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The Different Forms of Tomography and Their Applications
The medical community utilizes many imaging methods to aid in the diagnosis of, and subsequent treatment of, a variety of diseases and ailments. One of the oldest, most well-known methods used today is called computed tomography, also known as a CT scan. CT scans aid medical professionals by providing a non-invasive way to view the internal organs of a patient with the use of x-rays. However there are other types of CT scans that do not rely on x-rays to construct an image, such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET). Often times, these other CT imaging techniques are used outside of the medical field, though it is widely popular across many applications, including materials, archeology, and manufacturing. Each of these tomographic imaging techniques have their own advantages and disadvantages, relating to cost, safety, ease of execution, image resolution, and contrast.
Background and Early Technical Development
The technology prior to CT scans faced challenges. Technicians could not clearly see through the layers of tissue, bone, and organ in the human body because every layer was transposed on the next. This resulting image was frequently hard to decipher, with virtually no depth reference of what features are where. These methods had their beginnings when the first x-ray was produced and detected by Wilhelm Conrad Rontgen in 1895 (3). Using the information Rontgen discovered during his research, Johann Radon formulated a mathematical basis for CT in 1917, which later became known as the Radon transform (3). Radon’s formulation led to the first documented CT scan in 1971 and while these first images were not crisp and easily distinguishable, they were still a huge advancement in the development of current CT technology(3).
Introduction and Methods
With CT (or CAT, short for computed axial tomography), a narrow x-ray beam is shot onto the patient and rotated around him, and the x rays not absorbed by the patient are detected by x-ray detectors opposite the beam (figure 1). This process yields cross-sectional scans at different angles of the subject, which with the aid of computer software, can produce a complete 2 dimensional image or a detailed 3 dimensional image (romans). With every CT scan slice of a feature, that feature is broken into small cells called voxels (volume pixels). Each voxel has a number assigned to it corresponding with its permissibility of x-rays. The radiodensity scale (scale that shows opacity based on x-rays in the electromagnetic spectrum) that these numbers fall on is from +3071 to -1024; this scale is also known as the Hounsfield Scale (1). To provide more clarity on what these numbers mean, bone has a higher number because of its tendency to absorb x-rays due to its density. It requires a more intense x-ray beam to pass through the media (2). Soft tissue like skin, however, would have a lower number because it does not require as high of an intensity of x-ray to reach the detectors after passing through it. These numbers however are not known by the CT technician, but rather the computing technology itself. Through a series of scans and calculations, these numbers can construct an image.
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To get a better understanding of how an image is constructed, imagine a 2×2 cell box, with each cell having a different number; that box is the “patient.” As the x-ray beam rotates around the “patient,” a scan is taken at every angle. Per every angle, the detectors receive a composite of the numbers it passed through. The CT technology then uses an algorithm to identify the numbers in each cell, using the location of the detectors in reference to the number attained (2). The numbers and their locations are then translated into an image displaying the different materials (e.g. organs, bones, medical implants) and their placement in the body.
Figure 1: Schematic of Medical CT Scan (4)
The content of these voxels that created this image, which are the cells in the example, is related to the thickness of the CT slice. Each scan that is produced during the rotation of the x-ray beam is called a slice, representing a specific plane in the patient’s body (11). The thickness of the slice is determined by the CT technician’s settings in the CT scanner. When a thickness is selected, small shutters open and close to adjust the opening to the x-ray beam’s housing, limiting the total volume that the beam could pass through. This reduces scatter radiation and improves the quality of the resulting scan (11). The analogy most often used to explain 2d CT scans is a slices of a loaf of bread. The patient is the loaf, and the scan is one of the slices. The resulting scan concatenated by the CT scanner software is greyscale, with the more dense objects appearing white and opaque, and the softer, less dense objects appearing in shades of grey. To optimize visual contrast, sometimes CT technicians will narrow the range of the display, making values below a certain number display as black, or values above as white. Often times, when constructing a 3d image, the CT technician may add color to the structures in the scan for easier differentiation between anatomical features.
X-ray CT scans are the most popular tomographical imaging technique in the medical field, however it is not the only tomographical imaging technique used. Positron emission tomography (PET) is also used in diagnosing illnesses, both in humans and animals. PET gets its name from the tracer used during the imaging. A radioactive tracer is injected, inhaled, or swallowed, depending on which organ is begin studied (mayo) and emits gamma rays with positrons (1). To elaborate, positrons are positively charged particles that create gamma rays when they react with electrons (5). Nevertheless, some of the tracers used contain glucose, which was readily metabolized in brain tissue and malignant tumors; this metabolic activity would be shown as a bright spot on the resulting PET scan (1).
PET units, similar to CT machines, have a series of sensors arranged in a ring that take images as slices, but instead of detecting x-rays, it detects gamma rays in the form of photons emitted by the anatomical feature of interest, instead of an external beam. The imaging technique, with respect to the positioning and location of bodily elements, is very similar to the technique used in CT (Remember the 2×2 cell box). The denser portions of the body absorb more rays and are translated into different colors in the final scan. Many times, PET techniques are used in conjunction with CT scanners to produce more pronounced features in the scan. A patient may be injected with an isotope before being moved to the CT scanner. The patient may need to be anesthetized to prevent active muscles from taking up the isotope and showing metabolic activity not of interest (1).
