CT Flashcards
What “generation” are most CT scanners?
3rd generation - means the x-ray tube and the detectors spin around the patient in synchrony.
What is a 3rd generation CT scanner?
The x-ray tube and the detectors spin around the patient in synchrony.
Under what conditions are CT tubes designed to operate?
Reasonable voltages (between 80-140 kV) Very high tube currents - up to 1000 mA.
Difference between regular diagnostic x-rays and CT for mA, kVp, and focal spot
Regular Dx:
mA: 200-800
kVp: 50-120
Focal spot size: 1-1.2 mm
CT:
mA HIGH up to 1000
kVp: 80-120
Focal spot size: 0.6-1.2 mm
What filter mechanisms are used in CT?
Cu or Al (6 mm)- filter the x-ray beam.
Heavily filtered beam can have a half value thickness of up to 10 mm Al.
Bow Tie filters: compensate for uneven attenuation of the beam by the patient- attenuate less in the center and more on the edges - made of low Z materials - like Teflon (to reduce hardening differences).
What is a Bow-tie filter?
Compensate for uneven attenuation of the beam by the patient- attenuate less in the center and more on the edges - made of low Z materials - like Teflon (to reduce hardening differences).
Compensate for uneven filtration
Reduce scatter
Reduce dose
How is the x-ray tube anode-cathode axis positioned in CT?
Perpendicular to the imaging plane to reduce heel effect.
What is used for scatter reduction in CT?
Collimator is used at both the x-ray tube as well as the detector - shapes the x-ray beam - “defines the section thickness on a single slice.” - the collimator also reduces some scatter.
Use “anti-scatter septa” for additional scatter reduction
What defines the section thickness on a single slice?
Collimator that shapes the x-ray beam and reduces some scatter.
What does “filtered” in filtered back projection mean?
Sharpening of the projection data prior to back projection
Modern set ups use an “iterative reconstruction” math program that allows more noise, so you can have less dose at the same diagnostic quality.
What does multislice mean?
The numbers of detectors in the Z direction (head to toe) determines the number of slices that can be simultaneously acquired.
The MDCT can acquire images with “isotropic resolution” - can do non-axial reconstructions w/o stretching pixels.
What does “isotropic resolution” mean?
The MDCT can acquire images so that we can do non-axial reconstructions w/o stretching pixels.
What determines minimal slice thickness in modern CT?
Detector element aperture width.
How do modern scanners adjust dose?
Scout image- data in the scout is used to estimate density and adjust accordingly.
Continuous modulation as the scan happens
What is a “ray”?
A measure of total x-ray attenuation along a line from the focal point to a single detector
What is “projection”?
All rays at a given angle of the x-ray tube.
It’s a series of rays that pass through the patient - at the same orientation.
What is a “sinogram”?
Bunch of squiggly lines that represent the data from all the projections of all the tube angles (0-360)
What kind of x-rays are used with CT?
Highly filtered, high kV (average energy 75 keV).
What is the matrix size for CT? Each pixel is?
The matrix is 512 x 512 with each pixel representing 4096 possible shades of gray (12 bits)
2^12 = 4096
What is the relationship between pixel width and height to voxels?
They are the same. Pixel W x H = Voxel W x H.
The difference is a voxel has a 3rd dimension (depth), which represents the slice thickness.
A voxel is a cube, a pixel is a square.
How do you calculate pixel size?
FOV / Matrix Size
How do you improve spatial resolution?
Make the pixels smaller (matrix larger)
Pixel size = FOV / Matrix
Is mAs the same on CT as it is on plain film?
The conventional definition of radiographic mAs is not useful in spiral CT.
You have to use “effective mAs” - tube current (mA) x length of time that a given point in the patient is in the beam - exposure time.
Exposure time is going to be related to the collimated beam width and table speed.
What happens to your images if you turn down the mAs?
Images get noisier.