Computer Axial Tomography

A computerized axial tomography (CAT) scanner or a computed tomography is an imaging tool used in the medical practice to provide clear pictures of the body's internal structures. It uses a beam of x-rays together with a radiation detector to supply to supply the needed data to a computer, which uses it to construct a three dimensional image. CAT scanners are made up of many complex electronic components that are produced by a number of subcontractors and put together to form a complete unit by the manufacturers of scanners. The scanner was first developed in the early 1970s and has experienced steady improvements in technology, making it an invaluable device for radiologic diagnosis.

CAT scanner invention was made possible by Wilhelm Roentgen who was able to discover x-rays in 1885 by then many scientists were investigating how electrons move through a glass apparatus that was known as a Crookes tube.  Roentgen's aim was to capture the exact action of the electrons by wrapping his Crookes tube in a black photographic paper. On carrying out the experiment, he noticed that a plate that was covered in a fluorescent material that was lying near the tube glowed or fluoresced. This was not what he expected, as there was no visible light being emitted from the wrapped tube. However, on further investigation, he discovered that there was actually some invisible light that was produced by the tube and this light had the strength to penetrate many materials such as wood, some metals and even the human skin. This made him happy because he knew that he had just come up with an important discovery in the area of medicine.  He went further to find out that using x-rays could make it possible for the creation of structures found beneath the skin. He used this to publish the first x-ray of the hand of his wife. This discovery saw him receive the first Nobel Prize in physics in 1901. It is recorded that the first documented use of x-ray for an actual diagnosis in US happened in 1896. A certain doctor Gilman together with his brother, a physicist, used them to determine the injury extent suffered by a young boy who got injured while ice skating.  This took place in Dartmouth College laboratory.

This technology steadily improved over the years as the field of radiography expanded. The conventional x-rays had limitations, the major one being that they lacked depth and as such, many internal body structures were superimposed on each other. Scientist embarked on finding the solution to this problem with the use of computers. They came up with several methods one of them being the use of computed tomography (CT) or computerized axial tomography (CAT). In addition, as stated earlier, the first CAT scanner was successfully demonstrated by Godfrey Hounsfield and Allen Cormack in 1970. Major advances in scanner design followed in the next two decades resulting in the high quality imaging scanners in use today.

CAT scanners are just like any other x-ray machine and therefore use x-rays in their production of images of body structures inside the body. X-rays are just but one of the ionizing radiations that has the capability of penetrating solid materials in different measures depending on the material's density and thickness. Images in the conventional radiology are produced by placing a detector such as a photographic film behind the body of the patient and directing x-ray beam towards it. The radiation will pass through the body of the patient and interact with the film. Because x-rays striking the surface of the film produce dark areas after processing, those body structures that the x-rays can penetrate with ease such as the skin, will show up as dark regions. Other body structures such as the muscles, organs and soft tissue permit different amounts of x-ray through them and therefore appear as gray areas on the film. Hard structures like bones that do not permit at all x-rays to pass through them show as bright white areas on the film. As highlighted earlier on, images produced by the conventional film x-ray are usually blurry because of the superimposition of many of the internal structures on each other. This was the main reason for the development of tomography to reduce the unclearness and pave way for the imaging of specific areas inside the body. At first tomography, methods involved the moving of the x-ray generator and the detecting of in opposite directions simultaneously. As these two units moved horizontally, only those body structures lying in a specific geometric plane could allow x-ray to pass through consistently to the detector. This enables structures to clearly show up on the film as those outside the plane are blurred. This type of radiology produce images parallel to the long body axis.

CAT scanners represent a more improved and complex form of conventional tomography whereby images are produced by the rotation of the x-ray generator and detectors in a circle around the patient. The amounts of the long emitted radiations that remain are sent to a computer instead of recording them directly on the film. The computer's task is to now run a series of algorithms to construct the image that is then displayed on a computer monitor. X-rays are termed as ionizing radiations because of their ability to interact with and transform certain types of matter like body molecules. This has received much debate with people arguing that it is a health risk to human beings but its benefits in the medical field are overwhelming.