Another tomographic technique very similar to PET, is single-photon emission computed tomography, or SPECT. The main difference between PET and SPECT is where the radioactive isotopes end up in the body. While the isotopes in PET are absorbed into the surrounding tissues, SPECT isotopes stay in the bloodstream, reducing the area that can be seen in the resulting scan. This is also a technique that involves the use of the CT scanner, in order to detect the photons emitted and create a scan (5).
The instrumentation of CT scanners is very diverse. The most common type of CT scanner is called a spiral or helical CT. It uses the technique I described above, where a narrow x-ray beam is spun around an axis, the x-ray detectors are located directly across from the beam opening, and the patient in between them both on a bed that moves along the axis the detectors rotate around (14). However CT scanners did not start this way. Prior to 1989, the patient would be on a stationary bed, while the scanner rotated around him; the patient would need to be moved every so often to get another section of his body to image, resulting in longer scan times (14). Following the moving table advancement similar to what we use now, was the development of the Multi-detector Spiral CT (MSCT) scanner (14). This design consists of up to 4 detectors in the detection arc instead of the one, all offset at different angles. The addition of multiple detectors immensely increased the resulting images resolution while it also decreased the scan time even further (14).
Another design of CT scanner is called electron tomography (ET) scanner. Also known as electron beam computed tomography (EBCT) scanner, this tomographic imaging configuration relies on the use of electrons to create a 3 dimensional image. What differentiates this design from the traditional CT scanner’s is its massive x-ray tube and addition of deflection coils (10). The increased tube size is necessary for electrons spinning within the x-ray tube. However this design was not popular due to that very fact; it was found that the bulkiness of the machine resulted in decreased coverage of the patient (10).
Tomography has many uses in not only medicine, but also in material categorization. There have been instances where cargo has been imaged using CT. Because now more than ever thousands of tons of cargo are shipped every day, there becomes an increasing danger that these shipping containers may contain hazardous items, drugs, explosives, or many other potentially illegal items. To combat this, extra security measures were developed and mega-voltage (MV) cargo radiography was used to screen the contents of these containers (12). This imaging method utilized high-energy x-rays to penetrate the metal exteriors of the cargo containers quickly. However, although non-intrusive, this method did not produce clear images; the contents of the containers were superimposed on each other, making them hard to distinguish (12). This is a common shortcoming of this type of radiography, trying to produce a 2d image from a 3d structure with depth.
With CT being introduced as an option, image resolution and item contrast is better. A major obstacle of implementing this more commercially is the cost associated with building a gantry (CT scanner tube with x-ray components) long enough and wide enough to rotate around the cargo container (12). Additionally, implementing CT for imaging cargo interiors could be much slower than the MV method currently used. This is due to the option of creating single or multiple view scans with this new state-of-the-art tech (12). Multiple views create a need for larger x-ray sources and subsequently higher dose outputs, which directly decreases the linear scanning speeds of the cargo container in question (12). As a note, if designers were able to produce a cost effective, larger gantry to fit cargo units, the sheer bulk and inertia of the x-ray emitter and detectors would limit the speed the equipment can scan, as well. Despite these potential issues, CT as a means of material categorization for national security is feasible and highly favorable in providing another option to imaging cargo containers when more detailed information is necessary beyond what MV can provide (12).
Figure 2: Example of MV used in Airport Security (8)
Not only does tomography have uses in cargo inspection, but it also has uses in archaeology. In 1898, before the first CT scanner was constructed, archaeologists in Europe used x-rays to look inside of mummies and vases (14). This occurred only three years after x-rays themselves were discovered. Many times, archaeologists would use x-rays to study the human remains to determine the nature of bone fractures in the mummies (whether pre-mortem or post-mortem) and the state of their teeth. As an alternative, some archaeologists found it necessary to conduct an “autopsy” on the mummies when x-rays would not provide enough information. For example, one might x-ray a mummy and see a piece of jewelry on the scan. How were you to know whether the jewelry was in the cavity or on the surface? With x-rays, it is hard to tell because as mentioned before, structures are superimposed on each other. Lacking that information, they would have to remove the wraps to explore the mummy further (14). With the emergence of CT however, this was no longer a necessary reality. CT scanners are able to image organs, bones, and jewelry without disrupting the body.
Figure 3: Image of Sarcophagus on CT Scanner (14)
Often times, CT imaging of mummies is easier than living people, while also yielding better resulting images. This is because the water in living tissues slightly scatters the x-ray signal, creating noise and limiting the resolution and contrast. That makes CT the most suitable imaging technique for mummies and other old relics; magnetic resonance imaging (MRI) relies on high moisture content and ultrasound relies on sound waves in a homogenously dense media, which would not be the case for relics with many layers and wrappings of different types and thicknesses; the same goes for mummies as well (14).
