar products are made at Australia’s only nuclear reactor in Lucas Heights, Sydney, called OPAL. Certain medical isotopes such as for siliconchip.com.au Fig.26: MRI imagers don’t have to be huge. This is the “Swoop” model of bedside MRI from Hyperfine (https://hyperfine.io). It offers rapid imaging and turnover with minimal patient handling. Fig.27: the first MRI machine to ► produce a clinically useful wholebody image, in 1980. It was called the MRI Scanner Mark One. Source: Wikimedia user AndyGaskell. Fig.28: a diffusion MRI of a human brain; specifically, a diffusion tensor image depicting certain fibre tracts. Source: Wikimedia user Thomas Schultz. Some “fun” MRI videos At the end of its service life, during an emergency or certain maintenance procedures, the very expensive liquid helium that keeps the superconducting magnet coils of the MRI machine cooled to -269°C has to be vented. This is called a magnet quench. Some people have recorded these quenches and also put some objects into the magnet cavity before the decommissioning of these machines. See the video titled “Quenching an MRI Magnet” at https://youtu.be/4dbQxyrhZ2A The liquid dripping from the outside of the metal vent pipe is liquid air that has condensed on the pipe. Also see the video titled “How dangerous are magnetic items near an MRI magnet?” at https://youtu.be/6BBx8BwLhqg Australia’s electronics magazine August 2021  19 Fig.31 (above): a SPECT image showing slices through a normal human brain. The uptake of the radiotracer is greater in regions of higher metabolic activity. Source: Dr Bruno Di Muzio, radiopaedia.org ► Fig.29 (above): a whole-body bone scan using scintigraphy, showing the uptake of radiopharmaceutical in a normal skeletal structure. Source: Wikimedia user Myohan. Fig.30 (above): an Elscint VariCam scintigraphy machine circa 1995. It had a variable-angle dual-head gamma camera and was one of the first machines able to do 2D (planar) scintigraphy, SPECT scanning and PET scanning. GE took over Elscint, and this machine evolved to include CT scanning in the GE Discovery VG with the “Hawkeye option”. Source: Wikimedia user Arturo1299. 20  Silicon Chip ► Fig.32: the imaging principle of PET. A positron is emitted from an atomic nucleus of an injected radioactive compound which is annihilated when it collides with an electron, and two 511keV gamma-ray photons are emitted. These are detected in a coincidence detector ring along a line of response (LOR). An image is assembled from these by tomographic techniques. Source: Herman T. Van Dam (siliconchip.com.au/link/ab97). Fig.33: small PET scanners exist for laboratory animals (microPET). This shows disease progression and regression in response to therapy in a mouse using 18F-FDG as a radiotracer. It appears that the author has transposed scans 4 and 5. Source: University of Iowa, Small Animal Imaging Core Facility. Australia’s electronics magazine siliconchip.com.au PET imaging can also be made in a medical cyclotron. There are about 18 of these in Australia, at various hospitals and imaging centres (there is a list at siliconchip.com.au/link/ ab92). The imaging techniques applicable to gamma-ray imaging are scintigraphy, SPECT (Single-Photon Emission Computed Tomography) and PET (Positron Emission Tomography) Scintigraphy is a two-dimensional or planar technique using a gamma camera (see Figs.29 & 30). It gives an image equivalent to a 2D X-ray. SPECT imaging is much like scintigraphy, but it produces 3D images instead (see Fig.31). To achieve this, the gamma camera(s) are rotated about the patient (tomography) to create a series of 2D slices. The 3D image is generated with the appropriate mathematical transformations in a computer. SPECT scans have a resolution of about 1cm and use the same gammaemitting radiopharmaceuticals as in scintigraphy. PET imaging is similar to SPECT – SPECT radiotracer substances emit gamma rays directly, while those used for PET emit particles known as positrons (see Figs.32 & 33). A positron is the positively-charged antimatter equivalent of an electron. Positron emission occurs when a proton in a nucleus decays to give a neutron, a positron and a neutrino. In PET, gamma rays are emitted when the positron from this decay collides with a nearby electron, causing the annihilation of both particles and the emission of two gamma-ray photons in opposite directions. These are what is detected. The emission of two gamma-ray photons simultaneously in opposite directions and their “coincidence detection” gives more information about the exact location of the emission, and thus a higher image resolution than with SPECT. In coincidence detection, the emission event can be located anywhere along a line between the two detectors. Thus, it is necessary to generate a large set of data from multiple coincidence events with detectors at different angles in a “detector ring” to form an image, as in Fig.32. The data is mathematically filtered to remove likely false coincidences or single instances of emission. siliconchip.com.au Fig.34: the GE Discovery MI Gen 2, an example of a combined CT and PET scanner. Fig.35: a combined CT and PET image showing a lesion of interest in green and a cross-section through the neck on the left. The anatomical detail is captured with CT and the metabolic detail of the lesion with PET. For more information on coincidence detection, see siliconchip.com. au/link/ab93 PET scanners have a resolution of about 4mm-6mm, with dedicated brain scanners going down to about 2.5mm. The fundamental theoretical limit for PET resolution is about 2.4mm for practical devices. This is explained at siliconchip.com.au/ link/ab94 Different radiopharmaceuticals are needed for PET than for scintigraphy and SPECT. The