CT Principles part 1 Flashcards
Name the steps that the xrays go through from generation to what is seen at the console
Xrays are produced by the tube, emitted, filtered, and collimated. Then xray penetration occurs (through the patient), passes through post patient collimators, striking detectors. The computer system then converts that to a digital signal that the computer processes.
What is the relationship between the number of bits of the ADC and it’s accuracy?
Direct - the more bits the higher the accuracy
What is the term that describes the reduction in intensity of the x-ray beam as it passes through the patient by either absorption or scatter?
Attenuation
When discussing Beam Quality - how do homogeneous and heterogeneous differ?
A homogeneous beam is composed of photons with the same amount of energy while a heterogeneous beam possesses varying energies
Name a term that describes a quantitative measurement of attenuation per cm of an absorber (ie. bone or soft tissue)
Linear Attenuation Coefficient
What two main factors affect the Linear Attenuation Coefficient?
The energy of the beam and the atomic number of the absorber
What is another name for ‘CT numbers’?
Hounsfield Units
What is the HU of water?
zero
What is the HU of average bone?
1000
What is the HU of fresh blood?
20-50
What is the HU of Air?
-1000
What is the HU of fat?
-100
What is meant by “beam hardening”
This is when low energy, long wavelength photos are absorbed as the xray beam passes through an object (such as a filter or the patient) leaving the remaining beam with a higher average energy or penetrating power.
A single measurement of the x-ray attenuation along a path between the source and a detector is know as:
a ray or single ray
What is a view?
A collection of rays from one viewing angle
What is the equation for determining the total number of data samples?
Data samples = # of detectors x # of samples at each detector
What is the formula to calculate volume coverage aka Z axis coverage?
pitch x collimation x scan time / gantry rotation time.
The reduction in intensity of the x-ray beam as it passes through the patient by either absorption or scatter of photons from the beam is termed:
Attenuation
A quantitative measurement of attenuation per cm of an absorber for example bone or soft tissue is called:
Linear attenuation coefficient
The absorption of low energy, long wavelength x-ray photons within the anatomical structure with higher energy photons continuing through the body with an increasing mean beam energy or penetrating power is referred to as:
Beam Hardening
The movement of the tube & detectors with x-ray transmission is termed:
scanning
As a result of the scanning process, a single measurement of the x-ray attenuation along a path between the source and detector is termed:
ray
A view generates a ______ which is an electric signal relative to the amount of attenuation that has occurred as the x-ray is transmitted through the patient.
Profile
The number of detectors multiplied by the number of samples at each detector result in the:
Data Sample Total
What year was helical (spiral) scanning introduced? [think Taylor Swift]
1989
Gaps between slices, inter scan delay, variability of contrast media enhancement, inconsistent respiration or motion, and inaccurate 3D or reformations are all limitations of what kind of scanning?
Axial or step-and-shoot
What is the term for the geometric corrections that take place to convert the fan beam information to parallel beam information?
interpolation
What are three types of interpolation that are used in CT?
- Fan beam to parallel beam interpolation 2. helical to single slice interpolation 3. helical to multislice interpolation
Increasing pitch ______ coverage
increases
Increasing pitch ______ overall scan time
reduces
Increasing pitch ______ patient dose
reduces
Increasing pitch ______ resolution in the slice (Z) direction.
reduces
What is the definition of pitch in a single slice helical scan
PITCH = TABLE TRAVEL PER GANTRY ROTATION DIVIDED BY BEAM COLLIMATION
What is the definition of pitch in a multislice helical scan?
Pitch - table travel per gantry rotation divided by the number of channels times the detector size
How is pitch calculated differently in single slice CT vs MDCT?
Single slice is divided by the beam collimation where in MDCT you must divide by the (number of channels x detector size)
How do you calculate Z axis coverage?
pitch x section thickness x total scan time
What could be one negative consequence of reducing you scan time by increasing pitch?
You might miss small lesions or fractures.
Gapless examination of Anatomy, rapid volume scanning, reduced exam time, reduced motion artifacts, reconstruction of arbitrary slices, no mis-registration due to respiration level changes, less contrast required, uniform contrast enhancement, better multiplanar and 3D images, allows CT fluoro, CT angio, CT endoscopy, 3DCT ARE ALL ADVANTAGES OF WHAT KIND OF SCANNING (helical or axial)?
Helical
After x-rays are emitted from the CT x-ray tube, they pass through a ______ to shape the energy of the beam.
Beam filter
Slice thickness is impaced by (a. collimator system b. beam filter system c. DAC d. ADC)
a. collimator system
After the x-ray beam passes through the patient it is directed through the ______.
detector collimator
The x-ray beam is picked up by the _____ device that measures the amount of attenuation that occurs.
detector
