Data Acquisition Flashcards
What are the 3 methods of data acquisition?
- Preliminary/localiser scanning
- Axial/step-and-shoot scanning
- Helical/spiral scanning
What is a localiser scan?
Digital imaging used as a anatomical guide/map to plan the actual scans.
NOT cross sectional and can be either AP or lateral
How are localiser scans done?
Patient moves and tube does not, then the DFOV is chosen
What is DFOV?
Display Field Of View = box that hold relevant and necessary anatomy that will be what is included on the rest of scans
What is the X axis of table movements?
Left and right
If error: patient is not laying in the centre of the table
What is the Y axis of table movements?
Up and down or table height
If error: table/patient not centred midcoronal by tech
What is the z axis of table movements?
Depth of table into gantry
If error: centring incorrect (either too superior or distal)
What is step and shoot scanning?
Table moves to the desired location and takes a single axial slice (may not be done if not necessary)
MUST SUSPEND RESPIRATION!!
What are advantages of step and shoot scanning?
- Better image quality
- Contiguous or not
- Repeat an area at the same spot that needs to be redone (cine/dynamic method)
- Gapped imaging possible
What are disadvantages of step and shoot scanning?
- Increased scan time
- Not useful data for recons
- Cannot be done if patient cannot hold their breath
- Can cause mis regulation if the patient breathes differently for each breath hold
What is gapped imaging?
Skipping irrelevant areas or to especially focus on one region
Reduced patient dose
What is the largest allowable slice thickness?
10 mm aka must be smaller than detector width
What MDCT?
Multi Detector Computed Tomography
Parallel row scanners with a cone shaped beam source
What are advantages of MDCT?
- Produce multiple slices at once per 1 rotation
- Longer + faster z axis coverage
- Faster gantry rotation
- Less motion artefact
- Enhanced low contrast detectability
- Optimised iodinated contrast
What is dual source CT?
2 tubes and 2 corresponding detector array
First set: detector array covers entire scan FOV
Second set: detector limited to a more central and smaller FOV
What are advantages of dual source CT?
- Data acquisition time halved
- Dose not increased
- Dual energy and contrast differentiation possibilities
What is dual energy/spectral imaging CT?
Acquisition of data of 2 different kVp settings
What 3 things are required for helical scans?
- Continually rotating tube
- Constant x-ray output
- Uninterrupted table movement
What are advantages of helical scanning?
- Faster scan time
- Better image resolution
- Improved iodinated contrast concentration
- Less breathing and cardiac motion artefact
- Better 3-D reformation abilities
- Better diagnostic accuracy
What developments are needed in axial scans for helical scans?
- X-ray gantries with slip rings
- Increased tube cooling efficiency
- Higher x-ray output aka better mA abilities
- Smoother table movement
- Software to adjust for table motion
- Better raw data management
- Higher detector efficiency
What is helical interpolation?
Removes the tilt and blur from helical scans with statistical methodology to better resemble axial images
What is SDCT interpolation?
360LT or 180LT
What is MDCT interpolation and what are it’s effects on imaging?
180LT - corrects better to produce thinner slices, but creates more noise
360LT - worse for accuracy in slice thickness, better resolution
What should pitch be in between?
1 - 1.5
What is SDCT?
Table speed and slice thickness directly related to helical scan process.
What does changing pitch effect?
Increasing pitch = higher slant
Decreasing pitch = less amount of anatomy covered per rotation (higher patient dose)
<1, overlapping slices
>1.5, less sharp
What 3 qualities are important to image quality?
- Spatial resolution
- Contrast resolution
- Temporal resolution
What is spatial resolution?
Ability to distinguish between objects/structures that differ in density
Aka high contrast resolution
How is spatial resolution assessed?
Line pairs per mm phantom device
Modulation transfer function (MTF)
What is contrast resolution?
Ability to distinguish small changes in densities
Aka low contrast resolution or sensitivity
What is temporal resolution?
Time needed to acquire the data to generate an image
How do you improve spatial resolution?
- Smaller pixel size
- Thinner slices
- Bone algorithm
- mAs vs noise
How to improve contrast resolution?
- Larger pixel size
- Thicker slices
- Soft tissue algorithm
- mAs vs noise
How do you find the window range?
Divide window width in half
Upper range = add half of width from window level
Lower range = subtract half of width from window level