radioisotopes used are short-lived (eg, fluorine-18 with a 110-minute half-life or rubidium-82 at 76 seconds). This means that they must be prepared on-site with a cyclotron. This makes PET scans a very expensive procedure. SPECT is a cheaper imaging method than PET because of the more readilyobtained radioisotopes but gives poorer contrast and resolution. Combined CT, MRI and PET imaging Every method of scanning has inherent advantages and disadvantages. For example, CT and MRI give structural anatomical information while PET gives functional parameters such as metabolism, blood flow and compositional information. Combined images can be helpful to Australia’s electronics magazine relate structure and function. Images from single-mode machines can be combined by overlaying them in an alignment process called image registration. Still, better alignment can be obtained by acquired images using two or more modes from the same machine during the same scanning session. Scans from combined PET and CT (see Figs.34 & 35) have been shown to yield more accurate diagnoses than either type alone. Machines exist that combine PET and CT, or PET and MRI. Both combine structural and functional information; a combined CT and MRI machine has not yet been developed. Combined PET and CT is the more established technology. Medical ultrasonic imaging Ultrasonic imaging (or sonography) uses sound waves beyond the range of human hearing, and is similar to the process that bats and toothed whales use to navigate. The sound waves are typically in the range of 1-6MHz for deeper tissue penetration with less resolution, or 7-18MHz for shallower tissues with greater resolution. Higher frequencies may be used in some applications. Ultrasonic waves are produced by a piezoelectric transducer, which converts an electrical signal into August 2021  21 Fig.38: some of the wide variety of medical ultrasound imaging probes available. From left-to-right we have a linear, curvilinear, phased array, and all-in-one handheld probe. These are from Meraki Enterprises. Fig.36: this diagram shows how a piezoelectric transducer can convert an electrical signal into sound (upper) and also can generate an electrical signal when vibrated by a sound wave (lower). Fig.37: a basic ultrasonic transducer element for medical imaging. The matching layer provides an acoustic impedance match between the transducer and human tissue. Hundreds of such elements can be used in a transducer. Source: Dr Daniel J Bell and Dr Rachael Nightingale et al., radiopaedia.org Fig.39: in phased array ultrasound imaging, each piezoelectric element is fired with a slight delay so that the wavefronts of the individual beams join at an angle dependent upon the delay (θ (θ). T represents the transducer elements, TX is the oscillator signal, C the control system and φ the delay. Source: Wikimedia user Chetvorno. mechanical motion (see Figs.36 & 37). Ultrasonic waves are then transmitted through a sound-conducting medium and reflected back to the transducer. The time delay between the emission of the signal and its return represents the total distance travelled (or twice the distance to the target). It works like sonar (using sound waves) or radar (using radio waves), only on a much smaller scale. The same piezoelectric element used to create ultrasound when a voltage is applied can also generate a voltage signal when a signal is returned. Alternatively, two different transducers may be used. While quartz is a common piezoelectric material, medical devices generally use PZT (lead zirconate titanate) because of its high conversion efficiency. Piezoelectric polymers (plastics) such as PVDF also exist. A typical basic piezoelectric transducer is a disc with electrodes attached. Piezoelectric transducers for medical imaging must be sensitive and have the following properties: a) Good conversion efficiency between electrical and mechanical (sound) energy b) Be acoustically matched to the tissue, much like a radio antenna has to be impedance-matched c) Must be matched to the electronics Materials like PZT are good for (a) and (c) but not (b). Piezoelectric polymers are good for (b) but not (a) or (c). It has therefore been proposed to develop a composite material, having the best properties of both materials. In recent years, composite transducers have been introduced for medical ultrasound consisting of PZT rods embedded into a polymer matrix. Transducers for medical imaging have between 128 and 512 piezoelectric elements in either a linear or phased array (see Fig.38). With a linear array, one individual element is fired, then the next one in sequence and so on, to form a line image. In a phased array, the acoustic beam can be steered electronically by firing each element with a slight delay with respect to the previous ones (see Fig.39). Focusing is also possible by appropriate beam management. Some probes have a mechanically steered transducer array (see Fig.40). While traditionally ultrasound produced two-dimensional images (‘slices’), modern computing power means that ultrasound can now generate 3D images – see Fig.41. 