Computed Tomography (CT) PDF

Oral Radiology Computed Tomography (CT) OMD-725 What is the difference between Plain Radiography and Cross sectional imaging ? Plain radiography provide 2D image for a 3D subject Plain radiography is subjected to superimposition Plain radiography doesn’t...

Oral Radiology Computed Tomography (CT) OMD-725 What is the difference between Plain Radiography and Cross sectional imaging ? Plain radiography provide 2D image for a 3D subject Plain radiography is subjected to superimposition Plain radiography doesn’t demonstrate soft tissue Plain radiography acquires data manually  Tomography It is a specialized technique for producing radiographs showing only a section or slice of a patient. Each tomograph shows the tissues within that section sharply defined and in focus. This is to overcome the superimposition problem found in conventional radiography Hence CT employ tomography & computer processing to generate 3D images from 2D images using electronic detectors.  First CT scanner - Godfrey Hounsfield and Allan Cormack. 1971 First successful scan of a cerebral cyst 1979 Nobel Prize for Physiology and Medicine Current technology: Multislice CT, Dual-energy CT, and Spectral CT CT enables high resolution volumetric imaging for clinical and industrial applications. Recently, micro-CT has become popular for non-destructive imaging of samples at 0.5 – 150 μm spatial resolution. COMPUTED TOMOGRAPHY (CT) CT scanners use X-rays to produce slices as in conventional tomography, but: The radiographic film is replaced by very sensitive crystal or gas detectors. The X-ray tube-head rotates around the patient scanning one section at a time.  System Components 1.Computer 2.Gantry 3.Table 4.Operator’s Console  Computer &Operator’s Console The computer has four basic functions: 1.Control of data acquisition 2.Image reconstruction 3.Storage of image data 4.Image display  Gantry Gantry is a circular device that houses the Data Acquisition system (DAS) = Tube, Detectors, Filters, Collimators & Analog-to-Digital Converter (ADC)  Tube MDCT scanners operate at high tube voltage and tube current, and thus require special x-ray tubes to meet the high demands on heat production and cooling. MDCT units use x-ray tubes with rotating anodes. They typically operate at (range, 80 to 140 kVp) and at high tube currents (200 to 800 mA). {Periapical 2-12 mA} The high x-ray output minimizes exposure time and improves image quality by increasing the signal-to-noise ratio. To minimize patient exposure the beam is collimated to a thin fan beam before it enters the patient. Some of the x-ray photons interact with the patient and are scattered. To improve image quality the residual beam is again collimated to remove the scattered photons before it reaches the detector array.  Detectors Function as image receptors for remnant radiation, then converts the measurement into an electrical signal proportional to the radiation intensity. Two basic detector types are used: Scintillation (solid state) and Ionization (xenon gas) detectors. DETECTORS 1. Scintillation Crystals: Produce light when exposed to ionizing radiation Previous types had long afterglow Now replaced with silicon photodiodes solid state detectors eg Gadolinium Oxysulphide with no afterglow 2. Xenon Gas Ionization Chambers Inefficient Long chamber sizes Cannot be used in rotate-rotate scanners  Table Automated device linked to the computer and gantry Designed to move in increments after every scan according to the technologists scan program.  Evolution of CT Scanners First Generation Second Generation Third Generation Fourth Generation Pencil beam Small fan beam Large fan beam Large fan beam One detector Multiple detectors Multiple detectors Detector ring Source and detector Source and detector Source and detector Source Translation-Rotation Translation-Rotation Rotation-Rotation Rotation  Source-Detector Time to acquire 1 Generation Year Why Developed Anatomy Why it died? Movement image 1 st Gen 1971 To show CT works Head Only Translate-Rotate ~5 min Slow 2nd Gen 1974 Image Faster Head Only Translate-Rotate 20sec-2min Slow This Geometry 3rd Gen 1975 Image Faster All Anatomy Rotate-Rotate 1 sec won. Make images Expensive, not 4th Gen 1976 All Anatomy Rotate-Stationary 1 sec without rings good for scatter. Stationary- Cardiac specific, 5th Gen 1980s Fast Cardiac CT Cardiac Only 50 ms Stationary low x-ray flux. In 5th generation CT the x-tube in this design is a scanning x-ray tube, where the electrons are steered magnetically (like in old TVs) rather than physically moving the x-ray tube. This method allows for very fast acquisitions and is ideal for cardiac scanning. However, these scanners did not have full volumetric coverage. A) In First-generation CT scanners, to acquire every slice across the part of the body, the X ray tube and