Computer Tomography Flashcards
What EMR is used in CT?
x-ray
What is a gantry?
The gantry of a computed tomography scanner (CT) is a ring or cylinder, into which a patient is placed. The x-ray tube and x-ray detector spin rapidly in the gantry, as the patient is moved in and out of the gantry.
What is a voxel?
Voxel is the 3-D analogue of a pixel. Voxel size is related to both the pixel size and slice thickness
What is an isolcenter?
Isocenter is termed as the point in space through which the central rays of the radiation beams pass.
What is tomography?
A radiologic technique for obtaining clear X-ray images of deep internal structures by focusing on a specific plane within the body
what is the aperture?
Aperture is where the patient is (the hole)
What are the acquisition modes?
Helical
Axial
Factors of helical mode
- Patient moves constantly through gantry
- X-rays on constantly
- Path of X-ray tube looks like a “helix” or “spiral”
Higher dose, some parts overlap, however overlapping data makes better image
Factors of axial mode
- “Step and Shoot”
- Patient moves in increments.
- X-rays only one for one slice
- Patient then moves through to the next slice location.
CT x-ray tube design
Requires high power output
Normally high kV techniques
Has high demand on power supply
Scanners normally have big generators/transformers that sit within or outside of the gantry.
Heat management
CT detector array info?
Formed of multiple smaller detector elements
Each element is very small <1mm.
Arranged in rows
More rows allow more images per rotation.
How do CT detectors affect spatial resolution?
What is spatial resolution, the ability to distinguish between to different points adjacent to one another. More detectors and the smaller the detectors the better the spatial resolution, therefore smaller lesions can be seen.
How do the detectors work?
Most current scanners use detectors made of a scintillating material rather than an old style ionisation chamber/scintillator. The material produces light energy when an X-ray photon strikes it. This light energy is measured withing each individual detector element. It is then amplified by a photodiode. The amplified signal is then converted to a digital signal by an analogue/digital converter (ADC).
What makes a good detector material?
High detector efficiency
Short afterglow
High resolution
How does Iterative Reconstruction work?
Uses “iterations” of processing to achieve adequate images.
-Compares initial input data from scanner to a projected raw data volume.
Measures projected raw against model/statistical model.
If comparison is not good enough then reprocesses and re-compares
Does this as many times as it needs to.
Once the Comparison reaches a match, it produces the final image.
IR is able to remove image noise leading to a significant and direct reduction in radiation dose needed to produce a diagnostic image vs FBP.