22  Silicon Chip Australia’s electronics magazine Medical ultrasound development in Australia Australia was once a pioneer in medical ultrasound technology. Research began in 1959 with the establishment of an Ultrasound Research Section within the Commonwealth Acoustic Laboratories (CAL). That section became the Ultrasonics Institute in 1975, as a branch of the siliconchip.com.au Probe movement during acquisition of volume Central scan plane Acoustic window Coupling fluid Array Gear Motor Position sensing device Cables Housing Fig.40: an example of a mechanicallyscanned ultrasound transducer for medical imaging. Fig.42: the CAL Mark I Abdominal Echoschope at the Royal Hospital for Women, Sydney, in early 1962. The patient would stand with her abdomen pressed against the water bag on the right. Source: ASUM (www.asum.com.au). Fig.41: a 3D fetal ultrasound with a normal presentation. Source: Dr Servet Kahveci, radiopaedia.org Fig.43: fetal images obtained from Echoscope in 1962, considered the best in the world. The line drawings are manual annotations, not computer renditions; it was an entirely analog system. Source: ASUM. Commonwealth Department of Health. In 1989, the Institute was transferred to the CSIRO, and its staff were eventually integrated elsewhere within the organisation. In 1962, a system designed by CAL called the CAL Mark 1 Abdominal Echoscope (Fig.42) was installed at the Royal Hospital for Women in Sydney. It was designed by George Kossoff and David Robinson. The obstetric pictures obtained from this were acclaimed as possibly the best in the world (see Fig.43). The transducer ran at 2.5MHz and was a 25mm weakly-focused disc. All the original electronics were vacuum tubes, with a Hughes Tonotron storage CRT (as used in radar at the time) for image display using long-persistence phosphors. This was an entirely analog system with no computer (they were not sufficiently advanced at the time). Part of the motivation for developing obstetric ultrasound was the recognition of the hazards of fetal X-rays, the only alternative at the time. Apart from obstetric ultrasound, machines were also developed for the eye, breast and paediatric brain. One of the technical innovations made by the Australian group in 1969 was greyscale imaging, which yielded more and better quality imaging than black-and-white. The greyscales resulted from signal processing to extract more data, such as the distinction between liquid and solid tissue material. Existing Echoscopes were modified to operate in this mode. This development was credited to George Kossoff, David Carpenter, Michael Dadd, Jack Jellins, Kaye Griffiths and Margaret Tabrett. After many successes, in 1975, the work led to the development of siliconchip.com.au Australia’s electronics magazine a machine that was made commercially by Ausonics (part of the Nucleus Group, for whom I used to work) called the UI Octoson. More than 200 were made between 1976 and 1985 and sold in Australia and overseas. The machines sold for $100,000 each (see Fig.44 overleaf). The Octoson could acquire an image in one second. This work was credited to George Kossoff, David Robinson, David Carpenter, Ian Shepherd and George Radovanovich; it became obsolete with the development of realtime scanning. For a more comprehensive history of medical ultrasound in Australia, see siliconchip.com.au/link/ab95 & siliconchip.com.au/link/ab96 Endoscopy A modern endoscope is a flexible, steerable tubular instrument for August 2021  23 Fig.44: the Australian-made Ausonics UI Octoson from 1977. The patient lies on top of a water-filled membrane to conduct the ultrasonic waves. Source: ASUM. Fig.45: a flexible endoscope Source: Wikimedia user de:Benutzer:Kalumet. looking inside certain hollow or otherwise accessible parts of the body such as the colon, oesophagus, bladder, kidney, joints, abdomen and pelvis (see Fig.45). These instruments are usually specialised for the part of the body they are intended for. Inside the flexible tube, there are cables to help steer the instrument and bundles of optical fibres to transmit light into the body cavity as well simpler, safer and cheaper than conventional operations. as conduct light out to a camera. Each fibre optic bundle has about 50,000 individual fibres. Minor procedures can be performed with small instruments attached to the end of the device, to take tissue samples or remove small growths such as polyps. Lasers can also be directed down the tube to destroy diseased tissue. Endoscopic procedures can be much UV imaging of skin Photographing the skin in wavelengths of light other than ordinary visible light such as ultraviolet can reveal damage to the skin or underlying conditions not visible to the naked eye (Fig.46). Thermographic imaging Thermographic imaging is a technology for taking images of the human body in infrared light to examine medical conditions. It primarily reveals temperature anomalies due to variations in blood flow (see Fig.47). It is considered an aid to diagnosis rather than a direct diagnostic tool. It can also be used to measure body temperature in a non-contact manner, as is often done these days on entry to hospitals to ensure a visitor does not have a fever and is possibly infectious. Pill cameras Fig.46: imaging of the skin of a melanoma survivor in ultraviolet wavelengths reveals damage not visible in ordinary light. Source: University of Colorado Cancer Center. Fig.47: thermography of a patient’s legs after exposing the left foot to cold water to examine complex regional pain syndrome (CRPS). Source: Wikimedia user Thermadvocate. ► Fig.48: the PillCam by Given Imaging. It is swallowed and has a tiny camera on board to take pictures as it passes through the body. 24  Silicon Chip Australia’s electronics magazine Tiny ‘pill’ cameras exist which can be swallowed and take pictures throughout the alimentary canal (see Fig.48). This was described in detail in the August 2018 issue, in an article titled “Taking an Epic Voyage through your Alimentary Canal!” (siliconchip. com.au/Article/11187). Next month That’s all we have space for in this issue. Next month’s follow-up article will continue on the theme of imaging technology, but with non-medical applications. That includes investigating delicate archaeological objects, searching for contraband, checking structures for damage or defects and biometric access control. SC siliconchip.com.au