Exam (elaborations)
CT Physics and Instrumentation Questions and Answers 2024
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CT Physics and Instrumentation
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CT Physics and InstrumentationThe speed at which the CT data can be acquired. This speed is particularly important to
reduce or eliminate artifacts that result from object motion, such as those commonly
seen when imaging the heart. - answer temporal resolution
indicates width - answerX
indicates height - answerY
The thickness of a plane/determines the thickness of the slices: - answer Z axis
What is the most common matrix size? - answer512 (262,144 total pixels)
What quantifies the degree that a structure attenuates an x-ray beam? - answer
Hounsfield Units
True or False: the Hounsfield unit value is indirectly related to the linear attenuation
coefficient. - answerFalse: directly related, i.e., 1 HU equals a 0.1% difference between
the linear attenuation coefficient of the tissue as compared with the linear attenuation
coefficient of water.
Name 3 factors that contribute to an inaccurate Hounsfield measurement: - answer1.
poor equipment calibration
2. image artifacts
3. volume averaging
What artifact is the result of preferential absorption of the low energy photons, which
leaves higher-intensity photons to strike the detector array? - answerbeam hardening
What artifact appears as dark streaks or vague areas of decreased density - sometimes
called cupping artifacts? - answerbeam hardening
The process in CT by which different tissue attenuation values are averaged to produce
one less accurate pixel reading is called: - answervolume averaging
What determines pixel size? - answerthe matrix size and the FOV selected for display
The reconstruction that is automatically produced during scanning is often called
__________ reconstruction. The same raw data may be used later to generate new
images. This process is referred to as __________ reconstruction. - answerprospective
retrospective
,What plane is preferred for scanning the pituitary gland? Why? - answercoronal; in the
axial plane the number of streak artifacts and the partial volume effect are greater than
in the coronal plane
The _________ produces high voltage (or kV) and transmits it to the x-ray tube. -
answergenerator
Break down the CT process into three main segments: - answer1. data acquisition ->
get data
2. image reconstruction -> use data
3. image display -> display data
The range of the aperture size of a CT gantry is typically __ - __ cm. - answer70-90
The CT gantry can be tilted either forward or backward as needed to accommodate a
variety of patients and examination protocols. The degree of tilt varies among systems,
but __ to __ is usual. - answer0-30 degrees (some 0-15 degrees)
What component of the gantry permits the frame to rotate continuously, making helical
scan modes possible? - answerslip rings
The power capacity of the generator is listed in: - answerkilowatts (kW)
What is usually used as the CT anode target material? - answerTungsten (atomic # =
74; produces high intensity beam)
True or False: the intensity of the x-ray production is approximately proportional to the
atomic number of the target material. - answerTrue
What are the common focal spot sizes of a CT scanner? - answersmall: 0.5 mm
large: 1.0 mm
True or False: filtering the x-ray beam helps to reduce the radiation dose to the patient
and improves image quality. - answerTrue
Body-scanning filters or ___-___ filters are used to reduce the beam intensity at the
periphery of the beam, corresponding to the thinner areas of a patient's anatomy. -
answerbow-tie
What affects pt dose and determines how the dose is distributed across the slice
thickness? - answersource collimator
Scanners vary in the choices of slice thickness available, typical choices range from _._
- __ mm. - answer0.5 - 10 mm
, What is the primary functions of the predetector collimators? - answer- ensure beam is
proper width as it enters the detector
- prevent scatter radiation from reaching detector
What determines the size of the fan beam, which, in turn, determines the number of
detector elements that collect data? - answerThe scan field of view
What are the four optimal characteristics of a CT detector? - answer1. high detector
efficiency
2. low or no afterglow
3. high scatter suppression
4. high stability
Describe high detector efficiency: - answerthe ability of the detector to capture
transmitted photons and change them to electronic signals
_______ is a brief, persistent flash of scintillation that must be taken into account and
subtracted before image reconstruction in CT and is considered a disadvantage to
detector functionality. - answerAfterglow
What is considered high stability for a CT detector? - answerhigh stability allows the
system to be used without interruption of frequent calibration
Overall CT detector efficiency is a product of what five factors? - answer1. stopping
power of the detector material
2. scintillator efficiency (in solid-state types)
3. charge collection efficiency (in xenon types)
4. geometric efficiency, defined as the amount of space occupied by the detector
collimator plates relative to the surface area of the detector
5. scatter rejection
What is the ability with which the detector obtains photons that have passed through the
pt? - answercapture efficiency
What refers to the number of photons absorbed by the detector and is dependent on the
physical properties of the detector face (e.g. thickness, material)? - answerabsorption
efficiency
What is the time required for the signal from the detector to return to zero after
stimulation of the detector by x-ray radiation so that it is ready to detect another x-ray
event? - answerresponse time
What is the ratio of the maximum signal measured to the minimum signal the detectors
can measure? - answerdynamic range
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