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Imaging Ch.27
Compurted Tomography
| Question | Answer |
|---|---|
| Conventional Tomography | Considered axial tomography because the plane of the image is parallel with the long axis of the body. |
| Transaxial Tomography | - Also called transverse images are produced perpendicular with the long axis of the body. - Created by a computer therefore called computed tomography. |
| CT was developed by an engineer named _________________________ from england in the late 1960's. | Godfrey Newbold Hounsfield. |
| The first CT brain machine was used in clinical studies in _________. | 1971. |
| History of CT: | - First machine took lengthy periods of time to complete one slice scan. - It wasn't until the 1980's with the invention of supercomputers that CT was able to be used diagnostically. |
| Fifth Generation CT Scanners | - Utilizes many rows of detectors and constant movement of the tube along with advanced Interpolation and extrapolation algorithms to produce images. |
| Components of the CT System: | Gantry, Computer, & Operating console |
| Fifth Generation CT Scanners: | - Uses slip ring technology for continuous movement of the x-ray tube. - Also called multi-slice or helical tomography |
| Gantry | - The mounted framework that surrounds the patient in a vertical frame. - Must be able to tilt up to 30 degrees for exams. |
| Reconstruction: | Interpolation & Extrapolation. |
| Pre patient collimation | - Mounted on the x-ray tube housing. - Limits the area of the patient that intercepts the useful beam. - Determines patient dose. |
| Post patient / pre-detector collimation | - Restricts the x-ray beam viewed by the detector array. - Reduces scatter. |
| CT X-ray tube | - Must have very high heat capacities . - High speed rotors are used to dissipate heat Anodes have high cooling rates (1 MHU per minute). - Some tube incorporate an oil bath for heat dissipation. |
| CT X-ray tube: | - Air cooled systems are also used for heat dissipation. - Must have the ability to create multiple exposures quickly and accurately. - Small focal spots for high spatial resolution is required. |
| Window Width | Determines the maximum shades of gray for an exam. |
| Window Level | The center or midpoint of the range of CT numbers. |
| Windowing | A method by which the CT image gray scale can be manipulated using the CT numbers of the image. |
| Algorithms | - A set of rules or directions for getting a specific output from a specific input. - Mathematical calculation applied to raw data during image reconstruction. |
| Spatial Resolution | The systems ability to image small objects that have a high subject contrast. |
| Contrast Resolution | The systems ability to distinguish between images of similar tissue structures. |
| Slice Acquisition Rate | Measure of the efficiency of the multi-slice spiral CT imaging. |
| Linearity | The relationship of CT numbers to the linear attenuation coefficients of the objects to be imaged. |
| Noise in Radiography | Is the grainy or uneven appearance of an image as a result of an insufficient number of primary x-rays. |
| Noise in CT | Is a comparison of CT numbers in an image to a controlled material such as water. |
| Pitch | - The relationship between the patient couch movement and the x-ray beam width. - Examples are .5:1, 1:1,1.5:1,2:1. |
| Pitch: | - Smaller ______ results in overlapping images and higher patient doses. - The amount of tissue that may be imaged is determined by the beam width, the pitch and the imaging time. |
| Shaded Volume Display | - Volume display or volume rendering is the newest and most costly of all 3 D imaging techniques. |
| Shaded Volume Display: | - Includes 4 dimensions to create an image 1. Window level and width. 2. Opacity. 3. Brightness. 4. Accuracy. |
| Maximum Intensity Projection | - Reconstruction of an image through selection of the highest value pixels along any arbitrary line in the data set. - Only those pixels are exhibited. - The simplest form of 3 dimensional imaging. - Does not require special software. |
| Shaded Surface Display | - Computer aided technique that identifies a narrow range of values as belonging to the object to be imaged and displays that range. - The range displayed appears as an organ surface that is determined by the operator. |
| Shaded Surface Display: | - Originally used in bone imaging but now used in virtual colonoscopy. - It is also called volume rendered. |
| Multiplanar Reformation | - Spiral CT excels in 3-dimentional images because of MPR. - Process of stacking transverse images to form a 3 dimensional image. - Most often used MPRs. |
| MPRs: | - Maximum Intensity Projection (MIP). - Shaded Surface Display (SSD). - Shaded Volume Display (SVD). |
| Filtered Back Projection | - Process by which an image acquired during computed tomography and stored in computer memory is reconstructed. - Here filter refers to a mathematical function rather than a piece of metal used to reduce the useful beam. |
