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Flashcards in Notes From 3rd Weeks Readings Deck (75)
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-houses many of the components necessary to produce and detect x-rays
-components are mounted on a rotating scan frame


Slip rings permit the gantry frame to rotate _____, making _____ scan modes possible

Continuously, helical


Small focal spots in CT tubes produce _____ images (better spatial resolution)

Sharper images
-because they concentrate heat onto a smaller portion of the anode they cannot tolerate as much heat


High ___ is used to increase the intensity of the beam

-increasing its penetrating ability and thereby reducing patient dose
-high kV settings also help to reduce the heat load on the x-ray tube by allowing a lower mA setting


Filtering the x-ray beam helps to

-reduce radiation dose to the patient
-improves image quality (by reducing image artifacts that result from beam hardening)



-restrict x-ray beam to specific area, thereby reducing scatter radiation
-control the slice thickness by narrowing or widening the X-ray beam


reducing scatter improves _____ and decreases patient dose

contrast resolution


Source collimator aka prepatient collimation

-located near x-ray source and limits the amount of x-ray emerging to thin ribbons
-acts on x-ray beam before it passes through the patient
-affects patient dose and determines how the dose is distributed across the slice thickness
-resembles small shutters with an opening that adjusts, dependent on the operators selection of slice thickness


Pre detector collimation aka postpatient collimation

-located below the patient and above the detector array
-shapes the beam after it has passed through the patient
-ensures the beam is the proper width as it enters the detector
-prevent scatter radiation from reaching the detector


Scan field of view

-determines the size of the fan beam, which in turn, determines the number of detector elements that collect data


Capture efficiency

Ability with which the detector obtains photons that have passed through the patient


Absorption efficiency

The number of photons absorbed by the detector and is dependent on the physical properties of the detector face (ex thickness, material)


Response time

The time required for the signal from the detector to return to zero after stimulation of the detector by x-radiation so that it is ready to detect another x-ray event


Dynamic range

The ratio of the maximum signal measured to the minimum signal the detectors can measure


What is the most common material a detector is made out of?

Solid-state crystal variety


Solid state crystal detector aka scintillation detectors

-they used a crystal that fluoresces when struck by an x-ray photon
-a photodiode is attached to the crystal and transforms the light energy into electrical (analog) energy
-have high atomic numbers and density than gases, they have higher absorption characteristics
-absorb nearly 100% of the photons that reach them


Detector spacing

-detectors are separated using spacing bars
-this allows detectors to be placed in an arc or a circle
-measured from the middle of one detector to the middle of the neighbouring detector and accounts for the spacing bar
-ideally all detectors should be placed together as close as possible, so all x-rays are converted to data


A small detector is important for

-good spatial resolution
-scatter rejection


Characteristics of solid state crystal detectors

-high photon absorption
-sensitive to temperature, moisture
-solid material
-can exhibit afterglow
-no front window loss


Characteristics of pressurized xenon gas detectors (not used in newer models for CT)

-moderate photon absorption
-highly stable
-low-density material (gas)
-no afterglow
-losses attributable to front windows and the spaces taken up by the plates



Refers to the steadiness of the detector response time
-if the system is not stable, frequent calibrations are required to render the signals useful


Response time

Refers to the speed with which the detector can detect an x-ray event and recover to detect another event
-response times should be very short


Dynamic range

"Ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated"



The persistence of the image even after the radiation has been turned off
-CT detectors should have very low afterglow values


True/ false: MSCT scanners use gas ionization detectors

-because they have low quantum detection efficiency and low x-ray absorption


The detector elements of MSCT scanners use _______ materials

Solid-state materials


MSCT scanner should have what properties

-large dynamic range
-high quantum absorption efficiency
-high luminescence efficiency
-good geometric efficiency
-small after glow
-high precision machinability
-all detector elements must have a uniform response


What are the three steps for creating a CT image?

1. Data acquisition
2. Image reconstruction
3. Image display
-image post processing and image storage


Data acquisition

Refers to the method by which the patient is scanned to provide us with enough information to construct an image



Defined by the beam geometry used (size, shape, and motion of the beam and its path during the scan)
-the beam is shaped by special filters as it leaves the tube
-the beam is collimated to pass only the slice of interest
-the beam is attenuated by the patient