Transcript CTbushong2
Operational Modes Chapter 2 Stewart C. Bushong Major Early Developments Major early computed tomography developments were given the misnomer generation, as in genealogy Progress was rapid so that fourthgeneration CT imagers appeared in 1978, just 6 years after the first CT imager Unlike Hounsfield’s early experiments, the patient does not move during CT, except for spiral CT, rather, the x-ray source and the image receptor move First Generation Finely collimated x-ray beam (pencil beam) was used in first-generation CT imagers Fan-shaped x-ray beam (fan beam) is used in all current CT imagers Single radiation detector Translate-rotate motion First Generation 180 translation with 1 degree between translations Single image projection per translation Single image projection per translation Five minute image time Head imager only, not capable of body imaging Second Generation Fan-shaped x-ray beam Multiple radiation detectors (detector array) Translate-rotate motion Usually 18 translations with 10 degree rotation between translations Multiple image projections per translation Second Generation Approximately, 30 s imaging time Head and body imager Third Generation A fan beam x-ray source is used and it views the entire patient during imaging As many as several hundred radiation detectors are incorporated into the curvilinear detector array The curvilinear detector array provides constant distance between source and each detector, resulting in good image reconstruction Third Generation This development is based on 360 degree rotate-rotate motion. Both the x-ray source and the detector array rotate about the same axis Hundreds of image projections are acquired during each rotation, resulting in better contrast resolution and spatial resolution Third Generation Imaging time is reduced to 1s or less Various arc scans are possible in order to improve motion blur-half scan, full scan Ring artifacts are characteristic of third generation imagers Fourth Generation Fourth generation was developed principally to suppress ring artifacts The x-ray source is collimated to a fan beam as in third generation The detector array can contain several thousand individual detectors Fourth Generation The mechanical motion is rotation of the x-ray source around a fixed detector array (rotate-stationary) There is a modest sacrifice in geometry; however, the unattenuated leading edge and un attenuated trailing edge of the fan beam allows for individual detector calibration during each scan Fourth Generation Patient dose may be somewhat higher with fourth-generation scanners because of interspace between detectors When there is an interspace between detectors, some x-radiation falls on the interspace, resulting in a wasted dose As the fan beam passes across each detector, an image projection is acquired Fourth Generation Imaging time is 1s or less Various arc scan are available – half scan, full scan, over scan Electron Beam CT (EBCT) This CT imager was developed specifically for fast imaging Images can be obtained in less than 100ms, about the time of a radiograph The x-ray source is not an x-ray tube but rather a focused, steered, and microwave accelerated electron beam incident on a tungsten target EBCT The target covers one-half of the imaging circle; the detector array covers the other half The electron beam is steered along the curved tungsten target creating a moving source There are four targets, or focal tracks, and four detector arrays, resulting in four contiguous images simultaneously EBCT Electron beam CT is principally applied to cardiac imaging and frequently advertised as a heart scan Electron beam CT has no moving parts Electron beam CT uses a focused electron beam on a tungsten target ring as an xray source Heat dissipation is no problem in EBCT EBCT Electron beam CT can produce up to eight slices simultaneously Electron beam CT scan times as short as 50ms are possible Principal application for EBCT is cardiac imaging Spiral CT Spiral CT was introduced to clinical practice in 1989 and is now the standard CT imager If a third or fourth generation is CT imager is caused to continually rotate while the patient couch is moved through the imaging plane, spiral CT results Spiral CT The development of slip rings was the technology breakthrough that made spiral CT possible Spiral CT requires slip ring technology for data transfer from the rotating gantry Spiral CT requires either an on-board high voltage supply so that coiled high voltage cables are unnecessary or slip rings for high voltage transfer Spiral CT The principal advantage to spiral CT is the ability to image large volumes of anatomy in less time Single breath-hold imaging of the entire torso is possible with spiral CT Comparison First Gen Spiral CT Scan Time 300s Less than 1s Data/image 60kb 2 Mb Matrix Size 80x80 1024x1024 Energy/imag e Slice Thickness Spatial Res. 2kJ 60kJ 13mm 1-10mm 3 lp/cm 15 lp/cm