Forming CT Images Flashcards
What do you need to measure in order to produce a cross-sectional image?
Attenuation information through many projections
What are the three scanning methods used in CT?
Localizer
Conventional (Serial)
Helical
When would a localizer image (single projection image) be used?
To position the slices at the location where we want to collect the cross-sectional images
Which of the scanning methods used in CT produces cross-sectional images?
Conventional & Helical Scan
What is another term used for helical scans?
Spiral or Volumetric CT
What views may be collected in a localizer scan?
Lateral
Anterior/Posterior
Oblique
When would an AP localizer image be collected?
To specify transverse slices
When would a lateral localizer image be collected?
To specify oblique slices when the gantry is angle
What is the length of anatomical coverage of the localizer image dependent upon?
Speed of the patient table and amount of time the x-ray tube is on
What are the reasons as to why helical CT is most often used in CT studies performed today?
Speed
Ease of use
Ability to reconstruct images
Conventional CT yields the best image quality which is free from:
Helical artifacts
What are some reasons as to why conventional CT scans may be chosen over helical CT?
Neurological work involving fine detail
Obese patients
Cardiac calcium scoring
When performing a conventional scan on a single-row detector scanner, how is raw data obtained?
By a single rotation of the gantry from only one cross-sectional slice location
Why does a helical scan’s x-ray beam take on a helical path?
The patient table is moved through the gantry at the same time that the x-ray tube rotates around the patient’s body
Why would a patient be required to refrain from breathing in CT?
To minimize motion artifacts
To avoid skipping anatomy
What are some advantages of helical scanning?
Faster studies More coverage in a breath-hold No missed anatomy Less contrast injection needed Arbitrary slice positioning Ideal data for post-processing
In a helical scan, how is the path of the x-ray beam defined?
Thickness of the x-ray bream
Speed of the patient table
How is the thickness of the x-ray beam determined on a single-row detector scanner?
It equals the slice thickness of the final image
How is the thickness of the x-ray bream determined on a multi-row detector scanner?
It is divided by the number of rows of detectors being used
In which scanning method used in CT does not collect images in a single slice plane?
Helical
What must be done to the attenuation information in a helical scan?
It must be divided into raw data files for individual, planar slices
What are sharp reconstruction filters often used for?
Visualizing musculoskeletal detail
What are smooth reconstruction filters often used for?
Visualizing soft tissue contrast
On what kind of scanners would back projection be used?
Single-row detector
Multi-row detector
Why would retrospective reconstruction be commonly performed?
To apply a different reconstruction filter
Why might retrospective reconstruction be performed on MDCT scanners?
To vary the reconstructed slice thickness
Multiplanar reconstruction requires only the _____ data.
Image
3D surface rendering is performed on the _____ data.
Image
In which orientation would we position the scanner if we want to angle the slice away from the transverse plane and toward the coronal plane?
Lateral
What is used to store the actual attenuation data measured by the detectors prior to image reconstruction?
Raw data file
What is another term used for reconstruction filter?
“Kernel”
On scanners that are capable of collecting 64 slices in one rotation, which reconstruction is used?
Cone beam
On scanners that do not collect more than four slices per rotation, which reconstruction method is best to be used?
Back-projection
When performing retrospective reconstruction, what kind of data is needed?
Raw data
On a single-row detector, how is the slice thickness determined?
By the collimation of the x-ray beam
Which reconstruction process is used to reconstruct images in orientations other than the one in which they were acquired?
Multiplanar reconstruction
Overlapping slices with no gaps in-between generates the most diagnostic _____ reconstruction
Multi-planar
Which reconstruction method allows us to reconstruct images using different parameters than those that were originally reconstructed?
Retrospective reconstruction
Which reconstruction process is used to reconstruct cross-sectional CT images from the raw data?
Back-projection