CAT scanners design

A typical CAT scanner is made up of three major systems that include the operating console, the computer and the gantry. Each of these basic systems is again made up of many subcomponents the largest being the gantry. The gantry is made up of all equipments relating to the patient, these include the patient support, the mechanical supports, the positioning couch, and the scanner housing. The system carries the heart of the CAT scanner, the x-ray tube, and detectors, which generate and detect x-rays. The x-ray tube comes as a very special type of vacuum-sealed electrical diode designed so that it can emit x-rays. The tube is made up of two electrodes, the anode and the cathode. In order for it to produce x-rays, there is a filament in the cathode that is charged with electricity from a high voltage generator. This causes the hitting up of the filament and therefore emits electrons. Due to the law of natural attraction and a special focusing cup, the electrons move straight towards the positively charged anode. There follows an indiscriminate emission of x-rays when the electrons strike anode. The anode, which may be rotating, then passes electricity to the high voltage generator that then completes the circuit. The x-ray tube is placed in a protective housing in order to focus the x-rays into a beam. This protective housing  is lined with lead save for a small window at the bottom so that useful x-rays are able to escape through it as the lead prevent stray radiation from escaping in other directions.

Detectors in a CAT scanner do not measure x-rays directly as the radiological devices do. They measure the attenuated radiation from the structures of the body due to their interaction with x-rays. An ideal gas-filled detector is one type of detector in which when any of these detectors is struck by radiation, there is ionization of the gas thereby enabling the determination of a radiation level. The other important system is the computer, which is designed specially to collect and analyze all input from the detector. Computers used here are those with large capacity therefore able to perform many equations simultaneously. Reconstruction speeds and the quality of images all depend on the microprocessor and internal memory of a computer. A computer that is quick and fast, is very important in that it influences greatly the speed and efficiency of an examination. This specialty calls allocation of a room with a strictly controlled environment for the computer.

The third system that makes up a CAT canner is the operating console, which is the master control center of the entire CAT scanner. All the factors relating to taking a scan are input here. It is made of a computer, a key board, and a number of monitors. More often many facilities prefer two different control consoles, one is used by the operator of the CAT scanner and the other is used by the physician. The one used the operator controls variables like the thickness of the tissue that is imaged, the mechanical movement of the patient's couch, and many other radiographic technique factors. The physician's console allows the particular doctor to view the image without causing interference to the normal scanner operation. The manipulation of the image is also enabled here if required for diagnosis and image storage for use later on.

As mentioned the design of CAT scanners improved tremendously over the years. The original CAT scanner made use of a thin, pencil beam of x-rays and took only 180 readings one at each degree of rotation in a semicircle. The detector and x-ray generator moved horizontally for each scan and then rotated one degree to take the next scan. To generate two different images from each scan, two detectors were used. This had a disadvantage though, because the scanning time was too long. One scan alone could take up to five minutes. Nevertheless, as designs improved, more detectors were added and x-ray beam fanned out using a special filter. This reduced scanning time significantly to about 20 seconds. This was followed by another major improvement that so the elimination of the horizontal movements of the generator and the detector and making it be a rotate-only scanner. This did not stop here as more detectors were added and grouped into a curvilinear detector array. This was eventually made to be stationary reducing scan time to just one second.

The making of CAT scanners

In the construction of CAT scanners, a wide range of materials is used to make its components. These include glass, steel, and plastic. Compounds that are more specialized are found in the detector array, patient couch, and the x-ray tube. The patient couch is more often made from carbon fibre; this is to prevent from interfering with the transmission of the x-ray beam. Detectors arrays of modern scanners use tungsten plates, ceramic substrate, and xenon gas. The cathode and anode targets of the x-ray tube are also made of tungsten. There are also other materials found in the tube, these include Pyrex, glass copper, and tungsten alloys, and others.

