Chapter 4 ( Physics) Flashcards
describe the general process of CT imaging
Data acquisition
Data Reconstruction
Multidimensional Image display
image archival and communication
azimuth
the angle of the tube and detectors in relationship to the patient position during scout acquisition
What is housed in the CT gantry
the majority of the mechanical components including the generator, x-ray tube, assorted collimators, the data acquisition system (DAS), slip-rings, and the detectors
focal spot size range
0.5 to 1.2mm
flying focal spot technology
involves the electromagnetic steering of the electron beam emitted from the cathode. The beam is directed towards two seperate locations on the anode resulting in two sources of x-radiation
Photon fluence
quantity of x-radiation
photon flux
the rate at which the photon fluence passes through a unit area over unit time.
effective mAs
calculated mAs value per acquired slice. effective mAs= mAs/Pitch
streak artifact
occurs when x-rays pass through very dense parts. example:posterior fossa during head imaging
kVp range of CT tube
80-140
dual energy CT systems
as the single x-ray tube rotates around the patient, the kVp is varied at a very high rate.
dual source CT systems
systems consisting of two completely seperate x-ray tubes and detector arrays positioned 90 Degrees from each other.
beam hardening
as the beam traverses the patient, low energy photons are absorbed first increasing the average intensity of the beam as it travels along its path. This change in beam quality can have an artifactual result on the image.
What type of generators are used?
high frequency, power output = 60-100 kW
two distinct components of Multi Slice C T (MSCT)
beam collimation
detector collimation
beam collimation
restricts the beam to the are of interest. reduces dose, improves quality.
section width
also called the slice, directly controlled by beam collimation
section interval
describes the spacing between two adjacent CT images. Measure by the distance from the center of one section to the center of the next
contiguous images
acquired with equal section thickness and interval.. example: 5X5
overlapping images
section interval is less than section width. 1mm X 0.5mm =50% overlap
detector pitch formula
pitch= table feed per rotation/section width
beam pitch formula
beam pitch=table feed per rotation/total collimation
total collimation formula
total collimation= No. of sections X section width
what type of ionization chambers were primarily used for third-generation scanners?
gas
gas ionization chambers
detectors contain small high-pressure chambers of xenon gas. X-rays cause ionize the xenon gas and cause the chambers to discharge small electrical signals.
modern MSCT system detectors
solid state detectors consisting primarily of a scintillating crystal material. Crystal emits a quantity of light energy proportional to incident x-rays. light is measured by a photodiode device and converted to an electronic signal.
Crystal materials
cesium iodide, cadmium tungstate, bismuth germanate, rare earth materials…etc
First generation CT scanner
- prototypical head only CT scanner
- developed by Dir Godfrey Hounsefield
- put to clinical use in 1972
- two detectors positioned adjacently along z-axis
- pencil beam of x-rays
- translate-rotate
Second Generation CT scanner
- translate-rotate
- increase in number of detectors
- first use of fan beam to expose a wider detector array
- expanded clinical capabilities beyond head imaging
Third Generation CT scanner
- expanded curvilinear array of detectors rotates with the x-ray tube around the patient
- rotate-rotate geometry eliminating the need for translation
- larger fan beam used to expose the expanded detector array
Fourth Generation CT scanner
- co-evolved with third generation systems and based upon a rotate-stationary geometry.
- the gantry houses a stationary, circular ring of detectors and the x-ray tube rotates around the patient.
Uniform Matrix Array detector
utilizes multiple detectors in the longitudinal direction, each the same length.
adaptive-array detector
detectors configured with the thinnest widths at the center, surrounded by detectors of incrementally increasing widths along the z-axis
hybrid array detector
consists of two detector size. The narrower detectors are positioned midline, flanked by the wider detectors.
Cine CT acquisition
involves multiple axial scans obtained at a single anatomic level over a predetermined period. Example, perfusions and also bolus tracking.
most widely accepted standard for detector configuration ?
64-slice MSCT systems
Data Acquisition System (DAS)
responsible for measuring the transmitted x-radiation absorbed by the detectors.
parallel processing
the ability of the computer system to perform multiple tasks simultaneously.
