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Computed Tomography ARRT ( Image Production).
data acquisition - ANSWERthe measurement of the attenuation that occurs along each path through the patient from the x-ray tube to the detector data reconstruction - ANSWERcomputerized processing of the transmission measurements into the CT image multidimensional image display - ANSWERdisplay of the reconstructed grayscale image in two-dimensional and / or 3-D format ; representation of the attenuation that occurred across the scanned volume of tissue image archival and communication - ANSWERdisplay as well as storage, both short term and long term of images on computer workstations scout image acquisition (localizer or scanogram) - ANSWERthis survey scan is a digital radiograph obtained by keeping the x-ray tube and detector array in a fixed position while the patient bed is translated through the gantry azimuth - ANSWERthe angle of the tube and detectors in relationship to the patient position during scout acquisition steps from x-ray to actual image - ANSWERCT scans- x-rays hit detectors- attenuation measurements get turned into digital data- data goes to computer which process info- info is assigned a numerical value - forms image that we can see and store on PACS
why do we need a scout image? - ANSWERit is a localizer for the prescription of subsequent cross-sectional CT acquisition(s) what does the gantry house? - ANSWERgenerator, x-ray tube, assorted collimators , the data acquisition system , slip rings, and detectors before slip rings, what mechanism did CT machines use to rotate around the patient? - ANSWERthey were limited to hardwired cables , which prevented continuous rotation within the gantry, at the end of each rotation of the x-ray tube and/or detectors, the system would reverse course and travel in the opposite direction because the cables were of finite length slip ring technology - ANSWEReliminates the need for cables by utilizing a system of contact brushes that supply electricity to power the system and enable the passage of transmission data to the computer; also allowed for helical scanning which is standard today what are the technical elements of helical CT acquisition? - ANSWER1) continuous rotation of the gantry components (slip ring )
- a powerful x-ray tube capable of long exposure output
- continuous movement of the patient bed 4)specialized mathematical reconstruction techniques helical geometry allows for_________> - ANSWERvolumetric data acquisition ; the reconstruction of CT sections at any point along the scanned volume is possible as long as the section width is the same
what is the mA setting range? - ANSWER30mA-800 mA Automatic tube current modulation (ATCM) - ANSWERprogram that adjusts the mA throughout an acquisition to reduce patient radiation dose to a minimum, it can also alter the mA according to predetermined noise index that is acceptable for appropriate image quality exposure time is dependent on ____________ - ANSWERanatomic extent , detector configuration and table speed; exposure time can be from 30-60 s in length photon flux - ANSWERthe rate at which the photon fluence passes through a unit area over unit time effective mAs - ANSWERthe calculated mAs per acquired slice , the main controlling factor is the table speed pitch controls_______ - ANSWERthe speed at which the patient translates through the gantry for each tube revolution effective mAs= mAs/pitch as pitch increases, mAs decreases why are high kVp setting utilized? - ANSWERto decrease the streak artifact that may occur as the beam passes through very dense parts, such as those found in the posterior fossa during head imaging what is the kVp range used in CT? - ANSWER 70 - 150 kVP
ATVS - automated tube voltage selection - ANSWERan advanced feature that may be available on some modern CT exams, it modulates the tube potential based upon the changing patient attenuation along the scan acquisition range True/False Dual energy CT systems are capable of applying multiple x-ray energies during a single CT acquisition. - ANSWERTrue How does a dual energy CT system work? - ANSWERtransmission data is acquired from tissue interactions with both high and low kVp radiation. Differentiation and characterization of tissue composition are made possible by the difference in attenuation between the two x-ray spectra how can complex voltage-switching systems be use to achieve dual-energy CT scanning? - ANSWERthe single tube rotates around the patient. the applied peak kVp is switched at an extremely high rate for each successive projection how is a dual energy CT system constructed? - ANSWERhas 2 completely separate x-ray tubes and detector arrays that are 90 deg. from each other within the gantry. the two tubes acquire data simultaneously at different kVp rates How has dual energy CT systems expanded clinical opportunities? - ANSWER1) improved resolution of soft tissue structures during musculoskeletal studies ( ligaments/tendons)
- the ability to visualize atherosclerotic plaque within contrast-enhanced vasculature during cardiac CT studies
- contrast medium subtraction techniques for the demonstration of precontrast images from a singe scan acquired after the contrast injection
- characterization of the biochemical composition of urinary calculi