detectors has to be moved linearly before rotating the position of X ray tube to acquire images at different projection angles. It used a pencil beam of one degree. One of the disadvantages is that scanning time is between 25-30 mins. & resolution was very poor. The x-ray beam was collimated at the source and at the surface of the detector to minimize scatter radiation reaching the detector. B) In Second-generation CT scanners, it used a narrow fan beam of 10 degrees which results in a linear array of 30 detectors. Since it is a narrow fan beam, 2nd generation ct scanner still requires a Translate/Rotate process. Scanning time was improved than the first generation CT scanners. More number of detectors contributed to a more scattered radiation. It decreased the resolution of the image. C) In Third-generation CT scanners, both the x-ray source and the detector array rotate around the patient in a circular path. It used a larger fan beam of 40 – 60 degrees & Larger array of detectors (more than 800). The patient is moved incrementally between each rotation of the source. At the end of each revolution, the table advanced through the gantry, while the x-ray tube-detector assembly returned to its original position to unwind the attached wiring that supplied power and transferred data from the detector to the computer. D) In Fourth-generation CT scanners, the x-ray tube rotates around the patient, and the remnant beam is detected by a fixed circular array. to overcome detector drift (ring artifact in third generation). One of the disadvantage of 4th generation ct scanner is the lesser usage of detectors, only 1/4th of detectors are in use at a single time. Stationary detector requires a larger acceptance angle for radiation, therefore it is more sensitive to scattered radiation than third generation CT scanners. The table movement through the gantry, typically 1 to 5 mm, positioned the patient for imaging of the next slice. CT scanners that use this type of “step and shoot” movement for image acquisition are called incremental scanners. The final image set consists of a series of spaced or overlapping images in the axial plane. In the early 1990s, Helical (Spiral) CT imaging was introduced. In helical scanners, the patient is moved continuously through the gantry, and the x-ray source moves continuously around the patient in a circle. This was due to the use of “slip-ring” technology that eliminated the need for directed wired connections to the moving x-ray tube and detector. Markedly decreased scan time. In CT scanners, the x-ray source emits a collimated fan beam. Multislice CT (MSCT) One problem quickly encountered with single slice (single detector row) helical scanning (SSCT) was excess stress on the x ray tube. That is, the x ray tube would heat to extreme temperatures as very high energy was deposited onto the anode. This problem limited the ability to perform thin slice imaging necessary for acceptable imaging. The primary difference is SSCT uses a one dimensional detector arrangement where many individual detector elements are arranged in a single row. In MSCT, each detector in a single row is long enough in the slice thickness direction (z axis). Each individual detector in each row is then divided into multiple detector elements forming a two-dimensional array. By increasing the number of detector rows, the z axis coverage slab thickness increases thereby decreasing the number of gantry rotations necessary to image the selected field of view (scan length), theoretically reducing the strain on the x ray tube. Multi-detector CT (MDCT), also referred to as multi-slice CT (MSCT), was introduced in the late 1990s and has now become the most widely used CT scanner design across the world. (MDCT) array having 64 to 128 rows (could reach up to 640). In these scanners, all parts of the detector array arc are equidistant from the x-ray source. It has considerably reduced scan times, limiting motion artifact from breathing or heart contractions Third Generation Multidetector Helical CT (multi-slice/ multirow CT) In multi-detector CT, the gantry contains up to 256 rows of detectors instead of just one raw. Third Generation Multidetector Helical CT Advantages – Less exposure time – Less motion artifact from breathing, peristalsis or heart contractions; this is important for patients who cannot hold their breath for long periods and for pediatric and trauma patients. – Better quality of axial, reformatted, and three-dimensional images Third Generation Multidetector Helical CT Disadvantages – Higher patient dose than single- slice scanners. Equipment and Theory The circular gantry houses the X-ray tube-head and the detectors. The patient lies down with the part of the body to be examined within the gantry. The level and thickness of the section to be imaged are selected. Equipment and Theory The X-ray tube-head rotates around the patient scanning that section. Each set of detectors produces an attenuation or penetration profile. The computer calculates the absorption at points on a matrix formed by the intersection of all the attenuation profiles.  