| Operating Console | - 1 is located in the Physician viewing area. - Usually have 2 in CT area. - 1 for completing the procedure. |
| Operating Console: | - 1 for viewing and reconstructing images. -New technology allows technologist to complete data information through keyboard, mouse or touch screen. |
| Computer | - Are used to store date, construct images, and reconstruct extrapolated data to create 3-D images. |
| Computer: | - Must have a large computing capacity. - Must operate at high speeds. - Most systems use an array processor instead of a microprocessor for image reconstruction. |
| Mechanical Supports | Couch pedestal houses the mechanical and electrical components that facilitate vertical and horizontal couch movements. |
| High Voltage Generator | Operate on High frequency power. |
| Patient Support Couch | - Must support patient comfortably. - Must not interfere with x-ray beam transmission. - Has a low-Z number material for construction. |
| Patient Support Couch: | - Smoothly driven by a motor for accurate positioning. - Must be strong and rigid to support the patient's weight. |
| Other support devices include: | - Head holders, Table extenders, Pediatric cradles, Arm support, Knee supports, Head and hand rests. |
| Detector Array | - This may reduce patient dose per exam. - The design may determine the speed of the imaging system. - More detectors the faster the system. |
| Detector Array: | - Today's consist of a photodiode that converts light into an electronic signal. - Detectors must have a high conversion efficiency. - Approximately 90% of x-rays that are absorbed are converted into an electronic signal. |
| The X-ray source and the detector move synchronously for _____________________. | Computed tomography. |
| First-generation imaging system: | Translate and rotate, pencil beam, single detector, 5-minute imaging time. |
| Second-generation imaging system: | Translate and rotate, fan beam, detector array, 30-second imaging time. |
| Third generation imaging system: | Rotate and rotate, fan beam, detector array, sub second imaging time. |
| Fourth generation CT imaging system: | Rotate and stationary, fan beam, detector array, sub second imaging time. |
| Linear interpolation at ________ improves z-axis resolution. | 180°. |
| The ________________ allows the operator to view images before transferring them to the radiologist's viewing console. | Second monitor. |
| _____________ of images along coronal, sagittal, and oblique planes is also possible. | Reconstruction. |
| _____________ is the time from the end of imaging to appearance of the image. | Reconstruction time. |
| CT x-ray tubes are expected to last for at least _______________ exposures. | 50,000. |
| ________________ determines the radiation dose profile and patient radiation dose. | Pre-patient collimation. |
| The ________________ determines the sensitivity profile and improves image contrast. | Pre-detector collimaton. |
| ___________ eliminate the need for cables making possible continuous rotation resulting in multi-slice helical CT. | Slip rings. |
| When k is 1000, the CT numbers are called _____________________ and range from -1000 to +1000. | Hounsfield. |
| On most computer workstations, the image can be rotated to show striking ______________ features. | Three-dimensional. |
| Low spatial frequency represents ________________. | Large objects. |
| High spatial frequency represents ________________ | Small objects. |
| A graph of the image contrast to object contrast at each spatial frequency results in an ________ curve. | MTF. |
| ___________ for a CT image is limited to the size of the pixel. | Spatial resolution. |
| ____ is superior in CT principally because of x-ray beam collimation | Contrast resolution. |
| A large variation of pixel values represents high __________. | Image noise. |
| The resolution of __________ objects is limited by the noise of a CT imaging system. | Low-contrast. |
| The _____ value for water may drift from day to day or even hour to hour. | CT. |
| ______ allow imaging of greater tissue volume. | Wider multislices. |
| ________ size results in better spatial resolution. | Smaller detector. |
| _______ is assessed by imaging a wire or an edge to obtain the edge response function. | Spatial resolution. |
| All CT imaging systems should be capable of resolving 5-mm objects at ________ contrast. | 0.5%. |
| 1st Generation | 1°. |
| 2nd Generation | 10°. |
| 3rd Generation | Both directions |
| 4th Generation | Circle row. |
| 5th Generation | Multiple rows. |
| Computed Tomography Fluoroscopy | - Used primarily to localize lesions for biopsy proposes. - It is not dynamic like traditional fluoro however images are captured at high rates of speed seeming to by dynamic. |
| New advances in CT: | 1. 3D imaging. 2. Virtual Reality imaging. 3. Computed Tomography Fluoroscopy. |
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sassyrad
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