Manufacturing of CAT scanner is the assembly of the various components that outside manufacturers usually supply. Production of these major components involve processes, these include;

Gantry assembly components: the x-ray tube is made more or less like other electrical diodes. Individual components are put inside the envelope of the tube and the vacuum is then sealed. This tube is then located inside the protective housing that is then attached to the rotating portion of the frame of the scanner; there are many detector arrays available for CAT scanners one of them being the ideal-gas filled detector. Making of this detector involves placing tungsten strips 1 mm apart around a big metal frame. The strips are held in place by a ceramic substrate. This whole assembly is sealed hermetically and filled with pressure using an inert gas like xenon. Each small chamber that is formed by gaps between the tungsten plates form individual detectors. The resulting detector is also attached to frame of the scanner; in order to come up with enough voltage needed to produce x-rays, an autotransformer is used. This is a power supply device made by winding a wire around a core, and then tap connection are made at a number of points along the coil and connected to the main source of power. This device enables output voltage to be increased to about twice the input voltage; the computer and the control console are specially designed and supplied by manufacturers of computers. The major model building computer is programmed specifically with reconstruction algorithms that are needed to manipulate x-ray data coming from the gantry assembly. These control consoles are also programmed with the appropriate software that enable them control the administration of the CAT scan; finally there follows a custom process as the last assembly stage of a CAT scanner that takes place in the radiology imaging facility . Here rooms are made in such a way so that they can accommodate each component of the CAT scanner. This ensures that there is minimization of the possibility for excessive radiation exposure or even electric shocks. Specific facility plans an easy and facility installation of equipment and wiring of the whole CAT scanner system.

Importance of CAT scanners

CAT scanner enable doctor to carry out CT scans to analyze internal structures of the many parts of the body. These parts include the head where CT scans are done to identify traumatic injuries, tumors, and other infections. The spine can also be accurately examined especially in the evaluation of osteoporosis, which is the measuring the density of bones. Contrast materials such as x-ray dye can be used occasional to enhance the scan and many structural relationships between the spine, spinal cord, and its nerves. CT scans are also used to identify tumors, cysts, or infections in the chest by carrying out a chest x-ray. CT scans done on the abdomen are important in that they help in defining the anatomy of body organs such as the liver, pancreas, gall bladder, aorta, spleen, uterus, kidneys, and ovaries. They are used to ascertain the presence or absence of infection, tumors, abnormal anatomy, and body changes form trauma. CT scans are usually painless and provide more accurate images of structures of the body besides guiding radiologists in carrying out certain procedures like suspected cancer biopsies, removal of internal body fluids for medical tests, and draining of deep abscesses in the body. These processes have greatly reduced the need of surgery to achieve same results.

Risks involved in obtaining a CT scan

Any medical procedure has risks the only difference is that a CT scan is a low risk procedure. The most cited problem has always been reactions to the intravenous contrast materials that are used in the process. This is usually an iodine-based liquid that is administered through the vein to make internal body structures more visible on a CT scan. This can sometimes cause itching, rashes, hives, or sometimes a feeling of warmth in the whole body. Mostly these are self-limiting reactions that disappear within a short period. However, if they persist, antihistamines can be used to relief the symptoms. A much serious reaction to intravenous contrast is what doctors call anaphylactic reaction. A patient with this reaction experiences severe hives and sometimes extreme breathing difficulty. The reaction seldom happens, but poses a danger to one's life if not treated. Using medications such as antihistamines, corticosteroids, and epinephrine can help reverse the reaction. Another risk is toxicity to the kidneys that can cause kidney failure although this is extremely rare. Most cases of this risk occur in patients with diabetes, those who are dehydrated, or those with kidneys whose functions are already impaired. This complication has nearly been eliminated by the development of new intravenous contrast agents like Isovue. Radiations that patients usually receive are very minimal and therefore do not cause any adverse effects. However, there may be possible risks to the fetus if a pregnant woman is exposed to these radiations. Pregnant women are supposed to inform doctors about it so that other imaging methods that are not harmful are used.