what type of beam does a SSCT use? - ANSWERfan-shaped beam with a transverse (x,y)dimension corresponding to the size of the detector array and the maximum SFOV, the length of coverage is determined by the angle of the tube prepatient collimator - ANSWERin SSCT it controls the amount the tissues exposed to x-radiation as the tube rotates about the patient ; it directly limits the beam to the desired section width true/false You can make adjustments to the slice thickness after the data has been acquired in SSCT. - ANSWERfalse , you cannot make further adjustments SSCT beam collimation consists of : - ANSWERpre-patient collimation and post-collimation post-patient collimation - ANSWER(predetector) are used to improve image quality by limiting scatter radiation before it reaches the detectors what is the first component of the prepatient collimator? - ANSWERthe tube housing; acts as an initial collimator , the tube aperture roughly shapes the primary beam into a fan or cone shape for SSCT or MDCT what is the second component of the prepatient collimator? - ANSWERit had to be positioned just beyond the tube housing ; it restricts the beam along the z-axis , controlling slice thickness in the SSCT system in MDCT, how does post-patient collimation work? - ANSWERit occurs through a high resolution comb placed over the detector array, functioning
like a grid , this comb removes unwanted scatter radiation and off-axis photons what happens to the beam in MDCT if you increase the number of detectors grows? - ANSWERthe divergence of the cone needed to cover this amount has to increase the relationship between beam collimation and detector size is represented by - ANSWERd(mm)= D(mm)/ N d= detector collimation D= beam collimation N= number of detector rows how does collimation affect the volume of tissue measured for rotation of the tube? - ANSWERit directly affects it; wider collimation results in greater anatomic coverage with each tube rotation and allows for an increase in the overall acquisition volume over a given scan time Compared to SSCT, MDCT________. - ANSWERallows for faster and more extensive coverage along the z-axis while also providing thinner section widths for each gantry location section width - ANSWERdescribes the amount of tissue in the longitudinal or z axis that is represented by the 2D image SSCT is directly controlled by beam collimation while MDCT is controlled by_______> - ANSWERbeam and detector collimation
along the z-axis with each resolution of the gantry but at the cost of reduce image quality beam pitch equation - ANSWERbeam pitch= table feed per rotation/ total collimation total collimation - ANSWERnumber of sections x section width pitch less than 1 results in ______ - ANSWERreduces table speed and increases image quality BUT higher dose increasing pitch results in - ANSWERless patient dose , faster tablespeed, decreased image quality precise collimation in what axis results in decreased dose? - ANSWERz-axis to acquire data for interpolation during helical reconstruction what is necessary? - ANSWERaka z-axis overscanning- an additional half rotation ( deg) of the gantry components may be necessary at the initial (prespiral) and final ( postspiral) portion of each acquisition - results in unnecessary radiation adaptive collimation - ANSWERused to reduce z-axis overscanning by blocking a portion of the beam that would potentially expose tissues outside of the desired acquisition volume CT detector - ANSWERresponsible for measuring transmitted radiation and converting it into a proportionate electronic signal to be used for image reconstruction
what are some desired qualities in detectors? - ANSWER1) high efficiency
- rapid signal decay
- high dynamic range what does high efficiency mean for detectors? - ANSWERthe detector material must be efficient at absorbing the transmitted x-ray photons and converting them into a proportionate signal. Its efficiency is also determined by its overall size, density and atomic number and the amount of interspace material what does rapid signal decay mean? - ANSWERthe detector must be capable of measuring transmitted x-ray photons with an excellent response time and limited afterglow what is the response time? - ANSWERthe ability of the detector to quickly measure x-rays and then recover before the next measurement afterglow - ANSWERthe tendency of scintillation crystal detectors to continue to glow in response to x-rays once the exposure source has been terminated what does high dynamic range in detectors? - ANSWERThe CT detectors must be sensitive enough to measure a broad range of x-ray transmission data and accurate enough to modulate signal in response to small changes in radiation transmission what can CT detector be made of? - ANSWERgas ionization chamber or scintillation crystal
how did the 1st generation CTs take images? - ANSWERthe tube and detectors would translate across the patient's head , rotate 1 deg and then translate back in a method known as rectilinear or translate - rotate scanning what was the detector configuration of 2nd generation scanners? - ANSWERsimilar to 1st generation in that it used a translate - rotate scanning , but had more detectors which allowed for greater increments of rotation between translation and reduced scan time what type fo beam did 2nd generation scanners use? - ANSWERfan beam - to expose a wider detector array *allowed it to scan more than just heads what was the detector configuration of a 3rd generation scanner? - ANSWERit had an expanded curvilinear array of detectors that rotates with the x-ray tube around the patient. It had rotate-rotate geometry which eliminated the need for translation whats another name for a pencil beam? - ANSWERparallel beam, rectilinear beam scanning how many detectors did a 1st gen scanner have? - ANSWERusually one, but some had 2,
- a 3rd detector could be used for sampling what does translate-rotate motion mean? - ANSWERthe scanner would move across and rotate