Each point on the matrix is called a pixel Typical matrix sizes are 512 X 512 or 1024 X 1024 pixels. Smaller pixel size = greater image resolution The area being imaged by each pixel has a definite volume, depending on the thickness of the slice, called a voxel. COMPUTED TOMOGRAPHY (CT)  Matrix : is array of numbers arranged in a rows and columns.  Pixel : is a single square, or picture element, within the matrix.  Voxel: is the depth or the volume of pixels  Each voxel is given a CT number or Hounsfield unit between + 1000 and -1000, depending on the amount of absorption within that block of tissue. Air: -1000 (Black) Water: 0 Dense bone: +1000 (White) Each CT number is assigned a different degree of grayness, forming a visual image which is displayed on a television screen. - 1000 +1000 +800 - 600  Image manipulation Window level Window width The CT n° selected for the The range selected for the center of the range, various shades of grey, Depending on whether the A narrow range allows lesion under investigation is subtle differences between in soft tissue or bone. very similar tissues to be detected.  Image manipulation Bone window Soft tissue window Image manipulation 2. Image Reconstruction The information obtained from the original axial scan can be manipulated by the computer to reconstruct: Coronal cuts Sagittal cuts Cuts in any other plane 3-dimensional images. Axial cut Coronal cut Indications of CT Intracranial disease Bony fractures Developmental Anomalies Sinuses Oroantral fistula Cysts Follicular cyst maxilla Benign Tumours Malignant Tumours Infection sequestrum Salivary Gland Lesions  Preoperative assessment of alveolar bone height and thickness before inserting implants CT guided biopsy  Advantages over Conventional Film-based Tomography Very small amounts, and differences, in X-ray absorption can be detected. This enables: Detailed imaging of intracranial lesions Imaging of hard and soft tissues Excellent differentiation between different types of tissues, both normal and diseased  Advantages over Conventional Film-based Tomography Images can be manipulated Axial and coronal tomographic sections of the skulls are obtainable Reconstructed images can be obtained from information obtained in the axial plane Images can be enhanced by the use of IV contrast media providing additional information.  Disadvantages The equipment is very expensive Facilities are not widely available High dose of radiation Risks associated with IV contrast agents. Metallic objects, such as fillings may produce marked streak or star artifacts Disadvantages  Metal streaking artifacts occur because of the near-complete absorption of x-ray photons by metallic restorations. They appear as opaque streaks in the occlusal plane Ring artifacts: are a CT phenomenon that occurs due to the miscalibration or failure of one or more detector elements in a CT scanner. Less often, it can be caused by insufficient radiation dose or contrast material contamination of the detector cover. Partial Volume Artifact: when a single voxel represents tissues of differing densities (e.g., bone and soft tissue), the resulting CT number for that voxel is an intermediate value that does not accurately represent either tissue. This artifact will occur when the object being imaged is smaller than the size of an individual reconstructed voxel. Beam-Hardening Artifact: manifest as dark streaks between two highly attenuating structures, such as compact bone, dental implants, and dental restorations.  Dual Energy CT Scan A dual energy CT (DECT) scanner is fairly new technology that uses both the normal X-ray and also a second less powerful X-ray to make the images. This gives DECT additional advantages over standard CT for a wide range of tests and procedures  Dual Energy CT Scan It can selectively increase or decrease the effects of some chemical substances in the body, making some abnormalities clearer on the images taken; for example, iodine is a commonly used substance in X-ray contrast agents, and DECT can selectively increase its effects to produce better images of blood vessels (CT angiography) Images with and without contrast agents can be obtained using a single examination instead of two separate examinations. It can detect particular substances in the body that can be useful for patients with kidney stones to see what type of stone is present and assist in deciding the type of treatment required It can significantly improve image quality if patients have metal in the area being scanned, specially with metal artifact reduction software (MARS)  Faculty of Dentistry