Preparation and performing a CT scan

Patients who are about to undergo CT scans are usually advised to avoid food especially if a contrast material is to be used in the process. If the patient's history shows some allergic reactions to the contrast agent, then the doctor or physician should be made aware. Materials that are metallic and some types of cloths that can interfere with image clarity are usually removed from the patient. Patients are then put on a movable table that is then pushed into the middle of a large machine in the shape of a doughnut that then takes x-ray images around the body. The whole procedure can take from thirty minutes to one and a half hours. This can be more especially if specific tests are required. Body movements by the patient during the scan process are to be minimized to increase image clarity. Technologists usually tell patients when to breath or hold their breath, this is mostly when carrying out chest or abdomen scans. Technologists usually observe the patient's progress during the procedure through an observation window. This scans have increased largely the ability of doctors to diagnose many disease as early as possible with lower risks as what used to happen previously. Refinements are still evolving in CT scan technology with prospects of better quality of images and patient safety. For instance, new versions of CT scan called helical or spiral CT scans are able to give more rapid and precise visualization of the internal body organs.

Spiral or Helical CT scan

The development of spiral or helical CT scan is the major advance in CT technology in recent times. During a helical CT scan, there is a continuous rotation of x-ray tubes as the patient is moved smoothly through the x-ray scan field. Instead of producing separate data sets for each individual slice as it is one in standard CT, spiral CT produces just one continuous data set for the whole region that is scanned.  The advantage of this type of scan is that it is faster and therefore saves on the exam time. For instance, a chest scan or abdomen scan can be done just in one breath hold, roughly 30 seconds. This enables the obtaining clear images. There is also an improved detection of small lesions that may be bypassed in most of the standard CT scans.

Technical parameters for all scanner types

Image quality relates largely to the reliability of the CT numbers and the accurate reproduction of low contrast resolution and spatial resolution. Good imaging performance calls for sufficient image quality to meet clinical requirements for an examination and the same maintaining the patient's dose at the lowest level possible. Careful selection of technical parameters is therefore paramount in order to control patient exposure and image display, and regularly monitor the performance of the scanner as part of quality assurance. These parameters include nominal slice thickness; this is selected by the operator according to clinical requirements. It generally lies between 1 and 10 mm; generally the larger the thickness of the slice, the greater the low contrast resolution of an image; the smaller the thickness of the slice; the greater the spatial resolution; pitch factor or inter-slice distance, which is the couch distance between two slices. In spiral CT, it is the ratio of the couch distance for every rotation to thickness of the slice at the rotation axis. Inter-slice distance usually lies between 2 and 10 mm while the pitch factor between 1 and 2 mm. Generally, the smaller the pitch factor or inter-slice distance, the higher the integral dose and the local dose to the patient; investigation volume or imaging volume is the whole volume of the region being examined. This depends on the clinical needs, the greater the value, the higher the integral dose to the patient; exposure factors, this are settings of x-ray tube voltage, tube current and time of exposure; field view , which is the maximum diameter of the image that is reconstructed. This is usually decided upon by the operator; gantry tilt, is the angle between the vertical plane and the one having the x-ray tube, the detector array, and the x-ray beam. The degree of the gantry tilt is also decided upon basing on the clinical objective.

A computerized axial tomography scan is more often known in short as CAT scan. An x-ray procedure combining a number of x-ray images using a computer to come up with cross sectional views and three dimensional images of the body's internal organs and structures. A beam of x-rays in the shape of a fan is directed to an array of detectors that positioned relative to the source of x-rays. This x-ray beam rotates around a patient and directed at the detectors in a non-fixed relationship. As this x-ray source rotates around the patient, an x-ray slice data is generated; it is picked up x-ray sensors on the opposite side of the circle. Progressively, many data scans are taken as the object passes gradually through the gantry. These scans are combined using a mathematical procedure known as tomographic reconstruction to bring forward images that can then be studied to diagnose body complications. As it has been shown, scanners have greatly revolutionized the health industry. Procedure that took long for a doctor to diagnose a medication or those that required surgery have been replaced by CT scans that are efficient and less painful with low risk. Research is on going to come up with CAT scanners that will produce better quality images, the will reduce amounts of patient exposure, optimize the algorithms of computer reconstruction, and improve the designs of CAT scanners. Attempts at achieving these aims have already been made, to improve the quality of images; scanners have been made to include unique movements of the detector, x-ray tube or even both.