what does translation mean? - ANSWERused to describe one sweep of detectors-source assembly as the assembly moves across the patient what was the scan time of a 1st generation scanner? - ANSWER4.5-5. minutes , it was very slow and required water bags to compensate for drastic changes in attenuation what type of beam did a 3rd gen scanner use? - ANSWERlarger fan beam what as the detector array configuration of the 4th gen scanner? - ANSWERit had a rotate-stationary geometry and the gantry housed stationary circular rings of detectors and the x-ray tube would rotate around the patient electron beam computed tomography (EBCT or EBT) - ANSWERits configuration is devoid of moving parts and uses an adaptation of the 4th gen scanner known as stationary-stationary geometry which means that both the x-ray and source and detectors are fixed during scanning does EBCT have an x-ray tube? - ANSWERNo, a fan beam of x-radiation is produced by directing a beam of electrons toward stationary target rings of tungsten; each curved tungsten target emits radiation when bombarded by the electron beam. without moving parts, what can the EBCT accomplish? - ANSWERit is capable of extremely short exposure times (50msec or less); it is also ideal for cardiac imaging because of its high speed
uniform matrix array - ANSWERutilizes multiple detectors in longitudinal direction, each the same length ex: 4-slice MDCT system employs 16 detector elements , each 1.25mm adaptive arrays - ANSWERdetectors are configured with the thinnest widths at the center, surrounded by detectors of incrementally increasing widths along the z-axis hybrid array - ANSWERconsists of two detector sizes, the narrower detectors are positioned midline flanked by the wider detectors true/false , the quantity of data channels controls the number of sections the scanner can simultaneously acquire with each gantry rotation - ANSWERTRUE; a 16 slice MDCT has 16 channels at its disposal , so the maximum number of sections that can be constructed from each rotation is 16 true /false , the number of detectors in the array is less than the number of data channels - ANSWERfalse, it is equal to or greater a 16 slice MDCT can have 24 detectors binning - ANSWERthe electronic combination of signal from adjacent detectors to form a reconstructed slice that is thicker than the individual detector width how did the tube move for a 1st generation scanner? - ANSWERafter each translation the tube moves a an arc of 1 deg so more projections can be acquired, this repeated over and over for a 180 deg arc around the pt.
projection - ANSWERa series of measurements that is collected as the tube translates across the patient aka intensity profile what type of body parts could we scan with 2nd generation scanners? - ANSWERhead and body what type of beam did 2nd gen scanners use? - ANSWERfan beam aka Rectilinear multiple pencil beam scanning what was a pro of fan beam - ANSWERincreased speed of date acquisition what was a con of fan beam? - ANSWERincreased scatter radiation how many detectors did the 2nd gen scanner use? - ANSWER 5 - 30 detectors (helped reduce scan time) what sort of mtion did the 2nd gen scanner use? - ANSWERtranslate-rotate motion, but the rotation is typically 5 deg or more instead of 1 deg; a 180 deg arc of data is collected what was the scan time of 2nd gen scanners? - ANSWER20 secs to 3. minutes what sort of beam did a 3rd gen used? - ANSWERa continuously rotating fan beam that could angle from 30-40 deg ( rotate-rotate motion)
what is the scan time for a 4th gen scanner? - ANSWER0.8- 1 sec what were 5th generation scanners called? - ANSWERhigh speed scanners, they could acquire data in msec what were the 2 types of high speed scanners? - ANSWER1) EBCT ( ultrafast - Imatron and Evolution- Siemens)
- Dynamic Spatial reconstructor what made the 5th gen tube unique? - ANSWERit had not x-ray tube, it had an electron gun that generated a 130 kV beam how does a electron gun work? - ANSWERelectrons are accelerated , focused and deflected by electromagnetic coils to strike a tungsten target , there are 4 tracks - means 4 contiguous images simultaneously what is the configuration of the detectors on the 5the gen? - ANSWERthe bottom half of the circle are tungsten targets (210deg), the upper half are the detectors (216 deg arc of detectors)
- the detector array has 2 separate rings (1st- 864 detectors and 2nd- 432 detectors) is there any rotation in 5th generation scanners? - ANSWERNO! there is no mechanical movement of the tube or the detectors
what is the EBCT scanner good for? - ANSWERit has fast acquisition time so it can image the heart at 50-100msec; the goal is to produce high resolution images of moving organs that are free of artifact do EBCT scanner have heat dissipation issues? - ANSWERNo What kind of scanner is the 6th gen scanner? - ANSWERit is a dual source CT scanner, it is basically (2) 3rd generation tubes packed into one ; it was developed to eliminate rotation time of the x-ray tube and detectors to reduce artifacts what organ is the 6th generation tube ideal for? - ANSWERcardiac imaging how are the x-ray tubes and detectors placed inside the gantry? - ANSWERthey are offset by 90 deg. (each tube is perpendicular to its detector ) what are 7th generation scanners called? - ANSWERflat panel CT scanners, they are similar to DR but they were only prototypes ; in this generation, the x-ray tube and the flat panel detector( amorphous silicon thin film transistor) are coupled what are the advantages of 7th generation scanners? - ANSWERexcellent spatial resolution; great for CTA what is the disadvantage of 7th gen scanners? - ANSWERlacks good contrast resolution