Interestingly, CT in archaeology has also been used to decipher writing on decaying scrolls and bricks. A group of archaeologists from Isreal used CT to read faint writing scrawled on clay bricks that were enclosed by a series of clay envelopes (14). A group from Kentucky created a formula to extract and decipher writing from scroll that were still rolled up, as unraveling them would have surely damaged them more (14).
Beyond archaeology, CT has been used in the material development industry as a form of non-destructive material testing (NDT). A company called Eurocopter Deutschland used CT in conjunction with destructive testing of helicopter composite blades to assess their points of failure (6). Through the CT imaging, their designers were able to pinpoint areas of fatigue that may not have been apparent from the outside layer. Once their configurations were optimized, this company implemented CT as a quality control check in their manufacturing process (6). As the technology improved, the resulting images from the CT scan improved, in both resolution and contrast.While it proved to be good at detecting issues with bonding, laminate transitions, and delaminations within composite beams (6), one thing CT struggled with was detecting cracks in the fiber matrix. At times, the thickness of the fibers were so small that detecting cracks within them was outside of the resolution of the scanner.
Among the tomographical imaging techniques previously mentioned and highlighted in this piece, they all share a few advantages to other imaging methods. CT, PET, and SPECT are all non-invasive, meaning that in medical situations that call for internal exploration, medical professionals are able to see into the body with no slicing, and minimal to no downtime (9). CT scans are also one of the quickest imaging methods in the industry. CT does not rely on tracers or dyes to show the anatomical features of the patient while in the machine, that PET and SPECT use. These tracers can often take up to 45 minutes to move through the body enough to produce a sound image when the patient is placed into the scanner (1), although some may take as little as 10 to 20 minutes to reach the area of patient’s area of interest (5).
As mentioned earlier, some tomographical imaging methods can take a little longer to prepare for scanning. If one needs an immediate diagnosis, then PET and SPECT would not be the method to select, because of how long the isotope needs to be in the system of the patient. Additionally, the most of isotopes prepared for PET and SPECT have a short half-life of around 110 minutes (1). This means that these isotopes must be near the scanner, or must be able to be produced near the scanner. Some times, isotopes are specially ordered when tests are scheduled to ensure that they will not decay before use (1).
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Another issue faced by tomographical imaging techniques requiring tracers, is the adverse reactions afflicting some patients. Though rare, there have been cases where patients have become nauseus from the isotopes. Another biological reaction professionals are worried about is the risk of cancer. Because almost CT requires radiation, whether from x-rays or gamma rays, it is advised that pregnant women avoid getting this procedure done. It has been noted that the typical CT scan contains less radiation than an x-ray though (11).
A more simple but valid downside to current tomography technology is the actual structure of the gantry itself. The majority of gantry designs on the market are long, narrow tubes, which for a sizeable portion of the population that has claustrophobia, is not a viable option for medical imaging. Removing the medical applications from the assessment, the size of objects that can be imaged by any tomographic scanner is constrained by its size.
A unique disadvantage to SPECT specifically, lies in the nature of this technique. As mentioned earlier, the tracer media used in SPECT stays in the bloodstream and does not get absorbed into the surrounding tissue. Because of this, the resolution of the resulting scans is of lower quality compared to other tomography methods, and can often result in inaccurate test findings (13). Also, another unique disadvantage of SPECT lies in the fact it is not as effective when imaging larger patients; photons have a harder time moving through patients that have more body mass (13). Focusing on PET, while it does not have issues imaging different body types, it does have an extremely high cost. Most PET machines sold today are combined with CT, which adds to the exorbitance (13).
Tomography has proven to be a versatile, state-of-the-art imaging technology. It is one of the leading technologies in the medical industry in its various forms: computed tomography (CT), positron emission tomography (PET), and single-photon emission computed tomography (SPECT), to name a few. The main difference between these imaging techniques is their energy sources: traditional CT uses a narrow x-ray beam to generate an image based on the densities of the media the rays are passing through; PET requires the introduction of an isotope to the patient’s body to produce positrons which are interpreted the same way as the x-rays in CT; SPECT functions the same way PET does, except instead of a positron emitting isotope, the isotope emits gamma rays.
Most tomographical imaging scanners in the industry are comprised of gantry made of rotating detectors and a computer package. The detector types depend on the type of tomography, as stated above. Some have emitters built into the gantry, while with others, the object being imaged is the emitter. Some scanners have a moving bed for the object being imaged to sit on while being scanned. This accelerates the scan time and provides better resolution.
When not used in medicine, tomography is used in material disposition, archaeology, and cargo inspection. Implementing this in imaging method in material disposition has allows development engineers to gain a better understanding of the failure points in their designs when they may not be obvious. In archeology, this imagine technique can explore the interiors of mummies and their artifacts, all while maintaining their posterity. In cargo inspection, freight shipping operators are able to assess the security and contents of a cargo container without disturbing it, before bringing it into their receiving stations. Through and through, tomography is a cost effective, non-destructive, non-invasive imaging technique that has promise in future endevours.
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